Power stations produce the electricity that we use every day and in many parts of the world they are found at some distance away from the cities that they service. Examples of power stations are hydroelectric dams, as well as coal-fired, gas-fired and nuclear power plants. In each of these cases, turbines spin at high speeds to generate electricity. Other common examples of power sources are wind turbines and photovoltaic devices, commonly known as solar panels.
Electricity generation, transmission and distribution (GTD)
The electricity generation, transmission and distribution (GTD) chain starts at a power station, where electricity is generated. To deliver the electricity from the power station to the city, the voltage is usually raised to a very high level (about 245 kilovolts or more) by a transformer in a substation. This is done mainly to reduce losses (current is smaller when voltage is raised) and to reduce the size of line conductors. When electricity is transmitted over long distances it gradually loses voltage, so starting from high voltage ensures that the required amount of energy arrives at its destination. When the electricity reaches the city, it is stepped down to a lower distribution voltage (less than 72,5 kV). It is then transmitted in distribution networks and, when it reaches your neighbourhood, the voltage is reduced even further, to less than 1 kV.
The GTD chain terminates with the end user, where the electricity is used at medium and low voltages. End users fall into four broad categories:
- industrial (such as paper or steel mills);
commercial (such as a shopping centre, or a manufacturing company such as a home appliance factory);
infrastructure (city use such as streetlights); and
residential (offices, houses and apartments).
One crucial part of the electricity GTD chain is switchgear and controlgear. These are used to maintain or to break a connection on an electrical line. To explain this technology, let’s compare the flow of electricity in an electrical system with the flow of water in a river system that has tributaries and forks. Along the river and the streams that flow into it there are elements like dams, locks, and flood gates that regulate the flow of water and help to prevent flooding. All these elements perform definite functions, and they can do so either separately or together in assemblies: imagine a big dam to control the heavy floods with a flood gate that will limit the level of water in a lake by releasing a part of the water into a canal.
Under normal conditions the same flood gate and canal can divert the water away from the river so maintenance can be done. It is the same with switchgear: disconnectors are opened and the flow of electricity diverted to another branch of the network so that people can do maintenance on a line.
In case of abnormal conditions (a short-circuit on a line, for example) the circuit-breakers are opened to break the fault current, allowing the electricity to flow through other lines of the circuit.
Along the river you also have canal locks that are used to raise or lower the level of the river (to allow boats to pass, or to irrigate plantations). Transformers do the same thing in an electric network to raise or lower the voltage. A common objective is to distribute water in one case, or electricity in the other, to locations where it is needed.
The IEC prepares standards for switchgear and controlgear. Generally speaking, an IEC standard is a document that contains technical information about a product, a service in an area of electrotechnology. The kind of information you usually find in an IEC standard are definitions, characteristics and testing methods. Standards are important because they:
- help people to understand each other clearly on technological subjects by allowing them to “speak” the same language;
spell out the basic criteria for quality that everyone can agree upon (quality is often expressed in each standard in terms of safety and performance);
define in very clear terms the parameters that allow for testing;
provide a way to compare one product or service against another by using these defined parameters.
In between the power station and the end user, we find substations. They contain equipment for protection, for control, and for metering and monitoring. High voltage switchgear and controlgear protect the electricity transmission system from faults, allow the power supply to be restored quickly after an outage and regulate the system for optimum delivery of power. They protect equipment connected to the electricity supply chain, such as capacitor banks, transformers, electrical motors and distribution lines.
High voltage switchgear and controlgear consist of circuit breakers, contact boards (that open and close the power supply), sensors (for example temperature sensors, limit and proximity switches, and so forth) and disconnectors. The latter are used to isolate equipment from the power supply so that repair work can be carried out.
Low voltage switchgear and controlgear, which are an integral part of the electrical system of a building, consist of the same elements as their high voltage cousins (see above) and are used to control and protect processes. These processes are available to those end-users that fall into the four broad categories mentioned above: industrial; commercial; infrastructure; and residential. Examples of “processes” are air-conditioning in a shopping centre, where the process is to maintain a steady air temperature (climate control), or elevators in an office building where the process is to move people from one floor to another (transportation).
Low voltage switchgear and controlgear distribute power from the electricity network to the equipment that controls these processes, and at the same time protect this equipment. This includes such items as small electrical motors for elevators, ovens for steel mills and distribution centres for lighting and computers.
An assembly is an enclosure, such as a metal box, that holds a group of devices that together perform a complete function. These devices have to work together to ensure that the function occurs both properly as expected as well as safely. The devices are the various elements that are known as switchgear and controlgear. Assemblies include such elements as circuit breakers to protect against a short, contactors for remote control and overload relays to protect from overheating and burning.