Switc gear protection power point resentatiin

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1. PROTECTIVE RELAYS UNIT-2

2. IMPORTANT DEFINITIONS (OR) TERMS RELATED TO POWER SYSTEM PROTECTIVE RELAYS Relay : A relay is a device which detect the fault and initiates information to the circuit breaker which performs the function of circuit interruption. Protective Relay : A protective relay is a device which detect the fault and initiates information to the circuit breaker to isolate the defective element from the rest of the system. Pick-up Current ( Ip ) : It is the minimum value of current at which the relay picks-up (or) operation. Reset Value : It is the value of current (or) voltage below which a relay opens its contacts and resumes the original position. Reset Time : It is the time that lapses from the instant when the actuating quantity becomes less than the reset value to the instant at which relay contacts are closed.

3. Operating Time: It is the time taken from the instant when the actuating quantity exceeds the pick-up value to the instant when the relay contacts returns to its normal position. Drop-Out Current (Id) : It is the maximum value of current at which the relay drops out. Reset Ratio ( Ka ): It is the ratio of drop out current to pick-up current Primary Relay : It is the relay normally expected to take the initiative in case of a fault in the protected zone.  

4. Back-up Relay : It is a relay which operates usually after a certain delay, if the primary relay fails to operate . Causes of Failure of a Primary Relay : The primary relay may be inoperative due to failure of any one of the following : Supply to the relay Relay itself Circuit breaker itself Trip coil circuit Battery supply voltage in the trip coil circuit

5. Requirements of Relays Protective relay system should have the following basic requirements Selectivity (or) discrimination Reliability (or) system security Sensitivity Speed operation Stability Economy Adequateness

6. Selectivity (or) Discrimination: "Selectivity is the quality of protective relay system by which it is able to discriminate between a fault in the protected section under the normal condition and also it should be able to distinguish whether the fault lies within its zone of protection (or) outside the zone". When a fault occurs on a power system network, only the faulty part of the system should be isolated. No healthy part of the system should be deprived (disconnected) of electric supply and hence it should be left intact. The relay should also be able to discriminate between a fault and transient conditions like Power Surges or Inrush Magnetising currents of a transformer's because in case of large rating transformers the magnetising inrush currents are 5 to 7 times the full - load current hence the relay should compare with the fault current.When the generators of two interconnected power plants loss synchronism due to disturbances causes heavy currents flow through the equipment and lines. This condition is a like Short Circuit. The flow of heavy currents is known as Power Surge. Hence, the protective relay should able to distinguish between a fault (or) power surge either by its inherent characteristics (or) with the help of auxiliary relay.

7. Reliability: "It is the quality of protective relay system by which it must operate reliably when a fault occurs in its zone of protection under any kind of faults or conditions" A typical value of reliability of a protective relay system is 95%. Sensitivity : A protective relay should operate when the magnitude of the current exceeds the preset value. This value is called Pick-up current (I P ). The relay should not operate when the current is below its pick-up value. A relay should be sufficiently sensitive to operate when the operating current just exceeds its pick-up value. " Sensitivity of protective relay system refers to the smallest value of alternating quantity at which the protection starts operating in relation with the minimum pick-up value of fault current in the protected zone ".

8. Speed Operation: "It is the ability of the protective relay system by which it should be fast enough to isolate the faulty element of the system as quickly as possible to minimise damage to the equipment and to maintain the system stability" Now a days in a modern power system, stability criterion is very important and hence, the operating time of the protective system should not exceed the critical clearing time to avoid loss of synchronism. The operating time of protective relay is usually one- cycle. Half-cycle relays are also available. In case of distribution systems the operating time may be more than one cycle. Stability: "It is the quality of the protective relay system should remain stable even when a large currents is flowing through its protective zone due to an external fault, which does not lie in its zone ". The concerned circuit breaker is supposed to clear the fault. But the protective system will not wait indefinitely if the protective scheme of the zone in which faults has occurred fails to operate. After a preset delay the relay will operate to trip the circuit breaker.

9. According to principle of operation relays may be classified as ( i ) Electro-magnetic Attraction Type : Attracted Armature Type: In this type of relay, operation depends on the movement of an armature under the influence of attractive force due to magnetic field set up by current flowing through the relay winding. Solenoid Type: In this type of relay operation depends on the movement of an iron plunger core along the axis of a solenoid .

