A control method for a series connection networking type wind power full direct current transmission system

Abstract

The application provides a kind of wind power full DC sending-out system control method of series networking type. The sending-out system includes multiple groups of wind generators, PMSG machine side AC / DC converter, network side DC / DC converter and DC / AC converter, characterized in that DC bus between PMSG machine side AC / DC converter and network side DC / DC converter is connected with DC bus capacitor parallelly connected with the two converters respectively, each wind generator is connected in series through PMSG machine side AC / DC converter and network side DC / DC converter output side to form a single wind farm, wind farms are connected in parallel to form a field group, and the field group is transmitted to DC / AC converter through HVDC and connected to onshore AC power grid; the AC / DC converter is used to control single wind generator MPPT and stator side unit power factor.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of wind power transmission system, and particularly relates to a control method of a series networking type wind power full direct current transmission system. BACKGROUND

[0002] The statements in this section merely provide background information related to the present application and do not necessarily constitute the prior art.

[0003] Offshore wind power is an important technology in the power generation link to respond to the double carbon strategy, so offshore wind power has hot market demand and technical demand. In the existing offshore wind power transmission system, the technical scheme of alternating current convergence / alternating current transmission (full alternating current) and alternating current convergence / direct current transmission has been relatively mature, and has been realized in actual engineering applications. With the increasing capacity of wind farms and the increasing distance from the shore, the construction cost of offshore platforms under the above two schemes gradually increases, and the characteristics of long-distance and large-capacity power transmission of offshore wind power make the advantages of the direct current convergence / direct current transmission (referred to as full direct current) scheme prominent.

[0004] Compared with the high voltage AC transmission (HVAC) system, the high voltage DC transmission (HVDC) system has greater structural similarity, and thus the control method is more consistent. Generally, when the onshore converter station is connected to a high-strength power grid, the onshore converter adopts grid-type control. At this time, the converter can be equivalent to a controlled current source, providing support for the HVDC and offshore AC aggregation network. For example, the paper "Operation and Control of HVDC-Connected Wind Farm" (author S. M. Muyeen et al.) published in the journal IEEE Transactions on Sustainable Energy, Vol. 1, No. 1, pp. 30-37 in April 2010 divides the control strategy of the AC aggregation / DC transmission type offshore wind power transmission system into onshore converter control, offshore converter control, PMSG machine side converter control, and grid side converter control. Among them, the onshore converter controls the HVDC transmission voltage; the offshore converter provides voltage and frequency support for the offshore AC aggregation network on the basis of the onshore converter controlling the DC voltage; the PMSG grid side converter controls the DC link voltage between the PMSG two-stage converter and the reactive power output to the AC aggregation network; and the machine side converter completes the maximum power tracking control and PFC control. The above control strategy has good synchronous stability and small disturbance stability when the onshore AC power grid is strong. However, with the increasing proportion of new energy and power electronic converters in new power systems, the strength of the AC power grid is decreasing, so the onshore converter control method in the paper "Wind Farm-HVDC System Control Method with Real-Time Frequency Mirroring and Autonomous Grid Synchronization Capability" published in the journal Proceedings of the Chinese Society of Electrical Engineering, Vol. 37, No. 2, pp. 496-505 in January 2017 is grid-type control. At this time, the converter can be equivalent to a controlled voltage source. Its power synchronization method, which is different from the phase-locked loop (PLL) synchronization, makes the system have a broader application prospect in the weak grid scenario. The review paper "Offshore Wind Power DC Transmission and Grid Connection Technology Review" (author Cai Xu et al.) published in the journal Power System Automation, Vol. 45, No. 21 in 2021 summarizes the structure of the offshore wind power DC transmission system, but does not involve the coordinated control strategy of the whole system converter station. Only in the patent application number 202110606187.X, the corresponding control strategy is given for the offshore wind power DC transmission topology proposed by the patent. SUMMARY

[0005] The application proposes a series networking type wind power full DC sending-out system control method to achieve the control target of maximum wind energy output under a strong onshore AC power grid system and active support of HVDC transmission line voltage by the sending end system.

