A capacitor-free cascaded H-bridge STATCOM system and its 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 Buck multiplexing type H-bridge, N≥2; the Buck multiplexing type H-bridge comprises: an H-bridge composed of switch tubes S1-S4, a decoupling inductor L r , two film capacitors C d and C r , C r Passing by L r The midpoint of the second bridge arm of the H bridge is connected to form a bidirectional Buck circuit, and the bidirectional Buck circuit and the second bridge arm of the H bridge realize switch tube multiplexing; the midpoint of the first bridge arm of the H bridge is connected to the midpoint of the second bridge arm of the previous H bridge, and the three phases are connected in star shape. In the present invention, the bidirectional Buck circuit performs active power decoupling on the one hand, and the fluctuating power on the DC side of the H bridge is transmitted to the L r Transfer to auxiliary capacitor C r , thereby eliminating the double frequency fluctuation of the voltage on the DC side of the H-bridge, realizing the replacement of electrolytic capacitors with film capacitors, and at the same time assisting the first arm of the H-bridge to complete the power conversion on the AC and DC sides, thereby improving 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 three-phase filter inductor and a three-phase cascaded N Buck multiplexing H-bridge, N ≥ 2; characterized in that the Buck multiplexing H-bridge comprises: an H-bridge composed of switch tubes S1 to S4, a decoupling inductor L r , two film capacitors C d and C r , C r Passing by L rA bidirectional Buck circuit is formed by connecting 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 midpoint of the first bridge arm of the Buck multiplexing 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 Buck multiplexing H bridge in each phase is connected to the filter inductor, and the midpoint of the second bridge arm of the last Buck multiplexing H bridge is connected to the midpoint of the second bridge arm of the last Buck multiplexing H bridge of 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: Control the first bridge arm of each Buck multiplexing H bridge of phase x (x = A, B, C) to achieve AC and DC side power exchange;

[0010] Step 2: Control the second bridge arm of each Buck multiplexing H-bridge of the x-phase to achieve active power decoupling and suppress the double frequency fluctuation of the H-bridge DC side voltage.

[0011] Furthermore, the control method of the first bridge arm of each Buck multiplexed H bridge of the x phase in step 1 is:

[0012] Output current i for phase x ox Control to generate the output reference voltage u of this phase ox , according to u ox , the result of the phase voltage balance control Δu xj (j=1,2,…,N) and the modulation wave u of the second bridge arm ox2j Get the modulation wave u of the first bridge arm of each H bridge ox1j , according to u ox1j , based on the carrier phase shift modulation technology, the first bridge arm of each H bridge of this phase is driven.

[0013] Furthermore, the control method of the second bridge arm of each Buck multiplexed H bridge of phase x in step 2 is:

[0014] The decoupling inductor current i of each H-bridge of phase x Lrxj Control to obtain the modulation wave u of the second bridge arm ox2j , according to u ox2j The second bridge arm of each H-bridge of the x-phase is driven by using carrier phase shift modulation technology.

[0015] Furthermore, the modulation wave u of the first bridge arm of each H bridge of phase x is obtained. ox1j The specific method is:

[0016] The output current i of phase x ox With its given value The difference is sent to the quasi-PR controller, and the output of the quasi-PR controller is then compared with the grid voltage u sx After the difference is made, the zero-sequence voltage is separated to obtain u ox ; 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 Multiply the sign value of to get Δu xj ; will u ox , Δu xj After adding, divide by the DC side voltage u of each H bridge dcxj , and add u to the result ox2j The modulation wave u of the first bridge arm of each H bridge in this phase can be obtained ox1j .

[0017] Furthermore, the modulation wave u of the second bridge arm of each H bridge of phase x is obtained. ox2j The specific method is:

[0018] The average value u of the auxiliary capacitor voltage of each H-bridge of phase x cxv Perform PI control, the average current i of each H-bridge DC side dcxv After the multi-PR controller, the sum of the outputs of the PI controller and the multi-PR controller is used as the given value of each H-bridge decoupling current Will and the decoupling current i of the jth H-bridge module of this phase Lrxj The difference is passed through the multi-PR controller, and the output of the multi-PR controller is divided by the DC side voltage u of each H-bridge dcxj , we can get the modulation wave u of the second arm of the H bridge ox2j .

