WO2016177399A1

WO2016177399A1 - Converter arrangement

Info

Publication number: WO2016177399A1
Application number: PCT/EP2015/059780
Authority: WO
Inventors: Adam Ruszczyk; Marcin Szewczyk; Michal LAZARCZYK; Radoslaw Jez; Wojciech Piasecki
Original assignee: Abb Technology Ltd
Priority date: 2015-05-05
Filing date: 2015-05-05
Publication date: 2016-11-10

Abstract

A converter arrangement (60) is disclosed, which is configured to convey power from a first power system (61) to a second power system (62), or vice versa, wherein the second power system (62) has a nominal operating voltage frequency range and a nominal operating voltage amplitude range. The converter arrangement (60) comprises at least one converter module (63, 64, 65), which comprises at least one first converter unit (66, 73, 81) configured to, on basis of power conveyed from the first power system (61) and the nominal operating voltage frequency range, generate an amplitude-modulated voltage waveform having a selected frequency within the nominal operating voltage frequency range. The at least one converter module (63, 64, 65) comprises at least one transformer (67, 74, 82) configured to receive the amplitude-modulated voltage waveform and convert amplitude thereof on basis the nominal operating voltage amplitude range so as to generate an amplitude-modulated voltage waveform having the selected frequency within the nominal operating voltage frequency range and a selected amplitude within the nominal operating voltage amplitude range. The at least one converter module (63, 64, 65) comprises at least one second converter unit (68, 75, 83) configured to receive the amplitude-modulated voltage waveform generated by the at least one transformer (67, 74, 82) and on basis thereof generate a demodulated voltage waveform having a frequency within the nominal operating voltage frequency range and an amplitude within the nominal operating voltage amplitude range, wherein the at least one second converter unit (68, 75, 83) is electrically connected to the second power system (62).

Description

CONVERTER ARRANGEMENT

TECHNICAL FIELD

The present invention generally relates to the field of power transmission systems, which for example may include a High Voltage Direct Current (HVDC) power system. Specifically, the present invention relates to a converter arrangement configured to convey power from a first power system to a second power system, or vice versa. The first power system may comprise an alternating current (AC) power system or a direct current (DC) power system, such as a HVDC power system. The first power system may include one or more AC power systems and one or more DC power systems interconnected in an appropriate manner.

BACKGROUND

Power electronics converters and drives are used to supply power to a variety of loads. Power electronics converters and drives are usually employed in order to provide a desired or even required current and/or voltage to the loads, so as to facilitate or enable controlling operation of the loads, for example with respect to speed and torque of electrical motors, and voltage frequency of a power system or power grid. According to one example, power semiconductor elements may be employed in frequency converters which may be used to drive loads for example in the form of electrical motors in equipment such as pumps, compressors etc. Frequency converters in a medium voltage range and in high power range (about 0.1-100 MW) can be used to drive electrical motors by controlling the speed and torque thereof.

Recently there has been a growing interest in submersible electric equipment, for example in electrical installations which may be arranged at seabed, or sea floor or ocean floor, locations, which may be at depths from a few tens of meters up to several kilometers or even more. Such interest has recently been growing for example in the oil and gas industry. Subsea oil and gas production may require electric equipment such as drilling motors, pumps, compressors, etc. Such equipment is usually driven by means of frequency converters or the like located above the sea or ocean level, e.g. arranged on platforms which may be floatable or fixed to the sea floor. In order to convey power from the frequency converters to subsea electric equipment, so called umbilicals, or umbilical cables, are often used.

An example of a power supply system which is currently under development involves the concept of variable speed drives (VSDs). For example in subsea applications, it may be desired or even required that the VSD, and preferably the whole power supply system, exhibits a high reliability with respect to operation thereof. A relatively high degree of modularity of the VSDs and/or power electronics converters units is also desired or even required. For subsea loads, operation of high power electrical motors may be controlled within a voltage frequency range of about 10 Hz to about 300 Hz. That is, the electrical motors may have a nominal operating voltage frequency interval or range of about 10 Hz to about 300 Hz, in which voltage frequency range the electrical motors are designed to operate. It is therefore desired or even required that power can be supplied to the loads, e.g. electrical motors, in a controllable manner from the power source (which is usually located onshore or above the sea or ocean level) to which the VDS is connected by way of an AC and/or a DC transmission link, such that the voltage at the input to the load has a frequency within the nominal operating voltage frequency range of the load.

It may be the case that the preferred output voltage level of the VSD, as possibly required by the rated voltage of the load, is lower than the voltage level of the power source to which the VSD unit is connected. The voltage supplied to the load may therefore be required to be down-converted to a voltage level which may be required by the load, so that the inverter stage(s) of the VSD can operate efficiently. If the power source to which the VDS is connected is an AC power system (such an AC power distribution system), the voltage down-conversion stage may for example be implemented by way of a step-down transformer. If the VSD is intended to be connected directly to a DC power system such as a DC power distribution system, the voltage down-conversion stage may need to be implemented by way of a dedicated power electronics voltage converter. The voltage converter should preferably also provide galvanic insulation between the DC power system and the VSD unit.

SUMMARY

In subsea applications where power is desired or required to be conveyed to subsea loads located relatively far from the power source to which the VDS unit is connected (e.g., an onshore power grid), it may be preferred to employ a DC power transmission link for conveying of power from the power source to the VSD unit. The DC power transmission link may for example be an HVDC power transmission link. In such case, the VSD unit may require several power conversion stages. The power may then be supplied from the DC power transmission link to an inverter and a transformer, then via an AC power distribution system to the VSD unit, passing through a transformer of the VSD unit (reducing the AC voltage level), further on to a rectifier of the VSD unit, and subsequently to an inverter of the VSD unit via a DC link of the VSD unit, and then supplied to the load. The power transmission system for conveying power to the load may thus in general comprise a relatively large number of conversion stages, and at least two inverters may be required in order to provide a desired or required controllability of voltage at the input to the loads. The complexity of the power transmission system may thereby be relatively high.

For example in applications such as subsea applications, where maintenance of the power transmission system for conveying power to the load may be relatively difficult and/or relatively expensive (e.g., due to the fact that at least a part or portion of the power transmission system is arranged at a subsea location), it may be desired or even required to modify the power transmission system so as to reduce its complexity. For example, it may be desired or even required to modify the construction of a VSD unit so that it becomes less complex, e.g., so that it requires only a relatively small number of constituents or

components, thereby possibly requiring less or even substantially no maintenance.

Maintenance of power transmission systems for conveying power to subsea loads may be so demanding that the only way to increase and/or ensure availability of the power transmission system may be to increase and/or ensure a relatively high operational reliability thereof.

In view of the above, a concern of the present invention is to provide a power transmission system for conveying power to a power system, for example a load such as an asynchronous motor, or an AC power distribution system, which power system has at least a nominal operating voltage frequency range within which the power system is designed to operate, which power transmission system has a relatively low complexity, and possibly includes a relatively low number of components.

