This specification is based upon and claims the benefit of priority from United Kingdom Patent Application No. 2315701.9, filed on 13 Oct. 2023, the entire contents of which are incorporated herein by reference.
This disclosure relates to the control of DC:DC power converters in electrical power systems, particularly in response to faults.
A DC:DC power electronics converter may be used to interface between two DC parts of an electrical power system. For example, an electrical power system may have two DC electrical networks with different operating voltages, and a DC:DC converter may provide an interface between the two networks so that power can be exchanged between them. In another example, an electrical power system may include a DC electrical network that is supplied with power by an energy storage system (e.g., a battery). The terminal voltage of a battery typically decreases with its state of charge, so a DC:DC converter may be provided between the terminals of the energy storage system and the DC electrical network to stabilise the voltage supplied to the DC electrical network as the battery discharges.
In the event of a fault in a DC network connected to one side of a DC: DC converter, a zero or near-zero impedance is presented across the DC terminals facing the fault. This very low impedance may result in a very large current—referred to herein as the fault current-being fed from the healthy side of the converter to fault site. This may be particularly concerning where the healthy side of the converter interfaces with an energy storage system, which may store a very large amount of energy and have low internal resistance, resulting in a particularly high fault current capable of damaging the faulted DC network.
A know response to such a fault is illustrated in
According to a first aspect, there is an electrical power system, comprising: a DC:DC power electronics converter comprising a DC:AC converter circuit having a DC and an AC side; an AC:DC converter circuit having a DC side and an AC side; and an AC link connecting the AC side of the DC:AC circuit and the AC side of the AC:DC converter circuit, the AC link including a transformer having a first winding connected to the AC side of the DC:AC converter circuit and a second winding connected to the AC side of the AC: DC converter;
Each of the DC:AC converter circuit and the AC:DC converter circuit can take any suitable form. In an embodiment, each of the DC:AC converter circuit and the AC:DC converter circuit is an H-bridge circuit. In another embodiment, each of the DC:AC converter circuit and the AC:DC converter circuit is a 3-phase, 2-level converter circuit.
In an embodiment, modifying the switching operation of the first and/or second plurality of transistors comprises: modifying the switching operation of the first plurality of transistors so that a waveform of a voltage applied to the first winding of the transformer changes from a square wave to a quasi-square wave. In an embodiment, changing the square wave to a quasi-square wave comprises adding zero-voltage notches or portions between positive and negative voltage portions of the signal.
In an embodiment, modifying the switching operation of the first and/or second plurality of transistors comprises: modifying the switching operation of the first plurality of transistors so that a duty cycle of a waveform of a voltage applied to the first winding of the transformer changes. In an embodiment, the control system increases the duty cycle of the waveform, by increasing a fraction of each cycle during which the voltage is non-zero.
In an embodiment, modifying the switching operation of the first and/or second plurality of transistors comprises: alternately switching the second plurality of transistors between a fault feeding configuration and a crowbar configuration. In the fault feeding configuration, current is supplied to the DC electrical network through the AC:DC converter circuit. In the crowbar configuration, current is not supplied to the DC electrical network and is contained within the AC:DC converter circuit.
In an embodiment, in the fault-feeding configuration, a low-side transistor of a first half-bridge bridge of the AC:DC converter circuit and a high-side transistor of a second half-bridge of the AC:DC converter circuit are switched on while a high-side transistor of the first half-bridge and a low-side transistor of the second half-bridge are switched off. In the crowbar mode, only low-side transistors or only high-side transistors of the AC:DC converter circuit are switched on.
In an embodiment, the control system is further configured to control a fraction of a time period during which the second plurality of transistors are in the fault-feeding configuration to control the amount of current supplied from the DC power source to the DC electrical network.
In an embodiment, the control system is configured to control the fraction of the time period during which the second plurality of transistors are in the fault-feeding configuration to avoid a sum of an output impedance, Zout, of the DC:AC converter circuit and an impedance of the fault, Zfault, equaling zero.
