1. · web viewfailure of cts or pts operation secondary protection back-up protection is very...

LATHA MATHAVAN ENGINEERING COLLEGE, MADURAI

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

IV YEAR/VII SEMESTER

EE6702 PROTECTION AND SWITCHGEAR

TWO MARKS

UNIT-I

1. What are the difference types of faults occurring in the power system?(M/J-2014)

Types of Faults on a Three Phase System.

(A) Phase-to earth fault

(B) Phase-to-phase fault

(C) Phase-to-phase-to-earth fault

(D) Three phase fault

(E) Three phase-to-earth fault

2. Write the importance of ground wire. (M/J-2014)

This ground wire is an additional path for electrical current to return safely to the ground.

without danger to anyone in the event of a short circuit.

3. Differentiate between a fuse and a protective relay. (A/M -2010)

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

4. Define the following terms as related to protective relaying: (a) pick-up current, (b) reset value and (c) reset ratio. (A/M -2010)

pick-up current

If the actuating current in the relay coil increases gradually, the deflecting force in electro mechanical relay, is also increased. Once, the deflecting force crosses the controlling force, the moving parts of the relay initiate to move to change the position of the contacts in the relay. The current for which the relay initiates it operation is called pick up current of relay.

reset value

The limiting value of the characteristic quantity at which the relay returns to its initial position

Resetting ratio or resetting percentage can be given for relays only. It follows from the fact, that their operate (x o) and reset (x r) values are different. Fig. 6.2 illustrates the operating hysteresis of maximal relays (relays, with which the positive change of the measured value triggers operation). The resetting ratio of the relay:

5. What is the difference between a short circuit and overload?(N/D-2015)

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 , thecircuit 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.

6. why earth wire is provided in overhead transmission line? (N/D-2015)

Ground wires are bare conductors supported at the top of transmission towers. They serve to shield the line and intercept lighting stroke before it hits the current carrying conductors below. Ground wires normally do not carry current. Therefore, they are often made of steel. The ground wires are solidly connected to ground at each tower in transmission and distribution system.

7. What is the difference between primary and back up protection?(N/D – 2016)

if a fault happens on any line, it will be cleared by its relay and circuit breaker. This is called primary or main protection and acts as a first line defender.

Primary protection may fail due to the following reasons

· Failure of DC supply to the tripping Circuit

· Failure in relay operating current or voltage

· Failure in circuit breaker tripping mechanism

· Failure of main protective relay operation

· Failure in the wiring of relaying system

· Failure of CTs or PTs operation

Secondary Protection

Back-up protection is very important for stable and reliable power system As we know that, it is not possible to design a 100% secure and efficient system because there are possibilities of failure in the connected CTs, PTs, circuit breaker etc. in the system. If it happens, then it will destroy our whole switching system.If the primary protection operation falls into trouble, then secondary protection disconnects the faulty part from the system. Moreover, when we disconnect primary protection for testing or maintenance purpose, then secondary or back-up protection will act as primary protection. In the above fig, relay “X” (1 Sec time setting) provides backup protection for each of the four connected lines to the main bus.

In addition, a larger part is disconnected then when primary relaying functions correctly. Therefore, greater emphasis should be placed on the better maintenance of primary relaying which is economical.

Types of Secondary or backup protection

Relay Backup Protection

Breaker Backup Protection

Remote Backup Protection

Centrally Co-ordinate Backup Protection

8. what do you mean by dead spot in zones of protection? (N/D – 2016)

The section of Power System which is not covered under any zone of protection is called Dead Zone or Blind Zone and special kind of protection shall be provided to take care of fault in Dead Zone. Normally overcurrent element is used for the protection of Dead Zone with some suitable logic interlock. The logic interlock depends on the configuration of power system and the condition in which Dead Zone is created. Let us take an example to have more insight. Carefully observe the figure below.

9. What is Peterson coil? What protective functions are performed by this device?

(A/M -2010)

A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that the capacitive current in the earth fault is compensated by an inductive current passed by the Petersen Coil.

Although the standard ‘Peterson coil’ does not compensate the entire earth fault current in a network due to the presence of resistive losses in the lines and coil, it is now possible to apply ‘residual current compensation’ by injecting an additional 180° out of phase current into the neutral via the Peterson coil. The fault current is thereby reduced to practically zero. Such systems are known as ‘Resonant earthing with residual compensation’, and can be considered as a special case of reactive earthing.

16 MARKS:

1. Explain the importance of protective schemes employed in power system. (A/M -2010)

Protective Relaying is one of the several features of the power system design. Every part of the power system is protected. The factors affecting the choice of protection are type and rating of equipment, location of the equipment, types of funks, abnormal conditions and cost. The protective relaying is used to give an alarm or to cause prompt removal of any element of power system from service when that element behaves abnormally. The abnormal behavior of an element might cause damage or interference within effective operation of rest of the system.

The protective relaying minimizes the damage to the equipment and interruptions to the service

when electrical failure occurs. Along with some other equipment’s the relays help to minimize

damage and improve the service.

The relays are compact and self contained devices which can sense the abnormal conditions.

Whenever an abnormal condition exists the relay contacts get closed. This in tum closes the hip

circuit of a circuit breaker. The circuit breakers are capable of disconnecting a fatty element,

when they are called upon to do so by the relays. Thus entire process includes the operations like

fault, operation of relay, opening of a circuit breaker and removal of faulty element. This entire

process is automatic and fast, which is possible due to effective protector relaying scheme. The

protective relaying scheme includes protective current transformers, voltage transformers,

protective relays, time delay relays, auxiliary relays, secondary circuits, top circuits etc. Each

component plays its own role, which is very important in the overall operation of the scheme the

protective relaying is the team work of all these components. The protective relaying also

provides the indication of location and type of the fault.

2. Write the essential qualities of protection. (M/J-2014) (A/M -2010)

Protective Relaying

A protective relaying scheme should have certain important qualities. Such an essential qualities of protective relaying are,

1. Reliability

2. Selectivity and Discrimination

3. Speed and Time

4. Sensitivity

5. Stability

6. Adequateness

7. Simplicity and Economy

1 Reliability

A protective relaying should be reliable is its basic quality. It indicates the ability of the relay system to operate under the predetermined conditions. There are various components which go into the operation before a relay operates. Therefore every component and circuit which is involved in the operation of a relay plays an important role. The reliability of a protection system depends on the reliability of various components like circuit breakers, relays, current transformers (C.T.s), potential transformers (P.T.s), cables, trip circuits etc. The proper maintenance also plays an important role in improving the reliable operation of the system. The

reliability can not be expressed in the mathematical expressions but can be adjusted from the statistical data. The statistical survey and records give good idea about the reliability of the protective system. The inherent reliability is based on the design which is based on the long experience. This can be achieved by the factors like, i) Simplicity ii) Robustness

iii) High contact pressure iv) Dust free enclosure iv) Good contact material vi) Good workmanship and vii) Careful Maintenance

2 Selectivity and Discrimination

The selectivity id the ability of the protective system to identify the faulty part correctly and disconnect that part without affecting the rest of the healthy part of system. The discrimination means to distinguish between. The discrimination quality of the protective system is the ability to distinguish between normal condition and abnormal condition and also between abnormal condition within protective zone and elsewhere. The protective system should operate only at the time of abnormal condition and not at the time of normal condition. Hence it must clearly discriminate between normal and abnormal condition. Thus the protective system should select the fault part and disconnect only the faulty part without disturbing the healthy part of the system.

The protective system should not operate for the faults beyond its protective zone. For example, consider the portion of a typical power system shown in the Fig. 1

It is clear from the Fig. 1 that if fault F2 occurs on transmission line then the circuit breakers 2 and 3 should operate and disconnect the line from the remaining system. The protective system should be selective in selecting faulty transmission line only for the fault and it should isolate it without tripping the adjacent transmission line breakers or the transformer. If the protective system is not selective then it operates for the fault beyond its protective zones and unnecessary the large part of the system gets isolated. This causes a lot of inconvenience to the supplier and

users.

