CONTENTS

Chapter No. Topic Page No.Chapter 1

1.01.11.21.31.41.51.61.71.8

BASIC ASPECTS OF PROTECTIONPrinciples of RelaysSome Terms Associated with Protective RelayingFunctions of Protective RelayingThe Requirements of Protective Relaying Classification of RelaysOperating Principles of different types of RelaysTesting and Maintenance of Protective Relays Test Equipment Static Relying Concepts

Chapter 22.12.2

PROTECTIVE RELAYSIntroductionCharacteristic Curve

Chapter 33.13.23.33.43.53.63.7

MOTOR PROTECTIONOverload ProtectionSingle Phasing Protection or Unbalance ProtectionShort Circuit ProtectionStalling Protection (Lock Rotor Protection)Differential ProtectionEarth ProtectionUndervoltage Protection

Chapter 44.14.24.34.44.54.64.74.84.9

4.10

TRANSFORMER PROTECTIONSTransformer ProtectionsProtection against Internal FaultsPrinciples of Protection SystemGas DetectionOver Heating Protection Over Current & Earth Leakage Protection Percentage Bias Differential ProtectionOver Voltage ProtectionOver Fluxing ProtectionOver Differential Protection

CHAPTER 55.15.25.35.45.55.65.75.85.9

5.105.115.125.135.14

GENERATOR PROTECTIONIntroductionStator Earth FaultsRotor Earth Fault ProtectionGenerator Interturn Fault ProtectionGenerator Negative Phase Sequence Current ProtectionGenerator Loss of Excitation Protection Generator Minimum Impedance Protection Generator Differential ProtectionGenerator Overall Differential ProtectionGenerator Reverse Power ProtectionGenerator Over Frequency ProtectionGenerator Under Frequency protectionGenerator Thermal Overload ProtectionGenerator Overvoltage Protection

Chapter 6

6.16.26.36.46.5

BUS ZONE-PROTECTION AND LOCAL BREAKER

BACKUP PROTECTIONIntroductionBus Bar Protection RequirementsTypes of Busbar ProtectionLow Impedance Scheme (Biased)Local Breaker Back-up (LBB) Protection

Chapter 77.17.27.3

DISTRIBUTION FEEDER PROTECTIONIntroductionUnit ProtectionIDMT Overcurrent & Earth Fault Protection

Chapter 88.18.28.38.48.58.6

LINE PROTECTION (DISTANCE SCHEMES)IntroductionMeasuring CharacteristicsZones of ProtectionPhase Sequence Comparator for MHO CharacteristicAdditional features of Distance RelaysCarrier Aided Schemes

Chapter 99.19.29.3

CURRENT AND VOLTAGE TRANSFORMERIntroductionCurrent TransformersVoltages Transformers

Chapter 1010.110.210.3

DIGITAL RELAYINGIntroductionPC Based Schemes for Testing Protective RelaysTesting of a Distance Relay

CHAPTER 1

BASIC ASPECTS OF PROTECTION

1.0 Principles of RelaysEvery electrical equipment is designed to work under specified normal conditions. In case of short circuits, earth faults etc., an excessive current will flow through the windings of the connected equipment and cause abnormal temperature rise, which will damage the winding. In a power station, non-availability of an auxiliary, at times, may cause total shut down of the unit, which will result in heavy loss of revenue.

So, in modern power system, to minimise damage to equipment two alternatives are open to the designer, one is to design the system so that the faults cannot occur and other is to accept the possibility of faults and take steps to guard against the effect f these faults. Although it is possible to eliminate faults to a larger degree, by careful system design, careful insulation coordination, efficient operation and maintenance, it is obviously not possible to ensure cent percent reliability and therefore possibility of faults must be accepted; and the equipment are to be protected against the faults. To protect the equipment, it is necessary to detect the fault condition, so that the equipment can be isolated from the fault without any damage. This function is performed by a relay. In other words, protective relays are devices that detect abnormal conditions in electrical circuits by constantly measuring the electrical quantities, which are different under normal and faulty conditions. The basic quantities which may change during faulty conditions are voltage, current, frequency, phase angle etc. Having detected the fault relay operates to complete the trip circuit which results in the opening of the circuit breaker thereby isolating the equipment from the fault. The basic relay circuit can be seen in Fig. No. 1.1

FIG. 1.1

1.1 Some Terms Associated with Protective RelayingCircuit Breaker: It is an On-load switch, used to make or break an electrical circuit when it is carrying current.

Current Transformer: These are used for measuring protection purpose since it is not possible to measure very high currents directly, it will be

stepped down means of a current transformer to about 5A or 1A and the secondary current will be measured and monitored.

Voltage Transformer: These are also used for measuring purpose and protective relaying purpose. Since it is not practicable to measure and monitor high and extra high voltages they are stepped down to 110V and the secondary voltage is measured and monitored.

Relay time: It is the interval between the occurrence of the fault and closure of relay contact.

Pick Up: The operation of relay is called relay pick up. Pick up value or the level is the value of operating quantity at which the relay operates.

Back up Protection: A protective system intended to supplement the main protection in case the latter should be ineffective, or to deal with faults in those parts of the power system that are not readily included in the operating zones of the main protection.

Protected Zone: It is the portion of a power system protected by a given protective system.

Protective Gear: These are the apparatus, including protective relays, current/voltage transformers and ancillary equipment for use in a protective system.

Protective Relay: A relay is designed to initiate disconnection of a part of an electrical installation or to operate a warning signal, in case of a fault or other abnormal condition in the installation. A protective r3elay may include more than one unit electrical relay and accessories.

Rating: It is the nominal value of an energizing quantity which appears in the designation of a relay. The nominal value usually corresponds to the CT & VT secondary rating.

Resetting Value: It is value of the characteristic quantity at which the relay returns to its initial position.

Unrestricted Protection: It is a protection system which has no clearly defined zone of operation and which achieves selective operation only by time grading.

Basic Symbols: The equipments they represent are as given below:Sr. No.

Symbol Equipment Function

1. Circuit Breaker

Switching during normal and abnormal conditions, interrupt the fault currents.

2. Isolator Disconnecting a part of the system from live parts under no lad conditions.

3. Earth switch Discharging the voltage on the lines to the earth after disconnection.

4. LightingArrestor

Diverting the high voltage surges to earth.

5. Current Transformer

Stepping down the current for measurement, protection, and control.

6. VoltageTransformer

Stopping down the voltage for the purpose of protection, measurement and control.

1.2 Functions of protective Relaying To sound an alarm, so that the operator may take some corrective action

and/ or to close the trip circuit of circuit breaker so as to disconnect a component during an abnormal fault condition such as overload, under voltage, temperature rise etc.

To disconnect the faulty parts as quickly as possible so as to minimise the damage to the faulty art. Ex: If a generator is disconnected immediately after a winding fault only a few coils need replacement. If the fault is sustained, it may be beyond repairable condition.

To localize the effect of fault by disconnecting the faulty part from the healthy part, causing least disturbance to the healthy system.

To disconnect the faulty part as quickly as possible to improve the system stability and service continuity.

1.3 The requirements of protective relaying Speed: Protective relaying should disconnect a faulty element as quickly

as possible, in order to improve power system stability, decrease the amount of damage and to increase the possibility of development of one type of fault into other type. Modern high speed protective relaying has an operating time of about 1 cycle.

Selectivity: It is the ability of the protective system to determine the point at which the fault occurred and select the nearest of the circuit breakers, tripping of which leads to clearing of fault with minimum or no damage to the system.

Sensitivity: It is capability of the relaying to operate reliably under the actual minimum fault condition. It is desirable to have the protection as sensitive as possible in order that it shall operate for low value of actuating quantity.

Reliability: Protective relaying should function correctly at all times under any kind of fault and abnormal conditions of the power system for which it has been designed. It should also not operate on healthy conditions of system.

Simplicity: The relay should be as simple in construction as possible. As a rule, the simple the protective scheme, less the number of relays, and contacts it contains, the greater will be the reliability.

Economy: Cost of the protective system will increase directly with the degree of protection required. Depending on the situation a designer should strike a balance between with the degree of protection required and economy.

1.4 Classification of Relays1.4.1 Depending upon their principle of operation they are classified as:

Electromagnetic attraction type relays: These relays operate by the virtue of a plunger being drawn into a solenoid or an armature being attracted towards the poles of an electromagnet.

Induction type Relays: In his type of relay, a metal disc or cup is allowed to rotate or move between two electro-magnets. The fields produced by the two magnets are displaced in space and phase. The torque is developed by interaction of the flux of one of the magnets and the eddy current induced into the disc/up by the other.

