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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 135
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.
OCR for page 136
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.
