Direct-current side voltage balance control method for cascaded H-bridge energy storage converter

Abstract

The invention provides a three-phase cascaded H-bridge energy storage converter direct-current side voltage balance control method based on a super capacitor. The control method aims at solving the problems that multi-module direct-current side voltages are unbalanced, the dynamic response capability is insufficient and the system reliability is low in a high-fluctuation scene in a traditional scheme. Through the modular H-bridge topology design and the dynamic voltage-sharing control strategy, the power distribution efficiency is optimized, the dynamic response speed is improved, and finally the practical values of easy expansion, convenient voltage sharing and small switching loss are achieved. According to the method, an SVPWM adjusting method is applied, a three-dimensional space voltage vector is established, and in-phase direct current side voltage balance is achieved by simplifying a vector voltage-sharing SVPWM algorithm; and applying a maximum zero sequence voltage injection method to realize inter-phase direct current side voltage balance.

Description

Technical Field

[0001] The invention relates to a DC side voltage balancing control method of a cascaded H-bridge energy storage converter, and belongs to the field of power electronic converters and circuit control. Background Art

[0002] The cascaded H-bridge energy storage converter is an energy conversion device based on a modular multi-level topology, which is widely used in new energy systems, energy storage power stations and industrial drive fields. It achieves high-voltage output by cascading multiple H-bridge modules, and has the advantages of strong harmonic suppression and flexible expansion. In recent years, supercapacitors have been introduced into cascaded H-bridge systems as energy storage units due to their high power density, fast charging and discharging characteristics and long cycle life to cope with the instantaneous power fluctuation requirements in renewable energy power generation and grid frequency regulation. However, the existing cascaded H-bridge converters based on supercapacitors still face significant technical bottlenecks:

[0003] Due to differences in the parameters of the energy storage battery cells themselves, the parameters of the H-bridge circuit switch devices and the circuit parallel losses, the energy storage battery cells absorb or release unequal amounts of energy, resulting in capacity deviations of the energy storage cells, which are reflected in the output voltage as unequal voltages on the H-bridge DC side. If the DC side voltages of each module are unbalanced, it can lead to an increase in the harmonic content of the grid-side current, and in severe cases, it can cause the system to be unable to operate stably, so it is critical to balance the DC side voltages of each module.

[0004] The present invention aims to provide a control method for a cascaded H-bridge energy storage converter to solve the problems of unbalanced DC voltage on multiple modules, insufficient dynamic response capability, and low system reliability in traditional solutions under high volatility scenarios. Through modular H-bridge topology design and dynamic voltage balancing control strategy, the power distribution efficiency is optimized, the dynamic response speed is improved, and finally the practical value of easy expansion, good voltage balancing, and low switching loss is achieved. Summary of the invention

[0005] The present invention provides a DC side voltage balancing control method of a three-phase cascaded H-bridge energy storage converter based on supercapacitors. The circuit topology structure involved in the control method includes a cascaded H-bridge energy storage converter, a grid-side filter reactor L, an equivalent loss R and a three-phase power grid. The converter adopts a three-phase star-shaped wiring method, each phase is cascaded by n modules, and is connected to the AC power grid through a filter reactor to achieve a bidirectional flow of energy. At the same time, each module of the converter corresponds to an energy storage unit on the DC side, and the energy storage unit stores energy through a supercapacitor.

[0006] The voltage balancing method can achieve intra-phase voltage balancing of the cascaded H-bridge energy storage converter, and comprises the following steps:

[0007] Step 1: Number the cascade modules, regard the output levels of the three modules as the space vector basis, and establish the corresponding three-dimensional space voltage vector diagram

[0008] The present invention is based on the A-phase three-module cascade topology structure of the cascade H-bridge energy storage converter. dc is the DC side input voltage, then the phase voltage that the A phase 3 module converter can output is -3U dc To 3U dc , a total of seven levels; the output level number of the first module of phase A is U a1b1 / U dc For L a1 , the output level of the second module is U a2b2 / U dc For L a2 , the output level of the third module is U a3b3 / U dc For L a3 Since all voltage vectors are located at L a1 ,L a2 ,L a3 In the three-dimensional space of the three axes, a cube is formed, L a Vector points with equal values ​​can form a section, and the origin of the coordinate (0,0,0) is located at the center of gravity of the cube. Thus, a three-dimensional voltage vector diagram of the A-phase three-module cascade H-bridge energy storage converter is established.