Thermal Relay: They operate due to the action of heat generated by the passage of current through the relay element. The strip consists of two metals having different coefficients of expansions and firmly fixed together throughout the length so that different rates of thermal expansion of two layers of metal cause the strip to bend when current is passed through it.

Static Relays: It employs discrete electronic components like diodes, transistors, zenners, resistors/capacitors or Integrated circuits and use electronic measuring circuits like level detectors, comparators, integraters etc. to obtain the required operating characteristics.

Moving Coil Relays: In this relay a coil is free to rotate in magnetic field of a permanent magnet. The actuating current flows through the

FIG. 1.3

coil. The torque is produced by the interaction between the field of the permanent magnet and the field of the coil.

1.4.2 Relays can be classified depending upon their application also. Over voltage, over current and over over power relays, in which operation

takes place when the voltage, current or power rises above a specified value.

Under voltage, under current under frequencies low power relays, in which operation takes place when the voltage, current frequency or power fall below a specified value.

Directional or reverse power relays: In which operation occurs when the direction of the applied power changes.

Distance Relays: In this type, the relay operates when the ratio of the voltage and current change beyond a specified limit.

Differential Relays: Operation takes place at some specific phase or magnitude difference between two or more electrical quantities.

1.4.3 Relays can also be classified according to their time of operation Instantaneous Relay: In which operation takes place after negligibly

small interval of time from the incidence of the current or other quantity causing operation.

Definite time lag Relay: This operates after a set time lag, during which the threshold quantity of the parameter is maintained.

Inverse time lag Relay: This operates after a set time Lab, during which the operating quantity of the parameter is maintained above its operating threshold.

1.5 Operating Principles of different types of Relays:1.5.1 Introduction over current and earth fault relays:

These are quite commonly used relays. Schematic diagram of induction disc type relay is shown in Fig. No. 1.2

The output of the current transformer is fed to a winding (1) on the center limb of the E-shaped core, the second winding (2) on the limb

FIG. 1.4

FIG. 1.5

is connected to two windings on the poles of the E and U shaped cores. The magnetic flux across he air gap induce currents in the disc, which in conjunction with the flux produced by the lower magnet, produces a rotational torque. A magnet (3), is used to control the speed of the disc. The time of operation of the relay varies inversely with the current fed into it by the current transformer of the protected circuit. The permanent magnet used for breaking has a tendency to attract iron filings, which can

prevent operation. So care has to be taken while manufacturing this type of relays. Time-current characteristics induction type relays has been given in Fig. 1.3.

1.5.2 Balanced Beam Relays:It consists of a horizontal beam pivoted centrally, with one armature attached to either side. There are two coils one on each side. The current in one coil gives operating torque. The beam is given a slight mechanical bias by means of a spring so that under normal conditions trip contacts will not make and the beam remains in horizontal position. When the operating torque increases then the beam tilts and closes the trip contacts. In current balance system both coils are energised by current derived from CTs. In impedance relays, one coil is energized by current and other by voltage. In these relays the force is proportional to the square of the current, so it is very difficult to design the relay. This type of relay is fast and instantaneous. In modern relays electromagnets are used in place of coils (See Fig. 1.4.).

1.5.3 Permanent Magnet Moving Coil Relays:There are two general types of moving coil relays. One type is similar to that of a moving coil indicating instrument, employing a coil rotating between the poles of a permanent magnet. The other is, employing a coil moving at right angles to the plane of the poles of a permanent magnet. Only direct current measurement is possible with both the types.

FIG. 1.6

FIG. 1.7

The action of a rotating coil type is shown in the Fig. 1.5. A light rectangular coil is pivoted so that its sides lie in the gap between the two poles of a permanent magnet and a soft iron core. The passage of current through the coil produces a deflecting torque by the reaction between the permanent magnetic field and the field of the coil (See Fig. 1.5)

The moving contact is carried on an arm which is attached to the moving coil assembly. A phosper bronze spiral spring provides the resetting torque. Increasing the contact gap and thus increasing the tension of the spring permits variation in the quantity required to close the contacts.

Time current characteristic of a typical moving coil permanent magnetic relays is shown in Fig. 1.6.

1.5.4 Attracted armature relays:It is required to clear the faults in power system as early as possible. Hence, high-speed relay operation is essential. Attracted armature relays heave a coil or an electromagnet energised by a coil. The coil is energised by the operating quantity which may be proportional to circuit current or voltage. A plunger or a rotating vane is subjected to the action of magnetic field produced by the operating quantity. It is basically single actuating quantity relay.

Attracted armature relays respond to both AC and DC quantities. They are very fast in operation. Their operating time will not vary much with the amount of current. Operating time of the relay is as low as 10-15 m seconds and resetting time is as low as 30 m sec can be obtained in these relays. These relays are non-directional and are simple type of relays. Examples of attracted armature type relays are given in Fig. 1.7.

1.5.5 Time Lag Relays:These are commonly used in protection schemes as a means of time discrimination. They are also frequently used in control, delayed auto-reclosing and alarm scheme to allow time for the required sequence of operations to take place, and to measure the duration of the initial condition to ensure that it is not merely transient.

Various methods are used to obtain a time lag between the initiation of the relay and the operation of its contact mechanism. These includes gearing, permanent magnet damping, friction, thermal means or R.C. circuits. In some cases the time lag in operation of the contacts is achieved by a separate mechanism released by a voltage operated elements. The release mechanism may be an attracted armature or solenoid and plunger. The operating time of such relay is independent of the voltage applied to the relay coil. One of the simplest forms of time lag relay is provided by a mercury switch in which the flow of mercury is impeded by a constriction in the mercury bulb. The switch is tilted by a simple attracted armature mechanism. The time setting of such a relay is fixed by the design of the tube. Another method of obtaining short time delays is to delay operation

of a normally instantaneous relay by means of a device which delays the build up or decay of the flux in the operating magnet. The device consists of a copper ring (slug) around the magnet and can produce delay on pickup as well as delay on reset.

1.6 Testing and Maintenance of Protective Relays:Unlike other equipment, the protective relays remain without any operation until a fault develops. However, for a reliable service and to ensure that the relay is always vigilant, proper maintenance is a must. Lack of proper maintenance may lead to failure to operate.

It is possible for dirt and dust created by operating conditions in the plant to get accumulated around the moving parts of the relay and prevent it from operating. To avoid this, relays are to be cleaned periodically.

In general, overload relays sense over load by means f thermal element. Loose electrical connections can cause extra heat and may result in false operation of the relay. To avoid this, all the relay connections are to be tightened every now and then.

To confirm that the relay operation at the particular setting under particular conditions for which the relay is meant for operating, we should perform number of tests on the relays. Quality control is given foremost consideration in manufacturing of relay. Tests can be grouped into following five classes:1. Acceptance test2. Commissioning test3. Maintenance tests4. Repair tests5. Manufacturers tests

1.6.1 Acceptance testAcceptance tests are generally performed in presence of the customer in the laboratory or by the customer himself. These tests fall into two categories:1. Type tests such as High frequency disturbance, Impulse voltage test,

Fast transient test etc. on new relays. These tests are carried out to

prove the design and are not recommended for normal production relays.

2. Routine Tests like operating value check, operating characteristic on Relays which were used earlier and of proven design, requiring only minimum necessary checks.

After receiving the relays package, it should be visually examined for the damage in the transit. The following precautions are to be taken while removing the relay Care to be taken not to bend the light parts Avoid handling contact surface

Operating movement (disc, armature etc.) is to be checked for free movement after removing the packing pieces.

Do not take steel screw drivers near the permanent magnet.

1.6.2 Commissioning Tests:These are the field tests to prove the performance of the relay circuit in actual service. These are repeated till correct operations are obtained. These are performed by simulated tests with the secondary circuits energised from a portable test source; or simulated tests using primary load current or operating tests with primary energised at reduced voltage. The following steps are involved in commissioning tests.

Checking wiring on the basis of the circuit diagram

Checking C.T. polarity connections Measuring insulation resistance of circuits.

Checking C.T. Ratios Checking P.T. ratio, polarity and phasing

Conducting secondary injection test on relays. Conducting primary injection test

Checking tripping and alarm circuits. Stability check for balanced protections like differential/REF.

1.6.3 Maintenance TestsMaintenance tests are done in field periodically. The performance of a relay is ensured by better maintenance. Basic requirements of sensitivity, selectivity, reliability and stability can be satisfied only if the maintenance is proper.

The relay does not deteriorate by normal use; but other adverse conditions cause the deterioration. Continuous vibrations can damage the pivots or beatings. Insulation strength is reduced because of absorption of moisture; polluted atmosphere affects the relay contacts, rotating systems etc., Relays room, therefore, be maintained dust proof. Insects may cause mal-operation of the relay. Relay maintenance generally consists of:

a) Inspection of contactsb) Foreign matter removalc) Checking adjustmentsd) Checking of breaker operation by manual contact

closing of relays.e) Cleaning of cover etc.

