Power supply device

A power supply device for a variable rotation speed drive includes a free-running converter connected to a land-based power grid, and an inverter connected to the variable rotation speed drive. A direct-current cable electrically connects the DC side of the converter with the DC side of the inverter. The inverter includes a plurality of phase modules having an upper and a lower valve branches with least two series-connected, two-pole subsystems with distributed energy storage devices. The inverter is located on the seabed in immediate vicinity of the variable rotation speed drive. Signal electronics of the inverter is located on land. In this way, the distance between the power supply on land and the drive on the ocean floor can reach several hundred kilometers, with ocean depths of several kilometers.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is the U.S. National Stage of International Application No. PCT/EP2009/053654, filed Mar. 27, 2009, which designated the United States and has been published as International Publication No. WO 2009/135730, and which claims the priority of German Patent Application, Serial No. 10 2008 022 618.1, filed May 7, 2008, pursuant to 35 U.S.C. 119(a)-(d).

BACKGROUND OF THE INVENTION

The invention relates to a power supply device for a variable rotation speed drive which is arranged on the seabed.

Variable rotation speed drives, consisting of an electric motor and a pump or a compressor, are supplied by means of a power supply device such as this, when on the seabed, with power from an electrical power supply system on land. The distance between the feeder on land and the drive on the seabed may be several hundred kilometers, at sea depths of several kilometers.

Variable rotation speed drives for underwater applications, also referred to as subsea applications, are used, for example, for delivering oil and gas from the seabed. These variable rotation speed drives are, as is known, supplied with power from an electrical power supply system by means of a voltage intermediate-circuit converter.

The object of a power supply device for subsea applications is to supply a motor, which is located on the seabed, for a variable rotation speed drive, with a polyphase voltage system with a variable voltage and frequency. There are various fundamental embodiments in this case:

FIG. 1schematically illustrates a first known variant of a power supply device for subsea applications. In thisFIG. 1,2denotes a voltage intermediate-circuit converter,4an electric motor for a drive,6a power converter transformer and8a feeding power supply system. The voltage intermediate-circuit converter2has a power converter10and12on the power supply system side and on the load side, which are electrically connected to one another on the DC voltage side by means of a DC voltage intermediate circuit, which is not shown explicitly, for clarity reasons. The power converter on the load side, which is preferably a self-commutated pulse-controlled power converter, is linked to the motor4by means of a three-phase alternating-current cable14. In addition, this converter2has signal electronics16, which can be connected on the input side by means of a data line18to connections of the electric motor4, and this data cable18is therefore shown by means of a dashed line, and is connected on the output side to control connections of the self-commutated pulse-controlled power converter12. A transformer with two secondary windings20and22is provided as a power converter transformer6, of which windings the secondary winding20is connected in delta, and the secondary winding22is connected in star. Since the primary winding24is likewise connected in star, only the secondary winding20has a phase-shift angle with respect to the primary winding24of 30° electrical. The primary winding24is electrically conductively connected to the feeding power supply system8, in particular to a feed point26. A 12-pulse diode feed is provided as the power converter10on the power supply system side. This means that this diode feed10has two three-phase diode bridges, which are electrically connected in series on the DC voltage side. The 12-pulse embodiment of the diode feed10results in the current overshoots in the feeding power supply system8being small. This power supply device for subsea applications is arranged on land, or on a platform at sea. The transition from the land or platform to the sea is indicated by the wavy lines30. Therefore, only the drive, consisting of the motor4and a pump and/or a compressor, is located on the seabed. Of the drive, only the motor4is shown in any detail.

Since the capacitive charging power of the alternating-current cable14places a major requirement for reactive power on the voltage intermediate-circuit converter2, there can be only a limited distance between the converter2and the motor4. In addition, this power supply device does not allow a multiple-motor drive. Each motor4in a drive must be connected by means of its own alternating-current cable14to the voltage intermediate-circuit converter2.

