Star series H-bridge SVG, DC side voltage balance control method thereof and computer system

Abstract

The invention belongs to the field of power electronics and control thereof, and particularly relates to a star series H-bridge SVG, a direct current side voltage balance control method thereof and a computer system. The method comprises the following steps: firstly, generating a positive-sequence reactive current instruction and a negative-sequence active current instruction through the sum of positive-sequence reactive compensation current and three-phase negative-sequence current, generating a positive-sequence current instruction under the control of direct-current side voltage, and adding the negative-sequence active current instruction and the positive-sequence active current instruction to obtain a total positive-sequence active current instruction; the total positive-sequence current instruction generates a positive-sequence voltage through current control, the positive-sequence voltage and the scaled zero-sequence voltage injection value are subjected to dq inverse transformation to generate a three-phase transition voltage value, a three-phase output voltage is generated according to the three-phase transition voltage value, and the three-phase output voltage is utilized to generate a switching signal to control the SVG. According to the method, the power compensation capability and the dynamic performance of the SVG are improved by using the advantages that the zero-sequence voltage does not influence the power grid and the negative-sequence current is high in balancing capability and using a zero-sequence voltage and negative-sequence current mixed injection method.

Description

Technical Field

[0001] The invention belongs to the field of power electronics and control thereof, and in particular relates to a star-type series H-bridge SVG and a DC side voltage balancing control method and a computer system thereof. Background Art

[0002] With the continuous development of power electronic power systems, more and more renewable energy power generation is connected to the power grid, which puts forward higher requirements for power quality control. The star-type series H-bridge multi-level static VAR generator (SVG) has been widely used in the field of improving power quality due to its advantages of easy modular expansion, fast response speed, low harmonic content, independent inverter units, no need for multiple transformers, and fewer switching devices required at the same output level. At present, there are two main types of three-phase DC side voltage control, one is zero-sequence voltage injection control, and the other is negative-sequence current injection control. The zero-sequence voltage injection control method will not affect the power grid, but the power balancing ability of this method is weak. Under the condition of large imbalance, the zero-sequence voltage to be injected is too high, affecting the system output. The negative-sequence current injection control method has a strong power balancing ability. By injecting negative-sequence current, the power distribution of the three-phase DC side is balanced, but it will inject negative-sequence current into the power grid, which will affect the power quality of the power grid. Summary of the invention

[0003] The object of the present invention is to provide a star-type series H-bridge SVG and a DC side voltage balancing control method and a computer system thereof, so as to solve the problem that the zero-sequence voltage required to be injected is too high under severe unbalanced conditions, affecting the system output, or all negative-sequence current is injected, affecting the power quality of the power grid.

[0004] In order to solve the above technical problems, the present invention provides a DC side voltage balancing control method of a star-type series H-bridge SVG, comprising the following contents:

[0005] When the output voltage of any phase exceeds the set voltage threshold, the following steps are performed:

[0006] 1) Calculate the three-phase negative sequence current, obtain the sum of the three-phase negative sequence current and the three-phase positive sequence reactive compensation current, perform dq transformation on the sum to obtain the positive sequence reactive current command value and the negative sequence active current command; perform SVG DC side voltage control to generate the positive sequence active current command; add the negative sequence active current command and the positive sequence active current command to obtain the total positive sequence active current command;

[0007] 2) The total positive-sequence active current command and the positive-sequence reactive current command generate positive-sequence active voltage and positive-sequence reactive voltage after current control, and the positive-sequence active voltage, the positive-sequence reactive voltage and the scaled zero-sequence voltage injection value are subjected to dq inverse transformation to generate a three-phase transition voltage value; a three-phase output voltage is obtained according to the three-phase transition voltage value, and a switch signal is generated using the three-phase output voltage to control the SVG.

