The relay which operates based on the frequency and produces a tripping signal when the magnitude of frequency drops below a predetermined value is known as under frequency relay. Under frequency relays are either electromagnetic type or static type.
Construction of Under Frequency Relay
This relay consists of four coils wound on the four poles of stator. These poles act as electromagnets when the coil is energized. The rotor has cup type structure as shown in Figure 1.
Figure 1: Under Frequency Relay.
By using the impedances that are dependent of frequency, the coils on the stator are connected in parallel across supply voltage through a voltage transformer.
Operating Principle and Working of Under Frequency Relay
Under frequency relay regularly monitors the frequency and works on the principle of Ferraris measurement. At normal frequency. the impedances are in balance condition. Thus, the rotor does not experience any torque. But when frequency decreases, these impedances vary hence imbalance condition occurs and torque is applied on the rotor. Thus, the relay operates.
The power system operates at normal frequency when it is in stable condition. During this condition. the mechanical power input of generators is exactly equal to the algebraic sum of all load demands and power losses. But if excessive loading of generator takes place i.e., if the generator is forced to give more output power than its rated capacity, then imbalance occurs in the above power equation of generator and results in reduction of speed in generator. The relation between frequency and speed of generator is given by,
\[{{\text{N}}_{\text{S}}}=\text{ }\frac{120\text{ f}}{\text{P}}\]
Where,
f = System frequency
NS = Synchronous speed
P = Number of poles.
From the above equation. it is clear that under speeding caused due to increment in load leads to reduction of frequency. The rate at which frequency decreases depends on overload percentage, variations in load and generator and the duration of overload on system.
Applications of Under Frequency Relay
Under frequency relay is used to maintain the stability of the system and it also avoids damage to generator caused due to overloading. They are used in several load stations i.e., substations and power generating units to trip specified feeders and generators respectively. The setting of these relays is based on maximum permissible overloading conditions.