FIG. 2shows a further known power supply device for a variable rotation speed drive which is arranged on the seabed. This embodiment differs from the embodiment shown inFIG. 1in that the alternating-current cable14is linked by means of a transformer32to outputs of the self-commutated pulse-controlled power converter12in the voltage intermediate-circuit converter2. In addition, this alternating-current cable14is connected by means of a second transformer34to connections of the electric motor14which is arranged on the seabed. The transformer32transforms a generated converter voltage to a potential which is higher than the potential of the rated voltage of the electric motor4. After transmission, this potential is transformed back to the rated potential of the motor. The increased transmission voltage results in reduced resistive power losses. In addition, the alternating-current cable14may have a smaller cable cross section, thus allowing a better design of the cable14. This makes it possible to bridge a greater distance between the converter2and the motor4, in comparison to the embodiment shown inFIG. 1. These advantages are countered by the need for two transformers32and34, and the transformer34on the seabed must be encapsulated. When supplying power to a plurality of motors4on the seabed, two further transformers32,34must also be provided for each further motor4, as well as a further cable14.

In a further variant of the power supply device for a variable rotation speed drive on the seabed, the voltage intermediate-circuit converter2is arranged with the power converter transformer6on the power supply system side, as shown inFIG. 3, on the seabed in the immediate vicinity of the electric motor4for the variable rotation speed drive. A power supply system transformer36is provided on land, whose primary is electrically conductively connected to the feeding power supply system8, in particular to the feed point26, and whose secondary is electrically conductively connected to the alternating-current cable14. The use of the power supply system transformer36allows the AC voltage to be transmitted to be transformed to a value which is above the potential of the rated voltage of the electric motor4. This transmission voltage is transformed down again by the power converter transformer6.

In this power supply device, only the transformer36is still located on land or on a platform arranged at sea. The voltage intermediate-circuit converter2is now located adjacent to the motor4on the seabed, and can be directly linked to the motor4. This improves the drive performance, although the converter2now also has to be encapsulated. In comparison to the variant of the power supply device shown inFIG. 2, this changes nothing with regard to the distance between the feed point26and the motor4.

FIG. 4schematically illustrates a multiple-motor variant of the power supply device shown inFIG. 4. Each motor for a variable rotation speed drive is coupled to a voltage intermediate-circuit converter2by means of a power converter transformer6on the power supply system side. On the seabed, the alternating-current cable14is linked to an alternating-current busbar38, to which the plurality of power-converter-fed drives are connected. The alternating-current cable14can be provided with a further transformer40on the load side on the seabed, whose secondary is linked to the alternating-current busbar38. Since this further transformer40is not absolutely essential, it is illustrated by means of dashed lines. This further transformer40results in the AC voltage busbar38being at a potential below the potential of the transmission voltage, but above the potential of the rated voltage of the electric motor4for a variable rotation speed drive. In this variant as well, the distance between the feed point26on land and the drive on the seabed is still restricted, as in the case of the other variants shown inFIGS. 1 to 3. Furthermore, the number of installation parts on the seabed has increased many times. All the installation parts which are arranged on the seabed must be accommodated in an encapsulated form, in particular in each case in a pressure vessel.

The publication “Valhall Re-Development Project, Power from Shore” by Sverre Gilje and Lars Carlsson, printed in “ENERGEX 2006”, discloses a power supply device which connects a platform at sea to a feed point on land. The known “light” version of the high-voltage, direct-current transmission installation is provided as the power supply device. This HVDC light has two self-commutated pulse-controlled power converters, which are connected to one another on the DC voltage side by means of a direct-current cable. Each of these two self-commutated pulse-controlled power converters has an alternating-current filter on the AC voltage side, and a capacitor and a direct-current filter on the DC voltage side. The one self-commutated pulse-controlled power converter is arranged by means of a power supply system transformer at a feed point of a feeding power supply system on land while, in contrast, the second self-commutated pulse-controlled power converter is arranged on a platform at sea. A sea cable with a length of approximately 300 km is provided as the DC voltage cable. No communication is required between these two power converters. All that is required is the value of the DC voltage at both ends of the DC voltage cable. The power converter station on land controls the transmission voltage, and the power converter station on the platform at sea controls the real power. The distance between the feed point and the platform is likewise restricted with this power supply device, as well.

The invention is now based on the object of developing the known power supply device such that the distance between the feed point on land and the drive on the seabed is considerably greater.

