WO2009027520A2

WO2009027520A2 - Modular converter system with interchangeable converter modules

Info

Publication number: WO2009027520A2
Application number: PCT/EP2008/061419
Authority: WO
Inventors: Thomas Hjort
Original assignee: Vestas Wind Systems A/S
Priority date: 2007-08-31
Filing date: 2008-08-29
Publication date: 2009-03-05
Also published as: WO2009027520A3

Abstract

The present invention relates to an electronic power converter system comprising a plurality of power converter modules (1st - 9th module) operatively connected to a DC intermediate circuit (DC link infrastructure), the electronic converter system comprising first and second power converter modules being operable in the same plurality of modes of operation. The first and second power converter modules are operable in identical modes of operation thereby forming a pair of interchangeable power converter modules.

Description

MODULAR CONVERTER SYSTEM WITH INTERCHANGEABLE CONVERTER MODULES

FIELD OF THE INVENTION

The present invention relates to a modular converter system comprising a plurality of interchangeable power converter modules forming a reliable, scalable and redundant converter system. The present invention further relates to various devices applying such electronic converter system.

BACKGROUND OF THE INVENTION

Various types of modular converter system have been suggested in the patent literature over the years.

One particular interesting arrangement has been suggested by Aloys Wobben in US2006/0103137 where a number of independent rectifier modules may be combined with an equal number of inverter modules thus forming a complete frequency converter for a wind turbine.

In US2006/0103137 a number of rectifier modules can be by-passed whereas other rectifier modules may remain active. Similarly, a certain number of inverter modules can be by-passed whereas other inverter modules may remain active. In this way the number of active rectifier and inverter modules can be chosen to match specific demands, such as an amount of power to be delivered.

However, it is a disadvantage of the arrangement suggested by Aloys Wobben that total flexibility is not provided. A rectifier module can only be operated as a rectifier. Thus, a rectifier module can not replace an inverter module and vice versa.

It is an object of the present invention to provide a scalable power converter system offering full flexibility in terms of replacing one power converter module with another power converter module within the power converter system. It is a further object of the present invention to provide a redundant power converter system for reliable power supply.

SUMMARY OF THE INVENTION

The electronic converter system according to the present invention is directed towards a modular converter system that is well suited (but not limited to) large power systems. One field of application may be wind turbines, but many other systems could benefit from the solution suggested by the present invention.

In wind turbines the module weight is an issue in a service and production situation. Also the life time requirements for wind turbines are very high. This requires replacement of parts in due time to avoid downtime of a wind turbine. Service of wind turbines (especially offshore) is extreme costly and very time consuming so a better scheduling of service is desired. This implies that if a fault in an offshore facility occurs it is obviously a huge advantage if the wind turbine could maintain operating until the next scheduled service. As more and more wind turbines are placed in wind power plants and are perceived and operated as such new requirements have arisen for grid support. This means that the wind turbines must be able to deliver power at a very low power factor (PF). This normally requires a very overrated output of the converter system.

The above-mentioned objects and other objects are complied with by providing, in a first aspect, an electronic converter system comprising a plurality of power converter modules operatively connected to a DC intermediate circuit, the electronic converter system comprising

- a first power converter module being operable in a plurality of modes of operation, and

- a second power converter module being operable in a plurality of modes of operation, wherein both the first and second power converter modules are operable in overlapping modes of operation thereby forming a pair of interchangeable power converter modules if so required.

The expression overlapping modes of operation should be interpreted broadly. Thus, the first and second power converter modules are, preferably, operable in identical sets of modes of operation thereby forming a pair of fully interchangeable power converter modules. A set of modes of operation preferably comprises a plurality of modes of operation, such as for example an AC/DC mode, a DC/AC mode, a DC/DC mode etc. Alternatively, the first and second power converter modules may form interchangeable power converter modules only among a selected number of modes of operation.

Thus, it is an essential feature of the first aspect of the present invention that the power converter modules are interchangeable thereby providing a flexible, fully redundant and reliable electronic converter system. Obviously, fully redundancy requires at least three power converter modules. Even further, the electronic converter system according to the present invention is a scalable system in that the number of power converter modules may be adjusted to match for example the amount of power or current to be delivered. Thus, according to the present invention converter systems in the MW range may be provided as easily as converter systems in the kW range just by varying the number of power converter modules incorporated into the converter system.

