Patent Publication Number: US-2023136376-A1

Title: Electrical power system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This specification is based upon and claims the benefit of priority from United Kingdom Patent Application No. 2115513.0, filed on 28 Oct. 2021, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to an electrical power system. 
     BACKGROUND 
     An electrical power system may include apparatus for protecting components of the electrical power system from a fault current. The fault current may originate from, for example, an electrical energy storage device of the electrical power system or an electrical network of the power system. In particular, in an electrical power system comprising an electrical energy storage device with a very high electrical energy delivery capacity and a very low source impedance, a fault current which represents an effective short-circuit across a pair of terminals of the electrical energy storage device may lead to an extremely sharp rise in current. The fault current can cause damage to other components of the electrical power system. It is known to incorporate thermal fuses into an electrical power system in order to avoid such damage. However, thermal fuses must be replaced after every fault. 
     It is therefore desirable to provide an improved electrical power system. 
     SUMMARY 
     According to a first aspect there is provided an electrical power system comprising: an electrical power source, a power electronics converter, an electrical network, a current limiting diode and a controllable circuit interruption device, wherein: the current limiting diode is configured to limit a fault current passing between the electrical power source and the electrical network in a fault condition; and the controllable circuit interruption device is configured to interrupt the fault current in response to a determination that the electrical power system is in the fault condition. 
     The current limiting diode and the controllable circuit interruption device may combine synergistically to reduce a required current rating of the controllable circuit interruption device. This may in turn reduce the mass of the electrical power system, which may be a particular advantage in an aircraft system. For example, a controllable circuit interruption device (e.g., a contactor) may be unable to open in the presence of a very large fault current due to, e.g., contactor arcing unless the contactor is significantly overrated compared to the rating of the electrical power source. However, when combined with the current limiting diode, the fault current may be reduced to a lower level for an amount of time long enough for a controllable circuit interruption device with a relatively low current rating to interrupt the fault current. 
     Thus, a current rating of the controllable circuit interruption device may be relatively low. The current rating of the controllable circuit interruption device may be between one and five times a current rating of the electrical power source. The current rating of the controllable circuit interruption device may be less than four times, less than three times, or even less than 2.5 times the current rating of the electrical power source. The current rating of the controllable circuit interruption device may be between 1.25 and 2.5 times the current rating of the electrical power source, and may be between 1.5 and 2.0 times the current rating of the electrical power source. Such values may provide a sweet spot in a trade-off between system mass reduction and assurance against faults. 
     It may be that the controllable circuit interruption device comprises a controllable contactor and the electrical power system further comprises a controller configured to control the controllable contactor. In other embodiments the controllable circuit interruption device may comprise a semiconductor device such as a solid-state circuit breaker. 
     The current limiting diode may be provided with a heat sink. The heat sink provided to the current limiting diode may comprise a phase-change material configured to absorb heat from the current limiting diode in the fault condition. 
     It may be that the electrical power source is configured to provide an electrical potential difference to the electrical power system of at least 270 V. It may be that the electrical power source is configured to provide an electrical potential difference to the electrical power system of 270 V, 540 V, 800V or 1080 V or at least any of these values. 
     It may be that the electrical power source comprises a battery (e.g. a high-density battery). Other electrical power sources, including fuel cells, may be used. 
     The power electronics converter may comprise a DC to DC converter. The power electronics converter may comprise a DC to AC to DC converter. 
     It may be that the electrical power source is a first electrical power source, and wherein the electrical network comprises a second electrical power source. Further, it may be that the second electrical power source comprises an electrical generator and/or a capacitor. 
     In addition, it may be that the current limiting diode is a first current limiting diode, and wherein the electrical power system further comprises a second current limiting diode, the first current limiting diode and the second current limiting diode being electrically connected and together forming a bidirectional current limiting device. 
     According to a second aspect, there is provided an aircraft power and propulsion system comprising the electrical power system of the first aspect. 
     According to a third aspect, there is provided an aircraft comprising an electrical power system in accordance with the first aspect or an aircraft power and propulsion system in accordance with the second aspect. 
