IN recent years, with the development of online monitoring
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1 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 1, JANUARY A New Broadband Microcurrent Transducer for Insulator Leakage Current Monitoring System Weigen Chen, Chenguo Yao, Pan Chen, Caixin Sun, Lin Du, and Ruijin Liao Abstract In this paper, a type of high bandwidth microcurrent transducer designed for online monitoring of an insulator is described. According to the equivalent circuit model, the effects of transducer winding number, stray capacitance, and loading resistance on passbandwidth and sensitivity are discussed. The academic lower limit of the transducer is nearly zero and the upper limit is infinity, which are proven by the analysis of the electronic circuit and the simulation by saber. The test of the current transducer shows that the transducer can be used in most high-voltage insulations systems with broad passbandwidth, high linearity, high sensitivity, and noncontact. Index Terms Broadband, leakage current of polluted insulators, noncontact passing through microcurrent transducer, online monitoring system. I. INTRODUCTION IN recent years, with the development of online monitoring technology for power equipment, the microcurrent (, ma) transducer has been applied more broadly. Owing to the isolation characteristics between the primary winding and the secondary winding and no effect with the circuit connection in the power system, noncontact passing through a microcurrent transducer becomes the first choice of online monitoring for high-voltage (HV) electric equipment. Since the leakage current on polluted insulators of an HV transmission line varies from several microamperes to hundreds of amperes, a rather large dynamic range of the microcurrent transducer and its favorable linearity are required. Moreover, partial discharge on the insulators contains useful and plentiful impulse signals, so the broadband (from several hertz to tens of megahertz) microcurrent transducer and its excellent transient response are also required. Of course, the microcurrent transducer should possess high sensitivity, and better ability to resist electromagnetic interference and long-term stability. Unfortunately, there is a conflict between the sensitivity and the bandwidth of a traditional Rogowski current transducer as a result of the sensitivity of the whole winding being significantly increased, while the resonance frequency degrades [1], [2], so many improvements have been made toward a traditional transducer. Xiaohua Guo integrates two single-layer windings into a Manuscript received August 4, 2005; revised March 1, This work was supported in part by Chongqing Science and Technology Project (No. 2005AC3031) in China. Paper no. TPWRD The authors are with the Key Laboratory of High Voltage Engineering and Electrical New Technology, Ministry of Education, Chongqing University, Chongqing , China ( yaochenguo@cqu.edu.cn; yaochenguo@sohu.com). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TPWRD whole with one layer on top of the other. Each sub coil has its own counterwound turn to compensate for the errors caused by interference from external magnetic fields [3]. Papez developed a wide frequency band (from dc to several megahertz) current transducer with a resistive shunt realized from a wafer of sintered graphite on which copper strips are soldered using soft soldering with a lead-free solder [4]. D Antona and Lazzaroni propose an integration process of the electromotive-force (EMF) signal induced by the coil in their current transducer, using digital signal processing [5]. In fact, any choice of available current transducers represents a compromise between different performances, depending on the particular field of application [6], [7]. In this paper, the authors bring forward a novel noncontact passing through a microcurrent transducer to be used in online monitoring of leakage current on polluted insulators of an HV transmission line with an electronic circuit application. Through lots of simulations and experiments, it is proved that the transducer has satisfied performance and can meet the practical needs of engineering applications. II. PRINCIPLE OF BROADBAND MICROCURRENT TRANSDUCER A. Amplitude Frequency Characteristics Analysis of a Traditional Current Transducer According to leakage current in insulator online monitoring, not only should the power frequency leakage current be detected accurately, but the partial-discharge signal should be extracted and captured in leakage current. Therefore, a broadband transducer is required and its low cutoff frequency should be below the power frequency [8] [10]. The principle and equivalent circuit of the noncontact passing through the microcurrent transducer are shown in Fig. 1. In Fig. 1, M is the mutual inductance of the winding; is the inductance of the winding; is the equivalent stray capacitance of the winding; is the equivalent resistance of the winding; is the integral resistance of the winding, which composes an integral circuit with ; is the current of the primary winding; is the current of the secondary winding; is the potential generated by mutual inductance; and is the potential generated by the integral resistance of the winding. Thus, from the equivalent circuit Fig. 1(b), the equations can be expressed as (1) (2) (3) /$ IEEE
