8.9.3 Substation design
Since a power transformer interconnects two or more voltage levels, its location requires special
consideration in the design of a substation and the protection of all of the elements within it.
Usually the various voltage levels are contained within separate areas, each with its own bus configuration
and associated equipment and separated by considerable distances. Figure 8.18 shows a
345 kV/138 kV station with a full complement of circuit breakers. The protection is straightforward.
There are redundant transformer zones of protection, i.e. an internal differential (87T) and an
overall differential (870A). In addition to providing dependability the two zones allow ready identification
of the fault location, i.e. within the transformer itself or on the high- or low-voltage leads.
Variations of this configuration are very common. For instance, if the transformer and both circuit
breakers are relatively close to each other, the internal differential can be omitted and a pressure
relay used to indicate a fault within the transformer tank. If the low-voltage leads are very long a
lead differential may be added, as shown in Figure 8.19. This has the advantage of using a relay
more suited to a bus differential than a transformer differential, i.e. a short-time overcurrent relay
without harmonic restraint.
Since extra high voltage (EHV) breakers are expensive, they are often omitted in station designs
such as this. As with the tapped transformer, the transformer is tied directly to the EHV bus
through a MOAB. This is shown in Figure 8.19 with the associated zones of protection. The major
disadvantage is the fact that a transformer fault must be cleared by opening all of the 345 kV bus
breakers. This is usually acceptable since transformer faults are relatively rare.