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Institute of Electrical and Electronics Engineers (IEEE) defines protective relays “Relays whose primary function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action”.
What they do is- first, detect and then locate faults by measuring electrical quantities in the power system which acts differently when it is healthy and in intolerable condition. The most significant role of protective relaying is to protect customers and then the equipment like electrical switchboards.
In the second case, their task is to minimise the damage and expense caused by insulation breakdowns, which are known as ‘faults’ by the relay engineers. An added advantage with relays is that it is not required to operate during regular operation, but it immediately starts to activate while handling intolerable system conditions. This criterion is essential in order to avoid serious outages and damages to certain parts.
When theoretically speaking, a relay system should be capable enough to respond to an infinite number of issues that might take place.
However, practically, some compromises are made while comparing risks. As it is quite difficult to ensure the stability and security of the entire power system. If only the local measurements are implemented while monitoring, protection and control schemes. One of the promising ways is to develop system-wide protection and control mechanisms, complementary to the conventional local and zonal protection strategies.
If you are wondering what exactly qualifies as the performance and design criteria for system-protection? Then the answer to that question is -devices include reliability, selectivity, speed, economy, and simplicity.