DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING - DOC by Veu92qx4

VIEWS: 144 PAGES: 72

									    MONITORING AND OPERATION OF 220kV/132kV
                              SUBSTATION
        A Project Report submitted in partial fulfillment of the requirements

                          For the award of the degree of




              BACHELOR OF TECHNOLOGY
                         IN
       ELECTRICAL AND ELECTRONICS ENGINEERING

                                      by
                  P.NAGARJUNA REDDY (07281A0241)

                 MOMD.ZUBAIR                      (07281A0211)

                 S.TILAK                          (07281A0258)

                     Under the esteemed guidance of
                     Mr.S VIGNESHWAR
                                     Asst.Professor

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
    KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE
              (Affiliated to J.N.T.U, Hyderabad)
                Singapur, Karimnagar-505468




                                           1
         KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE
                              (Recognized by A.I.C.T.E)
                         (Affiliated to J.N.T.U, Hyderabad)
                    Singapur, Huzurabad, Karimnagar-505468


                                    CERTIFICATE




This is to certify that this project work entitled “MONITORING AND
OPERATION OF 220kV/132kV SUBSTATION” Warangal, Mulugu road ” is
the bonafide work carried out by P.NAGARJUNA REDDY(07281A0241),
MOMD.ZUBAIR(07281A0211), S.TILAK(07281A0258) in partial fulfillment of
the requirements for the award of the degree of Bachelor of technology in
Electrical & Electronics Engineering         students of final year B.Tech., EEE
Engineering (2007-2011) in partial fulfillment of the requirements for the award
of the Degree of B.Tech. Of the JNTU, HYDERABAD



      PROJECT GUIDE                                       HEAD OF DEPARTMENT

       S.VIGNESHWAR                                          Y.Y PUNDLIK
          (Asst. Professor)                                   (Assoc. Professor)




                                         2
                                  ACKNOWLEDGEMENT



 We wish to express our sincere thanks to our Project Guide Mr. S. VIGNESHWAR for having
been a source of constant inspiration, valuable guidance and generous assistance throughout the
period of our project work under his guidance.

 We express our sincere gratitude to our, Sri YOGESH.Y.PUNDLIK, Head of dept of
ELECTRICAL & ELECTRONICS ENGINEERING,                       for having been a source of constant
inspiration, valuable guidance and generous assistance throughout the period of our project work.
We deem it as a privilege to have worked under his able guidance.

 We wish to express our sincere thanks to our Principal, Dr. K. SHANKAR, for providing the
college facilities for completion of the project.

 We wish to express our sincere thanks to Sri T.RAJESHWAR RAO(A.D,MRT), who granted
permission to perform this project under the guidance of P.SANDEEP(A.E,MRT) and also
Mrs. SAJINI(A.E,TL&SS), who took active part in completion of this project.

 We express our gratitude for the encouraging remarks and valuable guide lines to all the
members of the Project Evaluation Committee.

           Finally, we would like to thank all the faculty members, supporting staff of the
Department of EEE and friends for their cooperation and valuable help for completing this
project.




                                                    Project associates

                                                P.NAGARJUNA REDDY (07281A0241)

                                                MOMD.ZUBAIR (07281A0211)

                                                S.TILAK (07281A0258)



                                                    3
                                  CONTENTS
                                                                          Page No

ABSTRACT                                                                      1

CHAPTER-1 INTRODUCTION                                                        2

     1.1   What is substation                                                 2
     1.2   Classification of substations
     1.3   220kV/132kV/33kV substation in Warangal                            3
     1.4   Single line diagram of 220kV/132kV/33kV substation                 5

CHAPTER-2 MAJOR EQUIPMENTS USED IN SUBSTATION                                 6

     2.1   Transformer                                                        6
           2.1.1   Introduction                                               6
           2.1.2   Principle and constructional features of transformer       7
           2.1.3   Name details of transformer                                10
           2.1.4   Transformer accessories                                    13
           2.1.5   Dissolved gas analysis (DGA)                               16
           2.1.6   Norms of protection for transformer                        17
           2.1.7   Instrument transformer introduction                        18
           2.1.8   current transformer and its specifications                 18
           2.1.9   current voltage transformer                                22
    2.2    Energy meter                                                       23
           2.2.1   Introduction                                               23
           2.2.2   Procedure for energy meter testing                         24
           2.2.3energy meter testing results                                  27
    2.3    Auxiliary AC and DC power supply                                   28
    2.4    Battery room                                                       29
           2.4.1   battery charger                                            29
           2.4.2   Theorey of operation of float charger                      32
           2.4.3   Theorey operation boost charger                            33


                                               4
CHAPTER-3 TYPES OF BUSBAR CONNECTIONS                                 36

    3.1   Introduction                                                36
    3.2   Single bus                                                  36
    3.3   Double bus with double breaker                              37
    3.4   Inspection bus                                              38
    3.5   Double bus with single breaker                              39
    3.6   Ring bus                                                    40
    3.7   Breaker and a half bus                                      41

CHAPTER-4 PROTECTION EQUIPMENT USED IN SUBSTATION                     42

    4.1   Circuit breakers                                            43
          4.1.1   Introduction                                        43
          4.1.2   Operation of circuit breaker                        44
          4.1.3   Different techniques used to extinguishes the arc   45
          4.1.4   Arc interruption of circuit breaker                 46
          4.1.5   Air-blast circuit breakers                          47
          4.1.6   SF6 circuit breakers                                47
          4.1.7   Vacuum circuit breakers                             49
          4.1.8   Advantages of circuit SF6 circuit breaker           49
    4.2   Lightening arrester                                         50
    4.3   Wave trap                                                   52
    4.4   Protective relays                                           52
          4.4.1   Introduction                                        52
          4.4.2   The basic requirement s for the relay               53
          4.4.3   Distance relay principle                            54
          4.4.4   Types of distance relays                            54
          4.4.5   Applications of distances relays                    58
    4.5   Substation earthing                                         60




                                               5
CHAPTER-5 SUMMARY AND CONCLUSSIONS   62

    5.1   Summary                    62
    5.2   Conclusions




REFERENCES                           63




                             6
                                   ABSTRACT




       Now a day’s everything is depending up on the power. So give the reliable

supply to the consumers. In distribution systems one of the major parts is

"SUBSTATIONS".




     An electrical substation is a subsidiary station of an electricity, Generation,

Transmission and distribution systems where the voltage is transformed from high to

low or reverse using the transformers .Electric power may flow through several

substations between generating plant and consumer and may be changed in different

voltage levels .the equipment used in substation are Transformer, Lightening

arresters, isolator, bus bar, protective devices, Battery charger, earth switches, earth

rods. So for of supply the regular maintenance and checking is necessary from that

we conclude weather it is suitable or not for the desired operation.




                                                 7
                                            CHAPTER-1


                                      INTRODUCTION

1.1 What is substation?

              An electrical substation is a subsidiary station of an electricity generation,
       transmission and distribution system where voltage is transformed high to low or the
       reverse using transformers. Electric power may flow through several substations between
       generation plant and consumer, and may be changed in voltage in several steps.




       The main equipment used in substation are transformers lighting arresters, Circuit
breakers PLCC, isolators, bus bars, protective relays, Battery charger, earth switches, earth rods.




1.2 Classification of substations

Classification of substations based on

        (1) Service requirements
        (2) Constructional features
   1. According to service requirements: According to service requirements substations are
       classified into:
              i.   Transformer Substations
             ii.   Switching Substations
            iii.   Power factor correction substations
            iv.    Frequency changer substations
             v.    Converting substations
            vi.    Industrial substations




                                                 8
      2. According to construction features: According to constructional features substations
         are classified as;
              i.   Indoor substations
            ii.    Outdoor substations
            iii.   Underground substations
            iv.    Pole mounted substations




1.3      220/132/33 kv sub-station warangal




            There are four incoming feeders named as B.pad-2, RSS-1 (Nagaram), RSS-2.There are
six 132 KV incoming feeders connected to 132 KV bus. There are 33 KV out going feeders
connected 33KV bus. There are three 220/132 KV power transformers and two 132/33 power
transformer




         Lighting arrestors is placed at the starting point of every feeder to every phase of the line.
Capacitive voltage transformer (CVT) follows the lightning arresters .Wave trap is placed next to
the CVT for collecting communication signals at higher frequencies




          Earth switch is there to transfer the induced voltage on the line when the isolator is open.
Then there is an isolator to open the line under maintenance. There is a current transformer to
step down the currents to the lower values for the purpose of protection and metering .Next to
CT circuit breaker is placed which functions under fault counditions.Then there is a another
isolator.



                                                   9
       There is 220 KV bus with large size conductors. The bus is very important in the
substation. The incoming feeders are connected to it through the control devices. There is an
isolator followed by a high voltage 220 KV circuit breaker. Then 220 KV feeders are connected
to primary side of power transformer through an isolatar.There are three power transformers of
each 100 MVA, 220/132 KV Size.




