Are you using the right CTs and PTs for your application John Levine, P.E. Levine Lectronics and Lectric E-mail: JohnL

1. Are you using the right CT's and PT's for your application?John Levine, P.E.Levine Lectronics and LectricE-mail: John@L-3.comGE Multilin/ Instrument Transformers, Inc.Manufacturers Representative

2. Why use Instrument Transformers? Circuit Isolation Reduce voltage and currents to reasonable working levels. Phasor combinations for summing and measuring power

3. Definitions Instrument Transformer (IT) - A high precision transformer designed to provide input into measurement and/or control equipment. Examples: Voltmeters Ammeters Watthour Meters Relays

4. Definitions Current Transformer (CT) An instrument transformer used to reflect a primary current into a secondary current through a magnetic medium. Always connected in series with the primary conductor. The nominal secondary current is often a 5 amp basis for ease of measurement. Construction can be one primary turn (Window, donut, or Bar type), or wound primary turns (usually for low ratios)

7. The `doughnut' fits over the primary conductor, which constitutes one primary turn. If the toroid is wound with 240 secondary turns, then the ratio of the C.T. is 240 : 1 or 1200 : 5A The continuous rating of the secondary winding is normally 5 AMPS in North America, and 1 AMP or 0.5 AMP in many other parts of the world. This type of `doughnut' C.T. is most commonly used in circuit breakers and transformers. The C.T. fits into the bushing `turret', and the porcelain bushing fits through the centre of the `doughnut'. Up to four C.T.'s of this type can be installed around each bushing of an oil circuit breaker. This arrangement is shown in the following diagram. The `doughnut' fits over the primary conductor, which constitutes one primary turn. If the toroid is wound with 240 secondary turns, then the ratio of the C.T. is 240 : 1 or 1200 : 5A The continuous rating of the secondary winding is normally 5 AMPS in North America, and 1 AMP or 0.5 AMP in many other parts of the world. This type of `doughnut' C.T. is most commonly used in circuit breakers and transformers. The C.T. fits into the bushing `turret', and the porcelain bushing fits through the centre of the `doughnut'. Up to four C.T.'s of this type can be installed around each bushing of an oil circuit breaker. This arrangement is shown in the following diagram.

8. A similar type of C.T. can be fitted over low voltage buswork. However, the C.T. must be insulated for the primary voltage level. A similar type of C.T. can be fitted over low voltage buswork. However, the C.T. must be insulated for the primary voltage level.

10. The other principal type of C.T. construction is the Free Standing, or Post type. These can be either Straight-Through or Hairpin construction. The toroid, wound with secondary turns, is located in the live tank at the top of the C.T. High voltage insulation must, of course, be provided, between the H.V. primary conductor, and the secondary winding, which operates at essentially ground potential. Current transformers of this type, for voltage levels of 44kV, 27.6kV, and 13.8kV are often called BEEHIVE C.T.'s The other principal type of C.T. construction is the Free Standing, or Post type. These can be either Straight-Through or Hairpin construction. The toroid, wound with secondary turns, is located in the live tank at the top of the C.T. High voltage insulation must, of course, be provided, between the H.V. primary conductor, and the secondary winding, which operates at essentially ground potential. Current transformers of this type, for voltage levels of 44kV, 27.6kV, and 13.8kV are often called BEEHIVE C.T.'s

