A method for coordinated control of the sending-end bipolar converter in a wind power grid connected to a flexible DC system.

Abstract

This invention discloses a coordinated control method for a sending-end bipolar converter in a wind power integrated into a flexible DC system, relating to the field of power system protection and control. The method includes the following steps: Step S1, setting the active power-frequency rise characteristic curve equations for the sending-end positive and negative converters; Step S2, setting the reactive power-voltage droop characteristic curve equations for the sending-end positive and negative converters; Step S3, setting the active power-frequency rise characteristic curves for the positive and negative converters under special operating conditions with limited capacity; Step S4, the active power balance and voltage stabilization adjustment process of the sending-end positive and negative converters during wind power output power fluctuations; Step S5, the frequency coordination process of the bipolar converters. This invention, employing the above-mentioned coordinated control method for a sending-end bipolar converter in a wind power integrated into a flexible DC system, can ensure the balance of active power between the positive and negative DC networks and the voltage and frequency stability of the AC bus voltage when there are control value data deviations in the control system.

Description

Technical Field

[0001] This invention relates to the field of power system protection and control, and in particular to a coordinated control method for a bipolar converter at the sending end of a wind power-to-flexible DC system. Background Technology

[0002] Developing renewable energy is a global consensus for addressing the fossil fuel depletion crisis and achieving energy structure transformation. Leveraging its resource advantages, the "sand and barren" regions of Northwest China are being prioritized for development into large-scale new energy bases to strongly support the large-scale development and utilization of new energy. These remote areas have historically experienced very low local loads, and their regional AC grids lack synchronous power sources such as traditional thermal power generators, making them typical islanded new energy systems requiring long-distance ultra-high-voltage transmission to reach eastern load centers. Against this backdrop, MMC-HVDC technology, with its stable voltage support capabilities and absence of commutation failure, is one of the mainstream technologies for efficient grid connection and inter-regional consumption of large-scale isolated wind power clusters. MMC-HVDC primarily employs two connection methods: pseudo-bipolar and true bipolar. Compared to the single converter configuration in pseudo-bipolar systems, true bipolar systems have two converters configured for each converter station according to the positive and negative poles. This not only increases the system's transmission capacity but also enhances the system's multi-objective coordinated control capabilities. It has the advantages of being economical, flexible, and highly reliable in large-capacity power transmission and has been widely adopted in DC transmission projects such as Zhangbei and Baihetan in my country.

[0003] In the scenario of transmitting isolated wind power to other regions, the control strategy of the sending-end MMC converter station faces two major technical challenges. On the one hand, the wind farm itself has weak voltage regulation capabilities, resulting in a high risk of AC bus voltage instability. On the other hand, the strong fluctuations in wind power output may cause power imbalance between the positive and negative DC lines. This power imbalance can lead to unbalanced currents in the metallic return line, increasing power losses and potentially inducing wideband oscillations and control failures, adversely affecting the voltage-frequency stability of the system. Therefore, it is necessary to study the voltage-frequency support method and power balance control strategy of the sending-end true bipolar converter station to ensure the safe and stable operation of the isolated wind power flexible DC transmission system. Thus, researching the voltage-frequency support method and power balance control strategy of the sending-end true bipolar converter station is of significant practical importance for ensuring the safe and stable operation of the isolated wind power transmission system via flexible DC transmission.

[0004] Currently, existing research on voltage-frequency support control strategies for sending-end converter stations mainly includes two categories: master-slave control and peer-to-peer control schemes. In the master-slave control scheme, one converter employs a constant AC voltage and frequency control strategy (VF control), while the other converter adopts an active-reactive power decoupling control strategy (PQ control). This type of control scheme suffers from insufficient stability when facing DC single-pole ground faults, and achieving power balance between the positive and negative DC lines is challenging under conditions of frequent wind power fluctuations. In the peer-to-peer control scheme, both converters employ control strategies with voltage-frequency support capabilities, such as VF control. However, when both poles use VF control, the requirements for data consistency are high in practical engineering. Even small control reference deviations can cause coordination imbalances between the positive and negative converters, leading to system stability risks.

[0005] Furthermore, existing research primarily focuses on operating conditions where the maximum transmission capacity of the sending-end bipolar converters is the same, and has not fully explored coordinated control strategies when the maximum transmission capacity of the bipolar converters differs due to limitations in their own equipment. Therefore, it is urgent to study coordinated control strategies between the sending-end positive and negative converters of a true bipolar flexible DC transmission system connected to a wind farm, addressing the stability issues arising from the lack of coordination between bipolar converters when there are deviations in the dual VF control objectives, and the need for bipolar converters to balance the power transmission between the positive and negative lines during wind fluctuations. This research aims to ensure the safe and stable operation of the power system under different operating conditions. Summary of the Invention

[0006] The purpose of this invention is to provide a coordinated control method for the sending-end bipolar converter of a wind power grid connected to a flexible DC system, which can ensure the balance of active power between the positive and negative DC networks and the stability of the AC bus voltage and frequency when there are control value data deviations in the control system.

