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					  Transformer Switching-A Current Update:Part 1
          Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin




                 Phil Hopkinson Introduction of Tutorial
                                 Panelists:
             •    Dr. Bob Degeneff: Chair C57.142
             •    Phil Hopkinson: 5-35 kV events and
                  thoughts about Mitigation and factory
                  tests
             •    Larry Coffeen: FRA measurements
                  and transients at 230 and 500 kV
             •    Nigel McQuin: Circuit Breakers
10/16/2007                    IEEE Minneapolis 10/15/07      1
  Transformer Switching-A Current Update:Part 1
          Background and New Findings
                  PJ Hopkinson, PE, R Degeneff, N McQuin




                 Tuesday Tutorials on Mitigation:
             •   Elements that must be prevented
             •   Concepts for possible mitigation
                 approaches.
             •   Factory Test to demonstrate
                 transformer immunity to switching.
             •   Discussion
10/16/2007                  IEEE Minneapolis 10/15/07      2
Transformer/Breaker/System
        Interaction


   Minneapolis, Minnesota
      October 15, 2007
       Robert C. Degeneff
                   Agenda
n   Background
    0CIGRE & IEEE Activity
n   Characteristics of Transformers
    0General
    0Single-Phase Response
    0Three-Phase Response
n   Summary



                    October 15, 2007   2
                 Background
n   A number of transformers failed
    dielectrically in field applications
    0Units passed all factory tests
    0Protected by arresters
    0Failed shortly after switching event
n   Well known that system can produce both
    aperiodic & oscillatory terminal voltages




                     October 15, 2007           3
              Aperiodic Voltages

n   Greater magnitude than system voltages
    0Switching transients
    0Lighting strokes
n   Addressed with ZnO, system arrangement &
    equipment design
n   Felt to be understood and under control
    0Standards
    0Industry rules of thumb




                            October 15, 2007   4
           Oscillatory Waveforms
n   When present generally of low magnitude
n   Traditionally felt not to be a major transformer
    insulation threat
    0Below arrester cutoff & “short” duration
n   Mentioned in standards
    0Not generally well understood
    0Potential to produce very large internal winding voltages
    0Several internal winding failures reported
n   Unexplained failures have raised industry interest



                         October 15, 2007                        5
         CIGRE JWG A2/A3/B3.21 Paper
Electrical Environment of Transformers
           Electa 218 pp 25-37
n   Historical Prospective
n   Field Case
n   Network Context
n   Transformers
n   Switching Devices
n   Phenomena of Interactions Between
    Transformers and Power Network
n   Transformer Risk Factors and Methodology for
    Risk Mitigation

                      October 15, 2007             6
            IEEE Working Group
n   Scope
    0Identify All Important Relationships
    0Provide Guidance/Education to Recognize
     Occurrences
    0Suggest and Quantify Mitigation Methods
    0Provide Clear Examples
n   Objective
    0Produce Application Guide – C57.142
     “A Guide To Describe The Occurrence And
     Mitigation Of Switching Transients Induced By
     Transformer, Switching Device, and System
     Interaction”



                      October 15, 2007               7
                     Summary
n   Transformers exhibit a complex response
    characteristics versus frequency
    0Terminal & Internal
n   Large internal voltages can be developed
    0Depend upon frequency, duration of excitation &
     overlap with transformer characteristic
    0Both to ground and within structure
n   These voltages can be higher than those
    produced by standard factory test wave forms

                        October 15, 2007               8
        Example
Transformer Cross Section




           October 15, 2007   9
      Information Normally Known
n Short circuit impedance
n Turns ratio voltage distribution

n Losses at low frequency

n Transient characteristics for full, chopped, &
  switching surge
n Capacitance



Characteristics at very high & intermediate
 frequencies normally not of interest

                    October 15, 2007               10
    Transformer Characteristics As a
         Function of Frequency

n Impedance
n Voltage distribution

n Amplification factor



     Normally not of primary interest




                    October 15, 2007    11
Turns Ratio




   October 15, 2007   12
Section of Transformer




        October 15, 2007   13
October 15, 2007   14
Voltage Distribution at 1st Zero




            October 15, 2007       15
2nd Resonance




   October 15, 2007   16
Amplification factor A




       October 15, 2007   17
Amplification factor B




       October 15, 2007   18
Effect of secondary termination on Z vs F
                                                       Impedance Vs. Frequency

                                          7


                                          6


                                          5




                Impedance (log10)
                                          4


                                          3


                                          2


                                          1


                                          0
                                    -10       0   10          20          30     40   50   60
                                                              Frequency (kHz)




