Method for controlling pulse width modulated inverter

Abstract

The invention relates to a method for controlling one or more PWM inverters. The method comprises determining first and second periods (P11, P12) of the first PWM inverter for operating the first PWM inverter with respective first and second switching modes, where the first and second switching modes have different 5-harmonic spectra, and operating the first PWM inverter by continuously applying the first and second switching modes during the respective first and second periods (P11, P12).

Description

Technical Field

[0001] The present invention relates to controlling a pulse width modulated inverter, such as a pulse width modulated inverter in a wind turbine. Background Art

[0002] A pulse width modulation (PWM) inverter connected to the network is used to convert DC power into AC power so that the AC electric power can be injected into the network. The inverter generates AC power by switching the DC power depending on a switching signal including a switching pattern.

[0003] For example, wind turbines generate DC power via an AC-DC converter, and solar farms generate DC power that needs to be converted to AC power.

[0004] The switching signals include high-frequency components which are inevitably injected into the network as harmonic current emissions.

[0005] Network regulations may set requirements for harmonic current emissions, such as harmonic current emission limits for each harmonic current frequency component.

[0006] Accordingly, there is a need to improve the PWM inverter system so that it can better meet the harmonic current emission requirements of network specifications. Summary of the invention

[0007] It is an object of the present invention to improve a PWM inverter or the control of a PWM inverter so that it can meet the harmonic current emission requirements of network regulations. Another object of the present invention is to provide a method for PWM inverter control in connection with harmonic current emission control, which method does not complicate such a control method. In particular, it is an object of the present invention to provide such a control method for a wind turbine and a wind turbine park.

[0008] In a first aspect of the invention, a method for controlling at least a first PWM inverter is proposed, the method comprising determining a first and a second period of the first PWM inverter for operating the first PWM inverter with respective first and second switching patterns, wherein the first and second switching patterns have different harmonic spectra, and

[0009] - operating the first PWM inverter with the first and second switching patterns applied continuously during respective first and second periods.

[0010] The method includes controlling at least first and second PWM inverters, and the method further includes determining first and second cycles of the second PWM inverter to operate the second PWM inverter using corresponding first and second switching patterns, and continuously applying the first and second switching patterns to operate the second PWM inverter during the corresponding first and second cycles of the second PWM inverter. The first cycle of the first PWM inverter and the first cycle of the second PWM inverter start at different start times.

[0011] Advantageously, by repeatedly operating the PWM converter with different switching patterns (such as alternatingly in the case of only two different switching patterns) during periods associated with the different switching patterns (the periods thus also being repeated continuously), different harmonic current emissions generated from the different switching patterns result in a reduction in harmonic current emissions at a given frequency or within a given frequency spectrum range (at least within a given time period, the given time period including the length of one of the first and second periods compared to operating the PWM converter with a single switching pattern).

[0012] Advantageously, the control method is easy to implement since only the switching method is modified, ie the method does not require modifications to the inverter itself or the network filter. For example, the switching pattern can be designed such that existing network filters can be used.

[0013] When different switching patterns are used to operate two or more PWM converters, the same advantages can be obtained by operating a single PWM converter using different switching patterns.

[0014] Accordingly, the first and second switching patterns are applied during the first and second periods, respectively, such that the first and second periods for the first PWM inverter are shifted in time relative to the first and second periods for the second PWM inverter. Advantageously, due to the application of the different switching patterns shifted in time, the different PWM inverters will generate different harmonic current emissions (at least during the time periods when the different inverters operate with the different switching patterns). Therefore, during such time periods, the harmonic current emissions will not increase in a cumulative manner if the different inverters operate with the same switching pattern.

[0015] The start times of the first cycles of the first and second PWM inverters, such as different start times, are predetermined or determined based on an externally generated trigger signal. Alternatively, the different start times of the first cycles of the first and second PWM inverters are determined from a random process.

