Converter system of wind turbine generator system, and wind turbine generator system and offshore wind farm

Abstract

Provided in the present disclosure are a converter system of a wind turbine generator system, and a wind turbine generator system and an offshore wind farm. The converter system comprises: a voltage source converter and a current source converter; and a converter control apparatus, which is configured to control the operation of the voltage source converter and the current source converter by means of taking into account an internal circulating current of a wind turbine generator system in voltage outer-loop control of the voltage source converter and by means of taking into account the internal circulating current in active and reactive power control stages of the current source converter.

Description

Converter system of wind turbine generator set, wind turbine generator set and offshore wind farm Technical Field

[0001] The present disclosure relates to the field of wind power generation, and more particularly, to a converter system of a wind turbine generator set, a wind turbine generator set, and an offshore wind farm. Background Art

[0002] A wind turbine is a device that converts wind energy into electrical energy, and outputs the electrical energy to a wind farm through a converter system installed therein. At present, converter systems can generally be divided into voltage source converters (i.e., grid-forming converters) and current source converters (grid-following converters). Voltage source converters have similar inertia characteristics to synchronous generators, which enables them to provide stable frequency support for offshore power grids. They have attracted much attention from the industry in recent years, but they are still in the theoretical stage and have hardly been put into practical application. Current source converters have fast power response capabilities and have been widely used, but they need to be synchronized with the power grid and generally do not have frequency support capabilities.

[0003] Summary of the Invention

[0004] Therefore, the present disclosure provides a converter system for a wind turbine generator set, a wind turbine generator set and an offshore wind farm, wherein the converter system for the wind turbine generator set combines a voltage source converter and a current source converter, thereby having the capabilities of both frequency support and rapid power response.

[0005] In a general aspect, a converter system for a wind turbine is provided, the converter system comprising: a voltage source converter and a current source converter, wherein the voltage source converter and the current source converter are connected in parallel between the output side of the generator of the wind turbine and the grid connection point of the wind turbine, the voltage source converter comprises a first machine-side converter and a first grid-side converter connected in sequence, and the current source converter comprises a second machine-side converter and a second grid-side converter connected in sequence; a converter control device is configured to control the operation of the voltage source converter and the current source converter by considering the internal circulating current of the wind turbine in the voltage outer loop control of the voltage source converter and by considering the internal circulating current of the wind turbine in the active power control link and the reactive power control link of the current source converter, wherein the internal circulating current of the wind turbine is determined based on the output current of the first grid-side converter and the output current of the second grid-side converter.

[0006] Optionally, the converter control device is configured to: determine the d-axis component of the converter output voltage reference value based on the active power reference value and actual value of the first grid-side converter; determine the q-axis component of the converter output voltage reference value based on the output current value and virtual angular frequency of the wind turbine generator set; determine the d-axis component and q-axis component of the first current reference value by performing voltage outer loop control on the d-axis component and q-axis component of the converter output voltage reference value, and based on the internal circulating current value of the wind turbine generator set; obtain the d-axis component and q-axis component of the modulation voltage reference value of the first grid-side converter by performing current inner loop control on the d-axis component and q-axis component of the first current reference value; control the operation of the first grid-side converter according to the virtual internal potential phase and the d-axis component and q-axis component of the modulation voltage reference value of the first grid-side converter; wherein, the output current of the wind turbine generator set is determined based on the output current of the first grid-side converter and the output current of the second grid-side converter.

[0007] Optionally, based on the active power reference value and actual value of the second grid-side converter and the output current value and internal circulating current value of the wind turbine generator set, the d-axis component and q-axis component of the second current reference value are determined; by performing current inner-loop control on the d-axis component and q-axis component of the second current reference value, the d-axis component and q-axis component of the modulation voltage reference value of the second grid-side converter are obtained; according to the virtual internal potential phase and the d-axis component and q-axis component of the modulation voltage reference value of the second grid-side converter, the operation of the second grid-side converter is controlled, wherein the output current of the wind turbine generator set is determined based on the output current of the first grid-side converter and the output current of the second grid-side converter.

[0008] Optionally, the converter control device is further configured to determine a virtual angular frequency and a virtual internal potential phase based on a voltage value of a filter capacitor of the first grid-side converter.

[0009] Optionally, the converter control device is further configured to: obtain a second proportional integral operation result by performing a proportional integral operation on the difference between the active power reference value and the actual value of the first grid-side converter; and determine the d-axis component of the converter output voltage reference value based on the second proportional integral operation result and a preset voltage feedforward compensation value of the wind turbine generator set.

