Partial Discharge Basics What is a Partial Discharge ? IEEE Definition

The present paper reports non-electrically intrusive partial discharge investigations on an aeronautic motor. Relevancy, robustness and repeatability of partial discharge testing procedures, both on insulating materials characterization and on operating aeronautic equipment are essential to ensure reliability of the aircraft systems. The aim of this paper is to be the very first step of defining such procedures and the associated test equipment. To do so, the paper will start by providing an understanding of partial discharge phenomena and will review typical more electrical aircraft architecture. Key characteristics causing partial discharge risk to increase will be highlighted. The impact of harness length, high performance power electronics and voltage level increase on insulation system is demonstrated. Then, an analysis is carried out from motor design perspective to find out which test procedure is relevant to assess each insulation system performance both at atmospheric and reduced pressures. The study helps to realize the benefits of using multiple test sample and procedure to assess insulation system performance level through the design process of the motor. It is shown that a good agreement observed between stator and representative samples. This paper will also review the ability of the non-intrusive test method to detect partial discharge with a full scope of voltage signal from AC to impulse voltage and under different pressures. The paper concludes with an analysis of results and thoughts about future work regarding motor design taking into account partial discharge risk and evolution of proposed procedure. Introduction Background Optimization of energy sources aboard aircraft and permanent improvements in more electric technologies are pushing the aeronautic industry to aim for the more electrical aircraft (MEA). Indeed, electric energy offers numerous benefits in aircrafts. First, power generation, distribution and conversion are easier because being more accurate and flexible than pneumatic or hydraulic energy control. Then, significant mass reduction seems more likely to be achieved than with traditional hydraulic systems needing large, heavy and maintenance intensive distribution system. But, this shift towards MEA is not without consequences on the electrical stress the insulation system has to withstand. Thus, with primary voltage increasing, power distribution architecture evolution and high power density power electronics, partial discharge is now a serious cause of concern for aircraft integrator, system designer and component manufacturer. Once partial discharges are occurring regularly, degradation of the insulation system until premature failure of the aircraft system is irremediable. Scope and goal of the work This topic of interest has already been discussed by several key actors of the aerospace industry, research center such as US Air force laboratory or international academic actors. All acknowledge that partial discharge risk in a harsh aeronautic environment where pressure, temperature and humidity variation, electrical stress created by inverter and harness length and weight increase. [1, 2, 3, 4] Among key issues of aeronautic companies that has to be dealt with at IRT Saint-Exupéry, two will be addressed in this paper and will be the main contribution. Non-Intrusive Partial Discharges Investigations on Aeronautic Motors Thibaut Billard, Cedric Abadie, and Bouazza Taghia IRT Saint-Exupery CITATION: Billard, T. , Abadie, C. , and Taghia, B. , "Non-Intrusive Partial Discharges Investigations on Aeronautic Motors," SAE Technical Paper 2016-01-2058, 2016, doi:10.4271/2016-01-2058. First, the ability to accurately detect partial discharge, from material characterization tests to full system analysis during operation, in an electrically non-intrusive way and under representative aeronautic environment. Secondly, determine and specify a relevant set of test procedure and characterization equipment to make informed decision during the design of electric component to take into account partial discharge risks prior to qualification tests and prototype manufacturing. In order to illustrate the full scope of the analysis, a typical 540VDC aeronautic stator has been designed. Outline First, partial discharge key points and its consequences on insulation system will be highlighted. In a second step, key features of more electrical aircraft will be recalled, especially those in relationship to the reliability of the insulation system. Then, details of the stator will be presented along with the typical insulation system used. An analysis will be made on the creation of electrical stress on the insulation system of the stator. Before providing test results and analysis, test procedure logic will be underlined as first step towards a robust characterization procedure. Finally, some discussions regarding results, modification of test procedure and future