Modular multilevel converter submodule capacitor voltage balancing control method based on space vector modulation

Abstract

This invention discloses a method for equalizing capacitor voltage in an MMC submodule based on space vector modulation. Firstly, in... α ' β The invention implements spatial vector modulation (MMC) in a coordinate system, locates the sector triangle containing the reference vector, calculates the action time of the three vertex vectors of the sector triangle that synthesize the reference vector, and determines the corresponding switching state sequence between the three vertex vectors of the sector triangle based on a five-segment algorithm. Then, based on the calculated number of submodules to be engaged in each phase's upper and lower arms, the direction of the arm current, and the maximum, minimum, second maximum, and second minimum capacitor voltage values ​​in the engaged and disengaged submodule groups, the specific engaged or disengaged submodules are determined. The capacitor voltage equalization control method proposed in this invention is simple to implement, unaffected by the number of arm submodules, avoids capacitor voltage sequencing, and ensures precise synchronization between spatial vector modulation and capacitor voltage equalization control.

Description

Technical Field

[0001] This invention relates to the field of space vector modulation of modular multilevel converters and corresponding submodule capacitor voltage equalization control, and specifically to a method for controlling the voltage equalization of MMC submodule capacitors based on space vector modulation. Background Technology

[0002] Modular Multilevel Converter (MMC) topologies are characterized by their modular structure, ease of redundancy design, and ease of expansion and control. They are widely used in high-voltage direct current (HVDC) transmission and flexible DC grids, medium- and high-voltage motor drives, static var compensators (SVCs), and energy storage converters. MMCs utilize the capacitors of sub-modules (SMs) as voltage support and energy buffer units. During modulation, the connection and disconnection of sub-modules generate complex internal power flows and bridge arm energy fluctuations, leading to sub-module capacitor voltage imbalance. This imbalance increases bridge arm circulating current, degrades the output power quality of the MMC, increases switching losses, reduces energy conversion efficiency, and in severe cases, can even affect the safe and stable operation of the converter. Different modulation strategies employed by the MMC result in different sub-module capacitor voltage balancing control methods and implementation difficulties. Therefore, to achieve sub-module capacitor voltage balancing control, a corresponding sub-module capacitor balancing control method must be designed based on the specific modulation strategy.

[0003] Compared to carrier phase-shift modulation and nearest-level modulation, space vector modulation (SVM) improves DC voltage utilization by approximately 15% and offers advantages such as low harmonic distortion, small voltage tracking error, low switching frequency, and low switching losses. In particular, when the number of bridge arm submodules in series is large, the SVM strategy can provide greater flexibility for MMC performance optimization. As the number of bridge arm submodules in the MMC increases, the sector triangle, switching states, and switching state sequences increase dramatically during the SVM strategy implementation. Traditional space vector modulation strategies employ... The coordinate system, with the space vector located at non-integer coordinate points, makes locating the sector triangle containing the reference vector and calculating the corresponding switching states of the space vector extremely difficult, hindering the implementation of the SVM strategy. Furthermore, the selection of the switching state sequence and the engagement and disengagement of submodules directly impact the implementation of the capacitor voltage equalization control strategy when switching between the three vertex vectors of a sector triangle or between adjacent sector triangles. Conventional capacitor voltage equalization control uses a sorting algorithm to determine the specific engagement or disengagement of submodules, requiring a capacitor voltage sorting process for each switch. Since the action time of the sector triangle vertex vectors synthesizing the reference vector is not fixed during space vector modulation, conventional sorting algorithms are time-consuming, sometimes even exceeding the vector action time, making synchronization between the sorting algorithm and the space vector modulation algorithm difficult.

[0004] Based on the problems existing in the space vector modulation and submodule capacitor voltage equalization control of MMC mentioned above, it is necessary to study a new MMC submodule capacitor voltage equalization control method based on space vector modulation. Summary of the Invention

[0005] The purpose of this invention is to address the aforementioned problems in the MMC space vector modulation and its corresponding submodule capacitor voltage equalization control by proposing a space vector modulation-based MMC submodule capacitor voltage equalization control method.

[0006] Firstly based on A coordinate system is used to implement spatial vector modulation of the MMC, simplifying the positioning of the sector triangle where the reference vector is located. The action time of the three vertex vectors of the sector triangle of the synthesized reference vector and their corresponding switching states are calculated. The switching state sequence corresponding to the switching between the three vertex vectors of the sector triangle is determined according to the five-segment algorithm. Then, when switching between the three vertex vectors of the sector triangle and between adjacent sector triangles, the specific submodule to be engaged or disengaged is determined based on the number of submodules to be engaged in each phase upper and lower bridge arm of the MMC corresponding to the obtained switching states, the direction of the bridge arm current, and the maximum, minimum, second maximum, and second minimum values ​​of the capacitor voltage in the engaged and disengaged submodule groups, thus realizing the equalization control of the submodule capacitor voltage.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] The MMC described in this invention is a three-phase converter composed of six bridge arms. Each phase consists of upper and lower bridge arms, and each bridge arm comprises n sub-modules and an inductor connected in series. The three phases share a single DC power supply. Each sub-module contains two IGBT power switches and a capacitor. Two IGBTs are connected in series to form a half-bridge. Each IGBT is connected in anti-parallel to a diode, i.e., the collector of the IGBT is connected to the cathode of the diode, and the emitter of the IGBT is connected to the anode of the diode. The emitter of the upper bridge arm IGBT is connected to the collector of the lower bridge arm IGBT as one output terminal of the sub-module, which is the positive terminal of the sub-module's output voltage. The collector of the upper half-bridge IGBT is connected to the positive terminal of the capacitor, and the emitter of the lower bridge arm IGBT is connected to the negative terminal of the capacitor as the other output terminal of the sub-module, which is the negative terminal of the sub-module's output voltage. Each bridge arm has n sub-modules connected in series, with the negative terminal of the previous sub-module connected to the positive terminal of the next sub-module. The positive terminal of the first sub-module of the upper bridge arm is connected to the positive terminal of the DC power supply, the negative terminal of the last sub-module of the upper bridge arm is connected to one end of the upper bridge arm inductor, the other end of the upper bridge arm inductor is connected to one end of the lower bridge arm inductor, the other end of the lower bridge arm inductor is connected to the positive terminal of the first sub-module of the lower bridge arm, and the negative terminal of the last sub-module of the lower bridge arm is connected to the negative terminal of the DC power supply. The connection point of the two inductors of the upper and lower bridge arms is the AC output terminal of that phase.

