Coordination control method for sending-end bipolar converter of wind power access flexible direct current system

Abstract

The invention discloses a coordinated control method for a transmitting-end bipolar converter of a wind power access flexible direct current system, and relates to the field of power system protection and control, and the method comprises the following steps: S1, setting an active power-frequency rise characteristic curve equation of a transmitting-end positive and negative converter; s2, setting a reactive power-voltage droop characteristic curve equation of the positive and negative transverters at the sending end; s3, setting an active power-frequency rising characteristic curve of the positive and negative current converters under a special working condition with limited capacity; s4, adjusting the active power balance and voltage stability of the positive and negative transverters of the sending end during the fluctuation period of the wind power output power; and step S5, a coordination process of the frequency of the bipolar converter. According to the coordinated control method for the sending-end bipolar transverter of the wind power access flexible direct current system, balance of active power between positive and negative direct current networks and stability of voltage and frequency of alternating current bus voltage when control value data deviation exists in the control system can be guaranteed.

Description

Technical Field

[0001] The present invention relates to the field of power system protection and control, and in particular to a coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system. Background Art

[0002] Vigorously developing renewable energy is a global consensus for addressing the fossil fuel depletion crisis and achieving energy structural transformation. Leveraging its resource advantages, the "Shagohuang" region in northwest China is being developed as a major new energy base to strongly support the large-scale development and utilization of new energy. The "Shagohuang" region is remote, with minimal local loads for a long time. The regional AC grid lacks synchronous power sources such as traditional thermal power generators, making it a typical sending-end, islanded new energy system. Power must be transmitted to eastern load centers via ultra-high voltage (UHV) long-distance transmission. Against this backdrop, MMC-HVDC technology, with its stable voltage support capabilities and lack of commutation failures, is a mainstream solution for the efficient grid connection and cross-regional consumption of large-scale, islanded wind farms. MMC-HVDC primarily utilizes two wiring methods: pseudo-bipolar and true bipolar. Compared with the single converter configuration scheme in the pseudo-bipolar system, each converter station in the true bipolar system is configured with two converters according to the positive and negative poles. While improving the system transmission capacity, it also enhances the system's multi-objective coordinated control capabilities. It has the advantages of economic flexibility and high reliability in large-capacity power transmission, and has been widely used in direct current transmission projects such as Zhangbei and Baihetan in my country.

[0003] In the transmission of isolated wind power, the control strategy of the sending-end MMC converter station faces two major technical challenges. First, the wind farm's inherent voltage regulation capability is weak, posing a high risk of AC bus voltage instability. Second, the high volatility of wind power output can cause power imbalance between the positive and negative DC lines. This power imbalance can lead to unbalanced currents in the metallic return lines, increasing power losses and potentially inducing broadband oscillations and control failures in the system, adversely affecting the system's voltage-frequency stability. Therefore, research on voltage-frequency support methods and power balance control strategies for the sending-end true bipolar converter station is necessary to ensure the safe and stable operation of the island wind power flexible direct current transmission system. Therefore, research on voltage-frequency support methods and power balance control strategies for the sending-end true bipolar converter station is of great practical significance for ensuring the safe and stable operation of the island wind power flexible direct current transmission system.

[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-pole converter adopts a constant AC voltage and frequency control strategy (VF control), while the other-pole converter adopts an active / reactive decoupling control strategy (PQ control). This type of control scheme is not stable enough in the face of DC single-pole grounding faults, and it is difficult to achieve power balance between the positive and negative DC lines when wind power fluctuates frequently. In the peer-to-peer control scheme, both bipolar converters adopt control strategies with voltage-frequency support capabilities, such as VF control. However, when both poles adopt VF control, the data consistency requirements are high in actual projects. A slight deviation in the control reference may cause an imbalance in the coordination of the positive and negative converters, thereby bringing system stability risks.