       The output is taken from the LV side of the transformer and connected to 132 KV bus bar
through the protecting devices. From every protecting control device cables are connevted to the
control center where the continuous monitoring and controlling is done through and relay panel.




       Five outgoing feeders are taken out from the 132 KV bus and transmitted the power at
lower voltage to the 132/33 KV substations which are located nearby load center.




                                               10
11
                                      CHAPTER-2

                MAJOR EQUIPMENTS USED IN SUBSTATON



   2.1 TRANSFORMERS

   2.1.1 Transformer introduction

       Transformers are static piece of apparatus by means of which electric power in one
circuit is transformed to electric power of the same frequency in another circuit. It can raise or
lower the voltage in a circuit but with a corresponding decrease or increase in current.

Transformers employed in a substation are

1. Power transformers

2. Instrument transformers

a) Current transformers
b) Potential transformers


3. Station transformers




Power transformer



       Power transformers convert power level voltages from one level or phase configuration to
another. They can include features for electric isolation, power distribution, and control and
instrumentation applications. Transformers typically relay on the principle of magnetic induction
between coils to convert voltage and /or current level.




                                                12
2.1.2 Principle and constructional features of transformer




                       Fig 2.1.2.1 Basic principle of transformer


       In its simplest form, it consists of two inductive coils, which are electrically separeted but
magnetically linked through a path of low reluctance. The two coils possess high mural
inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up
in the laminated core and it produces mutually induced emf.If the second coil circuit is closed,
current flows in it and so electric energy is transferred from the first coil to the second coil. The
first coil in which electric energy is fed from AC supply mains, is called primary winding, while
the second coils is known as secondary winding.




       The necessity of the transformer arises when voltages are required to b changed. For
example, the generated voltage of the alternators will b around 15KV.It is not economical to
have transmission and distribution systems at this voltage as, for the same power transmitted, the
current will b more when compare to high voltage transmission i.e.as the transmission voltage
increases, the current is reduced and their by the conductor diameter can b reduced resulting in

                                                 13
saving of conductor material and weight of structures (supporting)and power losses during the
conductor(I^2R losses).


       The efficiency of the transformer is very high as there are no moving parts and so there
are no mechanical losses. The only losses in transformers are
(a) Copper losses in primary in secondary winding
(b) Iron losses due to eddy current and hysteresis losses in the magnetic core




Constructional features




       The simple elements of a transformer consist of two coils and a laminated steel core.
The two coils are insulated from each other and from the steel coil. Other parts are


A) Suitable container for the assembled core and windings
B) Suitable medium for insulating the core in windings from the container and
C) Suitable bushings for insulating and bring out the terminals of the winding from the tank.


       The transformers are of two general types: distinguished from each other by the manner
in which the primary and secondary coils are placed around the laminator steel core. They are


   a) Shell type                     b) core type
Steel surrounding the coil is shell type of construction:
Coil surrounding the steel is core form type of construction
In the simplified diagram for the core type transformer the primary and the secondary winding
interleaved to reduced leakage fluxes. This is half the primary and half the secondary winding
will place side by side or concentrically on each limb. Not primary on one limb and the
secondary on other limb.




                                                 14
       Core type transformer




                       Fig 2.1.2.2 construction core type transformer


       The windings of transformers are made of copper wire or strip heavy current capacity
requires conductors of large cross section. To reduce the eddy current losses with in the
conductors, several small wire of parallel strips are preferable to one large strip. The coils used
are from wound and cylindrical type. The general form of this coils May be circular are
rectangular. But for large size core type transformers, because of their mechanical strength. Such
cylindrical coils are wound in helical layers, with the different layers insulated from each other
by paper, cloth etc.


       Figure shows the general arrangement of these coils with the core and from each other.
Since the low voltage windings are easiest to insulate it placed nearest to the core


                                                 15
Shell type Transformers


         In this case, the coils are wound but are multilayer disc type. The different layers of such
multi layer discs are insulated from each other by paper. The complete winding consists of
stacked discs with insulation spaces between the coils with the spaces forming horizontal cooling
and insulation ducts. A shell type transformer may have a simple rectangular form as shown in
figure




                        2.1.2.3 Construction of shell type transformer




2.1.3 Name plate details of the transformer




         Make                                   : Bharat bijlee limited
         Connection                             : YNyno
         K V A Rating                          : 25/5o kva


                                                  16
   Cooling                                  : ONAN/OFAF
   No load                                  : 132/33 kv
   Phases HV/LV                             : 3/3
   Frequency                                : 50 Hz
   %impedance volts at75

   At tap minimum                           : 13.6%
   At tap normal                            : 13.183%

   At tap maximum                           : 12.7%

   Untanking mass                           : 34450 kg

   Mass/oil volume                          : 17545/20400kg

   Total mass                               : 70500kg

   Transformation mass                      : 58000kg

   Maximum temperature rise over an ambient of 50

   Oil                                      : 50

   Winding                                  : 55

   Insulation levels P>F/Impulse

   HV                                       : 230kv/550kvp

   Hv(N)                                    : 38kv/95kvp

   LV                                        : 70KV/170KVP

   Short circuit duration current            : 2 sec




 % Impedance volts: IT is the percentage of HV volts required to create full load amperes in a
   shorted LV winding .It is also indicative of percentage of LV volts to b applied so that full load
   currents flow in a shorted HV winding.



                                               17
 Max. Ambient temperature: 50degree C. This means the transformer is designed to work in good
    condition when the surrounding temperature of the transformers increase up to a maximum of
    50


 Winding temperature rise 50        : this means that the winding temperature can go up to 55       over
    and above the atmospheric temperature




 Vector group Y No stands for :

Y: primary winding star connected

N: availability of primary bush ring at top plate.

Y: small letter representing secondary star connected

n: small letter representing availability of secondary neutral bush ring at top plate

O: this letter is zero indicating that the angle of voltages between primary and secondary is zero




 Insulation levels: for the safety point of view insulation will b provided for more than the
    operating voltages of the transformer.


 Total mass: it is the total weight of the transformer including the active part, oil, tank,
    bushing, all the spares etc. This is for the purpose of cranes capacity and transport facility
    required in case the full transformer is to be lifted and or transported from place to place.




 Transport mass: normally transportation is done when active part is mounted in the tank
    but without oil, bushings, all the spares etc. This is for the purpose of cranes capacity and
    tan sport facility required in case the full transformer is to be lifted and or transported
    from place to place.


 Untanking mass: weight of part plus top plate


                                                 18
2.1.4 Transformer accessories


    Conservator tank


             Conservator with the variation of temperature there is corresponding variation in the
oil volume. To account for this, an expansion vessel called conservator is added to the
transformer with a connecting pipe to the main tank. In small transformers this vessel is open
to atmosphere through dehydrating breather (to keep the air dry). In large transformers, an air
bag is mounted inside the conservator with the inside of bag open to atmosphere through the
breathers and the outside surface of the bag in contact with the oil surface.




    Breather

   Both the transformer oil and cellulosic paper are highly hygroscopic. Paper being more than
the mineral oil the moisture, if not excluded from the oil surface in conservator, thus will find its
way finally into the paper insulation and causes reduction insulation strength of transformer. To
minimize this, the conservator is allowed to breathe only through silica gel column, which
absorbs the moisture in air before it enters the conservator air surface.




    Pressure relief device/expansion vent

   Transformers tank is a pressure vessel as the inside pressure can group steeply whenever
there is a fault in the windings and the surroundings oil is suddenly vaporized. Tanks as such are
tested for a pressure with stand capacity of 0.35kg-cm^2.to preventing bursting of the tank and
thus catastrophe, these tanks are in addition provided with expansion vents with a thin diaphragm
made of Bakelite /copper/glass at the end. In present day transformers, pressure relief devices are
replacing the expansion vents. These are similar to safety valves on boilers.




                                                 19
     Buchholz’s relay

    Whenever a fault in a transformer develops slowly, heat is produced locally, which begins to
decompose solid of liquid insulating materials and thus to produce inflammable gas and oil flow
this relay is applicable only to conservator type transformers. Buchholz relay is connected in the
pipe leading to the conservator tank and detect the gas produced in transformer tank.




     Temperature indicators

Most of the transformers are provided with indicators that display oil temperature and winding
temperature. Oil temperature is that of the top oil, where as the winding temperature
measurement is indirect. This is done by adding the temperature rise due to heat produced in a
heater coil. When a current proportional to that flowing in windings is passed in it to that or top
oil. For proper functioning or OTI & WTI it is essential to keep the thermometers pocket clean
and filled with oil.

