When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?

When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?
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1 Answer

Answer :

No. We can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature).So, the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system. It should have both over voltage & over current relay.
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Related questions

When voltage increases then current also increases then why we need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?

Description : When voltage increases then current also increases then why we need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?

Last Answer : No. We cannot sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in ... , for a better protection of the system. It should have both over voltage & over current relay.

What are the advantages of static relay over electromagnetic relay?

Description : What are the advantages of static relay over electromagnetic relay?

Last Answer : o Low power consumption as low as 1mW o No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts o No gravity effect on operation of ... testing and servicing. Can convert even non-electrical quantities to electrical in conjunction with transducers.

What is the need of relay coordination?

Description : What is the need of relay coordination?

Last Answer : 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 ... relays, faults can always be isolated quickly without serious disturbance to the rest of the system.

What is reverse power relay?

Description : What is reverse power relay?

Last Answer : Reverse Power flow relay are used to stop the flow of power from grid to station generator when the generating units are off.

what is Buchholz relay and the significance of it in to the transformer?

Description : what is Buchholz relay and the significance of it in to the transformer?

Last Answer : Buchholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a transformer, the Buchholz ... circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.

What is a programmable relay?

Description : What is a programmable relay?

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

What is static relay?

Description : What is static relay?

Last Answer : 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.

What is REF relay?

Description : What is REF relay?

Last Answer : 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 ... of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

What are the limitations of Buchholz relay?

Description : What are the limitations of Buchholz relay?

Last Answer : (a) Only fault below the oil level are detected. (b) Mercury switch setting should be very accurate, otherwise even for vibration, there can be a false operation. (c) The relay is of slow operating type, which is unsatisfactory.

What are the types of graded used in line of radial relay feeder?

Description : What are the types of graded used in line of radial relay feeder?

Last Answer : Definite time relay and inverse-definite time relay.

What are the uses of Buchholz’s relay?

Description : What are the uses of Buchholz’s relay?

Last Answer : 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.

What are the features of directional relay?

Description : What are the features of directional relay?

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

What is an under frequency relay?

Description : What is an under frequency relay?

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

Mention any two applications of differential relay.

Description : Mention any two applications of differential relay.

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

Define resetting time of a relay.

Description : Define resetting time of a relay.

Last Answer : 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.

Define operating time of a relay.

Description : Define operating time of a relay.

Last Answer : It is defined as the time period extending from the occurrence of the fault through the relay detecting the fault to the operation of the relay.

How does the over voltage surge affect the power system?

Description : How does the over voltage surge affect the power system?

Last Answer : The over voltage of the power system leads to insulation breakdown of the equipments. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line.

Mention some of the applications of Directional Over-Current relays?

Description : Mention some of the applications of Directional Over-Current relays?

Last Answer : 1. Double end fed power system 2. Single end fed system of parallel feeders 3. Ring main feeder system

What are over and under current relays?

Description : What are over and under current relays?

Last Answer : 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 ... in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)

Name the different kinds of over current relays.

Description : Name the different kinds of over current relays.

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

What is meant by knee point voltage?

Description : What is meant by knee point voltage?

Last Answer : Knee point voltage is the voltage at which a CT gets saturated. It is one of the important factors taken into consideration for selecting a current transformer.

What is meant by recovery voltage?

Description : What is meant by recovery voltage?

Last Answer : The power frequency rms voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage.

What is restriking voltage?

Description : What is restriking voltage?

Last Answer : It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished.

Mention any two disadvantage of carrier current scheme for transmission line only.

Description : Mention any two disadvantage of carrier current scheme for transmission line only.

Last Answer : 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 .

What is inrush current?

Description : What is inrush current?

Last Answer : the initial value of flux is zero when the transformer is energized. The, the flux reaches a peak value in half a cycle. Since power transformers operate near the knee of the saturation curve, a flux ... the order of 8 to 30 times the full-load current. This current is known as inrush current.

Mention the withstanding current in our human body.

Description : Mention the withstanding current in our human body.

Last Answer : 9mA

What do you mean by current chopping?

Description : What do you mean by current chopping?

Last Answer : 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 ... This phenomenon of interruption of the current before its natural zero is called current chopping.

What are the merits of carrier current protection?

Description : What are the merits of carrier current protection?

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

What are the advantages of air blast circuit breaker over oil circuit breaker?

Description : What are the advantages of air blast circuit breaker over oil circuit breaker?