[0019] Furthermore, the x-phase output current reference value is obtained The specific method is:

[0020] The average voltage u of each H-bridge DC side of phase x dcxv Perform PI control and multiply the PI controller output by the sine value of the phase grid angle to obtain i ox Active part of the given value Extract reactive power from the x-phase load current to obtain i ox The reactive part of the given value i Lqx ;Will and i Lqx Add together to get i rx ; For three-phase i ra 、i rb and i rc Calculate the average value, and then multiply the average value by -1 to get the zero-sequence current i of each phase. zx ;change izx Superimpose to i rx The zero-sequence separation of a given current can be achieved, thus obtaining

[0021] Furthermore, the zero-sequence voltage is separated to obtain the x-phase output reference voltage u ox The specific method is:

[0022] x-phase grid voltage u sx Subtract the output of the phase current controller to get u rx , for the three-phase u ra 、u rb and u rc Calculate the average value, and then multiply the average value by -1 to get the zero-sequence voltage u of each phase zx ; will u zx Superimpose on u rx The zero-sequence separation of a given voltage can be achieved, thereby obtaining u ox .

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 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;

[0025] 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

[0026] Figure 1 This is the topology diagram of the traditional cascade H-bridge STATCOM circuit.

[0027] Figure 2 This is a topology diagram of the electrolytic capacitor-free cascade H-bridge STATCOM circuit of the present invention.

[0028] Figure 3 This is a control framework diagram of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention.

[0029] 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.

[0030] Figure 5 It is the DC side voltage waveform of each H-bridge module of the traditional cascade H-bridge STATCOM.

[0031] Figure 6 The waveforms of the DC side voltage and auxiliary capacitor voltage of each H-bridge module of the electrolytic capacitor-free cascade H-bridge STATCOM of the present invention. DETAILED DESCRIPTION

[0032] 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.

[0033] 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 three-phase filter inductor and a three-phase cascaded N Buck multiplexing H-bridge, N ≥ 2; wherein the Buck multiplexing H-bridge includes: an H-bridge composed of switch tubes S1 to S4, a decoupling inductor L r , two film capacitors C d and C r , C r Passing by L r Connected to the midpoint of the second arm of the H-bridge to form a bidirectional Buck circuit, the bidirectional Buck circuit and the second arm of the H-bridge realize switch tube multiplexing; the bidirectional Buck circuit is based on active power decoupling technology to transfer the double frequency fluctuation power on the DC side of the H-bridge to the auxiliary capacitor C r , so that the H-bridge DC side capacitor C d There is no fluctuating power on the upper side, realizing the suppression of the double frequency fluctuation of the DC side voltage of the H-bridge; wherein, the midpoint of the first bridge arm of the Buck multiplexing 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 Buck multiplexing H-bridge in each phase is connected to the filter inductor, and the midpoint of the second bridge arm of the last Buck multiplexing H-bridge is connected to the midpoint of the second bridge arm of the last Buck multiplexing H-bridge in the other two phases. Figure 2 The traditional cascaded H-bridge STATCOM system shown in the figure has an additional auxiliary capacitor C on the DC side of each H-bridge. r and a decoupling inductor L r , without adding additional power switching devices.

[0034] The working principle of the electrolytic capacitor-free cascade H-bridge STATCOM system of the present invention is as follows:

[0035] The control adopted by the present invention is overall phase-independent control, that is, three-phase independent control. The control of each phase has two goals. One is to complete the power exchange on the AC and DC sides and realize reactive compensation by controlling the opening and closing of the first bridge arm of each H-bridge module; the other is to realize active power decoupling and eliminate the double frequency fluctuation of the DC side voltage by controlling the opening and closing of the second bridge arm of each H-bridge module. The three phases are controlled separately, and a phase-independent control technology based on a combination of a given current zero-sequence separation method and a given voltage zero-sequence separation method is adopted. Each phase has an independent DC side voltage controller, an output current controller, an auxiliary capacitor voltage controller, and a decoupling current controller.

[0036] 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:

[0037]

[0038] Among them, U sx ,I ox u sx and i ox The amplitude of θ x for u sx The initial phase angle, φ is the x-phase output current i ox Relative to the grid voltage u sx When compensating for inductive loads, φ = π / 2, and when compensating for capacitive loads, φ = -π / 2. In this way, the instantaneous power on the AC side can be expressed as:

[0039]

[0040] 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. rxj So the auxiliary capacitor C rxj The voltage on is finally modulated into:

[0041] E3:u cxj =U c0 +U c sin(2ωt+δ x )

[0042] Among them, U c0 is the voltage u cxj The DC component, U c is u cxj Amplitude of the AC part, δ x is the initial phase angle of the AC part relative to the x-phase grid voltage. r The current on can be expressed as:

[0043] E4:

[0044] According to E3 and E4, the instantaneous power of the x-phase DC side can be calculated as:

[0045] E6:

[0046] 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 PR controller in the decoupling current set value extraction link.