[0016] According to one embodiment, the start time of the first cycle of the second PWM inverter is delayed relative to the start time of the first cycle of the first PWM inverter by the time length of the first cycle of the first PWM inverter. In this way, the first and second PWM inverters are operated in different sequences of different switching modes, so that when the first PWM inverter is operated in one switching mode, the second PWM inverter is operated in a different switching mode, so that no PWM inverter is operated in the same switching mode at the same time.

[0017] According to one embodiment, at least the first cycles of the first and second PWM inverters have equal lengths. However, in order to operate different PWM inverters with mutually different switching patterns at any time, all cycles should have the same length.

[0018] According to an embodiment, in response to a predefined operating condition, such as a network disturbance event, the method comprises switching from operating the at least first PWM inverter in the first and second switching modes to operating the at least first PWM inverter in only one of the first and second switching modes.

[0019] According to an embodiment, in response to another predefined operating condition, such as a network restoration event, the method comprises switching from operating the at least first PWM inverter in only one of the first and second switching modes, back to operating the at least first PWM inverter in the first and second switching modes.

[0020] The first and second switching patterns may be determined based on respective first and second switching frequencies, wherein the first and second switching frequencies are different.

[0021] A second aspect of the invention relates to a PWM controller configured for controlling at least a first PWM inverter, wherein the PWM controller comprises a data processor arranged to perform the method according to the first aspect.

[0022] A third aspect of the present invention relates to a PWM inverter system, comprising:

[0023] - A PWM controller according to the second aspect,

[0024] - A network filter for smoothing a voltage output of a PWM inverter, wherein the network filter is configured to operate at an operating frequency lying between maximum amplitude harmonics of different harmonics of different switching patterns (eg configured for optimal waveform smoothing).

[0025] A fourth aspect of the present invention relates to a wind turbine, comprising the PWM inverter system according to the third aspect.

[0026] A fifth aspect of the invention relates to an electric power generation farm comprising the first and second PWM inverter systems according to the third aspect or one or more wind turbines according to the fourth aspect.

[0027] A sixth aspect of the present invention relates to a computer program, the program comprising instructions, which, when a computer executes the program, cause the computer to execute the method of the first aspect.

[0028] In general, the various aspects and embodiments of the present invention may be combined and coupled in any possible manner within the scope of the present invention. These and other aspects, features and / or advantages of the present invention will be apparent from the embodiments described below and may be explained with reference to these embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Several embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0030] Figure 1A An electric power generation system is shown,

[0031] Figure 1B shows a wind power system,

[0032] Figure 1C A wind turbine is shown comprising a wind power system,

[0033] Figure 2 A plurality of PWM controllers showing a power generation field,

[0034] Figure 3-4 A timing diagram illustrating a method of controlling a PWM inverter is shown. DETAILED DESCRIPTION

[0035] Figure 1A An electric power generation system 100 is shown. The electric power generation system 100 includes a power supply 101 that generates DC power and supplies the DC power to a network-side inverter 102. The network-side inverter 102 here is a pulse width modulator (PWM) and is referred to as a PWM inverter 102. The PWM inverter 102 converts the DC power into AC power, which is injected into a network 104, such as an electric power distribution network. The AC power from the inverter 102 may be filtered by a network filter 103. In addition, a transformer may be included, for example between the network filter 103 and the network 104. The network filter 103 includes a filter inductor and a filter capacitor for waveform smoothing of the voltage output of the PWM inverter 102.

[0036] Although power from the PWM inverter 102 may be supplied directly to the network 104, an intermediate power network may be provided between the PWM inverter 102 and the network 104 to connect the output of the electric power generation system 100, such as one or more wind turbines, to the network 104. Such an intermediate network may include power lines, such as a medium voltage network, which connects the electric power generation system 100 or a plurality of such systems to a network 104, such as a high voltage transmission network 104, via a transformer.

[0037] Here, the network 104 may be any of a distribution network, a transmission network, a medium voltage network, a high voltage network, or other networks to which the output of the PWM inverter 102 is connected.

