WO2010079235A2

WO2010079235A2 - Load dependent converter switching frequency

Info

Publication number: WO2010079235A2
Application number: PCT/EP2010/050283
Authority: WO
Inventors: Thomas Hjort
Original assignee: Vestas Wind Systems A/S
Priority date: 2009-01-12
Filing date: 2010-01-12
Publication date: 2010-07-15
Also published as: WO2010079235A3

Abstract

The present invention relates to a method for providing fast and effective active power support, such as inertial support, the method comprising the steps of determining a fundamental frequency of the power supply grid, changing a switching frequency of a DC/AC converter in case the determined fundamental frequency falls outside a predetermined frequency dead-band of the power supply grid, and changing an amount of active power injected into the power supply grid in accordance with the changed switching frequency in order to bring the fundamental frequency within the frequency dead-band of the power supply grid.

Description

LOAD DEPENDENT CONVERTER SWITCHING FREQUENCY

FIELD OF THE INVENTION

The present invention relates to a method for providing fast and effective active power support, such as inertial support, to a power supply grid in case the fundamental frequency of the power supply grid falls outside a frequency dead-band.

BACKGROUND OF THE INVENTION

Overload capacities of power converters of wind turbine systems are typically established by overrating semiconductor switch components for higher continuous operating current. Alternatively, additional semiconductor switch components are added to provide a required overload capacity. However, additional switch components increase the overall losses of the converters.

It is a drawback of the above-mentioned conventional solutions that the costs of components and thereby the costs of manufacturing are high. Moreover, increased electric losses are related to the above-mentioned ways of establishing a required overload capacity. Also the addition of more components increases the risk of a component failure.

WO 2005/027301 deals with a method for handling low voltage ride through events where an DC/ AC converter is operated with a reduced switching frequency in order to pump additional current into a power supply grid in case the grid voltage drops significantly.

It may be seen as an object of embodiments of the present invention to provide a method for establishing overload capacities without increasing the complexity and the costs of manufacturing of a power converter.

It may be seen as a further object of embodiments of the present invention to provide a method for establishing fast and effective inertial support to a power supply grid.

DESCRIPTION OF THE INVENTION

The above-mentioned object is complied with by providing, in a first aspect, a method for providing active power support to a power supply grid operatively connected to a number of electrical power generating units, where at least one of said number of power generating units comprises a wind turbine facility supplying electrical energy to the power supply grid at least partly through a power converter module, the method comprising the steps of

- determining a fundamental frequency of the power supply grid,

- changing a switching frequency of a DC/ AC converter of the power converter module in case the determined fundamental frequency falls outside a predetermined frequency dead- band of the power supply grid, and

- changing an amount of active power injected into the power supply grid in accordance with the changed switching frequency in order to bring the fundamental frequency within the frequency dead-band of the power supply grid.

The term "active power support" is to be understood broadly thus also covering inertial support to the power support grid.

Inertial support to a power supply grid is concerned with the providing of active power to the power supply grid in a relative short time period in order to stabilize or even increasing the fundamental frequency of the grid. For example, if the fundamental frequency of the power supply grid falls below a frequency dead-band of said grid due to a heavy load acting on the grid or a power generator failure, active power may advantageously be injected into the grid in order to bring the fundamental frequency back into dead-band.

It is an advantage of the present invention that the time required for activating inertial support is considerable shorter than prior art system, where long mechanical time constants prevent a fast and effective response.

In one embodiment of the present invention the switching frequency of the DC/ AC converter of the power converter module may lowered in case the determined fundamental frequency is below the predetermined frequency dead-band. Following a lowering of the switching frequency the amount of active power injected into the power supply grid may be increased accordingly, i.e. the active power may be increased based on the detected fundamental frequency deviation. The active power may be increased proportionally to the detected fundamental frequency deviation.

The switching frequency of the DC/AC converter of the power converter module may be lowerable in discrete steps. Also, the amount of active power injected into the power supply grid may be increaseable in discrete steps. The amount of active power injected into the power supply grid may be increased by an amount which at least corresponds to the reduced switching losses achieved by lowering the switching frequency of the DC/ AC converter of the power converter module. Thus, a lowering of the switching losses in the DC/ AC converter may accomplish that additional active power may be injected into the power supply grid. The allowable amount of additional active power that can be injected into the power supply grid depends on the configuration of the wind turbine facility including the power capability of the devices/components constituting the wind turbine facility.