SUMMARY OF THE INVENTION

This object is achieved according to the invention with a power supply device for a variable rotation speed drive arranged on the seabed, which includes a free-running converter connected on an AC side to a power grid located on land, an inverter having a DC side connected to a DC side of the converter and a load side connected to the variable rotation speed drive, the inverter including a plurality of phase modules, with each phase module having an upper and a lower valve branch and each valve branch having at least two series-connected, two-pole subsystems with distributed energy storage devices, the inverter being located on the seabed in immediate vicinity of the variable rotation speed drive, a direct-current cable electrically connecting the DC side of the converter with the DC side of the inverter, and land-based signal electronics of the inverter.

Since a power converter with distributed energy stores is provided as the self-commutated power converter, on the load side, of the power supply device, the upper and lower valve branch of which power converter in each phase module has at least two two-pole subsystems which are electrically connected in series, the power supply device according to the invention no longer has an energy store in the DC voltage intermediate circuit, as a result of which the direct-current cable which electrically conductively connects the power converter on the power supply system side and the power converter on the load side of the power supply device according to the invention can bridge considerably greater distances. This allows the power converter with distributed energy stores in the power supply device according to the invention to be arranged adjacent to the motor being fed on the seabed and its converter on the power supply system side on land.

Because of the design of the intermediate circuit, which does not necessarily have a lower inductance, and because of the lack of the intermediate-circuit capacitor, an intermediate-circuit short is highly improbable in comparison to the situation in a voltage intermediate-circuit converter with an intermediate-circuit capacitor. The power converter valves in the power converter on the power supply system side of the power supply device according to the invention therefore need no longer be designed for a short-circuit current caused by a low-impedance intermediate-circuit short. Furthermore, the i2t arrangement of these power converter valves can be considerably reduced.

The use of a power converter having a multiplicity of two-pole subsystems as a power converter on the load side of the power supply device allows the voltage intermediate-circuit converter to be split between the land and the seabed. Only the load-side power converter with distributed energy stores of this power supply device is therefore still located on the seabed. Since the value of the converter output voltage, and therefore of the motor voltage, is governed by the number of two-pole subsystems for each valve branch in a phase module of the load-side power converter with distributed energy stores, there is no longer any need for a transformer on the seabed.

Furthermore, the finely graduated output voltage form of the load-side power converter with distributed energy stores for the power supply device according to the invention makes it possible to use motors that are suitable for under-water use with less stringent requirements for the winding insulation. Since a high motor voltage can nevertheless be set independently of the use of a transformer, connecting lines and bushings to the motor can be designed for lower currents. In addition, this makes it possible to avoid the need for motors with a plurality of winding systems for relatively high power levels.

Since the load-side power converter with distributed energy stores in the power supply device consists only of a number of two-pole subsystems, which are electrically connected in series, for each valve branch, the availability of the power supply device can be considerably improved by adding redundant two-pole subsystems.

In addition to the power converter on the power supply system side of the power supply device according to the invention, signal electronics for the load-side power converter, which is arranged on the seabed, with distributed energy stores are also arranged on land. These signal electronics are connected for signaling purposes by means of a data cable to control inputs of the load-side power converter with distributed energy stores on the seabed. Major components of the power supply device according to the invention are therefore accommodated on land or on a platform, thus considerably reducing the complexity for encapsulation of the components of the power supply device according to the invention.

In one advantageous embodiment of the power supply device according to the invention, the uncontrolled power converter on the power supply system side is electrically conductively connected by means of the direct-current cable to a DC voltage busbar which is arranged on the seabed. This DC voltage busbar can be connected to a multiplicity of load-side power converters with distributed energy stores, in each case with a motor on the output side, for a variable rotation speed drive. A power supply device according to the invention can therefore be produced at low cost for a multiple-motor drive.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 5schematically illustrates a first variant of the power supply device according to the invention. In this figure,42denotes a load-side power converter with distributed energy stores,44denotes a direct-current cable, and46denotes a control unit. The power converter10on the power supply system side and the power converter42on the load side are linked to one another on the DC voltage side by means of the direct-current cable44. The control unit46of this load-side power converter42with distributed energy stores is connected by means of a data cable18to the signal electronics16in the power supply device, which is associated with the power converter10on the power supply system side. The power converter10on the power supply system side, which is in the form of an uncontrolled power converter, is linked on the AC voltage side by means of the power supply system transformer36to the feed point26of the feeding power supply system8. In the power supply device according to the invention, only the load-side power converter42with distributed energy stores is arranged on the seabed. All the other installation parts of this power supply device are arranged on land. The transition from land to sea is likewise indicated in this figure by a wavy line30. A diode feed is provided as the power converter10on the power supply system side and, in the simplest case, is in the form of a 6-pulse feed. If the aim is, as far as possible, for there to be no current harmonics in the power supply system, and if then only with a small amplitude, then the diode feed10must be designed, for example, as a 12-pulse, 18-pulse or 24-pulse feed.