[0005] Furthermore, existing research has primarily focused on conditions where the maximum transmission capacity of the sending-end bipolar converters is the same. Coordinated control strategies for situations where the maximum transmission capacity of the bipolar converters differs due to inherent equipment limitations and other factors have not been fully explored. Therefore, it is urgent to address the stability issues caused by the lack of coordination between bipolar converters when there are deviations in the dual VF control targets, as well as the need for bipolar converters to balance the transmission power of the positive and negative lines during wind fluctuations. This requires research on coordinated control strategies between the sending-end positive and negative converters of true bipolar flexible HVDC transmission systems connected to wind farms, to ensure the safe and stable operation of the power system under different operating conditions. Summary of the Invention

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

[0007] The present invention provides a coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system, comprising the following steps:

[0008] Step S1, setting the active power-frequency rising characteristic curve equation of the positive and negative converters at the sending end;

[0009] Step S11, defining the positive direction of the active power of the positive and negative converters at the sending end;

[0010] Step S12: setting the active power-frequency rising characteristic curve of the positive and negative converters at the sending end;

[0011] Step S13: designing a control block diagram of active power-frequency rise characteristics of the positive and negative converters at the sending end;

[0012] Step S2, setting reactive power-voltage droop characteristic curve equations of the positive and negative converters at the sending end;

[0013] Step S21, defining the positive direction of reactive power of the positive and negative converters at the sending end;

[0014] Step S22, setting reactive power-voltage droop characteristic curves of the positive and negative converters at the sending end;

[0015] Step S23, designing a reactive power-voltage droop characteristic control block diagram of the positive and negative converters at the sending end;

[0016] Step S3, setting the active power-frequency rising characteristic curve of the positive and negative converters under special working conditions with limited capacity;

[0017] Step S31, setting an active power-frequency rising characteristic curve of a normal-pole converter under a special working condition with limited capacity;

[0018] Step S32, setting an active power-frequency rising characteristic curve of the capacity-limited pole converter under a special working condition with limited capacity;

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

[0020] Step S41, active power balancing process of the positive and negative converters at the sending end;

[0021] Step S42, AC bus voltage stabilization process of the positive and negative converters at the sending end;

[0022] Step S5: When there is an initial value deviation between the positive and negative polarity converter frequencies at the sending end, the frequency coordination process of the bipolar converter is carried out.

[0023] Preferably, in step S11, the active power of the sending-end bipolar converter is defined according to the active power transmitted from the wind farm to the sending-end converter station when the sending-end positive and negative converters are operating normally, with the active power flowing into the AC side of the sending-end positive and negative converters being 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 active power-frequency rising characteristic curve equations of the sending-end positive and negative converters are set, as shown in the following equation:

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

[0026] Among them, f1 and f2 are the actual AC frequencies of the positive and negative converters respectively; f 01 and f 02 are the initial values ​​of the positive and negative converter frequencies respectively; P1 and P2 are the actual active powers transmitted by the positive and negative converters respectively; P01 and P 02 are the initial values ​​of active power transmitted by the positive and negative converters respectively; k P1 and k P2 are the adjustment coefficients of the active power-frequency rising characteristic curve equations of the positive and negative converters, respectively, and are positive;

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

[0028]

[0029] Among them, f max is the maximum value allowed by the sending end system frequency; f min P is the minimum value allowed by the sending system frequency; maxi is the maximum value of active power that can be transmitted by the positive and negative converters; P mini The minimum value of active power that can be transmitted by the positive and negative converters;

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

[0031] Preferably, in step S21, the reactive power of the sending-end bipolar converter is defined based on the reactive power output by the sending-end positive and negative converters during normal operation to meet the reactive power demand of the system, with the reactive power flowing out of the AC side of the sending-end bipolar converter being the positive direction;

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

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

[0034] Where u ref1 and u ref2 are the reference values ​​of the AC bus voltage amplitude of the positive and negative converters respectively; u 01 and u 02 are the initial reference values ​​of the positive and negative AC bus voltage amplitudes respectively; Q1 and Q2 are the reactive powers actually output by the positive and negative converters respectively; Q 01 and Q 02 are the initial values ​​of reactive power output by the positive and negative converters respectively; k Q1 and k Q2are the adjustment coefficients of the reactive power-voltage droop characteristic curve equations of the positive and negative converters, respectively, and are positive;

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

[0036]

