Reliability of electrical insulation systems (EIS) in power distribution network and conversion chains

03/02/2015
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Reliability of electrical insulation systems (EIS) in power distribution network and conversion chains

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application/pdf Reliability of electrical insulation systems (EIS) in power distribution network and conversion chains Michel Dunand, Flavien Koliatene, Mounir Abdi
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Reliability of electrical insulation systems (EIS) in power distribution network and conversion chains

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Reliability of electrical insulation systems (EIS) in power distribution network and conversion chains Michel DUNAND (1), Flavien KOLIATENE (2), Mounir ABDI (3) 1: LPS Safran group, EWIS Eurasia Division, michel.dunand@safran-engineering.com 2: LPS Safran group, EWIS Eurasia Division, favien.koliatene@safran-engineering.com 3: LPS Safran group, Power Division, mounir.abdi@labinal-power.com Abstract: When the elevation of voltages levels (±270 VDC, 230VAC) was imposed in aeronautics for reducing the mass of electrical systems, Partial Discharges (PD) phenomena occurs. These PD can lead to the appearance of destructive arcs within the Electrical Insulation Systems (EIS). The generalization of the electrical power use for the supply of the aircraft consumers requires in parallel the deployment of static converters with high speed switches (dV/dt), required for the control of actuators placed in harsh environment (high temperature, low pressure) and connected through long power cables. All these topologies and parameters favor the emergence of PD. In order to develop reliable systems, LPS has to master the physics of PD and the mapping of electric fields and electrical potential differences in the systems, to put in place new rules for the design of EIS, to develop new technology for the isolation and to develop new procedures for systems testing and monitoring. Introduction The reliability of the electric systems was mainly due to the good performance and lifetime of the chemical structure of insulators submitted to a strong internal electric field. Dielectric materials with excellent performances, such as polyimide, were always used in aeronautics for insuring the insulating function. Almost flawless, these materials could be used in very thin layers for the insulation, thus contributing to the mass reduction of the EWIS (Electrical Wiring Interconnection System) [1]. Because of the electrification of systems previously pneumatic or hydraulic, the MEA (More electrical Aircraft) leads the aircraft manufacturers to raising the voltage levels of the networks up to ±270VDC and 230VAC, necessary for reducing the mass of EWIS. This revealed the phenomenon of electric discharge in gases (partial discharge (PD) or corona effect) previously unknown in aeronautics. This phenomenon is largely enhanced by typical aeronautical environments characterized by high temperature and low pressures. When occurring, the impact of these discharges engenders an erosion of dielectric materials which can quickly result in the loss of the insulation or even in the appearance of very destructive arcs (arcs tracking). MEA is also defined by the development of electrical systems, with their own integrated power conversion units (SAFRAN electrical systems: braking, steering and taxiing, nacelle de-icing, thrust reverser, flight control). These actuators placed in harsh environment (low pressure, high temperature much higher than 210°C in the engines) are connected through power cables to the converters placed in cabin environment. These new configurations, combined with the harsh environment, can lead to PD in the windings. In order to develop reliable systems, LPS has to master the physics of PD and the mapping of voltages in the systems, to put in place new rules for the design of EIS, to develop new technology for the isolation and to develop new procedures for systems testing and monitoring. Physics of PD The PD in an inter-electrode cavity occurs when the voltage level in the cavity boundaries is high enough to activate an avalanche phenomenon by accelerating the ions and the electrons placed in the electric field. Paschen’s law is used to approximate the partial discharge inception voltage level (PDIV). This law essentially states that the breakdown voltage of common gases in a uniform electric field is a function of the product pressure p - distance d. Recent studies allowed extending this law to the aeronautical environments (Fig.1) by taking into account the temperature and the humidity [2] [3]. Nevertheless, the