CASE STUDY- FAULT IN POWER TRANSFORMER AT LOKTAK POWER STATION - S K Mishra & S K Das NHPC Ltd O&M Division 1
PRESENTATION COVERS Introduction DESCRIPTION OF EVENTS INITIAL RESPONSE DETAILED INSPECTION FINDINGS RESTORATION R OF TRANSFORMER ANALYSIS OF EVENT AND ROOT CAUSE FINDINGS CONCLUSION RECOMMENDATION 2
INTRODUCTION The faulted transformer (Identification no. B5255) under study is 13.33 MVA, single phase, 11 KV/132 KV/ 3, ONAF and was manufactured by GEC in 1977 and under operation since 1983. The winding of the said transformer was replaced in 2011 at factory works of OEM. The repaired transformer was transported to Power House by road & put into service in 2014. The Power evacuation system at Loktak Power Station consists of four 132 KV transmission line. Due to hilly terrain and the lines are laid through dense forest, frequent fault occurs on transmission lines. The transformers installed at Loktak Power Station are subject to frequent short circuit current vis-à-vis electro-mechanical stress. 3
DESCRIPTION OF EVENTS On 07/08/2016 all the three units ( 3 x 35 MW) of Loktak Power Station were running at full load prior to the event. At 9:00 hrs on 07/08/2016, Buchholz relay alarm of Y-phase transformer of Unit#3 appeared. During inspection, gas accumulation in the Buchholz relay was observed and the unit was stopped at 11:10 hrs for further investigation. Here it is pertinent to mention that, no other protection relay started or operated even after operation of transformer under fault condition for more than two hours which indicates low energy fault. 4
INITIAL RESPONSE Tan Delta and capacitance test: The test was conducted at 2 KV, 5KV & 10 KV at HV side and 1 KV, 3KV & 5KV at LV side at winding/oil temperature at 52 0 C. The test result at 10KV (HV side) and 5KV (LV side) is presented below: Sl. No. Insulation Tested Capacitance (pf) Tan Delta (%) The above test results showed no abnormality in transformer winding insulation. ma Watts 1 HV to LV +GND 4151 0.166 13.01 0.215 2 HV-GND 1718 0.181 5.395 0.097 3 HV-LV 2434 0.150 7.639 0.114 4 LV to HV +GND 7462 0.209 11.71 0.122 5 LV to GND 5030 0.238 7.891 0.094 6 LV to HV 2433 0.152 3.807 0.029 5
INITIAL RESPONSE Breakdown Voltage (BDV) test: The BDV of transformer oil was tested as 47 KV at 2.5 mm gap, which was also within permissible limit. Therefore, transformer oil insulation property also found to be acceptable. Insulation Resistance (IR) Test: IR test of transformer winding was conducted at 46 0 C by using 5KV applied voltage. Winding IR at 5KV/60Sec (GΩ) PI (IR at 10 min/ir at 1 min) HV-LV 20.3 1.4 HV-LV+E 62.0 1.28 LV-HV+E 36.9 1.8 The above test result showed readings within permissible limit. In addition to above turn ratio and magnetizing current tests were also conducted and the results were within normal limit. 6
INITIAL RESPONSE Result of DGA test: The DGA test of transformer oil (after fault) was conducted. The recent DGA test result of transformer oil sample along with that of previous result is as under: Characteristics PPM (measured on 08/08/2016) PPM (measured on 25/05/2016) Methane 70 4 Ethane 18 1 Ethylene 184 7 Acetylene 3 ND Hydrogen 29 2 Oxygen 17133 15645 Nitrogen 63041 55238 Carbon monoxide 345 241 Carbon dioxide 3541 2042 7
INITIAL RESPONSE The DGA test conducted on 08/08/2016 showed increased content of hydro-carbon gases i.e. Methane, Ethane, Ethylene and Acetylene as compared to that of previous result. This clearly indicated that there can be an arc inside the transformer causing overheating of oil and successive generation of hydro-carbon gases. The transformer was removed from service for further investigation. Unit#3 was restored using spare transformer. 8
DETAILED INSPECTION FINDINGS The faulty transformer was thoroughly inspected on 25/08/2016 at site. After dismantling of all the accessories and top cover from the transformer, it was found that the core along with winding had shifted from its original position. Metal dust was also found on the transformer tank guiding rail area. The core along with winding was removed from the transformer tank to check the extent of damage caused and it was found that the insulation sheet at the bottom of the tank got shifted from its original position. However, no deformation of core/winding or any hot spot was observed over the core/winding. The Inspection at tap changer section reveals partial melting of contact finger from its edge 9
DETAILED INSPECTION FINDINGS Fig.1 Metal Dust at core foundation rail Fig.2 Displacement of core insulation 10
DETAILED INSPECTION FINDINGS DETAILED INSPECTION FINDINGS Fig. 3 Healthy winding Fig. 4 Hot spot at Tap Changer contact 11