3 Speed and Time

a protective system must disconnect the faulty system as fast as possible. If the faulty system is not disconnect for a long time then,

1. The devices carrying fault currents may get damaged.

2. The failure leads to the reduction in system voltage. Such low voltage may affect the motors and generators running on the consumer sude. The total time required between the instant of fault and the instant of final arc interruption in the circuit breaker is called fault clearing time. It is the sum of relay time and circuit breaker time. The relay time is the time between the instant of fault occurrence and the instant of closure of relay contacts. The circuit breaker times is the time taken by the circuit breaker to operate to open the contacts and to extinguish the arc completely. The fault clearing time should be as small as possible to have high speed operation of the protective

system.

Though the small fault clearing time is preferred, in practice certain time lag is provided. This is because,

1. To have clear discrimination between primary and backup protection

2. To prevent unnecessary operation of relay under the conditions such as transient, starting inrush of current etc.

Thus fast protective system is an important quality which minimises the damage and it improves the overall stability of the power system.

4 Sensitivity

The protective system should be sufficiently sensitive so that it can operate reliably when required. The sensitivity of the system is the ability of the relay system to operate with low value of actuating quantity. It indicates the smallest value of the actuating quantity at which the protection starts operating in relation with the minimum value of the fault current in the protected zone. The relay sensitivity is the function of the volt-amperes input to the relay coil necessary to cause its operation. Smaller the value of volt-ampere input, more sensitive is the relay. Thus 1 VA input relay is more sensitive than the 5VA input relay. Mathematically the sensitivity is expressed by a factor called sensitivity factor . It is the ratio of minimum

short circuit current in the protected zone to the minimum operating current required for the protection to start.

Ks = Is/Io

where Ks = sensitivity factor

Is = minimum short circuit current in the zone

Io= minimum operating current for the protection

5 Stability

The stability is the quality of the protective system due to which the system remains inoperative and stable under certain specified conditions such as transients, disturbance, through faults etc. For providing the stability, certain modifications are required in the system design. In most of the cases time delays, filter circuits, mechanical and electrical bias are provided to achieve stable operation during the disturbances.

6 Adequateness

There are variety of faults and disturbance those may practically exists in a power system. It is impossible to provide protection against each and every abnormal condition which may exist in practice, due to economical reasons. But the protective system must provide adequate protection for any element of the system. The adequateness of the system can be assessed by considering following factors,

1. Ratings of various equipments

2. Cost of the equipments

3. Locations of the equipments

4. Probability of abnormal condition due to internal and external causes.

5. Discontinuity of supply due to the failure of the equipment

1.7 Simplicity and Economy

In addition to all the important qualities, it is necessary that the cost of the system should be well within limits. In practice sometimes it is not necessary to use ideal protection scheme which is economically unjustified. In such cases compromise is done. As a rule, the protection cost should not be more than 5% of the total cost. But if the equipments to be protected are very important, the economic constrains can be relaxed The protective system should be as simple as possible so that it can be easily maintained. The complex system are difficult from the maintenance point of view. The simplicity and reliability are closely related to each other. The simpler system are

always more reliable.

3. Explain the protective schemes employed to protect formulating and switching effects. (M/J-2014)

Primary and Backup Protection:

The protection provided by the protective relaying equipment can be categorized with two types as 1. Primary protection 2. Backup protection The primary protection is the first line of defense and is responsible to protect all the power system elements from all the types of faults. The backup protection comes into play only when the primary protection fails. The backup protection is provided as the main protection can fail due to many reasons like,

1. Failure in circuit breaker

2. Failure in protective relay

3. Failure in tripping circuit

4. Failure in d.c tripping voltage

5. Leas of voltage or current supply to the relay. Thus it the backup protection is absent and

the main protection tails then there is a possibility of severe damage to the system. When the

primary protection is made inoperative for the maintenance purpose, the backup protection

acts like a main protection .The arrangement of back up protective scheme should be such that

the failure in main protection should not the failure in bark up protection as well This is

satisfied if back up relaying and primary relaying do not have anything common. Hence

generally backup protection is located at different stations from the primary protection. Front

the cast and economy point of sew. The backup protection is employed only for the protection

against short circuit and not for any other abnormal conditions.

4. Enumerate the basic concepts of insulation coordination. (M/J-2014)

Insulation Coordination is the process of determining the proper insulation levels of various components in a power system as well as their arrangements. It is the selection of an insulation structure that will withstand voltage stresses to which the system, or equipment will be subjected to, together with the proper surge arrester. The process is determined from the known characteristics of voltage surges and the characteristics of surge arresters.

5. A star connected, 3 phase, 10 MVA, 6.6kV alternator has a per phase reactance of 10%. It is protected by a Merz-Price circulating current protection which is set to operate for fault currents not less than 175 A. Calculate the value of earthing resistance to be provided in order to ensure that only 10% of the alternator winding remains unprotected. (16) (A/M -2010)

6. Explain the following: (A/M -2010)

(i) Earthing screen (8)

Earthing Screen

Earthing screen is generally used over electrical substation. In this arrangement a net of GI wire is mounted over the sub-station. The GI wires, used for earthing screen are properly grounded through different sub-station structures. This network of grounded GI wire over electrical sub-station, provides very low resistance path to the ground for lightning strokes. This method of high voltage protection is very simple and economic but the main drawback is, it can not protect the system from travelling wave which may reach to the sub-station via different feeders.

(ii) Overhead ground wires (8)

This method of over voltage protection is similar as earthing screen. The only difference is, an earthing screen is placed over an electrical sub-station, whereas, overhead earth wire is placed over electrical transmission network. One or two stranded GI wires of suitable cross-section are placed over the transmission conductors. These GI wires are properly grounded at each transmission tower. These overhead ground wires or earth wire divert all the lightning strokes to the ground instead of allowing them to strike directly on the transmission conductors.

7. Explain different types of earthing the neutral point of the power system. Derive an expression for the reactance of the Peterson coil in terms of capacitance of the protected line.(N/D 2015)

1. Solid grounding

When the neutral pointof a 3-phase system is directly connected to earth(i.e. soil) is called solid grounding or effective grounding.

When an earth fault occurs between earth and any one phase , the voltage to earth of the faulty phase becomes zero, but the healthy phases remains at normal phase values.

Fault current(IF) completely nullified by capacitive current(IC)

2. Resistance grounding

When the neutral point of a3-phase system(e.g.3-phasegenerator,3-phase transformer etc.)is connected to earth(i.e.soil)through a resistor, it is called resistance grounding.

(iii) Reactance grounding

In this system, a reactance is inserted between the neutral and ground

The purpose of reactance is to limit the earth fault current.

Disadvantages:

(i)Inthissystem, the fault current required to operate the protective device is higher than that of resistance grounding for the same fault conditions.

(ii)High transient voltages appear under fault conditions.

If inductance L of appropriate value is connected in parallel with the capacitance of the system, the fault current IF flowing through L will be in phase opposition to the capacitive current IC of the system. If L is so adjusted that IL=IC

then resultant current in the fault will be zero. This condition is known as Resonant Grounding.

When the value of L of arc suppression coil is such that the fault current IF exactly balances the capacitive current IC, it is called resonant grounding.

8. Explain the overlapping of protective zones with neat sketch(N/D 2015)

9. Explain in detail about different types of earthing scheme. (N/D- 2016)

Theprocessofconnectingthemetallicframe(i.e.noncurrentcarryingpart)ofelectricalequipmentorsomeelectricalpartofthesystemtoearth(i.e.soil)iscalledgroundingorearthing.

Grounding or earthing may be classified as:

(i)Equipment grounding

(ii)System grounding

Equipment Grounding

Theprocessofconnectingnon-current-carryingmetalpartsoftheelectricalequipmenttoearth.

System Grounding

The process of connecting some electrical part of the power system to earth (i.e.soil) is called system grounding.

Neutral Grounding

Connecting neutral point to earth (i.e.soil)either directly or some circuit element

(e.g.resistance,reactance,Petersoncoiletc.)iscalledneutralgrounding.

Neutral grounding provides protection to equipment.

(during earth fault ,the current path is completed neutral)

Advantages of Neutral Grounding(i)Voltages of the healthy phases do not exceed line to ground voltages i.e. they remain nearly constant.

(ii)The high voltages due to arcing grounds are eliminated.

