ARRANGEMENT PROVIDING A 3-PHASE OR 1-PHASE POWER STREAM

DESCRIPTION

Arrangement providing a 3-phase or 1-phase power stream

Field of invention

The present invention relates to an arrangement and a method for alternatively providing a 3-phase or a 1-phase power stream.

Art Background In a subsea installation or subsea application, such as an oil or gas exploration and conveying application, there may be a need for heating pipelines which convey the gas or the oil to prevent clogging due to the generation of hydrates within the conveyed carbohydrates including oil and/or gas. This clogging or aggregation of hydrates within the pipeline may occur both during operation, i.e. flow of the carbohydrates, and in particular at a steady state or at still- stand . Conventionally, to prevent the hydrates from being clogged chemicals have been injected into the manifold or the pipe^¬ line. Alternatively, a direct electrical heating (DEH) system has been installed, wherein the electric power has been pro^¬ vided from a topside facility, i.e. a facility which is lo- cated above the sea level. The direct electrical heating may heat a pipeline by running an electric current through the pipeline which evolves, when ends of the pipeline are con^¬ nected to two terminals of a 1-phase power source. Document WO 2006/075913 Al discloses a system for power sup^¬ ply to subsea installations, wherein a power supply cables are for direct electrical heating of a pipeline and the sys^¬ tem is further configured, to provide a 3-phase power supply to an electrically powered motor in a subsea installation unit, wherein means are arranged for switching so that the three conductors forming the power supply cables form paral^¬ lel conductors in a second circuit configuration, wherein the power supply cables supply power for direct heating of the pipeline .

It has been observed that supply of the 1-phase power stream from a topside facility to the direct electrical heating sys- tern is very cumbersome and difficult and involves hampering the performance of the system.

There may be a need for an arrangement and for a method for allowing supply of electric energy to a direct electrical heating system, in particular located at a bottom of the sea or at a ground of the sea, which is simpler in construction compared to conventional systems and which reduces installa^¬ tion and maintenance efforts.

Summary of the Invention

The need may be met by the subject-matter of the independent claims. The dependent claims specify particular embodiments of the invention.

According to an embodiment of the present invention, it is provided an arrangement for alternatively providing a 3-phase or a 1-phase power stream, comprising a 3-phase power source having a first, a second and a third power source output ter^¬ minal; a switching section adapted to selectively provide from the three power source output terminals of the 3-phase power source: a 3-phase power stream at three arrangement output terminals or a 1-phase power stream at two arrangement output terminals, different from the three arrangement output terminals, wherein power of the three input phases is evenly distributed to provide the 1-phase power stream. The arrangement may comprise the arrangement output terminals (for connection to a respective node, i.e. a 3-phase load or a 1-phase load) which may provide the 3-phase power stream and the 1-phase power stream. Thereby, the 3-phase power stream may be provided at three different arrangement output terminals and the 1-phase power stream may be provided at two different arrangement output terminals. The arrangement out^¬ put terminals may allow connection of a load thereto.

The 3-phase power source may provide a particular output voltage (which may be defined between pairs of the three phases) and the 3-phase power source may be provided with electric energy by another power source, such as another 3- phase power source. Thereby, the other power source, in par^¬ ticular the other 3-phase power source, may provide a lower output voltage than the 3-phase power source. In order to in^¬ crease the output voltage, the 3-phase power source may com^¬ prise a plurality of power source sections which may be at least partially connected in series in order to increase the output voltage compared with the output voltage provided by the other 3-phase power source. The 3-phase power stream may be provided at the first, the second and the third power source output terminals. The first, the second and the third power source output terminal may allow connection of a 3- phase load thereto.

The switching section may be used to switch between provision of a 3-phase power stream and a 1-phase power stream depend^¬ ing on the load intended to be supplied with electric energy, in particular depending on whether the load intended to be supplied with electric energy is a 3-phase load or a 1-phase load. Thereby, in particular, a pump or a motor may be a 3- phase load and a direct electrical heating (DEH) system may be a 1-phase load. Thereby, the arrangement supports that these different kinds of loads are supplied with electric en^¬ ergy. Thereby, a large flexibility is provided. In particu^¬ lar, in a typical subsea exploration or subsea application, conventionally a 3-phase power stream may be available but a 1-phase power stream may not be available. By using the available 3-phase power stream to power the 3-phase power source of the arrangement and by switching or by actuating the switching section to switch to the provision of a 1-phase power stream, also a 1-phase load may be supplied with elec^¬ tric energy originally using the 3-phase power stream.