11. The second kind of Free-Standing or Post type current transformer is the Hairpin construction as shown above: The HAIRPIN C.T. gets it's name from the shape of the primary conductor within the porcelain. With this type, the tank housing the toroid is at ground potential. The primary conductor is insulated for the full line voltage as it passes into the tank and through the toroid. Current transformers of this type are commonly used on H.V. transmission systems such as Ontario Hydro's 500kV and 230kV systems. Free standing current transformers are very expensive, and are only used where it is not possible to install `Doughnut' C.T.'s in Oil Breakers or transformer bushing turrets. As an example, C.T.'s cannot easily be accommodated in Air Blast circuit breakers, or some outdoor SF6 breakers. Free Standing current transformers must therefore be used with these types of switchgear. Current transformers often have multiple ratios. This is achieved by having taps on various points of the secondary winding, to provide the different turns ratios. Later in this section we will discuss the characteristics and testing of C.T's. The second kind of Free-Standing or Post type current transformer is the Hairpin construction as shown above: The HAIRPIN C.T. gets it's name from the shape of the primary conductor within the porcelain. With this type, the tank housing the toroid is at ground potential. The primary conductor is insulated for the full line voltage as it passes into the tank and through the toroid. Current transformers of this type are commonly used on H.V. transmission systems such as Ontario Hydro's 500kV and 230kV systems. Free standing current transformers are very expensive, and are only used where it is not possible to install `Doughnut' C.T.'s in Oil Breakers or transformer bushing turrets. As an example, C.T.'s cannot easily be accommodated in Air Blast circuit breakers, or some outdoor SF6 breakers. Free Standing current transformers must therefore be used with these types of switchgear. Current transformers often have multiple ratios. This is achieved by having taps on various points of the secondary winding, to provide the different turns ratios. Later in this section we will discuss the characteristics and testing of C.T's.

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13. If we have a 27.6 kV C.T. with a ratio of 1200 : 5A, the secondary winding is continuously rated for 5 Amps. If the maximum fault current that can flow through the C.T. is 12,000 Amps, then the C.T. must accurately produce a secondary current of 50 Amps to flow through the relay during this fault condition. This current will, of course, flow for only about 0.2 seconds, until the fault current is interrupted by the tripping of the circuit breaker. The C.T. must be designed such that the iron core does not saturate for currents below the maximum fault current. A magnetizing, or excitation curve for a typical C.T. is shown next.If we have a 27.6 kV C.T. with a ratio of 1200 : 5A, the secondary winding is continuously rated for 5 Amps. If the maximum fault current that can flow through the C.T. is 12,000 Amps, then the C.T. must accurately produce a secondary current of 50 Amps to flow through the relay during this fault condition. This current will, of course, flow for only about 0.2 seconds, until the fault current is interrupted by the tripping of the circuit breaker. The C.T. must be designed such that the iron core does not saturate for currents below the maximum fault current. A magnetizing, or excitation curve for a typical C.T. is shown next.

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15. CAUTION: When C.T.'s are in service they MUST have a continuous circuit connected across the secondary terminals. If the C.T. secondary is `open circuit' Whilst primary current is flowing, dangerously high voltages will appear across the C.T. secondary terminals. Extreme care must be exercised when performing `on load' tests on C.T. circuits, to ensure that a C.T. is not inadvertently `open circuited'. CAUTION: When C.T.'s are in service they MUST have a continuous circuit connected across the secondary terminals. If the C.T. secondary is `open circuit' Whilst primary current is flowing, dangerously high voltages will appear across the C.T. secondary terminals. Extreme care must be exercised when performing `on load' tests on C.T. circuits, to ensure that a C.T. is not inadvertently `open circuited'.

16. What is your application? If your application is metering , how high do I need to go in current? 2 times ? If your application is relaying, how high do I need to go in current? 20 times ?

17. CT Classification for relaying Over the years many standards for CT classification have been developed in North America and Europe. Protection class CT�s are assumed to be able to supply 20 times its rated secondary current to the relay. That means for a 5 amp rated secondary the CT must be able to supply 100 Amps of current, and for a 1 amp rated secondary the CT must be able to supply 20 Amps of current. The operating principals of CT�s are specified in a format such as this. The first number represents the maximum amount of error, listed in as a percentage, that this CT will produce. Therefore, the 10 in our example stands for no more then 10 percent error. The second item, which is always a letter, can either be a T, C, K, L, or H. If the letter is a T which stands for �test�, it means that the CT accuracy can only be determined by testing the CT. Current transformers with non-distributed windings fit in this category. If the letter is a C or a K which stands for �Calculated�, it means the CT accuracy can be determined by performing calculation using given excitation characteristics. CTs with fully distributed windings, (bushing CT�s for instance) fit in this category. If the letter is an L, this indicates that the CT has a �Low internal secondary impedance,� If the letter is an H, this indicates that the CT has a �high internal secondary impedance,�CT Classification for relaying Over the years many standards for CT classification have been developed in North America and Europe. Protection class CT�s are assumed to be able to supply 20 times its rated secondary current to the relay. That means for a 5 amp rated secondary the CT must be able to supply 100 Amps of current, and for a 1 amp rated secondary the CT must be able to supply 20 Amps of current. The operating principals of CT�s are specified in a format such as this. The first number represents the maximum amount of error, listed in as a percentage, that this CT will produce. Therefore, the 10 in our example stands for no more then 10 percent error. The second item, which is always a letter, can either be a T, C, K, L, or H. If the letter is a T which stands for �test�, it means that the CT accuracy can only be determined by testing the CT. Current transformers with non-distributed windings fit in this category. If the letter is a C or a K which stands for �Calculated�, it means the CT accuracy can be determined by performing calculation using given excitation characteristics. CTs with fully distributed windings, (bushing CT�s for instance) fit in this category. If the letter is an L, this indicates that the CT has a �Low internal secondary impedance,� If the letter is an H, this indicates that the CT has a �high internal secondary impedance,�

18. AC saturation occurs when the CT secondary excitation voltage rises above the knee point of the CT excitation curve. In North America the definition of the knee point is defined as the point on the excitation curve where a line drawn perpendicular to the curve is at an angle of 45 degrees with respect to the X axis. This definition comes from IEEE Std C37.110-1996. AC saturation occurs when the CT secondary excitation voltage rises above the knee point of the CT excitation curve. In North America the definition of the knee point is defined as the point on the excitation curve where a line drawn perpendicular to the curve is at an angle of 45 degrees with respect to the X axis. This definition comes from IEEE Std C37.110-1996.

19. Important:Instrument Transformer Accuracy is Always a Function of Applied Burden.Lead wires for CTs can be significant.

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22. Definitions Transformer ratio (TR) The ratio of a primary current or voltage to a secondary current or voltage. Examples: A 100:5 window CT will deliver 5 amperes of secondary current when 100 amperes of primary current is passed through a center window. Often expressed as XXX:1 for PTs

23. Accuracy Terminology Ratio Correction factor (RCF) The measure of transformer amplitude accuracy. I.E. the ratio of the marked (true) ratio to the actual (performance) ratio of a transformer. Primary = Secondary(measured) * Marked ratio * RCF

24. Accuracy Terminology Phase Angle (PA) The phase displacement between the primary and secondary circuit of an instrument transformer. Usually expressed in minutes.

25. Accuracy Terminology Transformer Correction factor (TCF) The ratio of true to measured watts or watt-hours divided by the marked ratio. The TCF is the product of the ratio correction factor and phase angle correction factor, so that it is determined by the ratio error, the phase angle shift and the power factor of the load.

27. Standard CT Burdens Standard CT burdens are defined in IEEE Std. C57-13-1993 Metering burdens are B0.1, B0.2, B0.5, B0.9, and B1.8 where each number represents the total impedance at a 0.9 power factor. VA for each burden is 2.5, 5.0, 12.5, 22.5, and 45. Relay burdens are B1, B2, B4 and B8 where each number represents the total impedance at a 0.5 power factor.

28. Standard CT Accuracy Metering accuracy classes are: 0.3%, 0.6% and 1.2% 0.3% is the traditional revenue metering class The limits of ratio and phase errors are defined by a parallelogram for accuracy from 0.6 to 1.0 metering load power factors. IEEE Std. C57-13-1993 requires that the performance be within the next lower relay class limits at 10% of rated current

31. This transformer meets 0.3 B0.1 accuracy class per IEEE C57.13

32. You may also see transformer accuracy performance presented in this format

33. Definition Polarity The primary and secondary transformer connections must be marked so that the relative instantaneous direction of current flow can be identified. The primary is often marked �H1� and the secondary �X1�. Another common practice is to use �polarity dots� to identify in phase terminations. Clarification: If you view a window type CT with the �H1� face toward you, then current flowing from your direction through the window is in phase with the �X1� terminal.