OLTC or ON-LOAD/OFF LOAD TAP CHANGER

In a transformer

Vp/Vs=Np/Ns OR Vs=Vp/Np since Ns is constant

Vp=voltage in primary

Vs=voltage in secondary

Np= number of turns in primary

Ns= number of turns in secondary

        As Ns is constant, Vs is proportional to Vp/Np. Whenever there is a decrease in Vp, Np
should also be increased in order to maintain Vs at constant levels. The action of
decrease/increase in Np whenever there is decrease/increase in Vp is done by the OLTC.

        In view of handling live currents, on load tap changers are provided in the H.V. side only
since current in H.V. side are much smaller in values of those of L.V. side. All off load tap

                                                20
changers are provided on L.V. side considering economics of construction and operation since it
works at no load and no voltage conditions.

       For the purpose as practiced at present the number of turns in H.V. is provided with
105% winding where 100% is the number of H.V turns derived for the required voltage class of
transformation.




    Cooling of transformers

   Heat is produced in the windings due to current flowing in the conductor (I-R) and in the
core on account of eddy current hysteresis losses. In oil immersed transformers heat is dissipated
by thermo-syphon system action. The oil serves as the medium for transferring the heat produced
inside the transformer to the outside atmosphere. Thermo-syphon action refers to the circulating
current set up in a liquid because of temperature difference between one part of the container and
other. When oil becomes hot it becomes lighter and therefore rises up, drawing in its wake colder
oil from below. This rising current of oil takes the heat away from the oil surfaces to the top of
the tank; from there it passes down the radiator tubes where the heat is radiated out into the
atmosphere. As the oil gets cooled it becomes heavier and sinks to the bottom.




   As the size of the transformer becomes large, the rating of oil circulating by thermo-syphon
action becomes insufficient to dissipate all the heat produced and an artificial means of
increasing the circulation have to be adopted; namely forced oil circulation by electric pumps,
providing large radiators with forced air draft cooling by electric fans which are automatically
switched on and off depending upon the loading of transformer. In very large transformers
special coolers with water circulation may have to be employed.




                                               21
2.1.5 Dissolved gas analysis




        Transformer undergoes electrical, chemical and thermal stresses during its service life
which may result in slow evolving incipient faults inside the transformer. The gases generated
under abnormal electrical or thermal stress are hydrogen, methane, ethane, ethylene, acetylene,
carbon monoxide, carbon dioxide, nitrogen and oxygen which get dissolved in oil. Collectively
these gases are known as FAULT gases, which are routinely detected and quantified at extremely
low level, typically in parts per million in dissolved gas analysis (DGA). Most commonly
method used to determine the content of these gases in oil is using a vacuum gas extraction
apparatus/head space sampler and gas chromatograph.

        DGA is a powerful diagnostic technique for detection of slow evolving faults inside the
transformer by analyzing the gases generated during the fault which gets dissolved in the oil. For
DGA to be reliable, it is essential that sample taken for DGA should be representive of lot, no
dissolved gas to be lost during transportation and laboratory analysis be precise and accurate.

        DGA can identify deterioration of insulation oil and hotspots, partial discharge, and
arcing. The health of oil is reflective of the health of the transformer itself. DGA analysis helps
the user to identity the reasons for gas formation and materials involved and indicate urgency of
corrective action to be taken.

.

    Fault type                                    Key gases
    Arcing                                        Acetylene, hydrogen
    Corona                                        Hydrogen
    Over heated oil                               Ethylene, methane
    Over heated cellulose                         Carbon monoxide and dioxide



                       Table 2.1.5.1 dissolved gas analysis




                                                22
2.1.6   Norms of protection for power transformer




   Voltage ratio & capacity   HV Side                    LV Side               Common relays

   i.   132/33/11KV upto 8 3 O/L relays + 1 2 O/L relays + 1 Buchholz, OLTC
        MVA                   E/L relay                  E/L relay
                                                                               Buchholz, OT, WT

   ii. 132/33/11KV above 3 O/L relays + 1 3 O/L relays + 1 Differential,
        8 MVA and below dir. E/L relay                   E/L relay
                                                                               Buchholz, OLTC
        31.5 MVA
                                                                               Buchholz, OT, WT

   iii. 132/33KV, 31.5 MVA 3 O/L relays + 1 3 O/L relays + 1 Differential, Overflux,
        & above               dir. E/L relay             E/L relay
                                                                               Buchholz, OLTC

                                                                               PRV, OT, WT

   iv. 220/33 KV, 31.5MVA 3 O/L relays + 1 3 O/L relays + 1 Differential, Overflux,
        &            50MVA dir. E/L relay                dir. relay
                                                                               Buchholz, OLTC
        220/132KV,      100
        MVA                                                                    PRV, OT, WT

   v. 400/220KV 315MVA        3 directional O/L 3 directional O/L Differential, Overflux,
                              relays            (with relays           (with
                                                                               Buchholz, OLTC
                              dir.highset)               dir.highset)+1
                                                         directional      E/L PRV, OT, WT and overload
                              +1 directional E/L
                                                         relays. Restricted (alarm) relay
                              relays. Restricted
                                                         E/F relay
                              E/F relay

                              + 3 Directional
                              O/L      relays     for
                              action



                                                        23
  2.1.7 Instrument transformers introduction



   Instruments transformer is a device used to the current and voltage in the primary system to
values suitable for the necessary instruments, meters, protective relays etc. they also serve the
purpose of isolating the primary system from the secondary system.




  2.1.8 Current transformer and it’s specifications



   A current transformer (CT) is a measurement device designed to provide a current in its
secondary coil proportional to the current flowing in its primary. Current transformer are
commonly used in metering and protective relaying in the electrical power industry where they
facilitate the safe measurement of large currents, often in the presence of high voltages. The
current transformer safely isolates measurements of large currents, often in the presence of high
voltages. The current transformer safely isolates measurement and control circuitry from the high
voltages typically present on the circuit being measured.




Rated primary current:

       The value of current which is to be transformed to a lower value. In CT parlance, the load
of the CT refers to the primary current.




Rated secondary current:

       The current in the secondary circuit and on which the performance of the CT is based.
Typical values of secondary current are 1A or 5A. in the case of transformer differential
protection, secondary currents of 1/root 3A and 5/root 3 A are also specified.


                                                24
Rated burden:

       The apparent power of the secondary circuit in volt-amperes expressed at the rated
secondary current and at a specific power factor (0.8 for almost all standards)



Accuracy class:

       In the case of metering CT’s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that
the errors have to be within the limits specified in the standards for that particular accuracy class.
The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and
100% of the rated burden at the specified power factor. In the case of protection CTs the CTs
should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as
well as composite error at the accuracy limit factor of the CT.




                            Fig 2.1.8.1 Current transformer




       The rms value of the difference between the instantaneous primary current and the
instantaneous secondary current multiplied by the turns ratio, under steady state conditions.




                                                 25
Accuracy limit factor:

        The value of primary current up to which the CT compiles with composite error
requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to
be within specified limits at 5, 10 or 15 times the rated primary current.




Short time rating:

        The value of primary current that the CT should be able to withstand both thermally and
dynamically without damage to the windings, with the secondary circuit being short-circuited.
The specified time is usually 1 or 3 seconds.




Instrument security factor:

        This typically takes a value of less than 5 or less than 10 though it could be much higher
if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error
of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This
means that heavy currents on the primary are not passed on to the secondary circuit and
instruments are therefore protected. In case of double ratio CT’s, FS is applicable for the lowest
ratio only.




Class PS/X CT

        In balance systems of protection, CT’s with a high degree of similarity in their
characteristics are required. These requirements are met by Class PS (X) CT’s. Their
performance is defined in terms of a knee-point voltage, and the resistance of the CT secondary
winding corrected to 75C. Accuracy is defined in terms of the turn’s ratio.




                                                 26
Knee point voltage:

       That point on the magnetizing curve where an increase of 10% in the flux density
(voltage) causes an increase of 50% in the magnetizing force (current)




        Name plate details

            Company                  :      W.S industry
           Highest saturation
           Voltage                   :      524kv
           Basic insulation level    :      460/1050kv
           I th                      :       40/1kA/sec
           I dyn                     :      10KAp
           Oil weight                :      360KG
           Total weight              :      1280Kg
           Ratio                     :       800-600-400/1-1-1-1
           Core number               :      1         2     3      4     5
           Rated primary current     :       800A
           Secondary current (A)     :       1        1     1      1     1
           O/p at 400/1(VA)          :       -        -     -      -     30
           Accuracy class            :       ps       ps    ps     ps    0.5
           ISF/ALF                   :       -        -     -      -     ≤5
           Turns ratio               :      2/600-1200-800
           Rct at 75 deg C at800/1   :      6         6     6      6     -
           KPV at 800/1v             :      2600      2600 720     720   -
           LEXC at KPV at 800/1mA:          100       100   30     30




                                                 27
  2.1.9 Current voltage transformer


   A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down
extra high voltage signals and provide low voltage signals either for measurement or to operate a
protective relay. In its most basic form the device consists of three parts: two capacitors across
which the voltage signal is split, an inductive element used to tune the device to the supply
Frequency and a transformer used to isolate and further step-down the voltage for the
Instrumentation or protective relay.
The device has at least four terminals, a high-voltage terminal for connection to the high voltage
signal, a ground terminal and at least one set of secondary terminals for connection to the
instrumentation or protective relay. CVTs are typically Single-phase devices used for measuring
voltages in excess of one hundred kilovolts where the use of voltage transformers would be
uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors
connected in series. This results in a large voltage drop across the stack of capacitors that
replaced the first capacitor and a comparatively small voltage drop across the second capacitor,
C2, and hence the secondary terminals.



