Docstoc

Transformer Protection

Document Sample
Transformer Protection Powered By Docstoc
					                                                                 Transformer Protection




                                                                                        GRID
                                                                             Technical Institute




This document is the exclusive property of Alstom Grid and shall not be
transmitted by any means, copied, reproduced or modified without the prior
written consent of Alstom Grid Technical Institute. All rights reserved.
                                Transformer Fault Categories




    1. Winding and terminal faults

    2. Sustained or uncleared external faults

    3. Abnormal operating conditions such as overload,
       overvoltage and overfluxing

    4. Core faults




2    > Transformer Protection
                                Transformer Protection (1)



     Transformer Connections
     Overcurrent Protection
     Directional Protection of Parallel Transformers
     Partial Differential Protection of Parallel Transformers
     Earth Faults on Transformer Windings
     Unrestricted Earth Fault Protection
     Restricted Earth Fault Protection
     Biased Differential Protection of 2 and 3 Winding
      Transformers



3    > Transformer Protection
                                Transformer Protection (2)




     Combined Differential and Restricted Earth Fault
      Protection
     Protection of Auto-Transformers
     Inter-Turn Faults and Buchholz Protection
     Overfluxing Protection
     Overload Protection
     Transformer Feeder Protection




4    > Transformer Protection
                               Transformer Connections




5   > Transformer Protection
                                     Transformer Protection (3)




                         I     A2              Ø



                    V           EP



                               A1




6   > Transformer Protection
                                             Transformer Protection (4)




        I                              A2                  a2



    V                                   EP                  ES



                                   A1                      a1




7           > Transformer Protection
                                              Transformer Protection (5)




        IP                              A2                  a2      IS



    V                                    EP                  ES



                                    A1                      a1




8            > Transformer Protection
                                                       Transformer Connections

                                    A                          a a2
                               C1        A2
                                                                      a1
                                                                 c1             b
                                              A1                      b1   b2
                      C2
                      C B1                B2 B               c2
                                                               c

                                                         “Clock face” numbers refer to
    A                                              a     position of low voltage phase -
           A2           A1 a1             a2             neutral vector with respect to high
                                                         voltage phase - neutral vector.
    B                                              b
           B2           B1         b1     b2             Line connections made to highest
                                                         numbered winding terminal available.

    C                                              c     Line phase designation is same as
          C2             C1         c1    c2             winding.


9       > Transformer Protection
                                         Transformer Vector Groups



        Group 1                                           Yy0
                                                          Dd0
                          0     Phase displacement        Zd0
        Group 2                                           Yy6
                                                          Dd6
                    180         Phase displacement        Dz6

        Group 3                                           Yd1
                                                          Dy1
                       30       Lag phase displacement    Yz1

        Group 4                                           Yd11
                                                          Dy11
                       30       Lead phase displacement   Yz11


10   > Transformer Protection
                                                          Transformer Connections

          “Clock Face” numbers refer to position of low voltage
           phase-neutral vector with respect to high voltage phase
           neutral vector
          Line connections made to highest numbered winding
           terminal available
          Line phase designation is same as winding

                                        Example 1 : Dy 11 Transformer
                                                High        Low Voltage
                                               Voltage       Windings
                                               Windings
                             A Phase
                             Windings
                                          A2          A1   a1        a2
                             B Phase
                             Windings
                                          B2          B1   b1        b2
                             C Phase
                             Windings
                                         C2          C1    c1       c2

                                Question : How to connect windings ?
11       > Transformer Protection
                                           Dy 11

                                      12

                                11




                                     30°




12   > Transformer Protection
                                                                Dy 11


     1. Draw Phase-Neutral Voltage Vectors



                                 A  Line Designation a


                                                           30
                                                           °

                                                                  b




        C                                  B
                                                       c


13    > Transformer Protection
                                                Dy 11


     2. Draw Delta Connection


                                    A
                                            a




                                                  b




     C                                  B   c


14       > Transformer Protection
                                                               Dy 11


     3. Draw A Phase Windings


                                    A
                                                     a
                                                     a2
                                        A2

                                                          a1
                                                                 b

                                             A
                                             1

     C                                           B   c


15       > Transformer Protection
                                                                   Dy 11


     4. Complete Connections (a)


                                    A
                                                     a
                                                         a2

                            C1          A2
                                                              a1
                                                          c             b
                                                              b1   b2
                                                          1
             C                               A
             2                               1
                                                     c
                       B                 B           2
     C                                           B
                       1                 2           c


16       > Transformer Protection
                                                         Dy 11


         4. Complete Connections (b)




     A                                                           a
                                     A    A    a1   a2
                                     2    1
     B                                                           b
                                     B    B    b1   b2
                                     2    1
     C                                                           c
                                     C2   C1   c    c
                                               1    2


17        > Transformer Protection
                                       11kV Distribution Transformers
                                                 Typical Fuse Ratings


