Apparatus Protection Using Relays for Engineering Students

Appartus Protection using Relay

1. UNIT-3 APPARATUS PROTECTION Current transformers and Potential transformers and their applications in protection schemes - Protection of transformer, generator, motor, bus bars and transmission line

2. CT and PT are collectively known as transducers and instrument transformer.

3. Current transformers and Potential transformers and their applications in protection schemes Current or voltage instrument transformers are necessary for isolating the protection, control and measurement equipment from the high voltages of a power system, and for supplying the equipment with the appropriate values of current and voltage - generally these are 1A or 5Α for the current coils, and 110 V for the voltage coils. The behaviour of current and voltage transformers during and after the occurrence of a fault is critical in electrical protection since errors in the signal from a transformer can cause maloperation of the relays.

4. CT PT

5. Symbol of CT PT

6. Current Transformer It performs two tasks Step down the heavy power system currents to low values that are suitable for relays and other measuring instruments (meters). It isolate the relays and meters circuits from high voltages of the power system. The standard current ratings of secondary is 1 A to 5 A . Depending on the applications, CTs are classified int o two categories Measuring CTs Protective CTs

7. Difference between measuring CT and protective CTs Measuring CT Measuring CT’s are required to give high accuracy for all load currents upto 125 % of the rated current. These CT’s may have a significant errors during fault conditions. These Measuring CT’s get saturated at about 1.25 times the full load current. Protective CT Protective CT’s should not saturate upto 20 to 50 times full load current. The DC offset should also be considered while designing the protective CT. The fault current for a secondary of 5 A rating could be 100 to 250 A. Therefore CT secondary having a continuous rating of 5 A should have a short time current rating of 100 A to 250 A , so that the same is not damaged.

8. Core Material of CTs

9. CT Burden It is defined as the load connected across the secondary of the CT . Which is usually expressed in VA. It can also be expressed in terms of impedance at the rated secondary current at a given power factor, usually 0.7 lagging. If the rated burden at rated secondary current I s amperes, the ohmic impedance of the burden can be calculated as follows: Calculate the VA output required for CT of 5 A rated secondary current when burden consists of relay requiring 7.5 VA at 5 A and connecting lead resistance of 0.08 ohm. Suggest the choices of the CT.

10. When the relay is set to operate at current different from the rated secondary current of the CT. The effective burden of the relay can be calculated as follows. Where , P e = Effective CT Burden P r = VA burden of Relay I s = rated secondary current I r = current setting of relay The rated VA output of the CT selected should be higher than standard value nearest to the calculated value.

11. For example: The rated secondary current of a CT is 5A, The plug setting of a relay is 3.75 A. The power consumption of the relay is 4 VA. Calculate the effective VA burden on the CT. CT error – magnitude or ratio error & phase angle error Phase angle Error: The phase difference between the primary and secondary phasor is zero. But actual transformer there is always a difference in phase between the two phasor. The phase difference between the primary current phasor and secondary current phasor is termed as phase angle error.

12. Phasor diagram

13. To minimize CT error: For achieving minimum error in current transformer, one can follow the following, Using a core of high permeability and low hysteresis loss magnetic materials. Keeping the rated burden to the nearer value of the actual burden. Ensuring minimum length of flux path and increasing cross-sectional area of the core, minimizing joint of the core. Lowering the secondary internal impedance.

14. Classification of CTs Based on technology Electromagnetic CTs Opto electronic CTs (optical current sensors) Rogowski Coil Based on Location Indoor CTs Outdoor CTs Based on Application Measuring or metering CTs Protective CTs

15. Electromagnetic current transformer

16. Optoelectronic based CT

17. Rogowski Coil CT

18. Why secondary of CTs should not be kept open ? Secondary of CT should not be kept open. Either it should be shorted or must be connected in series with the low R coil or ammeters. If it is left open, then current through secondary becomes zero hence ampere turns produced by secondary which generally oppose the primary ampere turns becomes zero. This cause high flux in the core, so heavy emf will be induced in the secondary side. This may damage the insulation of the winding.

19. Potential Transformer PT Voltage transformers (VT), also called potential transformers (PT), It performs two task: 1. It is used to reduce the power system voltages to standard lower values. 2. It isolate the relays and other instruments from the high voltages of the power system. They are designed to present negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering. The standard voltage rating of the secondary winding of PTs used in practice is 110 V line to line or 110 / √3 line to neutral.