A further concern of the present invention is to provide a converter arrangement or VSD unit for use in a power transmission system for conveying power to a power system, for example a load such as an asynchronous motor, or an AC power distribution system, which power system has at least a nominal operating voltage frequency range within which the power system is designed to operate, which converter arrangement or VSD unit facilitates or enables achieving a relatively low complexity in the power transmission system, and possibly facilitates or allows for the total number of components of the power transmission system to be relatively low.

To address at least one of these concerns and other concerns, a converter arrangement in accordance with the independent claim is provided. Preferred embodiments are defined by the dependent claims.

According to a first aspect, there is provided a converter arrangement which is configured to convey power from a first power system to a second power system, or vice versa. The second power system has a nominal operating voltage frequency range, or interval, and a nominal operating voltage amplitude range, or interval. The converter arrangement comprises at least one converter module. The at least one converter module comprises at least one first converter unit which is configured to, on basis of power conveyed from the first power system and the nominal operating voltage frequency range, generate an amplitude- modulated voltage waveform having a selected frequency within the nominal operating voltage frequency range. The at least one converter module comprises at least one transformer, which is configured to receive the amplitude-modulated voltage waveform generated by the at least one first converter unit, and convert amplitude of the received amplitude-modulated voltage waveform on basis the nominal operating voltage amplitude range, so as to generate an amplitude -modulated voltage waveform having the selected frequency within the nominal operating voltage frequency range and a selected amplitude within the nominal operating voltage amplitude range. The at least one converter module comprises at least one second converter unit which is configured to receive the amplitude- modulated voltage waveform generated by the at least one transformer, and on basis thereof generate a demodulated voltage waveform having a frequency within the nominal operating voltage frequency range and an amplitude within the nominal operating voltage amplitude range. The at least one second converter unit is electrically connected to the second power system.

The first power system may for example comprise a DC power system. The converter arrangement may for example be directly electrically connected to the first power system. In alternative or in addition, the first power system may comprise an AC power system. The second power system may for example comprise an asynchronous motor, an AC power distribution system, and/or an AC power generation system. For example in the case where the second power system comprises an AC electrical motor (e.g., an asynchronous motor), the converter arrangement may facilitate or enable drive functionality, or

controllability of voltage level and frequency at the input of the AC electrical motor so as to facilitate or enable controlling the speed and torque thereof. By 'integrating' the drive functionality and/or controllability with the voltage conversion as in the converter arrangement, a smaller number of conversion stages and/or inverters may be required in the converter arrangement compared to for example a VSD unit such as described in the foregoing.

According to a second aspect, there is provided an arrangement comprising a converter arrangement according to the first aspect, configured to convey power from a first power system to a second power system, or vice versa, wherein the second power system has a nominal operating voltage frequency range and a nominal operating voltage amplitude range. The arrangement may comprise a control unit configured to transmit control signals to the first converter unit and the transformer, respectively, by way of which control signal the at least one first converter unit and the transformer, respectively, are caused to generate the respective amplitude-modulated voltage waveforms.

According to a third aspect, there is provided a power transmission system comprising a first power system and a second power system. The second power system may have a nominal operating voltage frequency range and a nominal operating voltage amplitude range. The power transmission system comprises at least one converter arrangement according to the first aspect, configured to convey power from the first power system to the second power system, or vice versa.

In the context of the present application, by nominal operating voltage frequency range of a power system it is meant a voltage frequency range within which the power system is designed and/or desired or even required to operate.

In the context of the present application, by nominal operating voltage amplitude range of a power system it is meant a voltage amplitude range within which the power system is designed and/or desired or even required to operate.

The converter arrangement may comprise a plurality of electrically connected converter modules.

According to an embodiment of the present invention, the at least one first converter unit may be configured to, on basis of power conveyed from the first power system and the nominal operating voltage frequency range, generate a symmetrical amplitude- modulated voltage waveform having a selected frequency within the nominal operating voltage frequency range.

The at least one first converter unit may for example be configured to generate a carrier signal based on the power conveyed from the first power system and a selected carrier signal frequency. Based on the nominal operating voltage frequency range, an envelope voltage waveform may be generated. The amplitude-modulated voltage waveform may be generated based on the carrier signal and the envelope voltage waveform.

According to an embodiment of the present invention, the envelope voltage waveform may comprise only pulses of one type of polarity.

For example, the pulses of the envelope voltage waveform may be shaped in accordance with the shape of the positive or negative polarity pulses of a sine wave.

According to an embodiment of the present invention, the at least one second converter unit may be configured to generate the demodulated voltage waveform so that it comprises alternating positive polarity pulses and negative polarity pulses.

According to another embodiment of the present invention, the at least one second converter unit may be configured to rectify and frequency filter the received amplitude-modulated voltage waveform so as to generate the demodulated voltage waveform.

For example, the at least one second converter unit may be configured to rectify and frequency filter the received amplitude-modulated voltage waveform such that the rectified and frequency filtered voltage waveform comprises only pulses of positive polarity. The at least one second converter unit may be configured to invert polarity of every other pulse such that the demodulated voltage waveform comprises alternating positive polarity pulses and negative polarity pulses. For example, the at least one second converter unit may be configured to rectify and frequency filter the received amplitude-modulated voltage waveform such that the pulses of the rectified and frequency filtered voltage waveform are shaped in accordance with the shapes of the positive or negative polarity pulses of a sine wave.

According to another embodiment of the present invention, the at least one first converter unit, the at least one transformer, and the at least one second converter unit are configured according to a dual active bridge converter topology.

Further objects and advantages of the present invention are described in the following by means of exemplifying embodiments. It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the description herein. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

Figure 1 is a schematic circuit diagram of a power transmission system for supplying or conveying power from a DC power source or DC power system to an AC power system or electrical load.

Figure 2 is a schematic circuit diagram of a power transmission system comprising a converter arrangement according to an embodiment of the present invention.

Figure 3 is a schematic circuit diagram of a power transmission system comprising a converter arrangement according to another embodiment of the present invention.

Figure 4 is a graph of an envelope voltage waveform ui (in arbitrary unit) over two periods T generated by a voltage balancer of a converter arrangement in accordance with an embodiment of the present invention.

Figure 5 is a graph of carrier signal or control signal (in arbitrary unit) over two periods T, which carrier signal has been generated by an inverter of a converter arrangement in accordance with an embodiment of the present invention.

Figure 6 is a graph of an amplitude-modulated voltage waveform u₂ (in arbitrary unit) over two periods T at the output of an inverter of a converter arrangement in accordance with an embodiment of the present invention.

Figure 7 is a graph of an amplitude-modulated voltage waveform u₄ (in arbitrary unit) over two periods T at the output of a rectifier of a converter arrangement in accordance with an embodiment of the present invention. Figure 8 is a graph of a demodulated voltage waveform U5 (in arbitrary unit) over two periods T at the output of a polarity inverter of a converter arrangement in accordance with an embodiment of the present invention.

