A same-frequency control strategy based on an H-MMC motor driving system

Abstract

The application discloses a same-frequency control strategy based on an H-MMC motor driving system; the motor driving system comprises a permanent magnet synchronous motor, an H-MMC and an alternating current power grid; firstly, a double-loop control is adopted to obtain motor-side d-axis and q-axis voltage reference values, a phase angle superposition amount is calculated in real time, and the motor-side d-axis and q-axis voltage reference values are added to a motor terminal voltage phase angle obtained by a phase-locked loop to obtain a final motor phase angle so as to adjust the motor-side three-phase voltage reference values in real time; a double-loop control is adopted to obtain a grid-side three-phase voltage reference value; hierarchical capacitor voltage balance control and circulating current control are adopted to obtain a neutral point voltage reference value and a bridge arm voltage reference value superposition amount; voltage modulation wave superposition amount of a sub-module voltage is obtained through voltage balance control; finally, driving signals of each sub-module are generated through CPS-PWM after the modulation wave of each sub-module voltage is calculated according to the above control; through real-time adjustment of the phase angle of the motor-side three-phase voltage reference values, the application can realize that the bridge arm differential frequency power approaches to 0, and can effectively suppress the capacitor voltage fluctuation of the sub-module without increasing circulating current or injecting reactive power, so that the stable operation of the system under the same frequency condition is realized; and the application has the advantages of simple control and small loss.

Description

Technical Field

[0001] This invention belongs to the field of AC / AC converter control, and specifically relates to a synchronous frequency control strategy based on an H-MMC motor drive system. Background Technology

[0002] With increasingly stringent requirements for the quality of engineering construction and equipment operation, medium- and high-voltage variable frequency speed control technology has become increasingly important. Using medium- and high-voltage frequency converters to control the speed of high-power motors can significantly improve product quality and production performance, while also reducing production costs.

[0003] Limited by the manufacturing process of switching devices, traditional two-level frequency converters used in high-power medium- and high-voltage motor drives require the use of series-connected power devices for voltage boosting. This places high demands on the consistency of power devices, voltage and current equalization, and control strategies, and also results in problems such as high system losses and low reliability. H-MMC, as a derivative topology of modular multilevel converters, not only possesses the advantages of modular multilevel converters—easy scalability, high reliability, and low output voltage harmonics—but also offers the advantages of direct AC / AC conversion and good low-frequency characteristics. Therefore, it has broad prospects in the field of high-power variable frequency speed control.

[0004] When H-MMC drives a motor, if the motor's operating frequency is equal to or close to the grid frequency, theoretically, the differential frequency power component on the H-MMC bridge arm will equal to or approach infinity. This causes the bridge arm submodule capacitor voltage to increase or decrease rapidly, posing a significant threat to the system's stable operation. Currently, there is no specific synchronous frequency control strategy for H-MMC under this operating condition; only synchronous frequency control strategies have been proposed for modular multilevel matrix converters. For example, methods such as injecting additional circulating current and neutral point voltage can compensate for the power deviation between bridge arms, but this method not only increases system losses but may also lead to overmodulation. Another approach is to introduce circulating current and reactive power at the input to achieve bridge arm power balance, but this degrades the power quality of the grid. Summary of the Invention

[0005] To address the shortcomings and deficiencies described in the background art above, this invention proposes a synchronous control strategy based on an H-MMC motor drive system, which has the advantages of simple control, low loss, and the ability to achieve zero bridge arm differential frequency power component simply by adjusting the phase angle of the machine-side reference voltage.

[0006] The technical solution provided by this invention is as follows:

[0007] A synchronous control strategy based on an H-MMC motor drive system, characterized in that the H-MMC motor drive system consists of an H-MMC, a permanent magnet synchronous motor, and an AC power grid.

