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Voltage Transformers
Chapter 11
Voltage Transformers 11.1 INTRODUCTION This instrument is used to transfer the primary voltage to a secondary voltage proportional with the primary voltage. The secondary voltage value is suitable for measuring and protection devices. Voltage transformers (VTs) are used in: G G G G
Metering; Overvoltage protection, for example, no-load line; Under voltage protection, for example, overloads line; Discharging capacitor banks (wound).
11.2 PRINCIPLE OF OPERATION OF ELECTROMAGNETIC VOLTAGE TRANSFORMERS The electromagnetic VT is connected across the points at which the voltage is to be measured, and is therefore much like low-power transformers with secondary winding operating close to an open circuit. For such a no-load transformer the voltage transformation is in proportion to the primary (Np) and the secondary (Ns) turns: Vp Np 5 Vs Ns An inductive VT is ideally a transformer under no-load conditions where the load current is zero and the voltage drop is only caused by the magnetizing current and is thus negligible. In practice, the winding voltage drops are small, and the rated flux density in the core is designed to be well below the saturation density. Therefore, the exciting current is low and exciting impedance constant with a variation of applied voltage over the operating range including some degree of overvoltage. The parameters that define VT performance are voltage ratio error and phase displacement error. The ratio error is defined as: Ratio Error% 5
Kn Vs 2 Vp 3 100 Vp
Practical Power System and Protective Relays Commissioning. DOI: https://doi.org/10.1016/B978-0-12-816858-5.00011-3 © 2019 Elsevier Inc. All rights reserved.
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where Kn is the rated ratio and Vp and Vs are the primary and secondary terminal voltages. If the error is positive, the secondary voltage exceeds the rated value. The phase error is the phase difference between the secondary and the primary voltage phasors. It is positive when the secondary voltage leads the primary voltage phasor. According to the ratio and angle error, all VTs are required to comply with one of the classes as defined in IEC600442, measuring the VT accuracy class (0.1, 0.2, 0.5, 1, and 3) or protective VT accuracy class (3P or 6P). VTs should be of a sufficient size as to prevent measured disturbances from inducing saturation in the VT. For transients, this generally requires that the knee point of the VT saturation curve be at least 200% of the rated system voltage. It is always good practice to incorporate some allowance in the calculations for overvoltage conditions. The frequency response of standard metering and protection class VTs depend on their type and burden. In general, the burden should be very high impedance. This is generally not a problem with most monitoring equipment available today. Power quality monitoring instruments, intelligent electronic devices (IEDs), and other instruments all present very high impedance to the VT. With a high impedance burden, the response is usually adequate to at least 5 kHz. While working in energized V.T it is very dangerous to short circuit the secondary winding of the voltage transformer. Refer to Figs. 11.1A,B,C, 11.2, and 11.3 for examples of VTs. Refer to Fig. 11.3 for VT equivalent circuit.
FIGURE 11.1 (A) Inductive voltage transformers (VT) (wound); (B) medium-voltage dualbushing VT; (C) capacitive VT.
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125
FIGURE 11.2 Cut view of a double-pole voltage transformer.
FIGURE 11.3 VT equivalent circuit, where ZH is the primary leakage impedance; ZL is the secondary impedance; Rm and Xm are the core losses and exciting components, respectively; and n2 is the factor to refer ZH to the secondary side.
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FIGURE 11.4 Inductive VT.
11.3 PRINCIPLE OF OPERATION OF CAPACITIVE VOLTAGE TRANSFORMERS Due to the cost we use a voltage divide capacitance along with electromagnetic voltage transformer to form in total the capacitive voltage transformer (CVT) as shown in Fig. 11.5. This reduces the insulation requirements then later; as a result of this cost is also reduced. The total ratio of CVT 5 ððC1 1 C2Þ=C1Þ 3 T where T is the electromagnetic voltage transformer ratio. Fig. 11.6 shows the equivalent circuit of voltage transformer. The capacitance of the voltage divider in CVT and the reactance in electromagnetic VT form a resonance circuit called a Ferroresonance circuit. This circuit when subjected to a step change in voltage leads to an oscillation of nonlinear nature called Ferroresonance which in turn can be dangerous for the VT if it is left for a long time. Earth switch is therefore used to absorb this oscillation in Ferroresonance relay scheme.
11.4 BURDENS AND ACCURACY CLASSES There are two accuracy classes for protective voltage transformer and metering voltage transformer as per IEC 60044-2 as shown in Table 11.1.