                                                       Impedance Vs. Frequency

                                          9

                                          8

                                          7

                                          6
                Impedance (log10)




                                          5

                                          4

                                          3

                                          2

                                          1

                                          0
                                    -10       0   10          20          30     40   50   60
                                                             Frequency (kHz)




                                    October 15, 2007                                            19
Amplification factor At A                               Amplification Factor Between Nodes 63 and 85

                                               3


                                              2.5


                                               2




       Amplification Factor (log10)
                                              1.5


                                               1


                                              0.5


                                               0
                                      -10           0         10         20          30       40       50   60
                                             -0.5


                                               -1
                                                                         Frequency (kHz)




                                                        Amplification Factor Between Nodes 63 and 85

                                              2.5


                                               2


                                              1.5
       Amplification Factor (log10)




                                               1


                                              0.5


                                               0
                                      -10           0         10         20           30       40      50   60
                                             -0.5


                                               -1


                                             -1.5
                                                                         Frequency (kHz)




                                            October 15, 2007                                                     20
Effect of primary termination on Z vs F
                                                      Impedance Vs. Frequency

                                         7


                                         6


                                         5




               Impedance (log10)
                                         4


                                         3


                                         2


                                         1


                                         0
                                   -10       0   10          20         30      40    50    60
                                                            Frequency (kHz)


                                                      Impedance Vs. Frequency

                                         7


                                         6


                                         5
               Impedance (log10)




                                         4


                                         3


                                         2


                                         1


                                         0
                                   -10       0   10          20          30      40    50    60
                                                             Frequency (kHz)




                                   October 15, 2007                                               21
Amplification factor at A
                                                                                           Amplification Factor Between Nodes 63 and 85

                                                                             3


                                                                            2.5


                                                                             2




      Amplification Factor (log10)
                                                                            1.5


                                                                             1


                                                                            0.5


                                                                             0
                                                  -10                             0              10         20          30       40        50    60
                                                                           -0.5


                                                                             -1
                                                                                                            Frequency (kHz)




                                                                                            Amplification Factor Between Nodes 63 and 85

                                                                                  3


                                                                                  2
                                     Amplification Factor (log10)




                                                                                  1


                                                                                  0
                                                                    -10                0           10         20          30       40       50    60

                                                                                  -1


                                                                                  -2


                                                                                  -3


                                                                                  -4
                                                                                                             Frequency (kHz)




                                                                          October 15, 2007                                                             22
                   Summary


n All units have unique frequency
  characteristic
n Connection important

n Most analysis conducted single-phase

n Three-phase
    0Wye-Wye
    0Delta-Delta
    0Wye-Delta
    0Delta-Wye

                    October 15, 2007     23
Three-Phase Arrangements




         October 15, 2007   24
            Three-Phase Switching
n   Close A with
    B and C open &
    secondary unloaded
n   Close A with
    B open and C closed &
    secondary unloaded
n   Close A with
    B and C closed &
    secondary unloaded



                      October 15, 2007   25
Effect of Primary                                                          7
                                                                                          Impedance Vs. Frequency




  Terminaltion
                                                                           6


                                                                           5




                                               p d n e lo 1 )
                                             Ime a c ( g 0
                                                                           4




n   Close A with                                                           3


                                                                           2




    B and C open &                                               -10
                                                                           1


                                                                           0
                                                                               0    10           20            30    40    50    60



    secondary unloaded                                                                           Freque ncy (kHz)


                                                                                         Impedance Vs. Frequency

                                                                       8



n   Close A with                                                       7


                                                                       6




    B open and C closed
                            p d n e lo 1 )
                          Ime a c ( g 0
                                                                       5


                                                                       4




    & secondary
                                                                       3


                                                                       2


                                                                       1



    unloaded                                                    -10
                                                                       0
                                                                           0       10           20            30
                                                                                                Fre que ncy (kHz)
                                                                                                                    40     50    60




n   Close A with                                                       8
                                                                                         Impedance Vs. Frequency




    B and C closed &                                                   7


                                                                       6



    secondary unloaded
                          Im e a c (lo 1 )
                            pdne g0




                                                                       5


                                                                       4


                                                                       3


                                                                       2


                                                                       1


                                                                       0
                                                                -10        0       10           20           30     40    50    60

                          October 15, 2007                                                     Freque ncy (kHz)
                                                                                                                                     26
                                                                                                  Amplification Factor Between Nodes 63 and 254




Effect of Primary                                                                    2.5




                                                                                     1.5
                                                                                          2




                                                     mlif aio a t r lo 1 )
                                                    A p ic t nF co ( g 0
  Terminaltion                                                                       0.5
                                                                                          1




    Close A with
                                                                                          0


n
                                                                              -10             0          10         20           30      40       50   60
                                                                                    -0.5