[0008] The H-MMC consists of six identical bridge arms connected end-to-end to form a ring structure. Each bridge arm is composed of N cascaded full-bridge submodules and a reactor L. x The bridge is formed by connecting multiple bridge arms in series, where the subscript x represents the x-th bridge arm, and x = 1, 2, 3…6; the full-bridge submodule is denoted as SM. x_y The subscript y indicates the y-th full-bridge submodule, y = 1, 2, 3…N; four IGBTs S1, S2, S3, S4 and one capacitor C constitute a full-bridge submodule. The emitter of S1 and the collector of S3 are connected, and the connection point is called the positive terminal of the full-bridge submodule. The emitter of S2 and the collector of S4 are connected, and the connection point is called the negative terminal of the full-bridge submodule. The collectors of S1 and S2 are connected to the positive terminal of the capacitor, and the emitters of S3 and S4 are connected to the negative terminal of the capacitor; the bridge arm x is the first full-bridge submodule SM. x_1 The positive terminal is called the beginning of bridge arm x, SM x_1 The negative terminal and the second full-bridge submodule SM of bridge arm x x_2 Connect the positive terminals, and connect them sequentially according to this pattern. The Nth submodule SM of bridge arm x x_N The negative electrode and reactor L x One end is connected, L x The other end is called the tail end of bridge arm x; the Nth submodule SM of bridge arm x x_N The negative electrode and reactor L x One end is connected, L x The other end is called the tail end of bridge arm x; o1, o2, o3, o4, o5, and o6 are the six endpoints of H-MMC;

[0009] The three phases U, V, and W of the power grid are connected to points o4, o6, and o2 of the H-MMC, respectively; the three phases R, S, and T of the permanent magnet synchronous motor are connected to points o1, o3, and o5 of the H-MMC, respectively; the branch between points o1 and o2 is called bridge arm 1; the branch between points o2 and o3 is called bridge arm 2; the branch between points o3 and o4 is called bridge arm 3; the branch between points o4 and o5 is called bridge arm 4; the branch between points o5 and o6 is called bridge arm 5; and the branch between points o6 and o1 is called bridge arm 6.

[0010] The synchronous frequency control strategy based on the H-MMC motor drive system consists of the following steps:

[0011] (1) The speed n of the permanent magnet synchronous motor is detected in real time using an encoder. The reference value u of the d-axis voltage on the machine side is obtained through dual-loop control of the motor speed, namely the outer loop of speed and the inner loop of current. md_ref and q-axis voltage reference value u mq_ref ;

[0012] (2) Calculate the motor frequency f m :

[0013] f m = n×p / 60

[0014] Where p is the number of pole pairs of the permanent magnet synchronous motor;

[0015] (3) Calculate the phase angle superposition φ0:

[0016] φ0=

[0017] Where t is the control period;

[0018] (4) Obtain the phase angle φ of the motor terminal voltage through a phase-locked loop. m , will m Adding φ0 to obtain the adjusted motor phase angle φ m2 :

[0019] 𝜽 m2 = 𝜽 m +φ0

[0020] (5) Use the adjusted motor phase angle φ m2 For u md_ref and u mq_ref The reference value u of the three-phase voltage on the machine side is obtained by performing dq / abc transformation. mr_ref u ms_ref u mt_ref ;

[0021] (6) Detect the capacitor voltage u of all H-MMC submodules c_xy Calculate the average voltage u of the capacitor in the submodule. c_ave :

[0022] u c_ave =

[0023] (7) Set the reference value U of the submodule capacitor voltage. c_ref with u c_ave Subtracting the values, the difference is fed into the third PI controller to obtain the grid-side d-axis current reference value i. gd_ref :

[0024] i gd_ref =(U c_ref - u c_ave )×(K p3 + K i3 ×(1 / s))

[0025] Among them, K p3 and K i3 These are the proportional and integral coefficients of the third PI controller, respectively.

[0026] (8) Detect the three-phase voltage u on the grid sidegu u gv u gw The grid-side t-axis voltage u is obtained by performing an abc / tw transformation. g𝜶 and grid-side φ-axis voltage u g𝜷 And the phase angle φ of the power grid is obtained using a phase-locked loop. g ;

[0027] (9) Let the reference value of the q-axis current on the grid side be i gq_ref If i equals 0, then i gd_ref i gq_ref The reference value of the grid-side t-axis current i is obtained by performing dq / tq transformation. 𝜶_ref Reference value of grid-side φ-axis current i g𝜷_ref ;

[0028] (10) Detect the three-phase current i on the grid side u i v i w The actual value of the t-axis current on the network side, i, is obtained by performing an abc / tw transformation. g𝜶 Actual value of the φ-axis current on the grid side i g𝜷 ;