Last Answer : o The risk of fire is diminished o The arcing time is very small due to rapid buildup of dielectric strength between contacts o The arcing products are completely removed by the blast whereas oil deteriorates with successive operations

What are the disadvantages of MOCB over a bulk oil circuit breaker?

Description : What are the disadvantages of MOCB over a bulk oil circuit breaker?

Last Answer : o The degree of carbonization is increased due to smaller quantity of oil o There is difficulty of removing the gases from the contact space in time o The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization.

What are the advantages of MOCB over a bulk oil circuit breaker?

Description : What are the advantages of MOCB over a bulk oil circuit breaker?

Last Answer : • It requires lesser quantity of oil • It requires smaller space • There is a reduced risk of fire • Maintenance problem are reduced

What is over fluxing protection in transformer?

Description : What is over fluxing protection in transformer?

Last Answer : 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.

What are the causes of over speed and how alternators are protected from it?

Description : What are the causes of over speed and how alternators are protected from it?

Last Answer : Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.

In three pin plug why earth pin diameter is higher than other two pin?

Description : In three pin plug why earth pin diameter is higher than other two pin?

Last Answer : Because Current flow in the conductor is inversely proportional to the conductor diameter. So if any short circuits occur in the system first high currents bypassed in the Earthling terminal. (R=Pl/a area of the conductor increases resistance value decreases)

What is the difference between surge arrestor and lightning arrestor?

Description : What is the difference between surge arrestor and lightning arrestor?

Last Answer : Lightning arrestor is installed outside and the effect of lightning is grounded, whereas surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Where are the lighting arrestors placed in distribution lines?

Description : Where are the lighting arrestors placed in distribution lines?

Last Answer : Near distribution transformers and outgoing feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.

What is different between resistance grounding system and resistance earthing system?

Description : What is different between resistance grounding system and resistance earthing system?

Last Answer : In resistance grounding system the neutral point of the load is connected to the ground to carry the residual current in case of unbalanced conditions. Resistance earthing system is done in an electric equipment in order to protect the equipment in occurrence of fault in the system.

What is the difference between megger test equipment and contact resistance meter test instruments?

Description : What is the difference between megger test equipment and contact resistance meter test instruments?

Last Answer : Megger test equipment used to measure cable electric resistance, conductor continuity, phase identification. Contact resistance meter test instruments used to measure low resistance like relays, contactors.

What are the different types of faults in a transformer?

Description : What are the different types of faults in a transformer?

Last Answer : The internal faults include phase faults, ground faults, inter-turn faults, tap-changer failure, and leakage oil from tank. The external faults include system phase faults, system ground faults, over loads and over fluxing.

Explain differential protection?

Description : Explain differential protection?

Last Answer : Differential protection is based on the fact that any fault within an electrical equipment would cause the current entering it, to be different, from that leaving it. Thus, we can compare the two ... phase or both and issue a trip output if the difference exceeds a pre- determined set value.

What is dielectric test of a circuit breaker?

Description : What is dielectric test of a circuit breaker?

Last Answer : It consists of overvoltage withstand test of power frequency lightning and impulse voltages.Testa are done for both internal and external insulation with switch in both open and closed conditions.

How direct tests are conducted in circuit breakers?

Description : How direct tests are conducted in circuit breakers?

Last Answer : Using a short circuit generator as the source. Using the power utility system or network as the source.

Differentiate between a fuse and a circuit breaker.

Description : Differentiate between a fuse and a circuit breaker.

Last Answer : Fuse is a low current interrupting device. It is a copper or an aluminium wire. Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions.

Define single line diagram.

Description : Define single line diagram.

Last Answer : Representation of various power system components in a single line is defined as single line diagram.

Mention the inductance value of the peterson’s coil.

Description : Mention the inductance value of the peterson’s coil.

Last Answer : L=1/3wc2

State the different types of circuit breakers.

Description : State the different types of circuit breakers.

Last Answer : Air ,oil,vacuum circuit breakers.

State the various types of unsymmetrical faults.

Description : State the various types of unsymmetrical faults.

Last Answer : Line to ground ,line to line and double line to ground faults

State the different types of faults.

Description : State the different types of faults.

Last Answer : Symmetrical faults and unsymmetrical faults and open conductor faults.

Define negative sequence component.

Description : Define negative sequence component.

Last Answer : It has 3 vectors equal in magnitude and displaced from each other by an angle 120 degrees and has the phase sequence in opposite to its original phasors.

State the significance of double line fault.

Description : State the significance of double line fault.

Last Answer : It has no zero sequence component and the positive and negative sequence networks are connected in parallel.