On-site Measurement, Suppressing and Assessment of Inrush Currents in a 1000kV UHV Transformer, with Consideration of Core Saturation.

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1 21, rue d Artois, F PARIS A2_305_2012 CIGRE 2012 http : // On-site Measurement, Suppressing and Assessment of Inrush Currents in a 1000kV UHV Transformer, with Consideration of Core Saturation K. YOKOTSU Y. SHIRASAKA Tokyo Electric Power Co. Japan AE Power Systems Corp. Y. EBISAWA H. MURAKAMI Toshiba Corp. Mitsubishi Electric Corp. Japan SUMMARY This paper describes inrush current tests conducted during verification tests on a UHV transformer, and reviews and evaluates residual magnetic flux in the transformer s core. The UHV transformers had a voltage of 1000 kv and a capacity of 3000 MVA in a three-phase configuration. It had a far higher voltage and greater capacity than an ordinary substation transformer. When the transformer was energized into a transmission system, the inrush current exerted an extremely large impact on the transmission system, making it necessary to fully evaluate this impact. The inrush current tests were conducted by energizing a full-rating field test UHV transformer into a transmission system, using a circuit breaker on the 500kV secondary side. Tests were performed with these parameters: circuit breaker closing phase angle, and with closing resistor / without closing resister in a circuit breaker. The tests resulted in a good match between analysis results that accounted for residual magnetic flux and measured results. In addition, it was evident that inrush current can be reduced by with closing resistor and controlling the closing phase angle, and it was also evident that magnetic core residual flux levels in a UHV transformer are smaller than generally assumed. At the same time, a method to reduce residual flux in a magnetic core was developed, and tests were conducted to verify this method. It is believed that this was the first time in the world that a method to reduce magnetic core residual flux in a physical system was developed and subsequently verified in tests. The inrush current tests where residual magnetic flux was artificially controlled were also conducted, and we obtained excellent matching with analysis results. KEYWORDS 1000 kv transformer - UHV transformer - inrush current - core - core saturation - residual flux - closing resister - phase angle - verification test - on-site measuring yokotsu.kiichirou@tepco.co.jp

2 1. Introduction There are cases where short-time magnetic core saturation occurs when a transformer is energized into a transmission system, depending on conditions such as transformer core residual flux density before energizing and closing voltage phase angle on energizing. Large inrush current flows due to such magnetic core saturation phenomena are well known. Since UHV substation are installed relatively close to power generating stations and UHV transformer capacity is twice as great as that of a 500kV transformer, it is necessary to gain an adequate understanding of the influence exerted on systems, such as voltage fluctuations under the inrush current generating condition into a UHV transformer. In response to these factors, inrush current tests on a UHV transformer were conducted at the UHV equipment field test site of Tokyo Electric Power Company. A low frequency / low voltage demagnetization method was developed and applied. The inrush current tests were performed after residual magnetic flux was demagnetized, and a predetermined flux was applied. This method was also used to measure residual flux, after opening the circuit breaker. The above test results and EMTP (Electromagnetic Transients Program) analysis results were compared and evaluated, and a quantitative analysis of the newly obtained knowledge regarding inrush condition was performed. Obtained knowledge that was analyzed included the influence of residual magnetic flux, residual magnetic flux levels, and magnetic core saturation phenomena after circuit breaker opened. This paper describes the test results and the knowledge obtained. 2. Key Points of Tests We conducted inrush current tests on a UHV transformer installed at the UHV equipment field test site at the Shin-Haruna Substation of Tokyo Electric Power Company. Full rating test equipment configuration for the field tests is shown in Figure 1. [1][2][3] The UHV transformer was an auto-transformer with the following specifications: 1050/525/147kV; 3000/3000/1200 MVA; voltage regulator at loading (LVR) mounted on neutral point side; and tapping range at primary side + 7 % (27 taps). Figure kv field test equipment configuration The inrush current tests were conducted using a circuit breaker at the secondary side of the UHV transformer, which was connected to a 500 kv system. The outline of test circuit is shown in Figure 2. The exciting inrush current was measured at the 1000 kv side, the 500 kv side, the tertiary side, and the neutral 1

