Introduction to Power System Protection & Relays

					 P. GOPALA KRISHNA
ADE/400KV/APTRANSCO
Power Systems Protection -Introduction
Power Systems Protection -Introduction




       Power Evacuation Substation
       Transmission Substation

       Switching Substation

       Distribution Substation
Power Systems Protection -Introduction


             Power System Components
             1. Generators
             2. Transformers
             3. Transmission Lines
             4. Feeders
             5. Motors
             6. Capacitor Banks
             7. Bus Bars
             8. Reactors
Power Systems Protection -Introduction


    Protection – Purpose
        To detect abnormalities (faults).
        To eliminate such abnormality by
        isolating smallest portion of the system
        in a shortest period of time.
        To prevent injury to personnel.
        To prevent damage to Equipment.
Power Systems Protection -Introduction


     Protective Relay – What should it do ?
        Monitor system parameters continuously
        (V, I, P, F)
        Operate quickly when necessary
        (Dependability)
        Should not operate wrongly
        (stability, discrimination)

                To trip or not to trip ?
Power Systems Protection -Introduction


 Protective Relay Scheme – What is it ?
       A Protective Relay
       CT / CVT / PT(EMVT)
       Auxiliary Power Supply (24 V to 240 V AC/ DC)
       Switching Device ( Breaker/ Isolator / Contactor)
       Trip Coil
       Alarm / Trip contact
       Control Wiring
Power Systems Protection -Introduction

Power System - Faults
Current               Voltage             Frequency
Over Load             Over Voltage        Over Frequency
Over Current          Under Voltage       Under Frequency
Earth Fault           Voltage Unbalance   dF/dT
Current Unbalance     Neutral shift
Dir. Over Current                         Control/Management
Dir. Earth Fault                          Synchronising
                       Computed
                                          Load sharing
 Power                 Over fluxing
                                          Islanding
 Active power          Loss of field
                                          Load shedding
 Reactive power        Differential
 Over power            Over Current
 Under power           Restricted E/F        DC Relays
 Reverse power         Under Impedance
Power Systems Protection -Introduction


Protection – ANSI Codes




                                         Code numbers
                                         Used to denote
                                         Protections
                                         On a SLD
Power Systems Protection -Introduction


Power System – Fault Handling



   Trip & Isolate                Control & Regulate
                                 Breaker is not tripped
   Breaker is tripped on fault
                                 Corrective actions
   Faulty section is isolated    Generated on line
Power Systems Protection -Introduction


    Relay operation when a fault occurs

       Each Relay should Protect a specific Zone in
       the System.

       If fault is inside its Zone, Relay should
       operate and isolate the faulted Zone.

       If fault is outside zone, Relay should not
       operate,

       Some other Relay should operate and isolate
Power Systems Protection -Introduction


   What happens when a fault occurs

      Fault current flows through number of Relays.

      Some of these Relays will start to operate.

      Only one Relay related to particular fault
      should trip and interrupt the fault current.

      Remaining Relays will reset after above.
Power Systems Protection -Introduction



Protection Relays – Inputs / Out puts


  Inputs
                                         Outputs
  Current             CTs
  Voltage             PTs
                                         Trip Contact
  Frequency           PTs
  Power               CTs + PTs
Power Systems Protection -Introduction


Protection Relays – Settings

  Pick up setting           Low set         % of CT
                            Highset         Rating




  Time delay setting        Definite time       TMS
                                                Setting
                            Inverse time
Power Systems Protection -Introduction

Power System – Trip time characteristics




         t


   1.3 or
   3.0 sec




             1.4              10
                                         I/Is
             LS
Power Systems Protection -Introduction

Power System – Trip time characteristics




           t


    1.3 or
    3.0 sec


 50 msec

               1.4    6       10
                                         I/Is
               LS     HS
Power Systems Protection -Introduction