38. In this example the resistance of the C.T. secondary circuit, or C.T. burden is: C.T. Secondary Winding Resistance = 1 OHM Resistance of Cable from C.T. to Relay = 2 OHMS Resistance of Relay Coil = 2 OHMS Total Resistance of C.T. Secondary Circuit = 5 OHMS � If the fault current is 12,000 Amps, and the C.T. ratio is 1200 : 5A, then the C.T. secondary current is 50 Amps. At this secondary current and the above C.T. burden of 5 OHMS, the C.T. must produce a terminal voltage of 250 volts. For the C.T. to operate with good accuracy, without saturating for the maximum fault current, the knee point must be well above 250 volts. In this example the resistance of the C.T. secondary circuit, or C.T. burden is: C.T. Secondary Winding Resistance = 1 OHM Resistance of Cable from C.T. to Relay = 2 OHMS Resistance of Relay Coil = 2 OHMS Total Resistance of C.T. Secondary Circuit = 5 OHMS � If the fault current is 12,000 Amps, and the C.T. ratio is 1200 : 5A, then the C.T. secondary current is 50 Amps. At this secondary current and the above C.T. burden of 5 OHMS, the C.T. must produce a terminal voltage of 250 volts. For the C.T. to operate with good accuracy, without saturating for the maximum fault current, the knee point must be well above 250 volts.

39. In this example the resistance of the C.T. secondary circuit, or C.T. burden is: C.T. Secondary Winding Resistance = 1 OHM Resistance of Cable from C.T. to Relay = 2 OHMS Resistance of Relay Coil = 2 OHMS Total Resistance of C.T. Secondary Circuit = 5 OHMS � If the fault current is 12,000 Amps, and the C.T. ratio is 1200 : 5A, then the C.T. secondary current is 50 Amps. At this secondary current and the above C.T. burden of 5 OHMS, the C.T. must produce a terminal voltage of 250 volts. For the C.T. to operate with good accuracy, without saturating for the maximum fault current, the knee point must be well above 250 volts. In this example the resistance of the C.T. secondary circuit, or C.T. burden is: C.T. Secondary Winding Resistance = 1 OHM Resistance of Cable from C.T. to Relay = 2 OHMS Resistance of Relay Coil = 2 OHMS Total Resistance of C.T. Secondary Circuit = 5 OHMS � If the fault current is 12,000 Amps, and the C.T. ratio is 1200 : 5A, then the C.T. secondary current is 50 Amps. At this secondary current and the above C.T. burden of 5 OHMS, the C.T. must produce a terminal voltage of 250 volts. For the C.T. to operate with good accuracy, without saturating for the maximum fault current, the knee point must be well above 250 volts.

40. Potential Transformers

41. Definitions Voltage Transformer (VT) An instrument transformer used to reflect a primary voltage into a secondary voltage through a magnetic medium. Always connected in parallel with primary conductor across a circuit load. Secondary (measuring) voltage is usually 115 or 120 volts nominally. The secondary voltage level is selected for ease of measurement and safety. Control Power Transformer (CPT) Designed to provide power for contractors, relays and devices with high inrush currents, Regulation is not as critical.

43. Potential Transformers Potential transformers are used to isolate and step down and accurately reproduce the scaled voltage for the relay. The primary side of a PT needs to have the System voltage, symbolized as Vp, applied across the input terminals as shown here. The Secondary side of the PT will then accurately replicates a scaled down version the primary voltage over a defined voltage range . The secondary voltage is symbolized as Vs. There are two types of potential transformers: Electromagnetic voltage transformer and the capacitive voltage transformers. Potential Transformers Potential transformers are used to isolate and step down and accurately reproduce the scaled voltage for the relay. The primary side of a PT needs to have the System voltage, symbolized as Vp, applied across the input terminals as shown here. The Secondary side of the PT will then accurately replicates a scaled down version the primary voltage over a defined voltage range . The secondary voltage is symbolized as Vs. There are two types of potential transformers: Electromagnetic voltage transformer and the capacitive voltage transformers.

44. There are two types of potential transformers: Electromagnetic voltage transformer and the capacitive voltage transformers. Electromagnetic Voltage Transformers are usually used when accurate metering needs to be performed for lower voltage applications Capacitive voltage transformers are commonly used in high voltage transmission line applications where the voltage is higher that 66kv. There are two types of potential transformers: Electromagnetic voltage transformer and the capacitive voltage transformers. Electromagnetic Voltage Transformers are usually used when accurate metering needs to be performed for lower voltage applications Capacitive voltage transformers are commonly used in high voltage transmission line applications where the voltage is higher that 66kv.

