Transformer Failure Due to Circuit Breaker Induced Switching Transients
- 1. 1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 2: EMTP Modeling and Case Studies David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 Transient Analysis Tools • Electromagnetic Transients Program (EMTP) • Developed by Hermann Dommel brought to BPA • Major contributors: Scott Meyer & Dr. Liu at BPA • Alternative Transients Program (ATP) • Alternative to commercialized EMTP • Free to all if agree not to commercialize it • EMTP-RV • PSCAD EMT/DC
- 2. 2 3 3 ATP • ATP Draw interface • 3-phase modeling • Solution method - Trapezoidal integration • Robust for wide variety of modeling needs • Extensive library of models • Many options and features 4 4 Source Model • “3-phase wye cosine” • Resistance in ohms • Inductance in mili-Henries • Source Impedance • Z1 and X/R • Z0 and X/R SYSTEM SOURCE AT 13.8 KV XUTIL RUTIL VUTIL UN U UT
- 3. 3 5 5 Cable Model • “Pi Model” • Resistance in ohms • Inductance in mili-Henries • Cable charging in micro-Farads • ½ charging on sending end • ½ charning on receiving end • Multiple pi models in some cases CABLE 13.8KV C1 LCABLE RCABLE C2 C/2C/2 6 6 Breaker Model • “Switch” • Time dependent switch • Open or close at a specific time • Opens at current zero unless specify otherwise • Current dependent switch • Needed for current chopping • Define current at which to open switch • Independent Poles (A, B and C independent) • Data request • Vacuum or SF6 breaker nameplate • Current chop (ask manufacturer) • Transient Recovery Voltage Ratings – T2, E2 and RRV VCB BKR
- 4. 4 7 7 Transformer Model • “Three Phase Model” • Resistance in ohms • Inductance in mili-Henries • Magnetizing current • Winding capacitance • CH and CL for dry-types • CHL for oil-filled • More detailed modeling • Saturation • Hysteresis • Data Request • %Z and X/R, MVA • BIL TRANSFORMER RLRG LTRAN RTRANT1 T2 CH CL N:1 8 8 Complete System Model TRANSFORMER RLRG LTRAN RTRANT1 T2 CH CL N:1 CABLE 13.8KV C1 LCABLE RCABLE C2 C/2C/2 SYSTEM SOURCE AT 13.8 KV XUTIL RUTIL VUTIL UN U UT VCB BKR
- 5. 5 9 9 Time Step • Time Step – Integration Time Step • Choice depends on frequency of expected transient • Too large – miss high frequency effects • Too small – excessive simulation times • Nyquist Criteria • Minimum sample rate = 2 x frequency • Example 10-20KHz ring for VCB transient 10 10 Switching Transient Simulations • Switching transient simulations • EMTP and model requirements • Case studies • Data Center • Ferry Propulsion • Laddle Melt Furnace • Successful techniques/solutions
- 6. 6 11 11 Ferry Propulsion Highlights • 60K passengers per day • 20M passengers per year • 4160V distribution • 3 x 2865kW diesel gens • 2 x 1865KW forward propulsion motors • 2 x 1865KW reverse propulsion 12 12 Problem • Forward propulsion drives ferry • Upon reaching dock, reverse propulsion stops ferry • July 1, 2009, reverse propulsion was lost entering dock • lost power and hit a pier at full speed • one serious injury and nine minor injuries • 750 to 800 passengers were aboard • impact did not send any passengers overboard • 1185KVA rectifier transformer failed on reverse propulsion motor • Evidence of insulation failure on first few turns of primary winding • Investigation into source of such failure VCB switching
- 7. 7 13 13 Ferry - Oneline Vacuum Breaker Short Cable Dry-Type Transformer Forward Reverse 14 14 Ferry – Electrical Highlights • 3 x 2865kW diesel generators • 4160V, 3-bus system • Vacuum circuit breakers – 630A mains, ties, feeders • 2 x 1865kW motors (forward propulsion) • 2 x 1865kW motors (reverse propulsion) • 8 x 1185KVA dry-type transformers, 30kV BIL • 3-winding rectifier transformers • 12-pulse effective (6-pulse VFD per winding) • Feeder cable lengths of 50 feet each
- 8. 8 15 15 Worst Case Scenarios • Feeder cable lengths of 50 feet • Each 1185KVA transformer has one 4160V feeder • Need to examine both feeders for each transformer • “Worst Case” Switching Transient Simulations • Close 4160V VCB to transf. with 50ft. cable (model validation) • Close 4160V VCB to transf. with 50ft. cable (re-ignition) • Open 4160V VCB to transf. with 50ft. cable (current chop) • Repeat each case with Snubber 16 16 Measurements Transient Overvoltages at Primary of Rectifier Transformer – VCB Closes
