Electrical power system

Abstract

An electrical power system 100 for an electrically powered aircraft includes: a power converter arrangement 130 for receiving electrical power, converting the received electrical power, and providing

Description

Field and Background The present invention relates to electrical power systems for electrically powered aircraft. Electrically powered aircraft, for example electrically powered aircraft that use power source options other than typical fossil fuels (such as fuel cell arrangements), are not widespread. Indeed, such electrically powered aircraft are not at present used in commercial-sized aircraft (e.g. 48 or 96 or more passenger aircraft and / or CS-25 related aircraft). The systems and arrangements described herein may be directed toward use of green energy options with any sized aircraft. Electrically powered aircraft, which may in examples use cryogenic fuel as the primary fuel source, is an area of cutting-edge research; conventional electrical power systems are not suitable for dealing with the new complexities involved. Cryogenic fuel in particular presents many advantages over contemporary fuels, not least that they are more environmentally-friendly and provide a significant heat sink. However, the use of cryogenics can lead to electrical power system considerations. The electrical power systems discussed herein provide improved safety performance at a reduced weight for electrically powered aircraft. Summary Aspects of the invention are set out in the accompanying claims. Viewed from a first aspect, there is provided an electrical power system for an electrically powered aircraft, the system comprising: a power converter arrangement for receiving electrical power, converting the received electrical power, and providing the converted electrical power, the power converter arrangement having a pre-charge stage and a power stage; a first arrangement for providing electrical power to the power converter arrangement and; a second arrangement for receiving the converted electrical power from the power converter arrangement, wherein the precharge stage comprises a first isolation arrangement arranged in use to provide electrical isolation between the first arrangement and the second arrangement, and the power stage comprises a second isolation arrangement arranged in use to provide electrical isolation between the first arrangement and the second arrangement. The present inventors have identified that electrical power systems for electrically powered aircraft (for example an aircraft powered by a high density power source arrangement such as a fuel cell, or a hybridised power source arrangement having a fuel cell, battery arrangement, and super capacitor arrangement) require different considerations compared to conventional aircraft implementations that use conventional fuel sources. Indeed, such electrical power systems and electrically powered aircraft represent an area of cutting-edge technology and development. The present inventors have identified that existing approaches to protect against faults deployed in aircraft implementations using conventional fuel sources (such as fossil fuels) are not suitable for use in electrical power systems for electrically powered aircraft. For example, the present inventors have identified that different considerations arise in electrically powered aircraft implementations, as a result of electrical network architecture, component types, and aircraft requirements. In particular, electrically powered aircraft can use a high direct current (DC) network, and fault protection devices are typically larger and heavier for DC implementations than alternating current (AC) implementations because DC does not have a natural zero-crossing. Further, the weight and volume of typical fault protection devices for DC implementations can increase approximately linearly with current. Thus, at the high currents used in electrically powered aircraft, fault protection devices can be heavy and large. Furthermore, some certification specifications (such as CS-25) may place requirements on the arrangement and number of protection devices. This can therefore also increase the number of heavy protection devices that are to be included, which can significantly increase the weight of an aircraft. The present inventors have devised an improved electrical power system that provides improved fault protection (such as fault tolerance and component isolation) resulting in increased aircraft safety while reducing the weight of such electrical power systems. Hence, aircraft safety is increased without overly increasing the weight of the aircraft. Counter-intuitively, in examples discussed herein, components of a power converter arrangement itself are used to provide electrical isolation between a power providing arrangement and a power receiving arrangement. The present inventors have recognised that the power converter can itself be used to provide electrical isolation, for example because of contactors and / or switching devices that are contained therein. Thus, by using the power converter arrangement to provide controllable electrical isolation between power providing and power receiving arrangements (like fuel cells, propulsion motors, etc.), a significant weight saving can be realised because the need for additional fault protection devices is avoided. For example, the present inventors have estimated that the mass saving for a 19-person aircraft at an expected 2035 technology standard would be approximately 667Kg compared to an aircraft that did not use the present electrical power system techniques. A further advantage of the present electrical power system is that it provides a ‘ride-through’ capability. Ride-through capability refers to an ability of the system to provide time for a fault / failure to be assessed and corrective action to be performed (i.e. to ‘ride-through’ a fault scenario). In particular, as a result of the isolation arrangements (and in some examples the fault tolerance of components / arrangements discussed further below), the present electrical power system is able to, in a fault scenario, provide time for the fault to be assessed and corrective action to be performed, rather than just shutting down or allowing the fault to spread throughout the greater system. Hence, the present electrical power systems increase the safety of an electrically powered aircraft while reducing the weight of such systems. In some examples, the second isolation arrangement of the power stage is arranged in use to provide electrical isolation without operation of a circuit breaker. Counter-intuitively, a circuit breaker associated with the power converter may not be included because the need for a circuit breaker is avoided as a result of using the pre-charge stage and power stage of the power converter arrangement itself to provide electrical isolation. This represents a significant departure from previous techniques, which may use a dedicated circuit breaker arrangement with the power converter. By removing the circuit breaker, weight can be further reduced while maintaining electrical isolation capability. In some examples, the first and second arrangements comprise one or more of: a fuel cell arrangement; a super-capacitor arrangement; a battery arrangement; a propulsion motor arrangement; and a power distribution arrangement. Thus, the present techniques can be used to provide electrical isolation between a variety of components in an electrical power system for an electrically powered aircraft, and thus can be used to reduce weight and provide fault protection in various implementations and arrangements. In some examples, the electrical power system further comprises a plurality of different sensor arrangements arranged in use to measure one or more electrical parameters associated with the electrical power system. Hence, the electrical power system (and arrangements therein) can be provided with sensing ability to measure electrical parameters. Accordingly, the