Therefore, it may not be necessary anymore to use a particu^¬ lar cable from a topside facility located above the sea lev- el, in order to supply a 1-phase load with electric energy. Thereby, equipment may be saved, installation costs may be reduced and the whole subsea acceleration system may be sim^¬ plified . The power of the three input phases is evenly distributed to provide the 1-phase power stream. In particular, the 3-phase power source may provide at each of the three phases a par^¬ ticular power output and the three different power outputs may be evenly supplied and distributed to the 1-phase power stream, such that a same amount of power is drawn from every of the three phases of the 3-phase power source.

According to an embodiment of the present invention the switching section is adapted to selectively connect at least two power source output terminals of the three power source output terminals with each other and allow a connection thereto; or disconnect the at least two power source output terminals of the three power source output terminals from each other, wherein the at least two power source output ter- minals connected with each other provide a phase of the 1- phase power stream.

The at least two power source output terminals of the three power source output terminals which are connected in a second operation mode of the arrangement may provide the same volt^¬ age levels or may be synchronized, in order to avoid unin^¬ tended currents flowing from one of the output terminals to another one of the output terminals. In order to achieve the synchronization of the voltage at the output terminals which are connected to each other in the second operation mode, a particular control section may be provided which may control the 3-phase power source, in particular control transistors within the 3-phase power source.

In a first operation mode, all the three power source output terminals, i.e. the first, the second and the third power source output terminals, may be disconnected from each other, such as to provide a 3-phase power stream at the first, the second and the third power source output terminals.

The arrangement may address that in the future the need for various types of pumping installations in the subsea oil and gas fields may be evident. Thereby, at still-stand of the conveying of the oil and/or gas, normally no power for the pumping is required. In particular, there may be no need to supply a 3-phase power source with electric energy. In par^¬ ticular, at this situation, when the gas/oil is not conveyed within the pipeline, it may be necessary to avoid clogging of the gas/oil comprised within the pipeline. In this situation, the arrangement may advantageously be switched to provide a 1-phase power stream in order to supply a direct electrical heating system with power which is not needed for the pumps and/or motors. Thereby, the 3-phase power source may comprise a variable speed drive (VSD) which may be installed for oper^¬ ating a pump and the switching section (also referred to as change over switch) may be used to switch to the provision of the 1-phase power stream. In particular, when in the second operational mode the 1-phase power stream may be used as a high voltage power supply for the direct electrical heating. Thereby, the 3-phase power source may adopt a "perfect harmo^¬ ny topology", or in particular a star-like topology comprised of a plurality of variable speed drives which may form three strands connected at a neutral node, wherein each strand may comprise a series-connected plurality of variable speed drives (also referred to as 3-phase to 1-phase converters) . Further, a control system of the variable speed drives or the converters may be needed to be adapted to handle the first mode of operation (providing the 3-phase power stream) and the second mode of operation (providing the 1-phase power stream) .

Thereby, a combined use of the infrastructure for two purpos^¬ es is provided. Thereby, particular components of a topside installation may be redundant. Furthermore, it may be ex^¬ pected that many subsea installations may not have any top- side facility. Therefore, the arrangement enables the use of a direct electrical heating system also for this kind of oil field which do not have available any topside facility, and which in particular do not have available any 1-phase power stream at the bottom of the sea. The combined use may greatly reduce the installation costs and may increase the flexibil^¬ ity. Also the power source may be connected to multiple pipe^¬ lines, either in parallel or in sequence based on the specif^¬ ic requirements. The single phase power load from a DEH sys^¬ tem may be evenly distributed of the three input phases by the converter. Also the voltage and frequency control of the converter may enable a flexible use to connect to various lengths of pipelines requiring different voltage/frequency for the operation of the DEH system. According to an embodiment of the present invention, a phase of the 1-phase power stream is provided by the power source output terminal of the three power source output terminals which is not connected with the at least two power source output terminals connected with each other; or by a neutral node having a constant electrical potential and allowing a connection thereto.