[0009] Step 2: In the spatial voltage vector diagram established in step 1, calculate the level action time and obtain the PWM waveform when the corresponding adjacent vectors act alternately.

[0010] Converter phase voltage u a The corresponding reference vector is V ref , since the reference vector can be synthesized by two adjacent vectors, namely V up , V down , where V up The vector corresponding to the larger number of levels in the interval is called the large vector; V down The vector corresponding to the smaller number of levels in the interval is called a small vector.

[0011] V up 、V down The action time T up , T down They correspond to the action time of the level in the level interval respectively.

[0012] In the A-phase three-module cascade, let the output phase voltage be -3U dc ,-2U dc ,-U dc ,0,U dc ,2U dc ,3Udc The action time of the level is T a-3 ,T a-2 ,T a-1 ,T a0 ,T a1 ,T a2 ,T a3 , the switching period is T s , V ref The output modulus is V * ;

[0013] T a0 ,T a1 , for example, we have

[0014]

[0015] When the modulated wave is greater than or equal to the carrier wave, the control signal T PWM is 0, output U dc Level vector V down ; When the modulated wave is smaller than the carrier wave, the control signal T PWM 1, output 2U dc Level vector V up , thus obtaining V down 、V up Alternating PWM waveform.

[0016] Step 3: Determine the action time state, change the AC side output level, and use the vector selection method to make the phase balance by changing the AC side output level L a1 , L a2 and L a3 Balance the voltage of corresponding modules.

[0017] First, six level intervals are calculated based on the reference vector, and two synthetic magnitude vectors V of the current carrier cycle after the simplified vector are determined for each level interval. up 、V down ;

[0018] Then the time T of each level interval size vector is obtained by time calculation up , T down , and then perform three-stage time distribution to obtain T PWM ;

[0019] According to T PWM Determine the specific level of output in each level interval;

[0020] Different optimal voltage vectors are selected according to the current direction on the AC side and the voltage size on the DC side.

[0021] Step 4: Use the maximum zero-sequence voltage injection method to balance the phase-to-phase DC side voltage

[0022] V dca is the total voltage on the DC side of phase A, i dca represents the DC side current of phase A, C is the equivalent capacitance of the DC side supercapacitors of all modules in phase A, P a0 Indicates the additional fluctuating power of phase A. Take the zero-sequence voltage amplitude U 0 Maximum value.

[0023] When the energy storage system is charging and providing active power, U pa , U pb , U pc is the positive sequence voltage component of the converter output, U 0 is the zero-sequence voltage component.

[0024] When the positive sequence voltage and zero sequence voltage components are superimposed, the output voltage U of the CHB energy storage converter can be obtained. a , U b , U c , U amax , U bmax , U cmax is the output voltage U a , U b , U c At this time, it is necessary to make the three phases a, b, and c satisfy: U amax ≤V dca ,U bmax ≤V dcb ,U cmax ≤V dcc .

[0025] U a byU 0 with U pa It is formed by the superposition of vectors. According to the cosine theorem, we can get U a for:

[0026]

[0027] Solving the mathematical transformation of formula (2) yields:

[0028]

[0029] When the CHB energy storage system works at U a When the maximum value is obtained, equation (3) represents the constraint condition that the injected zero-sequence voltage can take the maximum value. dca (t) represents the total voltage on the DC side of phase A. Thus, equation (3) can be expressed as:

[0030]

[0031] Similarly, when the energy storage system works at Ub When the maximum value is reached, U 0 The maximum value of must satisfy the following constraints:

[0032]

[0033] When the energy storage system works at U c When the maximum value is reached, U 0 The maximum value of must satisfy the following constraints:

[0034]

[0035] The phase-to-phase maximum zero-sequence voltage injection method includes two operating stages:

[0036] Phase 1: In order to increase the voltage balancing speed, the system injects the maximum zero-sequence voltage U 0 , which can greatly shorten the time of phase voltage balancing;

[0037] Phase 2: When the interphase voltage of the CHB energy storage system is about to reach equilibrium, the control is switched to the traditional zero-sequence voltage injection according to the conversion formula (7), and the three-phase modulation voltage v a * 、v b * 、v c * With zero sequence voltage component U 0 The final modulation wave voltage v can be obtained by superposition. a *′ 、v b *′ 、v c *′ , where u ref is a given smaller baseline value.