[0007] This invention provides a coordinated control method for the sending-end bipolar converter of a wind power grid connected to a flexible DC system, comprising the following steps:

[0008] Step S1: Set the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter;

[0009] Step S11: Define the positive direction of active power of the sending-end positive and negative pole converters;

[0010] Step S12: Set the active power-frequency rise characteristic curve of the sending-end positive and negative pole converter;

[0011] Step S13: Design the active power-frequency rise characteristic control block diagram of the sending-end positive and negative pole converter;

[0012] Step S2: Set the reactive power-voltage droop characteristic curve equation for the sending-end positive and negative pole converters;

[0013] Step S21: Define the positive direction of reactive power of the sending-end positive and negative pole converters;

[0014] Step S22: Set the reactive power-voltage droop characteristic curves of the sending-end positive and negative pole converters;

[0015] Step S23: Design the control block diagram of the reactive power-voltage droop characteristic of the sending-end positive and negative pole converter;

[0016] Step S3: Set the active power-frequency rise characteristic curve of the positive and negative pole converter under special operating conditions with limited capacity;

[0017] Step S31: Set the active power-frequency rise characteristic curve of the normal pole converter under special operating conditions with limited capacity;

[0018] Step S32: Set the active power-frequency rise characteristic curve of the capacity-limited pole converter under special operating conditions with limited capacity;

[0019] Step S4: Active power balance and voltage stabilization regulation process of the sending-end positive and negative pole converters during wind power output power fluctuations;

[0020] Step S41: Active power balancing process of the sending-end positive and negative pole converter;

[0021] Step S42: Stabilization process of AC bus voltage of the sending-end positive and negative pole converter;

[0022] Step S5: The process of coordinating the frequency of the bipolar converter when there is an initial value deviation in the frequency of the sending-end positive and negative pole converters.

[0023] Preferably, in step S11, the active power of the sending-end bipolar converter is defined based on the active power received by the sending-end positive and negative pole converters from the wind farm to the sending-end converter station during normal operation, with the AC side flowing into the sending-end positive and negative pole converters as the positive direction.

[0024] In step S12, based on the correlation between the active power received by the sending-end positive and negative converters and the frequency, the equation for the active power-frequency rise characteristic curve of the sending-end positive and negative converters is set as follows:

[0025] f i =f 0i +k Pi (P i -P 0i (i=1,2) (1);

[0026] Where f1 and f2 are the actual AC frequencies of the positive and negative converters, respectively; f 01 and f 02 P1 and P2 are the initial values ​​of the frequencies of the positive and negative converters, respectively; P1 and P2 are the actual active power transmitted by the positive and negative converters, respectively; P01 and P 02 These are the initial values ​​of active power transmitted by the positive and negative converters, respectively; k P1 and k P2 These are the adjustment coefficients for the active power-frequency rise characteristic curve equations of the positive and negative pole converters, respectively, and their values ​​are positive.

[0027] The adjustment coefficient for the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter is shown in the following formula:

[0028]

[0029] Among them, f max f is the maximum allowable frequency for the sending system; min P is the minimum allowable frequency for the sending system. maxi P represents the maximum active power that the positive and negative pole converter can transmit; mini This is the minimum active power that the positive and negative pole converter can transmit;

[0030] In step S13, the active power-frequency rise characteristic control block diagram of the sending-end positive and negative pole converter is designed according to the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter shown in equation (1).

[0031] Preferably, in step S21, the reactive power of the sending-end bipolar converter is defined based on the reactive power output of the sending-end positive and negative pole converters meeting the reactive power requirements of the system during normal operation, with the AC side flowing out of the sending-end bipolar converter as the positive direction.

[0032] In step S22, based on the correlation between the reactive power output of the sending-end positive and negative converters and the voltage amplitude, the reactive power-voltage droop characteristic curve equation of the sending-end positive and negative converters is set as follows:

[0033] u refi =u 0i -k Qi ·(Q i -Q 0i (i = 1, 2) (3);

[0034] In the formula, u ref1 and u ref2 These are the reference values ​​for the AC bus voltage amplitude of the positive and negative converters, respectively; u 01 and u 02 Q1 and Q2 are the initial reference values ​​for the positive and negative AC bus voltage amplitudes, respectively; Q1 and Q2 are the actual reactive power outputs of the positive and negative converters, respectively; Q 01 and Q 02 These are the initial values ​​of reactive power output from the positive and negative converters, respectively; k Q1 and k Q2These are the adjustment coefficients for the reactive power-voltage droop characteristic curve equations of the positive and negative converters, respectively, and their values ​​are positive.