1.6.4 Maintenance Schedule:1. Continuous Supervision: Trip circuit supervision, pilot supervision,

relay, auxiliary voltage supervision, Battery supervision, CT circuit supervision.

2. Relay flags are to be checked and resetted in every shift.3. Carrier current protection testing is to be carried out once in a week.4. Six monthly inspections: Tripping tests, insulation resistance tests, etc.

Secondary injection tests are to be carried out at least once in a year.

The following tests are to be performed during routine maintenance:

Inspection: Before the relay cover is removed, a visual check of he cover is necessary. Excessive dust, dirt, metallic material deposited on the cover should be removed. Removing such material will prevent it from entering the relay when the cover is removed. Fogging of the cover glass should be noted and removed when the cover has been removed. Such fogging is due to volatile material being driven out of coils and insulating materials. However, if the fogging is excessive, cause is to be investigated. Since most of the relays are designed to operate at 40oC, a check of the ambient temperature is advisable. The voltage and current carried by the relay are to be checked with that of the name plate details.

1.6.5 Mechanical adjustments and Inspection:The relay connections are to be tight, otherwise it may cause overheating at the connections. It will cause relay vibrations also. All gaskets should be free from foreign matter. If any foreign matter is found gaskets are to be checked and replaced if required.

Contact gaps and pressure are to be measured and compared with the previous readings. Large variation in these measurements will indicate excessive wear, in which case worn contacts are to be replaced. Contacts alignment is to be checked for proper operation.

1.6.6 Electrical Tests and AdjustmentsContact function: Manually close or open the contacts and observe that they perform their required function.

Pick up: Gradually apply actuating quantity (current or voltage) to see that pickup is within limits.Drop out or reset: Reduce the actuating quantity (current or voltage) until the relay drops out or fully resets. This test will indicate excess friction.

Repair tests involve recalibration, and are performed after major repairs. Manufacturers tests include development tests, type and routine tests.

1.7 Test Equipment1.7.1 Primary current injection test sets:

Generally protective gear is fed from a current transformer on the equipments to be protected and primary current injection testing checks all parts of the protection system by injecting the test current through the primary circuit. The primary injection tests can be carried out by means of primary injection test sets. The sets are comprising current supply unit, control unit and other accessories. The test set can give variable output current. The output current can be varied by means of built-in auto transformer. The primary injection test set is connected to AC single phase supply. The output is connected to primary circuit of CT. the primary current of CT can be varied by means of the test set. By using this test one can find at what value of current the relay is picking up and dropping out.

1.7.2 Secondary current injection test set:It checks the operation of the protective gear but does not check the overall system including the current transformer. Since it is a rare

FIG. 1.8

occasion for a fault to occur in CT, the secondary test is sufficient for most routine maintenance. The primary test is essential when commissioning a new installation, as it checks the entire system. A simple test circuit is given in Fig. 1.8.

1.7.3 Test Benches:Test benches comprise calibrated variable current and voltage supplies and timing devices. These benches can be conveniently used for testing relays and obtaining their characteristics.

1.8 Static Relaying Concepts1.8.1 Introduction

Static Relay is a relay in which the comparison or measurement of electrical quantities is done by stationary network which gives a tripping signal when the threshold value is crossed. In simple language static relay is one in which there are no moving parts except in the output device. The static relay includes electronic devices, the output circuits of which may be electric, semiconductor or even electromagnetic. But the output device does not perform relay measurement, it is essentially a tripping device. Static relay employs electronic circuits for the purpose of relaying. The entity voltage, current etc. is rectified and measured. When the output device is triggered, the current flows in the trip circuit of the circuit breaker.

With the inventions of semiconductors devices like diodes, transistors, thyristors, zener diodes etc., there has been a tremendous leap in the field of static relays. The development of integrated circuits has made an impact in static relays. The static relays and static protection has grown into a special branch.

1.8.2 Advantages of Static Relays:The static relays compared to the electromagnetic relays have many advantages and a few limitations.

1.8.3 Low Power ConsumptionStatic relays provide less burden on CTs and PTs as compared to conventional relays. In other words, the power consumption in the measuring circuits of static relays is generally much lower than that for the electromechanical versions. The consumption of one milli-VA is quite common in static over current relay whereas as equivalent

electromechanical relay can have consumption of about 2-3 VA. Reduced consumption has the following merits.

a) CTs and PTs of less ratings are sufficientb) The accuracy of CTs and PTs are increasedc) Air gaped CTs can be usedd) Problems arising out of CT saturation are avoidede) Overall reduction in cost

1.8.4 Operating timeThe static relays do not have moving parts in their measuring circuits, hence relay times of low values can be achieved. Such low relay times are impossible with conventional electromagnetic relays.

By using special circuits the resetting times and the overshoot time can be improved and also high value of drop off to pick up ratio can also be achieved.

1.8.5 CompactStatic relays are compact. The use of integrated circuit have further reduced their size. Complex protection schemes may be obtained by using logic circuits or matrix. Static relays can be designed with good repeat accuracies. Number of characteristics can be obtained in a single execution, unlike in case of their Electro-mechanical counter parts.

Most of the components in static relays including the auxiliary relays in the output stage are relatively immune to vibrations and shocks. The risk of unwanted tripping is therefore less with static relays as compared to electromagnetic relays. So, these can be applied in earthquake prone areas, ships, vehicles, aeroplanes etc.

1.8.6 TransducersSeveral non-electrical quantities can be converted into electrical quantities and then fed to static relays. Amplifiers are used wherever necessary.

1.8.7 LimitationsAuxiliary voltage requirement: This disadvantage is not of any importance as auxiliary voltage can be obtained from station battery supply and conveniently stepped down to suit load requirements.

Static relays are sensitive to voltage spikes or voltage transients. Special measures are taken to overcome this difficulty. These include use of surge supressors and filter circuits in relays, use of screened cables in input circuits, use of galvanically isolated auxiliary power supplies like d.c./d.c. converters, use of isolating transformers with grounded screens for C.T./P.T. input circuits etc.

1.8.8 Temperature Dependence of Static RelaysThe characteristic of semiconductors are influenced by ambient temperatures. For example, the amplification factor o a transistor, the forward voltage drop of a diode etc., changes with temperature variation. This was a serious limitation of static relays in the beginning. Accurate measurement of relay should not be affected by temperature variation. Relay should be accurate over a wide range of temperature. (-20oC to +50oC) this difficulty is overcome bya) Individual components in circuits are used in such a way that change in

characteristic of component does not affect the characteristic of the complete relay.

b) Temperature compensation is provided by thermistor circuits. Extra precaution for quality control of the components has to be taken. As the failure rate is highest in early period of components life, artificial ageing of the components is normally done by heat soaking.

FIG. 1.9

FIG. 1.10

1.8.9 Level DetectorsA relay operates when the measured quantity changes, either from its normal value or in relation to another quantity. The operating quantity in most protective relays is the current entering the protected circuit. The relay may operate on current level against a standard bias or restrain, or it may compare the current with another quantity of he circuit such as the bus voltage or the current leaving the protected circuit (Fig. 1.9).

In a simple electromagnetic relay used as level detector, gravity or a spring can provide the fixed bias or reference quantity, opposing the force produced by the operating current in electromagnet. In static relays the equivalent is a D.C. voltage bias.

E.g. In the semiconductor circuit (See Fig. 1.10) the transistor is reverse biased in normal conditions. No current flows through the relay coil under fault conditions capacitor will be charged to +ve potential at the base side. If this potential exceeds that of the emitter, the B-E junction will be forward biased and transistor will conduct there by tripping the relay. Thus the comparison is made against the D.C. fixed bias.

1.8.10ComparatorsIn order to detect a fault or abnormal conditions of he power system, electrical quantities or a group of electric quantities are compared in magnitude or phase angle and the relay operates in response to an abnormal relation of these quantities. The quantities to be compared are fed into a comparators as two or more inputs; in complex relays each input is the vectorial sum or difference of two currents or voltages of the protected circuit, which may be shifted in phase or changed in magnitude before being compared.

1.8.11Types of comparators;Basically there are two types of comparators, viz.

FIG. 1.11

FIG. 1.12

FIG. 1.13

FIG. 1.14

a) Amplitude comparators, and b) Phase comparator

The amplitude comparator compares the magnitudes of two inputs by rectifying them and opposing them. If the inputs are A and B, the output of the comparator is A-B and this is positive if A is greater than B i.e. if the ratio of A/B is greater than one. Theoretically, the comparison should be purely scalar, i.e. the phase relation of the inputs should have no effect on the output, but this is usually so if at least one input is completely smoothened as well as rectified.

