Protective relays and monitoring relays detect or monitor for abnormal power system conditions. Protective relays detect defective lines, defective apparatuses, or other power system conditions of an abnormal or dangerous nature. The relay then initiates the 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 often have circuitry in them for the protecting function, as well as a relay to do the switching. Most are not simple, electromechanical devices like a circuit breaker but instead contain a circuit which measures some value and can be set at a tripping point. Some have visual displays such as lights or even screens for setting them up. The controls can be via programming or by knobs, or simpler ones may not be adjustable.

Protective Relays and Monitoring Relay Categories

Protective and monitoring relays could fall into one of several categories. Protective relays and monitoring relays may be categorized as a voltage sensitive relay, power (phase) sensitive relay, current sensitive relay, and frequency sensitive relay.

Voltage Sensitive Relay

Voltage relays identify overvoltage, undervoltage, or both. They can only detect an abnormal condition on the line side of where the relay is connected. This allows the device to provide pre-start protection. Voltage relays are easy to install, do not require current transformers, and are therefore less expensive. These require only voltage connections so that they may be applied independent of the system load.

Voltage Sensitve Relay. Image Credit: Wirthco

Undervoltage

Undervoltage relays trip when the voltage drops below a set point. An undervoltage is a sustained system voltage below transformer, motor, generator, or voltage ratings that can lead to equipment failure. They can be caused by a system overload or equipment failures. Special care should be given for undervoltages because many power systems loads are MVA loads (motors, uninterrupted power supplies, etc.). This means that as the voltage decreases, the load current increases while the power system transfer capability decreases. Undervoltage relays are usually instantaneous devices and should complete their function every time input voltage drops below the set point. Load transfer, voltage regulation, and motor protection are all applications for undervoltage protection relays. 

Overvoltage

Overvoltage relays trip when a voltage rises above a set point. An overvoltage is a sustained system voltage in excess of transformer capacitor, motor, generator, or reactor voltage rating. Overvoltages can lead to equipment failure or be due to equipment failure, such as failure of a load tapchanger controller or by a sudden loss of customer load.Overvoltage relays may be instantaneous or time-delayed devices. Voltage regulation, bus and back- up protection, and generator protection are applications for overvoltage protective relays. 

Differential

Differential voltage relays respond to the difference between incoming and outgoing voltages associated with the protected apparatus. The electrical quantities entering and leaving the system are compared by current transformers. If the net between the circuits is zero, then there is no fault or problem. If the net is not zero then an internal problem can be identified. This type of relay is applicable to all parts of the power system and is often the primary choice for protection. 

Power (Phase) Sensitive Relay

Power or phase-sensitive relays can monitor phase sequence, phase reversal, ground or earth fault, power factor, phase failure or loss, and phase unbalance.

Negative phase sequence relay. Image Credit: Circuitmaniac.com

Phase Failure (loss)- The relay monitors for voltage with the incorrect phase sequence, or one or more phases open. Failure may occur because of a blown fuse, a mechanical failure of the switching equipment, or if one of the power lines opens. Phase failure involves three phases where there are three wires. If a three-phrase motor is started on a single phase, the motor will not start. If one wire gets disconnected, it is identified as a loss of phase. It is suggested that a device monitoring phase failure be combined with a device that can detect phase angle displacement. This is because voltage sensing devices which monitor only the voltage magnitude may not provide protection when the motor is running.

Phase-reversal- Phase-reversal relays monitor for a change of one-half cycle or 180° in phase.A reversal in phase is often due to miswiring, faulty incoming power from modifications made to the power distribution system, or when power restoration results in a different phase sequence than before the power outage. This protection is required on all equipment transporting people, such as escalators or elevators.

Phase-sequence- Phase-sequence relays monitor for correct phase sequence if two wires have their connection reversed and become out of sequence. The device is used to ensure the sequence is correct when connecting three phase loads. If the phase sequence is incorrect, the relay will de-energize preventing the start of incorrectly connected machinery

Phase unbalance- The relay operates when the magnitude of one current excesses the magnitude of another current by a predetermined degree. Voltage balance operates in a similar manner.

Power factor- In AC power transmission and distribution, power factor is the cosine of the phase-angle between the voltage and the current. This deals with the different in real and apparent power. A bad power factor can lead to a distorted waveform and higher power use. 

Ground earth (fault)- Ground fault (earth) relays detect any undesired current path from a point of differing potential to ground.

Current Sensitive Relay

Protective relays and monitoring relays include current-sensitive relays. Current sensing relays offer an advantage over voltage sensitive relays because they do not respond to back electromotive force (EMF), which accompanies a phase failure on motor loads. They can detect a problem on either the line side or the load side in a branch circuit in which the relay is used.