Voltage Transformers Chapter | 11
FIGURE 11.5 Capacitive voltage transformer circuit.
FIGURE 11.6 Capacitive voltage transformer equivalent circuit.
TABLE 11.1 Voltage Transformer Accuracy Classes According to IEC 60044-2 Class
Ratio Error (%)
Phase Displacement Error (min)
0.1
0.1
5
0.2
0.2
10
0.5
0.5
20
1
1
40
3
3
3P
3
120
6P
6
240
127
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Practical Power System and Protective Relays Commissioning
11.5 TYPES AND THREE-PHASE CONNECTIONS OF VOLTAGE TRANSFORMERS The voltage transformer connection can be shown in Fig. 11.7.
11.6 OPTICAL CURRENT AND VOLTAGE TRANSFORMERS A new optical technology was introduced within a substation to replace conventional current and VTs with the advantage that technology being that it makes the new devices simple, compact, and reduces the Ferroresonance effect associated with conventional VTs, and also gives a direct connection digitally to the protection and automation system of the substation. The optical current transformer and the optical voltage transformer are used in substations which allow a direct connection from transducers to the substation communication network via IEC 61850.
11.6.1 Advantages of Optical Instruments Optical instruments is more suitable and matching to the new digital protection and automation.
FIGURE 11.7 Voltage transformer connection components.
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FIGURE 11.8 Optical HV voltage transformer.
Optical sensors (see Fig. 11.8 for an optical high-voltage and current transformer) provide several benefits over conventional CTs, VTs, and CCVTs, including the following: Better accuracy, with the accuracy being better in optical instruments; Output bandwidth—the bandwidth of optical instruments is wider than for conventional ones; Combined CT and VT as a compact unit includes the CT and VT; Greater safety as they are environmentally friendly, and there is no open circuit hazard for secondary winding of CT or Ferroresonance risks in VTs.
11.7 VOLTAGE TRANSFORMER TESTING 11.7.1 Visual Check The visual check is carried out as follows: 1. Check that the VT as per the manufacturer-provided drawings and manuals at the site. 2. Check VT nameplate ratings and terminal markings. 3. Check the primary connection is correct as per drawings especially in SF6 gas insulated system (GIS).
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4. Check secondary connections in the VT terminal box and in the local control cubicle (LCC), the tightness and cross-sectional area of the cables, and the color codes for phases and the ground cable. 5. Check the oil level in the outdoor PT and SF6 pressure in the SF6 GIS. 6. Check the MCB ratings of the VT secondary terminals in the LCC.
11.7.2 Insulation Resistance Test This test is carried out at 5000 V DC as follows: 1. between primary and secondary; 2. between primary and earth; and 3. between secondary and earth. Also, the voltage withstand test is performed at 2000 V AC 50 or 60 Hz for 30 seconds between the primary and secondary windings of the VT.
11.7.3 Polarity (Flick) Test This test is performed to confirm the correct marking on the VT primary and secondary as per the drawings, and it can be done by a flick DC test using a DC battery as shown in Fig. 11.9. The battery switch which is connected to the PT primary is instantaneously closed and opened and then the pointer is
FIGURE 11.9 Voltage transformer polarity test.
Voltage Transformers Chapter | 11
131
FIGURE 11.10 Voltage transformer ratio test.
checked for direction of movement in the galvanometer and if it is in the positive direction then the polarity is ok.
11.7.4 Voltage Transformer Ratio Test A variable AC voltage source advanced test set is used to inject voltages on the primary side and the secondary voltage is measured to check that the VT ratio is correct for each phase, as shown in Fig. 11.10.
11.7.5 Winding Resistance Test Using this test we can measure the secondary resistance of the VT for each core at the VT terminal box and at the LCC panels between phases and the neutral, and between phases and other phases, after opening the VT connection to relay panels or metering panels by opening all the VT secondary MCBs.
11.7.6 Loop Resistance Burden Test This test is done to ensure that the connected burden to VT is within the rated burden, identified on the nameplate. Using a secondary injection tester to inject the rated secondary voltage of the VT from the VT terminals toward the load side while isolating the VT terminals at the VT terminal blocks toward the VT and observe the voltage drop across the injection points as the burden can be found from the following equation:
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FIGURE 11.11 Voltage transformer burden test.
VAðburdenÞ 5 voltage drop 3 rated VT measured secondary current Refer to Fig. 11.11 for the VT burden test.