[0009] MMC space vector modulation three-phase reference voltage signal , , for:

[0010] (1)

[0011] In the formula, m represents the modulation coefficient, reflecting the utilization rate of the DC voltage, and n represents the number of series-connected submodules in the bridge arm; n is an even number. This indicates the angular frequency of the output voltage.

[0012] According to the definition of space vector modulation, Reference vector in coordinate system Represented as:

[0013] (2)

[0014] Indicates the reference vector. Represents the reference vector The coordinates.

[0015] (3)

[0016] Equation (3) shows the reference vector The trajectory is an ellipse.

[0017] Spatial vectors in coordinate system Represented as:

[0018] (4)

[0019] Represents a spatial vector. Represents a space vector The coordinates. In equation (4), , , These represent the output levels of phases A, B, and C of the MMC, respectively. , , The values ​​are 0, ±1, ±2, ±3, ..., ±n / 2, which correspond to n+1 levels for each level. Equation (4) indicates that the space vector is located at a unit integer coordinate point.

[0020] Satisfying equation (4) Called a space vector The switching state S can also be represented as .

[0021] Space vector modulation:

[0022] Step 1: Locate the reference vector The sector triangle it belongs to:

[0023] For three-phase reference voltage signal , , Sampling is performed, and the sampling results are substituted into equation (2) to obtain the reference vector. .

[0024] (5)

[0025] In the formula This represents the floor function.

[0026] Reference vector Located in space vector , , and The resulting unit square contains two sector triangles. When At that time, reference vector lie in , and In the type II sector triangle formed, It is the right-angle vertex vector of the type II sector triangle. When At that time, reference vector lie in , and In the type I sector triangle formed, It is the right-angle vertex vector of the type I sector triangle.

[0027] Step 2: Calculate the composite reference vector The duration of action of the vertex vector of the sector triangle

[0028] Reference vector lie in , and In the type II sector triangle formed, according to the volt-second balance principle, we get:

[0029] (6)

[0030] In the formula T s Indicates the sampling period.

[0031] Solving equation (6) yields the spatial vector. , and Corresponding action time , and :

[0032] (7)

[0033] Reference vector lie in , and In the type I sector triangle, according to the volt-second balance principle, we obtain:

[0034] (8)

[0035] Solving equation (8) yields the spatial vector. , and Corresponding action time , and :

[0036] (9)

[0037] It can be seen from equations (5) to (9) that The location of the sector triangle containing the reference vector in the coordinate system and the calculation of the action time of the vertex vector of the sector triangle only require simple arithmetic operations and judgments, which greatly reduces the amount of computation.

[0038] Step 3: Determine the switching sequence of the three vertices of the sector triangle corresponding to the switch states.

[0039] Based on the principle of closed switching paths, a five-segment switching method is adopted. For a type I sector triangle, from the spatial vector... Starting from this point, there are two switching paths, traversing the space vector. The corresponding switch state switching path for The corresponding switch state duration is Traversing space vectors The corresponding switch state switching path for The corresponding switch state duration is .

[0040] To improve the output power performance of the MMC, the switching state transitions between adjacent space vectors must adhere to the minimum switching principle. This means that each switching state transition must ensure that only one phase (A, B, or C) ±1 level is changed. Let the space vectors... The corresponding switching state with the minimum common-mode voltage is ,and Select the switch status switching path Space vector The corresponding switch state is Space vector The corresponding switch state is The switching path can be satisfied according to the principle of minimum switching, that is, the switching state switching path is:

[0041] ;

[0042] Select the switch status toggle path Space vector The corresponding switch state is Space vector The corresponding switch state is The switching path can be satisfied according to the principle of minimum switching, that is, the switching state switching path is:

[0043] .

[0044] Criteria , and Does it meet the requirements? If the conditions are not met, switch paths. Invalid, follow the switching path Switch to another device. Criteria , and Does it meet the requirements? If the conditions are not met, switch paths. Invalid, follow the switching path Switch. If and ,Compare , , , The size, if or If the maximum value is reached, switch paths. Invalid, follow the switching path Switch if or If the maximum value is reached, switch paths. Invalid, follow the switching path Switch to another device.

[0045] For a type II sector triangle, from the spatial vector... Starting from this point, there are two switching paths, traversing the space vector. The corresponding switch state switching path for The corresponding switch state duration is Traversing space vectors The corresponding switch state switching path for The corresponding switch state duration is .

[0046] Space vector The corresponding switching state with the minimum common-mode voltage is ,and Select the switch status switching path Space vector The corresponding switch state is Space vector The corresponding switch state is The switching path can be satisfied according to the principle of minimum switching, that is, the switching state switching path is:

[0047] ;

[0048] Select the switch status toggle path Space vector The corresponding switch state is Space vector The corresponding switch state is The switching path can be satisfied according to the principle of minimum switching, that is, the switching state switching path is:

[0049] .

