Method for improving negative sequence current compensation capability of cascaded H-bridge structure STATCOM

Abstract

The application discloses a method for improving the negative sequence current compensation capability of a cascade STATCOM, wherein the main circuit of the cascade STATCOM is composed of three-phase cascade H-bridges and a common DC bus circuit in series. Each phase of the three-phase cascade H-bridge is composed of N H-bridge modules in series, and the common DC bus circuit is a three-phase half-bridge structure. When compensating the negative sequence current, the common DC bus circuit superimposes the negative sequence voltage phase quantity opposite to the cascade H-bridge module through each phase to realize the re-distribution of active power. When the output active power capability of the common DC bus circuit is exceeded, the active power additionally required to be distributed is transferred by the zero sequence voltage. On the voltage instruction output by the current loop control, the common DC bus circuit superimposes the negative sequence voltage vector, the cascade H-bridge module superimposes the negative sequence voltage and the zero sequence voltage opposite thereto, and finally the instruction voltage of the common DC bus circuit and the cascade H-bridge module is obtained to realize the control of the converter on the negative sequence current and complete the improvement of the negative sequence current compensation capability of the cascade STATCOM.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a method for improving the negative sequence current compensation capability of a cascaded H-bridge structure STATCOM. Background Technology

[0002] With the expansion of the overall scale of the power system, the diversity of loads and power generation methods has brought severe challenges to system stability. The connection of a large number of loads such as welding machines, punch presses, and electric arc furnaces has reduced the power quality of the grid, specifically manifested in a decrease in power factor, three-phase load imbalance, and increased harmonic content. In particular, many high-precision equipment have strict parameter requirements for the three-phase imbalance indicator, making compensation for unbalanced current especially critical.

[0003] Given the limited power generation capacity of power plants, reactive power compensation devices are typically installed to improve power grid quality. Static Synchronous Compensator (STATCOM) is a commonly used reactive power compensation device. Its basic function is to compensate for reactive power in the power grid, and it also compensates for unbalanced currents. For medium and high voltage applications, STATCOM often adopts a multi-level topology to reduce voltage stress on individual switching devices, such as diode-clamped type, modular multi-level type, and cascaded H-bridge type. Among these, the cascaded H-bridge type is more widely used because it has fewer switching devices and no circulating current problem.

[0004] However, as the unbalanced current requiring compensation gradually increases, the cascaded H-bridge structure, lacking a common DC bus, leads to increased phase-to-phase unbalanced active power, causing severe phase-to-phase voltage imbalance. The current main solution is to transfer the unbalanced active power by injecting zero-sequence voltage. However, limited by the DC-side capacitor voltage, overvoltage occurs as the zero-sequence voltage increases when the current imbalance is high, preventing complete transfer of the unbalanced active power. To solve this problem, voltage utilization needs to be improved by adding the third harmonic voltage to the zero-sequence voltage, but the increased negative-sequence current compensation capability remains limited. Common DC bus circuits, due to their shared DC bus, do not suffer from phase-to-phase DC voltage imbalance under unbalanced output current conditions, but their DC bus faces higher voltage stress, making them unsuitable for high-voltage applications. Summary of the Invention

[0005] Objective: To address the above problems, this invention proposes a method to improve the negative sequence current compensation capability of a cascaded H-bridge STATCOM structure. By transferring unbalanced power between phases through a common DC bus, the injection of zero-sequence voltage into the cascaded H-bridge is reduced, thus compensating for more unbalanced current.