[0008] Furthermore, when the output voltage of any phase does not exceed the set voltage threshold, the following steps are performed:

[0009] a) Perform dq transformation on the three-phase positive sequence reactive compensation current to obtain the positive sequence reactive current command value; perform SVG DC side voltage control to generate the positive sequence active current command;

[0010] b) The positive-sequence active current command and the positive-sequence reactive current command are generated into the positive-sequence active voltage command and the positive-sequence reactive voltage command after current control; the positive-sequence active voltage command, the positive-sequence reactive voltage command and the zero-sequence voltage injection value are subjected to dq inverse transformation to generate the three-phase transition voltage value; the three-phase output voltage is obtained according to the three-phase transition voltage value, and the three-phase output voltage is used to generate the switch signal to control the SVG.

[0011] Furthermore, if the output voltage of any phase exceeds the set multiple of the rated voltage on the DC side, it is determined that a serious unbalanced condition has occurred; if the output voltage of any phase does not exceed the set multiple of the rated voltage on the DC side, it is determined that an unbalanced condition has occurred but the degree has not reached a serious level; the set multiple is greater than 0.7 and less than 1.

[0012] Furthermore, the process of controlling the SVG DC side voltage includes: collecting the DC side voltages of N H-bridge modules of each phase, calculating the average of the N DC side voltages of each phase, and then calculating the voltage average between each phase based on the obtained average of each phase, and subtracting the square of the voltage average from the square of the voltage reference value, and outputting it after a control link.

[0013] Furthermore, the scaling method is: becoming k times of the zero-sequence voltage injection value before scaling, 0<k<1.

[0014] Furthermore, the process of obtaining the output voltage according to the three-phase transition voltage is: performing a balanced control between each H-bridge module of each phase, and adding the balanced control result to the three-phase transition voltage to obtain the output voltage.

[0015] Furthermore, the zero-sequence voltage injection value u0 is:

[0016]

[0017] Where U n is the negative sequence voltage amplitude on the grid side, ω is the grid side angular frequency, γ is the initial phase of the positive sequence compensation current, is the initial phase of the negative sequence voltage on the grid side, and t represents time.

[0018] Furthermore, the control link is a PI control link.

[0019] To solve the above technical problem, the present invention further provides a computer system, including a processor, wherein the processor executes a computer program to implement the steps of the above method.

[0020] In order to solve the above technical problems, the present invention further provides a star-type series H-bridge SVG, comprising an SVG main circuit and an SVG controller, wherein the SVG controller comprises a processor, and the processor executes a computer program to implement the steps of the above method.

[0021] The beneficial effects are as follows: the present invention is an improved invention creation. The control method of mixed injection of zero-sequence voltage and negative-sequence current described in the present invention, as the negative-sequence voltage at the grid-connected point increases, the zero-sequence voltage injection value increases, significantly affecting the output voltage amplitude, the control strategy of mixed injection of zero-sequence voltage and negative-sequence current is utilized, combining the advantages of zero-sequence voltage not affecting the grid and strong negative-sequence current balancing capability, controlling the phase-to-phase voltage balance on the three-phase DC side, avoiding overmodulation, increasing power balancing capability, and improving the dynamic performance of SVG. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the main circuit of the star-type series H-bridge SVG of the present invention;

[0023] Figure 2 It is a control block diagram of the zero-sequence voltage separate injection of the present invention;

[0024] Figure 3 is a block diagram of the overall voltage control module of the present invention;

[0025] Figure 4 It is a control block diagram of the zero-sequence voltage and negative-sequence current mixed injection of the present invention;

[0026] Figure 5 It is a negative sequence current injection calculation control block diagram of the present invention;

[0027] Figure 6 It is the negative sequence current waveform output by SVG based on the negative sequence current separate injection control method of the present invention;

[0028] Figure 7 It is the voltage waveform of the three-phase DC side of SVG based on the negative sequence current separate injection control method of the present invention;

[0029] Figure 8 It is the negative sequence current waveform output by SVG based on the zero sequence voltage and negative sequence current mixed injection control method of the present invention;

[0030] Fig. 9 It is the voltage waveform of the three-phase DC side of SVG based on the zero-sequence voltage and negative-sequence current mixed injection control method of the present invention;