work will be presented. Partial Discharge Basics What is a Partial Discharge? IEEE Definition According to IEEE standard definition [5] a partial discharge (PD) is: An electrical discharge that only partially bridges the insulation between conductors. A transient gaseous ionization occurs in an insulation system when the electric stress exceeds a critical value, and this ionization produces partial discharges. Paschen’s Law One well known threshold for partial discharge inception is the so-called Paschen minimum at 320-330V peak in air at room temperature and standard humidity between parallel metallic electrodes. The so-called Paschen curve, plotted either experimentally or analytically after Townsend theory, represents the partial discharge inception voltage (self-sustained discharge) in aforementioned conditions. Under this curve, no self-sustained partial discharge could happen (figure 1). Consequences of Partial Discharge Partial discharge events have been known and studied for more than a century, starting with Paschen, Peek and Townsend among others. Partial discharges are feared because they contribute to the degradation of the insulation system and lead to its premature failure [6] • Chemical effect (ozone creation, nitric acid in the presence of moisture). • Thermal effect (hot needle on the insulation system) • Mechanical effect (ion and electron erosion, destruction of polymeric chemical bond) • UV creation Partial Discharge in More Electrical Aircraft In relation with aircraft network voltage, the Paschen minimum is 230V AC RMS. It means that, for conventional aircraft electric system using 28V DC and 115 VAC RMS, partial discharge risk was non-existent. But, with the recent increase in voltage from 115 V AC to 230 V AC or ±270 VDC on HVDC network, that risk has to be taken into account in designing equipment and in qualification process. As shown on figure 1, at 540V peak, partial discharge could occur in air at atmospheric pressure between parallel metallic electrodes at a distance of 40μm or less. At 100mbars, partial discharge could occur up to a distance of 0.35mm. Partial discharge risk is thus highly linked to pressure level. Obviously, air is not the only insulation system in aircraft equipment and geometry is highly non-uniform so direct calculation are hazardous. Still, there could be a combination of pressure, distance, voltage and electric field unfavorable enough to reach partial discharge inception voltage in air between insulation or in a void within the insulation system if the applied voltage is above Paschen minimum. Partial Discharge Inception and Extinction Voltage According to IEEE Standard [5], partial discharge inception voltage (PDIV) is The lowest voltage at which continuous partial discharges (PDs) above some stated magnitude (which may define the limit of permissible background noise) occur as the applied voltage is increased. Many factors may influence the value of PDIV, including the rate at which the voltage is increased as well as the previous history of the voltage applied to the winding or component thereof And partial discharge extinction voltage (PDEV) is: The highest voltage at which partial discharges (PDs) above some stated magnitude (which may define the limit of permissible background noise) no longer occur as the applied voltage is gradually decreased from above the inception voltage. Many factors may influence the value of PDEV including the rate at which the voltage is decreased as well as the previous history of the voltage applied to the winding or component thereof. In most cases, PDEV is less than PDIV. PDIV and PDEV could be expressed either at RMS or peak voltage depending on the applied voltage during test. During partial discharge test of equipment, PDIV and PDEV are usually a representation of the insulation system quality. In other words, it represents its ability to withstand a defined voltage level before partial discharge occurs. It is thus of a critical importance to that tests are robust, easy to reproduce and relevant for the foreseen application. It is also important to have an accurate PDIV and PDEV measurement methods that could be applied indistinctly on all the aforementioned configuration. From simple material characterization test at atmospheric pressure, to an operating system in a low pressure environment. Figure 1. Paschen curve for air at room temperature conditions between parallel metallic electrodes. Failure of reporting an equipment subject to partial discharge could have dramatic consequence on its reliability, leading to short-circuits creating electrical arcs. Safe level of target PDIV and PDEV should be set in order to ensure equipment are PD free during operational life of the system. More Electrical Aircraft Paradigm The MEA approach underlines the use of electrical system for non-propulsive application such as environmental control systems (ECS), electric driven hydraulic pump or flight control actuation. Other applications such as electrical taxiing o