Patent Description:
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.

United States Patent Application Publication <CIT> relates to a resonant converter system with over-current protection. A resonant converter receives an input voltage to generate an output voltage, and a buck converter provides the input voltage of the resonant converter and controls the input voltage to perform an over-current protection process.

"Bidirectional, Dual Active Bridge Reference Design for Level <NUM> Vehicle Charging Stations" by Texas Instruments provides an overview on the implementation of a single-phase Dual Active Bridge (DAB) DC/DC converter.

According to a first aspect there is provided an electrical power system comprising:.

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 transformer has a non-magnetic core and a leakage reactance of greater than <NUM> per unit.

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 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 <NUM> 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 <NUM> and <NUM> 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 <NUM> in response to a positive determination that the DC electrical network is in the fault condition.

The transformer may have a leakage reactance of greater than <NUM> per-unit.

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 transformer, a primary side comprising a plurality of transistors and connected between the DC voltage source and a primary winding of the transformer, and a secondary side comprising a plurality of transistors and connected between the DC electrical network and a secondary winding of the transformer; 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; and 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 transformer has a non-magnetic core and a leakage reactance of greater than <NUM> per unit.

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 <NUM> to <NUM> 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 <NUM> 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).

<FIG> shows a first example electrical power system <NUM> comprising a DC voltage source <NUM>, a DC electrical network <NUM>, a DC to AC to DC converter <NUM> and a controller <NUM>. The DC to AC to DC converter <NUM> has a primary side <NUM> connected to the DC voltage source <NUM> and a secondary side <NUM> connected to the DC electrical load network <NUM>. The controller <NUM> is configured to control the DC to AC to DC converter <NUM>. In the example of <FIG>, the DC electrical network is represented by a load <NUM>, an internal impedance of the DC voltage source <NUM> is represented by a resistor <NUM> and an electrical energy delivery capacity of the DC voltage source is represented by an ideal voltage source <NUM>. The DC voltage source <NUM> may comprise, for example, a battery, a supercapacitor and/or a DC capacitor.

In the example shown in <FIG>, the primary side <NUM> of the DC to AC to DC converter <NUM> comprises a plurality of primary transistors <NUM>. Likewise, the secondary side <NUM> of the DC to AC to DC converter comprises a plurality of secondary transistors <NUM>. Each of the transistors <NUM>, <NUM> may be provided with an anti-parallel diode which may be a separate semiconductor or an internal body diode. The transistors <NUM>, <NUM> 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 <NUM>, <NUM> 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 <NUM> comprises a transformer <NUM> connected between the primary side <NUM> and the secondary side <NUM>. The transformer <NUM> comprises a primary winding <NUM> connected to the primary side <NUM> and a secondary winding <NUM> connected to the secondary side <NUM>. It will be appreciated that each side of the DC to AC to DC converter <NUM> may comprise additional components, such as inductors or capacitors so as to provide a resonant/soft switching circuit.

In use, the primary side <NUM> receives an input DC voltage from the DC voltage source <NUM> and provides an input AC voltage to the transformer <NUM>. A frequency of the input AC voltage is dependent on a switching frequency of the primary side <NUM>. A root mean square of the input AC voltage is dependent on a duty cycle of the primary side <NUM>. Further, the transformer <NUM> receives the input AC voltage from the primary side <NUM> and provides an output AC voltage to the secondary side <NUM>. The secondary side <NUM> receives the output AC voltage from the transformer <NUM> and provides an output DC voltage to the DC electrical network <NUM>. A switching frequency of the secondary side <NUM> 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 <NUM> is shown as being a single-phase converter in the example of <FIG>, it will be appreciated that in other examples the DC to AC to DC converter <NUM> may be a multiple-phase (e.g., <NUM>-phase) converter. Other converter topologies, including multilevel topologies, may be used.