In an embodiment, the control system is further configured to:
In an embodiment, the AC link further comprises a capacitor connected in series between the AC side of the DC:AC converter circuit and the first winding of the transformer.
In an embodiment, the DC:DC converter further comprises a switch arrangement having a first state and a second state, and the control system is further configured to control the state of the switch arrangement. In the first state, the capacitor is connected in series between the AC side of the DC:AC converter circuit and the first winding of the transformer. In the second state, there is a current path between the AC side of the DC:AC converter circuit and the first winding of the transformer that does not include the capacitor.
In an embodiment, the control system is configured to control the state of the switch arrangement based on an operating condition of the DC electrical network. In an embodiment, the control system is configured to change the state of the switch arrangement in response to determining there has been a change in the operating condition of the DC electrical network.
In an embodiment, the switch arrangement is normally in the first state and the control system is configured to switch the switch arrangement into the second state in response to a determination that there is a fault in the DC electrical network. The expression “normally” is used here to refer to operation of the system as intended, without a fault in the DC network. Therefore, the DC: DC converter may normally be operated as a resonant LLC DC:DC converter and, if a fault in the DC network is detected, be reconfigured to operate as, e.g., a DAB DC:DC converter.
In an embodiment, the AC link further comprises a second capacitor connected in series between the AC side of the AC:DC converter circuit and the second winding of the transformer.
In an embodiment, the DC:DC converter further comprises a second switch arrangement having a first state and a second state, and the control system is further configured to control the state of the second switch arrangement. In the first state, the capacitor is connected in series between the AC side of the AC:DC converter circuit and the second winding of the transformer. In the second state, there is a current path between the AC side of the AC:DC converter circuit and the second winding of the transformer that does not include the capacitor.
In an embodiment, the first winding of the transformer has a first number of turns and the second winding of the transformer has a second number of turns different from the first number of turns.
In an embodiment, a turns ratio, defined as the ratio of the first and second numbers of turns, may be equal to a desired ratio of the voltages at the two DC sides of the DC:DC converter. In one specific example, the turns ratio between the first and second windings is about two.
In an embodiment, the DC power source is or comprises an energy storage system. In an embodiment, the DC power source is another DC electrical network. Where the DC power source is a further DC electrical network, the further DC electrical network may interface with an energy storage system.
The control system can take any suitable form. For example, the control system may be a single controller, or multiple distributed controllers. It may be implemented in hardware and/or software.
The electrical power system may be or form part of an aircraft power and propulsion system. The power and propulsion system may be a purely electric power and propulsion system, a hybrid propulsion (e.g., gas turbine and battery/fuel cell hybrid, or battery and fuel cell hybrid system), or a ‘more electric’ propulsion system having propulsive gas turbine engines that interface with an electrical power system through spool-coupled electrical machines.
According to a second aspect, there is an aircraft comprising the electrical power system of the first aspect.
According to a third aspect, there is a method of operating an electrical power system according to the first aspect. The method comprises: monitoring one or more operating parameters of the electrical power system; determining, based on the one or more operating parameters, whether there is a fault in the DC electrical network; and modifying a switching operation of a first plurality of transistors of the DC:AC converter circuit and/or a second plurality of transistors of the AC:DC converter circuit to supply a controlled amount of current from the DC power source to the DC electrical network.
In an embodiment, modifying the switching operation of the first plurality and/or the second plurality of transistors comprise one or more of:
In an embodiment, the supply of the controlled amount of current from the DC power source to the DC electrical network may charge one or more capacitors of the DC electrical network. In an embodiment, the one or more capacitors comprises a DC link capacitor of the AC:DC converter circuit.
In an embodiment, the method further comprises: while supplying the controlled amount of current from the DC power source to the DC electrical network via the DC:DC power electronics converter, isolating the fault in the DC electrical network by operating one or more protection devices.
In an embodiment, the method further comprises: after isolating the fault in the DC network and before supplying the controlled amount of current to charge the one or more capacitors, switching off each of the first plurality of transistors to block current flow from the DC power supply to the DC electrical network.
The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive, any feature described herein may be applied to any aspect and/or combined with any other feature described herein.
