A STATCOM system without electrolytic capacitor cascade H bridge and control method

Abstract

The present invention discloses a STATCOM system and a control method for a cascaded H-bridge without electrolytic capacitors, belonging to the field of power electronic control. The system comprises: a three-phase power grid, a three-phase reactive load, a filtering inductor and a three-phase cascaded N double-Buck multiplexed H-bridge, N≥2; the double-Buck multiplexed H-bridge comprises: four fully controlled switch tubes S1-S4, two decoupling inductors L r1 and L r2 , three film capacitors C r1 , C r2 and C d ; said C r1 Passing by L r1 Connected to the midpoint of the first bridge arm of the H bridge to form the first bidirectional Buck circuit. This bidirectional Buck circuit and the first bridge arm of the H bridge realize switch tube multiplexing. r2 Passing by L r2 The second bidirectional Buck circuit is connected to the midpoint of the second bridge arm of the H bridge, and the bidirectional Buck circuit and the second bridge arm of the H bridge realize switch tube multiplexing; the system as a whole adopts a star connection. The present invention utilizes multiplexed active power decoupling technology to eliminate the double frequency fluctuation of the H bridge DC side voltage, realizes the replacement of electrolytic capacitors with film capacitors, and improves the power density and reliability of the device.

Description

Technical Field

[0001] The present invention relates to the field of power electronic control, and in particular to a STATCOM system without electrolytic capacitor cascade H bridge and a control method thereof. Background Art

[0002] my country is a large country with a vast territory and uneven energy distribution. The abundant electricity in the northwest needs to be transported over long distances to reach the eastern region. On the other hand, with the development of science and technology and industry, many high-power reactive loads such as rolling mills, mine hoists, and large-capacity transformers have appeared in the power grid. If the reactive compensation in the power grid is insufficient, this long-distance transmission will have huge line losses, resulting in low power grid efficiency. In addition, with the exhaustion of traditional fossil energy, countries around the world are calling for more and more to strengthen the development and utilization of new energy, and a large number of new energy power generation devices are connected to the power grid, such as wind power and solar power generation. However, wind power and solar power generation have inherent intermittent and random characteristics, which will cause voltage fluctuations at the grid connection point, and in severe cases, cause off-grid operation. It is necessary to connect a fast reactive compensation device to the power grid to ensure the low voltage ride-through capability of the new energy grid. When the voltage at the grid connection point drops, the reactive compensation equipment can quickly inject reactive power into the grid connection point to maintain voltage stability. STATCOM is the most advanced reactive compensation device in modern times. It can both emit reactive power and absorb reactive power through different controls. It is a research hotspot for flexible AC transmission systems and smart grids. The cascaded H-bridge STATCOM is a product of the combination of multi-level technology and reactive power compensation technology. It is suitable for medium-high voltage and high-power applications. It has the characteristics of simple structure, modularization, high redundancy, small output harmonics and direct grid connection. It is one of the most studied STATCOM topologies at present.

[0003] The power transfer law of the H-bridge module in the cascaded H-bridge STATCOM is similar to that of the single-phase inverter. The instantaneous power on the AC side contains secondary fluctuation components, which leads to the double frequency fluctuation of the DC side voltage of the H-bridge module. In order to weaken the influence of fluctuations, the traditional method is to connect a large-capacitance electrolytic capacitor in parallel on the DC side of the H-bridge. However, electrolytic capacitors have problems such as easy volatile electrolyte, short life, low withstand voltage, and frequent replacement, which greatly affects the reliability of the STATCOM device. Therefore, people in the industry consider using film capacitors to replace electrolytic capacitors. Compared with electrolytic capacitors, film capacitors have lower power density. Under the same capacitance and fluctuation suppression effect, the volume of film capacitors is larger, which increases the overall volume of the STATCOM device and reduces the power density. In addition, it is difficult for film capacitors to reach a larger capacitance. Therefore, researchers consider suppressing the fluctuation of the DC side voltage of the H-bridge. In this way, in the case of small fluctuations, only a small-capacitance film capacitor needs to be connected in parallel on its DC side, thereby reducing the volume of the device while increasing the life and reliability of the device.

