Direct electrical heating arrangement comprising a power electronic converter

Field of invention

The present invention relates to an arrangement for providing an AC current to a load for direct electrical heating of a pipeline portion, and to a pipeline heating arrangement.

Background Direct Electrical Heating (DEH) of subsea pipelines for gas production is a widely used method for preventing hydrate plugs in gas pipelines. The method is based on injecting a single phase AC current direct through the gas steel pipe and back through a cable strapped on the top of the pipe. This method can limit the use of inhibitors as methanol. The con ^¬ ventional (state of the art) way to create a single phase power supply suitable for the load (pipeline) , is to use a 3 phase transformer with a balancing/compensating circuit on the transformer secondary windings. The balanc- ing/compensating circuit consists of two capacitors and one inductor connected between the three different phases. The transformer will be subjected to a symmetrical three phase load with power factor close to 1, if these three impedances are properly matched to the pipeline impedance.

Do to the mentioned properties of the state of the art DEH system, the balancing circuit have drawbacks as:

• Complex fine tuning of the circuit needs to be done dur- ing commissioning.

• High short circuit levels is seen at the single phase source terminals (danger of damage the pipeline by burn ^¬ ing hole in it in case of short circuit) • Transformer with tapping is needed to change the heating power .

• Only the grid frequency is available in all modes There may be a need for an arrangement for providing an AC current to a load and for a pipeline heating arrangement, wherein the above mentioned disadvantages are reduced, wherein in particular switch-on transients are reduced. Further there may be a need for an arrangement, in particular a power electronic unit, to heat up subsea gas pipes during production shut downs and tail gas production, without use of output transformer connected after the power electronic unit. Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

A power electronic circuit for DEH is suggested. The circuit will have the same advantages as the state of the art techni ^¬ cal solution and reduces some of the aforementioned disadvan ^¬ tages .

According to an embodiment of the present invention it is provided an arrangement for providing an AC current (alter ^¬ nating electrical current) to a load (in particular a pipe, a pipe system, a tube for transporting oil or gas) for direct electrical heating (in particular of the pipe) , wherein the arrangement is adapted to be used as a direct electrical heating power supply. Thereby, the arrangement comprises a (just one or two, three or even more) AC-DC-AC converter cell (in particular comprising controllable power electronics com- ponents such as controllable switches) , the converter cell having at least two converter input terminals (in particular exactly two or three) connected to at least two transformer output terminals, the converter cell having a first converter output terminal and a second converter output terminal (for outputting the AC voltage (in particular having a frequency different from the frequency of the AC voltage supplied to the transformer) for supplying energy to the load) , wherein the first converter cell output terminal is adapted to be connected to the load, wherein at all three input terminals of the transformer a symmetrical load is achieved by appro ^¬ priately controlling the AC-DC-AC converter cell.

Thereby, the power output of the arrangement may be con ^¬ trolled to in particular adjust a heating power for heating a gas or oil pipeline. Further, the frequency of the AC voltage output by the arrangement may be different from the AC volt- age provided at the inputs of the arrangement. Thereby, heat ^¬ ing efficiency of the pipeline may be improved. In particu ^¬ lar, it is not required to use a transformer with tabbing capability in order to change the heating power. Further, high short circuit levels at the load may be reduced or even avoided. Further, at all three input phases of the trans ^¬ former a symmetrical load may be achieved by appropriately controlling the AC-DC-AC converter cell.

Further a symmetric three phase load may be achieved on the transformer, thereby requiring a less complex transformer for changing heating power.

According to an embodiment of the present invention the di ^¬ rect electrical heating power supply comprises a transformer for transforming input voltages between three transformer input terminals connected to three primary windings (or four or five or six or seven or eight or even more, in particular wound around a ferromagnetic material having high magnetic permeability) to three transformer output voltages at three secondary winding portions (inductively coupled to respective primary windings) , the transformer having the at least two transformer output terminals (in particular three or six or four or five or six or seven or even more) ; According to an embodiment of the present invention the AC- DC-AC converter cell comprises an AC-DC (or rectifier) sec ^¬ tion (in particular comprising one or more controllable switches and associated control circuits) having two DC out ^¬ put nodes and being adapted to provide a DC voltage (in par ^¬ ticular a direct current voltage having an alternating current voltage overlaid, such as ripple voltage) between the two DC output nodes, when an AC voltage (having an input fre- quency) is applied between the at least two converter input terminals; and a DC-AC section (in particular comprising one or more controllable switches and associated control cir ^¬ cuits) having two DC input nodes connected to the two DC out ^¬ put nodes of the AC-DC section and being adapted to convert a DC voltage between the two DC output nodes to an AC voltage

(having an output frequency) between the first converter out ^¬ put terminal and the second converter output terminal .