Name plate details

Company                                :W.S industry

Manufacturaing                         : 1996

Weight                                 : 665 kg

Total o/p simulteanious                : 2560VA

Max.output                             : 750VA at 50

Rated voltage A-N                      : 220/     kv

Highest saturation voltage             : 245/     kv

Insulation level                       : 460/1050

                                                  28
Frequency                              : 50 hz

Nominal intermediate voltage A-N : 20/

Voltage factor                          : 1.2 cont. 1.5/30 sec

HF capacitance                          : 440 pf 10% - 5 %

Primary capacitance                     : 4840 pf 10% -5%

Secondary capacitance                    : 48400 pf 10% - 5%

Voltage ratio                            : 220/         110 /    —110/

Winding                                : 1a-1n      2a1-2a2 -2n

Voltage                                   : 100/        kV       110-100/   kv

Burden                                    : 150VA        100VA

Class                                     : 0.5                  3p




 2.2 Energy meter

 2.2.1 Introduction

        Electricity meters operate by continuously measuring the instantaneous voltage (volts) and
current (amperes) and finding the product of these to give instantaneous electrical power (watts)
which is then integrated against time to give energy used (joules, kilowatt-hours etc.). Meters for
smaller services ( such as small residential customers) can be connected directly in-line between
source and customer. For larger loads, more than about 200 amps of load, current transformers
are used, so that the meter can be located other than in line with the service conductors. The
meters fall into two basic categories, electromechanical and electronic.


                                                   29
     2.2.2 Procedure for energy meter testing



1      All the equipment is set up on a meter test bench capable of handling 10 No. meters at one
         time.

2        Change over switches for power factor are provided in each phase.

3        For upf phase sequence in RYB. For 0.5 pf phase sequence is BRY.

4        The current coils of both R.B.S. meter and MUT, i.e. meter under test are connected in
         series, and the potential coil in parallel.

5        Performance of meter should be checked up at full load upf, full load 0.5 pt lag, 1/10 full
         load upf.

6        For long range meters, say 5.20 Amps, Ib i.e., basic current is 5A. and Imax i.e.
         maximum current is 20 Amps.

7        The meter should start at 0.375% rated current for cyclometer type register and 0.25% of
         rated Current for dial and pointer type registers.

8        For full load upf test, pressure coils are connected across rated supply voltage and rated
         full load current at upto passed through current coils.

9       The position of brake magnet is adjusted (for full load-upf test) to bring the meter speed
         within the required limit.

10       The permissible errors are +2%.

11       The required calculation for evaluating the error is as follows :

         For a 3 Phase 10 Amp

         Meter constant is 240 rev/KWh.

         RSS constant is 100 rev

         MUT constant/RSS = 240/100 =

                                                       30
     i.e. for every revolutions of MUT disc, RSS should record 2.5 revolutions.

     If this is so, then MUT has no error.




12   Revolutions in RSS can be read upto second decimal place.

13   There is a red mark on MUT’s disc over a small length while counting the revolutions
     one has to select one and means where the beginning of the marking crosses the centre
     point or tie end of the marking crosses the centre point. This is to be followed for
     “CLICK ON” as well as “CLICK OFF”.

14   If suppose, corresponding to 6 revolutions of MUT (for 3 Phase, 10 Amps meter
     mentioned in (11) RSS makes 2.4 revolutions.




     Then % error = 2.5-2.4/2.5x100=4%

     This means MUT is faster.

     But RSS too has an error.

     Let RSS have an error of + 0.5%

     = Net error = +4+5 = +4.5%




15   If suppose, corresponding to 6 revolutions of above said MUT, RSS makes 2.65
     revolutions.

     Then % error = 2.5-2.65/2.5x100 = -6%

     This means MUT is slower.

     RSS error = +0.5%

     = Net error = -6+0.5 = -5.5%

                                             31
16   For full load, 0.5 pt lag, pressure coils are connected across rated supply voltage and
     rated full load current is passed through current coils at 0.5 pt lag.

17   Full load, 0.5 pt lag test, adjustment is made by means of loop on potential core or

     Resistance phase loop on current electromagnet with a clamp.

18   1/10 full load upf; rated supply voltage is applied across pressure coils and a very low

     current (1/10 of full load current) is passed through the meter at upto.

19   Creep test: All meters are to be tested for non-registration with the voltage ckts alone

     energized at 10% over voltage with current ckts open.

20   While conducting creep test, if the meter is found to be creeping then meter is to be

     adjusted. After the adjustments are done, errors at 1/10 full load are to be rechecked.

21   The two adjustments generally provided to prevent creeping of meters are (i) an iron
     wireon the shaft, near to the end of the potential core (ii) two diametrically opposite holes
     inthe disc.

22   Dial test is performed after full load-upf, full load 0.5pt lag, 1/10 full load upf, creep test
     are performed and meter adjusted to run within permissible limits of error.

23   During dial test, the load is to be light load at upf.

24    The initial reading of the meter during, dial test is usually 99997 or 99999 and the test is

     Continued till 00000 is obtained.

25   By the dial test it is ensured that apart from the meter revolutions being accurate for

     different loads and power factors, the drive is communicated to the cyclometer and that
     the final registration by the meter with reference to load connected is within limits of
     accuracy.

26   After conducting the tests discussed, potential links which were kept removed, should be


                                               32
        properly fixed.

27      Before proceeding with testing, it should be ensured that the disc rotates on all three

        phases.

28      Sometimes it so happens that the disc does not rotate on all three phases then the lower
        bearing is to be adjusted.

29      Sometimes it so happens that the disc rotates in two phases and does not rotate on the

        third phase. Then the terminal connections of that particular phase and connected coil

        should be checked.

30      If the disc stops rotating during the course of testing then lower

        bearing should be adjusted and all the tests should be repeated.




2.2.3   Energy meter Testing: (132kv feeder , Jammikunta)




        Make                           :      secure company

        E.M s. no                      :      Gec05782

        Type of E.M                    :      E3M021

        Class of accuracy              :      0.2

        CT ratio adopted               :       1200/1

        PT ratio adopted               :      33kv/110v


                                                 33
       E.M CT ratio adopted          :       800/1

       E.M PT ratio adopted          :       33kv/110v

       Error                         :       0.15 %




Evaluation of test results:




       From above results the error is less than the permissible limit

(“+ 2%), thus energy meter is in good working condition




 2.3 Auxiliary AC and DC power supply


       Auxiliary of AC power supply



AC supply in substations in generally made available through station auxiliary transformer or
through tertiary windings, if available in substation .However tertiary winding should be used
only to meet substation load requirement .substation load requirement in case of emergency.


       Auxiliary DC power supply


       Dc supply in substations is required mainly for operation of protection system and circuit
breaker operation .These virtual functions in a substation and hence reliable DC system should
be provided .for 132 kv substation there is requirement of one battery set with charger of
adequate capacity should be provided as correct operation is to be ensured taking into

                                                34
Consideration grid security .trip coils of circuit breakers of rating 220kv and above should be
necessarily wired to different sources to maintain reliable operation and faults clearance .line
protections also need to be wired from two different sources for redundancy




2.4 Battery room



2.4.1 Battery charger


Battery charger is intended to:


           1) Keep the lead acid battery on trickle or boost charge as required.


           2) Supply DC power to the plant load.


        The battery chargers consists of one float charger, one boost charger and a DC
distribution board the float charger converts the AC mains to the required DC and supplies load
current and float charging of the battery .The float charger out put is connected to the DC
distribution board through output isolator switch. During normal operation the float charger
supplies load current through DC distribution board and simultaneously trickle charges the
battery through interlocking contactors .In case of power failure the battery bank starts supplying
the load automatically through interlocking contactors




       The boost charger converts the 3phase AC supply to DC output and capable of boost
charging the battery up to max cell voltage of 2.3v/2.75v/cell.The charger output is connected to
the battery through output isolators switch .when boost charger is made on ,the float charger
output is automatically isolated from the battery .