10. (ii) Electro-magnetic Induction Type: In this type of relay, operation depends on the movement of a metallic disc or cylinder free to rotate by the inter-action of induced eddy currents and the alternating magnetic field producing them. (iii) Electro-dynamic Type: In this type of relay, moving member consists of a coil free to rotate in an electromagnetic field. (iv) Moving Coil Type: In this type of relay, moving member consists of a coil free to rotate in the air gap of a permanent magnet. (v) Thermal Type: In this type of relay, movement depends upon the action of heat produced by the current flowing through the element of the relay. (vi) Physico -Electric Relay : A relay controlled by a physical system and producing changes in one (or) several electrical circuits. Ex: Buchholz relay

11. 2. According to Timing Characteristics the Relays can be Divided into Following Classes : Instantaneous Relay : In these relays, complete operation takes place after a very short (negligible) duration from the incidence of fault current or other quantity resulting in operation. Definite Time Lag Relay: In these relays, the time of operation is sensibly independent of the magnitude of the current or of other quantity causing operation. Inverse Time Lag Relay : In these relays, the time of operation is approximately inversely proportional to the magnitude of the current or other quantity causing operation. Inverse Definite Minimum Time (IDMT) Lag Relay : In these relays, the time of operation is approximately inversely proportional to the smaller values of current or other quantity causing operation and tends to be a definite minimum time as the value increases without limit.

12. 3. According to Applications (or) Duty of Relays Over Voltage or Over Current or Over Power Relay : This relay operates when the voltage or current or power rises above a specified value. Under Voltage or Under Current or Under Power Relay : This relay operates when the voltage or current or power falls below a specified value . Directional or Reverse Current Relay : This relay operates when the applied current assumes a specified phase displacement with respect to the applied voltage and the relay is compensated for fall in voltage. Directional or Reverse Power Relay : This relay operates when the applied current and voltage assume specified phase displacement and no compensation is allowed for fall in voltage. Differential Relay : The relay operates when some specified phase or magnitude difference between two or more electrical quantities occurs. Distance Relay : In these relays, the operation depends upon the ratio of the voltage to the current.

13. PICK-UP CURRENT It is the minimum current in the relay coil at which the relay starts to operate. Current Setting: For any operation of the relaying equipment it is very desirable to adjust the pick-up current to any required value. This is known as current setting and is usually achieved by the use of tapping’s on the relay operating coil

14. Pick-up current = (Rates secondary current of C.T.) × (Current setting) The above figure shows the plug bridge and its tappign’s are brought outside. The plug bridge permits to alter the number of turns on the relay coil. This changes the torque on the disc and hence the time of operation of the relay. The taps are assigned to each tap and are expressed in terms of percentage full-load rating of C.T. will which the relay is associated and represents the value above which the disc rotate and finally closes the trip circuit.

15. Plug Setting Multiplier (P.S.M): It is the ratio of fault current in relay coil to the pick- up current .  

16. Time-Setting Multiplier (TSM) A relay which is generally provided with control to adjust the time of operation such adjustment is known as Time-setting multiplier . The time setting multiplier is calibrated from 0 to 1 in steps of 0.05 seconds as shown in Figure. These multipliers to be used to convert the time derived from time/PSM curve into actual operating time.

17. Graph shows the curve between time of operation and P.S.M of a typical relay. The X-axis is marked as P.S.M which represents the number of times the relay current is in excess of the current setting. The Y-axis is represented in terms of the time required for relay operation . From the graph for lower values of over current, time of operation is very high and varies inversely with current. This type of feature is necessary for higher rating relay .

18. INDUCTION TYPE OVER CURRENT RELAY An induction type over current relay works on the induction principle and it operates/ initiates when the current in the circuit exceeds the predetermined value.

19. Figure shows the induction type over current relay. Its construction is very similar to energy meter mechanism with slight modification to suit the required characteristics. It consists of a metallic ( aluminium ) disc which is free to rotate in between the poles of two electromagnets. The upper electromagnet has a tapped primary winding connected to the secondary winding of a current transformer (C.T) and also a part of the secondary winding whose circuit is completed through a winding on the lower electromagnet. The tappings are connected to a plug setting bridge by which the number of active turns on the relay operating coil can be varied, there-by giving the desired current setting. The secondary winding is energised by induction from primary winding and is connected in series with the winding on the lower magnet

20. The control torque is provided by a spiral spring. The spindle of disc carries a moving contact which may contact the two fixed contacts (connected to trip circuit) when the disc rotates through a pre-set angle. By adjusting the pre-set angle, the relay operating time can be varied. Principle of Operation: Under faulty conditions, the current flows through primary and secondary windings of the relay depending on the current setting. Due to this the fluxes 1 and 2 will be established in the upper and lower magnets and these are displaced from each other. Hence eddy currents are produced in the disc. These eddy currents produces a torque in the disc and hence the disc starts rotating.