[0050] Criteria , and Does it meet the requirements? If the conditions are not met, switch paths. Invalid, follow the switching path Switch to another device. Criteria , and Does it meet the requirements? If the conditions are not met, switch paths. Invalid, follow the switching path Switch. If and ,Compare , , , ,if or If the maximum value is reached, switch paths. Invalid, follow the switching path Switch if or If the maximum value is reached, switch paths. Invalid, follow the switching path Switch to another device.

[0051] Step 4: Determine the switching between two adjacent sector triangles

[0052] When two reference vectors obtained from two consecutive samples are located in the same sector triangle, the starting points of both switching paths are the switch states corresponding to the right-angle vertex vectors of that sector triangle, and there is no switch state switching during the transition between the two sampling periods. If the two reference vectors obtained from two consecutive samples are located in different sector triangles, there will definitely be a state switching during the transition between the two sampling periods, so the switching path between two adjacent sector triangles must be considered.

[0053] When two adjacent reference vectors are located in different sector triangles, one of the two adjacent sector triangles is Type I and the other is Type II. There are three combinations of adjacent Type I and Type II sector triangles: Combination 1: The Type I sector triangle is above the Type II sector triangle, and the two sector triangles share a right-angled side. Combination 2: The Type I sector triangle is to the right of the Type II sector triangle, and the two sector triangles share a right-angled side. Combination 3: The Type I sector triangle is to the left of the Type II sector triangle, and the two sector triangles share a hypotenuse. In Combinations 1 and 2, the right-angled vertex vectors of the two sector triangles are located on the common right-angled side. When switching between the two sector triangles, minimal switching can be achieved. Combination 1 only requires changing the C-phase ±1 level based on the last switching state of the previous space vector modulation cycle, and Combination 2 only requires changing the B-phase ±1 level based on the last switching state of the previous space vector modulation cycle. In combination 3, the two right-angled vertex vectors are located in different sector triangles. Minimal switching cannot be achieved between the two right-angled vertex vectors. The switching between the two sector triangles will simultaneously change the ±1 and / or ±2 levels of two phases based on the last switching state of the previous space vector modulation cycle.

[0054] Submodule capacitor voltage equalization control:

[0055] Step 1: While sampling the reference vector during this space vector modulation cycle, simultaneously start sampling the submodule capacitor voltage of each phase upper and lower bridge arm of the MMC, as well as sampling the current of the six bridge arms of the three phases. According to the submodule's on and off status, save the sampling data of the submodule capacitor voltage of each phase separately into on and off groups.

[0056] Step 2: Based on the sampling results, identify the four sub-modules with the largest and second largest, smallest and second smallest capacitor voltages in the six bridge arm input and cut-off sub-module groups, respectively, and use them as the selected objects for each bridge arm input and cut-off sub-module in this space vector modulation cycle.

[0057] Step 3: Based on the switching path and corresponding switching state obtained from space vector modulation. Calculate the number of submodules required for each of the six bridge arms of the MMC:

[0058] (10)

[0059] In the formula, , , , , , These represent the number of submodules that need to be deployed in the upper and lower arms of the three phases A, B, and C, respectively. The subscript p represents the upper arm, and the subscript l represents the lower arm.

[0060] Step 4: Based on the switching path obtained from space vector modulation and the direction of the bridge arm current, determine the specific sub-modules that need to be put into or removed on each bridge arm:

[0061] If the switching occurs within a sector triangle, each switch only changes the voltage level of one phase. If the switching state of that phase increases by one level compared to the previous switching state, then one submodule needs to be engaged in the upper bridge arm and one submodule needs to be disengaged in the lower bridge arm. Conversely, if the switching state of that phase decreases by one level compared to the previous switching state, then one submodule needs to be disengaged in the upper bridge arm and one submodule needs to be engaged in the lower bridge arm. Depending on the direction of the bridge arm current, if the current is charging the submodule capacitor, a PWM signal is allocated to control the engagement of the submodule with the lowest capacitor voltage in the disengaged group on that bridge arm, or to control the disengagement of the submodule with the highest capacitor voltage in the disengaged group on that bridge arm. If the current is discharging the submodule capacitor, a PWM signal is allocated to control the engagement of the submodule with the highest capacitor voltage in the disengaged group on that bridge arm, or to control the disengagement of the submodule with the lowest capacitor voltage in the disengaged group on that bridge arm.

[0062] If the reference vector of the current space vector modulation cycle and the reference vector of the previous modulation cycle are located in adjacent sector triangles, then a switch between adjacent sector triangles needs to be completed. Calculate the change in the switching state corresponding to the right-angle vertex vectors of the two sector triangles to determine the number of sub-modules that need to be added or removed in the corresponding phase bridge arm during the current modulation cycle. Based on the direction of the bridge arm current, if the current direction is charging the sub-module capacitor, and the bridge arm only needs to add or remove one sub-module, allocate a PWM signal to control the sub-module with the smallest capacitor voltage in the removal group on that bridge arm to be added, or control the sub-module with the largest capacitor voltage in the removal group on that bridge arm to be removed; if the current direction is discharging the sub-module capacitor, and the bridge arm only needs to add or remove one sub-module, allocate a PWM signal to control the sub-module with the largest capacitor voltage in the removal group on that bridge arm to be added, or control the sub-module with the smallest capacitor voltage in the removal group on that bridge arm to be removed; if the current... If the current direction is to charge the submodule capacitor, and the bridge arm needs to add or remove two submodules, a PWM signal is allocated to control the two submodules with the lowest and second lowest capacitor voltages in the cut-off group on the bridge arm to be put into operation, or to control the two submodules with the highest and second highest capacitor voltages in the cut-off group on the bridge arm to be cut out; if the current direction is to discharge the submodule capacitor, and the bridge arm needs to add or remove two submodules, a PWM signal is allocated to control the two submodules with the highest and second highest capacitor voltages in the cut-off group on the bridge arm to be put into operation, or to control the two submodules with the lowest and second lowest capacitor voltages in the cut-off group on the bridge arm to be cut out.