The phase comparator achieves a similar operation with phase angle; its output is positive if arg A-arg B is positive i.e. if arg A/B is less than

where is the angle determining the shape of the characteristic; = 90 for a circular characteristic.

Both types of comparators can be arranged either for direct comparison (instantaneous) or to integrate their output over each half cycle.

Amplitude Comparators: Fig.1.11 shows how two currents can be compared in magnitude only, using rectifiers and, in fig. 1.16 two voltages are compared. In current comparator, the rectifiers providing a limiting action so that the relay can be made more sensitive, the voltage across the rectifier bridge remains substantially constant and hence the rectifiers and the sensitive relay are protected at high currents. In voltage comparator, the increase of resistance at low voltage makes the relay less sensitive at low voltage and the rectifiers are not protected at high currents.

1.8.12Circulating Current ComparatorOperation: Normally the restraining current flow in the winding of the polarized relay in the blocking direction. If the restraining current is small and operating current is zero the flow of resultant current will be as shown in Fig. 1.12.

FIG. 1.15

FIG. 1.16

The voltage across the restraining coil is V, across the relay serves as a bias in the forward direction of bridge 1. if the restraining current Ir is further increased, the voltage drop the relay will rise to a value Vt, the threshold voltage of bridge 1 and I will then conduct, then the current paths will be shown in Fig. 1.13. The current through the relay consists of fairly flat topped half waves as shown in Fig. 1.14.

The reverse is true if Io flows alone: The voltage drop across relay will now be V and this will bias the restraint rectifier in its forward direction. When the voltage drop across the relay attains a value Vt, corresponding to the threshold voltage of two rectifiers in the series, the surplus current from bridge 1 is spilled through bridge 2. This corresponds to the case of io is greater than ir in the Fig. 1.14.

When both bridges are energised simultaneously the relay is responsive to small differences between io and ir without requiring a sensitive output relay. The composite characteristic (ideal) for the relay is shown in Fig.1.15.

Opposed Voltage Comparator: In this voltage comparator the voltage drop in the resistance connected externally in the bridge circuits is compared. The current directions are shown in Fig. 1.16. If the two drops are equal no current will flow through the relay coil and the relay will not operate. If he two voltages are not equal then unequal currents will flow through the resistances and a current will flow through the relay coil in a direction determined by the largest voltage drop in the resistor. That is, if the drop in the resistance of the operating bridge is more than that of the restraining bridge then a current will flow in the operating direction through the relay. The reverse is true if the drop across the restraining resistance is more than the operating resistance.

Phase Comparators: There are two main types of static phase comparators:

FIG. 1.17

FIG. 1.18

a) Those whose output is a D.C. voltage proportional to the vector product of the two A.C. input quantities:

b) Those which give an output whose polarity depends upon the phase relation of the inputs. The later are sometimes called coincidence type and can be direct acting or integrating.

1.8.13Operating Principles of Static Time Current Relays:Fig. 1.17 shows the block diagram of a static time current relay. The auxiliary C.T. has taps on the primary for selecting the desire pickup and current range. Its rectified output is supplied to a fault detector and an RC timing circuit. When the voltage of the timing capacitor has reached the value for triggering the level detector, tripping occurs.

Operation of a typical static time current relay: The current from the main C.T. is first rectified and smoothed by capacitor Cs and then passed

through he tapped resistor Rs so that the voltage across it is proportional to the secondary current. The spike filter RC protects the rectifier bridge against transient over voltages in the incoming current signal, Fig. 1.18.

1.8.14Timing CircuitThe rectified voltage across the Rs charges the capacitor Ct through resistor Rt. When the capacitor voltage exceeds the base emitter voltage Vt the transistor T2 in the Fig. 1.20 becomes conductive, triggering transistor T3 and operating the tripping relay.

Resetting circuit: In order that the relay shall have an instantaneous reset, the capacitor Ct must be discharged as quickly as possible. This is achieved by the detector as follows (Fig. 1.19).

The base of the transistor T1 is normally kept sufficiently positive relative to emitter to keep it conductive and hence short circuiting the timing capacitor Ct at YY in Fig. 1.20. When a fault occurs the over current through the resistor Rs makes the base of T1 negative and cuts it off leaving Ct free to be charged. When the fault is cleared the

FIG. 1.19

FIG. 1.20

current falls to zero and the negative bias on T1 disappears so that Ct is again short circuited and discharged immediately.

A weakness of very fast instantaneous units is the tendency to over sensitivity on off-set current waves. The instantaneous unit can be made insensitive to the D.C. off set component by making the auxiliary C.T. saturate jus above the pickup current value and connecting the capacitor and a resistor across the rectified input to the level detector. This prevents tripping until both halves of the current wave are above pickup valve. That is, until the off set has gone, the short delay thus entailed is acceptable with time current relaying.

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CHAPTER 2INDUCTION DISC TYPE IDMT

OVER CURRENT RELAYS

2.1 IntroductionInduction types are most widely used for protective relaying purposes involving A.C. quantities. Torque is produced in these relays when alternating flux reacts with eddy currents induced in a disc by another alternating flux of the same frequency but displaced in time and space. These relays are used as over current or earth fault relay. In its simplest form, such a relay consists of a metallic disc which is free to rotate between the poles of two electromagnets (Fig. 2.1).

The spindle of this disc carries a moving contact which bridges two fixed contacts when the disc rotates through an angle which is adjustable. By adjusting this angle the travel of the moving contact can be adjusted so that the relay can be given any desired time setting which is indicated by a pointer on a time setting dial. The dial is calibrated from 0 to 1. These figures do not represent the actual operating times but are multipliers to be used to convert the time known from he relay name plate curve into the actual operating time.

The upper electromagnet has a primary and a secondary winding. The primary is connected to the secondary of a C.T. in the line to be protected and is provided with tappings. These tappings are connected to a plug setting bridge which is usually arranged to give seven selections of tapping, the over current range being 50 per cent to 200 per cent in steps of 25% and the earth fault 10% to 40% or 20% to 80% in steps of 5% & 10% respectively. These values are percentages of the current rating of the relay. Thus a relay may have a current rating of 5A, indicating that it is suitable for use on CT having secondary current rating of 5A but with a setting of 50% the relay would start to operate at 2.5A. Similarly if set at 200% it would start to operate at 10A. Thus the relay can be set to pick-up at any

FIG. 2.1: NON DIRECTIONAL INDUCTION RELAY

FIG.2.2: TIME CURRENT CHRACTERISTIC OF A NON DIRECTIONAL INDUCTION DISC RELAY

Desired tapping and, therefore, current setting can be selected by inserting a pin spring-loaded jaws of the bridge type soccer at the appropriate tap value. When the pin is withdrawn for the purpose of changing the setting while the relay is in service, the relay automatically adopts a high setting, thus ensuring that the C.T. secondary is not open circuited and that the relay remains operative for faults during the process of changing the settings. The secondary winding surrounds the limbs of the lower electromagnet as well. The torque exerted on the disc is due to the interaction of eddy currents produced therein by means of the leakage flux from the upper electromagnet and the flux from the lower electromagnet: these two fluxed having a phase displacement between them.

2.2 Characteristic CurveA set of typical time current characteristic curves of his type of relay is shown in Fig. 2.2. The curve shows the relation between the operating current in terms of current setting multiplier along the x-axis and operating time in seconds along the y-axis. A current setting multiplier indicates the number of times the relay current is in excess of the current setting. The current setting multiplier is also referred to as plug setting multiplier (P.S.M.). Thus

P.S.M=

= Where, as is usually he case, the rated current of the relay is equal to the rated secondary current of C.T. From the figure the operating time, when current setting multiplier is 10 and he time multiplier is set at 1, is 3 seconds. This is sometimes called the basic 3/10 curve.

It is evident that at the same current setting but the time multiplier set at 0.8, the time of operation is 2.4 seconds. Thus o get the actual tie of operation against any particular time multiplier setting, multiply the time of operation of the basic curve by the multiplier

FIG. 2.3

Primary Current Primary Setting Current Primary Current

Primary Setting Current

Primary CurrentPrimary Current Setting XC.T.Ratio

setting. Thus in this example the time of operation is 3 x 0.8 = 2.4 secs.

The time current characteristics of Fig. 2.2 are the inverse definite minimum time (I.D.M.T.) type since the time of operation is approximately inversely proportional to smaller values of current and tends to a definite minimum time as the current increases above 10 times the setting current.

The D.M.T. characteristic is obtained by saturating the iron in the upper magnet so that there is practically no increase in flux after current has reached a certain value. This results in the flattening out of the current time curve.

