Partial discharge detection in electric apparatus fed by PWM like inverter in aircraft environment: a laboratory study

03/02/2015
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Partial discharge detection in electric apparatus fed by PWM like  inverter in aircraft environment: a laboratory study

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application/pdf Partial discharge detection in electric apparatus fed by PWM like inverter in aircraft environment: a laboratory study Cedric Abadie, Thibaut Billard, Sorin Dinculescu, Thierry Lebey
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Partial discharge detection in electric apparatus fed by PWM like  inverter in aircraft environment: a laboratory study

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Partial discharge detection in electric apparatus fed by PWM like inverter in aircraft environment: a laboratory study. Cedric Abadie (3), Thibaut Billard (1), Sorin Dinculescu (1), Thierry Lebey (1), 1 : Laplace Laboratory, 118 Route de Narbonne, 31062 Toulouse, France, thierry.lebey@laplace.univ-tlse.fr 3 : IRT Saint Exupéry, MRV, 118 Route de Narbonne, 31142 Toulouse Cedex 4, France, cedric.abadie@irt- saintexupery.com Abstract In forecoming “more electrical aircrafts”, aircrafts will become more compact and light, as pneumatic and mechanical systems will be replaced by electrical drives. To supply the required power, the network voltage is expected to increase, thus increasing the risk of partial discharges (PD). These discharges, known as the silent enemies, will affect the reliability of the system. Moreover, the environmental conditions of electrical equipments may vary, some of them being in unpressurized area and/or submitted to large temperature changes. These parameters have an influence on one hand, on discharge ignition voltage and on the other hand, on its nature. This paper deals with a new non-intrusive method to detect partial discharges in rotating machines under repetitive square voltage waveform at various pressures. 1 Introduction In an era of growing ecological concerns and cost- effectiveness policy, weight of aircraft is a key parameter to reduce fuel costs, expand commercial range and passengers capacity. Thus, the use of electrical equipment could allow a significant mass reduction of the aircraft and more and more pneumatic or mechanical systems could be replaced with electrical ones [1]. More directly, an increase of power density of machines is an objective when designing electrical system meant to be used in aircraft. As a result, in forecoming “more electrical aircrafts”, the embarked electrical power is expected to increase compared to current architectures and increasing the voltage has been decided. Such a voltage increase may lead to an increased risk of Partial Discharge (PD) appearance [2][3]. A PD is an electrical discharge that does not completely bridge the insulation, either because the electrical stress is not sufficient or because the insulating material strength is too large. These phenomena do not immediately destroy the device, but it is harmful because PD leads to a gradual deterioration of the insulation material that may ultimately lead to its premature failure. This degradation is due to the combination of different mechanisms:  Thermal constraint created by the discharge,  Chemical constraint. The discharge creates degradation products like ozone (or nitric acid in presence of moisture),  Mechanical erosion due to the bombardment by charged species (electrons, ions) In an aircraft, both DC and AC electrical networks are used with different levels of voltage and some electrical apparatus, such as electrical motors or actuators, must be fed with a three-phases electrical network. Nowadays, adjustable speed drives (ASD) are widely used in applications where the speed and/or the torque has to be varied and aircrafts are not exceptions. DC-AC converters implement the pulse width modulated (PWM) technique, which provides square voltage waveforms of the same voltage magnitude but not the same width, thus recreating a sine current waveform for the machine. The square width is modulated according to a sinusoidal law, with rise front up to some tens of kV/μs and pulse repetitions frequency up to some tens of kHz. A motor fed by PWM inverter is subjected to a very high number of fast rise time voltage pulses. The shortest the rise time, the most inhomogeneous the voltage distribution is along stator windings. In fact, most of the voltage is located within the first turns of the coil, thus a large voltage difference is localised between first turns of a coil and the rest of the same coil [4]. Furthermore if the motor is randomly wound, first turns could be adjacent to the last turns of the same coil and nearly 80 % of the DC bus voltage will be supported by the turn-to-turn insulation which is made of enamel and impregnating varnish. The turn- to-turn insulation has usually not been designed to withstand this constraint. Moreover, overvoltages could be created depending on impedance mismatch between the power supply and motors terminals, cable length and impulse rise time. In the worst case, the voltage at motor terminals can double the DC bus voltage. These severe electrical stresses increase the probability of PD occurrence in windings [5]. In aircraft applications, the electrical equipment and electronic systems must be designed to operate over a wide range of pressure and temperature: the former may change from atmospheric pressure to as low