[0035] The adjustment coefficient for the reactive power-voltage droop characteristic curve equation of the sending-end positive and negative pole converter is shown in the following formula:

[0036]

[0037] Among them, u max This is the maximum allowable voltage at the grid connection point during system operation; u min Q is the minimum allowable voltage at the grid connection point during system operation. maxi Q represents the maximum reactive power output of the positive and negative converters. mini This represents the minimum reactive power output of the positive and negative pole converters.

[0038] Since the reactive power output of the sending-end positive and negative converters is constrained by the converter's rated capacity and transmitted active power, the maximum and minimum values ​​of the reactive power output of the sending-end positive and negative converters are expressed as follows:

[0039]

[0040] Among them, S i P represents the rated capacity of the positive and negative pole converters. i The active power transmitted in real time by the positive and negative pole converters;

[0041] In step S23, based on the reactive power-voltage droop characteristic curve equation of the positive and negative pole converter shown in equation (3), the reactive power-voltage droop characteristic control block diagram of the sending-end positive and negative pole converter is designed.

[0042] Preferably, in step S31, according to equation (1), the active power-frequency rise characteristic curve equation of the normal pole converter under special operating conditions with limited capacity is as shown in the following equation;

[0043] f Z =f S +k PZ (P Z -P S (6);

[0044] Among them, f Z f is the actual AC frequency of the normal polarity converter. S P represents the initial frequency value for both the normal pole converter and the capacity-limited pole converter; Z P represents the actual active power transmitted by the normal pole converter. S The initial value for the active power transmitted by the normal pole converter and the capacity-limited pole converter; k PZThe adjustment coefficient is the equation of the active power-frequency rise characteristic curve of the normal pole converter.

[0045] According to equation (2), the adjustment coefficient of the active power-frequency rise characteristic curve equation of the normal pole converter is as follows:

[0046]

[0047] In the formula, P maxZ P represents the maximum transmission capacity of a normal polarity converter. min This represents the minimum transmission capacity of a normal polarity converter.

[0048] The initial value of active power transmitted by normal pole converters and capacity-limited pole converters is related to the maximum transmission capacity of the capacity-limited pole converter, as shown in the following formula;

[0049] P S =k S P maxF (8);

[0050] Where, k S P is the initial coefficient of the capacity-limited pole converter. maxF This represents the maximum transmission capacity of the capacity-limited polarity converter.

[0051] Preferably, in step S32, according to equation (1), the active power-frequency rise characteristic curve equation of the capacity-limited pole converter is as shown in the following equation;

[0052] f F =f S +k PF (P F -P S (9);

[0053] In the formula, f F P is the actual AC frequency of the capacity-limited pole converter. F k represents the actual active power transmitted by the capacity-limited pole converter. PF This is the adjustment coefficient for the active power-frequency rise characteristic curve equation of a capacity-limited pole converter.

[0054] Preferably, in step S32, the adjustment coefficient of the active power-frequency rise characteristic curve equation of the capacity-limited pole converter under special operating conditions with limited capacity is set, as shown in the following formula;

[0055]

[0056] Preferably, in step S41, according to the frequency-phase relationship shown in equation (11), the AC voltage phase of the positive converter will lead that of the negative converter, as shown in the following equation:

[0057]

[0058] In the formula: θ MMC1 and θ MMC2 θ0 represents the phase of the AC voltage of the positive and negative converters, respectively; θ0 is the initial phase of the AC voltage of the positive and negative converters; when the phase of the AC voltage of the positive converter leads that of the negative converter, the relationship between the active power and the phase of the AC voltage of the converter, as shown in equation (12), is as follows:

[0059]

[0060] Among them, U WF U is the voltage at the wind farm outlet. MCC1 and U MCC2 These are the AC voltages of the positive and negative converters, respectively; θ WF X1 represents the voltage phase at the wind farm outlet; X2 and X1 represent the equivalent reactance from the wind farm to the positive and negative converters, respectively.