     The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and/or combined with any other feature or parameter described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG.  1    is a circuit diagram which shows a first example electrical power system; 
         FIG.  2    is a circuit diagram which shows a second example electrical power system; 
         FIG.  3    is a circuit diagram which shows a third example electrical power system; 
         FIG.  4    is a circuit diagram which shows a fourth example electrical power system; 
         FIG.  5    shows an example aircraft power and propulsion system comprising an example electrical power system; and 
         FIG.  6    shows an aircraft comprising the example electrical power system. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. 
       FIG.  1    shows an electrical power system  100  according to a first example which comprises an electrical power source  110 , a power electronics converter  120 , an electrical network  130  connected between terminals  132  and  134 , a current limiting diode  140  and a controllable circuit interruption device  150 . In the example of  FIG.  1   , the electrical network is represented by a load  130 , an internal impedance of the electrical power source  110  is represented by a resistor  112  and an electrical energy delivery capacity of the electrical power source is represented by a voltage source  114 . 
     In the example shown in  FIG.  1   , the power electronics converter  120  comprises a DC to DC converter and the electrical network  130  is a DC electrical network. The DC to DC converter may comprise a plurality of transistors and an inductor coil  129 . In  FIG.  1   , the DC to DC converter comprises a first transistor  121 , a second transistor  122 , a third transistor  123  and a fourth transistor  124 . The power electronics converter  120  provides a regulation function to the electrical power system  100 , as is described in further detail below. In other examples, the power electronics converter  120  comprises a DC to AC converter and the electrical network  130  is an AC electrical network. 
     The current limiting diode  140  comprises a junction field-effect transistor (JFET) provided with a source, a drain and a gate, with the gate being shorted to the source and the drain being electrically connected to the source by an n-channel. Accordingly, the current limiting diode  140  is configured as a 2-terminal device which permits an electric current to be conducted through the n-channel between the source and the drain. The current limiting diode  140  has an electrical resistance which is primarily defined by a size of a depletion region of the n-channel. The depletion region is a region of the n-channel in which there are no free charge carriers. As a result, an electric current cannot be conducted through the depletion region and is instead conducted through a current-carrying path within the n-channel that is bounded by the depletion region. 
     As current flows through this current-carrying path, a potential difference is formed between the source and drain of the channel. As this potential increases with current eventually the channel is pinched off, i.e. the drain potential is greater than the threshold voltage. This causes the current to saturate and significant voltage to build up across the device and the electrical characteristic to transfer from a linear relationship between drain voltage and current to a constant current mode. 
     The size of the depletion region corresponds to a magnitude of an electric current being passed through the n-channel from the drain to the source. That is to say that if the magnitude of the electric current passing through the n-channel from the drain to the source is large, the size of the depletion zone will also be large and the electrical resistance of the current limiting diode  140  becomes large. However, the size of the depletion region does not correspond to a magnitude of an electric current being passed in the opposite direction from the source to the drain. 
     A relationship between the electrical resistance of the current limiting diode  140  and the magnitude of the electric current passing from the drain to the source is highly non-linear. In other words, the current limiting diode  140  operates in a manner which is analogous to a highly non-linear unidirectional resistor in that the electrical resistance of the current limiting diode  140  increases rapidly as the magnitude of the electric current passing from the drain to the source increases above a saturation threshold. Accordingly, the current limiting diode  140  “saturates” at a near constant current level. 
     In a normal operating condition of the first electrical power system  100 , a magnitude of an electric current being conducted through the current limiting diode  140  from the drain to the source is relatively low. Accordingly, the size of the depletion region within the current limiting diode  140  is relatively small and the electrical resistance of the current limiting diode  140  is also relatively small. Therefore, an amount of resistive heat dissipated by the current limiting diode  140  when the electrical power system is in the normal operating condition is low. As a result, a power insertion loss associated with the inclusion of the current limiting diode  140  within the electrical power system  100  is minimised in the normal operating condition. 