Figure 9 is a schematic circuit diagram of a power transmission system comprising several converter arrangements according to embodiments of the present invention.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate embodiments of the present invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments of the present invention set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

Figure 1 is a schematic circuit diagram of a power transmission system 1 for supplying or conveying power from a DC power source or DC power system 2 to an AC power system or electrical load 3, 4 in accordance with an example, which is relevant for example in subsea applications. The AC power system 3, 4 may for example comprise an asynchronous motor, i.e. an AC electrical motor, and/or an AC power distribution system. The asynchronous motor may be supplied with voltage having variable frequency, and the AC power distribution system may be supplied with voltage having a constant frequency.

According to the example illustrated in Figure 1 , the DC power system 2 is an HVDC power system 2. The power transmission system 1 comprises an inverter 5, a transformer 6, and an AC power distribution system, schematically indicated in Figure 1 at 7. The AC power distribution system 7 is electrically connected to several VSD units 8, 9 (of which only two are shown in Figure 1). However, the number of VSD units shown 8, 9 shown in Figure 1 is according to an example, and it is to be understood that the power transmission system 1 may comprise a single VSD unit, or more than two VSD units. Each VSD unit 8, 9 comprises a transformer 10, 11, a rectifier 12, 13, a DC transmission link 14, 15, and an inverter 16, 17, which may be electrically connected in that order such as illustrated in Figure 1. The VSD units 8, 9 are electrically connected to respective ones of the AC power systems 3, 4. In order to provide a voltage having for example a selected frequency at the respective inputs of the AC power systems 3, 4, several stages of voltage conversion may be required: voltage level conversion at transformer 6, voltage level conversion at transformers 10, 11, voltage frequency conversion at rectifiers 12, 13, and control of voltage level and frequency at the outputs of inverters 16, 17. The transformers 6, 10, 11 in addition provide galvanic isolation. A power transmission system 1 such as illustrated in Figure 1 hence comprises a relatively large number of conversion stages, and several inverters 5, 16, 17 may be required in order to provide a desired or required controllability of voltage at the input to the AC power systems or electrical loads 3, 4. The complexity of the power transmission system 1 may hence be relatively high.

Figure 2 is a schematic circuit diagram of a power transmission system 50 comprising a converter arrangement or VSD unit 20 according to an embodiment of the present invention. The converter arrangement 20 is configured to convey power from a first power system 21 to a second power system 22, or vice versa. The second power system 22 has a nominal operating voltage frequency range, e.g. a voltage frequency range within which the second power system 22 is designed and/or desired or even required to operate, and a nominal operating voltage amplitude range, e.g. a voltage amplitude range within which the second power system 22 is designed and/or desired or even required to operate.

The first power system 21 may for example comprise a DC power system, or a DC grid, or a HVDC power system or HVDC grid. The converter arrangement 20 may be directly electrically connected to the first power system 21. In alternative or in addition, the first power system 21 may comprise an AC power system.

The second power system 22 may for example comprise an asynchronous motor, which may be designed such that operation of the asynchronous motor may be controlled for example within a voltage frequency range of about 10 Hz to about 300 Hz. That is, the asynchronous motor may have a nominal operating voltage frequency range of about 10 Hz to about 300 Hz, within which voltage frequency range the asynchronous motor is designed to operate. The asynchronous motor may be supplied with voltage having variable frequency.

In alternative or in addition, the second power system 22 may for example comprise an industry load, an AC power distribution system and/or an AC power generation system, which may be supplied with voltage having a constant frequency. The AC power distribution system and/or AC power generation system may for example comprise a wind farm, or one or several electrically interconnected wind turbines, or other types of power generation devices. The second power system 22 may for example be arranged at a subsea location.

As will be further described in the following with reference to Figure 3, the converter arrangement 20 comprises at least one first converter unit, a transformer, and at least one second converter unit. The transformer is connected between the at least one first converter unit and the at least one second converter unit. The at least one second converter unit is electrically connected to the second power system 22. Power can be conveyed from the first power system 21 to the at least one first converter unit of the converter arrangement 20. The at least one first converter unit, a transformer, and at least one second converter unit of the converter arrangement 20 are configured such that the converter arrangement 20 is able to output a voltage waveform having a frequency within the nominal operating voltage frequency range, and an amplitude within the nominal operating voltage amplitude range.

Thus, the converter arrangement or VSD unit 20 may be able to be directly connected to a DC power system (which may constituted by or be included in the first power system 21) and interconnect the DC power system or first power system 21 with the second power system 22. As will be described in more detail with reference to Figure 3, within the converter arrangement or VSD unit 20 there is provided circuitry or components for providing galvanic separation between the first power system 21 and the second power system 22 and voltage level (i.e. amplitude) and frequency and amplitude conversion such that voltage at the input to the second power system 22 has a selected frequency within the nominal operating voltage frequency range (or at least a frequency within the nominal operating voltage frequency range) and a selected amplitude within the nominal operating voltage amplitude range (or at least an amplitude within the nominal operating voltage amplitude range). To that end, the at least one converter unit is configured to, on basis of power conveyed from the first power system 21 and the nominal operating voltage frequency range, generate an amplitude- modulated voltage waveform having a selected frequency within the nominal operating voltage frequency range. The at least one transformer is configured to receive the amplitude- modulated voltage waveform generated by the at least one first converter unit, and convert amplitude of the received amplitude -modulated voltage waveform on basis the nominal operating voltage amplitude range, so as to generate an amplitude-modulated voltage waveform having the selected frequency within the nominal operating voltage frequency range and a selected amplitude within the nominal operating voltage amplitude range. The at least one second converter unit which is configured to receive the amplitude-modulated voltage waveform generated by the at least one transformer, and on basis thereof generate a demodulated voltage waveform having a frequency within the nominal operating voltage frequency range and an amplitude within the nominal operating voltage amplitude range. The demodulated voltage waveform, which hence may have a frequency within the nominal operating voltage frequency range of the second power system 22 and an amplitude within the nominal operating voltage amplitude range of the second power system 22, can then be conveyed or supplied to the second power system 22. In this way, controllability of frequency and amplitude of voltage at the input of the second power system 22 may be achieved. For example in the case where the second power system 22 comprises an AC electrical motor (e.g., an asynchronous motor), the converter arrangement 20 may facilitate or enable drive functionality, or controllability of voltage level and frequency at the input of the AC electrical motor so as to facilitate or enable controlling the speed and torque thereof. By 'integrating' the drive functionality and/or controllability with the voltage conversion as in the converter arrangement 20, a smaller number of conversion stages and/or inverters may be required in the converter arrangement 20 compared to for example the VSD units 8, 9 described in the foregoing with reference to Figure 1.

