Very often fault clearing criteria for a power system specify that busbar faults must be cleared in the order of 5 cycles, and that only a few feeders are allowed to be tripped.
This may be the maximum allowed disturbance for a power system, in order to maintain stability of the remaining power system after fault clearing. Therefore phase to phase faults and phase to ground faults should be cleared within 5 cycles.
Typical power system busbar configurations are shown on Figure 3.2-1, Figure 3.2-2, and Figure 3.2-3. Busbar protection is typically based on differential current principles.
Busbar protections are mostly configured with zones, one or more zones for bus A and one or more zones for bus B. The busbar protection very often includes breaker failure protection, time delayed typically 5 to 9 power frequency cycles.
3.2.1 Two-breaker busbar configuration
Two-breaker power system busbar configuration is shown on Figure 3.2-1. With two current transformers in each bay, busbar protection functions (measuring and trip actions) are independent of isolator positions. Breaker failure protection is started from busbar protection, line protection and transformer protection.
Notation :
Bus-A is section A of the bus. CB-A-L1 is circuit breaker A for line 1. BP-A is bus protetion for bus zone A. CBFP-A-F1 is circuit breaker
failure protection for breaker A on feeder 1. Feeders may be lines, transformers or any other feeder.
Id-A is current differential protection for
bus zone A. Line 1 CBFP -B-L1 BP-B Id-B CBFP -A-L1 CB-B-L1 CB-A-L1 Id-A BP-A Bus A Bus B Line 2 CBFP -B-L2 CBFP -A-L2 CB-B-L2 CB-A-L2 Transf CBFP -B-T CBFP -A-T CB-B-T CB-A-T a c c d b
Figure 3.2-1: Two breaker busbar configuration
3.2.1.1 Normal fault clearing
For improving dependability or security, combinations of protection systems may be applied. The protection system has to detect faults and initiate actions on following faults:
Fault location a (b) :
CBFP-A (CBFP-B) trips bus A (B), and the fault is cleared. There is no need for telecommunication.
Fault location c :
This is a fault for line protection or transformer protection, see Chapter 3.1 and 3.3.2. Fault location between CB and CT, exemplified with fault location d :
Busbar protection zone A trips bus A. But the fault is not yet cleared - there is still infeed from bus B and Line 1. To obtain fast fault clearing, the breaker failure protection 'CBFP-A-L1' trips breaker B on Line 1 and must initiate tripping of the remote breaker(s) on Line 1. This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1 to trip the line at least at the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection executes the tripping, it should be three phase without initiation of automatic reclosing.
Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this fault is very seldom. If the line protection is performed by distance relays, transmitting a carrier signal to accelerate the 2nd zone of the line protection, at the remote line end, would provide a good solution.
3.2.1.2 Breaker failure
The following fault clearing procedures apply in case of a breaker failure. Fault location a (b):
For fault location a, if breaker CB-A-L1 is stuck, CBFP-A has to trip CB-B-L1 and initiate tripping of remote breaker(s) on Line 1. This can only be done by means of telecommunication as described in chapter 3.2.1.1 for fault location d.
Fault location c:
If breaker CB-A-L1 (CB-B-L1) is stuck, CBFP-A-L1 (CBFP-B-L1) has to trip bus A (B). There is no need for telecommunication in this case.
Fault location between CB and CT, exemplified with fault location d : The scenario is the same as described in 3.2.1.1.
3.2.2 One- and a half breaker busbar configuration
One- and a half breaker busbar configuration is shown on Figure 3.2-2. Busbar protection functions (measuring and trip actions) are independent of isolator positions. Breaker failure protection is started from busbar protection, line protection and transformer protection.
Notation :
Bus-A is section A of the bus.
T1 is transformer 1. CB-A-L1 is circuit breaker A for line 1.
CB_L1-L2 is circuit breaker between line 1 and line 2. BP-A is bus protetion for bus zone A.
CBFP-A-F1 is circuit breaker failure protection for breaker A on feeder 1. Feeders may be lines, transformers or any other feeder.