     On the other hand, in a fault condition of the first electrical power system  100 , a fault in the electrical power system  100  may lead to a magnitude of an electric current being conducted through the current limiting diode  140  from the drain to the source becoming extremely large in a very short period of time. 
     For example, if the internal impedance  112  of the electrical power source  110  is very low and the electrical energy delivery capacity  114  of the electrical power source  110  is very high, a fault in the electrical power system  100  which originates in the electrical network  130  or the power electronics converter  120  and which presents an effective short circuit across the electrical power source  110  may cause a magnitude of a fault current to be conducted through the electrical power system  100  which reaches an order of thousands of Amperes within a very short period of time. This may be because the magnitude of the fault current is only limited by the internal impedance  112  of the electrical power source  110 . If the magnitude of the fault current were not limited and/or interrupted, the magnitude of the fault current could rise beyond a tolerance limit of the power electronics converter  120  and/or a component of the electrical network  130 . The tolerance limit of the power electronics converter  120  may be associated with a switching capacity of at least one of the plurality of transistors. 
     In this example, the fault originates from the electrical network  130  or the power electronics converter  120  and the fault current is therefore conducted through the current limiting diode  140  from the drain to the source. If the fault current exceeds the saturation threshold of the current limiting diode  140 , the current limiting diode  140  almost instantaneously limits the magnitude of the fault current originating from the electrical network  130  or the power electronics converter  120  by providing a greatly increased resistance through the n-channel. However, in another example, it may be that a fault originates from the electrical power source  110  and a fault current is therefore conducted through the current limiting diode  140  from the source to the drain. 
     The size of the depletion region of the n-channel does not correspond to a magnitude of an electric current being passed in the opposite direction from the source to the drain. Therefore, the current limiting diode  140  is not able to limit the magnitude of a fault current caused by a fault originating from the electrical power source  110 . For this reason, an orientation of the current limiting diode  140  within the electrical power system  100  dictates whether the current limiting diode  140  is able to limit a fault current caused by a fault originating from the electrical network  130  or the power electronics converter  120  or a fault current caused by a fault originating from the electrical power source  110 . 
     In practice, the current limiting diode  140  is only able to respond to a development of the fault current caused by a fault originating from the electrical network  130  or the power electronics converter  120  and to limit the fault current within a response time period of the current limiting diode  140 . The response time period of the current limiting diode  140  may be, for example, of an order of a number of nanoseconds. Further, the current limiting diode  140  is configured to ensure that the magnitude of the fault current is maintained below the tolerance limit of the electrical power source  110 , the power electronics converter  120  and/or a component of the electrical network  130 . 
     However, while the current limiting diode  140  is able to respond to the development of the fault current almost instantaneously (being limited only by the response time period of the current limiting diode  140 ), the greatly increased electrical resistance provided by the current limiting diode  140  causes the resistive heat dissipated by the current limiting diode  140  to become extremely high. In practice, the resistive heat dissipated by the current limiting diode in the fault condition may be so high that a maximum internal temperature of the current limiting diode  140  rises above a threshold temperature at which damage to and loss of function of the current limiting diode  140  may occur. A duration between the development of the fault current and a time at which the maximum internal temperature exceeds the threshold temperature is referred to as a critical time period. The critical time period may be, for example, of an order of a number of milliseconds. 
     The controllable circuit interruption device  150  is configured to interrupt the fault current shortly after the development of the fault current in response to a determination that the electrical power system  100  is in the fault condition. The controllable circuit interruption device  150  may monitor an electrical current passing between the electrical power source  110  and the electrical network  130  in order to determine whether the electrical power system  100  is in the fault condition. However, the controllable circuit interruption device  150  is only able to respond to the development of the fault current and interrupt the fault current within a response time period of the controllable circuit interruption device  150 . The response time period of the controllable circuit interruption device  150  is longer than the response time period of the current limiting diode  140 . Accordingly, the current limiting diode  140  and the controllable circuit interruption device  150  are configured to co-operate in use so as to limit and then to interrupt a fault current passing between the electrical power source and the electrical network  130  in the fault condition. The current limiting diode  140  and the controllable circuit interruption device  150  provide the electrical power system  100  with a resettable protection system against a fault current passing between the electrical power source  110  and the electrical network  130  in the fault condition. 
