Power electronics unit

Abstract

The present disclosure is titled "Power electronics unit". The invention provides a power electronics unit 100 for an aircraft and a method of determining the integrity of the power electronics unit 100. The power electronics unit 100 comprises: a motor controller 120; a heat sink 105 arranged to conduct heat from the motor controller 120; a housing 106 comprising a sealed interior volume enclosing the motor controller 120 and heat sink 105; a dielectric liquid 107 partially filling the interior volume to cover the motor controller 120; a gas trap 108 located within the interior volume; a pressure sensor 110 arranged to measure the pressure of gas within the gas trap 108; a temperature sensor 117 arranged to measure the temperature within the interior volume; and a controller 118. The controller 118 is configured to receive signals from the pressure sensor 110 and temperature sensor 117, determine the pressure and temperature from the received signals, and provide an output signal in dependence on the determined temperature and pressure.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a power electronics unit for an aircraft. BACKGROUND

[0002] Aircraft high power electronics assemblies are typically mounted in a rack within a pressurized environment and tend to operate at voltages lower than the minimum corona inception voltage, which can be about 300 volts. In order to operate at higher voltages and power, for example in order to drive an electric propulsion unit, the electronics assembly can need to be hermetically sealed. In practice, such hermetically sealed units can develop leaks over time during multiple flights due to temperature and pressure changes. This problem is potentially exacerbated by mounting such units in inaccessible locations within the rack and / or in a non-pressurized environment.

[0003] For example, power electronics for controlling the operation of a hybrid engine can need to be located near or inside the engine in a relatively inaccessible and unpressurized environment. It is therefore desirable to prevent corona discharge by preventing or at least detecting any leaks of such high power electronics units. SUMMARY

[0004] According to a first aspect, there is provided a power electronics unit for an aircraft, the power electronics unit comprising:

[0005] a motor controller;

[0006] a heat sink arranged to conduct heat from the motor controller;

[0007] a housing comprising a sealed internal volume enclosing the motor controller and heat sink;

[0008] a dielectric liquid partially filling the internal volume to cover the motor controller;

[0009] a gas trap located within the internal volume;

[0010] a pressure sensor configured to measure a pressure of a gas within the gas trap;

[0011] a temperature sensor arranged to measure a temperature within the internal volume; and

[0012] a controller configured to receive signals from the pressure sensor and the temperature sensor, to determine a pressure and a temperature from the received signals, and to provide an output in dependence on the determined temperature and pressure.

[0013] By providing an air trap between the dielectric liquid and the upper internal surface of the housing, the power electronics unit can be more resilient to changes in temperature and external pressure, as the compressibility of the air trap will allow the dielectric liquid to expand and contract without needing to exert excessive pressure on the housing which could, over time, compromise the seal. Providing an output dependent on pressure and temperature indicates the integrity of the unit to be manufactured, such that if any leakage does occur, it can be detected by the pressure or temperature exceeding the change in pressure or temperature that would be expected in the absence of leakage.

[0014] The controller can be configured to provide an output signal indicating a leak of the dielectric liquid if the determined pressure or temperature deviates from a pre-calculated value.

[0015] The controller can compare the determined pressure to a calculated pressure for the determined temperature, and provide the output signal if the determined pressure is less than the calculated pressure by more than a threshold value (which can for example be about 5%, 10% or 20%). Alternatively, the controller can be configured to compare the determined temperature to a calculated temperature for the determined pressure, and provide the output signal if the determined temperature is greater than the calculated temperature by more than a threshold value (which can for example be about 10 degrees Celsius, 20 degrees Celsius or 30 degrees Celsius).

[0016] The air trap can be defined between the dielectric liquid and the upper internal surface of the housing, or can be positioned elsewhere within the internal volume.

[0017] The housing can comprise an upper lid portion enclosing the motor controller and heat sink, and a lower lid portion sealed against the bottom edge of the upper lid portion. This orientation allows the motor controller to be covered by the dielectric liquid for optimal insulation and thermal properties.

[0018] A seal ring can be provided to seal the lower lid portion against the bottom edge of the upper lid portion. Providing the seal at the bottom edge allows the seal to be against the dielectric liquid which is less likely to leak.

[0019] The air trap can be contained by a flexible membrane separating the dielectric liquid from the air trap. The flexible membrane allows the air trap to expand and contract, and changes in pressure and temperature of the internal volume of the housing. In some examples, the flexible membrane can be sealed against the upper lid portion of the housing. In other examples, the flexible membrane can form a sealed bladder containing the air trap.