A control unit 23 may be provided, which control unit 23 may be communicatively coupled to the converter arrangement 20 and possibly to the first power system 21 and/or the second power system 22. By two entities being communicatively coupled it is in the context of the present application meant that the two entities are coupled so as to facilitate or enable transmission of data, signals, commands, messages, etc., between the two entities. According to the embodiment of the present invention illustrated in Figure 2, the control unit 23 is communicatively coupled to the converter arrangement 20, the first power system 21, and to the second power system 22. For communicatively coupling the control unit 23 with the converter arrangement 20 and possibly the first power system 21 and/or the second power system 22, in principle any appropriate communication technique or means known in the art may for example be employed. For example, any appropriate

telecommunication, data transmission, digital transmission, or digital communication technique may be employed. The communication technique or means may be wired and/or wireless as known in the art.

The control unit 23 may be configured to transmit control signals to the first converter unit and the transformer, respectively. By way of the control signals, the at least one first converter unit and the transformer, respectively, may be caused to generate the respective amplitude-modulated voltage waveforms.

Information on for example the nominal operating voltage frequency range, the nominal operating voltage amplitude range and/or a carrier signal frequency (see the description in the following with reference to Figure 3), etc., or other information which may be used in operation of the converter arrangement 20 may be communicated to the control unit 23, e.g. transmitted by a power system controller or the like. In alternative or in addition, such information may be predefined and possibly stored in a memory in the control unit 23.

The control unit 23 may for example include any suitable central processing unit (CPU), microcontroller, Digital Signal Processor (DSP), Application Specific Integrated Circuit (ASIC), Field-Programmable Gate Array (FPGA), etc., or any combination thereof. The control unit 23 may optionally be capable of executing software instructions stored in a computer program product e.g. in the form of a memory. The memory may for example be any combination of Random Access Memory (RAM) and Read-Only Memory (ROM). The memory may comprise persistent storage, which for example can be a magnetic memory, an optical memory, a solid state memory or a remotely mounted memory, or any combination thereof. Figure 3 is a schematic circuit diagram of a power transmission system 100 comprising a converter arrangement or VSD unit 60 according to another embodiment of the present invention. The converter arrangement 60 is configured to convey power from a first power system 61 to a second power system 62, or vice versa.

The second power system 62 has a nominal operating voltage frequency range, e.g. a voltage frequency range within which the second power system 62 is designed and/or desired or even required to operate, and a nominal operating voltage amplitude range, e.g. a voltage amplitude range within which the second power system 62 is designed and/or desired or even required to operate.

The second power system 62 may for example be arranged at a subsea location.

In accordance with the embodiment of the present invention illustrated in Figure 3, the first power system 61 is a DC power system, to which the converter arrangement 60 is directly electrically connected. The DC power system may for example carry DC voltage having a voltage level of a few or tens of kV or an even larger voltage level.

However, according to other embodiments of the present invention, the first power system 61 may comprise an AC power system.

The converter arrangement 60 comprises three converter modules 63, 64, 65. In accordance with the embodiment of the present invention illustrated in Figure 3, the three converter modules 63, 64, 65 may for example be electrically connected in series, e.g.

between first and second DC poles or terminals of the first power system 61. However, this is according to a non-limiting example. According to another example, the three converter modules 63, 64, 65 may be electrically connected in parallel. It is to be understood that the number of converter modules shown in Figure 3 is according to an example. The converter arrangement 60 may comprise one, two, four, five, six, eight, or ten or more converter modules.

The second power system 62, which as indicated above may comprise an AC power system, and/or an AC electrical motor or an industry load, may be configured as a multi-phase arrangement or a single-phase arrangement. The second power system 62 may for example comprise an asynchronous motor, which may be designed such that operation of the asynchronous motor may be controlled for example within a voltage frequency range of about 10 Hz to about 300 Hz. That is, the asynchronous motor may have a nominal operating voltage frequency range of about 10 Hz to about 300 Hz, in which voltage frequency range the asynchronous motor is designed to operate. The asynchronous motor may be supplied with voltage having variable frequency. In alternative or in addition, the second power system 62 may for example comprise an AC power distribution system and/or an AC power generation system, which may be supplied with voltage having a constant frequency. The AC power distribution system and/or AC power generation system may for example comprise a wind farm, or one or several electrically interconnected wind turbines. In accordance with the embodiment of the present invention illustrated in Figure 3, the second power system 62 is configured as a three-phase arrangement or three- phase system, wherein each of the converter modules 63, 64, 65 corresponds to one of the phases. In general, in case the second power system 62 comprises a multi-phase AC power system, the converter arrangement 60 may comprise several converter modules (at least as many as the number of phases), and where each converter module may correspond to one of the phases of the multi-phase AC power system.

The converter module 63 comprises a first converter unit 66, a transformer 67, and a second converter unit 68. The transformer 67 is connected between the first converter unit 66 and the second converter unit 68. The second converter unit 68 is electrically connected to the second power system 62. Power - which in accordance with the embodiment of the present invention illustrated in Figure 3 for example may be DC power - can be conveyed from the first power system 61 to the first converter unit 66 of the converter module 63, and also to (first converter units of) the other converter modules 64, 65.

The first converter unit 66, the transformer 67, and the second converter unit

68 may for example be configured similarly or according to a dual active bridge (DAB) converter topology. In other words, the first converter unit 66, the transformer 67, and the second converter unit 68 may operate according to operating principles of a DAB converter system, e.g. such as known in the art. Operating the first converter unit 66, the transformer 67, and the second converter unit 68 according to operating principles of a DAB converter system may facilitate or allow for achieving bidirectional power transfer or conveyance by way of the converter arrangement 60 between the first power system 61 and the second power system 62. In alternative or in addition, the first converter unit 66, the transformer 67, and the second converter unit 68, and/or other components of the converter arrangement 60 may be configured according to another converter topology as known in the art for facilitating or allowing for the bidirectional power transfer. According to another example, the first converter unit 66 and the second converter unit 68, and/or other components of the converter arrangement 60 may in alternative or in addition be configured according to an H bridge circuit topology, an active bridge configuration, or a multi-level inverter configuration for facilitating or allowing for the bidirectional power transfer.

As will be further described in the following, in case of unidirectional operation of the converter arrangement 60, where power can be conveyed by the converter arrangement 60 from the first power system 61 to the second power system 62, the first converter unit 66 may comprise a first switching unit 71 for example in the form of an inverter 71 and possibly a voltage balancer 69, and the second converter unit 68 may comprise second switching unit 72 for example in the form of a rectifier 72, and a polarity inverter 73. In case of bidirectional operation of the converter arrangement 60, wherein power can be conveyed by the converter arrangement 60 also from the second power system 62 to the first power system 61 as well as from the first power system 61 to the second power system 62, the first converter unit 66 may in addition or in alternative comprise a polarity reverser or polarity inverter, which for example may be included in or be a part or portion of the voltage balancer 69 (which hence in such case may be referred to as a voltage balancer and polarity reverser or polarity inverter).

Further in case of bidirectional operation of the converter arrangement 60, the first switching unit 71 of the first converter unit 66, the transformer 67, and the second switching unit 72 of the second converter unit 68 may for example operate according to operating principles of a DAB converter system, as known in the art, for facilitating or allowing for bidirectional power transfer via the converter module 63. According to another example, the first switching unit 71 and the second switching unit 72, and/or other components of the converter arrangement 60 may in alternative or in addition be configured according to an H bridge circuit configuration, an active bridge configuration, or a multi-level inverter configuration, as known in the art, for facilitating or allowing for the bidirectional power transfer via the converter module 63.