         Transformer rating                              Fuse

                           Full load         Rated       Operating time at
         kVA
                          current (A)      current (A)     3 x rating(s)

          100                   5.25          16                 3.0
          200                   10.5          25                 3.0
          300                   15.8          36                10.0
          500                   26.2          50                20.0
         1000                   52.5          90                30.0




18   > Transformer Protection
                                         Traditional Small Transformer
                                                   Protection Package
                                               3.3k
                                               V
                                                           5   50
                                     200/5      51    50   N
                                                           1   N




                                              1MVA
                           1500/5
                                              3.3/0.44kV

                            51
                            N




                                64            1500/5


19   > Transformer Protection
                                     Traditional Medium Transformer
                                                  Protection Package
                                             11kV


                                               51   50

                                               64




                          1000/5            5MVA
                                            11/3.3kV
                            51
                            N




                                64           1000/5


                                              3.3kV
20   > Transformer Protection
                                Overcurrent Protection




21   > Transformer Protection
                                Transformer Overcurrent Protection




           Requirements

            Fast operation for primary short circuits

            Discrimination with downstream protections

            Operation within transformer withstand

            Non-operation for short or long term overloads

            Non-operation for magnetising inrush




22   > Transformer Protection
        Use of Instantaneous Overcurrent Protection



                                       HV
     Source                                      LV




                                  50

                                  51


     50 set to 1.2 - 1.3 x through fault level


23   > Transformer Protection
                                            Transient Overreach

     Concerns relay response to offset waveforms (DC
        transient)

                                               Definition

                                               I1 - I2
                                                       x 100
                                                  I2



                                      I2               I1 = Steady state
                                                            rms
     I1                              D.C                    pick up
                                                            current
                                        .              I2 = Fully offset
                                                            rms
                                                            pickup
                                                            current
24        > Transformer Protection
                                  Instantaneous High Set Overcurrent
                                       Relay Applied to a Transformer



                      51               51                             51
                     HV                HV                             L
                     2                 1                              V
                                                  HV2
                                            HV1
                                Time
                                        LV




                                                                           Current
                                                        IF(LV)    IF(HV
                                                          1.2IF(LV)
25   > Transformer Protection                             )
                                   2-1-1 Distribution (1)



                          I3Ø         I3Ø




                                0.866I3Ø
                         I3Ø




26   > Transformer Protection
                                                 2-1-1 Distribution (2)




                                           HV relay



                                            0.4 sec
                                LV relay




                                    0.866 I3Ø   I3Ø

27   > Transformer Protection
                                        Parallel Transformers
                                        Directional Relays (1)




                                                  51
                                   67
     Grid supply
                                                    Feeders
                                   51




                                   67              51


                                   51

28      > Transformer Protection
                                        Parallel Transformers
                                        Directional Relays (2)




                                                      51
                                   51       Bus
     Grid supply                            Section
                                                           Feeders
                                              51




                                   51                 51

29      > Transformer Protection
                                            Parallel Transformers
                                       Partial Differential Scheme

                                 Grid supply




                 P1         67                            67
                      S1
                                       51
                       S2
                 P2

                                        S2 S1
                                       P2      P1

                                                    51

                      51                                            51



                            Advantage : Reduced number of grading
30   > Transformer Protection              stages
                                Earth Fault Protection




31   > Transformer Protection
                                                          Transformer Earth Faults

                                          3 p.u. turns 1 p.u. turns



                                                           x
                                     IP


                                 PR
                                                      R        IF
                          Protective
                            Relay

                                                                                             
     ResistorlimitsE/Fcurrentto fullloadvalues                     primarycurrent,IP =
                                                               Thus,                              x .IF.L.
                                                                                              3
                                                                                             2
     For a fault at  : Faultcurrent =  . IF.L.                                         =        . IF.L.
                                                                                              3
                                                           If C.T. ratio (on primary side) is based on full
     Effectiveturnsratio = 3 :                            load current of transforme r, then C.T. secondary
                                                                      2
                                                            circuit =
                                                                       3
32        > Transformer Protection
                                                   Overcurrent Relay Sensitivity
                                                             to Earth Faults (1)
                                              If as multiple
                                                  of IF.L.

                                                    1.0
                                                    0.9                     Star Side
                                                    0.8
                                         IF
                                                   0.7
                                                    0.6
         51
                                                    0.5
     Overcurrent
                                                    0.4
       Relay                                                                                    Delta Side
                                                    0.3
                                                    0.2
                                                    0.1
                                                                                                       
                                                            0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
                                                                                                      p.u..
                                                          Overcurrent Relay Setting > IF.L.

33            > Transformer Protection
                                                   Overcurrent Relay Sensitivity
                                                             to Earth Faults (2)
                                              If as multiple
                                                  of IF.L.
                                                    10
                                                      9                      Star Side
                                                      8
                                         IF           7
                                   

                                                      6
         51                                           5
     Overcurrent                                      4
       Relay                                                                                    Delta Side
                                                      3
                                                      2
                                                      1
                                                                                                      
                                                           0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
                                                                                                     p.u..