In a fault condition, a fault in the electrical power system <NUM> may lead to a magnitude of an electric current passing between the DC voltage source <NUM> and the DC electrical network <NUM> becoming extremely large in a very short period of time. For example, if the internal impedance <NUM> of the DC voltage source <NUM> is very low and the electrical energy delivery capacity <NUM> of the DC voltage source <NUM> is very high, a fault in the electrical power system <NUM> which originates in the DC electrical network <NUM> and which presents an effective short circuit across the secondary side <NUM> of the DC to AC to DC converter <NUM> may cause a magnitude of a fault current to be conducted through the electrical power system <NUM> 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 <NUM> of the DC voltage source <NUM>.

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 <NUM> and/or a component of the DC electrical network <NUM>. The tolerance limit of the power electronics converter <NUM> may be associated with a switching capacity of at least one of the plurality of primary transistors <NUM> or at least one of the plurality of secondary transistors <NUM>.

The controller <NUM> is configured to monitor an electrical current passing between the DC voltage source <NUM> and the DC electrical network <NUM>. The controller <NUM> may monitor the electrical current being conducted through the electrical power system <NUM> between the DC voltage source <NUM> and the DC electrical network <NUM> 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 <NUM> is also configured to determine whether the DC electrical network <NUM> is in a fault condition based on the monitored electrical current. The controller <NUM> 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 <NUM>. In other examples, the controller <NUM> 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 <NUM> identifies that the voltage has collapsed beyond a threshold value (e.g., <NUM>%), then it may be determined that a fault is present.

As described above, the controller <NUM> is configured to control the DC to AC to DC converter <NUM>. In normal use, the controller <NUM> controls the switching frequency and the duty cycle of the primary side <NUM> and the secondary side <NUM>. The controller <NUM> controls the DC to AC to DC controller <NUM> by providing a plurality of pulse-width modulation (PWM) control signals to each of the primary side <NUM> and the secondary side <NUM>, each PWM control signal having a frequency and a duty cycle.

Each of the plurality of PWM control signals provided to the primary side <NUM> controls a switching frequency and a duty cycle of a respective transistor of the plurality of primary transistors <NUM>. Likewise, each of the plurality of PWM control signals provided to the secondary side <NUM> controls a switching frequency and a duty cycle of a respective transistor of the plurality of secondary transistors <NUM>. Accordingly, the controller <NUM> is configured to control the switching frequency and the duty cycle of the primary side <NUM> and the secondary side <NUM> by controlling the frequency and the duty cycle of each PWM control signal provided to the primary side <NUM> and the secondary side <NUM>, respectively.

The controller <NUM> is configured to increase the switching frequency of the primary side <NUM> of the DC to AC to DC converter <NUM> in response to a positive determination that the DC electrical network <NUM> is in the fault condition. The controller <NUM> increases the switching frequency of the primary side <NUM> of the DC to AC to DC converter <NUM> by increasing the frequency of each of the plurality of PWM control signals provided to the primary side <NUM>. However, when the DC electrical network <NUM> is in the fault condition, the plurality of secondary transistors <NUM> (or their anti-parallel diodes) are forced into conduction as a consequence of the effective short circuit across the secondary side <NUM> of the DC to AC to DC converter <NUM>. Accordingly, the controller <NUM> is not able to effectively control the operation of the secondary side <NUM> in the fault condition of the DC electrical network <NUM>.

In some examples, the controller <NUM> is further configured to modify the duty cycle of the primary side <NUM> of the DC to AC to DC converter <NUM> in response to a positive determination that the DC electrical network <NUM> is in the fault condition. The controller <NUM> modifying the duty cycle of the primary side <NUM> of the DC to AC to DC converter <NUM> by modifying the duty cycle of each of the plurality of PWM control signals provided to the primary side <NUM>.

