A closed-loop pre-charge control method for a fan grid-connected system based on a hexagon converter

Abstract

The application discloses a kind of based on hexagon converter's fan grid-connected system closed loop pre-charge control method.The fan grid-connected system includes: wind turbine, hexagon converter, direct-drive permanent magnet synchronous generator, contactor KM1, contactor KM2, current-limiting resistance R1, current-limiting resistance R2, current-limiting resistance R3 and alternating current network;Closed loop pre-charge control method is composed of uncontrolled charging stage and closed loop controllable charging stage, according to the capacity of converter, self-setting controllable charging stage charging power, bridge arm current reference value is calculated from charging power, quasi-PR controller is used to track bridge arm current reference value and obtain bridge arm voltage modulation wave, voltage control is used to obtain the superposition amount of modulation wave of full-bridge sub-module, bridge arm voltage modulation wave and the superposition amount of modulation wave of full-bridge sub-module are added to obtain the modulation wave of full-bridge sub-module, carrier phase-shifted modulation is used to obtain the switching signal of full-bridge sub-module, when the average value of full-bridge sub-module capacitor voltage reaches rated value, controllable charging is completed;The application provides a pre-charge topological structure of fan grid-connected system based on hexagon converter, and the closed loop pre-charge control method has the advantages of constant current charging, no inrush current, controllable charging time, simple control, and no need to additionally increase controller.

Description

Technical Field

[0001] This invention belongs to the field of AC / AC converter pre-charging, and specifically relates to a closed-loop pre-charging control method for wind turbine grid-connected systems based on hexagonal converters. Background Technology

[0002] Promoting energy transformation and establishing a new power system based on green and clean energy is imperative.

[0003] my country has a vast territory and abundant coastline, with high-quality wind energy resources in the Northeast, North China, and Northwest regions, as well as along the coast. As a sustainable and green clean energy source, wind power has become a research hotspot, with research on how to achieve stable and efficient grid connection gradually emerging.

[0004] Traditional wind power grid connection structures often use AC / DC / AC configurations. The electricity generated by the wind turbine usually needs to undergo two stages of conversion and a step-up transformer before it can be connected to the grid. The use of two-stage converters and step-up transformers increases system losses. Hexagonal converters are direct AC / AC converters that can enable wind power to be directly connected to the grid after one stage of conversion, eliminating the need for bulky and heavy step-up transformers. They also have good low-frequency characteristics and have broad application prospects in the wind power field.

[0005] Before a hexagonal converter can operate stably, all full-bridge submodule capacitors must be charged to their rated voltages. Otherwise, forced startup will cause overcurrent damage to components, and in severe cases, system failure. However, the dispersed and numerous submodule capacitors of a hexagonal converter present challenges for pre-charging. Currently, there is no specific pre-charging method for hexagonal converters. Existing pre-charging methods mainly focus on modular multilevel converters (MMCs) and modular multilevel converters (MMMCs). Among these, open-loop group pre-charging is the most common, but this method has a long charging time and is not suitable for applications with limited startup time. A few closed-loop pre-charging methods proposed for half-bridge MMCs can indeed achieve controllable charging current, but they require the design of an additional pre-charging controller, increasing costs. Summary of the Invention

[0006] To address the shortcomings and deficiencies described in the background art above, this invention proposes a closed-loop pre-charge control method for wind turbine grid-connected systems based on hexagonal converters. This method has the advantages of no surge charging current, controllable charging time, good consistency of sub-modules, and no need to add an additional pre-charge controller.

[0007] The technical solution provided by this invention is as follows:

[0008] A closed-loop pre-charge control method for a wind turbine grid-connected system based on a hexagonal converter is characterized in that the wind turbine grid-connected system based on the hexagonal converter consists of a wind turbine, a hexagonal converter, a direct-drive permanent magnet synchronous generator, contactors KM1 and KM2, current-limiting resistors R1, R2, and R3, and an AC power grid.

