Patent Publication Number: US-2023134788-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. 2115515.5, 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. The present disclosure also relates to a method of operating an electrical power system and a controller for controlling a DC to AC to DC converter. 
     BACKGROUND 
     The use of an Energy Storage System (ESS) is becoming an important part of advanced electrical power systems for aerospace, marine and automotive applications. The ESS typically employs a bulk energy storage medium such as a high-density battery which is connected through a DC to AC to DC power electronic converter to a DC electrical network. The DC to AC to DC converter provides a regulation function and allows the voltage at the terminals of the battery to change as it discharges its stored energy whilst maintaining a near-constant direct voltage at the electrical network it is supplying. 
     The ESS is generally used intermittently to provide high power for short periods of time such as for engine starting, rotating generator load-levelling (e.g., supplying load peak demands only) or during emergency conditions such as loss of a rotating generator. 
     The ESS is characterised by its very high energy storage capacity and very low source impedance meaning faults which present an effective short-circuit across the battery terminals can lead to a very high rate of change of current and extremely high peak current typically reaching thousands of Amperes. 
     In the case of a low impedance fault within the DC network, the first line of defence is to turn off all transistors that can extinguish the fault current before it rises beyond the capabilities of the semiconductors. If this fails, the back-up protection is commonly provided by a fuse which eventually blows, but in most cases this leads to destruction of the semiconductors and other converter components. 
     It is therefore desirable to provide other means for mitigating faults in such an electrical power system. 
     SUMMARY 
     According to a first aspect there is provided an electrical power system comprising: a DC voltage source, a DC electrical network, a DC to AC to DC converter having a primary side connected to the DC voltage source and a secondary side connected to the DC electrical network, and a controller configured to control the DC to AC to DC converter, wherein:
     the controller is configured to: 
   monitor an electrical current or voltage between the DC voltage source and the DC electrical network;   determine, based on the monitored electrical current or voltage, whether the DC electrical network is in a fault condition; and   increase a switching frequency of the primary side of the DC to AC to DC converter in response to a positive determination that the DC electrical network is in a fault condition.   
   

     The DC to AC to DC converter may be a single-phase converter or a multiple-phase converter. The DC voltage source may comprise a battery, a fuel cell, a supercapacitor and/or a DC capacitor. 
     It may be that controller is further configured to modify (e.g., decrease) a duty cycle of the primary side of the DC to AC to DC converter in response to a positive determination that the DC electrical network is in the fault condition. 
     It may also be that the primary side of the DC to AC to DC converter comprises a plurality of transistors, and wherein each of the plurality of transistors comprises Silicon Carbide or Gallium Nitride. 
     Further, it may be that the controller is configured to increase the switching frequency of the primary side of the DC to AC to DC converter by a factor of at least 5 in response to a positive determination that the DC electrical network is in the fault condition. The controller may be configured to increase the switching frequency of the primary side of the DC to AC to DC converter by a factor of between 5 and 10 in response to a positive determination that the DC electrical network is in the fault condition. 
     The controller may be configured to increase the switching frequency of the primary side of the DC to AC to DC converter to at least 50 kHz in response to a positive determination that the DC electrical network is in the fault condition. 
     It may be that the DC to AC to DC converter comprises a transformer. The transformer may have a leakage reactance of greater than 0.1 per-unit. The leakage reactance may preferably be greater than 0.2 per-unit. The leakage reactance may be more preferably greater than 0.3 per-unit. 
     It may be that the DC to AC to DC converter comprises a transformer with a non-magnetic core. The transformer may be an air-cored transformer. The transformer may have a high per-unit reactance. 
     It may be that the electrical power system further comprises an external inductor connected in series to a primary winding of the primary side of the DC to AC to DC converter and/or a secondary winding of the secondary side of the DC to AC to DC converter. 
     According to a second aspect, there is provided an aircraft power and propulsion system comprising the electrical power system in accordance with 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. 
     According to a fourth aspect, there is provided a method of operating an electrical power system comprising a DC voltage source, a DC electrical network, and a DC to AC to DC converter having a primary side connected to the DC voltage source and a secondary side connected to the DC electrical network; the method comprising:
     monitoring an electrical current or voltage between the DC voltage source and the DC electrical network;   determining, based on the monitored electrical current or voltage, whether the DC electrical network is in a fault condition;   increasing a switching frequency of the primary side of the DC to AC to DC converter in response to a positive determination that the DC electrical network is in a fault condition.   

     The method may further comprise modifying (e.g. decreasing) a duty cycle of the primary side of the DC to AC to DC converter in response to a positive determination that the DC electrical network is in the fault condition. 
