GB2478945A

GB2478945A - Aircraft DC power supply system

Info

Publication number: GB2478945A
Application number: GB1004915A
Authority: GB
Inventors: Adrian Shipley
Original assignee: GE Aviation Systems Ltd
Current assignee: GE Aviation Systems Ltd
Priority date: 2010-03-24
Filing date: 2010-03-24
Publication date: 2011-09-28
Also published as: GB201004915D0

Abstract

An aircraft DC power supply system 100 comprises a wiring harness 104 connecting a DC power source 102 to a load 106 and a circuit interruption device 108. An energy dissipation device 110 reduces the energy dissipated by the circuit interruption device 108 when it interrupts current in the wiring harness, disconnecting the load 106 from the DC source 102. The power source can be a high voltage supply. The wiring harness can have supply and return cables and comprise high tension wiring. The circuit interrupter 108 can be an electromechanical switch or a solid-state switch such as a transistor, FET or MOSFET. The energy dissipation device 110 can be a power diode, a solid state device or a MOSFET connected to approximate an ideal diode. The interruption device can be connected close to the switch 108, dividing the wiring harness into unequal inductance portions and dissipating switching energy arising in the lower inductance portion. The DC power supply system 100 provides improved reliability and operational lifetime for switching components particularly when used for high voltage power distribution which uses thinner wires having more inductance than wires used in low voltage systems.

Description

DC Power Systems for Aircraft

Field

The present invention relates generally to direct current (DC) power systems for aircraft. In particular, the present invention relates to improved reliability DC power supply systems for aircraft which use high voltage DC power distribution systems.

Background

Driven by a desire to reduce aircraft weight, aircraft power distribution systems have begun to use higher voltages than their predecessors, which generally operated using conventional 0/+28 volt DC power distribution systems.

By using higher voltage DC power supplies, thinner wires can be used in a wiring harness to transmit the same amount of power to various loads due to the lower current requirement. In turn, the use of such thinner wires advantageously leads to reduced weight for any particular wiring harness arrangement.

However, use of thinner wiring is not without problems. In particular, thinner wiring has inherently more inductance than thicker wiring, and this leads to an increase in the total amount of energy that needs to be dissipated as heat in circuits that include such wiring when switching power off, for example. This is because the total energy that needs to be dissipated as heat is proportional to the inductance of the wiring harness.

An increased need to dissipate energy stored in a circuit as the circuit is broken can lead to a reduced operational lifetime for various components. In particular, various switching components, such as electro-mechanical switches, for example, experience a reduced in-service lifetime and reliability problems because of the increased energy that needs to be dissipated.

Electro-mechanical switches are preferred for high voltage switching applications.

Such electro-mechanical switches typically include a pair of contactors that physically engage to close a circuit and are physically separated to break a circuit. As the contactors separate to break a circuit, an arc occurs. The arc is sustained until all the residual energy of the circuit is converted to heat.

The higher voltages used mean the arc heats the contactors to high temperatures (from about 5,000°C up to about 15,000°C) and causes vaporisation of contactor material which deforms the contactors thus giving rise to increased contact wear/degradation rates.

Moreover, material is transferred between contactors in a unidirectional manner due to the electric field produced by the DC power supply. Compared to AC supplies, this thus causes a further reduction in operating lifetime for such components, as in AC supplies the transfer of material between contacts occurs in both directions between the contacts thereby providing lower overall average contact wear rates.

Summary of the invention

Various aspects and embodiments of the present invention have thus been devised with a view to addressing the aforementioned problems associated with the use of high voltage DC power supplies in aircraft.

According to a first aspect of the present invention, there is provided a DC power supply system for an aircraft. The DC power supply system comprises a DC power source, a load for operating using power from the DC power source, a wiring harness connecting the DC power source to the load, at least one circuit interruption device for interrupting current in the wiring harness, and an energy dissipation device for reducing the energy dissipated by the circuit interruption device when the circuit interruption device disconnects the load from the DC power source.

By providing an energy dissipation device, the DC power supply system has improved reliability and operational lifetime compared to conventional aircraft power supply systems, particularly when used to distribute power at high voltage levels using reduced weight wiring harnesses.

Brief description of the drawing

Figure 1 shows a DC power supply system for an aircraft in accordance with various embodiments of the present invention.

Detailed description

Figure 1 shows a DC power supply system 100 for an aircraft in accordance with various embodiments of the present invention. The DC power supply system 100 comprises a DC power source 102. The DC power source 102 may supply power at a high voltage. For example, the DC power source 102 may supply power at greater than the 28 volts of conventional aircraft power supply units.