Thereby, a large flexibility is provided to provide the other phase of the 1-phase power stream. The two alternatives may be selected based on the requirements of the 1-phase load, in particular the requirements regarding voltage, current and/or power . According to an embodiment of the present invention, the three arrangement output terminals for the 3-phase power stream correspond to the three power source output terminals of the 3-phase power source and the two arrangement output terminals for the 1-phase power stream correspond to the two of three power source output terminals of the 3-phase power source. Thereby, a simple construction may be provided.

According to an embodiment of the present invention, the 3- phase power source comprises a first, a second and a third converter arrangement, being powered by another 3-phase power source and each having a first and a second converter ar^¬ rangement output terminal, wherein the first converter ar^¬ rangement output terminals of the three converter arrange- ments are connected to each other at the neutral node, where^¬ in the second converter arrangement output terminal of the first converter arrangement forms the first power source out^¬ put terminal, wherein the second converter arrangement output terminal of the second converter arrangement forms the second power source output terminal, wherein the second converter arrangement output terminal of the third converter arrange^¬ ment forms the third power source output terminal.

Each converter arrangement may comprise one or more convert- ers, in particular 3-phase to 1-phase converters. Thereby, the converters may be partially connected in series and/or partially connected in parallel. The 1-phase power output stream provided by each converter arrangement may be provided at the first and the second converter arrangement output ter- minals. When the first converter arrangement output terminals of the three converter arrangements are connected to each other at the neutral node, a star-like configuration may be achieved, which may be complying with the perfect harmony to^¬ pology. Other arrangements are possible. An operation mode of a control section of the arrangement may be different in the first operation mode of the arrangement and the second opera^¬ tion mode of the arrangement. According to an embodiment of the present invention, the oth^¬ er 3-phase power source comprises at least three secondary windings of a transformer. In particular, a plurality of other 3-phase power sources may be provided which supply the 3- phase power source with electric energy. In particular, the transformer may comprise one primary winding and nine second^¬ ary windings, wherein the secondary windings may provide the electric energy to the 3-phase power source. Thereby, conven^¬ tional equipment may be utilized advantageously in order to power the 3-phase power source. Also, when the 3-phase power source comprises the first, the second and the third convert^¬ er arrangement, conventionally available equipment may be utilized. Thereby, the installation cost and installation procedure may be simplified or reduced.

According to an embodiment of the present invention, at least one of the first, the second and the third converter arrange^¬ ment comprises at least two, in particular 3 to 30, further in particular 6 to 8, 3-phase to 1-phase converters which are series connected at their 1-phase output terminals.

Series connection of a number of 3-phase to 1-phase convert^¬ ers may increase an achievable voltage at the power source output terminal, which may allow to power particular 1-phase or 3-phase loads which require a relatively high voltage, which may in particular require a higher voltage than provided by the other 3-phase power source. Furthermore, conven^¬ tional modules may be utilized, thereby simplifying the con^¬ struction and reducing the costs.

According to an embodiment of the present invention, at least one 3-phase to 1-phase converter comprises an input section with six diodes for rectifying a 3-phase power stream; a capacitor for smoothing the rectified power stream; and an out- put section comprising four power transistors for providing a 1-phase output stream. The input section may alternatively or additionally comprise also a number of transistors. The six diodes may be arranged in three strands of pairs of diodes which are connected in series. A connection node between two diodes of each pair may be connected to a respective phase of the other 3-phase power source. Thereby, a simple rectifying process may be achieva^¬ ble. The capacitor may reduce ripples of a direct current power stream which is output by the input section of the 3- phase to 1-phase converter. The four transistors of the out- put section may be arranged in two strands, wherein each strand comprises a series connection of two transistors. Be^¬ tween the two transistors in each strand, a phase of the 1- phase power stream may be provided. Thereby, conventionally available equipment may be advantageously used for the con- struction of the arrangement according to this embodiment of the present invention.

According to an embodiment of the present invention, during switching between a 3-phase and 1-phase power stream transis- tors of the output section of the 3-phase to 1-phase convert^¬ er are controlled to adopt a non-conducting state.

When the transistors of the output section, in particular all output sections of all 3-phase to 1-phase converters com- prised in the 3-phase power source, adopt a non-conducting state, the 3-phase power source may not output any power stream. Thereby, the switching may be simplified and damages of components may be reduced or even avoided. According to an embodiment of the present invention, the first, the second and the third power source output terminals of the 3-phase power source provide synchronized voltages and are connected with each other to provide a phase of the 1- phase power stream and the neutral node provides another phase of the 1-phase power stream. Thereby, a particular con^¬ figuration, which is easily achievable is provided, in order to provide the 1-phase power stream in the second operation mode of the arrangement. This configuration may in particular be utilized, when the 1-phase load does not require very high supply voltage.