[0009] The hexagonal converter consists of six identical bridge arms connected end-to-end to form a hexagon; each bridge arm is composed of m cascaded full-bridge submodules and an AC reactor connected in series; the AC reactor is denoted as L. arm_k k = 1, 2, 3, 4, 5, 6, representing the k-th bridge arm; the full-bridge submodule is denoted as SM. k _ j The subscript j = 1, 2, ..., m indicates the j-th full-bridge submodule; each full-bridge submodule consists of four IGBTs S1, S2, S3, and S4 with anti-parallel freewheeling diodes and one DC floating capacitor C; the collectors of S1 and S3 and the positive terminal of the DC floating capacitor C are connected to form the positive terminal of the full-bridge submodule; the emitters of S2 and S4 and the negative terminal of the DC floating capacitor C are connected to form the negative terminal of the full-bridge submodule; the positive port of the full-bridge submodule is formed by the connection point of the emitter of S1 and the collector of S2, and the negative port of the full-bridge submodule is formed by the connection point of the emitter of S3 and the collector of S4;

[0010] The L1 port of contactor KM1 is connected to the U phase of the AC power grid, and the T1 port of contactor KM1 is connected to point o1 of the hexagonal converter; the L2 port of contactor KM1 is connected to the V phase of the AC power grid, and the T2 port of contactor KM1 is connected to point o5 of the hexagonal converter; the L3 port of contactor KM1 is connected to the W phase of the AC power grid, and the T3 port of contactor KM1 is connected to point o3 of the hexagonal converter.

[0011] One end of the current-limiting resistor R1 is connected to the L1 port of contactor KM1, and the other end of the current-limiting resistor R1 is connected to the T1 port of contactor KM1; one end of the current-limiting resistor R2 is connected to the L2 port of contactor KM1, and the other end of the current-limiting resistor R2 is connected to the T2 port of contactor KM1; one end of the current-limiting resistor R3 is connected to the L3 port of contactor KM1, and the other end of the current-limiting resistor R3 is connected to the T3 port of contactor KM1.

[0012] The wind turbine's shaft is coaxially connected to the rotor of the direct-drive permanent magnet synchronous generator. The three windings of the stator of the direct-drive permanent magnet synchronous generator are denoted as phases A, B, and C, respectively.

[0013] The L1 port of contactor KM2 is connected to the stator A of the direct-drive permanent magnet synchronous generator, and the T1 port of contactor KM2 is connected to point o6 of the hexagonal converter; the L2 port of contactor KM2 is connected to the stator B of the direct-drive permanent magnet synchronous generator, and the T2 port of contactor KM2 is connected to point o2 of the hexagonal converter; the L3 port of contactor KM2 is connected to the stator C of the direct-drive permanent magnet synchronous generator, and the T3 port of contactor KM2 is connected to point o4 of the hexagonal converter.

[0014] The bridge arm between points o6 and o1 of the hexagonal converter is denoted as bridge arm 1, the bridge arm between points o1 and o2 of the hexagonal converter is denoted as bridge arm 2, the bridge arm between points o2 and o3 of the hexagonal converter is denoted as bridge arm 3, the bridge arm between points o3 and o4 of the hexagonal converter is denoted as bridge arm 4, the bridge arm between points o4 and o5 of the hexagonal converter is denoted as bridge arm 5, and the bridge arm between points o5 and o6 of the hexagonal converter is denoted as bridge arm 6.

[0015] The aforementioned closed-loop pre-charge control method for a wind turbine grid-connected system based on a hexagonal converter consists of an uncontrolled charging stage and a closed-loop controllable charging stage.

[0016] The uncontrolled charging phase consists of the following steps:

[0017] (1) Disconnect contactor KM2 and contactor KM1, block the IGBT switch signal, and the AC power grid charges the hexagonal converter uncontrolled through the current limiting resistor;

[0018] (2) Calculate the target value U1 of the capacitor voltage of the full-bridge submodule of the hexagonal converter during the uncontrolled charging stage:

[0019] U1=0.93*sqrt(3)×U g / 2 / m

[0020] Where sqrt() represents the square root function, U g It is the amplitude of the phase voltage of the AC power grid;

[0021] (3) Real-time detection of the capacitor voltage of the full-bridge submodule of the hexagonal converter, and calculation of the average value U of the capacitor voltage of the full-bridge submodule of the hexagonal converter. ave , when U ave When the value is greater than or equal to the target value U1, the uncontrolled charging phase ends.