34            > Transformer Protection
                                          Overcurrent Relay Sensitivity
                                                    to Earth Faults (3)
                                     If as multiple
                                         of IF.L.

                                            10
                                             9            IF
                                IF
                                             8
                            
                                             7
                                             6                              IP
     IP                         IN           5
                                             4
                                             3
                                             2
                                                                       IN
                                             1
                                                                                         
                                                  0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
                                                  1.0                                   p.u..


35        > Transformer Protection
                                    Earth Fault on Transformer Winding



                                                    2
                                               I=
                                                    3

        I                                      For relay operation, I > IS
                                                                         2
                                               e.g. If IS = 20%, then         > 20% for operation
                                                                        3
                                                    i.e.  > 59%
                                Differential   Thus 59% of      winding is not protected
                                Relay
                                Setting = IS



                  Differential Relay Setting      % of Star Winding Protected
                              10%                             58%
                                    20%                            41%
                                    30%                            28%
                                    40%                            17%
                                    50%                            7%
36   > Transformer Protection
                                Unrestricted Earthfault Protection (1)




                                             51N      51 51 5
                                                            1




          Provides back-up protection for system
          Time delay required for co-ordination



37   > Transformer Protection
                                Unrestricted Earthfault Protection (2)




                                              51N     51 51 5
                                        51N                 1




          Can provide better sensitivity
                (C.T. ratio not related to full load current)
                (Improved “effective” setting)
          Provides back up protection for transformer and system

38   > Transformer Protection
                                               Star Winding REF




                                 Protected
                                   Zone



                                         RE
                                         F



          Relay only operates for earthfaults within protected zone.
          Uses high impedance principle.
          Stability level : usually maximum through fault level of
                             transformer
39   > Transformer Protection
                                Restricted E/F Protection
                                Low Voltage Windings (1)

                                                 A B C N




                                      LV restricted E/F
                                      protection trips
                                      both HV and LV breaker

                                Recommended setting : 10%
                                rated


40   > Transformer Protection
                                     Restricted E/F Protection
                                     Low Voltage Windings (2)

                                                         A B C N




          LV restricted E/F protection trips both HV and LV breaker
          Recommended setting : 10% rated



41   > Transformer Protection
                                 Delta Winding Restricted Earth Fault



         Source




                                        Protected zone

                                REF



          Delta winding cannot supply zero sequence current to
           system
          Stability : Consider max LV fault level
          Recommended setting : less than 30% minimum earth fault
           level

42   > Transformer Protection
                                                   High Impedance Principle

                                              Protected
                                               Circuit

                                Z   RC                              RC   Z
                                M   T                               T    M
                                         RL             IF     RL
                                                    I
                                              VS    RS
                                                    S
                                                    T
                                                   R
                                         RL                    RL

         Voltage Across Relay Circuit                        VS = IF (RCT + 2RL)
         Stabilising resistor RST limits spill current to IS (relay setting)
              RST = VS - RR           where RR = relay burden
                       IS
         CT knee point
                 VKP = 2VS = 2IF (RCT + 2RL)

43   > Transformer Protection
                                 Non-Linear Resistors (Metrosils)



          During internal faults the high impedance relay circuit
           constitutes an excessive burden to the CT‟s.
          A very high voltage develops across the relay circuit and
           the CT‟s.
                        Causes damage to insulation of CT, secondary
                         winding and relay.
          Magnitude of peak voltage VP is given by an approximate
           formula (based on experimental results)
                                VP = 2  2VK (VF - VK)
               Where VF = Swgr. Fault Rating in amps x Z of relay
               circuit                                    CT ratio
               @ setting

          Metrosil required if VP > 3kV


44   > Transformer Protection
                                      Non-Linear Resistors (Metrosils)

                                IO
                                P

                                                               RST
                                       V               VS
                                       M
                                                                R
                                                                R

                                     Metrosil Characteristic
                                           V = CI


      Suitable values of C &  chosen based on :
               Max secondary current under fault conditions
               Relay setting voltage

45   > Transformer Protection
                                                             REF Protection Example


                 1MVA
                  (5%)                          1600/1
            11000V     415V                     RCT =                    Calculate :
                                                 4.9
                                                                         1) Setting voltage (VS)
     80MV
     A                                                                   2) Value of stabilising
                                                                            resistor required
                                                                         3) Effective setting
                                                                         4) Peak voltage
            1600/1                            RS                            developed by CT‟s
            RCT =                                           MCAG14          for internal fault
             4.8                                         IS = 0.1 Amp

                                        2 Core 7/0.67mm
                                        (7.41/km)
                                        100m Long




46           > Transformer Protection
                                                               Solution (1)