Example: An I.D.M.T. over current relay has a current setting of 150% and had a time multiplier setting of 0.5. The relay is connected in the circuit through a C.T. having ratio 500:5 amps. Calculate the time of operation of the relay if the circuit carries a fault current of 6000 A. the relay characteristic is shown in Fig. 2.3.

Solution: Sec fault current 6000 x = 60A

Plug Setting multiplier (P.S.M.) = = = 8

Time from graph against this multiplier of 8 = 3.15 sec.Operating time = 3.15 x 0.5 = 1.575 sec.

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5 500

Actual Current in Relay Setting Current

605 x 1.5

CHAPTER 3MOTOR PROTECTION

Electrical Motor is an important component of an industry. Squirrel cage induction motor is most widely used in power stations and industries. To protect the motor from different faults condition various protection are provided, which are as listed below.

3.1 Overload ProtectionA motor may get overloaded during its operation because of excessive mechanical load; (b) Single phase; (c) Bearing fault. An overloaded motor draws overcurrent resulting in overheating of the winding insulation. A reasonable degree of overload protection can be provided by Bi-metallic thermal overload relay with setting, 15% for continuously rated motor and 40% for large motors. Modern Motor Protection Relays provide a I2 sensitive Thermal overload protection having a range of exponential current/time characteristics to match with the thermal withstand characteristic of motor.

3.2 Single Phasing Protection or Unbalance ProtectionWhen one of the supply fuse of a 3 phase motor blows off or a terminal connection comes out, single phasing at the motor may occur. In such case, motor may continue to rotate, but the two healthy phases may draw high current leading to thermal stress on the insulation.

Besides, the Negative Phase sequence (I2) component of the unbalanced current, produces a reverse reaction field which cuts the Rotor iron and winding at approximately double the speed, thereby inducing double frequency eddy currents, causing over heating of the rotor.

I2 based single phasing protection having Inverse or definite time delay is used to protect he motor against his eventuality. For small L.T. Motors, single phasing preventor (unbalanced voltage V2 or current I2 based) is used to detect single phasing and isolate the defaulting Motor.

3.3 Short Circuit ProtectionA short circuit in the winding or at the terminals of a motor, results in overcurrent and thus overheating/damage to the winding insulation.

An instantaneous high set over current relay with a setting sufficiently above the starting/locked Rotor current is used for this protection.

For contactor controlled motors (usually L.T.Motors of small ratings), the short circuit protection is provided by the backup fuse in view of the limited break rating of the contactor.

3.4 Stalling Protection (Lock Rotor Protection)A motor may stall during its operation because of excessive mechanical load resulting in overloading of the motor. A definite time over current relay, with a setting of 1.52 times the Motor rated current, is used to protect the Motor against stalling condition. The time delay set, is usually above the Acceleration time and below the stall withstand time.

For high inertia motors, having safe stall withstand time less than the starting time, the stalling protection is required to be controlled by a speed switch, mounted on the Motor shaft. During normal acceleration, the speed switch opens to disable the stalling protection, whereas during a genuine condition the speed switch remains closed, thereby enabling he stalling protection and disconnects the defaulting Motor within the safe stall withstand time.

3.5 Differential ProtectionTo protect the motor against internal faults, differential protection based on circulating current principle provided for large critical motors. The differential protection requires C.Ts of identical ratio and ratings (Class PS) on both line and neutral side of the Motor for each phase (i.e. 6 C.Ts in total). The differential relay is usually of high impedance type.

3.6 Earth Fault ProtectionA motor may suffer an earth fault because of damage to the winding insulation. Earth fault may occur in the connecting cable also. Usually two types of earth fault protections are in vogue.

a) Residually connected earth fault protection with a setting of 10% or above. No time delay is required except on contactor controlled motors where it is necessary to prevent earth fault element over riding the fuse, for infeeds above the break rating of the contactor. The relay is, however, required to

be used with a series stabilising resistor which impedes any unbalance current produced due to unequal errors in he CTs during starting transients.

b) C.B.C.T. operated earth fault relay with a setting of typically 1%, where low earth faults are expected, requiring very high sensitivity.

3.7 Undervoltage ProtectionA reduced supply to a motor will increase motor losses and overloading of he winding. An IDMT or definite time under voltage relay operated off Bus P.T. is used to protect the motor, the under voltage relay trips the motors connected to the Bus on upstream supply failure and eliminates possibility of co-incident starting of all motors together, when the supply is subsequently restored. Thus, prevents stressing of the supply source.

Composite Motors Protection Relays (Conventional analog types) provide following protection functions.

a) Thermal overload (Alarm/Trip) ITHb) Short circuit (ISC)c) Single Phasing (I2)d) Earth Fault (Io)e) Stalling (IIt)

Numerical versions are now available which offer following additional protection functions, besides those given above.

f) Prolonged starting timeg) Too many starth) Loss of load

The Numerical versions have data acquisition capabilities and provide useful service Data (such as load currents, I2/Io content in load current, thermal status etc.), historic data fault data on operation. These relays have programmable settings, programmable output relays and continuous self monitoring against any internal failures.

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CHAPTER 4TRANSFORMER PROTECTIONS

4.1 Transformer protections are provided -a) Against effects of faults in the system to which the transformer is

connected.b) Against effects of faults arising in the transformer itself.

4.1.2 Protections against faults in the Systema) Short Circuitsb) High Voltage, high frequency disturbancec) Pure Earth Faults.

4.1.3 System Short CircuitsA short circuit may occur across any to phases (phase to phase) or between any one line and earth neutral (phase to earth). The effect is excessive over current and electro-magnetic stresses proportional to square of short circuit current. For these type of faults additional reactance and additional bracing of the transformer winding and end leads is resorted to. This reactance may be incorporated in the design itself or separate series reactance with primary of transformer is provided.

4.1.4 High Voltage High frequency surges:These surges may be due to arching grounds, switching operation surges or atmospheric disturbances. These surges have very high amplitudes, steep wave front currents and high frequencies. Because of this, the breakdowns of the transformer turns adjacent to line terminals occurs causing short circuit between the turns.

To take care of this, the transformer winding is to be designed to withstand the impulse surge voltages as specified below and then protect it by surge divertors.

System Voltage KV (RMS)7.2 KV

12.5 KV

Impulse voltage withstand level (Peak value)60 KV75 KV

33 KV66 KV

145 KV245 KV400 KV

170 KV250 KV

550 KV900 KV

1350 KV

Surge divertors are provided from each line to earth. These consist of several spark gaps in series with a non-linear resistance. This spark gap breakdown when surge reaches the divertor and disturbance is discharged to earth through nonlinear resistance since at high voltage divertors resistance is low. These surge divertors should have rapid response, non-linear characteristics, high thermal capacity, high system flow current interrupting capacity and consistent characteristics under all conditions.

4.1.5 System Earth Faultsa) When neutral of the system is earthed: - It represents short circuit

across the phase. Hence, same protection as for short circuit can be provided.

b) When neutral is not earthed: - Surge divertor gears in front of transformer is used.

4.2 Protection against Internal Faultsa) Electrical faults which cause serious immediate damage but are detectable

by unbalance of current or voltage.i) Phase to Earth Fault or phase to phase faults on HV and LV external

terminalsii) Phase to earth faults or phase to phase faults on HV & LV

winding.iii) Short circuit between turns on HV & LV winding (inter turn faults)iv) Earth faults on tertiary winding or short circuit between turn of

tertiary winding.v) Problem in tap changer gear.

b) Incipient faults: These are initially minor but subsequently develops itself resulting into damage to the transformer. These may be due to i) Poor electrical connection of conductors due to breakdown of

insulation of laminations, core bolt faults, clampings, rings etc.ii) Coolant failureiii) Blocked oil flow causing local hot spot on winding.

iv) Continuous uneven load sharing between transformers in parallel causing overheating due to circulating current.

4.3 Principles of Protection SystemPrinciples used are

i) Overheatingii) Over currentiii) Un-restricted earth faultsiv) Restricted earth-faultsv) Percentage bias differential protectionvi) Gas detection due to incipient faultsvii) Over fluxingviii) Tank earth current detectionix) Over voltagex) Tap changer problems.

4.4 Gas Detectiona) Buchholz relay protectionb) Pressure relief valves/switches (for heavy internal faults)

4.4.1 Buchholz ProtectionThis is for two types of faults inside the transformer.a) For incipient faults because of

i) Core bolt insulation failureii) Short circuit in laminationsiii) Local over heating because of clogging of oiliv) Excess ingress of air in oil systemv) Loss of oil due to heavy leakagevi) Uneven load sharing between two transformers in parallel causing

overheating due to circulating current.These generate gases causing operation of upper float and energises the alarm circuits.

b) For serious faults inside the transformer due to i) Short circuit between phasesii) Winding earth faultsiii) Puncture on bushingiv) Tap changer problems

These types of faults are of serious nature and operate both the floats provided in the buchholz relay and trip out the transformer.