2 356 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 1, JANUARY 2008 The inductance and mutual inductance of the current transducer are defined in (9) and (10), respectively (9) (10) where is the relative magnetic permeability, is the turns of the secondary winding, is the winding height, and and are the internal and external semidiameters of the winding. Due to, the high or low cutoff frequency and the bandwidth can be derived from (8). High cutoff frequency Fig. 1. Principle and equivalent circuit of current transducer. If parameter in (2) and (3) is eliminated, one can obtain the following expression: Low cutoff frequency Bandwidth (11) (12) When (1) is substituted into (4), then it is transferred to (5) by Laplace transformation (5) Assuming the zero initial condition, the transfer function of an equivalent circuit can be expressed as Therefore, the sine steady-state signal response is given as follows: Then, the amplitude frequency response characteristics of the current transducer can be obtained (4) (6) (7) (8) (13) In order to obtain a broad bandwidth of the transducer, should be high enough and should be low enough. Therefore, from (11) and (12), should be as large as possible and should be as small as possible. The winding inductance has to do with the magnetic permeability of the winding magnetic core, the winding height, the internal and external semidiameters, and the turns of the winding according to (9). If the material of the magnetic core and the winding dimension are fixed, transducer bandwidth and sensitivity will be mainly determined by integral resistance and the turns. When, the turns of the winding, is increased, and the bandwidth of the current transducer will be enlarged, whereas the response sensitivity will decrease. In (11) (13), it is shown that the integral resistance affects the low cutoff frequency and the high cutoff frequency. If integral resistance is smaller, the low cutoff frequency becomes lower. Then, high cutoff frequency becomes higher. Consequently, the bandwidth becomes broader. But the reduction of integral resistance will result in the depression of output voltage; thus, it is hard to acquire reliable signals. So it is very difficult for the traditional transducer to achieve adequate sensitivity and sufficient bandwidth. B. Principle of Improved Current Transducer Based on the analysis from before, in order to obtain broader bandwidth, the integral resistance should be reduced; meanwhile, it has to be ensured that the output voltage is large enough. Therefore, the principle of the improved current transducer is shown in Fig. 2. The operational amplifier composes an - transfer circuit, which is equivalent to a current-controlled voltage source,
3 CHEN et al.: NEW BROADBAND MICROCURRENT TRANSDUCER 357 Fig. 2. Principle of improved current transducer. Fig. 3. Equivalent circuit of an improved current transducer. Fig. 4. Amplitude-frequency and phase-frequency characteristics of the traditional transducer. then the equivalent circuit of an improved current transducer is shown in Fig. 3. From Fig. 3 and (8), the amplitude frequency characteristics of an improved transducer can be given by (14) where is feedback resistance of the operational amplifier, is input resistance of the operational amplifier, which can be regarded as zero under ideal conditions. From (14), high cutoff frequency and low cutoff frequency of an improved transducer can be deduced as follows. High cutoff frequency Low cutoff frequency (15) (16) From (14) (16), the - transfer circuit is composed of an operational amplifier, so it is deduced that the output voltage can be adjusted conveniently by changing without any impact on the amplitude frequency characteristics and the - circuit is very small, which greatly extends the low cutoff frequency. Furthermore, is connected in parallel with the equivalent stray capacitance of the winding, whose bypass effects of are greatly reduced; thus, high cutoff frequency is also expanded enormously [11], [12]. When the frequency is very low, the self-inductance of the winding is comparable with the input resistance of an operational amplifier, which ensures a rather low cutoff frequency of the transducer with less turns. Fig. 5. Amplitude-frequency and phase-frequency characteristic of the improved transducer. Through lots of experiments, not only will there be higher sensitivity and broader bandwidth but also easier shielding can be obtained by choosing about 100 turns. Besides, owing to rather small, the material of the magnetic core has a weak influence on the performance of the transducer. It reduces the requirement of the magnetic core and the cost of the transducer. Therefore, if low-cost ferrite magnetic core is used or even the gap exists, it has been proven to be still satisfying for simulation and experiments. III. SIMULATION AND TESTING OF TRANSDUCER FREQUENCY CHARACTERISTICS A. Simulation of Frequency Characteristics In order to prove the analysis from before, the circuit models shown in Figs. 1 and 2 are utilized to finish ac small-signal simulation by Saber, the simulation parameters of,, and C in Fig. 2 are, respectively, 1 k,50, 1000 pf, and the turn number of the primary winding and the secondary winding of the transducer is, respectively, 1 and 95. The frequency characteristics of traditional and improved transducers are, respectively, shown in Figs. 4 and 5.