(iii)Life of insulation islong.

(iv)Theovervoltagesisreduced.

(v)Itprovidesgreatersafetytopersonnelandequipment.

(vi)Itprovidesimprovedservicereliability.

(vii)Operatingandmaintenanceexpendituresarereduced.

Methods of Neutral Grounding

(i) Solid or effective grounding

(ii) Resistance grounding

(iii) Reactance grounding

(iv) Peterson-coil grounding

(v) Voltage transformer earthing

10. A 132KV, 3phase, 50 cycles, overhead line, 50km long has a capacitance to earth for each line of 0.0157microfarad/km. Determine the inductance and KVA rating of the arc suppression coil. (N/D- 2016)

11. explain the essential qualities of protection and explain in details. (N/D- 2016)

1.1 Reliability

A protective relaying should be reliable is its basic quality. It indicates the ability of the relay system to operate under the predetermined conditions. There are various components which go into the operation before a relay operates. Therefore every component and circuit which is involved in the operation of a relay plays an important role. The reliability of a protection system depends on the reliability of various components like circuit breakers, relays, current transformers (C.T.s), potential transformers (P.T.s), cables, trip circuits etc. The proper maintenance also plays an important role in improving the reliable operation of the system. The

reliability can not be expressed in the mathematical expressions but can be adjusted from the statistical data. The statistical survey and records give good idea about the reliability of the protective system. The inherent reliability is based on the design which is based on the long experience. This can be achieved by the factors like, i) Simplicity ii) Robustness

iii) High contact pressure iv) Dust free enclosure iv) Good contact material vi) Good workmanship and vii) Careful Maintenance

1.2 Selectivity and Discrimination

The selectivity id the ability of the protective system to identify the faulty part correctly and disconnect that part without affecting the rest of the healthy part of system. The discrimination means to distinguish between. The discrimination quality of the protective system is the ability to distinguish between normal condition and abnormal condition and also between abnormal condition within protective zone and elsewhere. The protective system should operate only at the time of abnormal condition and not at the time of normal condition. Hence it must clearly discriminate between normal and abnormal condition. Thus the protective system should select the fault part and disconnect only the faulty part without disturbing the healthy part of the system.

The protective system should not operate for the faults beyond its protective zone. For example, consider the portion of a typical power system shown in the Fig. 1

It is clear from the Fig. 1 that if fault F2 occurs on transmission line then the circuit breakers 2 and 3 should operate and disconnect the line from the remaining system. The protective system should be selective in selecting faulty transmission line only for the fault and it should isolate it without tripping the adjacent transmission line breakers or the transformer. If the protective system is not selective then it operates for the fault beyond its protective zones and unnecessary the large part of the system gets isolated. This causes a lot of inconvenience to the supplier and

users.

1.3 Speed and Time

a protective system must disconnect the faulty system as fast as possible. If the faulty system is not disconnect for a long time then,

1. The devices carrying fault currents may get damaged.

2. The failure leads to the reduction in system voltage. Such low voltage may affect the motors and generators running on the consumer sude. The total time required between the instant of fault and the instant of final arc interruption in the circuit breaker is called fault clearing time. It is the sum of relay time and circuit breaker time. The relay time is the time between the instant of fault occurrence and the instant of closure of relay contacts. The circuit breaker times is the time taken by the circuit breaker to operate to open the contacts and to extinguish the arc completely. The fault clearing time should be as small as possible to have high speed operation of the protective

system.

Though the small fault clearing time is preferred, in practice certain time lag is provided. This is because,

1. To have clear discrimination between primary and backup protection

2. To prevent unnecessary operation of relay under the conditions such as transient, starting inrush of current etc.

Thus fast protective system is an important quality which minimises the damage and it improves the overall stability of the power system.

1.4 Sensitivity

The protective system should be sufficiently sensitive so that it can operate reliably when required. The sensitivity of the system is the ability of the relay system to operate with low value of actuating quantity. It indicates the smallest value of the actuating quantity at which the protection starts operating in relation with the minimum value of the fault current in the protected zone. The relay sensitivity is the function of the volt-amperes input to the relay coil necessary to cause its operation. Smaller the value of volt-ampere input, more sensitive is the relay. Thus 1 VA input relay is more sensitive than the 5VA input relay. Mathematically the sensitivity is expressed by a factor called sensitivity factor . It is the ratio of minimum

short circuit current in the protected zone to the minimum operating current required for the protection to start.

Ks = Is/Io

where Ks = sensitivity factor

Is = minimum short circuit current in the zone

Io= minimum operating current for the protection

1.5 Stability

The stability is the quality of the protective system due to which the system remains inoperative and stable under certain specified conditions such as transients, disturbance, through faults etc. For providing the stability, certain modifications are required in the system design. In most of the cases time delays, filter circuits, mechanical and electrical bias are provided to achieve stable operation during the disturbances.

1.6 Adequateness

There are variety of faults and disturbance those may practically exists in a power system. It is impossible to provide protection against each and every abnormal condition which may exist in practice, due to economical reasons. But the protective system must provide adequate protection for any element of the system. The adequateness of the system can be assessed by considering following factors,

1. Ratings of various equipments

2. Cost of the equipments

3. Locations of the equipments

4. Probability of abnormal condition due to internal and external causes.

5. Discontinuity of supply due to the failure of the equipment

1.7 Simplicity and Economy

In addition to all the important qualities, it is necessary that the cost of the system should be well within limits. In practice sometimes it is not necessary to use ideal protection scheme which is economically unjustified. In such cases compromise is done. As a rule, the protection cost should not be more than 5% of the total cost. But if the equipments to be protected are very important, the economic constrains can be relaxed The protective system should be as simple as possible so that it can be easily maintained. The complex system are difficult from the maintenance point of view. The simplicity and reliability are closely related to each other. The simpler system are

always more reliable.

12. Explain the method of calculating fault current using symmetrical components. (N/D- 2016)

UNIT-II

TWO MARKS

1. List out the different types of distance relay. (M/J-2014)

· Impedance relay

· Admittance relay

· Reactance relay

2. Write the merits of static relay. (M/J-2014)

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.

3. What are the different types of faults and abnormal conditions expected in an alternator? (A/M -2010)

The common faults are

1. Stator Faults

Phase-to-Phase FaultsPhase-to-Earth faults

Inter-turn Faults

2. Rotor Faults

Overloading

Reverse PowerUnderexcitationNegative Phase SequenceOvervoltage

Overspeeding

3. Operational Faults

4. What are the requirements of line protection? (A/M -2010)

· During fault, the only circuit breaker closest to the fault point should be tripped.

· If the circuit breaker closest the faulty point, fails to trip the circuit breaker just next to this breaker will trip as back up.

· The operating time of relay associated with protection of line should be as minimum as possible in order to prevent unnecessary tripping of circuit breakers associated with other healthy parts of power system.

5. In what way a distance relay is superior to over current protection for protection of transmission lines? (N/D-2015)

6. where is negative sequence relay employed? (N/D-2015)

In transmission lines, distribution lines and feeder.

7. what is the significance of PSM and TSM? (N/D – 2016)

Plug setting multiplier of relay is referred as ratio of fault current in the relay to its pick up current.

Suppose we have connected on protection CT of ratio 200/1 A and current setting is 150%.Hence, pick up current of the relay is, 1 × 150 % = 1.5 ANow, suppose fault current in the CT primary is 1000 A. Hence, fault current in the CT secondary i.e. in the relay coil is, 1000 × 1/200 = 5ATherefore PSM of the relay is, 5 / 1.5 =3.33

Time Setting Multiplier of Relay

The operating time of an electrical relay mainly depends upon two factors :

How long distance to be traveled by the moving parts of the relay for closing relay contacts and

How fast the moving parts of the relay cover this distance.

So far adjusting relay operating time, both of the factors to be adjusted.The adjustment of travelling distance of an electromechanical relay is commonly known as time setting. This adjustment is commonly known as time setting multiplier of relay.

8. A relay is connected to 400/5 ratio current transformer with current setting of 150%. calculate the plug setting multiplier when circuit carries a fault current of 400A. (N/D – 2016)

16 MARKS

1. Explain the construction and operating principle of over current relay with directional Scheme. (M/J-2014)

It is similar in construction to directional power relay.

it consists of upper magnet which is E-shaped and carries primary winding which is excited by voltage of the circuit to be protected through secondary of PT.

