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Home KSEB LBB Protection in Transformer Panel

LBB Protection in Transformer Panel

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LBBIntroduction:
The costliest equipment in a substation is transformer. Therefore, the protection of transformers in the substation requires most attention.
There are different relays functioning based on electrical and mechanical parameters for the protection of transformer. All these relays operate to open Circuit Breakers on either side of the transformer.
Normally, power flow through the transformers inteneded for distribution of power is from HV side to LV side as sources are situated at HV side for . In the case of power transformers in large substations, these transformers are having sources on HV and LV sides.
This note discusses about the importance of LBB protection to transformer and some suggestions to modify transformer panels accordingly.
(a) Uniqueness of Mechanical protections
Protections provided for transformer are of both mechanical and electrical types. In a substation, for feeder protection, there are no mechanical protections.
Mechanical protections may or may not carry fault currents. For example, a winding temperature very high fault condition may not carry very high fault currents. Similarly Buchholz relay trip may be or may not be associated by differential protection tripping. Differential protection tripping is very fast and buchholz operation is comparatively slow. Therefore, simultaneous operation of both protections or operation of buchholz protection alone show that the occurrence of sufficient fault current along with buchholz operation to operate the differential relay may not be there or the fault current may be reaching to the threshold level after a delay. Thus it cannot be expected operation of an electrical relay working based on fault current along with mechanical faults of the transformer.
The mechanical protections of transformer also operate the circuit breakers on the source sides. If the circuit breaker of the transformer fails to operate, the fault cannot be sensed by the relays provided in the feeders as those are sensing electrical parameters. So the fault will be aggravated until it turns out to be an electrical fault and then only the incoming feeders clear the fault electrically.
(b) Faults on the 11KV LV side (in substation transformers used for distribution )
11 KV feeders are fed through the transformer whose LV side is not having normally any source but radial feeders. If any fault occurs on the 11KV side, the fault current will be fed from the HV side of the transformer. Up to the 11KV cable end, the protection is through Restricted Earth fault protection and Differential protection. These are unit protections provided to get instantaneous fault clearance in order to minimize the damage. When these relays act, it will trip HV side breaker to isolate fault portion from the source.
If the HV side breaker fails to trip, this fault is to be cleared by the relays provided on the feeders. EHV feeders are provided with distance relays and directional relays which are looking in forward direction and not to the transformer side. There are some distance relays with reverse looking zone. This will be set with a delay of around 1 second. Besides it, this reverse zone is supposed to protect the bus and its reach will not cover the LV side of the transformer. Therefore, the distance relays provided on the remote ends of the feeders will not sense the fault on the LV side of the transformer at local ends. If there is any IDMT relay on the feeder, it may sense the fault. However, the delay for fault clearing will be depending upon the source impedance. Therefore, even if the fault is cleared, it will be a delayed action.
(c) Operation of breakers:
Control and Relay PanelThe feeder breakers trip during faults in lines as most of the feeders are of overhead type irrespective of the voltage level. Trippings of feeders upto the voltage level 220KV are not rare. At the same time, trippings of transformer breakers are very rare. Because, transformer breakers are supposed to operate for the faults in the transformer, lightning arrester, LV (or other ) side feeder upto next bus and also as back up to the faults in the bus on the LV (or other) side bus and on the feeders from that bus. These equipments are located well within substation area and are protected by sufficient clearances, lightning shields etc. This means the tripping of the transformer breakers are rare.
Similarly, the switching on and off operations of transformer breakers are also rare as it requires only during maintenance periods which are normally done by station authorities.
Thus it can be seen that the transformer circuit breakers are least operated circuit breakers. Sometimes, this leads to less attention to these breakers. There have been occasions where transformers have been charged with fuses on source side as the circuit breakers on the respective side became faulty. Such bad methods have not been adopted so far in the case of feeders.
There have been occasions where the transformer circuit breaker controls became faulty due to mechanical reasons, trip coil failures, absence of DC supply etc. One of the main reasons for the lack of proper maintenance of breaker is due to non availability of shut down of transformers as those are carrying at full load condition during day periods.
Suggestion for LBB protection:
LBB (Local Breaker Backup) protection can be used to extend the tripping to the transformer Circuit Breaker to the other breakers which comes as back up to the transformer breaker with an additional delay of 100 to 250 milliseconds. The LBB shall have two trips. First one with delay of 100 milliseconds shall issue trip to both trip coils of the same circuit breaker and second trip with delay of 250 milliseconds shall be issued to trip upstream side feeders.