[0010] Optionally, the converter control device is further configured to: obtain a first proportional operation result by performing a proportional operation on the output current value of the wind turbine generator set; and obtain the q-axis component of the converter output voltage reference value by performing a proportional operation on the difference between the first proportional operation result and the virtual angular frequency.

[0011] Optionally, the converter control device includes a first grid-side controller and a second grid-side controller communicating with each other, wherein the first grid-side controller is configured to control the operation of the first grid-side converter, and the second grid-side controller is configured to control the operation of the second grid-side converter.

[0012] Optionally, the wind turbine generator set is an offshore wind turbine generator set, and the generator is a six-phase permanent magnet synchronous wind turbine generator.

[0013] Optionally, the current source converter has the same capacity as the voltage source converter.

[0014] Optionally, the output current value of the wind turbine generator set is the sum of the output current value of the first grid-side converter and the output current value of the second grid-side converter, and the internal circulating current value of the wind turbine generator set is the difference between the output current value of the first grid-side converter and the output current value of the second grid-side converter.

[0015] In another general aspect, a wind turbine generator set is provided, comprising the wind turbine generator set converter system as described above.

[0016] In another general aspect, an offshore wind farm is provided, comprising a plurality of wind turbines as described above.

[0017] According to the converter system of the wind turbine generator set, the wind turbine generator set, and the offshore wind farm of the embodiment of the present disclosure, by combining a voltage source converter and a current source converter in the wind turbine generator set, using the voltage source converter to provide frequency support and using the current source converter to achieve rapid power response, the offshore low-frequency AC transmission system has the ability to simultaneously provide frequency support and rapid power response. In addition, in the control strategy of the converter system, a circulating current suppression strategy is applied to reduce the power imbalance problem caused by the circulating current between the voltage source converter and the current source converter, thereby eliminating the adverse effect of power imbalance on the power of the offshore low-frequency AC transmission system. In addition, in the wind turbine generator set, a six-phase permanent magnet synchronous wind turbine is used, which has the advantages of high rated power, small size, low harmonic content, and good economy, and supports the simultaneous connection of a voltage source converter and a current source converter to achieve frequency support and rapid power response. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and other objects and features of the present disclosure will become more apparent from the following description in conjunction with the accompanying drawings.

[0019] FIG1 is a block diagram illustrating a converter system of a wind turbine generator system according to an exemplary embodiment of the present disclosure.

[0020] FIG2A is a schematic diagram illustrating a control strategy of a voltage source converter according to an exemplary embodiment of the present disclosure, and FIG2B is a schematic diagram illustrating a control strategy of a current source converter according to an exemplary embodiment of the present disclosure.

[0021] FIG. 3 is a diagram showing an equivalent model at the outlet of a voltage source converter and a current source converter.

[0022] FIG4 is a topology diagram illustrating a DRU-based offshore low-frequency AC transmission system according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0023] The following detailed description is provided to help the reader gain a comprehensive understanding of the methods, devices and / or systems described herein. However, various changes, modifications and equivalents of the methods, devices and / or systems described herein will be clear after understanding the disclosure of the present application. For example, the order of operations described herein is merely an example and is not limited to those orders set forth herein, but can be changed as will be clear after understanding the disclosure of the present application, except for operations that must occur in a specific order. In addition, for greater clarity and conciseness, descriptions of features known in the art may be omitted.

[0024] The features described herein can be implemented in different forms and should not be construed as limited to the examples described herein. Rather, the examples described herein are provided to illustrate only some of the many possible ways to implement the methods, devices, and / or systems described herein, which will become clear after understanding the disclosure of this application.

[0025] As used herein, the term "and / or" includes any one of the associated listed items and any combination of any two or more.

[0026] Although terms such as "first," "second," and "third" may be used herein to describe various members, components, regions, layers, or portions, these members, components, regions, layers, or portions should not be limited by these terms. Instead, these terms are used solely to distinguish one member, component, region, layer, or portion from another member, component, region, layer, or portion. Thus, what is referred to as a first member, first component, first region, first layer, or first portion in the examples described herein may also be referred to as a second member, second component, second region, second layer, or second portion without departing from the teachings of the examples.

[0027] In the specification, when an element (such as a layer, region, or substrate) is described as being “on,” “connected to,” or “coupled to” another element, the element may be directly “on,” “connected to,” or “coupled to” the other element, or one or more other elements may be present therebetween. Conversely, when an element is described as being “directly on,” “directly connected to,” or “directly coupled to” another element, there may be no other elements present therebetween.