FIG. 6shows a block diagram of one advantageous embodiment of the power supply device according to the invention, in schematic form. According to this block diagram, the diode feed10has two 6-pulse diode bridges48and50, which are each connected on the AC voltage side to a secondary winding22and20of the power converter transformer6, and are electrically connected in series on the DC voltage side. The load-side power converter42with distributed energy stores has a plurality of phase modules52, which are electrically connected in parallel on the DC voltage side. A positive and a negative DC voltage busbar POWand NOWare provided for the parallel connection of these phase modules52. A DC voltage, which is not annotated in any more detail, is dropped between these two DC voltage busbars POWand NOW. Each phase module52has an upper and a lower valve branch T1, T2; T3, T4and T5, T6. Each valve branch T1, . . . , T6has at least two two-pole subsystems54.

In the illustrated embodiment, each valve branch has four two-pole subsystems54. The two-pole subsystems54are electrically connected in series. Exemplary embodiments of these two-pole subsystems54are illustrated inFIGS. 7 and 8. Each junction point between two valve branches T1, T2; T3, T4and T5, T6forms a respective connection L1, L2or L3on the AC voltage side. The electric motor4, which is shown inFIG. 5, is connected to these connections L1, L2and L3on the AC voltage side. The DC voltage busbars POWand NOWof the load-side power converter42with distributed energy stores and the DC voltage busbars POGand NPOGof the power converter on the power supply system side are electrically conductively connected to one another by means of the direct-current cable44.

FIG. 7shows a first embodiment of a two-pole subsystem54. This two-pole subsystem54has two semiconductor switches56and58which can be turned off, two diodes60and62and a unipolar energy storage capacitor64. The two semiconductor switches56and58which can be turned off are electrically connected in series, with this series circuit being electrically connected in parallel with the energy storage capacitor64. One of the two diodes60and62is electrically connected in parallel with each semiconductor switch56and58which can be turned off, such that this diode is connected back-to-back in parallel with the corresponding semiconductor switch56and58which can be turned off. The unipolar energy storage capacitor64in the two-pole subsystem54consists either of a capacitor or of a capacitor bank comprising a plurality of such capacitors, with a resultant capacitance C0. The junction point between the emitter of the semiconductor switch56which can be turned off and the anode of the diode60forms a connecting terminal X1of the subsystem54. The junction point between the two semiconductor switches56and58which can be turned off and the two diodes60and62forms a second connecting terminal X2of the two-pole subsystem54.

In the embodiment of the subsystem54shown inFIG. 8, this junction point forms the first connecting terminal X1. The junction point between the drain of the semiconductor switch58which can be turned off and the cathode of the diode62forms the second connecting terminal X2of the two-pole subsystem54.

FIG. 9schematically illustrates a second variant of the power supply device according to the invention. This second variant differs from the variant shown inFIG. 5by the provision of a DC voltage busbar66. Three load-side power converters42with distributed energy stores, and each with a load-side motor4for a variable rotation speed drive, are connected to this DC voltage busbar66. This DC voltage busbar66is linked by means of the direct-current cable44to the DC voltage connections of the power converter10on the power supply system side. In addition, a data busbar68is provided, to which, on the one hand, the control units46of the load-side power converters42with distributed energy stores, and on the other hand the data cable18, are connected. The signal electronics16, which are accommodated on land, for the power supply device according to the invention are therefore each connected for signaling purposes to a control unit46of a respective load-side power converter42, which is arranged on the seabed, with distributed energy stores. The use of a DC voltage busbar66reduces the cable costs for a multiple-motor drive, and reduces the installation effort.

This power supply device according to the invention allows power to be supplied from a feeding power supply system to variable rotation speed drives for subsea applications, for example oil and gas delivery installations, in which case the distance between the feed on land and the drive on the seabed may be several hundred kilometers at sea depths of several kilometers.