                                                 35
36
The DC distribution board consists of isolator switches and fuses and connects the DC output to
various loads .The float charger ,boost charger and DC distribution board are mounted on a
single Indoor floor mounting free standings type sheet steel cubicle.




Technical specifications


220 V battery chargers


1) AC input                                  415 volts, 3phase, 50 hz
2) DC output
Float charger                                220 v to 250v, 8 amps. Voltage can be varied by a
                                             Potentiometer
Boost charger                                198 to 297v, 16 amps .voltage can be varied by
                                             Coarse &fine rotary switch
3) Regulation from no load to full load         1.0%
4) Max error around the set point               1%
5) Ripple                                      2% peak to peak
6) Efficiency                                 75%at full load
7) Metering                                    moving coil voltmeter and ammeter for DC output
                                               Side
8) Cooling                                     natural air cooling .louvers provide for ventilation
9) Dimensions                                 1800 mm (H) X 600 MM(D) X 900 MM(W)
10) Weight                                    250 kg
11) Cable entry                               from the bottom cubicle
12) Temperature rise above ambient                   wound components--70
                                                         Diode           ---80
                                                         SCR              --80




                                                37
2.4.2 Theory of operation of float charger




The float charger converts the AC mains to regulated DC and supplies the load current and float
charging requirement of the battery .The float charger output is connected to DC distribution
Board through output isolators switches (sw1).




It is a thyristor controlled power supply transformer in double wound 3 phase, natural air cooled.
Rectifier bridge is 3 phase ,full wave half controlled .Each device is mounted on extruded heat
sink for efficient cooling .Each device is protected from voltage surge suppression net work and
from over load with fast acting HRC/semiconductor fuses.




       Float charger is basically designed for constant voltage opertion .Voltage can be set
between 220 to 250 V for 220V charger .Float chargers are provided with current limit .during
over load the charger output drop below set level, thereby allowing to take current surges .




       Float charger is provided with input MCB, contactor with overload relay, LED indication
lamps ; AC voltmeter, DC voltmeter and DC Ammeter and auto manual switch .All
disconnecting switches are normally closed and are for isolation purposes only.




                                                 38
2.4.3 Theory of operation boost charger




       The boost charger converts the input three phase AC supply to DC output and capable of
boost charging the battery up to a maximum cell voltage of 2.3/2.7v cell .the boost charger
output is connected to the battery through output isolator switch (SW3).when the boost charger is
made ON ,the float charger output and load is automatically isolated from the battery and
102nd/84th cell of the battery is connected to the distribution board .this provide a means to
supply continuous power upon a power failure during boost charging




The three phase AC input is applied through the MCB2 to the boost input contactor CON-2.
The boost charger transformer TX6 have primary taps which can be used as per the requirement
.The secondary has multiples with the four way rotary switches RS6& RS7 designed as coarse
and fine control for boost charging output .




       The AC voltage is applied after CON -2 is routed to the primary of TX6 through ballast
chokes CH2 to ch4 ,PL4 to PL6 indicates the healthiness of AC supply to boost charger .TX6
steps down the voltage down to suitable level.




                                                 39
40
The secondary voltage of TX6 as selected by the coarse, and fine control switches is applied to
the three phase full wave bridge rectifier consisting of six numbers Diodes D5 to D10,each diode
is protected by a HRC fuse (F10 to F15) and R-C surge suppressors RC8 to RC13.the dc output
is protected by HRC fuse F16 and F17.ammeter A2 with external shunt SH2 measures the boost
charger output current and BR2 is the bleeder resistor .


Boost charger is also provided with blocking diode and DC contactor CON3 for tapped cell
connection during operation of float boost charger .


DC contactor is inter locked with the input contactor, such that, when boost charger is switch on
DC contactor CON3 is demagnetized and battery will b connected to load circuit through diode
D12 maintain required voltage across load during boost charging.


Only the boost charger is provided with a contactor CON3 which is inter locked electrically with
its input contactor CON2.the DC contactor gets energized only when the boost charger is OFF
i.e.CON1 is OFF in which case full battery cell are floated across the float charger. When the
boost charger is switched ON, voltage from the tap cell gets connected to the float charger
through diode D12 tap cell diode D12 is protected through fuse F19.




                                                41
                                        CHAPTER-3

                                BUSBAR CONNECTIONS

3.1   Introduction




       Various factors affect the reliability of a substation or switchyard, one of which is the
arrangement of the buses and switching devices. In addition to reliability, arrangement of the
buses/switching devices will impact maintenance, protection, initial substation development, and
cost. There are six types of substation bus/switching arrangements commonly used in air
insulated substations:
1. Single bus
2. Double bus, double breaker
3. Main and transfer (inspection) bus
4. Double bus, single breaker
5. Ring bus
6. Breaker and a half


3.2 Single Bus


       This arrangement involves one main bus with all circuits connected directly to the bus.
The reliability of this type of an arrangement is very low. When properly protected by relaying, a
single failure to the main bus or any circuit section between its circuit breaker and the main bus
will cause an outage of the entire system. In addition, maintenance of devices on this system
requires the de-energizing of the line connected to the device. Maintenance of the bus would
require the outage of the total system, use of standby generation, or switching to adjacent station,
if available. Since the single bus arrangement is low in reliability, it is not recommended for
heavily loaded substations or substations having a high availability requirement. Reliability of
this arrangement can be proved by the addition of a bus tiebreaker to minimize the effect of a
main bus failure.


                                                42
                              Fig 3.1 single bus connection


3.3 Double Bus, Double Breaker


This scheme provides a very high level of reliability by having two separate breakers available to
each circuit. In addition, with two separate buses, failure of a single bus will not impact either
line. Maintenance of a bus or a circuit breaker in this arrangement can be accomplished without
interrupting either of the circuits. This arrangement allows various operating options as
additional lines are added to the arrangement; loading on the system can be shifted by connecting
lines to only one bus. A area for the substation to accommodate the additional equipment. This is
especially double bus; double breaker scheme is a high-cost arrangement, since each line has two
breakers and requires a larger true in a low profile configuration. The protection scheme is also
more involved than a single bus scheme.




                         Fig 3.2 double bus and double breaker connection




                                               43
3.4 Main and Transfer Bus


This scheme is arranged with all circuits connected between a main (operating) bus and a
transfer bus (also referred to as an inspection bus). Some arrangements include a bus tie breaker
that is connected between both buses with no circuits connected to it. Since all circuits are
connected to the single, main bus, reliability of this system is not very high. However, with the
transfer bus available during maintenance, de-energizing of the circuit can be avoided. Some
systems are operated with the transfer bus normally de-energized.


       When maintenance work is necessary, the transfer bus is energized by either closing the
tie breaker, or when a tie breaker is not installed, closing the switches connected to the transfer
bus. With these switches closed, the breaker to be maintained can be opened along with its
isolation switches. Then the breaker is taken out of service. The circuit breaker remaining in
service will now be connected to both circuits through the transfer bus. This way, both circuits
remain energized during maintenance. Since each circuit may have a different circuit
configuration, special relay settings may be used when operating in this abnormal arrangement.
When a bus tie breaker is present, the bus tie breaker is the breaker used to replace the breaker
being maintained, and the other breaker is not connected to the transfer bus. A shortcoming of
this scheme is that if the main bus is taken out of service, even though the circuits can remain
energized through the transfer bus and its associated switches, there would be no relay protection
for the circuits. Depending on the system arrangement, this concern can be minimized through
the use of circuit protection devices (reclosure or fuses) on the lines outside the substation. This
arrangement is slightly more expensive than the single bus arrangement, but does provide more
flexibility during maintenance. Protection of this scheme is similar to that of the single bus
arrangement. The area required for a low profile substation with a main and transfer bus scheme
is also greater than that of the single bus, due to the additional switches and bus.




                                                 44
                               Fig 3.3 main and transfer bus connection




3.5   Double Bus, Single Breaker


This scheme has two main buses connected to each line circuit breaker and a bus tie breaker.
Utilizing the bus tie breaker in the closed position allows the transfer of line circuits from bus to
bus by means of the switches. This arrangement allows the operation of the circuits from either
bus. In this arrangement, a failure on one bus will not affect the other bus. However, a bus tie
breaker failure will cause the outage of the entire system. Operating the bus tie breaker in the
normally open position defeats the advantages of the two main buses. It arranges the system into
two single bus systems, which as described previously, has very low reliability. Relay protection
for this scheme can be complex, depending on the system requirements, flexibility, and needs.
With two buses and a bus tie available, there is some ease in doing maintenance, but
maintenance on line breakers and switches would still require outside the substation switching to
avoid outages


.




