By Elizabeth Greenberg, Research Engineer & Technology Consultant
Design engineers look for form, fit and function when choosing components for their products. Choosing connectors is no different in general, but when specifying high-voltage connectors for AC, DC or RF applications, function drives the selection. This means choosing connectors suited to the task, preventing the breakdown of insulators and minimizing environmental effects within the connector.
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While this brief will touch on regulations and standards pertinent to high voltage applications, it will primarily discuss the atmospheric/environmental effects as well as geometric and material considerations that influence connector choice. With this understanding, design engineers can more easily specify the high voltage connector that will deliver maximum performance, optimum longevity, long-term reliability and ultimate safety.
IEC (International Electrotechnical Commission) 60038:2009 specifies standard voltage values to be uses as references for equipment and system design. However, what constitutes high voltage varies by industry and application. It is important to know the rules, regulations, and standards relevant to your product as well as what testing requirements are needed before your product can be released. All of these will be necessary if you are to choose the right connector for your application. In the end, the decision always comes down analyzing form, fit, and function as well as the environment in which the connector will be used.
Function Drives High Voltage Connector Selection
For selecting electrical connectors it is obvious that one must avoid shock for those handling the connectors, that the connector must be made of materials suited to the use and the environment, that the connector must be durable and not cause a fire. When specifying a high-voltage connector minimizing the risk of corona and arcing is the key consideration.
Corona and Arcing
Corona can initiate arcing which in turn can create fires. Corona results in power loss, thermal breakdown, and possible electromagnetic/RF interference. Corona may also cause the breakdown of insulators. The key criteria to minimize corona and arcing are atmospheric/environmental conditions and connector geometries and materials. Atmospheric conditions include temperature, pressure, particulates, and humidity. Geometrically the key considerations are spacing of the conductors to each other and other metal components as well as the effect of sharp corners. Since corona affects porous insulators more than non-porous this factor should also be considered.
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Atmospheric Conditions
The relationship of breakdown voltage between two conductors in gas as a function of pressure in gas and gap distance has been discussed extensively beginning with Paschen in 1889. Moongilan included the effects of temperature and humidity on corona inception voltage, showing that lower pressure and higher temperatures produce corona at lower voltages whereas relative humidity itself does not have much of an impact.1 However, even in an arid climate, a large change in temperature can result in contaminated condensation which must be prevented from entering the connector as it reduces the inception voltage for corona discharge.2
Geometry and Insulation Degradation
Given that the distance between conductors influences the inception voltage for corona, the goal is to maximize the distance between conductor pins and other metal parts inside the connector. By understanding a component’s response to partial discharge activity, material selection, geometries, and interfaces connectors can be optimized to increase endurance and reliability.
Dunbar et al discuss in-depth how partial discharge (corona is a type of partial discharge) destroys insulators. Whereas in AC and DC systems breakdown voltage is related more to environmental factors and spacing, in RF systems multipacting breakdown is a function of frequency (MHz) times spacing.3
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It is best to avoid sharp edges and try to use smooth and rounded surfaces as the former will also reduce inception voltages. Non-porous insulators fair better in general against breakdown but are not always practical inside a small space such as a connector. Incorporating insulating materials such as PEEK, Teflon and silicone rubber in the connector design will maximize creepage distance and air clearance.
In addition to corona, arcing, atmospheric effects, geometry and insulation degradation, you must also consider other variables such as pulsed voltages and voltage surges. High-voltage connector suppliers have already spent the time, energy and research on all these considerations to have their products meet the critical needs of their customers. Discuss all your requirements in detail and they will ensure that you choose the best connector for your particular high voltage application.
Learn more >> LEMO High Voltage Connectors
References
1 D. Moongilan, Corona and arcing in power and RF devices. 2009 IEEE Symposium on Product Compliance Engineering, Toronto, ON, 2009, pp. 1-7.
2 Y. Li et al. Effect of Water Droplets on the Corona Discharge Characteristics of Composite Insulators in Arid Areas. 2019 2nd International Conference on Electrical Materials and Power Equipment (ICEMPE), Guangzhou, China, 2019, pp. 467-472.
3 J. L. Rush, Partial discharge testing and AC characterization of HV connectors and terminations. 2010 IEEE International Power Modulator and High Voltage Conference, Atlanta, GA, 2010, pp. 521-525.
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