electrical power system supports an improved fault detection approach, because electrical parameters can be measured and used to diagnose faults and ensure that components / arrangements are operating as expected. Further, the provided sensor arrangements may be different (and in examples are independent), and so the measuring approach is more reliable and fault tolerant. For example, the likelihood that an error that affects one of the sensor arrangements will affect another one of the sensor arrangements is reduced (because the sensor arrangements are different and / or independent). In some examples, the plurality of different sensor arrangements comprises a voltage differential sensor arrangement for measuring a voltage differential, a current differential sensor arrangement for measuring a current differential, and an absolute current sensor arrangement for measuring an absolute current. Thus, a variety of measurement approaches are supported that may be chosen based on implementation. In some examples, the plurality of different sensor arrangements comprise one or more of: a first sensor arrangement arranged in use to measure an electrical parameter associated with the first arrangement, a second sensor arrangement arranged in use to measure an electrical parameter associated with the electrical input of the power converter arrangement, and a third sensor arrangement arranged in use to measure an electrical parameter associated with an electrical output of the power converter arrangement, a fourth sensor arrangement arranged in use to measure an electrical parameter associated with the second arrangement. The present inventors have identified that such a distribution of sensor arrangements is particularly advantageous because it supports fault detection in each of the first arrangement, second arrangement, and both the input and output of the power converter arrangement. Indeed, redundancy is provided that increases the likelihood that the electrical power system is able to detect and diagnose a fault in a fault scenario. In some examples, the first arrangement is configured in use such that a rated current of the first arrangement is approximately equal to 1 to 2 times a fault current of the first arrangement. As used herein, a fault-tolerant arrangement (such as a power providing / power source arrangement) is one in which the rated current is approximately equal to 1 to 2 times the fault current (within 1.25PU, per-unit, i.e. rated current = 1, fault current = 1.25PU, so that rated current = 1A and the fault current = 1,25A). Hence, the power providing / first arrangement may be fault-tolerant. As a result, the arrangement can withstand the fault current with little to no impact on power density. Thus, by utilising an arrangement having a fault current at 100A but having a rated current to withstand 125A, for example, the arrangement would function at 100A (its fault current) and would prevent the cascaded failure of other components under a fault. The present inventors have identified that existing implementations do not provide fault-tolerant arrangements or electrical machines. In some examples, the first arrangement is configured in use such that a rated current of the first arrangement is between 1 and the square root of 2 times a fault current of the first arrangement. As a result, an i2t (i.e. an expression of thermal energy that results from current flow) heating effect can be maintained below 2x and is thus particularly advantageous in cryogenic implementations. In some examples, the first arrangement is configured in use such that a rated current of the first arrangement is substantially equal to a fault current of the first arrangement (within approximately 1.25PLI, i.e. 25%). In some examples, the first arrangement is configured in use such that: rated current = A x fault current, where 1 <A <2, or preferably where 1 <A <square root(2). As discussed herein, so-called fault tolerance provides various advantages. In a fault scenario, assuming typical safety margins (assuming some life impact) of the electrical power network, the electrical power network / system could continue operating with such failures for the rest of a flight as a result of the fault-tolerance, thereby enabling sustained containment and operation under fault current. Indeed, with fault-tolerance, sustained operation can be achieved that provides time for fault diagnosis and resolution that would not have been possible without sustained operation (i.e. fault tolerance). Further, not only is the fault current reduced, but the power consumption of the fault current is limited so that there is power available on the electrical bus to perform any fault discriminating, locating and isolating as may be required. Hence, by using a fault-tolerant arrangement, an electrical power system having improved fault protection and safety is provided. As discussed herein, the present electrical power system is able to provide a ride-through capability as a result of its isolation arrangements and fault-tolerance. Example advantages of the present electrical power system are as follows: in a fault scenario, the electrical system / network can be maintained in an operational state until the fault has been diagnosed, and a solution to the fault has been analysed and implemented; the present electrical power system is able to provide adequate fault current to allow sensor arrangements to measure properties of the system (like electrical parameters) to support reactive diagnostic measurement functions; - the present electrical power system, as a result of its ability to isolate arrangements, supports processes to pro-actively diagnose the system to support development of a preferred fault solution; and the present electrical power system supports resetting of individual components and arrangements. In some examples, the first arrangement comprises a power distribution arrangement for providing electrical power to the pre-charge stage of the power converter arrangement, the second arrangement comprises a propulsion motor for receiving the converted electrical power from the power converter arrangement and for providing propulsion power, the first isolation arrangement is arranged in use to provide electrical isolation between the first arrangement and the power stage, and the second isolation arrangement is arranged in use to provide electrical isolation between the pre-charge stage and the second arrangement. Thus, in this example, electrical isolation can be provided between a power distribution arrangement (such as an electrical bus) and a propulsion motor. As a result, safety of the propulsion sub-system (i.e. a sub-system of the electrical power system) and indeed the greater system, can be increased while minimising weight. In some examples, the propulsion motor is configured in use such that a fault current less than two times or is substantially equal to a rated current of the propulsion motor. Thus, as discussed above, the propulsion motor may be fault-tolerant and realise the advantages associated therewith as discussed above. In some examples, the first arrangement comprises a fuel cell arrangement for providing electrical power to the power stage of the power converter arrangement, the second arrangement comprises a power distribution arrangement for receiving the converted electrical power from the pre-charge stage of the power converter arrangement, the first isolation arrangement is arranged in use to provide electrical isolation between the first arrangement and the pre-charge stage, and the second isolation arrangement is arranged in use to provide electrical isolation between the power stage and the second arrangement. Thus, in this example, electrical isolation can be provided between a fuel cell arrangement and a power distribution arrangement (such as an electrical bus). As a result, safety of the fuel source sub-system (i.e. a sub-system of the electrical power system), and indeed the greater system, can be increased while minimising