It is another advantage of the first aspect of the present invention that the power converter modules are capable of covering the same functionalities of the converter system. In this way the power converter modules become interchangeable and may thus replace each other. Thus, in case a given converter module, for example an AC/DC converter module, breaks down another converter module in the system may immediately be appointed an AC/DC converter module in replacement of the previous AC/DC converter module which may be repaired or physically replaced by another converter module. In this way the first aspect of the present invention provides a fully redundant converter system. The DC intermediate circuit may be implemented as a DC capacitor bank in a bus bar configuration, or as simple interconnection structure. Also overcurrent protective devices may be present in the DC intermediate circuit to limit fault propagation. The DC intermediate circuit may physically extend the boundary of the structure with the purpose of connecting more similar structures.

The electronic converter system according to the first aspect of the present invention may be used in connection with various applications. Thus, the converter systems according to the first aspect of the present invention may be applied as part of frequency converters for individual wind turbines or frequency converters for wind power plants. Since the converter system according to the present invention may also be configured as a DC/DC converter other applications may also be applicable.

In case of a frequency converter- 1 ike configuration a first mode of operation may involve operating the first power converter module as a AC/DC converter. Similarly, a second mode of operation may involve operating the second power converter module as a DC/ AC converter.

In case of a DC/DC converter both the first and the second power converter modules are operated as DC/DC converter modules.

The electronic converter system may further comprise one or more additional power converter modules also being operable in the plurality of modes of operation, the one or more additional power converter modules being operatively connected to the DC intermediate circuit.

In a preferred embodiment of the first aspect of the present invention at least two of the power converter modules each comprises a four quadrant power converter thereby allowing power conversion from AC to DC in a first mode of operation, and power conversion from DC to AC in a second mode of operation.

Controllable switching means may be provided in each of the power converter modules, said controllable switching means being adapted to control a power flow to and/or from each of the power converter modules. Thus, by operating the controllable switching means appropriately it may be controlled whether an individual converter module should provide power to the DC intermediate circuit or draw power from the DC intermediate circuit.

The controllable switching means may comprise IGBTs or other suitable switching means. The controllable switching means of the power converter modules may be switched with a frequency within the range 2-5 kHz.

The electronic converter system may further comprise an input filter for filtering input power signals to the electronic converter system. In addition, the electronic converter system may further comprise an output filter, such as a low-pass filter, for filtering power signals leaving the electronic converter system.

Each of the power converter modules may comprise control means adapted to communicate with a higher level central control module of the converter system.

In a second aspect the present invention relates to a power generating facility comprising means for generating AC power and an electronic converter system according to the first aspect of the present invention.

In a third aspect the present invention relates to a wind turbine comprising generator means for generating AC power, and an electronic converter system according to the first aspect of the present invention.

In a fourth aspect the present invention relates to a wind power plant comprising a plurality of generator means for generating AC power, and an electronic converter system according to the first aspect of the present invention.

In terms of implementation the electronic converter systems according to the second, third and fourth aspects of the present invention may be implemented and configured following the design routes mentioned in connection with the first aspect of the present invention.

In a fifth aspect the present invention relates to a method for operating an electronic converter system comprising a plurality of power converter modules operationally connected to a DC intermediate circuit, the method comprising the steps of

- providing a first power converter module being operable in a plurality of modes of operation and operating said first power converter module as an AC/DC converter, and

- providing a second power converter module being operable in a plurality of modes of operation and operating said second power converter module as an DC/ AC converter,

wherein the first and second power converter modules are operable in overlapping modes of operation thereby forming a pair of interchangeable power converter modules.

Again, the expression overlapping modes of operation should be interpreted broadly. Thus, the first and second power converter modules are, preferably, operable in identical sets of modes of operation thereby forming a pair of fully interchangeable power converter modules. A set of modes of operation preferably comprises a plurality of modes of operation, such as for example an AC/DC mode, a DC/ AC mode, a DC/DC mode etc. Alternatively, the first and second power converter modules may form interchangeable power converter modules only among a selected number of modes of operation.