Typical Inverse Time Delays
                TIME MULTIPLIER SETTING – TMS : 1.00
                       I/Is=2 I/Is=4 I/Is=6 I/Is=8 I/Is=10 I/Is=15 I/Is=20
 Normal Inv 3 sec      10.13   5.03   3.87   3.33   3.00    2.54     2.29
Normal Inv 1.3 sec     04.39   2.18   1.68   1.44   1.30    1.10     0.99

Very Inverse 1.5 sec   13.50   4.50   2.70   1.93   1.50    0.96     0.71
Extremely Inverse      26.66   5.33   2.28   1.27   0.81    0.36     0.20
Power Systems Protection -Introduction

Things you should know
                              Application
                              SLD representation
                              Front panel Controls
                              Back Panel terminations
                              Inputs / Outputs / Ratings
                              Settings (Inside)
                              Settings (on front panel)
                              Installation & wiring
                              Commissioning
                              Testing (Front panel)
                              Testing (SCITS)
                              Cat . No.
EVALUATION
    OF
PROTECTIVE
  RELAYS
           PREPARED BY
      GOPALA KRISHNA PALEPU
       ADE/MRT(PROTECTION)
          1 ST GENERATION
FIRST GENERATION RELAYS :
THESE ARE ELECTRO MAGNETIC RELAYS
IN THIS NO OF ELECTROMAGNETIC RELAYS PROVIDED FOR ARRIVING
A SPECIFIC FUNCTION i.e EACH FUNCTION OF THE SCHEME HAVING A
SEPARATE RELAY. COMBINING ALL IS CALLED ONE PROTECTION
FUNCTION. NO OF RELAYS ARE MORE AND SPACE OCUPATION IS
MORE AND INTER CONNECTION WIRING DIAGRAM IS MORE.
DISADVANTAGES: ELECTROMECHANICAL RELAYS HAVE A LOT OF
MECHANICAL PARTS, WHICH MAY BECOME CLOGGED WITH DIRT OR
CORRODED DUE TO ENVIRONMENTAL CONDITIONS, AFFECTING BOTH
OPERATION, CALIBRATION AND MOVEMENT OF THE DISKS. IT
REQUIRES PERIODICAL MAINTANENCE AND ADJUSTMENT.
         1 ST GENERATION
NEXT MODIFICATION :
ELECTRO MAGNETIC RELAYS WITH STATIC COMPONENTS
IN THIS SOME OF THE FUNCTIONS ARE DERIVED FROM STATIC
COMPONENTS PROVIDED ON THE PCB. IN THIS QUANTITY OF
ELECTROMAGNETIC RELAYS ARE REDUCED. COMBINING ALL IS
CALLED ONE PROTECTION FUNCTION. QUANTITY OF RELAYS ARE
REDUCED AND SPACE OCUPATION IS REDUCED SOME WHAT.
DISADVANTAGES: THE PCBs ARE MADE WITH TRANSISTORS AND ARE
BROUGHTOUT ITEMS WITH DIFFERENT MAKE. AFTER SOME TIME
THESE ARE TO BE REPLACED FOR CORRECT OPERATION WITHOUT
DRIFT. THE PCBs & COMPONENTS PERFORMANCE MAY BE AFFECTED
DUE TO DIST & DIRT. IT REQUIRES PERIODICAL MAINTANENCE AND
ADJUSTMENT.
            2 ND GENERATION
SECOND GENERATION RELAYS :
STATIC RELAYS WITH TRANSISTORS
In this all of the functions are derived from static components provided on the
PCB. In this initially each function is derived with separate static relay.
Quantity of static relays are more but space occupation is less. Combining all
is called one protection function. Inter connection wiring diagram is still not
reduced.
DISADVANTAGES: Static relays generally employ a lot of electronic
components made by other manufacturers. If these electronic components are
not tested with rigorous quality control, the chances of failure of components
during the relay life time may exist. A reliable DC power source within the
relay, to electronically measure circuits has to be generated from available
external power sources. Most of the static relays employ series, shunt, or
switched mode power supply designs. For a variety of reasons, if these power
supplies fail, the measuring circuits are inoperative and the relay is dead for
any measurements. No protection is available to the network. Most of the static
relays in use do not have the means to detect the failure of power supply and
initiate an alarm. It requires periodical Maintenance.
         2 ND GENERATION
NEXT MODIFICATION :
STATIC RELAYS ON SINGLE PCB
IN THIS ALL OF THE FUNCTIONS ARE DERIVED FROM STATIC
COMPONENTS PROVIDED ON THE PCB. IN THIS ALL STATIC RELAYS
ARE TAKEN TO ONE MASTER PCB AND EACH FUNCTION PCB WILL BE
ADD ON CARD TYPE. SO TOTAL RELAY IS ONE BUT FOR EVERY
FUNCTION IS PCB IS AVAILABLE AND CONNECTED TO MAIN PCB.
SPACE OCCUPATION IS LESS. COMBINING ALL IS CALLED ONE
PROTECTION FUNCTION. NO INTER CONECTION WIRING. THIS IS PART
OF MAIN PCB.
DISADVANTAGES:
DISADVANTAGES ARE SAME AS ABOVE.
         3 ND GENERATION
THIRD GENERATION RELAYS :
STATIC RELAYS WITH ICs
IN THIS NO OF COMPONENTS ARE REDUCED AND BROUGHT OR BUILT
IN ONE INTEGRATED CHIP. DUE TO THIS THE RELAY SIZE IS REDUCED
AND SOME OF THE FUNCTIONS ALSO TAKEN IN TO ONE INTEGRATED
CHIP. THIS ALSO BUILT IN ONE PCB. ALL OF THE FUNCTIONS ARE
DERIVED FROM STATIC COMPONENTS PROVIDED ON THE PCB. IN THIS
ONLY MASTER PCB IS AVAILABLE, NO SEPARATE PCB FOR EACH
FUNCTION , ALL ARE INTEGRATED IN ONE PCB. PROBLEMS ARE SOME
WHAT REDUCED. SPACE OCCUPATION IS LESS. ONE PROTECTION
FUNCTION ONLY ONE RELAY AND ALL FUNCTIONS ARE INTEGRATED.
NO INTER CONNECTION WIRING DIAGRAM.
       3 ND GENERATION