     Earth fault calculation :-
     Using 80MVA base
               Source impedance = 1 p.u.
               Transformer impedance = 0.05 x 80 = 4 p.u.
                                              1
            1 P.U.                                    Total impedance = 14
     p.u.                        1   4
                                          I1           I1 = 1 = 0.0714 p.u.
                                                             14
                                                      Base current = 80 x 106
                                     4                               3 x 415

                                          I2                = 111296 Amps
                                                IF   = 3 x 0.0714 x 111296
                                                      = 23840 Amps (primary)
                                     4                = 14.9 Amps (secondary)
                                          I0


                       Sequence Diagram

47    > Transformer Protection
                                                                       Solution (2)


     (1) Setting voltage
                     VS = IF (RCT + 2RL)
           Assuming “earth” CT saturates,
                     RCT = 4.8 ohms
                     2RL = 2 x 100 x 7.41 x 10-3 = 1.482 ohms
                      Setting voltage = 14.9 (4.8 + 1.482)
                                            = 93.6 Volts
     (2) Stabilising Resistor (RS)
                     RS = VS - 1
                          IS IS2           Where IS = relay current setting
                      RS = 93.6 - 1 = 836 ohms
                             0.1   0.12




48   > Transformer Protection
                                                                                                   Solution (3)



                                     1.6
     (multiply by Kv to obtain RMS
           Weber/m2 (Tesla)
            secondary volts)




                                     1.2
                                                                                                    Kv     Ki

                                                                                      Line &        158   0.341
                                     0.8                                              Neutral CT

                                                                                      Earth CT      236   0.275
                                     0.4




                                        0                0.04      0.08        0.12
                                                                AT/mm
                                                    (multiply by Ki to obtain total
                                                      exciting current in Amps)

49                                    > Transformer Protection
                                                                     Solution (4)

     (3) Effective setting IP = CT ratio x (IS + IMAG)
              Line & Neutral CTs
                  Flux density at 93.6V = 93.6 = 0.592 Tesla
                                          158
                  From graph, mag. Force at 0.592 Tesla = 0.015 AT/mm
                   Mag. Current = 0.015 x 0.341 = 0.0051 Amps
              „Earth‟ CT
                  Flux density at 93.6V = 93.6 = 0.396 Tesla
                                          236
                  From graph, mag. Force at 0.396 Tesla = 0.012 AT/mm
                   Mag. Current = 0.012 x 0.275 = 0.0033 Amps
                  Thus, effective setting = 1600 x (0.1 + [(4 x 0.0051) + 0.0033])
              Effective setting = 198 Amps
                  Transformer full load current = 1391 Amps
                   Effective setting = 198 x 100% = 14.2% x rated
                                        1391

50   > Transformer Protection
                                                                 Solution (5)


     (4)      Peak voltage = 22VK (VF - VK)
                     VF = 14.9 x VS = 14.9 x 936 = 13946 Volts
                                  IS
              For „Earth‟ CT, VK = 1.4 x 236 = 330 Volts (from graph)
               VPEAK = 22 x 330 x (13946 - 330)
                                  = 6kV
              Thus, Metrosil voltage limiter will be required.




51     > Transformer Protection
                                         Parallel Transformers

                                T     N A B C
                                1




                                                Bus Section




                                T2




                                       415 Volt
                                     Switchboard
52   > Transformer Protection
                                              Parallel Transformers
                                                         CT in Earth
                                            N A BC
                                T1




                                     51N




                                                     Bus Section
                                                     Open



                                T
                                2




                                     51N
                                            415 Volt
                                           Switchboar
                                                d

53   > Transformer Protection
                                           Parallel Transformers CT in
                                                      Earth and Neutral
                                               N A BC
                                T1




                                     51N




                                                        Bus Section
                                                        Open



                                T
                                2




                                     51N
                                               415 Volt
                                              Switchboar
                                                   d

54   > Transformer Protection
                                                       Parallel Transformers
                                                       Residual Connections

                                  T1                         NA BC
                                                                          F2




                                                                        Bus section

        Will maloperate if bus section is open
        for fault at F1 T2



                                                                          F1
                                                      415 volt switchboard
                                No maloperation for fault at F2 (but setting must be
                                greater than load neutral current)

55   > Transformer Protection
                                       Traditional Large Transformer
                                                  Protection Package
                                 33K
                                 V

                                       51 5
                         200/5            0




                     10MVA
                     33/11KV                     51
                                                 N      87
                                         600/5


                                                 64


                         600/5
                                                 5/5A




56   > Transformer Protection
                                Differential Protection




57   > Transformer Protection
                                            Differential Protection


      Overall differential protection may be justified for
       larger transformers (generally > 5MVA).
                Provides fast operation on any winding
      Measuring principle :
                Based on the same circulating current principle as the
                 restricted earth fault protection
                However, it employs the biasing technique, to maintain
                 stability for heavy through fault current
      Biasing allows mismatch between CT outputs.
      It is essential for transformers with tap changing
       facility.
      Another important requirement of transformer
       differential protection is immunity to magnetising in
       rush current.