[0061] Preferably, in step S42, when both the positive and negative converters at the sending end adopt a reactive power-voltage droop control strategy, the control objective expression for the voltage outer loop is as follows:

[0062]

[0063] Where: u sdref1 and u sdref2 These are the reference values ​​for the d-axis voltages of the positive and negative converters at the sending end, respectively; u sqref1 and u sqref2 These are the reference values ​​for the q-axis voltages of the positive and negative pole converters at the sending end, respectively.

[0064] According to the converter reactive power transmission equation shown in equation (14), the following equation is presented;

[0065]

[0066] Therefore, the above-mentioned coordinated control method for the sending-end bipolar converter of a wind power access flexible DC system can ensure the balance of active power between the positive and negative DC networks and the stability of the AC bus voltage and frequency when there are control value data deviations in the control system.

[0067] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0068] Figure 1 This is a schematic diagram of a dual-end MMC-HVDC system for wind farm access, based on the wind power access flexible DC system coordinated control method of the sending-end bipolar converter of the present invention.

[0069] Figure 2 The active power-frequency rise characteristic curve of the positive and negative pole converters at the sending end of the present invention is shown in the coordinated control method of the sending end bipolar converter of the wind power access flexible DC system.

[0070] Figure 3 This is a control block diagram of the active power-frequency rise characteristic of the sending-end positive and negative pole converters in the coordinated control method of the sending-end bipolar converter of the wind power access flexible DC system of the present invention.

[0071] Figure 4 The reactive power-voltage droop characteristic curves of the positive and negative pole converters at the sending end of the present invention are shown in the coordinated control method of the sending end bipolar converter in a wind power access flexible DC system.

[0072] Figure 5 This is a block diagram of the PF rise / QV droop control of the sending-end positive and negative pole converters in the coordinated control method of the sending-end bipolar converters of the wind power access flexible DC system of the present invention.

[0073] Figure 6 This invention provides a method for coordinated control of a bipolar converter at the sending end of a wind power grid connected to a flexible DC system, showing the active power-frequency rise characteristic curves of the positive and negative pole converters at the sending end under capacity-limited conditions.

[0074] Figure 7 This is a diagram illustrating the active power balance process of the positive and negative pole converters at the sending end of a wind power connection to a flexible DC system, according to the present invention.

[0075] Figure 8 This invention relates to the AC bus voltage stabilization process of the sending-end positive and negative pole converters in a coordinated control method for a wind power connection to a flexible DC system.

[0076] Figure 9 This is a diagram illustrating the frequency coordination process of the positive and negative pole converters at the sending end of a wind power grid connected to a flexible DC system, according to the present invention.

[0077] Figure 10 This is an overall flowchart of a coordinated control method for a bipolar converter at the sending end of a wind power grid connected to a flexible DC system, according to the present invention. Detailed Implementation

[0078] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0079] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0080] The terms "first," "second," and similar words used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0081] Example 1

[0082] like Figures 1-10 As shown, the present invention provides a coordinated control method for a bipolar converter at the sending end of a wind power grid connected to a flexible DC system, comprising the following steps:

[0083] Step S1: Set the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter;

[0084] Step S11: Define the positive direction of active power of the sending-end positive and negative pole converters;

[0085] In step S11, the active power of the sending-end bipolar converter is defined based on the active power received by the sending-end positive and negative pole converters from the wind farm during normal operation, with the AC side flowing into the sending-end positive and negative pole converters as the positive direction.

[0086] Step S12: Set the active power-frequency rise characteristic curve of the sending-end positive and negative pole converter;

[0087] In step S12, based on the correlation between the active power received by the sending-end positive and negative converters and the frequency, the equation for the active power-frequency rise characteristic curve of the sending-end positive and negative converters is set as follows: The corresponding rise characteristic curve is shown below. Figure 2 As shown;

[0088] f i =f 0i +k Pi (P i -P 0i (i=1,2) (1);

[0089] Where f1 and f2 are the actual AC frequencies of the positive and negative converters, respectively; f 01 and f 02 P1 and P2 are the initial values ​​of the frequencies of the positive and negative converters, respectively; P1 and P2 are the actual active power transmitted by the positive and negative converters, respectively; P 01 and P 02These are the initial values ​​of active power transmitted by the positive and negative converters, respectively; k P1 and k P2 These are the adjustment coefficients for the active power-frequency rise characteristic curve equations of the positive and negative converters, respectively, and their values ​​are positive.

[0090] The adjustment coefficient for the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter is:

[0091]

[0092] Among them, f max f is the maximum allowable frequency for the sending system; min P is the minimum allowable frequency for the sending system. maxi P represents the maximum active power that the positive and negative pole converter can transmit; mini This is the minimum active power that the positive and negative pole converter can transmit.