[0006] According to some embodiments, the application adopts the following technical solutions:

[0007] A series networking type wind power full DC sending-out system control method, comprising: multiple groups of wind generators, a PMSG machine side AC / DC converter, a network side DC / DC converter and a DC / AC converter, a DC bus connected between the PMSG machine side AC / DC converter and the network side DC / DC converter is connected with DC bus capacitors in parallel with the two converters respectively, each wind generator is connected in series with the PMSG machine side AC / DC converter and the network side DC / DC converter output side to form a single wind farm, the wind farms are connected in parallel to form a field group, and the field group is transmitted to the DC / AC converter by HVDC and connected to the onshore AC power grid; the AC / DC converter is used for controlling single wind generator MPPT and stator side unit power factor.

[0008] Compared with the prior art, the application has the following beneficial effects:

[0009] The application achieves the control target of maximum wind energy output under a strong onshore AC power grid system and active support of HVDC transmission line voltage by the sending end system. BRIEF DESCRIPTION OF DRAWINGS

[0010] The drawings accompanying the specification integrated into the application serve to provide further understanding of the application, and the illustrative embodiments of the application and the description thereof serve to explain the application, and do not constitute an improper limitation on the application.

[0011] Figure 1 is the overall framework diagram of the series networking type wind power full DC sending-out system shown in the application;

[0012] Figure 2 is the machine side AC / DC control diagram shown in the application;

[0013] Figure 3 is the DC / DC control diagram shown in the application;

[0014] Figure 4 is the onshore DC / AC control diagram shown in the application;

[0015] Figure 5 is the series networking type wind power full DC sending-out system control method framework diagram shown in the application. DETAILED DESCRIPTION

[0016] The application will be further described below in combination with the drawings and embodiments.

[0017] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0018] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, elements, components, and / or groups thereof, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof.

[0019] In the present application, the terms such as "connected", "linked" and the like should be interpreted broadly, which means that it can be fixedly connected, integrally connected or detachably connected; it can be directly connected or indirectly connected through an intermediate medium. For relevant researchers or technicians in the art, the specific meaning of the above terms in the present application can be determined according to the specific circumstances, and it should not be understood as a limitation on the present application.

[0020] The embodiment provides a series networking type wind power full direct current sending-out system control method.

[0021] The series networking type wind power full direct current sending-out system control method comprises the following steps: a plurality of wind power generators, a PMSG machine side AC / DC converter, a network side DC / DC converter and a DC / AC converter, a DC bus connected between the PMSG machine side AC / DC converter and the network side DC / DC converter is connected with a DC bus capacitor in parallel with the two converters respectively, each wind power generator is connected in series with each other through the output side of the PMSG machine side AC / DC converter and the network side DC / DC converter to form a single wind farm, the wind farms are connected in parallel with each other to form a farm group, and the farm group is transmitted to the DC / AC converter through HVDC and connected to an onshore alternating current power grid; the AC / DC converter is used for controlling single wind power generator MPPT and stator side unit power factor.

[0022] The series networking type offshore wind power full direct current sending-out system provided by the embodiment comprises the following steps: Figure 1), which is composed of three levels of structure, i.e. direct current convergence level, direct current transmission level and onshore grid-connected level. Among them, the direct current convergence level realizes wind energy convergence, and the specific implementation scheme is that a single wind turbine is rectified by a low-level AC / DC converter (referred to as a machine-side AC / DC converter) and then output by a one-stage isolation type DC / DC converter, and different wind turbines are connected in series at the output side of the DC / DC converter, thereby completing the direct current convergence of wind energy. The direct current output level realizes the direct current transmission of wind energy, and the specific implementation scheme is that the output of the direct current convergence level constitutes the input of this level, which is transmitted to the onshore grid-connected level by HVDC. The onshore grid-connected level realizes the transmission of wind energy into the grid, and the specific implementation scheme is that the HVDC output is connected to the onshore AC grid by a modular multilevel converter (MMC).