NEXT MODIFICATION :
SEMI NUMERIC RELAYS
IN THIS SOME FUNCTIONS ARE CAN BE PROGRAMMABLE AND
INTERFACE THROUGH PC. SOME ICs ARE HAVING THE FACILITY
TO INTERACT THROUGH COMMUNICATION PORT. IT IS SOME
WHAT MODIFICATION TO IC BASED RELAYS. IN THIS SOME
FUNCTION CAN BE ENABLED AND DISABLED, BASED ON THE
REQUIREMENT.
     4 ND GENERATION
MICROPROCESSOR BASED NUMERICAL RELAYS
 IN THIS ALL OF THE FUNCTIONS ARE BROUGHT ON
 ONE IC. THE FOURTH GENERATION PROCESSOR-
 BASED RELAYS, DO HAVE THE WATCHDOG FEATURE,
 WHICH FACILITATES THE CHECKING OF POWER
 SUPPLY FAILS, CLOCK FREQUENCIES, AND OTHER
 PATTERNS. MOST OF THESE RELAYS HAVE AUTO TEST
 FEATURES WHICH TEST THE ELECTRONIC CIRCUIT
 FUNCTIONS    AT    REGULAR     INTERVALS    &
 AUTOMATICALLY.
MICROPROCESSOR BASED RELAYS