[0038] The electric power generation system 100 may be a solar electric power generation field such as a photovoltaic system, a wind turbine generator, a battery storage system, a fuel cell system, or any other electric power generation system that generates DC output power.

[0039] The electric power generation system 100 further comprises a PWM controller 110 arranged to control the PWM inverter 102 to modulate the DC power into AC power according to a pulse width modulation scheme such as a switching pattern.

[0040] PWM inverter system 140 , as defined herein, includes PWM inverter 102 , PWM controller 110 , and network filter 103 .

[0041] Figure 1B An example is shown in which the electric power generation system 100 is a wind power system 150. The power source 101 of the wind power system 150 comprises a generator 151 of a wind turbine and a machine side converter 152, which is arranged to convert AC power from the generator into DC power. In general, the wind power system 150 comprises at least a PWM controller 110 or at least one PWM inverter system 140.

[0042] Figure 1C A wind turbine 160 is shown that includes a wind power system 150. The wind turbine also includes a tower 153 and a rotor 154, the rotor having at least one rotor blade 155, such as three blades. The rotor is connected to a nacelle 156 mounted on top of the tower 153 and is arranged to drive a generator 151 located in the nacelle via a drive train. The drive train includes a shaft connecting the rotor 154 to a gearbox or generator. The rotor 154 is rotatable under the action of the wind. The rotational energy of the rotor blades 155 induced by the wind is transmitted to the generator via the shaft. Therefore, the wind turbine 150 is able to convert the kinetic energy of the wind into mechanical energy by means of the rotor blades, and then into electrical energy by means of the generator. The wind turbine also includes a machine side converter 152 and a PWM controller and inverter 102, 110.

[0043] Figure 2 An electric power generation park 200 is shown that includes one or more electric power generation systems 100 and / or one or more wind power systems 150. The electric power generation park 200 may be a wind turbine park with a plurality of wind turbines 160, for example.

[0044] Each of the one or more electric power generation systems 100, such as one or more wind power systems 150, may be configured with a PWM inverter system 140, 141, 142, but it is also possible that a single electric power generation system 100 may include multiple PWM inverter systems 140, such as first and second PWM inverter systems 140, 141.

[0045] exist Figure 2 In the embodiment of the present invention, each of the PWM inverter systems 140 includes a PWM controller 110 , 111 , 121 configured to control at least one PWM inverter 102 .

[0046] The PWM controller 110 comprises a data processor arranged to execute an algorithm based on input data such as switching frequency f, start time t, period data P and / or trigger input y. The input data of the different PWM controllers 110 , 111 , 121 may be supplied and coordinated by the central power farm controller 201 .

[0047] A single PWM controller 110 need not be limited to controlling a single PWM inverter 102, but may be configured to control multiple PWM inverters, such as first and second PWM inverters 102, 112. For example, a single wind turbine 150 may include first and second PWM inverters 102, 112, and a single PWM controller 110 arranged to control both PWM inverters. Accordingly, a PWM inverter system may include one or more PWM inverters 102, 112.

[0048] In general, each PWM inverter system 140 includes a single network filter 103, 113, 123. The single network filter can be arranged to filter the output power from one or more PWM inverters (such as the first and second PWM inverters 102, 112 included in the PWM inverter system). The filtering provides smoothing of the voltage output from the PWM inverter. However, it is also possible that a single PWM inverter system 140 includes multiple network filters, such as the first and second network filters 103, 113.

[0049] The network filter may be an existing network filter designed for a switching frequency, for example, between the first and second switching frequencies. Thus, the same network filter may be used without modification when the first and second or further switching frequencies are located not too far from the original switching frequency.

[0050] As shown, the outputs from network filters 103 , 113 , 123 are supplied to network 104 .

[0051] Figure 3 Timing diagrams 301 , 302 are shown to illustrate an example of a method of controlling the one or more PWM inverters 102 , 112 , 122 based on different switching frequencies f1 , f2 .

