Relays: LEXIS OF RELAY

What Is Relay? How Do Relay Work?

 Relay a device that opens or closes a contact when energized. Relays are most commonly used in power systems, where their function is to detect defective lines or apparatus or other abnormal or dangerous occurrences and to initiate appropriate control action. 

When the voltage or current in a relay exceeds the specified “pickup” value, the relay contact changes its position and causes an action in the circuit breaker. A decision is made based on the information from the measuring instruments and relayed to the trip coil of the breaker, hence the name “relay.” Other relays are used as switches to turn on or off equipment.

 

If you look at these two figure, you will find the very first basic idea, how relay make automatic a circuit just using the control circuit.

Types of Relay

A relay is nothing but an open or closed switch, but for the purpose and place of used relay, it may be of a different type or name. The relay may be used in HVAC, Appliances, Security, Pool & Spa, Office Equipment, Telecommunications, Data-com, CP/OA, Office Automation, Process Control, Automotive, Energy Management Systems, Motor Drives/Controls, Appliance Controls, Industrial Control, Motors, Timers, Instrumentation, Test Equipment, etc.

Normally used relays are as follows:

Auxiliary Relay: a relay employed in power system protection schemes that do not directly sense fault presence and location. Typical auxiliary relays include lockout relays, reclosing relays, and circuit breaker anti-pump relays.

 

Bus Differential Relay a differential relay specifically designed to protect high-power buses with multiple inputs.

Computer Relay is a protective relay that digitizes the current and/or voltage signals and uses a microprocessor to condition the digitized signal and implement the operating logic.

Differential Relay A differential relay is a protective relay that measures current going into a device from all sources by means of a network of paralleled current transformers. Ideally, the operational current is zero for normal conditions and rises to a high value (proportional to fault current) when a fault comes on inside the differential zone. Differential relays are commonly applied in bus protection, transformer protection, generator protection, and large motor protection.

Digital Relay is a relay in which decisions are made by a digital computer, typically a microprocessor system.

Directional Overcurrent Relay an overcurrent relay that operates only for overcurrent flowing in the tripping direction. Direction sensing is typically done with respect to a voltage or current signal, which is not affected by fault location.

Directional Power Relay a protective relay that operates for power flow in a given direction. Applications are in cases where normal power flow is in one direction, including anti-motoring protection on a turbine generator and fault back-feed protection on parallel step-down transformers.

Distance Relay See impedance relay.

Electromechanical Relay is a protective relay that uses electrical, magnetic, and mechanical circuits to implement the operating logic.

Fourier Optics Relay Lens is a lens system that produces the exact Fourier transform of an image. Two such relay lenses will reproduce an image without any phase curvature.

Frequency Relay a protective relay which monitors the frequency of the electric power system.

Generator Differential Relay a generator differential relay is a differential relay specifically designed for the protection of electric power generators. Variations include allowances for split-phase winding machines.

Impedance Relay is a protective relay that senses the operational impedance at a location, i.e., the ratio of voltage to current at any given time. During fault conditions on the protected line, the impedance relay will sense the impedance (distance in ohms) between the location of the relay and the fault.  Typical impedance relay characteristics are mho and reactance. Impedance relays are widely used in sensing phase faults on transmission lines. Ground impedance relays are available that measure the distance between a single phase and the ground fault using a modified technique. Also known as distance relay.

Instantaneous Overcurrent Relay is an overcurrent relay that operates with no intentional delay following sensing of a power frequency overcurrent, i.e., a current above its set point.

A Lockout Relay is an auxiliary relay that is operated by the protective relay(s) that in turn opens the appropriate circuit breakers or other fault-clearing devices. The lockout relay will remain in the trip position until manually reset, and is used in protective zones where temporary faults are unusual and the potential for equipment damage is high.

Loss-of-Field Relay a protection relay used to trip a synchronous generator when the excitation system is lost. Loss of excitation causes the generator to run as an induction generator drawing reactive power from the system. This can cause severe system voltage reductions and damage to the stator due to excessive heating.

Master Control Relay (MCR) is used in programmable logic controllers to secure entire programs or just certain rungs of a program. An MCR will override any timer condition, whether it be time-on or time-off, and place all contacts in the program in a safe position whenever conditions warrant.