[0023] The autonomous operation control strategy of the whole system proposed in this embodiment can be divided into three types, i.e. PMSG machine-side low-level AC / DC converter control, grid-side DC / DC converter control and onshore grid-connected MMC DC / AC converter control, each of which undertakes the following control objectives:

[0024] The control objective of the PMSG machine-side control is MPPT control and reactive power control; the grid-side DC / DC converter is used to support the HVDC voltage, and the key point is to distribute the output voltage of the series grid-side DC / DC converter when the HVDC voltage is determined under normal operation and fault conditions, so voltage distribution control needs to be added to the control objective; the onshore MMC converter undertakes the control objective of sending out all MPPT wind energy, so the PQ control of the slave grid under strong grid support is adopted.

[0025] The meanings of the variables mentioned in this embodiment are shown in Table 1, and the control strategies of the converters of this embodiment are further described below in combination with the accompanying drawings.

[0026] Table 1

[0027]

[0028]

[0029] In the table, v gabc v gdq represent vg i (i = a, b, c) respectively, and the current is the same.C Xi represents the i th wind turbine generator unit in the wind farm, and n represents the number of wind turbine units in the wind farm;P * represents the output of the HVDC voltage controller;P * unit represents the output correction of the single wind turbine DC / DC controller; represents the output of the single wind turbine DC / DC controller; represents the single fan DC / DC controller correction output.

[0030] PMSG machine side AC / DC control strategy

[0031] Figure 2 For the full DC system PMSG machine side AC / DC converter control block diagram, the machine side AC / DC converter adopts double loop control of DC voltage, reactive power and current, the inner loop current is no different from the traditional control mode, the difference is the outer loop control mode. In this embodiment, the AC / DC DC side voltage reference value is given by the MPPT algorithm, so when the fan tracks the maximum power point, there will be a certain range of fluctuations rather than taking a fixed value. In addition to the machine side power factor correction (PFC) control is the same as the traditional control mode.

[0032] PMSG grid side DC / DC control strategy

[0033] In the traditional offshore wind power AC aggregation / DC transmission delivery system, different PMSG grid side DC / AC converters are connected in parallel to the AC aggregation network formed by the offshore converter, so that different aggregation units share the same AC aggregation network, but in the series network type full DC delivery system, the grid side DC / DC converter needs to be connected in series to support the HVDC voltage together, so there is a strong coupling between the grid side DC / DC converters of the same wind farm. Figure 3 In short, a new type of DC-DC converter coordinated control based on the active support of the sending end system DC transmission voltage is needed, and the specific control strategy is as follows.

[0034] 1) The top layer is the HVDC bus voltage control, as shown in Figure 3 , the controller outputs the active power reference signal to the N unit controllers; 2) the middle layer is the grid side DC / DC converter secondary side voltage balance control, the controller outputs are combined as the active power reference signal of this module; 3) the bottom layer is the individual converter controller to realize square wave frequency conversion or width modulation frequency conversion control.

[0035] It should be noted that the top layer, middle layer and bottom layer are the specific implementation process of the hierarchical control of the control method.

[0036] Onshore MMC DC / AC control strategy

[0037] The MMC type DC / AC converter currently used in engineering modeling and control mode is similar to the traditional low-level voltage source converter (VSC) (it should be noted that the voltage source converter here is determined by its topology, and the most prominent feature is that the DC side is a parallel capacitor, which is essentially different from the above-mentioned network type converter equivalent to a controlled voltage source), and a large number of sub-module bridge arms can be regarded as a voltage source for processing. The control target of the MMC converter in this embodiment is to send all the maximum wind energy of the offshore wind farm group, so the PQ type of the network type control should be used. At this time, the active reference value should be the sum of all MPPT outputs of the sending system, and the reactive power is given according to the demand of the onshore grid-connected AC system Figure 4 ).

