Busbar protection is needed to quickly detect and clear faults occurring on busbars to minimize damage. It must operate sensitively for internal faults and be stable for external faults. Differential relaying compares currents entering and leaving the busbar and operates if they differ, indicating an internal fault. On a double busbar system, two differential relays are used with overlapping zones and check zone relaying for reliability against CT switching failures. Current transformer circuits are switched depending on busbar disconnector position and circuit breaker failure protection operates if the primary breaker fails to clear the fault.

1. Said Salim Palayi
Telegram:
t.me/electrical_transmision
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
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

6. Single busbar Protection (Healthy condition)
BUSBAR
ZONE
P1
P1
P2
P2
S
2
S
2
S
1
S
1
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).

7. Single busbar Protection (fault within bus)
BUSBAR
ZONE
P1
P1
P2
P2
S
2
S
2
S
1
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
100 A
100
A
100 A
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
200 A
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
100 A
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
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
Feeder #1 Feeder #2
Trafo #1 Trafo #2
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 87-2
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.

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.