3 point side on the 1000kV transformer. The secondary of 500 kv side output current of the current transformer, where the exciting inrush current was measured, was converted into voltage, and processing was conducted to obtain the value of each winding current by A/D conversion using a digital recorder. (500kV circuit breaker) Figure 2. Outline of test circuit used in UHV transformer inrush current tests 3. Measurements and Analysis on Inrush Current Test 3.1 Measurement Conditions and Measured Results (1) Measurement Conditions While measuring inrush currents, we established two conditions as parameters: a circuit breaker with closing resistor (a resistor installed in a breaker and inserted in series in the circuit) or without closing resister, and a circuit breaker with various closing phase angles. The condition without closing resistor is defined as an electrically short circuit at both ends of the closing resistor inside the circuit breaker. The closing voltage phase angle control of the circuit breaker used a sequencer to control the circuit breaker closing command signals. During measurements using the parameter, with closing resistor / without closing resister, the impact was examined by varying tap positions on the validation equipment. The transformer was an auto-transformer, and magnetic core flux density differed according to tap position. The maximum voltage tap position was Tap No. 1, while the minimum voltage tap position was Tap No.27. Magnetic core flux density at the maximum voltage tap was approximately 1.7 Tesla. (2) Influence on with closing resistor / without closing resister in the circuit breaker Table 1. Measured results of inrush currents Figure 3. Voltage and inrush current waveforms at 500kV terminal The maximum value of a measured inrush current was 550A (0-P value) under with closing resistor, and 5920 A (0-P value) under without closing resistor. It is evident that the inrush current can be reduced to approximately 1/10 by the closing resistor. Table 1 shows the maximum current value measured and circuit breaker closing phases at that time. The closing phase angle was read out as a reference as zero (0) degrees for each phase voltage (sinusoidal waves). Figure 3 shows the voltage wave of the closed terminal (closed phase) in cases where the maximum current under without closing resistor was observed, and also shows the measured waveforms of exciting inrush currents at that time. 2

4 (3) Influence on voltage closing phase angle of circuit breaker Figure 4 shows the relationship between voltage closing phase angle of circuit breaker and inrush current (includes a comparison of the analysis results examined in section 3.2). The inrush current was the maximum at around closing phase angles 0 degree and 180 degree. The closing phase angle was controlled by the sequencer, but to be precise, the closing phase angle caused variations between the three phases due to the breaker s mechanical characteristics and pre-arcs between the breaker electrodes. For this reason, increases and decreases were observed in other phase magnetic fluxes, due to the influence of the advanced closing phase. This is because voltage was induced through the tertiary wiring even at the other phases on starting the energizing, due to variations between the phases during closed circuit operation when the advanced closing phase was ON. Actual measured results for inrush currents tended to be a little smaller than the analysis results discussed in the next section. It is considered that the voltage oscillation phenomenon at breaker open, discussed in section 4, was a contributing factor. Inrush Current without closing resistor at Red Phase (Tap No.-3) Measured Result Analysis Result (Upper Limit: r = + 40%) Analysis Result (Lower Limit: r = + 40%) Analysis Result (Upper Limit: r = + 20%) Analysis Result (Lower Limit: r = + 20%) Analysis Result (Upper Limit: r = 0%) Analysis Result (Lower Limit: r = 0%) Inrush Current (A) Note : r is meaning of residual flux rate in the magnetic core Voltage Closing Phase Angle (as a Red Phase) Figure 4. Relationship between closing phase angle and inrush current 3.2 Inrush Current Analysis (1) Modeling of Analyzing Circuit The analysis was conducted using an EMTP. The analyzing circuit is shown in Figure 5. Static capacitance is not shown in the analyzing circuit, but was taken into consideration. With regard to the exciting characteristics of the magnetic core, characteristics at saturation and equivalent circuit simulating non-saturated characteristics were used in order to consider the effect of a tertiary wiring delta connection circuit. By exciting characteristics, we established an equivalent circuit that simultaneously satisfied air-cored inductance and leakage inductance on the switching side and on the tertiary side. The inductance simulation circuit of the transformer is shown in Figure 6. Closing resistor Closing resistor Closing resistor (1000 ) Figure 5. Analyzing circuit Figure 6. Inductance simulation 3