[0029] (11) Change i g𝜶_ref and i g𝜶 The difference is fed into the first quasi-PR controller, and the output of the first quasi-PR controller is then multiplied by u. g𝜶 The summation yields the reference value u of the grid-side α-axis voltage. g𝜶_ref ;change i g𝜷_ref and i g𝜷 The difference is fed into the first quasi-PR controller, and the output of the first quasi-PR controller is then multiplied by u. 𝜷 The summation yields the reference value u of the grid-side φ axis voltage. g𝜷_ref ;

[0030] (12) will u g𝜶_ref and u g𝜷_ref The grid-side three-phase voltage reference value u is obtained by performing the φ / abc transformation. gu_ref u gv_ref u gw_ref ;

[0031] (13) The neutral point voltage reference value u is obtained by using hierarchical capacitor voltage balance control. st_ref The bridge arm voltage reference value superposition u is obtained by using hierarchical capacitor voltage balance control and circulating current control. cir_ref ;

[0032] (14) will u mr_ref u ms_ref u mt_ref u gu_ref ugv_ref u gw_ref u st_ref and u cir_ref Substituting into the following formula, the voltage modulation wave u of each bridge arm is calculated. x_ref :

[0033] u 1_ref = u gw_ref - u mr_ref - u st_ref + u cir_ref u 2_ref = u ms_ref - u gw_ref + u st_ref + u cir_ref u 3_ref = u gu_ref - u ms_ref - u st_ref + u cir_ref u 4_ref = u mt_ref - u gu_ref + u st_ref + u cir_ref u 5_ref = u gv_ref - u mt_ref - u st_ref + u cir_ref u 6_ref = u mr_ref - u gv_ref + u st_ref + u cir_ref

[0034] (15) Calculate the average capacitor voltage u of each bridge arm submodule respectively. x_ave ; will u x_ave The voltage u of the capacitor of the y-th submodule in bridge arm x is respectively compared with that of the bridge arm x. xy Subtracting the values ​​and substituting the difference into the third proportional controller, the output is multiplied by the sign function of the bridge arm current to obtain the superposition amount u of the modulated wave of the y-th submodule of bridge arm x. xy_add :

[0035] u xy_add = (u x_ave - u xy )×K p3 ×sign(i x )

[0036] Among them, K p9The proportional coefficient of the third proportional controller; sign() is the sign function;

[0037] (16) will u x_ref Divide by N and u xy_add The summation yields the modulation wave u of the y-th submodule of bridge arm x. xy_ref :

[0038] u xy_ref =1 / N × u x_ref + u xy_add

[0039] (17) For u xy_ref The switching signal of the IGBT is obtained by carrier phase shift modulation.

[0040] The beneficial effects of this invention are: 1) Taking the H-MMC motor drive system as the research object, a same-frequency control method is proposed, which effectively suppresses submodule fluctuations and expands the frequency range of system operation when the machine side and grid side are close to the same frequency; 2) The control is relatively simple, only requiring real-time adjustment of the phase of the machine side voltage reference wave to achieve the bridge arm differential frequency power close to 0; 3) The circulating current and neutral point voltage can be controlled within the normal value range under non-same-frequency operating conditions of the system, without increasing the injection amount of circulating current and neutral point voltage, reducing losses and improving system reliability. Attached Figure Description

[0041] Figure 1 This is a structural diagram of an H-MMC-based motor drive system;

[0042] Figure 2 This is a block diagram of the synchronous control of an H-MMC motor drive system.

[0043] Figure 3 This is the motor speed waveform;

[0044] Figure 4 The waveforms are the capacitor voltage waveforms of the submodules of bridge arm 1 and bridge arm 2. Detailed Implementation

[0045] 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.