                                                 45
                       Fig 3.4 double bus with single breaker connection
3.6 Ring Bus


       In this scheme, as indicated by the name, all breakers are arranged in a ring with circuits
tapped between breakers. For a failure on a circuit, the two adjacent breakers will trip without
affecting the rest of the system. Similarly, a single bus failure will only affect the adjacent
breakers and allow the rest of the system to remain energized. However, a breaker failure or
breakers that fail to trip will require adjacent breakers to be tripped to isolate the fault.
Maintenance on a circuit breaker in this scheme can be accomplished without interrupting any
circuit, including the two circuits adjacent to the breaker being maintained. The breaker to be
maintained is taken out of service by tripping the breaker, then opening its isolation switches.
Since the other breakers adjacent to the breaker being maintained are in service, they will
continue to supply the circuits. In order to gain the highest reliability with a ring bus scheme,
load and source circuits should be alternated when connecting to the scheme. Arranging the
scheme in this manner will minimize the potential for the loss of the supply to the ring bus due to
a breaker failure. Relaying is more complex in this scheme than some previously identified.
Since there is only one bus in this scheme, the area required to develop this scheme is less than
some of the previously discussed schemes. However, expansion of a ring bus is limited, due to
the practical arrangement of circuits.




                                                46
                                     Fig 3.5 ring bus




3.7 Breaker-and-a-Half


The breaker-and-a-half scheme can be developed from a ring bus arrangement as the number of
circuit’s increases. In this scheme, each circuit is between two circuit breakers, and there are two
main buses. The failure of a circuit will trip the two adjacent breakers and not interrupt any other
circuit. With the three breaker arrangement for each bay, a center breaker failure will cause the
loss of the two adjacent circuits. However, a breaker failure of the breaker adjacent to the bus
will only interrupt one circuit. Maintenance of a breaker on this scheme can be performed
without an outage to any circuit. Furthermore, either bus can be taken out of service with no
interruption to the service. This is one of the most reliable arrangements, and it can continue to
be expanded as required. Relaying is more involved than some schemes previously discussed.
This scheme will require more area and is costly due to the additional components.




                                                47
Fig 3.6 breaker-and- a half bus connection




                       48
                                         CHAPTER-4

            PROTECTION EQUIPMENT USED IN SUBSTATION

  4.1 circuit breakers

 4.1.1 Introduction

       Circuit breakers are mechanical devices designed to close or open contact members, thus
closing or opening of an electrical circuit under normal abnormal conditions.
       The circuit breakers are used to break the circuit if any fault occurs in any of the
instrument these circuit breaker breaks for a fault which can damage other instrument in the
station. For any unwanted fault over the station we need to break the line current. This is only
done
      Automatically by the circuit breaker. There are mainly two types of circuit breakers used
       for anySubstations. They are (a) SF6 circuit breakers; (b) spring circuit breakers. There
       are six types of circuit breakers depending up on the arc quenching media


   1. water circuit breakers
   2. oil circuit breakers
   3. air blast circuit breakers
   4. SF6 circuit breakers
   5. Air circuit breakers
   6. Vacuum circuit breakers


   The use of SF6 circuit breaker is mainly in the substations which are having high input kv
input, say above 220kv and more. The gas is put inside the circuit breaker by force i.e. under
high pressure. When if the gas gets decreases there is a motor connected to the circuit breaker.
The motor starts operating if the gas went lower than 20.8 bar. There is a meter connected to the
breaker so that it can be manually seen if the gas goes low. The circuit breaker uses the SF6 gas
to reduce the torque produce in it due to any fault in the line. The circuit breaker has a direct link
with the instruments in the station, when any fault occur alarm bell ring.


                                                  49
The spring type of circuit breakers is used for small kv stations. The spring here reduces the
torque produced so that the breaker can function again. The spring type is used for step down
side of 132kv to 33kv also in 33kv to 11kv and so on. They are only used in low distribution
side.




4.1.2 Operation of the circuit breaker



        All circuit breakers have common features in their operation, although details vary
substantially depending on the voltage class, current rating and type of the circuit breaker.



        The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is
usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are
usually arranged with pilot devices to sense a fault current and to operate the trip opening
mechanism. The trip solenoid that releases the latch is usually energized by a separate battery,
although some high-voltage circuit breakers are self-contained with current transformers,
protection relays, and an internal control power source.



        Once a fault is detected, contacts within the circuit breaker must open to interrupt the
circuit; some mechanically-stored energy (using something such as springs or compressed air)
contained within the breaker is used to separate the contacts, although some of the energy
required may be obtained from the fault current itself. Small circuit breakers may be manually
operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy
to the springs.



        The circuit breaker contacts must carry the load current without excessive heating, and
must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made
of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is

                                                 50
limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers are
usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit
breakers have replaceable contacts.



       When a current is interrupted, an arc is generated. This arc must be contained, cooled,
and extinguished in a controlled way, so that the gap between the contacts can again withstand
the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the
medium in which the arc forms.




4.1.3 Different techniques are used to extinguish the arc




   Lengthening of the arc
   Intensive cooling (in jet chambers)
   Division into partial arcs
   Zero point quenching (Contacts open at the zero current time crossing of the AC waveform,
    effectively breaking no load current at the time of opening. The zero crossing occures at
    twice the line frequency i.e. 100 times per second for 50Hz ac and 120 times per second for
    60Hz ac )
   Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore
power to the interrupted circuit.




                                                 51
4.1.4 Arc interruption of the circuit breaker



      Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings
will have metal plates or non-metallic arc chutes to divide and cool the arc magnetic
blowout coils deflect the arc into the arc chute. In larger ratings, oil circuit breakers rely upon
vaporization of some of the oil to blast a jet of oil through the arc.



      Gas (usually SF6) circuit breakers sometimes stretch the arc using a magnetic field, and
then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact
material), so the arc quenches when it is stretched a very small amount (<2–3 mm). Vacuum
circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.



     Air circuit breakers may use compressed air to blow out the arc, or alternatively, the
contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus
blowing out the arc. Circuit breakers are usually able to terminate all current very quickly:

typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been
tripped, depending upon age and construction of the device.




                                                  52
4.1.5 Air-blast circuit breakers


          Air is used as insulator in outdoor-type substations and for high-voltage transmission
lines. Air can also be used as extinguishing medium for current interruption. At atmospheric
pressure, the interrupting capability, however, is limited to low voltage and medium voltage
only. For medium voltage applications up to 50 kV, the breakers are mainly of the magnetic air-
blast type in which the arc is blown into a segmented compartment by the magnetic field
generated by the fault current. In this way, the arc length, the arc voltage, and the surface of the
arc column are increased. The arc voltage decreases the fault current, and the larger arc column
surface improves the cooling of the arc channel. At higher pressure, air has much more cooling
power, and air-blast breakers operating with compressed air can interrupt higher currents at
considerable higher-voltage levels.


          Air-blast breakers using compressed air can be of the axial-blast or the cross-blast type.
The cross-blast type air blast breaker operates similar to the magnetic-type breaker: compressed
air blows the arc into a segmented arc-chute compartment. Because the arc voltage increases
with the arc length, this is also called high-resistance interruption; it has the disadvantage that the
energy dissipated during the interruption process is rather high. In the axial-blast design, the arc
is cooled in axial direction by the airflow. The current is interrupted when the ionization level is
brought down around current zero. Because the arc voltage hardly increases this is called low-
resistance interruption. When operating, air-blast breakers make a lot of noise, especially when
the arc is cooled in the free air, as is the case with AEG’s free-jet breaker (Freistrahlschalter)
design.




4.1.6 SF6 circuit breakers


          The superior dielectric properties of SF6 were discovered as early as 1920. It lasted until
the 1940s before the first development of SF6 circuit breakers began, but it took till 1959 before
the first SF6 circuit breaker came to the market. These early designs were descendants of the
axial-blast and SF6 circuit breakers. Air-blast circuit breakers, the contacts were mounted inside

                                                  53
a tank filled with SF6 gas, and during the current interruption process, the arc was cooled by
compressed SF6 gas from a separate reservoir. The liquefying temperature of SF6 gas depends
on the pressure but lies in the range of the ambient temperature of the breaker. This means that
the SF6 reservoir should be equipped with a heating element that introduces an extra failure
possibility for the circuit breaker; when the heating element does not work, the breaker cannot
operate. Therefore the puffer circuit breaker was developed and the so-called double pressure
breaker disappeared from the market. In the puffer circuit breaker the opening stroke made by
the moving contacts moves a piston, compressing the gas in the puffer chamber and thus causing
an axial gas flow along the arc channel. The nozzle must be able to withstand the high
temperatures without deterioration and is made from Teflon. Presently, the SF6 puffer circuit
breaker is the breaker type used for the interruption of the highest short-circuits powers, up to
550 kV–63 kA per Interrupter made by Toshiba. Puffer circuit breakers require a rather strong
operating mechanism because the SF6 gas has to be compressed. When interrupting large
currents, for instance, in the case of a three-phase fault, the opening speed of the circuit breaker
has a tendency to slow down by the thermally generated pressure, and the mechanism (often
hydraulic or spring mechanisms) should have enough energy to keep the contacts moving apart.
Strong and reliable operating mechanisms are costly and form a substantial part of the price of a
breaker. For the lower-voltage range, self-blast circuit breakers are now on the market. Self-blast
breakers use the thermal energy released by the arc to heat the gas and to increase its pressure.




