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Essential considerations relating to partial discharge
Insulation voltage, test voltage, partial discharge and insulation strength – terms development engineers frequently have to deal with. But how are they related and which aspects and data are ultimately relevant to the developer?
By Werner Bresch, Tobias Gmelin and Erik Rehmann, GvA Leistungselektronik GmbH
The manufacturers’ data sheets normally provide information on the so-called insulation voltage for electronic components used in power electronics systems, which must insulate different electric potentials from each other safely and reliably in addition to providing the desired function. This value is often expressed in the following format: 'Uiso = 4 kV AC' (for example), usually with a defined test time of 60 s. This parameter should help the development engineer decide whether the component is suitable for the intended application in terms of insulation strength. Nevertheless the user still needs to know how to interpret this information, or be able to at least recognise the basic correlations and know the procedure for the practical design of insulation distances and test voltages.
In the process of insulation coordination for a power electronics system, the following basic standards generally apply as a start: IEC 60664-1 ('Insulation coordination for equipment within low-voltage systems – Part 1: Principles, requirements and tests') for operating voltages ? 1000 Vac or 1500 Vdc and IEC 60071-1 ('Insulation coordination – Part 1: Definitions, principles and rules') for high-voltage applications. The application of these standards is then described in more detail in corresponding product standards (e.g. DIN EN 61800-5-1 for electrical power drive systems). If there is no product standard for the intended application, the relevant basic standard can be used to define the insulation requirements.
Based on these standards, the process of insulation coordination as per DIN EN 61800-5-1 would proceed as described in the following, taking the conditions defined in the standard into account:
Whereas when developing power electronics systems, the development engineers have a great amount of freedom within the limits of the specifications in the standards regarding the design of clearances and creepage distances and the insulation materials used, this freedom is lost when it comes to power electronics components such as power semiconductor modules, current and voltage sensors, IGBT drivers and auxiliary power supplies. The insulation properties of these types of components are defined by the design and the selection of insulation materials by the manufacturer. The manufacturer usually provides evidence of the insulation resistance in an AC or DC voltage test (Uiso).
One such AC or DC voltage test (Uiso) must be considered as an inspection to ensure the proper installation of the components, in order to verify that the required clearances inside the components were observed during manufacture. However, this test does not provide any information about the permissible voltage which can be applied to the component or system in operation! Since the insulation resistance may already experience premature damage due to partial discharge when type testing using the AC or DC test voltage (Uiso) specified in the data sheet, the standard recommends conducting repeat tests only at a maximum of 80% of the specified test voltage. Essentially, the AC or DC voltage test (Uiso) only verifies that the tested component has survived the specified test time.
Therefore, the partial-discharge behaviour is more of a determining factor for the endurance strength, as partial discharge can weaken and damage a solid insulation already within short periods of time, ultimately causing a disruptive discharge. The aim of having a stable layout for an insulation system, both for components and for systems, is therefore to guarantee that all operating points are free of partial discharge. Passing the voltage withstand test (Uiso) cannot guarantee that no partial-discharge events occur during operation under worst-case conditions (maximum-duration voltage, temporary overvoltage, recurring peak voltage). Therefore, the specification of an 'insulation voltage' in the data sheet, which usually does not come with any additional commentary from the component manufacturer, is not a reliable indication, let alone verification, that the long-term electric strength is sustained for operation at this voltage under specific conditions.
For this reason, the IPSS or GPSS auxiliary power supply units developed by GvA specify the test voltage 'free of partial discharge' under the item ‘Insulation voltage’, too. This ultimately helps development engineers select the ideal product for their requirements and eliminates the need to address product failures for a fairly long time.