Using an Active Filter to Suppress Oscillation

Abstract

Using an active filter to suppress oscillations. A device (50) for controlling a converter (9, 15) of a power generation system, in particular a wind turbine (1), is described. The converter is connected to a connection point (21) to a public power grid (23). The device includes: a measurement section (31) adapted to provide measurement values indicating the values of the current (33) and voltage (35) at the connection point (21); a main converter controller (27) adapted to receive the measurement values (33, 35) and generate a main converter control signal (37) based on the measurement values; an active filter system (29) adapted to receive the measurement values (33, 35) and generate an active filter control signal (39) based on the measurement values; an adder element (30) adapted to add the main converter control signal (37) and the active filter control signal (39) and supply the sum signal (41) as a control signal to the converter (9, 15).

Description

Technical Field

[0001] The present invention relates to a device for controlling a converter of a power generation system, in particular a wind turbine, to a converter system, to a wind turbine, and further to a method for controlling a converter of a power generation system, in particular a wind turbine. Background Art

[0002] A wind turbine may include a wind turbine tower, a nacelle mounted on top of the tower, the nacelle housing a generator driven by a plurality of rotor blades connected to a rotating pivot. The generator may be coupled to a converter, for example having a generator side portion (such as an AC-DC converter portion) and having a grid side portion (in particular a DC-AC converter portion). The converter is configured to convert a variable frequency power flow delivered from the generator into a substantially fixed frequency power flow having desired electrical characteristics (such as in terms of frequency, voltage and / or power and / or active power and / or reactive power). The converter may include a plurality of controllable switches, such as IGBTs that may be switched at a high frequency (such as for example approximately 2.5 kHz). The controllable switches may be controlled at their respective gates by a pulse width modulation signal, which may be derived based on a control signal, such as a voltage control (e.g., reference) signal received by the converter from a corresponding converter controller. The converter may be connected to a public power grid via, for example, a wind turbine converter, in particular via a point of common coupling to which a plurality of other wind turbines are connected.

[0003] Due to the operation of the power converter, in particular a wind turbine power converter, a low-frequency harmonic voltage source may be generated, thereby causing undesirable oscillations at the output terminals of the power converter. There may also be low-frequency harmonic sources in the external public power grid. Suppressing unwanted resonances within a wind farm and / or between a wind farm (WPP) and the external grid may cause amplification of these harmonics (voltage and / or current) at one or more points of common coupling (PCC) of the wind farm. If the harmonic emission limits are exceeded, this may lead to grid code compliance issues, resulting in possible downtime of the wind farm. The worst-case scenario is when the resonance causes an uncontrolled growth of harmonic oscillations, leading to the wind turbine or the entire wind farm eventually disconnecting from the public power grid. These two scenarios may be encountered when connecting a conventional wind turbine to the grid.

[0004] The publication "Voltage Feedback based Harmonic Compensation for an Offshore Wind Power Plant" by Chaudhary, Sanjay K.; Lascu, Cristian Vaslie; Teodorescu, Remus; Kocewiak, Łukasz, published in the proceedings of the 2016 IEEE International Conference on Power Electronics, Drives and Energy Systems, discloses STATCOMs (additional hardware components) for power quality improvement in an offshore wind power plant. In it, the proposed control scheme is implemented in the wind turbine itself and thus does not require additional hardware investment.

[0005] The publication "Synthesis of Variable Harmonic Impedance in Inverter-Interfaced Distributed Generation Unit for Harmonic Damping Throughout a Distribution Network" by X. Wang, F. Blaabjerg and Z. Chen (IEEE Transactions on Industry Applications, vol. 48, no. 4, pp. 1407-1417, July 2020) utilizes an impedance-based active filter but for distributed generation embedded in a voltage control scheme. However, embedding an active filter in the main control loop is less desirable because the designs of the active filter and the main control loop must be considered together in combination. Additionally, when applying impedance shaping, this scheme also relies on band-pass filtering of current measurements to extract the correct phase sequence. Adding additional band-pass filtering to the control loop may be undesirable because more phase shifts occur outside the center frequency of the active filter. This may degrade the passivity (positive resistance at all frequencies) of the converter, resulting in worse performance and applicability of the active filter.

[0006] Accordingly, there may be a need for an apparatus for controlling a converter, for a converter system, for a wind turbine, and for a method of controlling a converter of a power generation system, in which at least the above-mentioned disadvantages or problems are reduced. In particular, therefore, an object of the present invention is to provide an apparatus for controlling a converter and a corresponding method of controlling a converter, in which undesired oscillations can be efficiently suppressed in a simple manner, especially without redesigning the entire control scheme. SUMMARY OF THE INVENTION

[0007] The device, converter system, wind turbine and method according to the invention can meet this need. Advantageous embodiments of the invention are also described.