[0093] Step S13: Design the active power-frequency rise characteristic control block diagram of the sending-end positive and negative pole converter;

[0094] In step S13, based on the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter shown in equation (1), the active power-frequency rise characteristic control block diagram of the sending-end positive and negative pole converter is designed. For example... Figure 3 As shown.

[0095] Step S2: Set the reactive power-voltage droop characteristic curve equation for the sending-end positive and negative pole converters;

[0096] Step S21: Define the positive direction of reactive power of the sending-end positive and negative pole converters;

[0097] In step S21, the reactive power of the sending-end bipolar converter is defined based on the fact that the reactive power output of the sending-end positive and negative pole converters meets the reactive power requirements of the system during normal operation, with the AC side flowing out of the sending-end bipolar converter as the positive direction.

[0098] Step S22: Set the reactive power-voltage droop characteristic curves of the sending-end positive and negative pole converters;

[0099] In step S22, based on the fact that the reactive power output of the sending-end positive and negative converters is mainly related to the voltage amplitude, the reactive power-voltage droop characteristic curve equation of the sending-end positive and negative converters is set as follows: The corresponding droop characteristic curve is shown below. Figure 4 As shown:

[0100] u refi =u 0i -k Qi ·(Q i -Q 0i (i = 1, 2)(3);

[0101] In the formula, u ref1 and u ref2 These are the reference values ​​for the AC bus voltage amplitude of the positive and negative converters, respectively; u 01 and u 02 Q1 and Q2 are the initial reference values ​​for the positive and negative AC bus voltage amplitudes, respectively; Q1 and Q2 are the actual reactive power outputs of the positive and negative converters, respectively; Q 01 and Q 02 These are the initial values ​​of reactive power output from the positive and negative converters, respectively; k Q1 and k Q2 These are the adjustment coefficients for the reactive power-voltage droop characteristic curve equations of the positive and negative converters, respectively, and their values ​​are positive.

[0102] The adjustment coefficient for the reactive power-voltage droop characteristic curve equation of the sending-end positive and negative pole converter is shown in the following formula:

[0103]

[0104] Among them, u max This is the maximum allowable voltage at the grid connection point during system operation; u min Q is the minimum allowable voltage at the grid connection point during system operation. maxi Q represents the maximum reactive power output of the positive and negative converters. mini This represents the minimum reactive power output of the positive and negative converters.

[0105] Since the reactive power output of the sending-end positive and negative converters is constrained by the converter's rated capacity and transmitted active power, the maximum and minimum values ​​of the reactive power output of the sending-end positive and negative converters are expressed as follows:

[0106]

[0107] Among them, S i P represents the rated capacity of the positive and negative pole converters. i This refers to the active power transmitted in real time by the positive and negative pole converters.

[0108] Step S23: Design the control block diagram of the reactive power-voltage droop characteristic of the sending-end positive and negative pole converter.

[0109] In step S23, based on the reactive power-voltage droop characteristic curve equation of the positive and negative pole converter shown in equation (3), the control block diagram of the reactive power-voltage droop characteristic of the sending-end positive and negative pole converter is designed. For example... Figure 5 As shown.

[0110] Step S3: Set the active power-frequency rise characteristic curve of the positive and negative pole converter under special operating conditions with limited capacity.

[0111] Step S31: Set the active power-frequency rise characteristic curve of the normal pole converter under special operating conditions with limited capacity.

[0112] In step S31, according to equation (1), the active power-frequency rise characteristic curve equation of the normal pole converter under the special operating condition of limited capacity is shown in equation (6); its corresponding rise characteristic curve is as follows: Figure 6 The black curve in the image is shown in the following formula:

[0113] f Z =f S +k PZ (P Z -P S (6);

[0114] Among them, f Z f is the actual AC frequency of the normal polarity converter. S P represents the initial frequency value for both the normal pole converter and the capacity-limited pole converter; Z P represents the actual active power transmitted by the normal pole converter. S The initial value for the active power transmitted by the normal pole converter and the capacity-limited pole converter; k PZ The adjustment coefficient is the equation of the active power-frequency rise characteristic curve of the normal pole converter.

[0115] According to equation (2), the adjustment coefficient of the active power-frequency rise characteristic curve equation of the normal pole converter is as follows:

[0116]

[0117] In the formula, P maxZ P represents the maximum transmission capacity of a normal polarity converter. min This represents the minimum transmission capacity of a normal polarity converter.

[0118] The initial value of active power transmitted by normal pole converters and capacity-limited pole converters is related to the maximum transmission capacity of the capacity-limited pole converter, as shown in the following formula:

[0119] P S =k S P maxF (8);

[0120] Where, k S P represents the initial coefficient for the capacity-limited pole converter, with a value ranging from 0.9 to 0.95. maxF This represents the maximum transmission capacity of the capacity-limited polarity converter.