Thereby, the arrangement may be simplified and may be assem- bled from known components, in particular comprising controllable power electronics components, such as diodes, transis ^¬ tors, thyristors and the like. Further, this provision may simplify controllability of power output, frequency of the power output and may avoid short circuits at the load.

According to an embodiment of the present invention the con ^¬ verter cell, in particular the AC-DC section of the at least one converter cell, comprises: a first controllable switch (a switch, wherein opening and closing may be controlled, in particular by a control signal) ; a second controllable switch, wherein the first controllable switch and the second controllable switch are connected in series between the two DC output nodes, wherein a first one of the at least two con ^¬ verter input terminals is connected between the first con- trollable switch and the second controllable switch.

Thereby, a controllable rectifier section may be provided, in particular using conventional power electronics components. Thereby, controllability of the arrangement may be improved and the arrangement may be simplified.

According to an embodiment of the present invention at least one of the first controllable switch and the second control ^¬ lable switch comprises a thyristor.

In particular, the first controllable switch comprises a thy ^¬ ristor and also the second controllable switch comprises a thyristor. Opening and closing the thyristor may be controlled by an appropriate signal supplied to the gate of the respective thyristor.

In particular, the thyristors may be controlled such that the current flowing to the DC output nodes may be at least ap ^¬ proximately constant, such that in particular the voltage across the DC capacitor bank fluctuates. In particular, the voltage between the two DC output nodes may fluctuate with a frequency corresponding to two times the output frequency of the AC voltage output by the converter cell. In particular, when increasing the output frequency, the heating efficiency for heating the pipeline may increase. Thereby, heating power and/or heating efficiency may be controlled. Further, conventional thyristors may be used to simplify the arrangement and also to reduce the costs of the arrangement.

According to an embodiment of the present invention the con ^¬ verter cell, in particular the AC-DC section, further comprises a capacitor (for storing an electrical charge) , in particular connected in parallel to the series connection of the first controllable switch and the second controllable switch .

Thereby, the voltage created by current flow through the thy- ristors (and in particular further controllable switches con ^¬ nected to other converter input terminals of the at least two converter input terminals) may be smoothed. In particular, the capacitor may comprise one or more capacitor units con- nected in series and/or connected in parallel between the two DC output nodes. Thereby, the rectifier section may be im ^¬ proved . According to an embodiment of the present invention the con ^¬ verter cell, in particular the AC-DC section, comprises a third controllable switch; and a fourth controllable switch, wherein the third controllable switch and the fourth control ^¬ lable switch are connected in series between the two DC out- put nodes, wherein a second one of the at least two converter input terminals is connected between the third controllable switch and the fourth controllable switch. In particular, the series connection of the first controllable switch and the second controllable switch is connected in parallel to the series connection of the third controllable switch and the fourth controllable switch.

Thereby, also the input voltage provided at the second one of the at least two converter input terminals may be rectified by the third controllable switch and the fourth controllable switch and a rectified voltage may be supplied to the two DC output nodes. Thereby, the power output of the arrangement may be increased. According to an embodiment of the present invention the three secondary winding portions of the transformer are serially conductively connected in an annular manner (such that the three secondary winding portions are connected in series forming a loop) , wherein the at least two transformer output terminals are formed by three transformer output terminals being provided between pairs of the three secondary winding portions (in particular such that each secondary winding portion provides one output terminal) , wherein the at least two converter input terminals are formed by three converter input terminals (for supporting three phases) , wherein the three transformer output terminals are connected to the three con ^¬ verter input terminals. Thereby, a converter cell having three converter input terminals for supporting three electrical phases may be provided.

In particular, the first controllable switch, the second con- trollable switch, the third controllable switch and/or the fourth controllable switch (and also the fifth controllable switch and/or the sixth controllable switch) may be replaced by diodes, which may therefore not be controlled but may adapt a conducting state depending on the voltage applied across the respective diode.

According to an embodiment of the present invention at least one of the third controllable switch and the fourth control ^¬ lable switch comprises a thyristor.

Thereby, the arrangement may further be simplified and re ^¬ duced in costs, since conventional components may be used.

According to an embodiment of the present invention the con- verter cell, in particular the AC-DC section, further comprises a fifth controllable switch comprising a thyristor; a sixth controllable switch comprising a thyristor, wherein the fifth controllable switch and the sixth controllable switch are connected in series between the two DC output nodes, wherein a third one of the three converter input terminals is connected between the fifth controllable switch and the sixth controllable switch, wherein in particular the series connec ^¬ tion of the first controllable switch and the second control ^¬ lable switch is connected in parallel to the series connec- tion of the fifth controllable switch and the sixth control ^¬ lable switch.

Thereby, an efficient rectifier section for supporting three phases may be provided using conventional electronic compo- nents.

