Power systems and methods for operating same

Abstract

A power system comprises a three phase open winding electrical machine 7 having three open windings 71, 72, 73, a first converter 10, and a second converter 20, wherein one end of the open windings is

Description

Field The present disclosure relates to power systems and methods for operating same. Background In next generation of aircrafts with more electric and hybrid propulsion systems, power electronics converters play a critical role. Power electronics converter such as DC / AC inverters, AC / DC rectifiers, and DC / DC converters are required to interface with electrical propulsion motors, turbo generators, fuel cell and battery energy storage systems. High performance power converter designs are always an attractive topic for different applications including aerospace, automotive, and other industrial applications. To meet high power / current requirements, two power converters may be arranged in parallel and connected to a three phase open winding electrical machine (motor or generator), wherein one end of the three phase open windings is connected to one of the power converters and the other end of the three phase open windings is connected to the other converter. There is a need to operate such power systems in an efficient manner, including a need to operate the electrical machine and the power converter over a large voltage range, optimize power delivery and include safety measures in case one of the converters has a fault condition. There is a need to provide power systems and methods that address the above mentioned needs or at least provide a useful alternative to known power systems or methods. Summary In a first aspect, a power system is provided that comprises a three phase open winding electrical machine having three open windings, a first converter, and a second converter, wherein one end of the open windings is connected to the first converter and the other end of the open windings is connected to the second converter. Each converter comprises a plurality of switching units which form three legs, each leg comprising a top switching unit connected to a positive voltage rail and a bottom switching unit connected to a negative voltage rail, and each leg configured to provide or receive a single phase current. The power system further comprises a controller which is configured to operate the first converter and the second converter in a first operating mode and in a second operating mode. In the first operating mode, the controller is configured to operate the first and second converters alternately in a first and second submode, wherein in the first submode only the first converter is operated to provide or receive current to / from the open winding electrical machine, wherein the second converter is in a crowbar configuration, and in the second submode only the second converter is operated to provide or receive current to / from the open winding electrical machine, wherein the first converter is in a crowbar configuration. In the second operating mode, both the first converter and the second converter are operated to provide or receive current to / from the open winding electrical machine. Aspects of the invention are thus based on the idea to operate the electrical machine in the first operating mode such that only one of the converters is used at a time, while the other converter is put into crowbar. By switching the converters between crowbar and inverter mode and thus operating them in rotation they can be effectively utilized. In the second operating mode, however, both converters feed the electrical machine or receive power from it and the electrical machine is operated as an open winding machine, which allows to have a higher voltage than with traditional designs. This is also beneficial in reducing the current in the electrical machine, resulting in smaller machine windings. Relevant applications of the invention are full electric propulsion units (EPU) and turbogenerator systems (TGS) in more electric and hybrid propulsion systems. Accordingly, the converters may be configured to generate an AC output from a DC input while providing power to an EPU, but may also be operated in reverse. When operated in reverse, the converters convert three-phase AC from a TGS into DC, acting as a rectifiers and providing power to a DC network or a battery. Accordingly, the power system of the present disclosure may be used in both directions, as a DC / AC converter and as an AC / DC rectifier (wherein the switching pattern of the semiconductor switches is adapted accordingly, as known to the skilled person). In some embodiments, the controller is configured to operate the first and second converters in the first mode if the electrical machine speed is below a predetermined speed level (low speed) and is further configured to operate the first and second converters in the second mode if the electrical machine speed is above the predetermined speed level (high speed). Accordingly, the electrical machine is operated by alternating converters at low speed. On the other hand, both converters are operated when the electrical machine is at high speed. Thereby, efficient use of the converters is implemented and the power system is optimized. In some embodiments, the controller is configured to change between the first and second submodes when the temperature of the operating converter (the one which is not in a crowbar configuration) exceeds a predetermined temperature and / or when a predetermined time has elapsed. Accordingly, temperature and or / time may determine the decision to alternate the controller. Generally, when operated in the crowbar configuration, the converter is disconnected from the power distribution system and does not contribute to providing power to / receiving power from the electrical machine. In particular, when operated in the crowbar configuration, it may be provided that the converters are operated such that the respective top three switching units or bottom switching units are switched on at the same time. In a second aspect, a another power system is provided. The power system comprises a three phase open winding electrical machine having three open windings, a first converter, and a second converter, wherein one end of the open windings is connected to the first converter and the other end of the open windings is connected to the second converter. Each converter comprises a plurality of switching units which form three legs, each leg comprising a top switching unit connected to a positive voltage rail and a bottom switching unit connected to a negative voltage rail, and each leg configured to provide or receive a single phase current. The power system further comprises a first three phase short-circuiting device arranged between the first converter and the electrical machine and a second three phase short-circuiting device arranged between the second converter and the electrical machine, wherein the first short-circuiting device comprises a short-circuit mode in which it is configured to short-circuit the three phases of the first converter, wherein the second short-circuiting device comprises a short-circuit mode in which it is configured to short-circuit the three phases of second converter, and wherein the first and second short-circuiting device are configured to operate independently from each other. This aspect of the invention is based on the idea to be able to disconnect a converter from the power distribution system without having to rely on the corresponding power converter’s ability to engage in a crowbar. Rather, an independent protection is provided for with increased reliability and allowing to operate the power system on a single converter mode in case of critical fault conditions, wherein the faulty converter is taken off the system. The short-circuiting devices may be three phase contactors. However, in principle, other devices for short-circuiting may be used as well, such as solid state power controllers (SSPCs). In some embodiments, the short-circuiting devices are switched to operate in the short-circuit mode upon detection or receipt of the fault information indicating that the respective converter has a fault. Accordingly, a faulty converter can effectively and independently be removed from the system. In a third aspect, a further power system is provided. The power system comprises that comprises a three phase open winding electrical machine having three open windings, a first converter, and a second converter, wherein one end of the open windings is connected to the first converter and the other end of the open windings is connected to the second converter. Each converter comprises a plurality of switching units which form three legs, each leg comprising a top switching unit connected to a positive voltage rail and a bottom switching unit connected to a negative voltage rail, and each leg configured to provide or receive a single phase current. The