58   > Transformer Protection
                                              Biased Differential Scheme


                                                  Differential
                                                   Current




                                                  I1 - I2
                   BIAS            BIAS I                              OPERATE
           I1                             2


                                I1 - I2
                                                                            RESTRAIN
            OPERATE



                                                                 I1 + I2   Mean Through
                                                                    2        Current




59   > Transformer Protection
                                                     Differential Protection


                                         PROTECTED
                                         ZONE
            HV                                                    LV




                                               R



        Correct application of differential protection requires CT ratio
        and winding connections to match those of transformer.
        CT secondary circuit should be a “replica” of primary system.
        Consider :
                  (1)           Difference in current magnitude
                  (2)           Phase shift
                  (3)           Zero sequence currents
60   > Transformer Protection
                                              Differential Connections



       P1               P2      A2       A1       a1       a2   P2   P1




                                     A        B        C




61   > Transformer Protection
                                                          Use of Interposing CT

                         P1     P2   A2           A1 a1      a2        P2        P1

                           S1   S2                                          S2    S1




                                             S2      S1 P1        P2




                                     R
                                         R
                                             R


         Interposing CT provides :
          Vector correction
          Ratio correction
          Zero sequence compensation
62   > Transformer Protection
                                                            Differential Protection
                                                  15MVA
                     150/5                      66kV / 11kV            800/5
                    P1     P2            A2         A1 a1       a2   P2      P1

                    S1           S2                                  S2    S1




                                              Dy1



     Given above: Need to consider -
         (1) Winding full load current     (2) Effect of tap changer (if any)
         (3) C.T. polarities
     Assuming no tap changer
         Full load currents:-       66kV: 131 Amp = 4.37 Amps secondary
                                    11kV: 787 Amp = 4.92 Amps secondary
     However, require 11kV C.T.‟s to be connected in 
     Thus, secondary current = 3 x 4.92 = 8.52A
                                       RATIO CORRECTION IS REQUIRED
63    > Transformer Protection
                                                                     Differential Protection
                                150/5                                        800/5
                           P1        P2   A2          A1 a1       a2        P2        P1

                                S1   S2                                          S2    S1



                                          4.37A                                  4.92A
                                                  S1      S2 P1        P2




                                          R           (2.56)   (5)
                                              R
                                                  R


     It is usual to connect 11kV C.T.‟s in    and utilise a    /   interposing C.T.
     (this method reduces lead VA burden on the line C.T.‟s)
     Current from 66kV side = 4.37 Amp
     Thus, current required from      winding of int. C.T. = 4.37 Amp
     Current input to    winding of int. C.T. = 4.92 Amp
     Required int C.T. ratio = 4.92 / 4.37 = 4.92 / 2.52
                                        3
     May also be expressed as : 5 / 2.56

64   > Transformer Protection
                                          Effect of Tap Changer


     e.g. Assume 66kV +5%, -15%
     Interposing C.T. ratio should be based on mid tap position
     Mid Tap (-5%)                 = 62.7 kV
     Primary current (15 MVA)      = 138 Amp
     Secondary current             = 4.6 Amp
      Interposing C.T. ratio required = 4.92 / 4.6
                                                3
                                             (    /    )
                                           = 4.92 / 2.66
     May also be expressed as : 5 / 2.7
     Compared with 5 / 2.56 based on nominal voltage




65    > Transformer Protection
                                                        Connections Check
                                              Arbitrary Current Distribution


                    P1          P2       A2        A1 a1        a2        P2         P1

                      S1        S2                                             S2   S1




                                              S2     S1    P1        P2




                                     R
                                         R
                                              R




66   > Transformer Protection
                                                         Connections Check
                                                   Add Delta Winding Current


                    P1          P2       A2        A1 a1        a2        P2         P1

                      S1        S2                                             S2   S1




                                              S2     S1    P1        P2




                                     R
                                         R
                                              R




67   > Transformer Protection
                                                  Connections Check
                                         Complete Primary Distribution


                    P1          P2       A2        A1 a1        a2        P2         P1

                      S1        S2                                             S2   S1




                                              S2     S1    P1        P2




                                     R
                                         R
                                              R




68   > Transformer Protection
                                                 Connections Check
                                     Complete Secondary Distribution


                    P1          P2       A2        A1 a1        a2        P2         P1

                     S1         S2                                             S2   S1




                                              S2     S1    P1        P2




                                     R
                                         R
                                              R




69   > Transformer Protection
                                     In-Zone Earthing Transformer
                                          a1   a2
                 P1        P2   A1   A2                        P2   P1

                                                               S2 S1




                                           S2 S1 T2 T1 P1 P2




70   > Transformer Protection
                                     In-Zone Earthing Transformer
                                          a2
                                                    Alternative (1)
                                          a1
                 P1        P2   A1   A2                      P2   P1

                                                             S2 S1




                                               S2 S1 P1 P2




71   > Transformer Protection
                                 In-Zone Earthing Transformer
                                                Alternative (2)

                                                      300/1




                                   EARTHING
                                    TRANSF.