[0037] Among them, u max The maximum value allowed for the grid connection point voltage when the system is running; u min The minimum value allowed for the grid connection point voltage when the system is running; Q maxi The maximum value of the reactive power output by the positive and negative converters; Q mini The minimum value of reactive power output by the positive and negative converters;

[0038] Since the reactive power output by the positive and negative converters at the sending end is constrained by the rated capacity of the converter and the transmitted active power, the maximum and minimum expressions for the reactive power output by the positive and negative converters at the sending end are as follows:

[0039]

[0040] Among them, S i is the rated capacity of the positive and negative converters; P i Active power transmitted in real time by the positive and negative converters;

[0041] In step S23, according to the reactive power-voltage droop characteristic curve equation of the positive and negative converters shown in formula (3), a reactive power-voltage droop characteristic control block diagram of the sending-end positive and negative converters is designed.

[0042] Preferably, in step S31, according to formula (1), the active power-frequency rising characteristic curve equation of the normal pole converter under the special working condition of limited capacity is as shown below:

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

[0044] Among them, f Z is the actual AC frequency of the normal pole converter; f S is the initial value of the frequency of the normal pole converter and the capacity limited pole converter; P Z is the actual active power transmitted by the normal pole converter; P S is the initial value of the active power transmitted by the normal pole converter and the capacity-limited pole converter; k PZis the adjustment coefficient of the active power-frequency rising characteristic curve equation of the normal pole converter;

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

[0046]

[0047] Where, P maxZ is the maximum transmission capacity of the normal pole converter; P min is the minimum transmission capacity of the normal pole converter;

[0048] The initial value of the active power transmitted by the normal pole converter and the capacity-limited pole converter 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] Among them, k S is the initial coefficient of the capacity-limited pole converter, P maxF is the maximum transmission capacity of the capacity-limited pole converter.

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

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

[0053] Where, f F is the actual AC frequency of the capacity-limited converter, P F is the actual active power transmitted by the capacity-limited pole converter; k PF It is the adjustment coefficient of the active power-frequency rising characteristic curve equation of the capacity-limited pole converter.

[0054] Preferably, in step S32, the adjustment coefficient of the active power-frequency rising characteristic curve equation of the capacity-limited pole converter under the special working condition of limited capacity is set as shown in the following formula:

[0055]

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

[0057]

[0058] Where: θ MMC1 and θ MMC2 are the phases of the AC voltages of the positive and negative converters, respectively; θ0 is the initial phase of the AC voltages of the positive and negative converters; 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:

[0059]

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

[0061] Preferably, in step S42, when both the sending-end positive and negative converters adopt the reactive power-voltage droop control strategy, the control target expression of the voltage outer loop is as shown in the following formula:

[0062]

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

[0064] According to the converter reactive power transfer equation shown in formula (14), it is as follows:

[0065]

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

[0067] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 A schematic diagram of a two-terminal MMC-HVDC system connected to a wind farm according to a coordinated control method of a sending-end bipolar converter of a wind power connected to a flexible direct current system of the present invention;

[0069] Figure 2 This is a graph showing the active power-frequency rise characteristic curve of the sending-end positive and negative converters in a coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to the present invention;

[0070] Figure 3 This is a control block diagram of the active power-frequency rise characteristic of the sending-end positive and negative converters of a sending-end bipolar converter coordinated control method for a wind power access flexible direct current system according to the present invention;

[0071] Figure 4 This is a graph showing reactive power-voltage droop characteristics of the sending-end positive and negative converters of a coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to the present invention;

[0072] Figure 5 This is a block diagram of the PF rise / QV droop control of the sending-end positive and negative converters of a coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to the present invention;

[0073] Figure 6 This is a graph showing the active power-frequency rising characteristic curve of the sending-end positive and negative converters under capacity-limited working conditions in a coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to the present invention;

[0074] Figure 7 This is a diagram of the active power balance process of the positive and negative converters at the sending end of a coordinated control method for bipolar converters at the sending end of a wind power access flexible direct current system according to the present invention;

[0075] Figure 8 This is a sending-end positive and negative converter AC bus voltage stabilization process of a sending-end bipolar converter coordinated control method for a wind power access flexible direct current system according to the present invention;