   BACK GROUND WORK

   ADVANTAGES & DISADVANTAGES

   OPERATIONS & ALGORITHMS IN
   MICROPROCESSOR

   FUNCTIONAL BLOCKS & OTHER HARDWARE OF
   MICROPROCESSOR BASED RELAY

   FILTERING TECHNIQUES

   TESTING OF MICROPROCESSOR RELAYS
MICROPROCESSOR BASED RELAYS - BACKGROUND WORK

 1960s
    A FEW CONCEPTS WERE PROPOSED

    HARDWARE WAS VERY EXPENSIVE

    BENEFITS OF MICROPROCESSORS FOR RELAYS
    WERE NOT CLEAR

    IEEE ARTICLE “FAULT PROTECTION WITH A
    DIGITAL COMPUTER” OUTLINED THE
    FEASIBILITY & PROBLEMS ASSOCIATED IN S/S
    PROTECTION WHEN A DIGITAL COMPUTER IS
    USED.
MICROPROCESSOR BASED RELAYS - BACKGROUND WORK
1970s
   TWO PAPERS WERE PUBLISHED
   “DIGITAL CALCULATION OF IMPEDANCE FOR TRANSMISSION
   LINE PROTECTION”
   “ 3 PH TRANSMISSION LINE PROTECTION WITH A DIGITAL
   COMPUTER”
   PROMINENT MANUFACTURERS LIKE WESTINGHOUSE, IBM
   STARTED INVESTIGATING S/S COMPUTER SYSTEMS
   PHILADELPHIA ELECTRIC & GE INITIATED PROJECTS ON
   DIGITAL TECHNIQUES FOR PROTECTION
   VARIOUS ALGORITHMS WERE DERIVED FOR DIGITAL
   CALCULATION OF PROTECTION PARAMETRS
   EXPERIMENTAL SYSTEMS WERE BUILT BY GE &
   WESTINGHOUSE TO CHECK ALGORITHMS
   FIRST GENERATION OF MICROPROCESSOR BASED RELAYS
   BUILT
MICROPROCESSOR BASED RELAYS - BACKGROUND WORK

 1980s
    MAJOR MANUFACTURERS LIKE ABB, GE, GEC , TOSHIBA,
    SIEMENS START DESIGN & SALES OF BROAD RANGE OF
    PRODUCTS FOR POWER SYSTEM PROTECTION
    MICROPROCESSOR IMPROVES PERFORMANCE SPECS FOR
    OPERATION IN INDUSTRIAL ENVIRONMENT
    MANY PLC BASED SYSTEMS ARE COMMISSIONED IN INDIA BY
    L&T, SIEMENS, ECIL ETC.
    MANY ELECTRICITY BOARDS & PROCESS PLANTS IN INDIA
    START USING MICROPROCESSOR BASED INSTRUMENTS

    THE WORD SCADA GETS POPULAR IN INDIA

    PGCIL GOES IN FOR MICROPROCESSOR BASED DISTANCE
    RELAYS IN INDIA
MICROPROCESSOR BASED RELAYS - BACKGROUND WORK
 1990s
    ABB  &   GEC   ALSTOM    INTRODUCED  RANGE   OF
    MICROPROCESSOR   BASED   RELAYS  FOR  ALL  UNIT
    PROTECTIONS
    MAJOR MANUFACTURERS LIKE ABB, GE, GEC , TOSHIBA
    STARTS DESIGN & SALES OF BROAD RANGE OF PRODUCTS
    FOR POWER SYSTEM PROTECTION

    MANY ELECTRICITY BOARDS & PROCESS PLANTS IN INDIA
    START USING MICROPROCESSOR BASED INSTRUMENTS

    PGCIL GOES IN FOR MICROPROCESSOR BASED DISTANCE
    RELAYS IN INDIA

    MICROCONTROLLERS / DSPs ARE INTRODUCED IN LATE 90s
    BY HARDWARE MANUFACTURERS WHICH HAVE IMPROVED
    THE SPEED OF OPERATION.
ADVANTAGES OF NUMERIC RELAYS

 PARAMETER               NUMERIC    CONVENTIONAL

 ACCURACY                1%         5% / 7.5%
 BURDEN                  < 0.5 VA   > 5 VA
 SETTING RANGES          WIDE       LIMITED
 MULTI FUNCTIONALITY     YES        NO
 SIZE                    SMALL      LARGE
 FIELD PROGRAMMABILITY   YES        NO
 PARAMETER DISPLAY       YES        NO
 SYSTEM FLEXILBILITY     YES        NO
 CO-ORDINATION TOOLS     MANY       TWO
 COMMUNICATION           YES        NO
 REMOTE CONTROL          YES        NO
 SPECIAL ALGORITHMS      MANY       LIMITED
 SPECIAL PROTECTIONS     YES        NO
 SELF DIAGNOSTICS        YES        NO
DIS-ADVANTAGES OF NUMERIC RELAYS




