Basics of busbar and LBB protection

Said Salim PalayiAssistant Executive EngineerElectrical Division, KSEB LimitedManjeri, Kerala, South [email protected]

Basics of Busbar Protection

Need for Busbar Protection

Need For Bus ProtectionIn 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 pointIndirect shock to connected equipments

like shaft of generator and windings of transformer

Requirements of Busbar ProtectionMust 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.

Types of Busbar ProtectionDifferential 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.

Differential Relay Principle

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 magnitude of currents, it is assumed that there is some internal fault and the differential relay operates.Protected object

Differential Relay

C.T C.T

Single busbar Protection (Healthy condition)

BUSBARZONE

P1

P1

P2

P2

S2

S2

S1

S1

Busbar

87

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).

Single busbar Protection (fault within bus)

BUSBARZONE

P1

P1

P2

P2

S2

S2

S1

S1 Busba

r87

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.

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.

BUSBARZONE

87

A DB C

Relay (R)

K

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 becomes dead.

This differential busbar protection scheme is also referred as current differential protection of busbar.

Double busbar system

Double bus system consists of two number of buses (Bus1 & Bus # 2 )

separated by a bus coupler.

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

remain stable as long as the buscoupler CB is open.

Feeder #1

Feeder #2

Trafo #1 Trafo #2

Bus #1

Bus #2

Bus Couple

r Open

87-1 87-

2

100 A

100 A

100 A

100 A

Busbar Protection Relay Bus-1


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

will operate. Hence, the bus coupler shall also be included in the protection scheme.

87-1

Feeder #1

Feeder #2

Trafo #1 Trafo #2

Bus #1

Bus #2

Bus Couple

r closed

87-2

100 A

100 A

200 A

Feeder #2 CB open

100 A



Buscoupler CTs

When the bus coupler bay is included in the bus bar protection scheme.

The relays will remain stable during normal condition and external fault.

Feeder #1

Feeder #2

Trafo #1 Trafo #2

Bus #1

Bus #2

Bus Coupler closed

87-187-2

100 A

100 A

200 A

Feeder #2 CB open

100 A 100

A



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 busbar protection

relays will not operate. So the scheme needs modification.

Feeder #1

Feeder #2

Trafo #1 Trafo #2

Bus #1

Bus #2

Bus Coupler fault

87-187-2

100 A

Busbar -1 Protection Zone


fault



Overlapping of Zones

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 buscoupler

bay.

Feeder #1

Feeder #2

Trafo #1 Trafo #2

Bus #1

Bus #2Bus

Coupler 87-1

87-2





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 auxiliary contacts at each busbars.

Bus -1

Bus -2

Check Zone Relay

Trafo #1 Trafo #2

Bus #1

Bus #2

Bus Couple

r

87-1

87-2

87-CH

87 CH- Check Zone Relay

Check Relay protection Zone

The figure above shows double bus bar protection scheme with a check zone relay.

Check Zone RelayFor 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.

An overall Check-Zone unit, fed from individual CT cores. This overall scheme does not include any switching of CT and therefore is more secure. Double bus system consists of 2 bus differential and a check zone relay.

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.

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 toggle the CT connections between bus 1 and bus 2 protection relays.

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.

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.

LBB/BFR FLOW CHART

MAIN PROTECTIONOPERATED

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.

Basics of busbar and LBB protection

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