[0008] According to an embodiment of the invention, there is provided a device for controlling a converter of a power generation system, in particular a wind turbine, the converter being connected to a connection point to a public power grid, the device comprising: a measurement section adapted to provide measurement values indicative of current and voltage values at the connection point; a main converter controller adapted to receive the measurement values and generate a main converter control signal based on the measurement values; an active filter system adapted to receive the measurement values and generate an active filter control signal based on the measurement values; an adder element adapted to add the main converter control signal and the active filter control signal and supply the sum signal as a control signal to the converter.

[0009] The device may be implemented in software and / or hardware. According to an embodiment, the device is implemented only (exclusively) in software. For example, the control software can simply be updated to include the (one or more) active filters. According to another embodiment, it is conceivable to use an additional piece of hardware to implement the active filter.

[0010] The different components of the device, such as the main converter controller and the active filter system, may be integrated in a combined control structure or may be present in separate modules. For example, the main converter controller may be or may include a conventional converter controller. In this case, the active filter system can be retrofitted as an additional component without changing the main converter controller.

[0011] The power generation system may be adapted to generate electrical power. The power generation system may particularly include at least one wind turbine or exactly one wind turbine. At a connection point such as a point of common coupling, for example, a plurality of wind turbines may be connected. The connection point may be connected to the public power grid particularly via a wind farm converter.

[0012] The measurement section may measure the values of current and / or voltage at the output terminals of the converter, or at the output terminals of the entire wind turbine, or at the grid side of the line inductor connected to the output terminals of the converter or near or at the point of common coupling. In particular, the measurement section may measure the values of current and / or voltage at a point between the wind turbine converter (connected to the output terminals of the converter) and the connection point or point of common coupling. When the values of current and voltage are determined not near or at the point of common coupling but at another point between the point of common coupling and the output terminals of the converter, the values of voltage and current at the point of common coupling may be inferred from the measurement values determined at another point of the transmission line using, for example, an analog or mathematical / physical model.

[0013] The main converter controller can be a conventional converter controller, particularly suitable for performing power control (e.g., active power control and / or reactive power control) and / or voltage control at a desired grid frequency of 50 Hz or 60 Hz. The main converter controller may not care about power control at frequencies different from the grid frequency. For example, when measurement values are provided as feedback signals to the main converter controller, all frequencies different from the desired grid frequency can be filtered out.

[0014] However, an active filter system is provided to suppress unwanted oscillations at one or more specific frequencies different from the grid frequency.

[0015] The active filter system can be used to eliminate the converter-generated harmonic voltage source generated by the wind turbine converter, and it can be used to suppress oscillations occurring in the WPP and / or the external grid due to resonance.

[0016] The active filter system can work in parallel with the main converter controller, whether it is voltage-controlled or current-controlled. Since the main control loop and the active filter can be decoupled in design and implementation, this may be a much easier method for implementation. Additionally, as a benefit, the proposed scheme may not require a band-pass filter, and thus can provide a significant simplification for (e.g., digital) implementation. The main benefit of not requiring an additional band-pass filter may be less phase shift outside the center frequency of the composite resonator. This may be important for maintaining the passivity of the converter (positive resistance at all frequencies).

[0017] The device for controlling a converter according to an embodiment of the present invention utilizes an existing voltage and current measurement system to develop an active filter control structure that amplifies the primary control scheme in a grid-connected converter.

[0018] The active filter system can achieve two goals simultaneously:

[0019] 1. The active filter system can eliminate the low-frequency harmonic voltage source generated by the wind turbine converter.

[0020] 2. The active filter system can allow impedance shaping of the converter impedance at specific frequencies where it is found that poor suppression resonance is about to occur.

[0021] Point 1 can allow the wind turbine converter to appear as an ideal voltage source at low frequencies. Therefore, the only harmonic sources may be those outside the wind power plant. Switching sideband harmonics may still exist, but they appear at higher frequencies and cannot be eliminated by active methods applied within the converter.

[0022] Point 2 may allow the wind turbine converter to act as a programmable impedance. This can then be used to suppress resonances occurring between the wind power plant and the external power grid.

[0023] Accordingly, a simple control device can be provided to effectively suppress undesired oscillations at the connection point.