- 9. 9 17 17 Study Cases • Case 1 – Closing the 4.16 kV feeder breaker feeding the three winding rectifier transformer with the distance of 50 feet. Simulated to match the Siemens measurements to ensure that the computer model is accurate. (Model Validation) • Case 2 – Same as Case 1, except with the RC snubber. • Case 3 - Closing the 4.16 kV feeder breaker feeding the three winding rectifier transformer, then the 4.16 kV feeder breaker opens during the inrush current. Shows the possibility of the vacuum breaker re-ignition. (Re-ignition) • Case 4 - Same as Case 3, except with the RC snubber. • Case 5 - Open the 4.16 kV feeder breaker feeding the three winding rectifier transformer under the light load condition. (Current Chop) • Case 6 – Same as Case 5, except with the RC snubber. 18 18 Case 1 – Model Validation Transformer primary voltage V max of 4.96kV < 30kV BIL Oscillation of 20,203Hz > 1000Hz
- 10. 10 19 19 Case 2 – Valid Model with Snubber Transformer primary voltage V max of 3.982kV < 30kV BIL Oscillation of < 1000Hz 20 20 Case 3 – Re-ignition Transient Recovery Voltage (TRV) at VCB
- 11. 11 21 21 Case 4 – Re-ignition with Snubber Transient Recovery Voltage (TRV) at VCB 22 22 Case 5 – Current Chop Transformer primary voltage V max of 31.9kV > 30kV BIL Oscillation of 958Hz ~ 1000Hz
- 12. 12 23 23 Case 6 – Current Chop with Snubber Transformer primary voltage V max of 8.9kV < 30kV BIL Oscillation of 299Hz < 1000Hz 24 24 Summary of Current Chop Cases
- 13. 13 25 25 Summary of Re-Ignition Cases 26 26 Recommendations • Install snubber at primary of each 1185KVA rectifier transformer • 40ohm resistor • Non-inductive • Peak voltage – 6 kVpeak • Peak energy – 2100 Joules • Average power – 190 Watt • 0.5uF capacitor • Rated voltage - 4.16kV
- 14. 14 27 27 Data Center Highlights • Tier III • LEEDS Certified • 12.5MVA Capacity • 13.2KV Ring Bus 28 28 Data Center - Oneline Vacuum Breaker Short Cable Cast-Coil Transformer
- 15. 15 29 29 Data Center – Electrical Highlights • 2 x 24.9 kV lines from Factory Shoals and Buzzard Roast • 2 x 12.5 MVA transformers • 13.2 kV “ring-bus” configuration • MSA and MSB and generator bus GPS • Vacuum circuit breakers – 600A mains & ties, 200A feeders • 3 x 2250 KW generators • 6 x 3750KVA cast coil transformers, 90kV BIL • 3 x DE Subs CSA, CSB and CSC • 3 x SE Subs MDSA, MDSB and MLB • Feeder cable lengths vary from 109 to 249 Feet 30 30 Worst Case Scenario • Feeder cable lengths vary from 109 to 249 Feet • Each 3750KVA transformer has two 13.2kV feeders • Need to examine both feeders for each transformer • Shortest of all 13.2kV cable runs to 3750KVA Transformers • MSB to Transformer CSC – 109 feet • MSA to Transformer CSC – 111 feet • “Worst Case” Switching Transient Simulation • Open 13.2 kV VCB at MSB to CSC with 109ft. cable • Open 13.2 kV VCB at MSA to CSC with 111ft. cable
- 16. 16 31 31 Worst Case – Study Case 13 • Study Case 13 - Open the 13.2 kV feeder breaker at Bus MSB feeding the 3,750 kVA dry type transformer CSC with the shorter distance of 109 feet. The result for this case will represent the “worst-case” condition, the other feeder from Bus MSA has a longer feeder distance of 111 feet. • Study Case 14 – Same as Case 13, except with the application of the 0.25 μF surge capacitor. • Study Case 15 – Same as Case 13, except with the application of the snubber circuit with 30ohm and 0.25 μF surge capacitor. 32 32 Case 13 – no surge protection Load current at 13.2kV VCB Load current of 10A Chopped current of 6A
- 17. 17 33 33 Case 13 – no surge protection Transformer CSC primary voltage VC max of 65.3kV > 95kV BIL VB max of 116kV > 95kV BIL VA max of 123kV > 95kV BIL Oscillation of 969Hz > 1000Hz 34 34 Case 14 – 0.25uF surge cap Load current at 13.2kV VCB Load current of 10A Chopped current of 6A
- 18. 18 35 35 Case 14 – 0.25uF surge cap Transformer CSC primary voltage VA max of 29.4kV < 95kV BIL VB max of 19.1kV < 95kV BIL VC max of 15.7kV < 95kV BIL Oscillation of 215Hz < 1000Hz 36 36 Case 15 – snubber 30ohm and 0.25uF Load current at 13.2kV VCB Load current of 10A Chopped current of 6A