[0022] The closed-loop controllable charging stage consists of the following steps:

[0023] (1) Close contactor KM1;

[0024] (2) Take the reference value i of the q-axis current of the power grid gq_ref =0, calculate the reference value i for the d-axis current of the power grid. gd_ref:

[0025] i gd_ref =2×P / 3 / U g

[0026] Where P is the power transmitted by the AC power grid, which can be set by the user;

[0027] (3) for i gd_ref i gq_ref = 0 Using dq / abc coordinate transformation, the reference values ​​of the three-phase currents of the AC power grid are obtained: i gu_ref i gv_ref and i gw_ref ;

[0028] (4) Change i gu_ref i gv_ref and i gw_ref Substitute into the following formula to calculate the current reference value i of bridge arm k. k_ref :

[0029] i 1_ref = i 6_ref = 1 / 3×(i gu_ref -i gv_ref )

[0030] i 2_ref = i 3_ref = 1 / 3×(i gw_ref -i gu_ref )

[0031] i 4_ref = i 5_ref = 1 / 3×(i gv_ref -i gw_ref )

[0032] (5) Sample the actual current value i of each bridge arm k , change i k_ref with i k The difference is substituted into the quasi-PR regulator to obtain the modulation wave U of bridge arm k. k_ref :

[0033] U k_ref = (i k_ref – i k ) ×(K p1 + K sc1 ×s / (s 2 +2×ω sc1 ×s+(100π) 2 ))

[0034] Where K p1 K is the proportional coefficient of the first quasi-PR regulator. sc1 ωsc1 These are the resonant coefficient and cutoff angular frequency of the first quasi-PR regulator, respectively.

[0035] (6) Calculate the average value U of the submodule capacitor voltage of bridge arm k. k_ave :

[0036] U k_ave =1 / m×(U k1 + U k2 + …+ U k6 )

[0037] Among them U kj The sampled value of the capacitor voltage of the j-th submodule of bridge arm k;

[0038] (7) Change U k_ave respectively with U kj After substituting the difference into the P controller, the output of the P controller is equal to i. k Multiplying them together yields the superposition of the modulated waves in the j-th submodule of bridge arm k. kj_add :

[0039] U kj_add = (U k_ave -U kj )×i k ×K p2

[0040] Where K p2 It is the proportional coefficient of the first P regulator;

[0041] (8) Will U k_ref and U kj_add The modulation wave U of the j-th submodule of bridge arm k is obtained by superposition. kj_ref :

[0042] U kj_ref = U k_ref + U kj_add

[0043] (9) Modulate the wave U kj_ref The switching signals of the full-bridge submodule IGBT are obtained by carrier phase-shift modulation;

[0044] (10) Real-time U ave Reference value U of the capacitor voltage of the full-bridge submodule c_ref When comparing, when U ave Greater than or equal to U c_ref At this time, the controllable charging phase ends; contactor KM2 is closed, the wind turbine is connected to the system, and wind turbine grid connection control based on hexagonal converter is executed;

[0045] The beneficial effects of this invention are: 1) It presents a pre-charging topology for a wind turbine grid-connected system based on a hexagonal converter, and proposes a closed-loop pre-charging control method on the AC side for this structure, which can achieve a constant charging current for the converter without surge current; 2) The charging power can be set according to the device capacity, and the charging time can be calculated when the charging power is determined, thus meeting the requirements of some applications with fast fault recovery; 3) The control is simple, and only a quasi-PR controller is needed to achieve closed-loop pre-charging, and this controller also works when the wind turbine is running in grid connection, without adding an additional controller. Attached Figure Description

[0046] Figure 1 Diagram of the pre-charging structure of a wind turbine grid-connected system based on a hexagonal converter;

[0047] Figure 2 This is a block diagram of the closed-loop pre-charge control for a wind turbine grid-connected system based on a hexagonal converter.

[0048] Figure 3 The current waveform of arm 1 of the hexagonal converter bridge;

[0049] Figure 4 This is the capacitor voltage waveform of the full-bridge submodule. Detailed Implementation

[0050] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific examples described herein are merely illustrative and not intended to limit the invention.