[0047] 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, the values ​​of the auxiliary capacitors are equal in the ideal case, that is, C rxj =C, we can solve:

[0048] E7:

[0049] 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 H-bridge DC side voltage.

[0050] Figure 3 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:

[0051] Step 1: Control the first bridge arm of each Buck multiplexing H bridge of phase x (x = A, B, C) to achieve AC and DC side power exchange;

[0052] Step 2: Control the second bridge arm of each Buck multiplexing H-bridge of the x-phase to achieve active power decoupling and suppress the double frequency fluctuation of the H-bridge DC side voltage.

[0053] Furthermore, the control method of the first bridge arm of each Buck multiplexed H bridge of the x phase in step 1 is:

[0054] Output current i for phase x ox Control to generate the output reference voltage u of this phase ox , according to u ox , the result of the phase voltage balance control Δu xj (j=1,2,…,N) and the modulation wave u of the second bridge arm ox2j Get the modulation wave u of the first bridge arm of each H bridge ox1j , according to u ox1j , based on the carrier phase shift modulation technology, the first bridge arm of each H bridge of this phase is driven.

[0055] Furthermore, the control method of the second bridge arm of each Buck multiplexed H bridge of phase x in step 2 is:

[0056] The decoupling inductor current i of each H-bridge of phase x Lrxj Control to obtain the modulation wave u of the second bridge arm ox2j , according to u ox2j The second bridge arm of each H-bridge of the x-phase is driven by using carrier phase shift modulation technology.

[0057] Furthermore, the modulation wave u of the first bridge arm of each H bridge of phase x is obtained. ox1j The specific method is:

[0058] The output current i of phase x ox With its given value The difference is sent to the quasi-PR controller, and the output of the quasi-PR controller is then compared with the grid voltage u sx After the difference is made, the zero-sequence voltage is separated to obtain u ox ; 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 Multiply the sign value of to get Δu xj ; will u ox , Δu xj After adding, divide by the DC side voltage u of each H bridge dcxj , and add u to the result ox2j The modulation wave u of the first bridge arm of each H bridge in this phase can be obtained ox1j .

[0059] Furthermore, the modulation wave u of the second bridge arm of each H bridge of phase x is obtained. ox2j The specific method is:

[0060] The average value u of the auxiliary capacitor voltage of each H-bridge of phase x cxv Perform PI control, the average current i of each H-bridge DC side dcxvAfter the multi-PR controller, the sum of the outputs of the PI controller and the multi-PR controller is used as the given value of each H-bridge decoupling current Will and the decoupling current i of the jth H-bridge module of this phase Lrxj The difference is passed through the multi-PR controller, and the output of the multi-PR controller is divided by the DC side voltage u of each H-bridge dcxj , we can get the modulation wave u of the second arm of the H bridge ox2j .

[0061] Furthermore, the x-phase output current reference value is obtained The specific method is:

[0062] The average voltage u of each H-bridge DC side of phase x dcxv Perform PI control and multiply the PI controller output by the sine value of the phase grid angle to obtain i ox Active part of the given value Extract reactive power from the x-phase load current to obtain i ox The reactive part of the given value i Lqx ;Will and i Lqx Add together to get i rx ; For three-phase i ra 、i rb and i rc Calculate the average value, and then multiply the average value by -1 to get the zero-sequence current i of each phase. zx ;change i zx Superimpose to i rx The zero-sequence separation of a given current can be achieved, thus obtaining

[0063] Furthermore, the zero-sequence voltage is separated to obtain the x-phase output reference voltage u ox The specific method is:

[0064] x-phase grid voltage u sx Subtract the output of the phase current controller to get u rx , for the three-phase u ra 、u rb and u rc Calculate the average value, and then multiply the average value by -1 to get the zero-sequence voltage u of each phase zx ; will u zx Superimpose on u rx The zero-sequence separation of a given voltage can be achieved, thereby obtaining u ox .

[0065] 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:

[0066] Table 1 Simulation parameters

[0067]

[0068] Figure 4 The DC side voltage waveforms of each H-bridge module of the electrolytic capacitor-free cascaded 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 the double frequency fluctuation of the DC side voltage is suppressed. The DC side voltage waveform is basically a straight line, with only about 2V high-order harmonics. Figure 5 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, and the fluctuation value is about 72V; Figure 6 The DC side voltage and auxiliary capacitor voltage of each H-bridge module of the cascaded H-bridge STATCOM without electrolytic capacitor of the present invention are given. It can be seen that the fluctuating power on the DC side of the cascaded H-bridge is transferred to the auxiliary capacitor C r Therefore, the auxiliary capacitor voltage contains double frequency fluctuations, and the DC side voltage has almost no fluctuations, which achieves the suppression of the low-frequency ripple of the DC side voltage, which is consistent with the previous analysis.