20. Phasor diagram I s - Secondary current. E s - Secondary induced emf . V s - Secondary terminal voltage. I p - Primary current. E p - Primary induced emf . V p - Primary terminal voltage K T - Turns ratio = Numbers of primary turns/number of secondary turns. I - Excitation current. I m - Magnetizing component of I . I w - Core loss component of I . Φ m - Main flux. β - Phase angle error.

21. PT error Voltage error or Ratio error: The difference between the ideal value V p /K T and actual value V s is the voltage error or ratio error in a potential transformer, it can be expressed as, Normally 2 % or 5% Phase Error or Phase Angle Error in Potential or Voltage Transformer The angle ′β′ between the primary system voltage V p and the reversed secondary voltage vectors K T .V s is the phase error. Normally 2 or 3 degrees To Minimize the overall error: The internal resistance and reactance to an appropriate magnitude. Minimum magnetising and loss components of exciting current required by the core

22. Types of PT’s Electromagnetic type PT It is used upto 132 kV. It is similar to coventional wound type with additional features to minimize errors. Coupling capacitor type PT At higher voltages above 132 kV, Electromagnetic type becomes very expensive. Capacitor voltage divider is used. CCVT Opto electronic type PT’s Opto electronics send a circular polarized light beam that travels through an optical fiber .

23. Electromagnetic type PT

24. Coupling capacitor type PT

25. Comparison between CT and PT CT PT Connected in series with power circuit. Connected in parallel with power circuit. Secondary works almost in short circuited condition. Secondary works almost in open circuit condition. Primary current depends on power circuit current. Primary current depends on secondary burden. Secondary is never be open circuited. Secondary can be used in open circuit condition.

26. Advantages of Instrument Transformers The normal range voltmeter and ammeter can be used along with these transformers to measure high voltage and currents The rating of low range meter can be fixed irrespective of the value of high voltage or current to be measured These transformers isolate the measurement from high voltage and current circuits. This ensures safety of the operator and makes the handling of the equipments very easy and safe. These can be used for operating many types of protecting devices such as relays or pilot lights. Several instruments can be fed economically by single transformer.

27. Disadvantages of Instrument Transformers The only disadvantage of these instrument transformers is that they can be used only for a.c . circuits and not for d.c. circuits.

28. Applications of Instrument Transformers Circulating current differential protection. Over current phase fault protection. Distance protection Intermediate CTs for feeding protective devices, measuring systems, relays etc.

29. Transformer Protection Small size distribution transformers (up to 100 kVA , 11 kV/400 V) only high voltage fuses are used as main protective device. A 250 MVA, 15 kV/400 kV generator transformer may be provided with very elaborate protection. This may consist of percentage differential protection , a protection against incipient faults and protection against over fluxing as primary protection. These will be backed up by over current protection.

30. Types of fault encountered in transformers External faults or through faults. Over load, Lightning, external short circuit unsymmetrical faults, etc Internal faults Short circuit in transformer windings and connections It is serious in nature, it includes LG, LL and inter turn faults on HV and LV windings. Incipient faults. It is minor in nature but slowly develop into major faults. It includes poor winding connections, core faults, failure of coolant, regulator faults and bad load sharing between transformers.

31. Possible Transformer Faults Overheating Winding faults Phase to phase faults Earth faults Interturn faults Open circuits (heating) Through faults (external faults) Over fluxing (V/f)

32. Transformer Protection Over current Protection. Percentage differential Protection (or) Merz - Prize Protection. Protection against Magnetic inrush current. Earth Fault protection.

33. Over current Protection of Transformer The pick up value of phase fault over current unit is set such that they do not pick up on minimum permissible overload, but are sensitive to smallest phase fault. The pick up value of earth fault relay on the other hand is independent of the loading. It is so sensitive. Normally time graded over current relays are provided for backup protection.

34. Connection between Power Transformer and CTs Power Transformer Connections C. T Connections Primary Secondary Primary Secondary Star Delta Delta Star Star Star Delta Delta Delta Star Star Delta Delta Delta Star Star

35. Power Transformer and CTs connections

36. Percentage differential protection of transformer or Merz -Price Protection Star- Delta Transformer

37. Star- Star Transformer

38. The basic requirements of the differential relay are, 1. The differential relay must not operate on load or external faults. 2. It must operate on severe internal faults. 3. The neutrals of C.T. star and power transformer stars are grounded.