[0006] Technical Solution: To achieve the objective of this invention, the technical solution adopted is: a method for improving the negative sequence current compensation capability of a cascaded H-bridge STATCOM structure, comprising:

[0007] The main circuit of the cascaded STATCOM consists of a three-phase cascaded H-bridge and a common DC bus circuit connected in series. Each phase of the three-phase cascaded H-bridge consists of N... H The common DC bus circuit is a three-phase half-bridge structure composed of H-bridge modules connected in series. When compensating for negative-sequence current, the unbalanced active power generated by the negative-sequence current is eliminated by the negative-sequence voltage and the zero-sequence voltage. The common DC bus circuit redistributes active power while ensuring that the negative-sequence current is not affected by superimposing negative-sequence voltage phasors opposite to those of the cascaded H-bridge modules on each phase. After exceeding the active power output capacity of the common DC bus circuit, the additional active power that needs to be allocated is transferred by the zero-sequence voltage. On the voltage command output by the current loop control, the cascaded H-bridge modules are superimposed with negative-sequence voltage and zero-sequence voltage, and the common DC bus circuit is superimposed with the opposite negative-sequence voltage, finally obtaining the command voltage of the cascaded H-bridge modules and the common DC bus circuit. The method includes six parts:

[0008] Part 1: Calculate the maximum negative sequence voltage of the superposition of all transferred unbalanced power;

[0009] Part Two: Calculating the power transfer coefficient K;

[0010] Part 3: Calculate the actual superimposed negative sequence voltage of the cascaded H-bridge;

[0011] Part Four: Calculate the maximum zero-sequence voltage of the superposition of all transferred unbalanced power;

[0012] Part 5: Calculate the actual superimposed zero-sequence voltage of the cascaded H-bridge;

[0013] Part 6: Calculate the command voltage for the cascaded H-bridge module and the common DC bus circuit.

[0014] Optionally, the first part includes:

[0015] The superimposed negative sequence voltage of the cascaded H-bridge can cancel the unbalanced power between phases. The maximum negative sequence voltage that the cascaded H-bridge needs to superimpose to cancel all the unbalanced power can be calculated by equation (1). Equation (1) is:

[0016]

[0017] In equation (1), U d_NSV U represents the d-axis component of the maximum negative sequence voltage superimposed on the cascaded H-bridges. q_NSV I represents the q-axis component of the maximum negative sequence voltage superimposed on the cascaded H-bridge. dp I represents the positive-sequence d-axis component of the output current. qpI is the positive-sequence q-axis component of the output current. dn I is the negative-sequence d-axis component of the output current. qn E represents the negative-sequence q-axis component of the output current. dp N represents the positive d-sequence component of the grid-side voltage. H N represents the number of cascaded H-bridge modules. T The rated DC side voltage of the common DC bus circuit and the rated voltage U of the H-bridge module. dc The ratio of .

[0018] Optionally, the second part includes:

[0019] First, considering the DC voltage limitation of the DC bus circuit, i.e., the output voltage of the common DC bus circuit cannot exceed the maximum allowable rated value, the cascaded H-bridge may not actually be able to superimpose the maximum negative sequence voltage and transfer all the unbalanced power. Therefore, the proportion of active power canceled by the negative sequence voltage to the phase-to-phase unbalanced power of the cascaded H-bridge is denoted as the power transfer coefficient K. The power transfer coefficient K needs to be determined:

[0020] The power transfer coefficients of each phase are calculated using equation (2):

[0021]

[0022] In equation (2), K Tm Here, m = a, b, c represent the power transfer coefficients of each phase in the common DC bus circuit, respectively, and U... dcT The DC side voltage rating of the common DC bus circuit is given by parameter G. mn n = 1, 2, 3, 4, as shown in equation (3):

[0023]

[0024] Among them, U Tdp U is the positive-sequence d-axis component of the output voltage of the common DC bus circuit. Tqp U is the positive-sequence q-axis component of the output voltage of the common DC bus circuit. Tdn U is the negative-sequence d-axis component of the output voltage of the common DC bus circuit. Tqn U is the negative-sequence q-axis component of the output voltage of the common DC bus circuit. Tdn and U Tqn Specifically, it can be expressed as equation (4):

[0025]

[0026] In equation (4), E qp The positive-sequence q-axis component represents the grid-side voltage, L is the grid-connected inductance value, and ω is the grid voltage angular frequency;

[0027] Considering that the output voltage of the three phases of the common DC bus circuit cannot exceed the rated value, the actual power transfer coefficient K is taken as the minimum value of the three phases, as shown in equation (5):

[0028] K = min(K) Ta ,K Tb ,K Tc ,1)(5)

[0029] In equation (5), min() means taking the minimum value among the multiple values ​​within the parentheses.