45. VOLTAGE TRANSFORMERS Voltage Transformers are used to step the power system primary voltage from, say 50 kV or 33 kV to 120 volts phase-to-phase, or 69 volts phase-to-ground. It is this secondary voltage that is applied to the fault detecting relays, and meters. The voltage transformers at these primary voltages of 50 kV and 33 kV are normally of the WOUND type. That is, a two winding transformer in an oil filled steel tank, with a turns ratio of 416.6:1 or 275:1. On higher voltage systems, such as 230kV and 500kV, CAPACITOR VOLTAGE TRANSFORMERS, (or CVT's) are normally used. A CVT is comprised of a capacitor divider made up from 10 equal capacitors, connected in series from the phase conductor to ground, with a voltage transformer connected across the bottom capacitor. This V.T. actually measures one-tenth of the line voltage, as illustrated in the diagram above: VOLTAGE TRANSFORMERS Voltage Transformers are used to step the power system primary voltage from, say 50 kV or 33 kV to 120 volts phase-to-phase, or 69 volts phase-to-ground. It is this secondary voltage that is applied to the fault detecting relays, and meters. The voltage transformers at these primary voltages of 50 kV and 33 kV are normally of the WOUND type. That is, a two winding transformer in an oil filled steel tank, with a turns ratio of 416.6:1 or 275:1. On higher voltage systems, such as 230kV and 500kV, CAPACITOR VOLTAGE TRANSFORMERS, (or CVT's) are normally used. A CVT is comprised of a capacitor divider made up from 10 equal capacitors, connected in series from the phase conductor to ground, with a voltage transformer connected across the bottom capacitor. This V.T. actually measures one-tenth of the line voltage, as illustrated in the diagram above:

46. Standard VT Burdens

48. Definition Thermal Rating Factor (TRF or RF) represents the maximum continuous thermal rating of an instrument transformer. For a CT, this is expressed as a number representing a multiplier applied to the nominal rating. For a VT, the rating is expressed applied VA. In both cases, the rating is incomplete unless the ambient temperature is defined! For example, a unit assigned a RF of 1.33 at 30 deg C, will be assigned a RF of 1.00 @ 55 deg C. What could happen under a single phase fault to ground if you are using a 50 to 5 Ground Fault CT?

52. Instrument Transformer Selection

53. Current Transformer Required Information Nominal System Voltage Basic Impulse Insulation Level Environment Accuracy Class Burden Physical mounting space requirements Load Current Over current capability

54. Voltage Transformer Required Information Nominal System Voltage Basic Impulse Insulation Level Environment Accuracy Class Burden Type of Circuit connection Physical mounting space requirements Fusing

55. Transformer Construction MOLDED Rubber, Epoxy, Polyurethane, Plastic Injection Under vacuum, at atmosphere, or under pressure PLASTIC CASE TAPE WRAPPED HYBRIDS Examples: Epoxy or polyurethane cast plastic case Rubber injection over epoxy cast coil

56. Medium Voltage VT core and coil assemblies ready to cast

57. Medium Voltage primary turns ready for cores and secondary windings to be added before casting

58. Medium Voltage Transformers just removed from a vacuum chamber were they were cast with polyurethane

59. The same transformer partially removed from the casting mold.

60. Winding a simple Window CT

61. A plastic molded case toroidal CT prior to case closure

62. Tape wrapped bushing CT ready for test

63. Routine Final Testing per IEEE C57.13 Dielectric testing between windings and windings and ground. Often called �HI Pot� testing. Induced Voltage Tests. Often called �Double Induced� Accuracy Tests Polarity Tests

64. Words of Caution CTs are intended to be proportional current devices. Very high voltages will result from open circuiting the secondary circuit of an energized CT. VTs are intended to be used as proportional voltage devices. Damaging current will result from short circuiting the secondary circuit of an energized VT.