5 (2) Comparison of Measured and Analyzed Values Figure 7 shows a comparison of measured waveforms without closing resistor and calculated waveforms. Peak values for both of the inrush currents and the waveforms match quite well with the measured values, as the result of assuming the red-phase residual magnetic flux to be 28% of the rated magnetic flux. The maximum residual magnetic flux was about 30% of the rated magnetic flux when the peak value of the inrush current was compared with waveforms in other test cases. However, the residual magnetic flux was about 20% for most other cases. In this analysis, the residual magnetic flux circuit is assumed with considering variations closing balances between three phases of the circuit breaker. Figure 7. Comparison of measured waveforms and calculated waveforms 4. Influence of Residual Magnetic Flux and Flux Reduction (Demagnetization) Method 4.1 Residual Magnetic Flux Reduction Method and Measured Example Generally, the transformer core is magnetized by a direct current (DC) when the winding resistance of the transformer is measured using a DC power supply. In addition, there is residual magnetic flux in the magnetic core when an operating transformer is de-energized from the system by a circuit breaker. It is advisable to demagnetize (degauss) the residual flux of the magnetic core before installing a transformer in a system because, if the transformer is installed in the system while there is residual flux in the magnetic core, large exciting inrush current flows could occur and the protection relay could malfunction. It was evident that a demagnetization effect occurs in voltage oscilation with polarity reversal when the circuit breaker was open. This will be discussed in section 4.3. The conventional method to demagnetize residual magnetic flux in a transformer core has been to reduce the applied voltage gradually after confirming the extent of demagnetization, based on the exciting current waveforms and the results of waveform analysis using rated frequency (commercial frequency) AC and increasing the applied voltage on the windings up to close to the rated voltage. A high voltage power source and testing equipment greater than the transformer exciting capacity are required for this purpose. The following relational expression is established for induced voltage E and magnetic core flux Φ in a transformer: E/f = 2 π n Φ (4.1) Where f: Frequency (Hz), and n: Number of windings Since magnetic flux Φ is proportional to E/f, easy demagnetization is possible by lowering the frequency in order to saturate magnetic core to feed the required flux, even with a low voltage power supply that does not use the previously discussed high voltage power source. Compared with the conventional demagnetization method, which uses AC with rated voltage and rated frequency, this low voltage/low frequency demagnetization method can demagnetize even with a small power source capacity, and the exciting characteristics of the magnetic core (I-Φ characteristics) and the residual flux before and after demagnetization can be confirmed as well, as shown in Figure 8 (a). If we let the residual flux before demagnetization of the transformer core be Φ 0, varying the voltage will cause the exciting characteristics of the magnetic core to vary over a hysteresis curve, and it is possible to make the residual magnetic flux value near zero (0), just like Φ 1. A working example of demagnetization by raising voltage and then decreasing it is shown in Figure 8 (b) and (c). A residual magnetic flux of 200 Wb is reduced to 50 Wb in this example. The 50 Wb corresponds to approximately 5%, if the saturated flux of the magnetic core is 920 Wb. 4

6 residual flux 0 before demagnetization residual flux 1 after demagnetization (a) Image of reducing (b) Measured example (c) Measured example hysteresis curve (Early half: Voltage rising) (Latter half: Voltage dropping) Figure 8. Image and working example of Φ I characteristic curve during demagnetization 4.2 Measurement of Inrush Current (Demagnetization and Residual Flux Magnetization) It was confirmed that the measured results match excellently with the analysis results, by assuming the residual magnetic flux values obtained from the inrush current test results in section 3 above. Next, demagnetization and residual flux magnetization were conducted before closing circuit breaker, and predetermined residual flux was measured, in order to conduct inrush current tests with residual fluxes in the transformer s magnetic core as the parameters. The low voltage / low frequency demagnetization method adopted for the tests is described below. Several voltage application steps could be considered for these tests, the voltage application step shown in Figure 9 was applied on the tertiary side (delta connection) of the transformer. The applied voltage was reduced with each application cycle, with a power frequency (1/Ts) of 0.01 Hz. Figure 9. Example of voltage application step under demagnetization Figure 10. Demagnetizer and measuring circuit The demagnetizer and its measuring circuit are shown in Figure 10. Demagnetizing for all three phases was possible while maintaining the delta connection, by demagnetizing every two phases and changing two phases of voltage application in order, in the circuit. The residual flux magnetization method will now be described. The relationship between voltage initial value V1 and the applied amount of flux in the DC voltage application pattern in Figure 11 was measured. With regard to the residual flux magnetization circuit, it was confirmed that an arbitrary residual flux magnetization is possible by varying V1 after application of predetermined fluxes into two phases, U and V, making a short circuit between u and w with switch 2a (as shown in Figure 10), and not applying flux to the remaining single phase. Voltage Applied Flux (%) Output Voltage Time (second) Voltage Initial Value, V1 (V) Figure 11. Flux application pattern and test results 5