Negative Sequence Overcurrent Relay a protective relay that senses and operates on negative sequence overcurrent. Typical applications include the sensing of unbalanced faults and the protection of synchronous and induction machines from rotor overheating.

Out-of-Step Relay is a protective relay that senses that a synchronous generator has pulled out of step, and is operating at a frequency different than the system frequency.

Overcurrent Relay a protective relay that operates when fed a current larger than its minimum pick-up value.

Overload Relay is a device designed to detect and interrupt motor overload conditions. Motor overload relays may be actuated by thermal (temperature), magnetic (current), or electronic (voltage and current) sensors.

Overvoltage Relay a protective relay that operates on overvoltage.

Phase Comparison Relay a phase comparison relay is a protective relay used on transmission lines which operates by comparing phase angles of signals generated at opposite ends of the line. They employ a dedicated communications channel to make the comparison. The signals compared are typically corresponding phase currents or sequence currents.

A Protective Relay is a device that monitors the condition of the electric power system and determines the presence of faults or other system anomalies. The protective relay monitors current flow, voltage level, or other parameter. When it operates due to a fault or other event, it initiates a trip signal intended to open the appropriate circuit breaker(s) or other protective devices.

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Reclosing Relay an auxiliary relay that initiates circuit breaker closing in a set sequence following fault clearing. Reclosing relays are typically employed on overhead lines where a high proportion of the faults are temporary.

Residual Overcurrent Relay an overcurrent relay that is connected to sense residual current. Residual current is the sum of the three phase currents flowing in a current transformer secondary circuit, and is proportional to the zero sequence current flowing in the primary circuit at that point.

Seal-in Relay an auxiliary relay that remains energized through one of its own contacts, which bypasses the initiating circuit until de-energized by some other device.

Solid State Relay a protective relay that employs analog electronics, logic electronics and magnetics to implement the operating logic.

Sudden Pressure Relay a protective relay that senses the internal pressure in a transformer tank, and operates on sudden changes in this pressure. These sudden pressure changes reliably indicate a fault inside the tank.

Synchro-Check Relay a device used to monitor the frequency and phase angle of the voltages across an open circuit breaker. Synchro-check relays are commonly used to prevent breaker closing or reclosing on excessive voltage or frequency difference.

Synchronizing Relay a relay that monitors the voltage across an open circuit breaker to determine the frequency and phase relationship of the voltage sources on either side of the breaker. Synchronizing relays are used on generator breakers to bring the generator to the system frequency and to match the phase angle between the generator and system prior to closing the breaker.

Time Overcurrent (TOC) Relay an overcurrent relay that has intentional, selectable, time delay. The time delay is chosen so that the relay will operate more slowly than downstream relays or fuses, and more quickly than upstream relays or transformer fuses. Relay and fuse curves are generally displayed on time-current curves.

Time-Delay Relay relay that responds with an intentional time delay.

1. in control circuits, time-delay relays are used to cause a time delay in the state of the relay when power is applied or removed to the relay actuator;

 

2. in power system protective relays, the response time usually depends on the magnitude of the measured value. If the measured value is a large multiple of the pickup value, then the relay operates or trips after a short time delay. For normaler multiples of pickup, the relay trips after a longer time delay.

Transformer Differential Relay a differential relay specifically designed to protect transformers. In particular, transformer differential relays must deal with current transformer turns ratio error and transformer inrush and excitation current.

Under-frequency Relay a protection device that curtails loads in an area that is deficient in generation. Lower generation compared to load demands give rise to lower frequency and a frequency threshold can be used by the relay to initiate load shedding in order to balance generation and demand.

Under-voltage Relay a protective relay that operates on low voltage or loss of voltage.

Voltage Regulating Relay a voltage regulating relay senses RMS voltage level and issues commands to devices such as load tap changers, which then adjust the tap position to bring the voltage back to the desired level.

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Relay Channel

a multi terminal channel in which the receiver observes the transmitted signal through two channels: one direct to the transmitter, the other via an intermediate transmitter/receiver pair.