The logic of protective relaying looks at a complex distribution system as an integration of various subsystems. In all cases, some common design criteria are applicable.

This consists of five basic elements: (1) selectivity, (2)speed of operation (3) reliability, (4) simplicity, and (5) economics. Sometimes a sixth criterion of maintainability is also added.

A protection system should operate so as to isolate the faulted section only. In a radial system of distribution, using inverse time relays as the primary protection, the desired selectivity is obtained by coordinating upstream relays with the downstream relays in steps, so that an upstream relay is slower than the downstream relay.

This may increase the fault clearance time toward the source depending upon relay characteristics and the fault current distribution. A separate zone of protection can be established around each system element so that a fault occurring in that zone will be instantaneously cleared without a time delay.

Normally, these zones are overlapped by proper location of the current transformers and protective relays so that there are no unprotected areas. This logically divides the system into protective zones for generators, transformers, buses, transmission lines, cables, and motors.

These are called unit protection schemes. The faults are cleared fast, with detection times of 1 to 2 cycles. The other relays, such as time overcurrent and directional relays, are still retained as backup protection.

The desired reliability may increase the system protection complexity and backup protection becomes necessary. Reliability and fast fault clearing dictate these unit protection schemes. In a network of interconnected lines and multiple generators, more than one breaker must be tripped to isolate the fault.

Fault damage to the system components, the stability between synchronous machines, and auto reclosing to restore power are to be considered in designing the speed of operation of the protective system. The total fault duration is the relay operating time plus the breaker interrupting time.

A reduction in power transferred to the loads will occur during a fault condition, the amount depending upon the type of fault. The shorter the fault clearance time, the greater the amount of power that can be transferred without a system separation and shutdown.

Dependability and security are the measures of reliability. The protection must be dependable and operate in response to system trouble within its required area and be secure against incorrect trips from all other conditions (i.e., voltage regulation due to load application and rejection, inrush currents, switching surges, and high magnitude of through fault currents).

Thus, these two objectives of reliability mutually oppose each other. Designing more flexibility into the system design (i.e., double-ended substation, duplicate feeders, auto-switching, and bus transfer schemes) will increase the complexity and hence reduce security of the protective system. Reliability should be viewed in terms of overall system performance—overprotection and underprotection may both jeopardize it.

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