     In the example of  FIG.  1   , the current limiting diode  140  is orientated within the electrical power system  100  such that the current limiting diode  140  and the controllable circuit interruption device  150  are configured to co-operate in use so as to limit and then to interrupt a fault current originating from the electrical network  130  or the power electronics converter  120 . However, it will be appreciated that the current limiting diode  140  may be otherwise orientated within the electrical power system  100  such that the current limiting diode  140  and the controllable circuit interruption device  150  are configured to co-operate in use so as to limit and then to interrupt a fault current originating from the electrical storage device  110 . 
     The controllable circuit interruption device  150  may comprise, for example, a contactor, any other type of mechanical switch or a semiconductor-type switch which is electrically controllable (e.g. a solid-state circuit breaker). Use of a contactor or another type of mechanical switch provides a physical circuit break (that is, Galvanic isolation) in the fault condition, which is associated with improved safety of the electrical power system  100 . 
     In the example shown in  FIG.  1   , the controllable circuit interruption device  150  comprises a controllable contactor and the electrical power system  100  is provided with a controller  190  configured to control the controllable contactor. The controller  190  may monitor an electric current being conducted through the electrical power system  100  between the electrical power source  110  and the electrical network  130  using a sensor. The sensor may comprise, for example, a Hall effect sensor, a fibre optic current sensor and/or a fluxgate sensor. However, it will be appreciated that the controllable circuit interruption device  150  may be configured to monitor the electrical current being conducted through the electrical power system  100  between the electrical power source  110  and the electrical network  130  and control its own operation based on the monitored electrical current. The controller  190  may therefore form part of the controllable circuit interruption device  150  itself. 
     If the response time period of the controllable circuit interruption device  150  is longer than the critical time period, the controllable circuit interruption device  150  is not able to interrupt the fault current before the critical time period has elapsed. As a consequence, damage to or loss of function of the current limiting diode  140  may follow. Subsequently, the current limiting diode  140  may no longer be able to limit the fault current, which in turn is liable to cause damage to other components of an electrical power system and pose a safety risk. A safety-time margin of the electrical power system  100  is defined as a difference between the response time period of the controllable circuit interruption device  150  and the critical time period. An elongated safety-time margin is associated with an improved safety of the electrical power system  100 . 
     It will be appreciated that a location of the controllable circuit interruption device  150  within the electrical power system  100  may be varied without affecting the intended function of the controllable circuit interruption device  150  as part of the electrical power system  100 . For illustration,  FIG.  1    shows a plurality of positions within the electrical power system  100  at which the controllable circuit interruption device  150  may be located. The plurality of positions includes but is not limited to a first position  150 A, a second position  150 B and a third position  150 C. In a similar way, it is to be appreciated that a location of the current limiting diode  140  within the electrical power system  100  may be varied without affecting the intended function of the current limiting diode  140  as part of the electrical power system  100 . In some examples, a plurality of controllable circuit interruption devices  150  may be provided in different locations within the electrical power system  100 , such as at some or all of positions  150 A- 150 C. 
     In this example, the current limiting diode  140  is provided with a heat sink  148 . The heat sink  148  is configured to absorb heat from the current limiting diode  140  in the fault condition. The heat absorbed by the heat sink  148  in the fault condition elongates the critical time period by reducing a rate of change of the maximum internal temperature of the current limiting diode  140 . Accordingly, the elongation of the critical time period increases the safety-time margin of the electrical power system  100  and is therefore associated with an improved safety of the electrical power system  100 . In addition, the provision of the heat sink  148  to the current limiting diode  140  may permit a controllable circuit interruption device  150  having a relatively long response time period which would otherwise render it unsuitable for use in conjunction with the current limiting diode  140  to be used as part of the electrical power system  100 . 