According to an embodiment of the present invention the three secondary winding portions are conductively isolated from each other (in particular formed by three separate wires wound around ferromagnetic material) , wherein the at least two transformer output terminals are formed by three pairs of transformer output terminals (the transformer thus having in total six output terminals) , wherein each of the three secon ^¬ dary winding portions provides one of the three pairs of transformer output terminals (each of the three secondary winding portions provides thus two transformer output termi ^¬ nals) , wherein the at least two converter input terminals are formed by just two converter input terminals, wherein the just two converter input terminals are connected to a pair of transformer output terminals of the three pairs of trans ^¬ former output terminals. In particular, in total, three converter cells each having exactly two converter input terminals may be connected to the six transformer output terminals. In particular, the three cells connected to the one transformer, may be connected in series at their output terminals. Thereby, the two cells at the two ends of the series connection of the three cells may provide two output terminals, which may be connected to the load .

According to an embodiment of the present invention it is provided at least one of the third controllable switch and the fourth controllable switch comprises a transistor, in particular a IGBT, wherein in particular a diode is connected in parallel to the third controllable switch and in particu ^¬ lar another diode is connected in parallel to the fourth con- trollable switch.

Providing a transistor for the third controllable switch and/or the fourth controllable switch may improve controlla ^¬ bility of the arrangement, in particular regarding power out- put and heating efficiency.

According to an embodiment of the present invention the ar ^¬ rangement (having the transformer with six output terminals) further comprises another AC-DC-AC converter cell (in particular configured as the AC-DC-AC converter cell as de ^¬ scribed) , the other converter cell having just two other converter input terminals connected to another pair of trans- former output terminals of the three pairs of transformer output terminals, the other converter cell having another first converter output terminal and another second converter output terminal; and still another AC-DC-AC converter cell (in particular configured as the AC-DC-AC converter cell as described) , the still other converter cell having just two still other converter input terminals connected to still an ^¬ other pair of transformer output terminals of the three pairs of transformer output terminals, the still other converter cell having still another first converter output terminal and still another second converter output terminal, wherein the load is connectable between the first converter output termi ^¬ nal and the still other second converter output terminal, wherein the second converter output terminal is connected to the first other converter output terminal, wherein the second other converter output terminal is connected to the still other first converter output terminal.

Thereby, a series connection of the converter cell, the other converter cell and the still other converter cell is provided for providing a single phase output (provided by two output terminals) to the load.

According to an embodiment of the present invention it is provided a series arrangement comprising at least a first and a last series connected arrangement as described above.

By providing a series arrangement, an output voltage for driving the load may be (linearly) increased, in particular to adapt the output voltage to the output voltage required by the load to heat the load. Thereby, a greater flexibility may be provided. According to an embodiment of the present invention the se ^¬ ries arrangement further comprises a compensating capacitor either connectable in series or in parallel to the load. By providing the capacitor, the power factor may be adjusted. In particular, the reactive power at the output may be com ^¬ pensated by the capacitor. Thereby, the capacitor may be placed either in series or in parallel with the load. If the capacitor is in series connected to the load, the output voltage may be needed to be lower and the current higher for the converter, compared to the case where the capacitor is connected in parallel with the load. If the capacitor is in parallel with the load, a small output choke may be put on the output of the converter, to limit the current in the ca- pacitor bank when switching.

According to an embodiment of the present invention it is provided a pipeline heating arrangement, comprising

an arrangement according to one of the embodiments as de- scribe above; and an electrically conductive pipeline con ^¬ nected to the arrangement as a load at a first longitudinal position and a second longitudinal position for electrical current flow through the pipeline from the first position to the second position for heating the pipeline.

In particular, the pipeline may be provided for transporting oil or gas. The pipeline may be manufactured from a metal, such as steel. The pipeline may comprise at least two termi ^¬ nals for connecting the output terminals of the heating ar- rangement to the first position and the second position, re ^¬ spectively.

Further, the arrangement for providing an AC current to a load, may comprise one or more gate control circuits for con- trolling at least one of the controllable switches, such that the electrical power provided at the converter output termi ^¬ nals, which is supplied to the load, satisfies desired prop ^¬ erties, in particular regarding power output or heating effi- ciency, frequency of the AC output voltage and the like. Fur ^¬ ther, the gate control circuits may be adapted to control, in particular the rectifier section of the converter cell, to achieve a at least approximately homogeneous or same load to the three primary windings of the transformer. Thereby, a load of the three primary winding portions may be balanced. Thereby, the efficiency of the arrangement for providing an AC current may be improved. According to an embodiment of the present invention it is provided an arrangement for providing an AC current to a load, in particular for direct electrical heating, the arrangement comprising: a transformer for transforming input voltages between three transformer input terminals connected to three primary windings to three transformer output volt ^¬ ages between three pairs of transformer output terminals, each pair of transformer output terminals being connected to a respective secondary winding inductively coupled to one of the three primary windings; a first, a second and a third AC- DC-AC converter cell, each cell having two converter input terminals connected to a pair of the transformer output ter ^¬ minals, each cell having a first converter output terminal and a second converter output terminal, wherein the load is connectable between the first converter output terminal of the first converter cell and the second converter output ter ^¬ minal of the third converter cell, wherein the second con ^¬ verter output terminal of the first converter cell is con ^¬ nected to the first converter output terminal of the second converter cell and wherein the second converter output termi- nal of the second converter cell is connected to the first converter output terminal of the third converter cell.

