1st International Conference on Electrical Materials and Power Equipment - Xi'an - China
Study on Turn-to-turn Insulation Detection Methods and Judgment of Dry-type Air-core Reactor S. Guo1,2, J. Ma1,2, Z. Niu1,2,T. Mao3, H. Li1,2
1
North China Electric Power Research Institute Co., Ltd 2 State Grid Jibei Electric Power Research Institute 3 State Grid Jibei Electric Power Co., Ltd No.1, Dizangannan Alley, Fuxingmenwai Street, Xicheng District, Beijing 10045, China the current research results and standards. Quantification criteria have not yet been established. In this paper, the diagnostic criteria based on oscillation frequency, attenuation coefficient and standard deviation of attenuation coefficient are proposed. A quantitative and standardized diagnosing process can be used to evaluate the turn-to-turn insulation condition of the dry-type air-core reactors and provide a reference for the operation, maintenance and fault diagnosis of the reactor equipment.
Abstract- Turn-to-turn insulation defects in dry-type air-core reactors are the main cause of equipment failure. At present, there is no effective detection method and diagnostic criteria on turn-to-turn insulation. In this paper, an AC voltage-high frequency oscillation combined partial discharge detection method is proposed. And combined partial discharge testing platform was developed. It can effectively test the insulation status of the series reactor. Moreover, the quantitative and standardized diagnostic criteria and judgment flow is formed, which can be used to assessment the turn-to-turn insulation of dry-type air-core reactor.
II. OPERATION CONDITION OF SERIES REACTOR Series reactor is located in the low-voltage side of the transformer and capacitor group, mainly used to limit the instantaneous current, and play a certain filtering effect. Operation voltage adjustment can be performed by adjusting the series reactors. In steady-state operation, the system voltage is mainly provided by the capacitor group, series reactor terminal voltage is very low. But in the transient process of the system, because the voltage of the capacitor group cannot be changed suddenly, the series reactor may appear high overvoltage when the system breaks down suddenly. With the increase of the operation times, the transient overvoltage has great influence on the turn-to-turn dielectric strength and life of the series reactor. The reactor with the inter-turn insulation defect is easy to breakdown, which causing the reactor failure.
I. INTRODUCTION In the transient process of inputting capacitor group or sudden failure in power system, the high transient overvoltage may appear on the series reactor. With the increase of operating frequency, the transient overvoltage of the series reactor will produce a detrimental effect on turn-to-turn insulation, which is likely to cause the turn-to-turn short circuit and faults on the series reactor. In recent years, drytype air-core reactor burnout has occurred in a majority of areas. According to statistics, turn-to-turn insulation defect is the main reason for equipment failure of the dry-type air-core reactor[1,2]. Currently high frequency oscillation test is widely used to detect the turn-to-turn insulation of the reactor[3,4]. The high frequency oscillation test of the reactor is recommended or required in the relevant international standards, national standards and national grid enterprise standards. Partial discharge test is also a widely used insulation detection method[5-7]. However, the high-frequency oscillation test has a higher test voltage but low energy. At the same time, the operating voltage of the series reactor is very low, so the test voltage of partial discharge test is also low. By using neither of these two methods alone can we effectively stimulate the turn-to-turn insulation defects in the reactor. This paper presents an AC voltage-high frequency oscillation combined partial discharge detection method, which utilizes the high voltage of the AC partial discharge test and the high voltage of the highfrequency oscillation turn-to-turn overvoltage test to excite the turn-to-turn insulation defects of the reactor. On the other hand, the diagnosis of turn-to-turn overvoltage test results remains in the stage of qualitative observation in
Fig. 1. Simulation of terminal voltage when inputting capacitor
978-1-5090-5736-8/17/$31.00 ©2017 IEEE 137
TABLE I RESULT OF TRANSIENT OVERVOLTAGE (p.u.) Transient process A phase B phase Inputting capacitor group 5.71 12.23 Single-phase-to-ground fault on 220 kV side Two-phase-to-ground fault on 220 kV side Three-phase-to-ground fault on 220 kV side Single-phase-to-ground fault on 500 kV side Two-phase-to-ground fault on 500 kV side Three-phase-to-ground fault on 500 kV side
test and high frequency oscillation test. The circuit diagram is show in Fig. 2. Lx is reactor. R is current limiting resistor. D is Electronic switch. C is resonant capacitor. P is frequency power. T is excitation transformer. C1 is shunt capacitor. L0 is filter reactor. The test signal is transmitted to the tester for recording and analysis by the detection unit.