        After the circuit breaker moving contacts are out of the arcing chamber, the heated gas is
released along the arc to cool it down. The interruption of small currents can be critical because
the developed arcing energy is in that case modest, and sometimes a small puffer is added to
assist in the interrupting process. In other designs, a coil carrying the current to be interrupted
creates magnetic field, which provides a driving force that rotates the arc around the contacts and
thus provides additional cooling. This design is called the rotating-arc circuit breaker. Both self-
blast breakers and rotating arc breakers can be designed with less powerful (and therefore
cheaper) mechanisms and are of a more compact design than puffer breakers.




                                                54
4.1.7 Vacuum circuit breakers


        Between the contacts of a vacuum circuit breaker a vacuum arc takes care of the
interruption process. As already discussed in the introduction to this chapter about the switching
arc, the vacuum arc differs from the high-pressure arc because the arc burns in vacuum in the
absence of an extinguishing medium. The behavior of the physical processes in the arc column of
a vacuum arc is to be understood as a metal surface phenomenon rather than a phenomenon in an
insulating medium. The first experiments with vacuum interrupters took place already in 1926,
but it lasted until the 1960s when metallurgical developments made it possible to manufacture
gas-free electrodes and when the fire.



4.1.8 Advantages SF6 circuit breaker




        Due to the superior arc quenching properties of SF6 gas, it has many advantages over the
oil circuit breakers

    1) circuit breakers have very short arcing time

    2) Since dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can

        interrupt much larger currents.

    3) There are no carbon deposits so that tracking and insulation problems are eliminated.

    4) There is no risk of fire in such breakers because SF6 gas is inflammable.

    5) The SF6 breakers have low maintenance cost, light foundation requirements and

        minimum auxiliary equipments.

    6) The closed gas enclosure keeps the interior dry so that there is no moisture problem.

    7) The SF6 circuit breakers give noiseless operation      due to its closed gas circuit and no

        exhaust to atmosphere unlike the air blast circuit breaker.

                                                55
4.2 Lightening arrester



        These lightening arrestors can resist or ground the lightening if falls on the incoming
feeders. The lightening arrestors can work in a angle of 30 around them. They are mostly used
for protection of the instruments used in the substation. As the cost of the instrument in the
station are very high to protect them from high voltage from lightening these lightening arrestors
are used.




       .
It is a device used on electrical power systems to protect the insulation on the system from the
Damaging effect of lightning. Metal oxide varistors (MOVs) have been used for power system
Protection since the mid 1970s. The typical lightning arrester also known as surge arrester has a
High voltage terminal and a ground terminal. When a lightning surge or switching surge travels
Down the power system to the arrester, the current from the surge is diverted around the
protected Insulation in most cases to earth.


Landscape suited for purpose of explanation: (1) Represents Lord Kelvin's "reduced" area of
The region, (2) Surface concentric with the Earth such that the quantities stored over it and under
It are equal; (3) Building on a site of excessive electrostatic charge density; (4) Building on a site
of low electrostatic charge density.

                                                 56
        In telegraphy and telephony, a lightning arrester is placed where wires enter a structure,
Preventing damage to electronic instruments within and ensuring the safety of individuals near
Them. Lightning arresters, also called surge protectors, are devices that are connected between
Each electrical conductor in a power and communications systems and the Earth. These provide
Short circuit to the ground that is interrupted by a non-conductor, over which lightning jumps. Its
Purpose is to limit the rise in voltage when a communications or power line is struck by
Lightning.


The non-conducting material may consist of a semi-conducting material such as silicon carbide
Or zinc oxide, or a spark gap. Primitive varieties of such spark gaps are simply open to the air,
But more modern varieties are filled with dry gas and have a small amount of radioactive
Material to encourage the gas to ionize when the voltage across the gap reaches a specified level.
Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow
Lamp) connected between the protected conductor and ground, or myriad voltage-activated solid
state switches called varistors or MOVs. Lightning arresters built for substation use are
Impressive devices, consisting of porcelain tube several feet long and several inches in
Diameter, filled with disks of zinc oxide. A safety port on the side of the device vents the
Occasional internal explosion without shattering the porcelain cylinder




Name plate details

Company                               : W.S industries

Rated voltage                         : 20kv (rms)

Long duration discharge class         :3

Frequency                             : 50Hz

Surge monitor type                    : CRM-SMX

Style                                 : SMX


                                                 57
Type                                   : zodiver

MCOV                                   : 120 kV

Normal discharge current               : 10 kA

Pressure relief current                : 40 kA

Y.O.M                                  : 1990




4.3 Wave trap


        A device used to exclude unwanted frequency components, such as noise or other
interference, of a wave. A device used to exclude unwanted frequency components, such as noise
or other interference, of a wave. Wave trap is an instrument using for tripping of the wave. The
function of this trap is that it traps the unwanted waves. Its function is of trapping wave. Its shape
is like a drum. It is connected to the main incoming feeder so that it can trap the waves which
may be dangerous to the Instruments here in the substation




4.4 Protective relays


4.4.1 Introduction


        It is to cause a prompt removal from service of any element of a power system when it
suffers a short circuit or when it starts to operate in any abnormal manner that might cause
damage or otherwise interfere with the effective operation of the rest of the system. The relaying
equipment is aided in this task by circuit breakers that are capable of disconnecting the faulty
element when they are called upon to do by the relaying equipment.



                                                   58
                        Fig 4.4.1 basic connection of a protective relay

4.4.2 The functional requirement of the relay



i) Reliability: The most important requisite of protective relay is reliability since they supervise
               the circuit for a long time before a fault occurs; if a fault then occurs, the relays
               must respond instantly and correctly.


ii) Selectivity: The relay must be able to discriminate (select) between those conditions for which
               prompt operation is required and those for which no operation, or time delayed
               operation is required.



iii) Sensitivity: The relaying equipment must be sufficiently sensitive so that it operates reliably
               when required under the actual conditions that produces least operating tendency.


iv) Speed:     The relay must operate at the required speed. It should neither be too slow which
               may result in damage to the equipment nor should it be too fast which may result
               in undesired operation.



                                                59
                 The general arrangement of protective zones




                      Fig 4.4.2 different types of protective zones

4.4.3 Distance relaying principle



       A distance relay compares the currents and voltages at the relaying point with Current
providing the operating torque and the voltage provides the restraining torque. In other words an
impedance relay is a voltage restrained over current relay.




       The equation at the balance point in a simple impedance relay is K1V2 = K2I2 or V/I = K3
where K1, K2 and K3 are constants. In other words, the relay is on the verge of operation at a
constant value of V/I ratio, which may be expressed as an impedance.



4.4.4 Types of distance relays

   (1) Impedance relay
   (2) Reactance relay
   (3) Mho relay
   (4) Modified impedance relay

                                                60
       (1)      Impedance relay:



Characteristics of an impedance relay on R-X diagram is shown in fig




                                Fig 4.4.3 impedance relay

The numerical value of the ratio of V to I is shown as the length of the radius vector, such as Z and
the phase angle between V and I determines the position of the vector, as shown.
        Operation of the impedance relay is practical or actually independent of the phase angle
between V and I. The operating characteristic is a circle with its center at the origin, and hence the
relay is non-directional.
Characteristic of Directional Impedance Relay:
Characteristic of a directional impedance relay in the complex R-X phase is shown in fig.




                            Fig 4.4.4 directional impedance relay

                                                  61
Along the line impedance locus line, the positive sequence impedance of the protected line as
seen by the relay between its location and different points along the protected line can be plotted.
The directional unit of the relay causes separation of the regions of the relay characteristic shown
in the figure by a line drawn perpendicular to the line impedance locus. The net result is that
tripping will occur only for points that are both within the circles and above the directional unit
characteristic.



    (2) The Reactance-type Distance Relay


Reactance relay measures V/I Sin Ø (i.e. Z sin Ø). Whenever the reactance measured by the
relay is less than the set value, the relay operates. The operating characteristic on R-X diagram
is indicated below:




                              Fig 4.4.5 reactance relay

The resistance component of impedance has no effect on the operation of reactance relay, the
relay responds solely to reactance component of impedance. This relay is inherently non-

                                                62
directional. The relay is most suitable to detect earth faults where the effect of arc resistance may
render other types of relays to detect faults with difficulty.