[0051] Figure 1 This is a pre-charging structure diagram of a wind turbine grid-connected system based on a hexagonal converter. In this invention, the wind turbine grid-connected system consists of a wind turbine, a hexagonal converter, a direct-drive permanent magnet synchronous generator, contactors KM1 and KM2, current-limiting resistors R1, R2, and R3, and an AC power grid. The hexagonal converter consists of six identical bridge arms connected end-to-end to form a hexagon. Each bridge arm consists of m cascaded full-bridge submodules and an AC reactor connected in series. The AC reactor is denoted as L. arm_k k = 1, 2, 3, 4, 5, 6, representing the k-th bridge arm; the full-bridge submodule is denoted as SM. k _ jThe subscript j = 1, 2, ..., m indicates the j-th full-bridge submodule; each full-bridge submodule consists of four IGBTs S1, S2, S3, and S4 with anti-parallel freewheeling diodes and one DC floating capacitor C; the collectors of S1 and S3 and the positive terminal of the DC floating capacitor C are connected to form the positive terminal of the full-bridge submodule; the emitters of S2 and S4 and the negative terminal of the DC floating capacitor C are connected to form the negative terminal of the full-bridge submodule; the positive port of the full-bridge submodule is formed by the connection point of the emitter of S1 and the collector of S2, and the negative port of the full-bridge submodule is formed by the connection point of the emitter of S3 and the collector of S4; the L1 port of contactor KM1 is connected to the U phase of the AC power grid, the T1 port of contactor KM1 is connected to point o1 of the hexagonal converter; the L2 port of contactor KM1 is connected to the AC power grid... Phase V is connected; the T2 port of contactor KM1 is connected to point o5 of the hexagonal converter; the L3 port of contactor KM1 is connected to phase W of the AC power grid; the T3 port of contactor KM1 is connected to point o3 of the hexagonal converter; one end of current-limiting resistor R1 is connected to the L1 port of contactor KM1, and the other end of current-limiting resistor R1 is connected to the T1 port of contactor KM1; one end of current-limiting resistor R2 is connected to the L2 port of contactor KM1, and the other end of current-limiting resistor R2 is connected to the T2 port of contactor KM1; one end of current-limiting resistor R3 is connected to the L3 port of contactor KM1. The other end of the current-limiting resistor R3 is connected to the T3 port of contactor KM1; the wind turbine shaft is coaxially connected to the rotor of the direct-drive permanent magnet synchronous generator, and the three windings of the stator of the direct-drive permanent magnet synchronous generator are denoted as phases A, B, and C respectively; the L1 port of contactor KM2 is connected to phase A of the stator of the direct-drive permanent magnet synchronous generator, and the T1 port of contactor KM2 is connected to point o6 of the hexagonal converter; the L2 port of contactor KM2 is connected to phase B of the stator of the direct-drive permanent magnet synchronous generator, and the T2 port of contactor KM2 is connected to point o2 of the hexagonal converter; contactor K... The L3 port of M2 is connected to the stator C phase of the direct-drive permanent magnet synchronous generator, and the T3 port of contactor KM2 is connected to point o4 of the hexagonal converter. The bridge arm between points o6 and o1 of the hexagonal converter is denoted as bridge arm 1, the bridge arm between points o1 and o2 of the hexagonal converter is denoted as bridge arm 2, the bridge arm between points o2 and o3 of the hexagonal converter is denoted as bridge arm 3, the bridge arm between points o3 and o4 of the hexagonal converter is denoted as bridge arm 4, the bridge arm between points o4 and o5 of the hexagonal converter is denoted as bridge arm 5, and the bridge arm between points o5 and o6 of the hexagonal converter is denoted as bridge arm 6.

[0052] In this example, the rated phase voltage U of the AC power grid g The voltage is 8165V, the DC floating capacitor C is 20mF, the number of full-bridge submodules in the hexagonal converter bridge arm m is 6, and the rated voltage U of the full-bridge submodule capacitor is... c_ref 2500V, AC reactor L arm_kThe power transmitted by the AC power grid is 10mH, P = 3MW, and the control period T is 0.0002s; R1 = 20, R2 = 20, R3 = 20.

[0053] Figure 2 This is a closed-loop pre-charge control block diagram for a wind turbine grid-connected system based on a hexagonal converter. The control method consists of an uncontrolled charging stage and a closed-loop controllable charging stage.

[0054] The uncontrolled charging phase consists of the following steps:

[0055] (1) Disconnect contactor KM2 and contactor KM1, block the IGBT switch signal, and the AC power grid charges the hexagonal converter uncontrolled through the current limiting resistor;

[0056] (2) Calculate the target value U1 of the capacitor voltage of the full-bridge submodule of the hexagonal converter during the uncontrolled charging stage:

[0057] U1=0.93*sqrt(3)×U g / 2 / m

[0058] Where sqrt() represents the square root function, U g It is the amplitude of the phase voltage of the AC power grid;

[0059] (3) Real-time detection of the capacitor voltage of the full-bridge submodule of the hexagonal converter, and calculation of the average value U of the capacitor voltage of the full-bridge submodule of the hexagonal converter. ave , when U ave When the value is greater than or equal to the target value U1, the uncontrolled charging phase ends.