An impedance of the transformer <NUM> is dependent on a resistance of the transformer <NUM> and a reactance of the transformer <NUM>. The reactance of the transformer <NUM> is dependent on, among other things, a reactance of the primary winding <NUM> of the transformer <NUM> and a reactance of the secondary winding <NUM> of the transformer <NUM>. In turn, the reactance of the primary winding <NUM> is related to a product of a self-inductance of the primary winding <NUM> and a frequency of the input AC voltage supplied to the transformer <NUM> by the primary side <NUM>. Consequently, as the frequency of the input AC voltage supplied to the transformer <NUM> increases, so does the impedance of the transformer <NUM>. The switching frequency of the primary side <NUM> of the DC to AC to DC converter <NUM> defines the frequency of the input AC voltage supplied to the transformer <NUM> by the primary side <NUM>. As a result, as the controller <NUM> increases the switching frequency of the primary side <NUM>, the reactance of the transformer <NUM> increases, which therefore brings about an increase in the reactance of the transformer <NUM>.

The magnitude of the electric current passing between the DC voltage source <NUM> and the DC electrical network <NUM> is moderated by the impedance of the transformer <NUM> in accordance with Ohm's law. More specifically, a ratio of the AC input voltage to the impedance of the primary winding <NUM> dictates a magnitude of an electric current passing through the primary winding <NUM>, which in turn moderates the magnitude of the electric current passing between the DC voltage source <NUM> and the DC electrical network <NUM>.

In other words, if the impedance of the transformer <NUM> is increased, the magnitude of the electric current passing between the DC voltage source <NUM> and the DC electrical network <NUM> 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 <NUM> and the DC electrical network <NUM> will decrease if the impedance of the transformer <NUM> is held constant. If the impedance of the transformer <NUM> 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 <NUM> will decrease further.

As a result, the increase in switching frequency of the primary side <NUM> of the DC to AC to DC converter <NUM> effected by the controller <NUM> in response to a determination that the DC electrical network <NUM> 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 <NUM> 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 <NUM> allows the DC voltage source <NUM> to be used in the fault condition to inject a limited current to a fault site within the DC electrical network <NUM>. This may enable a protection mechanism within the DC electrical network <NUM> to be safely triggered. In particular, the configuration of the electrical power system <NUM> enables a DC voltage source <NUM> having a very low internal impedance <NUM> and a very high electrical energy delivery capacity <NUM> to be safely used to provide the limited current to the fault site. In addition, the electrical power system <NUM> 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 <NUM> may be required to increase the switching frequency of the primary side <NUM> significantly in order to cause a sufficient increase in the impedance of the transformer <NUM>, which in turn is able to adequately limit the fault current. However, a maximum switching frequency of the primary side <NUM> is limited by a maximum switching speed of each of the primary plurality of transistors <NUM>.

In additional examples, each of the plurality of primary transistors <NUM> 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 <NUM> may be significantly increased. It follows that the controller <NUM> is able to increase the switching frequency of the primary side <NUM> 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 <NUM> is able to significantly increase the switching frequency of the primary side <NUM> of the DC to AC to DC converter <NUM> by a factor of <NUM>-<NUM>. For example, where the frequency of the input AC voltage is typically <NUM> during normal use, this may be increased to <NUM>-<NUM> during the fault condition. This provides a marked increase in the impedance of the transformer <NUM> which in turn provides more effective limitation of the fault current in the fault condition <NUM>.

As described above, the reactance of the primary winding <NUM> is related to a product of a self-inductance of the primary winding <NUM> and a frequency of the input AC voltage supplied to the transformer <NUM> by the primary side <NUM>. The self-inductance of the primary winding <NUM> is a result of an inherent leakage inductance of the transformer <NUM>. The inherent leakage inductance is a consequence of imperfect magnetic coupling within the transformer <NUM>, which arises as a consequence of an internal geometry of the transformer <NUM>. In further examples of the electrical power system <NUM>, the internal geometry of the transformer <NUM> may be specified so as to increase the inherent leakage inductance of the transformer <NUM>. Accordingly, the increase in the frequency of the input AC voltage supplied to the transformer <NUM> by the primary side <NUM> in the fault condition leads to a larger increase in the reactance of the primary winding <NUM>. Therefore, in such examples, the ability of the controller <NUM> to limit the fault current in the fault condition of the DC electrical network <NUM> is further increased.

