A second harmonic control method for cascaded three-phase medium voltage distribution power electronic system

Abstract

The present invention discloses a second harmonic control method suitable for a cascaded three-phase medium-voltage power distribution power electronic system, which is composed of a cascade converter and multiple isolated LLC converters. To address the problem of high current stress in the subsequent LLC converter when transmitting ripple power, a certain proportion of third harmonic components is superimposed on the duty cycle modulation signal of the cascaded H-bridge converter, so that the instantaneous power peak of each phase input is reduced by 25%, thereby reducing the current stress of the subsequent LLC converter. The isolated LLC converter performs closed-loop control of the bus voltage, so that the bus voltage can still be balanced when there is a certain error between the circuit parameters and the design values. On this basis, a second harmonic suppression module and a fourth harmonic suppression module are added to reduce the impedance of the subsequent LLC converter of the cascade unit to the second harmonic and the fourth harmonic, so that the ripple power is fully transmitted to the output side. The three-phase parallel connection realizes the superposition and cancellation of the power ripple components, significantly reducing the filter capacitance required on the DC side.

Description

Technical Field

[0001] The present invention relates to the technical field of medium-voltage distribution power electronic converters, and in particular to a control method for a three-phase distribution power electronic system composed of single-phase cascade converters. Background Art

[0002] The cascaded three-phase medium-voltage power distribution system is a popular power distribution system for DC loads. It typically consists of two low-voltage cascaded units. The first stage of the cascaded unit is an H-bridge converter, and the second stage is an isolated DC / DC converter. Multiple low-voltage cascaded units are connected in series with their inputs and in parallel with their outputs, forming a single-phase medium-voltage converter. This system is then connected to the medium-voltage grid using three single-phase medium-voltage converters in a three-phase star configuration. This solution has certain limitations. Each low-voltage cascaded unit is effectively a single-phase converter, receiving power in the form of double-frequency ripple power, while the DC load consumes constant DC power. Due to the mismatch between the instantaneous input and output power, large electrolytic capacitors are typically required on the DC side of the cascaded unit to absorb the double-frequency ripple power and maintain DC voltage stability. However, the large size of electrolytic capacitors hinders the power density of the cascaded unit. Furthermore, their short lifespan affects converter reliability.

[0003] In order to reduce the capacitance required on the DC side of the cascaded unit and improve the power density and reliability of the power electronic converter, it is necessary to process the input double frequency ripple power. Active power decoupling (APD) is a typical low-frequency ripple power processing method that transfers ripple power to the non-DC output port by adding additional switching devices and decoupling capacitors (An Overview of Capacitive DC-Links-Topology Derivation and Scalability Analysis," IEEE Transactions on Power Electronics, vol. 35, no. 2, pp. 1805-1829, 2020). This method can effectively reduce the filter capacitance required for the converter, but the required decoupling branch increases the cost, and the added switch tube increases the converter loss. For cascaded three-phase medium-voltage power electronic converters, another solution is to utilize the characteristics of the three-phase converter to process the doubled frequency ripple power only through control without adding hardware circuits. The basic idea is to synchronize the power input of each phase to the output side, and to cancel out the ripple components through superposition. The key to this is to allow the isolated DC / DC converter at the subsequent stage of the cascade unit to transmit the doubled frequency ripple power. Existing control methods can basically be divided into two categories:

[0004] Method 1: The post-stage isolated DC / DC converter adopts closed-loop control. This method usually aims to control the second harmonic of the intermediate bus voltage to zero, for example, "Voltage Control and Fluctuation Suppression of the Three-Phase SST with DC Bus in Dual Rotating Reference Frames," in 2016 IEEE 8th International Power Electronics and Motion Control Conference, pp. 1084-1087, 2016. After performing abc / dq coordinate transformation on the second harmonic component of the intermediate bus voltage, a proportional-integral (PI) controller is used to generate a compensation modulation signal. "Ripple Voltage Suppression and Control Strategy for CHB-Based Solid-State Transformer," IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 9, no. 1, pp. 1104-1118, 2021. A proportional-integral resonant (PIR) controller is directly used to suppress the second harmonic in the bus voltage. "Control of Three-Phase Solid-State Transformer with Phase-Separated Configuration for Minimized Energy Storage Capacitors," IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 9, no. 1, pp. 1104-1118, 2021. Selected Topics in Power Electronics, vol. 8, no. 3, pp. 3014-3028, 2020. Based on the PIR controller controlling the bus voltage, an input power feedforward link is added to improve control performance. This method is currently used for phase-shift control of the subsequent Dual-Active Bridge (DAB) converter.