[0038] |ΔU dca |≤u ref and|ΔU dcb |≤u ref and|ΔU dcc |≤u ref (7)

[0039] Beneficial effects: Since the supercapacitor energy storage unit and the H-bridge module constitute independent sub-modules, the system can be more easily modularized and more suitable for high-voltage, large-capacity energy storage system construction; it is easy to achieve high output voltage. By increasing the number of H-bridges per phase, the output voltage level can be increased, and high-voltage output can be obtained with low-voltage modules. When connected to the medium-voltage power grid, the output step-up transformer can be omitted; when the same number of level outputs is used, the cascaded H-bridge converter uses fewer power components and the cost is lower than other multi-level topologies. In the specific selection of the voltage-sharing vector, the simplified vector voltage-sharing SVPWM algorithm is used, which makes the voltage utilization rate high, easy to implement digitally, and the voltage-sharing speed is faster and the switching loss is smaller. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a flow chart of the control method;

[0041] Figure 2 It is a cascaded H-bridge module;

[0042] Figure 3 The three-dimensional voltage vector diagram of the A-phase converter is a simplified vector diagram;

[0043] Figure 4 The influence of different voltage vectors of phase A on the DC side voltage;

[0044] Figure 5 It is the flow chart of intra-phase voltage balancing control;

[0045] Figure 6 It is the vector diagram of the zero-sequence voltage injection circuit;

[0046] Figure 7 This is the maximum zero-sequence injection voltage control block diagram;

[0047] Figure 8 The DC side voltage waveform of each module of the three-phase under intra-phase balance;

[0048] Fig. 9 The charging voltage waveform of each module port of phase A of the converter;

[0049] It should be noted that these drawings and textual descriptions are not intended to limit the conceptual scope of the present invention in any way, but are intended to illustrate the concept of the present invention for those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0050] In order to more clearly understand the technical advantages of the present invention, the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods, and the specific implementation cases described are only used to explain the present invention and are not limited to the present invention.

[0051] Figure 1The overall flow chart of the control method is as follows. First, the cascade modules are numbered, the output levels of the three modules are regarded as the space vector basis, and the corresponding three-dimensional space voltage vector diagram is established; secondly, in the space voltage vector diagram, the level action time is calculated to obtain the PWM waveform when the corresponding adjacent vectors act alternately; then the action time state is determined, the output level of the AC side is changed, and the vector selection method is used to achieve phase balance; finally, the maximum zero-sequence voltage injection method is used to perform balanced control of the DC side voltage between phases.

[0052] Cascade H-bridge modules such as Figure 2 As shown, the cascaded H-bridge module consists of a DC power supply: U dc , 1 super capacitor: C, 4 insulated gate bipolar transistors: IGBT1, IGBT2, IGBT3, IGBT4, and two wiring terminals: a first wiring terminal (1) and a second wiring terminal (2).

[0053] In the present invention, all insulated gate bipolar transistors, capacitors, drive boards, and control boards are integrated into one module.

[0054] Each insulated gate bipolar transistor is not only of the same type, but can also be replaced by other fully controlled types of fully controlled devices, such as gate turn-off thyristors, power transistors, power field effect transistors, etc.