Voltage Transformers 11.1 INTRODUCTION This instrument is used to transfer the primary voltage to a secondary voltage proportional with the primary voltage. The secondary voltage value is suitable for measuring and protection devices. Voltage transformers (VTs) are used in: G G G G
Metering; Overvoltage protection, for example, no-load line; Under voltage protection, for example, overloads line; Discharging capacitor banks (wound).
11.2 PRINCIPLE OF OPERATION OF ELECTROMAGNETIC VOLTAGE TRANSFORMERS The electromagnetic VT is connected across the points at which the voltage is to be measured, and is therefore much like low-power transformers with secondary winding operating close to an open circuit. For such a no-load transformer the voltage transformation is in proportion to the primary (Np) and the secondary (Ns) turns: Vp Np 5 Vs Ns An inductive VT is ideally a transformer under no-load conditions where the load current is zero and the voltage drop is only caused by the magnetizing current and is thus negligible. In practice, the winding voltage drops are small, and the rated flux density in the core is designed to be well below the saturation density. Therefore, the exciting current is low and exciting impedance constant with a variation of applied voltage over the operating range including some degree of overvoltage. The parameters that define VT performance are voltage ratio error and phase displacement error. The ratio error is defined as: Ratio Error% 5
Kn Vs 2 Vp 3 100 Vp
Practical Power System and Protective Relays Commissioning. DOI: https://doi.org/10.1016/B978-0-12-816858-5.00011-3 © 2019 Elsevier Inc. All rights reserved.
123
124
Practical Power System and Protective Relays Commissioning
where Kn is the rated ratio and Vp and Vs are the primary and secondary terminal voltages. If the error is positive, the secondary voltage exceeds the rated value. The phase error is the phase difference between the secondary and the primary voltage phasors. It is positive when the secondary voltage leads the primary voltage phasor. According to the ratio and angle error, all VTs are required to comply with one of the classes as defined in IEC600442, measuring the VT accuracy class (0.1, 0.2, 0.5, 1, and 3) or protective VT accuracy class (3P or 6P). VTs should be of a sufficient size as to prevent measured disturbances from inducing saturation in the VT. For transients, this generally requires that the knee point of the VT saturation curve be at least 200% of the rated system voltage. It is always good practice to incorporate some allowance in the calculations for overvoltage conditions. The frequency response of standard metering and protection class VTs depend on their type and burden. In general, the burden should be very high impedance. This is generally not a problem with most monitoring equipment available today. Power quality monitoring instruments, intelligent electronic devices (IEDs), and other instruments all present very high impedance to the VT. With a high impedance burden, the response is usually adequate to at least 5 kHz. While working in energized V.T it is very dangerous to short circuit the secondary winding of the voltage transformer. Refer to Figs. 11.1A,B,C, 11.2, and 11.3 for examples of VTs. Refer to Fig. 11.3 for VT equivalent circuit.
FIGURE 11.1 (A) Inductive voltage transformers (VT) (wound); (B) medium-voltage dualbushing VT; (C) capacitive VT.
Voltage Transformers Chapter | 11
125
FIGURE 11.2 Cut view of a double-pole voltage transformer.
FIGURE 11.3 VT equivalent circuit, where ZH is the primary leakage impedance; ZL is the secondary impedance; Rm and Xm are the core losses and exciting components, respectively; and n2 is the factor to refer ZH to the secondary side.
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FIGURE 11.4 Inductive VT.
11.3 PRINCIPLE OF OPERATION OF CAPACITIVE VOLTAGE TRANSFORMERS Due to the cost we use a voltage divide capacitance along with electromagnetic voltage transformer to form in total the capacitive voltage transformer (CVT) as shown in Fig. 11.5. This reduces the insulation requirements then later; as a result of this cost is also reduced. The total ratio of CVT 5 ððC1 1 C2Þ=C1Þ 3 T where T is the electromagnetic voltage transformer ratio. Fig. 11.6 shows the equivalent circuit of voltage transformer. The capacitance of the voltage divider in CVT and the reactance in electromagnetic VT form a resonance circuit called a Ferroresonance circuit. This circuit when subjected to a step change in voltage leads to an oscillation of nonlinear nature called Ferroresonance which in turn can be dangerous for the VT if it is left for a long time. Earth switch is therefore used to absorb this oscillation in Ferroresonance relay scheme.
11.4 BURDENS AND ACCURACY CLASSES There are two accuracy classes for protective voltage transformer and metering voltage transformer as per IEC 60044-2 as shown in Table 11.1.
Voltage Transformers Chapter | 11
FIGURE 11.5 Capacitive voltage transformer circuit.