                                900/1




                                              TO DIFFERENTIAL
                                                   RELAY
72   > Transformer Protection
                                 In-Zone Earthing Transformer
                                                Alternative (3)

                                                         300/1




                                   EARTHING
                                    TRANSF.




                                300/1




                                              1/0.33

                                                 TO DIFFERENTIAL
                                                      RELAY
73   > Transformer Protection
                                            Combined Differential and
                                     Restricted Earth Fault Protection

                 A2             A1 a1     a2      P1        P2
                                                 S1          S2




                                P1
                                     S1                P2
                                          REF
                                     S2                P1
                                P2
                                                       S1

                                                       S2
                         To differential relay

74   > Transformer Protection
                                                   Combined Differential and
                                                      Earth Fault Protection

      Using Summation Auxiliary Current Transformer




                                Restricted earth
                                fault relay
                                                             64
                                     Bias windings



                                                                       Differential relay
                                                   87   87        87
                                                                       operating windings


75   > Transformer Protection
                                                            Z            T1                    X
                                        PHASE a

                                        PHASE b                               A


                                        PHASE c                               A

                                                                              A




                                           N                    B
                                                                              G   G G

                                   T1            BEF
                                                                                          T2
                                                                F



                                                                                           T3

                                                                              J   J   J



                                                                                          T4
                                            Rb
                                            2                                 H H H
                                                       Rb
                                                       2
                                                                    Rb
                                                                    2



                                           Ro          Ro           Ro


                                OVERALL DIFFERENTIAL
                                RELAY
76   > Transformer Protection
                                                        Three Winding Transformer

                                       63MV             25MV
                                       A                A
                   300/5               132KV            11KV       1600/5


                                                        50MV
                                                        A
                                                        33KV




                                        1000/5


                                4.59             5.51                       10.33
                                2.88              5                         2.88
                                                  5




                                                         All interposing C.T. ratio‟s refer to
                                                         common MVA base (63MVA)
77   > Transformer Protection
                                Traditional Use of Interposing CT
                                             Dy1(-30°)




                                                    Yd11(+30°)




                                 R
                                     R
                                         R


     Interposing CT provides :
     Vector correction
      Ratio correction
      Zero sequence compensation

78   > Transformer Protection
                                         Integral Vectorial and Ratio
                                                      Compensation

                                 Power transformer



                                                                  Ratio
                                        Numeric                 correction
                                         Relay

                                                                 Vectorial
                                                                correction
                                        Differential
     Virtual interposing CT              element
                                                       Virtual interposing CT




79    > Transformer Protection
                                          Transformer Magnetising
                                                    Characteristic


          Twice
          Normal
           Flux




             Normal
              Flux




                                Normal
                                No Load
                                Current
                                      No Load Current
                                      at Twice Normal
                                            Flux
80   > Transformer Protection
                                         Magnetising Inrush Current
                                                       Steady State




     + m                       V    
                                    Im




     - m




81   > Transformer Protection
                            Magnetising Inrush Current
             Switch on at Voltage Zero - No residual flux



                                    Im
         2 m

                                         
                                V




82   > Transformer Protection
                                 Transformer Differential Protection
                                      Effect of Magnetising Current



      Appears on one side of transformer only

      Seen as fault by differential relay

      Normal steady state magnetising current is less than
       relay setting

      Transient magnetising inrush could cause relay to
       operate




83    > Transformer Protection
                                Transformer Differential Protection
                                      Effect of Magnetising Inrush



      SOLUTION 1 : TIME DELAY
             Allows magnetising current to die away before relay can
                operate

             Slow operation for genuine transformer faults




84   > Transformer Protection
                                Transformer Differential Protection
                                      Effect of Magnetising Inrush



      SOLUTION 2 : 2ND (and 5TH) HARMONIC RESTRAINT
             Makes relay immune to magnetising inrush
             Slower operation may result for genuine transformer faults
                if CT saturation occurs

             Used in MiCOM P63x




85   > Transformer Protection
                                Transformer Differential Protection
                                      Effect of Magnetising Inrush



      SOLUTION 3 : GAP MEASUREMENT TECHNIQUE

             Inhibits relay operation during magnetising inrush
             Operate speed for genuine transformer faults unaffected by
                significant CT saturation

             Used in MBCH & KBCH relays




86   > Transformer Protection
                                    Typical Magnetising Inrush
                                                   Waveforms




                                A
                                B
                                C




87   > Transformer Protection
                                                             Detection of Typical
                                                        Magnetising Inrush (50Hz)

            Bias
     Differential                        Differential
                                         comparator
      Threshold




                                                T1 = 5ms        T2 = 22ms
                                                                            Trip




88            > Transformer Protection
                                                        Restraint for Transformer
                                                              Magnetising Inrush

