Relay the switches that aim at closing and gap the circuits electrically likewise as electromechanically. Its controls the gap and shutting of the circuit contacts of associate degree electronics circuit. Once the relay contact is open (NO), the relay isn’t energizing with the open contact. However, if it’s closed (NC), the relay isn’t energizing given the closed contact. However, once energy (electricity or charge) is provided, the states square measure vulnerable to modification.

Relay circuit

Relay square measure unremarkably utilized in the management panels, producing and building automation to regulate the ability alongside shift the smaller current values in an exceedingly feedback circuit.

However, the provision of amplifying result will facilitate management the massive amperes and voltages as a result of if low voltage is applied to the relay coil, an outsized voltage will be switched by the contacts. If preventive relays square measure is being employed, it will notice over current, overload, undercurrent, and reverse current to confirm the protection of equipment. Last however not the least; it’s wont to heat the weather, turn on loud alarms, switch the beginning coils, and pilots the lights.

1. Some Terms in protecting Relay

Which means and definitions of some terms involved with protecting relaying area unit given here, for the sake of familiarity.

1.1. Relay.

Relay may be a device by suggests that of that an electrical circuit (trip circuit of alarm circuit), is controlled, (closed), by modification in the alternative circuit. Relays area unit automatic. There area unit many varieties and applications of relays. Relays area unit essential elements of protecting systems.

1.2. Protecting Relay.

A protecting relay is Associate in nursing electrical relay used for protecting of electrical devices. It is a device that closes its contacts, once in operation amount reaches sure planned magnitude / section. Closing of relay contacts initiates Associate in alarm circuit or trip circuit.

1.3. Measuring Relay.

A measuring relay operates at planned price of in operation amount by performing the necessary measuring. A relay in that, the operation is independent f measuring is known as all-or-nothing relay. All-or-nothing relays, auxiliary relay, etc., area unit won to supplement the measuring relay.

1.4. Trip Circuit.

The circuit comprising trip coil, relay contacts, auxiliary switch, confine coil, battery provide, etc.; that controls the circuit-breaker for gap operation.

1.5. Current Transformers (CT).

This area unit used for measuring purpose and protecting relaying purpose. Accordingly, they area unit know as measuring CT and protecting CT. The current ratio of a CT is typically high. The secondary current ratings area unit of the order of5A, 1A, and 0.1A, the latter being used for static relays. Primary current ratings vary from ten to 1250 A or a lot of. Magnitude relation error and section angle area unit important aspects of CT’s. The CT’s play Associate in Nursing vital role in protecting relaying. The var rating of current transformers is low (5-150 VA) as compared therewith of power transformers (a few KVA to many MVA).

1.6. Voltage Transformers or Potential Transformers (VT).

The voltage transformers change of magnitude the primary voltage to a secondary voltage of lower price. The normal rated secondary voltage is a 110 V, 240,440 V. the volt ampere capability of Potential Transformers is tiny relative to that of power transformers.

The VT’s area unit used for measuring and protection. They area unit consequently known as as measuring instrument potential transformers and protecting potential transformers.

1.7. Auxiliary Switch.

A multipoint switch that operates in conjunction with circuit-breaker and connects / disconnects sure protecting, indicating and management circuits in each position, (open and close). It’s placed within the switch cubicles of circuit-breakers and isolators.

1.8. Fault Clearing Time.

Time advance between the moment of incidence of fault and instant of ultimate arc extinction in circuit breaker. It is expressed in milliseconds (ms) or cycles one cycle in fifty Hz system is reminiscent of 1/50 second, i.e. 0.02 second.

1.9. Relay Time.

Time interval between incidence of fault and closure of Relay contacts.

1.10. Breaker Time.

Time interval between closure of trip circuit and final arc interruption. Relay time and breaker time is adequate to fault clearing time.

1.11. Stability of facility.

Stability denotes a condition throughout that all the Synchronous machines in the system area unit in synchronization, i.e. in step with one another.

1.12. Earth Fault.

A fault that involves earth (ground) ; e.g., single line to ground fault, double line to ground fault, arcing grounds.

1.13. Section fault.

A fault that doesn’t involve earth; e.g., Line to line fault.

1.14. Fast Relay.

A quick relay having relay time of less than zero.  2 second and having no intentional interruption.

1.15. IDMT Relay.

Inverse definite minimum time relay is a relay having Associate in Nursing inverse characteristic of current vs. time, up to sure exaggerated price of current once which the time is definite.

1.16. Electro-mechanical Relay.

Conventional relay within which the measuring is performed by movable elements.