[0005] Method 2: The post-stage isolated DC / DC converter adopts open-loop control. This method is usually used in situations where a resonant converter (LLC / SRC) is used in the post-stage. In order to maintain the best soft switching performance, the resonant converter is operated at the resonant frequency with a fixed frequency and fixed duty cycle (Evaluation of Double-Line-Frequency Power Flow in Solid-State Transformers,"in2021IEEE 4th International Conference on DC Microgrids,pp.1-7,2021.High-Frequency-Link Current Stress Optimization of Cascaded H-Bridge-Based Solid-State Transformer with Third-Order Harmonic Voltage Injection,"IEEE Journal of Emerging and Selected Topics in Power Electronics,vol.9,no.1,pp.1027-1038,2021.). At this time, the resonant converter has a fixed voltage gain. When the output voltage is constant, the bus voltage will also remain constant. In order to reduce the current stress of the subsequent resonant converter, the third harmonic is injected into the AC input side to reduce the peak value of the input instantaneous power.

[0006] However, in actual situations, it is difficult for the switching frequency of the resonant converter to completely match its resonant frequency, and the voltage gain will change with its transmission power. Therefore, method 2 cannot achieve the ideal harmonic suppression effect. Summary of the Invention

[0007] The purpose of the present invention is to suppress the second harmonics on the DC side of a cascaded three-phase medium-voltage power distribution electronic system and reduce the current stress of the subsequent LLC resonant converter. On the one hand, the intermediate bus voltage is closed-loop controlled by the subsequent LLC resonant converter, so that the ripple power of each phase input is fully transmitted to the output side. By utilizing the characteristic that the ripple components of the three-phase power have a phase difference of 2π / 3, the ripple components of the power after the three phases are connected in parallel are superimposed and offset, significantly reducing the required filter capacitance and improving the power density of the converter. On the other hand, to address the problem that the peak power of the doubled frequency ripple is twice that of the DC component, a certain proportion of the third harmonic is injected into the duty cycle of the H-bridge converter to reduce the input power peak, thereby reducing the current stress of the subsequent LLC resonant converter.

[0008] The present invention is a second harmonic control method for a cascaded three-phase medium-voltage power distribution electronic system. Each phase of the system consists of multiple low-voltage cascade units with inputs connected in series and outputs connected in parallel. The three-phase inputs are connected in a star configuration, connected in series with a filter inductor, and then connected to a three-phase AC grid. The three-phase outputs are connected in parallel. Each low-voltage cascade unit includes a front-stage H-bridge converter and a rear-stage isolated LLC resonant converter. The method includes: performing closed-loop control of the intermediate bus voltage through the rear-stage LLC resonant converter to suppress the second and fourth harmonics in the bus voltage, automatically adjusting the LLC switching frequency, and ensuring that all ripple power input from each phase is transmitted to the output side. By utilizing the phase difference of the ripple components of the three-phase power by 2π / 3, the ripple components of the power after the three phases are connected in parallel are superimposed and canceled. Furthermore, the method includes: injecting a certain proportion of the third harmonic into the duty cycle of the front-stage H-bridge converter to reduce the input power peak, thereby reducing the current stress of the rear-stage LLC resonant converter.

[0009] The second harmonic control method is specifically implemented by providing a second controller and a first controller. The second controller controls the front-stage H-bridge converters in all cascaded units, while the first controller controls the rear-stage isolated LLC converters in the cascaded units. The second controller is used to control the output voltage and implement grid-side power factor correction, while the first controller is used to control the intermediate bus voltage of the cascaded units.

[0010] Optionally, the second controller includes a voltage control module, a current control module, a third harmonic injection module and a PWM modulation module. The voltage control module is used to control the output voltage V of the entire power distribution system. o The output signal it generates serves as the grid current reference value. The current control module controls the grid current to achieve power factor correction. Its output signal is the fundamental wave of the duty cycle modulation signal for the preceding H-bridge converters of all cascaded units. The third harmonic injection module generates the third harmonic of the duty cycle modulation signal, which is superimposed on the fundamental wave of the duty cycle modulation signal to serve as the duty cycle of the preceding H-bridge converter. The PWM modulation module generates switching pulses for each switch of all H-bridge converters.