The lower magnet is U-shaped carries secondary winding which is excited by current of the circuit to be protected through secondary of CT.

The secondary winding is extended to lower magnet primary winding as shown.

The trip contacts 1 & 2 are connected in series with secondary winding of lower magnet. Therefore for the relay to operate, at first directional element should be activated first.

2. Describe the principles of

(i) Negative sequence relay.

The negative relays are also called phase unbalance relay because these relays provide protection against negative sequence component of unbalanced current existing due to unbalanced loads or phase-phase faults.

The unbalanced currents are dangerous from generators and motors point of view as these currents can cause overheating. Negative sequence relays are generally used to give protection to generators and motors against unbalanced currents.

(ii) Under frequency relay. (M/J-2014)

Underfrequency refers to a condition where the frequency of the AC supply drops to a value that is lower than its defined value such as 50 Hz or 60 Hz. Underfrequency is usually caused by overloading a power source or problems with a prime mover such as engines or turbines.

The following are some of the effects of underfrequency

1. High flux density in electric machinery, thereby causing higher magnetizing currents.

2. High core loss and over-heating of the machines and possible failure.

3. Lower efficiency

4. Reduction in speed

The fault level increases due to reduced reactance

Underfrequency situations can be avoided by the installation of underfrequency relays which isolate systems and machinery in the event of an underfrequency.

3. With a neat schematic diagram, explain the protection of transformer with differential Protection scheme. (M/J-2014)

Inthissystem,twooverloadrelayandoneearthrelayareconnected.Thetwooverloadrelaysaresufficienttoprotectagainstphasetophasefaults.Thetripcontactsofoverloadrelaysandearthfaultrelayareconnectedinparallel.Thereforetheenergizingofeitheroneofthem,thecircuitbreakerwilltripped.

b) Write brief notes on

(i) Generator protection.

Complete Generator protection is divided into two category i.e.

Class –A Protection

Class –B Protection

CLASS –A: Protection where electrical isolation is an emergency.( Insulation failure, ,S.C. etc.). Trip the GCB/Turbine/Boiler without time delay or Generator automatic trips.

Class –A follows;

Gen. Differential Prot.

Gen. 100% E/F

Gen. SB E/F

Gen. NVD

Gen. O/C

Rotor 2nd stage E/F

Gen. Brg. Temp. high

4. Bus bar protection. (M/J-2014)

Protection Bus Differential: Current into bus must equal current out of bus

5. With a neat sketch explain the Merz-Price circulating current scheme for protection of alternators. (A/M -2010) (16)

This is most commonly used protection scheme for the alternator stator windings. The scheme is

also called biased differential protection and percentage differential protection. In this method,

the currents at the two ends of the protected section are sensed using current transformers. The

wires connecting relay coils to the current transformer secondary’s arc called pilot wires. Under

normal conditions, when there is no fault in the windings, the currents in the pilot wires fed (mm

C.T. secondary’s are equal. The differential current is - through the operating cods of the relay as

zero. Hence the relay is inoperative and system is said to be balanced. When fault occurs inside

the protected section ot the stator windings, the differential current i, - 12 flows through the

operating coils of the relay. Due to this current, the relay operates.. This trips the generator

circuit breaker to isolate the faulty section. The field is also disconnected and is discharged

through suitable impedance.

6. Explain the principle of a distance relay, stating clearly the difference between electromagnetic version of an impedance relay and a mho relay. Your answer should include typical schematics the characteristics of these relays in an R-X diagram. (A/M -2010)

i) impedance relay

+ve (operative)Torque by current element

-ve(restraining)Torque by voltage element

At normal condition operative torque = restraining torque

At fault operative torque > restraining torque

Also called voltage restrained over current relay.

ii) admittance relay

Induction cup type structure.

Operative Torque produced by V & I element.

Restraining Torque by Voltage element.

Also called Admittance relay.

iii) reactance relay

Operative Torque by current

o Restraining Torque by Current-Voltage Directional relay

+ve torque by over current element

-ve torque by directional unit

Directional element designed for maxi. Torque angle = 90 degree

7. Give the block diagram of a typical microprocessor based protective relay for motor protection. (A/M -2010) (8)

Based on the control action i.e. starting, stopping or reversal, controlling elements known in

electrical terms as switchgear are employed for the protection of induction motor. Generally two

basic protections viz short circuit protection and overload protection are provided for each motor.

The switchgear used for protection includes contactors with H.R.0 fuse and thermal overload

relays along with circuit breakers. If the rating of the motor is up to 150 kW then contactors and

fuses can be used while for motors having rating beyond 150 kW, circuit breakers are used. The

contactor is a kind of switch through which supply can be given to the motor when its coil is

energized. If the current to be interrupted is six times the rated current of the motor then

contactors can be used.

Abnormal Conditions and Failure in Case of Induction Motor: The three phase induction

motors are used in numerous industrial applications. Hence before studying the protection circuit

we have to consider the abnormal conditions and failure that may occur in case of induction

motor. If the motor is heavily loaded beyond its capacity then it will be overload condition of

motor in which case motor draws heavy current from the supply and there will be simultaneous

rise in temperature of winding and deterioration of the insulation resulting in damage of winding.

Hence the motor must be protected against tins mechanical overloading with overload protection

circuits. Normally thermal overload relays, over current relays or miniature circuit breaker with

built in trip coils may be used. It might be possible that the rotor is locked or starting lasts for

longer duration or rotor does not move because of excessive load (stalling) at start. In all these

cases motor draws heavy current from the supply and results in damage to the winding due to

overheating as stated above. In this case thermal relays or instantaneous overcurrent relays are

used.

If the supply conditions are abnormal such as loss of supply voltage, unbalanced supply voltage,

phase sequence reversal of supply voltage, over voltage, under voltage or under frequency then

also the performance of the motor is affected. With unbalanced supply voltage there will be

excessive heating while with under voltage the motor draws more current for the same load. For

under voltage protection, under voltage relays are used. With correct phase sequence, the motor

runs in one direction. With change in phase sequence of supply it runs in other direction which is

dangerous in some of the applications such as cranes, hoists or elevators. In such cases phase

reversal relay may be provided which will disconnect the supply to the motor through the circuit

breaker. Due to excessive temperature rise, the insulation may get damaged which may lead to

stator earth fault or stator phase to phase fault which are rare in nature. For low rating motors,

HRC fuses provide sufficient protection against these faults while for large motors, differential

protection may be used. Due to blowing of fuse in any phase or open circuit in one of the three

phases results in single phasing. In such case motor continues to run and if it is loaded to its rated

value then it will draw excessive current which will damage the rotor and eventually the motor

will be damaged due to excessive overheating. Normally thermal overload relays are used

against single phasing. Sometimes special single phase preventer may be provided.

8. Classify the different faults in power system. which of these are more frequent? (N/D 2015)

9. Explain the principle of working of distance relay. Describe with neat sketches the following types of relays (N/D 2015)

i) impedance relay






ii) admittance relay





iii) reactance relay






UNIT-III

TWO MARKS

1. Enumerate the concept of ring feeder. (M/J-2014)

The drawback of radial electrical power distribution system can be overcome by introducing a ring main electrical power distribution system. Here one ring network of distributors is fed by more than one feeder. In this case if one feeder is under fault or maintenance, the ring distributor is still energized by other feeders connected to it. In this way the supply to the consumers is not affected even when any feeder becomes out of service. In addition to that the ring main system is also provided with different section isolates at different suitable points. If any fault occurs on any section, of the ring, this section can easily be isolated by opening the associated section isolators on both sides of the faulty zone transformer directly.