Further, the electric potential at the neutral node may be constant and may be in particular a ground potential or earth potential and may for example be equal to a potential of a closing or a housing of the arrangement. In particular, the neutral node may then be connected to an end of the pipeline to be heated which is relatively close compared to another end of the pipeline which is to be connected to the phase of the 1-phase power stream which is provided by the connection of the first, the second and the third power source output terminals . According to an embodiment of the present invention, the first and the second power source output terminal of the 3- phase power source provide synchronized voltages and are con^¬ nected with each other to provide a phase of the 1-phase pow^¬ er stream and the third power source output terminal of the 3-phase power source provides another phase of the 1-phase power stream.

Thereby, an alternative configuration or switching characteristic is provided in order to generate or provide a 1-phase power stream. Compared to the first alternative described above, a higher voltage of the 1-phase power stream may be achievable, however, a lower current may be achievable. The two alternatives may be selected depending on the character^¬ istic of the 1-phase load which is to be supplied with elec- trie energy.

According to an embodiment of the present invention, the switching section comprises a mechanical and/or semiconductor based switch. The switching section may comprise a mechani- cal/electromechanical based switch. The switching section may be controlled by a controller which may receive control sig^¬ nals from a topside facility or from other control elements located at the sea ground. According to an embodiment of the present invention, the ar^¬ rangement further comprises a control section adapted to pro^¬ vide gate driver signals for the transistors of the convert- ers in the two different modes of operation of the arrange^¬ ment. The gate driver signals in the two different modes of operation may be different. In particular, the gate driver signal may be such that the voltage at those power source output terminals which are connected to each other are the same and are synchronized (when in the second operational mode) . When in the first operational mode, the voltages at the three power source output terminals may be different from each other, in particular different in amplitude and/or phase. However, all voltages at all three power source output terminals may oscillate with a same frequency. In particular, in all modes of operation, the voltages at the three power source output terminals may oscillate with a same frequency.

According to an embodiment of the present invention, the 3- phase power stream provides between 3 kV and 10 kV voltage, in particular 6.6 kV voltage.

Thereby, conventional equipment may be used and typical loads may be appropriately supplied with electric energy.

According to an embodiment of the present invention, it is provided a use of the arrangement according to one of the preceding embodiments in a subsea application, in particular in a depth 1000 m to 4000 m below sea level, for alternative- ly driving a 3-phase load, in particular a variable speed drive, such as a motor, a pump, or a 1-phase load, in partic^¬ ular a direct electrical heating system for heating a pipe.

Thereby, additional supply of a 1-phase power from a topside facility may not be necessary.

It should be understood that features which have been indi^¬ vidually or in any combination disclosed, described or men- tioned with respect to an arrangement for alternatively providing a 3-phase or a 1-phase power stream may also be ap^¬ plied individually or in any combination to a method of al^¬ ternatively providing a 3-phase or 1-phase power stream ac- cording to an embodiment of the present invention and vice versa .

According to an embodiment of the present invention, it is provided a method of alternatively providing a 3-phase or 1- phase power stream, comprising receiving at a first, a second and a third power source output terminal electrical power form a 3-phase power source; selectively providing from the three power source output terminals of the 3-phase power source: a 3-phase power stream at three arrangement output terminals or a 1-phase power stream at two arrangement output terminals, different from the three arrangement output termi^¬ nals. Further, the power of the three input phases is evenly distributed to provide the 1-phase power stream. The arrangement may in particular by adapted to selectively switching between connecting at least two power source output terminals of the three power source output terminals with each other; and disconnecting the at least two power source output terminals of the three power source output terminals from each other, wherein the at least two power source output terminals connected with each other provide a phase of the 1- phase power stream.

Embodiments of the present invention are now described with reference to the accompanying drawings. In the drawings, ele^¬ ments similar in structure and/or function are labelled with the same reference signs differing only in the first digit.

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi^¬ ment but to which the invention is not limited.

