Published by Mr. V.Veerapandi, Sr.Manager(ser)/SAS, Email:firstname.lastname@example.org
One of 200 MW old machine was experienced stator earth fault and the unit was tripped when the load was around 154 MW. The stator earth fault was attended on emergency basis and the unit was brought back with minimum possible time. This article covers observations, analysis of the problem and also suggests remedial measures to avoid recurrence of such a problem.
The 200MW TG set was brought back to grid after major overhauling of LP Turbine for a month and no work was carried out in Generator during this overhaul. Before synchronization of machine electrical test was carried out on generator and during the test, winding temperature marginal rise was observed in slot no.52. After confirmation of healthiness of stator winding during electrical test, the machine was synchronized and loaded to 154 MW. The set was tripped on stator earth fault after eight hours of service.
During the fact finding and thorough inspection, it has been observed that the following Relays were operated
• 64G1- Earth fault protection.
• 64G2- Standby earth fault protection.
To find out the cause for the stator earth fault, the following activities were carried out .
- IR value checked in Generator Bus duct + GT Primary + UAT Primary after isolating
of Generator from phase bushing. Value observed as 200 MΩ.
- Generator phase to phase PT voltage measured at Primary and the value observed
as RY – 2.024 V, YB – 2.08 V, BR – 2.05 V.
- Generator all three phase winding together Insulation Resistance value taken with
isolation of NGT and the value observed as – 770 KΩ (with 1 KV megger).
- Generator individual phase winding IR value taken with isolation of NGT and the
value observed as R- 486 KΩ, Y- 28.7 MΩ,B- 46.5 MΩ
- Generator individual Phase winding IR value recorded after cleaning of all bushing
and the value observed as R- 938 KΩ,Y- 28.28 MΩ,B- 46.5 MΩ
- R-phase bushing was inspected visually and no abnormality was found and heating
of phase bushing from bus duct side done for 24 hrs and the megger value observed
as R-1.19 MΩ,Y-40.4 MΩ,B- 117 MΩ.
- IR value of R-phase found improvement after cleaning & heating but the value
observed less than Y & B phase. Hence the existing R-phase bushing was replaced
by new one and the megger value observed as R-2.78 GΩ, Y-68.5 MΩ, B-79.7 MΩ
The above checks reveal that stator windings are in healthy condition and the following Electrical tests were carried out further confirmation.
• Generator DC high voltage applied for about 1 minute- R-12.7 KV,Y-12.6 KV,B- 12.6 KV and found ok.
• Tan delta test carried out at 10 KV and the results observed was in line with previous test values.
• DC hi pot test carried out at 16 KV for 1 minute and found ok.
All above electrical test results were found satisfactory and no significant fault was identified for the stator earth fault occurrence. Hence the following components & systems were visually checked for any abnormality.
Inspection of stator water system
Thorough visual inspection was made on stator core bar, stator water headers and all inlet and outlet Teflon hoses. Stator core bar and stator water headers are found in good condition. During the inspection of Teflon hose, a black spots was noticed in one of Teflon Tube connecting to bar no.52 Top and bar no.16 Bottom.
For further investigation, the particular Teflon hose was removed from the position and through inspection was made on Teflon hose. During inspection it was observed that the stator water inlet passageway of bar no16 bottom bar was completely blocked by small foreign material (rubber piece) as shown in Figure-1. The figure no.2 illustrates heavy carbon deposition inside the hose.
Root cause analysis for ingress of foreign materials into the stator water flow path:
• Stator water for cooling the stator winding, phase connectors and bushing is circulated in a closed circuit (see figure 3&4).
• The stator water system comprises the Expansion Tank, Stator water pumps, stator water coolers, stator water filters and Magnetic filter.
• The operating pump draws the water from expansion tank and feed the water to winding through water coolers, stator water filters and magnetic filter and returns back to the expansion tank.
• Makeup water is supplied for stator water is from DM makeup water and this makeup water line pipe inner surface is coated with rubber lining. If any deterioration in the rubber lining will be collected at stator water filters which are installed in the system. Hence there is no chance to enter foreign materials during operation.
Water path of stator winding and Terminals
Possibility of entering foreign material into the stator water system
During Generator inspection, Stator winding hydraulic test is one of the main test to check the healthiness of stator winding & Teflon hose connections. During overhauling the complete water path is subjected to rigorous hydraulic test. For performing hydraulic test, stator water circuit is to be isolated from stator winding by removing the spool pieces in the circuit as shown in Figure.5
In normal operating condition these spool piece joints are to be made with Teflon/ PVC gasket for long run operation and sturdy life. No rubber gaskets are to be used in the stator water circuit and during hydraulic test also.
In this case by oversight, over size rubber gasket might have been used during hydraulic test and it might leads to scratch/cut due to sharp collar in the spool piece mating flanges (see figure no.5). The damage /scratch pieces may travelled through stator water header, stator water Teflon hose and stuck up in the winding inlet.
Corrective action done
• Teflon tubes of slot no 52 top, 16 bottom and 14 top were replaced by new ones.
• Reverse water flushing done for thorough cleaning of stator winding.
• Hot stator water flushing done by installing heaters in stator water line.
• Flow measurement done from inlet of 52T and outlet of 16B and ensured proper flow in both the bar.
Performance of the Machine after corrective action
• Insulation Resistance measurement done with stator water in circuit and value observed as R phase-1.06 MΩ, Y phase-1.04 MΩ, B phase-1.08 MΩ
• Tan Delta test at 10 KV AC of stator winding done with stator water in circuit and found ok.
• Hydro Test of stator winding done at 6Kg/cm² pressure for 12 Hrs and found ok.
• OCC & SCC test on generator carried out during rolling and found ok.
Suggestion / Recommendations
1) Stator water chemistry is to be monitored thoroughly and conductivity to be maintained as per design.
2) Vacuum in stator water damper tank may be checked for any leakage of dust from outside to inside during running of vacuum pump.
3) Stator winding temperature may be monitored continuously.
4) During overhaul Generator Hot water Flushing / Reverse flushing may be carried out and stator water filter may be checked for debris collected after flushing. (Procedure is available in Generator Service Manual).
During first rolling slot number 52T temperature was raised due to blockage of water flow path at Bar no.16B. The carbon deposition observed inside the Teflon hose no.16B might be the main cause for flash over and earth fault operation in stator.