[0030] Optionally, the third part includes:

[0031] The actual superimposed three-phase negative sequence voltage u of the cascaded H-bridge (abc) is calculated using equations (6) and (7). a u b and u c Equations (6) and (7) are as follows:

[0032]

[0033]

[0034] In equations (6) and (7), U Hdn2 U represents the d-axis component of the negative sequence voltage actually superimposed on the cascaded H-bridge. Hqn2 Let t be the component of the negative sequence voltage actually superimposed on the cascaded H-bridge on the d-axis. t is time. The phase of the grid voltage phase a and its sine value sin(ωt) and cosine value cos(ωt) can be obtained through a phase-locked loop.

[0035] Optionally, the fourth part includes:

[0036] The maximum zero-sequence voltage that the cascaded H-bridge needs to superimpose to offset all unbalanced power is calculated using equation (8), which is:

[0037]

[0038] In equation (8), U d_ZSV U represents the d-axis component of the maximum zero-sequence voltage. q_ZSV This represents the q-axis component of the maximum zero-sequence voltage.

[0039] Optionally, the fifth part includes:

[0040] Considering that the negative sequence voltage cancels out K times the unbalanced power, the actual zero sequence voltage only needs to cancel out the remaining unbalanced power. The actual zero sequence voltage u0 that needs to be superimposed on the cascaded H-bridge is calculated using equations (9) and (10), where equations (9) and (10) are:

[0041]

[0042] u0 = U Hd0 cos(ωt)+U Hq0 sin(ωt)(10)

[0043] In equation (10), U Hd0 U represents the d-axis component of the zero-sequence voltage u0. Hq0 Let u0 be the q-axis component of the zero-sequence voltage.

[0044] Optionally, the sixth part includes:

[0045] The three-phase voltage modulation waves of converter abc output by the current control loop are denoted as u. ta u tb and u tc The voltage modulation wave v of the cascaded H-bridge module is calculated using equation (11). a v b and v c Equation (11) is:

[0046]

[0047] Next, the voltage modulation wave e of the common DC bus circuit is calculated using equation (12). a e b and e c The above (12) is:

[0048]

[0049] Beneficial effects: Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0050] The method of this invention transfers most of the interphase unbalanced active power by superimposing voltage phasors in the same or opposite direction as the corresponding phase current on the common DC bus circuit structure. This greatly reduces the pressure of zero-sequence voltage transfer and lessens the limitation of the DC side voltage level on the output zero-sequence voltage. Moreover, since the common DC bus circuit shares a DC bus, it does not need to transfer interphase unbalanced active power itself. Therefore, compared with a pure cascaded H-bridge structure, the converter can transfer more interphase unbalanced active power while maintaining the same DC side voltage level, thereby achieving the purpose of increasing the negative sequence current compensation capability of the cascaded H-bridge structure STATCOM. Attached Figure Description

[0051] Figure 1 This is a diagram of the hybrid topology of the cascaded H-bridge and common DC bus circuit proposed in this invention;

[0052] Figure 2 This is the negative sequence current compensation range of the method of the present invention. Detailed Implementation

[0053] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0054] To address the technical problems existing in related technologies, it has been found that cascaded H-bridges have the advantage of low DC-side voltage stress, while common DC bus circuit structures do not require the transfer of unbalanced active power between phases. Combining the advantages of the above two circuits, this invention provides a method to increase the negative sequence current compensation capability of cascaded H-bridge structures, thereby improving their negative sequence current compensation effect and aiming to solve the aforementioned problems. The following is a detailed description of this solution with reference to the accompanying drawings.