[0024] According to an embodiment of the invention, the active filter system is adapted to suppress or even substantially eliminate at least one low-frequency harmonic oscillation generated by the converter and / or allow impedance shaping of the converter impedance at at least one specific frequency. In particular, the active filter system can eliminate at least one low-frequency harmonic oscillation generated by the converter.

[0025] Suppression (increasing the resistive part of the converter impedance) is intended to reduce oscillations elsewhere in the wind power plant.

[0026] The low-frequency harmonic oscillations can for example be oscillations close to the desired grid frequency (such as 50 Hz), and can for example be in a range between for example 0 Hz and less than 50 Hz and / or greater than 50 Hz and less than 200 Hz. Other values are possible. The impedance shaping can be achieved by appropriately controlling the converter using for example a specific voltage reference, which is designed such that oscillations at a specific frequency are suppressed and / or the impedance is very high for these specific frequencies. Accordingly, effective suppression of undesired oscillations can be achieved.

[0027] It may be possible to tune the active filter system also at sub-synchronous (<50 Hz) frequencies, but this may not be the main application.

[0028] The main application can be in resonance suppression at harmonic orders of the fundamental frequency component, i.e., suppression at 250 Hz and / or 350 Hz and / or 550 Hz etc.

[0029] According to an embodiment of the invention, the active filter system controls at least one electrical quantity at a frequency different from and / or independent of any frequency at which the main converter controller controls an electrical quantity, where the main converter controller is specifically configured to control the power output and / or voltage output and / or current output of the converter at a frequency different from any center frequency of any active filter control part, in particular at the fundamental electrical frequency.

[0030] The active filter system can control the converter impedance and the harmonic voltage source at a frequency different from any frequency at which the main converter control controls the same electrical quantity.

[0031] Thus, the active filter system and the main converter controller can act independently and be decoupled from each other, so as not to interfere in a disturbing manner. As long as this property is satisfied, the main converter controller and the active filter system can be designed independently of each other without considering the interaction of the corresponding other control parts. In other embodiments, there may be an influence or interaction of the active filter on the main converter control, which can be evaluated and taken into account. The active filter can use the voltage and current feedback that is typically available for the main control of a wind turbine converter as feedback signals. The voltage and current measurements at the grid side of the inductor connected to the converter output terminals can be used as feedback.

[0032] According to an embodiment of the invention, the active filter system includes a plurality of active filter sections for a plurality of considered center frequencies, each active filter section outputting a - particularly transformed - frequency-specific active filter control signal, wherein the frequency-specific active filter control signals are added together to obtain the - particularly transformed - active filter control signal.

[0033] For each frequency for which suppression and / or impedance shaping is desired (or the frequencies around the corresponding center frequency), a corresponding active filter section can be present within the active filter system. All of these can act independently of each other, in parallel and decoupled from each other. Their respective output control signals, i.e., the frequency-specific active filter control signals, can be combined, particularly added together, to obtain the active filter control signal, which is then summed with the main converter control signal, and the sum can be supplied to the converter. Thus, converter impedance shaping can be enabled at a plurality of different specific frequencies, thereby allowing effective suppression of oscillations in the frequency ranges at all of these specific frequencies or around the corresponding center frequencies.

[0034] According to an embodiment of the invention, the plurality of active filter sections are configured to suppress or even eliminate at least one harmonic of the nominal grid frequency, particularly at least one of a plurality of odd harmonics.

[0035] In particular, supersynchronous frequencies (> 50 Hz), such as harmonics and interharmonics of the grid fundamental frequency, are intended to be filtered using the active filter system.

[0036] In particular, the voltage harmonics generated by the converter can actually be eliminated at the center frequencies of the active filter.

[0037] Suppression can refer to the impedance shaping aspect of the active filter, where the resistive part of the converter impedance increases.

[0038] In particular, by providing a respective active filter section with an appropriate center frequency for each of these harmonics, oscillations at all harmonics of the fundamental frequency and / or the mains frequency (less than half of the converter sampling frequency (Nyquist limit)) can be suppressed. Furthermore, by providing a respective active filter section with an appropriate center frequency for each of these harmonics, oscillations at frequencies of harmonics different from the fundamental frequency and / or the mains frequency can be suppressed.

[0039] According to an embodiment of the invention, the active filter system comprises: a transformation module for transforming the values of current and voltage into respective α and β components of an αβ frame, or into respective d and q components of a dq coordinate frame rotating at the base electrical frequency; an inverse transformation module adapted to inverse-transform the transformed active filter control signal to derive the active filter control signal.