RESTORATION OF TRANSFORMER Thorough cleaning of transformer core and winding was done. It was decided to bypass the tap changer section based on following consideration: The tap changer is not used to control reactive power supply vis-à- vis bus voltage at our Generator Bus (PV bus). The reactive Power supplied by the generator is sufficient to control the bus voltage within permissible limit. Fig. 5 Tap Changer Bypassed 12
RESTORATION OF TRANSFORMER The transformer core-winding was put into transformer tank properly. The IR test was conducted to ensure proper insulation between Channel to core, Tank to Core and Channel to Tank. The transformer was properly boxed up by using new gaskets. All the accessories except conservator were fitted and the transformer was filled with Nitrogen with a pressure of 0.2 Kg/sqcm to ensure leakage free boxing up of transformer. The main tank of the transformer was kept under partial vacuum (at 400 mmhg) for 12 hrs to facilitate release of moisture from paper insulator. Oil filling was started thereafter by oil filtration plant after installation of Conservator and radiator. All electrical tests were conducted and the test results showed the value within permissible limit. The work was completed in 6 days & the transformer is kept as spare. 13
ANALYSIS OF EVENT & ROOT CAUSE FINDINGS Two possibilities for the failure is envisaged. a) Displacement of core-winding section may have occurred during transport of transformer after repair. b) Due to frequent line fault the transformer was subjected to electromechanical stress vis-à-vis vibration. This may have further contributed towards misalignment of tap changer contact finger and successive low energy PD/fault Premature O&M failures are occurring due to accelerated ageing and/or weakening of short circuit withstand capability of transformer due to repeated short circuits in the network. The number of through faults seen by the transformers being high has a cumulative effect on the mechanical weakening of the winding supports and insulation & increases the probability of premature failure of the transformer. 14
ANALYSIS OF EVENT & ROOT CAUSE FINDINGS A recent, extensive survey of about 1000 major transformer failures by CIGRE WG A2.37 [8] showed that: Windings are the most common failure location mechanical failures account for over 20% of all failures of substation transformers, External short-circuit is the second largest known failure cause (after ageing). 15
CONCLUSION The weakening of winding foundation, if any during transportation is required to be confirmed before putting the transformer into service. The Sweep Frequency Response Analysis Test (SFRA) should necessarily be conducted both before despatch and after receiving/installing of transformer at site. Any deviation in frequency response signatures before and after transport should be observed carefully as it indicates mechanical displacement and deformation of winding occurred during transport. The frequent faults in transmission line also aggravate the situation and further accelerate the deformation/displacement of transformer winding. 16
CONCLUSION The Following facts are presented here as envisaged from above analysis: a) Transformers subjected to frequent short circuit (through fault) condition, may develop faults due to mechanical stress. However, no routine electrical test could specifically pin point such fault as the fault do not involve earth and the same is evident from the above test results. c) The Power Station should do periodic DGA tests of the GSU transformer oil as per present practice. A database consisting of all test records of individual transformer along with fault/failure history should be maintained. c) Immediate shutdown of unit should be ensured if Buchholz alarm appears. Restart of unit can only be considered after completion of thorough investigation. d) Buchholz-relays may operate unnecessarily due to the vibrations that occur during short-circuit conditions. Therefore, the behaviour of such relays for any transformer should carefully be monitored and recorded. 17
RECOMMENDATIONS The Power Station should consider carrying out (preferably include in the specification) Sweep Frequency Response Analysis Test (SFRA) on transformer both before transportation from workshop and after receiving at site. By comparing the above two signatures, physical displacement of transformer winding, if any, caused during transportation can be assessed. Further, the transformer should be fitted with impact recorder before transportation to record the mechanical impact on the transformer during transportation. 18
THANK YOU 19