- Plant situaiton
The plant Fueled by natural gas, the power generating block consists of three Siemens Westinghouse 501 F combustion gas turbine generators, three Deltak heat recovery steam generators, a single 517mw Toshiba steam turbine generator, and a wet mechanical draft cooling tower. The facility, with a nameplate capacity of 1,005 MW, which includes Selective Catalytic Reduction (SCR) systems for controlling nitrogen oxide exhaust emissions
Plant is on two shift operation, plant will start in the morning time 7:00 am and shutdown on at 11:00 pm , the three Deltak heat recovery steam generators includes HP, IP, LP three pressure steam loop, the HP boiler maximum allow working pressure 2700 psig , boiler heating surface 312038 square feet, steam capacity is 847000lbs/hr. three siemens 501F air cooling gas turbine and air cooling gas turbine provide the fast start up and stop function to support plant shift operation.
Figure 1, 3×1 power plant
2 Gas turbine generator
Figure 2, Generator nameplate
Three Siemens air cooling generator installed, the total capacity of the geerator is 249000KVa, the based load 230MW, the startor voltage is 1900KV and stator amperes is 7987 A, field voltage is 300 V and field amperers 1682A, manufacture at 2001 (figure 2), generator three phase lead line pass three the enclosure wall and install generator outle CT. then through the isophase bus to connect the step up transformer than connect to plant ring bus. three CT install at the generator outlet lead line, the CT ratio 10000/5 A, C4000/0.3B1.8 , 4 CT install at Generator nutral site. Two of CT used to provide the preminiary current for the malfunction relay, one of the CT provide the generaiton monitor.
The plant cooling tower includes 14 cells and was largest in the country at that time, located at the north side of the plant and statistic was showing the most of time the wind direction in the plant was towards north so the plume would not inpact the plant gas turbine operation.
3 Accident process and root cause
3.1 accident process
The acceidant date was the have heavy fogging and it was due to the wind direction from north toward south and push the cooling tower plume to cover the plant cause the humidity at plant reach 100%, high saturated air with cooling tower plume covered CT#3 air intacke of the aera, CTG#3 operation at based load 220 MW. Around 10:00 am, plant operator heard a big noise blast to the air, CTG #3 tripped. CTG#3 inspection found it was tripped due to three phase short circuit at generator outlet lead bus.
Figure 3, the red dot is short circuit location
Figure 4 the isophase bus connect to transformer
4 Root cause
Havey foggin air, 100% humidity was sucked through the air cooling generator, can cause the short of the cirucirt and plus the lead bus penetred panel has design fault cause the outside of the wall accumulted the water can pass through the panel to reach the lead bus of the generator which cause three phase short of the lead bus (Figure 5)
Figure 5 overview of the damaged CT
4 Damaged and calculation
4.1 fault current calculation
The calculation results are fault current is 79.87 kA
4.2 Equipment damaged
Inspection found this generator short circuit accident accuse the following damage:
- Generator copper lead line damage
The short circuit at outlet of the lead bus cause the arc flash to melt the copper bus (figure x) to burn the corner of the copper bus, the vaporized the copper metal was moved into generator winding
Figure 6 arc flash cause lead bus damaged
Figure 7 arc flash cause lead bus damaged
- Three CT damaged
Lead bus side three CT was overheated and burned, the surface was changed color to dark, the insulation was damaged
Figure 8 3 CTs damaged
- The panel wall damaged
The panel for the bus penetrated through was overheated and the surface line was pull off and vaporized to enter winding accumulated at the surface.
Figure 9 Lead bus panel damaged
- The arch flash causes the insulation damaged and move through the generator air cooling fan go inside of the winding
Figure 10 burned insulation
- Vapor copper also suck to the inside of the generator winding
Figure 11 inside of winding accumulated burned insulation
- The rotor winding under retaining ring found the copper element and overheated due to over current
5 Repair and back to operation
5.1 Rotor replaced
Borescope was showing vaporized copper bas was accumulated at the end winding underneath of the retaining ring, so it will cause the short and discharge during the operation, need to remove the retaining ring to do deep cleaning and repainting. Due to there is no tools and condition to remove the retaining ring, OEM decided to delivery a new generator rotor to site and replace the damaged one.
5.2 the penal for the lead bus penetrated through replace to new
The seriously damage due to arc flash, so damaged panel replace to new panel, the root cause found one of the contributors is the panel has design fault the water can penetrated through the gap to entire the inside of the wall, so new designed the lead bus penetration panel and changed all of three generators.
5.3 the three CT replaced
Three CT overheated and damaged relace it new panel, the fault current all pass neutral side 4 CTs test is showing the neutral side CT was not damaged, can be used.
5.4 the copper lead bus replaced
The copper lead bus damaged due to short circuits cause the arc flash over heated and part of them vaporized, replace it too new
5.5 Stator winding inspected and recoating
Stator winding inspected and tested, and DC resistance test was showing the generator insulation was not damaged, but the short circuit cause the mechanical force to impact the end ring, it is possible to vibrated and deformed, and the accumulation copper dust would cause the discharge to damage insulation after operation, so must be cleaning and inspected then test to make sure can put in service.
5.3 Install weather roof outside of the enclosure to protect the panel
CTG#1,2,3 isophase bus penetrated the generator enclosure though panel with bolts and washer installed with enclosure, in bad weather condition (heavy rain and snow), Rain goes penetrated though the panel and accumulated inside of panel which cause big risk of the generator lead line flashover. so install a removable weather roof will be a permanently solution to replace currently scaffolding cover.
6 Generator tests after repair
After two months working to replace the lead bus penetration panel, CT, lead copper bus and cleaning the generator winding and repainting the generator winding, craw through inside of the generator to do inspection, install a new rotor, the completed following test:
- Measurement of cold resistance of armature and field windings.
- Mechanical inspection.
- Insulation resistance of stator and rotor windings.
- High-potential tests.
- Dielectric tests of armature and field.
- Stator coil power factor tip-up tests for one out of every six coils.
- Resistance temperature detectors test.
- Stator core iron induction test at 1.0 Tesla or other stator core imperfection test
this test is required to verify the absence of hot spots in the
.
Figure 12 CT and Lead bus repaired
Figure 3: Generator winding repainting and inspected
7 Conclusion
The heavy fogging weather plus the cooling tower plume and design fault of the lead bus penetration panel cause three phase lead bus short circuit, seriously damaged the generator and cause generator rotor replaced and stator winding repainting, CT and lead bus replaced. After over 2 month of outage time. CTG#3 successfully back to service, the cast for this accident is over million dollars.