Corona is an area of electrical systems that many have some knowledge, but often only to the point of knowing that there is a voltage that should not be exceeded. It is common knowledge that factors, such as pressure, have an impact on when and how corona will happen, though the source of corona is not. This article describes what is corona and how to assess the susceptibility of materials designed to protect your aircraft electrical system.
Why Corona Matters in Aerospace Systems
Up until recently, corona inception and extinction has been generally ignored in the aerospace industry and for good reason. This is because most aircraft power systems are run on 115 VAC and/or 28 VDC systems (read the article about power systems here). These systems are virtually impervious to corona because the voltages are not high enough to generate an event. New aircraft systems are looking to move to higher voltages with the more-electric aircraft (MEA) movement. The MEA movement is looking to replace pneumatic and hydraulic systems with electric systems when possible to save cost and weight. These systems do a lot of the heavy lifting in aircraft and their electronic equivalents require a vast amount of power. This increase in power requirements of circuitry aboard aircraft essentially forces a move to higher voltages. However, there are practical limitations on the amount power that can be delivered with low voltage power systems.
Analysis has been done that concludes that without going to higher voltages, either weight from too large of a transmission system (large gauge wire) or transmission losses in the circuitry will be prohibitive. Higher voltages very soon get into the realm where corona and partial discharge become a problem.
What is Corona?
Use of higher voltages in aircraft electrical systems has significant benefits, but can lead to wire/cable insulation degradation and corona.
Corona is defined as ionization and partial discharge resultant from high voltages. The mechanism of corona hinges on a localized electric field that is strong enough to ionize an insulating medium such as a wire/cable insulation, an air gap, or a non-conductive coating. This phenomenon is distinct from arcing because the ionization from the high potential (voltage) does not extend far enough to reach a grounded voltage source, so generally only small leakage pulses to the surrounding air are witnessed.
Coronal ionization can happen around terminations within an insulated system (such as wires and cables) that are exposed to air or can happen within the insulation itself.
Corona Inception and Extinction
For all things, there is a start and stop; for corona, there are the inception and extinction voltages. Corona inception voltage is defined as the voltage at which corona begins to occur (note: since there can be a difference in the corona inception voltage depending on the rate of voltage increase, most tests are designed to find an inception voltage use a steadily raised voltage). It has been observed that when corona begins, it often continues until the voltage is lowered significantly lower than the corona inception voltage. This prompts the definition of another quantity. If corona is occurring, the corona extinction voltage is the voltage that corona ceases as the voltage is lowered. The corona extinction voltage is lower than the corona inception voltage by definition.
An Accepted Test Method
The ASTM D1868 test standard provides a method for testing an insulation systems corona inception and extinction voltages. This method defines several measurable quantities of interest when it comes to corona such as the effective power loss from corona pulses. It also describes ways of measuring these quantities. These methods use means of noise reduction to reduce noise from common sources such as Electromagnetic Interference (EMI) and the requirement of using a high voltage power supply. They also give measuring speed requirements to capture coronal fluctuations and provide ways of calibrating a test setup to appropriate levels of sensitivity.
The main purpose of this test method, however, is to be able to capture effectively when corona inception and extinction occurs. This requires, in general, highly sensitive equipment and tight controls in the test circuitry. A low corona inception voltage of an insulating material may be indicative of microscopic air bubbles trapped within the insulation.
Quantifying Long Term Insulation Damage
It is well known that corona causes degradation to insulation systems. A new SAE test method is on the horizon to address this: high voltage endurance test method. This effectively puts insulation under a high voltage above their corona inception voltages until a full discharge event occurs. The voltage the insulation is placed under is then varied to generate a time to failure against voltage applied curve.
The failure generally causes lasting damage so that the insulation can no longer withstand the voltages it could at the beginning of the test. A full discharge event is distinct from the partial discharges happening whenever corona is present. This is discussed more fully in this article.
Supporting New and Existing Systems
Long-term exposure to high voltage can have impacts on system components, as well as, the interconnection system. The processes of insulation degradation are understood and methods for its assessment already exist. As products are placed under the strain of higher voltages, it is necessary to assess and quantify component performance before they are place onto aircraft. Contact Lectromec to find out more about how Lectromec can help determine your component’s performance.
Tristan Epp Schmidt
Engineer, Lectromec
info@lectromec.comSince starting at Lectromec in early 2015, Tristan has been key in many of test and assessment wire systems assessment projects wire systems assessment. His attention to detail has lead to several key insights in Lectromec’s research initiatives.