[0055] Traditional cascaded H-bridge structures offer advantages such as fewer switching devices, multi-level operation, and no circulating current. However, they suffer from the disadvantage of lacking a common DC bus, leading to uneven DC-side voltage when the three-phase active power is uneven. To address this issue, the neutral point of the cascaded H-bridge structure is decoupled, and the three leads are connected to a common DC bus circuit. The circuit topology is shown in [reference needed]. Figure 1 .

[0056] First of all Figure 1 The circuit structure is explained below. Figure 1 The dashed box 1 shows a cascaded H-bridge structure, whose DC-side voltage sum is N. H times U dc The dashed box 2 represents the common DC bus circuit, and the sum of its DC side voltages is N. H times U dc The entire system is connected to the power grid via an inductor L.

[0057] Due to the imbalance of three-phase loads in the power grid, the static synchronizing compensator needs to output a corresponding unbalanced current to ensure three-phase symmetry of the grid-side output current. However, the unbalanced output current will cause an imbalance in inter-phase active power. Specifically, according to the symmetrical component method, the product of the positive-sequence and negative-sequence components of voltage and current will generate three-phase uneven active power, which has the following two characteristics:

[0058] (1) The active power values ​​of each phase are not the same and there are positive and negative values. In other words, there is a situation where one phase generates active power while another phase absorbs active power.

[0059] (2) The sum of the three-phase active power is zero. In other words, the converter does not absorb or generate active power to the grid side.

[0060] Because the common DC bus circuit has a common DC bus, the DC-side capacitor voltage can be stabilized while ensuring the total unbalanced active power of the three phases is equal. Based on this property, by controlling the output voltage of the common DC bus circuit, a portion of the unbalanced active power can be transferred through this circuit, thereby reducing the pressure on the active power transfer required by the original zero-sequence voltage. In other words, using... Figure 1 The topology shown allows the converter to transfer more active power while maintaining the same DC-side voltage level, thus expanding the unbalanced current compensation range. In summary, to achieve phase-to-phase power transfer, it is necessary to obtain the command voltage values ​​for the common DC bus circuit structure and the cascaded H-bridge. The method can be specifically divided into six parts:

[0061] Part 1: Calculate the maximum negative sequence voltage of the superposition of all transferred unbalanced power;

[0062] Part Two: Calculating the power transfer coefficient K;

[0063] Part 3: Calculate the actual superimposed negative sequence voltage of the cascaded H-bridge;

[0064] Part Four: Calculate the maximum zero-sequence voltage of the superposition of all transferred unbalanced power;

[0065] Part 5: Calculate the actual superimposed zero-sequence voltage of the cascaded H-bridge;

[0066] Part 6: Calculate the command voltage for the cascaded H-bridge module and the common DC bus circuit.

[0067] in:

[0068] Part 1: Calculating the maximum negative sequence voltage resulting from the superposition of all transferred unbalanced power:

[0069] The superimposed negative sequence voltage of the cascaded H-bridge can cancel the unbalanced power between phases. The maximum negative sequence voltage that the cascaded H-bridge needs to superimpose to cancel all the unbalanced power can be calculated by equation (1). Equation (1) is:

[0070]

[0071] In equation (1), U d_NSV U represents the d-axis component of the maximum negative sequence voltage superimposed on the cascaded H-bridges. q_NSV I represents the q-axis component of the maximum negative sequence voltage superimposed on the cascaded H-bridge. dp I represents the positive-sequence d-axis component of the output current. qp I is the positive-sequence q-axis component of the output current. dn I is the negative-sequence d-axis component of the output current. qn E represents the negative-sequence q-axis component of the output current. dp N represents the positive d-sequence component of the grid-side voltage. HN represents the number of cascaded H-bridge modules. T The rated DC side voltage of the common DC bus circuit and the rated voltage U of the H-bridge module. dc The ratio of .