power system further comprises a three phase short-circuiting device connected to the middle points of the windings of the open winding electrical machine, wherein the three phase short-circuiting device comprises a short-circuit mode in which it is configured to short-circuit the three phases of the electrical machine in the middle points, thereby splitting the electrical machine into two sets of three phase windings. In such case, one of the converters is operated to provide or receive current to / from one of the two sets and / or the other of the converters is operated to provide or receive current to / from the other of the two sets of three phase windings. This aspect of the invention is based on the idea to implement the open winding electrical machine as a three port machine, a first port connected to one of the converters, a second port connected to the other of the converters, and a third port connected to the short-circuiting device that is configured to short-circuit the third port of the electrical machine in the middle of the phase windings. By being able to split the three phase windings into two sets of three phase windings, the machine voltage can be further increased. Also, the power system can be further optimized by providing an additional mode of operation. In particular, in case one of the power converters fails, the three phase winding can be split and one resulting set of three phase windings can be operated with the other, operating power converter. The short-circuiting device may be three phase contactor. However, in principle, other devices for short-circuiting may be used as well, such as solid state power controllers (SSPCs). In some embodiments, the short-circuiting device is switched to operate in the short-circuit mode when the electrical machine speed is above a predetermined speed level. Accordingly, the short-circuit mode is initiated at high speeds of the electrical machine, the reason being that the short-circuit mode allows to increase the voltage up to two times the original baseline voltage. This also helps to reduce the current rating of the electrical machine. In some embodiments, the power system is configured to operate in a first and second operating mode, wherein in the first operating mode, the short-circuiting device is not switched to the short-circuit mode, wherein both the first converter and the second converter are operated to provide or receive current to / from the open winding electrical machine. In the second operating mode, the short-circuiting device is switched to the short-circuit mode, wherein one of the converters is operated to provide or receive current to / from one of the two sets and / or the other of the converters is operated to provide or receive current to / from the other of the two sets of three phase windings, wherein the first operating mode is activated if the electrical machine speed is below the predetermined speed level and the second operating mode is activated if the electrical machine speed is above the predetermined speed level. Such embodiments enable the electrical machine to operate efficiently at lower speed and at higher speed. In a further refinement of such embodiments, the power system is configured to further operate in a third operating mode, wherein in the third operating mode only one of the first and second converters is operated to provide or receive current to / from the open winding electrical machine, wherein the other converter is in a crowbar configuration (and wherein the first and second converters may alternate regarding operating and being in a crowbar configuration similar as per the first aspect), wherein the power system is configured to operate in the third operating mode if the electrical machine speed is below a further predetermined speed level, wherein the further predetermined speed level is below the predetermined speed level. Such embodiments enable the electrical machine to operate efficiently in three speed ranges, a low-speed range which corresponds to the third operating mode, a medium speed range which corresponds with the first operating mode, and a high speed range which corresponds with the second operating mode. In all of the above discussed embodiments, each of the first and second converters may be associated with a solid state power controller configured to control a power supply of the respective converter. The solid state power controller may be an integrated unit, thereby eliminating any need for additional cables and connectors. Also, in all of the above discussed embodiments, the converters may be configured to act as an inverter generating a three-phase AC output to the electrical machine from a DC input or as a rectifier converting a three-phase AC input from the electrical machine into a DC output. In a fourth aspect, a method of operating a power system is provided. The method is implemented in a structure that comprises a three phase open winding electrical machine having three windings, a first converter, and a second converter, wherein one end of the open windings is connected to the first converter and the other end of the open windings is connected to the second converter. The method comprises: operating the first converter and the second converter in a first operating mode and in a second operating mode, wherein: in the first operating mode, the first converter and the second converter are operated alternately in a first submode and in a second submode, wherein in the first submode only the first converter is operated to provide or receive current to / from the open winding electrical machine, wherein the second converter is in a crowbar configuration, and in the second submode only the second converter is operated to provide or receive current to / from the open winding electrical machine, wherein the first converter is in a crowbar configuration; and in the second operating mode, operating both the first converter and the second converter to provide all receive current to / from the open winding electrical machine. This method corresponds to the power converter of the first aspect. Accordingly, by switching the converters between crowbar and inverter mode and thus operating them in rotation, they can be effectively utilized. The first mode may be activated if the electrical machine speed is below a predetermined speed level and the second mode may be activated if the electrical machine speed is above the predetermined speed level. In a fifth aspect, another method of operating a power system is provided. The method is implemented in a structure that comprises a three phase open winding electrical machine having three windings, a first converter, and a second converter, wherein one end of the open windings is connected to the first converter and the other end of the open windings is connected to the second converter. The method comprises the steps of: in a first operating mode, operating both the first converter and the second converter to provide or receive current to / from the open winding electrical machine; and in a second operating mode, short-circuiting the three phases of the electrical machine in the middle points of the windings, thereby splitting the electrical machine into two sets of three phase windings, wherein one of the converters is operated to provide or receive current to / from one of the two sets of three phase windings and / or the other of the converters is operated to provide or receive current to / from the other of the two sets of three phase windings, wherein the power system is operated in the first operating mode if the electrical machine speed is below a predetermined speed level and the power system is operated in the second operating mode if the electrical machine speed is above the predetermined speed level. This method corresponds to the power converter of the third aspect. Accordingly, the electrical machine is configured as a three port machine, wherein short-circuiting the third port allows to split the three phase windings into two sets of three phase windings. Thereby, the machine voltage can be increased. In some embodiments of that method, there is further provided a third operating mode of the power system, wherein in the third operating mode only one of the first and seconds converters provide or receives current to / from the open winding electrical machine, wherein the other converter is in a crowbar configuration. The third operating mode may be activated if the electrical machine speed is below a further predetermined speed level, wherein the further predetermined speed level is below the predetermined speed level. In the third operating mode, if the converter feeding the open winding electrical machine is indicated to be faulty, it may be determined if that the converter can be operated in a crowbar configuration. If so, the converter is configured to operate in a crowbar configuration and further the other converter is configured to provide or receive current to / from