[0028] The terms used herein are intended only to describe various examples and are not intended to limit the disclosure. Unless the context clearly indicates otherwise, the singular is intended to include the plural. The terms "comprise," "include," and "have" indicate the presence of the recited features, quantities, operations, components, elements, and / or combinations thereof, but do not preclude the presence or addition of one or more other features, quantities, operations, components, elements, and / or combinations thereof.

[0029] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure pertains after understanding the present disclosure. Unless expressly defined as such herein, terms (such as those defined in general dictionaries) should be interpreted as having a meaning consistent with their meaning in the context of the relevant art and the present disclosure, and should not be interpreted in an idealized or overly formal manner.

[0030] Furthermore, in describing the examples, when it is deemed that a detailed description of well-known related structures or functions would cause ambiguous interpretation of the present disclosure, such detailed description will be omitted.

[0031] FIG1 is a block diagram illustrating a converter system for a wind turbine generator system according to an exemplary embodiment of the present disclosure. FIG2A is a schematic diagram illustrating a control strategy for a voltage source converter according to an exemplary embodiment of the present disclosure, and FIG2B is a schematic diagram illustrating a control strategy for a current source converter according to an exemplary embodiment of the present disclosure. The converter system for a wind turbine generator system and a control method thereof according to an exemplary embodiment of the present disclosure are described in detail below with reference to FIG1 to FIG2B .

[0032] 1 , a converter system 100 for a wind turbine generator includes a voltage source converter 110, a current source converter 120, and a converter control device 130. The voltage source converter 110 and the current source converter 120 are connected in parallel between the output side of the wind turbine generator and the wind turbine generator's grid connection point (i.e., first common connection point) PCC1. The voltage source converter 110 includes a first machine-side converter (MSC1) 111 and a first grid-side converter (GSC1) 112 connected in sequence. The current source converter 120 includes a second machine-side converter (MSC2) 121 and a second grid-side converter (GSC2) 122 connected in sequence. The converter control device 130 controls the operation of the voltage source converter 110 and the current source converter 120 by considering the internal circulating current of the wind turbine in the voltage outer loop control of the voltage source converter 110, and by considering the internal circulating current of the wind turbine in the active power control link and reactive power control link of the current source converter 120. Here, the internal circulating current of the wind turbine can be determined based on the output current of the first grid-side converter GSC1 112 and the output current of the second grid-side converter GSC2 122. According to embodiments of the present disclosure, the converter control device 130 can control the first generator-side converter MSC1 111 and the second generator-side converter MSC2 121 according to existing control strategies. Therefore, the converter control device 130 controlling the operation of the voltage source converter 110 and the current source converter 120 described herein refers to the converter control device 130 controlling the operation of the first grid-side converter GSC1 112 and the second grid-side converter GSC2 122. The first grid-side converter GSC1 112 is a voltage source GSC, and the second grid-side converter GSC2 122 is a current source GSC.

[0033] According to an embodiment of the present disclosure, the wind turbine generator set is an offshore wind turbine generator set, and the generator of the wind turbine generator set is a six-phase permanent magnet synchronous wind generator (PMSG). The six-phase PMSG has the advantages of high rated power, small size, low harmonic content, and good economy. At the same time, the three-phase output of the six-phase PMSG can be connected to the voltage source converter 110, and the other three phases can be connected to the current source converter 120, thereby achieving frequency support and rapid power response. On the other hand, in order to suppress the internal circulating current of the wind turbine generator set, the capacity of the voltage source converter 110 and the current source converter 120 can be the same, that is, the power ratio of the voltage source converter 110 and the current source converter 120 is 1:1.

[0034] According to an embodiment of the present disclosure, the converter control device 130 may be configured to calculate the active power reference value of the first grid-side converter GSC1112 based on the active power reference value of the first grid-side converter GSC1112. and the actual value P GFM , determine the d-axis component of the converter output voltage reference value Based on the output current value i of the wind turbine flow and virtual angular frequency ω wt , determine the q-axis component of the converter output voltage reference value By comparing the d-axis component of the converter output voltage reference value and q-axis component The voltage outer loop is controlled and based on the internal circulating current value i of the wind turbine generator set cir , determine the d-axis component of the first current reference value and q-axis component By performing current inner loop control on the d-axis component and the q-axis component of the first current reference value, the d-axis component of the modulation voltage reference value of the first grid-side converter GSC1 112 is obtained. and q-axis component According to the virtual internal potential phase theta and the d-axis component of the modulation voltage reference value of the first grid-side converter GSC1 112 and q-axis component Control the operation of the first grid-side converter GSC1 112. Here, the output current i flow The current is determined based on the output current of the first grid-side converter GSC1 112 and the output current of the second grid-side converter GSC2 122 .