1.17. Static Relay.

Relays within which relay measuring or comparison are performed by stationary (static) circuit.

1.18. Biased Relay.

A relay whose characteristic is changed by extra mechanical or electromagnetic provision like a bias-coil, magnet, etc.

1.19. Power Consumption of a Relay.

The value of power consumed expressed in VA (for AC) or watts (for DC ) under sure given conditions.

1.20. Pick-up.

The operation of relay is known as relay Pick-up. Pick-up price or level is the value of in operation amount that is on threshold (border) higher than that the relay operates and closes its contacts. 

Think about Associate in nursing over-current delay. During an injection check, suppose, the present is bit by bit exaggerated. At an exact price of current is understood as pick-up price. Commonly the relay setting corresponds to pick-up price.

1.21. Reset, drop-out.

The price of current / voltage etc.  Below that the relay resets and comes back to original position.

1.22. Over-current Relay.

A relay that responds to extend in current.

1.23. Earth-fault Relay.

A relay that sense earth fault.

1.24. Distance Protection.

A protection theme used for protection of transmission lines in that the relay measuring  is primarily based on measuring  V/I magnitude relation at relaying purpose that gives a live of distance between relay location and fault location.

1.25. Differential Protection.

A protecting system that responds to vector distinction (phase/ magnitude) between 2 or a lot of similar electrical quantities.

1.26. Protecting theme.

A set of protecting systems covering a specific protection zone, transmission line protection theme might comprise over current protection system, earth fault protection system. The schemes area unit counselled by the manufacturers of protecting gear. There area unit many different schemes from which final alternative is created.

1.27. Protecting System.

A combination of elements that along, performs the protecting relaying the elements embrace CTs pilot wires, measuring  relay, circuit, trip circuit etc. The CTs area unit connected in a very specific manner to obtain desired protection.

1.28. Auto-reclosure.

The process of automatic reclosing of fuse once its gap.

1.29. Power cable Carrier (PLC).

High frequency signals sent through the ability line conductors (for purpose of communication, observation and protection).

1.30. Carrier Current Protection.

Protection of cable by suggests that of power cable carrier signals.

1.31. Unit Protection.

Protection of cable by suggests that of power cable carrier signals.

1.32.  Reach.

(Off Distance Protection of Lines).  The limiting distances ‘covered by the protection, the faults beyond which are not within the reach of the protection and should be covered by other relay.

1.33.  Over-reach.

(Of Distance Protection). Operation of (distance) relay for a fault beyond its Set protected distance (say 130%).

1.34.  Under reach (of distance protection).

Failure  of  distance  relay  to  operate  within  the  set  protected  distance  (say90%).

1.35.  Carrier Transfer (Inter tripping).

Carrier signal sent from one end to other end of transmission line so as to trip the circuit-breaker at the other end.

1.36.  Carrier Blocking.

Carrier signal sent to the other end of transmission line so as to prevent the tripping of circuit-breaker at other end.

1.37.  Carrier Acceleration.

Carrier signal sent to other end of transmission line so as to reduce the relay time at that end by shunting the step timer. Distinction between Relay Unit, Protective Scheme and Protective System. Protected equipment (say, a Generator) comes in a particular protected zone. It is protected by a ‘Protection scheme’.  The protection scheme has a set of protective systems, e.g. a large generator may have a protection scheme comprises over current protection, differential protection, earth fault protection, and so many others. Hence, a protection scheme comprises set of protective systems and the protection scheme is named according to the protected equipment e.g.

–  Generator protection (scheme)

–  Transformer protection (scheme)

(The work ‘scheme’ is generally omitted).

The term Protective System or simply ‘Protection’ is named according to the principle Of operation or abnormal condition. Protective transformers and relays connected in a Particular fashion for giving protection against certain abnormal Conditions. The protective systems are named as follows:

Names based on abnormal condition:

–  Over-current protection (system)

–  Earth fault protection (system)1.

–  Reverse-power protection (system)

–  Under-voltage protection (system)

–  Under-frequency protection (system) etc.

Names based on principle of operation:

–  Differential protection (system)

–  Distance protection (system)

–  Power line carrier protection (system)

(The word system may be omitted).

Relay  unit  ‘Relay’  is  a  self-contained  unit  comprising  one  or  more  coils,  fixed  and movable sub-assemblies, or static circuits, provision for plug-setting, time-setting, etc. Relay unit is an important component of the protective system. It is generally named according to its type of construction / principle of operation.  It is either electromagnetic or static.

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