PROTECTIVE_RELAYS_Fjkknniinpnl.pptx1.pptx

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1. PROTECTIVE RELAYS Power systems 2 Lecturer : Eng S Masimba

2. SURNAME NAME REG NUMBER KAIRASORA WISDOM R213403A CHIBISA WENDY R213385E KUDZEREMA KELVIN R214247X CHINGUWO BRIGHTMORE R213394J MUTAMBASHORA CRISPEN R214252Z NCUBE SIBONGINKOSI R214251V PAWARINGIRA EMMANUEL R214237B CHANANDA CRAIG R214249S

3. INTRODUCTION In complex power systems, consisting of components such as generators, transformers, transmission lines, and distribution circuits , failures are inevitable over time. To maintain system integrity and reliability , it is crucial to have effective mechanisms in place for quick fault detection and disconnection . When a failure occurs, rapid detection and isolation of the fault are critical. This quick response serves two principal purposes: Minimize Service Interruption to customers . Rapid disconnection of faulted components reduces damage to equipment and prevents the fault from affecting the broader system.

4. Use of Fuses and Relays : For effective protection, the power system utilizes fuses and relays along with circuit breakers . Fuses : Automatically perform both detection and interruption but are typically limited to low-voltage applications . Relays and Circuit Breakers : In high-voltage systems (above 3.3 kV), relays work in conjunction with circuit breakers to provide robust fault detection and system protection. Relays detect the fault and instruct the circuit breakers to execute the circuit interruption .

5. WHAT ARE PROTECTIVE RELAYS? Protective Relays : - an essential device , designed to detect faults and command circuit breakers to isolate defective elements. Functionality : Detection : Relays monitor electrical quantities—voltage, current, frequency, and phase angle—which vary between normal and fault conditions . Any deviation in these parameters signals the presence, type, and location of a fault. Response : Upon detecting a fault, the relay activates the trip circuit of the associated circuit breaker. This action causes the breaker to open, disconnecting the faulty circuit from the rest of the electrical system.

6. Primary Winding of the Current Transformer (CT) : Connected in series with the line being protected and responsible for stepping down high currents . Secondary Winding and Relay Operating Coil : The secondary winding of the CT is linked to the relay coil, which becomes energized under fault conditions. Tripping Circuit : This can be AC or DC and includes a power source, the trip coil of the breaker, and the stationary contacts of the relay. Operational Scenario : When a fault like a short circuit occurs (e.g., at point F on a transmission line), it significantly increases the line current. This surge in current flows through the relay coil, prompting the relay to close its contacts. Consequently, this closes the trip circuit of the breaker, opening the breaker and isolating the fault.

7. Essential Qualities of Protective Relaying Systems Key Qualities of Protective Relays : Selectivity : Ability to precisely identify and isolate the part of the system experiencing issues without affecting the rest of the system. Speed : for minimizing equipment damage by swiftly disconnecting the faulty section to prevent prolonged exposure to fault currents. Maintains system voltage by preventing the spread of instability and potential shutdowns caused by persistent low voltage. Sensitivity : Defined by the relay’s ability to operate at low actuating quantities, ensuring that even minimal disturbances are detected and acted upon efficiently.

8. Reliability, Simplicity, and Economic Considerations in Protective Relaying 4 . Reliability : Essential for consistent operation under predetermined conditions, ensuring that the protection system is effective and not a liability. 5. Simplicity : Simplified systems are easier to maintain and typically more reliable, reducing the potential for errors and increasing operational trustworthiness. 6.Economy : Cost-effectiveness is crucial; protective systems should generally not exceed 5% of the total equipment cost. For critical components like generators or main transmission lines, reliability may take precedence over cost to ensure maximal protection .

9. BASIC RELAYS ROLES Essential for detecting and isolating faults within power systems. Operate by monitoring changes in current and voltage supplied by transformers connected to protected system elements. Fault Detection and Isolation: Variations in current and voltage signal fault presence, type, and location. Upon detection, relays activate the trip circuit, causing the circuit breaker to open and disconnect the faulty circuit. Types of Electro-Mechanical Relays: 1. Electromagnetic Attraction : Uses mechanical attraction to an electromagnet to operate. 2. Electromagnetic Induction : Operates through forces generated by electromagnetic fields induced by system currents.