Further power converter modules being operatively connected to the DC intermediate circuit and being operated as AC/DC or DC/ AC converter modules may be provided. A low-pass filter for filtering power signals leaving the electronic converter system may be provided. Each of the power converter modules may comprise control means adapted to communicate with a central control means of the system.

The power converter modules of the fifth aspect of the present invention may generally be implemented following the same design route as set forth in connection with the first aspect of the present invention.

BRIEF DESCRIPTION OF THE INVENTION

The present invention will now be described in further details with reference to the accompanying figures, where

Fig. 1 shows a power module,

Fig. 2 shows a modular power converter system according to a first embodiment, and

Fig. 3 shows a modular power converter system according to a second embodiment.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in details herein. It should be understood, however, that the invention is not intended to be limited to the particular form disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

In its most general aspect the present invention relates to a modular power converter system where a plurality of interchangeable power converter modules, such as AC/DC converters, DC/ AC converters, DC/DC converters, or a combination thereof are connected to a common DC intermediate circuit, such as a DC capacitor bank. Interchangeable means that the power converter modules can replace each other in terms of functionality. Thus, if a given converter module is operated as for example an AC/DC converter and this converter breaks down another converter can immediately be appointed AC/DC converter in order to take over from the broken down converter. In this way the converter system according to the present invention provides a fully redundant and very reliable converter system.

It is a characteristic of the power converter modules of the present invention that they may all be operated in a plurality of modes. Preferably, the power converter modules of the present invention are four quadrant semiconductor- based power converter modules since this type of power converter modules offer a high degree of flexibility. The active semiconductor components may be, but is not limited to, thyristors, IGCTs, GTOs, BJTs, mosFETs or IGBTs arranged in traditional rectifier, converter or booster bridge configurations.

It is an advantage of the power converter system according to the present invention that the system is scalable simply by varying the number of power converter modules constituting the converter system. Thus, if power in the MW range is to be delivered a relatively large number of power modules could be required. Contrary, if power in the kW range is to be delivered a significantly smaller number of power modules could be required. Obviously, the number of power converter modules in a given situation depends on the power handling capabilities of the chosen modules. Thus, the larger the modules the fewer modules are required.

Referring now to Fig. 1 a power converter module according to the present invention is depicted. As seen the power converter module comprises means for converting between AC and DC, or between two DC levels, an AC interface, appropriate filters, power semiconductors components (thyristors, IGCTs, GTOs, BJTs, mosFETs or IGBTs), DC capacitors, DC interface, drive circuits, and a local controller. An embodiment of the present invention in a wind turbine application is shown in Fig. 2. However, it should be noted that the present invention is by no means limited to wind turbine related or wind power plant related applications.

Beside the power converter system itself Fig. 2 depicts a drive train and a generator of a wind turbine. Moreover, a grid transformer and an associated power supply grid is depicted. Typically, the wind turbine generator, the grid transformer and the associated power supply grid are operated as three-phase AC elements. The drive train of the wind turbine involves the shaft to which the rotor blades are attached, the shaft to which the rotor of the generator is attached, and optionally a gear box inserted between the two shafts.

Fig. 2 shows power converter modules capable of converting between AC and DC using appropriate semiconductors components, such as thyristors, IGCTs, GTOs, BJTs, mosFETs or IGBTs. The power converter modules are all connected to a DC intermediate circuit and appropriate filters are provided at the output of the power converter system. Optionally an appropriate filter or filters may be provided at the input of the power converter system.

As depicted in Fig. 2 the DC-interfaces of the "n" power converter modules are all connected to the DC intermediate circuit - in Fig. 2 denoted DC link infrastructure. A high level control module provides control signals to each of the "n" power converter modules. Each of the "n" power converter modules is equipped with a local controller.