[0035] The following briefly describes the operation mode of controlling the first grid-side converter GSC1 112. The d-axis component of the modulation voltage reference value can be modulated according to the virtual internal potential phase theta. and q-axis component The voltage is converted into a three-phase voltage in the abc coordinate system or a two-phase voltage in the αβ coordinate system, and then input into a PWM (pulse width modulation) modulation module for modulation. The modulated signal after PWM can be input into GSC1 112 to control the switching of the IGBT devices in GSC1 112, thereby adjusting the output of GSC1 112 (e.g., output power, output current, output voltage, etc.). Since the above method for controlling the operation of the first grid-side converter GSC1 112 is well known to those skilled in the art, a detailed description thereof is omitted here.

[0036] 2A , an LC filter circuit is provided at the output end of the first grid-side converter GSC1 112. The current outputted from the first grid-side converter GSC1 112 to PCC1 via the LC filter circuit (i.e., the output current of the first grid-side converter GSC1 112) can be transformed from the abc coordinate system to the dq coordinate system by coordinate transformation (e.g., Park transformation). s_GFM Transformed to i sd_GFM and i sq_GFMThe filter capacitor voltage can be transformed from the abc coordinate system to the dq coordinate system by coordinate transformation, that is, u f Transformed to u fd and u fq The filter inductor current can be transformed from the abc coordinate system to the dq coordinate system by coordinate transformation, that is, i w Transformed to i wd and i wq .

[0037] According to an embodiment of the present disclosure, the converter control device 130 may also be configured to calculate the voltage value u of the filter capacitor of the first grid-side converter GSC1 112 based on the voltage value u of the filter capacitor of the first grid-side converter GSC1 112. f , determine the virtual angular frequency ω wt and the virtual internal potential phase theta.

[0038] Specifically, the virtual angular frequency ω can be determined by a phase-locked loop (PLL) wt That is, the voltage value of the filter capacitor u f The q-axis component u fq Perform an integral operation and add the result of the integral operation to the rated angular frequency ω0 of the power grid to obtain the virtual angular frequency ω wt ω wt The specific calculation method of is shown in the following equation (1). wt =k lp u fq +k li ∫u fq dt+ω0 (1)

[0039] Among them, k lp represents the proportionality coefficient, k li Represents the integral coefficient.

[0040] Then, the virtual angular frequency ω can be wt Perform integration operation to obtain the virtual internal potential phase theta.

[0041] According to an embodiment of the present disclosure, the converter control device 130 can adjust the active power reference value of the first grid-side converter GSC1 112 by and the actual value P GFM The difference between the proportional integral operation and the voltage feedforward compensation value u of the wind turbine generator set is calculated based on the second proportional integral operation result and the preset voltage feedforward compensation value u of the wind turbine generator set. f0 , determine the d-axis component of the converter output voltage reference value As shown in FIG2A , the converter control device 130 determines the d-axis component of the converter output voltage reference value by active power control. The specific calculation method of is shown in the following equation (2).

[0042] Among them, k pp_GFM represents the proportionality coefficient, k pi_GFM Represents the integral coefficient.

[0043] According to an embodiment of the present disclosure, the converter control device 130 can control the output current value i of the wind turbine generator set by flow Perform a scaling operation (in FIG2A , the scaling factor is shown as −k q ), obtain the first proportional operation result Then, by calculating the result of the first ratio With the virtual angular frequency ω wt The difference is proportional (in FIG2A , the proportionality factor is shown as k fq ), obtain the q-axis component of the converter output voltage reference value As shown in FIG2A , the converter control device 130 determines the virtual angular frequency ω by reactive power control and frequency closed-loop control. wt and the virtual internal potential phase theta and the q-axis component of the converter output voltage reference value

[0044] It is worth noting that in the conventional control strategy for wind turbine grid connection, the voltage amplitude of the grid is determined by the d-axis reference voltage (i.e., the d-axis component of the converter output voltage reference value). Regulation, and the q-axis reference voltage (i.e., the q-axis component of the above-mentioned converter output voltage reference value) Typically set to zero. The significance of setting it to zero is to assume that the phase-locked loop rotates completely according to the reference angular frequency and reference phase, requiring the grid voltage to be strongly synchronized with the grid voltage. However, in the control strategy of the voltage source converter (or voltage source wind turbine), there is no angular frequency supported by the grid. The angular frequency of the offshore wind farm is completely generated by the wind farm itself, so Setting it to zero is no longer appropriate. Since the ω measured by the phase-locked loop wt and theta reflects the output u of the offshore low-frequency AC transmission system fq , which indicates that the q-axis reference voltage It can be used to control the AC frequency. Therefore, the converter control device 130 can determine the q-axis reference voltage by the above-mentioned process. At the same time, by controlling the output current value i of the wind turbine generator set flow The proportional coefficient used in proportional operation is used to control This keeps the frequency of the wind turbine generator set stable at 20Hz, thereby realizing the grid-building function of the wind turbine generator set.