[0031] Fig.10 is the three-phase voltage waveform of the power grid in the dynamic simulation of the present invention;

[0032] Fig.11 is the SVG three-phase output current waveform in the dynamic simulation of the present invention;

[0033] Fig.12 is the negative sequence current waveform of the SVG output in the dynamic simulation of the present invention;

[0034] Fig.13 It is the voltage waveform of the three-phase DC side of SVG in the dynamic simulation of the present invention. DETAILED DESCRIPTION

[0035] The present invention generates a positive-sequence reactive current instruction and a negative-sequence active current instruction by the sum of the positive-sequence reactive compensation current and the three-phase negative-sequence current, and generates a positive-sequence current instruction by DC side voltage control, and the negative-sequence active current instruction and the positive-sequence active current instruction are added to obtain a total positive-sequence active current instruction; the total positive-sequence current instruction and the positive-sequence reactive current instruction are then controlled to generate a positive-sequence voltage (including a positive-sequence active voltage and a positive-sequence reactive voltage), and the positive-sequence voltage and the scaled zero-sequence voltage injection value are subjected to a dq inverse transformation to generate a three-phase transition voltage value, and a three-phase output voltage is generated according to the three-phase transition voltage value, and a switch signal is generated by the three-phase output voltage to control the SVG. Different from the prior art, the present invention utilizes the advantages of zero-sequence voltage not affecting the power grid and having a strong negative-sequence current balancing capability, and uses a zero-sequence voltage and negative-sequence current mixed injection method to improve the power compensation capability and dynamic performance of the SVG.

[0036] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all of the embodiments. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work belong to the scope of protection of the present invention.

[0037] An embodiment of a DC side voltage balancing control method for a star-type series H-bridge SVG:

[0038] This embodiment is directed to a star-type series H-bridge SVG. The main circuit structure of the star-type series H-bridge SVG is as follows: Figure 1 Each phase is composed of N H-bridge modules connected in series, and then connected to the grid between the load end and the generating end through the filter reactor L. The connection point is defined as the grid connection point. Figure 1 in u sa 、u sb and u sc are the three-phase grid voltages; i ca 、i cb and i cc They are SVG three-phase output current, L i (i=a, b, c) is the line inductance when SVG is connected to the grid; u dc_ik (i=a,b,c,k=1,2,…N) is the DC side voltage of the H-bridge circuit module; R dc_ik and C dc_ik (i=a, b, c,k=1, 2, ...N) are the equivalent loss and DC side capacitance value of each H-bridge module respectively.

[0039] The control strategies adopted by the SVG system are as follows: Figure 2 As shown in the control block diagram, it mainly includes three layers. The first layer is the SVG total DC voltage control, including the total voltage control module, load current detection module and current control module. The second layer is the phase-to-phase voltage balance control module. The third layer is the balance control between each H-bridge module in each phase.

[0040] The control block diagram of the overall voltage control module in the first layer structure is as follows: Figure 3 As shown, the DC side voltage of each phase of the N series-connected H-bridge modules is averaged, and the average DC side voltage of each phase is summed up for three phases and then averaged to obtain the voltage mean u ave , reference voltage value u ref The square of the voltage minus the mean voltage u ave The square of the positive sequence active current command i is generated by PI regulation. cdp * .

[0041] The control block diagram of the load current detection module can be seen Figure 4 , the text description is as follows:

[0042] When the zero-sequence voltage and negative-sequence current mixed injection control strategy is adopted, the detection module input current command (i ca * ,i cb * ,i cc * ) is the positive sequence reactive compensation current (i cap * ,i cbp * ,i ccp *) and the negative sequence current (i can * ,i cbn * ,i ccn * , the control block diagram of negative sequence current is shown in Figure 5 ) is transformed into the positive sequence reactive current command i by dq inverse transformation cq * and negative sequence active current command i cdn * , and the total positive sequence active current command i cd * The negative sequence active current command i cdn * Sum the positive sequence active current command i cdp * get;

[0043] When the zero-sequence voltage injection control strategy is adopted, there is no negative-sequence current for phase balancing, and the detection module input current command (i ca * ,i cb * ,i cc * ) is only the positive sequence reactive compensation current (i cap * ,i cbp * ,i ccp * ), input current command (i ca * ,i cb * ,i cc * ) After dq inverse transformation, only the positive sequence reactive current instruction i is generated cq * , does not generate i cdn * , then the total positive sequence active current command is composed of the positive sequence active current command i cdp * Get it alone.