         SOFTWARE INTENSIVE

         OBSOLESENCE RATE

         EMI / EMC PROBLEMS

         SERIAL NATURE
PROTECTION ALGORITHM




     MEASUREMENMT METHOD

     TRIP TIME CALCULATION

     GOOD FILTERING CHARACTERISTIC
     (HARMONICS, NOISE, DC SHIFT)

     FAST TRIP DECISION
FUNCTIONAL BLOCKS OF A NUMERIC RELAY



      ANALOG       D                  RAM
       INPUT       S
     SU-SYSTEM     P                  ROM
                         MICRO       EPROM
                       PROCESSOR     FLASH
       DIGITAL
        INPUT
     SUB-SYSTEM                      DIGITAL
                                     OUTPUT
                                   SUB-SYSTEM


    POWER SUPPLY   COMMUNICATION
                     INTERFACE
ANALOG INPUT SUB SYSTEM




         SURGE      ANALOG
 CT   SUPPRESSION   FILTER

                                      A/D        MICRO
                             MUX
                                   CONVERTER   PROCESSOR

         SURGE      ANALOG
 PT   SUPPRESSION   FILTER
MICROPROCESSORS Vs MICRO CONTROLERS


     Microprocessor          Micro controller

                                Address Register
         Address Register
     C                      C
     O                      O
     N     Data Register    N    Data Register
     T                      T
     R   Arithmetic Logic   R   Arithmetic Logic
     O         Unit         O        Unit
     L                      L
          Accumulator            Accumulator

                                 ROM  Timers
                            I/O RAM Counters
                                EPROM UART
RELAY HARDWARE

                    16 / 32 BIT




                                                     PC
                                                    MODEM


                                                    IRIG-B
                                                    RE/CC



    NORMALLY     400KV    RELAYS SUPPLIED    WITH   FOLLOWING
       CONFIGUARATION/HARDWARE
    1. MIN 4Nos MAX 8Nos COMMAND/TRIP OUTPUTS
    2. MIN 24Nos SIGNAL OUTPUTS
    3. MIN 14 LED INDICATIONS
    4. MIN 24 BINARY INPUTS
SELF DIAGNOSTICS - TECHNIQUES USED
RAM        Checked by computing a checksum of memory contents
           and comparing it against a stored factory value.

RAM        Checked by periodically writing a specific data and
           reading back the memory contents

A/D        Checked by inputing a known value of + / - voltage.
           Any off set at a given time, is software corrected.

SETTINGS   Checked by checksums or CRC values can be stored
           and compared. Often, 2 or 3 copies of settings are stored
           and compared.

POWER      Checked by monitoring power supply voltage values
SUPPLY     from A / D converter.
TYPES OF SIGNALS REQUIRED FOR PROPER PROTECTION



 Current, Voltage and Distance Relays :
 Require fundamental frequency component signals.
 All other signals will interfere with protection process.


 Harmonic Restraint Relays :
 Require both the fundamental & the Harmonic components ,
 each value separately, for decision making process.
   4 ND GENERATION
1ST DEVELOPMENT:
SOFTWARE DEVELOPMENT IS APPLICATION BASED
RELAYS i.e EACH PROTECTION FUNCTION HAVING
SEPARATE SOFTWARE & HARDWARE.

Example:
1. LINE PROTECTION,
2. TRANSFORMER PROTECTION,
3. BUSBAR PROTECTION,
4. GENERATOR PROTECTION
5. MOTOR PROTECTION
6. REACTOR PROTECTION
7. CAPACITOR PROTECTION
     4 ND GENERATION
2ND DEVELOPMENT:
SOFTWARE DEVELOPMENT IS SOME GROUP BASED
RELAYS i.e SOME PROTECTION FUNCTIONS ARE
TAKEN IN TO ONE FLATFORM AND PROVIDED
COMMON SOFTWARE.
FROM THIS INBUILT FACILITY OF EVENT RECORDER
AND DISTURBANCE RECORDER IS DEVELOPED.