     It may be that the switching frequency of the primary side of the DC to AC to DC converter is increased by a factor of between 5 to 10 in response to a positive determination that the DC electrical network is in the fault condition. 
     It may also be that the switching frequency of the primary side of the DC to AC to DC converter is increased to at least 50 kHz in response to a positive determination that the DC electrical network is in the fault condition. 
     According to a fifth aspect, there is provided a controller for controlling a DC to AC to DC converter in an electrical power system, wherein the controller is configured to:
     monitor an electrical current or voltage between a DC voltage source and a DC electrical network of the electrical power system;   determine, based on the monitored electrical current or voltage, whether the DC electrical network is in a fault condition; and   increase a switching frequency of a primary side of the DC to AC to DC converter in response to a positive determination that the DC electrical network is in a fault condition.   

     It may be that the controller is further configured to modify (e.g., decrease) a duty cycle of the primary side of the DC to AC to DC converter in response to a positive determination that the DC electrical network is in the fault condition. 
     The controller may be configured to increase the switching frequency of the primary side of the DC to AC to DC converter by a factor of between 5 to 10 in response to a positive determination that the DC electrical network is in the fault condition. 
     The controller may be configured to increase the switching frequency of the primary side of the DC to AC to DC converter to at least 50 kHz in response to a positive determination that the DC electrical network is in the fault condition. 
     The controller of any of the above aspects may be implemented as a single controller or multiple separate (e.g., distributed) controllers. Thus, the controller may be or may form part of a control system. The controller may be implemented in software, hardware or a combination of the two. The controller may be or may be a functional module of an Engine Electronic Controller (EEC) or a Full Authority Digital Engine Controller (FADEC). 
     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 an example electrical power system; 
         FIG.  2    shows an example aircraft power and propulsion system comprising an example electrical power system; 
         FIG.  3    shows an aircraft comprising the example electrical power system of  FIG.  1   ; 
         FIG.  4    is a flowchart which shows an example method of operating an 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 a first example electrical power system  100  comprising a DC voltage source  110 , a DC electrical network  130 , a DC to AC to DC converter  120  and a controller  190 . The DC to AC to DC converter  120  has a primary side  122  connected to the DC voltage source  110  and a secondary side  124  connected to the DC electrical load network  130 . The controller  190  is configured to control the DC to AC to DC converter  120 . In the example of  FIG.  1   , the DC electrical network is represented by a load  130 , an internal impedance of the DC voltage source  110  is represented by a resistor  112  and an electrical energy delivery capacity of the DC voltage source is represented by an ideal voltage source  114 . The DC voltage source  110  may comprise, for example, a battery, a supercapacitor and/or a DC capacitor. 
     In the example shown in  FIG.  1   , the primary side  122  of the DC to AC to DC converter  120  comprises a plurality of primary transistors  123 . Likewise, the secondary side  124  of the DC to AC to DC converter comprises a plurality of secondary transistors  125 . Each of the transistors  124 ,  125  may be provided with an anti-parallel diode which may be a separate semiconductor or an internal body diode. The transistors  124 ,  125  may be operated in a “synchronous rectification” mode where the transistor conducts in its reverse direction to support the anti-parallel diode conduction. Alternatively, the transistors  124 ,  125  may only conduct in their forward directions, with the anti-parallel diodes handling conduction in their forward directions for half of each AC cycle. 
     In addition, the DC to AC to DC converter  120  comprises a transformer  129  connected between the primary side  122  and the secondary side  124 . The transformer  129  comprises a primary winding  126  connected to the primary side  122  and a secondary winding  127  connected to the secondary side  124 . It will be appreciated that each side of the DC to AC to DC converter  120  may comprise additional components, such as inductors or capacitors so as to provide a resonant/soft switching circuit. 
     In use, the primary side  122  receives an input DC voltage from the DC voltage source  110  and provides an input AC voltage to the transformer  129 . A frequency of the input AC voltage is dependent on a switching frequency of the primary side  122 . A root mean square of the input AC voltage is dependent on a duty cycle of the primary side  122 . Further, the transformer  129  receives the input AC voltage from the primary side  122  and provides an output AC voltage to the secondary side  124 . The secondary side  124  receives the output AC voltage from the transformer  129  and provides an output DC voltage to the DC electrical network  130 . A switching frequency of the secondary side  124  is dependent on a frequency of the output AC voltage. A duty cycle of the secondary side is dependent on a root mean square of the output AC voltage. The frequency of the output AC voltage is approximately equal to the frequency of the input AC voltage. While the DC to AC to DC converter  120  is shown as being a single-phase converter in the example of  FIG.  1   , it will be appreciated that in other examples the DC to AC to DC converter  120  may be a multiple-phase (e.g., 3-phase) converter. Other converter topologies, including multilevel topologies, may be used. 
