2009 Annual Report Conference on Electrical Insulation and Dielectric Phenomena The voltage stresses of insulation systems under PWM inverter supplies 1 B. Florkowska(1), M. Florkowski(2), J. Furgaá(1), J. Roehrich(1), P. Zydron(1), University of Science and Technology, Department of Electrical Engineering and Electrical Power, Al. Mickiewicza 30, 30-059 Krakow, Poland 2 ABB Corporate Research , ul.StarowiĞlna 13A, 31-038 Kraków, Poland are much higher then at the sinusoidal voltage. Thus the voltage distribution within the coils and windings is highly non-linear and may cause large voltage stresses between two consecutive turns [4, 5]. Hence, testing and diagnosing methods of insulation system integrity subjected to high frequency stresses pose additional requirements. Those needs refer to the development of improved, unambiguous methods, which might be applied to both manufacturing and on-site diagnostics. Abstract- Exploitation stresses are causing degradation of high voltage insulation systems. The assessment of intensity and dynamics of these processes, being a consequence of local, working electric field strength is considered mainly at the sinusoidal voltage. However in applications where power electronics converters are used the voltage stress has usually a form of fast switching pulses composed of repetitive sequences. Such pulse trains pose usually a modulated width and fast riseand fall-times. Such conditions have essential influence on inception and development of partial discharges in insulating systems subjected to non-sinusoidal stimulus. The insulation degradation mechanism is especially important for cables and electrical machines subjected to non-sinusoidal waveforms. A novel time-resolved partial discharge (PD) surge pulse acquisition has been described. The method is based on very fast PD registration during repetition of HV surge pulses on insulating material and visualization if form of PRPD like pattern. Comparison between PD patterns obtained at surge pulse and subjected to power frequency sine and trapezoidal is shown. II. Assessment of insulation systems designed for operation under PWM like stress creates new challenges as conventional methods are not appropriate. In case of partial discharge (PD) based assessment the main problem is related with detection of PD pulses and simultaneous influence of the supply voltage [e.g. 6]. a) I. INTRODUCTION 2.0 1 2 u, pu Voltage source pulse with modulated (PWM) inverters are the most common type of drives, which are currently in use to supply electric machines. PWM uses a square wave whose pulse width is modulated, which allows for variation and control of the average value of the waveform. PWM based supply voltage results in a sine-like current in a magnetic circuit of an electric motor. The smoothness of the waveform can be adjusted by the width and number of modulated pulses. In the past special attention was paid to voltage distortion caused by harmonics, whereas last decade the effect of step waveform introduced by converters with fast switches like IGBT (Insulated Gate Bipolar Transistor) started to be treated as even more dangerous. In those drives the control unit is based on pulse-width modulated train of fast pulses with slew rate up to 100 kVμs-1 and frequency repetition up to 100 kHz. In consequence large spikes, overshoots and oscillations are visible on motor terminals. The electrical stress caused by such distorted supply voltage may lead to the insulation system degradation and breakdown of electrical machines and cables [e.g. 1, 2, 3]. The problem may not be meaningful under sinusoidal supply at power frequency (50/60Hz), as the turnto-turn voltage is relatively small. However in case of PWM inverter based supply also the steepness of the applied voltage 978-1-4244-4559-2/09/$25.00 © 2009 IEEE PULSE BASED ASSESSMENT METHOD 1.5 1.0 0.5 0.0 0 2 4 6 8 t, μs 10 Fig. 1. Overvoltages at the motor terminal and at the inverter output for different rise time (dotted red – approximation by a surge pulse): 1 - rise time 300ns, 2 - rise time 1 μs overvoltage at the motor terminal, voltage at the inverter output In frequency domain, both the spectrum of the PWM power supply with fast rise time and PD spectrum are overlapping preventing application of easy filtering and separation of both components. In consequence, the PD detection during fast 372 slopes of the stimulus is much more difficult, due to the significant content of that signal at the receiver input. Additional difficulty in phase/time-resolved registration is the repetition rate of the PWM voltage, which can reach up to 100kHz. This paper presents novel method based on time resolved PD acquisition under repetitive surge stimulus. In drive fed motors, both insulation systems of a motor and cables are subjected to overvoltages, whose crest value depend on the configuration, length of cables, impedance matching etc.. The exemplary overvoltages at the motor terminal and at the inverter output are shown in Figure 1 for two different rise time values of supply voltage: 300 ns and 1 μs. Such additional stress can be approximated by a surge pulse with a fast rise time and relatively slow decay time (dotted red line in Fig.1). Applying the repetitive train of such pulses to the insulation system, the partial discharges are registered in timeresolved mode. The surge pulse can be treated