10. ELECTROMAGNETIC ATTRACTION RELAYS Electromagnetic attraction relays are among the simplest and most widely used relays in power systems. They operate based on the principle of electromagnetic attraction, where a movable part is attracted to a fixed electromagnet when current passes through the coil. When current flows through the relay coil, it generates a magnetic field which attracts a movable iron armature, causing the relay contacts to either open or close, depending on the relay design. The operation is fast, making these relays suitable for applications where speed is critical.

11. Types of Electromagnetic Attraction Relays 1 . Attracted Armature Relay : The armature is attracted to the electromagnet, which moves to open or close the contacts. 2 . Solenoid-Type Relay: A solenoid pulls a plunger into the coil, moving the contacts. 3 . Balanced Beam Type Relay: Similar to the solenoid type but used for higher current applications; the plunger's movement changes the state of the contacts.

12. Induction Relays These function on the principle of electromagnetic induction. Alternating current in the relay's coils creates a rotating magnetic field which induces a current in a movable conductor, such as a disc or cup, generating a torque that causes it to rotate. The rotation shifts the relay contacts, leading to the opening or closing of the circuit .

13. T hree types of structures are commonly used for obtaining the phase difference in the fluxes and hence the operating torque in induction relays 1. Shaded Pole Structure uses a copper ring to create a phase-shifted magnetic field, producing the necessary torque for moving the relay's disc or cup (simple and often used for applications requiring a smaller torque). 2. Watt hour Meter or Double Winding Structure This structure is employed in energy meters, consisting of two windings—one for voltage and one for current—placed at right angles. 3. Induction Cup Structure uses four or more coils to create a rotating magnetic field that induces eddy currents in a cup, generating torque. This structure offers precise operation and is used in differential and directional protection.

14. Relay Timing ensures the proper sequence of operations in power system protection to prevent unnecessary tripping of protective devices and allowing only the faulty section to be isolated. Relay timing characteristics 1.Instantaneous Relay responds immediately to fault conditions without any intentional time delay designed for situations where rapid fault isolation is critical, typically operating within a few milliseconds. 2.Inverse Time Relay feature an operating time that decreases as the fault current increases allowing for faster response to severe faults while maintaining coordination with other devices. 3. Definite Time Lag Relay have a fixed operating delay, regardless of the fault current magnitude. This delay is preset to ensure that the relay operates after a specific time, allowing for coordination with other protection devices.

15. IMPORTANT TERMS Pick-up current- the minimum current in the relay coil at which the relay starts to operate. For currents less than this current ,the breaker controlled by it remains in the closed position. However, when the relay coil current is equal to or greater than the pickup value, the relay operates to energise the trip coil which opens the circuit breaker Current setting- It is often 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 tappings on the relay operating coil. The taps are brought out to a plug bridge .

16. Pick-up current = Rated secondary current of C.T. × Current setting For example, suppose that an overcurrent relay having current setting of 125% is connected to a supply circuit through a current transformer of 400/5. The rated secondary current of C.T. is 5 amperes. Therefore, the pick-up value will be 25% more than 5 A i.e. 5 × 1·25 = 6·25 A. It means that with above current setting, the relay will actually operate for a relay coil current equal to or greater than 6·25 A. The current plug settings usually range from 50% to 200% in steps of 25% for overcurrent relays and 10% to 70% in steps of 10% for earth leakage relays. The desired current setting is obtained by inserting a plug between the jaws of a bridge type socket at the tap value required. (iii) Plug-setting multiplier (P.S.M.)- It is the ratio of fault current in relay coil to the pick-up current i.e. For example, suppose that a relay is connected to a 400/5 current transformer and set at 150%. With a primary fault current of 2400 A, the plug-setting multiplier can be calculated as below Pick-up value = Rated secondary current of CT × Current setting = 5 × 1·5 = 7·5 A Fault current in relay coil = ×5 = 30 A ∴ P.S.M = 30/7·5 = 4 P.S.M value has no units since it is a ratio.  

17. (iv) Time-setting multiplier- A relay is generally provided with control to adjust the time of operation. This adjustment is known as time-setting multiplier. The time-setting dial is calibrated from 0 to 1 in steps of 0.05 sec as shown in the diagram These figures are multipliers to be used to convert the time derived from time/P.S.M. curve into the actual operating time. Thus if the time setting is 0·1 and the time obtained from the time/P.S.M. curve is 3 seconds, then actual relay operating time = 3 × 0·1 = 0·3 second. For instance, in an induction relay, the time of operation is controlled by adjusting the amount of travel of the disc from its reset position to its pickup position. This is achieved by the adjustment of the position of a movable backstop which controls the travel of the disc and thereby varies the time in which the relay will close its contacts for given values of fault current. A so-called “time dial” with an evenly divided scale provides this adjustment. The actual time of operation is calculated by multiplying the time setting multiplier with the time obtained from time/P.S.M. curve of the relay.

