Basics of busbar protection
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Basics of busbar protection
- 1. Basics of Busbar Protection
- 2. Need for Busbar Protection Need For Bus Protection In its absence fault clearance takes place in zone II of distance relay by remote end tripping. This means slow and unselective tripping and wide spread black out. Effect of delayed clearance Greater damage at fault point Indirect shock to connected equipments like shaft of generator and windings of transformer
- 3. Requirements of Busbar Protection Must have short tripping time as possible. Must be able to detect internal faults (sensitivity). Must be absolutely stable to external faults (stability). Must be able to detect and trip only faulty part of busbar system (selectivity) Must be secure against maloperation due to auxilliary contact failure.
- 4. Types of Busbar Protection Differential type of busbar protection is divided into two groups. Low impedance scheme : Low impedance scheme uses biased differential relay. High impedance Scheme: High impedance scheme uses a stabilizing resistor in series with the differential relay.
- 5. Differential Relay Principle ault and the differential rsome internal f elay operates.Protected object Differential Relay Busbar protection relays works on the differential principle i.e. comparing the currents entering and leaving a protected object. If those currents matches the protected object is assumed to be in healthy condition and relay remains stable (non operating) . If there is a difference in magnitudCe.Tof currents, it is assumed that thCe.Treis
- 6. Single busbar Protection (Healthy condition) BUSBAR ZONE P1 P1 S P2 2 S 2 S 1 P2 S 1 Busbar87 Current entering the bus Current leaving the bus Under healthy condition the current entering the busbar and leaving the busbar will be same and the CT secondary current circulates thru the secondaries. No current flows thru the relay. Hence the relay will remain restrained (non operating).
- 7. Single busbar Protection (fault within bus) BUSBAR ZONE P1 P1 S P2 2 S 1 P2 S 2 S 1 Busbar 87 Current entering to the bus Current enters from the remote bus Under faulty condition all remote busbars feed into fault and the direction CT secondary currents becomes additive and flows though the rlay.
- 8. Single busbar Protection scheme Here, in the figure above we assume that at normal condition feed, A, B, C& D, carries current IA, IB, IC, and ID . Now, according to Kirchhoff's current law, at node K, IA + IB+ IC+ID = 0 So, it is clear that under normal condition there is no current flows through the busbar protection tripping relay. Now, say fault is occurred at any of the feeders, outside the protected zone. In that case, the faulty current will pass through primary of the CT of that feeder. This fault current is contributed by all other feeders connected to the bus. So, contributed part of fault current flows through the corresponding CT of respective feeder. Hence at that faulty condition, if we apply KCL at node K, we will still get, iR = 0. BUSBAR ZONE 87 A D B C Rela y (R) K
- 9. When fault is occurred on the bus itself. The fault current is contributed by all feeders connected to the bus. Hence, at this condition, sum of all contributed fault current is equal to total fault current. The sum of all secondary currents is no longer zero. It is equal to secondary equivalent of fault current. So at this condition current starts flowing through 87 relay and it makes trip the circuit breaker corresponding to all the feeders connected to this section of the busbar. As all the incoming and outgoing feeders, connected to this section of bus are tripped, the bus
- 10. Double busbar system Double bus system consists of two number of buses (Bus1 & Bus # 2 ) separated by a bus coupler.
