RELAYS

RELAY: Relay is a device by means of which an electrical circuit (trip circuit. Or alarm circuit) is controlled (closed) change in the other circuit. Relays are automatic. There are several types and application of relays. Relays are essential component of protective system.

Instantaneous:  When the operating time lapsed from the instant of energization to the instant of closing of contacts is very small then the relay is said to be instantaneous relay.

IDMT Relay: Inverse define minimum time relay, is a relay having an inverse characteristic of current v/s, time up to certain increased value of current after which the time is definite.

Over Current Relay: A relay which respects to increase in current.

Earth fault relay: A relay which sense earth fault (ground) i.e. single line to ground fault, double line to ground fault arcing ground.

Differential protection: Protective system which responds to vector difference (phase/magnitude) between two or more similar quantities.

Static relay: Relays in which relay measurement of comparison is performed by stationary (static) circuit.

Biased relay: A relay whose characteristics is modified by additional mechanical or electromagnetic procession such as a bias coil, magnet etc.

Trip circuits: The circuit comprising trip coil relay contacts. Auxiliary switch, seal in coil etc which controls the circuit breaker for opening operation.

Auxiliary switch: A multi-point switches which operates in conjunction with circuit breaker and connect/disconnects certain protective, indicating and control circuits in each position (open and close). It is placed in the switch cubicles of circuit breakers and isolators.

Protective relay: The relay is an electrical device place in between the main electrical power circuit and the circuit breaker in such a manner that any fault in the circuit actuates the relay which in turn energizes the trip circuit and causes circuit breaker to open and thus protects the circuit from any severe damage.

Difference between fuse and relay: The difference is that a fuse performs both detection and interruption functions automatically whereas a relay detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

Protective zone in a power system: The different protective zones in a power system are as follows:

a) Generators, b) Transformers, c) Bus bars, d) Transmission and Distribution lines, e) Motors, f) Some bulk power consuming apparatus.

Function of the protective relay in the power system: The functions of protective relay area to sense the fault in the system, compare the signal so obtained with the reference signal under normal condition of operation and amplify the error signal such that the trip coil of the circuit breaker is energized and the faulty section of the system is disconnected from the rest of the system thus ensuring the safety of the circuit equipments.

Use of C.T. in the relay circuit: The C.T. is used in the relay circuit for reducing the large current of power circuit to a suitable value for relay operation.

Burdon of a protective relay: The burden of a protective relay is the power absorbed by the circuit of the relay.

Types of protection of a power system: There are two types of protection called as (i) the main or primary protection and (ii) the back up or secondary protection.

The main or primary protection is the essential protection provided for protecting each section of an electrical installation which is the first line of defense and ensures quick acting and selective clearing of faults.

The back-up or secondary protection is the second line of defense acting as additional protection with precautionary measure which functions to isolate a faulty part from the healthy system in case the main protection fails to function properly.

Causes of failure of main protection: The main protection can fail due to the failure of any one of the components of the protective system such as (i) relay due to non availability of voltage or current supply, (ii) auxiliary relay, (iii) CT, (iv) PT, (v) Trip circuit due to non availability od DC supply, (vi) circuit breaker itself etc.

Important elements of protective relays: (i) Sensing element, (ii) Comparing element and (iii) Control element.

Main circuits connected with a relay for controlling a circuit breaker: The connections are divided into three main circuits e.g.(i) The primary circuit which is to be controlled and with which the primary of CT is connected, (ii) The secondary circuit of the CT to which the relay operating coil is connected, (iii) The tripping circuit consisting of a source of supply, the tripping coil and the relay contacts.

Pick-up current of the relay: Pick-up current is the minimum current in the relay coil at which the relay starts to operate.

Current setting of the relay: Current setting means the adjustment of pick-up current of a relay to any required value expressed in terms of percentage full load rating of CT.

                                             Pick-up current

Current Setting =  ---------------------------------------

                                Rated secondary current of CT

Plug setting multiplier(P.S.M) of a relay: Plug setting multiplier is the ration of fault current in the relay coil to the pick up current i.e.

 

                                             Fault current in the relay coil

Current Setting =  --------------------------------------------------------------

                               Rated secondary current of CT x Current setting

Time setting multiplier of a relay: Time setting multiplier is a controlling arrangement to adjust the time of relay operation. This adjustment is provided with the position of a movable back stop on a so called time setting dial calibrated from 0 to 1 in steps of 0.05 evenly divided. The actual time of operation is calculated by multiplying the time setting multiplier with the time obtained from Time/PSM curve of the relay.

Pick-up value is greater than the reset value because of the greater air gap length during picking up as compared to that while resetting.

Shunt trip and series trip: When the tripping energy is obtained from a separate source (generally a battery) the relay is said to be ‘shunt trip’ relay and when the tripping energy is obtained from the AC system itself through a CT, the relay is said to be ‘series trip’ relay.

Classification of relays according to their function: (i) Over current or over voltage or over power or over frequency relay, (ii) Under current or under voltage or under power or under frequency relays, (iii) Directional relays (the Reverse current or reverse power relay), (iv) Differential relays, Iv) Distance relays, (vi) Earth fault relay.

Over current protection: Over current protection is that protection in which the relay operates automatically, when the magnitude of the current exceeds a predetermined value, to make the system free from that abnormal condition. The basic element of the over current protection is an over current relay. Over current relays are connected to the system by means of current transformers. Every over current relay should operate to remove the fault with smallest possible disturbance to the whole system. Sow quicker operation is needed for the relay near the fault than that near the source which is done by necessary time grading of respective relay. Therefore, an over current relay always operates with a time grading.