                                                                                      -1



    B and C open &                                                                  -1.5
                                                                                                                    Freque ncy (kHz)




    secondary unloaded                                                              2.5
                                                                                                  Amplification Factor Between Nodes 63 and 254




    Close A with
                                                                                      2


n                                                                                   1.5




                                       mlif aio a t r lo 1 )
                                      A p ic t nF co ( g 0
                                                                                      1



    B open and C closed                                                             0.5


                                                                                      0




    & secondary
                                                                             -10              0          10         20           30      40       50   60
                                                                                    -0.5


                                                                                     -1




    unloaded                                                                        -1.5
                                                                                                                   Fre que ncy (kHz)




                                                                                                  Amplification Factor Between Nodes 63 and 254

n   Close A with                                                                      3




    B and C closed &                                                                  2
                            A p a n F c r (lo 1 )
                                              g0




                                                                                      1



    secondary unloaded
                             m lific tio a to




                                                                                      0
                                                                             -10           0            10         20            30      40       50   60

                                                                                     -1


                                                                                     -2


                                                                                     -3


                                                                                     -4
                                                                                                                   Frequency (kHz)
                          October 15, 2007                                                                                                             27
Protection / Mitigation Techniques
n Surge Arresters
n Surge Capacitors

n Snubber (Shunt Capacitor & Resistor)

n ZORC (Snubber & Arrester)

n Series Inductance - Shunt Capacitor




                October 15, 2007         28
                   Summary
n   Transformer failures rare
n   However, number of internal dielectric failures
    observed
n   Risk factors for breaker-transformer interaction
    identified
    0Coincidence of exciting frequency & transformer
     natural frequency
    0Duration of excitation
    0Low system damping
n   Mitigation methods straightforward
n   Should be applied as routinely as an arrestor

                      October 15, 2007                 29
  Transformer Switching-A Current Update:Part 1
          Background and New Findings
                      PJ Hopkinson, PE, R Degeneff, N McQuin

                                 Phil Hopkinson:
             •       Review of early events: Degeneff/Hopkinson
                     seminar of 2003
                 –     1996 Data Center
                 –     1997 Steel Rolling Mill
                 –     1998 Multiple transformer failure event
                 –     2001 Light Rail Switching event.
             •       New events examined from 2007
                 –     Another Data Center
                 –     Zinc Plating Plant
                 –     Oil Refinery transformer


10/16/2007                      IEEE Minneapolis 10/15/07         3
  Transformer Switching-A Current Update:Part 1
          Background and New Findings
               PJ Hopkinson, PE, R Degeneff, N McQuin



                        Data Center
                      •Vacuum Breaker
                         •150’ Cable
         •15 kV 3000 kVA Dry Open Vent Transformer
          with Disk-wound primary and sheet wound
                         secondary
             •UPS Load with 11th harmonic filters




10/16/2007               IEEE Minneapolis 10/15/07      4
            Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin

        Data Center
          Breaker


          Cable


          Transformer


          Inductive
          Load


          Low Power
          Factor
    Current Chops &
    Breaker reignitions
10/16/2007                  IEEE Minneapolis 10/15/07        5
            Transformer Switching-A Current Update:Part 1
                        Background and New Findings
                   PJ Hopkinson, PE, R Degeneff, N McQuin

        Data Center




               Failure point




    Current Chops &
    Breaker reignitions
10/16/2007                 IEEE Minneapolis 10/15/07        6
              Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin



             Data Center Switching Event w/o snubbers
         9000 amps at 700 kHZ Measured!



Current
Chops &
Breaker
reignitions
make 900
kHZ


10/16/2007                  IEEE Minneapolis 10/15/07         7
              Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin


       Data Center Tap-Tap Voltage during Breaker closing with 700 kHz




Current
Chops &
Breaker
reignitions
make 700
kHZ


10/16/2007                  IEEE Minneapolis 10/15/07                    8
              Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin


       Data Center Line-Gnd Voltage during Breaker opening with
       snubbers in place- - note absence of high frequency transients




Current
Chops &
Breaker
reignitions
make 700
kHZ


10/16/2007                  IEEE Minneapolis 10/15/07                   9
             Transformer Switching-A Current Update:Part 1
                        Background and New Findings
                   PJ Hopkinson, PE, R Degeneff, N McQuin


                 Data Center Measured Voltages
 Location         Maximum Measured Voltage
                  w/o snubbers with snubbers   Reduction
 Line-Gnd               30 kV       15 kV       50%
 Tap-Gnd                75 kV       10 kV       87%
 Tap-Tap                65 kV       1.2 kV      98%
Current
Chops &
Breaker
reignitions
make 900                 Conclusions:
  1.
kHZ Tap-Tap voltage w/o snubbers was 18 times normal BIL Test
     2. Present Design/Test Standards inadequate for this duty!