Any one of the first converter unit 66, the second converter unit 68, the first switching unit 71, and the second switching unit 72 may for example be based on or include active (controllable) switching devices or switching elements such as transistors, thyristors, etc., or passive switching devices or switching elements such as diodes. The first converter unit 66 is configured to, on basis of power conveyed from the first power system 61 and the nominal operating voltage frequency range, generate an amplitude-modulated voltage waveform, which for example may be periodical and/or symmetrical, and which has a selected frequency within the nominal operating voltage frequency range. The selected frequency may for example be a fundamental frequency of the second power system 62.

The first converter unit 66 may be configured to receive the power conveyed from the first power system 61. The power from the first power system 61 may for example be conveyed directly from the first power system 61 to the first converter unit 66, or via some intermediate unit(s) or component(s). The power conveyed from the first power system 61 may be generated by the first power system 61. According to another example, power may be generated by some entity other than the first power system 61, and the first power system 61 may be configured to distribute or convey that power to other entities such as the converter arrangement 60.

According to an exemplifying implementation, the first converter unit 66 may be configured to generate a carrier signal or control signal based on the power conveyed from the first power system 61 and a selected or predefined carrier signal frequency. The first converter unit 66 may be configured to generate an envelope voltage waveform based on the nominal operating voltage frequency range. The first converter unit 66 may be configured to generate the amplitude-modulated voltage waveform based on the carrier signal and the envelope voltage waveform.

A so called voltage balancer 69 which may be included in the first converter unit 66 may be configured to generate the envelope voltage waveform based on the nominal operating voltage frequency range.

As mentioned in the foregoing, according to other embodiments of the present invention, the first power system 61 may comprise or be an AC power system, such that power conveyed or supplied to the converter arrangement 60 at the first converter unit 66 side is AC power. For example, the converter arrangement 60 may be supplied at its first converter unit 66 side with sinusoidal AC voltage. The first converter unit 66 side of the converter arrangement 60 may for example be directly electrically connected to AC power system. For example in such case, generation of an envelope voltage waveform (and the voltage balancer 69) may be omitted. The first converter unit 66 may then be configured to generate the amplitude-modulated voltage waveform for example by way of modulating the received AC voltage signal or waveform with a modulation signal based on the nominal voltage frequency range.

In case of bidirectional operation of the converter arrangement 60, i.e. wherein power can be conveyed by the converter arrangement 60 both from the first power system 61 to the second power system 62 as well as from the second power system 62 to the first power system 61, the voltage balancer 69 may operate in conjunction with polarity reverser or polarity inverter (which hence may be included in the first converter unit 66). According to another example, the polarity reverser or polarity inverter may be included in or be a part or portion of the voltage balancer 69. However, for example in case the first power system 61 comprises or is an AC power system, such that power conveyed or supplied to the converter arrangement 60 at the first converter unit 66 side is AC power, both the voltage balancer 69 and the polarity reverser may be omitted, and the first converter unit 66 may be configured to generate the amplitude-modulated voltage waveform for example by way of modulating the received AC voltage signal or waveform with a modulation signal based on the nominal voltage frequency range.

Figure 4 shows an envelope voltage waveform ui (in arbitrary unit) over two periods T generated by the voltage balancer 69 in accordance with an embodiment of the present invention, which envelope voltage waveform has a (selected) frequency within the nominal operating voltage frequency range, e.g. between 0 Hz and about 300 Hz as indicated in Figure 4, or between about 10 Hz and about 300 Hz. The envelope voltage waveform ui is the voltage waveform present at the output of the voltage balancer 69. In accordance with the embodiment of the present invention illustrated in Figure 4, the envelope voltage waveform may for example comprise only pulses of positive polarity. In general, the envelope voltage waveform may comprise only pulses of one polarity, or one type of polarity (either positive or negative). Further in accordance with the embodiment of the present invention illustrated in Figure 4, pulses of the envelope voltage waveform may be shaped in accordance with the shape of the positive polarity pulses of a sine wave. According to another embodiment, the pulses of the envelope voltage waveform may be shaped in accordance with the shape of the negative polarity pulses of a sine wave. The (selected) frequency of the envelope voltage waveform will govern the frequency of the voltage waveform which is output by the second converter unit 68 (described further in the following) and which can be conveyed or supplied to the second power system 62. As mentioned in the foregoing, the selected frequency may for example be a fundamental frequency of the second power system 62 (e.g., including or being constituted by an AC power system).

Referring now again to Figure 3, the first converter unit 66 may comprise a first switching unit 71 for example constituted by or including an inverter 71 , which may be configured to generate the amplitude-modulated voltage waveform based on the carrier signal and the envelope voltage waveform.

According to the embodiment illustrated in Figure 3, the inverter 71 may comprise at least one electrical energy storage element in the form of a capacitor(not shown in Figure 3), and a switching element block(not shown in Figure 3) electrically connected to the electrical energy storage element or capacitor, for example in parallel. The switching element block may for example comprise at least one transistor-diode pair. The transistor may for example comprise an insulated gate bipolar transistor (IGBT). It is however to be understood that this is according to an example, and that other types of switching element blocks can be used. Also, other types of electrical energy storage elements than capacitors may be employed. The inverter 71 may be configured as a half-bridge circuit, wherein the switching element block may comprise two switches or switching elements connected in series across the electrical energy storage element, with a midpoint connection between the switches or switching elements and one of the electrical energy storage element terminals as external connections. However, it is to be understood that such a configuration of the inverter 71 is according to a non-limiting example and that variations are possible. For example, the inverter 71 could be configured as a full -bridge circuit. Configuring the inverter 71 as a full-bridge circuit may allow for or facilitate insertion of the electrical energy storage element into the circuit in either polarity.

An alternative way to describe the inverter 71 is that it may comprise a multilevel converter cell, i.e. a converter cell that is configured so as to be capable of providing a multiple of (two or more) voltage levels, which may be used in forming an (modulated) AC voltage waveform. The multi-level converter cell may for example comprise a half -bridge, or two-level, cell or a full-bridge, or three-level, cell. The multi-level converter cell may for example comprise at least one capacitor, and/or another type of electrical energy storage element, electrically connected, e.g. in parallel, with a series connection of switching elements, e.g. including Integrated Gate-Commutated Transistor (IGBT)-diode pairs, each IGBT-diode pair comprising one or more IGBTs and a diode arranged in parallel or anti- parallel fashion with respect to the IGBT(s).

As mentioned in the foregoing, the first converter unit 66 may be configured to generate a carrier signal or control signal based on the power conveyed from the first power system 61 and a selected carrier signal frequency, and the first converter unit 66 may be configured to generate the amplitude-modulated voltage waveform based on the carrier signal and the envelope voltage waveform.