Example:
ABB: 1. REX 5xx SERIES FLATFORM
     2. REX 316 SERIES FLATFORM
     3. REX 670 SERIES FLATFORM
     4. RED 500 SERIES FLATFORM
     5. RED 600 SERIES FLATFORM
     4 ND GENERATION
3RD DEVELOPMENT:
UNIVERSAL SOFTWARE FOR ALL TYPES OF RELAYS
FOR PARTICULAR MANUFACTURER. i.e. ONE
SOFTWARE ONE MANUFACTURER.

Example:
1. SIEMENS: SIPROTEC SERIES – DIGSI
2. GE MULTILIN: ENERVISTA
3. AREVA : MICOM S1
     4 ND GENERATION
4TH DEVELOPMENT:
UNIVERSAL HARDWARE FOR ALL TYPES OF RELAYS
FOR PARTICULAR MANUFACTURER. ONE HARDWARE
FOR ONE MANUFACTURER.
BUT IT IS MODULAR DESIGN.
RELAY IS COMMON HARDWARE BASED ON
PROTECTION FUNCTION, PARTICULAR CARD IS
ADDED.

Example:
1. GE MULTILIN: UR SERIES & SR SERIES
     4 ND GENERATION
5TH DEVELOPMENT:
EACH        MANUFACTURER             ADOPTING    THEIR
PROPERITIERY          BASED           PROTOCOL     FOR
COMMUNICATION, INTERFACING, NETWORKING AND
AUTOMATION. SOME UTILTIES ARE REQUESTED
MANUFACTURERS          TO     SUIT      THEIR  ADOPTED
PROTOCOL.
Example:
1. UCA – Utility communication architecture
2. LON
3. SPA
4. PROFIBUS
5. MODBUS
6. DNP
7. FIELDBUS
8. MVB
9. IEC 60870
    4 ND GENERATION
6TH DEVELOPMENT:
BASED ON THE EXPERIENCE WITH DIFFERENT
PROTOCOLS, NEED FOR UNIFORMITY AND KNOW –
HOW FOR GLOBAL CONSIDERATION
 ONE WORLD
ONE TECHNOLOGY
ONE STANDARD
A UNIVERSAL PROTOCOL FOR COMMUNICATION,
INTERFACING AND NETWORKING IS DEVELOPED.
ALL MANUFACTURERS ARE FORM A GROUP AND
PROTOCOLS ARE STANDARDIZED. ANY RELAY CAN BE
COMMUNICATED FOR COMMON COMMUNICATION
PROTOCOL, i.e INTEROPERATABULITY. THIS IS
SPECIAL FOR AUTOMATION OF STATIONS.
Example:
1. IEC 61850
     BENIFITS OF UNIVERSAL PROTOCOL
One Protocol                            ETHERNET & TCP/IP
  for all the needs in the substation     Adopted worldwide
                                          Scalable technology
 flexible configuration
                                          Common use of infrastructure
 no gateways required