[0044] In the current control module, the positive sequence active current command i cd * Subtract the positive sequence active current i cd The difference is adjusted by PI and then compared with the positive sequence active voltage U sd The result of the addition is then added to the grid side angular frequency ω, the incoming line inductance L connecting SVG to the grid, and the positive sequence reactive current i cq The product of ωLi cq The sum is the voltage command Ucmdd ;

[0045] Positive sequence reactive current command i cq * Subtract the positive sequence reactive current i cq The difference obtained after PI adjustment and the positive sequence reactive voltage U sq The sum of the two, minus the grid side angular frequency ω, the incoming line inductance L connecting SVG to the grid, and the positive sequence active current i cd The product of ωLi cd Get voltage command U cmdq .

[0046] The second layer structure is phase voltage balance control, U cmdq , U cmdd Together with the zero-sequence voltage injection value u0, the three-phase transition voltage (U oa * , U ob * , U oc * ).

[0047] The third layer is the balanced control between each H-bridge module in each phase. The balanced output voltage (U oab , U obb , U ocb ) and three-phase transition voltage (U oa * , U ob * , U oc * ) and the three-phase output voltage (U oa , U ob , U oc ), the three-phase output voltage (U oa , U ob , U oc ) Use PWM to convert into switching signal to control the star series H-bridge SVG system.

[0048] Specifically, SVG adopts a segmented control strategy of zero-sequence voltage injection and mixed injection of zero-sequence voltage and negative-sequence current. Specifically:

[0049] 1) Under unbalanced conditions, when any phase output voltage (U oa , U ob , U oc ) When the amplitudes do not exceed 80% (i.e. the set multiple) of the rated voltage on the three-phase DC side, the zero-sequence voltage is injected separately into the control strategy. At this time, an unbalanced condition occurs but the degree is not serious. The zero-sequence voltage injection value u0 is calculated as follows:

[0050] Calculate the grid connection point voltage using the following formula:

[0051]

[0052] Where U p is the positive sequence voltage amplitude on the grid side, U n is the negative sequence voltage amplitude on the grid side, ω is the angular frequency on the grid side, is the initial phase of the negative sequence voltage on the grid side, and t represents time;

[0053] Calculate the SVG three-phase output current using the following formula:

[0054]

[0055] In the formula, I p is the positive sequence compensation current amplitude, γ is the positive sequence compensation current initial phase;

[0056] Calculate the active power of the grid connection point voltage and the three-phase output current. The calculation formula is as follows:

[0057]

[0058] Assume that the zero-sequence voltage injection value u0 is:

[0059]

[0060] Where, δ is the assumed initial phase of zero-sequence voltage;

[0061] Ignoring the SVG device loss, the active power obtained by the SVG three-phase branch from the power grid should be zero, and the formula is as follows:

[0062] P ap +P a0 =0

[0063] P bp +P b0 =0

[0064] Where P a0 , P b0 The active power generated by zero-sequence voltage injection into phases A and B;

[0065] According to the above constraints, the zero-sequence voltage injection value u0 can be obtained as:

[0066]

[0067] 2) When any phase output voltage (U oa , U ob , U oc) When the amplitude exceeds 80% of the rated voltage of the three-phase DC side, it indicates that a serious unbalanced condition occurs, and the zero-sequence voltage and negative-sequence current mixed injection control strategy is entered. The attenuation coefficient k is introduced to reduce the zero-sequence voltage injection (equivalent to scaling the injection value of the zero-sequence voltage). The attenuation coefficient needs to be set according to the severity of overmodulation and can be set between (0.7, 1). For example, the value is 0.8. At this time, the zero-sequence voltage injection value changes as follows:

[0068]

[0069] The remaining power deviation is balanced by negative sequence current injection. The negative sequence current injection calculation block diagram is as follows: Figure 5 At this time, the current detection module inputs the current command (i ca * ,i cb * ,i cc * ) is the positive sequence reactive compensation current (i cap * ,i cbp * ,i ccp * ) and the three-phase negative sequence current (i can * ,i cbn * ,i ccn * ) and.

