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2016 China International Conference on Electricity Distribution (CICED 2016) Xi’an, 10-13 Aug, 2016 Impacts of the Frequency Effects on Partial Discharge Characteristics of the High Frequency Power Transformer Insulation Han Shuai1, Wang Feng1, Bi Jiangang1, Zhang Bowen1, Li Qingmin2 1.China Electric Power Research Institute 2.North China Electric Power University Abstract—The High-frequency power transformer (HFPT) is a new type of intelligent distribution equipment. It is easy to fail under high frequency voltage, the main cause of which is partial discharge (PD). A comprehensive test system is established to study the impacts of the frequency on PD characteristics. The PD inception voltage (PDIV), PD times, and PD mean amplitude of polyimide film are measured under the frequency 1~50 kHz. The adaptive algorithm is established to decompose and reduce the noise so as to acquire the pure PD signal and achieve the effective characteristics. Also the insulation lifetime is recorded to illustrate the relationship between PD and aging state. Furthermore, the frequency-dependent characteristics of several typical PD parameters are elucidated in details using the method of equivalent microcircuit which can explain the PD activities under high frequency voltage. The experimental results show that with the increase of frequency, PD mean amplitude and times of polyimide film see downward trends after first increase while the PDIV keeps roughly constant with frequency variation. Through the analysis, the frequency-induced space charge accumulation effect plays a dominant role in the above physical changes under high frequency, whereas, the frequency stress changes the insulation permeability as well as the tangential E-field vector, which executes a prominent impact on the PD development under low frequency. It is worth noting that the lifetime is greatly shortened as the frequency increase which is different form the trend of PD characters, showing that there is no direct relationship between PD and the insulation lifetime. Index Terms— high-frequency power transformer, partial discharge, insulation system, space charge, frequency effect I. INTRODUCTION Intelligent high voltage electrical equipment in substation is an important physical basis for the realization of smart grid which not only relies on the CICED2016 Session 1 Paper No CP0146 intelligent monitoring of running state of electrical equipment but also the intelligent operation and control of electrical equipment itself [1]. With the development of semiconductor devices and power electronic technology, a new type of intelligent substation equipment is coming into the world—High-frequency Transformer (HFT) or Power electronic transformer (PET), resolving most of the problems and drawbacks that traditional transformer has (e.g. large volume and high loss factor). Moreover, PET has realized the flexible control over energy to meet the various needs of power grid and proved to be the most intelligent transformer fundamentally [2]. The HFT usually works under sinusoidal voltage with the frequency 1-50 kHz. Despite the advantages illustrated above, the insulation system of HFT which is composed of polymer and air is fragile under high frequency voltage [3]. Partial discharge (PD) is considered to be the dominant factor to the insulation failure under strong electrical press in all power transformers. PD properties under power frequency and square-wave voltage are widely studied. The PD pulses are more likely to emerge on the rising edge of the voltage whereas few on the falling edge in positive or negative respectively [4]. The PD times and amplitude remain substantially constant with frequency under square wave voltage [5]. However, the unique characteristics of HF sinusoidal voltage is obvious which have sharper rising edge than the normal power frequency voltage, no platform area compared with square wave voltage, and much higher frequency than those waveforms. Therefore, the impact as well as the mechanism of the high frequency sinusoidal voltage should be further explored. In this paper, frequency based experiment ranging from 1 kHz to 50 kHz is carried out. PD times, and mean PD amplitude of polyimide film are measured. Several frequency-dependent characteristics of PD are elucidated in details using the method of equivalent microcircuit in order to find out their intrinsic relationship from the physics point of view. Page 1/4 2016 China International Conference on Electricity Distribution (CICED 2016) II. Experimental Setup The schematic diagram of PD mesure system is shown in Fig. 1. The adopted HF voltage source can generate the sinusoidal voltage (0-15kV) with frequency 1 k- 50 kHz. Stick-plate electode according to IEC 60343 is used to model the field disitrabution of the real insulation system. The photomultiplier (PMT) is employed to detect the emitted light so as to standardize the PD inception voltage (PDIV). Considering the unique characteristics of high frequency PD, ETS-93686 high frequency current transducer (HFCT) is adopted to detect PD pulses. Polyimide films (PI) of 75 μm are chosen as the insulation