[0063] Step 5: Switch the path according to the switch state, and repeat steps 3 and 4 to complete one cycle T. sSynthesis of internal reference vectors and equalization control of submodule capacitor voltage.

[0064] When a control cycle T s After completion, repeat all the steps of space vector modulation and submodule capacitor voltage equalization control to finally complete the space vector modulation and submodule capacitor voltage equalization control of the MMC.

[0065] Beneficial effects:

[0066] This invention discloses a method for equalizing capacitor voltage in MMC submodules based on space vector modulation. First, space vector modulation of the MMC is implemented in the α'β' coordinate system to locate the sector triangle containing the reference vector. The action time of the three vertex vectors of the sector triangle is calculated, and the corresponding switching state sequence between the three vertex vectors of the sector triangle is determined according to a five-segment algorithm. Then, based on the calculated number of submodules to be engaged in each phase's upper and lower arms, the direction of the arm current, and the maximum, minimum, second maximum, and second minimum capacitor voltage values ​​in the engaged and disengaged submodule groups, the specific engaged or disengaged submodules are determined. The capacitor voltage equalization control method proposed in this invention is simple to implement, unaffected by the number of arm submodules, avoids capacitor voltage sequencing, and ensures precise synchronization between space vector modulation and capacitor voltage equalization control. Attached Figure Description

[0067] Figure 1 This is a schematic diagram of a three-phase MMC main circuit, where each bridge arm consists of n sub-modules.

[0068] Figure 2 This is a block diagram of the capacitor voltage equalization control structure of the MMC submodule based on space vector modulation.

[0069] Figure 3 This is a schematic diagram of the MMC space vector distribution and sector triangle combination, where each bridge arm consists of 6 sub-module strings.

[0070] Figure 4 This is a schematic diagram illustrating the triangular positioning principle of the sector where the reference vector is located;

[0071] Figure 5 This is a schematic diagram of the switch state switching sequence and the allocation of action time. Detailed Implementation

[0072] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0073] Taking an MMC consisting of 6 sub-modules connected in series in each bridge arm as an example, the main circuit schematic is attached. Figure 1 As shown in the figure, n=6, SM xyj(x represents phases a, b, and c; y represents the upper and lower bridge arms; y = p and l, where p represents the upper bridge arm and l represents the lower bridge arm; j represents the submodule number) represents the submodule, and L is the bridge arm inductance. It is the input DC voltage. Indicates the corresponding submodule SM xyj The capacitor voltage, This represents the current on the x-phase y-arm of the bridge. The block diagram of the MMC submodule capacitor voltage equalization control structure based on space vector modulation is attached. Figure 2 As shown in the figure, the space vector modulation module and the sub-module capacitor voltage equalization control module are synchronously sampled by a synchronization signal.

[0074] Space vector modulation:

[0075] The three phase reference voltage signals are respectively , , The reference voltage signal is sampled, and the sampling period is... Substitute the sampling results into equation (2) to calculate Reference vector in coordinate system The reference vector trajectory and spatial vector distribution are as follows: Figure 3 As shown in the figure, the ellipse is the trajectory of the reference vector, and the black dots are the spatial vectors located at integer coordinate points.

[0076] Reference vector coordinate components and Substituting into equation (5), the spatial vector is calculated. Reference vector Located in space vector , , , Within the unit square formed. Spatial vectors. , , Forming a type I sector triangle, spatial vector , , This forms a type II sector triangle. The positioning principle of the sector triangle containing the reference vector is as follows: Figure 4 As shown in the diagram, the space vector below corresponds to the switch state, i.e. Corresponding switch state , Corresponding switch state , Corresponding switch state , Corresponding switch state .

[0077] if At that time, based on the volt-second balance principle, using spatial vectors , , Synthetic reference vector Calculate according to formula (9) , , Duration of action , , .if At that time, based on the volt-second balance principle, using spatial vectors , , Synthetic reference vector Calculate according to formula (7) , , Duration of action , , .

[0078] For a type I sector triangle, from the spatial vector... Starting from this point, there are two switching paths, traversing the space vector. The corresponding switch state switching path for The corresponding switch state duration is Traversing space vectors The corresponding switch state switching path for The corresponding switch state duration is Let spatial vectors be defined. The corresponding switching state with the minimum common-mode voltage is ,and Select the switch status switching path Space vector The corresponding switch state is Space vector The corresponding switch state is The switching path can be satisfied according to the principle of minimum switching, that is, the switching state switching path is:

[0079] ;

[0080] Select the switch status toggle path Space vector The corresponding switch state is Space vector The corresponding switch state is The switching path can be satisfied according to the principle of minimum switching, that is, the switching state switching path is:

[0081] .

[0082] Criteria , and Does it meet the requirements? If the conditions are not met, switch paths. Invalid, follow the switching path Switch to another device. Criteria , and Does it meet the requirements? If the conditions are not met, switch paths. Invalid, follow the switching path Switch. If and ,Compare , , , The size, if or If the maximum value is reached, switch paths. Invalid, follow the switching path Switch if or If the maximum value is reached, switch paths. Invalid, follow the switching path Switch to another device.

[0083] For a type II sector triangle, from the spatial vector... Starting from this point, there are two switching paths, traversing the space vector. The corresponding switch state switching path for The corresponding switch state duration is Traversing space vectors The corresponding switch state switching path for The corresponding switch state duration is .