[0004] At present, the research hotspot of the method of suppressing the double frequency fluctuation of the DC side voltage of the H-bridge module is the active power decoupling technology. By connecting an active power decoupling circuit, i.e., a DC active power filter, in parallel to the DC side of the H-bridge, the double frequency fluctuation of the DC side of the H-bridge is transferred to the auxiliary energy storage element of the decoupling circuit. In this way, there is no double frequency fluctuation of the DC side voltage of the H-bridge, and its DC side capacitor mainly supports the DC voltage, which only requires a very small film capacitor. However, this requires the addition of additional power devices, which will increase the cost and volume of the overall system. A small number of scholars have studied the multiplexed active power decoupling technology based on single-phase grid-connected inverters without adding power switch tubes. The second arm switch tube of the H-bridge is reused with the switch tube of the active power decoupling circuit. There are problems such as complex control and difficult control parameter design. At the same time, the research background is mostly for a single H-bridge. At present, there is almost no research on the multiplexed active power decoupling control of the three-phase cascade H-bridge. Although the multiplexed active power decoupling control of the three-phase cascade H-bridge is similar to that of the single H-bridge, there are many differences. There are still many problems to be solved in the multiplexed active power decoupling control of the three-phase cascade H-bridge. Therefore, the research on the multiplexed active power decoupling control of the cascade H-bridge is very necessary. The present invention takes the cascade H-bridge STATCOM as the research background, and suppresses the double frequency fluctuation of the H-bridge DC side voltage based on the multiplexed Buck type active power decoupling technology. Its control method can be extended to other cascade multi-level converters. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a non-electrolytic capacitor cascaded H-bridge STATCOM system and control method based on active power decoupling technology without adding power switch tubes. The specific technical implementation plan is as follows:

[0006] A non-electrolytic capacitor cascade H-bridge STATCOM system and control method, the system specifically comprising:

[0007] A three-phase power grid, a three-phase reactive load, a filter inductor and a three-phase cascaded N double-Buck multiplexed H-bridge, N ≥ 2; the double-Buck multiplexed H-bridge includes: four fully controlled switch tubes S1-S4, two decoupling inductors L r1 and L r2 , three film capacitors C r1 , C r2 and C d ; said C r1 Passing by L r1 Connected to the midpoint of the first bridge arm of the H bridge to form the first bidirectional Buck circuit. This bidirectional Buck circuit and the first bridge arm of the H bridge realize switch tube multiplexing. r2 Passing by L r2The film capacitor C is connected to the midpoint of the second bridge arm of the H bridge to form a second bidirectional Buck circuit, which realizes switch tube multiplexing with the second bridge arm of the H bridge; d It is used to replace the traditional electrolytic capacitor on the DC side of the H-bridge; the midpoint of the first bridge arm of the double-Buck multiplexed H-bridge in each phase is connected to the midpoint of the second bridge arm of the previous H-bridge, the midpoint of the first bridge arm of the first double-Buck multiplexed H-bridge in each phase is connected to the filter inductor, and the midpoint of the second bridge arm of the last double-Buck multiplexed H-bridge is connected to the midpoint of the second bridge arm of the last double-Buck multiplexed H-bridge in the other two phases.

[0008] Furthermore, a non-electrolytic capacitor cascaded H-bridge STATCOM system and control method are provided, wherein the control method specifically comprises the following steps:

[0009] Step 1: Output current i of phase x (x = A, B, C) ox Control to complete the AC and DC power exchange to achieve compensation for load reactive current;

[0010] Step 2: Decouple the inductor current i of each dual-Buck multiplexed H-bridge of phase x Lrxj1 with i Lrxj2 The mean value of is controlled to achieve active power decoupling and suppress the double frequency fluctuation of the H-bridge DC side voltage.

