The 460-volt, double-ended secondary unit substations are supplied by two 3-phase, 750-kv.-a., 4,160 to 480-volt transformers located at each end.

Figures 202 through 205 show single-line diagrams for the 4,160-volt switchgear and the 460-volt distribution system.

(b) 4,160-Volt Intermediate Voltage Switchgear.-The 4,160-volt 4,160-volt intermediate voltage switchgear consists of a double-bus, double-ended unit substation with a supply breaker on each end, a bus-tie breaker, and a group of 4,160-volt feeder breakers. Since the unit substation is normally supplied from two different end sources, it could be possible to have unsynchronized sources connected to this switchgear. To avoid this possibility, the control to the two supply breakers and the tie-breaker is so arranged that only two of the three breakers can be closed simultaneously under manual or automatic mode of operation.

Each bus is protected by individual sets of bus differential, undervoltage, overcurrent, and overfrequency relays. Each feeder breaker is protected by individual sets of phase overcurrent and overcurrent ground relays.

Under normal operating conditions, the station-service load is divided and connected to each of the two end supplies. In the event that one end supply breaker trips by operation of the undervoltage or overfrequency relays, the tie breaker will close automatically and sound an alarm, provided the mode selector switches are in the "Automatic" position. The transfer is blocked when a supply breaker is tripped by its bus differential relay. When this transfer is made, the entire station-service load is connected to one end. If the transformer feeding this end is overloaded, an inverse time overcurrent relay will trip the end supply breaker. To restore station service due to this overloading, the 25-kilovolt switchyard supply can be used or some of the heating load can be dropped to reduce the overload condition. Restoration of this switchgear to normal operation is accomplished manually. When an end supply breaker and the bus-tie breaker are closed, the other end supply breaker can be closed manually through a synchronism check relay. Before this end supply breaker is closed, the bus-tie breaker will be tripped to prevent temporary paralleling of the two end supplies. The bus-tie breaker cannot be closed manually when both end supply breakers are closed.

The 25-kilovolt switchyard supply can be manually connected to one bus or the other bus

through feeder breakers. The control of these feeder breakers is arranged so that only one breaker is closed at any one time. These breakers are interlocked with the bus differential relays and will be tripped on a bus differential and cannot be closed on the faulted bus. Since this operation is manual, the feeder breakers are not interlocked with the end supply or bus-tie breakers. The 25-kilovolt switchyard supply could be connected to a hot bus out of synchronism. Connection of this supply to the switchgear is left to the discretion of the operator.

(c) 460-Volt Secondary Unit Substations No. 1 and 2.-The 460-volt secondary unit substations No. 1 and 2 consist of a double-bus, double-ended unit substation with a supply breaker on each end, a bus-tie breaker, and a group of 460-volt feeder breakers. Since the unit substation is normally supplied from two different end sources, it could be possible to have unsynchronized sources connected to this switchgear. To avoid this possibility, the control to the two supply breakers and the bus-tie breaker is so arranged that only two of the three breakers can be closed simultaneously under manual or automatic mode of operation.

Each bus is protected by individual undervoltage relays. The feeder lines are protected by the circuit breakers having static-type trip devices. The feeder breakers which feed the alternating-current unit auxiliary powerboards (D1A through D8A) are part of a selective trip feeder breaker control system to provide coordination with the feeder breakers on the auxiliary powerboards, so that a fault on the load side of the powerboard feeder breaker will trip the powerboard feeder breaker and not trip the respective secondary unit unit substation feeder breaker. The instantaneous elements on these breakers to the unit auxiliary powerboards have elements which can be set between 5 and 40 times the pickup setting, whereas the regular dual static overcurrent trip devices only have a range of 5 to 15 times that setting. The time delay characteristics of the selective static trip devices also reaches out to the 40 times. By setting the instantaneous elements higher, more time delay characteristics are used. At high multiples of the pickup current, the time delay trip time becomes faster. This feature is used to provide the selective trip characteristics.

Under normal automatic operating conditions, the secondary unit substation load is divided and connected to each of two end supplies. In the event that one end supply breaker trips by operation of the undervoltage relays, the end supply breaker will trip