Following are some modifications shall be incorporated suitably in the tranformer LBB circuit:
1. Provide a separate DC supply from charger to the LBB protection. Otherwise, failure of the dc supply in the transformer panel due to fuse blowing off etc. may block operation of not only the relay, master trip relay and circuit breaker but also operation of LBB.
2. NC contacts of DC supervision relays of Breaker panels shall be wired to energize LBB protection to operate LBB when DC supply fails in transformer panel when the transformer is in service.
In this case, there is a risk of unwanted tripping of upstream side breakers and complete loss of supply for a DC fuse fail inside transformer panel. This becomes critical when the substation is large.
Therefore, a fault sensing condition may be added in series with the DC supervision NC contacts. The numerical relays like Areva make P642 type have both differential and back up overcurrent and earth fault protection. So this relay may be programmed for back up overcurrent and earth fault protection too. Install an electro mechanical overcurrent and earth fault relay like CDG 31 and connect its one contact to trip the transformer breaker and connect another contact to trip the upstream side feeder breakers in series with NC contact of DC supervision relay. The electro mechanical relay will operate even when there is no DC supply in panel as the relay does not require power supply.
3. LBB may programmed in the numerical relay like P642 to incorporate mechanical protections like buchholz relay trip in LBB scheme to trip upstream side feeder breakers after a delay if the transformer breaker does not open within the prescribed time delay. In this case, the DC supply to the LBB is same as used for the numerical relay and therefore the proposal given in item (1) above cannot be satisfied. So, if panel DC fails, this LBB scheme will not be effective. At that time the electromechanical relay in series with NC contact of DC supervision will take care of tripping HV side breakers as mentioned in item (2) above. Simultaneous occurrence of panel DC failure and buchholz trip operation are very rare and such a chance can be neglected.
4. Normally, lock out relay or trip relay (86) is used to initiate LBB. Failure of this 86 relay will lead to non operation of not only circuit breaker but also LBB. Therefore, each individual relay contacts shall be wired to LBB.
5. Wiring of individual relays to LBB relays may sometimes be very difficult as it requires a lot of additional wiring. Therefore the suggestion provided in item 4 above may be difficult to implement. Healthiness of 86 relay can be supervised by supervision circuit. Similarly, an additional 86 relay may be connected in parallel with which the problem of non tripping due to failure of coil of 86 relay can be avoided.
Cost Implication:
Compared to the transformer relay panels now available, there is no additional relay requirement if a numerical relay like P642 type are used along with electro mechanical overcurrent and earth fault relays are used in the panel. Therefore, additional cost is not much due to the modification work for additional wiring to trip upstream side feeder breakers.
Conclusion:
1. In KSEB, most of the transformer panels are not having LBB protection. The LBB protections shall be provided for transformer CBs and the LBB scheme now existing in the transformer panels may be modified as proposed above to make it reliable in the adverse conditions which may affect smooth of operation of LBB.
2. Two trip coils of circuit breakers are to be wired to separate DC sources or to the single DC supply with separate fuses.
3. At Kalamassery 220 KV substation, 10MVA, 66/11KV transformer became damaged years ago due to non operation of HV side breaker. Similarly, 11 KV panels at Vaikom 110KV substation were burnt out for cable end box fault in 11 KV incomer which was not cleared by operation of HV side breaker. In these situations presence of LBB with external power supply or programming of numerical relay like P642 as described above , use of electro mechanical back up overcurrent and earth fault relay along with extending trip to upstream side breakers through DC supervision NC contacts, duplication of 86 relay and supervision of 86 relay might save these precious equipments and would avoid huge revenue loss to KSEB.
4. As pointed out earlier, the transformer faults which leads to operation of Buchholz trip or Winding Temperature High trip may not sometimes carry sufficient fault current to operate differential, REF or back up overcurrent or earth fault protection. Sometimes, buchholz relay may operate when the transformer is charged at no load. Therefore, if HV side breaker fails to operate during Buchholz operation, the LBB protection may fail to operate due to absence of sufficient fault current or LBB protection will act delayed after the fault was developed into more severe one which is having sufficient fault current. Therefore, as mentioned above, numerical relays may be programmed to operate upstream side feeder breakers in such cases too.
5. In stations like Kalamassery we have a situation where the 10 MVA 110/11 KV transformers are fed through large capacity 220/110 KV transformers and other 110 KV lines which are operated in parallel most of the time. Failure of the 110 KV breaker of these 10 MVA Transformers will not initiate back up relay tripping of the 220/110 Transformers as its full load current settings are above maximum fault current through 10MVA transformer and have to be cleared by remote end of 220 KV feeders. This has been reported as occurred at least twice . The 10 MVA transformers were literally exploded causing wide spread damage to nearby 110 KV equipments and even the bushings of 220 KV transformers which are quite far away. This shows the importance of LBB protection in such situations.
 

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