4 358 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 1, JANUARY 2008 Fig. 6. Frequency characteristics testing of current transducer. The test signal is (a) a low-frequency square waveform, (b) impulse signal, (c) 10-Hz sinusoidal waveform, (d) 1-MHz sinusoidal waveform, and (e) microcurrent of 5 A. As shown in Figs. 4 and 5, through the - transfer circuits, both the low cutoff frequency and high cutoff frequency of the amplitude frequency characteristic curve are extended largely. The low-limit frequency is less than 10 Hz, and the high-limit frequency reaches 10 MHz without obvious attenuation. The phase frequency characteristic is nearly zero-phase contrast in considerably broad bandwidth and has greatly improved. B. Testing of Frequency Characteristics The ring-shaped ferrite magnetic core is adopted in the testing and the number of turns is 95. Function generator CFG280 and oscilloscopetds3032b(tektronix Corp.) are used. In order to examine high-frequency and low-frequency characteristics and sensitivity of the transducer, many experiments have been conducted with an impulse signal, 30-Hz low-frequency square signal, 10-Hz sine signal, high-frequency signal, and a microcurrent sine signal in the laboratory. The testing results are shown in Fig. 6, in which Ch1 and Ch2 are, respectively, labeledinavertical axistoshowthefunctiongeneratorinputvoltage signal and output voltage signal. As shown in Fig. 6, a 10-Hz ultra-low-frequency sine response and 30-Hz square response of the transducer have not
5 CHEN et al.: NEW BROADBAND MICROCURRENT TRANSDUCER 359 Fig. 7. Block diagram of the polluted insulators test. Fig. 9. Partial-discharge current waveform. Fig. 8. Leakage current waveform. been attenuated, which shows that the low cutoff frequency is just several hertz. Meanwhile, the response of a 1-MHz high-frequency signal is consistent with that of the low-frequency signal, which indicates that the high cutoff frequency of the transducer is above 1 MHz. In fact, for the 20-MHz high-frequency signal, the transducer still has a perfect high-frequency response with the simulation test in the laboratory. The 5- A current testing signal is shown in Fig. 6(e). Though the external interference is quite serious, it is certain that the transducer does obtain the microcurrent signal. IV. SIGNAL MEASURING OF POLLUTED INSULATORS Using the designed transducer and leakage current online monitoring system, the actual measurement of leakage current and partial discharge signal of 10-kV polluted synthetic insulators were performed in a climate chamber. The testing arrangement is shown in Fig. 7. The leakage current and measured partial discharge signal are shown in Figs. 8 and 9, respectively. The results show that the polarity, phase, and amplitude of the leakage current and partial discharge signal of insulators can be transmitted by the transducer accurately, and the degree of waveform distortion is also very small. Due to broad working bandwidth, it is very necessary to adopt a better filter technique and appropriate antiinterference measures to prevent the interfering signal from affecting the monitoring signal. As shown in Fig. 8, the leakage current evidently appears when the voltage rises to 1 kv. But there is not an obvious partial-discharge impulsive current. When the voltage rises to 5 kv, the humid dirt bed is partially parched and the partial-discharge impulsive current evidently appears, which is shown as a mass of dense impulse in Fig. 9(a). After the dirt bed is ulteriorly parched, the partial discharge weakens and the leakage current amplitude also reduces under the rated 10-kV voltage as shown in Fig. 9(b). V. CONCLUSION Based on theoretical derivation mentioned before, the greatest effecting factors in pass-bandwidth of the microcurrent transducer are load resistance and stray capacitance, while the turn number of winding and load resistance make a great impact on the transducer sensitivity. A broad bandwidth microcurrent transducer used for online insulation measuring can be equivalent to the mutual inductance element with load resistance. Therefore, the proposed scheme not only extends the passbandwidth of the transducer, but also ensures enough high sensitivity by designing the electronic circuit and choosing the appropriate turn number of windings, when the size of the winding and magnetic core stuff is determined. The transducer can accurately transmit the signal monitored by experimental verification, which offers a reliable method for online leakage current monitoring and discharge pattern recognition of insulators. REFERENCES [1] R. Bartnikas, Detection of partial discharges (corona) in electrical apparatus, IEEE Trans. Elect. Insul., vol. 25, no. 1, pp , Feb [2] L. Kojovic, Rogowski