4.4.2 Principles of Buchholz Relay Operation (Fig. 4.1)This relay is provided in the connecting pipe from transformer tank to conservator. Two floats are provided inside the relay and are connected to mercury switches. Normally the relay is full of oil and in case of gas collection the floats due to their buyopancy rotate on their supports until they engage their respective stops. Initially fault develops slowly and heat is produced locally which begins to decompose solid or liquid insulating material and thus produce inflammable gases. Gas bubbles are collected in relay causing oil level to lower down. The upper float rotates as he oil level in the relay goes down and when sufficient oil id displaced the mercury switch contacts close and initiates alarm. For serious faults as described above, gas generation is more violent and the oil displaced by gas bubbles flows through connecting pipe to conservator. This abnormal flow of oil causes deflection of both float and trip out the transformer. Recently the dissolved gas analysis technique (gas chromatography) is in use for pre-detection of type of slowly developing faults inside the transformer which helps to decide whether the transformer maintenance/internal inspection is required to be

FIG. 4.1 CIRCUIT DIAGRAM OF BUCHHOLZ RELAY

carried out or otherwise, and thus helps to predict transformer damage in future.

4.4.3 Dissolved Gas AnalysisThe inflammable gases dissolved in the transformer oil are mainly hydrocarbon gases (methane CH4, Ethane C2H6, Ethylene C2H4. Acetylene C2H2, Propane, hydrogen, carbon monoxide and carbon dioxide). With the help of dissolved gas analysis equipment the concentration of these gases in PPM can be known and can be cross checked with the IS standard. Also with the help of Rogers ratio method, the type of probable incipient fault can be judged and corrective action can be taken in advance to prevent failure of the transformer (Ref. Annex.1 &II).

4.5 Over Heating ProtectionProtection is mainly required for continuous over load of the transformer.

a) Protection is based on measurement of winding temperature which is measured by thermal image technique.

b) Thermal sensing element is placed in small pocket located near the top transformer tank in the hot oil. A small heater fed fro a current transformer (winding temp. C.T.) in the lower voltage terminal of one phase, is also located in this pocket and produces a local temp. rise, similar to that of main winding and proportional to copper losses, above general temp. of oil.

c) Winding temperature high alarm/rip is provided through mercury switches in the winding temp. indicators.

d) By thermo-meters, mercury switches heat sensing silicon resistance are also used for sensing the temp. rise.

e) Thermisters are provided manly in the dry type transformers for temperature sensing.Temperature of 55o above ambient of 50oC is generally provided for tripping.

4.6 Over Current & Earth leakage protection4.6.1 Earth Leakage Protection

In case of transformer earthed through resistance or earthed through impedance.

Resistance Grounding: The earth fault current in faulty winding in resistance grounded transformer depends on voltage between neutral and fault point and is inversely proportional to neutral resistance.

Iy =

Where Iy is earth fault current; P= percentage of winding to be protected; KV line to line voltage and Rn = Neutral Grounding resistance. Suitable earth fault relay can be provided across C.T. in the neutral of the transformer depending upon the minimum earth fault current to be detected.

Impedance Grounding; Transformer neutral is connected to the primary of neutral grounding transformer. The suitable resistance is connected in parallel with the secondary of this neutral grounding transformer. The earth fault relay (neutral displacement relay) is connected across this resistance. The earth fault relay can be set at about 2.5 percent of maximum neutral voltage. The relay is time delayed for transient free operation.

10kv x p Sq. root of 3.Rn

4.6.2 Over current protectioni) HRC fuses are provided for small distribution transformer.ii) Over current relays are used for power transformers, considering the

following:a) IDMT relays should be chosenb) Discrimination with circuit protection of secondary side should

be provided;c) Instantaneous trip facility for high speed clearance of terminals

short circuit should be provided.d) Setting depends on transformer reactance or percentage

impedance, faults MVA, type of relay used.e) Setting of over current relays can be slightly higher than rated

full load current (say 120 percent of FL) with proper discrimination.

4.6.3 Combined over current and un-restricted E/F Protection(Ref. Fig. 4.2)a) Typical over current/earth fault protection is shown for a Delta/ start

transformer in Fig. 4.2.b) IDMT O/C elements on delta and star side, primarily serve as back up

protection against downstream short circuits and are time co-ordinated with downstream O/C protections.

c) The high set instantaneous O/C elements on Delta side (connected to source) are provided to detect severe terminal short circuits and quickly isolate the transformer. These are set over and above the maximum short circuit current infeeds of the transformer for star side faults.

d) The start side earth fault protection (IDMT) serves as a backup against downstream earth faults and is required to be suitably time graded. This can either be residually connected across the phase C.Ts or operated off a C.T. in the Neutral Earth connection (standby earth fault relay). The latter is considered to be advantageous since it can detect star winding earth faults, beside providing backup for downstream earth faults. Since the neutral C.T. ratio is not tied up with the load current, a lower C.T. ratio consistent with the maximum E/F current limited by NGR can be provided. This renders good sensitivity for the standby E/F protection.

e) The E/F protection on delta side is inherently restricted to delta winding earth faults and does not respond to earth faults on the star side, due

to zero sequence isolation provided by the delta connection. The delta side E/F protection, therefore, assumes the form of REF

FIG. 4.2: COMBINED 0/C AND E/F PROTECTION CKT

FIG. 4.3: RESTRICTED E/F PROTECTION CKT

protection, enabling sensitive setting and instantaneous operation. The relay is connected in high impedance mode with a series stabilizing resistor, as shown.

4.6.4 Restricted Earth Fault Protection: (Ref. Fig. 4.3)a) REF protection is used to supplement the differential protection,

particularly here star neutral of the transformer is grounded through a neutral rounding resistor to limit the earth fault current. REF protection provides increased coverage to the star winding against earth faults.

b) The REF protection operates on the principle of Kirchoffs law and requires CTs of identical ratio and ratings as the phases and neutral earth connection. The relay is connected across the parallel combination of the CTs in High Impedance mode.

c) For external earth fault, the associated CTs have dissimilar polarities forming a series connection. Thus, the resulting current through the relay is negligible. For internal fault, however, the CTs have similar polarities, forming a parallel connection, thus adding up the current in the relay branch. This ensures positive operation of the relay.

4.7 Percentage Bias Differential Protectiona) In this protection, operating current is a function of differential current.b) The value of restraining current depends on 2nd and 5th harmonic

component of differential current during magnetic inrush and over excited operation.

c) Bias current is a function of through current (external fault current) and stabilizes the relays against heavy external fault.

4.7.1 Basic Consideration for differential protectiona) Transformer ratio: the current transformers should match to the rated

currents of the primary windings.

b) Transformer Connection: In delta star connected transformer, the phase shift of 30oC between primary and secondary side current exist. Also zero sequence current flowing on the star side will not produce the reflected current in the delta on the other side. To eliminate zero sequence component on star side the current transformer must be connected in delta and the current transformer of delta side must be connected in start.

c) For star / star transformer CTs on both sides should be connected in delta.

d) In order that secondary currents from two groups of CTs may have the same magnitude (i.e. primary side CTs and secondary side CTs). The ratio of star connected CTs if 5 Amp, then those of delta connected group may be 5 / Sq. Root of 3 = 2.89 Amps.

e) The operating current is a appropriate percentage of reflected through fault current in the restraining (bias) coils and the ratio is termed as percentage slope.

f) Operating coil is provided with vectors sum of the currents in the transformer windings and the bias coil sees the average scaler sum of the reflected through fault current. Spill current required to operate the relay is expressed as percentage of through current.

g) The relay is also provided with an unrestrained differential high set, to protect against heavy faults which are enough to saturate the line current transformers. The setting of this high set unit is kept above the maximum in rush current magnitude. This will operate in typically one cycle for heavy internal faults.

4.7.2 Operating Principles for Internal fault & external faults During external fault condition (through fault) (Fig. 4.4):Current in pilot wires would pass through whole of bias coils and only out of balance current due to mis-match caused by OLTC and C.T. errors

FIG. 4.4

FIG. 4.5

Would flow through operating coil. Under this condition biasing effect pre-dominates and prevents the relay operation.

During internal faults: (Fig. 4.5)In this case, the reflected current flows through only one half of bias coil and the operating coil and back to CT neutral connection. Here the operating quantity pre-dominates resulting into operation of the relay.