the open winding electrical machine. If not, the second operating mode is activated, thereby splitting the electrical machine into two sets of three phase windings. Subsequently, the converter that has been operating with the open winding electrical machine so far is shut down and the other converter is activated and provides or receives current to / from one of the sets of three phase windings. Accordingly, even if one of the converters has a fault, the other of the converters can provide or receive current to / from one of the three phase windings of the split winding. In some embodiments, in case a fault of the electrical machine is detected, the steps may be implemented: activate the second operating mode, if not already present; monitor and analyze the phase currents for each one of the two sets of three phase windings; determine if there is a winding fault; and if so, identify the faulty side of the two sets of three phase windings and shutdown of the converter feeding the faulty side. Such embodiments enable the electrical machine top operate in case a fault of the electrical machine is detected, wherein the faulty side is shut down and the other side is operated. In some embodiments, when the electrical machine is operated in the first operating mode in which both the first converter and the second converter provide or receive current to / from the open winding electrical machine and an indication is received that one of the converters is faulty, the steps may be implemented: activate the second operating mode; shut down and disconnect the faulty converter; and continue to modulate the other converter. Such embodiments enable the electrical machine to operate in case a fault of one of the converters is detected when the electrical machine is operated in the first operating mode. 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 The invention will be explained in more detail on the basis of exemplary embodiments with reference to the accompanying drawings in which: FIG. 1 is an embodiment of a power system that comprises first and second converters and an open winding electrical machine, wherein the power system is configured to operate in different modes and wherein the modes include alternately using one or the other of the converters; FIG. 2 is a flowchart of a method for operating a power system such as the power system of FIG. 1; FIG. 3 is a flowchart of a more detailed embodiment of the method of FIG. 2; FIG. 4 is an embodiment of a power system that comprises first and second converters and an open winding electrical machine, wherein the power system further comprises three phase short-circuiting devices arranged between the converters and the open winding electrical machine, wherein the short-circuiting devices are configured to short-circuit the three phases of the respective converter; FIG. 5 is an embodiment of a power system that comprises first and second converters and an open winding electrical machine, wherein the power system further comprises a three phase short-circuiting device connected to the middle points of the windings of the open winding electrical machine, wherein the short-circuiting device when short-circuited splits the electrical machine into two sets of three phase windings; FIG. 6 is a flowchart of a method for operating a power system such as the power system of FIG. 5; FIG. 7 is a flowchart of a more detailed embodiment of the method of FIG. 6; FIG. 8 is a flowchart of a method to operate the power system of FIG. 5 in case of a converter fault during low-speed operation; FIG. 9 is a flowchart of a method to operate the power system of FIG. 5 in case of an electrical machine winding fault during low-speed operation; FIG. 10 is a flowchart of a method to operate the power system of FIG. 5 in case of different fault conditions; and FIG. 11 shows a power inverter providing a three phase alternating current. Detailed description Before discussing embodiments of the present disclosure with respect to FIGS. 1 to 10, the background of the disclosure is discussed with respect to FIG. 11 to provide for a better understanding of the present disclosure. FIG. 11 depicts a DC power system having a positive voltage rail 3 and a negative voltage rail 4, wherein a DC voltage is present between the positive voltage rail 3 and the negative voltage rail 4. The DC voltage may be provided by a DC power source (not shown) such as a DC battery, a DC / DC converter or a rectifier. The positive voltage rail 3 and the negative voltage rail 4 form a high-voltage power bus. A power converter is arranged between the positive voltage rail 3 and the negative voltage rail 4, wherein the power converter comprises six switching units S1-S6 which are arranged in three parallel legs 61, 62, 63, wherein each leg comprises a top switching unit S1, S3, S5 connected to the positive voltage rail 3 and a bottom switching unit S2, S4, S6 connected to the negative voltage rail 4. Each leg 61, 62, 63 provides one phase of an alternating current which is provided to a load 7 such as an electric propulsion motor or other electrical machine. The power converter further comprises a controller 5 which provides control signals 52 to the control terminals (i.e., the gates) of the semiconductor switches S1-S6. The controller 5 receives input signals 51 regarding characteristics of the three phase alternating current provided to the load 7. For example, current sensors 8 are present that provide the respective current values into the controller 5. However, this is to be understood as an example only and the controller 5 may receive further and / or others input signals 51. Further, a filtering capacitor Cl is arranged in parallel to the power converter 6 and extends between the positive voltage rail 3 and the negative voltage rail 4. There is further provided a bidirectional solid state power controller 9, in the following referred to as SSPC, which comprises switching units S101, S103 in the positive voltage rail 3 and switching units S102, S104 in the negative voltage rail 4. The SSPC 9 is configured to connect loads to the high-voltage bus 3, 4 and protect the power converter against overload and short circuits. The SSPC may further comprise a controller and one or several gate drivers for providing the necessary switching signals to the switching units S101-S104. Both the switching units S1-S6 of the power converter and the switching units S101-S104 of the SSPC 9 may each comprise a semiconductor switch with an antiparallel bypass diode, as indicated in FIG. 1. In other embodiments, the switching units S1-S6 of the power converter and / or the switching units S101-S104 of the SSPC 9 may each comprise a plurality of semiconductor switches arranged in parallel. The semiconductor switches may be a MOSFET (metal-oxide-semiconductor fieldeffect transistor), GaN (Gallium Nitride), SiC (Silicon Carbide) or IGBT (Insulated Gate Bipolar Transistor) switch. The power converter as shown in FIG. 11 may be operated as an inverter to provide a three phase alternating current to the load 7, but may also be operated in reverse, wherein it receives a three phase alternating current from the load 7 and converts it into a DC current to be output on the power bus 3, 4. FIG. 1 shows a power system which comprises a first converter 10 and a second converter 20 which both power an open winding electrical machine 7 (generator or electric motor). The first converter 10 and the second converter 20 are arranged in parallel, each located between a positive voltage rail 3 and a negative voltage rail 4. The design of the first and second converters 10, 20 is similar to the design of the converter of FIG. 11, such that a reference is made to the description of FIG. 11. Accordingly, the first converter 10 forms three legs 61-63 with each leg comprising a top switching unitS11, S13, S15 connected to the positive voltage rail 3 and a bottom switching unit S12, S14, S16 connected to the negative voltage rail. Each of the legs 61-63 provides an AC phase current to the electrical machine 7. Similarly, the second converter 20 forms three legs 64-66 with each leg comprising a top switching unit S21, S23, S25 connected to the positive voltage rail 3 and a bottom switching unit S22, S24, S26 connected to the negative voltage rail. Each of the legs 64-66 provides an AC phase current to the electrical machine 7. The electrical machine 7 may be an electric motor or a generator. It is configured as an open winding electrical machine and, accordingly, comprises three open windings 71,72, 73. The windings 71,72, 73 being open and the electrical machine 7 being an open winding electrical machine means that, for each of the windings 71, 72, 73, 1, one end is connected to the first converter 10 and the other end is connected to the second converter 20, as indicated in FIG. 1. Accordingly, one end of open winding 71 is connected to the phase current provided / received by leg 61 of converter 10 and the other end of open