     The heat sink  148  may comprise a phase-change material. The phase-change material is configured to absorb heat from the current limiting diode  140  in the fault condition by changing phase. In particular, the phase-change material is configured to absorb heat energy from the current limiting diode  140  by means of a substantially constant-temperature heat absorption process. Consequently, the phase-change material can absorb a significant amount of heat from the current limiting diode  140  without necessitating a rise in the maximum internal temperature of the current limiting diode  140 . The heat absorbed by the phase-change material in the fault condition by means of the substantially constant-temperature heat absorption process further elongates the critical time period. 
     The electrical power source  110  is configured to provide a first electrical potential difference to the power electronics converter  120 . The power electronics converter  120  is configured to convert and/or regulate the electrical potential difference provided by the electrical power source  110  and provide a second electrical potential difference to the electrical network  130 . Accordingly, the power electronics converter  120  provides a regulation function to the electrical power system  100 . In particular, the power electronics converter  120  allows the first electrical potential difference to change while providing a substantially constant second electrical potential difference to the electrical network  130 . The electrical network  130  may comprise at least one load component which benefits from being driven by a substantially constant electrical potential difference. In addition, the electrical network  130  may require the supply of a pre-determined electrical potential difference for optimal operation thereof. The pre-determined electrical potential difference is referred to as the electrical network  130  voltage demand. 
     The power electronics converter  120  may be configured to provide a second electrical potential difference to the electrical network  130  which is greater than the first electrical potential difference provided by the electrical power source  110  in a boost regulation mode of the power electronics converter  120 . A ratio of the second electrical potential difference provided to the electrical network  130  by the power electronics converter  120  and the first electrical potential difference provided by the electrical power source  110  in the boost regulation mode is governed by a boost duty cycle of the power electronics converter  120 . 
     A magnitude of the boost duty cycle is associated with an efficiency of the power electronics converter  120  in the boost regulation mode. By way of example, if the power electronics converter  120  is configured to provide a second electrical potential difference to the electrical network  130  which is significantly greater than the first electrical potential difference provided by the electrical power source  110  in the boost regulation mode, the magnitude of the boost duty cycle of the power electronics converter  120  will increase. An increase in the magnitude of the boost duty cycle of the power electronics converter is associated with a decrease in the efficiency of the power electronics converter  120  in the boost regulation mode. 
     Additionally or alternatively, the power electronics converter  120  may be configured to provide a second electrical potential difference to the electrical network  130  which is lower than the first electrical potential difference provided by the electrical power source  110  in a buck regulation mode of the power electronics converter  120 . A ratio of the second electrical potential difference provided to the electrical network  130  by the power electronics converter  120  and the first electrical potential difference provided by the electrical power source  110  in the buck regulation mode is governed by a buck duty cycle of the power electronics converter  120 . 
     A magnitude of the buck duty cycle is associated with an efficiency of the power electronics converter  120  in the buck regulation mode. By way of example, if the power electronics converter  120  is configured to provide a second electrical potential difference to the electrical network  130  which is significantly lower than the first electrical potential difference provided by the electrical power source  110  in the buck regulation mode, the magnitude of the buck duty cycle of the power electronics converter  120  will increase. An increase in the magnitude of the buck duty cycle of the power electronics converter is associated with a decrease in the efficiency of the power electronics converter  120  in the buck regulation mode. 
     Consequently, in order to optimise the efficiency of the power electronics converter  120  (and therefore the efficiency of the electrical power system  100 ), the first electrical potential difference provided to the power electronics converter  120  by the electrical power source  110  should not be significantly greater than nor significantly lower than the electrical network  130  voltage demand of the electrical network  130 . 
     The electrical power source  110  may be configured to provide an electrical potential difference that is within 20% of the voltage demand so as to optimise the efficiency of the power electronics converter  110 . In other examples, the electrical power source is configured to provide an electrical potential difference which is within 10% or within 5% of the voltage demand. In examples, the electrical network  130  voltage demand may be 270 V, 540 V, 800V or 1080 V. 