10/16/2007                 IEEE Minneapolis 10/15/07              10
             Transformer Switching-A Current Update:Part 1
                        Background and New Findings
                   PJ Hopkinson, PE, R Degeneff, N McQuin



                    Steel Rolling Mill
                   SF6 Breaker
                    1000’ Cable
  35 kV 10 MVA Liquid Filled Transformer with
    Layer-Wound primary and sheet wound
                    secondary.
  Load is (4) 2500 kVA substation transformers
            with d-c secondary drives.


10/16/2007                 IEEE Minneapolis 10/15/07         11
             Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin

             Steel Rolling mill-Liquid filled transformer




10/16/2007                  IEEE Minneapolis 10/15/07        12
             Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin

             Steel Rolling mill-Liquid filled transformer




               Layer-Layer puncture led to telescoping coil failure


10/16/2007                  IEEE Minneapolis 10/15/07                 13
             Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin

             Steel Rolling mill-Liquid filled transformer




               Layer-Layer puncture led to telescoping coil failure
10/16/2007                  IEEE Minneapolis 10/15/07                 14
             Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin

             Steel Rolling mill-Liquid filled transformer


       Cause of failure uncertain between Low-Side
       Vacuum and Hi-Side SF6 Breaker switching
       • Both produced current chops
       • Both switched into inductive loads
       • Shielded cable on both sides of the transformer




     R-C Snubber designed for 5 kV LV side of transformer- - suspecting
     the Vacuum breaker more than the 34.5 kV SF6 Breaker



10/16/2007                  IEEE Minneapolis 10/15/07                 15
              Transformer Switching-A Current Update:Part 1
                          Background and New Findings
                     PJ Hopkinson, PE, R Degeneff, N McQuin

Steel Rolling mill-Liquid filled transformer-switched by HV Air Switch
measuring HV Line-Ground Volts




             No LV Snubber                     LV R-C Snubber connected



 Note: LV Snubber seen by primary and protects both windings


10/16/2007                   IEEE Minneapolis 10/15/07                    16
               Transformer Switching-A Current Update:Part 1
                            Background and New Findings
                       PJ Hopkinson, PE, R Degeneff, N McQuin



               Triple Jeopardy-three transformer
               failures simultaneously


      •All three transformers protected by Station Class Arresters
      •One initiator
      •Two by collateral damage
             •Line end failures
             •Phase –Phase barrier flashover
             •Line end disk-disk flashover




10/16/2007                        IEEE Minneapolis 10/15/07          17
             Transformer Switching-A Current Update:Part 1
                        Background and New Findings
                   PJ Hopkinson, PE, R Degeneff, N McQuin


        Triple Jeopardy-three transformer failures simultaneously




     Collateral damage flashover over phase-phase barriers
10/16/2007                 IEEE Minneapolis 10/15/07                18
             Transformer Switching-A Current Update:Part 1
                        Background and New Findings
                   PJ Hopkinson, PE, R Degeneff, N McQuin


        Triple Jeopardy-three transformer failures simultaneously




     Collateral damage flashover disk to disk at line end
10/16/2007                 IEEE Minneapolis 10/15/07                19
              Transformer Switching-A Current Update:Part 1
                           Background and New Findings
                      PJ Hopkinson, PE, R Degeneff, N McQuin



        Light Rail Coil Flashover from Taps to each Line Terminall




Only high differential stress could track line to line and leave windings intact!
10/16/2007                      IEEE Minneapolis 10/15/07                           20
             Transformer Switching-A Current Update:Part 1
                          Background and New Findings
                     PJ Hopkinson, PE, R Degeneff, N McQuin



              New Data Center Transformer problems in 2007l




16 separate disk-disk flashovers from one event!

10/16/2007                    IEEE Minneapolis 10/15/07       21
             Transformer Switching-A Current Update:Part 1
                          Background and New Findings
                     PJ Hopkinson, PE, R Degeneff, N McQuin

             New Data Center Transformer problems in 2007l

                                                          Close-up view of one
                                                          flashover out of 16.