to some extent as an analog of a lightning/switching pulse test performed on MV/HV insulation systems. The following parameters of the surge pulse can be adjusted with relation to the PWM supply voltage: rise time, decay time, repetition rate of the train of pulses and amplitude. The block diagram of the measuring system is shown in Fig.2. The test object is subjected to repetitive train of pulses from the high voltage surge generator with controlled amplitude and rise/decay time. No of repetitions surge decay time [μs] Fig. 3. Exemplary time-resolved PD pattern obtained for repetitive train of fast surge stimulus III. TEST SAMPLES The investigations have been performed on the stator-bar samples of the motor insulation, which have been subjected to surge pulses with steep front as well as to slow sine and trapezoidal stimulus at power frequency. The stator bar samples represent the form-wound type of insulation. The main insulation is based on mica type insulation. All measurements have been performed at room temperature. The test voltage level for sine and trapezoidal case has been adjusted to 1.25 inception voltage for sine supply. The ground electrode has been formed from the aluminum foil. IV. RESULTS In order to compare the impact of PWM based inverter supply on insulation system, the measurement of partial discharges at three different waveforms has been curried out. The PD have been registered at sine (50Hz), trapezoidal (rise time 140μs, 50Hz) and surge voltage (rise time 100ns, decay time 100μs repetition in the range from ms to seconds). The comparison of the steepness dU/dt and the rise time for these waveforms has been presented in Table I. The rise time (tr) has been measured between the ground level and the crest value of the corresponding waveform. Fig. 2. Measuring setup of the time-resolved surge acquisition TABLE I Waveform The rise time was selected in the range between 50ns and 1000ns and fall time between 20-1000μs. The partial discharge signal can be obtained from 3 sources: ultra wide band current transformer, coupling impedance in form of high pass filter or antenna. Fast PD acquisition was performed with a sampling rate 2GS/s. The PDs have been recorder during surge pulse exposition. The conversion of the PD record to the time-resolved pattern has been done by the dedicated software. The communication to the host computer is provided by GPIB interface. Typical measurement consists of certain number of surge impacts, typically 100 to 10000. In order to obtain consistent pattern, the acquisition of all surges has been synchronized by a trigger point on the surge pulse wavefront. The repetition rate has been set between ms to 5 seconds. dU/dt [kV/μs] Rise time tr [μs] 0.0019 5000 0.044 140 90 0.1 The PD patterns obtained at sine and trapezoidal voltage are shown in Fig 4. The voltage in both cases is 1.25Uinc, where Uinc is the inception voltage at sine. In case of sine pattern, the structure of discharges reminds typical void distribution in the stator bar insulation with many sources of discharges. Asymmetry of the phase-resolved images within a period of 373 occurring on the fast rising slope were not acquired. PD inception level defined in such a way for surge voltage stimulus has been observed at lower voltage level comparing to sine/triangle case. Surge patterns obtained for motor insulation at voltage range Uinc_surge - 1.5⋅Uinc_surge are shown in Fig. 5. the test voltage indicate that some of the sources adhere to the conductor. a) Amp. [au] Uinc_surge PHASE [deg] b) Amp. [au] 1.25⋅Uinc_surge PHASE [deg] 1.5⋅Uinc_surge Fig. 4. The PD patterns at: a) sinusoidal voltage b) trapezoidal voltage, tr=140μs, 1.25Uinc_sin=6.25kV, freq=50Hz The PD pattern obtained for trapezoidal waveform consists of two groups of PD pulses in each half of the period: the first group represents discharges appearing on the rising / falling slope of the voltage and the second one corresponds to the flat part of the waveform [7, 8]. During flat part of the trapezoidal voltage the rare activity of discharges results from stable dU/dt inside the discharge source. PD activity concentrated on the rising and falling slope comprises narrow phase range (ca. 12 deg), while at sine wave it stretches to approximately 90 deg. PD patterns show that the maximal value of PD magnitude is larger at trapezoidal wave. This escalation may results from remaining and not neutralized space charge on the surface of discharge source [9, 10]. Hence, PD activity depends on voltage steepness, this problem has been investigated applying fast surge impulses with a rise time 100 ns and a decay time 100 μs. For PD visualization, the PRPD like, time-resolved acquisition was used. In case of the fast surge stimulus (tr=100ns), the quasi PD inception (Uinc_surge) was defined as a presence of discharges on the tail part of the waveform. The potential discharges time =70μs Fig. 5. Surge (PRPD like) patterns obtained for motor insulation up to 1.5⋅Uinc_surge (surge pulse rise time 100ns, decay 100μs), Y-axis amplitude on the plots is in arbitrary units While increasing the crest value of the surge, the PD group shows the tendency to appear earlier. One can also observe increase in the PD magnitude and number of pulses. Surge voltage with very short rise time reflects the stress voltage generated at PWM conditions including the effect of overvoltages. 