winding 71 is connected to the phase current provided / received by leg 66 of converter 20. Similarly, one end of open winding 72 is connected to the phase current provided / received by leg 62 of converter 10 and the other end of open winding 72 is connected to the phase current provided / received by leg 65 of converter 20. Further, one end of open winding 73 is connected to the phase current provided / received by leg 63 of converter 10 and the other end of open winding 73 is connected to the phase current provided / received by leg 64 of converter 20. As discussed with respect to FIG. 11, each switching unit S11-S16, S21-S26 comprises a semiconductor switch with an antiparallel diode. Alternatively, a plurality of semiconductor switches may be arranged in parallel. A controller 5 is provided which receives input signals 51 from current sensors 8 and / or other sensors and / or a higher level controlling unit and provides control signals 52 to the switching units of the first and second converters 10, 20. The control signals are provided to the respective gate / base terminals of the semiconductor switches and control the switching of the semiconductor switches to provide for an alternating current. The control signals 52 may be pulsed PWM signals. In FIG. 1, an SSPC 91 is further provided at the input side of the first converter 10 and an SSPC 92 is provided at the input side of the second converter 20. The SSPC 91 is bidirectional both on the positive voltage rail 3 and on the negative voltage rail 4 and comprises switching units S111, S112 on the positive voltage rail and switching units S113, S114 on the negative voltage rail. The SSPC 92 is unidirectional both on the positive voltage rail 3 and the negative voltage rail 4 and comprises switching units S121, S122. In other embodiments, the SSPCs 91, 92 may comprise switching units in only one of the positive voltage rail 3 and the negative voltage rail 4. It is to be noted that the positive voltage rail 3 that leads to SSPC 92 and the second converter 20 branches off between switching units S111 and S112 on the positive voltage rail 3 and that the negative voltage rail 4 that leads to SSPC 92 and the second converter 20 branches off between switching units S113 and S114 on the negative voltage rail 4. The SSPCs 91,92 may be designed as integrated units of the power converters 10, 20. As discussed before, the converters 10, 20 may be configured to act as an inverter generating a three-phase AC output to the electrical machine 7 from a DC input which in the following is referred to as EPU case (EPU - electric propulsion system). Alternatively, they may be configured to act as a rectifier converting a three-phase AC input from the electrical machine 7 into a DC output on power bus 3, 4 which in the following is referred to as TGS case (TGS - turbine generator system). In particular, the TGS case may regard an aerospace starter generator system. Also, the system may be able to be switched between the inverter and rectifier modes, wherein the control signals of the converter 5 are adapted accordingly (when operated in reverse as rectifiers, the switching signals for the converters 10, 20 change as is well known to the skilled person). In the power system of FIG. 1, the first converter 10 and the second converter 20 can be operated by the controller 5 in a first operating mode and in a second operating mode. In the first operating mode, the first and second converters 10, 20 are operated alternately in a first and second submode, wherein, in the first submode, only the first converter is operated to provide current to the electrical machine 7 (in the EPU case) or receive current from the open winding electrical machine 7 (in the TGS case), wherein the second converter is in a crowbar configuration. In the second submode, only the second converter is operated to provide or receive current to / from the electrical machine, wherein the first converter is in a crowbar configuration. Such alternate use of the converters allows to operate the first and second converters 10, 20 in an efficient manner. The first operating mode is implemented when the speed of the electrical machine 7 is below a predetermined level. On the other hand, if the speed of the electrical machine 7 is above the predetermined level, the first and second converters 10, 20 are operated in the second mode, wherein both the first converter 10 and the second converter 20 provide or receive a current to / from the electrical machine 7. In the following, the power system of FIG. 1 is considered in more detail with respect to the TGS case and the EPU case. Regarding the TGS case, generally, the ability to generate / deliver power in large speed range and ability to provide high pulse power when operating at lower speed is a typical demanding requirement from airframers. In general, due to fault tolerant requirements and to be able to manage ripple currents, low inductance generators designs are not acceptable for aerospace applications. This limits the ability to design a lower weight machine and converter for such a system. These problems are addressed by having different modes of operation as discussed above which allow higher back emf which reduces the current in the machine and hence the size and weight. The TGS system operates in two modes when the generator operates in the lower speed where the back emf is low. When one of the converters 10, 20 is in crowbar operation, the top semiconductor switches S11, S13, S15 or S21, S23, S25 are turned on. Further, optionally, the DC side is disconnected from the network wherein the electrical machine 7 is operated in star configuration. The electrical machine 7 back emf may be designed to be higher than traditional values with pre-defined back emf value or speed, such as 13500 rpm. The back emf may be optimized to generate 270V with the star configuration. In this mode only one of the converters 10, 20 contributes to rectification and the other converter acts as crowbar. Once this speed is crossed the other converter 20, 10 is put into PWM rectification mode as well and the bus voltage will be controlled to 270V before connecting to the main distribution network. Then, the machine will operate in open winding configuration delivering equal power from both the converters 10, 20. Optionally the converters are modulated in interleaved PWM modulations to avoid zero sequence currents. This allows having large operating range without increasing the current rating of the converters and the machine. When the machine speed is low the back emf of the machine is lower resulting in requiring large current to match with power requirements under this condition. Moreover, both converters 10, 20 are operated in two submodes in the low speed range to optimize the performance and improve the thermal performance. At start one of the converters 10 will be the primary converter and the other converter 20 will operate as crowbar by clamping the top three switches. After a predetermined period the other converter 20 is switched to become the primary converter and converter 10 will be put into crowbar by turning ON top three switches. Optionally this can be done after a pre-determined period or by monitoring the temperature of the converters, the latter being more effective when high power needs are to be delivered at low speeds where higher current needs to be supplied, this leading to heating of the converter. In addition, the crowbar can also be alternated to prevent overheating of the device and to have symmetrical degradation of the converters which can give extra benefits in terms of device utilization and resulting in symmetrical aging of the components. Regarding the EPU case, one of the main design challenges lies in that the propulsion motor and inverter system must be designed to operate in a large voltage range. Typically, a battery system will have large voltage variation for example from 550V to 850V. It follows that the converter and the EPU motor are able to deliver most demanding power and torque requirements at both ends. Due to this, the design will be biased to cater the lower voltage point and the back emf of the machine will be constrained at the lowest DC link voltage and inductance of the machine will be constrained by the power factor and fault current requirements. Such conditions may result in a suboptimum design. The power losses on the converter will be highest at lower voltage and capacitor requirement on the DC link. Similarly, the electric motor needs be designed to carry very high current leading to high current density and thermal challenges. This would make both the power electronics and electric machine heavy. The challenges are also addressed by having different modes of operation as discussed above. When the battery voltage is high (e.g. 700V to 850V), one of the converters 10, 20 acting as primary converter is operated in PWM switching mode and the other converter 20, 10 is put into the crowbar