[0070] It should be noted that when the zero-sequence voltage injection control strategy is entered, there is no three-phase negative-sequence current, and the positive-sequence current command generated by the current detection module only contains the positive-sequence reactive current command; when the zero-sequence voltage and negative-sequence current mixed injection control strategy is entered, due to the existence of three-phase negative-sequence current, the current command generated by the current detection module contains the positive-sequence reactive current command and the negative-sequence active current command. This difference is Figure 2 and Figure 4 There are specific manifestations in it.

[0071] A simulation model is constructed in Matlab / Simulink simulation software to perform simulation verification to illustrate the effect of the method of the present invention. The simulation construction parameters are shown in Table 1:

[0072] Table 1 Simulation parameters

[0073]

[0074] The feasibility of mixed injection of zero-sequence voltage and negative-sequence current is proved through simulation experiments. Fig. 9It can be seen that the three-phase DC side voltage is stable and consistent with the command value of 800V, proving that the hybrid injection method can effectively achieve three-phase DC side voltage balance. Figure 6 and Figure 8 By comparison, it can be found that the negative sequence current required to be injected by the control method of mixed injection of zero sequence voltage and negative sequence current is less than that of the control method of injecting negative sequence current alone; Figure 7 and Fig. 9 Due to the use of zero-sequence voltage feedforward, the zero-sequence voltage and negative-sequence current mixed injection control method has a faster voltage balancing control speed than the single negative-sequence current injection control method. The system is dynamically analyzed. At 0.5s, an unbalanced voltage is generated at the grid connection point. The dynamic waveform of the system is as follows: Figure 10-13 As shown in the figure, according to the simulation results, it can be seen that when the grid-connected point voltage suddenly changes unbalanced, the three-phase DC side voltage quickly recovers to the reference value of 800V, with a faster dynamic response speed.

[0075] A computer system embodiment:

[0076] A computer system embodiment of the present invention includes a memory, a processor, an internal bus and a computer program stored in the memory, and the processor and the memory communicate and exchange data with each other through the internal bus. The processor executes the computer program to implement the steps of the method described in the embodiment of the DC side voltage balancing control method of a star-type series H-bridge SVG of the present invention. The processor can be a processing device such as a microprocessor MCU, a programmable logic device FPGA, etc.; the memory can be various memories that use electrical energy to store information, such as RAM, ROM, etc., or it can be a memory using other methods.

[0077] A star-type series H-bridge SVG embodiment:

[0078] A star-type series H-bridge SVG embodiment of the present invention comprises an SVG main circuit and an SVG controller, wherein the SVG controller comprises a processor, and the processor executes a computer program to implement the steps of the method described in an embodiment of a method for controlling a DC side voltage balance of a star-type series H-bridge SVG of the present invention. The processor may be a processing device such as a microprocessor MCU or a programmable logic device FPGA.

[0079] Compared with the prior art, the present invention adopts a zero-sequence voltage and negative-sequence current mixed injection method to effectively realize three-phase DC side voltage balance control, thereby improving the power balance capability and dynamic performance of SVG.

[0080] Specific implementation methods are given above, but the present invention is not limited to the described implementation methods. The basic idea of ​​the present invention lies in the above basic scheme. For ordinary technicians in this field, it does not take creative work to design various deformed models, formulas, and parameters according to the teachings of the present invention. Changes, modifications, substitutions, and variations of the implementation methods without departing from the principles and spirit of the present invention still fall within the scope of protection of the present invention.