[0055] Figure 3 The three-dimensional voltage vector diagram of the A-phase three-module cascade H-bridge energy storage converter. The A-phase 3-module converter can output a phase voltage of -3U dc To 3U dc , a total of seven levels. The output level number of the first module of phase A is U a1b1 / U dc For L a1 , the output level of the second module is U a2b2 / U dc For L a2 , the output level of the third module is U a3b3 / U dc For L a3 Since all voltage vectors are located at L a1 ,L a2 ,L a3 In the three-dimensional space of the three axes, a cube is formed, L a Vector points of equal value form a tangent plane, with the origin (0,0,0) at the center of the cube.

[0056] Figure 4 The influence of different voltage vectors of phase A on the DC side voltage can be obtained by simplifying the vector L. a =0 and L a = ±3, there is only one voltage vector, no selection is required, reducing the system burden.a = ±1 and L a = ±2, the optimal vector is selected according to the voltage magnitude and current direction. The influence of the voltage vector at different output levels after phase A simplifies the vector on the DC side voltage is as follows: Figure 4 As shown in the figure, "↓" indicates voltage reduction, "↑" indicates voltage increase, and "+" indicates the input current i of phase A. a Positive, “-” indicates the input current i of phase A a Is negative. 5 [U dc ,2U dc ] interval as an example, when the A phase output level L a Located in R 5 When the interval is , the method of selecting the vector can be explained by assuming the following moments:

[0057] 1) At the initial moment t, L a =1 means output U dc Level, when the voltage vector is (1,0,0);

[0058] 2) If at time t+1 i a <0,U dca2 minimum, then L a =2, the output is (1,0,1), according to Figure 4 Will make U dca1 and U dca3 Reduce, U dca2 Unchanged, meeting the balance requirement;

[0059] 3) If at time t+2 i a <0,U dca1 Maximum, then L a =1, the output is (1,0,0), according to Figure 4 Will make U dca1 Reduce, U dca2 and U dca3 Unchanged, meeting the balance requirement;

[0060] 4) If at time t+3 i a >0,U dca3 Maximum, then L a =2, the output is (1,1,0), according to Figure 4 Will make U dca1 and U dca2 Increase, U dca3 Unchanged, meeting the balance requirement.

[0061] Figure 5 This is the flow chart of the voltage balance control within the phase. By changing the AC side output level L a1 , L a2 and L a3The voltage of the corresponding module is balanced. First, six level intervals are calculated based on the reference vector, and two synthetic magnitude vectors V of the current carrier cycle after the simplified vector are determined for each level interval. up 、V down ; Then the time T of each level interval size vector is obtained by time calculation up , T down , and then perform three-stage time distribution to obtain T PWM ; Then according to T PWM Determine the specific level of each level interval output; select different optimal voltage vectors according to the current direction of the AC side and the voltage size of the DC side.

[0062] Figure 6 It is the zero sequence voltage injection circuit vector diagram. pa , U pb , U pc is the positive sequence voltage component of the converter output, U 0 is the zero-sequence voltage component. During the charging process of the energy storage system, Figure 6 (a) is the condition where the voltage on the DC side of phase A is the lowest and the deviation is the largest. At this time, the zero-sequence voltage U injected into the system is 0 With positive sequence voltage component U pa The smallest angle, U a Maximum, the charging power of phase A is also the maximum; Figure 6 (b) The voltage on the DC side of phase B is the lowest, and the zero-sequence voltage U injected into the system 0 With positive sequence voltage U pb The smallest angle, U b Maximum, the charging power of phase B is also the maximum; Figure 6 (c) The voltage value of the DC side of phase C is the lowest, and the zero-sequence voltage U injected into the system 0 With positive sequence voltage U pc The smallest angle, U c The maximum, the charging power of phase C is also the maximum. The same is true when the energy storage system is discharging.

[0063] Figure 7 This is the maximum zero-sequence injection voltage control block diagram. dca is the total voltage on the DC side of phase A, i dca represents the DC side current of phase A, C is the equivalent capacitance of the DC side supercapacitors of all modules in phase A, P a0 Indicates the additional fluctuating power of phase A. Take the zero-sequence voltage amplitude U 0 Maximum value.

[0064] When the energy storage system is charging and providing active power, U pa , U pb , U pc is the positive sequence voltage component of the converter output, U 0is the zero-sequence voltage component.

