Protective Relays and Monitoring Relays

Protective relays and monitoring relays detect or monitor for abnormal power system conditions. Protective relays detect defective lines, defective apparatus, or other power system conditions of an abnormal or dangerous nature, and initiate appropriate control circuit actions. Monitoring relays are used to verify conditions in the power system or in the protective system. Monitoring relay functions include fault detection, voltage checking, and direction-sensing that confirms power system conditions, but does not directly sense the fault or problem. Both protective relays and monitoring relays may be sensitive to voltages, power or phase, current, or frequency. 

Protective relays and monitoring relays may be categorized as undervoltage, overvoltage, differential, power or phase-sensitive, phase-sequence, phase-reversal, and ground earth (fault). Undervoltage relays trip when the voltage drops below a set point. Overvoltage relays trip when a voltage rises above a set point. Differential voltage relays respond to the difference between incoming and outgoing voltages associated with the protected apparatus. Power or phase-sensitive relays can monitor phase sequence, phase reversal, ground or earth fault, power factor, phase failure or loss, and phase unbalance. Phase-sequence relays monitor for correct phase sequence. Phase-reversal relays monitor for a change of one-half cycle or 180°  in phase. Ground fault (earth) relays detect any undesired current path from a point of differing potential to ground.  

Selecting protective relays and monitoring relays requires an analysis of power factor, phase failure, phase unbalance, and voltage balance. Power factor is the cosine of the phase angle between the voltage and current in AC power transmission and distribution applications. Phase failure relays detect voltages with the incorrect phase sequence, or with one or more phases open. Phase unbalance relays operate when the magnitude of one current exceeds the magnitude of another current by a predetermined degree. Voltage balance operates in a similar manner.

Protective relays and monitoring relays include current-sensitive relays which can detect undercurrent, overcurrent, and differential current conditions. Undercurrent relays trip when the current drops below a set point. Overcurrent relays trip when a current rises above a set point. Differential current relays respond to the difference between incoming and outgoing currents associated with the protected apparatus. 

Frequency-sensitive relays are protective relays and monitoring relays with under-frequency, overfrequency, and differential frequency capabilities. Under-frequency relays respond to a decrease in the frequency of an alternating electrical input quantity. By contrast, overfrequency relays respond to a frequency increase. Differential frequency relays respond to the difference between incoming and outgoing frequencies associated with the protected apparatus. Other protective functions include time delay and synchronous check.

Important sensing and measuring specifications to consider when searching for protective relays and monitoring relays include voltage sensing range, current sensing range, line voltage range, line voltage mode (line-to-line or line-to-neutral) and frequency sensing range.  The power supply voltage range is also important to consider. Common features for protective relays and monitoring relays include programmable time delay, automatic reset, visual indicators, and latching controls.  Operating temperature is an important environmental parameter. 

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