[0011] Optionally, the value of the third harmonic injection coefficient k3 is selected based on the principle of minimizing the peak value of the instantaneous power input to each phase in the system.

[0012] Optionally, the first controller includes a bus voltage control module, a second harmonic suppression module, a fourth harmonic suppression module, and a pulse frequency modulation module. The intermediate bus voltage v b_mn is the input signal of the controller, where m = A, B, C, n = 1, 2, ... N, N is the number of cascaded units in phase m. The bus voltage control module is used to generate the reference switching frequency f of the LLC converter. (0)The second harmonic suppression module and the fourth harmonic suppression module are used to suppress the second harmonic and fourth harmonic in the bus voltage respectively. The modulation signals generated are the second harmonic compensation amount f of the LLC converter switching frequency respectively. (2) And the fourth harmonic compensation f (4) The two compensation quantities are simultaneously added to the reference switching frequency f (0) The switching frequency f of the LLC converter is obtained mn , and is used as the input signal of the pulse frequency modulation module. After pulse frequency modulation, the switching pulse is obtained and sent to each switching tube of the subsequent LLC resonant converter.

[0013] This invention implements closed-loop control of the LLC converter (LLC) in the rear stage of a cascaded three-phase medium-voltage power distribution power electronics system. Through second and fourth harmonic suppression modules, the low-frequency ripple power input to each phase is fully transmitted to the output side, achieving superposition and cancellation of ripple components, significantly reducing the filter capacitance required on the DC side. Furthermore, to address the problem of the peak power of the doubled-frequency ripple being twice that of the DC component, the third harmonic is injected into the duty cycle modulation signal of the preceding H-bridge converter, reducing the instantaneous input power peak to 1.5 times that of the DC component, thereby reducing the current stress of the rear-stage isolated LLC converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a cascaded three-phase medium voltage distribution power electronic system architecture.

[0015] Figure 2 It is a schematic diagram of the front-stage control system of the present invention.

[0016] Figure 3 It is a schematic diagram of the post-stage control system of the present invention. DETAILED DESCRIPTION

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The embodiments are only used to illustrate the specific implementation and corresponding effects of the present invention and are not intended to limit the present invention.

[0018] Figure 1This is a schematic diagram of the cascaded three-phase medium-voltage power distribution electronic system and its second harmonic control method described in the present invention. It includes low-voltage cascade units 1, controller 2, and controller 3. Each phase consists of multiple cascade units 1, with inputs connected in series and outputs connected in parallel. The three-phase inputs are connected in a star configuration, connected in series with a filter inductor 4, and then connected to a three-phase AC grid. The three-phase outputs are connected in parallel to maintain a stable DC voltage. The low-voltage cascade units consist of a front-stage H-bridge converter 101 and a rear-stage isolated LLC resonant converter 102-105. The rear-stage LLC converter consists of a high-frequency inverter 102, an LC resonant network 103, a high-frequency isolation transformer 104, and a rectifier 105. The output port of H-bridge converter 101 is connected to the input port of high-frequency inverter 102, which is connected to the input port of LC resonant network 103. The output port of LC resonant network 103 is connected to one end of high-frequency isolation transformer 104, the other end of which is connected to the input port of rectifier 105. The output of rectifier 105 is connected in parallel with the outputs of all cascaded units 1. The H-bridge converter 101 at the front stage of each cascaded unit processes energy from the medium-voltage grid, reducing the equivalent input voltage of each cascaded unit through series connection and converting it into a lower DC bus voltage. This energy is then fed into the subsequent LLC resonant converter. After high-frequency isolation, the outputs of all cascaded units are connected in parallel to provide energy for the DC load.