     In a fault condition, a fault in the electrical power system  100  may lead to a magnitude of an electric current passing between the DC voltage source  110  and the DC electrical network  130  becoming extremely large in a very short period of time. For example, if the internal impedance  112  of the DC voltage source  110  is very low and the electrical energy delivery capacity  114  of the DC voltage source  110  is very high, a fault in the electrical power system  100  which originates in the DC electrical network  130  and which presents an effective short circuit across the secondary side  124  of the DC to AC to DC converter  120  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 DC voltage source  110 . 
     If the magnitude of the fault current were not limited, the magnitude of the fault current could rise beyond a tolerance limit of the power electronics converter  120  and/or a component of the DC 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 primary transistors  123  or at least one of the plurality of secondary transistors  125 . 
     The controller  190  is configured to monitor an electrical current passing between the DC voltage source  110  and the DC electrical network  130 . The controller  190  may monitor the electrical current being conducted through the electrical power system  100  between the DC voltage source  110  and the DC electrical network  130  using a current sensor. The current sensor may comprise, for example, a Hall effect sensor, a fibre optic current sensor and/or a fluxgate sensor. The controller  190  is also configured to determine whether the DC electrical network  130  is in a fault condition based on the monitored electrical current. The controller  190  is configured to make a positive determination to the effect that the DC electrical network is in the fault condition when the monitored electrical current is indicative of a presence of a fault current which is caused by a fault in the DC electrical network  130 . In other examples, the controller  190  may monitor the voltage at the DC electrical network using a suitable voltage sensor in order to identify a fault condition. For example, if the controller  190  identifies that the voltage has collapsed beyond a threshold value (e.g., 50%), then it may be determined that a fault is present. 
     As described above, the controller  190  is configured to control the DC to AC to DC converter  120 . In normal use, the controller  190  controls the switching frequency and the duty cycle of the primary side  122  and the secondary side  124 . The controller  190  controls the DC to AC to DC controller  120  by providing a plurality of pulse-width modulation (PWM) control signals to each of the primary side  122  and the secondary side  124 , each PWM control signal having a frequency and a duty cycle. 
     Each of the plurality of PWM control signals provided to the primary side  122  controls a switching frequency and a duty cycle of a respective transistor of the plurality of primary transistors  123 . Likewise, each of the plurality of PWM control signals provided to the secondary side  124  controls a switching frequency and a duty cycle of a respective transistor of the plurality of secondary transistors  125 . Accordingly, the controller  190  is configured to control the switching frequency and the duty cycle of the primary side  122  and the secondary side  124  by controlling the frequency and the duty cycle of each PWM control signal provided to the primary side  122  and the secondary side  124 , respectively. 
     The controller  190  is configured to increase the switching frequency of the primary side  122  of the DC to AC to DC converter  120  in response to a positive determination that the DC electrical network  130  is in the fault condition. The controller  190  increases the switching frequency of the primary side  122  of the DC to AC to DC converter  120  by increasing the frequency of each of the plurality of PWM control signals provided to the primary side  122 . However, when the DC electrical network  130  is in the fault condition, the plurality of secondary transistors  125  (or their anti-parallel diodes) are forced into conduction as a consequence of the effective short circuit across the secondary side  124  of the DC to AC to DC converter  120 . Accordingly, the controller  190  is not able to effectively control the operation of the secondary side  124  in the fault condition of the DC electrical network  130 . 
     In some examples, the controller  190  is further configured to modify the duty cycle of the primary side  122  of the DC to AC to DC converter  120  in response to a positive determination that the DC electrical network  130  is in the fault condition. The controller  190  modifying the duty cycle of the primary side  122  of the DC to AC to DC converter  120  by modifying the duty cycle of each of the plurality of PWM control signals provided to the primary side  122 . 
     An impedance of the transformer  129  is dependent on a resistance of the transformer  129  and a reactance of the transformer  129 . The reactance of the transformer  129  is dependent on, among other things, a reactance of the primary winding  126  of the transformer  129  and a reactance of the secondary winding  127  of the transformer  129 . In turn, the reactance of the primary winding  126  is related to a product of a self-inductance of the primary winding  126  and a frequency of the input AC voltage supplied to the transformer  129  by the primary side  122 . Consequently, as the frequency of the input AC voltage supplied to the transformer  129  increases, so does the impedance of the transformer  129 . The switching frequency of the primary side  122  of the DC to AC to DC converter  120  defines the frequency of the input AC voltage supplied to the transformer  129  by the primary side  122 . As a result, as the controller  190  increases the switching frequency of the primary side  122 , the reactance of the transformer  129  increases, which therefore brings about an increase in the reactance of the transformer  129 . 