Claims

1. A DC side voltage balancing control method for a star-type series H-bridge SVG, characterized in that: The method includes: When a serious unbalanced condition occurs, proceed as follows: 1) Calculate the three-phase negative sequence current, obtain the sum of the three-phase negative sequence current and the three-phase positive sequence reactive compensation current, perform dq transformation on the sum to obtain the positive sequence reactive current command value and the negative sequence active current command; perform SVG DC side voltage control to generate the positive sequence active current command; add the negative sequence active current command and the positive sequence active current command to obtain the total positive sequence active current command; 2) The total positive-sequence active current command and the positive-sequence reactive current command generate positive-sequence active voltage and positive-sequence reactive voltage after current control, and the positive-sequence active voltage, the positive-sequence reactive voltage and the scaled zero-sequence voltage injection value are subjected to dq inverse transformation to generate a three-phase transition voltage value; a three-phase output voltage is obtained according to the three-phase transition voltage value, and a switch signal is generated using the three-phase output voltage to control the SVG.

2. The DC side voltage balancing control method of the star-type series H-bridge SVG according to claim 1 is characterized in that: When an unbalanced condition occurs but is not severe, proceed as follows: a) Perform dq transformation on the three-phase positive sequence reactive compensation current to obtain the positive sequence reactive current command value; perform SVG DC side voltage control to generate the positive sequence active current command; b) The positive-sequence active current command and the positive-sequence reactive current command are generated into the positive-sequence active voltage command and the positive-sequence reactive voltage command after current control; the positive-sequence active voltage command, the positive-sequence reactive voltage command and the zero-sequence voltage injection value are subjected to dq inverse transformation to generate the three-phase transition voltage value; the three-phase output voltage is obtained according to the three-phase transition voltage value, and the three-phase output voltage is used to generate the switch signal to control the SVG.

3. The DC side voltage balancing control method of the star-type series H-bridge SVG according to claim 2 is characterized in that: If the output voltage of any phase exceeds the set multiple of the rated voltage on the DC side, it is determined that a serious unbalanced condition has occurred; if the output voltage of any phase does not exceed the set multiple of the rated voltage on the DC side, it is determined that an unbalanced condition has occurred but the degree has not reached a serious level; Set the multiplier to be greater than 0.7 and less than 1.

4. The DC side voltage balancing control method of the star-type series H-bridge SVG according to claim 1 or 2, characterized in that: The process of controlling the DC side voltage of SVG includes: collecting the DC side voltage of N H-bridge modules of each phase, calculating the average value of the N DC side voltages of each phase, and then calculating the voltage average value between each phase based on the average value of each phase, and subtracting the square of the voltage average value from the square of the voltage reference value, and outputting it after a control link.

5. The DC side voltage balancing control method of the star-type series H-bridge SVG according to claim 1 is characterized in that: The scaling method is: becoming k times the zero-sequence voltage injection value before scaling, 0<k<1.

6. The DC side voltage balancing control method of the star-type series H-bridge SVG according to claim 1 or 2, characterized in that: The process of obtaining the output voltage according to the three-phase transition voltage is as follows: performing a balanced control between each H-bridge module of each phase, and adding the balanced control result to the three-phase transition voltage to obtain the output voltage.

7. The DC side voltage balancing control method of the star-type series H-bridge SVG according to claim 1 or 2, characterized in that: The zero-sequence voltage injection value u0 is: Where U n is the negative sequence voltage amplitude on the grid side, ω is the grid side angular frequency, γ is the initial phase of the positive sequence compensation current, is the initial phase of the negative sequence voltage on the grid side, and t represents time.

8. The DC side voltage balancing control method of the star-type series H-bridge SVG according to claim 4 is characterized in that: The control link is a PI control link.

9. A computer system comprising a processor, characterized in that: The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 8.

10. A star-type series H-bridge SVG, comprising an SVG main circuit and an SVG controller, wherein the SVG controller comprises a processor, characterized in that: The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 8.

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