Figure 5 shows an example of a carrier signal or control signal (in arbitrary unit) over two periods T in accordance with an embodiment of the present invention, which carrier signal has been generated by the inverter 71. As seen in Figure 5, the carrier signal may exhibit a square waveform. As indicated in the foregoing, the inverter 71 can for example be configured as a half -bridge (two- state) circuit or a full-bridge (three-state) circuit, which can be used in order to generate the square waveform which is shown in Figure 5, e.g. by way of switching between two voltage levels of different polarities. The carrier signal frequency may for example be approximately 2 kHz but it is to be understood that variations are possible.

Figure 6 shows an example of an amplitude-modulated voltage waveform u₂ (in arbitrary unit) over two periods T in accordance with an embodiment of the present invention, which amplitude-modulated voltage waveform u₂ has been generated based on the carrier signal shown in Figure 5 and the envelope voltage waveform ui shown in Figure 4. According to the embodiment of the present invention illustrated in Figure 6, the amplitude- modulated voltage waveform u₂ has been generated by multiplying the carrier signal shown in Figure 5 and the envelope voltage waveform ui shown in Figure 4. As illustrated in Figure 6, the amplitude-modulated voltage waveform u₂ should preferably be symmetrical (in Figure 6, the amplitude-modulated voltage waveform u₂ is symmetrical with respect to the horizontal axis in Figure 6). In that way, any occurrence of low frequency currents in the transformer 67 may be avoided or at least reduced.

Referring now again to Figure 3, the converter module 63 comprises a transformer 67 (and/or some other galvanic isolator for example such as known in the art) configured to receive the amplitude-modulated voltage waveform generated by first converter unit 66. The transformer 67 is configured to convert amplitude of the received amplitude- modulated voltage waveform, on basis the nominal operating voltage amplitude range, so as to generate an amplitude-modulated voltage waveform having the selected frequency within the nominal operating voltage frequency range and a selected amplitude within the nominal operating voltage amplitude range. The transformer 67 may for example be implemented by way of a step-down (or step-up) transformer, which down-converts (up-converts) amplitude of the received amplitude-modulated voltage waveform by a selected or predefined transformer ratio so as to attain a selected amplitude of the amplitude-modulated voltage waveform output from the transformer 67 that is within the nominal operating voltage amplitude range, or at least so as to attain an amplitude of the amplitude-modulated voltage waveform output from the transformer 67 that is within the nominal operating voltage amplitude range.

The transformer provides galvanic isolation between the first power system 61 and the second power system 62.

As indicated in Figure 3, the transformer 67 (and also the transformers 74, 82 of the converter modules 64, 65) may be single-phase transformers, or single-phase winding transformers. By way of such transformers, modularization of the converter arrangement 60 may be facilitated or even enabled.

According to embodiments of the present invention, the transformer 67 comprises a transformer having a relatively high nominal operating frequency. According to embodiments of the present invention, the transformer 67 may comprise a medium- frequency transformer, or a high-frequency transformer. In general, the higher the nominal operating frequency of the transformer, the smaller the size and lower the weight of the transformer, due to the relatively low volume of the magnetic core of the transformer.

In the context of the present application, by a medium-frequency transformer it is meant a transformer having a nominal operating frequency between about 300 Hz and about 20 kHz, or between about 500 Hz and 5 kHz. In the context of the present application, by a high-frequency transformer it is meant a transformer having a nominal operating frequency higher than about 5kHz, or higher than about 20 kHz.

The transformer 67 is connected between the first converter unit 66 and the second converter unit 68. The second converter unit 68 is configured to receive the amplitude- modulated voltage waveform which has been generated by the transformer 67, and on basis thereof generate a demodulated voltage waveform, which has a frequency within the nominal operating voltage frequency range and an amplitude within the nominal operating voltage amplitude range. The demodulated voltage waveform can then be conveyed or supplied to the second power system 62. The second converter unit 68 may be configured to generate the demodulated voltage waveform so that it comprises alternating positive polarity pulses and negative polarity pulses over time, for example so that it comprises a periodic and/or a sinusoidal shape (i.e. a sine wave shape). The second converter unit 68 may be configured to rectify and frequency filter the amplitude-modulated voltage waveform received from the transformer 67 so as to generate the demodulated voltage waveform.

In accordance with the embodiment of the present invention illustrated in Figure 3, the second converter unit 68 may be configured to rectify and possibly frequency filter the amplitude-modulated voltage waveform received from the transformer 67 such that the rectified and possibly frequency filtered voltage waveform comprises (possibly only) pulses of positive polarity. For example, the second converter unit 68 may be configured to rectify and frequency filter the amplitude-modulated voltage waveform received from the transformer 67 such that the pulses of the rectified and frequency filtered voltage waveform are shaped in accordance with the shapes of the positive polarity pulses of a sine wave. In accordance with another embodiment, the amplitude-modulated voltage waveform received from the transformer 67 may be rectified and possibly frequency filtered such that the rectified and possibly frequency filtered voltage waveform comprises (possibly only) pulses of negative polarity.

A second switching unit 72, which for example may be constituted by or including a rectifier 72, and which hence may be included in the second converter unit 68, may be configured to carry out the rectification and possibly also the frequency filtering. By way of frequency filtering, components of relatively high frequency may be removed from the signal which is input into the second converter unit 68 or second switching unit (e.g., rectifier) 72. The second switching unit or rectifier 72 may be a passive device, wherein switching device(s) or switching element(s) in the second switching unit or rectifier 72 includes diode(s) or the like. In alternative, the second switching unit or rectifier 72 may be configured to carry out active rectification, e.g., be controllable, wherein switching device(s) or switching element(s) in the second switching unit or rectifier 72 includes controllable switching device(s) or switching element(s) such as transistor(s), for example comprising at least one metal oxide semiconductor field-effect transistor (MOSFET), IGBT, or the like.

Figure 7 shows an example of an amplitude-modulated voltage waveform u₄ (in arbitrary unit) over two periods T in accordance with an embodiment of the present invention at the output of the second switching unit (e.g., a rectifier) 72. As can be seen in Figure 7, the pulses of the amplitude-modulated voltage waveform u₄ at the output of the rectifier 72 are shaped in accordance with the shapes of the positive polarity pulses of a sine wave. The amplitude-modulated voltage waveform u₄ shown in Figure 7 is a result of the bottom half of the amplitude-modulated voltage waveform u₂ shown in Figure 6, i.e. the portion of the amplitude-modulated voltage waveform u₂ shown in Figure 6 below the horizontal axis, being full-wave rectified. In the context of the present application, by full- wave rectifying it is meant converting the whole of an input waveform to an output waveform of constant polarity (positive or negative). Full- wave rectification can for example be achieved by means of a so called diode bridge or bridge rectifier, as known in the art.

