Two-Source Primary— Secondary Selective System

This system uses the same principle of duplicate sources from the power supply point using two primary main breakers and a primary tie breaker. The two primary main breakers and primary tie breaker being either manually or electrically interlocked to prevent closing all three at the same time and paralleling the sources. Upon loss of voltage on one source, a manual or automatic transfer to the alternate source line may be used to restore power to all primary loads. 

Each transformer secondary is arranged in a typical double ended unit substation arrangement as shown in Figure 1.1-37. The two secondary main breakers and secondary tie breaker of each unit substation are again either mechanically or electrically interlocked to prevent parallel operation. Upon loss of secondary source voltage on one side, manual or automatic transfer may be used to transfer the loads to the other side, thus restoring power to all secondary loads. 

This arrangement permits quick restoration of service to all loads when a primary feeder or transformer fault occurs by opening the associated secondary main and closing the secondary tie breaker. If the loss of secondary voltage has occurred because of a primary feeder fault with the associated primary feeder breaker opening, then all secondary loads normally served by the faulted feeder would have to be transferred to the opposite primary feeder. 

This means each primary feeder conductor must be sized to carry the load on both sides of all the secondary buses it is serving under secondary emergency transfer If the loss of voltage was due to a failure of one of the transformers in the double-ended unit substation, then the associated primary fuses would open taking only the failed transformer out of service, and then only the secondary loads normally served by the faulted transformer would have to be transferred to the opposite transformer. 

In either of the above emergency conditions, the in-service transformer of a double-ended unit substation would have to have the capability of serving the loads on both sides of the tie breaker. For this reason, transformers used in this application must have equal kVA ratings on each side of the double-ended unit substation. The transformers are sized so the normal operating maximum load on each transformer is typically about 2/3 base nameplate kVA rating. 

Typically these transformers are furnished with fan-cooling and/or lower than normal temperature rise such that under emergency conditions they can continuously carry the maximum load on both sides of the secondary tie breaker. Because of this spare transformer capacity, the voltage regulation provided by the double-ended unit substation system under normal conditions is better than that of the systems previously discussed. 

The double-ended unit substation arrangement can be used in conjunction with any of the previous systems discussed, which involve two primary sources. Although not recommended, if allowed by the utility, momentary re-transfer of loads to the restored source may be made closed transition (anti-parallel interlock schemes would have to be defeated) for either the primary or secondary systems. 

Under this condition, all equipment interrupting and momentary ratings should be suitable for the fault current available from both sources. 

For double-ended unit substations equipped with ground fault systems special consideration to transformer neutral grounding and equipment operation should be made—see Grounding/Ground Fault Protection section of this Design Guide. Where two single-ended unit substations are connected together by busway or external tie conductors, it is recommended that a tie breaker be furnished at each end of the tie conductors. The second tie breaker provides a means to isolate the interconnection between the two single-ended substations for maintenance or servicing purposes.

Figure 1.1-37. Two-Source Primary—Secondary Selective System