374 accumulated surge patterns reflect the superposition of partial discharges occurring in the consecutive periods of the PWM waveform. It has been noticed the influence of the wave-front rise time on the partial discharges activity occurring on the surge impulse voltage wave-tail. Employing of the surge pulse with decaying part instead of square waveform results in more dynamic PD occurrence in the wave-tail due to changing dU/dt. The application of the surge mimics the partial discharges appearing on the flat part of the square waveform in real inverter-motor system, in case when overvoltage on motor terminal are incorporated. The presented results demonstrate the coherent PD pattern obtained for surge pulse. The comparison of results for the stator-bar insulation of formwound motor subjected to sine, trapezoidal and surge stress reveal the decrease of PD inception voltage while increasing the wave-front steepness. Presented approach may be applied for test and diagnostics of insulation system integrity subjected to steepfront voltage. Thus a surge pulse may be treated as an approximation of the square wave with superimposed transients. The surge patterns illustrate the accumulated in a time-resolved mode set of PD pulses, which reflect the condition on an insulating system subjected to high du/dt voltage stress. V. PD AT VARIOUS dU/dt STEEPNESS The comparison of PD dynamics on form-wound motor insulation subjected to the following three cases of voltage stimulus has been performed: sine at power frequency, trapezoidal waveform with the rise time 140μs and surge pulse with a steepness 90kV/μs. Relationship of the PD inception voltage versus wave-front voltage steepness in stator bar motor insulation has been illustrated in Fig. 6. The transition from sine to trapezoidal waveform indicated the decrease of the inception voltage. Further, this trend is also confirmed for very fast surge. In sine/trapezoidal cases the PD inception voltage refers to the crest value of the waveform. Whereas for surge pulse the inception voltage has been defined as appearance of the stable PD group on the decaying part of the surge impulse. The time proceeding the PD set on the wavetail is related to the formation of the PD channels in voids and to the time constant of the voltage build-up on the voltage source in response to fast du/dt. Increasing the surge voltage one can notice higher steepness on the wave-tail, which is influencing also the PD dynamics. ACKNOWLEDGMENT The work described in the paper was partly carried out in project NR 01 0019 04 sponsored by the Polish Ministry of Science and Higher Education. REFERENCES [1] M. Kaufhold, H. Aninger, M. Berth, J. Speck, M. Eberhardt, “Electrical stress and failure mechanism of the winding insulation in PWMinverter-fed low voltage induction motors”, IEEE Trans. Industr. Electron., vol.47, pp. 396-402, 2000. [2] S. Grzybowski, P. Shresta, I. Cao, Electrical, “Aging phenomena of XLPE and EPR cable insulation energized by switching impulses”, Proc. of 2008 Int. Conf. on High Voltage Eng. and Appl., pp.422-425, Chongqing, China, November 9-13, 2008. [3] S.U.Haq, S.H.Jayaram, E.A.Cherney, Insulatiopn problems in mediumvoltage stator coils under fast repetitive voltage pulses, IEEE Trans. Industry Applications, vol 44, no 4, 2008, pp. 1004-1012 [4] F. Gustavino, G. Coletti, A. Ratto, E. Torello, “A study about partial discharge measurements performed applying to insulating systems square voltages with different rise time”, IEEE CEIDP’2005 Annual Report, pp 418-421. [5] IEC TS 60034-18-41, Rotating electrical machines, qualification and type tests for Type I electrical insulation system used in rotating electrical machines fed from voltage converters, 2006 [6] IEC TS 61934, Electrical insulating materials and systems – electrical measurement of partial discharges (PD) under short rise time and repetitive voltage impulses, 2006 [7] B. Florkowska, P. Zydron, “Analysis of conditions of partial discharges inception and development at non-sinusoidal testing voltages”, IEEE CEIDP’2006 Annual Report, pp. 648-651, October 2006. [8] B. Florkowska, M. Florkowski, R. Wlodek, P. Zydron, Mechanisms, measurements and analysis of partial discharges in diagnostics of high voltage insulating systems (written in Polish), Ed. IPPT PAN, ISBN 83910387-5-0, Warszawa, 2001. [9] D. Fabiani, G.C.Montanari, A.Cavallini, G.Mazzanti, “Relation between space charge accumulation and partial discharge activity in enameled wires under PWM-like voltage waveforms”, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 11, 2004, pp. 393 - 405 [10] B.Florkowska, M.Florkowski, J.Furgaá, P.ZydroĔ, Influence of different voltage waveforms on PD formation in HV insulation systems, Electrical Insulation Conference (EIC), Montreal, 2009 Fig. 6. Relationship of the PD inception voltage vs. wave-front voltage steepness in stator bar motor insulation VI. CONCLUSIONS Paper presents the results of investigations of voltage steepness influence on the PD mechanism in order to verify the impact of the waveforms used is PWM based inverters. The insulation assessment approach based on repetitive train of fast surges has been shown. The individual surge is approximating the typical PWM based inverter supply with overvoltages. The method of time-resolved PD acquisition resulting in PRPD like patterns has been described. The 375