C phase 7.73
2.65
1.03
2.73
7.78
7.85
3.10
7.87
14.81
8.78
1.22
1.03
1.43
4.14
4.57
1.57
4.22
8.22
4.51
Fig. 2. Circuit diagram of combined test
In order to study the terminal voltage of the series reactor in the transient process, the terminal voltage of the 35kV series reactor in the 500kV substation is simulated when inputting capacitor group, 220kV side fault and 500kV side fault. As shown in Fig. 1, the series reactors undergo transient oscillation decay and eventually reach a steady state during the process of inputting the capacitor group. Table 1 shows the results of the calculation of the terminal voltage of the series reactor at the time of inputting the capacitor group and the occurrence of a ground fault. It can be seen that the transient overvoltage peak can reach more than 10 times of the operating voltage and the oscillation period is about 43μs. In the repeated switching process of the capacitor group, severe overvoltage shock will bring serious threat to turn-to-turn insulation of the series reactor. In the event of a system failure, the series reactor will also experience the process of instantaneous oscillation. In addition to the effects of transient overvoltage, temperature rise and transient electrical forces can also affect the solid insulation of the series reactors. Temperature rise of series reactors during long-term operation can cause aging. The loss of mechanical strength of the insulation material in the role of instantaneous power will vibrate, resulting in turnto-turn insulation material cracks and powder, and ultimately the formation of conductive path breakdown. The method of simulating the transient process of series reactors is the necessary means to detect and diagnose the fault of the reactor. III. COMBINED PARTIAL DISCHARGE TEST Partial discharge detection technology has been widely used in the insulation testing of transformer equipment, which is very effective for detecting defects such as bubbles, burrs and metal particles. However, the test voltage for partial discharge detection is typically 1.3 or 1.5 times the maximum operating voltage. Even 1.5 times the operating voltage is also far below the partial discharge test voltage requirements. It is difficult to find the turn-to-turn insulation defect of the series reactor by simply carrying out the partial discharge test. In this paper, an AC voltage-high frequency oscillation combined partial discharge detection method is proposed, which combines the characteristics of AC partial discharge
The partial discharge system and the oscillation wave system are superimposed and integrated. Since the test voltage of the oscillating wave system may have an effect on the variable frequency power supply of the partial discharge detection system, a series of filter reactors are added in the middle of the two systems in order to reduce the influence of the oscillation wave on the variable frequency power supply and to reduce the influence of the parallel resonant capacitor. System platform is shown in Fig. 3.
Fig. 3. Test system platform
Through the further design of the detection platform, the partial discharge equipment is uniformly installed on the fixed platform. The excitation transformer connector and capacitor connections could be changed to achieve the system resonant requirements. The devices are individually packaged in separate components. The whole test platform weighs 4 tons, which can be loaded on the truck to carry on the experiment with 380V power supply.
Fig. 4. Flow chart of test voltage
Fig. 4 shows the pressurized flow of the combined test. According to the capacity of the sample inductor, configure the compensation capacitance, excitation change and no discharge power. Resonant frequency is configured in 150200Hz.
138
After square-wave calibration, the reactor partial discharge detection system and the oscillation wave detection system are respectively started. Boost to U2, corresponding to the system operating voltage stage, observe the partial discharge changes. The oscillation wave test voltage is applied to U 1, corresponding to the system transient overvoltage stage. Observe the oscillation frequency and test waveform changes, continuous test 1min. Reduce the test voltage to U2, duration 3min, corresponding to the transient process after the end of the system operating voltage stage. Criteria for the test results include: Compare the calibration voltage and the test voltage waveforms. Observe the attenuation cycle, frequency and inductance whether there is significant change; The partial discharge of the test voltage U2 under the oscillation wave before and after the test whether there is significant change; Ultrasonic, infrared thermal imaging and other means were used to test whether there are abnormal samples, such as smoke, fire and so on.
concrete judgment method and the criterion of quantification are given. Oscillation frequency During the test, the oscillating waveform data at the calibration voltage and the test voltage are recorded. Calibration voltage is usually below 30% the factory withstand value. The test voltage should be high enough to ignite the turn-to-turn insulation defect as much as possible. Test voltage is generally selected 100% factory withstand value. The difference between the frequency at the test voltage and the calibration voltage divided by the frequency at the calibration voltage is defined as the frequency deviation. 10kV reactor frequency deviation statistics is shown in Fig. 5.
