Handbook of Electrical Engineering

316 HANDBOOK OF ELECTRICAL ENGINEERING

at the generator is used to measure the severity of the fault. The greater the fall in voltage the more
severe is the fault. The measured voltage is used to modify the characteristic of the relay. Electronic
relays use a function generator and a multiplying element to achieve the required characteristic.
The voltage signal is used to automatically reduce the time setting, which is often called the ‘Time
Multiplier Setting’. There are several methods frequently used, two of which are:-

a) Two definite levels of voltage.

b) Continuously variable between two limiting values of voltage.

This type of relay is called a ‘Voltage Restrained Overcurrent Relay’ (51 V). It often has a
definite minimum time limitation built into its design.

A typical multiplying function for the continuously acting voltage restraint is:-

Kv=

(

1. 333 ×V− 6. 667

100

)

per unit

For V in the range 20% to 80%, Figure 12.4 shows the voltage restraint function.

The ‘unrestrained’ operation of these relays is used as back-up overcurrent protection for
downstream relays in case they fail to respond.

Figure 12.4 Voltage restraint characteristic for a 51 V relay.