Power System Protection Protection Coordination
Protection coordination
Relay operating characteristics and their setting must be carefully coordinated in order to achieve selectivity. The aim is basically to switch off only the faulted component and to leave the rest of the power system in service in order to minimize supply interruptions and to assure stability.
Sensitivity
Protection should be as sensitive as possible to detect faults at the lowest possible current level. At the same time, however, it should remain stable under all permissible load, overload and through-fault conditions.
Phase-fault relays
The pick-up values of phase o/c relays are normally set 30% above the maximum load current, provided that sufficient shortcircuit current is available. This practice is recommmended in particular for mechanical relays with reset ratios of 0.8 to 0.85. Numerical relays have high reset ratios
near 0.95 and allow therefore about 10% lower setting. Feeders with high transformer and/or
motor load require special consideration.
Transformer feeders
The energizing of transformers causes inrush currents that may last for seconds, depending on their size (Fig. 113).
Transformer feeders
The energizing of transformers causes inrush currents that may last for seconds, depending on their size (Fig. 113).
Relay operating characteristics and their setting must be carefully coordinated in order to achieve selectivity. The aim is basically to switch off only the faulted component and to leave the rest of the power system in service in order to minimize supply interruptions and to assure stability.
Sensitivity
Protection should be as sensitive as possible to detect faults at the lowest possible current level. At the same time, however, it should remain stable under all permissible load, overload and through-fault conditions.
Phase-fault relays
The pick-up values of phase o/c relays are normally set 30% above the maximum load current, provided that sufficient shortcircuit current is available. This practice is recommmended in particular for mechanical relays with reset ratios of 0.8 to 0.85. Numerical relays have high reset ratios
near 0.95 and allow therefore about 10% lower setting. Feeders with high transformer and/or
motor load require special consideration.
Transformer feeders
The energizing of transformers causes inrush currents that may last for seconds, depending on their size (Fig. 113).
The energizing of transformers causes inrush currents that may last for seconds, depending on their size (Fig. 113).
Selection of the pickup current and assigned time delay have to be coordinated so that the rush current decreases below the relay o/c reset value before the set operating time has elapsed.
The rush current typically contains only about 50% fundamental frequency component.
Numerical relays that filter out harmonics and the DC component of the rush current can therefore be set more sensitive. The inrush current peak values of Fig. 113 will be nearly reduced to one half in this case.
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