mode, wherein the other converter is disconnected from the main DC link and its top switches or bottom switches are kept in ON state. Similar to the description of the TGS case, the converters 10, 20 are switched alternately between crowbar mode and PWM operating mode to optimize the performance. When the battery voltage drops below a predetermined value, e.g. 700V, the other converter is removed from crowbar mode and also operated in the PWM control mode. The EPU motor will then be driven as open winding machine sharing equal power from each converter 10, 20. One advantage of the power system of FIG. 1 and the method it implements lies in that it allows optimization of a TGS or EPU system with reduced weight and increased efficiency. This is achieved by allowing the machine to have higher back emf and reducing the current when delivering the rated power or transient over loading. Back emf of the machine can increase by factor of 1.731 and result is significant reduction of current. This is also beneficial in order to reduce the size of the converters as power modules and capacitors are the most dominant components contributing to weight. FIG. 2 shows the steps of the general method implemented in the system of FIG. 1. According to step 201, the first converter and the second converter may be operated in a first operating mode and in a second operating mode. In the first operating mode, the first and second converters are alternately operated in a first and second submode, step 202, wherein in the first submode only the first converter is operated to provide or receive current to / from the open winding electrical machine, wherein the second converter is in a crowbar configuration, step 203. On the other hand, in the second submode, only the second converter is operated to provide or receive current to / from the open winding electrical machine, wherein the first converter is in a crowbar configuration, step 204. In the second operating mode, both the first converter and the second converter are operated to provide a receive current to / from the open winding electrical machine. FIG. 3 is a more detailed embodiment of the method of FIG. 2, wherein the TGS case is considered, but which is applicable accordingly to the EPU case. The method starts at step 301. The system is started by, e.g., providing a start command to a starter generator. Further, one of the converters (converter 1) is modulated while the other converter (converter 2) is put in a crowbar configuration. In step 302, the DC bus is precharged under control of the SSPCs 91, 92 and connected to a distribution network. In step 303, the engine speed and the DC link voltage are monitored, wherein the engine speed and the voltage generated by the converter are proportional. In step 304, it is determined if the engine speed is smaller than a predetermined speed. In particular, the predetermined speed indicates the upper boundary of a low speed range of the electric machine 7. If the engine speed is not smaller than the predetermined speed, the system is configured in step 305 to operate in an open winding note, meaning that both converters (converters 10, 20 of FIG. 1) are operated. If the engine speed is smaller than the predetermined speed, it is determined in step 306 if a specific condition is met, the specific condition being either that the temperature of the operating converter is larger than a predetermined temperature value or that a specific time duration has reached a predetermined value (such that a predetermined time has elapsed). If the specific condition is not met, the method returns to step 303. Otherwise, in step 307, the converters are alternated in that the other converter (converter 2) is now operated as primary converter, wherein the first converter (converter 1) is put in a crowbar configuration. Again, in step 308, the engine speed and the DC link voltage or monitored. If, in step 310, the engine speed is not smaller than the predetermined speed, the system is configured in step 310 to operate in an open winding note, meaning that both converters (converters 10, 20 of FIG. 1) are operated. If the engine speed is smaller than the predetermined speed, it is determined in step 311 if a specific condition is met, the specific condition being either that the temperature of the operating converter (now converter 2) is larger than a predetermined temperature value or that a specific time duration has reached a predetermined value (such that a predetermined time has elapsed). If the specific condition is not met, the method returns to step 308. Otherwise, in step 312, the converters are alternated again in that the first converter (converter 1) is now operated again as primary converter, wherein the second converter (converter 2) is put in a crowbar configuration. The method then returns to step 303. FIG. 4 shows a power system which is similar to the power system of FIG. 1 except that, additionally, a three phase short-circuiting device 15 is arranged between the first converter 10 and electrical machine 7 and a second three phase short-circuiting device 16 is arranged between the second converter 20 and the electrical machine 7. The short-circuiting advises 15, 16 are configured as three-phase contactors 150, 160. When in the short-circuit mode, the contactors 150, 160 short-circuit the three phases of the first converter 10 and the second converter 20, respectively. By implementing short-circuiting devices 15,16, an improved fault management can be provided for. As background, some fault management is already available without these short-circuiting devices 15, 16. For example, if the electrical machine 7 is faulty, then both converters 10, 20 may be switched such that the top switches are connected to the positive rail or negative rail, this making sure that the current is circulated only within the machine 7 and does not flow out into the converters 10, 20. In another example, if one of the converters 10, 20 or its DC link has a fault, but its semiconductor switches are still operational, then that converter can be isolated from the DC link via SSPCs 91, 92. Subsequently, the healthy switches of the converter can be controlled to create a crowbar allowing the machine 7 to operate in star connected mode. However the operation capability is limited to low speed of the generator in such case. However, due to poor reliability of power electronics there is a risk that one of the converters 10, 20 fails completely and becomes inoperable. This vulnerability is eliminated by the topology of FIG. 4. The added contactors 150, 160 provide for a short circuiting capability at each of the ends of machine 7 without having to rely on the corresponding power converter’s 10, 20 ability to engage a crowbar. Due to its independent operability, the reliability of the system is increased. This allows the system to operate with a single converter mode under some of the critical fault conditions with lower power rating. In the context of the TGS case the configuration of FIG. 7 allows low speed operation of the TGS with only one of the converters 10, 20 operating. In the context of the EPU case the DC Link capability is not changed when putting contactors 150, 160 in the short-circuit mode, as such mode does not have an impact on the battery voltage. However, the EPU may need to be operated with lower speed when delivering power from only single converter only. One advantage of the power system of FIG. 4 thus lies in that it allows improved fault management and provides independent reconfiguration capabilities. FIG. 5 shows a power system which is similar to the power system of FIG. 1 except that, additionally, a three phase short-circuiting device 17 is provided that is connected to the middle points of the windings 71, 72, 73 of the open winding electrical machine 7. The three phase short-circuiting device 17 comprises a short-circuit mode in which it is configured to short-circuit the three phases of the electrical machine 7 in the middle points (the neutral point), thereby splitting the electrical machine 7 into two sets 701, 702 of three phase windings. When there are two sets 701, 702 of three phase windings, one of the converters 10 provides or receives current to / from one of the two sets 701, and / or the other of the converters 20 provides or receives current to / from the other of the two sets 702. The short-circuiting device 17 may be implemented by three-phase contactors 170. Accordingly, in the topology of FIG. 5, the electrical machine 7 has three ports, wherein two ports are connected to first converter 10 and second converter 20, and a third port is connected to the set of contactors 170 which are configured to short-circuit all three phases of the machine winding exactly at the middle. This allows the machine 7 and the converters 10, 20 to operate in a large speed and voltage range and allows the machine 7 to have a lower current density or high-power density at the system level. The