[0076] Figure 9 This is a diagram of the frequency coordination process of the positive and negative converters at the sending end of a coordinated control method for bipolar converters at the sending end of a wind power access flexible direct current system according to the present invention;

[0077] Figure 10 This is an overall flow chart of a coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to the present invention. DETAILED DESCRIPTION

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

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

[0080] The words "first", "second" and similar terms used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Words such as "include" or "comprise" mean that the elements or objects preceding the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative position relationships. When the absolute position of the object being described changes, the relative position 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 sending-end bipolar converter of a wind power access flexible direct current system, comprising the following steps:

[0083] Step S1, setting the active power-frequency rising characteristic curve equation of the positive and negative converters at the sending end;

[0084] Step S11, defining the positive direction of the active power of the positive and negative converters at the sending end;

[0085] In step S11, the active power of the sending-end bipolar converter is defined according to the active power transmitted from the wind farm to the sending-end converter station when the sending-end positive and negative converters are operating normally, with the active power flowing into the AC side of the sending-end positive and negative converters as the positive direction.

[0086] Step S12: setting the active power-frequency rising characteristic curve of the positive and negative converters at the sending end;

[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 active power-frequency rising characteristic curve equation of the sending-end positive and negative converters is set as shown below: The corresponding rising characteristic curve is as follows: Figure 2 As shown;

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

[0089] Among them, f1 and f2 are the actual AC frequencies of the positive and negative converters respectively; f 01 and f 02 are the initial values ​​of the positive and negative converter frequencies respectively; P1 and P2 are the actual active powers transmitted by the positive and negative converters respectively; P 01 and P 02are the initial values ​​of active power transmitted by the positive and negative converters respectively; k P1 and k P2 They are the adjustment coefficients of the active power-frequency rising characteristic curve equations of the positive and negative pole converters, respectively, and are positive.

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

[0091]

[0092] Among them, f max is the maximum value allowed by the sending end system frequency; f min P is the minimum value allowed by the sending system frequency; maxi is the maximum value of active power that can be transmitted by the positive and negative converters; P mini It is the minimum value of active power that can be transmitted by the positive and negative converters.

[0093] Step S13: designing a control block diagram of active power-frequency rise characteristics of the positive and negative converters at the sending end;

[0094] In step S13, according to the active power-frequency rising characteristic curve equation of the sending-end positive and negative converters shown in formula (1), the active power-frequency rising characteristic control block diagram of the sending-end positive and negative converters is designed. Figure 3 shown.

[0095] Step S2, setting reactive power-voltage droop characteristic curve equations of the positive and negative converters at the sending end;

[0096] Step S21, defining the positive direction of reactive power of the positive and negative converters at the sending end;

[0097] In step S21, the reactive power of the sending-end bipolar converter is defined based on the reactive power output by the sending-end positive and negative converters during normal operation to meet the reactive power demand of the system, with the reactive power flowing out of the AC side of the sending-end bipolar converter being the positive direction.

[0098] Step S22, setting reactive power-voltage droop characteristic curves of the positive and negative converters at the sending end;

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

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

[0101] Where u ref1 and u ref2 are the reference values ​​of the AC bus voltage amplitude of the positive and negative converters respectively; u 01 and u 02 are the initial reference values ​​of the positive and negative AC bus voltage amplitudes respectively; Q1 and Q2 are the reactive powers actually output by the positive and negative converters respectively; Q 01 and Q 02 are the initial values ​​of reactive power output by the positive and negative converters respectively; k Q1 and k Q2 They are the adjustment coefficients of the reactive power-voltage droop characteristic curve equations of the positive and negative pole converters, respectively, and are positive.

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

[0103]

[0104] Among them, u max The maximum value allowed for the grid connection point voltage when the system is running; u min The minimum value allowed for the grid connection point voltage when the system is running; Q maxi The maximum value of the reactive power output by the positive and negative converters; Q mini It is the minimum value of reactive power output by the positive and negative converters.

[0105] Since the reactive power output by the positive and negative converters at the sending end is constrained by the rated capacity of the converter and the transmitted active power, the maximum and minimum expressions for the reactive power output by the positive and negative converters at the sending end are as follows:

[0106]

[0107] Among them, S i is the rated capacity of the positive and negative converters; P i It is the active power transmitted in real time by the positive and negative converters.

