PROTECTION RELAY;
Relay is a protective device which senses the faulty condition in a power circuit line and initiate the operation of the circuit breaker to isolate the fault. Apart from the rest of the healthy power system many form of electrical protection are available which are designed to protect the distribution system. protection relay are used to monitor overcurrent, over under voltage, overload, under or over frequency , Earth leakage and balance loading over temperature, reverse power etc.
No matter how well design and operation there is always the possibility of developing an electrical equipment fault, which can develop due to natural wear and tear in-correct operation and accidental damage.
Three reason why protection equipment is essential in electrical distribution system . 1 to disconnect the faulty equipment in order to maintain the power supply to the remaining healthy circuit .2 to prevent damage to equipment from thermal and magnetic force that occurs during short circuit and overload faults 3. three to protect Personnel from electrical shock.
Protection Scheme consists of circuit breaker, fuses, contractor. Relay detects the fault in circuit and then initiate the trip action. Relay detect the faulty current and then signals the breaker to cut the supply of current to the faulty equipment.
The protection relays are as follows;
1.Overcurrent relay(initiates at 150% curent)- overcurrent is due to the short circuit faults in distribution circuit .
The types include magnetic relay( embedded in ACB ,MCCB,MCB and initiates trip action in ACB of generator,MCCB,MCB of distribution circuits). Magnetic relay construction is similar to that of a domestic water metre or reverse power relay.
Relay is made up of light weight non-magnetic aluminium disc between two soft laminated iron core Electromagnets. the upper magnet is wound with voltage coil which is supplied from one phase and artificial neutral of generator output. The current coil connects to the same phase as the voltage in the upper electromagnet sense voltage coil has more number of turns so it has more inductive value .the current coil has less number of turns so less value of Inductive and less induced current that lag less.
As we know current carrying conductor produces magnetic field so both upper and lower section produces magnetic field but induced current in voltage coil lags more than current coil so magnetic field produces and upper section will be weaker than the lower section and both magnetic field will have a difference of 90 degree. When both field pass through the aluminium disc it produces Eddy current which makes the disc to rotate . Accessing current due to short circut causes rotation against the spring and moving contact so that the tripping circuit is closed.
2.OVERLOAD RELAY(initiates at 110%load) it can be due to mechanical overloading of motor and for ex if 1 generator is running and too many loads are given to the generator. The types of overload relay are
1.dashpot type(embedded in ACB): it allows preferential trip in running generator and slowly shutting off the non essential load if still overload in generator it trips the air circuit breaker of generator in 15 sec time delay.
2.Thermal type;(embedded in MCB ,MCCB and contractor of motor etc)
3.electronic type overload relay; Magnetic type overload relay operate by sensing the strength of magnetic field produced by the current flowing to the motor .The greatest difference between the magnetic type and thermal type overload relay is that magnetic type is not sensitive to ambient temperature type overload relay are generally used in areas that exhibit extreme changes in ambient temperature. electronic overload relay employs a current transformer to sense the motor current the conductor passes through the core of a toroid Transformer. it induces voltage in directly proportional to the amount of current flows to the motor .The voltage induced into a toroid transformer is transmitted through a connected electronic interface that provides a time delay necessary to to permit the motor to start.
3.under voltage relay: (embedded in ACB ,all motor stator in contractor with setting 0.8 of rated voltage)-- undervoltage release mechanism is fitted to all generator breaker its main function is to trip the breaker when a severe voltage dip around 50% occurs this is achieved by lifting the mechanical latch which keeps the contact close to allow the trip spring to function which opens the breaker . The UV release on a generator circuit breaker also prevent it being closed when the generator voltage is very low or absent .UV relay may be magnetic or electronic. It also provide backup protection to short circuit protection as an example suppose during generator paralleling procedure your attempt was made to close the wrong circuit breaker if the circuit breaker was close the dead generator would be the equivalent of a short circuit fault on the bus bar and cause a blackout. The under voltage relay prevent the closure of circuit breaker of the Dead generator. The UV protection is also provided for motor starters the starter contractor thermally provide this protection as it drops out when the supply voltage is lost or is drastically reduced it prevent motor from restarting automatically when power is restored after blackout.
4.Negative phase sequence relay
5.earth leakage relay
6.under and over frequency relay.
So all this relay will send a tripping current to circuit breaker to open the contacts and disconnect the faulty generator or circuit.
Various type of circuit breaker closing mechanism may be fitted:-
1.Independent manual spring used in mcb and mccb
2.motor driven stored charge spring: used in ACB -closing spring is charged by a motor gearbox unit spring recharging is automatic following closure of a breaker which is initiated by a push button this may be a direct mechanical release of the charged spring or more usually it will be released electrically that is a solenoid latch.
Now lets talk about circuit breaker and see its different type practically;
1.MCB :- they are small air circuit breaker fitted in moulded plastic covers they have current rating of 5- 200A and generally thermal over current and magnetic short circuit protection. They have very limited breaking capacity up to 3000 ampere and are commercially used in final distribution board instead of fuses.
2.MCCB:- used in industrial as well as commercial mccb rating is up 1000A with breaking capacity of 210 KA- 100KA. There are many functions such as magnetic overcurrent trip function, thermal overload trip function ,undervoltage trip function, under frequency trip function etc they are small compact air circuit breaker fitted in a molded plastic case they have lower normal current rating 50-1500 ampere then main breaker and lower breaking capacity they usually have an adjustable thermal overcurrent setting and an adjustable overcurrent trip for short circuit protection built into the case and undervoltage trip coil may also be included within the case . Operation to close is usually by hand operated Lever but motor charge spring closing can also be used . Mccb are reliable trouble free and require negligible maintenance . If the breaker operates in the on position for long period it should be tripped and close the few times to free the mechanism and clean the contacts.