     The magnitude of the electric current passing between the DC voltage source  110  and the DC electrical network  130  is moderated by the impedance of the transformer  129  in accordance with Ohm’s law. More specifically, a ratio of the AC input voltage to the impedance of the primary winding  126  dictates a magnitude of an electric current passing through the primary winding  126 , which in turn moderates the magnitude of the electric current passing between the DC voltage source  110  and the DC electrical network  130 . 
     In other words, if the impedance of the transformer  129  is increased, the magnitude of the electric current passing between the DC voltage source  110  and the DC electrical network  130  will decrease if the root mean square of the AC input voltage is held constant. Conversely, if the root mean square of the AC input voltage is decreased, the magnitude of the electric current passing between the DC voltage source  110  and the DC electrical network  130  will decrease if the impedance of the transformer  129  is held constant. If the impedance of the transformer  129  is increased and the root mean square of the AC input voltage is decreased, the magnitude of the electric current passing between the DC voltage source and the electrical network  130  will decrease further. 
     As a result, the increase in switching frequency of the primary side  122  of the DC to AC to DC converter  120  effected by the controller  190  in response to a determination that the DC electrical network  130  is in a fault condition has the effect of limiting the magnitude of a fault current without requiring the interruption of the fault current. In examples in which the controller is further configured to decrease the duty cycle of the primary side  122  in response to a positive determination that the DC electrical network is in a fault condition, the effect of limiting the magnitude of the fault current is further enhanced. Alternatively, if the increase in the switching frequency provides sufficient limitation of the fault current, the controller may not modify the duty cycle or may use the modification of the duty cycle to provide finer control of the current (e.g., by selectively decreasing or increasing the duty cycle according to requirements). 
     Accordingly, the configuration of the electrical power system  100  allows the DC voltage source  110  to be used in the fault condition to inject a limited current to a fault site within the DC electrical network  130 . This may enable a protection mechanism within the DC electrical network  130  to be safely triggered. In particular, the configuration of the electrical power system  100  enables a DC voltage source  110  having a very low internal impedance  112  and a very high electrical energy delivery capacity  114  to be safely used to provide the limited current to the fault site. In addition, the electrical power system  100  is provided with means for limiting a fault current without a need to provide additional hardware components. 
     If the magnitude of the fault current is extremely large, the controller  190  may be required to increase the switching frequency of the primary side  122  significantly in order to cause a sufficient increase in the impedance of the transformer  129 , which in turn is able to adequately limit the fault current. However, a maximum switching frequency of the primary side  122  is limited by a maximum switching speed of each of the primary plurality of transistors  123 . 
     In additional examples, each of the plurality of primary transistors  123  comprises an advanced semiconductor material such as Silicon Carbide (SiC) or Gallium Nitride (GaN). The maximum switching speed of a transistor comprising a Silicon Carbide or a Gallium Nitride based semiconductor material may be significantly higher than conventional semiconductor materials. Consequently, in such additional examples, the maximum switching frequency of the primary side  122  may be significantly increased. It follows that the controller  190  is able to increase the switching frequency of the primary side  122  more significantly, and therefore is able to more effectively limit a fault current having an extremely large magnitude in the fault condition of the DC electrical network. 
     Particularly with such advanced semiconductor materials, the controller  190  is able to significantly increase the switching frequency of the primary side  122  of the DC to AC to DC converter  120  by a factor of 5-10. For example, where the frequency of the input AC voltage is typically 10 kHz during normal use, this may be increased to 50-100 kHz during the fault condition. This provides a marked increase in the impedance of the transformer  129  which in turn provides more effective limitation of the fault current in the fault condition  120 . 
     As described above, the reactance of the primary winding  126  is related to a product of a self-inductance of the primary winding  126  and a frequency of the input AC voltage supplied to the transformer  129  by the primary side  122 . The self-inductance of the primary winding  126  is a result of an inherent leakage inductance of the transformer  129 . The inherent leakage inductance is a consequence of imperfect magnetic coupling within the transformer  129 , which arises as a consequence of an internal geometry of the transformer  129 . In further examples of the electrical power system  100 , the internal geometry of the transformer  129  may be specified so as to increase the inherent leakage inductance of the transformer  129 . Accordingly, the increase in the frequency of the input AC voltage supplied to the transformer  129  by the primary side  122  in the fault condition leads to a larger increase in the reactance of the primary winding  126 . Therefore, in such examples, the ability of the controller  190  to limit the fault current in the fault condition of the DC electrical network  130  is further increased. 
