Practically all electrical measurements and relaying decisions are derived from current and voltage signals. Since relaying hardware works with smaller range of current (in amperes and not kA) and voltage (volts and not kV), real life signals (feeder or transmission line currents) and bus voltages have to be scaled to lower levels and then fed to the relays. This job is done by current and voltage transformers (CTs and VTs). CTs and VTs also electrically isolate the relaying system from the actual power apparatus. The electrical isolation from the primary voltage also provides safety of both human personnel and the equipment. Thus,
CT and VTs are the sensors for the relay.
CT and VT function like ‘ears' and the ‘eyes' of the protection system. They listen to and observe all happening in the
external world. Relay itself is the brain which processes these signals and issues decision commands implemented by circuit breakers, alarms etc.
Clearly, quality of the relaying decision depends upon ‘faithful' reproduction on the secondary side of the transformer. In this module, we will learn a lot more about these devices. In particular, we will answer the following questions:
How is a CT different from the normal transformer?
How to decide the CT specifications?
How to ascertain that CT is functioning as desired i.e., performance analysis?