As previous stated, each of the "n" power converter modules are preferably a four quadrant semiconductor-based power converter module. A number of controllable switches, in Fig. 2 denoted transfer switches, allow power to be provided to and/or from each of the "n" power converter modules. In Fig. 2 the upper transfer switch for each power converter module is adapted to provide AC power to the module when said module is operated as an AC/DC converter, whereas the lower transfer switch for each converter module is adapted to lead AC power away from the power converter module when said converter module is operated as an DC/ AC converter. Depending on the desired mode of operation, AC/DC or DC/AC, the controllable transfer switches may be activated/deactivated accordingly.

As previously stated the converter system is a fully redundant system where the mode of operation of each of the "n" power converter modules can be varied in order to fulfil specific demands. Thus, a specific power converter module being operated as an AC/DC converter may be appointed to be operated as a DC/ AC converter if demands so require. Also, if a converter module suddenly fails or breaks down the redundancy of the system ensures that another converter module immediately takes over the functionality of the faulty converter module thereby ensuring a reliable power delivery. The converter module taking over could be a spare converter module or it could be a converter module being operated in another mode of operation. Thus, if a converter module operating in AC/DC mode breaks down it could be replaced by a module operating in either AC/DC or DC/ AC mode, or even a spare converter module. This selection depends on the required power and the available modules.

At the input and output ports of the converter system appropriate input and output filters are arranged. As depicted in Fig. 2 the input filter is connected to the generator of the wind turbine, said generator being driven by the drive train. Similarly, the output filter is connected to a grid transformer, said grid transformer being adapted to match the output voltage level of the converter system with the voltage of an associated power grid. The input and output filters are both connected to the AC sides of the converter system. Depending on the operating mode (AC/DC or DC/ AC) of the converter modules and the load on the AC side of the converter system different types of filter may be required.

The input and output filters may each comprise a high-pass and a low-pass filter part. The high-pass filter part may comprise EMC-filters and/or dv/dt filters whereas the low-pass filter part may comprise switch harmonic filters. The power converter modules can be cooled by various means. In one embodiment liquid cooling can be used. The cooling liquid for all power converter modules can circulate in a shared cooling system or in a system that is divided into two or more liquid systems. Alternatively, each of the individual power converter modules may have their own cooling system including a pump for circulating a cooling liquid for each converter module. In this way the redundancy of the system also includes the cooling arrangement. This is in contrast to conventional converter systems which purely rely on a shared cooling system having only a single or just a few common cooling pumps. Each converter module may include a complete cooling system, where parts of the cooling system is placed in an suitable environment for dissipating the heat (i.e. outdoor)

In Fig. 2 additional modules such as means for obtaining advanced grid operations (AGOs), dump resistors, means for static VAr compensation, batteries or other energy storage means for power backup may be connected to the DC intermediate circuit. These functionalities may be performed by converter modules. For example, converter modules being operated as a DC/ AC converter module can be appointed to fill out an AGO functionality.

As previously stated the functionality of a faulty converter module may be taken over by another converter module. This means that the converter system may be kept running even though one or more converter modules are closed down for service, repair or replacement. The switching between converter modules is done on the fly, i.e. without interruption, in order to avoid short shut-down periods.

The generator of the wind turbine may be of various types, including a full scale solution as depicted in Fig. 2 or a doubly fed generator arrangement as depicted in Fig. 3. The only difference between the doubly fed configuration of Fig. 3 and the full scale configuration of Fig. 2 is a third transformer winding of the grid transformer and its connection to the wind turbine generator. As depicted in Fig. 3 the connection between the grid transformer and generator comprises a filter and a stator switchgear.

The flexibility of the power converter systems shown in Fig. 2 and 3 is evident. In most typical cases a plurality of converter modules are connected to the input side of the converter system, and thus operated as AC/DC rectifiers. Similarly, a plurality of converter modules are connected to the output of the converter system, and thus operated as DC/ AC inverters.

Depending on the load situation the following adjustments to the converter system configuration may typically occur:

1. If more output current is desired an extra converter module is connected to the output side

2. If there is a fault in an input converter module an extra converter module is switched to the input side

3. If there is a fault in an output converter module an extra converter module is switched to the output side

4. If there is an overload on a converter module an extra converter module is provided - for example an input converter module may temporary be switched over to the output side

5. If there is a need for fast demagnetizing of the generator, more converter modules may be switched over to the input side

6. If a converter module approaches its expected end of life time a redundant module may take over its functionality, and the converter may request Converter Module replacement at the next service (alternatively accept reduced performance until next service).