39. Problems encountered in differential protection Unmatched characteristics of CTs Ratio change due to tap change Difference in lengths of pilot wires Magnetizing current inrush

40. Protection against magnetic inrush current.

41. 1. When a unloaded transformer is switched on, it draws initial magnetising current which may be several times the rated current of the transformer. 2. This initial magnetising current is called the magnetising inrush current. 3. As the inrush current flows only in primary winding, the differential protection will see this inrush current as an internal fault. 4. The inrush current contents are 40 to 60 % dc component, second harmonic 30 to 70 %, third harmonic 10 to 30 %. 5. This feature can be utilize to distinguish between inrush current and fault current

42. 6. The third harmonic do not appear in CT leads as these harmonics circulate in the delta winding or delta connected CT. 7. Tuned circuit X c , X L allows only fundamental frequency current to operating coil. 8. Second harmonic currents are diverted into the restraining coil

43. Earth fault protection Earth-fault usually involves a partial breakdown of winding insulation to earth and the vector sum 3-phase is no longer zero. The resultant current sets up flux in the core of the CT which induces e.m.f. in the secondary winding.

44. Buchholz relay Gas operated relay, it is used to detect incipient faults which are initially minor faults but may cause major fault in due course of time. It is slow actuating device, the , the minimum operating time is 0.1 sec, the average time 0.2 sec.

45. Advantages of Buchholz Relay Normally a protective relay does not indicate the appearance of the fault. It operates when fault occurs. But buchholz relay gives an indication of the fault at very early stage, by anticipating the fault and operating the alarm circuit. Thus the transformer can be taken out of service before any type of serious damage occurs. It is the simplest protection in case of transformers.

46. Disadvantages of Buchholz Relay Can be used only for oil immersed transformers having conservator tanks Only faults below oil level are detected Setting of the mercury switches can not be kept too sensitive otherwise the relay can operate due to bubbles, vibration, earthquakes mechanical shocks etc. The relay is sloe to operate having minimum operating time of 0.1 seconds and average time of 0.2 seconds.

47. Applications of Buchholz Relay The following type of transformers fault can be protected by the Buchholz relay and are indicated by alarm: Local overheating Entrance of air bubbles in oil Core bolt insulation failure Short circuited laminations Loss of oil and reduction in oil level due to leakage Bad and loose electrical contacts Short circuit between phases Winding short circuit Bushing puncture Winding earth faults

48. Generator Protection Generator is accompanied by prime mover, excitation system, voltage regulator, cooling system, etc its protection becomes very complex and elaborate. The protection scheme for generator involves the consideration of more possible failures or abnormal operating conditions than any other system equipment. Possible faults in generator. Stator fault. Phase to earth fault (occur in armature slots) Phase to phase fault (S.C. two phase winding, melting Cu) Inter turn fault ( multiturn coil, due to high voltage, single turn ) Rotor fault. Field ground fault ( mechanical and thermal stresses on field winding) Rotor over heating Loss of excitation

49. Abnormal running condition. Over loading (temperature rise) Over speed (sudden loss of load, turbogoverner ) Unbalanced loading (- ve sequence current, rotor heating, unsymmetrical faults, failure of C.B) Over Voltage (due to over speed, faulty operation of volt. Regulators) Failure of prime mover (motoring action, draw active power, reverse power protection by directional relay) Loss of excitation (fault in exciter, increase in speed, work as induction gen., draw reactive power, overheating stator winding) Cooling system failure (overheating, insulation failure) Excessive vibration Difference in expansion between rotor and stator parts Loss of synchronism

50. Generator Protection schemes Stator protection Percentage differential protection Protection against stator inter turn faults Stator over heating protection Rotor protection Field ground fault protection Loss of excitation protection Protection against rotor overheating Miscellaneous protection Over voltage protection Over speed protection Protection against motoring

51. Protection against vibration Bearing overheating protection Protection against voltage regulator failure.

52. Generator Protection Stator protection: Percentage Differential protection. Earth fault protection. Rotor protection. Rotor Earth Fault protection Protection against Loss of excitation Protection against Loss of prime mover

53. Basic differential Protection scheme for Generator

54. Basic percentage differential Protection scheme for Generator

55. Percentage differential protection of generator or Merz price protection of generator Star connected Generator This diagram is same for star connected motor

56. Percentage differential protection of generator or Merz price protection of generator Delta connected Generator This diagram is same for delta connected motor

57. Advantages of Merz -Price Scheme Very high speed operation with operating time of about 15 msec. It allows low fault setting which ensures maximum protection of machine windings In ensures complete stability under the most severe through and external faults It does not require current transformers with air gap or special balancing features

58. Earth Fault protection Usually neutral is grounded to protect the complete winding against ground faults. When the neutral is solidly grounded, it is possible to provide protection to complete winding of the generator However, the neutral i s grounded through resistance to limit the ground fault currents. But it is not possible to protect the complete winding against ground faults.

