Key Takeaways
- High voltage power systems are coming to more aircraft and require an evaluation of how the power is transmitted through the aircraft.
- The separation distance between and from wiring system components is not straight forward. Testing and simulations are necessary to find the safe separation distance.
- The longevity of wiring system components is reduced with higher-voltage/higher-frequency applications.
For aerospace applications, high-voltage power is a rapidly growing interest and being addressed across several industry technical committees. The basic idea is that the power generation is increasing the supplied voltage and creating a need for the electrical wiring interconnects systems (EWIS) to have components designed to sustain these higher voltages for the entire length of the aircraft life. Just as it should not be expected for a connector to operate in a 300°C environment if it is only rated to 150°C, a connector should not be expected to perform perfectly with voltages exceeding its voltage rating.
The first challenge is to determine the delineation between low and high voltage systems. While the utility industry set the bar for high-voltage power systems at tens of kilovolts, the aerospace industry currently is debating whether high-voltage should be defined as voltages above 300 V or above 550 V (a proposal at a recent committee meeting suggested setting the high voltage threshold as anything above 120V). The reason for this discussion is that the current EWIS components have only been tested in power systems views up to 115 V AC or three-phase power with a voltage difference of 208 V. Deployed solutions using 270 V or greater now must rely upon custom solutions.
By setting the bar for high voltage expectations, the technical committees can start work on identifying and developing specifications and component configurations that can meet the challenges of systems that go beyond the low-voltage threshold.
Wires
Up until 2012, all SAE wires that were under the family AS22759 were rated to 600 V. That changed when the committee realized that there was not a test to verify this voltage rating. For the last six years, through literature and research, the committee has come closer to a point that would help users select wire for high voltage applications. Research has found this to not simply be a component issue, but rather a system-level issue.
The basic configuration of the high voltage endurance test setup is for the wire to be wrapped around a grounded mandrel and a high voltage to be placed on the conductor. This testing continues until there is a breakdown of the dielectric and a high current flow is detected. Testing performed above the Corona inception level of the dielectric has found a rapid degradation and short longevity of the material. When voltages below the Corona extinction voltage are used, the longevity of the dielectric is much greater and increases logarithmically as the voltage decreases.
The separation distance between and from wiring system components is not straight forward. Testing and simulations is necessary to find the safe separation distance. Capture from a electrical arcing test performed with a 270 V DC wire. In this test, the wire was pre-damaged and swung into a grounded aluminum tube. After a brief duration of shorting to the aluminum tube, electrical arcing initiated and pushed the wire more than 2.5 cm from the grounded tube. Source: Lectromec/FAA/USAF testing.
The research into using this technique has found the process to be stable and has found that as the frequency of the power system increases, the long-term reliability of the component decreases. In the example provided here, 60 Hz and 600 Hz. The research found that the long-term reliability at the 600 Hz frequency was 10% of the long-term reliability at 60 Hz. While this phenomenon has been established for devices such as coil windings for Variable Frequency Drives (VDFs), research into the impact on parallel routed aircraft wires has been limited.
When considering this with the push for pulse width modulated (PWM) power systems, the use of PWM technology will greatly strain the electrical components. The main degradation mechanism associated with the EWIS components attached to these systems is the dV/dt. As the PWM rapidly turns on and off a DC power source to a load to maintain high precision control, the rapid voltage change places a high strain on components.
While high voltage cables and power systems are nothing new, applying the systems at high altitudes and the low pressures at which aircraft operate are the new challenge. The lower pressure creates scenarios in which corona can occur at much lower voltages.
Testing performed by Lectromec has shown that those wire constructions with extruded insulations (e.g. AS22759/34 XL-ETFE) perform better than tape-wrapped constructions (e.g. AS22759/87 PTFE-Polyimide construction). Tape-wrapped constructions have more voids in the insulation construction and thus a greater likelihood of partial discharge between those gaps. This result suggests an important fact of materials that the aerospace industry has relied upon for high-temperature applications: the tape-wrapped constructions may not be suitable for airframe wire at higher voltage applications. With this in mind, current materials available for aerospace applications and are used by wire manufacturers, the maximum operational temperature for wiring will be below 200°C.