Brief Description of the Drawings

Fig. 1 schematically illustrates an arrangement for alterna^¬ tively providing a 3-phase or a 1-phase power stream accord^¬ ing to an embodiment of the present invention;

Fig. 2 schematically illustrates a 3-phase to 1-phase con^¬ verter which is used in a 3-phase power source of an arrange^¬ ment for alternatively providing a 3-phase or a 1-phase power stream according to an embodiment of the present invention; and

Fig. 3 schematically illustrates an arrangement for alterna^¬ tively providing a 3-phase or a 1-phase power stream accord^¬ ing to an embodiment of the present invention.

Detailed Description

The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical ele^¬ ments are provided with the same reference signs or with ref^¬ erence signs, which are different from the corresponding ref^¬ erence signs only within the first digit. The arrangement 100 for alternatively providing a 3-phase or a 1-phase power stream illustrated in Fig. 1 comprises a 3- phase power source 103 which has a first 105, a second 107 and a third 109 power source output terminal. Further, the arrangement 100 comprises a switching section 111 which is adapted to connect the first, the second and the third power source output terminals 105, 107, 109 with each other to the branch 113 and allow a connection thereto at a terminal 115 to provide a phase of a 1-phase power stream. In the illus- tration of Fig. 1, the arrangement 100 is in a first opera^¬ tion mode, wherein the switching section 111 is switched such that the first, the second and the third power source output terminals 105, 107, 109 provide a 3-phase power stream to the branches or terminals 117, 119, 121.

In a second operation mode of the arrangement 100 illustrated in Fig. 1, the switching section 111 provides a 1-phase power stream at the terminals 115, 123 by connecting the first, the second and the third power source output terminals 105, 107, 109 to the branch 113 and by connecting the neutral node 125 to the branch connected to the terminal 123. Thereby, four individual switching actions are performed using the switches 127, 129, 131, 133.

The arrangement 100 further comprises another 3-phase power source 135 which provides via a not illustrated connection electric energy to the 3-phase power source 103. The 3-phase power source 103 comprises a first 137, a second 139 and a third 141 converter arrangement which are powered by the oth^¬ er 3-phase power source 135 by not illustrated connections, wherein each of the first, the second and the third converter arrangement 137, 139, 141 has a first converter arrangement output terminal 143, 145, 147, respectively, and a second converter arrangement output terminal 149, 151, and 153, re^¬ spectively. Thereby, the respective first converter arrange^¬ ment output terminals 143, 145, and 147 are connected to each other at a neutral node 125. Further, the second converter arrangement output terminals 149, 151, and 153 form or are connected to the first, the second and the third power source output terminals 105, 107, 109, respectively.

Each of the first, the second and the third converter ar^¬ rangements 137, 139, 141 comprises three 3-phase-to-l-phase converters 155 which are connected in series. The arrangement 100 further comprises a control section 157 which provides via a control line 159 control signals to each of the con^¬ verters 155, as will be in more detail described below. As an illustrative example, the 1-phase output terminals 115, 123 of the arrangement 100 are connected to a pipeline 161 for direct electrical heating. In particular, the phase pro- vided at terminal 123 which is connected to the neutral node 125 carries a constant potential and is connected to a side 163 of the pipeline 161 which is closer to the output termi^¬ nal or the arrangement 100 than a side 165 of the pipeline 161 to which the terminal 115 is connected to which the ar- rangement 100 supplies an oscillating AC voltage.

The control section 157 is adapted to control the transistors 281 comprised within each of the converters 155 by providing appropriate gate driver signals. In particular, the control section 157 is adapted, when the arrangement 100 is in the first operational mode, to control the transistors 281 such that voltages output at the branches 117, 119, and 121 have the same frequency but different phase. Further, the control section 157 is adapted, when the arrangement 100 is in the second operational state, to control the transistors to have a same frequency and a same phase, in order to avoid unin^¬ tended current and damages of the components.