65. Hot Topics �High Temperature� pad mount CT applications Extended Range CTs High accuracy CTs (0.15%, IEEE std.)

66. Thank You

67. Relay Class IEEE relay class is defined in terms of the voltage a CT can deliver at 20 times the nominal current rating without exceeding a 10% composite ratio error. �For example, a relay class of C100 on a 1200:5 CT means that the CT can develop 100 volts at 24,000 primary amps (1200*20) without exceeding a 10% ratio error. Aside: This C100 is associated with a 1 ohm burden in the standard because: 5 amps secondary * 20 times over-current * 1 ohm = 100 volts. So C100, C200, C400, and C800, corresponds to 1, 2, 4, and 8 ohms, respectively. �A relay class assignment alone provides limited information. More information for relay calculations can be provided in an excitation curve�

68. Voltage Transformer Appendix

73. Testing

74. TESTING OF CURRENT TRANSFORMERS During field commissioning, the following tests are required for Current Transformers: A. C.T. Excitation Curves The purpose of this test is to verify that the C.T. meets the specifications, and will not saturate during maximum fault conditions. The C.T. characteristics will have been specified by the designer of the protection scheme. The C.T. excitation test is performed as follows: The voltage applied to secondary terminals of the C.T. is varied in steps of, say 50 volts, and the C.T. magnetizing current is measured in milli-amps, up until the C.T. saturates. The results obtained should be similar to those specified in manufacturer's test data, and also to the results for similar C.T.'s. NOTE: The C.T. primary must be `open circuit' when performing excitation tests. TESTING OF CURRENT TRANSFORMERS During field commissioning, the following tests are required for Current Transformers: A. C.T. Excitation Curves The purpose of this test is to verify that the C.T. meets the specifications, and will not saturate during maximum fault conditions. The C.T. characteristics will have been specified by the designer of the protection scheme. The C.T. excitation test is performed as follows: The voltage applied to secondary terminals of the C.T. is varied in steps of, say 50 volts, and the C.T. magnetizing current is measured in milli-amps, up until the C.T. saturates. The results obtained should be similar to those specified in manufacturer's test data, and also to the results for similar C.T.'s. NOTE: The C.T. primary must be `open circuit' when performing excitation tests.

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76. The C.T. polarity can be verified by a very simple test, known as the FLICK TEST. An analogue meter, on the d.c. milli-amp range, is connected across the C.T. secondary terminals, with the positive lead to `spot' or X1. A 1.5 volt `D' cell is then used to pass a current through the C.T. primary. As the connection is made to the `D' cell, to pass current from the cell positive, to the C.T. primary `spot' or H1, then the d.c. milli-ammeter will deflect or `flick' in a positive direction. As the connection from the `D' cell is removed, the milli-ammeter will deflect in a negative direction. If a ratiometer is used to check the C.T. ratio, then the correct polarity will be indicated by that meter. The C.T. polarity can be verified by a very simple test, known as the FLICK TEST. An analogue meter, on the d.c. milli-amp range, is connected across the C.T. secondary terminals, with the positive lead to `spot' or X1. A 1.5 volt `D' cell is then used to pass a current through the C.T. primary. As the connection is made to the `D' cell, to pass current from the cell positive, to the C.T. primary `spot' or H1, then the d.c. milli-ammeter will deflect or `flick' in a positive direction. As the connection from the `D' cell is removed, the milli-ammeter will deflect in a negative direction. If a ratiometer is used to check the C.T. ratio, then the correct polarity will be indicated by that meter.

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78. One field test that is sometimes performed is to energise the V.T. from the secondary terminals, and measure the magnetizing current at the rated voltage of 67 volts. DURING THIS TEST THE PRIMARY TERMINALS WILL BE AT FULL PRIMARY RATED VOLTAGE. e.g. 44kV, 33kV or 27.6kV etc. The purpose of this test is to record the magnetizing current, and compare it with the manufacturer's test data, and to record it for future reference. This test is of questionable value, and may not be worth performing, in view of the risks associated with the very high voltages. One field test that is sometimes performed is to energise the V.T. from the secondary terminals, and measure the magnetizing current at the rated voltage of 67 volts. DURING THIS TEST THE PRIMARY TERMINALS WILL BE AT FULL PRIMARY RATED VOLTAGE. e.g. 44kV, 33kV or 27.6kV etc. The purpose of this test is to record the magnetizing current, and compare it with the manufacturer's test data, and to record it for future reference. This test is of questionable value, and may not be worth performing, in view of the risks associated with the very high voltages.

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