[0011] Furthermore, in step 1, the x-phase output current i ox The control method is:

[0012] The output current i of phase x ox With its given value i ox * The difference is sent to the PR controller, and the grid voltage u sx Subtract the output of the PR controller to get the output voltage set value u of the phase containing the zero-sequence voltage rx ; for u rx The reference output voltage u of the phase is obtained by separating the zero-sequence voltage ox ;Use u ox Add the intra-phase voltage balance control result Δu xj (j=1,2,…,N), and the sum is divided by the total voltage u of the x-phase DC side dcx The result is the reference modulation wave u of the j-th dual-Buck multiplexed H-bridge of phase x. oxj ;Use u oxj Plus the active power decoupling control modulation wave u of the jth H-bridge module of phase x xcj The result is the modulation wave u of the first bridge arm of the jth H-bridge module in phase x. oxj1 ; for u oxj Negate to get -uoxj , use -u oxj Plus the active power decoupling control modulation wave u of the jth H-bridge module of phase x xcj The result is the modulation wave u of the second bridge arm of the jth H-bridge module in phase x. oxj2 ; According to u oxj1 、u oxj2 , the four switch tubes of each H bridge are driven by carrier phase-shift SPWM technology.

[0013] Furthermore, the rx Perform zero-sequence voltage separation to obtain the x-phase reference output voltage u ox The specific method is:

[0014] Find u ra 、u rb and u rc The average value of each phase can be obtained by multiplying the average value by -1. zx ; will u zx Superimpose on u rx The zero-sequence separation of a given voltage can be achieved, thereby obtaining u ox .

[0015] Furthermore, the intra-phase voltage balancing control is performed to obtain Δu xj The specific method is:

[0016] The DC side voltage u of each H-bridge of phase x dcxj The average voltage u of the DC side of this phase dcxv The difference is then passed through the P controller and then compared with i ox Multiplying the sign value of xj .

[0017] Furthermore, the x-phase output current given value i is obtained. ox * The specific method is:

[0018] The average voltage u of each H-bridge DC side of phase x dcxv With its given value u dcxv * The difference is sent to the PI controller, and the PI controller output is multiplied by the sine value of the phase grid angle to obtain i ox Active part of given value i dx * ; Extract reactive power from the x-phase load current to obtain i ox The reactive part of the given value i Lqx ;change i dx * and i Lqx Add together to get i rx ; Find ira 、i rb and i rc The average value of each phase can be obtained by multiplying the average value by -1. zx ;change i zx Superimpose to i rx The zero-sequence separation of a given current can be achieved, thereby obtaining i ox * .

[0019] Furthermore, in step 2, the decoupling current i of each H-bridge module of the x phase is Lrxj1 with i Lrxj2 The control method of the mean is:

[0020] For the two decoupling currents i of the jth H-bridge module Lrxj1 with i Lrxj2 Find the average value to get i Lrxvj ;change i Lrxvj With its given value i Lrxv * The difference is sent to the PR controller, and the output of the PR controller is divided by the DC side voltage u of each H-bridge module. dcxj The result is the active power decoupling control modulation wave u of the jth H-bridge module in phase x. xcj .

[0021] Furthermore, the given value i of the decoupling current mean is obtained Lrxv * The specific method is:

[0022] The DC side current i of each H-bridge module of phase x dcxj Find the average value to get i dcxv ;change i dcxv The output of the multi-R controller is the given value i of the decoupling current mean of the jth H-bridge module of phase x. Lrxv * .

[0023] Furthermore, the multi-R controller specifically includes:

[0024] A resonant controller with a resonant frequency of 200π and a resonant controller with a resonant frequency of 400π.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] 1) The present invention is based on the multiplexed active power decoupling technology to realize the sharing of switch tubes of the cascaded H-bridge and the Buck type active power decoupling circuit. Without increasing the power switch tube, the double frequency fluctuation of the DC side voltage of the H-bridge can be well suppressed, thereby replacing the traditional electrolytic capacitor with a small-capacitance film capacitor, thereby increasing the reliability of the device;

[0027] 2) The present invention combines cascaded multi-level technology, active power decoupling technology and three-phase independent control technology, and proposes a phase-independent control method that combines a given current zero-sequence separation method with a given voltage zero-sequence separation method. While ensuring the advantages of traditional cascaded H-bridge multi-level technology, it can greatly reduce the double frequency fluctuation of the H-bridge DC side voltage, and has strong adaptability to three-phase unbalanced power grids. In addition, the control is simple and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a topology diagram of the electrolytic capacitor-free cascade H-bridge STATCOM circuit of the present invention.