21. The speed of the disc depends on the magnitude of fault current. When the deflecting torque is produced on the disc and due to the disc carries a moving contact which closes two fixed contacts of the trip circuit after disc has rotated through a pre-set angle. The trip circuit operates the circuit breaker which isolates the faulty section .

22. Principle of Obtaining Directional Property for a Over Current Relay Incase of induction type over current relay (Non-directional relay), the relay will be operated due to occurrence of fault flow in either direction. In order to achieve operation for the fault flowing in a specific direction it is necessary to add a directional element (or) reverse power relay. When power flows in the normal direction, the relay will not operate but if the power flows in the reverse direction, the fluxes set up by the actuating quantities in the two windings develop positive operating torque and the relay contacts will be closed. In this relay it contains two magnets, ie , upper and lower. The voltage winding is placed on the upper magnet and the current winding is placed on the lower magnet, with provision of tappings on current winding. The relay can be made very sensitive by providing a light control spring so that even a small power flow in the reverse direction will operate the relay.

23. Directional Over Current Induction Relay Construction: It consists of two relays elements mounted on a common case, namely. Over current element (non-directional element) Directional element Over Current Element: It consists of a metallic ( aluminium ) disc which is free to rotate in between the poles of two magnets. The upper magnet has a tapped primary winding connected to the secondary winding of a current transformer (C.T). The tappings are connected to a plug setting bridge by which the number of active turns on the relay operating coil can be varied. The spindle of the disc carries a moving contact which closes the fixed contacts (trip circuit contacts) after the operations of directional element. Directional Element: Potential coil of this element is connected through a potential transformer (P.T) to the system voltage. The current coil of the element is energised through a C.T. by circuit current. This winding is carried over the upper magnet of the Non-directional element. The trip contacts (1 and 2) of the directional element are connected in series with the secondary circuit of the over current element. Hence, the non-directional element cannot start to operate until its secondary circuit is completed. In other words, the directional element must operate first (i.e., contacts 1 and 2 should close) in order to operate the overcurrent element .

24. Operation: Under normal operating conditions, power flows in the normal direction causes the directional element (reverse power relay) does not operate thereby keeping the overcurrent element e nergised . Hence, for the operation of the relay the directional element (reverse power relay) must operate first. Under short-circuit conditions, there is a tendency for the current (or) power to flow in the reverse direction causes the disc of the directional element will rotate and as a result it closes the trip circuit contacts thereby completing the secondary circuit of the over current relay unit. However, the disc of the over current unit will not rotate unless the current in the protected line exceeds the preset value. If current exceeds the preset value the overcurrent unit will also be energised and its trip coil circuit will be completed by closing its contacts and hence the circuit breaker to operate and isolate the feeder

25. Impedance Relay The operation of the relay is governed by the ratio of applied voltage to current in the protected circuit. Such relays are called "Impedance Relay“ The voltage element in the relay is excited through a potential transformer (PT) from the line to be protected. The current element is excited from current transformer (C.T) in series with the line. The torque produced by a current element is opposed by the torque produced by a voltage element and the relay operate when the ratio V/l is less than pre-determined value. Consider the portion AB of the line is the protected zone. Under normal operating conditions, the impedance of the protected zone is Z L . The relay is designed so that it closes the trip circuit contacts when ever impedance of the protected section falls below the pre-determined value of the line Z L i.e ., it operates when the ratio is less than pre-determined value.  

26. When ever a fault occurs in the portion of protected zone (AB) at point F₁ . The impedance Z between the point where the relay is connected and the point of fault will be less than the predetermined value of Z L and hence the relay operated. Consequently the circuit breaker will open the circuit so that the line is protected. When ever a fault occurs beyond the protected zone at point F2. The impedance Z between the point where the relay is connected and the point of fault will be more than the pre-determined value Z L and hence the relay is not operated.

27. Distance Relays The increase in the transmission line lengths and the power transmitted has introduced a number of new problems in relation to protection. New techniques have become necessary to handle these problems. Firstly, it has become necessary to see that lines are not unnecessarily disconnected, but at the same time shorter operating times from protective relays are demanded on faulted sections to preserve the system stability. Two major types of transmission line protection schemes are developed extensively to meet modern requirements, namely (a) Distance relaying system (b) Carrier current system (or) pilot wire system Pilot wire systems of protection are not applicable for long over head transmission lines because of high cost of pilot wires and unreliability .