[0046] Figure 1 This is a structural diagram of an H-MMC-based motor drive system. The H-MMC-based motor drive system in this invention consists of an H-MMC, a permanent magnet synchronous motor, and an AC power grid. The H-MMC is a ring structure formed by connecting six identical bridge arms end-to-end. Each bridge arm consists of N cascaded full-bridge submodules and a reactor L.x The bridge is formed by connecting multiple bridge arms in series, where the subscript x represents the x-th bridge arm, and x = 1, 2, 3…6; the full-bridge submodule is denoted as SM. x_y The subscript y indicates the y-th full-bridge submodule, y = 1, 2, 3…N; four IGBTs S1, S2, S3, S4 and one capacitor C constitute a full-bridge submodule. The emitter of S1 and the collector of S3 are connected, and the connection point is called the positive terminal of the full-bridge submodule. The emitter of S2 and the collector of S4 are connected, and the connection point is called the negative terminal of the full-bridge submodule. The collectors of S1 and S2 are connected to the positive terminal of the capacitor, and the emitters of S3 and S4 are connected to the negative terminal of the capacitor; the bridge arm x is the first full-bridge submodule SM. x_1 The positive terminal is called the beginning of bridge arm x, SM x_1 The negative terminal and the second full-bridge submodule SM of bridge arm x x_2 Connect the positive terminals, and connect them sequentially according to this pattern. The Nth submodule SM of bridge arm x x_N The negative electrode and reactor L x One end is connected, L x The other end is called the tail end of bridge arm x; the Nth submodule SM of bridge arm x x_N The negative electrode and reactor L x One end is connected, L x The other end is called the tail end of bridge arm x; o1, o2, o3, o4, o5, and o6 are the six terminals of H-MMC; the three phases U, V, and W of the power grid are connected to points o4, o6, and o2 of H-MMC respectively; the three phases R, S, and T of the permanent magnet synchronous motor are connected to points o1, o3, and o5 of H-MMC respectively; the branch between points o1 and o2 is called bridge arm 1; the branch between points o2 and o3 is called bridge arm 2; the branch between points o3 and o4 is called bridge arm 3; the branch between points o4 and o5 is called bridge arm 4; the branch between points o5 and o6 is called bridge arm 5; and the branch between points o6 and o1 is called bridge arm 6.

[0047] In this example, the permanent magnet synchronous motor has 4 pole pairs (p) and a reference speed (n). ref The value is 750, the motor voltage is 3.3kV, the AC mains voltage is 10kV, and the H-MMC reactor L... x The capacitor C is 20mF, the number of full-bridge submodules N is 6, and the rated voltage U of the full-bridge submodule capacitors is 10mH. c_ref It is 2500V.

[0048] Figure 2 This is a block diagram of synchronous frequency control based on the H-MMC motor drive system. The control method consists of the following steps.

[0049] (1) The speed n of the permanent magnet synchronous motor is detected in real time using an encoder. The reference value u of the d-axis voltage on the machine side is obtained through dual-loop control of the motor speed, namely the outer loop of speed and the inner loop of current. md_ref and q-axis voltage reference value u mq_ref ;

[0050] (2) Calculate the motor frequency f m :

[0051] f m = n×p / 60

[0052] Where p is the number of pole pairs of the permanent magnet synchronous motor;

[0053] (3) Calculate the phase angle superposition φ0:

[0054] φ0=

[0055] Where t is the control period;

[0056] (4) Obtain the phase angle φ of the motor terminal voltage through a phase-locked loop. m , will m Adding φ0 to obtain the adjusted motor phase angle φ m2 :

[0057] 𝜽 m2 = 𝜽 m +φ0

[0058] (5) Use the adjusted motor phase angle φ m2 For u md_ref and u mq_ref The reference value u of the three-phase voltage on the machine side is obtained by performing dq / abc transformation. mr_ref u ms_ref u mt_ref ;

[0059] (6) Detect the capacitor voltage u of all H-MMC submodules c_xy Calculate the average voltage u of the capacitor in the submodule. c_ave :

[0060] u c_ave =

[0061] (7) Set the reference value U of the submodule capacitor voltage. c_ref with u c_ave Subtracting the values, the difference is fed into the third PI controller to obtain the grid-side d-axis current reference value i. gd_ref :

[0062] i gd_ref =(U c_ref - u c_ave )×(Kp3 + K i3 ×(1 / s))

[0063] Among them, K p3 and K i3 These are the proportional and integral coefficients of the third PI controller, respectively.