[0084] Space vector The corresponding switching state with the minimum common-mode voltage is ,and Select the switch status switching path Space vector The corresponding switch state is Space vector The corresponding switch state is The switching path can be satisfied according to the principle of minimum switching, that is, the switching state switching path is:

[0085] ;

[0086] Select the switch status toggle path Space vector The corresponding switch state is Space vector The corresponding switch state is The switching path can be satisfied according to the principle of minimum switching, that is, the switching state switching path is:

[0087] .

[0088] Criteria , and Does it meet the requirements? If the conditions are not met, switch paths. Invalid, follow the switching path Switch to another device. Criteria , and Does it meet the requirements? If the conditions are not met, switch paths. Invalid, follow the switching path Switch. If and ,Compare , , , ,if or If the maximum value is reached, switch paths. Invalid, follow the switching path Switch if or If the maximum value is reached, switch paths. Invalid, follow the switching path Switch to another device.

[0089] by Figure 4 Middle reference vector For example, let the spatial vector The corresponding switching state with the minimum common-mode voltage is ,and Space vector The corresponding switch state is Space vector The corresponding switch state is ,if ,and , , , Of the four numbers, or The maximum value satisfies the minimum switching principle, and the switch state switching path is as follows:

[0090] .

[0091] Based on the switch state switching sequence and action time allocation obtained from the switching path, as follows: Figure 5 As shown.

[0092] When two adjacent sampling points are located in different sector triangles, there is a switching between the two vertex vectors of the sector triangle. For example... Figure 3 The diagram shows different combinations of type I and type II sector triangles. In combinations 1 and 2, the two sector triangles share a right-angled side, and the right-angled vertex vectors of the two sector triangles are located on the common right-angled side. When switching between the two sector triangles, minimal switching can be achieved. Combination 1 only requires changing the ±1 level of phase C based on the last switching state of the previous spatial vector modulation cycle, and Combination 2 only requires changing the ±1 level of phase B based on the last switching state of the previous spatial vector modulation cycle. In combination 3, the two right-angled vertex vectors are located in different sector triangles, and minimal switching cannot be achieved between the two right-angled vertex vectors. Switching between the two sector triangles will simultaneously change the ±1 and / or ±2 levels of two phases based on the last switching state of the previous spatial vector modulation cycle.

[0093] Submodule capacitor voltage equalization control:

[0094] While sampling the three-phase reference voltage, the submodule capacitor voltage of each phase upper and lower bridge arm is started. Sampling, and sampling the current of the six arms of the three-phase bridge. Based on the activation and switching status of the submodules, the sampled data of the capacitor voltage of each phase submodule are saved separately according to the activation group and the deactivation group.

[0095] Based on the sampling results, the four sub-modules with the largest and second largest, smallest and second smallest capacitor voltages in the six bridge arm input and cut-off sub-module groups were identified as the selected objects for each bridge arm input and cut-off sub-module in this space vector modulation cycle.

[0096] Based on the switch state switching path and corresponding switch state obtained by space vector modulation The number of submodules required for each of the six bridge arms of the MMC can be calculated using equation (10). , , , , , .

[0097] Based on the switching path obtained from space vector modulation and the bridge arm current The direction determines the specific sub-modules that need to be added or removed on each bridge arm:

[0098] If the switching occurs within a sector delta, each switch only changes the voltage level of one phase. If the switching state of that phase increases by one voltage level compared to the previous switching state, then one submodule needs to be engaged in the upper bridge arm and one submodule needs to be disengaged in the lower bridge arm. Conversely, if the switching state of that phase decreases by one voltage level compared to the previous switching state, then one submodule needs to be disengaged in the upper bridge arm and one submodule needs to be engaged in the lower bridge arm. This is based on the bridge arm current. If the current direction is positive, a PWM signal is allocated to control the submodule with the smallest capacitor voltage in the cut-off group on that bridge arm to be put into operation, or to control the submodule with the largest capacitor voltage in the cut-off group on that bridge arm to be put into operation; if the current direction is negative, a PWM signal is allocated to control the submodule with the largest capacitor voltage in the cut-off group on that bridge arm to be put into operation, or to control the submodule with the smallest capacitor voltage in the cut-off group on that bridge arm to be put into operation.

[0099] If the reference vector of the current space vector modulation cycle and the reference vector of the previous modulation cycle are located in adjacent sector triangles, then a switch between adjacent sector triangles needs to be completed. Calculate the change in the switching state corresponding to the right-angle vertex vectors of the two sector triangles to determine the number of submodules that need to be added or removed in the corresponding phase arm of the current modulation cycle. This is based on the arm current. If the current direction is positive and the bridge arm only needs to add or remove one sub-module, a PWM signal is allocated to control the sub-module with the smallest capacitor voltage in the cut-off group to be put into operation, or to control the sub-module with the largest capacitor voltage in the cut-off group to be put into operation. If the current direction is negative and the bridge arm only needs to add or remove one sub-module, a PWM signal is allocated to control the sub-module with the largest capacitor voltage in the cut-off group to be put into operation, or to control the sub-module with the smallest capacitor voltage in the cut-off group to be put into operation. If the current direction is positive and the bridge arm needs to add or remove two sub-modules, a PWM signal is allocated to control the two sub-modules with the smallest and second smallest capacitor voltages in the cut-off group to be put into operation, or to control the two sub-modules with the largest and second largest capacitor voltages in the cut-off group to be put into operation. If the current direction is negative and the bridge arm needs to add or remove two sub-modules, a PWM signal is allocated to control the two sub-modules with the largest and second largest capacitor voltages in the cut-off group to be put into operation, or to control the two sub-modules with the smallest and second smallest capacitor voltages in the cut-off group to be put into operation.