18. TIME/P.S.M CURVE The diagram below shows the curve between time of operation and plug setting multiplier of a typical relay. The horizontal scale is marked in terms of plug-setting multiplier and represents the number of times the relay current is in excess of the current setting. The vertical scale is marked in terms of the time required for relay operation. If the P.S.M. is 10, then the time of operation (from the curve) is 3 seconds. The actual time of operation is obtained by multiplying this time by the time-setting multiplier. It is evident from the Time/P.S.M Curve that for lower values of overcurrent, time of operation varies inversely with the current but as the current approaches 20 times full-load value, the operating time of relay tends to become constant. This feature is necessary in order to ensure discrimination on very heavy fault currents flowing through sound feeders.

19. Calculation of Relay Operating Time I n order to calculate the actual relay operating time, the following things must be known : (a) Time/P.S.M. curve (b) Current setting (c) Time setting (d) Fault current (e) Current transformer ratio The procedure for calculating the actual relay operating time is as follows : ( i ) Convert the fault current into the relay coil current by using the current transformer ratio. (ii) Express the relay current as a multiple of current setting i.e. calculate the P.S.M. (iii) From the Time/P.S.M. curve of the relay, read off the time of operation for the calculated P.S.M. (iv) Determine the actual time of operation by multiplying the above time of the relay by time-setting multiplier in use

20. TUTORIAL QUESTION

21. Functional type relays The classification of protective relays is done based on the functionality of the relay in the power system. some common and important special-function relays are : Induction type overcurrent relays Induction type reverse power relays Distance relays Differential relays Translay scheme

22. Induction type overcurrent relay (non-directional) Works on the induction principle and initiates the operation when the current exceeds the pick-up current. The actuating source is the current from the CT These relays are used on a.c circuits only and can operate in either direction.

23. Constructional diagram Consists of an aluminium metal disc that is free to rotate between the poles of the two electromagnets. The primary is connected to secondary of the CT via a plug-setting bridge for tap changing purposes thus allowing for current setting of the relay . The spindle carries a moving contact which bridges two fixed contacts (connected the trip circuit ) when the disc rotates through a preset angle. The time setting can be adjusted by adjusting the angle of rotation.

24. The driving torque on the disc is set up following the principle discussed for the watthour-meter structure relay. The torque is opposed by the restraining torque from the spring. Under normal conditions the restraining torque is dominant. Thus the relay contacts remain open. However , the instant the relay coil current exceeds the pick-up current, the driving torque becomes dominant thus rotating the disc to initiate the isolation process.

25. Induction type directional power relay Operates when power in the circuit flows in a specific direction. Designed in such way that its operating torque is obtained from the interaction of the magnetic fields from both voltage and current source of the circuit it protects. Basically a watt meter and torque direction is dependent on that of the current relative the associated voltage . Constructional details Employs a similar structure to that of the watt-hour meter relay. The upper electromagnet carries a winding (potential coil) on the central limb connected through a potential transformer to the voltage circuit. The lower magnet thus have a separate winding (current coil) connected to the CT

26. The current coil is provided with the tap setting connected to the plug setting bridge for current setting. The restraining torque is provided by the spiral spring. The disc is rotated through a preset angle and the adjustment of this angle of rotation results in time-setting.

27. OPERATION The flux in the potential coil will lag behind applied voltage, V by nearly 90 whereas the flux in the current coil will nearly be in phase with the operating current I (see phasor diagram) From the expression deduced below it can be noted that the direction of the driving torque depends upon the direction of the power flow Interaction of the fluxes to produce the driving force As long as power flows in the normal direction, the driving torque and the restraining act to keep the contacts open.

28. Operation cont ’ The instant the current changes direction ( reverses) it reverses the driving torque direction on the disc. When driving torque is large enough the disc moves to close the relay contacts. This initiates the isolation process of the faulty section.