[0029] Figure 2 This is a control framework diagram of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention.

[0030] Figure 3 It is the DC side voltage waveform of each H-bridge module of the traditional cascade H-bridge STATCOM.

[0031] Figure 4 This is the DC side voltage waveform of each H-bridge module of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention.

[0032] Figure 5 These are the voltage waveforms of two auxiliary capacitors of the H-bridge module of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention.

[0033] Figure 6 It is the FFT analysis result of the auxiliary capacitor voltage of each H-bridge module of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. However, it should be understood that the present invention can be implemented in various forms, and some exemplary and non-limiting embodiments shown in the accompanying drawings and described below are not intended to limit the present invention to the specific embodiments described.

[0035] Figure 1 The main circuit topology of the electrolytic capacitor-free cascade H-bridge STATCOM system of the present invention is shown, including: a three-phase power grid, a three-phase reactive load, a filter inductor and a three-phase cascaded N double-Buck multiplexed H-bridge, N ≥ 2; the double-Buck multiplexed H-bridge includes: four fully controlled switch tubes S1-S4, two decoupling inductors L r1 and L r2 , three film capacitors C r1 , C r2 and C d ; said C r1Passing by L r1 Connected to the midpoint of the first bridge arm of the H bridge to form the first bidirectional Buck circuit. This bidirectional Buck circuit and the first bridge arm of the H bridge realize switch tube multiplexing. r2 Passing by L r2 The film capacitor C is connected to the midpoint of the second bridge arm of the H bridge to form a second bidirectional Buck circuit, which realizes switch tube multiplexing with the second bridge arm of the H bridge; d It is used to replace the traditional electrolytic capacitor on the DC side of the H-bridge; the midpoint of the first bridge arm of the double-Buck multiplexed H-bridge in each phase is connected to the midpoint of the second bridge arm of the previous H-bridge, the midpoint of the first bridge arm of the first double-Buck multiplexed H-bridge in each phase is connected to the filter inductor, and the midpoint of the second bridge arm of the last double-Buck multiplexed H-bridge is connected to the midpoint of the second bridge arm of the last double-Buck multiplexed H-bridge in the other two phases.

[0036] The working principle of the electrolytic capacitor-free cascade H-bridge STATCOM system of the present invention is as follows:

[0037] The control adopted by the present invention is overall phase-by-phase independent control, that is, three-phase independent control. The control of each phase has two objectives, one is to control the output current i of the x phase ox Complete the power exchange on the AC and DC sides and realize reactive power compensation; second, the two decoupled inductor currents i of each H-bridge module Lrxj1 with i Lrxj2 The mean value is controlled to achieve active power decoupling and eliminate the double frequency fluctuation of the DC side voltage. The three phases are controlled separately, and the phase-separated independent control technology based on the given current zero-sequence separation method and the given voltage zero-sequence separation method is adopted. Each phase has an independent DC side voltage controller, output current controller, and decoupling current mean value controller.

[0038] Ideally, the parameters of the components of the three-phase H-bridge modules are the same, and the control principles are similar. The difference is that the grid angles referenced by each phase are different, with phase B lagging behind phase A by 120°, and phase C lagging behind phase B by 120°. Therefore, to simplify the analysis, only one phase is used as an example to derive and analyze the control principle. Without considering the active power loss of the converter, the grid connection point voltage and STATCOM output current expressions can be expressed as:

[0039] E1:

[0040] Among them, U sx ,I ox u sx and i ox The amplitude of θ x for u sx The initial phase angle, is the x-phase output current i oxRelative to the grid voltage u sx Angle, when compensating for inductive loads, When compensating for capacitive loads, In this way, the instantaneous power on the AC side can be expressed as:

[0041] E2:

[0042] From E2, we can see that the instantaneous power on the AC side of STATCOM contains a double frequency fluctuation component. According to the law of energy conservation, this fluctuating power will cause the voltage on the DC side of the H-bridge to fluctuate at a double frequency after being transferred to the DC side of the H-bridge. In order to suppress the double frequency fluctuation of the voltage on the DC side of the H-bridge, this part of the fluctuating power needs to be transferred to the auxiliary capacitor C. r1 and C r2 So the auxiliary capacitor C r1 and C r2 The voltage on is finally modulated into:

[0043] E3:

[0044] Among them, U dc is the DC side voltage of the H-bridge module u dcxj The DC component, U B is the amplitude of the double frequency fluctuation voltage, δ x is the initial phase angle of the double frequency fluctuation voltage relative to the x-phase grid voltage. r The current on can be expressed as:

[0045] E4:

[0046] Ignore the decoupling inductor L r1 and L r2 The impact of C r1 =C r2 =C r According to E3 and E4, the instantaneous power of the x-phase DC side can be calculated as:

[0047] E6:

[0048] It can be seen from E6 that the DC side power contains secondary and quartic fluctuation components at this time. The secondary fluctuation component can be used to balance the AC part in E2, thereby suppressing the double frequency fluctuation of the DC side voltage of the H-bridge. In addition, it can be found that additional quartic fluctuation power is introduced in E6, which can be eliminated by adding a 4-fold frequency R controller in the decoupling current set value extraction link.

[0049] In order to eliminate the double frequency voltage on the DC side of the H-bridge, the secondary fluctuation part in E6 is made equal to the secondary fluctuation part in E2. At the same time, considering that the values ​​of each auxiliary capacitor are equal in an ideal situation, the solution is:

[0050]

[0051] By combining E7 and E3, the auxiliary capacitor voltage of the H-bridge is modulated into the form shown in E3 to achieve active power decoupling and eliminate the double frequency fluctuation of the DC side voltage of the H-bridge.

[0052] Figure 2 The control framework diagram of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention is a control method of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention, and the specific implementation includes the following steps:

[0053] Step 1: Output current i of phase x (x = A, B, C) ox Control to complete the AC and DC power exchange to achieve compensation for load reactive current;

[0054] Step 2: Decouple the inductor current i of each dual-Buck multiplexed H-bridge of phase x Lrxj1 with i Lrxj2 The mean value of is controlled to achieve active power decoupling and suppress the double frequency fluctuation of the H-bridge DC side voltage.

[0055] Furthermore, in step 1, the x-phase output current i ox The control method is:

[0056] The output current i of phase x ox With its given value i ox * The difference is sent to the PR controller, and the grid voltage u sx Subtract the output of the PR controller to get the output voltage set value u of the phase containing the zero-sequence voltage rx ; for u rx The reference output voltage u of the phase is obtained by separating the zero-sequence voltage ox ;Use u ox Add the intra-phase voltage balance control result Δu xj (j=1,2,…,N), and the sum is divided by the total voltage u of the x-phase DC side dcx The result is the reference modulation wave u of the j-th dual-Buck multiplexed H-bridge of phase x. oxj ;Use u oxj Plus the active power decoupling control modulation wave u of the jth H-bridge module of phase x xcj The result is the modulation wave u of the first bridge arm of the jth H-bridge module in phase x. oxj1 ; for u oxjNegate to get -u oxj , use -u oxj Plus the active power decoupling control modulation wave u of the jth H-bridge module of phase x xcj The result is the modulation wave u of the second bridge arm of the jth H-bridge module in phase x. oxj2 ; According to u oxj1 、u oxj2 , the four switch tubes of each H bridge are driven by carrier phase-shift SPWM technology.

[0057] Furthermore, the rx Perform zero-sequence voltage separation to obtain the x-phase reference output voltage u ox The specific method is:

[0058] Find u ra 、u rb and u rc The average value of each phase can be obtained by multiplying the average value by -1. zx ; will u zx Superimpose on u rx The zero-sequence separation of a given voltage can be achieved, thereby obtaining u ox .