2. List out the applications of current transformer. (M/J-2014)

Current Transformers : C.T. are used for measurement and Protection. Its step down the current from high value to low current value. Their ratio is constant for given range of Primary & Secondary Current

3. Define (a) arc voltage, (b) restriking voltage, (c) recovery voltage and (d) breaking capacity of a circuit breaker. (A/M -2010)

As soon as the Breaker contacts open, an arc is formed between the contacts of the Circuit Breaker. The voltage which appears across the contacts of the Breaker during this arcing period is called the Arc Voltage. Its value is low but when the value of arc current reaches to zero, arc voltage will shoot up to its peak value which in turn will try to main the arc across the contacts. So here we come to a voltage which shoots up to peak when the current crosses to its zero. Actually this is the origination of Restriking Voltage.

As the arcing current crosses zero, a high frequency transient voltage appears across the contacts of the Circuit Breaker. This Transient voltage is known as Restriking Voltage.

Recovery Voltage is the normal frequency RMS voltage that appears across the contacts of the Circuit Breaker after final arc extinction. It is equal to the system voltage.

Breaking capacity of the circuit breaker refers to the maximum current in rms value the circuit breaker can interrupt. This is also in the order of kA.

4. What is the importance of arc resistance? On which factor does it depend? (A/M -2010)

High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished _ Current zero method:-The arc is interrupted at current zero position that occurs100 times a second in case of 50Hz power system frequency in ac.

5. What are the short comings of differential protection scheme as applied to power transformer? (N/D-2015)

· The faults occur in the transformer inside the insulating oil can be detected by Buchholz relay. But if any fault occurs in the transformer but not in oil then it can not be detected by Buchholz relay. Any flash over at the bushings are not adequately covered by Buchholz relay. Differential relays can detect such type of faults. Moreover Buchholz relay is provided in transformer for detecting any internal fault in the transformer but Differential Protection scheme detects the same in more faster way.

· The differential relays normally response to those faults which occur in side the differential protection zone of transformer.

· At maximum through fault current, the spill output produced by the small percentage unbalance may be substantial. Therefore, differential protection of transformer should be provided with a proportional bias of an amount which exceeds in effect the maximum ratio deviation.

6. Give the examples for unit and non-unit system for protection. (N/D-2015)

Unit Protection Scheme: Unit type schemes protect a specific area of the system i.e. a transformer, transmission line, generator or bus bar. The unit protection schemes are based on Kirchhoff’s Current Law – the sum of the currents entering an area of the system must be zero. Any deviation from this must indicate an abnormal current path. In these schemes, the effects of any disturbance or operating condition outside the area of interest are totally ignored and the protection must be designed to be stable above the maximum possible fault current that could flow through the protected area.

Non-unit Protection Scheme: The non-unit schemes, while also intended to protect specific areas, have no fixed boundaries. As well as protecting their own designated areas, the protective zones can overlap into other areas. While this can be very beneficial for backup purposes, there can be a tendency for too great an area to be isolated if a fault is detected by different non unit schemes.

7. What is over fluxing? How it affect transformer? (N/D – 2016)

Over fluxing is a dangerous situation in which the magnetic flux density increases to extremely high levels. The high flux density can induce excessive eddy currents in the windings and in other conductive parts inside the transformers. The heat generated by these eddy currents can damage the windings and the insulation. The high flux density also causes magneto striction inside the transformer core and produces noise. The powerful magnetostrictive forces can also cause damage. The winding temperatures may also increase due to the heat produced.

Overfluxing is usually encountered in Transformers which are directly connected to the generator. It usually occurs when the generator is being started or stopped. As the rpm of the generator and consequently the frequency of the power falls, the same system voltage induces a higher magnetic flux. Modern Automatic Voltage regulators are equipped with V/Hz limiters which limit the voltage in accordance with the frequency.

8. Write two protection schemes used for protection of bus-bar. (N/D – 2016)

Differential Busbar Protection

Current Differential Protection

Differential Protection of Sectionalized Bus

Voltage Differential Protection of Busbar

16 MARKS

1. with neat diagram explain the construction and operation of induction type directional over current relay. (N/D 2015)

From the previous discussionT = V I COSθUnder abnormal condition under abnormal conditions voltage in the circuit is too low. Therefore the driving torque becomes abnormally too small .Hence the relay does not operate.ie., the directional power relay is not suitable for short circuit conditions.This problem can be overcome by directional over current relay.

Directional overcurrentrelay makes use of two relaysi) directional power relay ( directional element)ii) Non directional current relay (non-directional element)Construction:

1) Directional element :It is similar in construction to directional power relay.




The trip contacts 1 & 2 are connected in series with secondary winding of lower magnet. therefore for the relay to operate, at first directional element should be activated first.

2. Explain the current carrier protection scheme. Describe the carrier phase comparison relay with neat sketches. (N/D 2015)

For long overhead lines the power line itself may be used as the interconnecting channel between the terminal equipments. Carrier-current protection is the most widely used scheme for the protection of Extra High Voltage (EHV) and Ultra High Voltage (UHV) power lines. The carrier signal is directly coupled to the power line itself which is to be protected. Carrier-current protection is faster and superior to distance protection schemes and is more reliable when used for long transmission lines, although the terminal equipments are more expensive and complicated. In addition to protection the carrier signals can also be used for communication, supervisory control and telemetering.

In carrier-current protection or any other unit protection, the circuit breakers at both the ends of the line trip simultaneously when a fault occurs at one of the ends of the protected line sections. This helps in improving the stability. The carrier signals can be used either to initiate or to prevent the tripping of a protective relay according to which they are classified. When a carrier signal is used to initiate tripping of relay, the scheme is known as carrier inter-tripping, or transfer tripping or permissive tripping scheme. The scheme is known as carrier-blocking scheme when the carrier signals are used to prevent the operation of a relay.

Different operating techniques used in carrier-current protection

The two operating techniques mainly used in carrier-current protection are:

1. Phase comparison technique, and

2. Directional comparison technique.

In phase comparison technique, the phase angle of the current entering at one end is compared with the phase angle of the current leaving the other end of the protected section. During normal operating conditions or in case of an external fault, the currents at both the ends of the protected line are in phase. In case of an internal fault i.e. fault in the protected section, the currents at the two ends will be 180o out of phase.

The direction of power flow at the two ends of the protected sections is compared in the directional comparison technique. During normal conditions or external faults, the power flows into the protected section at one end and leaves at the other end. During internal faults, the direction of power flow is inwards at both the ends.

The signals generated in a carrier-current protection scheme are at a frequency between 50 and 500 kHz. Below 50 kHz the size and cost of the coupling equipments would be too high and above 500 kHz the line losses and therefore the signal attenuation would be too high on long lines. Carrier-current protection can be used only on overhead lines and cannot be used for underground cables as the capacitance of a cable would attenuate the carrier signals appreciably.

3. why is the harmonic restrained differential relay required to be used for protecting a large size transformer? describe the construction and working of such a relay. (N/D 2015)

It is experienced, that the three phase harmonic restraint is more secure [8] . The cumulative restraint defines the combined currents as sums of the appropriate quantities over three phases.

Also, instead of the real RMS, only low order harmonics can be used. In such approach, the combined differential signal is composed. Where the highest harmonic measured (usually the fifth harmonic used for restraining the relay during stationary over excitation conditions).

Finally, a modern method used both the magnitude of the dominant harmonics compared to the fundamental frequency as used in old versions of this method in addition to the phase shift of these harmonics to the fundamental one. This method gives a more reliable criterion above the others in blocking the differential relay during inrush current case since it considers the drop of the second harmonic magnitude momentarily to less than 20% of its magnitude. Depending on the exact formula employed for the combined harmonic and differential signals, the setting assumes slightly different values. Generally, however, the setting is at about 0.15 - 0.20 (15% - 20%)

The harmonic restraint in general, regardless of the method of composing the combined harmonic and differential signals, displays certain limitations.

· First, the estimator of the harmonic component (usually the second harmonic only) needs a certain amount of time for accurate estimation of the amplitude. Even if the harmonic is not present in the differential signal at all, the ratio of the harmonic current to the differential current is initially significantly overestimated (until the fault data fills out the estimator data window). This means that the harmonic restraint usually will not permit tripping for the time approximately equal to the data window length of the estimators (typically one cycle).

· Second, in modern transformers the amount of higher harmonics in the magnetizing current may drop well below 10% (the second harmonic as low as 7%, while the total harmonic content at a level of 7.5%. Under such circumstances, the setting should be adjusted below 7%. This may lead, however, to delayed or even missing operations of the relay due to the harmonics in the differential currents during internal faults accompanied by saturation of the current transformers. Cross-restraint or time-controlled threshold provide only a partial solution to this problem.

