CN102739071A - Method for controlling direct current capacitor voltage of modular multi-level converter based on circulating current decoupling - Google Patents

Abstract

The invention discloses a method for controlling DC capacitor voltage of a modular multilevel converter based on circulating current decoupling, which is characterized in that the active current absorbed by the modular multilevel converter from a power grid is used to control the total DC bus voltage, Use the DC component in the circulating current to balance the DC voltage between the three phases, use the decoupled fundamental wave component in the circulating current to control the balance of the DC bus bars of the upper and lower bridge arms of each phase, and finally fine-tune the output voltage of each module along the current direction of the bridge arm Realize the balance of the DC capacitor voltage among the various modules inside the bridge arm. This control method can well realize the balanced control of the DC bus voltage between the upper and lower bridge arms of each phase; coupled with the other three levels of voltage control loops, the DC side voltage of each module is well balanced and stabilized at Given value; this method is correct and reliable, and provides a good reference value for engineering applications.

Description

DC Capacitor Voltage Control Method for Modular Multilevel Converter Based on Circulating Current Decoupling

technical field

The invention relates to the research fields of a high voltage power quality controller in a modular multilevel topology (MMC) and a high voltage direct current transmission (HVDC), and in particular relates to a DC bus voltage balance control between modular multilevel bridge arms.

Background technique

With social progress and industrial development, modern power systems have two major characteristics: large transmission and distribution systems, and increased reactive power and nonlinear load capacity. First of all, the demand for electricity in modern society is also increasing. In order to meet the growing demand of users for electricity, the power system has become larger and larger, and its coverage has become wider and wider. This brings challenges to the stability of the power system. At the same time, different power systems often need to be interconnected to enhance the reliability of the entire power supply system, but there are asynchronous problems when different power systems are interconnected. Secondly, the loads of modern power systems also have new features: power electronic devices have superior performance and have been widely adopted by industry and agriculture as well as consumer industries. However, as a nonlinear load, power electronic devices will inject reactive power and harmonics into the grid. With the increase of the nonlinear load capacity, its impact on the power distribution system is also increasing, making the system unsafe and unstable. Stability hidden dangers [1]. Reactive power and harmonic compensation for medium and high voltage transmission and distribution systems can effectively improve the stability of the power system [2] [3]. The interconnection of different power supply systems through the high-voltage direct current transmission system (HVDC) can solve the problem of system asynchrony and prevent faults from spreading between systems. It is a feasible method to improve system stability and reliability.

Modular Multilevel Converter (MMC) has been extensively studied by scholars and strongly concerned by engineers since it was proposed. Modular multilevel converters have many advantages: modular design, low switching frequency, low power consumption, high-quality spectrum characteristics, etc. [4]. These advantages bring great convenience to the manufacture, installation and maintenance of the modular multilevel converter, and also make it directly connected to the medium and high voltage power grid without a grid-side transformer. Modular multilevel converters have been applied to HVDC power transmission systems and medium and high voltage power quality control systems, and have become an effective method to improve the stability and reliability of power transmission and distribution systems [5]-[9].

In the application of HVDC power transmission, medium and high voltage static var generator, medium and high voltage harmonic compensator, and medium and high voltage variable frequency speed control system, many chopper units are connected in series to form a bridge arm, and the six bridge arms are connected by reactance The converters are connected into a double star structure to become a three-phase converter. The DC side of each chopper unit in each bridge arm needs to be incorporated into an electrolytic capacitor, without adding an independent DC voltage source. At present, experts at home and abroad have conducted a lot of research on modular multilevel converters and proposed some control methods.

However, in practical applications, the loss of each chopper unit is different, and the delay of the switching signal generated by the control circuit is also different. In addition, the positive and negative half-cycle asymmetry of the current waveform during the dynamic process of load mutation will cause The imbalance of the DC side voltage between bridge arms or between modules. If it is not corrected in real time, the voltage on the DC side of some modules will become higher and higher, and the module will work in an excess state, causing the working life to be shortened or even directly blown up; the voltage on the DC side of some modules will be lower and lower, and the module will work in an underrated state. , cannot perform as expected. Therefore, the problem of controlling the DC side capacitor voltage of each module has become a key issue in the application of modular multilevel variable transformers, and it is also a difficult issue [7].

For the DC bus voltage control problem of modular multilevel converters, there are many solutions: such as module wiring method, bridge arm energy estimation method, circulating current configuration method, negative sequence current balance method, etc. [10] -[17]. These methods are only proposed for a specific application, not universal and systematic, and have their own shortcomings and deficiencies. A good control strategy for modular multilevel converters has not been seen yet. For this reason, it is also necessary to conduct systematic research on the DC side capacitor voltage control problem of modular multilevel converters.