29. Time-Distance Impedance Relay Is the one which automatically adjusts its operating time according to the distance of the relay from the fault point i.e. Operating time, T ∝ V/I ∝ Z ∝ distance CONSTRUCTION It consists of a current driven induction element similar to the double winding type induction overcurrent relay. The spindle carrying the disc of this element is connected by means of a spiral spring coupling to a second spindle which carries the bridging piece of the relay trip contacts. The bridge is normally held in the open position by an armature held against the pole face of an electromagnet excited by the voltage of the circuit to be protected .

30. OPERATION

31. DIFFERENTIAL RELAY operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined value . Thus it compares the current entering a section of the system with the current leaving the section. Under normal operating conditions, the two currents are equal but as soon as a fault occurs, this condition no longer applies. The difference between the incoming and outgoing currents is arranged to flow through the operating coil of the relay. If this differential current is equal to or greater than the pickup value, the relay will operate and open the circuit breaker to isolate the faulty section . A ny type of relay when connected in a particular way can be made to operate as a differential relay. There are two fundamental systems of differential or balanced protection ( i ) Current balance protection (ii) Voltage balance protection

32. Current Differential Relay Fig below shows an arrangement of an overcurrent relay connected to operate as a differential relay. A pair of identical current transformers are fitted on either end of the section to be protected (alternator winding in this case). The secondaries of CT’s are connected in series in such a way that they carry the induced currents in the same direction. The operating coil of the overcurrent relay is connected across the CT secondary circuit. This differential relay compares the current at the two ends of the alternator winding. OPERATION Under normal operating conditions, suppose the alternator winding carries a normal current of 1000 A. Then the currents in the two secondaries of CT’s are equal . These currents will merely circulate between the two CT’s and no current will flow through the differential relay. Therefore, the relay remains inoperative.

33. OPERATION If a ground fault occurs on the alternator winding as shown in Fig. 21.24 ( i ), the two secondary currents will not be equal and the current flows through the operating coil of the relay, causing the relay to operate. The amount of current flow through the relay will depend upon the way the fault is being fed. ( i ) If some current (500 A in this case) flows out of one side while a larger current (2000 A) enters the other side as shown in Fig. 21.24 , then the difference of the CT secondary currents i.e. 10 − 2·5 = 7·5 A will flow through the relay. (ii) If current flows to the fault from both sides as shown in Fig. 21.24 then sum of CT secondary currents i.e. 10 + 5 = 15 A will flow through the relay

34. DISADVANTAGES ( i ) The impedance of the *pilot cables generally causes a slight difference between the currents at the two ends of the section to be protected. If the relay is very sensitive, then the small differential current flowing through the relay may cause it to operate even under no fault conditions. ( ii) Pilot cable capacitance causes incorrect operation of the relay when a large through-current flows . (iii) Accurate matching of current transformers cannot be achieved due to pilot circuit impedance. The above disadvantages are overcome to a great extent in biased beam relay

35. BIASED BEAM RELAY is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected section . Fig. shows the schematic arrangement of a biased beam relay. It is essentially an overcurrent balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force . Under normal and through load conditions, the bias force due to restraining coil is greater than the operating force. Therefore, the relay remains inoperative. When an internal fault occurs, the operating force exceeds the bias force. Consequently, the trip contacts are closed to open the circuit breaker. The bias force can be adjusted by varying the number of turns on the restraining coil.

36. EQUIVALENT CIRCUIT OF BIASED BEAM RELAY The equivalent circuit of a biased beam relay is shown in Fig. 21.26. The differential current in the operating coil is proportional to i2 − i1 and the equivalent current in the restraining coil is proportional to *(i1 + i2)/2 since the operating coil is connected to the mid-point of the restraining coil. It is clear that greater the current flowing through the restraining coil, the higher the value of current required in the operating winding to trip the relay. Thus under a heavy load, a greater differential current through the relay operating coil is required for operation than under light load conditions. This relay is called percentage relay because the operating current requried to trip can be expressed as a percentage of load current.

37. Voltage balance differential Relay T wo similar current transformers are connected at either end of the element to be protected (e.g. an alternator winding) by means of pilot wires. The secondaries of current transformers are connected in series with a relay in such a way that under normal conditions, their induced e.m.f.s ’ are in opposition. Under healthy conditions, equal currents (I1 = I2) flow in both primary windings. Therefore, 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. I1 ≠ I2) 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.