[0121] Step S32: Set the active power-frequency rise characteristic curve of the capacity-limited pole converter under special operating conditions with limited capacity.

[0122] In step S32, according to equation (1), the active power-frequency rise characteristic curve equation of the capacity-limited pole converter is shown in equation (9); its corresponding rise characteristic curve is as follows: Figure 6 The red curve in the image is shown in the following formula:

[0123] f F =f S +k PF (P F -P S (9);

[0124] In the formula, f F P is the actual AC frequency of the capacity-limited pole converter. F k represents the actual active power transmitted by the capacity-limited pole converter. PF This is the adjustment coefficient for the active power-frequency rise characteristic curve equation of a capacity-limited pole converter.

[0125] In step S32, the adjustment coefficient of the active power-frequency rise characteristic curve equation of the capacity-limited pole converter under the special operating condition of limited capacity is set, as shown in the following formula:

[0126]

[0127] Step S4: Active power balance and voltage stabilization regulation process of the sending-end positive and negative pole converters during wind power output power fluctuations.

[0128] Step S41: Active power balancing process of the sending-end positive and negative pole converters.

[0129] In step S41, during the transient process, when the active power received by the positive converter is higher than that of the negative converter, its frequency will be higher than that of the negative converter. According to the relationship between frequency and phase shown in equation (11), the phase of the AC voltage of the positive converter will lead that of the negative converter, as shown in the following equation:

[0130]

[0131] In the formula: θ MMC1 and θ MMC2 θ0 represents the phase of the AC voltage of the positive and negative converters, respectively; θ0 is the initial phase of the AC voltage of the positive and negative converters. When the phase of the AC voltage of the positive converter leads that of the negative converter, according to the relationship between active power and the phase of the AC voltage of the converter shown in equation (12), the positive converter receives less active power due to the increase in phase, while the negative converter receives more active power due to the decrease in phase. The power of the positive and negative converters gradually tends to be balanced, as shown in the following equation:

[0132]

[0133] Among them, U WF U is the voltage at the wind farm outlet. MCC1 and U MCC2 These are the AC voltages of the positive and negative converters, respectively; θ WF X1 represents the voltage phase at the wind farm outlet; X2 and X1 represent the equivalent reactance from the wind farm to the positive and negative converters, respectively.

[0134] Step S42: Stabilization process of AC bus voltage of positive and negative converter at the sending end.

[0135] In step S42, both the sending-end positive and negative converters adopt the following... Figure 5 When implementing the reactive power-voltage droop control strategy, the control objective expression for the outer voltage loop is as follows:

[0136]

[0137] Where: u sdref1 and u sdref2 These are the reference values ​​for the d-axis voltages of the positive and negative converters at the sending end, respectively; u sqref1 and u sqref2 These are the reference values ​​for the q-axis voltages of the sending-end positive and negative converters, respectively.

[0138] According to the converter reactive power transmission equation shown in equation (14), as shown in the following equation, the rise of the AC voltage of the positive converter will promote its reactive power output, and the decrease of the AC voltage of the negative converter will reduce its reactive power output. The reactive power of the positive and negative converters gradually becomes the same. According to equation (13), the reference values ​​of the AC bus voltage of the positive and negative converters tend to be consistent. The AC bus voltage of the positive and negative converters tends to be the same value and reaches stability.

[0139]

[0140] Step S5: The frequency coordination process of the bipolar converter when there is an initial value deviation in the frequency of the sending-end positive and negative pole converters.

[0141] Step S51: When the initial value of the AC side frequency of the positive converter is greater than that of the negative converter, according to equations (11) and (12), the phase advance of its AC voltage will lead to a decrease in the active power transmitted from the wind farm to the positive converter and an increase in the active power transmitted to the negative converter.

[0142] Step S52: According to equation (1), the AC side frequency of the positive converter will gradually decrease as its active power decreases, while the AC side frequency of the negative converter will increase as its active power increases. The AC side frequencies of the two gradually become consistent, and the system reaches a stable operating state.