[0038] As shown in Figure 5 , the embodiment is a unified control method of a global converter of a series network type offshore wind power full DC transmission system. The control method is divided into three types of converter control, and the main feature is to support the HVDC voltage of the sending system and the maximum power transmission of the wind farm group. Under the unified control method, the control requirements of the three types of converters are as follows:

[0039] The PMSG machine side AC / DC converter controls the single wind turbine MPPT and the stator side unit power factor. Unlike traditional MPPT control, the MPPT algorithm used in this embodiment is based on voltage oriented control (VOC), which changes the AC / DC DC side voltage reference value by collecting system variables, so as to track the single machine maximum power, so the DC side voltage U X exists a certain degree of fluctuation. In addition, the current reference value is set to 0, so as to realize the unit power factor.

[0040] On the basis of the PMSG machine side AC / DC control U X , the network side DC / DC converter realizes three control targets, which are 1) HVDC voltage control, thereby supporting the HVDC network; 2) due to the series DC / DC unified control of the HVDC voltage, the DC / DC secondary side currents are the same, and when the power transmission of different wind turbines is inconsistent, the secondary voltage reference value needs to be coordinated and allocated, and for this purpose, the series DC / DC secondary voltage balancing control is needed; 3) on the basis of the coordinated allocation of the series DC / DC secondary voltage, the single DC / DC completes the respective secondary voltage control.

[0041] The HVDC voltage is controlled by the sending end DC / DC, and the onshore DC / AC converter controls the maximum power transmission of the whole offshore wind farm group, so a PQ type control strategy is adopted. The active reference value is the maximum power point of the offshore wind farm group, and the reactive power reference value needs to consider the demand of the connected AC grid to determine.

[0042] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. The present application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A control method of a wind power full-direct-current transmission system in a series network, comprising: Multiple groups of wind power generators, PMSG machine side AC / DC converter, network side DC / DC converter and DC / AC converter, characterized in that DC bus connected between PMSG machine side AC / DC converter and network side DC / DC converter has DC bus capacitor connected in parallel with the two converters respectively, each wind power generator is connected through PMSG machine side AC / DC converter and network side DC / DC converter output side in series to form a single wind farm, wind farms are connected in parallel to form a field group, and the field group is transmitted to the DC / AC converter by HVDC and connected to the onshore AC power grid; the AC / DC converter is used for controlling single wind power generator MPPT and stator side unit power factor; The DC / DC converter is used for coordinated control and support HVDC voltage, and the specific control strategy is as follows: 1) the top layer is HVDC bus voltage control; 2) the middle layer is network side DC / DC converter secondary side voltage balance control; 3) the bottom layer is square wave frequency conversion or width modulation frequency conversion control realized by single converter controller. 2.The control method of the series-connected wind power full-DC transmission system according to claim 1, characterized in that, The PMSG machine side AC / DC converter is used for controlling single wind power generator MPPT and stator side unit power factor, specifically including: using MPPT algorithm based on voltage dominant control, changing AC / DC converter DC side voltage reference value by collecting system variables, so that the reference value tracks the maximum power of single wind power generator.

3. The control method of the series-connected wind power full-DC transmission system according to claim 2, characterized in that, On the basis of the PMSG machine side AC / DC converter controlling AC / DC DC side voltage, the network side DC / DC converter is used for controlling HVDC voltage to realize HVDC network support.

4. The control method of the series-connected wind power full-DC transmission system according to claim 3, characterized in that, The network side DC / DC converter connected in series with three-phase line is used for controlling DC / DC converter secondary voltage balance.

5. The control method of the series-connected wind power full-DC transmission system according to claim 4, characterized in that, On the basis of the network side DC / DC converter connected in series with three-phase line, the network side DC / DC converter coordinates and distributes series DC / DC secondary voltage, and each secondary voltage is controlled by single DC / DC.

6. The control method of the series-connected wind power full-DC transmission system according to claim 1, characterized in that, The DC / AC converter adopts PQ type control strategy to control the maximum power transmission of offshore wind farm group.

7. The control method of the series-connected wind power full-DC transmission system according to claim 1, characterized in that, The DC / AC converter is used for controlling active power and reactive power of offshore wind farm group grid connection.

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