Reducing greenhouse gases, saving energy resources and mass optimization require technological changes towards increasingly electric vehicles. At the same time, performance improvement of semiconductor and dielectric materials further promotes electronic components confinement, resulting in a significant increase of embedded power densities. In the particular case of future hybrid propulsion aircrafts, electrical power that intended to supply reactors would be converted through power electronics components mounted on power busbars and insulated by solid dielectrics materials. These dielectrics materials have to respond to various electrical constraints of use (HVDC), in spite of environment change of aircraft parameters such as low pressure, temperature and thermal cycles, humidity... Unfortunately, partial discharges phenomenon is the most problem within electrical insulation system (EIS). Based on a topological model of power busbars designed for power converters dedicated to hybrid aircraft, partial discharge studies were conducted by simulation in various charging conditions of a PTFE insulator. Simulation results, which focus on electric field thresholds criteria of partial discharge inception voltage in air, reveal a net sensitivity of a space charge accumulation and distribution on dielectrics behaviour even for low space charge density, depending on their location in dielectrics. Compared to the behaviour observed with implanted homocharges, when by increasing homocharges density from 0.5 C/m 3 to 2 C/m 3 we observe a decrease of electric field by 450%, simulation results show a highest risk of partial discharge inception when heterocharges are accumulated inside dielectrics. Their accumulation increases the electric field in triple points beyond electric field thresholds of partial discharge inception in air. The simulated electric field reaching 22 kV/mm with only 2 C/m 3 of heterocharges density accumulated in dielectric/busbars interfaces.
Increase environmental impact of greenhouse gas emissions induced by air transport requires substituting pollutant fuel propulsion by cleanest propulsion systems. Hybrid propulsion systems expected for short-term time and entirely electric propulsion systems expected for long-term time represent a new technological challenge [
However, in many case, triple material regions appear in such powered systems, with many “conductor-air-insulator” junction points. These triple points play an important role in electric field enhancement [
In power busbars application case, the triple point is constituted of busbars-insulator-air junction point. De facto, the both confinement and power densities increasing expose electric insulation systems to more partial discharge risks. Added to materials defects (structural and chemical defects, electronic traps distributions, trapping level…), these partial discharge inceptions locally increase the electric field within the dielectric material. This local increase in electric field changes the intrinsic material properties and reliability, with the consequence of an irreversible insulators damage of dielectric that can affect the insulation system and then expose the power converters on which the propulsion depends on [
Based on a busbar topology from a power electronic converters, electric field cartographies were simulated around the most exposed triple points to partial discharge risks. Sandwiched between two copper busbars, a non-charged PTFE insulator is firstly subjected to a DC electric field of 2.5 kV/mm. In this case, two geometrical configurations are studied to highlight partial discharge risks to which the dielectric would be exposed. Secondly, hypothesis of a non-charged insulator is then replaced by an inhomogeneous accumulation of space charges through dielectrics: firstly, by homocharges and then by heterocharges. The impact of charge maximum density on implanted charges within dielectric as well as their spatial distribution on partial discharge risks at triple points are exposed and discussed.
this study focuses on the estimation of partial discharges risks through electric field assessment in this triple point to characterize the insulation behaviour of busbars for this applied electric specification. Based on simulation results highlighted around this triple point, partial discharge risks incurred on this one would also reveal those expected on other triple points, since other triple points support lower electric potential differences.
A common way to evaluate if partial discharges occur between two conductor materials consists in comparison of Paschen theoretical and experimental curves obtained in considered gas where Paschen’s assumptions are applicable (uniformity of the electric field, plan/plan geometry of conductors, preponderance of electronic avalanche mechanism...) [
The electric field is computed through finite elements method by solving Poisson’s equation in both air and dielectrics domains:
∂ E ( x , y ) ∂ x ∂ y = ρ ( x , y ) ε 0 ε r (1)
where E is related to electric potential V such as:
E = − ∇ V (2)
ρ is the space charge density, ε 0 and ε r the vacuum and relative permittivity, respectively. A zero flux was applied in air boundaries according to relation:
n ⋅ ( ε 0 ε r E ) = 0 (3)
where n is surface unit vector. The calculation domain was non-uniformly meshed. To increase calculation precision of electric field and charge at the interfaces, refinement was added to both boundaries and in triple points as shown in
The model was implemented in finite element numerical method through Comsol Multiphysics® 5.2a software (CM). The simulation was based on Partial Differential Equation (PDE) sub-module embedded in the mathematics module. Firstly, the 2D-geometry was built to reproduce calculation domain shown in
In a first step, the conventional configuration keeping dielectric and busbars in same lengths was compared with a curved busbars configuration (curving radius ~1 mm).