Further in accordance with the embodiment of the present invention illustrated in Figure 3, the second converter unit 68 may be configured to invert polarity of every other pulse of the rectified and frequency filtered voltage waveform, such that the demodulated voltage waveform comprises alternating positive polarity pulses and negative polarity pulses, for example so that it comprises a periodic and/or a sinusoidal shape or a sine wave shape. A polarity inverter 73 which may be included in the second converter unit 68 may be configured to carry out the polarity inversion. In alternative or in addition, the polarity inverter 73 may further be configured to carry out the frequency filtering.

Figure 8 shows an example of a demodulated voltage waveform us (in arbitrary unit) over two periods T in accordance with an embodiment of the present invention at the output of the polarity inverter 73. In accordance with the embodiment of the present invention illustrated in Figure 8, switching element(s) of the polarity inverter 73 operate only twice per output voltage period T, at the zero-crossings of the waveform u₄. Therefore it is envisaged that relatively slow switching elements may be employed, such as, for example, integrated gate-commutated thyristors (IGCTs). Such slow switching elements may under zero voltage switching conditions provide zero switching losses, and thereby a relatively high efficiency. The demodulated voltage waveform us, which hence may have a frequency within the nominal operating voltage frequency range of the second power system 62 and an amplitude within the nominal operating voltage amplitude range of the second power system 62, can then be conveyed or supplied to the second power system 62. In this way, controllability of frequency and amplitude of voltage at the input of the second power system 62 may be achieved. For example in the case where the second power system 62 comprises an AC electrical motor (e.g., an asynchronous motor), the converter arrangement 60 may facilitate or enable drive functionality, or controllability of voltage level and frequency at the input of the AC electrical motor so as to facilitate or enable controlling the speed and torque thereof.

The two other converter modules 64, 65 may include components which are similar or identical to the components of the converter module 63 described in the foregoing. The respective components of the converter modules 64, 65 may include components which may have the same or similar functionality as the components of the converter module 63 such as described in the foregoing.

The converter module 64 includes a first converter unit 73, a transformer 74, and a second converter unit 75. The first converter unit 73 may include a voltage balancer 76, and a first switching unit 78 for example constituted by or including an inverter 78, which may comprise at least one electrical energy storage element for example in the form of a capacitor (not shown in Figure 3), and a switching element block (not shown in Figure 3) electrically connected to the electrical energy storage element or capacitor. The second converter unit 75 may include a second switching unit 79 for example constituted by or including a rectifier 79, and a polarity inverter 80.

The converter module 65 includes a first converter unit 81, a transformer 82, and a second converter unit 83. The first converter unit 81 may include a voltage balancer 84, and a first switching unit 86 for example constituted by or including an inverter 86, which may comprise at least one electrical energy storage element in the form of a capacitor (not shown in Figure 3), and a switching element block (not shown in Figure 3) electrically connected to the electrical energy storage element or capacitor. The second converter unit 83 may include a second switching unit 87 for example constituted by or including a rectifier 87, and a polarity inverter 88.

As mentioned in the foregoing, according to other embodiments of the present invention, the first power system 61 may comprise or be an AC power system, such that power conveyed or supplied to the converter arrangement 60 at the first converter unit 66, 73, 81 side is AC power. For example, the converter arrangement 60 may be supplied at its first converter unit 66, 73, 81 side with sinusoidal AC voltage. The first converter unit 66, 73, 81 side of the converter arrangement 60 may for example be directly electrically connected to AC power system. For example in such case, generation of an envelope voltage waveform

(and the voltage balancers 69, 76, 84) may be omitted. The first converter unit 66, 73, 81 may then be configured to generate the amplitude-modulated voltage waveform for example by way of modulating the received AC voltage signal or waveform with a modulation signal based on the nominal voltage frequency range.

In case of bidirectional operation of the converter arrangement 60, wherein power can be conveyed by the converter arrangement 60 also from the second power system 62 to the first power system 61 as well as from the first power system 61 to the second power system 62, each of the first converter units 73, 81 may in addition or in alternative comprise a polarity reverser or polarity inverter, which for example may be included in or be a part or portion of the voltage balancer 76, 84 (which hence in such case may be referred to as a voltage balancer and polarity reverser or polarity inverter).

Further in case of bidirectional operation of the converter arrangement 60, the first switching unit 78 of the first converter unit 73, the transformer 74, and the second switching unit 79 of the second converter unit 75 may for example operate according to operating principles of a DAB converter system for facilitating or allowing for bidirectional power transfer via the converter module 64. In alternative or in addition, the first switching unit 78 of the first converter unit 73 and the second switching unit 79 of the second converter unit 75, and/or another component of the converter arrangement 60 may in alternative or in addition be configured according to an H bridge circuit configuration, an active bridge configuration, or a multi-level inverter configuration, as known in the art, for facilitating or allowing for the bidirectional power transfer via the converter module 64. Similarly, the first switching unit 86 of the first converter unit 81 , the transformer 82, and the second switching unit 87 of the second converter unit 83 may for example operate according to operating principles of a DAB converter system for facilitating or allowing for bidirectional power transfer via the converter module 65. In alternative or in addition, the first switching unit 86 of the first converter unit 81 and the second switching unit 87 of the second converter unit 83, and/or another component of the converter arrangement 60 may in alternative or in addition be configured according to an H bridge circuit configuration, an active bridge configuration, or a multi-level inverter configuration, as known in the art, for facilitating or allowing for the bidirectional power transfer via the converter module 65.

Any one of the first converter units 73, 81, the second converter units 75, 83, the first switching units 78, 86, and the second switching units 79, 87 may for example be based on or include active (controllable) switching devices or switching elements such as transistors, thyristors, etc., or passive switching devices or switching elements such as diodes.

The envelope voltage waveform which is generated by the converter module 64 (possibly by the voltage balancer 76) may be shifted in phase by a selected or predefined phase shift, for example by approximately 120 degrees, with respect to the envelope voltage waveform generated by converter module 63, (possibly by the voltage balancer 69). Further, the envelope voltage waveform which is generated by converter module 65 (possibly by the voltage balancer 84) may be shifted in phase by a selected or predefined phase shift, for example by approximately 120 degrees, with respect to the envelope voltage waveform which is generated by the converter module 64 (possibly by the voltage balancer 76). Thereby, the sum of the three envelope voltage waveforms as generated by the respective first converter units 66, 73, 81 (possibly by the respective voltage balancers 69, 76, 84) of the three converter modules 63, 64, 65 may be approximately constant, and the input voltage across the input of the converter arrangement 60 may be maintained substantially constant. Thus, voltage at the input to the second power system 62 may for example be three-phase AC voltage having a frequency within the nominal operating voltage frequency range of the second power system 62 wherein the phases are shifted by e.g. 120 degrees.

It is to be understood that various components which are not illustrated in Figure 3 may be included in the converter arrangement 60. Such components, which thus are not shown in Figure 3, may for example include resistors, capacitors, smoothing circuits, filters, additional transformers and/or other auxiliary elements.