[0040] (For example, a single) transformation module can provide the respective transformed current and voltage values for all active filter sections, in particular each having two components. The transformation module can allow for a simplification of the calculation, since the three electrical phases (e.g., A, B, C) each requiring three voltage and current components are transformed into respective two components, thus simplifying the calculation. The inverse transformation module can be applied after summing the active filter control signals specific to all frequencies, so that only a single inverse transformation module is required. Thus, the arrangement can be simplified.

[0041] According to an embodiment of the invention, at least one, in particular each, active filter section is characterized by: a center frequency; a desired resistance; and a desired reactance, and outputs a, in particular transformed, center frequency-specific voltage signal configured to achieve the impedance of the converter at the center frequency defined by the desired resistance and the desired reactance.

[0042] Each active filter section can be characterized by a specific transfer function or frequency response, which in turn can be characterized by the center frequency, the desired resistance, and the desired reactance. Each or at least one active filter section can at least comprise a complex multiplication element, an optional complex phase shift element, and a complex resonator. An additional complex phase shift element can compensate for any sampling and calculation delays that may be present due to the digital control system. Different phase shifts can be applied to the voltage and current, since generally they may use different sampling techniques. Thus, the device can be constructed from conventionally known elements or modules.

[0043] According to an embodiment of the present invention, at least one, and in particular each, active filter section comprises: a multiplication element adapted to multiply a transformed value of a current by a complex number representing a desired resistance and a desired reactance (e.g., to be achieved at a connection point); an addition (or difference) element adapted to derive a voltage error (or sum or difference) between the transformed complex value of the current and the transformed value of the voltage; a composite resonator tuned at a specific center frequency, adapted to receive the voltage error and output a voltage signal specific to the center frequency, in particular a transformed one.

[0044] The output of the addition element can be supplied to the composite resonator via a sign conversion element. Thus, the composite resonator can receive the negative of the output of the addition element.

[0045] The composite resonator can act as a controller that ideally adjusts the output such that the input is zero. If the input of the composite resonator is zero, the converter effectively has an impedance equal to the desired resistance and desired reactance defined in the complex number of the multiplication element at the considered center frequency.

[0046] According to an embodiment of the present invention, at least one composite resonator has a transfer function or frequency response characterized by a response amplitude and a response phase, wherein the response amplitude has a peak at the corresponding center frequency, and for frequencies less than the corresponding center frequency, the response phase is equal to 90°, and / or for frequencies greater than the corresponding center frequency, the response phase is equal to -90°, and / or the phase is exactly zero at the center frequency, wherein the transfer function is particularly characterized by:

[0047] k / (s - jω1),

[0048] where k is a constant,

[0049] s is the Laplace variable,

[0050] j is the imaginary unit, and

[0051] ω1 is the corresponding center frequency.

[0052] This is the continuous-time transfer function of the composite resonator. In this case, "s" is the complex Laplace variable. Variants of the composite resonator, in particular discrete-time composite resonators (e.g., for digital implementation), can also be used. The discrete-time composite resonator can be derived from the continuous-time composite resonator, so the continuous-time composite resonator is shown generally.

[0053] The transfer function can be defined in the Laplace domain. The frequency response of the transfer function can be calculated in the frequency domain.

[0054] Therefore, the composite resonator can change the amplitude of the signal and also change the phase of the signal. Accordingly, the output of the composite resonator can correspond to an input with a changed amplitude and a changed phase.

[0055] The composite resonator can be designed in the frequency domain. This implementation is based on using the time domain of the sampled input signal.

[0056] According to an embodiment of the present invention, a control signal is provided to the grid side part of the converter, in particular the DC-AC part, where the control signal is used as the voltage reference of the converter, and a pulse width modulation signal for the controllable switch is derived based on this voltage reference.

[0057] The converter can particularly include a generator side part, a DC link, and a grid side part. Only the grid side part can be controlled by this device. The voltage reference can define the desired voltage at the output terminals of the converter. The voltage reference can include a plurality of frequency components, in particular one for each active filter part included in the active filter system, and at least one frequency part generated by the main converter controller. The pulse width modulation signal can define in which small part of the pulse width modulation period the corresponding controllable switch of the converter is assumed to be in the on state or the off state. Thus, a conventional converter generator design can be supported.

[0058] According to an embodiment of the present invention, the device further includes at least one passive filter, which is implemented as: at least one inductor connected in series between the output terminals of the converter and the connection point; and / or at least one capacitive shunt connected to the connection point.

[0059] The passive filter can be different from the active filter system in that they do not include any control logic different from the active filter system. The passive filter can supplement the overall filter capacity of the device.

