Very Fast Transients (VFT) in Gas Insulated Substations (GIS)

Report
Les Recksiedler
Background – Transformers with a Wye
and Delta secondary winding
- Similar transformers not gassing or failing
-Increased harmonic currents
-No gassing during factory load run
-No increase in gassing during 2 day Field test load run
-One supplier known for “cookie cutter” approach
-Failure of three winding transformers in USA analyzed to be due to
harmonics.
-All transformer in each HVDC system either gassing or not.
-Data is sparse
Case A– Three Phase Transformers with
a Wye and Delta secondary winding
- HVDC System one year earlier all transformers not gassing
-All Transformer one year later gassing badly
-One transformer failed after 5 years
-3 transformer failures so far in 26 years
-Reports not available for transformer repairs
-No gassing during factory load run assumed to be 12 hours
-No appreciable gas increase during 2 day Field load run.
-Appreciable gassing within two months of in-service
Case A– Gassing Results
- Duval’s Triangle – about 700 oC
-No Furans one test only
Hydrogen
H2
Methane
CH4
Acetylene
C2H2
Ethylene
C2H4
Ethane
C2H6
CO
CO2
TDCG ppm
CO2/CO
Years
37
Normal
385
Low
5
Normal
350
High
293
High
187
Normal
5248
Medium
1257
Low
28.6
Abnormal
27
77
Normal
443
Medium
2
Normal
579
High
172
High
1154
Medium
5300
Medium
2427
Medium
4.59
Abnormal
16
247
Low
3139
High
20
Normal
3955
High
1245
High
140
Normal
5724
Medium
8746
High
40.89
Abnormal
4
Case A – Field Load Run Tests – Two Days
 Hydrogen 117 ---- 133
 Methane
273-----259
 Ethane
95------99
 Ethylene
305----311
 Acetylene
3--------3
 CO
438---- 419
 CO2
1671----1689
 TDCG
1231----1224
Case B – Single Phase Transformers with a Wye
and Delta Secondary winding
 All transformers in one HVDC system gassing
 One unit has acetylene and is being repaired
 Similar HVDC scheme all transformers not gassing
 Same supplier for both.
 Non gassing transformers produced 11 years earlier
 Remaining units are acceptable ????
Case B – Gassing Results – Two worst units
excluding unit with Acetylene

Unit 1
 Methane
800
 Ethylene
780
 Acetylene < 5
Unit 2
500
650
<5
Condition/Risk
Medium
High
Normal
 Extreme gassing above 70% loading
 Gassing 100 to 1000 times the daily gas volume in IEC
60599 table A3 at high loading
Case C – Gassing and Failures linked to
Harmonics
 IEEE transactions Volume 19, No 1, pp208-213 dated
January 2004, “Thermal problems caused by Harmonic
Frequency Leakage Fluxes in 3 Phase 3 winding
Converter Transformers”
 Magnetic fluxes must be managed
 Use CTC cable
 Leads arranged to cancel fluxes
 Increased distances
 Non-magnetic structural framework
Conclusions
 Factory Testing cannot ensure a reliable converter
transformer?
 Design Review a possible solution?
 What are critical factors to consider? Winding
configuration, lead routing, flux cancellation?
 Should warranty consider gassing results? If so at what
level?
 QUESTIONS & ANSWERS

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