Preferably, the additional amount of active power should be up to 25% of the nominal power level of the wind turbine facility. Thus, according to the present invention inertial support up to around 125% of the nominal power level of the wind turbine facility may be provided.

The amount of active power injected into the power supply grid may be maintained at an increased power level - for example 125% of nominal power - over a time period of several seconds, such as for example 10 seconds.

As previously mentioned it is an advantage of the present invention that inertial support may be provided very fast. Thus, the ramp-up time from a nominal power level to 125% of nominal is very short. The increased active power level may be reached over a time period of less than 100 ms, such as less than 50 ms, such as less than 25 ms, such as less than 10 ms after detection of the needs for active power support, such as for example inertial support. Thus, the method underlying the present invention provides that an active power boost may be provided to the power supply grid much faster than traditional power plants can offer.

The amount of active power injected into the power supply grid while operating the DC/ AC converter of the power converter module with a lowered switching frequency may, as already mentioned, exceed a nominal power level of the power converter module. The increased power level of active power may be injected into the power supply grid until the fundamental frequency of the power supply grid is brought back within the predetermined frequency dead- band. Alternatively, the amount of active power injected into the power supply grid may be continuously increased until the fundamental frequency of the power supply grid is brought back.

The DC/ AC converter may be fed from a DC-link comprising a capacitor bank. The capacitance of this capacitor bank may be chosen so that the capacitor bank may be used as an energy reservoir/buffer. Electrical energy from the capacitor bank may be injected into the power supply grid in order to generate a fast inertial support. In a second aspect the present invention relates to a wind turbine facility adapted to deliver electrical energy to an associated power supply grid, the wind turbine facility comprising

- generator means for converting mechanical energy to electrical energy, the generator means being mechanically coupled to a set of rotor blades optionally via a gearing arrangement, and

- an electric power converter electrically coupled to the generator means and the associated power supply grid, optionally through a grid transformer, the electric power converter comprising a number of controllable switching elements being switchable with a variable or changeable switching frequency

wherein the switching frequency of the controllable switching elements is selected in accordance with an electrical load generated by the associated power supply grid on the wind turbine facility. Thus, the load generated by the associated power supply grid determines the switching frequency of the converter.

It should be noted that the term wind turbine facility should be interpreted broadly. Thus, the term wind turbine facility covers a single wind turbine or a group of wind turbines forming a wind turbine farm.

The generator means comprises a power generator. Similarly, the electric power converter comprises an AC/DC and/or DC/ AC converter. Preferably, the power generator, the AC/DC and/or DC/ AC converter and the optional grid transformer are all configured to operate in a three-phase configuration.

The power generator may deliver a three-phase AC output power voltage having a voltage level in the range 0.4-6 kV. Moreover, the power generator may be capable of generating and delivering AC power in a wide power range, i.e. from a few kW to several MW. The nominal frequency of the generated power supplied to the power supply grid may be 50 Hz or 60 Hz.

The power generator may be a synchronous generator applying external magnetisation or, alternatively, a synchronous generator applying permanent magnets for magnetisation. Alternatively, an induction generator or doubly fed induction generator may be applied. However, the generator type is not limited to the before-mentioned types. Thus, the present invention may be implemented in connection with basically any type of electrical generator. Also, the present invention is applicable on both full-scale and doubly fed arrangements, the latter involving that the electric power converter is at least partly coupled the generator through the rotor circuit. In a full-scale system all power generated by the power generator is passed through the AC/DC and/or DC/ AC converter and the optional grid transformer.

The controllable switching elements of the electric power converter may be any traditional switching element available for such purposes, such as for example IGBTs. Moreover, the controllable switching elements may be arranged in various known bridge configurations in respect to the number phases in the system.