coils suit relay protection and measurement of power systems, IEEE Comput. Appl. Power, vol. 10, no. 3, pp , Jul [3] X. Guo and L. Jingsheng, An improved frequency characteristic Rogowski current transducer, in Proc. IEEE CCECE Canadian Conf., 2003, vol. 1, pp [4] V. Papez and V. Benda, A wide-band current sensor for power electronics, in Proc. 7th Int. Conf. Power Electronics Variable Speed Drives, 1998, no. 456, pp [5] G. D Antona and M. Lazzaroni, AC current-to-voltage transducer based on digital processing of Rogowski coils signal, in Proc. Sensors Industry Conf., 2002, pp [6] K. Ivansson, G. Sinapius, and S. Middelhoek, Measuring current, voltage and powers, in Handbook of Sensors and Actuators. New York: Elsevier, 1999, vol. 7. [7] W. F. Ray and C. R. Hewson, High performance Rogowski current transducers, in Proc. Industry Applications Conf., 2000, vol. 5, pp [8] D. A. Tziouvaras, P. Mclaren, and G. Alexander, Mathematical models for current, voltage and coupling capacitor voltage transformers, IEEE Trans. Power Del., vol. 15, no. 1, pp , Feb
6 360 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 1, JANUARY 2008 [9] J. D. Ramboz, Machinable Rogowski coil design and calibration, IEEE Trans. Instrum. Meas., vol. 45, no. 2, pp , Apr [10] C. N. Richards and J. D. Renowden, Development of a remote insulator contamination monitoring system, IEEE Trans. Power Del., vol. 12, no. 1, pp , Jan [11] W. F. Ray, The use of Rogowski coils for low amplitude current waveform measurement, in Inst. Elect. Colloq., Measurement Techniques for Power Electronics, [12] C. Oates, Tools for HV semiconductor measurement, in Inst. Elect. Colloq. Measurement Techniques for Power Electronics, 1992, no Weigen Chen was born in Shaoxing, Zejiang, China, on August 22, He received the Ph.D. degree in electrical engineering from Chongqing University, Chongqing, China, in He became Professor at Chongqing University in 2003 and is now with the Key Laboratory of High Voltage Engineering and Electrical New Technology, Ministry of Education, Chongqing University. His research interests are online monitoring and fault diagnosis of electrical equipment. Chenguo Yao was born in Nanchong, Sichuan, China, on February 1, He received the Ph.D. degree in electrical engineering from Chongqing University, Chongqing, China, in He became Professor at the College of Electrical Engineering at Chongqing University in His current works include the online monitoring of insulation condition and insulation fault diagnosis for high-voltage apparatus. Caixin Sun was born in Chongqing, China, on December 13, He received the electrical engineering degree from Chongqing University, Chongqing, in He became Professor at Chongqing University in He has been Vice-President of Chongqing University since He is a Standing Director of the China Electrotechnical Society and one of directors of the Chinese Society for Electrical Engineering. He is the Director of the Key Lab of High Voltage Engineering and Electrical New Technology, Ministry of Education. His current research includes online detection of insulation condition and insulation fault diagnosis for high-voltage apparatus, discharge mechanism in a complicated environment, a geography information system, and a high-voltage technique applied to biomedicine. He is an author of more than 200 publications and inventions. Prof. Sun was elected an Academician of the Chinese Academy of Engineering in Lin Du was born in Dazhou, Sichuan, China, on June 7, He received the Ph.D. degree in electrical engineering from Chongqing University, Chongqing, China, in He became Associate Professor at the College of Electrical Engineering at Chongqing University in His current works include online monitoring of insulation condition for high-voltage apparatus. Pan Chen received the B. S. degree in electrical engineering from Huazhong University of Science and Technology, Wuhan, China, in Currently, he is a graduate student at the Key Laboratory of High Voltage Engineering and Electrical New Technology, Ministry of Education, Chongqing University. His research has focused on online monitoring of insulation conditions for high-voltage apparatus. Ruijin Liao was born in Suining, Sichuan, China, on July 28, He received the Ph.D. degree in electrical engineering from Chongqing University, Chonqing, China, in He became Professor of Chongqing University in He has been Dean of College of Electrical Engineering at Chongqing University since His current research includes the online detection of insulation condition and insulation fault diagnosis for high-voltage (HV) apparatus and high-voltage testing technique. He is an author of more than 100 publications and inventions.
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