· Third, the second harmonic ratio may temporarily (for several cycles) drop below the safe 20% due to transients.

4. With the neat sketch explain the working of a directional over current relay. Derive the torque equation and also explain about directional relay connection. (N/D- 2016)

This type of relay works on the induction principle and initiates corrective measures when current in the circuit exceeds the pre determined value.

The actuating source is a current in the circuit supplied to the relay from a current transformer. These relays are used on a.c. circuits only and can operate for fault current flow in either direction.

5. from the universal torque equation determine the condition of operation for impedance relay, reactance relay and admittance relay. (N/D- 2016)

i) Impedance relay






ii) Admittance relay





iii) Reactance relay






6. Draw and explain the protective scheme of AC induction motor. (N/D- 2016)

· THERMAL OVERLOAD

· SHORT CIRCUIT

· EARTH FAULT

· UNBALANCE

· BLOCKED ROTOR/STALLING PROTECTION

Differential Protection

Differential protection may be considered the first line of protection for internal phase-to-phase or phase-to-ground faults.

Summation method with six CTs:

If six CTs are used in a summing configuration, during motor starting, the values from the two CTs on each phase may not be equal as the CTs are not perfectly identical and asymmetrical currents may cause the CTs on each phase to have different outputs.

The running differential delay can then be fine tuned to an application such that it responds very fast and is sensitive to low differential current levels.

What is:-Aground fault is a fault that creates a path for current to flow from one of the phases directly to the neutral through the earth bypassing the load

Ground faults in a motor occur:

When its phase conductor’s insulation is damaged for example due to voltage stress, moisture or internal fault occurs between the conductor and ground

To limit :-thelevel of the ground fault current connect an resistance known as stablising resistance Ground Fault Protection

Main causes of current unbalance

Blown fuses

Loose connections

Stator turn-to-turn faults

System voltage distortion and unbalance

Faults

Effects

Motor winding overheating

Excessive vibrations

Cause motor insulation/winding/bearing damage

Motor Protection Stalling

What is:-It happens when motor circuits are energized, but motor rotor is not rotating. It is also called locked rotor.

Effects:

This will result in excessive currents flow given the same load. This will cause thermal damage to the motor winding and insulation.

7. Explain the protection scheme of transformer (N/D- 2016)

Faults occurring in Transformers

Open-Circuit faults

Earth faults

Phase-to-Phase faults

Inter-Turn faults

Buchholz Protection

Also known as gas accumulator relay, commonly used on all oil-immersed transformer provided with conservator.

Working Principle:

Wheneverafaultoccurinsidethetransformer,theoilofthetankgetsoverheatedandgasesaregenerated.Theheatgeneratedbythehighlocalcurrentcausesthetransformeroiltodecomposeandproducegaswhichcanbeusedtodetectthewindingfaults

Overheating

Core-Balance Leakage Protection

This system is used to provide protection against earth faults on high voltage winding. When earth fault occurs, the sum of the three currents is no longer zero and a current is induced in the secondary of the CT causing the trip relay to operate and isolate the transformer from the bus-bars.

UNIT-IV

TWO MARKS

1. How do you quench an arc in a circuit breaker? (M/J-2014)

The arc produced not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself.

Therefore main problem in a c.b is to extinguish the arc within the shortest possible time so the heat generated by it may not reach a dangerous value.

2. What are the demerits of resistance switching? (M/J-2014)

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.

3. A three-phase oil circuit breaker is rated at 1500 A, 1000MVA and 33kV Find (a) rated symmetrical breaking current, (b) making capacity. (A/M -2010)

4. Suggest a suitable choice of circuit breakers for the following voltage ranges: (A/M -2010)

(a) 3.3kV to 33kV, (b) 400kV to 760kV.

5. what is meant by making capacity of circuit breaker? (N/D-2015)

The making capacity is expressed as a peak value as the dc offset during fault conditions is taken into account. Breaking capacity of the circuit breaker refers to the maximum current in rms value the circuit breaker can interrupt.

6. why current chopping is not common in oil circuit breaker? (N/D-2015)

In oil circuit breaker, the chance of arc restriking again is less.

But in other CB, the chance of restriking is more. particularly when transformer fault is concerned, the fault current (magnetising current ) is low. this low current is turned off before it can reach natural zero.so there is chance of restriking voltage.

The current is turned off again by CB action. but if the transient voltage is sufficient , the arc restrikes again. after every current chopping, the value of current decreases.in the end, the chopped current is low and no restrike takes place.

Hence current chopping in not present in oil CB.

7. Write two application of static relay. (N/D – 2016)

One of the primary advantages of SSR’s and I/O modules is their compatibility with low-level, solid state logic. Any logic gate, buffered or not, capable of delivering the required cur rent and voltage within its max i mum power dissipation rating can be used to control an SSR or I/O module

8. State the difference between conventional relay with numerical relay. (N/D – 2016)

Electro Mechanical Relay • Memory of previous Fault records, Actual Value of Fault Current, etc. are not possible to store • Only one Mechanical Flag is shown for Relay Operation & Phase Indication. Electro-Mechanical Relays are non-communicable • These relays cannot be used for Sub Station Automation & SCADA system Numerical Relay •

Numerical Relay • Numerical Relays have the Memory to store the events & last 5 faults with Date & Time Stamping • This information is very important to diagnose Fault Pattern, by which R Numerical Relays are Communicable with all Standard Protocols • Numerical Relays can be used for Sub Station Automation & SCADA system effectively elay can be reset so that such fault possibility could be removed/eliminated •

16 MARKS

1. How will you synthesis a mho relay using static phase comparator? (N/D- 2016)

Phase comparison technique is the most widely used one for all practical directional, distance, differential and carrier relays.

If the two input signals are S1 and S2 the output occurs when the inputs have phase relationship lying within the specified limits.

Both the input must exist for an output to occur. The operation is independent of their magnitudes and is dependent only on their phase relationship. The figures below show that the phase comparator is simple form. The function is defined by the boundary of marginal operation and represented by the straight lines from the origin of the S-plane.

The condition of operation is β1 < θ < β2.

θ is the angle by which S2 lands S1. If β1 = β2 =90o, the comparator is called cosine comparator and if β1=0 and β2=180o, it is a sine comparator.

In short, a phase comparator compares two input quantities in phase angle (vertically) irrespective of the magnitude and operates if the phase angle between them is < 90o.

There are two types of phase comparators:

1. Vector product comparator

2. Coincidence type phase comparator.

Vector Product comparator

This comparator recognizes the vector product or division between the two or more quantities. Thus, the output is A, B or A/B

Coincidence Comparator

Consider two signals S1 and S2. The period of Coincidence of S1and S2 will depend on the phase difference between S1 and S2. The fig below shows the coincidence of S1 and S2 when S2 lags S1 by less than π/2 ie., θ.

The period of coincidence of S1 and S2 with a phase difference of θ is Ψ = 180o – θ. Different techniques are used to measure the period of coincidence. Two of the important types are

1. Bloke Spike Method (Direct Phase Comparison) and

2. Coincidence type – Integrating phase comparator

2. Explain the numerical over current protection and numerical transformer differential protection. (N/D- 2016)

A microchip Relay is a computerized electronic relay, which determines its qualities by method for a pre-programmed algorithm and counts (calculations), in light of the chose settings and the deliberate current and/or voltage signals.

BASIC PRINCIPLE: In computerized transfers Digital Signals are utilized for information preparing rather than Analog signs. Simple signs are Continuous Signals and can't be prepared effectively as a result of their few constraints when contrasted with advanced signs. Computerized signs are in type of coded square heartbeats which speaks to discrete components of data. In computerized framework, the signs are in double shape just two discrete qualities alluded to as paired coefficients 0 and 1 or consistent values genuine and false. The quantity of paired digits expected to encode the different discrete components of data affects the outline of an advanced framework. The advanced framework by and large works on gatherings of 8 to 32 bits of data without a moment's delay. The scope of the computerized arrangement of encoding the data by a n bit gathering is 2 n. Thus advanced frameworks with bigger piece working gathering can prepare a more extensive scope of encoded data. The data to be handled may be literary, numerical and legitimate

Current transformer: Current transformer gives output at its secondary terminal from the load side. Current to voltage converter: Convert current input to analog voltage input to microprocessor. Microprocessor: take input analog voltage does sampling, calculate Root Mean square value and compare with given algorithm if condition is fulfilled then output signal to trip (Breaker or solid state switch)

3. Differential Protection

Differential protection may be considered the first line of protection for internal phase-to-phase or phase-to-ground faults.