                                                          Normal disk-disk
                                                          stress ~18 kV for 150
                                                          kV BIL


                                                          Breakdown >60 kV



Transformer passed full and chop wave retest at the factory
Standing waves from TRV-Reignitions-Chops the most likely cause

10/16/2007                    IEEE Minneapolis 10/15/07                      22
             Transformer Switching-A Current Update:Part 1
                        Background and New Findings
                   PJ Hopkinson, PE, R Degeneff, N McQuin

                    New 2007 Occurrence

                                        •Large 25 MVA Open Vent Dry
                                        Transformers at 35 kV, 150 kV BIL
                                        •14 years successful operation with
                                        oil breaker
                                        •New Vacuum breakers in 2007
                                        flashed over 2 transformers with
                                        multiple disk-disk flashovers
                                        •R-C snubbers fixed problems



                                       Reignitions and Current Chops on
                                       TRV most likely cause of flashovers


10/16/2007                 IEEE Minneapolis 10/15/07                        23
             Transformer Switching-A Current Update:Part 1
                        Background and New Findings
                   PJ Hopkinson, PE, R Degeneff, N McQuin

                    New 2007 Occurrence
                                                       Liquid Filled
                                                       Transformer at Oil
                                                       Refinery




10/16/2007                 IEEE Minneapolis 10/15/07                        24
                          Transformer Switching-A Current Update:Part 1
                                                        Background and New Findings
                                                   PJ Hopkinson, PE, R Degeneff, N McQuin

                                                       New 2007 Occurrence
        1.1 LEGENDA:
                                                                                                                Liquid Filled
                     A        = CIR CUIT BREAKER

                     F        = CON TAC TOR                                                                     Transformer at Oil
                              = SH IEL DED CABLE
                                                                                                                Refinery- - Switching
                              = TR AN SFORMER
                                                                                                                LV vacuum breaker
        1.2 ELECTRIC NETWORK
                                                                                                                resulted in flashover to
                         SS 44         34.5 K V
                                                                                                                HV winding
                          L                        B



                                                                   HOVEN SA
                                                                   TECH NI P

         XFR-4901                                      XFR-4902                                      Opening 5 kV Tie Breaker
                                                                                                     “E” killed transformer on
                          C                        D
                                                                                                     left
             BUS A                                     SS 48           4.16 KV         BUS B
                                      E

                                                                               LS         HS
                                                       HS      H           F          G


                                                                                               Circuit Breaker Reignitions and Chops
                                                               C M-4901A
                                                                                               on TRV likely cause of failure



10/16/2007                                                                          IEEE Minneapolis 10/15/07                         25
               Transformer Switching-A Current Update:Part 1
                          Background and New Findings
                     PJ Hopkinson, PE, R Degeneff, N McQuin




              Current Chops and Breaker reignitions
             Appears most likely common denominator
               to all failures reported in 2003-present




10/16/2007                   IEEE Minneapolis 10/15/07         26
             Transformer Switching-A Current Update:Part 1
                            Background and New Findings
                       PJ Hopkinson, PE, R Degeneff, N McQuin

 The Damaging Switching Event: TRV, Reignitions, and Current Chops

                              Ping test #5 at Tap 4-5 & 9.5 amps   Feb. 1. 06
                                      Island Park Substation Unit #2
                                            Line 1 to Gr. Voltage
                                 With 20:1 Attenuators & Arc Gap@ 5.5"
                                                                     3 . Current decays a nd is chopped out
                                   60                                of conduct ion and voltage osc illates to
                                   40                                zero.
                                   20
                                       0
                       -200       -20 0                200            400              600             800            1000
                  kV




                                  -40
                                  -60
                                  -80
                                 -100
                                 -120
                                                                Microseconds


                1. Circuit breake r        2 . Contacts reignite ba ck int o   4. Second TRV                 5. Contacts
                contacts open and          c onduction, current ris es to      volt age rises to (-)80       open
                transient recov ery        peak and de cays to chopping                                      sufficiently to
                voltage ( TRV) rise s      level, induc ing oscillat ory
                                                                               kV , the n breaker            prevent
                                           t ransie nt. Voltage rises by       reignites, raising            re ignition
                by Ldi/dt t o (-)100
                                                                               volt age by +120 kV           current and
                kV in ~90 µ sec.           +145 kV in <1 µ sec., then
                                           oscillates to zero.                 in <1   µs ec., et c.         interruption is
                                                                                                             completed




10/16/2007                                 IEEE Minneapolis 10/15/07                                                           27
                 Transformer Switching-A Current Update:Part 1
                            Background and New Findings
                       PJ Hopkinson, PE, R Degeneff, N McQuin

 The Damaging Switching Event: TRV, Reignitions, and Current Chops




             Two dominant frequencies observed
             •   10-20 kHZ Transient Recovery Voltage (TRV)
                 associated with total system impedance
             •   300-1,000+ kHZ Circuit Breaker chops/reignitions,
                 driven mostly by cable impedance



        The superposition of TRV and Cable resonance produce
        standing waves that kill transformers


10/16/2007                     IEEE Minneapolis 10/15/07             28
                 Transformer Switching-A Current Update:Part 1
                            Background and New Findings
                       PJ Hopkinson, PE, R Degeneff, N McQuin