     It is usual to select a transformer with a low leakage reactance, for example a leakage reactance of the order 0.1 per unit (such that, for 1.0 per unit current, about 10% of the voltage across the transformer is lost) or even lower. However, as explained above, it may be desirable to select a transformer with a higher-than-normal leakage reactance to enhance the fault current limiting effect. For example, a transformer with a leakage reactance of above 0.1 per unit, above 0.2 per unit or even above 0.3 per unit may be selected. This increase can be achieved through suitable selection of transformer geometry, as will be understood by those skilled in the art. Additionally or alternatively, a transformer with a non-magnetic core (e.g., an air-cored transformer) may be used to achieve a higher per unit leakage reactance. 
     In the example of  FIG.  1   , a first external inductor  128  is connected in series with the primary winding  126  of the transformer  129  and a second external inductor  128 ′ is connected in series with the secondary winding  127  of the transformer  129 . It will be appreciated that in other examples of the electrical power system  100 , only the first external inductor  128  or the second external inductor  128 ′ are provided, or it may be that neither the first external inductor  128  nor the second external inductor  128 ′ are provided. The provision of the first external inductor  128  and/or the second external inductor  128 ′ increases an effective inductance of the transformer  129  as part of the DC to AC to DC converter  120 , and thereby increases the impedance of the transformer  129  independently of the frequency of the input AC voltage supplied to the transformer  129 . Thus, an increase in the frequency of the input AC voltage supplied to the transformer  129  by the primary side  122  in the fault condition leads to an even larger increase in the impedance of the transformer  129 . Therefore, the inclusion of the first external inductor  128  and/or the second external inductor  128 ′ may provide even more effective limitation of the fault current in the fault condition of the DC electrical network  130 . 
       FIG.  2    shows an example aircraft power and propulsion system  200  comprising an electrical power system  100 . The electrical power system  100  may be in accordance with any of examples of the electrical power system  100  described above and/or with respect to  FIG.  1   . 
       FIG.  3    shows an aircraft  300  comprising an electrical power system  100 . The electrical power system  100  may be in accordance with any of examples of the electrical power system  100  described above and/or with respect to  FIG.  1    or the aircraft power and propulsion system  200  of  FIG.  2   . 
       FIG.  4    shows a flowchart of a method  400  of operating an electrical power system. The electrical power system may be in accordance with any of examples of the electrical power system  100  described above and/or with respect to  FIG.  1   . The electrical power system therefore comprises a DC voltage source, a DC electrical network, and a DC to AC to DC converter having a primary side connected to the DC voltage source and a secondary side connected to the DC electrical network. 
     The method  400  begins at block  402 , which includes monitoring an electrical current passing between the DC voltage source and the DC electrical network. The electrical current may be monitored, for instance, using a controller operatively connected to a current sensor. The sensor may comprise, for example, a Hall effect sensor, a fibre optic current sensor and/or a fluxgate sensor. 
     The method  400  proceeds to block  404 , which comprises determining, based on the monitored electrical current, whether the DC electrical network is in a fault condition. In block  404 , a positive determination to the effect that the DC electrical network is in the fault condition is made when the monitored electrical current is indicative of a presence of a fault current which is caused by a fault in the DC electrical network. The determination may be made using a controller or a processor. If no positive determination to the effect that the DC electrical network is in the fault condition is made in block  404 , the following method blocks are not executed. 
     The method  400  further includes block  406 , comprising increasing a switching frequency of the primary side of the DC to AC to DC converter in response to a positive determination that the DC electrical network is in a fault condition. In some examples, the primary side of the DC to AC to DC converter comprises a plurality of transistors, and wherein at least one of the plurality of transistors comprises Silicon Carbide or Gallium Nitride. In which case, it may be that block  406  comprises increasing the switching frequency of the primary side of the DC to AC to DC converter so as to produce an input AC voltage on the primary side of the DC to AC to DC converter having a frequency of at least 50 kHz in response to a positive determination that the DC electrical network is in the fault condition. 
     Optionally, the method further includes block  408 . Block  408  comprises modifying (e.g. decreasing) a duty cycle of the primary side of the DC to AC to DC converter in response to a positive determination that the DC electrical network is in the fault condition. 
     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.