According to an embodiment of the invention a converter is employed in the DEH power supply which employs power elec- tronics for conversion (e.g. Thyristors or IGBTs) and symmet- risation providing a similar function as a conventional sym- metrisation unit as disclosed in WO 2010/031626. By means of the converter, in particular its intermediate DC link, the single phase load of the pipeline section is transformed into a symmetric three phase load on the transformer. Further it is possible of using a less complex transformer for changing the heating power, reduced short circuit currents at the sin- gle phase load.

A further embodiment of the invention provides a direct elec ^¬ trical heating power supply, comprising a converter comprising at least one AC-DC-AC converter cell, the converter cell having an AC-DC section, a DC link and a DC-AC section. The direct electrical heating power supply further comprises a first, a second and a third AC converter input, wherein the first converter input is connectable to a first secondary winding of a three phase transformer, the second converter input is connectable to a second secondary winding of the three phase transformer, and the third converter input is connectable to a third secondary winding of the three phase transformer, and an AC converter output connectable to an electrically conductive pipeline section forming a single phase load for direct electrical heating of the pipeline sec ^¬ tion. The converter is configured supply AC electric power to the single phase load and to distribute the load equally be ^¬ tween the first, the second and the third AC converter in ^¬ puts .

In an embodiment of the direct electrical heating power sup ^¬ ply, the converter may comprise at least three power cells, one power cell providing the first AC converter input, one power cell providing the second AC converter input and one power cell providing the third AC converter input. The output of the at least three power cells may be connected in series to provide the AC converter output. The converter can be con ^¬ figured to synchronize the generation of an AC voltage by the DC-AC sections of the at least three power cells to provide a single phase AC output at the AC converter output. For this purpose, a controller may be provided which controls the syn ^¬ chronization of the DC-AC sections of the at least three power cells. In a further embodiment of the direct electrical heating power supply, the power cell may comprise three AC inputs which provide the first, the second and the third AC con- verter inputs. The AC-DC section of the converter cell can be configured to convert a three phase AC voltage received on the first, the second and the third AC converter inputs, in particular from the transformer's secondaries, into a common DC voltage on the DC link. Thereby, the load on the first, the second and the third AC converter inputs may be sym ^¬ metrized.

In other embodiments of the direct electrical heating power supply, the converter may be provided by the arrangement for providing an AC current in any of the above outlined embodi ^¬ ments and configurations.

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 pro ^¬ viding an AC current to a load according to an embodiment of the present invention;

Fig. 2 schematically illustrates an arrangement for pro ^¬ viding an AC current to a load comprising three AC-DC-AC con ^¬ verter cells, such as the converter cell illustrated in Fig. 1, according to an embodiment of the present invention;

Fig. 3 schematically illustrates a pipeline heating ar ^¬ rangement for providing an AC current to a pipeline for heat- ing the pipeline according to an embodiment of the present invention, comprising five seriously connected arrangement as illustrated in Fig. 2 ; Fig. 4 schematically illustrates an arrangement for pro ^¬ viding an AC current to a load according to an embodiment of the present invention;

Fig. 5 schematically illustrates an arrangement for pro- viding an AC current to a load according to a still other em ^¬ bodiment of the present invention;

Fig. 6 and Fig. 7 schematically illustrate heating arrange ^¬ ments for providing an AC current to a pipeline comprising a plurality of AC-DC-AC converter cells according to embodi ^¬ ments of the present invention; and

Fig. 8 schematically illustrates a pipeline heating ar ^¬ rangement according to an embodiment of the present inven- tion, comprising plural AC-DC-AC converter cells.

Detailed Description The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical ele ^¬ ments may be provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit.

Fig. 1 schematically illustrates an arrangement 100 for pro ^¬ viding an AC current to a load, wherein the arrangement 100 comprises a transformer portion 101, an AC-DC section 103, a capacitor arrangement or DC-link 105 and a DC-AC section 107. The arrangement 100 can form a direct electrical heating

(DEH) power supply, and the AC-DC section 103, the DC-link 105 and the DC-AC section 107 can form a power cell 133 of a converter . The transformer portion 101 comprises a secondary winding portion 109, 110, which is inductively coupled to a not il ^¬ lustrated primary winding of a not completely illustrated transformer for transforming an AC input voltage to a higher AC output voltage. The portion 101 represents only a portion of a transformer, the complete transformer having three transformer input terminals connected to three primary wind ^¬ ings, wherein each of the three primary windings is induc- tively coupled to a corresponding secondary winding portion 109, 110.