[0052] The switching frequencies f1, f2 define a carrier frequency used to modulate a desired voltage reference waveform into a modulation wave or switching pattern used to control switching of the PWM inverter 102. The carrier wave may be a triangular wave.

[0053] Alternatively, different switching patterns may be predefined and, for example, stored in the PWM controllers 110, 111, 121. It is also possible to determine the switching patterns as needed, for example during operation, and store them. For example, a selective harmonic cancellation PWM algorithm may be used to determine the switching patterns.

[0054] When the switching mode is applied to the PWM inverter, the different switching modes, such as the first and second switching modes, generate different harmonic current spectra. The different harmonic current spectra have different maximum amplitude harmonics. The maximum amplitude harmonic is the harmonic, i.e., the frequency, of the signal component of the switching mode with the maximum voltage or power amplitude. For the purpose of explanation, the maximum amplitude harmonic corresponds to different switching frequencies f1, f2. The different switching modes can be determined based on the corresponding first and second switching frequencies f1, f2 or any different switching frequencies, for example, so that the maximum amplitude harmonic approaches different switching frequencies.

[0055] Here, the examples are explained based on different switching frequencies f1, f2, but can also be equivalently based on different switching modes. Accordingly, any examples or descriptions involving different switching frequencies f1, f2 are not limited to different switching frequencies, but are based on different switching modes with different maximum amplitude harmonics in general.

[0056] The first timing diagram 301 is associated with the first PWM inverter 102. The first timing diagram 301 defines first and second periods P11, P12 defining periods in which the first PWM inverter 102 operates based on different first and second switching patterns (equivalently based on first and second switching frequencies f1, f2).

[0057] Similarly, the second timing diagram 302 is associated with the second PWM inverter 112. Here, the first and second periods P21, P22 similarly define periods in which the second PWM inverter 112 operates based on different first and second switching patterns (equivalently based on first and second switching frequencies f1, f2).

[0058] The first and second switching patterns or switching frequencies f1 , f2 are used for both the first and second PWM inverters 102 , 112 .

[0059] The first period P11 associated with the first PWM inverter 102 and the first period P21 associated with the second PWM inverter 112 start at start times t11, t21, respectively. The start times t11, t21 of the first and second PWM inverters may be referred to as first and second start times t11, t21.

[0060] The first and second start times t11, t21 may be equal, i.e., such that the first and second periods P11, P21 start at the same time. Alternatively, the first and second start times t11, t21 may be equal, i.e., such that the first and second periods P11, P21 start at different times. Thus, the second start time t21 may be shifted relative to the first start time t11 by a shift time ΔT.

[0061] In one example, a single PWM inverter, such as the first PWM inverter 102, is controlled based on first and second periods P11, P12 and corresponding different first and second switching frequencies f1, f2. As shown, the periods P11, P12 are applied alternately.

[0062] As shown, the second PWM inverter 112 may be controlled according to first and second periods P21 , P22 that are switched relative to the first and second periods P11 , P12 of the first PWM inverter 102 by a switching time ΔT.

[0063] In the case of a switching time ΔT=0, the start times t11, t21 of the first periods P11, P21 are equal. Accordingly, the first and second PWM inverters 102, 112 operate at the same switching frequency. In this case, when measured in a period that is at least longer than one of the first periods P11, P21, the harmonic current emissions of the first and second PWM inverters 102 are spread over different frequency spectra.

[0064] In the case where the switching time ΔT>0 but is less than the length of one of the first periods P11, P21, then at least for some periods of time, i.e. during the period when the first PWM inverter 102 operates at one switching frequency f1 and the second PWM inverter 112 operates at another switching frequency f2, the same switching frequency or the same or the sum of the maximum harmonics does not reach the same level as if all PWM inverters were operated at different switching frequencies or non-switching cycles of the switching pattern. Therefore, by starting the first period P11 of the first PWM inverter and the first period P21 of the second PWM inverter at different starting times t11, t21, the cumulative emission of harmonic current emissions from multiple PWM inverters at a given moment in time, rather than just over a period of time, can be reduced.