[0045] According to an embodiment of the present disclosure, the converter control device 130 can output the d-axis component of the converter voltage reference value by and q-axis component The voltage outer loop is controlled to obtain the d-axis component and q-axis component of the voltage outer loop output current reference value. The voltage outer loop is a control method well known to those skilled in the art, and its detailed description is omitted here. On the other hand, the converter control device 130 can determine the internal circulating current value i of the wind turbine generator set. cir After obtaining the d-axis component and q-axis component of the voltage outer loop output current reference value and determining the internal circulating current value i of the wind turbine generator set cir After the d-axis component and q-axis component of the voltage outer loop output current reference value are calculated, the converter control device 130 can determine the d-axis component of the first current reference value based on the d-axis component of the voltage outer loop output current reference value and the d-axis component of the internal loop current value. And based on the q-axis component of the voltage outer loop output current reference value and the q-axis component of the internal loop current value, the q-axis component of the first current reference value is determined As shown in FIG2A , the converter control device 130 may determine the sum of the d-axis component of the voltage outer loop output current reference value and the d-axis component of the internal loop current value as The sum of the q-axis component of the voltage outer loop output current reference value and the q-axis component of the internal loop current value can be determined as In this way, the voltage outer loop control shown in FIG2A may be a voltage outer loop control with circulating current control added.

[0046] Alternatively, the converter control device 130 can convert the internal circulating current value i of the wind turbine generator set into cir Transformed into the first d-axis component i in the dq coordinate system cird and the first q-axis component i cirq By integrating the first d-axis component (in FIG. 2A , the integration coefficient is shown as −k cird ), the d-axis component of the internal circulation value is obtained; by integrating the first q-axis component (in FIG. 2A , the integral coefficient is shown as −k cirq ), and obtain the q-axis component of the internal circulation value.

[0047] Due to asynchronous control strategies for voltage source converter 110 and current source converter 120, or due to inconsistent hardware parameters for voltage source converter 110 and current source converter 120, internal circulating currents are unavoidable within the wind turbine. This internal circulating current in the wind turbine can lead to power imbalance within the wind turbine. Therefore, the internal circulating current is used as a feedforward control variable, and circulating current control is incorporated into the voltage outer loop control system shown in FIG2A to eliminate phase fluctuations caused by asynchrony between voltage source converter 110 and current source converter 120.

[0048] FIG3 is a diagram showing an equivalent model at the outlet of a voltage source converter and a current source converter. w_GFM represents the voltage source converter, u w_GFL represents a current source type converter, and the output current value and the internal circulating current value of the wind turbine generator set can be determined by the following equation (3).

[0049] That is to say, the output current value i of the wind turbine generator set flow is the output current value i of the first grid-side converter GSC1 112 s_GFM The output current value i of the second grid-side converter GSC2 122 is s_GFL The sum of the internal circulation value of the wind turbine i cir is the output current value i of the first grid-side converter GSC1 112 s_GFM The output current value i of the second grid-side converter GSC2 122 is s_GFL difference.

[0050] In this way, the voltage outer loop control with circulating current control added can be expressed by the following equation (4).

[0051] Among them, k vp is the proportionality coefficient, k vi is the integral coefficient, and C is the capacitance of the filter capacitor.

[0052] According to an embodiment of the present disclosure, the converter control device 130 may be based on the active power reference value of the second grid-side converter GSC2122. and the actual value P GFL And the output current value i of the wind turbine generator set flow and the internal circulation value i cir , determine the d-axis component of the second current reference value and q-axis component By calculating the d-axis component of the second current reference value and q-axis component Perform current inner loop control to obtain the d-axis component of the modulation voltage reference value of the second grid-side converter GSC2 122 and q-axis component According to the virtual internal potential phase theta and the d-axis component of the modulation voltage reference value of the second grid-side converter GSC2 122 and q-axis component Control the operation of the second grid-side converter GSC2 122. As mentioned above, the output current i flow The virtual internal potential phase theta is determined based on the output current of the first grid-side converter GSC1 112 and the output current of the second grid-side converter GSC2 122. Here, the virtual internal potential phase theta is as described above. That is, the converter control device 130 can be based on the filter capacitor voltage value u of the first grid-side converter GSC1 112. f , determining the virtual internal potential phase theta. The operation of the second grid-side converter GSC2 122 may be controlled in a manner similar to the operation of the first grid-side converter GSC1 112 described above, and a detailed description thereof is omitted here.