[0069] 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 three-phase filter inductor and a three-phase cascaded N Buck multiplexing H bridge, N ≥ 2; the characteristic of the Buck multiplexing H bridge is that the H bridge includes: an H bridge composed of switch tubes S1 to S4, a decoupling inductor L r , two film capacitors C d and C r , C r Passing by L r The midpoint of the second bridge arm of the H-bridge is connected to form a bidirectional Buck circuit, and the bidirectional Buck circuit and the second bridge arm of the H-bridge realize switch tube multiplexing; the midpoint of the first bridge arm of the Buck multiplexing 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 Buck multiplexing H-bridge in each phase is connected to the filter inductor, and the midpoint of the second bridge arm of the last Buck multiplexing H-bridge is connected to the midpoint of the second bridge arm of the last Buck multiplexing H-bridge of the other two phases; The system adopts the following control method based on the non-electrolytic capacitor cascade H-bridge STATCOM system, including the following steps: Step 1: Control the first bridge arm of each Buck multiplexing H-bridge of the x-phase to realize AC and DC side power exchange; Step 2: Control the second bridge arm of each Buck multiplexing H-bridge of the x-phase to achieve active power decoupling and suppress the double frequency fluctuation of the H-bridge DC side voltage; The control method of the first bridge arm of each Buck multiplexed H bridge of phase x in step 1 is: Output current i for phase x ox Control to generate the output reference voltage u of this phase ox , according to u ox , the result of the phase voltage balance control Δu xj And the modulation wave u of the second bridge arm ox2j Get the modulation wave u of the first bridge arm of each H bridge ox1j , according to u ox1j , based on the carrier phase shift modulation technology, the first bridge arm of each H bridge of the phase is driven; The modulation wave u of the first bridge arm of each H bridge of phase x is obtained ox1j The specific method is: The output current i of phase x ox With its given value i ox * is used to make a difference, and the difference is sent to the quasi-PR controller. The output of the quasi-PR controller is then compared with the grid voltage u sx After the difference is made, the zero-sequence voltage is separated to obtain u ox ; x-phase H-bridge DC side voltage u 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 Multiply the sign value of to get Δu xj ; will u ox , Δu xj Add and divide by u dcxj , and add u to the result ox2j The modulation wave u of the first bridge arm of each H bridge in this phase can be obtained ox1j ; The control method of the second bridge arm of each Buck multiplexed H bridge of phase x in step 2 is: The decoupling inductor current i of each H-bridge of phase x Lrxj Control to obtain the modulation wave u of the second bridge arm ox2j , according to u ox2j The second bridge arm of each H bridge of the x phase is driven by using the carrier phase shift modulation technology; The modulation wave u of the second bridge arm of each H bridge of phase x is obtained ox2j The specific method is: The average value u of the auxiliary capacitor voltage of each H-bridge of phase x cxv Perform PI control, the average current i of each H-bridge DC side dcxv After the multi-PR controller, the sum of the outputs of the PI controller and the multi-PR controller is used as the given value i of each H-bridge decoupling current Lrxj * ;change i Lrxj * and i Lrxj The difference is passed through the multi-PR controller, and the output of the multi-PR controller is divided by the DC side voltage u of each H-bridge dcxj , we can get the modulation wave u of the second arm of the H bridge ox2j .

2. The electrolytic capacitor-free cascade H-bridge STATCOM system according to claim 1, characterized in that: The x-phase output current reference value i is obtained ox *The specific method is: The average voltage u of each H-bridge DC side of phase x dcxv Perform PI control and multiply the PI controller output 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 ; For three-phase i ra 、i rb and i rc Calculate the average value, and then multiply the average value by -1 to get the zero-sequence current i of each phase. zx ;change i zx Superimpose to i rx The zero-sequence separation of a given current can be achieved, thereby obtaining i ox *.

3. The electrolytic capacitor-free cascade H-bridge STATCOM system according to claim 1, characterized in that: The zero-sequence voltage is separated to obtain the x-phase output reference voltage u ox The specific method is: x-phase grid voltage u sx Subtract the output of the phase current controller to get u rx , for the three-phase u ra 、u rb and u rc Calculate the average value, and then multiply the average value by -1 to get the zero-sequence voltage u of each phase zx ; will u zx Superimpose on u rx The zero-sequence separation of a given voltage can be achieved, thereby obtaining u ox .

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