samples, and they are all cleaned by ethanol and dried in 60℃ for 10 hours before the test. The test is carried out under the standard atmospheric condition (20 ℃, 0.1 MPa ). In this test, the voltage is increased rapidly until the PD happens and then it will be set to zero. Such step is repeated 5 times to obtain the PDIV. The PD test voltage is chosen to be 2kV which is the 1.5 times of the PDIV, and the voltage is applied until the breakdown happens. Xi’an, 10-13 Aug, 2016 The process of PD signal extraction is shown in Figure 2. Figure 2(a) displays the original signal in which many interfering signals exist including the environmental noise. Firstly, the adaptive threshold method is used to remove the noise signal in the original, and the waveform can be obtained by reconstructing the remaining signal, as is shown in Figure 2(b). To prove the effectiveness of the de-noising method, the partial discharge signal energy is calculated. The energy ratio between the de-noised and the original signal is 93.56%, which means that fine filtering effect can be achieved without distorting the partial discharge signal. Then according to the principle that the bipolar is discharge and the unipolar is interference, the real discharge signal can be acquired by excluding the unipolar pulse interference from the reconstructed signal, and the waveform of the result is shown in Fig. 2(c). Finally, for the reason that a complete discharge is an oscillation, the phase and amplitude of partial discharge signal cannot be directly obtained from the signal. As a result, the open-window method by separating the different discharge signals and selecting the maximum in every oscillation is used to process the real discharge signal. The processed result is shown in Figure 2(d) and the final patterns of PD pulses can be seen in Fig. 3. III. Experimental Result Fig. 1. Schematic diagram of test system Fig. 4 shows that when the frequency is below 10 kHz, the PD times will see an increasing trend at first. During the period 10~50 kHz, the PD times decrease slightly and it end up with 1400 which is close to the 6 kHz point. The similar situation also takes place to the curve of PD mean amplitude which is shown in Fig. 5, however, it is not a monotone decrease until the frequency reaches the 30 kHz (0.19V) and then it rises slightly. Both these two curves reach their maximum values at about 8~ 10 kHz and that is also the critical point between HF and LF in the traditional sense. As can be seen in Fig. 6, the PDIV keeps roughly constant with frequency variation which is around 1.3kV. The trends of the phases of the pulses in the test are exhibited in Fig.7. It can be seen that the phases of both the positive and negative PD pulses decline with the frequency rises and their tendencies are basically the same. It is worth to notice that the minimum value of both the positive and negative PD phase is below 0°when the frequency is above 30kHz, and such phenomena cannot be seen under power frequency atmosphere. Fig. 2. The extraction of PD Signals CICED2016 Session 1 Paper No CP0146 Page 1/4 2016 China International Conference on Electricity Distribution (CICED 2016) 0.5 0 -0.5 -1 0 100 200 300 Phase /( deg) 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 0 (a) 1 kHz 250 Possitive pulse Negative pulse 200 Phase of the PD pulse / ° PD Amplitude /( V) PD Amplitude /( V) 1 Xi’an, 10-13 Aug, 2016 100 200 300 Phase /( deg) 150 100 50 0 (b) 10 kHz -50 Fig. 3. PD patterns at different frenquencies 0 10 20 30 Frequency / kHz 40 50 Fig. 7. Trend of the PD pulse Phase 1700 1600 IV. Analysis and Discussions PD Times 1500 1400 1300 1200 1100 0 10 20 30 Frequency / kHz 40 50 Fig. 4. PD times under different frenquencies The whole insulation-electrode system can be modeled as an equivalent circuit (Figure 8). Cd and Cg are the dielectric and air gap equivalent capacitor respectively. Before the breakdown occurs, the two capacitors are in series. When the air gap breakdown, the switch will be closed and Cg will be in parallel with the plasma resistance which varies with time. Obviously, the voltage in the air gap Ua(t) can be illustrated by equation (1). 0.21 (1) PD mean amplitude 0.2 0.19 0.18 0.17 0.16 0.15 Cd Ud 0 10 20 30 Frequency / kHz 40 50 Id Ua Gap Breakdown Fig. 5. PD mean amplitude under different frenquencies Ug Rs Cg Fig.8 The equivalent microcircuit for partial discharge of the gas-solid insulation Fig. 6. Behavior of PDIV value with frequency CICED2016 Session 1 Paper No CP0146 Wherein, Ua(t) and Ud(t) are the applied voltage and the voltage applied on the insulation dielectric. As the field taken by air gap is much higher than that of dielectric, the gap will breakdown first when the voltage reach the certain value Ui. After the breakdown event, Ug(t) will decrease dramatically below Ui until the PD current extinguishes. Because of the existing plasma resistance Rs, the charges in capacitor will continue to decline, finally leaving a residual voltage Ures which will hinder the PD when the voltage polarity gets reversed [6]. Meanwhile, the Page 1/4 2016 China International Conference on Electricity Distribution (CICED 2016) current Id increases rapidly, charging