weight. The present inventors have identified that a fuel cell may be inherently fault tolerant. For example, a fuel-cell may be unable to sustain a current much above its rated current. This enables sustained containment under fault current. Indeed, reduced fuel and air in the fuel cell means that the concentration region occurs at a lower current density. Therefore, at reduced fuel and air levels the maximum output current is less than the rated current. In some examples, the power converter arrangement is configured in use to withstand a maximum fuel cell voltage and current. Thus, the fault tolerance of the fuel cell can further be supported by the power converter. The power converter arrangement may thus be fault-tolerant. In some examples, the first arrangement comprises a super-capacitor arrangement for providing electrical power to the power stage of the power converter arrangement, the second arrangement comprises a power distribution arrangement for receiving the converted electrical power from the pre-charge stage of the power converter arrangement, the first isolation arrangement is arranged in use to provide electrical isolation between the first arrangement and the pre-charge stage, and the second isolation arrangement is arranged in use to provide electrical isolation between the power stage and the second arrangement. Thus, in this example, electrical isolation can be provided between a super-capacitor arrangement and a power distribution arrangement (such as an electrical bus). As a result, safety of the supercapacitor sub-system (i.e. a sub-system of the electrical power system) can be increased while minimising weight. In some examples, the power converter arrangement is configured in use to withstand a maximum super capacitor voltage and current. Thus, the power converter arrangement may be fault-tolerant. In some examples, the first arrangement comprises a battery arrangement for providing electrical power to the power stage of the power converter arrangement, the second arrangement comprises a power distribution arrangement for receiving the converted electrical power from the pre-charge stage of the power converter arrangement, the first isolation arrangement is arranged in use to provide electrical isolation between the first arrangement and the pre-charge stage, and the second isolation arrangement is arranged in use to provide electrical isolation between the power stage and the second arrangement. Thus, in this example, electrical isolation can be provided between a battery arrangement and a power distribution arrangement (such as an electrical bus). As a result, safety of the battery subsystem (i.e. a sub-system of the electrical power system) and indeed the greater system, can be increased while minimising weight. In some examples, the electrical power system may comprise a third isolation arrangement arranged in use to provide electrical isolation between the first arrangement and the power converter arrangement. Thus, a further line of isolation may be provided to further increase the fault protection of the first and second arrangements. In some examples, the third isolation arrangement comprises a circuit breaker or fuse arrangement. In some examples, the third isolation arrangement comprises a cryogenic fuse. A cryogenic fuse may be a fuse arrangement that comprises both a cryogenic conduit for transporting cryogenic material and a current conductor, wherein the current conductor is in thermal communication with the cryogenic conduit. A fusible portion of the current conductor may be configured in use to structurally alter in response to a detected change in condition (current, voltage, temperature for example) of the cryogenic fuse to prevent the flow of current through the fusible portion. In some examples, the current conductor may also include a main portion arranged to not structurally alter in response to detection of the change in condition. Such a cryogenic fuse provides great levels of control in response to safety events. For example, the safety event may be that there is a change in condition of the cryogenic fuse that may have further impact on the cryogenic fuse. The effect of this safety event can be detected and responded to in such a way that the system is protected from greater levels of damage. In particular, this system prevents safety events from spiralling into creating large scale damage and disruption to the greater system around the cryogenic fuse. In some examples, the electrical power system further comprises an uninterruptible power supply for supplying back-up electrical power. This capability ensures that secondary aircraft distribution systems remain unaffected during a time when the primary distribution system experiences a fault. In some examples, the electrical power system further comprises a battery and a circuit breaker arranged in use to interrupt a power supply from the battery. Some example batteries may not be fault tolerant and may actually have the capability to supply significantly more current than its nominal operation. This could lead to battery thermal runaway and unwanted cascaded fire failure. Thus, a circuit breaker may be provided to interrupt a power supply from the battery. This circuit breaker may be arranged to trigger / respond to a rate of increase in current exceeding a predetermined threshold. Thus, great control in response to battery failures is provided. In examples, the aircraft is at least partially electrically powered. It may be particularly advantageous for the arrangement discussed herein to be used in a partially or fully electrically powered aircraft. There are efforts underway to reduce harmful emissions from aircraft and at least partially electrically powered aircraft have reduced harmful emissions from more typical fuels such as kerosene or the like. The electrical propulsion may be provided by a fuel cell arrangement or a gas turbine arrangement that uses, e.g., warmed cryogenic material. For example, a system may store hydrogen as a cryogen and may provide this hydrogen (in a warmed state) to at least one of a fuel cell and a gas turbine for use in electrical propulsion. In some examples, the electrical power system comprises a fuel cell for providing electrical power. This may be provided in combination with a gas turbine arrangement for example or otherwise. In some examples, the electrical power system comprises a cryogenic fuel arrangement arranged to provide cryogenic fuel for power generation. For example, a system may store hydrogen as a cryogen and may provide this hydrogen (in a warmed state) to a fuel cell arrangement for use in electrical power generation. According to a second aspect there is provided an at least partially electrically powered aircraft comprising the electrical power system as discussed herein. According to a third aspect, there is provided method of fault isolation in an electrical power system for an electrically powered aircraft, the electrical power system having a plurality of different isolation arrangements arranged to isolate electrical power between a plurality of arrangements, the arrangements comprising a power converter arrangement having a pre-charge stage and a power stage and a one or more of: a fuel cell arrangement, a super-capacitor arrangement, a propulsion motor arrangement; a battery arrangement and a power distribution arrangement, the method comprising: detecting a fault associated with the electrical power system; and isolating electrical power in the electrical power system using one or more of the different isolation arrangements based on the detected fault. In some examples, the method further comprises determining measurement data using a plurality of different sensor arrangements, wherein detecting the fault is based on the measurement data. According to a fourth aspect, there is provided a controller arrangement for an electrical power system of an electrically powered aircraft, the controller arrangement comprising one or more processors configured to