[0064] (8) Detect the three-phase voltage u on the grid side gu u gv u gw The grid-side t-axis voltage u is obtained by performing an abc / tw transformation. g𝜶 and grid-side φ-axis voltage u g𝜷 And the phase angle φ of the power grid is obtained using a phase-locked loop. g ;

[0065] (9) Let the reference value of the q-axis current on the grid side be i gq_ref If i equals 0, then i gd_ref i gq_ref The reference value of the grid-side t-axis current i is obtained by performing dq / tq transformation. 𝜶_ref Reference value of grid-side φ-axis current i g𝜷_ref ;

[0066] (10) Detect the three-phase current i on the grid side u i v i w The actual value of the t-axis current on the network side, i, is obtained by performing an abc / tw transformation. g𝜶 Actual value of the φ-axis current on the grid side i g𝜷 ;

[0067] (11) Change i g𝜶_ref and i g𝜶 The difference is fed into the first quasi-PR controller, and the output of the first quasi-PR controller is then multiplied by u. g𝜶 The summation yields the reference value u of the grid-side α-axis voltage. g𝜶_ref ;change i g𝜷_ref and i g𝜷 The difference is fed into the first quasi-PR controller, and the output of the first quasi-PR controller is then multiplied by u. 𝜷 The summation yields the reference value u of the grid-side φ axis voltage. g𝜷_ref ;

[0068] (12) will u g𝜶_ref and u g𝜷_ref The grid-side three-phase voltage reference value u is obtained by performing the φ / abc transformation. gu_ref u gv_ref u gw_ref ;

[0069] (13) The neutral point voltage reference value u is obtained by using hierarchical capacitor voltage balance control.st_ref The bridge arm voltage reference value superposition u is obtained by using hierarchical capacitor voltage balance control and circulating current control. cir_ref ;

[0070] (14) will u mr_ref u ms_ref u mt_ref u gu_ref u gv_ref u gw_ref u st_ref and u cir_ref Substituting into the following formula, the voltage modulation wave u of each bridge arm is calculated. x_ref :

[0071] u 1_ref = u gw_ref - u mr_ref - u st_ref + u cir_ref u 2_ref = u ms_ref - u gw_ref + u st_ref + u cir_ref u 3_ref = u gu_ref - u ms_ref - u st_ref + u cir_ref u 4_ref = u mt_ref - u gu_ref + u st_ref + u cir_ref u 5_ref = u gv_ref - u mt_ref - u st_ref + u cir_ref u 6_ref = u mr_ref - u gv_ref + u st_ref + u cir_ref

[0072] (15) Calculate the average capacitor voltage u of each bridge arm submodule respectively. x_ave ; will u x_ave The voltage u of the capacitor of the y-th submodule in bridge arm x is respectively compared with that of the bridge arm x. xy Subtracting the values ​​and substituting the difference into the third proportional controller, the output is multiplied by the sign function of the bridge arm current to obtain the superposition amount u of the modulated wave of the y-th submodule of bridge arm x. xy_add :

[0073] u xy_add = (u x_ave - u xy )×K p3 ×sign(i x )

[0074] Among them, K p9 The proportional coefficient of the third proportional controller; sign() is the sign function;

[0075] (16) will u x_ref Divide by N and u xy_add The summation yields the modulation wave u of the y-th submodule of bridge arm x. xy_ref :

[0076] u xy_ref =1 / N × u x_ref + u xy_add

[0077] (17) For u xy_ref The switching signal of the IGBT is obtained by carrier phase shift modulation.

[0078] In the above steps, the control period t is 0.0001s; the machine-side speed loop parameter is: K p1 =0.8, K i1 =1.2; Machine-side current loop parameters are: K p2 =150, K i2 =300; The parameters of the third PI controller are: K p3 =2、K i3 =20; The first quasi-PR controller parameter is: K p4 =100, K sc1 =2000、ω sc1 =2 rad / s; The hierarchical capacitor voltage balance control parameters are: K p5 =1、K i4 =5、 K p6 =6、K p7 =2e-5; the circulating control parameter is: K p8 =30、K i5 =300, K sc2 =1、ω sc2 =1 rad / s, K sc3 =1、ω sc3 =2 rad / s; the parameters of the third proportional controller are: K p9 =0.03.

[0079] Figure 3The waveform of the motor speed shows that after the motor starts, the speed gradually increases. When the machine-side frequency increases to 47Hz at 1.84s, the proposed same-frequency control strategy is switched. After a transient adjustment process of about 0.8s, the speed stabilizes at the rated speed of 750r / min, realizing full-frequency operation from 0 to the rated frequency.

[0080] Figure 4 The waveforms of the submodule capacitors in bridge arm 1 and bridge arm 2 are shown. As the machine-side frequency gradually approaches 50Hz, the differential frequency power continuously increases, causing the submodule ripple to continuously increase, reaching as high as 10.8%. If left uncontrolled, the ripple will continue to increase and damage the capacitors. After 1.84s, when the same frequency control strategy is adopted, the submodule ripple is effectively suppressed, and it gradually stabilizes at the rated voltage of 2500V at 4s, with the ripple less than 0.8%, proving that the same frequency control strategy has a good ripple suppression effect.