[0108] Step S23: Design a reactive power-voltage droop characteristic control block diagram for the positive and negative converters at the sending end.

[0109] In step S23, according to the reactive power-voltage droop characteristic curve equation of the positive and negative converters shown in formula (3), the reactive power-voltage droop characteristic control block diagram of the sending-end positive and negative converters is designed. Figure 5 shown.

[0110] Step S3: setting the active power-frequency rising characteristic curves of the positive and negative converters under special working conditions with limited capacity.

[0111] Step S31 : setting an active power-frequency rising characteristic curve of a normal-pole converter under a special working condition with limited capacity.

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

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

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

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

[0116]

[0117] Where, P maxZ is the maximum transmission capacity of the normal pole converter; P min is the minimum transmission capacity of the normal pole converter.

[0118] The initial value of the active power transmitted by the normal pole converter and the capacity-limited pole converter 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] Among them, k S is the initial coefficient of the capacity-limited pole converter, ranging from 0.9 to 0.95; P maxF is the maximum transmission capacity of the capacity-limited pole converter.

[0121] Step S32: setting an active power-frequency rising characteristic curve of the capacity-limited pole converter under a special working condition of limited capacity.

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

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

[0124] Where, f F is the actual AC frequency of the capacity-limited converter, P F is the actual active power transmitted by the capacity-limited pole converter; k PF It is the adjustment coefficient of the active power-frequency rising characteristic curve equation of the capacity-limited pole converter.

[0125] In step S32, the adjustment coefficient of the active power-frequency rising characteristic curve equation of the capacity-limited pole converter under the special working 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 positive and negative converters at the sending end during wind power output power fluctuations.

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

[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 as shown in formula (11), the AC voltage phase of the positive converter will lead that of the negative converter, as shown in the following formula:

[0130]

[0131] Where: θ MMC1 and θ MMC2 are the phases of the AC voltages of the positive and negative converters, respectively; θ0 is the initial phase of the AC voltages of the positive and negative converters. When the AC voltage phase of the positive converter leads that of the negative converter, according to the relationship between active power and the AC voltage phase 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 approaches balance, as shown in the following equation:

[0132]

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

[0134] Step S42: The AC bus voltage of the sending-end positive and negative converters is stabilized.

[0135] In step S42, the positive and negative converters at the sending end are both Figure 5 When the reactive power-voltage droop control strategy is used, the control target expression of the voltage outer loop is as follows:

[0136]

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

[0138] According to the reactive power transmission equation of the converter shown in formula (14), as shown in the following formula: the increase 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 formula (13), the AC bus voltage reference values ​​of the positive and negative converters tend to be consistent, and the positive and negative converters control the AC bus voltages to tend to the same value, achieving stability.

[0139]

[0140] Step S5: When there is an initial value deviation between the positive and negative polarity converter frequencies at the sending end, the frequency coordination process of the bipolar converter is carried out;

[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 leading AC voltage phase will cause the active power transmitted by the wind farm to the positive converter to decrease, while the active power transmitted to the negative converter to increase;