The secondary winding 109, 110 is here illustrated as com ^¬ prising two sections 109, 110, but may also comprise only one section, in particular comprising a wire wound around a ferromagnetic material. Optionally, the secondary winding por ^¬ tion 109, 110 may comprise a capacitor 112 connected between the transformer output terminals 111, 113. The portion 109 is configured as an inductance representing a secondary winding of a transformer, while the portion 110 represents an addi ^¬ tional inductor. The capacitor 112 will be a filter capacitor. The choke and the capacitor 112 will not be absolute needed and may be omitted in other embodiments. The transformer portion 101 has two transformer output terminals 111 and 113.

The AC-DC section 103 comprises a first controllable switch 115 implemented as a thyristor; a second controllable switch 117, also implemented as a thyristor, wherein the first of the transformer output terminals 111 is connected between the thyristors 115, 117, which are connected in series between two DC output nodes 119, 121. The second output terminal 113 of the two transformer output terminals is connected between a third controllable switch 123 and a fourth controllable switch 125, which are connected in series between the two DC output nodes 119, 121. The third controllable switch and the fourth controllable switch 123, 125 are implemented each as isolated gate bipolar transistor (IGBT) . In parallel to the IGBTs 123, 125, diodes 127 are connected .

The capacitor section 105 comprises two series connections of capacitors 129, wherein the two series connections of capaci ^¬ tors 129 are connected in parallel between the two DC output nodes 119, 121.

The DC-AC section 107 comprises four transistors (in particu ^¬ lar IGBTs) 131, wherein a series connection of two IGBTs 131 is connected between the two DC output nodes 119, 121. In parallel to the IGBTs 131 diodes 127 are arranged. Two series connections of two IGBTs 131 each are connected in parallel.

The AC-DC section 103, the capacitor section 105 and the DC- AC section 107 together form an AC-DC-AC converter cell 133. The AC-DC-AC converter cell 133 has a first converter output terminal 135 (connected between a pair of serially connected IGBTs 131) and a second converter output terminal 137 (con ^¬ nected between another pair of serially connected IGBTs 131) .

In particular Figure 1 illustrates a converter cell topology with a single phase, switch mode rectifier 103.

The input transformer has one common primary winding and one secondary winding 109, 110 for each cell 133. Each cell 133 has a single phase input 111, 113. Therefore the cell con- verter should consist of a multiple of 3 cells in order to give symmetrical impact on the mains. The input IGBT's 123, 125 are pulse width modulation (PWM) controlled, and an input filter may be needed (similar to an Active Front End on a mo ^¬ tor drive) .

Using thyristors 115, 117 instead of diodes in the other phase 111 gives the possibility of soft start and cell isola ^¬ tion in the case of breakdown on the cell. Since this rectifier section 103 will produce a pulsating power to the capacitor section or DC link 105 with the double mains frequency, the DC capacitor 105 must be capable to withstand the sum of the pulsating power from the inverter and the rectifier.

Fig. 2 schematically illustrates an arrangement 200 for pro ^¬ viding an AC current to a load according to an embodiment of the present invention comprising three converter cells 133a, 133b, 133c, as illustrated in Fig. 1.

The arrangement 200 illustrated in Fig. 2 comprises a trans ^¬ former 240 having three transformer input terminals 241, 242, 243. The three transformer input terminals 241, 242, 243 are connected to three primary windings 244. Thereby, the wind ^¬ ings 244 are connected in series. The primary windings 244 comprise wires wound around ferromagnetic material comprised in the transformer 240.

Inductively coupled to the three primary windings 244 are secondary winding portions 109, wherein each secondary winding portion 109 is comprised of a wire wound around ferromag ^¬ netic material and inductively coupled to a respective pri- mary winding 244. The secondary windings 109 are isolated from each other. Thereby, the transformer 240 has in total six transformer output terminals, each of the three secondary winding portion providing two terminals 111, 113. The arrangement 200 for proving an AC current to a load fur ^¬ ther comprises three AC-DC-AC converter cells 133 (denoted as 133a, 133b, 133c) , wherein each of the converter cells 133 receives two of the six transformer output terminals 111, 113, wherein each cell 133 receives the output terminals 111, 113 belonging to a particular secondary winding 109. The cells 133a, 133b, 133c illustrated in Fig. 2 are constructed and configured at the cell 133 illustrated in Fig. 1. Each cell 133 has two converter cell output terminals 135 and 137. As can be taken from Fig. 2, the first converter cell output terminal 135 of the first converter cell 133a is con- nectible to a load and also the second converter cell output terminal 137 of the third converter cell 133c is connectible to a load, in order to provide a single phase output to a load, such as a pipeline.