7 Table 2 shows three typical cases of the closing phase angle, inrush currents, and residual magnetic fluxes, for inrush current tests where residual magnetic flux was controlled through demagnetization and residual flux magnetization before closing the circuit breaker. The closing phase was read out as a reference as sinusoidal waves at zero degrees for each phase. A maximum inrush current value of 5100A and residual magnetic flux of -38 ~ 43% of the rated flux were measured. When calculating inrush current under simultaneous closing of three phase breakers, the assumed residual magnetic flux was set to about 80% of the formerly observed rated magnetic flux, in consideration of uncertain factors. It was found in this test that the residual magnetic flux in the analysis is sufficient to estimate about 50% of the rated magnetic flux. Table 2. Examples of measured inrush peak current and measured residual magnetic flux with a condition of residual flux magnetization after demagnetization (Three typical cases) Black Phase Red Phase White Phase LVR tap Phase ( ) Magnetization Current Phase Magnetization Current Phase Magnetization Residual flux (A) ( ) Residual flux (A) ( ) Residual flux 12% 30% -7% % 43% -23% 9% 25% -23% % -38% -5% 24% -22% 12% % -3% -28% 4.3 Demagnetizing Effect When Circuit Breaker Open Current (A) The mechanism under which residual magnetic flux in the inrush current test circuit becomes less than around 50% of the rated magnetic flux was examined as follows. Figure 12 shows a measured example of voltage waveforms at the 1000kV and 500kV terminals immediately after circuit breaker opened, and also shows an output waveform (generated sound) obtained from an ultrasound sensor installed in a transformer tank, and a simplified equivalent circuit of the test circuit. Voltage dropped suddenly after shutdown in the peak vicinity, and the polarity was inverted and finally reached zero with gradual vibrations. This phenomenon is considered to be as follows (using the equivalent circuit as a reference): 0 Immediately before CB opened 0 After CB opened L : Excited inductance C : Transformer and GIS capacitance Figure 12. Voltage waveform and sound generation after breaker opened, and it equivalent circuits The electrical charge, which was charged in a capacitance after circuit breaker opened, is discharged through the transformer windings. The transformer is excited with this current. The large static capacity of the UHV transformer is the reason for this. When the current flows continuously, magnetic flux increases, and finally the magnetic core saturates, the current increases at a great rate, and voltage polarity is reversed at the same time. The exciting inductance becomes an empty inductance due to magnetic core saturation. Then, the transformer is excited in the reverse direction by the discharging of the charge, which had been charged in reversed polarity, and the magnetic core saturates again. As these phenomena are repeated, the voltage gradually drops mainly due to circuit loss, and then reaches zero potential with gradual attenuation. It is considered that the residual magnetic flux is degaussed during this process. In addition, sounds were generated from the transformer tank. These are assumed to be magnetostrictive vibrations caused by saturation of the magnetic core during the time of polarity reversal with voltage. 6

8 For the above reasons, it is considered that there can be a demagnetization effect in the polarity reversal with voltage after breaker opened. Voltage waveform and voltage integrated waveform before and after the circuit breaker opened are shown in Figure 13, and the relationship between the circuit breaker open phase and voltage integrated values (is equal to a magnetic flux) are shown in Figure 14. Voltage integration values at 36 and 42 msec after breaker opened were obtained through analysis, and are shown in Figure 14. In this analysis, the times of 36 and 42 msec after breaker opened were selected as the agreeable times between measured residual flux value and it analysis value. The excellent correlation between measured results and analysis results is evident. Voltage Integral Open start phase: 127 Time Residual flux -9 % Figure 13. Example of voltage waveform and voltage integrated waveform before and after circuit breaker opened 5. Conclusion Voltage Integrated Value (%) Open Start Phase (Tap-14) Figure 14. Relationship between circuit breaker open phase and voltage integrated values after opened The 1000kV UHV transformer used in the field tests had a three phases configuration consisting of three single-phase units with a capacity of 3000 MVA. It had a far higher voltage and greater capacity than an ordinary substation transformer. Because the impact of an inrush current into a system is serious, it is important to conduct an evaluation also from the view point of magnetic core characteristics. The inrush current tests were conducted by energizing a full rating field test transformer into a system, using a circuit breaker from the 500kV secondary side. Tests were performed with these parameters: circuit breaker closing phase angle, and with / without closing resistor with connection in series to the system. The tests resulted in a good match between analysis results that accounted for residual magnetic flux and measured results. In addition, it was evident that the inrush current can be reduced by applying the closing resistor and/or controlling the closing phase angle, and it was also evident that magnetic core residual flux levels in a UHV transformer are smaller than generally assumed. At the same time, a method to reduce residual flux in a transformer magnetic core was developed, and tests were conducted to verify this method. It is believed that this was the first time in the world that a method to reduce magnetic core residual flux in a physical system was developed and subsequently verified in tests. Inrush current tests where residual magnetic flux was artificially controlled were also conducted, and we obtained excellent matching with analysis results. It is believed that these verification test results and the analysis evaluation method will be applied effectively in the future operation of UHV transformers. Measured results for residual flux Analysis results (36 ms after opened) Analysis results (42 ms after opened) BIBLIOGRAPHY [1] Y. Yamagata et.al., Development and Field Test of 1000 kv 3000 MVA Transformer (CIGRE Paris session 1998, ) [2] Y. Yamagata et.al., Field Test of 1000 kv Gas Insulated Switchgear (CIGRE Paris session 2000, ) [3] T. Kawamura et.al., Development and Long Term Field Tests for UHV, 3000 MVA Transformer in Japan (IEC/CIGRE UHV Symposium Beijing, China July 2007, Session 2-6-5) 7