We have a 15 MW power plant and we are evacuating power to CSPDCL through 132 KV switch yard. In our switch yard the metering and protection potential transformers are bursting out frequently (5 times in 3 years). We discussed with consultants and improved earthing of switch yard. But still the problem continues. During heavy lightning and rain the PTs are bursting. Can any one tell me what will be the reason for this and how to prevent the same?
Thiagarajan Sriram... what do your consultants say are the problem?
1) Please provide pertinent equipment parameters!
2) What is the failure frequency?
3) Are failures occurring on a specific phase? Or are all three phases involved?
4) Are failures random?
5) Are failures more prevalent in one area of the switch yard?
6) Have you produced a "FAM", Forensic Analysis Matrix?
7) What are site conditions, e.g., normally dry (until storm)?
8) Do you have an Isokeraunic map of the site?
9) Have surge or lightning arresters shown evidence of discharge?
10) Are PTs from the same manufacturer?
11) Is 132kV yard static-line protected?
Regards, Phil Corso
> Thiagarajan Sriram... what do your consultants say are the problem?
As per their guidance we provided earth electrodes of 50 feet depth at both the diagonal corners of the switch yard and connected them to the earth grid.
the neutral of the primary is separately connected to earth instead if body earth.
> 1) Please provide pertinent equipment parameters!
Ours is 132 KV/rt3/110/rt3-110/rt3 PT, Accuracy class: 0.2 and 3P, burden 50 VA , it is connected to three Nos of Electronic tariff trivector meters only. (i.e. metering purpose) The protection core is not used. The highest system voltage is 145 KV. Generally our grid voltage will be 135 - 138 KV.
> 2) What is the failure frequency?
The PTs were failing during heavy rain and lightning.
> 3) Are failures occurring on a specificphase? Or are all three phases involved?
The failures occurred in R and B phase only. Y phase no failures.
> 5) Are failures more prevalent in one area of the switch yard?
Tariff Metering PTs failed three times protection PT failed once.
> 6) Have you produced a "FAM", Forensic Analysis Matrix?
> 7) What are site conditions, e.g., normally dry (until storm)?
Coal and ash Dust pron, surrounded by forest range, Normally dry
> 8) Do you have an Isokeraunic map of the site?
> 9) Have surge or lightning arresters shown evidence of discharge?
Every time the surge arrester counter reading found changed. Always the SAs were showing discharge current0.3 - 0.5 mA
> 10) Are PTs from the same manufacturer?
Yes and all are purchased in the same lot.
> 11) Is 132kV yard static-line protected?
Yes Micom Static relay is provided.
sir pl clarify the following doubts.
While reviewing the history cards i noticed that before commissioning there was leakage found in the PTs near drain plug and the same was arrested. PT was filled with transformer oil for maintaining the level by opening the oil filling point .But some electricity board persons told that if we do so the nitrogen filled at the top of the PT will get released and pressure will be created in the PT during heavy surge ,finally it may result in bursting out. Is it correct?
T. Sriram... your response indicates, to me, that your problems and observations could be due to an inadequate earthing-plane (or earthing-grid! Following are additional comments:
1. How many PT failures have occurred? And, in what time period?
2. Have insulators (other then PTs) also occurred?
3. Earth electrodes (assume vertical rods) are not very effective in "dry" soil installations. Has testing confirmed their efficacy? How far apart are they located?
4. The frequency of PT failures in just two phases indicates physical-asymmetries in earthing wire installation. Have you checked for major differences in routing or perhaps method? For example are earthing conductors passing through steel (magnetic) conduit without "bonding" the earthing-wire to both ends of the conduit?
5. Have earthing-wires "fused" meaning obliterated by fault-currents?
6. Has the failure-rate been proportional to operating voltage, that is, higher for 145kV operation? It seems to me that operating 132kv-rated PTs at 145kV is not the prudent thing to do!
7. How often are the insulators "washed?"
8. Are the insulators designed for coal-dust or ash environments?
9. Are transmission-line insulators vertical or horizontal-type?
10. Lastly, have you also lost major assets, like the HT-side of the grid transformer(s)? Anything damaged on the LT-side of the grid xfmr?
Regarding your comments related to nitrogen blanketing... while possible, I question how the pressure is built up in an inert environment. How does the theory explain the presence of fault-current in an insulated medium, within the sealed housing?
Regards, Phil Corso