The second converter cell output terminal 137 of the first cell 133a is connected with the first converter output termi ^¬ nal 135 of the second cell 133b. The second converter cell output terminal 137 of the second converter cell 133b is con ^¬ nected to the first converter cell output terminal 135 of the third converter cell 133c. Thereby, the cells 133a, 133b and 133c are connected in series.

According to an embodiment of the present invention a master control system (not illustrated) is provided which e.g. con ^¬ trols synchronization of the three single phase converters 133a, 133b, 133c illustrated of Figure 2. The master control system may provide pulse width modulation signals to semiconductor switches comprised in the converters in order to achieve a symmetrical input load on the three transformer in ^¬ put terminals 241, 242, 243.

Fig. 3 schematically illustrates a pipeline heating arrange ^¬ ment 300 comprises five arrangements 200 illustrated in Fig. 2. Each arrangement 200 for providing an AC current to a load 350 comprises three transformer input terminals 241, 242 and 243, which are connected to an energy supply providing the electric energy in three electrical phases 351, 352, 353.

As can be taken from Fig. 3, the arrangements 200 illustrated in Fig. 3, are connected in series, wherein in particular the respective second output terminal of the arrangements 200

(the output terminal 138) is connected to a respective first output terminal (the output terminal 136) of the next ar ^¬ rangement 200 in the series. The first output terminal 136 of the first arrangement 200 and the second output terminal 138 of the last arrangement 200 are connected to a compensating capacitor 355, which is connected to a first position 357 and a second position 359 of a pipeline 350. Thereby, the ar- rangement 300 provides an AC current through the pipeline 350 flowing from the first position 357 to the second position 359.

Fig . 4 schematically illustrates an arrangement 400 for pro- viding an AC current to a load, wherein the arrangement 400 comprises a transformer portion 401, an AC-DC section 403, a capacitor section or DC-link 405 and a DC-AC section 407. The arrangement 400 can form a direct electrical heating (DEH) power supply, and the AC-DC section 403, the DC-link 405 and the DC-AC section 407 can form a power cell 433 of a converter .

In contrast to the embodiment illustrated in Fig. 1, the ar ^¬ rangement 400 comprises a transformer portion 401, which com- prises three secondary windings 409, which are connected in series, wherein each of the secondary winding portions 409 is inductively coupled to a respective primary winding of a transformer, such as the transformer 240 illustrated in Fig. 2.

Each of the secondary winding portions 409 provides a voltage to the three converter cell input terminals 411, 412 and 413. Thus, the converter cell 433 comprises three converter cell input terminals 411, 412, 413 to support three phases.

The arrangement 400 further comprises a first thyristor 415 and a second thyristor 417, which are connected in series be ^¬ tween two DC output nodes 419, 421. The first converter cell input terminal 411 is connected between the first thyristor 415 and the second thyristor 417.

The arrangement 400 further comprises a third thyristor 423, a fourth thyristor 425, a fifth thyristor 428 and a sixth thyristor 430, wherein the third thyristor 423 and the fourth thyristor 425 are connected in series between the DC output nodes 419, 421 and also the fifth thyristor 428 and the sixth thyristor 430 is connected in series between the two DC out- put nodes 419, 421. Further, the second converter cell input terminal 412 is connected between the third thyristor 423 and the fourth thyristor 425. Further, the third converter cell input terminal 413 is connected between the fifth thyristor 428 and the sixth thyristor 430.

The capacitor section 405 comprises capacitors 429 arranged and connected as in the embodiment illustrated in Fig. 1.

Further, the DC-AC section 407 comprises IGBTs 431 and diodes 427 configured and arranged as in the embodiment illustrated in Fig . 1.

The arrangement 400 has a first converter cell output termi ^¬ nal 435 and a second converter cell output terminal 437. A load, such as the pipeline 350, illustrated in Fig. 3 may be connected (directly or via one or more compensating capaci ^¬ tors) to the converter cell output terminals 435 and 437 or to a series connection of several serially connected arrange ^¬ ments 400. In particular, the arrangements 200 illustrated in Fig. 3 may be replaced by the arrangements 400 illustrated in Fig. 4 or by the arrangements 500 illustrated in Fig. 5 (de ^¬ scribe below) to supply electric energy for heating the load 350. In particular Figure 4 illustrates a cell topology with a thyristor rectifier section 403. The main advantages with the thyristors are that they can perform a soft start, shut down locally, if the cell should fail and they can keep a constant DC voltage, if the input voltage is varying. In addition, the thyristors can be controlled to give a symmetrical input cur ^¬ rent even if the DC link voltage has a load side frequency dependant ripple. Fig. 5 schematically illustrates an arrangement 500 for pro ^¬ viding an AC current to a load, which shows similarities to the arrangement 400 illustrated in Fig. 4. Elements similar in structure and/or function in Figs. 4 and 5 are labelled by reference signs differing only in the first digit.