Fig. 5. Statistical results of frequency deviation
In 10kV reactors, the frequency deviation of No. 32, 33, and 53 reactors exceeded ± 2%, which were out of service and turn-to-turn insulation defects were identified in subsequent diagnostic tests. Frequency deviation of No. 27, 41, 43, 67, and 74 is more than ± 1% and no more than ± 2%, in which No. 74 reactor has been out of operation and to confirm the existence of turn-to-turn insulation defects. The statistical results of 35kV reactors are similar. In the statistical results, all of the reactors with oscillation frequency deviations of more than 2% showed turn-to-turn insulation defects. According to the parallel resonant circuit frequency calculation formula, when the oscillation frequency changes 2%, the equivalent inductance changes about 4%. At the same time according to the calculation formula of the hollow inductance, when the inductance changes 4%, the equivalent reactor turns change about 2%. Usually dry-type air-core reactor turns in more than 100 turns. It can be considered that when the oscillation frequency changes by more than 2%, the reactor has appeared turn-to-turn insulation damage. Attenuation coefficient The difference between the attenuation coefficient at the test voltage and the calibration voltage divided by the attenuation coefficient at the calibration voltage is defined as the attenuation coefficient deviation. 10kV reactor attenuation coefficient deviation statistics is shown in Fig. 6.
IV. QUANTITATIVE DIAGNOSTIC CRITERIA During the test, if there is a short circuit in the reactor, the inductance and loss of the reactor will change. Turn-to-turn short circuit is equivalent to reduce the number of reactor coil turns, resulting in the inductance of the entire reactor to reduce. From the relationship between the oscillation frequency and the inductance, the oscillation frequency will increase. Due to the mutual inductance, non-short-circuit turns will generate induced electromotive force in the short-circuit turns. The induced electromotive force generates a large circulation in the short circuit. The direction of the magnetic field generated by the circulating current is opposite to the direction of the overall magnetic field of the reactor, thereby reducing the total magnetic field in the entire reactor coil. The reduction of the magnetic field corresponds to a decrease in the inductance, and the pulse oscillation frequency is increased. Circulation within the short-circuit coil will consume a lot of energy to speed up the reactor in the energy decay rate, so that the reactor voltage and current decay rate increases. The magnetic field inside the reactor coil is changed, resulting in a change in the current in the coils of the respective layers. Reactor coil layer will have a circulation, which will also increase the loss of the reactor and the deceleration speed of voltage and current. In summary, when the reactor short-circuit fault occurs, the oscillation circuit voltage, current, frequency and decay rate will be significantly changed. Therefore, by detecting the variation of the voltage waveform of the reactor, it can be preliminarily determined whether there is turn-to-turn short circuit in the reactor coil, but there is still a lack of quantitative basis for judging. Based on this, we carried out a large number of series reactor test. Based on the analysis and statistics of the data and the calculation results in the previous section, the
Fig. 6. Statistical results of attenuation coefficient
In 10kV reactors, the attenuation coefficient deviation of No. 53 reactors exceeded 10%, which were out of service and turnto-turn insulation defects were identified in subsequent
139
diagnostic tests. Attenuation coefficient deviation of 8 reactors is more than 5% and no more than 10%, in which 3 reactors have been out of operation and to confirm the existence of turn-to-turn insulation defects. The statistical results of 35kV reactors are similar. Standard deviation rate Through the test results of the reactor single-phase judgments can be part of the reactor turn-to-turn insulation defects to judge. However, for a reactor with a stable shortcircuited turn, there is little difference in the test results between the nominal voltage and the test voltage, but there is a large gap with the adjacent phase. Based on this, this paper presents the phase-to-phase dispersion of the same group of reactors. The standard deviation is a relative indication of the magnitude of the risk included in the expected return per unit, and is particularly applicable to the comparison of the degree of program risk with different expected values. Under normal circumstances, the larger the standard deviation, the greater the relative risk, and standard deviation is smaller, the relative risk is smaller.10kV reactor standard deviation rate statistics is shown in Fig. 7.