[0100] by Figure 4 Middle reference vector For example, let the spatial vector The corresponding switching state with the minimum common-mode voltage is Based on the previous assumptions, the switch state switching path is as follows:

[0101] .

[0102] If the reference vector obtained in the previous sampling period and the reference vector obtained in the current sampling period are located in the same sector triangle, such as sector I in the figure, then the starting point for the current switch state transition is the same as the last switch state of the previous sampling period. Therefore, there is no switch state change, meaning there is no need to change the input and output states of the submodule. In the switch state... Switch to At this time, since the states of phases A and B remain unchanged, only the level of phase C is reduced by one level. This means the upper bridge arm needs to have one submodule disconnected, and the lower bridge arm needs to have one submodule connected. If the current of the upper bridge arm of phase C... If positive, allocate a PWM signal to cut off the submodule with the largest capacitor voltage among the submodules already engaged in the upper arm of phase C; if the current of the upper arm of phase C is positive... If the current is negative, allocate a PWM signal to cut off the submodule with the lowest capacitor voltage among the submodules already engaged in the upper arm of phase C. If the current in the lower arm of phase C is negative... If positive, allocate a PWM signal to connect the submodule with the lowest capacitor voltage in the submodule group that has been disconnected in the lower arm of phase C; if the current in the lower arm of phase C is positive... If the value is negative, a PWM signal is allocated, and the submodule with the largest capacitor voltage in the submodule group that has been disconnected in the lower arm of phase C is switched on. Subsequent switching states then change from... Switch to ,from Switch to ,from Switch to All of these can be achieved by following the switching rules above.

[0103] If the reference vector obtained in the previous sampling period and the reference vector obtained in the current sampling period are located in different sector triangles, such as Figure 4 As shown, the reference vector obtained from the previous sampling is located in a type II sector, and the reference vector for this sampling is located in a type I sector. Therefore, the last switching state of the previous sampling period is... The first switch state in this sampling period is Therefore, this sampling needs to be completed from arrive This switching process. Based on the principle of space vector modulation, space vector... The corresponding switching state with the minimum common-mode voltage is It can be calculated Corresponding switch state There are three possible states: , , Criteria Does it meet the requirements? If it does, It is a valid switch state; otherwise, it is an invalid switch state. Criterion Does it meet the requirements? If it does, It is a valid switch state; otherwise, it is an invalid switch state. Does it meet the requirements? If it does, A valid switching state is determined by its common-mode voltage; otherwise, it is considered an invalid switching state. The common-mode voltage corresponding to each valid switching state is compared, and the switching state with the lowest common-mode voltage is the final switching state.

[0104] Assuming switch state To achieve the final switching state, the process of switching between the two sector triangles must be completed. arrive Switching between on / off states. Switch to When phase A increases by one level and phase C decreases by two levels, one submodule needs to be added to the upper arm of phase A, and one submodule needs to be removed from the lower arm. Similarly, two submodules need to be added to the upper arm of phase C, and two submodules need to be removed from the lower arm. If the current in the upper arm of phase A... If positive, allocate a PWM signal to connect the submodule with the lowest capacitor voltage in the submodule group that has been disconnected from the upper bridge arm of phase A; if the upper bridge arm current is positive... If the current is negative, allocate the PWM signal and connect the submodule with the largest capacitor voltage in the submodule group that has been disconnected from the upper bridge arm of phase A. If the lower bridge arm current is negative... If positive, allocate a PWM signal to cut off the submodule with the largest capacitor voltage among the submodules already engaged in the lower bridge arm of phase A; if the lower bridge arm current is positive... If the current is negative, allocate the PWM signal to cut off the submodule with the smallest capacitance voltage among the submodules already engaged in the lower arm of phase A. If the current in the upper arm of phase C is negative... If positive, allocate the PWM signal and connect the two submodules with the lowest and second lowest capacitor voltages in the submodule group that have been disconnected from the upper bridge arm of phase C; if the upper bridge arm current is positive... If the current is negative, allocate the PWM signal and connect the two submodules with the largest and second largest capacitor voltages in the submodule group that have been disconnected in the upper arm of phase C. If the lower arm current... If positive, allocate a PWM signal to cut off the two sub-modules with the largest and second largest capacitor voltages in the sub-module group already engaged in the lower arm of phase C; if the lower arm current... If the value is negative, a PWM signal is allocated to cut off the two sub-modules with the smallest and second smallest capacitive voltages in the sub-module group that have been put into operation in the lower arm of phase C.

[0105] Based on the switching path of the switch state, repeat steps three and four to complete one cycle T. sSynthesis of internal reference vectors and equalization control of submodule capacitor voltage.

[0106] When a control cycle T s After completion, repeat all the steps of space vector modulation and submodule capacitor voltage equalization control to finally complete the space vector modulation and submodule capacitor voltage equalization control of the MMC.