[0053] 2B , similar to FIG2A , an LC filter circuit is provided at the output end of the second grid-side converter GSC2 122. The current outputted from the second grid-side converter GSC2 122 to PCC1 via the LC filter circuit (i.e., the output current of the second grid-side converter GSC2 122) can be transformed from the abc coordinate system to the dq coordinate system by coordinate transformation (e.g., Park transformation). s_GFL Transformed to i sd_GFL and i sq_GFL The filter capacitor voltage can be transformed from the abc coordinate system to the dq coordinate system by coordinate transformation, that is, u f Transformed to u fd and u fq The filter inductor current can be transformed from the abc coordinate system to the dq coordinate system by coordinate transformation, that is, i w Transformed to i wd and i wq .

[0054] According to an embodiment of the present disclosure, the converter control device 130 can adjust the active power reference value of the second grid-side converter GSC2 122 by and the actual value P GFL The difference between the proportional integral operation and the voltage feedforward compensation value u of the wind turbine generator set is calculated based on the third proportional integral operation result and the preset voltage feedforward compensation value u of the wind turbine generator set. f0 , determine the d-axis current reference value The output current value i of the wind turbine generator set is converted into flowTransformed into the d-axis component and q-axis component in the dq coordinate system By adjusting the output current value i of the wind turbine generator set flow The q-axis component is proportional (in FIG. 2B , the proportionality factor is shown as −k q ), obtain the q-axis current reference value; determine the internal circulation value i of the wind turbine generator set cir The d-axis component and q-axis component of the current are based on the d-axis current reference value. The d-axis component of the second current reference value is determined by combining the d-axis component of the internal circulating current value And based on the q-axis current reference value The q-axis component of the second current reference value is determined by combining the q-axis component of the internal circulating current value As shown in FIG2B , the d-axis component of the second current reference value is determined by adding active power control to the circulating current control. And the q-axis component of the second current reference value is determined by adding the reactive power control of the circulating current control In addition, the converter control device 130 can set the d-axis current reference value The sum of the d-axis component of the internal circulating current value is determined as the d-axis component of the second current reference value. And the q-axis current reference value can be The sum of the q-axis component of the internal circulating current value is determined as the q-axis component of the second current reference value.

[0055] Here, the d-axis current reference value can be calculated by the following equation (5): The d-axis component of the second current reference value can be determined by the following equation (6):

[0056] Among them, k pp_GFL 、k cird and k cirq represents the proportionality coefficient, k pi_GFL Represents the integral coefficient.

[0057] As described above, the converter control device 130 can transform the internal circulating current value i of the wind turbine generator set into cir Transformed into the first d-axis component i in the dq coordinate system cird and the first q-axis component i cirq By integrating the first d-axis component (in FIG. 2B , the integration coefficient is shown as −k cird ), the d-axis component of the internal circulation value is obtained; by integrating the first q-axis component (in FIG. 2B , the integral coefficient is shown as −k cirq ), and obtain the q-axis component of the internal circulation value.

[0058] According to an embodiment of the present disclosure, the converter control device 130 may include a first grid-side controller (not shown) and a second grid-side controller (not shown) that communicate with each other. The first grid-side controller may control the operation of the first grid-side converter, and the second grid-side controller may control the operation of the second grid-side converter. In other words, the operations described above with reference to FIG. 2A may all be performed by the first grid-side controller, while the operations described above with reference to FIG. 2B may all be performed by the second grid-side controller.

[0059] According to the converter system of a wind turbine generator set according to an embodiment of the present disclosure, by combining a voltage source converter and a current source converter in the wind turbine generator set, the voltage source converter is used to provide frequency support, and the current source converter is used to achieve rapid power response. This enables the offshore low-frequency AC transmission system to simultaneously provide frequency support and rapid power response. Furthermore, by applying a circulating current suppression strategy to the converter system's control strategy, the power imbalance caused by circulating current between the voltage source converter and the current source converter is reduced, thereby eliminating the adverse effects of power imbalance on the power of the offshore low-frequency AC transmission system.

[0060] 4 , a wind turbine generator set and an offshore low-frequency AC transmission system based on a diode rectifier unit (DRU) according to an embodiment of the present disclosure will be described.

