The basic form of this relay is shown in Figure 11.17. The armatures at the ends of the beam are attracted by electromagnets which are operated by the appropriate parameters, usually voltage and current. A slight mechanical bias is incorporated to keep the contacts open, except when operation is required.
The pulls on the armatures by the electromagnets are equal to KjV2 and K2I2, where K and K2 are constants, and for operation (that is, contacts to close), that is.
Figure 11.17 Schematic diagram of balanced-beam relay
This shows that the relay operates when the impedance it 'sees' is less than a predetermined value. The characteristic of this impedance relay, when drawn on an R and jX axes, is a circle as shown in Figure 11.18.
The balanced-beam relay, because it measures the impedance of the protected line, effectively measures distance to the fault. Two other relay forms also may be used for this purpose:
- 1. The reactance relay which operates when — sin f < constant, having the straight line characteristic shown in Figure 11.18.
- 2. The mho or admittance relay, the characteristic of which is also shown in Figure 11.18.
Figure 11.18 Characteristics of impedance, reactance, and admittance (mho) relays shown on the R-Xdiagram
These relays operate with any impedance phasor lying inside the characteristic circle or below the reactance line. In practice modern distance relays calculate the operating characteristics using digital micro-processors and some manufacturers offer the ability to create more complex shapes (for example, trapezoids).