59. Restricted earth fault protection

60. The usual practice is to protect 80 to 85 % of the generator winding against ground fault. The remaining 15-20 % from neutral end is left unprotected.

61. Let, p % of the winding from the neutral remains unprotected. Then (100-p) % of the winding is protected. The ground fault current I f is given by Where V is the line to neutral and R n is the neutral grounding resistance.

62. Unrestricted earth fault protection

63. Balanced earth fault protection Small rating alternators

64. 100% Earth fault protection

65. Stator Protection against Interturn Faults

66. Rotor earth Fault Protection Method 1

67. Rotor earth Fault Protection Method 2

68. Protection against loss of excitation (field failure)

69. Detection of turn-turn fault

70. Negative Sequence Protection

71. Reverse Power Protection (Loss of Prime Mover Protection)

72. Motor Protection Possible fault The types of faults in motors are similar to those of generators Stator Faults Phase fault Ground fault Inter turn fault Rotor Faults Number of induction motors are being used in the industry, It is not possible to make any general statements about the protection of induction motor, since the protection scheme depends on the size (horsepower/kW rating) of the motor and its importance in the system. 3. Prolonged Over load 4. Unbalanced supply voltages including single phasing. 5. Under voltage 6. Reverse of phases. 7. Rotor jam’ 8. Failure of bearings

73. S. No. Abnormal Condition Choice of Protection Circuit to be employed 1 Mechanical overload Overload release, thermal overload relay, over current relay, MCB with built-in trip coil 2 Stalling or prolonged starting of motor Thermal relays, instantaneous overcurrent relay 3 Under voltage Under voltage release, under voltage relay 4 Unbalanced voltage Negative phase sequence relays 5 Reverse phase sequence Phase reversal relay 6 Phase to phase fault or phase to earth HRC fuse, instantaneous overcurrent relays. For large motors, differential protection may be employed for economy 7 Single phasing Thermal overloading relays, single phase preventer

74. Protection circuit for Induction Motor

75. Different types of Motor Protection Schemes Percentage differential protection of Motor (or) Merz Price Protection Phase fault and ground fault protection of motor. Protection of motor against unbalance in supply voltage. Protection against overheating

76. Percentage differential protection of Motor This diagram is same for delta connected motor

77. Single Phasing Preventer

78. Ground Fault Protection

79. Ground Fault Protection

80. Ground Fault Protection by ZSCT

81. Phase fault protection (short circuit protection) of motor Protection against phase faults as well as ground faults can be provided using either fuses , or over-current relays depending upon the voltage rating and size of the motor . (fault current 8 -10 times, attracted armature relay setting is 4 -5 times)

82. Protection of motor against unbalance in supply voltage. Large induction motors are very sensitive to unbalances in supply voltage. The negative sequence component, which comes into picture because of the unbalance in the supply, is particularly troublesome

83. Motor protection against over heating using RTD Relays These relays operate from one or more RTDs that monitor the temperature of the machine winding, motor or load bearings or load case. They are usually applied to large motors of 1500 HP and above.

84. Bus bar protection The word bus is derived from the Latin word omnibus which means common for all. Bus bars are the nerve-centres of the power system where various circuits are connected together. Busbars are located in switchyards, substations, Weakening of insulation because of ageing, corrosion because of salty water, Breakdown of insulation because of overvoltages , foreign objects, and so on. For example, lizards and snakes are known to have caused bus bar faults in remote unmanned substations. The causes of faults experienced on busbars are:

85. Busbar Faults Failure of insulation due to material deterioration Failure of circuit breaker Earth fault due to failure of support insulator Flashover due to sustained excessive over voltages Errors in the operation and maintenance of switchgear Earthquake and mechanical damage Accidents due to foreign bodies falling across the busbars Flashover due to heavily polluted insulator

86. There is a large concentration of short-circuit capacity at the bus bars. A fault on the bus bar, though rare, causes enormous damage. When protective relays operate to isolate the bus bar from the system, What type of protection is best suited for busbar ? Differential protection will suit this situation best because the ends (terminals) of the system are physically near to each other. Thus, by installing CTs on the two sides, we can simply compare the current entering the busbar with that leaving it. Any discrepancy between the two will immediately signal an internal fault.