FIGURE 11.6 Capacitive voltage transformer equivalent circuit.
TABLE 11.1 Voltage Transformer Accuracy Classes According to IEC 60044-2 Class
Ratio Error (%)
Phase Displacement Error (min)
0.1
0.1
5
0.2
0.2
10
0.5
0.5
20
1
1
40
3
3
3P
3
120
6P
6
240
127
128
Practical Power System and Protective Relays Commissioning
11.5 TYPES AND THREE-PHASE CONNECTIONS OF VOLTAGE TRANSFORMERS The voltage transformer connection can be shown in Fig. 11.7.
11.6 OPTICAL CURRENT AND VOLTAGE TRANSFORMERS A new optical technology was introduced within a substation to replace conventional current and VTs with the advantage that technology being that it makes the new devices simple, compact, and reduces the Ferroresonance effect associated with conventional VTs, and also gives a direct connection digitally to the protection and automation system of the substation. The optical current transformer and the optical voltage transformer are used in substations which allow a direct connection from transducers to the substation communication network via IEC 61850.
11.6.1 Advantages of Optical Instruments Optical instruments is more suitable and matching to the new digital protection and automation.
FIGURE 11.7 Voltage transformer connection components.
Voltage Transformers Chapter | 11
129
FIGURE 11.8 Optical HV voltage transformer.
Optical sensors (see Fig. 11.8 for an optical high-voltage and current transformer) provide several benefits over conventional CTs, VTs, and CCVTs, including the following: Better accuracy, with the accuracy being better in optical instruments; Output bandwidth—the bandwidth of optical instruments is wider than for conventional ones; Combined CT and VT as a compact unit includes the CT and VT; Greater safety as they are environmentally friendly, and there is no open circuit hazard for secondary winding of CT or Ferroresonance risks in VTs.
11.7 VOLTAGE TRANSFORMER TESTING 11.7.1 Visual Check The visual check is carried out as follows: 1. Check that the VT as per the manufacturer-provided drawings and manuals at the site. 2. Check VT nameplate ratings and terminal markings. 3. Check the primary connection is correct as per drawings especially in SF6 gas insulated system (GIS).
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Practical Power System and Protective Relays Commissioning
4. Check secondary connections in the VT terminal box and in the local control cubicle (LCC), the tightness and cross-sectional area of the cables, and the color codes for phases and the ground cable. 5. Check the oil level in the outdoor PT and SF6 pressure in the SF6 GIS. 6. Check the MCB ratings of the VT secondary terminals in the LCC.
11.7.2 Insulation Resistance Test This test is carried out at 5000 V DC as follows: 1. between primary and secondary; 2. between primary and earth; and 3. between secondary and earth. Also, the voltage withstand test is performed at 2000 V AC 50 or 60 Hz for 30 seconds between the primary and secondary windings of the VT.
11.7.3 Polarity (Flick) Test This test is performed to confirm the correct marking on the VT primary and secondary as per the drawings, and it can be done by a flick DC test using a DC battery as shown in Fig. 11.9. The battery switch which is connected to the PT primary is instantaneously closed and opened and then the pointer is
FIGURE 11.9 Voltage transformer polarity test.
Voltage Transformers Chapter | 11
131
FIGURE 11.10 Voltage transformer ratio test.
checked for direction of movement in the galvanometer and if it is in the positive direction then the polarity is ok.
11.7.4 Voltage Transformer Ratio Test A variable AC voltage source advanced test set is used to inject voltages on the primary side and the secondary voltage is measured to check that the VT ratio is correct for each phase, as shown in Fig. 11.10.
11.7.5 Winding Resistance Test Using this test we can measure the secondary resistance of the VT for each core at the VT terminal box and at the LCC panels between phases and the neutral, and between phases and other phases, after opening the VT connection to relay panels or metering panels by opening all the VT secondary MCBs.
11.7.6 Loop Resistance Burden Test This test is done to ensure that the connected burden to VT is within the rated burden, identified on the nameplate. Using a secondary injection tester to inject the rated secondary voltage of the VT from the VT terminals toward the load side while isolating the VT terminals at the VT terminal blocks toward the VT and observe the voltage drop across the injection points as the burden can be found from the following equation:
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Practical Power System and Protective Relays Commissioning
FIGURE 11.11 Voltage transformer burden test.
VAðburdenÞ 5 voltage drop 3 rated VT measured secondary current Refer to Fig. 11.11 for the VT burden test.