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• OPENING of single capacitor BANK
• Closing of one capacitor bank against ANOTHER

When interrupting low inductive currents such as magnetizing currents of the TRANSFORMER, shunt reactor, the rapid deionization of the contact space and blast effect MAY CAUSE the current to be interrupted before the natural current zero. This PHENOMENON of INTERRUPTION of the current before its natural zero is called current chopping.

When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping.

It is the METHOD of connecting a resistance in PARALLEL with the contact space(arc). The resistance reduces the restriking voltage frequency and it diverts part of the arc current. It assists the CIRCUIT breaker in interrupting the MAGNETIZING current and capacity current.

It is the method of connecting a resistance in parallel with the contact space(arc). The resistance reduces the restriking voltage frequency and it diverts part of the arc current. It assists the circuit breaker in interrupting the magnetizing current and capacity current.

• Low power consumption as low as 1mW
• No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts
• No gravity effect on OPERATION of static relays. Hence can be used in vessels ie, ships, aircrafts etc.
• A SINGLE relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks. This is not possible in ELECTROMAGNETIC relays
• Static relay is compact
• Superior operating characteristics and accuracy
• Static relay can think , programmable operation is possible with static relay
• Effect of vibration is nil, hence can be used in earthquake-prone areas o Simplified TESTING and servicing. Can convert even non-ELECTRICAL quantities to electrical in conjunction with transducers.

It is combined PROTECTION, monitoring and CONTROL SYSTEM INCORPORATED in the static system.

It is combined protection, monitoring and control system incorporated in the static system.

A STATIC RELAY may have one or more programmable units such as microprocessors or MICROCOMPUTERS in its CIRCUIT.

A static relay may have one or more programmable units such as microprocessors or microcomputers in its circuit.

During SWITCHING of lines or wrong synchronization SURGES of real and reactive power flowing in TRANSMISSION line causes severe oscillations in the VOLTAGE and current vectors. It is represented by curves originating in load regions and TRAVELING towards relay characteristics.

During switching of lines or wrong synchronization surges of real and reactive power flowing in transmission line causes severe oscillations in the voltage and current vectors. It is represented by curves originating in load regions and traveling towards relay characteristics.

It is a RELAY in which measurement or COMPARISON of electrical QUANTITIES is made in a STATIC network which is DESIGNED to give an output signal when a threshold condition is passed which operates a tripping device.

It is a relay in which measurement or comparison of electrical quantities is made in a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device.

1. Large number of circuits, DIFFERENT current levels for different circuits for external faults.
2. Saturation of CT CORES due to dc component and ac component in short circuit CURRENTS. The saturation introduces ratio error.
3. Sectionalizing of the bus makes circuit complicated.
4. Setting of relays need a CHANGE with large LOAD changes.

• Failure of support INSULATOR resulting in earth fault
• FLASHOVER across support insulator during over VOLTAGE Heavily polluted insulator causing flashover EARTHQUAKE, mechanical damage etc.

When a fault occurs in an alternator winding EVEN though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself. Hence the field circuit breaker is opened and STORED energy in the field winding is DISCHARGED through another resistor. This method is known as field SUPPRESSION.

When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself. Hence the field circuit breaker is opened and stored energy in the field winding is discharged through another resistor. This method is known as field suppression.

RATIO ERROR

PERCENTAGE ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100 The value of transformation ratio is not equal to the TURNS ratio.

Phase angle error:

Phase angle =180/π[(ImCos δ-I1Sin δ)/NIS]

Ratio error

Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100 The value of transformation ratio is not equal to the turns ratio.

Phase angle error:

Phase angle =180/π[(ImCos δ-I1Sin δ)/nIs]

FAST OPERATION, AUTO re-closing POSSIBLE, easy discrimination of SIMULTANEOUS faults .

Fast operation, auto re-closing possible, easy discrimination of simultaneous faults .

Fault level at busbar is high.

The STABILITY of the SYSTEM is affected by the faults in the bus zone.

A fault in the bus bar causes interruption of SUPPLY to a large PORTION of the system network.

Fault level at busbar is high.

The stability of the system is affected by the faults in the bus zone.

A fault in the bus bar causes interruption of supply to a large portion of the system network.

If the TURNS ratio of the TRANSFORMER is more than 1:1, there will be HIGHER CORE loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is CALLED over fluxing.

If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing.

It is RESTRICTED earth fault RELAY. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low CURRENT. Hence the zone of protection in the WINDING of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

DIFFERENCE in lengths of pilot wires on EITHER sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.

Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.

Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.

Magnetizing inrush current appears wherever a transformer is energized on its primary side PRODUCING harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will BLOCK it when the transformer is energized.

Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.

Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.

Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.

Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.

Only FAULT below the oil level are detected.

Mercury switch setting should be very accurate, otherwise even for vibration, there can be a FALSE operation.

The relay is of SLOW OPERATING type, which is unsatisfactory.

Only fault below the oil level are detected.

Mercury switch setting should be very accurate, otherwise even for vibration, there can be a false operation.

The relay is of slow operating type, which is unsatisfactory.

OIL level GUAGE, sudden pressure DELAY, oil TEMPERATURE indicator, WINDING temperature indicator .

Oil level guage, sudden pressure delay, oil temperature indicator, winding temperature indicator .