[0059] Furthermore, the intra-phase voltage balancing control is performed to obtain Δu xj The specific method is:

[0060] The DC side voltage u of each H-bridge of phase x dcxj The average voltage u of the DC side of this phase dcxv The difference is then passed through the P controller and then compared with i ox Multiplying the sign value of xj .

[0061] Furthermore, the x-phase output current given value i is obtained. ox * The specific method is:

[0062] The average voltage u of each H-bridge DC side of phase x dcxv With its given value u dcxv * The difference is sent to the PI controller, and the PI controller output is multiplied by the sine value of the phase grid angle to obtain i ox Active part of given value i dx * ; Extract reactive power from the x-phase load current to obtain i ox The reactive part of the given value i Lqx ;change i dx * and i Lqx Add together to get i rx; Find i ra 、i rb and i rc The average value of each phase can be obtained by multiplying the average value by -1. zx ;change i zx Superimpose to i rx The zero-sequence separation of a given current can be achieved, thereby obtaining i ox * .

[0063] Furthermore, in step 2, the decoupling current i of each H-bridge module of the x phase is Lrxj1 with i Lrxj2 The control method of the mean is:

[0064] For the two decoupling currents i of the jth H-bridge module Lrxj1 with i Lrxj2 Find the average value to get i Lrxvj ;change i Lrxvj With its given value i Lrxv * The difference is sent to the PR controller, and the output of the PR controller is divided by the DC side voltage u of each H-bridge module. dcxj The result is the active power decoupling control modulation wave u of the jth H-bridge module in phase x. xcj .

[0065] Furthermore, the given value i of the decoupling current mean is obtained Lrxv * The specific method is:

[0066] The DC side current i of each H-bridge module of phase x dcxj Find the average value to get i dcxv ;change i dcxv The output of the multi-R controller is the given value i of the decoupling current mean of the jth H-bridge module of phase x. Lrxv * .

[0067] Furthermore, the multi-R controller specifically includes:

[0068] A resonant controller with a resonant frequency of 200π and a resonant controller with a resonant frequency of 400π.

[0069] The control method is designed according to the above process, and a simulation experiment is performed using Matlab / Simulink to verify the effectiveness of the present invention. The simulation parameters are shown in Table 1:

[0070] Table 1 Simulation parameters

[0071]

[0072] Figure 3 The DC side voltage waveforms of each H-bridge module of the traditional cascade H-bridge STATCOM are given. It can be seen that the three-phase DC side voltage reaches a balanced state at this time, and the mean values ​​of the three-phase DC side voltages are equal, but there is a large double frequency fluctuation with an amplitude of about 260V.

[0073] Figure 4 The DC side voltage waveforms of each H-bridge module of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention are given. It can be seen that under the control of the method of the present invention, the three-phase DC side voltages are equal, and at the same time, the double frequency fluctuation of the DC side voltage is suppressed. The DC side voltage waveform is basically a straight line, with only about 3V of high-order harmonics, and its DC side voltage fluctuation is about 1% of that of the traditional cascade H-bridge STATCOM.

[0074] Figure 5 The two auxiliary capacitor voltage waveforms of the H-bridge module of the electrolytic capacitor-free cascaded H-bridge STATCOM of the present invention are given. It can be seen that the fluctuation parts of the two voltages have equal amplitudes and opposite phases, which is consistent with the previous analysis.

[0075] Figure 6 The FFT analysis results of the auxiliary capacitor voltage of the H-bridge module of the system of the present invention are given. It can be seen that the auxiliary capacitor voltage contains a fundamental frequency fluctuation and a double frequency fluctuation, which is consistent with the previous theoretical analysis. The fundamental frequency part is used to generate the AC side sinusoidal voltage of the H-bridge module, and the double frequency part is used to balance the double frequency fluctuation power of the AC side of the H-bridge module, thereby suppressing the double frequency fluctuation of the DC side voltage of the H-bridge.

