Protection of Power System

PROTECTION OF POWER SYSTEM

A great demand for electrical energy is a notable feature of modern civilization. In general the electric energy is needed for lighting, heating, domestic appliances, industrial electrical machinery and electric traction. It is desirable to protect the power system from harm during fault conditions and to ensure maximum continuity of supply. For this purpose, all electrical equipments and system under both normal and abnormal conditions must be provided to switch on or off. This is achieved by an apparatus called switch gear. A switch gear essentially consists of switching and protecting devices such as fuses, switches, circuit breaker, relays, lighting arresters etc.

1. Switch gear

The apparatus used for switching, controlling and protecting the electrical apparatus and equipment is known as switch gear. During normal conditions, switch gear permits to switch on or off generators, transmission lines, distributors and other electrical equipment. When short circuit or fault occurs on any part of power system, a heavy 'current flows through the equipment, threatening damage to the equipment and interruption of service to the consumers. However, the switch gear detects the fault and disconnects the unhealthy section from the system.

The switch gear consists of switches, fuses, circuit breakers, relays and other equipments. The switches may be classified into air switches and oil switches. The contacts of the former are opened in air and that of the latter are opened in oil.

A fuse is a short piece of wire or thin strip which 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. When a over load or fault occurs, the current through the fuse element increases beyond its rated capacity. This raises the temperature and the fuse element melts, disconnecting the circuits protected by it.

2. Circuit breaker

A circuit breaker is an equipment which can open or close a circuit under normal and abnormal conditions. Under normal conditions, it can be operated manually. Under normal conditions, it can be operated automatically.

Fig 7.4 shows the circuit breaker with relay connection. The relay coil is connected to the secondary of a current transformer. The primary carries the line current of the phase that has to be protected. If the line current exceeds a preset limit, the secondary current will cause the relay contacts C₁ C₂ to close. When C₁ C₂ contacts get closed, the tripping coil is energised by an auxiliary d.c source. This causes the main contacts to open, thus interrupting its circuit.

3. Relay

A relay is a device which detects the fault and supplies information to the breaker for circuit interruption. Fig. 7.5 shows the typical relay circuit. The relay circuit consists three major parts.

In an current transformer, the primary winding is connected in series with the circuit to be protected. The secondary winding of current transformer is connected to the relay operating coil. The tripping coil circuit consists of a source of supply, trip coil of circuit breaker and relay contacts.

Under normal conditions, the emf induced in the secondary winding of CT is small and the current flowing in the relay operating coil is insufficient to close the relay contacts. The contacts of the circuit breaker remain closed and it carries normal load current. When a fault occurs, a large current flows through the primary of CT. This increases the secondary emf and hence the current through the relay operating coil. The relay contacts are closed and trip the coil of the circuit breaker is energised to open the contacts of the circuit breaker.