Claims

1. A synchronous frequency control strategy based on an H-MMC motor drive system, characterized in that... The H-MMC-based motor drive system consists of H-MMC, a permanent magnet synchronous motor, and an AC power grid. The H-MMC consists of six identical bridge arms connected end-to-end to form a ring structure. Each bridge arm is composed of N cascaded full-bridge submodules and a reactor L. x The bridge is formed by connecting multiple bridge arms in series, where the subscript x represents the x-th bridge arm, and x = 1, 2, 3…6; the full-bridge submodule is denoted as SM. x_y The subscript y indicates the y-th full-bridge submodule, y = 1, 2, 3…N; four IGBTs S1, S2, S3, S4 and one capacitor C constitute a full-bridge submodule. The emitter of S1 and the collector of S3 are connected, and the connection point is called the positive terminal of the full-bridge submodule. The emitter of S2 and the collector of S4 are connected, and the connection point is called the negative terminal of the full-bridge submodule. The collectors of S1 and S2 are connected to the positive terminal of the capacitor, and the emitters of S3 and S4 are connected to the negative terminal of the capacitor; the bridge arm x is the first full-bridge submodule SM. x_1 The positive terminal is called the beginning of bridge arm x, SM x_1 The negative terminal and the second full-bridge submodule SM of bridge arm x x_2 Connect the positive terminals, and connect them sequentially according to this pattern. The Nth submodule SM of bridge arm x x_N The negative electrode and reactor L x One end is connected, L x The other end is called the tail end of bridge arm x; the Nth submodule SM of bridge arm x x_N The negative electrode and reactor L x One end is connected, L x The other end is called the tail end of bridge arm x; o1, o2, o3, o4, o5, and o6 are the six endpoints of H-MMC; The three phases U, V, and W of the power grid are connected to points o4, o6, and o2 of the H-MMC, respectively; the three phases R, S, and T of the permanent magnet synchronous motor are connected to points o1, o3, and o5 of the H-MMC, respectively; the branch between points o1 and o2 is called bridge arm 1; the branch between points o2 and o3 is called bridge arm 2; the branch between points o3 and o4 is called bridge arm 3; the branch between points o4 and o5 is called bridge arm 4; the branch between points o5 and o6 is called bridge arm 5; and the branch between points o6 and o1 is called bridge arm 6. The synchronous frequency control strategy based on the H-MMC motor drive system consists of the following steps: (1) The speed n of the permanent magnet synchronous motor is detected in real time using an encoder. The reference value u of the d-axis voltage on the machine side is obtained through dual-loop control of the motor speed, namely the outer loop of speed and the inner loop of current. md_ref and q-axis voltage reference value u mq_ref ; (2) Calculate the motor frequency f m : f m = n×p / 60 Where p is the number of pole pairs of the permanent magnet synchronous motor; (3) Calculate the phase angle superposition φ0: φ0= Where t is the control period; (4) Obtain the phase angle φ of the motor terminal voltage through a phase-locked loop. m , will m Adding φ0 to obtain the adjusted motor phase angle φ m2 : 𝜽 m2 = 𝜽 m +φ0 (5) Use the adjusted motor phase angle φ m2 For u md_ref and u mq_ref The reference value u of the three-phase voltage on the machine side is obtained by performing dq / abc transformation. mr_ref u ms_ref u mt_ref ; (6) Detect the capacitor voltage u of all H-MMC submodules c_xy Calculate the average voltage u of the capacitor in the submodule. c_ave : u c_ave = (7) Set the reference value U of the submodule capacitor voltage. c_ref with u c_ave The difference is fed into the third PI controller to obtain the grid-side d-axis current reference value i. gd_ref : i gd_ref =(U c_ref - u c_ave )×(K p3 + K i3 ×(1 / s)) Among them, K p3 and K i3 These are the proportional and integral coefficients of the third PI controller, respectively. (8) Detect the three-phase voltage u on the grid side gu u gv u gw The grid-side t-axis voltage u is obtained by performing an abc / tw transformation. g𝜶 and grid-side φ-axis voltage u g𝜷 And the phase angle φ of the power grid is obtained using a phase-locked loop. g ; (9) Let the reference value of the q-axis current on the grid side be i gq_ref If i equals 0, then i gd_ref i gq_ref The reference value of the grid-side t-axis current i is obtained by performing dq / tq transformation. 