38. Disadvantages A multi-gap transformer construction is required to achieve the accurate balance between current transformer pairs. The system is suitable for protection of cables of relatively short lengths due to the capacitance of pilot wires. On long cables, the charging current may be sufficient to operate the relay even if a perfect balance of current transformers is attained. These disadvantages have been overcome in Translay (modified) balanced voltage system.

39. TRANSLAY SYSTEM This system is the modified form of voltage-balance system. Although the principle of balanced (opposed) voltages is retained, it differs from the above voltage-balance system in that the balance or opposition is between voltages induced in the secondary coils wound on the relay magnets and not between the secondary voltages of the line current transformers. Since the current transformers used with Translay scheme have only to supply to a relay coil, they can be made of normal design without any air gaps. This permits the scheme to be used for feeders of any voltage. The relays used embrace the function of transformer as well as relay. Hence the name Translay .

40. Construction Fig. 21.28 shows the simplified diagram illustrating the principle of Translay scheme. It consists of two identical double winding induction type relays fitted at either end of the feeder to be protected. The primary circuits (11, 11a) of these relays are supplied through a pair of current transformers. The secondary windings (12, 13 and 12a, 13a) of the two relays are connected in series by pilot wires in such a way that voltages induced in the former opposes the other. The compensating devices (18, 18a) neutralise the effects of pilot-wire capacitance currents and of inherent lack of balance between the two current transformers.

41. Types of Protection When a fault occurs on any part of electric power system, it must be cleared quickly in order to avoid damage and/or interference with the rest of the system. It is a usual practice to divide the protection scheme into two classes. Primary Protection . It is the protection scheme which is designed to protect the component parts of the power system. Thus referring to Fig. 21.29, each line has an overcurrent relay that protects the line. If a fault occurs on any line, it will be cleared by its relay and circuit breaker. This forms the primary or main protection and serves as the first line of defence. The service record of primary relaying is very high with well over ninety percent of all operations being correct.

42. ii ) Back-up protection. It is the second line of defence in case of failure of the primary protection. It is designed to operate with sufficient time delay so that primary relaying will be given enough time to function if it is able to. Thus referring to Fig. 21.29, relay A provides back-up protection for each of the four lines. If a line fault is not cleared by its relay and breaker, the relay A on the group breaker will operate after a definite time delay and clear the entire group of lines. It is evident that when back-up relaying functions, a larger part is disconnected than when primary relaying functions correctly. Therefore, greater emphasis should be placed on the better maintenance of primary relaying .

43. SUMMARY BASIC PROTECTIVE RELAYS RELAY TIMING Instantaneous relay- no intentional time delay is provided Inverse-time relay- operating time approximately inversely proportional to the magnitude of the actuating quantity Definite time lag relay- a definite time elapse between the instant pickup and closing of relay contacts Electromagnetic attraction Electromagnetic induction Attracted armature type Solenoid type Balanced beam type Shaded-pole structure Watthour-meter structure Induction cup structure

44. SUMMARY FUNCTIONAL RELAY TYPES Induction type overcurrent relay Induction type reverse power relay Distance relays Differential relays Translay scheme TYPES OF PROTECTION PRIMARY PROTECTION- protection scheme designed to protect the component parts of the power system BACK-UP PROTECTION- second line of defense in case of failure of primary protection

45. APPLICATION OF PROTECTIVE RELAYS Overcurrent Protection : Protects transformers, generators, and feeders from overload conditions and short circuits by Detecting excessive current flow and trips the circuit to prevent damage . 2) Differential Protection : Commonly used for transformers, generators, and busbars. Function: Compares the current entering and leaving a device; if a difference is detected, it indicates a fault. 3) Distance Protection : Used in transmission lines Function: Measures the impedance to determine the distance to a fault, allowing for selective tripping. 4) Voltage Protection : Protects equipment from overvoltage and undervoltage conditions. Function: Monitors voltage levels and can trip circuits to prevent damage. 5) Ground Fault Protection : Used in industrial facilities to detect ground faults. Function: Monitors current imbalance and trips the circuit when a ground fault is detected. 6) Motor Protection : Used for industrial motors. Function: Combines various protective functions (overload, phase failure, etc.) to protect motors.

46. CONCLUSION Protective relays are essential for maintaining reliability of power systems in industrial applications by preventing equipment damage and ensuring safety by quickly detecting and isolating faults. THANK YOU