Substation

Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Substation varies in size and configuration. Between the generating station and consuming point, electric power may flow through several substations at different voltage levels.

A transmission substation connects two or more transmission lines. The simplest case is where all the transmission lines have the same voltage. In such case, substation contains high voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station normally has transformers to convert between two transmission voltages, voltage control, power factor correction devices such as capacitors, reactors or static VAR compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems.

Transmission substations can range from simple to complex. A small ‘switching station’ normally consists of a bus plus some circuit breakers. The large transmission substations usually are accommodated in a large area (several hectares) and have multiple voltage levels, many circuit breakers and a large amount of protection and control equipment (voltage and current transformers, relays, and SCADA systems). Modern substations are installed as per international standards such as IEC Standard 61850.

Substations vary in size and configuration but may cover several acres; they are cleared of vegetation and typically surfaced with gravel. They are normally fenced, and are reached by a permanent access road. In general, substations include a variety of structures, conductors, fencing, lighting, and other features that result in an ‘industrial’ appearance.

Transmission towers

Transmission towers are the most visible component of the power transmission system. They are used in high voltage AC and DC systems. A transmission tower is normally a tall steel structure. Its function is to keep the high-voltage conductors (power lines) separated from their surroundings and from each other. A wide variety of tower shapes, sizes, and designs exist that generally employ an open lattice work or a monopole, but generally they are very tall with height ranging from 15 m to 55 m and cross arms as much as 30 m wide. In addition to steel, other materials may be used, including concrete and wood.

There are four major categories of transmission towers. They are suspension, terminal, tension, and transposition. Some transmission towers combine these basic functions.

The towers must be designed to carry three (or multiples of three) conductors. The towers are usually made of steel lattices or trusses. The insulators are either glass or porcelain discs assembled in strings or long rods whose lengths are dependent on the line voltage and environmental conditions.

Typically, one or two ground wires, also called ‘guard’ wires, are placed on top to intercept lightning and harmlessly divert it to ground. Towers for high and extra high voltage are usually designed to carry two or more electric circuits.