system of FIG. 5 may be operated in a first and second operating mode, as shown in the method of FIG. 6. According to step 601, in the first operating mode, both the first converter 10 and the second converter 20 are operated to provide and receive current to / from the open winding electrical machine 7. In the second operating mode, the three phases of the electrical machine 7 are short-circuited in the middle points of the windings, thereby splitting the electrical machine into two sets of three phase windings, step 602. Subsequently, in step 603, one of the converters is operated to provide or receive current to / from one of the two sets, and / or the other of the converters is operated to provide or receive current to / from the other of the two sets. According to step 604, the system is operated in the first operating mode if the electrical machine speed is below a predetermined speed level and operated in the second operating mode the electrical machine speed is above the predetermined speed level. The system of FIG. 5 may be operated additionally in a third operating mode. In the third operating mode, only one of the first and second converters is operated to provide or receive current to / from the open winding electrical machine, wherein the other converter is in a crowbar configuration, similar to the embodiment of FIG. 1. The third operating mode is activated if the electrical machine speed is below a further predetermined speed level which is below the predetermined speed level. In other words, at low speed, the third operating mode is implemented, at medium speed, the first operating mode is implemented, and at high speed, the second (short-circuit) operating mode is implemented. A more detailed embodiment of the method of FIG. 6 is depicted in FIG. 7, wherein the TGS case is considered, but which is applicable accordingly to the EPU case. The method starts at step 701. The system is started by, e.g., providing a start command to a starter generator. Further, one of the converters (converter 1) is modulated while the other converter (converter 2) is put in a crowbar configuration. In step 702, the DC bus is precharged under control of the SSPCs 91, 92 and connected to a distribution network. In step 703, the engine speed and the DC link voltage are monitored. If the machine base output voltage is V, then open winding would increase the voltage by factor more than 1.7 and split star would allow increase up to 2 times the original base line voltage. This help to reduce the current rating of the machine. In step 704, it is determined if the engine speed is smaller than a predetermined first speed level. If the engine speed is smaller than the predetermined first speed level, the method goes back to step 703. Only one of the first and second converters is operated to receive current from the electrical machine 7, wherein the other converter is in a crowbar configuration, similar to the embodiment of FIG. 1. If the engine speed is not smaller than the predetermined first speed level, the system is configured in step 705 to operate in an open winding note, meaning that both converters (converters 10, 20 of FIG. 5) are operated. In step 706, again the engine speed and the DC link voltage are monitored. In step 707, it is determined if the engine speed is smaller than a predetermined second speed level. If the engine speed is smaller than the predetermined second speed level, the method goes back to step 706. If the engine speed is not smaller than the predetermined second speed level, the system is configured in step 708 that the short-circuiting device 17 of FIG. 5 is switched into the short-circuit mode, wherein the contactors 170 short-circuit the windings of the electrical machine in the middle. Subsequently, in step 709, both of the converters (converters 10, 20 of FIG. 5) are operated to modulate and extract power from one of the two sets of three phase windings produced by the short-circuiting. In the TGS case, the provision of three operating modes allows to optimize the performance of an aerospace starter generator system by providing the options of a low speed range, a medium speed range and a high-speed range. Also, this allows to operate the machine 7 even during fault condition in the machine or in the power converter 10, 20 and further allows operation with partial power. The topology of FIG. 5 allows to make a certifiable product for aerospace as it provides power electronics independent protection solution. In the EPU case, the machine 7 is driven from variable voltage battery, wherein three different operating areas may be defined, namely, a high voltage operating area (750V-850V), a medium voltage operating area (650V-750V), and a low voltage (550V-650V). The EPU can be operated optimally in accordance with the three modes. With the system of FIG. 5, different logics may be used for configuring the system to allow fault tolerant operation in the low speed, medium speed, or high speed, as next discussed with respect to FIGS. 8 to 10. In case the system operates in the low-speed range, the method of FIG. 8 may be implemented. According to step 801, the system is operated at low speed, wherein one converter is modulating and the other converter in crowbar. At step 802, it is determined if the modulating converter is faulty. If this is not the case, the method goes back to step 801. If the modulating converter is faulty, it is determined if that converter can be operated in a crowbar configuration, step 803. If this is the case, step 805, the function of the converters is alternated, wherein the other converter is now operating and modulating current received from the electrical machine 7, and wherein the so far operating converter is now operated in a crowbar configuration. If this is not the case, step 804, the second operating mode is activated in that the contactors 170 of FIG. 5 are turned on. Accordingly, the three phases of the electrical machine are short-circuited in the middle points of the winding. At the same time, the so far modulating and now faulty converter is shut down and disconnected using the SSPC and the other converter is operated and modulating current received from the electrical machine 7. If the system operates in the low, medium, or high speed range, the method of FIG. 9 may be implemented. In step 901, the systems operates in the low, medium or high speed range. It is then determined in step 902 if the machine 7 is faulty. If this is not the case, the method goes back to step 901. If a fault of machine 7 is indicated, the second operating mode is activated in step 903 in that the contactors 170 of FIG. 5 are turned on. At the same time, both converters 10, 20 are modulated. It is then monitored and analysed the phase currents for each of the two sets of three phase windings, step 904. Next, it is determined in step 905 if there is a winding fault. For example, there may be a turn-to-turn short-circuit of the machine windings. If this is the case, the faulty side of the two sets of three phase windings is identified and the converter feeding the faulty side is shut down, wherein the other converter continues to modulate the received current. If there is no winding fault, and an alarm issues in step 906 as there must be another fault in such case. The method of FIG. 9 allows limited operation in case of a winding fault, wherein only the healthy side of machine continues to operate to provide power, wherein half of the power can be delivered to machine. FIG. 10 shows a method that can be implemented when the system operates in the medium range. According to step 1001, the systems operates in the medium speed range with both converters 10,20 modulating. It is determined in step 1002 if one of the converters is faulty and in step 1004 if the other of the converters is faulty. If so, respectively, the second operating mode is activated in that the contactors 170 of FIG. 5 are turned on. Accordingly, the three phases of the electrical machine are short-circuited in the middle points of the winding. At the same time, the so far modulating and now faulty converter is shut down and disconnected using the SSPC and the other converter is operated and modulating current received from the electrical machine 7. In step 1006, it is determined if the machine is faulty. If so, the method continues with the method of FIG. 9, wherein step 1007 to 1011 correspond to steps 903 to 907. It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in any way. Also, those skilled in the art will appreciate that other aspects of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. Various features of the various embodiments disclosed herein can be combined in different combinations to create new embodiments within the scope of the present disclosure. In particular, the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. Any ranges given herein include any and all specific values within the range and any and all sub-ranges within the given range.