[0072] Part Two: Calculating the Power Transfer Coefficient K

[0073] First, considering the DC voltage limitation of the DC bus circuit, i.e., the output voltage of the common DC bus circuit cannot exceed the maximum allowable rated value, the cascaded H-bridge may not actually be able to superimpose the maximum negative sequence voltage and transfer all the unbalanced power. Therefore, the proportion of active power canceled by the negative sequence voltage to the phase-to-phase unbalanced power of the cascaded H-bridge is denoted as the power transfer coefficient K. The power transfer coefficient K needs to be determined:

[0074] The power transfer coefficients of each phase are calculated using equation (2):

[0075]

[0076] In equation (2), K Tm Here, m = a, b, c represent the power transfer coefficients of each phase in the common DC bus circuit, respectively, and U... dcT The DC side voltage rating of the common DC bus circuit is given by parameter G. mn n = 1, 2, 3, 4, as shown in equation (3):

[0077]

[0078] Among them, U Tdp U is the positive-sequence d-axis component of the output voltage of the common DC bus circuit. Tqp U is the positive-sequence q-axis component of the output voltage of the common DC bus circuit. Tdn U is the negative-sequence d-axis component of the output voltage of the common DC bus circuit. Tqn U is the negative-sequence q-axis component of the output voltage of the common DC bus circuit. Tdn and U Tqn Specifically, it can be expressed as equation (4):

[0079]

[0080] In equation (4), E qp The positive-sequence q-axis component represents the grid-side voltage, L is the grid-connected inductance value, and ω is the grid voltage angular frequency;

[0081] Considering that the output voltage of the three phases of the common DC bus circuit cannot exceed the rated value, the actual power transfer coefficient K is taken as the minimum value of the three phases, as shown in equation (5):

[0082] K = min(K) Ta ,K Tb ,KTc ,1)(5)

[0083] In equation (5), min() means taking the minimum value among the multiple values ​​within the parentheses.

[0084] Part Three: Calculating the actual superimposed negative sequence voltage of the cascaded H-bridge:

[0085] The actual superimposed three-phase negative sequence voltage u of the cascaded H-bridge (abc) is calculated using equations (6) and (7). a u b and u c Equations (6) and (7) are as follows:

[0086]

[0087]

[0088] In equations (6) and (7), U Hdn2 U represents the d-axis component of the negative sequence voltage actually superimposed on the cascaded H-bridge. Hqn2 Let t be the component of the negative sequence voltage actually superimposed on the cascaded H-bridge on the d-axis. t is time. The phase of the grid voltage phase a and its sine value sin(ωt) and cosine value cos(ωt) can be obtained through a phase-locked loop.

[0089] Part 4: Calculating the maximum zero-sequence voltage of the cascaded H-bridges when transferring all unbalanced power:

[0090] The maximum zero-sequence voltage that the cascaded H-bridge needs to superimpose to offset all unbalanced power is calculated using equation (8), which is:

[0091]

[0092] In equation (8), U d_ZSV U represents the d-axis component of the maximum zero-sequence voltage. q_ZSV This represents the q-axis component of the maximum zero-sequence voltage.

[0093] Part 5: Calculation of the actual superimposed zero-sequence voltage of the cascaded H-bridge:

[0094] Considering that the negative sequence voltage cancels out K times the unbalanced power, the actual zero sequence voltage only needs to cancel out the remaining unbalanced power. The actual zero sequence voltage u0 that needs to be superimposed on the cascaded H-bridge is calculated using equations (9) and (10), where equations (9) and (10) are:

[0095]

[0096] u0 = U Hd0 cos(ωt)+U Hq0 sin(ωt)(10)

[0097] In equation (10), U Hd0 U represents the d-axis component of the zero-sequence voltage u0. Hq0 Let u0 be the q-axis component of the zero-sequence voltage.

