11 kV Feeder Earth-Fault Restoration with a Critical Water Plant
At a 33/11 kV substation serving an industrial area near Bhilwara, 11 kV feeder F-3 trips on earth fault three times in 48 hours during dust storms. F-3 is 6.4 km long, carries a peak of 1.2 MVA, and supplies 18 distribution transformers; one 630 kVA transformer at LT-7 feeds a water-treatment plant that cannot be shut for more than 90 minutes. The earth-fault relay records 4.8 A primary and 0.3 s. With all transformers isolated, a 5 kV megger test gives 1.2 MΩ phase-to-earth for the feeder, but LT-7 reads 0.4 MΩ. The only alternate source is a tie from adjacent feeder F-5, which has about 0.8 MVA spare capacity; the tie switch has not been operated for two years. The assistant engineer proposes charging F-3 immediately after isolating LT-7. The JE must decide the switching sequence, testing, and whether to use the tie.
1. Recommend the immediate isolation, testing and restoration sequence. Justify each step by safety and continuity.
Read the worked answer
Do not charge F-3 immediately after isolating only LT-7. The feeder itself reads 1.2 MΩ, which is low for an 11 kV feeder and suggests more than one weak point. The safe sequence is:
- Open the F-3 breaker, rack it out, lock and tag it, verify dead, and apply portable earths. This protects the line crew from backfeed, including from the F-5 tie.
- Isolate LT-7 and all other transformers from the feeder. Sectionalise F-3 using the available isolators into at least three sections.
- Megger each section phase-to-earth and phase-to-phase at 5 kV. Record the readings and, where possible, polarisation index. The low overall reading means the fault may be in a cable joint, a pin insulator, or a transformer connection, not just LT-7.
- Restore healthy sections first. Keep LT-7 isolated until it is cleaned and tested.
- Use the F-5 tie only for the critical water plant, not for the whole 1.2 MVA feeder. The tie has about 0.8 MVA spare, so a 630 kVA transformer plus losses is already close to the limit.
Continuity for the water plant should be obtained through a verified tie or a temporary generator, not by energising a feeder with suspect insulation. The earth-fault relay operation at 4.8 A also shows that the protection is sensitive; repeated charging without locating the defect risks a second fault and equipment damage.
2. Interpret the megger results and state whether LT-7 can be recharged after cleaning. What additional tests and acceptance logic would you apply?
Read the worked answer
A reading of 0.4 MΩ at 5 kV on an 11 kV class transformer is not acceptable. It usually indicates moisture and dust on bushings, a damaged cable termination, contaminated oil, or degraded winding insulation. Cleaning may improve the reading, but cleaning alone is not proof of health.
After cleaning and drying, I would:
- disconnect the cable and surge arresters so that the transformer alone is tested;
- measure insulation resistance at 5 kV for 1 minute and 10 minutes and calculate the polarisation index;
- measure winding resistance on all phases and compare phase-to-phase and with previous records;
- check turns ratio and magnetic balance;
- for an oil-filled transformer, test oil breakdown voltage and moisture, and inspect for sludge or carbon;
- inspect bushings, breathers, silica gel and the cable box for tracking.
Acceptance should not rest on one number. The transformer should show a stable reading several times higher than 0.4 MΩ, a satisfactory polarisation index, and no downward drift during the test. If the reading remains low or the oil tests are poor, LT-7 should not be recharged; it should be replaced or sent for repair. The feeder's 1.2 MΩ reading also needs sectional testing before any full restoration.
3. Evaluate the tie-line backfeeding option. List the checks before closing and explain the protection coordination problem.
Read the worked answer
The tie can be useful, but only as a controlled emergency supply. Before closing:
- inspect and megger the tie cable and switchgear; an idle switch for two years may have moisture, insect nests or a stiff mechanism;
- verify phase sequence and phasing across the tie;
- confirm the 0.8 MVA spare capacity and compare it with the water-plant load plus losses; do not pick up other F-3 load;
- check earth-fault protection on F-5, because backfeeding changes the direction and split of fault current;
- confirm that the tie breaker or switch has fault-making capacity and that no unsynchronised parallel operation will occur;
- operate the switch no-load first, then carry out a controlled charge.
The protection problem is that the earth-fault current from an F-3 fault may now be supplied from F-5 and may not be seen by the F-3 relay in the same way. The F-5 overcurrent setting may also be too high for the smaller backfed load. A temporary setting group, directional earth-fault element, or a dedicated tie protection scheme may be needed. Because the tie cannot carry the full 1.2 MVA feeder, it must not be presented as a general restoration solution.