[0065] When the positive sequence voltage and zero sequence voltage components are superimposed, the output voltage U of the CHB energy storage converter can be obtained. a , U b , U c , U amax , U bmax , U cmax is the output voltage U a , U b , U c At this time, the three phases a, b, and c need to satisfy: U amax ≤V dca , U bmax ≤V dcb , U cmax ≤V dcc .

[0066] Figure 8 The DC side voltage waveforms of each module of the three-phase abc using the simplified vector voltage balancing SVPWM algorithm. Taking phase A as an example, it can be seen that the energy storage unit with a large initial voltage during charging has the slowest charging speed, and the energy storage unit with a small initial voltage has the fastest charging speed. The control algorithms are different when the voltage difference is large and the voltage difference is small, and the control accuracy is also different; when the voltage difference is small, the difference between the maximum voltage and the minimum voltage is less than 1V, which meets the requirements. As the imbalance decreases, it is finally charged at the same speed at 0.705s. When discharging, its discharge speed is opposite to that of charging, and finally reaches equilibrium at 0.46s. Although the initial voltage values ​​of each energy storage unit in each phase are different, their voltages can still reach equilibrium in a short time, and charge and discharge at the same charging speed, which proves that the proposed intra-phase voltage balancing algorithm has a strong voltage balancing ability.

[0067] Fig. 9 The charging voltage waveform of each module port of phase A when the voltage difference of the DC side of the converter is large. Fig. 9 (a) Under the traditional method, the levels of modules 1, 2, and 3 all have over-level jumps. The output voltage u of phase A on the AC side a When the level changes, the levels of the three modules also change at the same time. Since the vector is not simplified, the module actions are frequent and the switching loss is also large. And because the initial voltages of the supercapacitors of modules 1, 2, and 3 are set to 60V, 80V, and 100V respectively when charging phase A, module 1 needs to charge faster to obtain more power to maintain balance, followed by module 2, and finally module 3. Therefore, module 1 needs a high level to obtain more power, so Fig. 9 i in (a) a>0, the +1 level is maintained. However, when the level changes without simplifying the vector, there are still cases where the levels of the three modules change at the same time, such as Fig. 9 (a) Voltage amplification at (1) and (2). Fig. 9 (b) is the voltage of each module port of phase A of the proposed method. a When the level changes, each module does not jump to the next level, presenting a step wave, and only one of the three modules changes its level. This shows that the switching device actions of each module of the converter under the simplified vector voltage-balanced SVPWM algorithm are small, the voltage stress is low, and the switching loss is small.

Claims

1. A DC side voltage balancing control method for a cascaded H-bridge energy storage converter, characterized in that: The following steps are involved: Step 1: Number the cascade modules, regard the output levels of the three modules as the space vector basis, and establish the corresponding three-dimensional space voltage vector diagram; Step 2: In the spatial voltage vector diagram established in step 1, calculate the level action time and obtain the PWM waveform corresponding to the alternating action of adjacent vectors; Step 3: Determine the action time state, change the AC side output level, and use the vector selection method to achieve intra-phase balance; Step 4: Use the maximum zero-sequence voltage injection method to balance the phase-to-phase DC side voltage.

2. The DC side voltage balancing control method of a cascaded H-bridge energy storage converter according to claim 1 is characterized in that: The method in step 1 is based on the A-phase three-module cascade topology structure of the cascaded H-bridge energy storage converter, U dc is the DC side input voltage, then the phase voltage that the A phase 3 module converter can output is -3U dc To 3U dc , a total of seven levels; the output level number of the first module of phase A is U a1b1 / U dc For L a1 , the output level of the second module is U a2b2 / U dc For L a2 , the output level of the third module is U a3b3 / U dc For L a3 ; Since all voltage vectors are located at L a1 ,L a2 ,L a3 In the three-dimensional space of the three axes, a cube is formed, L a Vector points with equal values ​​can form a section, and the coordinate origin (0,0,0) is located at the center of gravity of the cube; thus, a three-dimensional voltage vector diagram of the A-phase three-module cascaded H-bridge energy storage converter is established.