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

[0143] Therefore, the present invention adopts the above-mentioned coordinated control method for the sending-end bipolar converter of a wind power access flexible direct current system. Considering that the sending-end positive and negative converters connected to the wind farm receive active power and output reactive power during normal operation, the positive flow direction of the active power and reactive power of the sending-end positive and negative converters is defined respectively. The active power distribution of the bipolar converter is realized by establishing a response equation for the active power-frequency rise characteristic based on the relationship between active power transmission and voltage phase. The reactive power distribution of the bipolar converter is realized by establishing a response equation for the reactive power-voltage droop characteristic based on the relationship between reactive power transmission and voltage amplitude. Furthermore, for special operating conditions where the capacity of a certain pole converter at the sending-end converter station is limited, the proposed active power-frequency rise / reactive power-voltage droop control method can effectively avoid converter capacity exceeding the 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 rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can still be modified or replaced by equivalents, and these modifications or equivalent replacements 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 bipolar converters at the sending end of a wind power access flexible direct current system, characterized in that: The following steps are involved: Step S1, setting the active power-frequency rising characteristic curve equation of the positive and negative converters at the sending end; Step S11, defining the positive direction of the active power of the positive and negative converters at the sending end; Step S12: setting the active power-frequency rising characteristic curve of the positive and negative converters at the sending end; Step S13: designing a control block diagram of active power-frequency rise characteristics of the positive and negative converters at the sending end; Step S2, setting reactive power-voltage droop characteristic curve equations of the positive and negative converters at the sending end; Step S21, defining the positive direction of reactive power of the positive and negative converters at the sending end; Step S22, setting reactive power-voltage droop characteristic curves of the positive and negative converters at the sending end; Step S23, designing a reactive power-voltage droop characteristic control block diagram of the positive and negative converters at the sending end; Step S3, setting the active power-frequency rising characteristic curve of the positive and negative converters under special working conditions with limited capacity; Step S31, setting an active power-frequency rising characteristic curve of a normal-pole converter under a special working condition with limited capacity; Step S32, setting an active characteristic curve of the capacity-limited pole converter under a special working condition with limited capacity; Step S4: Active power balance and voltage stabilization regulation process of the positive and negative converters at the sending end during wind power output power fluctuations; Step S41, active power balancing process of the positive and negative converters at the sending end; Step S42, AC bus voltage stabilization process of the positive and negative converters at the sending end; Step S5: When there is an initial value deviation between the positive and negative polarity converter frequencies at the sending end, the frequency coordination process of the bipolar converter is carried out.

2. The coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to claim 1 is characterized in that: In step S11, the active power of the sending-end bipolar converter is defined according to the active power transmitted from the wind farm to the sending-end converter station when the sending-end positive and negative converters are operating normally, with the active power flowing into the AC side of the sending-end positive and negative converters being 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 active power-frequency rising characteristic curve equations of the sending-end positive and negative converters are set, as shown in the following equation: f i =f 0i +k Pi (P i -P 0i ) (i=1,2) (1); Among them, f1 and f2 are the actual AC frequencies of the positive and negative converters respectively; f 01 and f 02 are the initial values ​​of the positive and negative converter frequencies respectively; P1 and P2 are the actual active powers transmitted by the positive and negative converters respectively; P 01 and P 02 are the initial values ​​of active power transmitted by the positive and negative converters respectively; k P1 and k P2 are the adjustment coefficients of the active power-frequency rising characteristic curve equations of the positive and negative converters, respectively, and are positive; The adjustment coefficient of the active power-frequency rise characteristic curve equation of the positive and negative converters at the sending end is shown in the following formula: Among them, f max is the maximum value allowed by the sending end system frequency; f min P is the minimum value allowed by the sending system frequency; maxi is the maximum value of active power that can be transmitted by the positive and negative converters; P mini is the minimum value of active power that can be transmitted by the positive and negative converters; In step S13, according to the active power-frequency rise characteristic curve equation of the sending-end positive and negative converters shown in formula (1), the active power-frequency rise characteristic control block diagram of the sending-end positive and negative converters is designed.

3. The coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to claim 1 is characterized in that: In step S21, the reactive power of the sending-end bipolar converter is defined based on the reactive power output by the sending-end positive and negative converters during normal operation to meet the reactive power demand of the system, with the reactive power flowing out of the AC side of the sending-end bipolar converter being the positive direction; In step S22, based on the correlation between the reactive power output by the sending-end positive and negative converters and the voltage amplitude, the reactive power-voltage droop characteristic curve equations of the sending-end positive and negative converters are set, as shown in the following equation: u refi =u 0i -k Qi ·(Q i -Q 0i ) (i=1,2) (3); Where u ref1 and u ref2 are the reference values ​​of the AC bus voltage amplitude of the positive and negative converters respectively; u 01 and u 02 are the initial reference values ​​of the positive and negative AC bus voltage amplitudes respectively; Q1 and Q2 are the reactive powers actually output by the positive and negative converters respectively; Q 01 and Q 02 are the initial values ​​of reactive power output by the positive and negative converters respectively; k Q1 and k Q2 are the adjustment coefficients of the reactive power-voltage droop characteristic curve equations of the positive and negative converters, respectively, and are positive; The adjustment coefficient of the reactive power-voltage droop characteristic curve equation of the positive and negative converters at the sending end is shown in the following formula: Among them, u max The maximum value allowed for the grid connection point voltage when the system is running; u min The minimum value allowed for the grid connection point voltage when the system is running; Q maxi The maximum value of the reactive power output by the positive and negative converters; Q mini The minimum value of reactive power output by the positive and negative converters; Since the reactive power output by the positive and negative converters at the sending end is constrained by the rated capacity of the converter and the transmitted active power, the maximum and minimum expressions for the reactive power output by the positive and negative converters at the sending end are as follows: Among them, S i is the rated capacity of the positive and negative converters; P i Active power transmitted in real time by the positive and negative converters; In step S23, according to the reactive power-voltage droop characteristic curve equation of the positive and negative converters shown in formula (3), a reactive power-voltage droop characteristic control block diagram of the sending-end positive and negative converters is designed.

