Protection system is required for power system. An electrical energy becomes a part and parcel of over daily life. Electrical energy generate in bulk to meet the demands of consumers. The energy is required for lighting, heating, industrial electrical machinery and many other purposes. In present days, we totally rely on electrical energy. So it is desirable to protect the power system from damage during fault conditions such as in short circuit conditions. The protection is achieved by an apparatus known as switchgear. A switchgear consist of switch, fuse, circuit breaker, relays etc.
During normal operation, the protecting device or switchgear allow the switch on or off the electrical equipment machinery, transmission lines, generators etc. When faults occur on any part of power system the value of current is raised from the current flowing during the normal condition. The switchgear detects this increased fault current and disconnect the unhealthy section from the power system. In this way, the switchgear protects power system from adverse effects of faults and maintains the continuity of supply.
What is switchgear? or definition of switchgear
Switchgear usually consists of switches, fuses circuit breakers and relays used for switch on or off, control the equipment, transmission lines, protect the system from faults.
The switchgear equipment essentially used for switch on or off and interrupting currents either under normal or abnormal conditions. The simple switch with fuse is a simplest form of switchgear used fro protection of domestic load and offices. For higher rating circuit, a high-rupturing capacity fuse is used in conjunction with switch. For very high rating circuits, this protection system is not advisable. In this case automatically operated circuit breakers are used to protect the electrical circuits.
Simple switchgear show in figure. The circuit consists of trip coil, battery, relay coil current transformer and circuit breaker. Under normal operating conditions, the contacts remain closed and the circuit breaker carries the load current. The protective relay is connected in the secondary of a current transformer. When fault occurs on a system heavy current flows through the current transformer which increase the secondary current. This current when flows through relay coil, it closes the trip coil circuit. A battery is connected in series with trip coil and current start flowing through the trip coil and trip coil gets energized and disconnect the unhealthy section from the healthy section.
Basic requirements of switchgear
It should be reliable. Reliability means that the switchgear must be ready to operate, correctly at all times and under all conditions of a fault and abnormal conditions of the power system fro which it has been designed.
It should have sensitive. Sensitivity means the protection system sense the minimum value of fault quantity when it occurs in a power system. If the sensitivity is increased the cost is also increased.
Selectivity and Discrimination
Selectivity means switchgear should be such it select correctly that part of the system in which fault is occurred and disconnect or isolate the faulty part without disturbing the rest of the system. Discrimination means it must be able to discriminate healthy and unhealthy part of the power system.
The switchgear would isolate the unhealthy section as quick as possible. The speed of operation of switchgear reduces the amount of damaged incurred.
Large number of equipment assembled in switchgear which used for switching and interrupting currents under normal and abnormal conditions. The equipment used in switchgear is switches, fuses, circuit breakers, relays and other equipment.
The switch is a device which is used for make or breaks an electrical circuit. The switch is used in low voltage and small current circuits. When it is used for heavy current and high voltage circuits on the breaking time of circuit, arc is produced between two contacts of the switch. For large power operations circuit breakers are used. The switches can be used under full load or no load conditions but it cannot interrupt the fault current. The switches may be divided into following types
- Air-Breaker switch
- Oil switches
A fuse is a short piece of wire which melts when excessive current flows through it. Fuse is always connected in series with the circuit to be protected. Under normal conditions, it carry normal current without overheating. When faults occurs on the system excessive current start flows through the fuse and it melts and disconnect the faulty section from the healthy section.
A circuit breaker is a protecting device which can make or break the circuit under no-load, full-load and fault conditions. It is so designed that it can be operated manually under normal conditions and automatically under fault conditions.
Relays are the device which sense the faults and send signal to trip coil or circuit breakers.