Electrical Power Plant Design: Distribution System



Distribution: Substation Design: Functions

Sections: Functions | Types | Switching Schemes | Components

Switching Schemes

Main Bus Connections The substation scheme selected determines the electrical ad physical arrangement of the switching equipment. Different bus schemes are dependent on the factors of reliability, economy, safety, and simplicity as warranted by the function and importance of the substation. The bus scheme more often used as (a) Single Bus, (b) Double bus, double breaker (c) Main and transfer bus (d) Double bus, single breaker (e) Ring bus, and (f) Breaker and a half. Some of these schemes maybe modified by the addition of bus-tie breakers, bus sectionalizing devices, breaker bypass facilities and extra transfer buses.

Single Bus is not normally used for major substations. Dependence on one main bus can cause serious outage in the event of breaker or bus failure. The station must be deenergized in order to carry out bus maintenance or add bus extensions. Although protective relaying is relatively simple, it is considered inflexible and subject to complete outage.

Double Bus, Double Breaker requires two circuit breakers for each feeder circuit. Any breaker can be taken out of service for maintenance. Normally each circuit is connected to both buses. In some cases, half of the circuits could operate on each bus. For each cases bus or breaker failure would cause the loss of half of the circuits. The location of the main buses must be such as to prevent faults spreading to both buses. This scheme is expensive, however, reliability is high when all circuits are connected to operate on both buses.

Main and Transfer Bus adds a transfer bus to the single-bus scheme. An extra bus-tie circuit breaker is provided to tie the main and the transfer buses together. When a circuit breaker is removed from service for maintenance, the bus-tie circuit breaker is used to keep that circuit energized. Unless the protective relays are also transferred, the bus-tie relaying must be capable of protecting transmission lines or generators. If the main bus is ever taken out of service for maintenance, no circuit breakers remain to protect any of the feeder circuits. Failure of any breaker or failure of the main bus can cause complete loss of service of the station. Although this scheme is low in cost and enjoys great popularity, it does not provide the high degree of reliability and flexibility required.

Double Bus, Single Breaker uses two main buses, and each circuit includes two bus selector disconnect switches. A bus-tie circuit connects to the two main buses and when closed, allows transfer of a feeder from one bus to the other bus without deenergizing the feeder circuit by operating the bus selector disconnect switches. The circuit may all operate from the No. 1 main bus, or half of the circuits maybe operated off either bus. In the firs case, the station will be out of service for bus or breaker failure. In the second case, half the circuits would be lost for bus or breaker failure. Bus-tie breaker failure takes entire substation out of service. Although, it permits some flexibility with two operating buses, thus either bus maybe isolated for maintenance, it is poor in reliability and not normally used for important substations

Ring Bus breakers are arranged in a ring with circuits connected between breakers. There are a number of circuits as there are breakers. There is no main bus. During normal operation, all breakers are closed. For a circuit fault, two breakers are tripped, and in the event one of the breaker fails to operate to clear fault, an additional circuit will be tripped by the operation of breaker-failure backup relays. During breaker maintenance, the ring is broken, but all lines remain in service. The circuits connected to the ring are arranged so that sources are alternated with loads. The scheme is economical in cost, has good reliability, is safe for operation, and normally suitable for important substations up to a limit of five circuits. Protective relaying and automatic reclosing are more complex than other schemes.

Breaker and a Half, also called the Three-Switch scheme is usually developed from the Ring Scheme, has three breakers in series between the main buses. Two circuits are c onnected between the three breakers. This pattern is repeated along the main buses so that one and a half breakers are used for each circuit. Under normal operating conditions all breakers are closed and both buses are energized. A circuit is tripped by opening the two associated circuit breakers. Tie breaker failure will trip one additional circuit, but no additional circuit is lost if a line trip involves failure of line breaker. Either bus maybe taken out of service at any time with no loss of service. With sources connected opposite loads, it is possible to operate both buses out of service. Breaker maintenance can be done with no loss of service, no relay changes, and simple operation of the breaker disconnects. It is more expensive than the other schemes, except the double-breaker-double-bus scheme. It is, however, superior in flexibility, reliability, and safety. Protective relaying and automatic reclosing are more complex than other schemes.

Physical Arrangements

Once the switching scheme is determined, it is necessary to consider the station arrangement, as follows: (a) Conventional outdoor open-type bus-and-switch, (b) Inverted-bus substation and (c) Sulfur hexaflouride gas mini-type metal-clad substation.

 Outdoor open-type bus-and-switch arrangements consist essentially of open-bus construction using either rigid- or strain-bus design or combination of rigid and strain bus. The buses are arranged to run the length of the station and are located toward the outside of the station. Transmission-line exits cross over the main bus and are dead-ended on takeoff tower structures. The line drops into the bay in the station and connects to the the disconnecting switches and circuit breakers. It requires three distinct levels of bus to make the necessary crossovers and connections to each substation bay. Typical dimensions of these levels at 230kV, for example, are 16 ft for the first level above ground, 30 ft high for the main bus location and 57 ft for the highest level of bus. It required a minimum land area per bay and relative ease of maintenance, and is ideally suited to a transmission-line through connection where a substation must be cut into a line right-of-way.

Inverted Bus uses the breaker-and-a-half scheme, usually used for EHV substations. All outgoing circuit takeoff towers are located in the outer perimeter of the substation, eliminating the crossover of line and exit facilities. Main buses are located in the middle of the substation, with all disconnecting switches,circuit breakers, and all bay equipment located outboard of the main buses. It offers advantages such as beauty due to very low profile and aesthetic qualities.

 Distribution: Substation Design: Components

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