[0098] Part 6: Calculating the command voltage for the cascaded H-bridge module and the common DC bus circuit:

[0099] The three-phase voltage modulation waves of converter abc output by the current control loop are denoted as u. ta u tb and u tc The voltage modulation wave v of the cascaded H-bridge module is calculated using equation (11). a v b and v c Equation (11) is:

[0100]

[0101] Next, the voltage modulation wave e of the common DC bus circuit is calculated using equation (12). a e b and e c The above (12) is:

[0102]

[0103] As can be seen from the foregoing embodiments of the present invention, by superimposing voltage phasors in the same or opposite direction as the corresponding phase currents on the common DC bus circuit structure, most of the interphase unbalanced active power can be transferred, thereby greatly reducing the pressure of zero-sequence voltage transfer and reducing the limitation of the DC side voltage level on the output zero-sequence voltage. Moreover, because the common DC bus circuit shares a DC bus, it itself does not need to transfer interphase unbalanced active power. Therefore, compared with a pure cascaded H-bridge structure, the converter can transfer more interphase unbalanced active power while maintaining the same DC side voltage level, thereby achieving the purpose of increasing the negative sequence current compensation capability of the cascaded H-bridge structure STATCOM. The negative sequence current compensation range of the method of the present invention is as follows: Figure 2 .

[0104] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. The invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0105] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

[0106] It should be noted that in this invention, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. The terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0107] The methods and apparatus provided in the embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the embodiments above are only for the purpose of helping to understand the methods and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for improving the negative sequence current compensation capability of a cascaded H-bridge STATCOM structure, characterized in that, The main circuit of the cascaded H-bridge structure STATCOM is composed of a three-phase cascaded H-bridge and a common DC bus circuit connected in series. Each phase of the three-phase cascaded H-bridge consists of N H The method comprises several H-bridge modules connected in series, wherein the common DC bus circuit is a three-phase half-bridge structure; the method includes: When compensating for negative sequence current, the unbalanced active power generated by negative sequence current is eliminated by both negative sequence voltage and zero sequence voltage. Considering the output voltage limitation of the common DC bus circuit, the negative sequence voltage cancels out K times the unbalanced power, and the remaining 1-K times the unbalanced power is canceled out by the zero sequence voltage. On the voltage command output by the current loop control, the cascaded H-bridge module superimposes the negative sequence voltage and the zero sequence voltage, and the common DC bus circuit superimposes the opposite negative sequence voltage vector. While ensuring that the output negative sequence current is not affected, the active power is redistributed, and finally the command voltage of the cascaded H-bridge module and the common DC bus circuit is obtained. The negative sequence voltage calculation process includes: calculating the maximum negative sequence voltage of the cascaded H-bridges when all unbalanced power is transferred. The cascaded H-bridges superimpose negative sequence voltages to cancel the unbalanced power between phases. The maximum negative sequence voltage that the cascaded H-bridges need to superimpose to cancel all the unbalanced power is calculated using equation (1), which is: （1） In equation (1), U d_NSV U represents the d-axis component of the maximum negative sequence voltage superimposed on the cascaded H-bridges. q_NSV I represents the q-axis component of the maximum negative sequence voltage superimposed on the cascaded H-bridge. dp I represents the positive-sequence d-axis component of the output current. qp I is the positive-sequence q-axis component of the output current. dn I is the negative-sequence d-axis component of the output current. qn E represents the negative-sequence q-axis component of the output current. dp N represents the positive d-sequence component of the grid-side voltage. H N represents the number of cascaded H-bridge modules. T The rated DC side voltage of the common DC bus circuit and the rated voltage U of the H-bridge module. dc The ratio of .