 The Damaging Switching Event: TRV, Reignitions, and Current Chops




             Significance of Breaker Reignitons
             •   Results in rapid winding voltage reversals
             •   Typical voltage reversals exceeding BIL <0.6
                 microsecond.
             •   Winding stresses exceed design requirements


        The superposition of TRV and Cable resonance produce
        standing waves that kill transformers


10/16/2007                     IEEE Minneapolis 10/15/07             29
             Transformer Switching-A Current Update:Part 1
                          Background and New Findings
                     PJ Hopkinson, PE, R Degeneff, N McQuin

 The Damaging Switching Event: TRV, Reignitions, and Current Chops

         Current Chops
         •   Ability to quickly extinguish current
         •   Chopping current limits
              •   ~6 amps for vacuum
              •   3-15 amps for SF6
         •   Switching time constants
              •   0.1 microseconds for vacuum
              •   1 microsecond for SF6

     Current Chops on reignition events redouble damaging stresses


10/16/2007                   IEEE Minneapolis 10/15/07               30
             Transformer Switching-A Current Update:Part 1
                         Background and New Findings
                    PJ Hopkinson, PE, R Degeneff, N McQuin

 The Damaging Switching Event: TRV, Reignitions, and Current Chops

         Part 1 Summary: New Understanding of
             2003 events attributed to
         •   Breaker Reignitions
         •   Current Chops




         All failures low current low power factor
         •Probable current <6 amps
         •Power Factor <0.1

10/16/2007                   IEEE Minneapolis 10/15/07               31
          Switching Transients on a Southern
             Company 500 / 230 kV Auto-
        transformer: Examples from Project to
                     Present Date

             IEEE Transformers Committee Meeting
                       Minneapolis, MN
                          10/15/2007

                           Larry Coffeen
                   NEETRAC High Voltage Laboratory

             Presentation courtesy of The Southern Company

                  Contributions from JMX Services, Inc


10/16/2007              IEEE Minneapolis 10/15/07            32
             Switching transients at 500 / 230 kV
                      auto-transformer

• Laboratory withstand & calibration tests on a field
  installed laboratory grade voltage divider
• Transients due to de-energizing 950 ft. of 500 kV bus
  with motorized disconnect switch
• On-line transfer function of 500 / 230 kV auto using
  same disconnect switch transient
• Show comparison of on-line transfer function to baseline
  off-line transfer function
• Show transients for switching 500 / 230 auto trans out of
  service and switching auto back in service
• Show frequency content of a low level area lightning
  strike typically used for on-line FRA calculation

10/16/2007              IEEE Minneapolis 10/15/07         33
Wideband Voltage Divider
Near 230 kV X1 Bushing




     10/16/2007            IEEE Minneapolis 10/15/07   34
                        Laboratory
                        FRA Trailer




    10/16/2007                        IEEE Minneapolis 10/15/07   35
500 / 230 kV, Phase 3
                                                             500 kV




                                      230 kV




             Neutral




                        N . Filter




                                      High P ass          High Pass
                                         Filter              Filter
                                      with 60 Hz          with 60 Hz
                                      Separation          Separation


                  PF
                 Live                                                  12 Bit P icoScope
                                                                           Digitizers




                                                                         OnLine FRA
               Laptop
                                                                           Control
                Data
                                                                          Computer
             Connection




                                     ON-Line FRA System Layout
10/16/2007                            IEEE Minneapolis 10/15/07
                                          (a typical phase)
                                                                                           36
                                               Figure 1
      Laboratory Tests Performed
• ac 60 Hz ratio 40 kV - 300 kV (75,007:1)
• Partial Discharge @ 300 kV (2 PC)
• Withstand of 3+ & 3-, 1 MV lightning
  impulses (72,973:1 compared to
  laboratory 2.1 MV impulse divider)
• Time domain comparisons 45 kV - 200 kV
  impulse with laboratory reference voltage
  divider (76,015:1)
10/16/2007     IEEE Minneapolis 10/15/07   37
                                           Time Domain Calibration Check of BPA Voltage Divider


                             50

                             45

                             40

                             35
Ap p lied V o lt ag e




                             30
        ( kV)




                             25

                             20

                             15

                             10

                             5

                             0
                        -5        5   15         25         35          45        55           65   75   85        95
                                                                     Tim e
                                                          (m icroseconds) @ 10MS/sec.

