Relaying Quantities

A relay chooses between tolerable and intolerable system conditions by sensing some electrical system quantity or combination of quantities. That is, the relay must be given a set of parameters that define acceptable system operation. In certain situations, they may not exist, or it may be very costly to measure them.

Some of the parameters commonly used for relaying are:

1. A line current, a combination or comparison of several currents, or a harmonic component of a current.
2. A voltage or comparison of several voltages, or a sequence component of voltages, or the frequency of a voltage.
3. Power flow - amount or comparison, or direction of flow.
4. Impedance seen at a point in the system.
5. One or more non-electrical parameters such as temperature or pressure, etc.
6. Time, in combination with the above quantities.

Some examples of these might include a current magnitude relay which senses the sum of the currents flowing in all conductors of a feeder. Any net current increase could indicate there was a ground fault on the system. A voltage relay across a system grounding resistor would also indicate a ground fault. A reverse power relay could be used on a generating station with two or more generators operating in parallel - the relay would operate when either prime mover lost power. A bearing temperature relay would signal an alarm or shut down a large motor if the bearing started to run hot. As you can imagine, the number of combinations is almost limitless.