- At higher altitudes, a lower applied voltage is required for partial discharge to occur.
- The altitude-dependent air density directly affects the mean-free path of air molecules present within an insulation gap.
- Molecules with a longer mean free path are able to collide at higher energy than those with a short mean free path.
Partial Discharge (PD) is a phenomenon that causes long-term degradation over time within the imperfections of a wire/ cable insulation. A recent Lectromec article provides valuable information about the phenomenon and the impact it can have on aerospace Electrical Wiring Interconnection Systems (EWIS).
PD is not a simple phenomenon to model as there are many environmental factors that have influence. Factors such as heat, humidity, pressure, are some that can change the voltages required to initiate and extinguish PD within a wire/cable’s insulation. This article specifically discusses the influence of altitude on partial discharge events. The consideration of the operational altitude effect on partial discharge is particularly necessary for aerospace applications as aircraft tend to spend a lot of operating time at high altitudes.
Years of partial discharge testing (at Lectromec) have shown that PD inception voltage and extinction voltage (PDIV and PDEV) of any wire/cable’s construction decrease at higher altitudes. But why and how does the altitude affect the partial discharge voltages of wire/ cable?
What’s Happening Inside the Gap?
As discussed in a previous article, partial discharge occurs when a low current “jumps” the gap between two points with a voltage differential across or through an insulation. These gaps, or voids, are typically air bubbles within the wire’s insulation that occur during wire/cable fabrication. The voltage differential between the two points creates an electric field inside the gap polarizing the air molecules within. Naturally, the electric field strength increases as greater voltage is applied to the wire, further polarizing the air molecules in the gap.
As the electric field grows stronger, the molecules within the insulation gap are more apt to collide and ionize, that is, to become either positively or negatively charged due to the loss or gain of an electron, respectively. The ionized particles are then accelerated by the surrounding electric field, further colliding and ionizing the gas within. Thus, the ionized particles form a conductive path by which current can “jump” across the gap resulting in partial discharge. This mechanism remains the same regardless of altitude, so why are the extinction and inception voltages lower at higher altitudes? The change in PDIV with altitude is accounted for by the air pressure/density and the resulting mean free path of air particles within the gap.
Mean Free Path
Mean free path is a fancy term for the average distance a particle travels before experiencing a collision (i.e., the average distance any air molecule in the insulation gap will travel before hitting another.) As altitude increases, air pressure decreases, meaning that air density is reduced at high altitudes and increased at low altitudes. This is also true of the air present in the gap(s) within a wire’s insulation.
At low altitudes, the higher density of the air within an insulation gap results in more frequent collisions, meaning that the mean free path is shorter; following the same logic, there are fewer collisions at high altitudes where the air density is reduced resulting in a longer mean free path.
Putting it All Together
At first glance, one may infer that more frequent collisions within a gap of greater air density lead to a higher rate of ionization and that a partial discharge inception voltage should be easier to reach at low altitudes and high pressures. In fact, the opposite is true.
The collisions of molecules in a high-pressure (low altitude) environment occur more frequently (because of the shorter mean free path) but due to the high density of air molecules and the shorter mean free path, each collision occurs with relatively low energy. For ionization to occur, the energy of a collision must surpass a minimum threshold so that electrons may be transferred. Without sufficient energy in collisions, the air molecules are unable to ionize and do not create a conductive path across the gap.
At high altitudes (low-pressure), despite fewer collisions, the longer mean free path allows the particles to accelerate over a greater distance and collide with significantly greater kinetic energy. These higher energy collisions are more likely to facilitate ionization. So, in reality, the rate of ionization at higher altitudes is higher than the rate near sea level, thus a lower applied voltage can create an electrical path within the air gap.
A Simple Analogy
A useful way to picture this is to consider a group of 100 blindfolded people trying to run around a gymnasium. Due to the density of people, each person is not able to build up much speed (or in this case, kinetic energy) without bumping into someone else. Because of the low speed of collision, the people are unlikely to lose their balance and fall during a collision.
However, if the number of people in the gymnasium is reduced to 10, the blindfolded runners can accelerate to a higher speed without bumping into one another. The runners will bump into each other less frequently, but when they do, it is with significantly greater energy and a higher likelihood of both falling.
Thus, as the density of people in the gymnasium decreases, the probability of falling increases. Similarly, as the density of air molecules in the insulation gap decreases, the probability of ionization increases. The probably of ionization increases until the the air pressure is sufficiently low to reduce the collision frequency. This is shown in the graph above where low pressure-distance values correspond to very high breakdown voltages.
The mean free path of air molecules in the induced electric field within an insulation gap has a significant impact on the rate of ionization within. Therefore, the voltage required to initiate (and subsequently extinguish) partial discharge within the insulation of a wire/cable decreases with an increase of altitude (to a point). This concept is clearly an important consideration to make when choosing EWIS components for an aircraft as each component must be fully functional at high altitudes where partial discharge is more likely to occur. Contact Lectromec today for help evaluating the partial discharge performance of your wire/ cable construction.