perform the method described herein. Other aspects will also become apparent upon review of the present disclosure, in particular upon review of the Brief Description of the Drawings, Detailed Description and Claims sections. Brief Description of the Drawings Examples of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic view of an example electrical power system as discussed herein; Figure 2 shows a schematic view of an example electrical power system as discussed herein; Figure 3 shows a schematic view of an example electrical power system having a propulsion motor arrangement as discussed herein; Figure 4 shows a schematic view of an example electrical power system having a fuel cell arrangement as discussed herein; Figure 5 shows a schematic view of an example electrical power system having a super-capacitor arrangement as discussed herein; Figure 6 shows a schematic view of an example electrical power system having a battery arrangement as discussed herein; and Figure 7 shows steps for isolating a fault as discussed herein. While the disclosure is susceptible to various modifications and alternative forms, specific example approaches are shown by way of example in the drawings and are herein described in detail. It should be understood however that the drawings and detailed description attached hereto are not intended to limit the disclosure to the particular form disclosed but rather the disclosure is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention. It will be recognised that the features of the above-described examples of the disclosure can conveniently and interchangeably be used in any suitable combination. Detailed Description An invention described herein relates to electrical power systems for electrically powered aircraft. The controller arrangements herein may be used with any vehicles however it is a particularly advantageous arrangement for use in aircraft. A particular use for this invention may be in an aircraft with an electrically drivable motor or a drivable motor that is at least partially electrically driven. For example, the propulsion in the aircraft in which the electrical power systems disclosed herein are used may be fully or partially electrically powered. Indeed, it will be appreciated that the term ‘electrically powered aircraft’ refers to an aircraft that is at least partially electrically powered (and may also be at least partially combustion powered). Partially powered aircraft may use thrust provided in part by electrical means and in part by combustion means. This invention may be used in a fully or partially combustion powered aircraft. The electrical and combustion aspects may be provided by one or by a few fuels. In some examples a cryogenic fuel may be used. For example, a high energy density power source arrangement may comprise a cryogenic fuel arrangement arranged to provide fuel for power generation. Figure 1 shows a schematic view of an electrical power system 10 for an electrically powered aircraft according to an example of the present disclosure and in which the present techniques and arrangements may be implemented. It will be appreciated that electrical power system 10 may include additional components not shown in figure 1, and that components shown in figure 1 may be omitted. Electrical power system 10 includes a power distribution arrangement 12, such as an electrical bus or busbar, for distributing electrical power between components of the electrical power system 10. It will be appreciated that the arrangement and connections of the various components is not particularly limited and that the form shown in figure 1 is an example. Electrical power system 10 includes a number of PECs 14 (i.e. power converter arrangements as discussed herein) for receiving electrical power, converting the received electrical power, and providing the converted electrical power. Each PEC may include a pre-charge stage and a power stage (not shown), which are discussed in more detail further below. A PEC may convert DC to DC, or DC to AC or the like. The PEC may be referred to herein as a power converter arrangement or inverter. It will be appreciated that PECs having different properties (such as ratings or the like) 10 may be provided with the motor 18, fuel cell 16. super-capacitor 20, and battery 22. Thus, although shown as PEC 14, it will be appreciated that the configuration of the PEC may vary depending on implementation. It will also be appreciated that various fuel cell technologies may be used for the fuel cell 16, including for example proton-exchange membrane fuel cells. Electrical power system 10 also includes a fuel cell arrangement 16 for providing electrical power. The fuel cell arrangement 16 may include a plurality of fuel cells, such as a fuel cell stack or series of stacks of the like. The fuel cell arrangement 16 may take as input air and fuel (such as hydrogen) and output electrical power. The hydrogen may be stored in a fuel container (not shown) as a liquid cryogen. Electrical power system 10 also includes a motor arrangement (i.e. propulsion arrangement) 18 for providing propulsive power. Motor arrangement 18 may include a plurality of propulsive motors for providing propulsive power to the aircraft. Electrical power system 10 includes a super-capacitor arrangement 20. Super-capacitor arrangement 20 may be configured to store energy and provide electrical energy to the motor arrangement 18. Further, the super-capacitor arrangement 20 may be configured to receive electrical energy from the fuel cell arrangement 16 and store that energy. The super-capacitor arrangement 20 can provide a burst of power over a relatively short period of time, and thus may be advantageous during safety events or during short periods of high power demand. Supercapacitor arrangement 20 may include a plurality of super-capacitors. In some cases, a supercapacitor arrangement may be omitted from the electrical power system 10. The electrical power system 10 also includes a battery arrangement 22. Battery arrangement 22 may be configured to store energy and provide electrical energy to the motor arrangement 18. Further, the battery arrangement 22 may be configured to receive electrical energy from the fuel cell arrangement 16 and store that energy. The battery arrangement 22 may include one or more battery cells. The battery arrangement 22 can provide peak power relative to the fuel cell arrangement 16. In some examples, the battery arrangement 22 may be omitted from the electrical power system 10. As discussed further below in relation to figures 3 to 6, the first and second arrangements described herein may correspond to the fuel cell arrangement 16, super-capacitor arrangement 20, propulsion motor arrangement 18, battery arrangement 22 and power distribution arrangement 12. Figure 1 also shows a number of example electrical power systems 24, 26, 28, 30 (i.e. subsystems of the electrical power system 10). Electrical power system 24 includes the motor arrangement 18, a PEC 14, and the power distribution arrangement 12. Electrical power system 26 includes the fuel cell arrangement 16, a PEC 14, and the power distribution arrangement 12. 