The electrical potential cartographies show apparent similarities with both simulated configurations. The geometric change on busbars (see
From a partial discharges point of view, for a DC 2.5 kV/mm applied voltage, both configurations lead to a much higher electric field than the breakdown field admitted in air at atmospheric pressure and at ambient temperature ( ≫ 3 kV/mm) [
Taking into account the advantages (electric field decrease at triple points) obtained with curved geometrical configuration, another simulation was performed to more decrease electric field in air, and thus limit partial discharge risks at triple points, by ranging dielectric length relative to conductor length. Figures 4(a)-(d) show the electric field cartographies obtained for four distinct dielectric heights: 2.5 mm (
Reducing length of dielectrics sandwiched between busbars is accompanied by a drastic reduction of the electric field in the air, and more sensitively within the triple points. By changing from a dielectric length of 2.5 mm (see
This electric field reduction also decreases the electric field confinement within triple points, thereby decrease potential gradient at these locations. Thus, by decreasing confinement regions, it is also possible to reduce electric field reinforcement in air. However, although this dielectric length reduction decreases, electric field between busbars, it represents a high partial discharge risk due to resulting electric field amplitude near of air breakdown threshold (2.76 kV/mm - 3kV/mm), since both conductors (busbars) will be separated only by air gap. This geometric configuration is therefore not better to prevent partial discharges around busbars. The elongation from
geometrical configuration, since the two conductors are separated by a solid insulator having a larger breakdown voltage threshold than air.
Now the assumption of a perfectly non-charged dielectric is replaced by a more realistic charged one, considering an initial charge state due to homocharges deposition (charges of same sign than electrode polarities) at busbars/dielectrics interfaces then polarized by a DC 2.5 kV/mm electric field.
・ Simulated electric field due to gradual homocharges densities:
Homocharge density [C/m3] | 0 | 0.01 | 0.1 | 0.5 | 1 | 2 |
---|---|---|---|---|---|---|
Emax in air [kV/mm] | 6.52 | 6.44 | 5.73 | 2.57 | −1.37 | −9.25 |
Emax in PTFE [kV/mm] | 3.01 | 2.98 | 2.66 | 2.65 | 3 | 5.08 |
For densities of charges lower than 0.1 C/m3 (Figures 6(a)-(c)), simulated electric field is maximum in air and higher than the breakdown threshold (>3 kV/mm). Beyond that, for charge densities exceeding 0.5 C/m3 (Figures 5(d)-(f)), maximum of electric field is located within the dielectrics bulk. The maximum of electric field decreases by 450% when amplitude of charge density increases from 0.5 C/m3 (
More homocharges accumulation near dielectric surface seems to decrease electric field at the triple points (for densities greater than 1 C/m3). Although such considered charge densities of homocharges accumulated in dielectrics bulk could prevent partial discharge risks in air, it could however generate a gradual accumulation of homocharges that would increase the electric field to upper dielectric/air interface. This increase could create premature electron/hole pairs and generate others ionization processes in favour of partial discharges.
・ Simulated electric field due to gradual homocharges distribution: Now the charge density was fixed to 0.01 C/m3. In this case, only spatial distribution of homocharges through the dielectrics bulk is ranged from 1 to 300 μm.
Even with weaker distribution of homocharges, the electric field remains important within triple points whereas it is weak in the dielectrics bulk. When charge
distribution through dielectric bulk increase from 1 μm to 300 μm, the maximum of electric field is almost halved. This decrease is observed especially at triple points while the electric field continues to grow in volume. The transport of homocharges from both dielectrics interfaces creates a recombination zone of heterocharges in dielectrics bulk, that increase electric field in insulator material. As observed with previous results under variation of homocharges densities presented in
As well for static implantation near busbars/insulator interfaces as for spatial ranging of their distribution inside insulator, accumulation of homocharges within insulating material sandwiched between busbars would qualitatively and quantitatively affects distribution electric field in the dielectric. It would seem that for a charge density below 0.5 C/m3, the electric field due to homocharges would not be sufficient to impact the total electric field in material and consequently at the triple points, although this threshold would not be the unique conditions to generate a partial discharge in air since other physical and electronic processes should also be met [
From charge generation and transport mechanisms point of view, polarization of a polymeric solid insulators such as PTFE under a DC electric field between copper conductive busbars would favour a preponderance of homocharges injection on other generation mechanisms. This would seem theoretically favourable to reduce partial discharge risks if heterocharges zones are not generate at the interfaces of busbar/insulator. Increasing strong homocharges injection at busbar/insulator interfaces would thus be possible to decrease interfacial electric field, and consequently reduce electric field in triple points of circuits and systems, which are moreover areas most exposed to partial discharges risks under these topological conditions.