7. If there is a need for a lower PF on the output of the system, modules could be transferred from the generator side to the output, maximizing the capability of the system as a whole.

8. If there is an overload of the output side, modules could be transferred from the input the output to help provide the current needed to activate available overcurrent protective devices. 9. If an equal lifetime for the converter modules is decidable the converter system may be configured to ensure that the converter modules experience an essentially equal load profile over their life.

10. When an AGO functionality is needed converter modules can be transferred from either the input side (AC/DC) or the output side (DC/ AC) of the converter system to an AGO dump resistor.

11. A full rated AGO functionality could be established if the output converter modules (DC/ AC) are transferred to AGO functionalities in case of a power grid failure. This ensures that the full load on the mechanical system of a wind turbine may be maintained. When the power grid failure is no longer present the converter modules being operated to provide AGO functionalities are transferred to output converter modules (DC/AC). To ensure a smooth transfer the converter modules are transferred one at a time thereby enabling a fast reconnect of the wind turbine to the power grid. Pitch systems and the rest of the mechanical system of the wind turbine remain unaffected by the power grid dropout.

Claims

1. An electronic converter system comprising a plurality of power converter modules operatively connected to a DC intermediate circuit, the electronic converter system comprising

- a second power converter module being operable in a plurality of modes of operation,

wherein both the first and second power converter modules are operable in overlapping modes of operation thereby forming a pair of interchangeable power converter modules if so required.

2. An electronic converter system according to claim 1, wherein the first and second power converter modules, in a first mode of operation, are adapted to be operated as AC/DC converter modules.

3. An electronic converter system according to claim 1 or 2, wherein the first and second power converter modules, in a second mode of operation, are adapted to be operated as DC/AC converter modules.

4. An electronic converter system according to any of claims 1-3, wherein the first and second power converter modules, in a third mode of operation, are adapted to be operated as DC/DC converter modules.

5. An electronic converter system according to any of the preceding claims, further comprising one or more additional power converter modules being operable as AC/DC, DC/AC or DC/DC power converter modules, the one or more additional power converter modules being operatively connected to the DC intermediate circuit.

6. An electronic converter system according to any of the preceding claims, wherein controllable switching means is provided in each of the power converter modules, said controllable switching means being adapted to control a power flow to and/or from each of the power converter modules.

7. An electronic converter system according to any of the preceding claims, further comprising input filter means for filtering input power signals to the electronic converter system.

8. An electronic converter system according to any of the preceding claims, further comprising output filter means for filtering power signals leaving the electronic converter system.

9. An electronic converter system according to claim 8, wherein the output filter means comprises a low-pass filter.

10. An electronic converter system according to any of the preceding claims, wherein each of the power converter modules comprises control means.

11. An electronic converter system according to claim 10, further comprising a central control module adapted to communicate with control means of the power converter modules.

12. A power generating facility comprising means for generating AC-power, said means for generating AC-power being operationally connected to an electronic converter system according to any of the preceding claims.

13. A wind turbine comprising a generator for generating AC-power, said generator being operationally connected to an electronic converter system according to any of claims 1-11.

14. A wind power plant comprising a plurality of wind turbines according to claim 13.

15. A method for operating an electronic converter system comprising a plurality of power converter modules operationally connected to a DC intermediate circuit, the method comprising the steps of

- providing a first power converter module being operable in a plurality of modes of operation and operating said first power converter module as an

AC/DC converter, and

16. A method according to claim 15, wherein further power converter modules being operated as AC/DC or DC/ AC converter modules are provided, the further power converter modules being operatively connected to the DC intermediate circuit.

17. A method according to claim 15 or 16, wherein input filter means for filtering input power signals to the electronic converter system is provided.

18. A method according to any of claims 15-17, wherein a output filter means for filtering power signals leaving the electronic converter system is provided.

19. A method according to any of claims 15-18, wherein each of the provided power converter modules comprises control means.

20. A method according to claim 19, wherein a central control module adapted to communicate with control means of the power converter modules is provided.