Patent Description:
Permanent magnet electrical generators are used for their high efficiency and high power density. Their output is often supplied to a rectifier to produce a direct current output.

One problem with permanent magnet generators is that unless they are physically stopped from rotating, they continue to drive current. This can be an issue if, in the arrangement described above, a fault occurs in the dc network. In many implementations, it is not possible to instantaneously stop the generator from being rotatably driven: either the driving apparatus has a certain degree of angular momentum, or it is not possible for safety reasons or otherwise to mechanically sever the connection between the generator and the driving apparatus.

In such circumstances, it has been a requirement to implement a separate crowbar circuit, in which the approach is to short circuit the phases of the generator. However, such an approach imposes a severe overcurrent on the power electronics in the rectifier, which cannot be guaranteed to survive the process. This may not be satisfactory in circumstances where, once the dc fault is cleared, the generator is required to be re-connected to the dc network and continue to provide power.

International patent application <CIT> discloses a VSC-converter for converting alternating voltage into direct voltage and is controlled so that semiconductor devices of turn-off type connected in antiparallel with diodes are controlled to be turned on and turned off so that an alternating voltage phase line is alternatively connected to a midpoint of a direct voltage side of the apparatus, the plus pole and the minus pole of the direct voltage side for generating a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the apparatus. The midpoint of the direct voltage side is connectable to the phase output through two different so called zero states. The semiconductor devices are controlled so that both zero states are each assumed at least once during a half period during which the fundamental frequency component of a voltage set value of the phase output is positive or negative.

The invention is directed towards an electrical converter and methods of operation therefor.

In one aspect of the invention, an electrical converter is provided having an active diode-clamped multilevel topology in which each clamping diode is connected in antiparallel with a switch.

The converter comprises polyphase supply phases each of which are connected via a respective phase leg to dc rails and a dc-link capacitor. The dc-link capacitor including a plurality of series-connected capacitors.

In another aspect of the invention, a method of controlling such a converter is provided, comprising, for each phase leg, activates a combination of switches therein to form a pair of parallel conduction paths to a midpoint between two capacitors in the dc-link capacitor, thereby connecting each phase to the same node.

An example application of an embodiment is illustrated in <FIG>. An aircraft <NUM> is shown in which a series-hybrid propulsion system is employed. A pair of electric propulsion units <NUM> are installed below the wings of the aircraft <NUM>. The electric propulsion units <NUM> are connected via a power distribution bus <NUM> to a source of electrical power located in the fuselage of the aircraft <NUM>. In the present example, the source of electrical power is an internal combustion engine configured to drive an electric machine. In the specific embodiment of <FIG>, the source of electrical power is turboelectric generator <NUM>, in which the internal combustion engine is a gas turbine engine. In an alternative embodiment, the internal combustion engine may be a piston engine or similar. The source of electrical power could alternatively by a fuel cell, battery, etc..

Each electric propulsion unit <NUM> includes an electric machine configured to drive a propulsive fan. In the present example, the electric propulsion units <NUM> are ducted fans, although it will be appreciated that in alternative embodiments the electric propulsion units <NUM> may be configured as open propellers (a type of propulsive fan), or any other configuration able to produce thrust by causing a pressure rise in the incident airflow.

<FIG> is a block diagram of the propulsion system of the aircraft of <FIG>.

In the present example, therefore, a gas turbine engine <NUM> drives a first machine <NUM> operating as a generator via an interconnecting shaft, thus providing a source of electrical power in the form of alternating current. In the present example, the alternating current is converted to direct current for distribution on the bus by way of a first electrical converter <NUM> operating as a rectifier.

Power is drawn from the bus <NUM> by a second electrical converter <NUM> operating as an inverter for supplying a second electric machine <NUM>, which operates as a motor. This in turn drives a propulsive fan <NUM> via an interconnecting shaft, thereby providing propulsion.

It will be appreciated that depending upon the configuration of the aircraft (for example aircraft <NUM>) further propulsive fans, electric machines and electrical converters may be provided.