Summation method with six CTs:

If six CTs are used in a summing configuration, during motor starting, the values from the two CTs on each phase may not be equal as the CTs are not perfectly identical and asymmetrical currents may cause the CTs on each phase to have different outputs.

The running differential delay can then be fine tuned to an application such that it responds very fast and is sensitive to low differential current levels.

What is:-Aground fault is a fault that creates a path for current to flow from one of the phases directly to the neutral through the earth bypassing the load

Ground faults in a motor occur:

When its phase conductor’s insulation is damaged for example due to voltage stress, moisture or internal fault occurs between the conductor and ground

To limit :-thelevel of the ground fault current connect an resistance known as stablising resistance Ground Fault Protection

4. Explain the current carrier protection scheme. Describe the carrier phase comparison relay with neat sketches. (N/D 2015)

For long overhead lines the power line itself may be used as the interconnecting channel between the terminal equipments. Carrier-current protection is the most widely used scheme for the protection of Extra High Voltage (EHV) and Ultra High Voltage (UHV) power lines. The carrier signal is directly coupled to the power line itself which is to be protected. Carrier-current protection is faster and superior to distance protection schemes and is more reliable when used for long transmission lines, although the terminal equipments are more expensive and complicated. In addition to protection the carrier signals can also be used for communication, supervisory control and telemetering.

In carrier-current protection or any other unit protection, the circuit breakers at both the ends of the line trip simultaneously when a fault occurs at one of the ends of the protected line sections. This helps in improving the stability. The carrier signals can be used either to initiate or to prevent the tripping of a protective relay according to which they are classified. When a carrier signal is used to initiate tripping of relay, the scheme is known as carrier inter-tripping, or transfer tripping or permissive tripping scheme. The scheme is known as carrier-blocking scheme when the carrier signals are used to prevent the operation of a relay.

Different operating techniques used in carrier-current protection

The two operating techniques mainly used in carrier-current protection are:

1. Phase comparison technique, and

2. Directional comparison technique.

In phase comparison technique, the phase angle of the current entering at one end is compared with the phase angle of the current leaving the other end of the protected section. During normal operating conditions or in case of an external fault, the currents at both the ends of the protected line are in phase. In case of an internal fault i.e. fault in the protected section, the currents at the two ends will be 180o out of phase.

The direction of power flow at the two ends of the protected sections is compared in the directional comparison technique. During normal conditions or external faults, the power flows into the protected section at one end and leaves at the other end. During internal faults, the direction of power flow is inwards at both the ends.

The signals generated in a carrier-current protection scheme are at a frequency between 50 and 500 kHz. Below 50 kHz the size and cost of the coupling equipments would be too high and above 500 kHz the line losses and therefore the signal attenuation would be too high on long lines. Carrier-current protection can be used only on overhead lines and cannot be used for underground cables as the capacitance of a cable would attenuate the carrier signals appreciably.

5. with neat diagram explain the construction and operation of induction type directional over current relay. (N/D 2015)

Under abnormal condition T = V I COSθ

under abnormal conditions voltage in the circuit is too low. Therefore the driving torque becomes abnormally too small .Hence the relay does not operate.ie., the directional power relay is not suitable for short circuit conditions. This problem can be overcome by directional over current relay.

Directional overcurrentrelay makes use of two relaysi) directional power relay ( directional element)ii) Non directional current relay (non-directional element)Construction:

1) Directional element :It is similar in construction to directional power relay.




The trip contacts 1 & 2 are connected in series with secondary winding of lower magnet. therefore for the relay to operate, at first directional element should be activated first.

UNIT-V

TWO MARKS

1. Write the difference between the fuse and circuit breaker. (M/J-2014)

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.

2. Define the operating time of circuit breaker. (M/J-2014)

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.

3. Differentiate between a surge diverter and surge absorber. (A/M -2010)

Surge Absorber: It is a protective device used to reduce the steepness of the wave front of a surge and absorbs energy contained in the travelling wave.

Surge Diverter: It will divert excess voltages from an electrical surge to earth. It measures the volts coming in and once it gets above a set amount (normally 260 volts), will divert the excess volts to earth. An Electrical Surge Diverter is a great way to ensure adequate lightning protection for your valuable electronic equipment. Unlike the more common Surge Protector Power boards that simply switch off if there is spike in volts, a Surge Diverter will just divert the excess volts away. It is also installed on your main switchboard, thereby protecting all power points.

4. how does a circuit breaker differ from a switch? (N/D-2015)

5. Name the materials used for contacts of vaccum circuit breaker. (N/D-2015)

Cu/Cr is the most ideal material to make VCB contacts. Vacuum interrupter technology was first introduced in the year of 1960. The moving contact is connected to the control mechanism by stainless steel bellow. The arc shields are supported o the insulating housing such that they cover on these shields and is prevented from condensing on the insulating enclosure. The possibility of a leak is eliminated due to permanent sealing of vacuum chamber for that a glass vessel or ceramic vessel is used as the outer insulating body.

6. what is the difference between re-striking voltage and recovery voltage? (N/D – 2016)

Recovery voltage is the voltage at the terminal of CB, after the circuit interruption.Restriking voltage is what appears across the contacts at current zero during arc period & will have higher frequency.

7.state the difference between DC and AC circuit breaking. (N/D – 2016)

DC circuit breakers and AC breaker main difference is the ability to arc. Because the exchange of each cycle, have had zero, zero easy to extinction in the past, but has not been zero DC switching, arc extinguishing ability is poor, so to add additional interrupter device. DC arc is generally difficult, but the exchange has zero, breaking easily. Exchange can be derived for the DC circuit breaker protection, attention to three changes: 1, overload and short circuit protection.

16 MARKS

1. with a help of neat block diagram, explain the construction, operating principle and

Advantages of SF6 circuit breaker. (M/J-2014), (N/D 2015), (N/D 2016)

· It contains an arc interruption chamber containing SF6gas.

· In closed position the contacts remain surrounded bySF6gas at a pressure of 2.8 kg/cm2.

· During opening high pressure SF6 gas at 14 kg/cm2from its reservoir flows towards the chamber by valve mechanism.

· SF6rapidly absorbs the free electrons in the arc path to form immobile negative ions to build up high dielectric strength.

· It also cools the arc and extinguishes it.

· After operation the valve is closed by the action of a set of springs.

· Absorbent materials are used to absorb the byproducts and moisture.

Advantages:

· Very short arcing period due to superior arc quenching property ofSF6.

· Can interrupt much larger currents as compared to other breakers.

· No risk of fire.

· Low maintenance, light foundation.

· No over voltage problem.

· There are no carbon deposits.

· SF6breakers are costly due to high cost ofSF6.

· SF6gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.

2. Briefly describe the different types of testing schemes of circuit breakers. (M/J-2014)

There are mainly two tests classified:

1)Typetest

2) Routine Test

1) Type Tests:

The purpose of type tests is to prove design features and the quality of circuit breaker. Type tests are not conducted on each circuit breaker. This is done to prove the capabilities and to confirm the rated characteristics of the circuit breakers.

2)Routine Tests:

Routine test is performed before circuit breaker dispatch to ensure the product. This gives result about defects in materials and construction of circuit breaker. We can check quality of material of circuit breaker by performing Routine Test.

3. Discuss the various methods of arc quenching in circuit breakers. (10) (A/M -2010)

1. Lengthening the Arc:

2. Splitting the arc:

In this method the arc is elongated and splitted by arc splitters. These are made with plates of resin bonded fiber gas. These are placed perpendicular to arc and arc is pulled into them by electromagnetic forces.

