Distribution Substations, Summaries of Construction

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Distribution Substations

CHAPTER 6

Electrical Substation

An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers.

Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps.

A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution.

Transmission substation

A transmission substation connects two or more transmission lines.

In case where all transmission lines have the same voltage: the substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance.

A transmission station may have:

1. Transformers to convert between two transmission voltages, 2. Voltage control/power factor correction devices such as capacitors, reactors or static VAR compensators 3. Phase shifting transformers to control power flow between two adjacent power systems.

Transmission substations can range from simple to complex. The large transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large amount of protection and control equipment.

Distribution substation

A distribution substation transfers power from the transmission system to the distribution system of an area.

The input for a distribution substation is typically at least two transmission or subtransmission lines.

Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility.

Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems.

Distribution substations may also be the points of voltage regulation , although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side.

Switching substation

A switching substation is a substation which does not contain transformers and operates only at a single voltage level.

Switching substations are sometimes used as collector and distribution stations.

Sometimes they are used for switching the current to back-up lines or for paralellizing circuits in case of failure.

Design

The main considerations taking into account during the design process are:

1. Reliability
2. Cost (sufficient reliability without excessive cost)
3. Expansion of the station, if required.

Selection of the location of a substation must consider many factors:

1. Sufficient land area
2. Necessary clearances for electrical safety
3. Access to maintain large apparatus such as transformers.
4. The site must have room for expansion due to load growth or planned transmission additions.
5. Environmental effects( drainage, noise and road traffic effects.
6. Grounding must be taking into account to protect passers-by during a short- circuit in the transmission system
7. The substation site must be reasonably central to the distribution area to be served.

Layout

Following the switching components, the lines are connected to one or more buses.

An electrical bus , derived from bus bar, is a common electrical connection between multiple electrical devices.

Symbolic representation of a bus: The thick line is the bus, which represents three wires. The slash through the bus arrow and the "3" means that the bus represents 3 wires

The arrangement of switches, circuit breakers and buses used affects the cost and reliability of the substation.

For important substations a ring bus or double bus. Substations feeding only a single industrial load may have minimal switching provisions.

Once having established buses for the various voltage levels, transformers may be connected between the voltage levels. These will again have a circuit breaker in case a transformer has a fault.

A substation always has control circuitry to operate the various breakers to open in case of the failure of some component.

Layout

Load

The size of the load to be served determines the capacity of the substation.

The load must be distributed such that it can be served with reasonable feeder loss or more.

Critical loads (industrial districts) are served by more complex substations, designed for maximum reliability and speed of power restoration compare to the ones used in residential areas where a short time power loss is usually not a disaster.

Other substations in the area influence the design of a new substation.

The presence of the other substations will increase the overall power capacity and as a result can satisfy the demand for heavy loads.

Substations for critical loads usually use more than one transformer so that the load is served even if one transformer is out. Otherwise a single large three-phase transformer is used because it costs less per kVA of capacity, and requires less room, bussing, and simpler protective relaying.

Distribution Substation Protection Needs

This substation can be used to serve a small commercial area. It has a circuit breaker as well as a primary fuse for back up, and more disconnect switches for isolation during maintenance. The circuit breaker will operate from relays that require a metal clad enclosure, instrument transformers, and a de power supply for the trip circuit. The increased speed of fault removal supplied by the circuit breaker for this substation has substantially increased its cost.

Both substations are simple single-source, single-transformer, single-feeder types. The cost differences increase with the size of the substation, and the size and number of transformers used.

DISTRIBUTION SUBSTATION CONSTRUCTION METHODS

Four basic methods exist for substation construction:

1. Wood
2. Steel lattice
3. Steel low profile
4. Unit.

Wood pole substations are inexpensive, and can easily use wire bus structures. Wood is suitable only for relatively small, simple substations because of the difficulty of building complex bus and switch gear support structures from wood.

Lattice steel provides structures of low weight and high strength. Complex, lattice steel is reasonably economical and is the preferred material for substation construction whenever possible.

Solid steel low profile substations are superior to lattice or wood constructed substations. However, low profile construction is more expensive than either wood or lattice steel, and requires more land because multilevel bus structures cannot be used.

The unit substation is a relatively recent development. A unit substation is factory built and tested, then shipped in modules that are bolted together at the site. Unit substations usually contain high and low voltage disconnect switches, one or two three-phase transformers, low voltage breakers, high voltage fusing, bus work, and relays.

2. Single bus substation

This is the one line of a single bus substation fed by a single radial subtransaission line.

Each feeder must has its own overcurrent protection.

The primary switch must be able to break the transformer excitation current.

The transformer may have differential relaying that trips all of the feeder breakers in the event of a fault. Each distribution voltage the substation supplies must have its own bus. The possibility of a subtransmission circuit fault is much higher than a transformer fault. Two sources allow service to be restored more quickly upon a subtransmission circuit fault.

2. Single bus substation

This is the one line of a double throw switch on the primary side which allows the transfer to be made quickly from one subtransmission circuit to another.

The switch is interlocked with the transformer breaker so it cannot be opened under load.

The switch can be replaced by two manual high voltage breakers that can break the load, and expected fault current.

The transformer secondary breaker makes possible very effective differential bus protection to detect faults internal to the bus. The bus relays then trip all of the circuit breakers connected to the bus upon a bus fault.