87. Bus-Bar Protection Percentage differential protection of bus bar Frame leakage protection High Impedance relaying scheme.

88. Percentage differential protection of bus bar The algebraic sum of all the currents entering and leaving the bus bar zone must be zero. Unless there is a fault therin . The relay is connected to trip all the circuit breakers. In case of a bus fault the algebraic sum of currents will not be zero and relay will operate.

89. Selection of CT Ratios Frame leakage protection

90. High Impedance Differential Protection of Busbar

91. Difficulties in Busbar Protection Current levels for different circuits are different Large number of circuits to be protected Saturation of cores of current transformers due to d.c. component in short circuit current is possible which produces ratio error Due to various bus sections, the scheme becomes complicated With large load changes, relay settings need to be changed

92. Transmission line protection Possible faults in a transmission line: Symmetrical Faults. Unsymmetrical Faults. Lightning Faults. Protection Schemes: Over current Protection of transmission line Distance protection of transmission line Pilot relaying protection scheme of transmission line

93. Over current Protection scheme of transmission line

94. Over current protection scheme of transmission line. 1.Current grading scheme 2. Time grading scheme 3. Combined current ant time grading scheme

95. Distance Protection scheme of transmission Line Distance protection schemes are commonly employed for providing the primary or main protection and backup protection for AC transmission line and distribution line  against three phase faults, phase-to-phase faults, and phase-to-ground faults . The distance relay of impedance relay or reactance relay or mho relay can be employed in this protection scheme. Distance Relay Transmission line

96. Pilot relaying protection scheme Pilot relaying schemes are used for the protection of transmission lines. They fall into the category of unit protection. In these scheme some electrical quantities at the two ends of the transmission line are compared and hence they require some sort of interconnected channel (pilot) over which information can be transmitted from one end t other. Different types of pilot scheme based on the channels are Wire pilot protection (distance upto 30 km ) Translay Scheme of protection Carrier current pilot (high freq signal 50kHz to 700 kHz) Micro wave pilot. (very high freq signal 450 MHz to 10000MHz)

97. Wire pilot protection The pilot wires are used to connect the relays. Under normal working condition, the two currents at both ends are equal and pilot wires do not carry any current, keeping relays inoperative. Under fault conditions, The two currents at two ends are no longer same, this causes circulating current flow through pilot wires

98. Translay scheme This system is the modified form of voltage-balance system . In the event of fault on the protected feeder, current leaving the feeder will differ from the current entering the feeder. Consequently, unequal voltages will be induced in the secondary windings of the relays and current will circulate between the two windings, causing the torque to be exerted on the disc of each relay.

99. Carrier Current protection scheme This type of protection is used for protection of transmission lines Carrier currents of the frequency range 30 to 200 kc /s in USA and 80 to 500 kc /s (kHz) in UK are transmitted and received through the transmission lines for the purpose of protection.

100. Coupling capacitor : coupling capacitors allows carrier frequency signals to enter the carrier equipment but does not allow 50 Hz power frequency currents to enter the carrier equipment. It offers low reactance (1/ ωC ) to carrier frequency but high reactance power frequency. Line trap It has a low impedance (less than 0.1 ohm) to 50 Hz and high impedance to carrier frequencies. This unit prevents high frequency signals from entering the neighbouring line, This unit prevents high frequency signals from entering the neighbouring line, and the carrier currents flow only in the protected line.

101. Microwave pilot protection scheme 1 . Microwave scheme uses space as the channel. 2. It uses Ultra transmitter receiver system high frequency (450 MHz to 10000 MHz) for connecting the relaying equipment at the terminals of the protected line.

102. Summary of transmission line protection Lines or feeders can be protected by several methods. Each method has some advantages and some limitations. The classes of protective relays used for line protection ; roughly in ascending order of cost and complexity are : Instantaneous overcurrent                  - Time- overcurrent Directional overcurrent                     Distance Pilot (pilot wire, power line carrier, or microwave).