𝜶_ref Reference value of grid-side φ-axis current i g𝜷_ref ; (10) Detect the three-phase current i on the grid side u i v i w The actual value of the t-axis current on the network side, i, is obtained by performing an abc / tw transformation. g𝜶 Actual value of the φ-axis current on the grid side i g𝜷 ; (11) Change i g𝜶_ref and i g𝜶 The difference is fed into the first quasi-PR controller, and the output of the first quasi-PR controller is then multiplied by u. g𝜶 The summation yields the reference value u of the grid-side α-axis voltage. g𝜶_ref ;change i g𝜷_ref and i g𝜷 The difference is fed into the first quasi-PR controller, and the output of the first quasi-PR controller is then multiplied by u. 𝜷 The summation yields the reference value u of the grid-side φ axis voltage. g𝜷_ref ; (12) will u g𝜶_ref and u g𝜷_ref The grid-side three-phase voltage reference value u is obtained by performing the φ / abc transformation. gu_ref u gv_ref u gw_ref ; (13) The neutral point voltage reference value u is obtained by using hierarchical capacitor voltage balance control. st_ref The bridge arm voltage reference value superposition u is obtained by using hierarchical capacitor voltage balance control and circulating current control. cir_ref ; (14) will u mr_ref u ms_ref u mt_ref u gu_ref u gv_ref u gw_ref u st_ref and u cir_ref Substituting into the following formula, the voltage modulation wave u of each bridge arm is calculated. x_ref : in 1_ref = in gw_ref - in mr_ref - in st_ref + in cir_ref in 2_ref = in ms_ref - in gw_ref + in st_ref + in cir_ref in 3_ref = in gu_ref - in ms_ref - in st_ref + in cir_ref in 4_ref = in mt_ref - in gu_ref + in st_ref + in cir_ref in 5_ref = in gv_ref - in mt_ref - in st_ref + in cir_ref in 6_ref = in mr_ref - in gv_ref + in st_ref + in cir_ref (15) Calculate the average capacitor voltage u of each bridge arm submodule respectively. x_ave ; will u x_ave The voltage u of the capacitor of the y-th submodule in bridge arm x is respectively compared with that of the bridge arm x. xy Subtracting the values ​​and substituting the difference into the third proportional controller, the output is multiplied by the sign function of the bridge arm current to obtain the superposition amount u of the modulated wave of the y-th submodule of bridge arm x. xy_add : in xy_add = (in x_ave - in xy )×K p3 ×sign(i x ) Among them, K p9 The proportional coefficient of the third proportional controller; sign() is the sign function; (16) will u x_ref Divide by N and u xy_add The summation yields the modulation wave u of the y-th submodule of bridge arm x. xy_ref : u xy_ref =1 / N × u x_ref + u xy_add (17) For u xy_ref The switching signal of the IGBT is obtained by carrier phase shift modulation.

2. The same-frequency control strategy based on an H-MMC motor drive system according to claim 1, characterized in that... The permanent magnet synchronous motor has 4 pole pairs (p) and a reference speed (n). ref The value is 750, the motor voltage is 3.3kV, the AC mains voltage is 10kV, and the H-MMC reactor L... x The capacitor C is 20mF, the number of full-bridge submodules N is 6, and the rated voltage U of the full-bridge submodule capacitors is 10mH. c_ref The voltage is 2500V; the control cycle t is 0.0001s; the machine-side speed loop parameter is: K p1 =0.8, K i1 =1.2; Machine-side current loop parameters are: K p2 =150, K i2 =300; The parameters of the third PI controller are: K p3 =2、K i3 =20; The first quasi-PR controller parameter is: K p4 =100, K sc1 =2000、ω sc1 =2 rad / s; the hierarchical capacitor voltage balance control parameters are: K p5 =1、K i4 =5、 K p6 =6、K p7 =2e-5; the circulating control parameter is: K p8 =30、K i5 =300, K sc2 =1、ω sc2 =1 rad / s, K sc3 =1、ω sc3 =2 rad / s; the parameters of the third proportional controller are: K p9 =0.03.

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