3. The DC side voltage balancing control method of a cascaded H-bridge energy storage converter according to claim 1 is characterized in that: In step 2, the converter phase voltage u a The corresponding reference vector is V ref , since the reference vector can be synthesized by two adjacent vectors, namely V up , V down , where V up The vector corresponding to the larger number of levels in the interval is called the large vector; V down The vector corresponding to the smaller number of levels in the interval is called a small vector; V up 、V down The action time T up , T down They correspond to the action time of the level in the level interval respectively; In the A-phase three-module cascade, let the output phase voltage be -3U dc ,-2U dc ,-U dc ,0,U dc ,2U dc ,3U dc The action time of the level is T a-3 ,T a-2 ,T a-1 ,T a0 ,T a1 ,T a2 ,T a3 , the switching period is T s , V ref The output modulus is V * ; T a0 ,T a1 , for example, we have: When the modulated wave is greater than or equal to the carrier wave, the control signal T PWM is 0, output U dc Level vector V down ; When the modulated wave is smaller than the carrier wave, the control signal T PWM 1, output 2U dc Level vector V up , thus obtaining V down 、V up Alternating PWM waveform.

4. The DC side voltage balancing control method of a cascaded H-bridge energy storage converter according to claim 1 is characterized in that: In step 3, by changing the AC side output level L a1 , L a2 and L a3 Balance the voltage of the corresponding module; first calculate 6 level intervals based on the reference vector, and determine the two synthetic size vectors V of the current carrier cycle after simplifying the vector for each level interval up 、V down ; Then the time T of each level interval size vector is obtained by time calculation up , T down , and then perform three-stage time distribution to obtain T PWM ; Then according to T PWM Determine the specific level of the output in each level interval; finally, select different optimal voltage vectors according to the current direction on the AC side and the voltage size on the DC side.

5. The DC side voltage balancing control method of a cascaded H-bridge energy storage converter according to claim 1, characterized in that: V in step 4 dca is the total voltage on the DC side of phase A, i dca represents the DC side current of phase A, C is the equivalent capacitance of the DC side supercapacitors of all modules in phase A, P a0 Indicates the additional fluctuating power of phase A; takes the maximum value of zero-sequence voltage amplitude U0; When the energy storage system is charging and providing active power, U pa , U pb , U pc is the positive sequence voltage component output by the converter, and U0 is the zero sequence voltage component; When the positive sequence voltage and zero sequence voltage components are superimposed, the output voltage U of the CHB energy storage converter can be obtained. a , U b , U c , U amax、 U bmax , U cmax is the output voltage U a , U b , U c At this time, it is necessary to make the three phases a, b, and c satisfy: U amax ≤V dca ,U bmax ≤V dcb ,U cmax ≤V dcc ; U a By U0 and U pa It is formed by the superposition of vectors. According to the cosine theorem, we can get U a for: Solving the mathematical transformation of formula (2) yields: When the CHB energy storage system works at U a When the maximum value is obtained, equation (3) represents the constraint condition that the injected zero-sequence voltage can take the maximum value; V dca (t) represents the total voltage on the DC side of phase A. Thus, equation (3) can be expressed as: Similarly, when the energy storage system works at U b When the maximum value is obtained, the constraints that the maximum value of U0 must satisfy are: When the energy storage system works at U c When the maximum value is obtained, the constraints that the maximum value of U0 must satisfy are: The method comprises two operating phases: Stage 1: In order to increase the voltage balancing speed, the system can greatly shorten the phase-to-phase voltage balancing time by injecting the maximum zero-sequence voltage U0; Stage 2: When the phase-to-phase voltage of the CHB energy storage system is about to reach equilibrium, according to the following conversion formula: |ΔV dca |≤u ref and|ΔV dcb |≤u ref and|ΔV dcc |≤u ref Switch to the traditional zero-sequence voltage injection control, the three-phase modulation voltage v a * 、v b * 、v c * The final modulation wave voltage v can be obtained by superimposing it with the zero-sequence voltage component U0. a *′ 、v b *′ 、v c *′ , where u ref is a given smaller baseline value.

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