The relevant literature retrieved is given below

[1] He Xiangning, Chen Alian. Theory and Application Technology of Multilevel Converter. Beijing: Mechanical Industry Press. 2006

[2] Li Yongdong, Xiao Xi, Gao Yue, etc. Large-capacity multilevel converter - principle, control, application. Beijing: Science Press. 2005

[3] Wang Zhaoan, Yang Jun, Liu Jinjun. Harmonic suppression and reactive power compensation [M]. Beijing: Machinery Industry Press, 2004.

[4] Hirofumi Akagi. "Classification, Terminology, and Application of the Modular Multilevel Cascade Converter (MMCC)," IEEE Trans. Power Electron., vol.26, no.11, pp.3119-31305, Jul.2011.

[5] Saeedifard, M., Iravani, R. "Dynamic Performance of a Modular Multileve Back-to-BackHVDC System," IEEE Trans. Power Delivery., vol.25, no.4, pp.2903-2912, Jul. 2010.

[6] Hirofumi Akagi. "New Trends in Medium-Voltage Power Converters and MotorDrives," Industrial Electronics (ISIE), 2011 IEEE International Symposium.pp.5-14, 2011.

[7] H.Mohammadi P., M.Tavakoli Bina. "A Transformerless Medium-Voltage STATCOMTopology Based on Extended Modular Multilevel Converters," IEEE Trans.Power Electron.,vol.26,no.5,pp.1534–1545, Jul. 2011.

[8] Xiaofeng Yang, Jianghong Li, Wenbao Fang, et al. "Research on Modular Multilevel Converter Based STATCOM," Industrial Electronics and Applications (ICIEA), 2011 6th IEEE Conference, pp.2569-2574.2011.

[9] B. Gemmell, J. Dorn, D. Retzmann, and D. Soerangr, “Prospects of multilevel VSC technologies for power transmission,” in Proc. Rec. IEEE TDCE, Chicago, IL, 2008, pp.1–16.

[10] Qing rui Tu, Zheng Xu. "Impact of Sampling Frequency on Harmonic Distortion for ModularMultilevel Converter," IEEE Trans.Power Delivery.,vol.26,no.1,pp.298–306,Ju1.2011.

[11] Chun Gao, Jianguo Jiang, Xingwu Yang, "A Novel Topology and Control Strategy of Modular Multilevel Converter (MMC)," Electrical and Control Engineering (ICECE), 2011International Conference, pp.967-971, sept.2011.

[12]D.Soto-Sanchez, T.C.Gree. "Control of a modular multilevel converter-based HVDC transmission system," Power Electronics and Application (EPE 2011), Proceedings of the2011-14th European Conference, pp.1-10.Aug. /Sept.2011.

[13] Makoto Hagiwara, Ryo Maeda, Hirofumi Akagi. "Control and Analysis of the ModularMultilevel Cascade Converter Based on Double-Star Chopper-Cells (MMCC-DSCC)," IEEETrans.Power Electron., vol.26, no.6, pp.1649–1658, Ju1.2011.

[14] Lanhua Zhang, Guangzhou Wang. "Voltage Balancing Control of a Novel Modular Multilevel Converter," Electric Utility Deregulation and Restructuring and Power Technologies (DRPT), 2011 4th International Conference, pp.109-114, Conf.2011.

[15] Makoto Hagiwara, Hirofumi Akagi. "Control and Experiment of Pulsewidth-Modulated Modular Multilevel Converters," IEEE Trans. Power Electron., vol.24, no.7, pp.1737–1746, Jul.2009.

[16] Zhao Xin, Zhao Chengyong, Li Guangkai. Modular multilevel converter capacitor voltage balance control using carrier phase shift technology [J]. Chinese Journal of Electrical Engineering, Vol. 31, No. 21, 48-55, 2011 .

[17] Guo Jie, Jiang Daozhao, Zhou Yuebin. Control method of modular multilevel converter with controllable AC and DC side currents. Automation of Electric Power Systems, Volume 35, Issue 7, 42-47, 2011.

Contents of the invention

The purpose of the present invention is to propose a method for controlling the DC capacitor voltage of a modular multilevel converter based on circulating current decoupling. Specifically, it is to use the active current absorbed by the modular multilevel converter from the grid to control the total DC bus voltage, use the DC component in the circulating current to balance the DC voltage between the three phases, and use the decoupled fundamental wave in the circulating current The components control the balance of the DC bus bars of the upper and lower bridge arms of each phase, and finally fine-tune the output voltage of each module along the current direction of the bridge arm to achieve the balance of the DC capacitor voltage between the modules inside the bridge arm. The key point of the present invention is the control part of the circulation decoupling part.