[0076] The above embodiments are preferred embodiments of the present invention and are possible examples of embodiments, which are presented only for a clear understanding of the principles of the present invention. Many changes and modifications can be made to the above embodiments without departing from the spirit and principles of the technology described in the present invention. For those skilled in the art, several improvements and modifications can be made without departing from the principles of the present invention, and these changes and modifications should also be considered as the scope of protection of the present invention.

Claims

1. A STATCOM system without electrolytic capacitor cascade H bridge, characterized in that: The system includes: a three-phase power grid, a three-phase reactive load, a filter inductor and a three-phase cascaded N double-Buck multiplexed H-bridge, N ≥ 2; the double-Buck multiplexed H-bridge includes: four fully controlled switch tubes S1-S4, two decoupling inductors L r1 and L r2 , three film capacitors C r1 , C r2 and C d ; said C r1 Passing by L r1 Connected to the midpoint of the first bridge arm of the H bridge to form the first bidirectional Buck circuit. This bidirectional Buck circuit and the first bridge arm of the H bridge realize switch tube multiplexing. r2 Passing by L r2 The film capacitor C is connected to the midpoint of the second bridge arm of the H bridge to form a second bidirectional Buck circuit, which realizes switch tube multiplexing with the second bridge arm of the H bridge; d It is used to replace the traditional electrolytic capacitor on the DC side of the H-bridge; the midpoint of the first bridge arm of the dual-Buck multiplexed H-bridge in each phase is connected to the midpoint of the second bridge arm of the previous H-bridge, the midpoint of the first bridge arm of the first dual-Buck multiplexed H-bridge in each phase is connected to the filter inductor, and the midpoint of the second bridge arm of the last dual-Buck multiplexed H-bridge is connected to the midpoint of the second bridge arm of the last dual-Buck multiplexed H-bridge in the other two phases; The system control method comprises the following steps: Step 1: Output current i for phases x = A, B, C ox Control to complete the AC and DC power exchange to achieve compensation for load reactive current; Step 2: Decouple the inductor current i of each dual-Buck multiplexed H-bridge of phase x Lrxj1 with i Lrxj2 The mean value of is controlled to achieve active power decoupling and suppress the double frequency fluctuation of the H-bridge DC side voltage; In step 1, the x-phase output current i ox The control method is: The output current i of phase x ox With its given value i ox * The difference is sent to the PR controller, and the grid voltage u sx Subtract the output of the PR controller to get the output voltage set value u of the phase containing the zero-sequence voltage rx ; for u rx The reference output voltage u of the phase is obtained by separating the zero-sequence voltage ox ;Use u ox Add the intra-phase voltage balance control result Δu xj (j=1,2,…,N), and the sum is divided by the total voltage u of the x-phase DC side dcx The result is the reference modulation wave u of the j-th dual-Buck multiplexed H-bridge of phase x. oxj ;Use u oxj Plus the active power decoupling control modulation wave u of the jth H-bridge module of phase x xcj The result is the modulation wave u of the first bridge arm of the jth H-bridge module in phase x. oxj1 ; for u oxj Negate to get -u oxj , use -u oxj Plus the active power decoupling control modulation wave u of the jth H-bridge module of phase x xcj The result is the modulation wave u of the second bridge arm of the jth H-bridge module in phase x. oxj2 ; According to u oxj1 、u oxj2 , the four switch tubes of each H bridge are driven by carrier phase-shift SPWM technology.