“Relay time” and “Circuit breaker time” of a circuit breaker: Relay time is the time lapsed between the instant of occurrence of fault and the instant of closure of relay contacts i.e. closure ot trip circuit in a circuit breaker.

Circuit breaker time is the time lapsed between the instant of closure of trip circuit and the instant of final arc extinction i.e. the time required for operating mechanism to open contact and the arcing time in a circuit breaker.

Fault clearing time: It is the time lapsed between the instant of the occurrence of a fault and the instant of final arc extinction i.e. the sum of relay time and circuit breaker time in a circuit breaker.

The fault clearing time is usually expressed in cycles. One cycle of 50 Hz AC supply system is equal to

1/50 second.

Fault clearing times are (i) ½ cycle for HRC fuse, (ii) a few second for over current or earth fault protection & (iii) 1 to 10 cycle for distance protection.

Time limit fuse over current protection scheme: In the time limit fuse over current protection scheme the fuse is connected across the trip coil of the circuit breaker. The sequence of tripping time of fuse connected at generating station down to various substations is in descending order. In this scheme the tripping coil will carry fault current after occurrence of fault and blowing off fuse.

Earth fault protection: It is that protection in which the relay operates automatically, as soon as an earth fault or leak develops to make the system free from danger.

Restricted earth fault relay: When a fourth CT is added in the neutral connection to earth and the secondary of which is connected in parallel with the secondary of three line CTs in order to discriminate between faults external to the protected zone and those within the zone, then it is said to be restricted earth fault protection.

In case of multiple feeder lines the restricted earth fault protection is applied to the incoming feeder of transformer or generator which gives supply to a group of outgoing circuits where as unrestricted earth fault protection is applied to the outgoing feeders.

In restricted earth fault protection the relay will operate only when the earth fault occurs within the protective zone.

Difference between earth relay and over current relay: An earth relay operates under earth or leakage faults only having low current setting whereas the over current relays area arranged to operate against faults between the phases having high current setting.

Under voltage relay: It means the relay which operates to open main contacts automatically when the voltage across the coils fails falls below a specified value.

Zero voltage protection: It means that the relay operates to open main contactors automatically when the voltage closely approaches zero usually below 10% of rated voltage.

Directional over current relay: It means the relay which trips the circuit breaker when the direction of power flow under fault condition is in the reverse direction to that of the power flow during normal condition of operation and the magnitude of the current exceeds a predetermined value above normal current.

Since these relays respond not only to the direction of the fault currents but also to the magnitude of the currents in the protected circuits, these are used for parallel feeder protection usually at the load ends.

Differential relay: It is that type of relay which responds to vector difference between two or more similar electrical quantities exceeding a predetermined value.

Most of the differential relays are current differential relays. Under normal condition the current entering at one end of the protected equipment is identical both in phase and magnitude with that of leaving the other end. When a fault is occurred this balance is altered and the difference of the two current quantities works as the actuating quantity of the relay. This vector difference is achieved in the relay by suitable connections of current transformer or voltage transformer secondary’s. Being energized when the relay operates the trip circuit becomes closed so as to trip the circuit breaker.

Application of differential relays: The differential relay are applied for the protection of the following apparatus:

a) Generator, Generator-transformer units of rating 1000 kVA and above.

b) Transformers of capacity 10 MVA and above.

c) Feeder line (Transmission line) by pilot wire differential protection.

d) Transmission line by phase comparison carrier current protection.

e) Large motors.

f) Bus zone.

It is not possible to protect any electrical circuit against external fault by differential relay because under external fault conditions the sum of the currents entering the bus is equal to those leaving it as in the case of normal load conditions and no current flows through the operating coil of the relay. The differential relay will operate only when a fault occurs within the protected zone.

Percentage differential relay: When the differential relay consists of an additional restraining coil with the circulating current system in which current induced in both CTs flows and relay operating coil is fed from toppings on restraining coil, then it is said to be percentage differential relay. The advantage is that it operates only when the current in the operating coil exceeds a specified value and prevents the chance of incorrect operation of relay during the occurring any short circuit fault beyond the protected zone.

Distance relay: It is a double actuating quantity relay with one coil energized by the local voltage and the other coil energized by the local current of the phase of a system which under fault actually measures the distance of fault from the relay location by correctly assessing the ration of voltage and current. This relay operates whenever the ration of the local voltage of the fault current falls below a certain value.

Distance relays are used universally for the protection of long high voltage a.c. transmission line and sectionalized radial distribution lines, against 3-phase faults, phase to phase fault and phase to earth faults.

There are three main types in use e.g. (i) Impedance type measuring Z, (ii) Reactance type measuring X, and (iii) Mho type measuring admittance Y.

The Mho type distance relay supersedes other two types due to its more stability on power swings.

Classification of relays according to timing characteristics:

(a) Instantaneous Relay: Instantaneous relay means the high speed relay in which complete operation takes place after a very short period almost instantaneously from the occurrence of fault. Electronic and Electro-magnetic relays are usually of instantaneous type.

(b) Definite Time Lag Relay:  Definite Time Lag Relay means the relay in which the operation takes place after a definite time lag from he occurrence of fault. The time lag is obtained by an oil dash pot or an air escapement chamber or a clockwise mechanism or by placing a fuse in parallel with the relay.

(c) Inverse Time Lag Relay: It means that type of relay in which the operating time from the occurring of fault is inversely proportional to the magnitude of the fault current.

(d) Inverse Definite Minimum Time Lag Relay (IDMTL): It means that type of relay in which the operating time required for the relay to close is inversely proportional to the fault current and it will not close before a definite minimum time whatever may be the magnitude of the fault current. All induction type relays have an inverse time with definite minimum characteristics.

In India: AC supply is 33Kv, 66Kv, 132Kv,220Kv, 400Kv