                10/16/2007                            IEEE Minneapolis 10/15/07                               38
                                                         HV Reference Divider    BPA Divider
                                         Time Domain Calibration Check of BPA Voltage Divider
                                                [zoom in on front & peak of impulse]


                            50

                            45

                            40

                            35
Ap p lied Vo lt ag e




                            30
        ( kV)




                            25

                            20

                            15

                            10

                            5

                            0
                       -1        0   1      2         3           4           5          6      7   8   9        10
                                                                      Tim e
                                                          (m icroseconds) @ 10 MS/sec.

               10/16/2007                           IEEE Minneapolis 10/15/07                               39
                                                       HV Reference Divider       BPA Divider
                  De-energize 500 kV Bus Section


                     H1 Bushing Tap, HP Filtered




                     X1 Bushing T ap, HP Filter ed




                     X1 W ideband Divider, H P Filter ed




                    Phase 1 Neutral, HP Filtered




                    FFT of Divider W aveform




10/16/2007   IEEE Minneapolis 10/15/07                     40
                  De-energize 500 kV bus section


                  H1 Bush Tap, HP Filtered




                     FFT of H1 Bush Tap W aveform




10/16/2007   IEEE Minneapolis 10/15/07              41
               De-energize 500 kV bus section



                 H3 Bush Tap, HP Filtered




                 X3 Bush T ap, HP Filter ed




                Divider not on Phase 3, noise sho wn




                Phase 3 Neutral, HP Filtered




                      FFT of H3 Bush Tap W aveform




10/16/2007   IEEE Minneapolis 10/15/07                 42
               On-line FRA of H3-H0X0 using 500 kV switching
               transient from pr evious slide




10/16/2007   IEEE Minneapolis 10/15/07                         43
                        Off-line FRA test of all thr ee H W indings.
                        Shows similar fr equency peaks to onlin e results
                        from disconnect switch tran sient (previous slid e)




10/16/2007   IEEE Minneapolis 10/15/07                                        44
             Off-line test, Lo w Fr equency Rang e
             On-line has promin ent peak here. On-
             line energ y is much gr eat er at this
             resonant frequ ency (about 175 kHz)




10/16/2007                 IEEE Minneapolis 10/15/07   45
                 De-energize 230 kV Bus Section


                               H1 Bush Tap HP Filter ed




                            X1 Bush T ap HP Filtered




                            Noise Interferen ce




                  Phase 1 Neutral Bush HP Filtered




                         FFT of X1 Bush Tap




10/16/2007   IEEE Minneapolis 10/15/07                    46
             De-energize 230 kV bus section




                      X2 Bush T ap, HP Filter ed




                            Divider not on X2, noise sho wn




                      Phase 2 Neutral Bush, HP Filter ed




                                             FFT of X2 Bush Tap




10/16/2007                  IEEE Minneapolis 10/15/07             47
             De-energize 230 kV bus section

                   H3 W aveform Clipped




                X3 W aveform Clipped




               Divider not on X3, noise sho wn




                      Phase 3 Neutral Bush, HP Filter ed




                           FFT of Phase 3 N eutral W aveform




10/16/2007       IEEE Minneapolis 10/15/07                     48
                  Pick up load on re-energize


                          H1 Bush Tap, HP Filtered




                         X1 Bush T ap, HP Filter ed




                         X1 full b and width on divider




                          Phase 1 Neutral, HP filt ered




                        FFT of X1 Bush Tap W aveform




10/16/2007   IEEE Minneapolis 10/15/07                    49
             Pick up load on re-energize




               X2 Bushing T ap, HP Filter ed




                Divider not on X2, noise sho wn




                   Phase 2 Neutral, HP Filtered




                  FFT of X2 Bush Tap W aveform




10/16/2007   IEEE Minneapolis 10/15/07            50
                Area Lightning Strike, Phase 1



                       H1 Bushing Tap, off scale




                        X1 Bushing T ap, off scale




                        WB divider X1 w/ HP filter




                       Phase 1 Neutral, HP filt ered




                    FFT of W B divider waveform




10/16/2007   IEEE Minneapolis 10/15/07                 51
                Area Lightning Strike, Phase 2



                H2 Bush Tap, Clipped




                X2 Bush T ap, HP Filter ed




               Divider not on X2, noise sho wn




                   Phase 2 Neutral Bush, HP Filter ed




                  FFT of X2 Bush Tap W aveform




10/16/2007   IEEE Minneapolis 10/15/07                  52
                      Area Lightning Strike, Phase 3


                 H3 Bush Tap, HP Filtered




                X3 Bush T ap, HP Filter ed




             Divider not on X3, noise sho wn




                Phase 3 Neutral Bush, HP Filter ed




                      FFT of H3 Bush Tap W aveform




10/16/2007    IEEE Minneapolis 10/15/07                53
Switching Transient Observations on 500 / 230 kV
        Auto-transformer Project to Date


• 500 / 230 kV systems can have transients present with
  frequency content into the MHz region.
• These transients can be generated by local switching
  and / or lightning.
• High frequencies can pass through a 500 / 230 kV auto
  with significant energy content to excite resonant
  frequencies in the windings.
• The TRV capacitors on the 230 kV low side breaker at
  the test site reduced significant transients at the
  transformer bushings for the normal dropping of load.
• We need to increase our dynamic range of transient
  recording input for the most severe transients.