[0060] According to an embodiment of the present invention, a converter system is provided, including: a converter having at least one grid side part; and a device according to one of the foregoing embodiments, which is connected to control the grid side part of the converter.

[0061] According to an embodiment of the present invention, a wind turbine is provided, including: a generator to which a rotor having a plurality of rotor blades is connected; and a converter system according to the foregoing embodiment connected to the output terminals of the generator.

[0062] It should be understood that the features disclosed, described, explained, or applied to the device for controlling the converter of the power generation system, either individually or in any combination, can also be applied or provided, either individually or in any combination, to the method for controlling the converter of the wind turbine according to the embodiments of the present invention, and vice versa.

[0063] According to an embodiment of the present invention, there is provided a method for controlling a converter of a power generation system, particularly a wind turbine, the converter being connected to a connection point to a public power grid, the apparatus comprising: providing measured values indicating current and voltage values at the connection point; receiving the measured values and generating a main converter control signal based on the measured values; receiving the measured values and generating an active filter control signal based on the measured values; adding the main converter control signal and the active filter control signal, and supplying the sum signal as a control signal to the converter.

[0064] The aspects defined above and further aspects of the present invention will be apparent from the examples of embodiments to be described hereinafter and will be explained with reference to the examples of embodiments. The present invention will be described in more detail hereinafter with reference to the examples of embodiments, but the present invention is not limited to the examples of embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 Schematically illustrates a wind turbine according to an embodiment of the present invention;

[0066] Figure 2 Schematically illustrates an active filter system according to an embodiment of the present invention, which may be included, for example, in the Figure 1 wind turbine illustrated in;

[0067] Figure 3 and Figure 4 illustrates the characteristics of a composite resonator of an active filter section as used according to an embodiment of the present invention; and Figure 5 and Figure 6 illustrates an example of a converter impedance implemented according to an embodiment of the present invention. DETAILED DESCRIPTION

[0068] Figure 1 The wind turbine 1 schematically illustrated in includes a rotating pivot 3 to which a plurality of rotor blades 4 are connected. The rotating pivot 3 is coupled to a generator 5 (such as a permanent magnet synchronous machine), which outputs a power flow, for example, provided in three different phases A, B, C, at the output terminals 7.

[0069] The three-phase power flow is supplied to a converter 9, which includes a generator-side section 11, particularly an AC-DC converter section, a DC link 13, and a grid-side converter section 15, particularly a DC-AC converter section. The generator-side section 11 converts the variable-frequency power flow generated by the generator 5 into a (substantially) DC power flow at the DC link 13. The grid-side converter section 15 converts the DC power flow into a power flow at a fixed frequency (e.g., grid frequency 50 Hz or 60 Hz) at the output terminals 17 of the converter 9.

[0070] An inductor 19 (an example of a passive filter) is present between the output terminals 17 of the converter and the common coupling point 21 to which a plurality of other (not shown) wind turbines may be connected. The public power grid 23 is connected to the common coupling point via an optional wind farm converter. Additionally, in other embodiments, the wind turbine converter may be arranged between the output terminals of the converter 9 and the common coupling point 21.

[0071] The wind turbine 1 is an example of a power generation system which, in other embodiments, may be or include a photovoltaic power generation system, a hydroelectric power generation system, a steam turbine power generation system, and so on. Embodiments of the present invention may be applied to any one of these power generation systems, or even other power generation systems. The wind turbine 1 includes at least a generator 5 and a converter system 25 which at least includes a grid side part 15 of the converter 9, a main converter controller 27, and an active filter system 29, as well as a measurement section 31 and an adder element 30.

[0072] The device 50 according to an embodiment of the present invention is formed by the measurement section 31, the main converter controller 27, and the active filter system 29, and by the adder element 30. Herein, the measurement section 31 is adapted to provide measurement values indicative of the (one or more) value(s) 33 of the current and the (one or more) value(s) 35 of the voltage at the common coupling point 21, in particular measurement values continuously measured / provided over time. The main converter controller 27 is adapted to receive the measurement values 33, 35 and generate a main converter control signal 37 based on the measurement values 33, 35.

[0073] The active filter system 29 is also adapted to receive the measurement values 33, 35 and generate an active filter control signal 39 based on the measurement values 33, 35. The adder element 30 is adapted to and connected to add the main converter control signal 37 and the active filter control signal 39 and supply the sum signal 41 as a control signal to the converter 9, in particular to the grid side converter part 15.