[0060] The closed-loop controllable charging stage consists of the following steps:

[0061] (1) Close contactor KM1;

[0062] (2) Take the reference value i of the q-axis current of the power grid gq_ref =0, calculate the reference value i for the d-axis current of the power grid. gd_ref :

[0063] i gd_ref =2×P / 3 / U g

[0064] Where P is the power transmitted by the AC power grid, which can be set by the user;

[0065] (3) for i gd_ref i gq_ref = 0 Using dq / abc coordinate transformation, the reference values ​​of the three-phase currents of the AC power grid are obtained: i gu_ref i gv_ref and i gw_ref ;

[0066] (4) Change i gu_ref i gv_ref and i gw_ref Substitute into the following formula to calculate the current reference value i of bridge arm k. k_ref :

[0067] i 1_ref = i 6_ref = 1 / 3×(i gu_ref -i gv_ref )

[0068] i 2_ref = i 3_ref = 1 / 3×(i gw_ref -i gu_ref )

[0069] i 4_ref = i 5_ref = 1 / 3×(i gv_ref -i gw_ref )

[0070] (5) Sample the actual current value i of each bridge arm k , change i k_ref with i k The difference is substituted into the quasi-PR regulator to obtain the modulation wave U of bridge arm k. k_ref :

[0071] U k_ref = (i k_ref – i k ) ×(K p1 + K sc1 ×s / (s 2 +2×ω sc1 ×s+(100π) 2 ))

[0072] Where K p1 K is the proportional coefficient of the first quasi-PR regulator. sc1 ω sc1 These are the resonant coefficient and cutoff angular frequency of the first quasi-PR regulator, respectively.

[0073] (6) Calculate the average value U of the submodule capacitor voltage of bridge arm k. k_ave :

[0074] U k_ave =1 / m×(U k1 + U k2 + …+ U k6 )

[0075] Among them U kj The sampled value of the capacitor voltage of the j-th submodule of bridge arm k;

[0076] (7) Change U k_ave respectively with U kj After substituting the difference into the P controller, the output of the P controller is equal to i. k Multiplying them together yields the superposition of the modulated waves in the j-th submodule of bridge arm k. kj_add :

[0077] U kj_add = (U k_ave -U kj )×i k ×K p2

[0078] Where K p2 It is the proportional coefficient of the first P regulator;

[0079] (8) Will U k_ref and U kj_add The modulation wave U of the j-th submodule of bridge arm k is obtained by superposition. kj_ref :

[0080] U kj_ref = U k_ref + U kj_add

[0081] (9) Modulate the wave U kj_ref The switching signals of the full-bridge submodule IGBT are obtained by carrier phase-shift modulation;

[0082] (10) Real-time U ave Reference value U of the capacitor voltage of the full-bridge submodule c_ref When comparing, when U ave Greater than or equal to U c_ref At this time, the controllable charging phase ends; contactor KM2 is closed, the wind turbine is connected to the system, and wind turbine grid connection control based on hexagonal converter is executed;

[0083] In the above steps, K p1 =90、K sc1 =1.2、ω sc1 =6 rad / s; K p2 = 0.1.

[0084] Figure 3The waveform of the current in arm 1 of the hexagonal converter bridge is shown. From 0s to 1s, it is in the uncontrolled charging stage. The charging current reaches its maximum at the beginning of charging and then gradually decreases as the submodule capacitor voltage increases. Due to the current-limiting resistor, the uncontrolled charging current is less than the rated current of the hexagonal converter. From 1s to 1.6s, it is in the controlled charging stage, with a charging current amplitude of 141A. Except for a slight overshoot at the beginning of the controlled charging stage, the tracking of the reference current is good, achieving constant current charging. From 1.6s to 2.5s, it is in the rated operating condition of the wind turbine connected to the grid.

