F

Substation Configurations

In each of the four figures in this appendix, the bus work or node is depicted as a solid line. The squares represent circuit breakers that open the electrical circuit under load or short-circuit conditions. The switches, which have limited ability to interrupt current, serve to isolate components and bus sections.

MAIN AND TRANSFER BUS CONFIGURATION

A main and transfer bus configuration consists of two independent buses, one of which, the main bus, is normally energized. Under normal operating conditions, all incoming and outgoing circuits are fed from the main bus through their associated circuit breakers and switches. If it becomes necessary to remove a circuit breaker from service for maintenance or repairs, circuit operation can be maintained through use of the isolating switches and bus transfer equipment. The circuit breaker to be maintained and its switches are opened, the bus transfer switches are closed, the switch from the transfer bus to the circuit is closed, and then the bus transfer breaker is closed to re-energize the circuit. The circuit is then protected by the bus transfer breaker. Figure F.1 shows the typical configuration of a main and transfer bus scheme.

The main advantages of this scheme include:

•   Accommodation of circuit breaker maintenance while maintaining service and line protection;

•   Low cost—essentially one breaker per line or transformer;

•   Fairly small land area; and

•   Easily expandable.

The primary disadvantages of this scheme include the following:

•   Failure of a circuit breaker or a bus fault causes loss of the entire bus with outage of all circuits.

•   An additional circuit breaker is required for bus tie.

•   Since the bus tie breaker has to be able to be substituted for any line breaker, its associated relaying may be complicated.

•   Complicated switching is required to remove a circuit breaker from service for maintenance.

The main and transfer bus scheme, which has the potential for a major outage of all circuits, is mainly used in older stations, most often at voltages of 230 kV and below. For large stations, the bus may be broken into two or three sections, with bus-sectionalizing circuit breakers. A bus fault or breaker failure then affects only one section of bus, with the opening of the sectionalizing breakers preventing outages on other bus sections. It is important to distribute circuits onto bus sections in a balanced way, so that sufficient transmission network conductivity remains with a bus section outage.

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FIGURE F.1 One-line diagram of main and transfer bus scheme. In normal operation, the main bus is energized and the transfer bus is de-energized. In the bottom bay, the breaker and switches are open. In the top three bays, the switches on the left are open with the breakers and other switches closed.



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OCR for page 134
F Substation Configurations In each of the four figures in this appendix, the bus work An additional circuit breaker is required for bus tie. or node is depicted as a solid line. The squares represent Since the bus tie breaker has to be able to be substi- circuit breakers that open the electrical circuit under load or tuted for any line breaker, its associated relaying may short-circuit conditions. The switches, which have limited be complicated. ability to interrupt current, serve to isolate components and Complicated switching is required to remove a circuit bus sections. breaker from service for maintenance. The main and transfer bus scheme, which has the poten- MAIN AND TRANSFER BUS CONFIGURATION tial for a major outage of all circuits, is mainly used in older A main and transfer bus configuration consists of two stations, most often at voltages of 230 kV and below. For independent buses, one of which, the main bus, is normally large stations, the bus may be broken into two or three sec- energized. Under normal operating conditions, all incoming tions, with bus-sectionalizing circuit breakers. A bus fault or and outgoing circuits are fed from the main bus through their breaker failure then affects only one section of bus, with the associated circuit breakers and switches. If it becomes neces- opening of the sectionalizing breakers preventing outages on sary to remove a circuit breaker from service for maintenance other bus sections. It is important to distribute circuits onto or repairs, circuit operation can be maintained through use of bus sections in a balanced way, so that sufficient transmis- the isolating switches and bus transfer equipment. The circuit sion network conductivity remains with a bus section outage. breaker to be maintained and its switches are opened, the bus transfer switches are closed, the switch from the transfer bus to the circuit is closed, and then the bus transfer breaker is closed to re-energize the circuit. The circuit is then protected by the bus transfer breaker. Figure F.1 shows the typical configuration of a main and transfer bus scheme. TRASNSFER MAIN The main advantages of this scheme include: BUS BUS Accommodation of circuit breaker maintenance while maintaining service and line protection; Low cost--essentially one breaker per line or transformer; Fairly small land area; and Easily expandable. The primary disadvantages of this scheme include the following: FIGURE F.1 One-line diagram of main and transfer bus scheme. In normal operation, the main bus is energized and the transfer bus Failure of a circuit breaker or a bus fault causes loss is de-energized. In the bottom bay, the breaker and switches are of the entire bus with outage of all circuits. open. In the top three bays, the switches on the left are open with the breakers and other switches closed. 134