[0065] In the case of a switching time ΔT=ΔTP, where ΔTP is the length of the first period P11, and all periods of the first and second PWM inverters 102, 112 are equal, a special case occurs. In this case, the first and second PWM inverters are operated at mutually different switching frequencies or different switching modes at any time. Therefore, by delaying the start time t21 of the first period P21 of the second PWM inverter 112 relative to the start time t11 of the first period P11 of the first PWM inverter 102 by the time length ΔTP of the first period P11 of the first PWM inverter 102, the first period P11 of the first PWM inverter 102 is operated simultaneously with the second period P22 of the second PWM inverter 112, and the second period P12 of the first PWM inverter 102 is operated simultaneously with the first period P21 of the first PWM inverter 102.

[0066] The start times t11, t21 of the first periods P11, P21 of the first and second PWM inverters, the start times t11, t22 of the respective first and second periods P11, P22 of the first and second PWM inverters, or the start times t12, t21 of the respective second and first periods P12, P21 of the first and second PWM inverters may be predetermined or determined based on a common externally generated trigger signal. For example, a predetermined timer signal t may be supplied to the PWM controllers 110, 111, 121 by the central power generation farm controller 201, wherein the timer signal comprises the start times t11, t12, t21, t22 of the different periods P11, P12, P21, P11. Alternatively, an externally generated trigger signal, such as a commonly generated trigger signal y (from the power farm controller 201) may be supplied to the PWM controllers 110, 111, 121, wherein the PWM controllers are configured to determine the timing of changes between periods P11, P12, etc. (i.e., changes between different switching frequencies f1, f2). For example, the PWM controllers 110 , 111 may include internal timers to determine the switching frequency or transitions between switching modes.

[0067] Similarly, the central power farm controller 201 may supply period information P, such as the period lengths of the first and second periods P11 , P12 , P21 , P22 , to the PWM controller.

[0068] Alternatively, the start time t11, t21 of the first period P11, P21 of the first and second PWM inverters may be determined by a random generator comprised by each PWM controller 110, 111, 121. Subsequent changes in the switching frequency may be determined by an internal timer relative to the randomly determined start time.

[0069] In general, different PWM inverters can be based on the following control: a sequence of different switching frequencies is applied in a sequence of cycles of different or equal lengths, with a distinguishable switching frequency for each cycle. For different PWM inverters, the cycle sequence can be shifted in time so that two or more PWM inverters operate with different switching frequencies at a given time. The time shift can be equal to or less than one cycle. In general, the cycles for operating the first and second PWM inverters with different switching cycles can have the same length, but can also have different lengths, or randomly varying lengths.

[0070] The length of any of the first period P11, P21 and / or the second period P12, P22, or the overall length of the period in which the PWM converter operates at distinguishable and mutually different switching frequencies, should be longer than a multiple of the basic period of the network frequency. For example, the length of the period should be longer than 0.1 seconds, such as longer than 1 minute, longer than 2 minutes or even longer than 4 minutes.

[0071] The switching frequencies may be chosen such that the difference between the first and second switching frequencies f1 , f2 , or the overall difference between different switching frequencies or different maximum amplitude harmonics is at least 100 Hz, such as at least 200 Hz, for example 400 Hz.

[0072] The power farm controller 201 may be configured to stop operating the PWM inverter at a different switching frequency in response to a predefined operating condition, such as a network disturbance event. In this case, the PWM inverter will only operate at one of the first and second switching frequencies or switching modes. Similarly, the power farm controller 201 may be configured to automatically return to operating the PWM inverter at a different switching frequency in response to detecting another predefined operating condition, such as a network recovery event.