3.AIR CIRCUIT BREAKER:- they have many trip function like magnetic type overcurrent relay trip, dashpot type overload trip, under voltage trip, reverse power trip ,low and high frequency trip etc.
Relay is a protective device which senses the faulty condition in a power circuit line and initiate the operation of the circuit breaker to isolate the fault. Apart from the rest of the healthy power system many form of electrical protection are available which are designed to protect the distribution system. protection relay are used to monitor overcurrent, over under voltage, overload, under or over frequency , Earth leakage and balance loading over temperature, reverse power etc.
No matter how well design and operation there is always the possibility of developing an electrical equipment fault, which can develop due to natural wear and tear in-correct operation and accidental damage.
Three reason why protection equipment is essential in electrical distribution system . 1 to disconnect the faulty equipment in order to maintain the power supply to the remaining healthy circuit .2 to prevent damage to equipment from thermal and magnetic force that occurs during short circuit and overload faults 3. three to protect Personnel from electrical shock.
Protection Scheme consists of circuit breaker, fuses, contractor. Relay detects the fault in circuit and then initiate the trip action. Relay detect the faulty current and then signals the breaker to cut the supply of current to the faulty equipment.
The protection relays are as follows;
1.Overcurrent relay(initiates at 150% curent)- overcurrent is due to the short circuit faults in distribution circuit .
The types include magnetic relay( embedded in ACB ,MCCB,MCB and initiates trip action in ACB of generator,MCCB,MCB of distribution circuits). Magnetic relay construction is similar to that of a domestic water metre or reverse power relay.
Relay is made up of light weight non-magnetic aluminium disc between two soft laminated iron core Electromagnets. the upper magnet is wound with voltage coil which is supplied from one phase and artificial neutral of generator output. The current coil connects to the same phase as the voltage in the upper electromagnet sense voltage coil has more number of turns so it has more inductive value .the current coil has less number of turns so less value of Inductive and less induced current that lag less.
As we know current carrying conductor produces magnetic field so both upper and lower section produces magnetic field but induced current in voltage coil lags more than current coil so magnetic field produces and upper section will be weaker than the lower section and both magnetic field will have a difference of 90 degree. When both field pass through the aluminium disc it produces Eddy current which makes the disc to rotate . Accessing current due to short circut causes rotation against the spring and moving contact so that the tripping circuit is closed.
2.OVERLOAD RELAY(initiates at 110%load) it can be due to mechanical overloading of motor and for ex if 1 generator is running and too many loads are given to the generator. The types of overload relay are
1.dashpot type(embedded in ACB): it allows preferential trip in running generator and slowly shutting off the non essential load if still overload in generator it trips the air circuit breaker of generator in 15 sec time delay.
2.Thermal type;(embedded in MCB ,MCCB and contractor of motor etc)
3.under voltage relay: (embedded in ACB ,all motor stator in contractor with setting 0.8 of rated voltage)-- undervoltage release mechanism is fitted to all generator breaker its main function is to trip the breaker when a severe voltage dip around 50% occurs this is achieved by lifting the mechanical latch which keeps the contact close to allow the trip spring to function which opens the breaker . The UV release on a generator circuit breaker also prevent it being closed when the generator voltage is very low or absent .UV relay may be magnetic or electronic. It also provide backup protection to short circuit protection as an example suppose during generator paralleling procedure your attempt was made to close the wrong circuit breaker if the circuit breaker was close the dead generator would be the equivalent of a short circuit fault on the bus bar and cause a blackout. The under voltage relay prevent the closure of circuit breaker of the Dead generator. The UV protection is also provided for motor starters the starter contractor thermally provide this protection as it drops out when the supply voltage is lost or is drastically reduced it prevent motor from restarting automatically when power is restored after blackout.
4.Negative phase sequence relay
5.earth leakage relay
6.under and over frequency relay.
So all this relay will send a tripping current to circuit breaker to open the contacts and disconnect the faulty generator or circuit.
Various type of circuit breaker closing mechanism may be fitted:-
1.Independent manual spring used in mcb and mccb
2.motor driven stored charge spring: used in ACB -closing spring is charged by a motor gearbox unit spring recharging is automatic following closure of a breaker which is initiated by a push button this may be a direct mechanical release of the charged spring or more usually it will be released electrically that is a solenoid latch.
1.MCB :- they are small air circuit breaker fitted in moulded plastic covers they have current rating of 5- 200A and generally thermal over current and magnetic short circuit protection. They have very limited breaking capacity up to 3000 ampere and are commercially used in final distribution board instead of fuses.
2.MCCB:- used in industrial as well as commercial mccb rating is up 1000A with breaking capacity of 210 KA- 100KA. There are many functions such as magnetic overcurrent trip function, thermal overload trip function ,undervoltage trip function, under frequency trip function etc they are small compact air circuit breaker fitted in a molded plastic case they have lower normal current rating 50-1500 ampere then main breaker and lower breaking capacity they usually have an adjustable thermal overcurrent setting and an adjustable overcurrent trip for short circuit protection built into the case and undervoltage trip coil may also be included within the case . Operation to close is usually by hand operated Lever but motor charge spring closing can also be used . Mccb are reliable trouble free and require negligible maintenance . If the breaker operates in the on position for long period it should be tripped and close the few times to free the mechanism and clean the contacts.
3.AIR CIRCUIT BREAKER:- they have many trip function like magnetic type overcurrent relay trip, dashpot type overload trip, under voltage trip, reverse power trip ,low and high frequency trip etc.
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