            Bias                                                               Trip
     Differential                        Differential
                                                        T1 = 5ms   T2 = 22ms
                                         comparator
      Threshold



              Differential input


              Comparator output

              T1

                                              Trip
              T2
                                           Reset

89            > Transformer Protection
                                               Operation for Transformer Faults


            Bias                                                               Trip
     Differential                        Differential
                                                        T1 = 5ms   T2 = 22ms
                                         comparator
      Threshold



              Differential input


              Comparator output

              T1

                                              Trip
              T2
                                           Reset

90            > Transformer Protection
                                     Protection of Auto-Transformer by
                                     High Impedance Differential Relays

      (a) Earth Fault Scheme


      A


      B


      C




                                                    High impedance
                                             87
                                                    relay



91        > Transformer Protection
                                  Protection of Auto-Transformer by
                                  High Impedance Differential Relays

      (b) Phase and Earth Fault Scheme


           A

           B

           C

           a
           b
           c

                                                87 87 87



                           n

92     > Transformer Protection
                                Inter-Turn Fault Protection




93   > Transformer Protection
                                                              Inter-Turn Fault




                                CT
                  E
                                                                        Load
                                                         Shorted
                                                         turn




                                Nominal turns ratio   - 11,000 / 240
                                Fault turns ratio     - 11,000 / 1
                                Current ratio         - 1 / 11,000


                                     Requires Buchholz relay


94   > Transformer Protection
                                         Interturn Fault Current / Number
                                                 of Turns Short Circuited


                                   100                                     10


                                             Fault current in short
                                   80                                      8
                                             circuited turns

     Fault current                 60                                      6    Primary current
     (multiples of                                                              (multiples of
                                              Primary input current
     rated current)                                                             rated current)
                                   40                                      4


                                   20                                      2


                                    0
                                         5      10     15     20      25
                             Turn short-circuited (percentage of winding)


95      > Transformer Protection
                                         Buchholz Relay Installation


                                            3 x internal pipe
                                                                    Conservator
                                            diameter (minimum)
                    5 x internal pipe
                    diameter (minimum)




                                                        Oil conservator

                                            3 minimum


                    Transformer




96   > Transformer Protection
                                                            Buchholz Relay



                                                     Petcock

     Alarm bucket                                               Counter balance
                                                                weight


     Mercury switch
                                                                Oil level




             To oil                                             From transformer
             conservator


     Trip bucket
                                                                Aperture adjuster


                                 Drain plug   Deflector plate


97    > Transformer Protection
                                Overfluxing Protection




98   > Transformer Protection
                                                            Overfluxing

      Generator transformers
      Grid transformers
         Usually only a problem during run-up or shut down, but can be
         caused by loss of load / load shedding etc.

         Flux   V
                   f
      Effects of overfluxing :
                 Increase in magnetising current
                 Increase in winding temperature
                 Increase in noise and vibration
                 Overheating of laminations and metal parts (caused by
                  stray flux)
      Protective relay responds to V/f ratio
            Stage 1 - lower A.V.R.
            Stage 2 - Trip field
99    > Transformer Protection
                                        Overfluxing Basic Theory



                                           2m
                              
                         V = kf
                                             m

       CAUSES
                                                                          Ie
              Low frequency
              High voltage
              Geomagnetic disturbances
       EFFECTS
              Tripping of differential element (Transient overfluxing)
              Damage to transformers (Prolonged overfluxing)


100   > Transformer Protection
                                             V/Hz Overfluxing Protection



                                            V  K
                                             f

      Trip and alarm outputs for clearing prolonged overfluxing

      Alarm : Definite time characteristic to initiate corrective action

      Trip : IDMT or DT characteristic to clear overfluxing condition

                Settings

                              Pick-up 1.5 to 3.0 i.e.   110V x 1.05 = 2.31
                                                        50Hz
                              DT setting range 0.1 to 60 seconds


101    > Transformer Protection
                                                                  V/Hz Characteristics


        Enables co-ordination with plant withstand characteristics

                                         t = 0.8 + 0.18 x K
                                                        2
                                                  (M - 1)
                         1000
                                                                                  K = 63
                            100                                                   K = 40
      Operating                                                                   K = 20
      time (s)                                                                    K=5
                              10
                                                                                  K=1

                                 1
                                     1   1.1   1.2      1.3     1.4   1.5   1.6
                                                     M = V Hz
                                                        Setting

102       > Transformer Protection
                                         Overfluxing Relay




            Ex
                                 G



                                         VT




         AVR                         R
                                     L




103   > Transformer Protection
                                          Application of Overfluxing Relay



                        Circuit breaker
                        position repeat relay
                                                       VAA relay

                          RL1-1                       Lower AVR
      DC                                                                        DC

                                                      Inhibit AVR
                                                      raise

                                                      Alarm

                             RL2-1

                                                      Generator field circuit
                                            Alarm     breaker trip coil
                                  RL2-2




104    > Transformer Protection
                            Thermal Overload Protection