[0020] A temperature sensor can be arranged to measure the temperature of the dielectric liquid. The temperature of the liquid will tend to be more stable than the temperature of the air trap, and so will provide a more stable reading.

[0021] According to a second aspect, a method for determining the integrity of a power electronic device unit according to the first aspect is provided, the method comprising:

[0022] The pressure inside the air trap is determined based on the signal obtained from the pressure sensor.

[0023] The temperature is determined based on the signal obtained from the temperature sensor; and

[0024] The output signal is provided based on the determined temperature and pressure.

[0025] If the temperature or pressure deviates from the pre-calculated value by a greater than predetermined amount, the output signal can indicate leakage of the dielectric liquid.

[0026] According to the ideal gas law, pV = nRT, where P is pressure (in Pa), V is volume (in cubic meters), n is the number of moles of gas, R is the ideal gas law constant (in Joules / Kelvin / moles), and T is absolute temperature (in Kelvin). When the unit is sealed to prevent leakage of the dielectric liquid, the volume of the gas trap will vary according to the thermal properties of the relatively incompressible dielectric liquid, which will also tend to have a lower coefficient of thermal expansion than the gas within the gas trap. If the volume of the gas trap increases beyond the expected volume at any given temperature (indicated by a pressure lower than expected), this will tend to indicate leakage of the dielectric liquid.

[0027] Those skilled in the art will understand that, unless mutually exclusive, features described with respect to any of the foregoing aspects, with appropriate modifications, can be applied to any other aspect. Furthermore, unless mutually exclusive, any feature described herein can be applied to any aspect and / or combined with any other feature described herein. Attached Figure Description

[0028] The implementation scheme will now be described by way of example only, referring to the accompanying drawings, which are schematic diagrams and not drawn to scale, and in the accompanying drawings:

[0029] Figure 1 This is a schematic cross-sectional view of an exemplary hermetically sealed power electronic device unit;

[0030] Figure 2 This is a schematic diagram of a power electronics unit connected to a controller used to monitor pressure and temperature and provide alarm signals;

[0031] Figure 3 This shows the detection of dielectric liquid from Figure 1 and Figure 2 A schematic flowchart illustrating an exemplary method for addressing leakage in power electronic device cells of the type shown; and

[0032] Figure 4 This is a pressure graph as a function of temperature for an exemplary unit with a gas volume, showing the change of pressure with temperature in the presence of different levels of dielectric liquid. Detailed Implementation

[0033] Figure 1 An exemplary power electronics unit 100 for an aircraft is shown. This unit 100 includes a motor controller, which can be a controller for an electric motor or a generator. For example, the motor controller could be a motor controller for an electric propulsion unit driving an aircraft. The motor controller includes power semiconductor modules 101a, 101b, a set of DC chain support capacitors 102, and an output filter 103, all of which are mounted to a busbar 104. A control and monitoring PCB is separated from the busbar 104 by a perforated EMC shield 115. A heat sink 105, attached to the upper surface of the motor controller, is arranged to conduct heat away from the motor controller. The motor controller and heat sink 105 are enclosed within a sealed internal volume of a housing 106, which includes an upper cover portion 111 and a lower cover portion 112. The lower cover portion 112 is sealed against the bottom edge 113 of the upper cover portion 111, and the upper cover portion surrounds the motor controller and heat sink 105.

[0034] A sealing ring 114 extends around the bottom edge 113 of the upper housing, thereby providing a seal between the bottom edge 113 of the lower cover portion 112 and the upper cover portion 111.

[0035] The internal volume of the housing 106 is filled with a dielectric liquid 107, which covers the motor controller and at least partially covers the heat sink 105. A portion of the internal volume is filled with an air trap 108, defined between the dielectric liquid 107 and the upper inner surface 109 of the housing 106. The air trap 108 may be defined by a flexible diaphragm 116 separating the dielectric liquid from the air trap 108. Figure 1 In the example shown, the diaphragm 116 seals against the upper cover portion 111 of the housing 106. In an alternative embodiment, the air trap 108 may be located in other locations surrounding the internal volume of the housing, such as on the side or bottom of the housing. The flexible diaphragm 116 may form a sealed bladder that encloses the air trap 108 within the internal volume, or it may seal a portion of the internal volume against an internal surface.

[0036] Pressure sensor 110 is arranged and configured to measure the pressure of the gas within air trap 108. Temperature sensor 117 is also provided to measure the temperature within the internal volume. Temperature sensor 117 may be positioned within dielectric liquid 107, or in some embodiments, within air trap 108. Positioning temperature sensor 117 within dielectric liquid tends to produce more stable temperature readings that will quickly equalize with the temperature of the gas within air trap 108. Signals from pressure sensor 110 and temperature sensor 117 may be received by a sensing module on a control and monitoring PCB or controller 118.

