Protection Relays – Operating Principles

Protection Relays – Functions

All relays will operate in response to one or more electrical quantities to open or close contacts. Electromechanical relays have been used for years and have established a reputation for simplicity, reliability, security, low maintenance, and long life. However, in recent years, solid state relays are being used advantageous in some applications. Many of the protection functions can be accomplished by either electromechanical or numerical relays.

Electromechanical relays have 2 different operating principles:

1. Electromagnetic attraction – operates by having either a plunger drawn by a solenoid or an armature drawn to a pole of an electromagnet. This type of relay will operate either from ac or a dc voltage source.
2. Electromagnetic induction – uses the principle of the induction motor, whereby the torque is developed by induction into a rotor. This principle is used in watt-hour meter where the rotor is a disk. The force developed on the rotor is a result of the interaction of the electromagnetic flux applied and the flux produced by eddy currents that are induced in the rotor.

Few are the list of commonly used relay device protection function:

Protection function	Device function No.
Distance	21
Synchronizing	25
Undervoltage	27
Directional Power	32
Loss of Field Excitation (field)	40
Phase balance (current balance, negative sequence I	46
Phase sequence voltage (reverse phase voltage)	47
Thermal ( Thermal overload)	49
Instantaneous Overcurrent	50
Time-overcurrent	51
Over voltage	59
Voltage balance ( between two circuits)	60
Directional Overcurrent	67
Frequency	81
Lockout	86
Differential	87

In this area we will try to give a brief detail of each protection relay function. Just take note of this section as I will attached few samples of protection relays in the future.

1. Distance relays – this type of relay is commonly used for primary and backup protection on transmission lines where high speed relaying is required. Distance relay is a term applied to ohmic relays that use voltage and current inputs to provide an output signal if there is a fault within a predetermined distance from the relay location. The major advantage of a distance relay is that it responds mainly to system impedance instead of the magnitude of currents.
2. Synchronizing relays – these relays are applied when two or more sources of power are to be interconnected to a common bus. This relay is also called synchronism check relay or sychro-verifier. Automatic or manual closing of a circuit breaker only when the systems on each side of the circuit breaker are very nearly in synchronism.
3. Undervoltage relays – this relay is calibrated on decreasing voltage to close a set of contacts at a specified voltage. Typical uses of this relays are bus undervoltage protection, backup functions, timing applications etc. Undervoltage relays may either be solid-state or electromechanical design.
4. Directional Power relay – the function of this relay is when the real component (watts) flow in a circuit that exceeds a preset level in a specified direction. Uses of this relay are Source Power Flow Control, Antimotoring of generators and reverse power flow.
5. Loss of Excitation – this relay is used to protect a generator against loss of excitation. Common protection used for smaller motors are two types. Instantaneous direct-undercurrent relay that monitors field current or a relay that monitors the relative angle between voltage and current.
6. Phase Balance Current Relay – this relay provides motor and generator protection against unbalanced phase currents that are caused by and open fuse or conductor in a motor branch circuit, an open fuse or conductor in the primary of a delta-wye connected transformer serving a group of motors, generators. Unbalanced load condition or single phase switching in the distribution/ transmission systems.
7. Phase Sequence Voltage Relay – the phase sequence relays are used to protect ac machines from undervoltage and to prevent starting on open or reverse phase sequence. This type of relays may also provide overvoltage protection and often monitor the bus voltage, thus protecting a group of motors.
8. Transformer Thermal Relay – this type of relays are used to protect motors, generators, and transformers from damage due to excessive long-term overloads.
9. Time-overcurrent and Instantaneous Overcurrent Relays ­­– this is the most commonly used protective relays. They are used a both primary and backup protective devices and are applied in every protective zone in the system. Time-overcurrent relays is selected to give a desired time-delay tripping characteristic versus applied current, whereas instantaneous overcurrent relays are selected to provide high-speed tripping (0.5 – 2 cycles).
10.  Overvoltage relay – an overvoltage relay is one that is calibrated on increasing voltage to close a set of normally open contacts at a specified voltage. Two basic functions for this relay are as follows: Simple Overvoltage Bus Protection and Ground Fault Detection.
11. Voltage Balance Relay – a voltage balance relay function is to block relays or other devices that will operate incorrectly when a voltage transformer fuse blows.
12. Directional Overcurrent Relay – the function of this relay is to provide sensitive tripping for fault currents in one direction and nontripping for load or fault currents in the normal direction.
13. Frequency Relays – is a device that functions on a predetermined value of frequency, either under or over normal system frequency or rate of change of frequency. When it is used to function on a predetermined value below nominal frequency, it is generally called underfrequency relay, and when it functions on a predetermined value above nominal, it is called overfrequency relay.
14. Lockout Relay – this relay is not a protective relay but it is widely used in conjunction with relaying schemes. This relay is a high speed, multicontact, manual or electrically reset auxillary relay for multiple contacts, increasing contact rating, isolating circuits, and tripping and locking out breakers.