same curves have to be established for the electrode nature and gas inter-electrodes dependencies. Fig. 1: PDIV at T=80°C (d=1 mm) (3) The electrodes shape may lead to an intense electric field reinforcement around edges or pins and can cause electric discharges ignition localized around these sharp objects. Then, this phenomenon is called corona effect. In order to avoid all DP in a cavity, it is necessary to avoid thus on one hand the presence of sharp objects, and on the other hand an inter- electrodes voltage Vg higher than the acceptable minimal PDIV. Fig. 2: simplify equivalent model Fig.2 illustrates the dependency between the value of the inter-electrode voltage Vg, and thus of the electrical field’s value, and the insulators’ thicknesses: more the insulating material is thick, less the inter- electrode voltage is high. If Vo is the voltage at the boundaries of the system, then the voltage Vg at the boundaries of the cavity will depend on the insulating materials’ thickness and permittivity. Mapping of the electrical constraints Vo in the electrical systems The maximal electrical constraint Vo(t) capable of initiating PD, in any part of the system, must be determined; these values give the insulators’ thicknesses necessary for a PD free system. These over-voltages, occurring on the first turns of the motor windings, may get values as high as the double of the BUS voltage, and are a function of: the dV/dt of the converter voltage, the cable and load impedances [4]. A better modeling of the power supply chain, taking into account the characteristics of the cables and the voltages distribution within the windings is an important research axis. The PDs occur only in gases and mainly in zones where the inter-electrodes gaps are very small; as a consequence, the local uniform field hypothesis can be used. This allowed us to develop a tool for the design the EIS that maps high potential PD ignition risk areas (Fig. 3). It computes the values of the inter- electrode voltage and electrical potential differences exceeding the PDIV in aeronautical environment [3]. Fig. 3: Location of the discharges a)in a cavity inter-turns, b)between two cables New technology and rules for the isolation In the case of windings requiring very low thicknesses of insulating material, the studies concentrate on diminishing the voltage between turns in the design of the winding, adapted to the given motor topology [5], on the development of impregnation techniques capable of excluding gaseous cavities, and the study of wires partial discharges resistant, and last, but not least, on the DP resistant electrical wires. With the actuators’ normal functioning temperature rise above 210°C, new EIS solutions have to be developed. Different solutions are studied with targets at 270°C (Polyimide), 340°C (Silicone) and 450°C (Ceramic, Fiberglass, Mica). Some of them could be applicable for standard temperature range and offer high reliability. New procedures for systems testing and monitoring The best guarantee of reliability of the system will be based on a checking procedure verifying the abscence of PD in the electric systems. The methods of "on line" DP detection [6] must be further studied to ensure their efficiency in an aeronautical environment. These techniques of in situ diagnostic will allow determining the quality of the electric insulating of the systems and could be used during the conception, the design, the manufacturing or the installation, thus ensuring the reliability of the electric systems. Conclusions Substantial progress has been made in understanding the phenomena and their modeling in aeronautical environment, leading today to the development of new technologies and the improvement of insulating design rules. The studies led on the test methods and on-line characterizations will allow the design of mastered reliability electrical systems. References 1 FAR 25 subpart H CS25 2 F.Koliatene : Contribution à l’étude des DP dans le système de l’avionique. Thesis January 2009 3 E.Sili : Etude de caractérisation des DP et du vieillissement du polyimide en environnement aéronautique. Thesis December 2012 4 AF.Moreira, TA.Lipo, G.Venkataramanan, S.Bernet : High-Frequency Modeling for Cable and Induction - Motor Overvoltage Studies in Long Cable Drives. IEEE Transaction on industry applications, Vol 38, N°5, sep/oct 2002 5 J.Moeneclaey : Conception des bobinages des actionneurs de l’avion plus électrique. JCGE 2013 6 T.Billard, F.Fresnet, M.Makarov, T.Lebey, P.Castelan, P.Bidan, S.Dinculescu : Partial Discharges monitoring in twisted pair fed with PWM inverter using non-intrusive sensors. IEEE International Conference on Solid Dielectrics, Bologna, Italy, June 30 – July 4, 2013