Embodiments will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which:
Each of the DC:AC converter circuit 110 and the AC:DC converter circuit 120 can take any form suitable for the desired system functionality, and for this reason no circuit structure is shown in
The converter circuits 110, 120 could, however, take other forms, for example:
The normal operation of a DAB DC:DC converter 100 under the control of a control system 150 is illustrated in
A DAB DC:DC converter 100 has several benefits over some other known types of DC:DC converter (e.g., buck-boost converters), including:
A DAB DC:DC converter 100 also has inherent DC fault blocking capability, illustrated in
While the approach shown in
The present disclosure provides ways of supplying a controlled amount of current to a DC electrical network following a fault, e.g., a short circuit fault. In each case, a control system responds to a fault by modifying a switching operation of a first plurality of transistors 111-L, 111-H, 112-L, 112-H of the DC:AC converter circuit 110 and/or a second plurality of transistors 121-L, 121-H, 122-L, 112-H of the AC:DC converter circuit 120.
When the fault occurs, the voltage V2 collapses to zero or near-zero. The transformer 135 now supports the whole of the alternating voltage V1 produced by the healthy converter circuit 110. If the transistors 121-L, 121-H, 122-L, 122- of the AC:DC converter circuit 120 continue to be switched to produce a 50:50 duty cycle square wave voltage waveform, a fully rectified version of the transformer current is transferred to the faulted network. In other words, the amount of current supplied to the faulted DC network will depend on the AC link current. It is noted that a similar transfer of current can occur if its transistors are turned off, leaving the anti-parallel diodes to conduct instead.
According to the first embodiment, to control the AC link current, the control system 150 responds to the fault by modifying the switching signals supplied to the transistors 111-L, 111-H, 112-L, 112-H so that the voltage V1 applied to the first side of the transformer 135 is a quasi-square wave. Those skilled in the art will appreciate this to mean that the voltage waveform V1 has zero-voltage notches, or portions, between positive and negative voltage portions of waveform, reducing the volt-time area applied to the transformer leakage inductance. In the case of an H-bridge circuit 110, this may be achieved by switching on the two upper transistors or the two lower transistors of the H-bridge 110. By changing the converter switching pattern in this way, the magnitude of the AC link current, and consequently the amount of fault current supplied to the DC network, can be controlled.
It can be seen from
In the third embodiment, the control system 150 responds to the fault by alternately switching the converter circuit facing the fault, i.e., the AC: DC converter circuit 120, between a fault-feeding configuration and a crowbar configuration. In the fault-feeding configuration, the AC:DC converter circuit 120 rectifies the AC link current and transfers it to the faulted DC network. In the crowbar configuration, current from the AC link is contained within (e.g., circulates within) the DC:AC converter circuit 120 so that no current is fed to the faulted DC network. Consequently, the time-averaged amount of current supplied to the faulted DC network is controlled, with the amount depending on the fraction of time spent in the crowbar configuration.
While a DAB DC:DC converter 100 with H-bridge circuits is illustrated other converter circuits (e.g., two-level, three-phase circuits) could be used instead. For example, in the case of a two-level, three-phase converter, the fault-feeding configuration would have either two high-side and one low-side transistor switched on or one high-side and two low-side transistors switched on. The crowbar configuration would have only high-side or only low-side transistors switched on.
The third embodiment may be used in isolation or in combination with the first and/or second embodiment. In other words, the AC link current may not be controlled, with all control of the DC network fault current being provided by alternate switching of the AC:DC converter circuit 120 between the fault-feeding and crowbar configuration. Alternatively, however, further control of the DC network current may be provided by also modifying the switching operation of the transistors of the DC:AC converter circuit 110 to change the AC link current, in accordance with the first and second embodiments.
The alternate switching between the fault-feeding and crowbar configurations to control the amount of fault current supplied to the DC network may also be approached from an equivalent circuit perspective. To this end,
The system 10 can be represented as an input stage with input current Iin and input impedance Zin, output current Iout and variable impedance Zout. Zout is the impedance presented by the DC:DC converter 100 to the DC electrical network 12. Iin and Iout are regulated via the input and output impedances for a given input and output voltage Vin and Vout. The input and output impedances Zin and Zout are controlled by varying the switching pattern of the transistors in the converter 100.