[0074] The main converter control signal 37 may be adapted to mainly or specifically control electrical quantities such as power or active power and / or reactive power and / or voltage at a desired grid frequency (e.g., 50 Hz or 60 Hz). However, the active filter system 29 is adapted to eliminate, at least suppress, at least one low-frequency harmonic oscillation generated by the converter 9, and / or allow impedance shaping of the converter impedance of the converter 9 at at least one specific frequency different from the grid frequency.

[0075] Figure 2 A block diagram schematically illustrates an example of an active filter system 29 according to an embodiment of the present invention which may, for example, be used within the device 50 and thus in the wind turbine 1. Figure 2The active filter system 29 illustrated in the schematic block diagram includes a plurality of active filter sections, of which only two active filter sections 43_1 and 43_2 are illustrated. In other embodiments, the active filter system 29 may include only one or more than two active filter sections, such as three, four, five, six or even more than six filter sections, depending on the number of frequencies to be suppressed.

[0076] The active filter system 29 includes input terminals 49 for receiving the values 33 and 35 of current and voltage from the measurement system 31. The active filter system 29 further includes a corresponding transformation module 47 for each current value 33 and voltage value 35. Thus, the transformation module 47 is adapted to transform the current and voltage values in the abc frame into the corresponding α components and corresponding β components of the αβ frame that is static in the abc frame of the electrical phases A, B, C.

[0077] In another embodiment, the transformation module 47 is adapted to transform the current and voltage values in the abc frame into the corresponding d components and corresponding q components of a dq frame that rotates, for example, at the base electrical frequency.

[0078] Each active filter section receives, respectively, at corresponding input terminals 45_1, 45_2, the transformed values 33' of the current 33, where the transformed values 33' of the current value 33 are derived by the transformation module 47. Each of the active filter sections 43_1, 43_2 includes a corresponding multiplication element 51_2, 51_2, which multiplies the transformed value 33' of the current by a complex number that represents the desired resistance and desired reactance to be achieved at the common connection point 21. In the illustrated embodiment, the complex multiplication elements multiply the transformed current values 33' by R1 + jX1 and R2 + jX2, respectively, where R1, R2 are the corresponding resistance values and X1, X2 are the corresponding desired reactance values.

[0079] Each of the active filter control sections 43_1, 43_2 further includes corresponding phase shift elements 54_1, 54_2 for multiplying the current by a complex number (e.g., characterized as exp(jθi1), exp(jθi2), where θi1, θi2 are phase shift angles), which are adapted to apply a phase shift to the current to compensate for any sampling and calculation delays that may exist due to the digital control system. In addition, each of the active filter control sections 43_1, 43_2 further includes corresponding phase shift elements 56_1, 56_2 for the voltage (e.g., characterized as exp(jθv1), exp(jθv2), where θv1, θv2 are phase shift angles), which are adapted to apply a phase shift to the voltage. Different phase shifts can be utilized for the voltage and the current because, generally speaking, they may use different sampling techniques.

[0080] Each of the active filter control sections 43_1, 43_2 further includes addition elements 53_1, 53_2, and the addition elements 53_1, 53_2 are adapted to derive voltage errors 55_1, 55_2 of the phase-shifted complex multiplication transform values 52_1, 52_2 of the current and the transform value 35' of the voltage value 35. The voltage errors 55_1, 55_2 are further multiplied by -1 by sign conversion elements 57_1, 57_2 respectively.

[0081] The outputs of these conversion elements are supplied as inputs to the corresponding complex resonators 59_1, 59_2, the inputs are tuned at a specific center frequency, and corresponding transformed voltage signals 61_1, 61_2 specific to the center frequency are output, and they are all added together using an addition element 63 (from different active filter control sections). The result of the addition is the transformed active filter control signal 65.

[0082] The active filter system 29 further includes an inverse transform module 67, which is adapted to inverse transform the transformed active filter control signal 65 to derive the active filter control signal 39, particularly the voltage control signal V abc， AF.

[0083] Each of the active filter control sections 43_1, 43_2 is characterized by: a center frequency (such as ω1, ω2 for filter sections 43_1, 43_2 respectively), a desired resistance (such as R1, R2), and a desired reactance (such as X1, X2).

[0084] In particular, the active filter system 29 includes a transformation module 47 from the "abc" frame components of voltage and current to a two-component (which can be stationary or rotating) frame, further includes a complex gain "R + jX" and corresponding complex resonators. The active filter system allows programming the impedance to a desired value at a given frequency. Multiple active filter sections can be connected in parallel to target different frequencies.