Claims

1. A method for voltage balancing control of a sub-module capacitor of a space vector modulation based MMC, characterized in that, Three-phase reference voltage signal for space vector modulation of an MMC , , is: （1）； In the formula, m represents a modulation coefficient, reflecting the utilization rate of the direct current voltage, n represents the number of series sub-modules of the bridge arm, and n is an even number, ω represents the angular frequency of the output voltage; According to the definition of space vector modulation, Reference vector on coordinate system is expressed as: （2）； represents a reference vector, represents a reference vector coordinates of the vector; The reference vector trajectory is: （3）； spatial vector on a coordinate system is represented as: （4）； represents a spatial vector, represents a spatial vector coordinates; in the formula, , , respectively represent the output level of the A phase, the B phase and the C phase of the MMC, , , take 0, ±1, ±2, ±3, …, ±n / 2, that is, n+1 levels corresponding to each phase; The space vector modulation refers to adopting The space vector modulation of the MMC is realized by a coordinate system Step one: Position the reference vector Sector triangle; Step two: Calculate the synthesis reference vector of the sector triangle vertex vector; Step three: determine the switching sequence of the switching states corresponding to the three vertex vectors of the sector triangle; According to the sector triangle of type I and type II, taking the switching state corresponding to the right-angle vertex vector of the sector triangle as the reference, the switching sequence of the switching states corresponding to the three vertex vectors of the sector triangle is determined by using the five-segment algorithm according to the minimum switching principle and the requirement of closed switching path; Step four: determine the switching between adjacent sector triangles, that is, complete the switching between the switching states corresponding to the two right-angle vertex vectors of the sector triangle of type I and type II; In the switching between the three vertex vectors of the sector triangle and the switching between adjacent sector triangles, the specific input or cut-off sub-modules are determined according to the number of sub-modules to be input on the upper and lower bridge arms of each phase of the MMC, the bridge arm current direction, and the maximum value, minimum value, second maximum value and second minimum value of the capacitor voltage in the input sub-module group and the cut-off sub-module group, the PWM signals are distributed, the input and cut-off of the corresponding sub-modules are controlled, and the sub-module capacitor voltage balancing control is realized; The sub-module capacitor voltage balancing control refers to determining the specific input or cut-off sub-modules according to the number of sub-modules to be input on the upper and lower bridge arms of each phase, the bridge arm current direction, and the maximum value, minimum value, second maximum value and second minimum value of the capacitor voltage in the input sub-module group and the cut-off sub-module group calculated according to the obtained switching state.

2. The space vector modulation based MMC sub-module capacitor voltage balancing control method of claim 1, wherein, The MMC is a modular multilevel converter; the modular multilevel converter is a three-phase converter composed of six bridge arms, each phase is composed of an upper and a lower bridge arm, each bridge arm is composed of n sub-modules and an inductor in series, and the three phases share one DC power supply; each sub-module includes two power switches IGBT and one capacitor, two power switches IGBT are connected in series to form a half-bridge, and each IGBT is respectively connected in anti-parallel with a diode, that is, the collector of IGBT is connected with the cathode of diode, and the emitter of IGBT is connected with the anode of diode; the emitter of the upper bridge arm IGBT and the collector of the lower bridge arm IGBT are connected as one output terminal of the sub-module, which is the positive terminal of the sub-module output voltage, the collector of the upper half-bridge IGBT and the positive electrode of the capacitor are connected, and the emitter of the lower bridge arm IGBT and the negative electrode of the capacitor are connected as the other output terminal of the sub-module, which is the negative terminal of the sub-module output voltage; n sub-modules of each bridge arm are connected in series, that is, the negative terminal of the previous sub-module is connected with the positive terminal of the next sub-module, the positive terminal of the first sub-module of the upper bridge arm is connected with the positive electrode of the DC power supply, the negative terminal of the last sub-module of the upper bridge arm is connected with one end of the upper bridge arm inductor, the other end of the upper bridge arm inductor is connected with one end of the lower bridge arm inductor, the other end of the lower bridge arm inductor is connected with the positive terminal of the first sub-module of the lower bridge arm, and the negative terminal of the last sub-module of the lower bridge arm is connected with the negative electrode of the DC power supply; the connection point of the two inductors of the upper and lower bridge arms is the AC output terminal of the phase.

3. The space vector modulation based MMC sub-module capacitor voltage balancing control method of claim 1, wherein, In step one, the positioning reference vector The method for locating the sector triangle is: to a three-phase reference voltage signal , , sampling, the reference vector is obtained ; computing (5); In the formula denotes a floor function; Reference vector Located in the unit square consisting of , , and , the unit square contains 2 sector triangles; Case 1: When , the reference vector is located in the type II sector triangle composed of , and , is the right-angled vertex vector of the type II sector triangle; Case 2: When , the reference vector is located in the type I sector triangle composed of , and , is the right-angle vertex vector of the type I sector triangle.

4. The space vector modulation based MMC sub-module capacitor voltage balancing control method of claim 3, wherein, Step two: Compute the synthesis reference vector Sector triangle vertex vector's time of action: Reference vector Located , And In the type II sector triangle consisting of, according to the volt-second balance principle: （6）； In the formula, T s denotes the sampling period; Solving the above equations gives the space vectors , and corresponding action times , and : （7）； Reference vector Located , And The sector triangle of type I, according to the principle of volt-second balance: （8）； Solving the above equations gives the space vectors , and corresponding action times , and : （9）。 5. The space vector modulation based MMC sub-module capacitor voltage balancing control method of claim 4, wherein, Step three: determine the switching sequence of the switching states corresponding to the three vertex vectors of the sector triangle: Based on the principle of closed switching path, the five-segment switching method is adopted, Case 1: for sector 1 triangle, let the space vector The corresponding minimum common-mode voltage switching state is , and The switching path of the switching state is : ； Switching state switching path For: ； criteria , and whether if not, switching path is invalid, switching is made according to switching path ; criteria , and whether if not, switching path is invalid, switching according to switching path ; If and , compare , , , , if or is the largest, switch path is invalid, switch according to switch path , if or is the largest, switch path is invalid, switch according to switch path ; Case 2: For sector triangle of type II, the space vector The switching state with the minimum corresponding common-mode voltage is , and The switching state switching path is: ； Switching state switching path For: ； criteria , and are fulfilled , if not, the switching path is invalid, switching is performed according to the switching path ; criteria , and are fulfilled , if not, the switching path is invalid, switching is performed according to the switching path ; If and , compare , , , if or is maximum, switch path is invalid, switch according to path if or is maximum, switch path is invalid, switch according to path .