[0019] Each phase of this cascaded three-phase converter is effectively equivalent to a single-phase converter. Its power is input in the form of doubled-frequency ripple power, while the DC-side load absorbs constant DC power. This mismatch in instantaneous input and output power can generate second-harmonic voltages on the DC side of the cascaded unit. Suppressing second-harmonic voltages in the cascaded unit is one of the primary objectives of the present invention. Through closed-loop control of the subsequent LLC, all ripple power input from each phase is transmitted to the output side. By leveraging the phase difference of the ripple components of the three-phase power by 2π / 3, the ripple components superimpose and cancel each other after the outputs are connected in parallel, thus unaffecting the DC-side voltage from low-frequency ripple power. This objective is achieved by controller 3. When transmitting the same active power, the peak power when transmitting doubled-frequency ripple power is twice that when transmitting DC power, resulting in greater current stress on the subsequent LLC. Therefore, another objective of the present invention is to reduce the peak instantaneous power input from each phase by injecting a third harmonic into the duty cycle modulation signal of the preceding H-bridge converter, thereby reducing the current stress on the subsequent LLC. This objective is achieved by controller 2.

[0020] Optionally, the controller 2 includes four parts: a voltage control module 201, a current control module 202, a third harmonic injection module 203 and a PWM modulation module 204. Figure 2 The voltage control module 201 is used to control the output voltage V of the entire power distribution system. oThe specific implementation is: using the output voltage reference value V o_ref Subtract the output voltage sampling value V o The obtained error value is calculated by a voltage controller, and the generated output signal is used as the input signal of the current control module 202, wherein the voltage controller generally adopts, but is not limited to, a proportional integral controller (PI controller).

[0021] The current control module 202 is used to control the grid current to achieve power factor correction. The specific implementation method can be: obtain the grid voltage phase information through the phase-locked loop, perform abc / dq coordinate transformation on the three-phase grid current according to the grid voltage phase to obtain the d-axis component and the q-axis component, and use the output signal of the voltage control module 201 as the reference value i of the grid current d-axis component d_ref , and the reference value of the q-axis component is 0, the d-axis and q-axis reference values ​​are respectively subtracted from the d-axis component and q-axis component of the grid current, and the errors are respectively passed through the current controller to generate the d-axis component m of the fundamental modulation signal d and the q-axis component m q , and then m d and m q Perform dq / abc coordinate transformation according to the grid voltage phase to obtain the duty cycle fundamental modulation signal m of the three-phase front-stage H-bridge converter. a (1) 、m b (1) 、m c (1) , wherein the current controller generally adopts, but is not limited to, a proportional integral controller (PI controller).

[0022] The third harmonic injection module 203 is used to generate the third harmonic component of the duty cycle modulation signal of the front-stage H-bridge converter. The specific implementation method can be: the d-axis component m of the duty cycle fundamental modulation signal is injected into the d-axis component m of the duty cycle fundamental modulation signal. d The amplitude of the harmonic modulation signal is obtained by multiplying it by the proportional link and the third harmonic injection coefficient k3, and then multiplying it by the cosine value of 3 times the grid voltage phase to obtain the third harmonic component m h (3) .

[0023] The PWM modulation module 204 is used to generate the switching pulses of all the switching tubes of the H-bridge converter 101, and convert the three-phase duty cycle fundamental wave modulation signal m a (1) 、m b (1) 、m c (1) Subtract the harmonic modulation signal m h (3) , get the actual three-phase duty cycle modulation signal m a 、m b 、mc , and compared with the high-frequency carrier signal, a square wave pulse signal whose duty cycle changes with the modulation signal is obtained, which is used as the switching signal of each switch tube of the three-phase H-bridge converter.

[0024] Optionally, the value of the third harmonic injection coefficient k3 is selected based on the principle of minimizing the peak value of the instantaneous power input to each phase in the system, and the specific selection method is as follows:

[0025] Due to the three-phase symmetry, only a single phase is used as an example. Assuming that the ratio of the amplitude of the third harmonic injected by the duty cycle to the amplitude of the fundamental wave is k3, the duty cycle d is recorded as:

[0026] d=D(cosωt-k3cos3ωt)

[0027] Where D is the amplitude of the fundamental component of the duty cycle, ω is the grid voltage angular frequency, and t is time. Then the equivalent input voltage v in for:

[0028] v in =DV bus (cosωt-k3cos3ωt)

[0029] Where V bus is the bus voltage. When the power factor is 1, the input current i in In phase with the fundamental wave of the input voltage, that is:

[0030] i in =I in cosωt

[0031] Among them I in is the input current amplitude, then the input instantaneous power p in The expression is:

[0032]

[0033] The steps to further obtain k3 are: 1. Obtain p in For the maximum value of time t, the relationship between the input power peak and the third harmonic injection amount k3 is obtained: max (k3); 2. Find p max The minimum value of k3 and its corresponding k3.

