What is switchgear?
The importance of electric supply in everyday life has reached such a stage that it is desirable to protect the power system from harm during electrical fault conditions and to ensure maximum continuity of supply. For this purpose, means must be provided to switch on or off generators, transmission lines, distributors, and other equipment under both normal and abnormal conditions. This is achieved by an apparatus called switchgear. We can say that the apparatus used for switching, controlling, and protecting the electrical circuits and equipment in the power system is known as switchgear. Switchgear essentially consists of switching and protecting devices such as switches, fuses, circuit breakers, relays, lighting arresters, etc.
Important Features of switchgear
(i) Complete reliability. With the continued trend of interconnection and the increasing capacity of generating stations, the need for reliable switchgear has become of paramount importance. When the fault occurs on any part of the power system, the switchgear must operate to isolate the faulty section from the remaining circuit.
(ii) Absolutely certain discrimination. When a fault occurs on any section of the power system, the switchgear isolates the faulty section from the system without affecting the healthy section which will ensure continuity of supply.
(iii) Quick operation. When the fault occurs on any part of the power system, the switchgear must operate quickly so that no damage is done to generators, transformers, and other equipment by the short-circuit currents.
(iv) Provision for manual control and instruments. Switchgear must have provision for manual control and instruments. In case the electrical (or electronics) control fails, the necessary operation can be carried out through manual control with various instruments.
Switchgear covers a wide range of equipment concerned with switching and interrupting currents under both normal and abnormal conditions. It includes switches, lightning arresters, fuses, circuit breakers, relays, and other equipment.
(i) Air-break switch. It is an air switch and is designed to open a circuit under load. In order to quench the arc that occurs on opening such a switch, special arcing horns are provided. Arcing horns are pieces of metal between which an arc is formed during the opening operation. As the switch opens, these horns are spread farther and farther apart. Consequently, the arc is lengthened, cooled, and interrupted. Air-break switches are generally used outdoors for circuits of medium capacity such as lines supplying an industrial load from a main transmission line or feeder.
(ii) Isolator or disconnecting switch. It is essentially a knife switch and is designed to open a circuit under no load. Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line. Such switches are generally used on both sides of circuit breakers in order that repairs and replacement of circuit breakers can be made without any danger.
(iii) Oil switches. As the name implies, the contacts of such switches are opened under oil, usually transformer oil. These switches are used for circuits of high voltage and large current-carrying capacities.
A fuse is a short piece of wire or thin strip that melts when excessive current flows through it for sufficient time. It is inserted in series with the circuit to be protected. Under normal operating conditions, the fuse element is at a temperature below its melting point. Therefore, it carries the normal load current without overheating. However, when a short circuit or overload occurs, the current through the fuse element increases beyond its rated capacity. This raises the temperature and the fuse element melts (or blows out), disconnecting the circuit protected by it.
A circuit breaker is equipment that can open or close a circuit under all conditions viz. no-load, full load, and fault conditions. It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions.
Fig. a-(i) shows the parts of a typical oil circuit breaker whereas Fig. a-(ii) shows its control by a relay circuit. The circuit breaker essentially consists of moving and fixed contacts enclosed in a strong metal tank and immersed in oil, known as transformer oil. Under normal operating conditions, the contacts remain closed and the circuit breaker carries the full-load current continuously. In this condition, the e.m.f. in the secondary winding of the current transformer (C.T.) is insufficient to operate the trip coil of the breaker but the contacts can be opened (and hence the circuit can be opened) by manual or remote control. When a fault occurs, the resulting overcurrent in the C.T. primary winding increases the secondary e.m.f. This energizes the trip coil of the breaker and moving contacts are pulled down, thus opening the contacts and hence the circuit. The arc produced during the opening operation is quenched by the oil. It is interesting to note that the relay performs the function of detecting a fault whereas the circuit breaker does the actual circuit interruption.
A relay is a device that detects the fault and supplies information to the breaker for circuit interruption. Fig. (a)-ii shows a typical relay circuit. It can be divided into three parts viz.
- The primary winding of a *current transformer (C.T.) which is connected in series with the circuit to be protected. The primary winding often consists of the main conductor itself.
- The second circuit is the secondary winding of C.T. connected to the relay operating coil.
- The third circuit is the tripping circuit which consists of a source of supply, the trip coil of a circuit breaker, and the relay stationary contacts.
Under normal load conditions, the e.m.f. of the secondary winding of C.T. is small and the current flowing in the relay operating coil is insufficient to close the relay contacts. This keeps the trip coil of the circuit breaker unenergised. Consequently, the contacts of the circuit breaker remain closed and it carries the normal load current. When a fault occurs, a large current flows through the primary of C.T. This increases the secondary e.m.f. and hence the current through the relay operating coil. The relay contacts are closed and the trip coil of the circuit breaker is energized to open the contacts of the circuit breaker.