TRANSMISSION OF ELECTRIC POWER

Transmission of electric power is a process by which the electric power produced at power plants is transported in bulk quantities over long distances for eventual use by consumers. Electric power is sent from generating power plants to the end consumer by transmission lines. Transmission lines, when interconnected with each other, become transmission networks.

This transmission network along with power stations and substations is known as ‘transmission grid’ or simply ‘grid’. A typical transmission grid is shown in Fig 1. The transmission networks which are interconnected at the national level are known as ‘National grid’. Energy is usually transmitted within a grid with three phase alternating current (AC). Due to the involvement of large quantity of electric power and due to the properties of electricity, transmission involving long distances normally takes place at high voltage (33 kV or above). Electric power is usually transported to a substation near the consuming point which is either a populated area or an industrial complex. At the substation, the high voltage electric power is converted to lower voltages suitable for consumer use, and then transported to the end users through low voltage electric distribution line

Fig 1 Typical transmission grid

Transmission efficiency and transmission losses

Transmitting electricity at high voltage reduces the fraction of energy lost to resistance, which varies depending on the specific conductors, the current flowing, and the length of the transmission line. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. Transmission efficiency is improved by increasing the transmission voltage using a step-up transformer which has the effect of reducing the current in the conductors, whilst keeping the power transmitted nearly equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since, according to Ohms law, the losses are proportional to the square of the current, halving the current results in a four-fold decrease in transmission losses. Reduced current means lesser I2R (square of the current I multiplied by the conductor resistance R) loss in the system, less cross sectional area of the electrical conductor cable means less capital involvement and decreased current causes improvement in voltage regulation of power transmission system and improved voltage regulation indicates quality power. Because of these three reasons electrical power is mainly transmitted at high voltage level.

Hence electric power to be efficiently transported to long distances need high voltages. This voltage can be 33 kV, 66 kV, 110 kV, 132 kV, 220 kV, 400 kV or even higher. The generator voltage of a power plant usually ranges from 11 kV to 25 kV. The generated electric power is first transported from the generator to a transformer at the power plant. The transformer increases the voltage to the voltage of the grid. The generator is then synchronized with the grid and the generated power is transmitted to the consumer end. At the consuming point end the transmission lines are connected to a substation. Here the transformers of substation change the voltage of the electric power from high voltage to a lower level. From substation electrical power of lower voltage is distributed to the consumers of the electrical power through distribution lines.

The main components of an electric power transmission grid are as follows.

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.

Transmission lines

Electric power is transmitted at high voltage (110 kV or above) to reduce the energy lost in long-distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.

Transmission lines normally use high voltage three-phase alternating current. High voltage direct current (HVDC) technology is used for greater efficiency in very long distances (typically several hundreds of kilometers).  HVDC links are also used to stabilize against control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems and cascading failures.

Generally, several conductors are strung on a transmission tower for each electrical circuit. Conductors are constructed primarily of twisted metal conductors. High voltage overhead conductors are not covered by insulation. The conductor material is usually aluminum conductor steel reinforced (ACSR) which is a specific type of high-capacity, high-strength stranded conductor.  The outer strands are made from hard drawn aluminum wire manufactured from not less than 99.5% pure electrolytic aluminum rods of EC grade and copper content not exceeding 0.04%. High purity aluminum alloy is chosen for its excellent conductivity, low weight and low cost. The center strands are of steel for the strength required to support the weight without stretching the aluminum due to its ductility. This gives the conductor an overall high tensile strength. Copper was earlier used for overhead transmission but aluminum is lighter, yields only marginally reduced performance and costs much less. 

Rights of way and access roads

The rights of way (ROW) for a transmission corridor includes land set aside for the transmission line and associated facilities, needed to facilitate maintenance, and to avoid risk of fires and other accidents. It provides a safety margin between the high voltage lines and surrounding structures and vegetation. Some vegetation clearing may be needed for safety and/or access reasons. A ROW generally consists of native vegetation or plants selected for favorable growth patterns (slow growth and low mature heights). However, in some cases, access roads constitute a portion of the ROW and provide more convenient access for repair and inspection vehicles. The width of a ROW varies depending on the voltage rating of the transmission line. Access roads to transmission line structures for both line construction and maintenance are normally required, and may be paved or gravel.