     The electrical power source  110  may comprise, for example, a battery, a capacitor and/or an ultracapacitor. The electrical power source  110  may comprise a high-density battery. 
     In the example shown in  FIG.  1   , the first example electrical power system  100  comprises a plurality of additional components. In  FIG.  1   , the plurality of additional components includes a first DC capacitor  172 , a second DC capacitor  174 , a third DC capacitor  176  and an earth ground  178 . It will be appreciated that these components are shown and described for the purpose of illustration and are not to be construed as essential elements of the first electrical power system  100 . 
       FIG.  2    shows a second example electrical power system  200 . Many of the components of the second electrical power system  200  are similar to or identical to the components of the first electrical power system  100 , with like reference numerals indicating similar or identical components. 
     In the example shown in  FIG.  2   , the power electronics converter  120  comprises a DC to AC to DC converter. The DC to AC to DC converter comprises a plurality of transistors  121 - 128  and a transformer  129 . The transformer  129  provides magnetic coupling (i.e. inductive coupling) between the electrical power source  110  and the electrical network  130  while maintaining a Galvanic isolation therebetween. The Galvanic isolation between the electrical power source  110  and the electrical network  130  is associated with an improved safety of the electrical power system  200 . 
       FIG.  3    shows a third example electrical power system  300 . Many of the components of the second electrical power system  300  are similar to or identical to the components of the first electrical power system  100  and the second electrical power system  200 , with like reference numerals indicating similar or identical components. 
     In the example shown in  FIG.  3   , the electrical power source  110  is a first electrical power source, and the electrical network  130  connected between terminals  132  and  134  comprises a second electrical power source  136 . By way of example,  FIG.  3    shows the second electrical power source  136  as comprising an electrical generator  137  and a capacitor  138 . However, it will be appreciated than the second electrical power source may comprise only one of or neither of the electrical generator  137  and the capacitor  138 . 
       FIG.  4    shows a fourth example electrical power system  400 . Many of the components of the second electrical power system  400  are similar to or identical to the components of the first electrical power system  100 , the second electrical power system  200  and the third electrical power system  300 , with like reference numerals indicating similar or identical components. 
     In the example of  FIG.  4   , the current limiting diode  140  is a first current limiting diode. The example electrical power system  400  is also provided with a second current limiting diode  142 . The first and second current limiting diodes  140 ,  142  form a current limiting device  160 . Like the first current limiting diode  140 , the second current limiting diode  142  comprises a junction field-effect transistor (JFET) provided with a source, a drain and a gate, with the gate being shorted to the source and the drain being electrically connected to the source by an n-channel. Each current limiting diode may have any of the features described in relation to the current limiting diode  140  as shown in  FIGS.  1 - 3   . For example, each current limiting diode and/or the current limiting device  160  as a whole may be provided with a heat sink  148  as described with respect to the first electrical power system  100 . 
     The source of the first current limiting diode  140  and the source of the second current limiting diode  142  are electrically connected in series. As such, the first current limiting diode  140  and the second current limiting diode  142  are electrically connected so that the current limiting device  160  is bidirectional. Accordingly, the bidirectional current limiting device  160  is configured as a  2 -terminal device which permits an electric current to be conducted between the drain of the first current limiting diode  140  and the drain of the second current limiting diode  142  through the n-channel of each current limiting diode. 
     A relationship between an electrical resistance of the bidirectional current limiting device  160  and a magnitude of an electric current passing from the drain to the source of the first current limiting diode  140  is highly non-linear. Similarly, a relationship between the electrical resistance of the bidirectional current limiting device  160  and a magnitude of an electric current passing from the drain to the source of the second current limiting diode  142  is highly non-linear. 
     In other words, the bidirectional current limiting device  160  operates in a manner which is analogous to a highly non-linear bidirectional resistor in that the electrical resistance of the bidirectional current limiting device  160  increases rapidly as the magnitude of an electric current passing through the bidirectional current limiting device  160  in either direction increases above a saturation threshold. Accordingly, the bidirectional current limiting device  160  “saturates” at a near constant current level. 