[0061] FIG4 is a topology diagram illustrating a DRU-based offshore low-frequency AC transmission system according to an embodiment of the present disclosure.

[0062] 4 , an offshore low-frequency AC transmission system 400 may include n wind turbines 100 ′ connected in parallel, a step-up transformer 410 , and a DRU 420 , where n is an integer greater than 2. The n wind turbines 100 ′ connected in parallel may constitute an offshore wind farm.

[0063] Each wind turbine generator system 100' may include the converter system described above, and may also include a generator (i.e., a six-phase PMSG) 140 and a wind turbine transformer 150. The converter system includes a voltage source converter 110' and a current source converter 120'. The output terminals of the voltage source converter 110' and the current source converter 120' may be provided with LC filtering circuits and connected to the primary side of a wind turbine transformer 150 (e.g., but not limited to, a 690V / 35kV transformer) via a first common connection point (i.e., PCC1). The secondary side of the wind turbine transformer 150 is connected to the primary side of a step-up transformer 410 (e.g., but not limited to, a 35kV / 220kV transformer) via a second common connection point (i.e., PCC2). The secondary side of the step-up transformer 410 is connected to a DRU 420 via a low-frequency AC submarine cable 430. The low-frequency AC submarine cable 430 can transmit the low-frequency AC power output from the wind farm to the DRU 420. The DC power output by the DRU 420 can be transmitted to an onshore converter station via a submarine cable and converted into AC power, which is then connected to the grid via an onshore substation. The structure and connection method of the DRU unit are well known to those skilled in the art, and a detailed description thereof is omitted here.

[0064] Optionally, the offshore low-frequency AC transmission system 400 may further include a filter and reactive power compensation system 440 and a converter transformer 450. The primary side of the converter transformer 450 (for example, but not limited to, a 220 kV / 194 kV / 194 kV converter transformer) is connected to the secondary side of the step-up transformer 410 via a low-frequency AC submarine cable 430. The secondary side of the converter transformer 450 is connected to the DRU 420. Furthermore, the primary side of the converter transformer 450 is also connected to the filter and reactive power compensation system 440.

[0065] Here, it should be explained that since the onshore MMC (modular multilevel converter) controls the DC voltage output by the DRU 420 to a rated value, it can be equivalent to a DC voltage source. Therefore, the transmitted active power is largely determined by the DC voltage generated by the DRU 420. The DC voltage U generated by the DRU 420 is dc_DRU As shown in the following equation (7).

[0066] Where, X is the reactance of DRU 420, I dc_DRU is the DC current output by DRU 420, U r is the offshore AC grid voltage (ie, the AC voltage of the low-frequency AC submarine cable 430). Therefore, the active power transmitted by the wind farm can be determined by the offshore AC grid voltage U r and the filter capacitor voltage value u of the voltage source converter fIn view of this, the active power control link of the voltage source grid-side converter as shown in FIG2A can be constructed so that its output is the d-axis component of the converter output voltage reference value, that is, the amplitude of the offshore AC voltage.

[0067] As described above, since the wind turbines in the offshore wind farm are equipped with both voltage source converters and current source converters, the voltage source converters can be used to provide frequency support and the current source converters can be used to achieve rapid power response, so that the offshore low-frequency AC transmission system 400 has the capabilities of both frequency support and rapid power response.

[0068] The control strategy of the voltage source converter and the control strategy of the current source converter as described above can be written as a computer program and stored on a computer-readable storage medium. When the computer program is executed by a processor, the control strategy of the voltage source converter and the control strategy of the current source converter as described above can be implemented. Examples of computer-readable storage media include: read-only memory (ROM), random access programmable read-only memory (PROM), electrically erasable programmable read-only memory (EEPROM), random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, non-volatile memory, CD-ROM, CD-R, CD+R, CD-RW, CD+RW, DVD-ROM, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-RAM, BD-ROM, BD-R, BD-R LTH, BD-RE, Blu-ray or optical disk storage, hard disk drive (HDD), solid state drive (SSD), card storage (such as, multimedia card, secure digital (SD) card or ultra fast digital (XD) card), magnetic tape, floppy disk, magneto-optical data storage device, optical data storage device, hard disk, solid state disk and any other device, any other device is configured to store the computer program and any associated data, data files and data structures in a non-transitory manner and provide the computer program and any associated data, data files and data structures to a processor or computer so that the processor or computer can execute the computer program. In one example, the computer program and any associated data, data files and data structures are distributed on a networked computer system so that the computer program and any associated data, data files and data structures are stored, accessed and executed in a distributed manner by one or more processors or computers.