Referring now to Figure 9, there is shown a schematic circuit diagram of a power transmission system 200 comprising N converter arrangements 110-1, 110-2, 110- N according to embodiments of the present invention, each converter arrangement 110-1, 110-2, ..., 110-N being configured to convey power from a first power system 120 to a respective one of N second power systems 130-1, 130-2, ..., 130-N, or vice versa. N may in principle be any integer. Each of the converter arrangements 110-1, 110-2, ..., 110-N may for example be configured such as described in the foregoing with reference to Figure 2 or Figure 3.

Each of the second power systems 130-1, 130-2, 130-N may have a nominal operating voltage frequency range, e.g. a voltage frequency range within which the respective one of the second power systems 130- 1, 130-2, ..., 130-N are designed and/or desired or even required to operate, and a nominal operating voltage amplitude range, e.g. a voltage amplitude range within which the respective ones of the second power systems 130-1, 130-2, ..., 130-N are designed and/or desired or even required to operate.

In accordance with the embodiment of the present invention illustrated in Figure 9, the first power system 120 is a DC power system, to which each of the converter arrangements 110-1, 110-2, ..., 110-N is directly electrically connected. The DC power system may for example carry DC voltage having a voltage level of a few or tens of kV or an even larger voltage level. However, according to other embodiments of the present invention, the first power system 120 may be an AC power system. In accordance with the embodiment of the present invention illustrated in Figure 9, the N converter arrangements 110-1, 110-2, ..., 110-N may for example be electrically connected in series, e.g. between first and second DC poles or terminals of the first power system 120. However, this is according to a non- limiting example, and other types of electrical connections between the converter

arrangements 110-1, 110-2, ..., 110-N and the first power system 120 are contemplated.

Each of the second power systems 130-1, 130-2, 130-N may, as indicated in Figure 9, comprise an AC power system, and/or an AC electrical motor, and may be configured as a multi-phase arrangement or a single-phase arrangement. In accordance with the embodiment of the present invention illustrated in Figure 9, each of the second power systems 130-1, 130-2, ..., 130-N is configured as a three-phase arrangement or three-phase system. In case the second power systems 130-1, 130-2, 130-N comprise multi-phase AC power systems, the respective ones of the converter arrangements 110-1, 110-2, 110-N may comprise several converter modules (at least as many as the number of phases), and where each converter module may correspond to one of the phases of the multi -phase AC power system.

While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A converter arrangement (20; 60; 110-1, 110-2, ..., 110-N) configured to convey power from a first power system (21; 61) to a second power system (22; 62), or vice versa, wherein the second power system has a nominal operating voltage frequency range and a nominal operating voltage amplitude range;

the converter arrangement comprising:

at least one converter module (63, 64, 65) comprising:

at least one first converter unit (66, 73, 81) configured to, on basis of power conveyed from the first power system and the nominal operating voltage frequency range, generate an amplitude-modulated voltage waveform having a selected frequency within the nominal operating voltage frequency range;

at least one transformer (67, 74, 82) configured to receive the amplitude-modulated voltage waveform and convert amplitude thereof on basis the nominal operating voltage amplitude range so as to generate an amplitude- modulated voltage waveform having the selected frequency within the nominal operating voltage frequency range and a selected amplitude within the nominal operating voltage amplitude range; and

at least one second converter unit (68, 75, 83) configured to receive the amplitude-modulated voltage waveform generated by the at least one transformer and on basis thereof generate a demodulated voltage waveform having a frequency within the nominal operating voltage frequency range and an amplitude within the nominal operating voltage amplitude range, wherein the at least one second converter unit is electrically connected to the second power system.

2. A converter arrangement according to claim 1, wherein the at least one first converter unit is configured to, on basis of power conveyed from the first power system and the nominal operating voltage frequency range, generate a symmetrical amplitude-modulated voltage waveform having a selected frequency within the nominal operating voltage frequency range.

3. A converter arrangement according to claim 1 or 2, wherein the at least one first converter unit is configured to: generate a carrier signal based on the power conveyed from the first power system and a selected carrier signal frequency;

generate an envelope voltage waveform based on the nominal operating voltage frequency range; and

generate the amplitude-modulated voltage waveform based on the carrier signal and the envelope voltage waveform.

4. A converter arrangement according to claim 3, wherein the envelope voltage waveform comprises only pulses of one type of polarity.

5. A converter arrangement according to claim 4, wherein the pulses of the envelope voltage waveform are shaped in accordance with the shape of the positive or negative polarity pulses of a sine wave.

6. A converter arrangement according to any one of claims 1-5, wherein the at least one second converter unit is configured to generate the demodulated voltage waveform so that it comprises alternating positive polarity pulses and negative polarity pulses.

7. A converter arrangement according to any one of claims 1-6, wherein the at least one second converter unit is configured to rectify and frequency filter the received amplitude-modulated voltage waveform so as to generate the demodulated voltage waveform.

8. A converter arrangement according to claim 7, wherein the at least one second converter unit is configured to rectify and frequency filter the received amplitude-modulated voltage waveform such that the rectified and frequency filtered voltage waveform comprises only pulses of positive polarity, and wherein the at least one second converter unit is further configured to invert polarity of every other pulse, such that the demodulated voltage waveform comprises alternating positive polarity pulses and negative polarity pulses.

9. A converter arrangement according to claim 8, wherein the at least one second converter unit is configured to rectify and frequency filter the received amplitude-modulated voltage waveform such that the pulses of the rectified and frequency filtered voltage waveform are shaped in accordance with the shapes of the positive or negative polarity pulses of a sine wave.

10. A converter arrangement according to any one of claims 1-9, wherein the at least one first converter unit, the at least one transformer, and the at least one second converter unit are configured according to a dual active bridge converter topology.

11. A converter arrangement according to any one of claims 1-10, wherein the converter arrangement comprises a plurality of electrically connected converter modules (63, 64, 65).

12. A converter arrangement according to any one of claims 1-11, wherein the first power system comprises a DC power system, and the converter arrangement is directly electrically connected to the first power system.

13. A converter arrangement according to any one of claims 1-12, wherein the first power system comprises an AC power system.

14. A converter arrangement according to any one of claims 1-13, wherein the second power system comprises an asynchronous motor, an AC power distribution system, and/or an AC power generation system.

15. An arrangement comprising:

a converter arrangement (20; 60; 110-1, 110-2, 110-N) according to any one of claims 1-14 configured to convey power from a first power system (21, 61, 120) to a second power system (22; 62; 130-1, 130-2, 130-N), or vice versa, wherein the second power system has a nominal operating voltage frequency range and a nominal operating voltage amplitude range; and

a control unit (23) configured to transmit control signals to the first converter unit and the transformer, respectively, by way of which control signal the at least one first converter unit and the transformer, respectively, are caused to generate the respective amplitude-modulated voltage waveforms.

16. A power transmission system (50; 100; 200) comprising:

a first power system (21; 61; 120);

a second power system (22; 62; 130-1, 130-2, ..., 130-N) having a nominal operating voltage frequency range and a nominal operating voltage amplitude range; and at least one converter arrangement (20; 60; 110-1, 110-2, 110-N) according to any one of claims 1-14 configured to convey power from the first power system to the second power system, or vice versa.