New wires constructions on the market today that seek to use high-temperature extruded materials for entry into the high-voltage market often rely on materials that are susceptible to carbon arc tracking phenomenon. This creates a dueling competition between requirements of high-voltage performance and high-voltage arc track resistance.
Hazards of High Voltage Arcing
One of the significant hazards with high-voltage DC arcing events is that once the event is started it is very difficult to terminate the event. In this example, a wire with the insulation breach makes contact with the aluminum tube. After a short duration of contact between the exposed conductor of the wire and the tube, the electrical arcing event is initiated. The force from the electrical arcing event pushes the wire away from the tube to a distance of 1.5 inches at which time the circuit protection trips and halts the event.
Electrical discharges on More Electric Aircraft (MEA) or All Electric Aircraft (AEA) are a potential risk that should be mitigated through careful design and implementation. Electrical discharges include tracking, partial, and disruptive discharges. Voltages beyond the 327V threshold increase the likelihood of an electrical discharge event, which may lead to significant failures in high-voltage systems. Since the impact of an electrical discharge is significant, it is vital to design systems that eliminate the potential for its occurrence. Key design decisions to minimize the likelihood of an electrical discharge include careful spacing of components and subassemblies and controlling the manufacturing process.
An important design factor that affects MEA/AEA is solid insulation thickness. The purpose of solid insulation is to minimize the probability for an electrical discharge to occur. Insulation capability testing is conducted through the application of an electric field and determination of dielectric breakdown. It can be concluded that the strength of the insulation is dependent on the voltage stress applied to it. As the voltage frequency increases, the breakdown voltage decreases.
Long-Term Reliability
Aircraft require long-term performance of the wiring system as full rewires of aircraft are highly uncommon. However, the long-term performance cannot be assumed with the greater strains on the systems. Ideally, any aircraft wire/cable must have:
- Insulating materials stable for the entire aircraft service life
- Conductors must not degrade or corrode
- Be used within design limits
- The ability to withstand typical in-service shocks
To evaluate each of these requires thorough testing. Some of the test methods exist, and some need to be developed. Without a doubt, the long-term reliability of these components must be assessed prior to installation on the vehicle.
The option does exist to use the same components used for low voltage for the high-voltage applications. The consequences of using low-voltage wire/cable for high-voltage applications have been demonstrated. It is will become necessary to identify wiring system components as life limited and require replacement every one or two heavy maintenance cycle (D check). In the short term, how high-voltage testing impacts wiring reliability must also be considered.
If the industry chooses to use life-limited low voltage parts and the wiring system cannot be assumed reliable for the entire life of the vehicle, then this must be identified early. The consequences of not doing so are unacceptable.
Conclusion
From a practical standpoint, these are challenges that the aerospace industry has brought upon itself. The drive has been for use of smaller components that weigh less. In recent years, the push has been for more electric aircraft driving the need for higher current and voltage levels within these vehicles. A ground-based utility in a similar situation would use larger cables and separate the high-voltage components. Both of these options are unlikely for modern aircraft applications. Consideration of thicker materials is almost unheard of and to increase the weight in order to achieve an objective is something that is left to only critical systems were no alternative exists.
If the industry is truly interested and willing to and sets on moving forward with high-voltage applications, then the wiring system cannot be ignored. Additional space will be necessary for wire separation distances. Larger connectors will be necessary to route the power through the vehicle without creating hazards to the rest of the aircraft systems or to airworthiness.
Lectromec’s lab has the capability to perform a wide range of tests for high-voltage systems for both voltage endurance and electrical arcing.
Michael Traskos
President, Lectromec
michael.traskos@lectromec.comMichael has been involved in wire degradation and failure assessments for more than a decade. He has worked on dozens of projects assessing the reliability and qualification of EWIS components. Michael is an FAA DER with a delegated authority covering EWIS certification and the chairman of the SAE AE-8A EWIS installation committee.