Current Relay. Image Credit: ChipDipvideo / CC BY-SA 4.0

Under-current- Under-current relays trip when the current drops below a set point. Under-currents can occur if there is a fault with the power supply, or if a loaded motor becomes unloaded. Often an overvoltage situation will cause under-current and can cause damages to the equipment.

Over-current- Overcurrent relays trip when a current rises above a set point. Over-current can be caused by either the load or the supply such as a sudden increase in load due to faulty electronics or physical load on a motor. Additionally, a drop in voltage could also cause an over-current situation. 

Differential current conditions- Differential current relays respond to the difference between incoming and outgoing currents associated with the protected apparatus. The principle of differential relays is the same for a station bus and for generators; the device monitors for the sum of all the currents into and out of the bus or generator to be equal to zero. If there is a fault then there will be a net flow of current and the differential relay will be triggered.

Frequency Sensitive

Frequency-sensitive relays are protective relays and monitoring relays with under-frequency, over-frequency, and differential frequency capabilities. Frequency changes are generally related to the power being supplied. Power from a power company is unlikely to change, however if the power is generated on site from an inverter, back-up type system, or alternative energy, than there is more likely to be issues with frequency. Frequency is important because many electronic devices rely on it for timing. For example, the speed of an AC induction motor is dependent on frequency. An increase or decrease in the frequency could result in an increase or decrease of the motor, causing a problem in an industrial process. Frequency is ultimately dependent on the generator and how fast it is spinning, or in the case of an inverter, the timing circuit in the inverter. 

• Under-frequency relays respond to a decrease in the frequency of an alternating electrical input quantity.
• Over-frequency relays respond to a frequency increase. They fall under the categories of instantaneous and time-overcurrent relays.
• Differential frequency relays respond to the difference between incoming and outgoing frequencies associated with the protected apparatus.

Protection Relay Reference Table

Chart adapted from Littelfuse

Performance Criteria

Specifications

Important sensing and measuring specifications to consider when searching for protective relays and monitoring relays include:

• Voltage sensing range- Voltage-sensing range applies to power (phase), voltage, voltage/frequency, and paralleling (synchronous) sensing relays.
• Current sensing range- Current-sensing range applies to power (phase) and current-sensing relays.
• Line voltage range-Line-voltage range applies to power (phase) sensing relays.
• Line voltage mode-(line-to-line or line-to-neutral)
• Frequency sensing range- The range of frequencies that the relay can act on. Typical frequencies would be 50hz, 60hz or 400hz.
• Power supply voltage range
• Operating temperature is an important environmental parameter. This is the full-required range of ambient operating temperature. This represents the surrounding air temperature limits.

Additional Functions

Other protective functions include:

Time delay- Time delay, where the relay can have different timing functions, such as delay from the time a fault is detected to trip, or delay in time it takes to reset. Time delay is set based on fault cleaning time or motor damage time. There are five different versions, defined by the steepness of the time-overcurrent characteristics: definite time, moderately inverters, inverse, very inverse, extremely inverse.

Image Credit: xnet.rrc.mb.ca

Synchronous check- Synchronous check is for two power sources, like two generators or a generator and wall power, where a connection or switch between them will have both connected at the same time. This type of relay will check to make sure the phase is aligned so the user can perform this switching.

Features

Common features for protective relays and monitoring relays include:

• Programmable time delay- The relay has a programmable time-delay feature.
• Automatic reset- The relay resets automatically after conditions return to normal.
• Visual indicators- The relay has visual indicator such as an LED for identifying various conditions of the system.
• Latching controls- Relays are used for latching applications (e.g. latching limit controllers). Latches retain the last defined state before a loss of power. If a latch is not included the system needs to be designed for fail-safe or acceptable "stand-by" conditions in the event of controller power loss.

Standards

BS EN 50216-3 - Power transformer and reactor fittings - Part 3: protective relay for hermetically sealed liquid-immersed transformers and reactors without gaseous cushion

IEEE C37.113 - Guide for protective relay applications to transmission lines

MIL-PRF-32484 - Protective relays and attachments, medium voltage vacuum circuit breaker applications

Resources

Sleva, Anthony F. Protective Relay Principles. Boca Raton: CRC, 2009. Print.

Van Cortlandt Warrington, Albert R. Protective Relays: Their Theory and Practice. Vol.2. London: Chapman & Hall, 1978. Print.

Steven Engineering - Phase Failure Relays

Image credits:

Grainger | Phoenix Contact USA | Newark element14 | GE