In steady-state, fault-free conditions, Zout tends to be large, with a value depending on the droop defined by the output power (or alternatively output current) versus output voltage characteristic of the DC-DC converter 100. When a fault occurs, Zout is regulated to a low value approaching the fault impedance Zfault to limit the fault contribution of the converter 100 into the fault site. The regulation between zero (short-circuit) and infinity (open circuit) defines the amount of current presented to the DC network 12.
By using the Norton and Thevenin analogy, the output stage of the converter can be represented by the circuit shown in
The fault current becomes unstable when the series impedance value Zout+Zfault resonates to produce a near-zero impedance value (i.e., when the denominator of Equation 1 approaches zero). This instability can be avoided by controlling the transistor switching pattern to provide a minimum positive damping (positive resistive part of Zout) to limit the fault current. This is an example of a Nyquist Stability Criterion, where a resonance induced by two series impedances that can lead to uncontrolled currents or unwanted sustained oscillations is avoided.
In practice, the instability can be avoided and the fault current, Ifault, limited to a desired amount by controlling the amount of time the AC:DC converter circuit 120 spends in the crowbar configuration. This can be understood from the following equation:
In Equation 2, “d”, which may be referred to as the control duty cycle, represents the fraction of a time period, T, during which the AC:DC converter circuit 120 is in the fault-feeding configuration. Zmin is the output impedance in the fault-feeding configuration and has a value close to zero. Zmax is the output impedance in the crowbar configuration and has some higher value. Zout_avg is then the average output impedance over the time period, T. From Equations 1 and 2, it can be seen that the fault current IFault is controlled by selecting the value of “d”. The value should be selected so that:
As discussed above, the ability to supply a controlled amount of fault current to a faulted DC network may advantageously allow a fault to be identified and isolated. For example, the controlled current may be used to activate a switch (e.g., a DC contactor or SSCB) to isolate the faulted part of the network. In accordance with another aspect of the present disclosure, the described fault current control techniques may also be used after a fault has been isolated.
A consequence of a fault in a DC network is that the network voltage collapses to zero or near-zero. This typically causes capacitors in the faulted network to discharge. For example, referring briefly to
The process is illustrated in
The use of the above-described current control techniques to pre-charge the capacitors may provide further advantages, including the optional omission of pre-insertion resistors (PIRs) in some or all of the power system. PIRs are conventionally fitted to converters to protect the capacitors and converter diodes against high inrush currents at converter start-up, which can degrade the capacitors and semiconductors and limit their lifetime. However, by controlling the current as described herein, the inrush current can be controlled and the PIRs omitted. In particular, the converter 100 may be operated using a lower-than-normal frequency signal, generated by using pulse-width modulation (PWM) of the transistors at one side of the DC:DC converter 100. At the side of the converter where the capacitors are being charged, synchronous rectification is performed to provide a smooth current waveform, charging the capacitors with minimal voltage ripple and extending their lifetime. The use of a lower-than-normal frequency in a DAB DC:DC converter would ordinarily necessitate a transformer with a larger cross-section, however by only using this mode to transfer low powers (as is the case with capacitor charging) a relatively small core can be maintained without reaching transformer saturation.
The AC link 130 of the DC:DC converter 100 of
In Equation 3, Cr is the capacitance of the capacitor 131 and Lr is the inductance of the transformer 135. Lr includes both the series contribution (“leakage inductance”) and the parallel contribution (“magnetizing inductance”) of the transformer 135, with the leakage inductance usually dominating the value of Lr.
The right-hand graph of
One potential drawback of the resonant LLC converter topology is that it may be difficult to effectively implement the fault current control techniques described above. This is because the steep and non-linear rise in impedance away from F0 limits the control bandwidth of the resonant LLC converter. To overcome this drawback, the converter 100 shown in
In the depicted example, the switch arrangement 132 takes the form an AC switch connected in parallel with the capacitor 131. The AC switch 132 may be a mechanical AC switch, for example an AC contactor. Alternatively, a semiconductor switch, for example a pair of reverse-series transistors may be used. Switches and switch arrangements of types other than a parallel-connected switch are contemplated and will occur to persons skilled in the art.