   (3) Mho relay


This is a directional impedance relay, also known as admittance relay. Its characteristic on R-X
diagram is a circle whose circumference passes through the origin as illustrated in figure
showing that the relay is inherently directional and it only operates for faults in the forward
direction.




                                      Fig 4.4.6 mho relay




   (4) Modified impedance relay


Also known as offset Mho relay whose characteristic encloses the origin on R-X diagram as
indicated below:




                                                  63
                               Fig 4.4.7 offset mho relay

This offset mho relay has three main applications: -
       i)   Bus bar zone backup
       ii) Carrier starting unit in distance/carrier blocking schemes.
       iii) Power Swing blocking.




4.4.5 Application of distance relaying



Relay Setting:
Since the distance relays are fed from the secondaries of line CTs and bus PTs/line CVTs, the
line parameters are to be converted into secondary values to set the relay as per requirements.

Zsecy = Zpri/Impedance ratio
(where Impedance ratio = P.T.Ratio/C.T.Ratio)


It is to be noted that C.T Ratios (and P.T Ratios) and relay settings are inter-related. Hence any
changes in C.T .ratio have to be effected along with revision of relay settings only.


For the lines, the impedance in Ohms per KM is approximately as under:




                                                   64
               KV                      Z1 (=Z2)             Line Angle

               132 KV                  0.4                  60 to 70 Deg

               220 KV                  0.4                  70 to 80 Deg

               400 KV                  0.3                  80 to 85 Deg




A distance relay is either 3 zones or 4 zones to provide protection.


To ensure proper coordination between distance relays in power system, it is customary to
choose relay ohmic setting as follows: -


       S.No.    Zones                Reactance                               Time

       1.       Zone-1               80% of ZL                               Instantaneous

                                                                             (no intentional

                                                                             time delay)

       2.       Zone-2               100% of ZL + 40-50% of ZSL              0.3 to 0.4 seconds

       3.       Zone-3               100% of ZL + 120% of ZSL                0.6 to 0.8 seconds

       4.       Zone-4               100% of ZL + 120% of ZLL                0.9 to 1.5 seconds




       Where ZL = Positive sequence impedance of line to be protected.

               ZSL = Positive sequence impedance of adjacent shortest line.

               ZLL = Positive sequence impedance of adjacent longest line.




                                                  65
Note:

   i)       Where a three zone relay only is available, the zone 3 will be set to cover the adjacent
            longest line.
   ii)      The zonal timings will be carefully selected to properly grade with the relays       on
            adjoining sections




4.5 Substation earthing


The substation grounding system is an essential part of the overall electrical system. The proper
grounding of a substation is important for the following two reasons:


1. It provides a means of dissipating electric current into the earth without exceeding the
operating limits of the equipment.
2. It provides a safe environment to protect personnel in the vicinity of grounded facilities from
the dangers of electric shock under fault conditions
         The grounding system includes all of the interconnected grounding facilities in the
substation area, including the ground grid, overhead ground wires, neutral conductors,
underground cables, foundations, deep well, etc. The ground grid consists of horizontal
interconnected bare conductors (mat) and ground rods. The design of the ground grid to control
voltage levels to safe values should consider the total grounding system to provide a safe system
at an economical cost.
         Modern substation earthing system has buried horizontal mesh of steel rods and vertical
elctr5odes welded to the mesh further the vertical rises and the galvanizes steel grounding strips
or copper bars etc are connected between the grounding mesh and the points to be grounded.


         The conventional criterion of “low earth resistance” and low current earth resistance
measurement continues tomb in practice for substation and power stations up to and including
220kV.

                                                 66
        The following information is mainly concerned with personnel safety. The information
regarding the grounding system resistance, grid current, and ground potential rise can also be
used to determine if the operating limits of the equipment will be exceeded.


Safe grounding requires the interaction of two grounding systems:


1. The intentional ground, consisting of grounding systems buried at some depth below the
earth’s surface
2. The accidental ground, temporarily established by a person exposed to a potential gradient in
the vicinity of a grounded facility


        It is often assumed that any grounded object can be safely touched. A low substation
ground resistances not, in itself, a guarantee of safety. There is no simple relation between the
resistance of the grounding system as a whole and the maximum shock current to which a person
might be exposed. A substation with relatively low ground resistance might be dangerous, while
another substation with very high ground resistance might be safe or could be made safe by
careful design.


    The substation earthing is provided for the fallowing reasons.


     Safety of operational and maintenance staff
     Discharge electrical charges to ground
     Grounding of overhead shielding wires
     Electro-magnetic interferences




                                                67
                                      CHAPTER-5
                       SUMMARY AND CONCLUSSIONS


5.1 Summary


       Transmission and distribution stations exist at various scales throughout a power system.
In general, they represent an interface between different levels or sections of the power system,
with the capability to switch or reconfigure the connections among various transmission and
distribution lines. On the largest scale, a transmission substation would be the meeting place for
different high-voltage transmission circuits. At the intermediate scale, a large distribution station
would receive high-voltage transmission on one side and provide power to a set of primary
distribution circuits. Depending on the territory, the number of circuits may vary from just a few
to a dozen or so.




       The major stations include a control room from which operations are coordinated.
Smaller distribution substations follow the same principle of receiving power at higher voltage
on one side and sending out a number of distribution feeders at lower voltage on the other, but
they serve a more limited local area and are generally unstaffed. The central omponent of the
substation is the transformer, as it provides the effective in enface between the high- and low-
voltage parts of the system. Other crucial components are circuit breakers and switches. Breakers
serve as protective devices that open automatically in the event of a fault, that is, when a
protective relay indicates excessive current due to some abnormal condition. Switches are
control devices that can be opened or closed deliberately to establish or break a connection. An
important difference between circuit breakers and switches is that breakers are designed to
interrupt abnormally high currents (as they occur only in those very situations for which circuit
protection is needed), whereas regular switches are designed to be operable under normal
currents. Breakers are placed on both the high- and low-voltage side of transformers. Finally,
substations may also include capacitor banks to provide voltage support.

                                                 68
5.2 conclusions




    % Impedance volts: IT is the percentage of HV volts required to create full load
       amperes in a shorted LV winding .It is also indicative of percentage of LV volts to
       b applied so that full load currents flow in a shorted HV winding.




    Max. Ambient temperature: 50degree C. This means the transformer is designed
       to work in good condition when the surrounding temperature of the transformers
       increase up to a maximum of 50


    Winding temperature rise 50        : this means that the winding temperature can go
       up to 55   over and above the atmospheric temperature



    Insulation levels: for the safety point of view insulation will b provided for more
       than the operating voltages of the transformer.


    Major insulation: between primary and secondary phase to phase and inter coil to
       core, this is achieved by Bakelite, wooden blocks, cellulosic paper cylinder.



    Transformer oil: this is derivate of petroleum crude. This has a good dielectric
       strength and improves the dielectric strength of transformer when filled under
       vacuum by displacing air from all cavities .this is also a good cooling medium and
       absorb heat from the windings in adequate movement of cooling medium.




                                        69
 Thus mineral oil has a flash point of 140deg c and 160dec c five point. This also
   can sustain the combusting with its own energy, one its catches fire. Thus this is
   unsuitable for the transformer


 The indoor transformers are filled with a synthetic liquid know as silicate liquid.
   This is fire assistant and has flash point well above 300oc.as hauls
   polychlorinated biphenyls, which were quite popular with indoor transformers
   earlier have since been banned all over the world due to bioaccumulation
   properties.


 The insulation resistances values of the circuit breaker are in the order of gaga
   ohms then it is suitable for the desired operation.


 The lightening arrester discharges the high voltages to ground through nonlinear
   resistance. How many times surges are diverted to the ground is displayed on the
   screen. Lightening arrester having two regions, they are green and red. if needle
   comes under the green it is working else damaged in red region.


 In substations the energy meter testing is performed quart alley, the permissible
   amount of error is     %. if it is exceeds the limit ,instead of old one replaced by
   new one.


 Wave trap blocks the high frequency carrier waves are in the range of 24KHz to
   50KHz, let power waves passes (50KHz -60KHz) through it.




                                     70
REFERENCES




  Operation manual of MRT, APTRANSCO “relays principle, working and applications of distances

    relays”


  Hand book of MRT engineers, APTRANSCO “SF6 circuit breakers description”

  Power system by J.B Gupta, S.K katria& sons publications, Tenth Edition “substation

    earthing”

  Electric power substations engineer by John D.Mcnonald “bus bar arrangements and

    its related matter”

  Principles of power system by V.K Mehta& Rohit Mehta, first multicolor edition,

    “substation introduction and its classifications”

  www.wikipedia.com “ circuit breakers, energy meter and some description about

    transformers”




                                            71
72

								
To top