As explained above, each of the first, the second and the third converter arrangements 137, 139, 141 comprises several 3-phase-to-l-phase converters 155 which are connected in se^¬ ries. According to an embodiment, each of the first, the se^¬ cond and the third converter arrangements 137, 139, 141 may be connectable (e.g. via the neutral node 125 and an end of the respective converter arrangement) to a associated pipe^¬ line segment and each of the first, the second and the third converter arrangements 137, 139, 141 may be operated (inde^¬ pendently from each other) with an associated (individual) frequency and/or voltage which are adapted to the respective pipeline segment characterized e.g. by a particular length, resistance, etc. Thereby, several pipeline segments may be heated, in particular using optimized voltage and/or frequency for each individual pipeline segment. In this case the control section 157 provides via a control line 159 control signals to each of the converters 155 of each of the first, the second and the third converter ar- rangements 137, 139, 141 individually, to achieve the indi^¬ vidual voltage and/or frequency. In this embodiment the con^¬ verter arrangements 137, 139, 141 are connected in a star like manner each being connected to the neutral node.

Fig. 2 schematically illustrates a 3-phase-to-l-phase con^¬ verter 255 which may be used as a converter 155 in the arrangement 100 illustrated in Fig. 1 and also may be used as a converter 355 in the arrangement 300 illustrated in Fig. 3 and described below.

The converter 255 is adapted to convert a 3-phase power stream received at an input section 267 to a 1-phase power stream provided at an output section 269. At the input sec^¬ tion 267, six diodes 271 are arranged in three strands, wherein each strand comprises two series connected diodes 271. At a midpoint between two diodes in each strand, ends of secondary windings 273 of a not completely illustrated trans^¬ former are connected. The transformer comprises a not illus^¬ trated primary winding which is inductively coupled to the secondary windings 273. Thereby, a 3-phase power stream is input to the converter 255.

The input section 267 rectifies the AC 3-phase power stream and provides it at DC terminals 275 and 277 between which a capacitor 279 is connected which smoothes the voltage gener^¬ ated by the diodes 271 to filter out some remaining AC compo^¬ nents .

The output section 269 comprises four transistors 281 which are arranged in two strands, each having two series connected transistors 281. At midpoints between two transistors in each strand, a first converter output terminal 283 and a second converter output terminal 285 are provided for providing a 1- phase power stream between the converter output terminals 283, 285. For series connection of two or more of the con^¬ verters 255 illustrated in Fig. 2, a second converter output terminal 285 is connected to a first converter output termi- nal 283 of the respective adjacent or next converter in the series connected converters, such as the converter arrange^¬ ments 137, 139, and 141 illustrated in Fig. 1 and the respec^¬ tive converter arrangements 337, 339, and 341 illustrated in Fig. 3.

Fig. 3 schematically illustrates another arrangement for al^¬ ternatively providing a 3-phase or a 1-phase power stream 300 according to an embodiment of the present invention. As the arrangement 100 illustrated in Fig. 2, the arrangement 300 comprises a 3-phase power source 303 having a first 305, a second 307 and a third 309 power source output terminal. The 3-phase power source 303 illustrated in Fig. 3 may be simi^¬ larly or equally constructed as the 3-phase power source 103 illustrated in Fig. 1.

Further, the arrangement 300 comprises a switching section 311 which is however differently constructed than the switch^¬ ing section 111 of the arrangement 100 illustrated in Fig. 1. In particular, the switching section 311 comprises three switches 387, 389, and 391 which may be controlled such as to set the arrangement 300 in a first operation mode where a 3- phase power stream is provided at branches or terminals 317, 319, 321 (or arrangement output terminals) or in a second op^¬ erational mode, where a 1-phase power stream is provided at terminals 315, 323.

In Fig. 3, the first operational state is illustrated, where^¬ in the three third power source output terminals 305, 307, 309 are connected to the branches 317, 319, and 321, respec- tively. To adopt the second operational state, the switches

389, 391 would be switched such as to connect the second pow^¬ er source output terminal 307 to the terminal 315 and to con^¬ nect the third power source output terminal 309 to the termi- nal 323. The output terminals 315, 323 may for example be used for supply electric energy to a direct electrical heat^¬ ing system and the 3-phase output terminals 317, 319, 321 may for example be utilized for supplying electric energy to a pump or a motor.

The control section 357 illustrated in Fig. 3 is adapted, when the arrangement 300 is in the second operational state or mode, to control the transistors 281 comprised in each of the converters 355 such that the voltage provided at the pow^¬ er source output terminals 307, 309 to be different in phase and equal in frequency and also equal in amplitude.

The converter 255, 155, 355 may also be denoted as a power cell.

It should be noted that the term "comprising" does not ex^¬ clude other elements or steps and "a" or "an" does not ex^¬ clude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con^¬ strued as limiting the scope of the claims.