External fault or through fault

Internal fault

1. SHORT circuit in transformer WINDING and CONNECTION

2.Incipient or slow DEVELOPING FAULTS

External fault or through fault

Internal fault

1. Short circuit in transformer winding and connection

2.Incipient or slow developing faults

In order to LIMIT the flow of CURRENT through NEUTRAL and earth a resistor is introduced between them.

In order to limit the flow of current through neutral and earth a resistor is introduced between them.

Bucholz relay is used to give an alarm in case of incipient( slow-developing) FAULTS in the TRANSFORMER and to connect the transformer from the supply in the event of severe INTERNAL faults. It is USUALLY used in oil immersion transformers with a RATING over 750KVA.

Bucholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer and to connect the transformer from the supply in the event of severe internal faults. It is usually used in oil immersion transformers with a rating over 750KVA.

Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes IMPOSSIBLE to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the voltage may be INSUFFICIENT to operate the relay. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase difference between the primary and the secondary QUANTITIES and a possibility of current through the relay EVEN when there is no fault.

Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase difference between the primary and the secondary quantities and a possibility of current through the relay even when there is no fault.

FAULT between phase and GROUND; fault between PHASES and inter-turn fault INVOLVING turns of the same phase WINDING.

Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same phase winding.

HIGH speed OPERATION; high sensitivity; ability to operate at low voltages; adequate short-time THERMAL ratio; burden MUST not be excessive.

High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive.

The PROGRAM time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the RESPONSE time of BAND pass filter; capacitance phase-shift of the TRANSMISSION line .

The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response time of band pass filter; capacitance phase-shift of the transmission line .

Pilot WIRES refers to the wires that connect the CT’s placed at the ends of a POWER transmission line as part of its PROTECTION SCHEME. The resistance of the pilot wires is usually LESS than 500 ohms.

Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.

An under frequency RELAY is one which operates when the frequency of the SYSTEM (usually an ALTERNATOR or transformer) falls below a CERTAIN value.

An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value.

LOADING; STATOR WINDING FAULT 

loading; stator winding fault 

In a POWER transformer, currents in the PRIMARY and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition. Also, there is usually a PHASE difference between the primary and secondary currents of three phase transformers. Even CT’s of PROPER turn-ratio are used, the differential current may FLOW through the relay under normal condition.

In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay under normal condition.

The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the SYSTEM. Also, if a relay fails to operate, there should be SUFFICIENTLY quick BACKUP protection so that the REST of the system is protected. By COORDINATING relays, faults can always be isolated quickly without serious disturbance to the rest of the system.

The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system.

The biased beam relay is designed to respond to the differential current in TERMS of its fractional relation to the current flowing through the PROTECTED zone. It is essentially an over-current BALANCED beam relay type with an ADDITIONAL RESTRAINING coil. The restraining coil produces a bias force in the opposite direction to the operating force.

The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.

PROTECTION of GENERATOR & generator transformer UNIT; protection of large MOTORS and busbars .

Protection of generator & generator transformer unit; protection of large motors and busbars .

Overcurrent relays are those that operate when the CURRENT in a line exceeds a predetermined VALUE. (eg: INDUCTION TYPE non-directional/directional overcurrent relay, differential overcurrent relay)whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)

Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg: Induction type non-directional/directional overcurrent relay, differential overcurrent relay)whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)

It is defined as the time TAKEN by the relay from the instant of isolating the FAULT to the MOMENT when the fault is REMOVED and the relay can be reset.

It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.

It is DEFINED as the time PERIOD extended from the occurrence of the fault through the RELAY DETECTING the fault to the operation of the relay.

It is defined as the time period extended from the occurrence of the fault through the relay detecting the fault to the operation of the relay.

It REFERS to the CURRENT or voltage which is USED to activate the relay into OPERATION.

It refers to the current or voltage which is used to activate the relay into operation.

INDUCTION type non-DIRECTIONAL over CURRENT RELAY,Induction type directional over current relay & current differential relay.

Induction type non-directional over current relay,Induction type directional over current relay & current differential relay.

Is the second LINE of defence , which OPERATES if the primary PROTECTION FAILS to activate within a definite time delay.

Is the second line of defence , which operates if the primary protection fails to activate within a definite time delay.

Is the PROTECTION in which the FAULT OCCURRING in a line will be CLEARED by its own relay and circuit breaker.It serves as the first line of defence.

Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker.It serves as the first line of defence.

A unit protective system is ONE in which only faults OCCURRING within its protected ZONE are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A NON unit system is a protective system which is activated EVEN when the faults are external to its protected zone.

A unit protective system is one in which only faults occurring within its protected zone are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A non unit system is a protective system which is activated even when the faults are external to its protected zone.

Are those which are directly protected by a protective SYSTEM such as RELAYS, FUSES or switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme DEDICATED to that particular zone.

Are those which are directly protected by a protective system such as relays, fuses or switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.

Whenever a short-circuit OCCURS, the current flowing through the coil increases to an enormous value. If protective relays are present , a HEAVY current also flows through the relay coil, causing it to operate by CLOSING its contacts.The trip circuit is then closed , the circuit breaker opens and the fault is ISOLATED from the REST of the system. Also, a low voltage may be created which may damage systems connected to the supply.

Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts.The trip circuit is then closed , the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.

ANSWER :To detect the FAULT and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, THEREBY protecting the system from damages CONSEQUENT to the fault.