[0069] While some embodiments of the present disclosure have been shown and described, it will be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined by the claims and their equivalents.

Claims

1. A converter system for a wind turbine generator set, the converter system comprising: A voltage source converter and a current source converter, wherein the voltage source converter and the current source converter are connected in parallel between the output side of the generator of the wind turbine generator set and the grid connection point of the wind turbine generator set. The voltage source converter includes a first machine-side converter and a first grid-side converter connected in sequence, and the current source converter includes a second machine-side converter and a second grid-side converter connected in sequence; A converter control device configured to control the operation of the voltage source converter and the current source converter by considering the internal circulating current of the wind turbine generator set in the voltage outer loop control of the voltage source converter and by considering the internal circulating current of the wind turbine generator set in the active power control link and the reactive power control link of the current source converter, wherein the internal circulating current of the wind turbine generator set is determined based on the output current of the first grid-side converter and the output current of the second grid-side converter.

2. The converter system according to claim 1, wherein The converter control device is configured to: Determine the d-axis component of the converter output voltage reference value based on the active power reference value and the actual value of the first grid-side converter; Determine the q-axis component of the converter output voltage reference value based on the output current value of the wind turbine generator set and the virtual angular frequency; Perform voltage outer loop control on the d-axis component and the q-axis component of the converter output voltage reference value, and determine the d-axis component and the q-axis component of the first current reference value based on the internal circulating current value of the wind turbine generator set; Obtain the d-axis component and the q-axis component of the modulation voltage reference value of the first grid-side converter by performing current inner loop control on the d-axis component and the q-axis component of the first current reference value; Control the operation of the first grid-side converter according to the virtual internal potential phase and the d-axis component and the q-axis component of the modulation voltage reference value of the first grid-side converter; wherein the output current of the wind turbine generator set is determined based on the output current of the first grid-side converter and the output current of the second grid-side converter.

3. The converter system according to claim 1, wherein The converter control device is configured to: Determine the d-axis component and the q-axis component of the second current reference value based on the active power reference value and the actual value of the second grid-side converter, the output current value of the wind turbine generator set, and the internal circulating current value; By performing current inner loop control on the d-axis component and the q-axis component of the second current reference value, obtain The d-axis component and the q-axis component of the modulation voltage reference value of the second grid-side converter; Control the operation of the second grid-side converter according to the virtual internal potential phase and the d-axis component and the q-axis component of the modulation voltage reference value of the second grid-side converter, wherein the output current of the wind turbine generator set is determined based on the output current of the first grid-side converter and the output current of the second grid-side converter.

4. The converter system according to claim 2 or 3, wherein, The converter control device is further configured to: determine the virtual angular frequency and the virtual internal potential phase based on the filter capacitor voltage value of the first grid-side converter.

5. The converter system according to claim 2, wherein, The converter control device is further configured to: Obtain a second proportional integral operation result by performing proportional integral operation on the difference between the active power reference value and the actual value of the first grid-side converter; Determine the d-axis component of the converter output voltage reference value based on the second proportional-integral operation result and the pre-set voltage feed-forward compensation value of the wind turbine generator set.

6. The converter system according to claim 2, wherein The converter control device is further configured to: Obtain a first proportional operation result by performing proportional operation on the output current value of the wind turbine generator set; Obtain the q-axis component of the converter output voltage reference value by performing proportional operation on the difference between the first proportional operation result and the virtual angular frequency.

7. The converter system according to claim 2 or 3, wherein The converter control device includes a first grid-side controller and a second grid-side controller that communicate with each other. Among them, the first grid-side controller is configured to control the operation of the first grid-side converter, and the second grid-side controller is configured to control the operation of the second grid-side converter.

8. The converter system according to claim 1, wherein, The wind turbine generator set is an offshore wind turbine generator set, and the generator is a six-phase permanent magnet synchronous wind generator.

9. The converter system according to claim 1, wherein, The capacities of the current source type converter and the voltage source type converter are the same.

10. The converter system according to claim 2 or 3, wherein The output current value of the wind turbine generator set is the sum of the output current values of the first grid-side converter and the second grid-side converter, and the internal circulating current value of the wind turbine generator set is the difference between the output current values of the first grid-side converter and the second grid-side converter.

11. A wind turbine generator set, the wind turbine generator set includes a converter system of the wind turbine generator set according to any one of claims 1-10.

12. An offshore wind farm, the offshore wind farm includes multiple wind turbine generator sets according to claim 11.

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