In various embodiments, the DC power source 102 is operable to supply power at high tension (HT) levels as a single or dual power supply operating from about 270 volts to about 540 volts. In various preferred embodiments a HI dual ±440 volt DC supply, or a HI dual ±270 volt DC supply, or a single power rail HI 270 volt DC supply is provided.

The DC power supply system 100 also includes a load 106 for operating using power obtained from the DC power source 102, a wiring harness 104 connecting the DC power source 102 to the load 106, at least one circuit interruption device 108 for interrupting current in the wiring harness 104, and an energy dissipation device 110 for reducing the energy dissipated by the circuit interruption device 108 when the circuit interruption device 108 disconnects the load 106 from the DC power source 102.

The load 106 may include various electrical loads for use in an aircraft. For example, the load 106 may include one or more of: electric motors, emergency power services, lighting devices, electro-mechanical actuators, auxiliary services, etc. The load 106 may also include voltage converters and!or regulators for stepping down the voltage of the DC power source 102 to lower voltage DC levels.

There is at least one circuit interruption device 108 provided for interrupting current flow from the DC source 102 so that the load 106 can be powered down. Various components can be used to provide such a circuit interruption device 108. For example, circuit interruption device 108 may comprise one or more electro-mechanical switches, solid state switches (e.g. transistor, FET, MOSFET, etc.), or the like.

The wiring harness 104 connects the DC source 102 to the circuit interruption device 108, the circuit interruption device 108 to the load 106, and the load 106 to the DC source 102 to complete a circuit. The wiring harness 104 has inherent inductance continuously distributed along its entire length, which is illustrated schematically in Figure 1 by the discrete inductors LI, L2, L3 and L4.

In various preferred embodiments of the present invention, the wiring harness 104 uses a light-weight high voltage arrangement for distributing high voltage, or high tension, DC power to the load 106. The wiring harness 104 may use two cables (supply and return) rather than four cables (three power phases and neutral). By using a wiring harness 104 with fewer cables, the weight of the wiring harness can be further reduced.

Also by moving to high voltage DC systems in aircraft (e.g. operating at voltage V), thinner wiring can be used to transmit a fixed power P, as a lower current I is needed (given that P = V.]). For example, in the Joint Strike Fighter (JSF) a high voltage power supply system operating at V = 270 volts is used with nitrogen filled electro-magnetic contactor units provided as circuit interruption devices. The thinner wiring has a reduced weight, but it also has the disadvantage of a relatively high intrinsic inductance.

The energy dissipation device 110 is connected in parallel with the load 106. The energy dissipation device 110 electrically partitions the DC power supply system 100 into two separate circuit loops 112, 114. The relative position of the energy dissipation device 110 determines the relative inductance of the separate circuit ioops 112, 114.

For example, if the energy dissipation device 110 is connected in the wiring harness 104 proximal to the circuit interruption device 108, it is possible to partition the inductance of the wiring harness 104 into two unequal inductance portions with the circuit interruption device 108 being provided in a portion forming a lower inductance circuit loop 112 when compared to a relatively higher inductance circuit loop 114 that includes the load 106 (i.e. Ll + L3 <L2 + L4 in Figure 1).

In various embodiments, the inductance (Li + L3) of first circuit ioop 112 may be less than half of the total circuit inductance (L0 = Li + L2 + L3 + L4) of the wiring harness 104 in the absence of the energy dissipation device 110. For example, (Li + L3)/ may be about 1%, 2%, 5%, 10%, 20%, 25%, 33%, 40%, etc. up to 50%.

Since switching energy in the respective circuit portions is proportional to the inductances of the respective circuit loops 112, 1i4, the energy that needs to be dissipated by the circuit interruption device 108 is reduced when Li + L3 <L2 + L4.

This effect can in turn be used to increase the operational lifetime of the circuit interruption device 108, as the energy that needs to be dissipated by the circuit interruption device 108 itself when the circuit interruption device 108 disconnects the load 106 from the DC power source 102 is reduced.

In certain embodiments, the circuit interruption device 108 includes an electro-magnetic switch. The electro-magnetic switch has moving contacts that physically touch to close a circuit and physically separate to break a circuit. Such contacts may be housed in an inert gas environment to minimise arcing, but nevertheless will degrade over time when used with a DC supply since any material that melts from the electrical contacts during switching transfers between contacts in a single preferred direction. As mentioned above, this kind of unidirectional switch contact material transfer/build up degradation is less of a problem for AC systems, as material there can transfer back and forth to a degree as the poiarity of the AC source changes direction.