2. The method for improving the negative sequence current compensation capability of a cascaded H-bridge structure STATCOM according to claim 1, characterized in that, The negative sequence voltage calculation process also includes: calculating the power transfer coefficient K: First, based on the DC voltage limit of the DC bus circuit, that is, the output voltage of the common DC bus circuit does not exceed the maximum allowable rated value, the cascaded H-bridge does not superimpose the maximum negative sequence voltage, and transfers all the unbalanced power. The proportion of the active power canceled by the negative sequence voltage to the phase unbalanced power of the cascaded H-bridge is denoted as the power transfer coefficient K. The power transfer coefficient of each phase is calculated by equation (2): （2） In equation (2), K Tm Here, m = a, b, c represent the power transfer coefficients of each phase in the common DC bus circuit, respectively, and U... dcT The DC side voltage rating of the common DC bus circuit is given by parameter G. mn n=1,2,3,4, as shown in equation (3): （3） Among them, U Tdp U is the positive-sequence d-axis component of the output voltage of the common DC bus circuit. Tqp U is the positive-sequence q-axis component of the output voltage of the common DC bus circuit. Tdn U is the negative-sequence d-axis component of the output voltage of the common DC bus circuit. Tqn U is the negative-sequence q-axis component of the output voltage of the common DC bus circuit. Tdn and U Tqn Specifically, it is expressed as equation (4): （4） In equation (4), E qp The positive-sequence q-axis component represents the grid-side voltage, L is the grid-connected inductance value, and ω is the grid voltage angular frequency; the power transfer coefficient K is taken as the minimum value of the three phases, as shown in equation (5): （5） In equation (5), min() means taking the minimum value among the multiple values ​​within the parentheses.

3. The method for improving the negative sequence current compensation capability of a cascaded H-bridge structure STATCOM according to claim 2, characterized in that, The negative sequence voltage calculation process also includes: calculating the actual superimposed negative sequence voltage of the cascaded H-bridge: The actual superimposed three-phase negative sequence voltage u of the cascaded H-bridge (abc) is calculated using equations (6) and (7). a u b and u c Equations (6) and (7) are: （6） （7） Among them, U Hdn2 U represents the d-axis component of the negative sequence voltage actually superimposed on the cascaded H-bridge. Hqn2 The negative sequence voltage component on the d-axis is the actual superimposed negative sequence voltage of the cascaded H-bridge. t is time. The phase of the grid voltage phase a and its sine value sin(ωt) and cosine value cos(ωt) are obtained through a phase-locked loop.

4. The method for improving the negative sequence current compensation capability of a cascaded H-bridge structure STATCOM according to claim 3, characterized in that, The zero-sequence voltage calculation process includes: calculating the maximum zero-sequence voltage of the cascaded H-bridges when all unbalanced power is transferred. The maximum zero-sequence voltage that the cascaded H-bridge needs to superimpose to offset all unbalanced power is calculated using equation (8), which is: （8） In equation (8), The component of the maximum zero-sequence voltage on the d-axis. This represents the q-axis component of the maximum zero-sequence voltage.

5. The method for improving the negative sequence current compensation capability of a cascaded H-bridge structure STATCOM according to claim 4, characterized in that, The zero-sequence voltage calculation process also includes: calculating the actual superimposed zero-sequence voltage of the cascaded H-bridge: The negative sequence voltage cancels out K times the unbalanced power, while the actual zero sequence voltage only cancels out the remaining unbalanced power. The actual superimposed zero sequence voltage u0 of the cascaded H-bridge is calculated using equations (9) and (10), where equations (9) and (10) are: （9） （10） In equation (10), U Hd0 U represents the d-axis component of the zero-sequence voltage u0. Hq0 Let u0 be the q-axis component of the zero-sequence voltage.

6. The method for improving the negative sequence current compensation capability of a cascaded H-bridge structure STATCOM according to claim 5, characterized in that, The calculation process for the command voltage of the cascaded H-bridge module and the common DC bus circuit includes: The three-phase voltage modulation waves of converter abc output by the current control loop are denoted as u. ta u tb and u tc The voltage modulation wave v of the cascaded H-bridge module is calculated using equation (11). a v b and v c Equation (11) is: （11） Next, the voltage modulation wave e of the common DC bus circuit is calculated using equation (12). a e b and e c The above (12) is: （12）。

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