3. Cooling of arc

It causes recombination of ionized particles. Cooling remove the heat from the Arc. Efficient cooling may be obtained by gas blast directed along Arc .

Low resistance (or) current zero interruption:

This method is used for Arc Extinction in A.C circuit breakers. In this method the resistance kept low until current is zero.

4. For a 132KV system, the reactance up to the fault location and bushing capacitance are 3 ohms and 0.015µF respectively. Calculate the maximum value of RRRV. (8) (A/M -2010)

5. In a 132kV systems, the reactance and capacitance up to the location of the circuit breaker is 5O and 0.02µF respectively. A resistance of 500 O is connected across the contacts of the circuit breaker. Determine (A/M -2010)

(i) natural frequency of oscillations (4)

(ii) frequency of damped oscillations (4)

(iii) critical value of resistance (4)

(iv) the value of resistance which will give frequency of damped oscillations which is equal to 1/4th the natural frequency (4)

(ii)

(iii)critical resistance :

6. Describe the construction and the operation of metal oxide surge arrester?(10)

(A/M -2010)

Introduction: The voltage wave having magnitude more than its normal value and which remains

for a very short duration are called overvoltage surges or transmit over voltages. For any

electrical equipment, its insulation requirements are decided by these transient over voltages. The

over voltages in the system occur due to various reasons such as lightning surges, switching

surges, faults and travelling waves. There is high rate of rise and high peak value in transient

over voltages which are dangerous for the Insulation and hence protection is rewired against

these over voltages.

ii) What are its advantages over conventional arrester and its drawbacks? (6) (A/M -2010)

To protect the generators from surge voltages, the surge arresters and surge capacitors are often used. At the time of re striking across the contacts of circuit breakers, the transient over voltages get generated such surges are called switching surges and can be limited by the uses of modem

circuit breakers RC surge suppressors also help in reducing switching surges. Another situation,

when the transient over voltages are generated, is when the arcs are pounded. During arcing

grounds, the transient voltages having amplitudes five times more than the normal line to neutral

peak amplitude are generated Such transient voltages are dangerous and can be reduced by using

resistance earthing.

7. Interruption of capacitive current (N/D- 2016) (M/J 2014)

The interruption of capacitive current produces high voltage transients across the gap of the circuit breaker.

At the instant M

The capacitive current is 0.

System voltage is maximum

If interruption occurs Capacitor CLremains charged at the maximum value of system voltage. After the instant M Voltage across the breaker gap is the difference of Vcand VcL.

At the instant N

The voltage across the gap is twice the maximum

Value of Vc.

If the breaker restrikes The voltage across the gap become partially zero.

Voltage falls from 2Vcmaxto zero.

A severe high frequency oscillation occurs (about the point S)

Interrupted again.( if restriking current=0)

The capacitor CL at the voltage -3emax.

At the instant P

The system voltage reaches its positive maximum.(point T)

Voltage across the gap becomes 4emax.

The capacitive current reaches zero again and there may be an interruption.

The transient voltage oscillates between -3emaxand +5emax. (point P—Q)

8. Derive the expression for restriking voltage and maximum RRRV. (N/D- 2016), Derive an expression for restriking voltage and rate of rise of restriking voltage (RRRV) in terms of system voltage, inductance up to the fault location and bushing to earth capacitance of the circuit breaker. (8) (A/M -2010)

When the circuit breaker is closed, the short circuit current flows through R, L and the contacts of the circuit breaker, the capacitance C being short-circuited by the fault.

When the circuit breaker contacts are opened and the arc is extinguished, the current is diverted through the capacitance C, resulting in a transient condition.

The voltage across the capacitance which is the voltage across the contacts of the circuit breaker can be calculated in terms of L, C, fn and system voltage. The mathematical expression for transient condition (neglecting resistance) is as follows.

9. Explain current chopping phenomenon. (6) (A/M -2010), (M/J 2014)

In power systems capacitor banks are used in the network which supplies reactive power at

leading power factors there are various aspects like long transmission where it is required interrupt the capacitive current which is difficult. To understand this difficulty let us consider a

simple circuit shown in the Fig. The value of load capacitance CL is greater than C. The voltage across a capacitor cannot change instantaneously. The currents supplied to the capacitor are generally small and interruption of such currents take place at first current zero. Also at the beginning, the rate of rise of recovery voltage is low and increases slowly. Whenever such circuit is opened a charge is trapped in the capacitance Ct The voltage across the load capacitance will hold the same value when circuit was opened. This voltage is making but peak of supply voltage as power factor angle is nearly 90° leading. After opening the circuit the voltage Vc across the capacitance C oscillates and approaches a new steady value. But due to small value of capacitance C. the value attained is close to the supply voltage. The recovery voltage Cr is nothing but difference between and CL. Its initial value is zero as the circuit breaker will be closed and increases slowly in the beginning. When Vc reverses after half cycle, the recovery voltage is about twice the normal peak value. Therefore it is possible that at this instant arc may restrike as the electrical strength between the circuit breaker contacts is not sufficient. The circuit will be reclosed and et oscillates at a high frequency. The supply voltage at this instant will be at its negative peak; therefore a high frequency oscillation takes place. At the instant of rest rucking the arc, the recovery voltage V, is zero. The voltage across the load capacitance reaches - times the peak value of normal supply voltage. The recovery voltage then starts increasing. If again restriking of arc takes place, a high frequency of oscillation of CL takes place. Such several repetitions of the restriking cycle will increase the voltage across load capacitance to a dangerously high value. In practice this voltage is limited to 4 times the normal peak of the voltage.

10. Explain the physics of arc phenomena and interruption.

By high voltage gradient at the cathode resulting into field emission.

By increase of temperature resulting into thermo ionic emission

· High temperature of the medium around the contacts caused by high current densities, with high temp the kinetic energy gained by moving electrons increased.

· The field strength or volt. gradient which increases the kinetic energy of the moving electrons and increases the chances of detaching electrons from neutral molecule.

· An increase in mean free path-the distance through which the electron moves freely.

· Energy Balance or Cassie Theory

· Thistheorystatesthatiftherateofheatdissipationbetweenthecontactsisgreaterthentherateatwhichheatisgenerated, the arc will be extinguished, otherwise it will restrike.

· Recovery rate or Slepian’s Theory

· Thistheorystatesthatiftherateatwhichtheionsandelectronscombinetoformorreplacedbyneutralmolecules.

11. Describe the operating principle of DC circuit breaker. (M/J-2014)

D.C. Circuit Breaking The breaking in case of d.c. can be explained as follows. For this, we will consider a circuit which will consist of generator with voltage E, resistance R. inductor L and the circuit breaker as shown in the Fig.

The voltage-current relationship can be represented as shown in the graph it could be seen that curve AB represents the voltage E - iR, i is nothing but current at any instant. The curve XY represents the voltage-current characteristics of the arc for decreasing currents.

12. Discuss in detail the different types of rating of circuit breaker bringing out clearly their physical significance (N/D 2015)

1. Air Circuit Breaker

This circuit breaker will operate in the air; the quenching medium is an Arc at atmospheric pressure. In many of the countries air circuit breaker is replaced by oil circuit breaker. About oil circuit breaker we will discuss later in the article. Thus the importance of ACB is still preferable choice to use an Air circuit breaker up to 15KV. This is because; oil circuit breaker may catch fire when used at 15V

The two types of air circuit breakers are

· Plain air circuit breaker

· Air blast Circuit Breaker

Plain Air Circuit Breaker

Plain air circuit breaker is also called as Cross-Blast Circuit Breaker. In this, the circuit breaker is fitted with a chamber which basically surrounds the contacts. This chamber is known as arc chute.

Advantages of Air-Blast Circuit Beaker

· It is used where frequent operation is required because of lesser arc energy.

· It is risk free from fire.

· Small in size.

· It requires less maintenance.

· Arc quenching is much faster

· Speed of circuit breaker is much higher.

· The time duration of the arc is same for all values of current.

Disadvantages of Air-Blast Circuit Breaker

· It requires additional maintenance.

· The air has relatively lower arc extinguishing properties

· It contains hi

1. · web viewfailure of cts or pts operation secondary protection back-up protection is very...

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