In contrast to the arrangement 400 illustrated in Fig. 4, the arrangement 500 illustrated in Fig. 5 does not comprise thy- ristors 415, 417, 423, 425, 428, 430 connected between DC output nodes but diodes 514, wherein two diodes 514 are seri ^¬ ally connected between the DC output nodes 519 and 521.

Thereby, three pairs of the serially connected diodes 514 are connected in parallel. Each of the three input terminals 511, 512, 513 of the converter cell 533 is connected between two serially connected respective diodes 514.

The arrangement 500 further includes fuses 560, which are connected between the input terminals 511, 512, 513 and the secondary winding portions 509, in order to protect the ar- rangement 500 from over-current.

In particular Figure 5 illustrates a cell topology with diode rectifier 503. The input transformer has one primary winding and one secondary winding for each cell 533. The different secondary phases are phase shifted to give a high pulse number impact on the mains. In the case of a breakdown in the cell compo ^¬ nents, the input fuses 560 will blow and isolate the faulty cell 533, while the others can continue operating.

The rectifier 503 is a simple diode bridge and a precharging circuit on one transformer winding will be necessary.

The output IGBT inverter 507 in H-bridge configuration is controlled in a Pulse Width Modulation mode to give a con ^¬ trollable fundamental sine wave output. The DC capacitor 505 is dimensioned to buffer the second har ^¬ monic pulsating power to a single-phase system with a limited voltage ripple. A short circuit contactor at the output will in case of cell failure isolate and short the faulty cell and allow the rest of the converter operate with reduced peak power capacity.

Fig. 6 schematically illustrates another pipeline heating ar ^¬ rangement 600 comprising a number of converter cells such as cells 100, 400 or 500 illustrated in Figs. 1, 4 or 5. A transformer 640 comprises for example a number of six secondary winding portions 609, which provide either two phases or three phases to each of the cells 633. The cells 633 are se ^¬ rially connected to provide an appropriate voltage to the load 650, wherein the capacitor 655 acts as a compensating capacitor .

In particular Figure 6 illustrates a 6-cell converter with series capacitor compensation.

In this configuration, the load reactance is balanced with a series connected capacitor 655. When tuned correctly, the load impedance seen from the converter will be resistive, and the converter output components will be used to produce ac- tive power only.

The transformer 640 has to be specifically made for the cell converter with separate secondary windings, with medium volt insulation, for each cell. If necessary, it may of cause be divided into 2 or 3 separate units. A converter with 6 series connected blocks will be capable of 4.1 kV and 1600 A ( 6.6 MW) .

Fig. 7 illustrates a pipe heating arrangement 700 according to an embodiment of the present invention, wherein three transformers 740 are employed, each transformer 740 providing electric energy to six secondary winding portions 709, wherein each secondary winding portion 709 is connected (via two or three terminals) to a cell 733, such as cells 133, 433, 533 illustrated in Figs. 1, 4, 5, respectively. Since the arrangement 700 provides a three times higher voltage compared to the arrangement 600 illustrated in Fig. 6, a com- pensating capacitor 655 may be avoided.

It is possible to avoid the series connected capacitor by us ^¬ ing a converter for full voltage and full current. Since each cell 733 may give e.g. a maximum of 690 V AC, 18 cells may be series connected to give 12 kV. Instead of the tree separate transformers shown on Fig. 7, a single transformer with 18 secondary windings could also be used.

In this case, the cell should be made for low cos φ, with a transformer and rectifier part rated for 300 kW pr cell. The inverter and DC-link capacitor has to be rated for 1600 A as for the capacitor compensated system.

Fig. 8 schematically illustrates a pipeline heating arrange- ment 800 for heating a pipeline 850 by direct electrical heating using a series connection of cells 833, such as cells 133, 433 or 533 illustrated in Figs. 1, 4 or 5, respectively.

The cells 833 are powered by not illustrated transformers. The thyristors 115, 117, 415, 417, 423, 425, 428, 430 illus ^¬ trated in Fig. 4, and the IGBT 123, 125 illustrated in Fig. 1 are controlled regarding their conductivities by a pulse with modulation control 860, which supplies control signals to corresponding gates of the thyristors and/or IGBTs. Further, the pipeline heating arrangement 800 comprises a measuring system 861 for measuring voltage and/or current flowing through the pipeline 850 or between different positions of the pipeline or the whole system. Further, the arrangement 800 comprises a feedback system 863, which receives measure- ment data of the measurement system 861 and controls the pulse with modulation control, in particular via a closed loop control 865 using set point information 867 for control ^¬ ling the pulse with modulation control 860, in order to achieve desired heating power, desired voltage or desired current flow through the load 850. Thereby, the load is moni ^¬ tored by a monitoring system 869. The voltage and current in the power circuit is transformed to signal level and used as feedback to the closed loop con ^¬ troller. The same signals are used for load circuit monitor ^¬ ing. The controller can be run in sine wave pulse pattern mode (no AC feedback) or in sine wave current mode (AC cur- rent feedback) . In sine wave current mode, the short circuit current will be equal to the actual load current.