At higher DC voltages, use of such electro-mechanical switches are nevertheless preferred over solid state devices because of their higher power handling capacities.

This causes even greater contact wear as the higher voltages give rise to higher arcing temperatures (5,000 -15,000°C) and a higher amount of stored residual energy that needs to be dissipated by the circuit interruption device 108.

For an electro-mechanical switch, an arc is produced between the contacts when they are moved apart. The arc sustains until all the energy in the circuit Etojai is converted to heat, where: -1 L j2 -Equation 1 Eoiai --total and I is the current in the circuit when the contacts are separated.

The circuit remains unbroken until E,0,,1 decays to zero, and without an energy dissipation device 110, the circuit interruption device 108 has to dissipate this amount of energy, thus rendering it liable to increased wear and reduced useable lifetime.

However, for a circuit in which an energy dissipation device 110 is provided, the circuit interruption device 108 has to dissipate an amount of energy E112 in the first circuit ioop 112, where: E =I(L1+L3)12 -Equation2 112 2 Hence, with energy dissipation device 110, the energy dissipated by the circuit interruption device 108 is reduced by a factor A, where: A(I13) -Equation3 Lb101 For example, where the inductance in the first loop 112 is 7 jiH and the inductance in the second ioop 114 is 50 tH, A = 0.1228. However, in various embodiments, the inductance ratio may be about 1:5 (20%, i.e. A = 0.2) or 1:7 (14%, i.e. A 0.14) providing increased switch contact lifetime (for an electromechanical device) of about or 7 times the conventional rated switch lifetime.

The energy dissipation device 110 may include various solid state devices, such as, for example, one or more of: diodes, metal-oxide-silicon filed effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), etc. For example, a power diode rated at a high current rating (e.g. 1000 Amperes) might be used, for example.

Alternatively, or additionally, the energy dissipation device 110 may comprise one or more MOSFET devices. Such MOSFET devices may be provided with feedback control to provide a close approximation to an ideal diode. Advantageously, this provides fast switching and improved energy dissipation characteristics.

Various aspects and embodiments of the present invention thus provide improved operational lifetime for circuit interruption devices. Moreover, this advantage can be obtained using an elegant, cheap and simple solution for providing enhanced reliability.

The lack of complexity of various embodiments is particularly advantageous in the aviation industry where profit margins are usually tight and regulatory requirements strict. Moreover, the simple design of various embodiments also makes aspects of the invention ideally suited as a retrofit to existing aircraft wiring.

Whilst the present invention has been described in accordance with various aspects and preferred embodiments, it is to be understood that the scope of the invention is not considered to be limited solely thereto and that it is the Applicant's intention that all variants and equivalents thereof also fall within the scope of the appended claims.

For example, although shown as a single rail power distribution system in Figure 1, it would be understood by those skilled in the art that embodiments of the present invention are not so limited, and may include, for example, dual rail DC power supplies.

Claims

CLAIMS: 1. A DC power supply system for an aircraft, the DC power supply system comprising: a DC power source; a load for operating using power from the DC power source; a wiring harness connecting the DC power source to the load; at least one circuit interruption device for interrupting current in the wiring harness; and an energy dissipation device for reducing the energy dissipated by the circuit interruption device when the circuit interruption device disconnects the load from the DC power source.
2. The DC power supply system of claim 1, wherein the DC power source comprises at least one of: a high voltage power supply and a high tension (HT) power supply system.
3. The DC power supply system of any preceding claim, wherein the wiring harness comprises high tension (HT) wiring.
4. The DC power supply system of any preceding claim, wherein the wiring harness comprises two power cables, said power cables being a power supply cable and a power return cable, respectively.
5. The DC power supply system of any preceding claim, wherein the circuit interruption device comprises one or more electro-mechanical switches.
6. The DC power supply system of any preceding claim, wherein the circuit interruption device comprises one or more solid-state switches.
7. The DC power supply system of any preceding claim, wherein the energy dissipation device is connected in the wiring harness proximal the circuit interruption device so as to partition the inductance of the wiring harness into two unequal inductance portions, the circuit interruption device being configured to dissipate switching energy arising in a first lower inductance portion of the wiring harness.
8. The DC power supply system of any preceding claim, wherein the energy dissipation device comprises one or more power diodes.
9. The DC power supply system of any preceding claim, wherein the energy dissipation device comprises one or more solid state devices.
10. The DC power supply system of claim 9, the energy dissipation device comprising one or more MOSFET devices respectively configured to approximate an ideal diode.
11. A DC power supply system substantially as hereinbefore described with reference to the accompanying drawing.