In the event of a fault in the dc part of the network, i.e. in the bus <NUM>, it is of course advantageous to be able to isolate the electric machine <NUM> and converter <NUM>. However, on the basis that it may be possible to clear the fault in the dc part of the network, it is a requirement that the method used to isolate the electric machine <NUM> and converter <NUM> be reversible. In the present example, this is to guarantee safety of the aircraft <NUM>. Thus, the electric machine <NUM> must remain drivingly connected with the gas turbine engine <NUM>, and the converter <NUM> must remain in a condition to provide continued rectification.

<FIG> illustrate the approach adopted by the present embodiment of the electrical converter <NUM>. As shown in <FIG>, in the present embodiment the electric machine <NUM> is connected to the converter <NUM> via a polyphase supply. In the present example, three phases A, B, C are connected with the converter. Being a polyphase supply, each phase is <NUM> degrees out-of-phase with the other two.

The converter <NUM> is an active diode-clamped multilevel converter. Such a converter topology is substantially the same as a diode-clamped multilevel converter, except that the clamping diodes are connected in antiparallel with a switch to facilitate load sharing and thus prevent elevated temperatures. The topology of converter <NUM> will be described in further detail with reference to <FIG>, and indeed such arrangements will be familiar to those skilled in the art.

The converter <NUM> produces a dc output between a pair of dc rails <NUM> and <NUM>. The converter <NUM> operates under the control of a controller <NUM>, which in the present example is coupled with the switches in the converter <NUM> to generate a dc output from the polyphase supply. Such standard mode of operation will be understood by those skilled in the art.

In the present example, the controller <NUM> is a field-programmable gate array (FPGA) having been configured with appropriate control logic (e.g. a hardware description) to operate the converter <NUM>. In alternative embodiments the controller <NUM> may instead be an application-specific integrated circuit or a microcontroller, etc..

In the event that a fault <NUM> occurs on the dc side of the converter <NUM>, an event signal may be received by the controller <NUM>. The generation of such event signals will be within the competence of those skilled in the art.

As illustrated in <FIG>, in the present example the controller <NUM> is configured to, after receiving the event signal, enter the converter <NUM> into a crowbar mode of operation so as to isolate the electric machine <NUM> from the dc network. The crowbar mode of operation will be described further with reference to <FIG>.

Once the fault has been cleared, the controller <NUM> may return the converter <NUM> to the normal mode of operation.

A circuit diagram of a specific embodiment of the converter <NUM> is shown in <FIG>.

As described previously, the converter <NUM> has what is termed active diode-clamped multilevel topology. In such an arrangement, each clamping diode is connected in antiparallel with a switch. As can be seen in <FIG>, each switch in the converter <NUM> is provided by an insulated gate bipolar transistor (IGBT). This is due to the high levels of power produced by the electrical machine <NUM>, which in the present embodiment is rated at <NUM> megawatts continuous for propulsion of the aircraft <NUM>. It will be appreciated, however, that other transistors such as BJTs or MOSFETs, or indeed other types of switching devices may be used instead depending upon the intended application of the converter.

The polyphase supply comprising phases A, B, and C are connected via a respective phase leg <NUM>, <NUM>, and <NUM> to the dc rails <NUM> and <NUM>. It will be appreciated that other numbers of phases and phase legs may be implemented depending upon the configuration of the electric machine <NUM>.

In the present embodiment, converter <NUM> is a three-level converter, and so includes a dc-link capacitor comprising two series-connected capacitors <NUM> and <NUM>. The midpoint <NUM> between the capacitors <NUM> and <NUM> is therefore a neutral point.

It will be appreciated that more than three levels may be provided in other embodiments, for example four or greater. For example, an alternative embodiment of the converter will be described in <FIG>, in which the converter is a five-level converter. Odd numbers of levels will result in there being one midpoint that is neutral, whilst even numbers of levels will not have a midpoint that is a neutral.

Referring again to <FIG>, The phase legs each include a plurality of switches composed of an IGBT-antiparallel diode pair, and which are labelled Snp, where n is the number of the switch and p is the attendant phase of the phase leg in which it is located, e.g. S1A is the upper outer IGBT-antiparallel diode pair in phase leg <NUM>. Each leg includes four series-connected IGBT-antiparallel diode pairs {S1p. S4p}, and two clamping IGBT-antiparallel diode pairs S5p and S6p.