In one embodiment of the present invention the electric power converter may be configured to lower the switching frequency when the electrical load generated by the associated power supply grid and the consumers connected thereto exceeds a predetermined value. The predetermined value may correspond to a nominal power level of the converter. Alternatively, the switching frequency may be lowered when the electric power converter exhibits a given set of load dependent characteristics.

Electric power converters for wind turbine related applications are typically dimensioned in such a way that the electric losses in the semiconductor switching elements are split approximately evenly between conduction and switching losses. Thus, if the switching frequency is reduced to 50% of a given initial value the switching losses are lowered with approximately 25%.

In the case of an overload situation the wind turbine facility is, when operated with a reduces switching frequency, capable of delivering sufficient power even though the required power level exceeds the nominal power level of the facility. Since an overload situation is considered an abnormal mode (non stationary) of operation EMC standards do generally not apply.

Thus, the switching frequency may be lowered up to 50% (or even more) when the electrical load generated by the associated power supply grid exceeds the nominal power level of the converter. In terms of absolute frequency values the nominal switching frequency of an electric power converter is typically around a few kHz.

The wind turbine facility may further comprise converter filters for filtering electric power leaving the electric power converter, i.e. electric power being generated to the power supply grid. The implementation of converter filters is preferably identical for all phases. Thus, if one phase is equipped with one or more tuned LC-filters for suppressing selected harmonics the remaining phases are preferably equipped with identical filters. Converter filters may, alternatively, be implemented as active filters. Preferably, the active filters are operated with a switching frequency being significantly higher than the switching frequency of the converter itself. The active filters may be implemented as voltage-source converters.

By lowering the switching frequency in a doubly fed system, the power rating may be increased and therefore the wind turbine facility can provide more reactive power via both available converters for reactive power or via only one of the converters.

In a third aspect, the present invention relates to a method of operating an electric power converter of a wind turbine facility in response to load variations provided by an associated power supply grid, wherein the wind turbine facility comprises generator means for converting mechanical energy to electrical energy and an electric power converter electrically coupled to the generator means and the associated power supply grid, optionally through a grid transformer, the electric power converter comprising a number of controllable switching elements being switchable with a variable or changeable switching frequency, wherein the switching frequency of the controllable switching elements is selected in accordance with an electrical load generated by the associated power supply grid on the wind turbine facility. Thus, the load generated by the associated power supply grid determines the switching frequency of the converter.

In yet another aspect, the present invention relates to a method of operating an STATCOM for supplying reactive power to an associated power supply grid, wherein the STATCOM is installed as part of the wind turbine facility. The STATCOM consists of an electric power converter comprising a number of controllable switching elements being switchable with a switching frequency, wherein the switching frequency of the controllable switching elements normally is fixed. However, by reducing the switching frequency the reactive power rating may be increased due to a reduction in switching losses, and therefore the wind turbine facility can provide more reactive power via the STATCOM.

Again, to reduce switching losses the switching frequency may be lowered when the electrical load generated by the associated power supply grid in combination with consumers connected thereto exceeds a predetermined value, such as the nominal power level of the converter. In this way the electric power converter may be operated above its nominal power level in a short time period.

As previously mentioned the switching frequency may be lowered with up to 50% (or even higher) in overload situations where the electrical load generated by the associated power supply grid exceeds the nominal power level of the converter. The method according to the third aspect of the present invention may also imply that the switching frequency is selected or chosen so as to approximately match load dependent characteristics of converter filters in that such converter filter characteristics may be dependent on the electrical load generated by the associated power supply grid. Converter filters are typically designed for being operated at a specific switching frequency. However, since converter filters contain magnetic materials (iron, iron powder or ferrite cores) the characteristics of the filters will change with the load applied to the filters. Thus, in order to utilize the converter filters in an optimal manner, the switching frequency of the power converter may be continuously adjusted so as to follow the behaviour of the converter filters.

Preferably, the converter filters comprise band-pass filters. Thus, the converter filter characteristics may involve the frequency values of the centre frequencies of such band-pass converter filters.

Also, the third aspect of the present invention implies that the switching frequency of the electric power converter may be varied or adjusted so as to compensate for degradation or tolerances of the components of the wind turbine facility as a whole. In particular capacitors degrade over time.

