The Use of PD Patterns to Evaluate the Wear-Resistance and Manufacturing Quality to Estimate Expected Lifetime of Ignition Coils

Paper in proceeding, 2023

Partial discharges (PDs) are localized flashovers that occur within electrical insulation, when the electric field exceeds its dielectric strength. The electrical insulation, which covers the distance between conductive parts is also referred to as the dielectricum. In ignition coils, typically two different insulation materials are used: epoxy and transformer oil. Epoxy has the advantage of making the construction independent of positioning but its dielectric strength is sensitive to the potting process. Using oil as electrical insulation is less process sensitive than epoxy but the construction becomes more challenging as it must encapsulate its insulation material. An additional disadvantage is that an oil-insulated ignition coil can generally only be operated in certain positions, but it gains the advantage of being self-healing after dielectric breakdowns within the oil. Epoxy-insulated ignition coils lack this property as electrical trees are created within the insulation. These trees are gas-filled cavities that grow in size with the number of occurred PDs. In high performance ignition coil design, the maximum field strength should not exceed the dielectric strength of the insulation material. However, impurities locally decrease the dielectric strength and increase the electric field level. Thus, PDs are initiated at these impurities or defects and with time, the electric trees will continue to grow until the ignition coil breaks due to insulation breakdown. In epoxy-insulated ignition coils, a defect can be introduced as a crack due to internal forces caused by differences in thermal expansion coefficients of the insulation system materials, even at normal operational temperatures. Therefore, measuring how homogeneous, free from impurities like gas bubbles, cracks and charred compounds, the epoxy is after the potting process could be a vital tool for monitoring manufacturing quality. An important aspect for an ignition coils performance is how temperature-wear-resistant, the resistance to cracking induced by a set of temperature cycles, it is. By measuring PDs, there is a possibility to quantify these quantities at a great resolution. Generally, an increase in the number of temperature cycles is observed to cause an enlarged PD activity within the insulation system, which is believed to be related to possible detrimental temperature differences within a single temperature cycle. This, in turn, is related to a reduction in insulation lifetime. The goals of this study are to fit the measured data to an inverse power law model for estimating lifetime and investigate whether we can detect which failure mode caused an ignition coil to break.