11 Electrical power system 28 includes the super-capacitor arrangement 20, a PEC 14, and the power distribution arrangement 12. Electrical power system 28 includes the battery arrangement 22, a PEC 14, and the power distribution arrangement 12. Figure 2 shows a schematic view of an electrical power system 100 for an electrically powered aircraft according to an example of the present disclosure. Electrical power system 100 may correspond to part or all of the electrical power system 10 of figure 1 or a sub-system thereof. Thus, electrical power system 100 may correspond to any of electrical power systems 24, 26, 28, 30 of figure 1. will be appreciated that electrical power system 100 may also include additional components not shown in figure 2. Electrical power system 100 includes a first arrangement 110 configured to provide electrical power to a power converter arrangement 130 (also referred to as a power electronic converter, PEC). The power converter arrangement 130 may correspond to the PEC 14 of figure 1. The power converter arrangement 130 is configured to receive the electrical power from the first arrangement 110, convert the received electrical power, and provide the converted electrical power to a second arrangement 120. The second arrangement 120 is configured to receive the converted electrical power from the power converter arrangement 130. The power converter arrangement 130 includes a pre-charge stage 150 (also known as a precharge circuit). The pre-charge stage 150 may be configured in use to limit or control an inrush (or initial) current, for example to controllably charge a downstream capacitance. The pre-charge stage 150 may be configured to charge a capacitor. For example, the pre-charge stage 150 may be configured to charge an input capacitor as part of start-up and also discharge during shutdown. In some examples, the pre-charge stage 150 comprises a contactor. Pre-charging can increase the lifespan of electric components. In a fuel cell implementation, the pre-charge stage 150 may be configured to mitigate against inrush current drawn by a fuel cell converter capacitor during a charging process. The power converter arrangement 130 also includes a power stage 160 (also known as a power circuit). The power stage 160 may be configured to convert voltage and / or current for use by the second arrangement. The power stage 160 may include switching devices. For example, the power stage 160 may be configured to convert input voltage and current from a fuel cell to a voltage and current for use by the second arrangement 120. In some examples, the power stage 160 is a converter comprising semiconductor switches. It will be appreciated that the power converter arrangement 130 (and indeed power stage 160) may convert DC to DC, or DC to AC, depending on implementation. In some examples, the power converter arrangement is a power inverter. Various configurations of the power converter arrangement are envisaged, and will depend on implementation. For example, the configuration (output voltage / current, etc.) of the power converter arrangement may depend on the demand of the second arrangement for which the power converter arrangement is converting power. A path of electrical power communication is shown by electrical power path 140. This shows how the electrical power path 140 may start at the first arrangement 110, pass through the power converter arrangement 130, where it is converted, to the second arrangement 120. Pre-charge stage 150 includes a first isolation arrangement 150 arranged in use to provide electrical isolation between the first arrangement 110 and the second arrangement 120. Power stage 160 includes a second isolation arrangement 180 arranged in use to provide electrical isolation between the first arrangement 110 and the second arrangement 120. Thus, each of the first and second isolation arrangements 170,180 may be arranged in use to interrupt the electrical power path 140 to provide electrical isolation between the first and second arrangements 110, 120. The first isolation arrangement 170 may correspond to a contactor of the pre-charge stage 150. For example, the contactor may be controllable to cause an open circuit (for example by opening a switch of the contactor) and thus provide electrical isolation. The second isolation arrangement 180 may be arranged to provide electrical isolation without operation of a circuit breaker. As discussed herein, the present inventors have identified that the power stage 160 itself may act as a fault isolation switch, as a result of the switching devices that are included in the power stage 160 (for the purpose of converting the power). Hence, in the event of a fault, the switching devices (i.e. the second isolation arrangement 180) within the power stage 160 may be controllable to provide electrical isolation. For example, if all the power stage 160 switches are opened (off) then current cannot flow, thereby isolating the input of the power stage 160 from the output of the power stage 160. It will be appreciated that even when in open circuit there may be an extremely small current leakage through an open power stage device. Thus, reference to electrical isolation refers to electrical isolation taking into account that a negligible amount of current leakage may still occur, i.e. that the component provides substantially total electrical isolation. Example fault scenarios that may be experience by the electrical power system 100 will now be described. In a first scenario, a fault occurs with the first arrangement 110. The first isolation arrangement 170 of the pre-charge stage 150 may be controlled to provide electrical isolation between the first arrangement 110 and the power stage 160 and second arrangement 120. For example, a contactor of the pre-charge stage 150 may be opened to provide the electrical isolation. As a 13 result, the fault is isolated to the first arrangement 110 and damage to other components of the electrical power system 100 can be minimised. This control may be provided by a controller that signals the contactor to open. In a second scenario, a fault occurs with the second arrangement 120. The second isolation arrangement 180 of the power stage 160 may be controlled to provide electrical isolation between the second arrangement 120 and the pre-charge stage 150. For example, switching devices of the power stage 160 may be opened to provide the electrical isolation. As a result, the fault is isolated to the second arrangement 120 and damage to other components of the electrical power system 100 can be minimised. This control may be provided by a controller that signals the switching devices to open. In a third scenario, a fault may occur with the pre-charge stage 150 and so the power stage 160 can be controlled to provide electrical isolation, thereby isolating the fault from the second arrangement 120. Similarly, in a fourth scenario, a fault may occur with the power stage 160 and so the pre-charge stage 150 can be controlled to provide electrical isolation, thereby isolating the fault from the first arrangement 110. Hence, the electrical power system 100 provides improved fault isolation while minimising weight (by using components of the power converter rather than dedicated circuit breakers or DC protection devices, for example). In some examples, the electrical power system 100 is optionally provided with a third isolation arrangement 190 arranged in use to provide electrical isolation between the first arrangement 110 and the power converter arrangement 130. The third isolation arrangement 190 may comprise a fuse. For example, a cryogenic fuse as discussed herein. The third isolation arrangement 190 may further isolate a fault occurring at the first arrangement 110 from affecting the rest of the components of the electrical power system 100. Furthermore, the third isolation arrangement 190 may isolate a fault occurring with an electrical power conduit that is arranged to communicate the electrical power from the first arrangement 110 to the second arrangement 120. Although not shown in figure 2, the electrical power system 100 may be provided with a plurality of different sensor arrangements to determine electrical parameters associated with the electrical power system 100. For example, each of the first arrangement 110, second arrangement 120, and input and output of the power converter arrangement 130 may be provided with a sensing arrangement to measure an electrical parameter associated with the respective arrangement. The present inventors have identified that by providing a plurality of different sensor arrangements, redundancy can be provided and the likelihood that a fault can be detected and diagnosed is increased. Thus, corrective action can be taken more effectively. The first and second arrangements 110, 120 may take a variety of forms. For example, the first