- 11. Double busbar Protection Scheme (explained step-by-step) Two number of bus bar protection relays are required for protection of the double bus system , one for each bus. The relays will Feeder #1 Feeder #2 Trafo #1 Trafo #2 Bus #1 Bus #2 Bus Coupler Open 87-1 87-2 100 A 100A 100 A 100A Busbar Protection Relay Bus- 1 Busbar Protection Relay Bus- 2
- 12. Bus coupler closed condition When bus coupler CB closed and feeder#2 switched- off condition, all the load current is coming through feeder #1. There will be unbalance current in the relays and both relays 87-1 Feeder #1 Feeder #2 Trafo #1 Trafo #2 Bus #1 Bus #2 Bus Coupler closed 87-2 100 A 100 A 200A Feeder #2 CB open 100 A Busbar Protection Relay Bus- 1 Busbar Protection Relay Bus- 2
- 13. Buscoupler CTs When the bus coupler bay is included in the bus bar protection scheme. The relays will remain stable during normal condition and Feeder #1 Feeder #2 Trafo #1 Trafo #2 Bus #1 Bus #2 Bus Coupler closed 87-1 87-2 100 A 100 A 200 A Feeder #2 CB open 100 A 100A Busbar Protection Relay Bus-1 Busbar Protection Relay Bus-2
- 14. Fault at bus coupler In case of a fault in busbar heavy fault current flows but bus coupler CB is not covered by any bus bar protection zones. So the Feeder #1 Feeder #2 Trafo #1 Trafo #2 Bus #1 Bus #2 Bus Coupler fault 87-1 87-2 100 A Busbar -1 Protection Zone Busbar -2 Protection Zone fault Busbar Protection Relay Bus-1 Busbar Protection Relay Bus-2
- 15. Overlapping of Zones Feeder #1 Feeder #2 Trafo #1 Trafo #2 Now the protection zones of Bus-1 and Bus-2 overlaps to include the buscoupler CB, So both Relays operates for a fault in the Bus #1 Bus #2 Bus Coupler 87-1 87-2 Busbar -1 Protection Zone Busbar -2 Protection Zone Busbar Protection Relay Bus-1 Busbar Protection Relay Bus-2
- 16. CT Switching CT –Circuits are switched depending upon the position of busbar disconnectors. The current is either connected to busbar-1’s or busbar 2’s differential protection. Switching is performed by using repeat relays controlled via two Bus -1 Bus -2
- 17. Check Zone Relay Trafo #1 Trafo #2 Bus #1 Bus #2 Bus Coupler 87-1 87-287- CH 87 CH- Check Zone Relay Check Relay protection Zone The figure above shows double bus bar protection scheme with a check zone relay.
- 18. Check Zone Relay For a double busbar arrangement, two different high impedance units are required. In this case, the current must be switched between the two different measuring units by connecting auxiliary switches to the busbar isolator contacts. In some cases the auxiliary switches did not operate correctly. This causes the busbar Protection to trip the busbar. For this reason, a safety precaution was introduced. Check zone is a safety precaution to avoid tripping of bus bars due to defective CT Switching relays.
- 19. Double bus with Check Zone - Trip Logic Trip 1 Trip 2 Trip 87-1 Trip 87-2 Trip 87-CH The TRIP command is issued only when both discriminating and check-zone system operates. It is also called two-out-of- three (2/3) logic.
- 20. Busbar protection- CT Switching Relays In double bus system all the feeders could be connected to either bus 1 or bus 2 through disconnectors. The auxilairy contacts of the disconnectors decide to which protection relays(i.e. bus 1 or bus 2 protection relays) the CT inputs from the specific feeder should be feeding. So the aux.contacts of the disconnectors helps in activating the switching relays to
- 21. CT wire Supervision Relays This is a three phase monitoring device designed to provide continuous supervision of the bus wires in high impedance type bus wire protection schemes. The relay will detect open circuited bus wires as well as open circuited main current transformers. 3-5seconds time lag is provided to ensure that the protection would not be interfered with.
- 22. Breaker Failure Protection (LBB) In modern networks the critical fault clearing time may be less than 200ms. Hence, if the fault is not cleared due to failure of the primary protective relays or their associated circuit breaker, a fast acting back-up protective relay must clear the fault. LBB is a protection designed to clear a system faulty by initiating tripping other circuit breaker(s) in the case of failure to trip of the appropriate circuit breaker.
- 23. LBB/BFR FLOW CHART MAIN PROTECTION OPERATED YES YES TRIP MAIN BREAKER INITIATE BFR WAIT FOR FAULT CLEARENCE FAULT CLEARED YES NO RESET BREAKER FAILURE SCHEME TRIP BACK-UP BREAKERS & LBB trip is given to all breakers in the bus (to which the failed circuit breaker is connected) and incoming CBs in the remote station via communication channel to isolate the CB completely.