105   > Transformer Protection
                                   Effect of Overload on Transformer
                                                       Insulation Life


                                 100
                                        With ambient of 20 C.
                                        Hot spot rise of 78 C is
                                        design normal.
                                        A further rise of 6 C
                                        doubles rate of
             Relative            10     using life.
             rate
             of using
             life

                                 1.0

                                             98
                                 0.1
                                       80   90 100 110 120 130 140
                                                  Hot spot temp C
106   > Transformer Protection
                                                          Overheating Protection


                                   I load                 Trip

                                                            Alarm
                               TD
                             setting
                                                          On
      Top oil of
                                   I load                        Fan
      power                                                      control
      transformer
                                                          Off

                                                          On
                                                                 Pump
                                                                 control
                                                          Off
      Heater                                                        Temp. indication
                                                      Local
                Thermal                Temperature                                     Remote
                replica                sensing resistor



107     > Transformer Protection
                                                 Overload Protection




       Overcurrent protection designed for fault condition
       Thermal replica provides better protection for overload
               Current based
               Flexible characteristics
               Single or dual time constant
                                               Time
               Reset facility
               Non-volatile



                                                            Current


108    > Transformer Protection
                                                           Thermal Overload
                                                     Oil Filled Transformers

      Trip time (s)
        10000
                                                                     Single
                                                                     characteristic:
                                                                      = 120 mins

          1000


                                                                     Dual
            100                                                      characteristic



                                                                     Single
               10                                                    characteristic:
                       1        2       3        4      5        6    = 5 mins
                           Current (multiple of thermal setting)

109       > Transformer Protection
                                                              Thermal Trip Time


                                       I 2          
                                       I  -q 
                                                
                      TripTime = ln 
                                       REF      P 
                                                       
                                        I 
                                              2
                                            -K qTRIP
                                      I REF
                                                    
                                                       


                        where           = heating time constant
                                    I = actual current measured by relay
                                  IREF = continuous current rating of protected plant
                                   qP    = previous thermal state
                                 qTRIP   = trip threshold
                                    K = multiplier (for actual temperature)

110   > Transformer Protection
                                 Transformer Feeders




111   > Transformer Protection
                                                                                       Protection of Parallel
                                                                                       Transformer Feeders
                                                                       Higher -
                                                                       voltage
                                                                       busbar

                                            Z         OC                                OC        Z




                                                     FTS                                FTS

                                     REF                                                              REF


                                     DP                                                               DP


                                                     Bh           WT              WT    Bh


                                     REF                                                              REF




                                            SBEF                                          SBEF
                         2       1         2 stage                                                              2
                                                                                         2 stage            1




                                 DOC            OC                                           OC         DOC




                                                                       Lower -
                                                           Load        voltage
                                                           Load
112   > Transformer Protection
                                                                       busbar
                                        Protection of Transformer Feeders




                                                    CTs      CTs         HV LV
                             CTs                                                 CTs


      TRIP




                      Feeder               PILOTS           Feeder        Transformer
                    Differential                          Differential     Differential
                    Protection                            Protection       Protection

                                                                   UNSTABILISE      TRIP

                                                                                     TRIP




113          > Transformer Protection
                                           Transformer Feeders


                                               FEEDE
                                               R



                                             PILOT
                                  PW                   P
                                             S
                                                       W

       For use where no breaker separates the transformer from the
        feeder.
       Transformer inrush current must be considered.
       Inrush is a transient condition which may occur at the instant
        of transformer energisation.
       Mag. Inrush current is not a fault condition
         Protection must remain stable.
       MCTH provides a blocking signal in the presence of inrush
        current and allows protection to be used on transformer
        feeders.
114    > Transformer Protection
                                                           Transformer Feeder Protection

                                                                    Dy11
              P1                     P2                                                   P2            P1
      A                                                I                   i                                 A
                  S1            S2                                                                S2   S1
      B                                                II                  ii                                B

      C                                                III                 iii                               C




                                      23   MCTH                                      MCTH 23
                                      24      17
                                                                                     17      24
                                      25
                                                                                             25
                                      26
                                      27                                                     26
                                              19                                             27
                                      28                                             19
                                                                                             28

                                      23                                             17
                                              17                                             23
                                      24
                                                                                             24
                                      25
                                           MBCI                             PILOTS    MBCI   25
                                      26
                                              18               18 17                 18      26
                                      27
                                              19                                             27
                                      28                       19
                                                                                             28
                                                        + MVTW
                                     27            1
                                           MFAC            02
                                     28    14                  13
                                                   3           14
                                                           -

115       > Transformer Protection
                                             P541/ P542 - Protection
                                             of Transformer Feeders


                                  Power transformer



                                                                 Ratio
                                           P540                correction
                                          Scheme
                                                                Vectorial
                                                               correction

      Virtual interposing CT                          Virtual interposing CT




116    > Transformer Protection

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:6
posted:10/17/2013
language:
pages:116