Protection Relays serve to protect equipment and circuits from abnormalities such as overcurrent, overvoltage, overloading and under reactance. The input to the protection Relays are the current and the voltage of the system. Using these two basic parameters, the relays are able to calculate a host of values such as kW, kVAr, pf, etc.
Thus, every protection relays needs an input from the Potential Transformer or the current transformer or both. When the relay is connected to the circuit of an instrument transformer, it becomes a load. The Potential or Current Transformer acts as the source.
When designing the Protection Scheme, we must ensure that the Potential Transformer or the Current Transformer does not get overloaded. This is done by adding the VA burden of each protection relay in the system. Every Relay will have the VA burden mentioned in the manual. The total VA burden imposed by all the relays should be calculated. The VA capacity of the instrument transformer should be greater than this.
While designing a system, the instrument transformer should have an excess capacity of 10% of the present load. This is an allowance for future relays which may be added to the system.
If the transformer is overloaded, the voltage and current signals will not be accurate.
The wires used in the protection system should be of sufficient thickness so as not to added unnecessary burden on the transformers.