2. The control method of a non-electrolytic capacitor cascaded H-bridge STATCOM system as claimed in claim 1, characterized in that: The system control method comprises the following steps: Step 1: Output current i for phases x = A, B, C ox Control to complete the AC and DC power exchange to achieve compensation for load reactive current; Step 2: Decouple the inductor current i of each dual-Buck multiplexed H-bridge of phase x Lrxj1 with i Lrxj2 The mean value of is controlled to achieve active power decoupling and suppress the double frequency fluctuation of the H-bridge DC side voltage; In step 1, the x-phase output current i ox The control method is: The output current i of phase x ox With its given value i ox * The difference is sent to the PR controller, and the grid voltage u sx Subtract the output of the PR controller to get the output voltage set value u of the phase containing the zero-sequence voltage rx ; for u rx The reference output voltage u of the phase is obtained by separating the zero-sequence voltage ox ;Use u ox Add the intra-phase voltage balance control result Δu xj (j=1,2,…,N), and the sum is divided by the total voltage u of the x-phase DC side dcx The result is the reference modulation wave u of the j-th dual-Buck multiplexed H-bridge of phase x. oxj ;Use u oxj Plus the active power decoupling control modulation wave u of the jth H-bridge module of phase x xcj The result is the modulation wave u of the first bridge arm of the jth H-bridge module in phase x. oxj1 ; for u oxj Negate to get -u oxj , use -u oxj Plus the active power decoupling control modulation wave u of the jth H-bridge module of phase x xcj The result is the modulation wave u of the second bridge arm of the jth H-bridge module in phase x. oxj2 ; According to u oxj1 、u oxj2 , the four switch tubes of each H bridge are driven by carrier phase-shift SPWM technology.

3. The control method of a non-electrolytic capacitor cascaded H-bridge STATCOM system as claimed in claim 2, characterized in that: To u rx Perform zero-sequence voltage separation to obtain the x-phase reference output voltage u ox The specific method is: Find u ra 、u rb and u rc The average value of each phase can be obtained by multiplying the average value by -1. zx ; will u zx Superimpose to u rx The zero-sequence separation of a given voltage can be achieved, thereby obtaining u ox .

4. The control method of a non-electrolytic capacitor cascaded H-bridge STATCOM system as claimed in claim 2, characterized in that: Perform phase voltage balancing control to obtain Δu xj The specific method is: The DC side voltage u of each H-bridge of phase x dcxj The average voltage u of the DC side of this phase dcxv The difference is then passed through the P controller and then compared with i ox Multiplying the sign value of xj .

5. The control method of a non-electrolytic capacitor cascaded H-bridge STATCOM system as claimed in claim 2, characterized in that: Get the x-phase output current given value i ox * The specific method is: The average voltage u of each H-bridge DC side of phase x dcxv With its given value u dcxv * The difference is sent to the PI controller, and the PI controller output is multiplied by the sine value of the phase grid angle to obtain i ox Active part of given value i dx * ; Extract reactive power from the x-phase load current to obtain i ox The reactive part of the given value i Lqx ;change i dx * and i Lqx Add together to get i rx ; Find i ra 、i rb and i rc The average value of each phase can be obtained by multiplying the average value by -1. zx ;change i zx Superimpose to i rx The zero-sequence separation of a given current can be achieved, thereby obtaining i ox * .

6. The control method of a non-electrolytic capacitor cascaded H-bridge STATCOM system as claimed in claim 2, characterized in that: In step 2, the decoupling current i of each H-bridge module of the x phase is Lrxj1 with i Lrxj2 The control method of the mean is: For the two decoupling currents i of the jth H-bridge module Lrxj1 with i Lrxj2 Find the average value to get i Lrxvj ;change i Lrxvj With its given value i Lrxv * The difference is sent to the PR controller, and the output of the PR controller is divided by the DC side voltage u of each H-bridge module. dcxj The result is the active power decoupling control modulation wave u of the jth H-bridge module in phase x. xcj .

7. The control method of a non-electrolytic capacitor cascaded H-bridge STATCOM system as claimed in claim 6, characterized in that: Get the given value i of the decoupled current mean Lrxv * The specific method is: The DC side current i of each H-bridge module of phase x dcxj Find the average value to get i dcxv ;change i dcxv The output of the multi-R controller is the given value i of the decoupling current mean of the jth H-bridge module of the x-phase. Lrxv * .

8. The control method of a non-electrolytic capacitor cascaded H-bridge STATCOM system as claimed in claim 7, characterized in that: The multi-R controller specifically includes: A resonant controller with a resonant frequency of 200π and a resonant controller with a resonant frequency of 400π.

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