6. The space vector modulation based MMC sub-module capacitor voltage balancing control method of claim 5, wherein, Step four: determine the switching between adjacent sector triangles: When the two adjacent reference vectors are located in different sector triangles, one of the two adjacent sector triangles is type I and the other is type II; the adjacent type I and type II sector triangles have three combinations, combination 1 is that the type I sector triangle is above the type II sector triangle, and the two sector triangles share a right-angle side; combination 2 is that the type I sector triangle is right to the type II sector triangle, and the two sector triangles share a right-angle side; combination 3 is that the type I sector triangle is left to the type II sector triangle, and the two sector triangles share a slant side; In combination 1 and combination 2, the right-angle vertex vectors of the two sector triangles are located on the common right-angle side, and when switching between the two sector triangles, the minimum switching can be realized, and in combination 1, only the C-phase ±1 level needs to be changed on the basis of the last switching state of the previous space vector modulation period, and in combination 2, only the B-phase ±1 level needs to be changed on the basis of the last switching state of the previous space vector modulation period; in combination 3, the two right-angle vertex vectors are located in different sector triangles, and the minimum switching cannot be realized between the two right-angle vertex vectors, and the switching between the two sector triangles will change the ±1 and / or ±2 levels of certain two phases on the basis of the last switching state of the previous space vector modulation period.

7. The space vector modulation based MMC sub-module capacitor voltage balancing control method of claim 6, wherein, Submodule capacitor voltage equalization control: Step 1: when sampling the reference vector in the current space vector modulation period, the sampling of the submodule capacitor voltage of the upper and lower bridge arms of each phase of the MMC and the sampling of the currents of the three-phase six bridge arms are started synchronously, and according to the input and switching state of the submodule, the sampling data of the capacitor voltage of each phase submodule is saved in the input group and the cut-off group respectively; Step 2: according to the sampling results, four submodules with the largest and second largest capacitor voltages and the smallest and second smallest capacitor voltages in the input submodule group and the cut-off submodule group of the six bridge arms are found out as the selected objects of the input and cut-off submodules of each bridge arm in the current space vector modulation period; Step 3: Switching path and corresponding switch states according to the space vector modulation The number of sub-modules needed to be put into each of the six bridge arms of the MMC is calculated: （10）； In the formula, , , , , , respectively represent the number of sub-modules required to be put into the upper and lower bridge arms of the three-phase A, B and C, and the subscript p represents the upper bridge arm and the subscript l represents the lower bridge arm. Step 4: according to the switching path obtained by the space vector modulation and the bridge arm current direction, the specific submodules that need to be input or cut off on each bridge arm are determined: If the switching is within a sector triangle, only one level of a phase is changed each time, if the switching state of the phase is increased by one level on the basis of the previous switching state, a submodule needs to be input on the upper bridge arm and cut off on the lower bridge arm of the phase, if the switching state of the phase is reduced by one level on the basis of the previous switching state, a submodule needs to be cut off on the upper bridge arm and input on the lower bridge arm of the phase; according to the direction of the bridge arm current, if the current direction is to charge the capacitor of the submodule, the PWM signal is distributed to control the submodule with the smallest capacitor voltage in the cut-off group on the bridge arm to be input, or to control the submodule with the largest capacitor voltage in the input group on the bridge arm to be cut off; If the current direction is to discharge the capacitor of the submodule, the PWM signal is distributed to control the submodule with the largest capacitor voltage in the cut-off group on the bridge arm to be input, or to control the submodule with the smallest capacitor voltage in the input group on the bridge arm to be cut off; If the reference vector of the current space vector modulation period and the reference vector of the last modulation period are located in adjacent sector triangles, switching between adjacent sector triangles needs to be completed; The variation of the switch state corresponding to the vectors of the right-angle vertex of the two sector triangles is calculated to determine the number of sub-modules that need to be newly added and removed in the corresponding phase bridge arm of the current modulation period; according to the direction of the bridge arm current, if the current direction is to charge the sub-module capacitor, and only one sub-module needs to be added or removed in the bridge arm, the PWM signal is distributed to control the sub-module with the minimum capacitor voltage in the removal group of the bridge arm to be put into, or the sub-module with the maximum capacitor voltage in the input group of the bridge arm to be removed; if the current direction is to discharge the sub-module capacitor, and only one sub-module needs to be added or removed in the bridge arm, the PWM signal is distributed to control the sub-module with the maximum capacitor voltage in the removal group of the bridge arm to be put into, or the sub-module with the minimum capacitor voltage in the input group of the bridge arm to be removed; If the current direction is to charge the sub-module capacitor, and two sub-modules need to be added or removed in the bridge arm, the PWM signal is distributed to control the two sub-modules with the minimum and second minimum capacitor voltages in the removal group of the bridge arm to be put into, or the two sub-modules with the maximum and second maximum capacitor voltages in the input group of the bridge arm to be removed; If the current direction is to discharge the sub-module capacitor, and two sub-modules need to be added or removed in the bridge arm, the PWM signal is distributed to control the two sub-modules with the maximum and second maximum capacitor voltages in the removal group of the bridge arm to be put into, or the two sub-modules with the minimum and second minimum capacitor voltages in the input group of the bridge arm to be removed; Step 5: according to the switching path of the switch state, repeating step 3 and step 4, completing a sampling period T s Synthesis of inner reference vector and sub-module capacitor voltage balance control; When one sampling period T s After the end of one sampling period T All steps of space vector modulation and submodule capacitor voltage balancing control are repeated, and space vector modulation and submodule capacitor voltage balancing control of MMC are finally completed.

Images