[0037] The gas trap 108 may be filled with air or alternatively with a gas such as nitrogen.

[0038] Sealed connectors or terminal blocks 119a, 119b located on unit 100 are provided to connect unit 100 to a power source and a motor, and control signal connections are provided for driving controller 118 and for outputting diagnostic signals and other signals from controller 118.

[0039] One or more additional air traps 121 may be provided to allow the dielectric liquid to have further elasticity in terms of expansion and contraction within the operating temperature range.

[0040] exist Figure 2 A simplified schematic diagram of the power electronics unit 100 is shown. A controller 118 controls the operation of the motor controller 120 and receives signals from a pressure sensor 110 and a temperature sensor 117. The controller 118 determines the pressure within the air trap 108 based on the pressure sensor signal and determines the temperature of the internal volume of the unit 100 based on the temperature sensor signal. The controller 118 outputs a signal based on the ratio of temperature to pressure. The signal output by the controller 118 can, for example, be provided to an aircraft engine management system to provide an indication of the integrity of the unit 100. The signal can be a simple binary signal indicating positive or negative integrity according to the ratio, or it can be a signal indicating a calculated value, such as the volume V determined by the ideal gas law, a given prior knowledge of the amount of gas present in the air trap, or a leakage calculated based on the determined pressure and temperature.

[0041] The determination of pressure and temperature can be performed by controller 118 within unit 100, or in an alternative example by controller outside unit 100.

[0042] Figure 3A method for determining the integrity of a power electronic device unit of the type described herein is illustrated schematically. In a first step 301, controller 118 determines the pressure within air trap 108 based on a signal obtained from pressure sensor 110, and determines the temperature based on a signal obtained from temperature sensor 117. In a second step 302, controller 118 compares the pressure and temperature to nominal values, for example, by determining what pressure should be for the measured temperature or what temperature should be for the measured pressure. At step 303, if the pressure and temperature are within the expected range, the method returns to step 301 and continues to monitor the temperature and pressure. However, if the pressure and temperature are outside the expected range, the method proceeds to step 304 and provides an output signal indicating that the integrity of unit 100 has been compromised, for example, an alarm signal indicating that the dielectric liquid 107 has leaked.

[0043] Figure 4 An exemplary graph of pressure as a function of temperature is shown for an exemplary unit comprising a dielectric liquid and a gas trap. In this example, the total volume of liquid and gas in a unit with a total internal volume of 40 liters is 6 liters, with 4.8 liters of liquid and 1.2 liters of gas at 20 degrees Celsius and standard atmospheric pressure (101 kPa). In this example, the dielectric liquid is Novec (RTM) 7700 (available from 3M Company of Maplewood, Minnesota, USA) with a coefficient of thermal expansion of approximately 0.11% / Kelvin, and the gas is air. The required gas volume over a given temperature range can be determined by the amount by which the liquid will expand within that temperature range and the maximum pressure the sealed unit is designed to withstand. The gas volume will need to be greater than the volume difference that the liquid will occupy within that temperature range, and can be, for example, selected to be more than twice that volume difference, while still allowing the liquid to cover components within the unit that require cooling and protection against discharge.

[0044] As the temperature increases from 20 degrees Celsius, the dielectric liquid expands and the pressure in the gas volume increases, which is consistent with the thermal expansion coefficient of the dielectric liquid and the ideal gas law. This can be calculated as following... Figure 4Curve 401 shows the pressure in the gas volume rising from 1 atmosphere (101 kPa) at 20 degrees Celsius to over 2 atmospheres (243 kPa) at 120 degrees Celsius. Therefore, the controller can be configured to calculate or look up the ideal pressure for the measured temperature and compare the calculated pressure with the measured pressure. If the measured pressure is less than the calculated pressure by a threshold amount, for example, by more than about 5%, 10%, or 20%, an output signal can be triggered to indicate that a leak has occurred. Alternatively, the calculation can be arranged such that the measured pressure is used to calculate or look up the temperature at which the gas volume should be, and this can be compared with the measured temperature. If the measured temperature is higher than the calculated temperature by a threshold amount, for example, by more than about 10, 20, or 30 degrees Celsius, an output signal can be triggered to indicate a leak. The controller can perform this calculation or alternatively use a lookup table providing pre-calculated values ​​of pressure and / or temperature for comparison with the measured pressure and / or temperature.