Id-A is current differential
protection for bus zone A. Line 2 CBFP -B-L1 BP-B Id-B CBFP -A-L1 CB-B-L2 CB-A-L1 Id-A BP-A Bus A Bus B Line 4 CBFP -B-L2 CBFP -A-L2 CB-B-L4 CB-A-L3 T2 CBFP -B-T2 CBFP -A-T1 CB-B-T2 CB-A-T1 a c e b Line 1 Line 3 T1 c h g f d d CB-T1-T2 CB-L3-L4 CB-L1-L2
Figure 3.2-2: 1½ breaker busbar configuration
3.2.2.1 Normal fault clearing
For improving dependability or security, combinations of protection systems may be applied. The protection system has to detect faults and initiate actions on following faults :
Fault location a (b) :
CBFP-A (CBFP-B) trips Bus A (B), and the fault is cleared. There is no need for telecomm- unication.
Fault location c and d :
This is a fault for line protection or transformer protection, see Chapters 3.1 and 3.3.2. Fault location between CB and CT, exemplified with fault location e (h) :
Busbar protection zone A trips Bus A. But the fault is not yet cleared - there is still infeed from bus B and Line 1.
To obtain fast fault clearing, the breaker failure protection 'CBFP-A-L1' trips breaker CB-L1-L2 and must initiate tripping of remote breaker(s) on Line 1. This remote tripping can be executed either by direct intertripping of breakers, or by ’commanding’ or helping line protection systems on Line 1 to trip the line at least at the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection execute the trip, it should be three phase without initiation of automatic reclosing.
Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this type of fault is rare in practice.
If the line protection is performed by distance relays, transmitting a carrier signal to accelerate the 2nd zone of the line protection, at the remote line end, would provide a good solution.
Fault location f and g:
If current measurement for line protection of Line 1 and Line 2 crosses, this is a fault for line protection or transformer protection. See ’Fault location c and d’ above.
3.2.2.2 Breaker failure
The following fault clearing procedures apply in case of a breaker failure. Fault location a (b):
For fault location a, if breaker CB-A-L1 is stuck, CBFP-A-L1 has to trip CB-L1-L2 and initiate tripping of remote breaker(s) on Line 1. This can only be done by means of telecommunication as described in Chapter 3.2.1.1 for fault location d.
Fault location c :
If breaker CB-A-L1 (CB-B-L1) is stuck, CBFP-A (B) has to trip Bus A (B). There is no need for telecommunication in this case.
If breaker CB-L1-L2 is stuck, the breaker failure protection of that breaker has to initiate tripping of remote breaker(s) of Line 1 (2). This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1 (2) to trip the line at least in the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection execute the trip, it should be three phase without initiation of automatic reclosing.
Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this type of fault is rare in practice.
Fault location f or g :
If current measurement for line protection of Line 1 and Line 2 ’crosses’, this is similar to ’Fault location c’ above.
Fault location between CB and CT, exemplified with fault location e or h :
The probability of this fault location in combination with stuck breakers is very low. Normally no breaker failure protection is applied.
3.2.3 Two zones / one breaker configuration
Notation :
Bus-A is section A of the bus. T1 is transformer 1.
CB-L1 is circuit breaker for line 1. CB-T1 is circuit breaker for transformer 1. Coupler is coupler between bus section A
and B.
I-A-L1 is isolator A for line 1.
Id-A is current differential protection for bus zone A. Line 1 BP-B Id-B I-B-L1 I-A-L1 Id-A BP-A Bus A Bus B CB-L1 Coupler-A-B a c b T1 CB-T1 d
Figure 3.2-3: Two protection zones / one breaker busbar configuration
3.2.3.1 Normal fault clearing Fault location a and b:
The busbar protection trips the bus, and the fault is cleared. If a line breaker fails, the second zone of the line protection ( Z< ) at the opposite line end serves as back-up protection.
Fault location c:
Busbar protection zone A and/or B trips bus A and/or B dependent of isolator positions. But the fault is on the line side of the breaker. Therefore, the fault is not cleared. To achieve fast fault clearing, trip command from busbar protection - dependent of isolator position - must initiate tripping of remote breaker(s) of Line 1(n). This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1(n) to trip the line at least in the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection executes the trip, it should be three phase without initiation of automatic reclosing. Fault clearing time will not necessarily exceed 5 cycles.
Fault location d:
The protection initiates a trip command, but the fault is not yet cleared. In order to clear the fault busbar protection zone B is designed to trip bus B if receiving a signal from zone A for more than 5 cycles.