5%, the reactor may have turn-to-turn insulation defects. It is a state of attention and requires further diagnosis and confirmation. c. If the standard deviation rate is more than 30%, the reactor has a turn-to-turn insulation defect and is in a serious state. If the offset does not exceed 30% but exceeds 10%, the reactor may have turn-to-turn insulation defects. It is a state of attention and requires further diagnosis and confirmation. For the reactors under attention state, the AC voltage-high frequency oscillation combined partial discharge test should be superimposed. In the combined partial discharge test, it is necessary to compare the partial discharge quantity before and after the oscillatory wave test. At the same time, the discharge and the fever of the test sample should be judged synthetically by means of ultrasonic and infrared. V. CONCLUSION For turn-to-turn insulation test of dry-type air-core reactor, this paper presents an AC voltage-high frequency oscillation combined partial discharge detection method, which combines the high voltage of the oscillation wave with the high energy of the operating voltage to simulate the operating condition and effectively stimulate turn-to-turn insulation defects. An integrated turn-to-turn insulation test platform for reactor was developed to facilitate the on-site diagnostic test. Based on the statistics and analysis of a large number of field test data, a quantitative judgment criterion based on oscillation frequency, attenuation coefficient and standard deviation is proposed, and a standardized diagnosing process is formed. According to the criterion and process, it can effectively distinguish the turn-to-turn insulation condition of the reactor into the normal state, the attention state and the serious state, which has certain guiding significance for the equipment operation and maintenance.
Fig. 7. S Statistical results of standard deviation rate
In 10kV reactors, the standard deviation rate of No. 32, 38, 44, 53 and 56 reactors exceeded 30%, which were out of service and turn-to-turn insulation defects were identified in subsequent diagnostic tests, in which No. 38 and 44 were not found by the single-phase judgment. Standard deviation rate of 6 reactors are more than 10% and no more than 30%, in which No. 74 reactor has been out of operation and to confirm the existence of turn-to-turn insulation defects. The statistical results of 35kV reactors are similar. Thus, it is possible to accurately determine the insulation state of the reactor by the above-described quantization criterion. Diagnosing process The results of each test were judged in order. When all the criteria do not exceed the criterion, the turn-to-turn insulation of the reactor can be considered good. a. If the oscillation frequency deviation is more than 2%, the reactor has a turn-to-turn insulation defect and is in a serious state. If the offset does not exceed 2% but exceeds 1%, the reactor may have turn-to-turn insulation defects. This is a state of attention and requires further diagnosis and confirmation. b. If the attenuation coefficient deviation is more than 10%, the reactor has a turn-to-turn insulation defect and is in a serious state. If the offset does not exceed 10% but exceeds
REFERENCES [1] [2] [3] [4] [5] [6] [7]
140
H. Zhang, Y. Cai, “Fault analysis on dry-type air-core reactor in 500kV substation,” Power Capacitor & Reactive Power Compensation, vol. 37(1), pp.47-50, 2016. G. Wang, Y. Li, and S. Yang, “Failure analysis and preventive measures to dry-type air-core reactor,” Power Capacitor & Reactive Power Compensation, vol. 36(6), pp.82-85, 2015. M. Liao, X. Cheng, and Y. Zhai, “Simulation and experiment on impulse oscillating turn-to-turn insulation test system of dry-type air core reactor,” High Voltage Engineering, vol. 37(6), pp.1343-1348, 2011. L. Xu, Y. Lin, and Y. Wang, “Technique research on turn-to-turn overvoltage test for dry-type air-core reactors,” High Voltage Apparatus, vol. 48(7), pp.71-75, 2012. “IEC 60076-6-2007 Power transformers part 6: reactors,” IEC, 2007. “IEEE Std C57.21-2008 IEEE standard requirements, terminol-ogy, and test code for shunt reactors over 500kVA,” Printed in the United States of America, 2008. “GB/T 1094.6—2011 Power transformers part 6: reactors,” Beijing, China: China Standard Press, 2011.