and second arrangements 110, 120, may comprise one or more of: a fuel cell arrangement; a super-capacitor arrangement; a battery arrangement; a propulsion motor arrangement; and a power distribution arrangement. Examples of different forms of the first and second arrangements 110, 120 will now be discussed with reference to figures 3 to 6. Figure 3 shows an example electrical power system 200, where the first arrangement 110 is a power distribution arrangement (for example an electrical bus) 210 and the second arrangement 120 is a propulsion motor 220. Thus, electrical power system 200 may correspond to electrical power system 24 of figure 1. As for figure 1, electrical power system 200 includes a power converter arrangement 130 having a pre-charge stage or circuit 150, a power stage circuit 160, a third isolation arrangement 190 and an electrical power path 140 (the first and second isolation arrangements 170, 180 are not shown in figure 3). In addition, electrical power system 200 includes a plurality of sensor different arrangements 230. The plurality of different sensor arrangements 230 are configured to measure one or more electrical parameters associated with the electrical power system 200. For example, the plurality of different sensor arrangements 230 comprises: a first sensor arrangement 230a arranged in use to measure an electrical parameter associated with the first arrangement 210, a second sensor arrangement 230b arranged in use to measure an electrical parameter associated with the electrical input of the power converter arrangement 130, and a third sensor arrangement 230b arranged in use to measure an electrical parameter associated with an electrical output of the power converter arrangement 130. In some examples, a fourth sensor arrangement (not shown) arranged in use to measure an electrical parameter associated with the second arrangement 220 may be provided. The propulsion motor 220 may be fault-tolerant. In other words, the propulsion motor 220 may be configured in use such that a rated current is substantially equal (i.e. within 0.25) to 1 to 2 times a fault current of the propulsion motor 220. Electrical machines (i.e. motors) may be designed to be fail-safe. When designed to be fail-safe fault currents of 6PU (to 10PLI) may be experienced. The nominal performance of a synchronous machine can be enhanced by operating with a low power factor (and thus low stator reactance). This can reduce size of the machine and increase efficiency. However, a problem with a low stator reactance is with a terminal short-circuit failure, the back-emf is seen across the Q axis. The only impedance in this circuit is the (low) reactance resulting in a high fault current. A fault tolerant machine is designed with a high stator reactance (and thus power factor). Because of the power factor, a fault tolerant machine may use a power converter (i.e. PEC). With a PEC, the controlled switching of the current with back-emf of the machine can deliver improved nominal performance. An advantage of a high reactance is that under a terminal short-circuit the machine fault current is close to 1PU, i.e. the rated current is approximately equal to the fault current. For a non-fault-tolerant machine, with a power factor of 0.1, the fault current may be 10x the rated current and so the fault energy may be 100x. For a fault-tolerant machine, with a power factor of 1, the fault current may be 1x the rated current and so the fault energy may be 1x. This illustrates how a fault-tolerant electrical machine can support sustained containment under a fault current and hence ride-through as discussed herein. Electrical power system 200 corresponds to a so-called propulsion feeder arrangement. The electrical power system 200 is provided with three fault detecting arrangements (the three sensor arrangements 230a, 230b, and 230c) and three isolation arrangements (the first, second, and third isolation arrangements 170, 180, 190). Additionally, fault discrimination may be performed by the three different and independent sensor arrangements 230a, 230b, and 230c, using different methods, e.g. voltage differential, current differential, and absolute current measurements. The present inventors have identified that some existing aircrafts that include a power stage (i.e. a converter formed of semiconductors) which requires its own circuit breaker. In contrast, in the present electrical power system, the power stage 160 itself acts as a fault isolation switch (or circuit breaker), thereby removing the circuit breaker and reducing weight. In the present example of the propulsion feeder arrangement, the isolation arrangements are: the power stage 160; the pre-charge circuit (providing galvanic isolation) 150; and a cryogenic fuse 190. Example fault scenarios will now be discussed with reference to the example of figure 3. A first scenario is where a fault occurs in the power stage 160. The fault is isolated by the precharge circuit 150, combined with the fault tolerance of the propulsion motor 220. A second scenario is where a fault occurs in the pre-charge circuit 150. This fault is isolated by the cryogenic fuse (also called a cryo-conduit fuse) 190 and the switches in the power stage 160. A third scenario is where the fault occurs on an electrical conduit between 190 and 150 (i.e. on the electrical power path 140). This can be isolated by opening the contactor of the pre-charge circuit 150 and by activating the cryo-conduit fuse 190. Thus, an electrical power system 200 that provides redundancy and electrical isolation is realised. In particular, each propulsion feeder can be isolated from the propulsion bus following any component failure within that propulsion feeder and each PEC (power converter) offers integrated protection functionality not only with the switching devices but also with the pre-charge contactors. This enables continued operation with reduced capability but without multiple heavy DC circuit breakers. Thus, fault isolation and hence safety can be increased while reducing weight. Figure 4 shows an example electrical power system 300, where the first arrangement 110 is a fuel cell arrangement 310 and the second arrangement 120 is a power distribution arrangement (such as an electrical bus) 320. Thus, electrical power system 300 may correspond to electrical power system 26 of figure 1. As for figures 1 and 2, electrical power system 300 includes a power converter arrangement 130 having a pre-charge stage or circuit 150, a power stage circuit 160, a third isolation arrangement 190 and an electrical power path 140 (the first and second isolation arrangements 170, 180 are not shown in figure 3). In this example, the order of the power stage 160 and the pre-charge circuit stage 150 are swapped. The fuel cell arrangement 310 is fault-tolerant. In other words, the fuel cell 310 cannot exceed its maximum current or voltage rating. The power converter arrangement 130 in this example is configured in use to withstand a maximum fuel cell voltage and current. As discussed herein, a fuel cell is unable to sustain current above its rated current and thus may be considered inherently fault tolerant. This can support sustained containment under a fault current. At a reduced fuel and air level in the fuel cell, a concentration region of a polarization curve occurs at a lower current density. Therefore, at reduced fuel and air level the maximum output current is less than rated current. Hence, the fuel cell can be considered fault-tolerant. Some fuel cell dc / dc converters (an example of a power convert arrangement) have an output filter that serves two functions. The first is to provide an AC (switching) return path across the power modules and the second function is to regulate the output voltage to create a stable supply. For a voltage regulating fuel cell dc / dc converter the second function dominates the output filter capacitor rating. Though the energy of a capacitor bank is limited its ability to supply a fast rate of rise of fault current is