28. Operation of Distance Relay: The operation of all distance relays depends on the basic fact that on the occurrence of a fault, the distance between any point in the power system and the fault is proportional to the ratio of voltage to current at the point. These are used at various points on the system to give a measure of the distance (or) length of the line to the fault and by arranging the relays such that those nearest to fault operate quicker than those at a farther distance. As the measured quantity is proportional to the distance along the line, the measuring relay is called distance relay Family of Distance Relays : A distance protection scheme is a non-unit system of protection. The following are the family of distance relays (a) Impedance relays (b) Reactance relays (c) MHO relays (d) Angle impedance relays (e) Quadrilateral relays (f) Elliptical and other conic section relays Features of Distance Relays : 1. These are used for protection of transmission and sub transmission lines 2. These are operated at higher speed to clear the fault.

29. 3. These are mainly used where the over current relays becomes slow, and there is difficulty in grading time-over current relays. 4. The measured quantity is proportional to the line-length between the location of the relay and the point where the fault has occurred. Working of Distance Relay Using 3-Zone Protection Scheme: The first zone unit which is set to cover usually between 80 to 90% of the first section and is operated with instantaneous high speed relay. The second zone unit which is set to cover about 25% of the second section and the third zone unit which is set to cover upto the end of second section with time-delay operated relays of T2 and T 3 respectively. Setting in the first zone for less than 100% ( ie , 80 to 90%) of the length is made to avoid reach of the relay into adjacent section.

30. (a) Errors in the relay (b) Errors in the CT's and PT's (c) Errors in the data on which the impedance settings are made The main purpose of second zone unit is to provide protection to the end zone of the first section and also to give remote back-up to the next section up to about 25% of its length. The time delay (T2) operated in second zone is normally between 0.2 and 0.5 seconds. The third zone unit provides back-up protection for faults in the adjoining line sections. As far as its reach should extend beyond the end of the largest adjoining line section under condition that causes the maximum amount of under reach. The time delay (T3) operated in third zone is usually between 0.4 seconds and 1.0 seconds.

31. Differential Relays This type of relays operates when the vector difference of two (or) more similar electrical quantities ( Eg : Voltage (or) current) exceeds a pre-determined value. For the operation of differential relay it should have 1. Two (or) more similar electrical quantities 2. These quantities should have phase displacement (Normally approx. 180°). There are two types of differential relays, namely (a) Current balance differential relay (b) Voltage balance differential relay

32. Current Balance Differential Relay: It contains a pair of identical C. T's are connected on either side of the section (or) line to be protected. The protected line is represented in dotted line and it may be a transformer, an alternator, a bus line etc. The secondaries of C.T's are connected in series in such a way that they carry the induced currents in the same direction. The relay operating coil is connected between the mid-points (equipotential points) of the pilot wire.

33. Un der normal operating conditions, the currents in the two secondaries of C.T's are equal and no current will flow through the operating coil (O.C) of the differential relay Therefore, the relay remains inoperative. For an internal fault , when the circuit is fed from one end (a) and when the circuit is fed from both the ends (b). It is seen that in both the cases a current will flow through the operating coil (O.C) of the relay and hence it will operate. This form of protection is known as " Merz -Price Protection”.

34. Disadvantages of Merz -Price Differential Protection: The current transformers (C.T's) used in this protection is not having 100% similar ratings (or) characteristics. Due to this under normal operating conditions they differ in their magnetic properties slightly in terms of different amounts of residual magnetism (or) in terms of unequal burden on the two C.T's, one of the C.T's will saturate earlier during short circuit currents. This disadvantage is overcome by modifying the Merz -price differential protection and is known as "Percentage differential relay (or) biased differential relay".

35. Percentage Differential Protection: It consists of an operating coil and a restraining coil. The operating coil is connected to the mid point of the restraining coil. Total number of ampere-turns on the restraining coil   From the above the current flowing through the restraining coil is . This current produces a pull preventing the trip contacts from closing under normal condition as well as under external fault condition.  

36. Operating Characteristics of Percentage Differential Relay

37. Voltage Balance Differential Relay: It consists of two similar current transformers are connected at either end of the section to be protected. The protected line is represented in dotted line and it may be a transformer, an alternator, a bus line etc. The secondaries of current transformers are connected in series with a relay in such a way that under normal conditions, their induced emf's are in opposition .

38. Under healthy conditions, equal currents flow in both primary windings. Hence , the secondary voltages of the two transformers are balanced against each other and no current will flow through the relay operating coil. When a fault occurs in the protected zone, the currents in the two primaries will differ from one another (i.e., ) and their secondary voltages will no longer be in balance. This voltage difference will cause a current to flow through the operating coil of the relay which closes the trip circuit.