As explained above, the present disclosure may be implemented in an aircraft electrical power system. Exemplary power and propulsion systems are shown in
At the start of the method 200, the electrical power system 200 is operating in a normal condition. In other words, it is functioning as intended with no faults.
At 210, the control system 150 monitors one or more operating parameters of the electrical power system. For example, the control system 150 may monitor the voltage across the DC link capacitor 125 that faces the DC electrical network, or it may monitor one or more other voltages or currents in the DC electrical network.
At 220, the control system 150 determines, based on the monitored operating parameter(s), whether there is a fault in the DC electrical network. For example, the control system may monitor the voltage across the DC link capacitor 125 and make the determination based on whether there is a change in the voltage. Other examples will occur to those skilled in the art. If no fault is detected, the method 200 returns to 210 and the control system 150 continues to monitor the electrical power system. If a fault is detected (e.g., if the monitored voltage drops), the method proceeds to 230.
At optional step 230, the control system 150 responds to the fault by turning off a first plurality of transistors 111-L, 111-H, 112-L, 112-H of the DC:AC converter circuit 110 to block all current from being supplied from the healthy side of the system to the fault site. By doing so, the risk of the fault site being subject to a very high and harmful current spike may be reduced. Alternatively, the method 200 may proceed directly to step 240.
At 240, the control system 150 modifies a switching operation of the first plurality of transistors of the DC:AC converter circuit 110 and/or a second plurality of transistors of the AC:DC converter circuit to supply a controlled amount of current from the DC power source to the DC electrical network. The control system 150 may implement any one of the first, second or third embodiment described above, or any combination thereof. For example, the control system 150 may:
In some examples, the method 200 may end at step 240. For example, the control system 150 may continue to supply the controlled amount of current to the DC network so that one or more loads can continue to operate. As another example, the control system 150 may determine that the fault in the DC network cannot be isolated and may therefore isolate the entire DC network from the DC:DC converter 100 and possibly utilize a redundant system if one is available.
In other examples, the method 200 proceeds to 250. At optional step 250, while supplying the controlled amount of current to the DC network, the control system operates one or more protection devices to isolate a fault in the DC network. For example, the control system 150 may operate one or more switch devices (e.g., a mechanical DC contactor, hybrid relay or Solid-State Circuit Breaker) to isolate the fault. It may not be possible to operate such protection devices at zero network current and risky to operate them at full fault current (e.g., due to contactor arcing), however in accordance with the present disclosure such devices can be operated at a controlled current level.
Having isolated the fault, the method may proceed to optional step 260. Here, in response to isolating the fault and/or determining that the fault has been cleared (e.g., due to a measured increase in the network voltage), the control system 150 turns off a first plurality of transistors 111-L, 111-H, 112-L, 112-H of the DC:AC converter circuit 110 to block all current from being supplied from the cleared DC network. In some examples, step 260 may be omitted and proceed straight to step 270, for example where an already low or zero current was being supplied to the DC network.
At 270, the control system 150 prepares the electrical power system for a restart by recharging one or more capacitors (e.g., DC link capacitor 125) that has discharged a consequence of the fault. To do so, the control system 150 implements any one of the first, second or third embodiment described above, or any combination thereof, to supply a controlled, low current to the DC network to charge the capacitor(s).
Having charged the capacitor(s), the electrical power system restarts and begins normal operation. The method 200 therefore proceeds back to 210 where the control system 150 monitors and provides normal control of the system.
Various examples have been described, each of which feature various combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein.
It will also be appreciated that while the invention has been described with reference to aircraft and aircraft propulsion systems, the techniques described herein could be used for many other applications. These include, but are not limited to, automotive, marine and land-based applications.
Number | Date | Country | Kind |
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2315701.9 | Oct 2023 | GB | national |