[0073] Figure 4 Timing diagrams 401 - 403 are shown to illustrate the control of the one or more PWM inverters 102 , 112 , 122 based on three different switching frequencies f1 , f2 , f3 .

[0074] Therefore, if Figure 4 As shown, the PWM converter 102 may alternatively be controlled based on: three or more periods P11 , P12 , P13 associated with different switching frequencies f1 , f2 , f3 , wherein the first, second and third periods are supplied sequentially and the same pattern of different switching frequencies is continuously repeated.

[0075] Since the periods P11, P12, P13 with different switching frequencies are applied successively, when the measuring period is at least longer than the first period P11, the harmonic current emission of the PWM 102 is spread over different frequency spectra. The maximum amplitude harmonics during different periods may be the actual switching frequencies of the different switching frequencies. Similarly, for the case of different switching modes, the harmonic current emission of the PWM 102 will be spread over different frequency spectra, and the maximum amplitude harmonics during different periods will vary according to the different maximum amplitude harmonics.

Claims

1. A method for controlling at least first and second PWM inverters (102, 112, 122), the method comprising: - determining a first and a second period (P11, P12) of the first PWM inverter for operating the first PWM inverter using a corresponding first and a second switching pattern, wherein the first and the second switching pattern have different harmonic spectra, and - operating the first PWM inverter with the first and second switching patterns being applied successively during respective first and second periods (P11, P12), - determining a first and a second period (P21, P22) of a second PWM inverter (112) for operating the second PWM inverter using corresponding first and second switching patterns, and - operating the second PWM inverter (112) by applying the first and second switching patterns successively during respective first and second periods (P21, P22) of the second PWM inverter, wherein: - The first period (P11) of the first PWM inverter and the first period (P21) of the second PWM inverter start at different start times (t11, t21).

2. The method according to claim 1, wherein: The start times (t11, t21) of the first cycles of the first and second PWM inverters are predetermined or determined based on an externally generated trigger signal.

3. The method according to claim 1 or 2, wherein: The start time (t21) of the first period (P21) of the second PWM inverter is delayed relative to the start time (t11) of the first period of the first PWM inverter by the time length (ΔTP) of the first period (P11) of the first PWM inverter.

4. The method according to claim 1, wherein: The different start times (t11, t21) of the first cycles of the first and second PWM inverters are determined from a random process.

5. A method according to any one of the preceding claims, wherein: At least the first periods (P11, P21) of the first and second PWM inverters have equal lengths.

6. A method according to any one of the preceding claims, wherein: In response to a predefined operating condition, such as a network disturbance event, the method includes switching from operating the at least first PWM inverter in first and second switching modes to operating the at least first PWM inverter in only one of the first and second switching modes.

7. The method according to claim 6, wherein: In response to another predefined operating condition, such as a network restoration event, the method includes switching from operating the at least first PWM inverter in only one of the first and second switching modes back to operating the at least first PWM inverter in the first and second switching modes.

8. A method according to any one of the preceding claims, wherein: The first and second switching patterns are determined based on respective first and second switching frequencies (f1, f2), wherein the first and second switching frequencies are different.

9. A plurality of PWM controllers (110, 111, 121) configured to control at least first and second PWM inverters (102, 112, 122), wherein: The PWM controller comprises a data processor arranged to perform a method according to any one of the preceding claims.

10. At least two PWM inverter systems (140), each comprising: - A PWM controller (110) according to claim 9, - PWM inverter (102), - a network filter (103) for smoothing the voltage output from the PWM inverter, wherein the network filter is configured to operate at an operating frequency located between the maximum amplitude harmonics of the different harmonics of the different switching patterns.

11. A wind turbine (160), comprising: The PWM inverter system (140) according to claim 10.

12. An electric power generation field (200), comprising: The first and second PWM inverter systems (140) according to claim 10 or one or more wind turbines according to claim 11.

13. A computer program comprising instructions, which, when the program is executed by a computer, cause the computer to execute the method according to any one of claims 1 to 8.

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