the insulation dielectric capacitor, and increases the charges accumulating the Cd. During the discharge process, the air gap voltage Ug(t) can be expressed by the equation (2): (2) Owing to the space charge effect, once the polarity gets reversed, the space charge is still captured in deep traps of the PI film, making the stay at a higher value. When the applied voltage decreases to a value low enough, air gap breakdown will occur by the voltage . The higher the frequency is, the smaller charge dissipation time will be, and also the larger will become because of the unchanged constant . That is why the phases change with the frequency and fall below 0° in Fig. 7. When the frequency is below 10 kHz, the residual space charge in the air gap will reduce to a low value as the cycle is long enough for the dissipation of charge. The frequency changes the insulation permeability as well as the tangential E-field vector [7], so that the PD can easily take place in larger area. The compression of energy density of the applied voltage together with the enlarged discharge area will make the PD times and PD mean amplitude rise in lower frequency. However, when the frequency becomes higher, charge dissipation time t will be shortened in half cycle and Ua(t)+Ud(t)-Ug(t) will become smaller, leading to the reduce of PD parameters. Limited by the applied voltage and charge density that the electrons emit, the enlarged PD area will no longer be in the dominant place. Moreover, due to the constriction of cycle caused by frequency increasing, the chance that Ug attain Ui has been largely reduced, thus the PD times decrease. In order to better study the properties of PD, statistics of the average lifetime of the samples are obtained, see TABLE I. It is obvious that samples under lower frequency have longer lifetime which is different from the trend of PD parameters. As a result, the main cause of aging will not be PD, but the diametrical damage caused by the high frequency effect. In order to study the potential relationship between PD characteristic and frequency, 1-50 kHz high frequency voltage and a series of signal extraction methods are chosen for the experimental arrangements. The results indicate that with the increase of frequency PD both the amplitude and times of PI film see downward trends after first increase in the frequency range of 1~50 kHz while the PDIV keeps roughly 1.3kV. Through the analysis, the frequency-induced space charge accumulation effect plays a dominant role in the above physical changes, in the meanwhile, the frequency stress changes the insulation permeability as well as the tangential E-field vector, which executes a prominent impact on the PD development. It is worth noting that the lifetime is greatly shortened as the frequency increase which is different from the trend of PD characters, showing that PD is not the main cause of aging, but the diametrical damage caused by the frequency effect. The research above can provide theoretical and technological basis for the development of the high-frequency power transformer to be equipped in smart grid, which presents academic significance and application prospects. ACKNOWLEDGMENT The authors would here convey their appreciation for the support by the technology project of the State Grid Corporation of China (GY71-15-045). REFERENCES [1] [2] [3] [4] TABLE I. RELATIONSHIP BETWEEN LIFETIME AND FREQUENCY Frequency Lifetime 1 kHz 16121s 5 kHz 5090s 10 30 50 kHz kHz kHz 2805s 1632s 1214s [5] [6] [7] V. CICED2016 Session 1 Conclusions Paper No CP0146 Xi’an, 10-13 Aug, 2016 Seunghun Baek, Yu Du, Gangyao Wang. Design Considerations of High Voltage and High Frequency Transformer for Solid State Transformer Application. IECON 2010-6th Annual Conference on IEEE Industrial Electronics Society, North Carolina State, 7-10 Nov. 2010, pp.421-426. Lothar Heinemann. An Actively Cooled High Power, High Frequency Transformer with High Insulation Capability. IEEE Applied Power Electronics Conference and Exposition, Dallas, TX, Mar 10-14, 2002, pp.352-357. E. Sili, J P,Cambronne, et al. Polyimide lifetime under partial discharge aging: effects of temperature, pressure and humidity. IEEE Transactions on Dielectrics and Electrical Insulation, 2013, vol. 20, pp.435-442. Zhou Liren, Wu Guangning, Gao Bo, et al. Charge Transport Mechanism and Space Charge Characteristic in Polyimide Film. Transactions Of China Electrotechnical Society, 2009, vol.24, pp.6-11. K. Wu, T. Okamoto and Y. Suzuoki. Effects of discharge area and surface conductivity on partial discharge behavior in voids under square voltages. IEEE Transactions on Dielectrics and Electrical Insulation, 2007, vol.14, pp.461-470. D. Fabiani, G. C. Montanari, et al. Relation between space charge accumulation and partial discharge activity in enameled wires under PWM-like voltage waveforms. IEEE Transactions on Dielectrics and Electrical Insulation, 2004, vol.11, pp. 393-405. N. Hayakawa, H. Inano, Y. Nakamura, et al. Time variation of partial discharge activity leading to breakdown of magnet wire under repetitive surge voltage application. IEEE Transactions on Dielectrics and Electrical Insulation, 2008, vol.15, pp.1701-1706. Page 1/4