A special frequency controller part tunes the frequency to give a power factor better than 0.95 on the cell converter part (before the compensating capacitor) . Other control functions like constant power output and power input limit may be added according to demands. The output of the closed loop controller 865 is the PWM signal to the cells through optical fibre .

The load circuit impedance is monitored and checked against limits. Warnings and stop signal are given when limits are exceeded. The cooling system is monitored with respect to wa ^¬ ter level, water conductivity, water flow and water tempera ^¬ ture. Transformer input current and temperature is monitored.

Each cell 833 may have its own intelligence in order to mini ^¬ mize the communication on the optical fiber. Auxiliary power and phase information may be picked up from the 3 phase input power side. The thyristor rectifier is automatically controlled by its own intelligence which takes care of: soft start, constant DC capacitor voltage, input current symmetry, shutdown at short circuit, overload indication. The inverter part will take its control signal from the fiber optics.

On each cell 833 there may be a monitoring function for: Heat sink temperature, DC capacitor voltage, thyristor current symmetry, possible thyristor fault and IGBT switching func- tion. The monitoring status is transferred through a separate optic fiber from each cell to a central unit.

Most power components like power semiconductors, DC capaci- tors, filter inductors and resistors in the cell are water cooled with de-ionised water. Therefore the power density in the cell may be high, and the margins in current rating of the components are kept low. The DC capacitor may be an Aluminium Electrolytic Capacitor, designed for base plate cooling.

When using diodes instead of thyristors, the DC voltage will vary according to the input voltage. The rated inverter volt- age may have to be reduced in order to keep the maximum DC working voltage below maximum rating of the DC capacitor. Therefore the cell has to be derated with a factor propor ^¬ tional to the voltage variation range. It is convenient that the input fuses are the standard 690 V type. The transformer output voltage should therefore not exceed 700 V.

The compensating capacitor may be connected in series with the cable in order to reduce the voltage seen from the con ^¬ verter. When the capacitor is tuned to give the same reac- tance as the cable at the operating frequency, the converter will work at a power factor close to unity and the stress and loss in the converter components are minimized. If the system is to be used with variable frequency or on different cables (pipe lines) , the capacitor value has to be varied accord- ingly.

Advantages of the cell converter system according to embodi ^¬ ments of the invention may be:

• The output power and output frequency is continuously variable (Compensating capacitor must be tuned to fre ^¬ quency) The system is tolerant to harmonics on the mains

The converter cell is standardized, cells can be stacked up to the necessary power or voltage

The losses of the cells are dissipated to water, the air condition capacity can be reduced

High reliability due to redundancy, possibility to by ^¬ pass one cell on the case of cell failure.

Possible to use the same converter on different pipes (easy power regulation)

Rapid electronic switch-off in the case of load-cable breakdown .

It may be possible to implement power setting without steps and soft start.

By using power electronics it is possible to vary the output frequency. There are indications that a higher frequency than 50Hz or 60Hz will improve the power efficiency on the heating system. If the output frequency is to be varied, the compen ^¬ sating capacitor has to be varied accordingly.

The state of the art system must be tuned to match the spe ^¬ cific pipe line impedance when it is installed. The power electronic system can be used on different pipe lines that demand approximately the same energy for heating. If the re ^¬ actances of the different pipe lines are different, the com ^¬ pensating capacitor may have to be retuned, or different fre ^¬ quencies for the different pipe lines may be used.

This new power electronic circuit for DEH according to an embodiment of the invention may fit in the available room on an oil or gas platforms leg 8 m x 3 m and may be divided in units small enough to be hoisted down trough a hatch in the floor with the size 2.35 m x 1.65 m. As an alternative space on the pipe deck may also be used with a container solution. The modularity of the cell converter may enable to make a re ^¬ dundant system where a cell may be bypassed in the case of breakdown without the loss of heating power. The arrangement for a DEH pipeline heating system is supposed to perform three different tasks: Convert 3 phase power to single phase power; Provide optimum output frequency and power for the load; and Compensate for the low power factor of the load, which may approximately be 0.25.

The DC current link converter using thyristors may have a unit power rating of 2.5 MW with an operating AC voltage of 1500 V. Several units may easily be paralleled, but are not so easily series connected. It will therefore be necessary to use an output transformer to match the impedance in the load circuit .

Several DC voltage link converters using IGBT's may be series connected for power and voltage increase into a multi cell converter. This converter can work with series capacitor compensation, and an output transformer may be omitted.

It should be noted that the term "comprising" does not ex ^¬ clude other elements or steps and "a" or "an" does not ex- elude 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.