[0085] Figure 4 The waveform of the submodule capacitor voltage is shown, with the first full-bridge submodule capacitor voltage U of bridge arm 1 as an example. 11 And the capacitor voltage U of the first full-bridge submodule of bridge arm 2 21 For example, during the uncontrolled charging phase, the submodule voltage is charged to 1096V in 1 second, transitioning to controlled charging. During the controlled charging phase, the submodule capacitor voltage rises at a roughly fixed slope to the rated value of 2500V. This is also consistent with... Figure 3 Corresponding to the constant current charging, since the charging power P provided by the AC grid is 3MW, the theoretical value and actual value of the charging time calculated based on the law of conservation of energy, 0.6s, are basically consistent. When the voltage of the full-bridge submodule capacitor is charged to the rated value in 1.6s, the system switches to the rated operating condition of the wind turbine connected to the grid.

Claims

1. A closed-loop pre-charge control method for a wind turbine grid-connected system based on a hexagonal converter, characterized in that... The wind turbine grid-connected system based on hexagonal converter consists of a wind turbine, a hexagonal converter, a direct-drive permanent magnet synchronous generator, contactors KM1 and KM2, current-limiting resistors R1, R2, and R3, and an AC power grid. The hexagonal converter consists of six identical bridge arms connected end-to-end to form a hexagon; each bridge arm is composed of m cascaded full-bridge submodules and an AC reactor connected in series; the AC reactor is denoted as L. arm_k k = 1, 2, 3, 4, 5, 6, representing the k-th bridge arm; the full-bridge submodule is denoted as SM. k _ j The subscript j = 1, 2, ..., m indicates the j-th full-bridge submodule; each full-bridge submodule consists of four IGBTs S1, S2, S3, and S4 with anti-parallel freewheeling diodes and one DC floating capacitor C; the collectors of S1 and S3 and the positive terminal of the DC floating capacitor C are connected to form the positive terminal of the full-bridge submodule; the emitters of S2 and S4 and the negative terminal of the DC floating capacitor C are connected to form the negative terminal of the full-bridge submodule; the positive port of the full-bridge submodule is formed by the connection point of the emitter of S1 and the collector of S2, and the negative port of the full-bridge submodule is formed by the connection point of the emitter of S3 and the collector of S4; The L1 port of contactor KM1 is connected to the U phase of the AC power grid, and the T1 port of contactor KM1 is connected to point o1 of the hexagonal converter; the L2 port of contactor KM1 is connected to the V phase of the AC power grid, and the T2 port of contactor KM1 is connected to point o5 of the hexagonal converter; the L3 port of contactor KM1 is connected to the W phase of the AC power grid, and the T3 port of contactor KM1 is connected to point o3 of the hexagonal converter. One end of the current-limiting resistor R1 is connected to the L1 port of contactor KM1, and the other end of the current-limiting resistor R1 is connected to the T1 port of contactor KM1; one end of the current-limiting resistor R2 is connected to the L2 port of contactor KM1, and the other end of the current-limiting resistor R2 is connected to the T2 port of contactor KM1; one end of the current-limiting resistor R3 is connected to the L3 port of contactor KM1, and the other end of the current-limiting resistor R3 is connected to the T3 port of contactor KM1. The wind turbine's shaft is coaxially connected to the rotor of the direct-drive permanent magnet synchronous generator. The three windings of the stator of the direct-drive permanent magnet synchronous generator are denoted as phases A, B, and C, respectively. The L1 port of contactor KM2 is connected to the stator A of the direct-drive permanent magnet synchronous generator, and the T1 port of contactor KM2 is connected to point o6 of the hexagonal converter; the L2 port of contactor KM2 is connected to the stator B of the direct-drive permanent magnet synchronous generator, and the T2 port of contactor KM2 is connected to point o2 of the hexagonal converter; the L3 port of contactor KM2 is connected to the stator C of the direct-drive permanent magnet synchronous generator, and the T3 port of contactor KM2 is connected to point o4 of the hexagonal converter. The bridge arm between points o6 and o1 of the hexagonal converter is denoted as bridge arm 1, the bridge arm between points o1 and o2 of the hexagonal converter is denoted as bridge arm 2, the bridge arm between points o2 and o3 of the hexagonal