10/16/2007          IEEE Minneapolis 10/15/07             54
     Transformer, Circuit-breaker
       and System Interactions
                       Nigel P. McQuin
                McQuin Electrical Power Consulting, Inc.

             IEEE TR/C57 meeting, Minneapolis, 10/15/2007



10/16/2007                IEEE Minneapolis 10/15/07         55
 Transformers, circuit breakers and the
              system -1
• At all times on the electrical network, all
  connected parts are inter-acting with:

     – Steady state electrical voltage/current
     – Transient lightning/geo-technical disturbances
     – Transient switching disturbances




10/16/2007          IEEE Minneapolis 10/15/07       56
 Transformers, circuit breakers and the
              system - 2
• Standard design methods and testing aims
  to provide a satisfactory life expectation
  when electrical equipment is subjected to
  normal service voltage/current conditions.

• If service conditions may include unusual
  stressing events, these requirements
  should be added to the design/test
  procedures.
10/16/2007      IEEE Minneapolis 10/15/07     57
             Switching events
• Huge range of potential studies:
     – Sources - circuit switching devices
     – Propagation – system configuration
       dependent, and line/cable dependent
     – Equipment behavior – detailed
       design/construction dependent.

     – Who has performed transient
       voltage/switching studies?
10/16/2007         IEEE Minneapolis 10/15/07   58
             Potential sources of
                disturbances
• Circuit breaker – nothing new, >120 years
  of experience on both AC and DC circuits.

• Electronic controls, drive systems, UPS,
  active filters, etc – NEW.




10/16/2007        IEEE Minneapolis 10/15/07   59
   Circuit breaker characteristics
• All circuit breakers produce switching
  transients irrespective of the technology

• The character of the transient is a
  combination of the switching device and
  the connected circuit.



10/16/2007      IEEE Minneapolis 10/15/07     60
             Circuit breaker closing
• Inrush currents in
  low-power factor
  loads (inductors,
  transformers,
  capacitors)
• Starting transients
  in motor/generator
  circuits


10/16/2007          IEEE Minneapolis 10/15/07   61
         Circuit breaker opening - I




10/16/2007       IEEE Minneapolis 10/15/07   62
        Circuit breaker opening - II




10/16/2007       IEEE Minneapolis 10/15/07   63
       Circuit breaker opening - III




10/16/2007      IEEE Minneapolis 10/15/07   64
             Circuit breaker opening - IV




10/16/2007          IEEE Minneapolis 10/15/07   65
       Circuit breaker opening - V
• Qualification test characterization for:

     – Interrupting capabilities
     – Current zero instability
     – Re-ignitions
     – Re-strikes
     – Current chopping


10/16/2007            IEEE Minneapolis 10/15/07   66
     Electronic controls, UPS, etc
• Can produce large harmonic content
    (IEEE 519 limits)

• Can produce very specific frequencies

• Can produce high current-chopping
  capabilities, even with equipment
  unloaded.
10/16/2007     IEEE Minneapolis 10/15/07   67
             What’s important?
• Frequency content
• Voltage/current magnitude
• Persistence/repetition rates

• What is the system propagation response?
• What is the internal equipment response?

10/16/2007       IEEE Minneapolis 10/15/07   68
             System response
•   Line/cable propagation characteristics
•   Load power-factor
•   System damping effects
•   System/equipment capacitances

• Take measures to fix the cause, rather
  than deal with the outcome.

10/16/2007        IEEE Minneapolis 10/15/07   69
             Counter-measures
• Add capacitance to slow rate of transient
  voltage response
• Add selective damping/loss to dissipate
  the transient energy ASAP.

• If the equipment is important, add a
  snubber circuit to the system.

10/16/2007       IEEE Minneapolis 10/15/07    70
             Snubber circuit




10/16/2007      IEEE Minneapolis 10/15/07   71
               Conclusions
• The types of circuit switching devices has
  substantially increased.
• Modern circuit breakers, irrespective of
  technology have more favorable characteristics
  than old/early types.
• The use of low capacitance, low loss electrical
  components, increases the chances of initiating
  and sustaining switching transients.
• If the equipment is important, take measures to
  mitigate the possible source of disturbances.

10/16/2007        IEEE Minneapolis 10/15/07         72

				
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