Also, the switching frequency of the frequency converter may be dependent on the speed of rotation of a rotor of a power generator. Thus, it may be advantageous to increase the switching frequency when the speed of rotation of the rotor is changed significantly. It has been found by the inventors that during such changes a higher switching frequency over a period of maybe 1-10 seconds makes the control of power generator more easy. When full control over the power generator has been regained the switching frequency returns to its normal value.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. Ia shows a first example of a relationship between a load and the switching frequency,

Fig. Ib shows a second example of a relationship between a load and the switching frequency, and

Fig. Ic shows a third example of a relationship between a load and the switching frequency. While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the figures and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms 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 DRAWINGS

In its broadest aspect the present invention relates to a method involving adjustment of a switching frequency of a power converter in order to meet specific demands, such as to reduce losses in overload situations, to match load dependent characteristics of converter filters operationally connected to the power converter, or to provide inertial support to the power supply grid in a fast and effective manner. The embodiments of the present invention are, in particular, useful in wind power related applications. However, the embodiments of the present invention are not limited to such applications.

Referring now to Fig. Ia an example of a relationship between a switching frequency (Tsw) of a power converter and a load is depicted. As depicted in Fig. Ia the power converter is operated at a nominal switching frequency (PU = I) until the load reaches a level corresponding to nominal load (100%). Above nominal load, i.e. at overload conditions, the switching frequency is reduced linearly until the load reaches a level corresponding to 125% of the nominal load. The converter is closed down if the load increases further. The linear dependency between switching frequency and load may differ from the depiction shown in Fig. Ia. Similarly, the close down level may differ from the 125% level depicted in Fig. Ia.

As previously mentioned, electric power converters are typically dimensioned in such a way that the electric losses in the semiconductor switching elements are split approximately evenly between conduction and switching losses. Thus, if the switching frequency is reduced with 50% the switching losses are lowered with 25%. The reduced losses facilitate that the converter is operable in an overload mode of operation. Typically, the nominal switching frequency of power converters is around 5 kHz.

Fig. Ib and Ic depict other relationships between the switching frequency (Tsw) and the load. As seen various dependencies both below and above the nominal load level are possible.

Claims

1. A method for providing active power support to a power supply grid operatively connected to a number of electrical power generating units, where at least one of said number of power generating units comprises a wind turbine facility supplying electrical energy to the power supply grid at least partly through a power converter module, the method comprising the steps of

- determining a fundamental frequency of the power supply grid,

2. A method according to claim 1, wherein the switching frequency of the DC/ AC converter of the power converter module is lowered in case the determined fundamental frequency is below the predetermined frequency dead-band, and wherein the amount of active power injected into the power supply grid is increased accordingly.

3. A method according to claim 2, wherein the switching frequency of the DC/ AC converter of the power converter module is lowerable in discrete steps.

4. A method according to claim 3, wherein the amount of active power injected into the power supply grid is increaseable in discrete steps.

5. A method according to any of claims 2-4, wherein the amount of active power injected into the power supply grid is increased by an amount which at least corresponds to the reduced switching losses achieved by lowering the switching frequency of the DC/ AC converter of the power converter module.

6. A method according to any of claims 2-5, wherein the amount of active power injected into the power supply grid is maintained at an increased power level over a time period of at least 1 second, such as 2 seconds, such as 5 seconds, such as 10 seconds.

7. A method according to any of claims 2-6, wherein the increased active power level is reached over a time period of less than 100 ms, such as less than 50 ms, such as less than 25 ms, such as less than 10 ms after detection of the needs for active power support.

8. A method according to any of claims 2-7, wherein the amount of active power injected into the power supply grid while operating the DC/ AC converter of the power converter module with a lowered switching frequency exceeds a nominal power level of the power converter module.

9. A method according to any of claims 2-8, wherein an increased power level of active power is injected into the power supply grid until the fundamental frequency of the power supply grid is brought back within the predetermined frequency dead-band.

10. A method according to any of claims 2-8, wherein an amount of active power injected into the power supply grid is continuously increased until the fundamental frequency of the power supply grid is brought back within the predetermined frequency dead-band.