converter is denoted as bridge arm 3, the bridge arm between points o3 and o4 of the hexagonal converter is denoted as bridge arm 4, the bridge arm between points o4 and o5 of the hexagonal converter is denoted as bridge arm 5, and the bridge arm between points o5 and o6 of the hexagonal converter is denoted as bridge arm 6. The aforementioned closed-loop pre-charge control method for a wind turbine grid-connected system based on a hexagonal converter consists of an uncontrolled charging stage and a closed-loop controllable charging stage. The uncontrolled charging phase consists of the following steps: (1) Disconnect contactor KM2 and contactor KM1, block the IGBT switch signal, and the AC power grid charges the hexagonal converter uncontrolled through the current limiting resistor; (2) Calculate the target value U1 of the capacitor voltage of the full-bridge submodule of the hexagonal converter during the uncontrolled charging stage: U1=0.93*sqrt(3)×U g / 2 / m Where sqrt() represents the square root function, U g It is the amplitude of the phase voltage of the AC power grid; (3) Real-time detection of the capacitor voltage of the full-bridge submodule of the hexagonal converter, and calculation of the average value U of the capacitor voltage of the full-bridge submodule of the hexagonal converter. ave , when U ave When the value is greater than or equal to the target value U1, the uncontrolled charging phase ends. The closed-loop controllable charging stage consists of the following steps: (1) Close contactor KM1; (2) Take the reference value i of the q-axis current of the power grid gq_ref =0, calculate the reference value i for the d-axis current of the power grid. gd_ref : and gd_ref =2×P / 3 / U g Where P is the power transmitted by the AC power grid, which can be set by the user; (3) for i gd_ref i gq_ref = 0 Using dq / abc coordinate transformation, the reference values ​​of the three-phase currents of the AC power grid are obtained: i gu_ref i gv_ref and i gw_ref ; (4) Change i gu_ref i gv_ref and i gw_ref Substitute into the following formula to calculate the current reference value i of bridge arm k. k_ref : i 1_ref = i 6_ref = 1 / 3×(i gu_ref -i gv_ref ) i 2_ref = i 3_ref = 1 / 3×(i gw_ref -i gu_ref ) i 4_ref = i 5_ref = 1 / 3×(i gv_ref -i gw_ref ) (5) Sample the actual current value i of each bridge arm k , change i k_ref with i k The difference is substituted into the quasi-PR regulator to obtain the modulation wave U of bridge arm k. k_ref : U k_ref = (i k_ref – i k ) ×(K p1 + K sc1 ×s / (s 2 +2×ω sc1 ×s+(100π) 2 )) Where K p1 K is the proportional coefficient of the first quasi-PR regulator. sc1 ω sc1 These are the resonant coefficient and cutoff angular frequency of the first quasi-PR regulator, respectively. (6) Calculate the average value U of the submodule capacitor voltage of bridge arm k. k_ave : IN k_ave =1 / m×(U k1 + In k2 + …+ In k6 ) Among them U kj The sampled value of the capacitor voltage of the j-th submodule of bridge arm k; (7) Change U k_ave respectively with U kj After substituting the difference into the P controller, the output of the P controller is equal to i. k Multiplying them together yields the superposition of the modulated waves in the j-th submodule of bridge arm k. kj_add : IN kj_add = (In k_ave -IN kj )×i k ×K p2 Where K p2 It is the proportional coefficient of the first P regulator; (8) Will U k_ref and U kj_add The modulation wave U of the j-th submodule of bridge arm k is obtained by superposition. kj_ref : IN kj_ref = U k_ref + In kj_add (9) Modulate the wave U kj_ref The switching signals of the full-bridge submodule IGBT are obtained by carrier phase-shift modulation; (10) Real-time U ave Reference value U of the capacitor voltage of the full-bridge submodule c_ref When comparing, when U ave Greater than or equal to U c_ref At this time, the controllable charging phase ends; contactor KM2 is closed, the wind turbine is connected to the system, and the wind turbine grid connection control based on the hexagonal converter is executed.

2. The closed-loop pre-charge control method for a wind turbine grid-connected system based on a hexagonal converter according to claim 1, characterized in that... AC power grid phase voltage rating U g The voltage is 8165V, the DC floating capacitor C is 20mF, the number of full-bridge submodules in the hexagonal converter bridge arm m is 6, and the rated voltage U of the full-bridge submodule capacitor is... c_ref 2500V, AC reactor L arm_k The power transmitted by the AC power grid is 10mH, P = 3MW, and the control period T is 0.0002s; R1 = 20, R2 = 20, R3 = 20, K p1 =90、K sc1 =1.2、ω sc1 =6 rad / s; K p2 = 0.1.

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