4.8 Over Voltage Protectiona) Two stage protection is providedb) The delayed trip is set at 110 percent of the rated voltage with two

second time delay (typical).c) Instantaneous setting is kept at 115 120 percent of the rated voltaged) During voltage fluctuations the AVR (Automatic Voltage Regulator) will

take care to avoid over voltage condition if fluctuations are within its operating limits (for Generator step-up transformer).

4.9 Over Fluxing Protectiona) This protection is commonly used for Generator Transformers and

large inter connecting transformers in the Grid.

b) This condition arises during abnormal operating conditions i.e. heavy voltage fluctuations at lower frequency conditions. This condition is experienced by the transformer during heavy power swings, cascade tripping of the generator sets and HT line in the Grid, interstate system separation conditions and due to AVR malfunctioning during start-up or shutting down in case of Generator Transformers.

c) The power frequency over voltage cause both stress on insulation and proportionate increase in the magnetizing flux inside the transformer due to which the iron losses area increased and the core bolts get maximum component of flux, thereby rapidly heating and damaging its own insulation and coil insulation. Reduction in frequency during high voltage fluctuation has the same effect.

FIG. NO. 4.6

d) Transformer should be isolated within one or two minutes or as recommended by the manufacturer.

e) The core flux V/f where V impressed voltage and f frequency. He index of over fluxing is, therefore, V/f. Over fluxing relays having variable V/f setting and time delays are used for this protection.

4.10 Overall Differential Protectiona) This is provided for complete protection of generator and generator

transformer and as such is a compound overall differential protection.b) In addition to normal differential protection of generator, overall biased

differential protection relay is connected to protect the unit as shown in Fig. 4.6.

c) 20% pickup and 20% bias setting is provided. (The values are typical).d) This is a supplementary protection for individual differential protection

of the generator.e) Unit auxiliary transformers are provided with separate differential

protection

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ANNEXURE IPERMISSIBLE CONCENTRATIONS OF

DISSOLVEDGASES IN THE OIL OF HEALTHY

TRANSFORMER(TRANSFORMERS UNION AG)

Gas Less than fourYear in service

4-10 years inservice

More than 10Years in service

Hydrogen 100/150 ppm 200/300 ppm 200/300 ppmMethane 50/70 ppm 100/150 ppm 200/300 ppmAcetylene 20/30 ppm 30/50 ppm 100/150 ppmEthylene 100/150 ppm 150/200 ppm 200/300 ppmEthane 30/40 ppm 100/150 ppm 800/1000 ppmCarbon monoxide

200/300 ppm 400/500 ppm 600/700 ppm

Carbon dioxide

3000/3500 ppm 4000/5000 ppm 9000/12000ppm

ANNEXURE IICODE FOR EXAMINING ANALYSIS OF GAS DISSOLVED IN

MINERAL OIL AS PER CBIP TECHNICAL REPORT 62.

Ratio of Characteristic gases0.1

0.1-11-33

Code of Range ratioC2H2C2H4

CH4H2

C2H4C2H6

0112

1022

0012

Case No. Characteristic fault Typical Example0 No fault 0 0 0 Normal ageing1 Partial discharge

of low energy density

0but not signify-

cant

1 0 Discharge in gas-filled cavities resulting from incomplete impregnation, or super-saturation or cavita-tion or high humidity.

2 Partial discharge of high energy density

1 1 0 As above, but leading to cracking or perforation of solid insulation

3 Discharge of low energy

1-2 0 1-2 Continuous sparking in oil between bad connections of different potential. Breakdown of oil between solid materials.

4 Discharges of high energy

1 0 2 Discharges with power flow through. Arcing-breakdown of oil between winding or coils or between coils to earth. Selector breaking current.

5 Thermal fault of low temperature (150oC)

0 0 1 General insulated conductor overheating

6 Thermal fault of low temperature range

0 2 0 Local overheating of the core due to concentrations of flux,

150o-300oC increasing hot sot temperature varying from small spots in core, shorting links in core.

7 Thermal fault of medium temperature range 300o-700oC

0 2 1 Overheating of copper due to eddy currents, bad contacts/ joints (pyrolitic carbon formation) upto core and tank circulating current.

8 Thermal fault of high temperature 150o-300oC

0 2 2 - do -

CHAPTER 5GENERATOR PROTECTIONS

5.1 IntroductionGeneration are designed to run at a high load factor for a large number of years and permit certain incidences of abnormal working conditions. The machine and its auxiliaries are supervised by monitoring devices to keep the incidences of abnormal working conditions down to a minimum. Despite of this monitoring, electrical and mechanical faults may occur, and the generators must be provided with protective relays, which in case of a fault, quickly initiate a disconnection of the machine from the system and, if necessary, initiate a complete shut-down of the machine.

The following are the various types of protections provided for a 200/210 MW Generator.

1. Stator ground (earth) fault protection a) 95% stator ground fault protectionb) 100% stator ground fault protection

2. Rotor earth fault protectiona) First rotor earth fault protectionb) Second rotor earth fault protection

3. Generator Interturn fault protection4. Generator Negative phase sequence protection5. Generator Loss of excitation protection6. Generator Minimum Impedance (MHO backup) protection

7. Generator Differential protection8. Generator Overall differential protection9. Generator Reverse power protection10. Generator Over frequency protection11. Generator Under frequency protection12. Generator Thermal overload protection13. Generator Over voltage protection14. Generator out of sep (Pole slipping) protection

FIG. 5.1

FIG. 5.2

FIG. 5.35.2 Stator Earth Faults:

In most countries, it is a common practice to ground the generator neutral through a Grounding Transformer having a loading resistor across its secondary. This method of earthing is called High Impedance earthing where the earth fault current is limited to 510 Amps. Tuned reactor which limit the ground fault current to less than 1.0A are also used.

The generator grounding resistor normally limits the neutral voltage transmitted from the high voltage side of the unit transformer in case of a ground fault on the H.V. side to maximum 2-3% of rated generator phase voltage.

Short circuits between the stator winding in the slots and the stator core are the most common electrical fault in Generators. Interturn faults, which normally are difficult to detect, will quickly develop into a ground fault and will be tripped by the stator ground fault protection.

5.2.1 95% Stator Ground fault Relay for Generator (Fig.5.1)For generators with unit transformer and with high impedance grounding of the neutral, a neutral voltage relay with harmonic immunity and independent time delay is used. The relay is normally set to operate at 5% of maximum neutral voltage with a time delay of 0.3 0.5 second. With this voltage setting, it protects approximately 95% of he Stator winding.

It also covers the generator bus, the low voltage winding of the unit transformer and the high voltage winding of the unit aux. Transformer.

Relay details: 64 A / B - Neutral Displacement Relay having IDMT or definite time characteristic.

5.2.2 100% Stator Ground fault protection for GeneratorGround faults caused by mechanical damage may occur close to the generator neutral. Today there is a distinct trend towards providing ground fault protection for the entire stator winding (100% stator ground fault protection).

The principle diagram of the relay is shown in Fig. 5.2. The 100% stator ground fault scheme includes a 95% unit (1), which covers the stator winding from 5% of the neutral and a 3rd harmonic voltage measuring unit (2) which protects the rest of the stator winding.

For generators with 3rd harmonic voltage less than 1%, a filter is available with a damping factor of more than 100.

When the generator is running and here is no ground fault near the neutral, the third harmonic voltage unit (2) and the voltage check unit (4) are both activated and the relay contact used in alarm/trip circuit is open. If a round fault occurs close to the generator neutral, the third harmonic voltage unit will reset, operating relay contact will close and alarm or tripping is obtained.

The voltage check unit is included to prevent faulty operation of the relay at generator standstill or during the machine running up or running down period.

Generators which produce more than 1% third harmonic voltage under all service conditions, can have the entire stator winding up to and including the neutral point protected by the 100% stator ground fault relay.

5.3 Rotor Earth Fault Protection (64R1/64R2):The field circuit of generator (i.e. rotor winding) is a isolated D.C. circuit and not earthed anywhere. The field circuit can be exposed to abnormal mechanical or thermal stresses due to vibrations, excessive currents or choked cooling medium flow. This may result in a breakdown of the insulation between the field winding and the rotor iron at one point where the stress has been too high.

A single earth fault in the field winding or its associated circuits, therefore, gives rise to a negligible fault current and does not represent any immediate danger. If, however a second ground fault should occur, heavy fault current and severe mechanical unbalance may quickly arise and lead to serious damage. It is essential therefore that any occurrence of insulation failure is discovered and that the machine is taken out of service as soon as possible. Normally the machine is tripped instantly on occurrence of second rotor earth fault. Three methods are available to detect this type of faults (First Rotor earth fault protection) 64R1.a) Potentiometer methodb) A.C. injection methodc) D.C. injection method

5.3.1 Potentiometer Method (Fig. 5.3)In this scheme, a center tapped resistor is connected in parallel with the main field winding as shown in Fig. 5.3. The center point of the resistor is connected to earth through a voltage relay. An earth fault on the field winding will produce a voltage across the relay. The maximum voltage occurring for faults at the ends of the winding.