[0143] Therefore, this invention employs the aforementioned coordinated control method for the sending-end bipolar converter in a wind power-to-flexible DC system. Considering that the sending-end positive and negative converters connecting to the wind farm receive active power and output reactive power during normal operation, the positive flow directions of active and reactive power in the sending-end positive and negative converters are defined respectively. By establishing the relationship between active power transmission and voltage phase, a response equation for the active power-frequency rise characteristic is established to achieve active power allocation in the bipolar converter. Similarly, by establishing the relationship between reactive power transmission and voltage amplitude, a response equation for the reactive power-voltage droop characteristic is established to achieve reactive power allocation in the bipolar converter. Furthermore, for special operating conditions where the capacity of a certain pole converter in the sending-end converter station is limited, the proposed active power-frequency rise / reactive power-voltage droop control method can effectively prevent converter capacity over-limit and ensure the stable operation of the power system.

[0144] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A coordinated control method for a sending-end bipolar converter in a wind power grid connected to a flexible DC system, characterized in that, Includes the following steps: Step S1: Set the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter; Step S11: Define the positive direction of active power of the sending-end positive and negative pole converters; Step S12: Set the active power-frequency rise characteristic curve of the sending-end positive and negative pole converter; Step S13: Design the active power-frequency rise characteristic control block diagram of the sending-end positive and negative pole converter; Step S2: Set the reactive power-voltage droop characteristic curve equation for the sending-end positive and negative pole converters; Step S21: Define the positive direction of reactive power of the sending-end positive and negative pole converters; Step S22: Set the reactive power-voltage droop characteristic curves of the sending-end positive and negative pole converters; Step S23: Design the control block diagram of the reactive power-voltage droop characteristic of the sending-end positive and negative pole converter; Step S3: Set the active power-frequency rise characteristic curve of the positive and negative pole converter under special operating conditions with limited capacity; Step S31: Set the active power-frequency rise characteristic curve of the normal pole converter under special operating conditions with limited capacity; Step S32: Set the active power characteristic curve of the capacity-limited pole converter under special operating conditions with limited capacity; Step S4: Active power balance and voltage stabilization regulation process of the sending-end positive and negative pole converters during wind power output power fluctuations; Step S41: Active power balancing process of the sending-end positive and negative pole converter; In step S41, according to the frequency-phase relationship shown in equation (11), the AC voltage phase of the positive converter will lead that of the negative converter, as shown in the following equation: (11)； In the formula: and These represent the phases of the AC voltages of the positive and negative converters, respectively. This represents the initial phase of the AC voltage in the positive and negative converters. and The positive and negative converters are the actual AC frequencies, respectively. When the AC voltage phase of the positive converter leads that of the negative converter, the relationship between the active power and the AC voltage phase of the converter, as shown in equation (12), is as follows: (12)； in, This refers to the voltage at the wind farm's outlet. and These are the positive and negative AC voltages of the converter, respectively. This refers to the voltage phase at the wind farm outlet. and These are the equivalent reactances from the wind farm to the positive and negative pole converters, respectively; and These represent the actual active power transmitted by the positive and negative converters, respectively. Step S42: Stabilization process of AC bus voltage of the sending-end positive and negative pole converter; In step S42, when both the positive and negative converters at the sending end adopt the reactive power-voltage droop control strategy, the control objective expression for the voltage outer loop is as follows: (13)； in: and These are the sending-end positive and negative pole converters, respectively. Reference value for shaft voltage; and These are the sending-end positive and negative pole converters, respectively. Reference value for shaft voltage; This serves as the initial reference value for the amplitude of the AC bus voltage at both the positive and negative poles. and These are the adjustment coefficients for the reactive power-voltage droop characteristic curve equations of the positive and negative converters, respectively, and their values ​​are positive. and These represent the actual reactive power output of the positive and negative converters, respectively. This represents the initial value of the reactive power output from the positive and negative converters. According to the converter reactive power transmission equation shown in equation (14), the following equation is presented; (14)； Step S5: The process of coordinating the frequency of the bipolar converter when there is an initial value deviation in the frequency of the sending-end positive and negative pole converters.

2. The method for coordinated control of the sending-end bipolar converter in a wind power grid connected to a flexible DC system according to claim 1, characterized in that, In step S11, the active power of the sending-end bipolar converter is defined based on the active power received by the sending-end positive and negative pole converters from the wind farm to the sending-end converter station during normal operation, with the AC side flowing into the sending-end positive and negative pole converters as the positive direction. In step S12, based on the correlation between the active power received by the sending-end positive and negative converters and the frequency, the equation for the active power-frequency rise characteristic curve of the sending-end positive and negative converters is set as follows: （1）； in, and These are the actual AC frequencies of the positive and negative converters, respectively. and These are the initial values ​​of the frequencies of the positive and negative converters, respectively. and These represent the actual active power transmitted by the positive and negative converters, respectively. and These are the initial values ​​of active power transmitted by the positive and negative converters, respectively. and These are the adjustment coefficients for the active power-frequency rise characteristic curve equations of the positive and negative pole converters, respectively, and their values ​​are positive. The adjustment coefficient for the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter is shown in the following formula: (2)； in, This is the maximum allowable frequency for the sending system. This is the minimum allowable frequency for the sending system. This represents the maximum active power that the positive and negative pole converter can transmit. This is the minimum active power that the positive and negative pole converter can transmit; In step S13, the active power-frequency rise characteristic control block diagram of the sending-end positive and negative pole converter is designed according to the active power-frequency rise characteristic curve equation of the sending-end positive and negative pole converter shown in equation (1).