4. The coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to claim 1, characterized in that: In step S31, according to formula (1), the active power-frequency rising characteristic curve equation of the normal pole converter under the special working condition of limited capacity is as shown below: f Z =f S +k PZ (P Z -P S ) (6); Among them, f Z is the actual AC frequency of the normal pole converter; f S is the initial value of the frequency of the normal pole converter and the capacity limited pole converter; P Z is the actual active power transmitted by the normal pole converter; P S is the initial value of the active power transmitted by the normal pole converter and the capacity-limited pole converter; k PZ is the adjustment coefficient of the active power-frequency rising characteristic curve equation of the normal pole converter; According to formula (2), the adjustment coefficient of the active power-frequency rising characteristic curve equation of the normal pole converter is as follows: Where, P maxZ is the maximum transmission capacity of the normal pole converter; P min is the minimum transmission capacity of the normal pole converter; The initial value of the active power transmitted by the normal pole converter and the capacity-limited pole converter is related to the maximum transmission capacity of the capacity-limited pole converter, as shown in the following formula; P S =k S P maxF (8); Among them, k S is the initial coefficient of the capacity-limited pole converter, P maxF is the maximum transmission capacity of the capacity-limited pole converter.

5. The coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to claim 1 is characterized in that: In step S32, according to formula (1), the active power-frequency rising characteristic curve equation of the capacity-limited pole converter is as shown below: f F =f S +k PF (P F -P S ) (9); Where, f F is the actual AC frequency of the capacity-limited converter, P F is the actual active power transmitted by the capacity-limited pole converter; k PF It is the adjustment coefficient of the active power-frequency rising characteristic curve equation of the capacity-limited pole converter.

6. The coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to claim 1, characterized in that: In step S32, the adjustment coefficient of the active power-frequency rising characteristic curve equation of the capacity-limited pole converter under the special working condition of limited capacity is set, as shown in the following formula; 7. The coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to claim 1, characterized in that: In step S41, according to the relationship between frequency and phase as shown in formula (11), the AC voltage phase of the positive converter will lead that of the negative converter, as shown in the following formula: Where: θ MMC1 and θ MMC2 are the phases of the AC voltages of the positive and negative converters, respectively; θ0 is the initial phase of the AC voltages of the positive and negative converters; 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: Among them, U WF is the voltage at the outlet of the wind farm; U MCC1 and U MCC2 are the positive and negative converter AC voltages respectively; θ WF is the voltage phase at the outlet of the wind farm; X1 and X2 are the equivalent reactances from the wind farm to the positive and negative converters, respectively.

8. The coordinated control method for a sending-end bipolar converter of a wind power access flexible direct current system according to claim 1 is characterized in that: In step S42, when both the positive and negative converters at the sending end adopt the reactive power-voltage droop control strategy, the control target expression of the voltage outer loop is as shown in the following formula: Where: u sdref1 and u sdref2 are the reference values ​​of the d-axis voltage of the positive and negative converters at the sending end; u sqref1 and u sqref2 are the reference values ​​of the q-axis voltage of the positive and negative converters at the sending end, respectively; According to the converter reactive power transfer equation shown in formula (14), it is as follows:

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