[0085] Figure 3 and Figure 4 illustrates an embodiment of the frequency response of a complex resonator (such as the complex resonators 59_1, 59_2 of the active filter system 29 illustrated in Figure 2 ). Thus, Figure 3 illustrates the magnitude on the ordinate 69 with the abscissa 70 of the marked frequency, where the magnitude is depicted as a curve 71. As can be appreciated from Figure 3 the magnitude 71 has a peak at a specific frequency ω1, and this specific frequency ω1 represents the center frequency of the corresponding complex resonator.

[0086] Figure 4The phase is marked on the vertical axis 72 and the frequency is marked on the horizontal axis 70. Curve 73 represents the phase response that describes the steady-state phase shift of the transfer function to be applied to the input signal as introduced by the corresponding composite resonator. At the center frequency, the phase shift is precisely zero. It can be seen that for frequencies less than the center frequency ω1, the phase (shift) is 90°, while for frequencies greater than the center frequency ω1, the phase (shift) is -90°.

[0087] As a complex (coefficient) filter, the composite resonator has an asymmetric amplitude response with respect to positive and negative frequencies (with respect to the center frequency). This can allow infinite gain to be applied only at one frequency (specifically the center frequency ω1), thus facilitating the control of complex exponential input signals.

[0088] Figure 5 and Figure 6 The graph is illustrated, where the horizontal axis 70 marks the frequency, and Figure 5 the vertical axis 74 in Figure 6 marks the magnitude of the impedance Z, and Figure 5 the vertical axis 76 of Figure 6 marks the phase of the converter impedance Z. When controlled by the control signal 41 which also includes the active filter control signal 39, the converter impedance Z is the programmed impedance of the converter 9 (including the line inductor L, as a voltage / current used in the active filter, which is a feedback measured at the grid side of the inductor or related to the grid side of the inductor), particularly the converter section 15. Figure 5 The solid curve 75 in Figure 6 marks the converter impedance magnitude without the active filter control signal 39, while the dashed curve 77 illustrates the converter impedance magnitude when the active filter control signal 39 is included in the converter control signal 41. The same names are given in Figure 6 for the phase of the converter impedance Z.

[0089] As Figure 5 and Figure 6 can be seen, the magnitude and phase of the converter impedance with and without impedance shaping at the frequency ω1 can be significantly different. The original (without impedance shaping, i.e., not considering the active filter control signal 39) converter impedance is defined by the main converter controller. This original is programmed and / or reshaped by the complex gain R1 + jX1 (thus with impedance shaping, curve 77) to have a greater magnitude and a smaller phase shift at the center frequency ω1. Thus, the oscillation at this center frequency ω1 is effectively suppressed because the resistive part of the converter impedance has increased.

[0090] Embodiments of the present invention utilize an active filter system to allow the converter impedance to be programmed to a desired value at a desired frequency through a control structure that is collocated in parallel with the main converter controller. This can allow the main controller loop to be designed independently of the active filter system. Additionally, embodiments of the present invention can enable the voltage harmonics generated by the converter to be inherently compensated at the frequencies targeted by the active filter system. Further, as a benefit, embodiments of the present invention can eliminate the need for a bandpass filter, allowing for a simple overall structure.

[0091] A major benefit of eliminating the need for an additional bandpass filter can be that less phase shift occurs outside the center frequency of the complex resonator. This can be important for maintaining the passivity of the converter (positive resistance at all frequencies).

[0092] It should be noted that the term "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Also, elements described in connection with different embodiments can be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

1. Apparatus (50) for controlling a converter (9, 15) of a power generation system, the converter being connected to a connection point (21) to a public power grid (23), the apparatus comprising: A measurement section (31) adapted to provide measurement values indicative of values of current (33) and voltage (35) at the connection point (21); A main converter controller (27) adapted to receive the measurement values (33, 35) and generate a main converter control signal (37) based on the measurement values; An active filter system (29) adapted to receive the measurement values (33, 35) and generate an active filter control signal (39) based on the measurement values; A first adder element (30) adapted to add the main converter control signal (37) and the active filter control signal (39) and supply the sum signal (41) as a control signal to the converter (9, 15); Wherein the active filter system (29) is adapted to suppress at least one low-frequency harmonic oscillation generated by the converter and allow impedance shaping of the converter impedance (Z) of the converter (9, 15) at at least one frequency where it is found that poor suppression resonance is to occur, wherein the active filter system (29) includes a plurality of active filter sections (43_1, 43_2) for a plurality of considered center frequencies (ω1, ω2).