Based on a reverse reasoning, homocharges is now replaced by heterocharges implantation (charges of opposite sign to the electrode polarities). First, the heterocharges are implanted at the same place within the dielectric material, for six different densities (see
・ Simulated electric field due to gradual heterocharges densities: The simulated electric field cartographies corresponding to heterocharges implantation with charge densities ranging (
Heterocharge density [C/m3] | 0 | 0.01 | 0.1 | 0.5 | 1 | 2 |
---|---|---|---|---|---|---|
Emax in air [kV/mm] | 6.52 | 6.60 | 7.31 | 10.5 | 14.4 | 22.3 |
Emax in PTFE [kV/mm] | 3.01 | 3.04 | 3.36 | 4.76 | 6.59 | 10.2 |
Electric field massively increase in air despite small implanted heterocharge densities. Resulting electric field in air is seven times higher than breakdown threshold necessary to initiate partial discharge [
・ Simulated electric field due to gradual heterocharges distribution: In the last simulation, the heterocharges distribution within dielectric was changed according to
When heterocharges are extended over 1 μm from busbar/insulator interfaces (
Various mechanisms could cause heterocharges accumulation in such solid insulating material polarized between busbars in a DC electric field. These heterocharges could be generated through natural traps within insulator, trapping distribution in dielectrics bulk, electrode/dielectrics interface defaults but also directly by extraction of charges towards busbars conductors due to a favourable internal electric field, chemical defaults or by detrapping processes induced by thermal activation through potential barrier. All these physical mechanisms that would make possible heterocharges accumulation would therefore be favourable to partial discharges inception in busbars environment [
These simulations are a contribution to the study of partial discharge appearance, through analyse of space charges impact in a solid dielectric (PTFE) used for insulation of power busbars dedicated to hybrid aeronautical systems. Not only this study confirms that presence of space charges within a solid insulator generates an internal increase of electric field that impacts total electric field distribution, but it also highlights that for an applied electric field of 2.5 kV/mm, accumulation of small charges density in dielectrics (~0.01 C/m3), induced by example by some chemical defaults, thermal, pressure or mechanical stress, would be sufficient to anticipate partial discharges in air recovering busbars. Moreover, according to the sign of these charges, the risks of discharges would be differently appreciable. In case of homocharges accumulation at the busbars/insulator interfaces, electric field would tend to strengthen in dielectrics. This is less advantageously the case when these homocharges are transported in the volume of the dielectrics. On the contrary, heterocharges accumulation at busbar/insulator interfaces would reinforce the total electric field at insulator surface, leading to the greater risks of partial discharges resulting to the confinement of electric field at triple points. This situation would be accentuated as well by surface heterocharges density as their transport within the insulator bulk. Although PTFE is considered among the lower charging insulators, this study, applicable to other polymer insulators, reveals above all that charge accumulation of a few C/m3 would generate various mechanisms sufficient to age or activate degradation of insulators [
Complementary experimental study is simultaneously underway to better understand the charge generation and transport mechanisms which could better help to predict partial discharge inception in organic solid dielectrics used for the isolation power converters busbars dedicated to hybrid propulsion. Results of this study will be the topic of other forthcoming investigations.
This project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation program under grant agreement No. 715483.
The authors declare no conflicts of interest regarding the publication of this paper.
Banda, M.E., Malec, D. and Cambronne, J.-P. (2018) Simulation of Space Charge Impact on Partial Discharge Inception Voltage in Power Busbars Dedicated to Future Hybrid Aircrafts. Circuits and Systems, 9, 196-212. https://doi.org/10.4236/cs.2018.911018