In normal operation, the controller <NUM> activates the required combination of switches {S1A. S6C} to generate a dc output from the polyphase input. As will be familiar to those skilled in the art, each leg has four possible combinations {<NUM>. <NUM>} for connection of the phase to the neutral midpoint node <NUM>. The well-established switch logic for this type of converter topology is set out in Table <NUM>:.

It will be understood that these current paths are acceptable during normal operation, in terms of not exceeding the rating of the devices in the converter <NUM>.

However, when a fault occurs in the dc network such as described with reference to <FIG>, the voltage between the dc rails <NUM> and <NUM> nominally becomes zero. This results in a high continuous current flow. For example, in the present embodiment the electric machine <NUM> is optimised such that it exhibits an inductive impedance of <NUM> per-unit. Given a generated ac voltage of <NUM> per-unit, this would result in, unchecked, a fault current of <NUM> per-unit which would quickly cause thermal damage in the converter <NUM>.

Thus, in order to implement a crowbar mode of operation (state C), in the present embodiment the controller <NUM> activates a combination of switches in each phase leg <NUM>, <NUM>, and <NUM> that forms a pair of parallel conduction paths to the midpoint <NUM>, as set out in Table <NUM>:.

In this way, each phase A, B, C is connected to the same node and is thereby isolated from the dc network.

It will be appreciated that the division of current between the two parallel conduction paths means that each path only carries <NUM> per-unit current. Consequently, the converter <NUM> may remain in this mode of operation indefinitely if required, as the current in any particular device is below its rating. Thus it will be appreciated that this mode of control is particularly advantageous for power conversion from electrical machines having an inductive impedance of from <NUM> to <NUM> per-unit.

An alternative embodiment of the converter <NUM> is illustrated in <FIG>, and is identified by reference numeral <NUM>'. For simplicity, only one phase A and associated phase leg <NUM> is shown.

The converter <NUM>' has a five-level active diode-clamped topology and thus has a dc-link capacitor between the dc rails <NUM> and <NUM> that comprises four series-connected capacitors <NUM>, <NUM>, <NUM>, <NUM>. Three midpoints <NUM>, <NUM>, and <NUM> are therefore defined at the nodes between any adjacent two of the capacitors <NUM> to <NUM>.

In a similar way to converter <NUM>, phase leg <NUM> corresponding to phase A comprises a plurality of series-connected IGBT-antiparallel diode pairs {S1A. S8A} which are connected between the dc rails <NUM> and <NUM>. The connection with phase A is at a node between IGBT-antiparallel diode pairs S4A and S5A. Upper clamping IGBT-antiparallel diode pair S9A and S10A clamp to midpoint <NUM>, neutral clamping IGBT-antiparallel diode pair S11A and S12A clamp to midpoint <NUM>, and lower clamping IGBT-antiparallel diode pair S13A and S14A clamp to midpoint <NUM>. This topology and its normal mode of operation will be understood by those skilled in the art and therefore will not be described further.

There are therefore three different parallel conduction paths that may achieve crowbar operation. The switching states C1, C2, and C3 are set out in Table <NUM>:.

In an embodiment, the controller <NUM> is configured to intermittently switch between the crowbar states C1, C2, and C3. This may be as part of a load balancing strategy. For example, the temperature in some of the activated IGBTs may after a certain time begin to exceed a threshold, for example due to conduction losses therein. In such a case, the present crowbar state, say C1, may be switched for another, say C2 or C3. Such a decision may be taken based upon the temperature of the devices to be activated in the other states (C2 or C3 in this example).

Claim 1:
An electrical converter (<NUM>) having an active diode-clamped multilevel topology in which each clamping diode (S5P, S6P) is connected in antiparallel with a switch (S5A, S5B), the converter comprising:
polyphase supply phases (A, B, C) each of which is connected via a respective phase leg (<NUM>, <NUM>, <NUM>) to dc rails (<NUM>, <NUM>) and a dc-link capacitor, the dc-link capacitor including a plurality of series-connected capacitors (<NUM>, <NUM>); and
characterized by:
a controller (<NUM>) configured to, in response to an event signal indicative of a fault which has occurred on a dc side of the converter, for each phase leg, activate a combination of switches therein to form a pair of parallel conduction paths to a midpoint (<NUM>) between two capacitors in the dc-link capacitor, thereby connecting each phase to the same node and isolating an input of the converter from the DC side of the converter.