[0034] When the power factor is 1 (the grid voltage and grid current are in phase), the value of k3 is 1 / 3, which can reduce the input power peak by 25%.

[0035] Optionally, the controller 3 includes four parts: a bus voltage control module 301, a second harmonic suppression module 302, a fourth harmonic suppression module 303, and a pulse frequency modulation module 304. Figure 3 As shown. The intermediate bus voltage of the cascade unit v b_mn It is an input signal of the isolated LLC converter control system 3, wherein m=A, B, C; n=1, 2, ... N, and N is the number of m-phase cascade units.

[0036] The bus voltage control module 301 is used to generate the reference switching frequency f of the LLC converter. (0) , the specific implementation method can be: set the bus voltage reference value V b_ref Subtract the bus voltage sampling value v b_mn The error is calculated by the DC voltage controller to obtain the reference switching frequency f (0) , wherein the DC voltage controller may include but is not limited to a proportional integral controller (PI controller).

[0037] The second harmonic suppression module 302 is used to suppress the second harmonic in the bus voltage and generate the second harmonic compensation value f of the LLC converter switching frequency. (2) The specific implementation method can be as follows: the bus voltage is delayed by 1 / 6 and 1 / 3 of the power frequency cycle respectively, and the bus voltage is used as a three-phase signal to perform abc / dq coordinate transformation, wherein the phase of the coordinate transformation is twice the phase of the grid voltage; the d-axis component and q-axis component obtained by the coordinate transformation are respectively filtered through a low-pass filter to remove ripples, and the d-axis component v of the second harmonic of the bus voltage is obtained. d (2) and the q-axis component v q (2) ; Subtract v from 0 d (2) and v q (2) , the obtained error is passed through the second harmonic controller to calculate the d-axis component f of the second harmonic compensation of the switching frequency d (2) and the q-axis component f q (2) , wherein the second harmonic controller may include but is not limited to a proportional integral controller (PI controller); f d (2) and f q (2) Perform dq / abc coordinate transformation and obtain the first phase as the switching frequency second harmonic compensation amount f (2) , where the phase of the coordinate transformation is twice the phase of the grid voltage.

[0038] The fourth harmonic suppression module 303 is used to suppress the fourth harmonic in the bus voltage and generate the fourth harmonic compensation value f of the LLC converter switching frequency. (4)The specific implementation method can be as follows: the bus voltage is delayed by 1 / 12 and 1 / 6 of the power frequency cycle respectively, and the bus voltage is used as a three-phase signal to perform abc / dq coordinate transformation, wherein the phase of the coordinate transformation is 4 times the phase of the grid voltage; the d-axis component and q-axis component obtained by the coordinate transformation are respectively filtered through a low-pass filter to remove ripples, and the d-axis component v of the fourth harmonic of the bus voltage is obtained. d (4) and the q-axis component v q (4) ; Subtract v from 0 d (4) and v q (4) , the obtained error is passed through the fourth harmonic controller to calculate the d-axis component f of the fourth harmonic compensation of the switching frequency d (4) and the q-axis component f q (4) , wherein the fourth harmonic controller may include but is not limited to a proportional integral controller (PI controller); f d (4) and f q (4) Perform dq / abc coordinate transformation and obtain the first phase as the fourth harmonic compensation amount f of the switching frequency (4) , where the phase of the coordinate transformation is 4 times the phase of the grid voltage.

[0039] Reference switching frequency f (0) and the second harmonic compensation f (2) , fourth harmonic compensation amount f (4) The LLC converter switching frequency f is obtained by superposition mn , as the input signal of the pulse frequency modulation module 304, used to generate a frequency-dependent mn The changing square wave pulse signal is used as the switching signal for each switch tube of the subsequent LLC converter.