A blind spot exists at the center of the field winding, this point being at a potential equal to that of the tapping point on the potentiometer. To avoid a fault at this location remaining undetected, the tapping point on the potentiometer is varied by a push button or switch. It is essential that station instructions be issued to make certain that the blind spot is checked at least once per shift. The scheme is simple in that no auxiliary supply is needed. A relay with a setting 5% of the exciter voltage is adequate. The potentiometer will dissipate about 60 volts.

5.3.2 A.C. Injection Method (Fig. 5.4)This scheme is shown in Fig. 5.4. It comprises of an auxiliary supply transformer, the secondary of which is connected between earth and one side of he field circuit through an interposed capacitor and a relay coil.

The field circuit is subjected to an alternating potential at the same level through out, so that an earth fault anywhere in the field system will give rise to a current which is detected by the relay. The capacitor limits the magnitude of the current and blocks the normal field voltage, preventing the discharge of a large direct current through the transformer.

FIG. 5.4

FIG. 5.5

This scheme has an advantage over the potentiometer method in that there is no blind spot in the supervision of the field system. It has the disadvantage that some current will flow to earth continuously through the capacitance of the field winding. This current may flow through the machine bearings, causing erosion of the bearing surface. It is a common practice to insulate the bearings and to provide an earthing brush for the shaft, and if this is done the capacitance current would be harmless.

5.3.3 D.C. Injection Method (Fig. 5.5)The capacitance current objection to the a.c. injection scheme is overcome by rectifying the injection voltage as shown in Fig. 5.5. The d.c. out put of a transformer rectifier power unit is arranged to bias the positive side of the field circuit to a negative voltage relative to earth. The negative side of the field system is at a greater negative voltage to earth, so an earth fault at any point in the field winding will cause current to flow through the power unit. The current is limited by including a high resistance in the circuit and a sensitive relay is used to detect the current.

The fault current varies with fault position, but this is not detrimental provided the relay can detect the minimum fault current and withstand the maximum.

The relay must have enough resistance to limit the fault current to a harmless value and be sufficiently sensitive to respond to a fault which at the low injection voltage may have a fairly high resistance. The relay must not be so sensitive as to operate with the normal insulation leakage current, taking into account of the high voltage to earth at the negative end of the winding and any over voltage due to field forcing and so on.

5.3.4 (a) Second Rotor Earth Fault Protection 64R2 (Fig. 5.5 a)In this test system is replaced by a replica field system in the form of potential divider, two 1K potentiometers in parallel with station D.C. is used as shown in Figure 5.5 (a) with SW1 at 1st rotor E/F position. Close switch S1 check that 1st rotor E/F relay VAEM (64R1) operated.

FIG. 5.5 a

FIG. 5.5 b

Shift SW1 to Balance. Obtain balance on the mA meter (Galvanometers) by coarse / fine adjustment of potentiometer. Shift SW1 on Test position, check operation of relay 64R2 by closing switch S2 thus creating an unbalance which simulates second E/F.

5.3.4 (b) Rotor Earth Fault (Fig. 5.5.b)The Scheme to detect Rotor Earth Fault in case of Brushless excitation system is shown in Fig. 5.5.(b). In this case, Rotor Earth Fault relay forms the three arms of a bridge whose fourth arm is the field winding capacitance o Rotor body. During Rotor earth fault, this capacitance gets shorted and the bridge becomes unbalanced operating the relay. Main exciter winding, rotating diodes and Generator field winding is protected by this relay.

5.4 Generator Interturn Fault Protection (95A,B, C) (Fig. 5.6)Interturn faults have commonly been disregarded on the basis that if they occur they will quickly develop into earth faults. This is probably true if the fault is in the slot portion but will take a little longer in the region of the end connection. An approach of this kind is never attractive and may be entirely unjustified. There is a possibility of the machine being very seriously damaged before the fault evolves to a condition that can be detected by the longitudinal system.

Modern medium size and large size turbo generators have the stator winding designed with only one turn per phase per slot. For these machine Interturn faults can only occur in case of double ground faults or as a result of severe mechanical damage of the stator end winding. The latter is considered rather unlikely to occur.

It is generally considered difficult to obtain a reliable protection against short- circuiting of one turn if the stator winding has a large number of turn per phase.

For generators with split neutrals, the conventional inter-turn fault protective scheme comprises a time delayed low set over-current relay which senses the current flowing in the connection between the neutrals of

the stator winding. The fault current can be extensively large in case of Interturn fault, hence the time delay must be short,

FIG. 5.6

FIG. 5.7

0.2 to 0.4 sec and the over current relay must be set higher than the maximum unbalanced current in case of external faults and the minimum unbalanced current for single turn short circuit have to be obtained from the manufacturer of the machine.

Due to the difficulties in obtaining a reliable and secure interturn protection, it is in most cases omitted. It is assumed that the Interturn fault will lead to a single phase ground fault at the faulty spot, and the machine will then be ripped by the ground fault relay within 0.3 0.4 secs.

Relay is show for one phase only. Similar connections are for other two phases. Time delay of 200 sec. is provided to avoid operation of relay in system disturbance condition.

5.5 Generator Negative Phase sequence Current Protection (46) (FIG. 5.7)

When the generator is connected to a balanced load, the phase currents are equal in magnitude and displaced electrically by 120o. The ampere turn wave produced by the stator currents rotate synchronously with the rotor and no eddy currents are induced in the rotor parts.

Unbalanced loading gives rise to a negative sequence component in the stator current. The negative sequence current produces an additional ampere turn wave which rotates backwards, hence it moves relatively to the rotor at twice the synchronous seed. The double frequency eddy currents induced in the rotor may cause excessive heating, primarily in the surface of cylindrical rotors and in the damper winding of rotors with salient poles.

The approximate heating effect on the rotor of a synchronous machine for various unbalanced fault or severe load unbalance conditions is determined by the product 2I2t = K, where

I2 = Negative sequence current expressed per unit of stator current (PU)t = Time in secondsK = a constant depending on the heating characteristic of the machine

(rotor) i.e. type of machine and the method of cooling adopted for rotor.

The capability of machine to withstand continuously unbalanced currents is expressed as negative sequence current in percent of rated stator current. Typical values for generators are given in table.

Type of generator Max. permitted2

I2.t

Max. permittedcontinuous I2

(%)A Cylindrical rotor

Indirectly cooled Directly cooled

30(5 10)

10(5 8)

B Salient pole rotor with Damper windingWithout damper winding

4040

105

Single phase and specially two phase short circuits give rise to large negative sequence currents. The faults are however, cleared by other relays in a tie much shorter than the operate time of the negative sequence relay.

A two phase short circuit with fault current equal to 3.46 (2 Sq.rt of 3) time rated generator current implies a negative sequence current component equal to twice the rate current (2 p.u.). Hence a negative sequence relay with the setting.

2I2t = 10s would trip with a time delay of

10 = 2.5 sec.22

Example on load dissymetries which give rise to negative sequence currents in the generator are -

1. Unbalanced single phase loads-Traction loads and induction furnaces.2. Transmission line dissymetries due to capacitors, non-transposed

phase wire or open conductors (C.B. pole failure)

An open conductor may give rise to a considerable negative sequence current, as a maximum of more than 50% of rated machine current. The combination of two or more of the above mentioned dissymetries case give rise to harmful negative phase sequence current, even if each of them gives rise to a relatively small unbalance. The Fig. 5.7 will illustrate the C.T. and circuit.

5.6 Generator Loss of Excitation Protection (40G) (Fig. 5.8)A complete loss of excitation may occur as a result of a. Unintentional opening of the field breakerb. An open circuit or a short circuit of the main fieldc. A fault in the automatic voltage regulator (AVR) with the result that the

field current is reduced to zero

When a generator with sufficient active load looses the field supply, it goes out of synchronization and starts to run a synchronously at a speed higher than the system, absorbing reactive power (VAR) for its excitation from the system, operates as an induction generator.

The maximum active power that can be generated without loss of synchronism when the generator losses its excitation depends on the difference between the direct axis and quadrature axis synchronous reactance. For generators with salient poles, the difference is normally sufficiently large to keep the machine running synchronously; even with an active load of 15=25% of rated load.

For cylindrical turbo generators, the direct ad quadrature axis reactance are practically equal, and the machine falls out of synchronism even with a very small active load