OCR for page 134
APPENDIX F 135 BREAKER-AND-A-HALF CONFIGURATION The breaker-and-a-half configuration, typically used at extrahigh-voltage (EHV) stations, consists of two buses, each normally energized. Electrically connected between the buses are three circuit breakers and, between each two break- ers, a circuit, as shown in Figure F.2. In this arrangement, three circuit breakers are used in a bay for two independent circuits; hence, each circuit shares the common center circuit breaker, so there are 1.5 circuit breakers per circuit. The breaker-and-a-half configuration provides for circuit breaker maintenance, since any breaker can be removed from service FIGURE F.3 One-line diagram for ring bus configuration. and isolated without interrupting any circuit. Additionally, faults on either of the main buses cause no circuit interrup- tions. Failure of a circuit breaker results in the loss of two RING BUS CONFIGURATION circuits if a common breaker fails and only one circuit if an For stations having three to five circuits, a ring bus is outside breaker fails. It is important to balance circuits in the often used. As more circuits are added, the configuration may bays, for example, source lines coming into the right-hand evolve to a breaker-and-a-half arrangement. Figure F.3 shows side of bays and load lines leaving the left-hand side of bays. a three-circuit ring bus that is based on Figure F.2 but with The main advantages of this scheme include the following: the bottom bay and three breakers and one bay-two circuit removed. A maintenance outage of a circuit breaker or circuit A bus fault does not interrupt service on any circuit, causes an "open ring." For open-ring operation, a subsequent and circuit breaker failure causes loss of only one or circuit outage may cause outage of additional circuits. two circuits; The advantages of this scheme include: Flexible operation; High reliability; and Low cost--only one circuit breaker per circuit; and Double feed to each circuit. Flexibility to evolve to a breaker-and-a-half arrange- ments as more circuits are added. The primary disadvantages of this scheme include the following: The disadvantages of this scheme include: One-and-a-half breakers are required per circuit; Reduced reliability in open-ring operation; and Relaying is complex, since the center breaker has to Temptation to add circuits without evolution to a respond to faults of either of its associated circuits, breaker-and-a-half arrangement. and since currents from two sources must be mea- sured for all circuits; and Each circuit must have its own potential source for DOUBLE BREAKER-DOUBLE BUS CONFIGURATION relaying. The double breaker-double bus configuration consists of two main buses, each normally energized. Electrically connected between the buses are two circuit breakers and, between the breakers, one circuit, as shown in Figure F.4. Two circuit breakers are required for each circuit. In the double breaker-double bus configuration, any cir- cuit breaker can be removed from service without interrup- tion of any circuits. Faults on either of the main buses cause no circuit interruptions. Circuit breaker failure results in the loss of only one circuit. Because of high cost, the double breakerdouble bus configuration is usually limited to large generating stations. The additional reliability afforded by this arrangement over the breaker-and-a-half scheme usually cannot be justified for conventional transmission or distribution substations. Occasionally, at a generating station, one bay of a breaker- FIGURE F.2 One-line diagram of breaker-and-a-half bus and-a-half arrangement is used as a double breaker-double configuration.

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136 TERRORISM AND THE ELECTRIC POWER DELIVERY SYSTEM The main advantages of this scheme include: Flexible operation, Very high reliability, Isolation of either main bus for maintenance without disrupting service, Isolation of any circuit breaker for maintenance with- out disrupting service, Double feed to each circuit, No interruption of service to any circuits from bus FIGURE F.4 One-line diagram of double breakerdouble bus fault, configuration. Loss of only one circuit for breaker failure, and All switching with circuit breakers. bus arrangement for a generator terminal to provide equal access to either main bus. The primary disadvantage of this scheme is high cost because two circuit breakers are required for each circuit.