very high. To achieve a ride-through scheme the power converter would need to be able to withstand the output capacitor high power delivery capability (even for a short duration). This is enable by two design variations: a. The removal of the second regulate function. By removing the second regulate function the fuel cell dc / dc converter is naturally able to withstand the impact of the remaining capacitance. b. The consequence of (a) above is that the converter becomes a current regulating dc / dc converter. The super-capacitor and battery arrangements now become voltage regulating units. As for the example of figure 3, the electrical power system 300 includes a plurality of different sensor arrangements 230 (230a, 230b, 230c, and 230d). The example fault scenarios discussed above may similarly occur in the present example and may be similarly isolated as discussed above. Figure 5 shows an example electrical power system 400, where the first arrangement 110 is a super-capacitor arrangement 410 and the second arrangement 120 is a power distribution arrangement (such as an electrical bus) 420. Thus, electrical power system 400 may correspond to electrical power system 30 of figure 1. As discussed in relation to previous examples, electrical power system 400 includes a power converter arrangement 130 having a pre-charge stage or circuit 150, a power stage circuit 160, a third isolation arrangement 190 and an electrical power path 140 (the first and second isolation arrangements 170, 180 are not shown in figure 5). In this example, the order of the power stage 160 and the pre-charge circuit stage 150 are swapped. In some implementations, the third isolation arrangement 190 is not present when the cable length (between the super-capacitor 410 and the power converter arrangement 130, and separately between the power converter arrangement 130 and the power distribution arrangement 420) is less than 1.5m. When either of these cable lengths is longer than 1.5m, a third isolation arrangement 190 is provided between the pre-charge circuit and the power distribution arrangement 420. This is because the risk of cable failure for short-length cables is sufficiently reduced. In this example, the power converter arrangement 130 is configured in use to withstand a maximum super-capacitor voltage and current. Thus, the power converter arrangement 130 is fault-tolerant. As for the examples above, the electrical power system 400 includes a plurality of different sensor arrangements 230 (230a, 230b, 230c, and 230d). The example fault scenarios discussed above may similarly occur in the present example and may be similarly isolated as discussed above. Figure 6 shows an example electrical power system 500, where the first arrangement 110 is a battery arrangement 510 and the second arrangement 120 is a power distribution arrangement (such as an electrical bus) 520. Thus, electrical power system 500 may correspond to electrical power system 28 of figure 1. As discussed in relation to previous examples, electrical power system 500 includes a power converter arrangement 130 having a pre-charge stage or circuit 150, a power stage circuit 160, a third isolation arrangement 190 and an electrical power path 140 (the first and second isolation arrangements 170, 180 are not shown in figure 4). In this example, the order of the power stage 160 and the pre-charge circuit stage 150 are swapped. As for the examples above, the electrical power system 500 includes a plurality of different sensor arrangements 230 (230a, 230b, 230c, and 230d). Further, a third isolation arrangement 190 may be provided. In addition, a circuit breaker 530 may be provided. In this example, the battery is not fault-tolerant. Thus, the circuit breaker 530 will trip if a current / time rating is exceeded. Thus, the third isolation arrangement 190 (the fuse) and the circuit breaker 530 will trip before the battery and power converter arrangement 130 ratings are exceeded., thereby providing fault isolation. Some example batteries may not be fault tolerant and may actually have the capability to supply significantly more current than its nominal operation. This could lead to battery thermal runaway and unwanted cascaded fire failure. Thus, the circuit breaker 530 may be provided to interrupt a power supply from the battery. This circuit breaker 530 may be arranged to trigger / respond to a rate of increase in current exceeding a predetermined threshold. The example fault scenarios discussed above may similarly occur in the present example and may be similarly isolated as discussed above. It will be appreciated that the examples of figures 3 to 6 may be combined such that the electrical power system comprises a number of different electrical power sub-systems (such as one or more of a propulsion sub-system, fuel cell sub-system, battery sub-system, and super-capacitor subsystem). An example fault isolation method 700 will now be discussed with reference to figure 7. Method 700 may be performed by a controller arrangement of the electrical power system discussed herein, for example the electrical power system 10 of figure 1, or any of the electrical power systems of figures 3 to 6. At 710, a fault associated with the electrical power system is detected. For example, a fault may be detected with the power converter arrangement having a pre-charge stage and a power stage, a fuel cell arrangement, a super-capacitor arrangement, a propulsion motor arrangement; a battery arrangement and a power distribution arrangement, such as those discussed with reference to figures 1 to 6. The fault may be detected based on measurement data, the measurement data having been determined using a plurality of different sensor arrangements as discussed herein, such as the plurality of different sensor arrangements 230. At 720, electrical power in the electrical power system is isolated using one or more of the different isolation arrangements based on the detected fault. In some examples, one or more of the different isolation arrangements are controllable and thus can be controlled to isolate electrical power. Thus, in some examples, step 720 may include signalling the one or more different isolation arrangements to isolate the fault. For example, the switching devices of the power stage or the contactor of the pre-charge stage of the power converter arrangement may be controlled to open, so as to isolate electrical power. Thus, faults can be efficiently and effectively isolated, thereby increasing aircraft safety. Method 700 may be performed by an electronic device. The electronic device may have processing circuitry 710 for performing data processing in response to program instructions and data storage 720 for storing data and instructions to be processed by the processing circuitry 710. In some examples, the processing circuitry 710 includes one or more caches for caching recent data or instructions. The device can further include a communication interface 740 which can be used, for example, to obtain / receive information relating to the measurement data and the fault isolation. Method 700 may be performed under control of a computer program executing on a device. Hence a computer program may comprise instructions for controlling a device to perform any of the methods discussed above. The program can be encoded in a computer-readable medium. A computer-readable medium may include non-transitory type media such as physical storage media including storage discs and solid state devices. A computer-readable medium may also or alternatively include transient media such as carrier signals and transmission media. A computer-readable storage medium is defined herein as a non-transitory memory device. A memory device includes memory space within a single physical storage device or memory space spread across multiple physical storage devices. Although illustrative teachings of the disclosure have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise teachings, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.