as about 100 mbar whereas the later will range between -60°C and up to 400°C in some cases. Changes in the dielectric strength of gases from high pressures to vacuum have been the subject of a very extensive literature [6].The pressure has not only an influence on the discharge inception voltage (DIV) but can also modify the nature of the discharge too. As regards PD, their nature may be Pulseless, Silent, Glow, Streamer or Townsend type. Fig. 1: The different nature of discharges The aim of this paper is to investigate the impact of pressure not only on the DIV but also on the nature of the discharge created by a one phase home made PWM inverter on different samples like twisted pair of enamel wire or electric motor. This paper describes the tests realized on an electrical stator supply by a PWM voltage waveform at different air pressure. Partial discharges measurements were realized using a nonintrusive sensor. 2 Experimental set-up The experimental set-up used was built in Laplace and the details may be found in [7]. The sensor is a coaxial cable stripped to expose its core on 1cm, then to expose the inner insulator on 1 cm without any ground shield. In order to feed a stator, a homemade PWM inverter is associated to a high voltage power source (up to 1.7 kV DC and current peaks up to 50 A). This bipolar pulses generator allows testing low voltage electric motor in quasi-real situation. The voltage is increased until discharges are detected (ie when the Partial Discharge Inception Voltage (PDIV) is reached). All the different parameters of the generator may be modified. These are pulse duration, duty cycle, switching frequency or even create a full PWM period. This power supply allows us to test specifically one phase by connecting it through the neutral point. As a result, even off-line tests are very close to online tests. A high-voltage differential probe measures the impulse voltage at the stator terminals. In order to detect PD for pulse voltage presenting short rise time, it is necessary to improve the signal to noise ratio (S/N). As a matter of fact, PDs appear as small amplitude and high frequencies signals amongst large amplitude and lower frequencies signals associated to voltage application. It is therefore obvious that the use of a filter is mandatory to detect PDs during rise time and switching. PD signal spectrum ranges, at least, up to 1GHz whereas noise generated by the PWM inverter does not seem to exceed 100 MHz (in our equipment). That is to say that a different cut-off frequency may be needed depending on the rise time of switching. Obviously, the shortest the rise time, the higher the cut-off frequency should be. In our experiments, a cut-off frequency of 400MHz has been chosen for the high- pass filter directly connected to the sensor placed in the vicinity of power cable near motor terminals. The data are displayed and recorded using a Tektronix MSO 5204 Digital Oscilloscope with a 2 GHz numerical bandwidth and 5 GS/s sampling rate. All signals coming from high voltage differential probe or sensors are displayed simultaneously. An example of PD detection is given in Figure 2. Fig. 2: One-Phase testing – With filtering The stator is installed in a climatic chamber, which allows measuring partial discharge activity under combined stresses caused by the atmospheric parameters corresponding to the aeronautical environment. In this study, only the influence of the pressure is investigated. The different results regarding the detection of PD in such an environment will be discussed and detailed in the final paper. However, it is worth to notice that PD when they exist appear at a lower voltage when the pressure is low thus proving that the discharges are mainly associated to surface or open defects (interturn insulation in the end-winding) References [1] O. Langlois, E. Foch, X. Roboam, and H. Piquet, “De l’avion plus électrique à l’avion tout électrique : état de l’art et prospective sur les réseaux de bord,” J3eA, vol. 4, no. HORS SÉRIE 1, 2005. [2] R. Rui and I. Cotton, “Impact of low pressure aerospace environment on machine winding insulation,” in Conference Record of the 2010 IEEE International Symposium on Electrical Insulation (ISEI), 2010, pp. 1–5. [3] F. Koliatene, “Contribution à l’étude de l’existence des décharges dans les systèmes de l’avionique,” phd, Université de Toulouse, Université Toulouse III - Paul Sabatier, 2009. [4] J. P. Bellomo, T. Lebey, J. M. Oraison, and F. Peltier, “Characterisation of voltage shapes acting on the insulation of rotating machines supplied by inverters,” in , Proceedings of the 4th International Conference on Properties and Applications of Dielectric Materials, 1994, 1994, vol. 2, pp. 792–795 vol.2. [5] A. Mbaye, J. P. Bellomo, T. Lebey, J. M. Oraison, and F. Peltier, “Electrical stresses applied to stator insulation in low-voltage induction motors fed by PWM drives,” Electr. Power Appl. IEE Proc. -, vol. 144, no. 3, pp. 191–198, May 1997. [6] F. Paschen, “Sur la différence de potentiel nécessaire à la formation d’arc électrique dans de l’air, de l’hydrogène et du gaz carbonique sous différentes pressions (trad. Über die zum Funkenübergang in Luft, Wasserstoff and Kohlensäure bei verschiedenen Drücken erforderliche Potentialdifferenz),” Wied Ann. Phys., vol. 37, pp. 69–96, 1889. [7] T. Billard, T. Lebey, and F. Fresnet, “Partial discharge in electric motor fed by a PWM inverter: off-line and on-line detection,” IEEE Trans. Dielectr. Electr. Insul., vol. 21, no. 3, pp. 1235–1242, Jun. 2014.