[0045] Figure 4 Additional curves 402 and 403 illustrate how pressure changes with temperature when the dielectric fluid loss is 5% and 10%, respectively. For a 5% fluid loss, the pressure measured at 120°C is approximately 60 kPa lower than the pressure without fluid loss, which should be easily detectable during implementation. The measured pressure can be compared, for example, to a calculated pressure, and an output signal is provided if the measured pressure is less than the calculated pressure by more than 5%, 10%, or 20%. The threshold used to trigger the alarm signal can be set to allow thermal fluctuations within the unit to reach an average, since the measured temperature will not be perfectly uniform throughout the unit. For example, if the temperature within the unit is known to vary by approximately ±10°C, the pressure threshold can be selected based on a worst-case scenario, which would be close to the higher end of the calculated range. A 10°C error at the higher temperature end would roughly correspond to an error of approximately 10% in pressure, so the pressure threshold can be set higher than this error to avoid triggering incorrect alarm signals.

[0046] It should be understood that the present invention is not limited to the embodiments described above, and various modifications and improvements can be made without departing from the concepts described herein. Unless mutually exclusive, any feature may be used alone or in combination with any other feature, and this disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

[0047] Various examples have been described, each characterized by a variety of combinations of features. Those skilled in the art will understand that any feature may be used alone or in combination with any other feature unless they are obviously mutually exclusive, and that the invention extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

1. A power electronics unit (100) for an aircraft, the power electronics unit comprising: a motor controller (120); a heat sink (105) arranged to conduct heat from the motor controller (120); a housing (106) comprising a sealed interior volume enclosing the motor controller (120) and heat sink (105); a dielectric liquid (107) partially filling the interior volume to cover the motor controller (120); a gas trap (108) within the interior volume; a pressure sensor (110) arranged to measure a pressure of a gas within the gas trap (108); a temperature sensor (117) arranged to measure a temperature within the interior volume; and a controller (118) configured to receive signals from the pressure sensor (110) and the temperature sensor (117), determine a pressure and a temperature from the received signals, and provide an output signal in dependence on the determined temperature and pressure, wherein the output signal is indicative of an integrity of the power electronics unit.

2. The power electronics unit (100) of claim 1, wherein the controller is configured to provide the output signal indicative of a leak of the dielectric liquid (107) if the determined pressure or temperature deviates from a pre-calculated value.

3. The power electronics unit (100) of claim 2, wherein the controller is configured to compare the determined pressure to a calculated pressure for the determined temperature, and provide the output signal if the determined pressure is less than the calculated pressure by more than a threshold value.

4. The power electronics unit (100) of claim 3, wherein the threshold value is about 5%, 10%, or 20%.

5. The power electronics unit (100) of claim 2, wherein the controller is configured to compare the determined temperature to a calculated temperature for the determined pressure, and provide the output signal if the determined temperature is greater than the calculated temperature by more than a threshold value.

6. The power electronics unit (100) of claim 5, wherein the threshold value is about 10 degrees Celsius, 20 degrees Celsius, or 30 degrees Celsius.

7. The power electronics unit (100) of any one of claims 1-6, wherein the gas trap (108) is defined between the dielectric liquid (107) and an upper interior surface (109) of the housing (106).

8. The power electronics unit (100) of any one of claims 1-6, wherein the housing (106) comprises an upper cover portion (111) enclosing the motor controller (120) and heat sink (105), and a lower cover portion (112) sealed against a bottom edge (113) of the upper cover portion (111). ​ 9. The power electronics unit (100) according to claim 8, comprising a sealing ring (114) sealing the lower cover part (112) against the bottom edge (113) of the upper cover part (111).

10. The power electronics unit (100) according to any of claims 1-6, wherein the gas trap (108) is contained by a flexible membrane (116) separating the dielectric liquid (107) from the gas trap (108).

11. The power electronics unit (100) according to claim 10, wherein the flexible membrane (116) is sealed against an upper cover part (111) of the housing (106).

12. The power electronics unit (100) according to any of claims 1-6, wherein the temperature sensor (117) is arranged to measure the temperature of the dielectric liquid (107).

13. A method of determining the integrity of a power electronics unit according to any preceding claim, the method comprising: determining a pressure within the gas trap (108) from a signal obtained from the pressure sensor (110); determining a temperature from a signal obtained from the temperature sensor (117); and providing an output signal in dependence on the determined temperature and pressure, wherein the output signal is indicative of the integrity of the power electronics unit.

14. The method according to claim 13, wherein the output signal is provided to indicate a leakage of the dielectric liquid (107) if the pressure or temperature deviates from a pre-calculated value by more than a predetermined amount. ​

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