[0040] The present invention aims to suppress the second harmonic problem of the cascaded three-phase medium-voltage power distribution power electronic system by utilizing the characteristic that the ripple components of the three-phase power can be superimposed and offset. First, the bus voltage is closed-loop controlled by isolating the LLC converter, so that the bus voltage balance can be maintained when there is a certain error between the circuit parameters and the design values. On this basis, the second harmonic suppression module and the fourth harmonic suppression module are added to reduce the impedance of the LLC of the cascade unit to the second harmonic and the fourth harmonic, so that the ripple power is fully transmitted to the output side. The power ripple components are superimposed and offset by the three-phase parallel connection, which significantly reduces the filter capacitance required on the DC side and is conducive to improving the power density of the converter. At the same time, in order to solve the problem of large current stress when the LLC of the latter stage transmits low-frequency ripple power, a simple third harmonic duty cycle injection method is adopted to perform harmonic injection control on the H-bridge converter of the former stage, which can reduce the instantaneous power peak of each phase input, thereby reducing the current stress of the LLC of the latter stage.

Claims

1. A second harmonic control method for a cascaded three-phase medium-voltage power distribution electronic system, wherein each phase of the system is composed of a plurality of low-voltage cascade units (1) having input ends connected in series and output ends connected in parallel, the three-phase input ends being connected in a star manner, connected in series with a filter inductor and then connected to a three-phase AC power grid, and the three-phase output ends being connected in parallel, and each low-voltage cascade unit comprising a front-stage H-bridge converter (101) and a rear-stage isolated LLC resonant converter (102-105); characterized in that The method includes: performing closed-loop control on the intermediate bus voltage through a subsequent LLC resonant converter, suppressing the second harmonic and the fourth harmonic in the bus voltage, and realizing automatic adjustment of the LLC switching frequency, so that the ripple power inputted by each phase is fully transmitted to the output side, and utilizing the characteristic that the ripple components of the three-phase power have a phase difference of 2π / 3, so that the ripple components of the power after the three phases are connected in parallel are superimposed and offset.

2. The second harmonic control method applicable to a cascaded three-phase medium voltage distribution power electronic system according to claim 1, characterized in that: The method further includes: injecting a certain proportion of third harmonics into the duty cycle of the front-stage H-bridge converter to reduce the input power peak, thereby reducing the current stress of the rear-stage LLC resonant converter.

3. The second harmonic control method applicable to a cascaded three-phase medium voltage distribution power electronic system according to claim 1, characterized in that: A first controller (3) is provided for each low-voltage cascade unit, comprising a bus voltage control module (301), a second harmonic suppression module (302), a fourth harmonic suppression module (303), and a pulse frequency modulation module (304); the intermediate bus voltage v of the cascade unit is b_mn is the input signal of the first controller (3), wherein m=A, B, C, n=1, 2, ... N, N is the number of cascade units in phase m; the bus voltage control module (301) is used to generate the reference switching frequency f of the subsequent LLC resonant converter (0) The second harmonic suppression module (302) and the fourth harmonic suppression module (303) are used to suppress the second harmonic and the fourth harmonic in the bus voltage respectively, and the generated modulation signals are respectively the second harmonic compensation amount f of the LLC converter switching frequency (2) And the fourth harmonic compensation f (4) , the two compensation quantities are simultaneously added to the reference switching frequency f (0) The switching frequency f of the LLC converter is obtained mn , and serves as an input signal of the pulse frequency modulation module (304), obtains a switching pulse through pulse frequency modulation, and sends it to each switching tube of the subsequent LLC resonant converter.

4. The second harmonic control method applicable to a cascaded three-phase medium voltage distribution power electronic system according to claim 1, characterized in that: A second controller (2) is provided for the system, comprising a voltage control module (201), a current control module (202), a third harmonic injection module (203) and a PWM modulation module (204); the voltage control module (201) is used to control the output voltage V of the entire power distribution system o , the output signal generated by it is used as the input signal of the current control module (202); the current control module (202) is used to control the grid current to achieve power factor correction, and its output signal is the fundamental wave of the duty cycle modulation signal of the front-stage H-bridge converter of all cascaded units; the third harmonic injection module (203) is used to generate the third harmonic of the duty cycle modulation signal, subtract it from the fundamental wave component output by the current control module (202), and use it as the duty cycle modulation signal of the front-stage H-bridge converter; The PWM modulation module (204) is used to generate switching pulses for all switching tubes of the H-bridge converter (101).

5. The second harmonic control method applicable to a cascaded three-phase medium voltage distribution power electronic system according to claim 4, characterized in that: The value of the injection coefficient k3 of the third harmonic injection module (203) is selected based on the principle of minimizing the peak value of the instantaneous power input to each phase in the system.

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