     In one example, it may be that a fault originates from the electrical network  130  or the power electronics converter  120  and a fault current is therefore conducted from the drain to the source of the first current limiting diode  140 . If the fault current exceeds the saturation threshold of the first current limiting diode  140 , the first current limiting diode  140  almost instantaneously limits the magnitude of the fault current caused by the fault originating from the electrical network  130  or the power electronics converter  120  by providing a greatly increased resistance through the n-channel of the first current limiting diode  140 . 
     In another example, it may be that a fault originates from the electrical power source  110  and a fault current is therefore conducted from the source to the drain of the second current limiting diode  142 . If the fault current exceeds the saturation threshold of the second current limiting diode  142 , the second current limiting diode  142  almost instantaneously limits the magnitude of the fault current caused by the fault originating from the electrical power source  110  by providing a greatly increased resistance through the n-channel of the second current limiting diode  142 . 
     Consequently, the bidirectional current limiting device  160  is configured to limit the magnitude of a fault current caused by a fault originating from the electrical network  130  and/or to limit the magnitude of a fault current caused by a fault originating from the electrical power source  110 . Accordingly, the current limiting device  160  and the controllable circuit interruption device  150  are configured to co-operate in use so as to limit and then to interrupt a fault current caused by a fault originating from the electrical network  130  and/or a fault current caused by a fault originating from the electrical power source  110 . In other words, the bidirectional current limiting device  160  is able to limit a fault current caused by a fault originating from the electrical network  130  or the power electronics converter  120  or a fault current caused by a fault originating from the electrical power source  110  irrespective of an orientation of the bidirectional current limiting device  160  within the electrical power system  400 . Consequently, the bidirectional current limiting device  160  and the controllable circuit interruption device  150  provide the electrical power system  400  with a more versatile resettable protection system against a fault current passing between the electrical power source  110  and the electrical network  130  in the fault condition. 
     A further advantage of the arrangements described herein may be that circuit interruption devices  150  with relatively low current ratings may be selected for use. In the absence of the current limiting diodes/devices  140 ,  142 ,  160 , circuit interruption devices  150  are difficult to rate because of the high fault currents and high pulse currents drawn by some converters  120 . This difficulty in rating would invariably result in over-rating which may be associated with e.g., increased component mass and increased switching time. However, with the combination of a current limiting diode/device  140 ,  142 ,  160  and a controllable circuit interruption device  150 , a controllable circuit interruption device  150  with a relatively low current rating may be selected because the fault current level and pulse current level at which the circuit interruption device  150  must be operable is both reduced and subject to less uncertainty. 
     The current rating of the circuit interruption device  150  will be at least equal to the normal current rating of the electrical power source  110  and is preferably be less than five times the normal current rating of the electrical power source  110 . In one particular example the current rating of the circuit interruption device  150  is between 1.5 and 2.0 times the current rating of the electrical power source  110 . Where the electrical power source  110  is a battery, the normal current rating will be understood to refer to the current output by the battery to a connected load in the absence of a fault and at full battery charge. 
     In all examples described above, the channel that connects the source and drain of each current limiting diode is an n-channel. Those skilled in the art will appreciate that the channel could, in principle, instead be a p-channel, doped such that the majority carriers are ‘holes’ instead of electrons. The mobility of charge carriers in n-channel devices is typically significantly higher than those in p-channel devices, however, such that n-channel devices are generally preferred. 
       FIG.  5    shows an example aircraft power and propulsion system  500  comprising an electrical power system  100 . The electrical power system  100  may be in accordance with any of the example electrical power systems described with respect to  FIGS.  1 - 4   . 
       FIG.  6    shows an aircraft  600  comprising an electrical power system  100 . The electrical power system may be in accordance with any of the example electrical power systems described with respect to  FIGS.  1 - 4    or the aircraft power and propulsion system  500  of  FIG.  5   . 
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.