Current transformers are used for protection, instrumentation, metering and control. It is only the first function that has any bearing on the location of the current transformer.
Ideally the current transformers should be on the power source side of the circuit breaker that is tripped by the protection so that the circuit breaker is included in the protective zone.
In many circuits the power flow can be in either direction and it then becomes necessary to decide which location of fault is most important or likely and to locate the current transformers on the side of the circuit breaker remote from those faults. In the case of generator (and some transformer) circuits it is necessary to decide whether the protection is to protect against for faults in the generator or to protect the generator against system faults.
Current transformers can often be located in the generator phase connections at the neutral end and will then protect the generator from the system faults and to a large degree give protection for faults in the generator.
When current transformers can be accommodated within the circuit breaker, they can in most cases be accommodated on both sides of the circuit breaker and the allocation of the current transformers should give the desired overlapping of protective zones.
With some designs of circuit breaker the current transformer accommodation may be on one side only and it may be necessary to consider the implications of the circuit breaker position in the substation before deciding on the electrical location of the current transformers.
However the risk of a fault between the current transformers and the circuit breaker and within the circuit breaker itself is very small and so the economics of accommodating the current transformers may have an important influence on their location.
Where separate current transformer accommodation has to be provided, the cost of separately mounted current transformers and also the extra substation space required almost always results in them being located only on one side of the circuit breaker. In practice this is generally on the circuit side of the circuit breaker.
Often it may be possible to accommodate current transformers on the power transformer bushings or on through wall bushings. When this is done it is usually for economic reasons to save the cost of, and space for, separately mounted current transformers.
Transformer mounted current transformers have minor disadvantages in that a longer length of conductor and, more especially, the bushing is outside the protected zone, and in the event of the transformer being removed then disconnections have to be made to the protective circuits.
Note that the arrangement of the individual current transformers within a unit should preferably be arranged that any protective zones overlap and that current transformers for other functions are included within the protected zone.
Under by-pass conditions (where this is provided) the circuit is switched by the bus coupler circuit breaker.
The location of the current transformers is determined by whether the protective relaying and current transformers are provided by the bus coupler circuit, or whether the protective relaying and current transformers of the circuit are used with the tripping signal being routed to the bus coupler circuit breaker during by-pass. If the latter method is used then the current transformers must be separately mounted on the line side of the by-pass isolator.
The advantage of this method is that the circuit protection is unchanged to the possibly inferior protection of the bus coupler circuit. On the other hand the circuit would have to be taken out of service to work on the current transformers.
The need for continued metering of the by-passed circuit needs also to be considered.
Possible locations of current transformers
Figures 1 (a), (b) and (c) show possible locations of current transformers in a portion of mesh substation.
In arrangement (a) the current transformers are summed to equate to the feeder current and to operate the circuit protection.
The protection also covers a portion of the mesh and, with overlapping current transformers as shown, the whole mesh is included in discriminative protective zones. Because the feeder current may be significantly smaller than the possible mesh current, the ratio of the mesh current transformers may be too high to give the best feeder protection.
In arrangement (b) the current transformers are in the feeder circuit and so their ratio can be chosen to give the best protection.
However there is now no discriminative protection for the mesh. Note that the current transformers can be located either inboard or outboard of the feeder isolator, the choice being dependent on the ease of shutting down the feeder circuit and the undesirability of opening the mesh if maintenance of the current transformer were required.
The arrangement shown in (c) is a combination of (a) and (b) with, if necessary, different ratio current transformers in the feeder circuit. This arrangement however requires three sets of current transformers as opposed to two and one in arrangements (a)and (b).
Similar arrangements are possible with breaker-and-a-half substations with the slight difference that at the end of the diameter the protection becomes protection for the busbar instead of a feeder. All the diameter currents are summed for the bus zone protection.
Reference: Substation design/application guide – V AYADURAI BSC, C.Eng, FIEE Engineering Expert