Quicker project execution               Separation from Application &
  comprehensive data model              Communication
  clear, standardised project-            data and application stay secure
and equipment description                 independent from communication
                                        systems
  Configuration of substation in XML      unconstrained further development of
                                        the technology
Efficient maintenance                   Innovation & Expansion
 robust data modelling                    firm rules for the description of new
 self-descriptive equipment             data- objects and functions
 automation-configuration in XML          Interoperability is maintained
INTEROPERATABULITY WITH ABB, AREVA & SIEMENS
            TECHNOLOGY COMPARISION
   SUBJECT       ELECTROMECHANICAL             STATIC             NUMERICAL
Measuring       Induction Discs,        Discrete R L C,      Microprocessors,
Elements &      Electromagnets,         Transistors,         Digital ICs, Digital
Hardware        Induction cups etc      Analogue ICs,        Signal Processors
                                        Comparators
Measuring       Electrical Quantities   Level Detectors      Analogue to Digital
Method          converted into          Comparison with      Conversion,
                Mechanical Force &      Reference Value in   Numerical
                Torque                  Analogue             Algorithms,
                                        Comparators          Techniques,
                                                             Evaluation Trip
                                                             Criteria
Timing Function Mechanical Clock        Static Timers        Counters
                  works, Dashpot
Visual Indication Flags, Targets        LEDs                 LEDs & LCD
                                                             Display
Trip Command    Additional Trip         Additional Trip      Trip Relays are
                Relay Required          Relay Required       Inbuilt
Contacts &      Fixed                   Fixed                Freely Configurable
Assignments
          TECHNOLOGY COMPARISION
    SUBJECT        ELECTROMECHANICAL            STATIC               NUMERICAL
Sequence of       Not Possible           Not Possible           Available
Events
Hardware Size     Bulky                  Modular, Compact       Most Compact
Parameter         Plug Setting, Dial     Thumb Wheel,           Human Machine
Setting           Setting                Potentiometers, DIP    Interface, Softwares
                                         Switches
Binary Input & Not Available             Not Available          Available & Freely
Output                                                          Configurable
Self Supervision Not Available           Partially Available    Available
Calibration      Frequently Required     Required as settings   Not required as
                 as settings drift due   drift due to ageing    settings are stored
                 to ageing                                      permanently in
                                                                Memory in Digital
                                                                Format
Multiple          Not Possible           Not Possible           Possible
Characteristics
Multiple          Not Possible           Not Possible           Possible
Integrated
Protection func
          TECHNOLOGY COMPARISION
    SUBJECT       ELECTROMECHANICAL           STATIC            NUMERICAL
Range of         Limited              Moderate             Wide
Settings
Service value    Not Possible         Not Possible         Possible
Indication
Disturbance      External Hardware    External Hardware    Inbuilt
Recording
Communication    Not Possible         Not Possible         Available
facility
Burden on CTs,   Higher               Lower Than          Lower
PTs & CVTs                            Electromechanical &
                                      Moderate
Protection       Only Protection      Protection &        Protection Control &
Control &                             Monitoring          Monitoring
Monitoring
Solution         Fixed                Fixed                Extension and New
                                                           development
                                                           Possible and Open
                                                           Architecture
Fault History    Not possible         Not Possible         Stored In Memory
         Relay ANSI Numbers (IEEE C37.2)
NUMBER                    DEVICE              NUMBER                      DEVICE
  2      Time delay relay                       61     Density Switch or Sensor
  3      Interlocking relay                     63     Pressure Switch
  21     Distance Relay                         64     Restricted Earth Fault Relay
  24     Volts per Hertz Relay                  67     Directional Over Current Relay
  25     Synchronism Check Relay                68     Blocking/Locking Relay
  27     Under Voltage Relay                    72     DC Circuit Breaker
  29     Isolating Contactor                    74     Alarm Relay
  30     Annunciation relay                     76     DC Over-Current Relay
  32     Directional Power Relay                78     Phase Angle measuring or out of step Relay
  37     Under Current / Power Relay            79     AC Auto Reclosure Relay
  40     Field failure (loss of excitation)     81     Frequency Relay
  46     Negative phase sequence Relay          83     Automatic selective control or Transfer Relay
  49     M/C or T/F Thermal Relay               84     Operating Mechanism
  50     Inst Over-Current Relay                85     Carrier or Pilot wire Receive Relay
  51     AC IDMT Over-Current Relay             86     Lockout/Tripping Relay
  52     AC Circuit Breaker                     87     Differential Relay
  53     Exciter or DC Generator Relay          89     Isolator or Disconnector
  55     Power Factor Relay                     91     Voltage Directional Relay
  56     Field Application Relay                92     Voltage or Directional Power Relay
  59     Over-Voltage Relay                     95     Trip circuit supervision Relay
  60     Voltage / Current balance Relay        99     Over-Flux Relay