2. The apparatus according to claim 1, Wherein the active filter system (29) controls at least one electrical quantity at a frequency different from and / or independent of any frequency at which the main converter controller controls an electrical quantity, Wherein the main converter controller (27) is configured to control the power output and / or voltage output and / or current output of the converter at a frequency different from any center frequency (ω1, ω2) of any active filter section (43_1, 43_2).

3. The apparatus according to claim 1 or 2, wherein the measurement section (31) is adapted to measure values of current and voltage (33, 35) at or near the converter output terminals (17) or at or near the connection point (21).

4. The device according to claim 1 or 2, wherein Each active filter section outputs a transformed center-frequency specific voltage signal (61_1, 61_2), wherein the transformed center-frequency specific voltage signals are added together to obtain a transformed active filter control signal (65).

5. The apparatus according to claim 1 or 2, wherein the plurality of active filter sections (43_1, 43_2) are configured to suppress at least one harmonic of the fundamental frequency.

6. The apparatus according to claim 1 or 2, wherein the active filter system (29) includes: A transformation module (47) for transforming the values of current and voltage (33, 35) into corresponding α and β components in an αβ frame or into corresponding d and q components in a dq coordinate frame rotating at the base electrical frequency; An inverse transformation module adapted to inverse transform the transformed active filter control signal (65) to derive the active filter control signal (39).

7. The apparatus according to claim 1 or 2, wherein at least one active filter section (43_1, 43_2) is characterized by: Center frequencies (ω1, ω2); Desired resistances (R1, R2); and Desired reactances (X1, X2), and output a voltage signal specific to the center frequency, the voltage signal being configured to achieve the impedance of the converter at the center frequency defined by the desired resistance and the desired reactance.

8. The apparatus according to claim 1 or 2, wherein at least one active filter section (43_1, 43_2) comprises: Multiplication elements (51_1, 51_2) adapted to multiply a transformed value of the current (33’) by a complex number (R1 + jX1, R2 + jX2) representing the desired resistance and the desired reactance; Second addition elements (53_1, 53_2) adapted to derive a voltage error (55_1, 55_2) between the complex multiplied transformed value of the current (52_1, 52_2) and the transformed value of the voltage (35’); Complex resonators (59_1, 59_2) tuned at the center frequencies (ω1, ω2) adapted to receive the voltage error and output a transformed voltage signal specific to the center frequency (61_1, 61_2).

9. The apparatus according to claim 8, wherein at least one complex resonator (59_1, 59_2) has a transfer function or frequency response (71, 73) characterized by a response amplitude and a response phase, wherein the response amplitude has a peak at the respective center frequency (ω1, ω2), and for frequencies less than the respective center frequency (ω1, ω2), the response phase is equal to 90°, and for frequencies greater than the respective center frequency (ω1, ω2), the response phase is equal to -90°, and is exactly zero degrees at the center frequency (ω1, ω2), wherein the transfer function is characterized by: k / (s - jω1), where k is a constant, s is the Laplace variable, j is the imaginary unit, and ω1 is the respective center frequency.

10. The apparatus according to claim 1 or 2, wherein a control signal (41) is provided to the grid side part (15) of the converter (9), the control signal being used as a voltage reference for the converter, and a pulse width modulation signal for a controllable switch is derived based on the voltage reference.

11. The apparatus according to claim 1 or 2, further comprising at least one passive filter (19) implemented as: At least one inductor (L) connected in series between the output terminals of the converter and the connection point; and / or At least one capacitive shunt connected to the connection point.

12. A converter system (25), comprising: A converter (9) having at least a grid side part (15); and An apparatus (50) according to one of claims 1 - 11, which is connected to control the grid side part of the converter.

13. A wind turbine (1), comprising: A generator (5) to which a rotor (3) having a plurality of rotor blades is connected; and A converter system (25) according to claim 12, which is connected to the generator output terminals.

14. A method of controlling a converter (9, 15) of a power generation system, the converter being connected to a connection point (21) leading to a public power grid (23), the method comprising: Provide measured values (33, 35) indicating the current and voltage values at the connection point (21); Receive the measured values (33, 35) and generate a main converter control signal (37) based on the measured values; Receive the measured values (33, 35) and generate an active filter control signal (39) based on the measured values; Add the main converter control signal (37) and the active filter control signal (39); and Supply the sum signal (41) as a control signal to the converters (9, 15); wherein at least one low-frequency harmonic oscillation generated by the converters is suppressed and impedance shaping of the converter impedance (Z) of the converters (9, 15) is allowed at at least one frequency where it is found that poor suppression resonance is to occur.

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