Claims

1. A power system comprising:a three phase open winding electrical machine (7) having three open windings (71, 72, 73);- a first converter (10) comprising a plurality of switching units (S11-S16), the switching units (S11-S16) forming three legs (61-63), each leg (61-63) comprising a top switching unit (S11, S13, S15) connected to a positive voltage rail (3) and a bottom switching unit (S12, S14, S16) connected to a negative voltage rail (4), and each leg (61-63) configured to provide or receive a single phase current;- a second converter (20) comprising a plurality of switching units (S21-S26), the switching units (S21-S26) forming three legs (64-66), each leg (64-66) comprising a top switching unit (S21, S23, S25) connected to the positive voltage rail (3) and a bottom switching unit (S22, S24, S26) connected to the negative voltage rail (4), and each leg (64-66) configured to provide or receive a single phase current, wherein one end of the open windings (71, 72, 73) is connected to the first converter (10) and the other end of the open windings (71, 72, 73) is connected to the second converter (20); anda controller (5) configured to operate the first converter (10) and the second converter (20) in a first operating mode and in a second operating mode, wherein in the first operating mode, the controller (5) is configured to operate the first and second converters (10, 20) alternately in a first and second submode, whereino in the first submode only the first converter (10) is operated to provide or receive current to / from the open winding electrical machine (7), wherein the second converter (20) is in a crowbar configuration, ando in the second submode only the second converter (20) is operated to provide or receive current to / from the open winding electrical machine (7), wherein the first converter (10) is in a crowbar configuration, and- in the second operating mode, both the first converter (10) and the second converter (20) are operated to provide or receive current to / from the open winding electrical machine (7).

2. The power system of claim 1, wherein the controller (5) is configured to operate the first and second converters (10, 20) in the first mode if the electrical machine speed is below a predetermined speed level, and is configured to operate the first and second converters (10, 20) in the second mode if the electrical machine speed is above the predetermined speed level.

3. The power system of claim 1 or 2, wherein the controller (5) is configured to change between the first and second submodes when the temperature of the operating converter (10,20) exceeds a predetermined temperature and / or when a predetermined time has elapsed.

4. The power system of any preceding claim, wherein when operated in the crowbar configuration, the first and second converters (10, 20) are operated such that the respective top three switching units (S11, S13, S15, S21, S23, S25) or bottom switching units (S12, S14, S16, S22, S24, S26) are switched on at the same time.

5. A power system comprising:- a three phase open winding electrical machine (7) having three open windings (71, 72, 73);a first converter (10) comprising a plurality of switching units (S11-S16), the switching units (S11-S16) forming three legs (61-63), each leg (61-63) comprising a top switching unit (S11, S13, S15) connected to a positive voltage rail (3) and a bottom switching unit (S12, S14, S16) connected to a negative voltage rail (4), and each leg (61-63) configured to provide or receive a single phase current;a second converter (20) comprising a plurality of switching units (S21-S26), the switching units (S21-S26) forming three legs (64-66), each leg (64-66) comprising a top switching unit (S21, S23, S25) connected to the positive voltage rail (3) and a bottom switching unit (S22, S24, S26) connected to the negative voltage rail (4), and each leg (64-66) configured to provide or receive a single phase current, wherein one end of the open windings (71, 72, 73) is connected to the first converter (10) and the other end of the open windings (71, 72, 73) is connected to the second converter (20); anda first three phase short-circuiting device (15) arranged between the first converter (10) and the electrical machine (7) and a second three phase short-circuiting device (16) arranged between the second converter (20) and the electrical machine (7), o wherein the first short-circuiting device (15) comprises a short-circuit mode in which it is configured to short-circuit the three phases of the first converter (10), o wherein the second short-circuiting device (16) comprises a short-circuit mode in which it is configured to short-circuit the three phases of second converter (20), ando wherein the first and second short-circuiting devices (15, 16) are configured to operate independently from each other.

6. The power system of claim 5, wherein the short-circuiting devices (15, 16) are three phase contactors (150, 160).

7. The power system of claim 5 or 6, wherein the short-circuiting devices (15, 16) are switched to operate in the short-circuit mode upon detection or receipt of a fault information indicating that the respective converter (10, 20) has a fault.

8. A power system comprising:a three phase open winding electrical machine (7) having three open windings (71, 72, 73);- a first converter (10) comprising a plurality of switching units (S11-S16), the switching units (S11-S16) forming three legs (61-63), each leg (61-63) comprising a top switching unit (S11, S13, S15) connected to a positive voltage rail (3) and a bottom switching unit (S12, S14, S16) connected to a negative voltage rail (4), and each leg (61-63) configured to provide or receive a single phase current;a second converter (20) comprising a plurality of switching units (S21-S26), the switching units (S21-S26) forming three legs (64-66), each leg (64-66) comprising a top switching unit (S21, S23, S25) connected to the positive voltage rail (3) and a bottom switching unit (S22, S24, S26) connected to the negative voltage rail (4), and each leg (64-66) configured to provide or receive a single phase current, wherein one end of the open windings (71, 72, 73) is connected to the first converter (10) and the other end of the open windings (71, 72, 73) is connected to the second converter (20); and- a three phase short-circuiting device (17) connected to the middle points of the windings (71, 72, 73) of the open winding electrical machine (7), wherein the three phase short-circuiting device (17) comprises a short-circuit mode in which it is configured to short-circuit the three phases of the electrical machine (7) in the middle points, thereby splitting the electrical machine (7) into two sets (701, 702) of three phase windings, wherein one of the converters (10) is operated to provide or receive current to / from one of the two sets (701, 702) of three phase windings and / or the other of the converters (20) is operated to provide or receive current to / from the other of the two sets (701, 702) of three phase windings.

9. The power system of claim 8, wherein the short-circuiting device (17) is a three phase contactor (170).

10. The power system of claim 8 or 9, wherein the short-circuiting device (17) is switched to operate in the short-circuit mode when the electrical machine speed is above a predetermined speed level.