Claims

1. An electrical power system for an electrically powered aircraft, the system comprising: a power converter arrangement for receiving electrical power, converting the received electrical power, and providing the converted electrical power, the power converter arrangement having a pre-charge stage and a power stage;a first arrangement for providing electrical power to the power converter arrangement and;a second arrangement for receiving the converted electrical power from the power converter arrangement, whereinthe pre-charge stage comprises a first isolation arrangement arranged in use to provide electrical isolation between the first arrangement and the second arrangement, andthe power stage comprises a second isolation arrangement arranged in use to provide electrical isolation between the first arrangement and the second arrangement.

2. The system of claim 1, wherein the second isolation arrangement of the power stage is arranged in use to provide electrical isolation without operation of a circuit breaker.

3. The system of any preceding claim, wherein the first and second arrangements comprise one or more of: a fuel cell arrangement; a battery arrangement; a super-capacitor arrangement; a propulsion motor arrangement; and a power distribution arrangement.

4. The system of any preceding claim, further comprising a plurality of different sensor arrangements arranged in use to measure one or more electrical parameters associated with the electrical power system.

5. The system of claim 4, wherein the plurality of different sensor arrangements comprises a voltage differential sensor arrangement for measuring a voltage differential, a current differential sensor arrangement for measuring a current differential, and an absolute current sensor arrangement for measuring an absolute current.

6. The system of claims 4 to 5, wherein the plurality of different sensor arrangements comprise one or more of: a first sensor arrangement arranged in use to measure an electrical parameter associated with the first arrangement, a second sensor arrangement arranged in use to measure an electrical parameter associated with the electrical input of the power converter arrangement, and a third sensor arrangement arranged in use to measure an electrical parameter associated with an electrical output of the power converter arrangement, a fourth sensor arrangement arranged in use to measure an electrical parameter associated with the second arrangement.

7. The system of any preceding claim, wherein the first arrangement is configured in use such that a rated current of the first arrangement is approximately equal to 1 to 2 times a fault current of the first arrangement.

8. The system of any preceding claim, wherein:the first arrangement comprises a power distribution arrangement for providing electrical power to the pre-charge stage of the power converter arrangement,the second arrangement comprises a propulsion motor for receiving the converted electrical power from the power converter arrangement and for providing propulsion power,the first isolation arrangement is arranged in use to provide electrical isolation between the first arrangement and the power stage, andthe second isolation arrangement is arranged in use to provide electrical isolation between the pre-charge stage and the second arrangement.

9. The system of claim 8, wherein the propulsion motor is configured in use such that a fault current is substantially equal to a rated current of the propulsion motor.

10. The system of any of claims 1 to 7, wherein:the first arrangement comprises a fuel cell arrangement for providing electrical power to the power stage of the power converter arrangement,the second arrangement comprises a power distribution arrangement for receiving the converted electrical power from the pre-charge stage of the power converter arrangement,the first isolation arrangement is arranged in use to provide electrical isolation between the first arrangement and the pre-charge stage, andthe second isolation arrangement is arranged in use to provide electrical isolation between the power stage and the second arrangement.

11. The system of claim 10, wherein the power converter arrangement is configured in use to withstand a maximum fuel cell voltage and current.

12. The system of any of claims 1 to 7, wherein:the first arrangement comprises a super-capacitor arrangement for providing electrical power to the power stage of the power converter arrangement,the second arrangement comprises a power distribution arrangement for receiving the converted electrical power from the pre-charge stage of the power converter arrangement,the first isolation arrangement is arranged in use to provide electrical isolation between the first arrangement and the pre-charge stage, andthe second isolation arrangement is arranged in use to provide electrical isolation between the power stage and the second arrangement.

13. The system of claim 12, wherein the power converter arrangement is configured in use to withstand a maximum super-capacitor voltage and current.

14. The system of any of claims 1 to 7, wherein:the first arrangement comprises a battery arrangement for providing electrical power to the power stage of the power converter arrangement,the second arrangement comprises a power distribution arrangement for receiving the converted electrical power from the pre-charge stage of the power converter arrangement,the first isolation arrangement is arranged in use to provide electrical isolation between the first arrangement and the pre-charge stage, andthe second isolation arrangement is arranged in use to provide electrical isolation between the power stage and the second arrangement.

15. The system of any preceding claim, further comprising a third isolation arrangement arranged in use to provide electrical isolation between the first arrangement and the power converter arrangement.

16. The system of claim 15, wherein the third isolation arrangement comprises a cryogenic fuse.

17. The system of any preceding claim, further comprising an uninterruptible power supply for supplying back-up electrical power.

18. The system of any preceding claim, further comprising a battery and a circuit breaker arranged in use to interrupt a power supply from the battery.

19. The system of any preceding claim, further comprising a fuel cell for providing electrical power.

20. The system of any preceding claim, further comprising a cryogenic fuel arrangement arranged to provide cryogenic fuel for power generation.

21. An at least partially electrically powered aircraft comprising the electrical power system of any preceding claim.

22. A method of fault isolation in an electrical power system for an electrically powered aircraft, the electrical power system having a plurality of different isolation arrangements arranged to isolate electrical power between a plurality of arrangements, the arrangements comprising a power converter arrangement having a pre-charge stage and a power stage and a one or more of: a fuel cell arrangement, a super-capacitor arrangement, a propulsion motor arrangement; a battery arrangement and a power distribution arrangement, the method comprising:detecting a fault associated with the electrical power system; andisolating electrical power in the electrical power system using one or more of the different isolation arrangements based on the detected fault.

23. The method of claim 22, further comprising determining measurement data using a plurality of different sensor arrangements, wherein detecting the fault is based on the measurement data.

24. A controller arrangement for an electrical power system of an electrically powered aircraft, the controller arrangement comprising one or more processors configured to perform the method of claims 22 or 23.

25. A computer-readable medium comprising instructions which, when executed by a controller arrangement comprising one or more processors, cause the one or more processors to perform the method of claims 22 or 23.

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