It is usual to select a transformer with a low leakage reactance, for example a leakage reactance of the order <NUM> per unit (such that, for <NUM> per unit current, about <NUM>% 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 <NUM> per unit, above <NUM> per unit or even above <NUM> 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>, a first external inductor <NUM> is connected in series with the primary winding <NUM> of the transformer <NUM> and a second external inductor <NUM>' is connected in series with the secondary winding <NUM> of the transformer <NUM>. It will be appreciated that in other examples of the electrical power system <NUM>, only the first external inductor <NUM> or the second external inductor <NUM>' are provided, or it may be that neither the first external inductor <NUM> nor the second external inductor <NUM>' are provided. The provision of the first external inductor <NUM> and/or the second external inductor <NUM>' increases an effective inductance of the transformer <NUM> as part of the DC to AC to DC converter <NUM>, and thereby increases the impedance of the transformer <NUM> independently of the frequency of the input AC voltage supplied to the transformer <NUM>. Thus, an increase in the frequency of the input AC voltage supplied to the transformer <NUM> by the primary side <NUM> in the fault condition leads to an even larger increase in the impedance of the transformer <NUM>. Therefore, the inclusion of the first external inductor <NUM> and/or the second external inductor <NUM>' may provide even more effective limitation of the fault current in the fault condition of the DC electrical network <NUM>.

<FIG> shows an example aircraft power and propulsion system <NUM> comprising an electrical power system <NUM>. The electrical power system <NUM> may be in accordance with any of examples of the electrical power system <NUM> described above and/or with respect to <FIG>.

<FIG> shows an aircraft <NUM> comprising an electrical power system <NUM>. The electrical power system <NUM> may be in accordance with any of examples of the electrical power system <NUM> described above and/or with respect to <FIG> or the aircraft power and propulsion system <NUM> of <FIG>.

<FIG> shows a flowchart of a method <NUM> of operating an electrical power system. The electrical power system may be in accordance with any of examples of the electrical power system <NUM> described above and/or with respect to <FIG>. 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 <NUM> begins at block <NUM>, 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 <NUM> proceeds to block <NUM>, which comprises determining, based on the monitored electrical current, whether the DC electrical network is in a fault condition. In block <NUM>, 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 <NUM>, the following method blocks are not executed.

The method <NUM> further includes block <NUM>, 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 <NUM> 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 <NUM> in response to a positive determination that the DC electrical network is in the fault condition.

Claim 1:
An electrical power system (<NUM>) comprising:
a DC voltage source (<NUM>),
a DC electrical network (<NUM>),
a DC to AC to DC converter (<NUM>) having a transformer (<NUM>), a primary side (<NUM>) comprising a plurality of transistors (<NUM>) connected between the DC voltage source (<NUM>) and a primary winding (<NUM>) of the transformer (<NUM>), and a secondary side (<NUM>) comprising a plurality of transistors (<NUM>) connected between the DC electrical network (<NUM>) and a secondary winding (<NUM>) of the transformer (<NUM>), and
a controller (<NUM>) configured to control the DC to AC to DC converter (<NUM>),
wherein the controller (<NUM>) is configured to:
monitor an electrical current or voltage between the DC voltage source (<NUM>) and the DC electrical network (<NUM>);
determine, based on the monitored electrical current or voltage, whether the DC electrical network (<NUM>) is in a fault condition; and
increase a switching frequency of the primary side (<NUM>) of the DC to AC to DC converter (<NUM>) in response to a positive determination that the DC electrical network (<NUM>) is in a fault condition,
characterized in that the transformer (<NUM>) has a non-magnetic core and a leakage reactance of greater than <NUM> per unit.