11. The power system of claim 10, wherein the power system is configured to operate in a first and second operating mode, wherein:- in the first operating mode, the short-circuiting device (17) is not switched to the short-circuit mode, wherein both the first converter (10) and the second converter (20) are operated to provide or receive current to / from the open winding electrical machine (7); and- in the second operating mode, the short-circuiting device (17) is switched to the short-circuit mode, wherein one of the converters (10) is operated to provide or receive current to / from one of the two sets (701, 702) of three phase windings and / or the other of the converters (20) is operated to provide or receive current to / from the other of the two sets (701, 702) of three phase windings, wherein the first operating mode is activated if the electrical machine speed is below the predetermined speed level and the second operating mode is activated if the electrical machine speed is above the predetermined speed level.

12. The power system of claim 11 wherein the power system is configured to further operate in a third operating mode, wherein in the third operating mode only one of the first and second converters (10, 20) is operated to provide or receive current to / from the open winding electrical machine (7), wherein the other converter (20, 10) is in a crowbar configuration, wherein the power system is configured to operate in the third operating mode if the electrical machine speed is below a further predetermined speed level, wherein the further predetermined speed level is below the predetermined speed level.

13. The power system of any preceding claim, wherein each of the first and second converters (10, 20) is associated with a solid state power controller (91, 92) configured to control power supply of the respective converter (10, 20).

14. A method of operating a power system that comprises:a three phase open winding electrical machine (7) having three open windings (71, 72, 73);- a first converter (10) comprising a plurality of switching units (S11-S16), the switching units (S11-S16) forming three legs (61-63), each leg (61-63) comprising a top switching unit (S11, S13, S15) connected to a positive voltage rail (3) and abottom switching unit (S12, S14, S16) connected to a negative voltage rail (4), and each leg (61-63) configured to provide or receive a single phase current; and a second converter (20) comprising a plurality of switching units (S21-S26), the switching units (S21-S26) forming three legs (64-66), each leg (64-66) comprising a top switching unit (S21, S23, S25) connected to the positive voltage rail (3) and a bottom switching unit (S22, S24, S26) connected to the negative voltage rail (4), and each leg (64-66) configured to provide or receive a single phase current;wherein one end of the open windings (71, 72, 73) is connected to the first converter (10) and the other end of the open windings (71, 72, 73) is connected to the second converter (20),the method comprising the steps of:- operating (201) the first converter (10) and the second converter (20) in a first operating mode and in a second operating mode, wherein:in the first operating mode, operating (202) the first converter (10) and the second converter (20) alternately in a first submode and in a second submode, wherein o in the first submode only the first converter (10) is operated (203) to provide or receive current to / from the open winding electrical machine (7), wherein the second converter (20) is in a crowbar configuration, ando in the second submode only the second converter (20) is operated (204) to provide or receive current to / from the open winding electrical machine (7), wherein the first converter (10) is in a crowbar configuration, andin the second operating mode, operating both the first converter (10) and the second converter (20) to provide or receive current to / from the open winding electrical machine (7).

15. The method of claim 14, wherein the first mode is activated if the electrical machine speed is below a predetermined speed level and in that the second mode is activated if the electrical machine speed is above the predetermined speed level.

16. A method of operating a power system that comprises:a three phase open winding electrical machine (7) having three open windings (71, 72, 73);a first converter (10) comprising a plurality of switching units (S11-S16), the switching units (S11-S16) forming three legs (61-63), each leg (61-63) comprising a top switching unit (S11, S13, S15) connected to a positive voltage rail (3) and a bottom switching unit (S12, S14, S16) connected to a negative voltage rail (4), and each leg (61-63) configured to provide or receive a single phase current; anda second converter (20) comprising a plurality of switching units (S21-S26), the switching units (S21-S26) forming three legs (64-66), each leg (64-66) comprising a top switching unit (S21, S23, S25) connected to the positive voltage rail (3) and a bottom switching unit (S22, S24, S26) connected to the negative voltage rail (4), and each leg (64-66) configured to provide or receive a single phase current, wherein one end of the open windings (71, 72, 73) is connected to the first converter (10) and the other end of the open windings (71, 72, 73) is connected to the second converter (20);the method comprising the steps of:in a first operating mode, operate (601) both the first converter (10) and the second converter (20) to provide or receive current to / from the open winding electrical machine (7); andin a second operating mode, short-circuit (602) the three phases of the electrical machine (7) in the middle points of the windings (71, 72, 73), thereby splitting the electrical machine (7) into two sets (701, 702) of three phase windings, wherein one of the converters (10) is operated to provide or receive current to / from one of the two sets (701, 702) of three phase windings and / or the other of the converters (20) is operated to provide or receive current to / from the other of the two sets (701, 702) of three phase windings,wherein the power system is operated (604) in the first operating mode if the electrical machine speed is below a predetermined speed level and the power system is operated in the second operating mode if the electrical machine speed is above the predetermined speed level.

17. The method of claim 16, wherein a third operating mode of the power system, wherein in the third operating mode only one of the first and seconds converters (10, 20) provides or receives current to / from the open winding electrical machine (7), wherein the other converter (20, 10) is in a crowbar configuration, wherein the third operating mode is activated if the electrical machine speed is below a further predetermined speed level, wherein the further predetermined speed level is below the predetermined speed level.

18. The method of claim 17, wherein in the third operating mode, if the operating converter (10, 20) is indicated to be faulty, it is determined (803) if that converter (10, 20) can be operated in a crowbar configuration, and:if so, configure (805) the converter (10, 20) to operate in a crowbar configuration and further configure the other converter (20, 10) to provide or receive current to / from the open winding electrical machine (7); andif not, activate (804) the second operating mode, shut down the operating converter (10, 20) and activate the other converter (20, 10).

19. The method of any one of claims 16 to 18, wherein in case a fault of the electrical machine is detected (902):activate (903) the second operating mode, if not already present;monitor and analyze (904) the phase currents for each one of the two sets (701, 702) of three phase windings;- determine (905) if there is a winding fault; and- if so, identify (907) the faulty side of the two sets (701,702) of three phase windingsand shutdown of the converter (10, 20) feeding the faulty side.

20. The method of any one of claims 16 to 19, wherein when the electrical machine (7) is operated in the first operating mode in which both the first converter (10) and the second converter (20) provide or receive current to / from the open winding electrical machine (7) and an indication (1002, 1004) is received that one of the converters (10, 20) is faulty:activate the second operating mode (1003, 1005);shut down and disconnect (1003, 1005) the faulty converter (10, 20); and- continue (1005, 1007) to modulate the other converter (20, 10).27

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