3. The method for coordinated control of the sending-end bipolar converter in a wind power grid connected to a flexible DC system according to claim 1, characterized in that, In step S21, the reactive power of the sending-end bipolar converter is defined based on the fact that the reactive power output of the sending-end positive and negative pole converters meets the reactive power requirements of the system during normal operation, with the AC side flowing out of the sending-end bipolar converter as the positive direction. In step S22, based on the correlation between the reactive power output of the sending-end positive and negative converters and the voltage amplitude, the reactive power-voltage droop characteristic curve equation of the sending-end positive and negative converters is set as follows: (3)； In the formula, and These are reference values ​​for the AC bus voltage amplitude of the positive and negative converters, respectively. and These are the initial reference values ​​for the amplitude of the AC bus voltage at the positive and negative poles, respectively. and These represent the actual reactive power output of the positive and negative converters, respectively. and These are the initial values ​​of reactive power output from the positive and negative converters, respectively. and These are the adjustment coefficients for the reactive power-voltage droop characteristic curve equations of the positive and negative converters, respectively, and their values ​​are positive. The adjustment coefficient for the reactive power-voltage droop characteristic curve equation of the sending-end positive and negative pole converter is shown in the following formula: (4)； in, This is the maximum allowable voltage at the grid connection point during system operation; This is the minimum allowable voltage at the grid connection point during system operation; This represents the maximum reactive power output of the positive and negative converters. This represents the minimum reactive power output of the positive and negative pole converters. Since the reactive power output of the sending-end positive and negative converters is constrained by the converter's rated capacity and transmitted active power, the maximum and minimum values ​​of the reactive power output of the sending-end positive and negative converters are expressed as follows: (5)； in, This refers to the rated capacity of the positive and negative pole converters. The active power transmitted in real time by the positive and negative pole converters; In step S23, based on the reactive power-voltage droop characteristic curve equation of the positive and negative pole converter shown in equation (3), the reactive power-voltage droop characteristic control block diagram of the sending-end positive and negative pole converter is designed.

4. The method for coordinated control of the sending-end bipolar converter in a wind power grid connected to a flexible DC system according to claim 2, characterized in that, In step S31, according to equation (1), the active power-frequency rise characteristic curve equation of the normal pole converter under special operating conditions with limited capacity is as follows; (6)； in, This is the actual AC frequency of the normal polarity converter; These are the initial values ​​for the frequency of the normal pole converter and the capacity-limited pole converter; This refers to the actual active power transmitted by the normal pole converter; The initial value for the active power transmitted by the normal pole converter and the capacity-limited pole converter; The adjustment coefficient is the equation of the active power-frequency rise characteristic curve of the normal pole converter. According to equation (1), the adjustment coefficient of the active power-frequency rise characteristic curve equation of the normal pole converter is as follows: (7)； In the formula, This represents the maximum transmission capacity of a normal polarity converter. This represents the minimum transmission capacity of a normal polarity converter. The initial value of active power transmitted by normal pole converters and capacity-limited pole converters is related to the maximum transmission capacity of the capacity-limited pole converter, as shown in the following formula; (8)； in, These are the initial coefficients for the capacity-limited pole converter. This represents the maximum transmission capacity of the capacity-limited polarity converter.

5. The method for coordinated control of the sending-end bipolar converter in a wind power grid connected to a flexible DC system according to claim 4, characterized in that, In step S32, according to equation (1), the active power-frequency rise characteristic curve equation of the capacity-limited pole converter is as follows; (9)； In the formula, The actual AC frequency of the capacity-limited pole converter. The actual active power transmitted by the capacity-limited pole converter; This is the adjustment coefficient for the active power-frequency rise characteristic curve equation of a capacity-limited pole converter.

6. The method for coordinated control of the sending-end bipolar converter in a wind power grid connected to a flexible DC system according to claim 5, characterized in that, In step S32, the adjustment coefficient of the active power-frequency rise characteristic curve equation of the capacity-limited pole converter under the special operating condition of limited capacity is set as shown in the following formula; (10)。

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