- 19. 19 37 37 Case 15 – snubber 30ohm and 0.25uF Transformer CSC primary voltage VB max of 28.6kV < 95kV BIL VA max of 19.4kV < 95kV BIL VC max of 15.9kV < 95kV BIL Oscillation of 215Hz < 1000Hz 38 38 Case 15 – snubber 30ohm and 0.25uF Snubber Current – important for duty on resistor and capacitor IC peak of 7.7A IB peak of 8.0A IA peak of 6.8A
- 20. 20 39 39 Comparison of Results No Snubber 0.25uF surge cap Snubber R=30ohm C=0.25uF 40 40 Measured Transients with Snubber
- 21. 21 41 41 Recommendations • Install snubber at primary of each 3750KVA cast coil transformer 30ohm and 0.25uF • Install surge caps 0.25uF at each emergency generator • Install surge arresters at the following locations: • both incoming power transformers • every distribution dry type transformer • every generator • line side of both main circuit breakers • line side of the three generator circuit breakers • load side of every feeder breaker and every tie circuit breaker 42 42 Laddle Melt Furnace Highlights • Retiring 3 x 38MW EAFs • Adding 1 x 155MVA EAF • Adding 2 x 138kV lines to new EAF • Adding SVC at 34.5kV • Adding 1 x 20MW LMF
- 22. 22 43 43 LMF & EAF - Oneline Vacuum Breaker Short Bus Oil-Filled Transformer Existing EAF New LMF 44 44 138KV UTILITY 4713MVA 3PH SC 9.26 X/R 50/66/83MVA 135.3/26.4KV 7.5%Z SF-6 BREAKER 2000A 1600A 27KV 13OHM AUTO LTC 56MVA 27-10KV 3.3%Z ALUMINUM IPS BUS 53FEET VACUUM BREAKER 1200A LMF XFMR 50/56MVA 25/.53KV 2.5%Z HEAVY DUTY COPPER PIPE 28FEET LMF 20MW LMF Vacuum Breaker Short Bus Oil-Filled Furnace Transformer New LMF SF6 Breaker Short Bus Oil-Filled Auto-Regulating Transformer
- 23. 23 45 45 LMF Circuit – Electrical Highlights • 50MVA Power Transformer 135/26.4kV • 27kV system • SF6 circuit breaker – 2000A • Bus length of 53 feet • 56MVA autoregulating transformer, 200kV BIL • Vacuum circuit breaker – 1200A • Bus length of 28 feet • 50MVA oil-filled LMF transformer, 200kV BIL • 20MW LMF 46 46 Worst Case Scenarios • Examine switching transients at both transformers • “Worst Case” Switching Transients for Auto-Reg Transf • Open SF6 breaker to transf. with 53ft. bus (6A current chop) • 3 cycles after energizing Auto-Reg Tran, open SF6 bkr to transf. with 53ft. bus (re-ignition) highly inductive current • “Worst Case” Switching Transients for LMF Transf • Open VCB to transf. with 28ft. bus (6A current chop) • 3 cycles after energizing LMF Tran, open VCB to transf. with 28ft. bus (re-ignition) highly inductive current • Repeat each case with Snubber
- 24. 24 47 47 Case 1 – Open VCB LMF Transformer primary voltage V max of 386kV > 200kV BIL Oscillation of 1217Hz > 1000Hz 48 48 Case 2 – Open VCB with Snubber LMF Transformer primary voltage V max of 56.4kV < 200kV BIL Oscillation of 200Hz < 1000Hz
- 25. 25 49 49 Case 3 – Open SF6 Breaker Auto-regulating Transformer primary voltage V max of 23kV < 200kV BIL No oscillating frequency 50 50 Case 4 – Open SF6 Breaker with Snubber Auto-regulating Transformer primary voltage VB max of 54.7kV < 200kV BIL Oscillation of 197Hz < 1000Hz
- 26. 26 51 51 Case 5 – VCB Re-ignition Transient Recovery Voltage for VCB 52 52 Case 5 – Highly Inductive Current Inrush current to LMF transformer 6000A Peak Inrush VCB Opens VCB Closes
- 27. 27 53 53 Case 6 – VCB Re-ignition & Snubber Transient Recovery Voltage for VCB 54 54 Case 7 – SF6 Breaker Re-ignition Transient Recovery Voltage for Siemens SF6
- 28. 28 55 55 Case 7 – Highly Inductive Current Inrush current to Auto-Regulating transformer 14000A Peak Inrush SF6 CB Opens SF6 CB Closes 56 56 Case 8 – SF6 Breaker Re-ignition & Snubber Transient Recovery Voltage for SF6 Breaker
- 29. 29 57 57 Summary of Current Chop Cases 58 58 Summary of Re-Ignition Cases
- 30. 30 59 59 Recommendations • Install snubber (100ohm and 0.15uF) at primary of each transformer • 100ohm resistor • Non-inductive • Peak voltage – 38 kVpeak • Peak energy – 17,500 Joules • Average power – 1000 Watt • 0.15uF, 34.5kV surge capacitor (not available) • 1-pole, 24kV, 0.13uF • 2-pole, 14.4kV, 0.5uF • Series combination gives 0.103uF