In order to achieve the above object, the present invention adopts the following technical solutions:

A DC capacitor voltage control method for modular multilevel converters based on circulating current decoupling. The active current absorbed by the modular multilevel converters from the power grid is used to control the total DC bus voltage, and the DC component in the circulating current is used to make the The DC voltage balance between the three phases is controlled by the decoupled fundamental wave component in the circulating current to control the balance of the DC bus bars of the upper and lower bridge arms of each phase, and finally the output voltage of each module is fine-tuned along the current direction of the bridge arm to realize the connection between each module inside the bridge arm. Balance the DC capacitor voltage between them.

In order to achieve the above object, the present invention can also adopt the following technical solutions:

A method for controlling DC capacitor voltage of a modular multilevel converter based on circulating current decoupling, comprising the following steps:

Step 1, three-phase total DC bus voltage control

Step 1.1, detect the DC side voltages v _api , _vani , v _bpi , v _bni , v _cpi , v _cni of all three-phase chopper units of the modular multilevel converter; where i=1,2...N, where N is Natural number; calculate the DC side voltage and Use this method to sequentially calculate the total DC side voltage of the lower bridge arm of phase A and the bridge arms of the two phases B and C $v_{c,\mathrm{na}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{ani}},$ $v_{c,\mathrm{pb}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{bpi}},$ $v_{c,\mathrm{nb}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{bni}},$ $v_{c,\mathrm{pc}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{cpi}},$ $v_{c,\mathrm{nc}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{cni}},$ Find the total DC side voltage v _{ph, a} =v _{c, pa} +v _{c, na} , v _{ph, b = v c, pb} +v _{c, nb} , v _{ph, c} = v of the upper and lower two bridge arms of each phase _{c, pc} +v _{c, nc} and three-phase average voltage Finally, calculate the average DC side voltage of each bridge arm module $\stackrel{\‾}{V_{\mathrm{cell},\mathrm{pa}}}=v_{c,\mathrm{pa}}/N,$ $\stackrel{\‾}{V_{\mathrm{cell},\mathrm{na}}}=v_{c,\mathrm{na}}/N,$ $\stackrel{\‾}{V_{\mathrm{cell},\mathrm{pb}}}=v_{c,\mathrm{pb}}/N,$ $\stackrel{\‾}{V_{\mathrm{cell},\mathrm{nb}}}=v_{c,\mathrm{nb}}/N,$ $\stackrel{\‾}{V_{\mathrm{cell},\mathrm{pc}}}=v_{c,\mathrm{pc}}/N,$ $\stackrel{\‾}{V_{\mathrm{cell},\mathrm{nc}}}=v_{c,\mathrm{nc}}/N.$

Step 1.2, the average value of the three-phase DC bus voltage and DC side voltage given value Send it to the single-way subtractor for calculation, and the calculation result is sent to the single-way proportional-integral regulator for adjustment, and its output value is used as the active current component exchanged between the modular multi-level converter and the AC grid As an additional amount injected into the d-axis of the current inner loop based on dq decoupling control to control the active power absorbed by the whole modular multilevel converter from the grid.

Step 2, synthesize the instruction of the DC component in the circulating current

Step 2.1, using the amount detected in step 1.1 And v _{ph, a} , v _{ph, b} , v _{ph, c} do the difference, and send it to three single-channel PI regulators to generate

Step 2.2, generated by the previous operation As a command for the DC component in the circulating current.

Step 3, synthesize the instruction of the AC component in the circulation

Step 3.1, detecting three-phase grid voltage v _{s, a} , v _s,b , v _{s, c} ;

In step 3.2, the three-phase grid voltage is transformed from the three-phase stationary coordinate system to the two-phase rotating coordinate system, and the value of the transformed d-axis is sent to a single-channel low-pass filter for filtering, and the output of the filter is recorded as V _pd ; Then exchange the detected B and C two-phase grid voltages, and then carry out the conversion operation from the three-phase stationary coordinate system to the two-phase rotating coordinate system. The filter performs filtering, and the numerical values of the d-axis and the q-axis are respectively denoted as V _nd and V _nq after filtering; the transformation matrix in this step is:

${\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; abc</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; dq</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; 3</span>}}\left[\begin{array}{ccc}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>\\ \mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>\end{array}\right]$

In step 3.3, substitute the V _pd , V _nd and V _nq generated in step 3.2 into the following relational formula to generate the inductive and capacitive reference directions of the grid side voltage of each phase:

v _s,a,1 ＝V _pd cosωt+V _nd cosωt+V _nq sinωt

v _s,a,-1 ＝-V _pd cosωt-V _nd cosωt-V _nq sinωt

${\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nq</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}$

${\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nq</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}$

${\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nq</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}$

${\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nq</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}$

In step 3.4, the v _s,b,-1 and v _s,c,1 generation generated in step 3.3 are given the following relationship -v _s,a =A(a) v _s,b,-1 +B(a) · v _{s, c, 1} According to the coefficients of sinωt and cosωt on the left and right sides of the equation are equal, obtain the coefficients A(a) and B(a); similarly, according to -v _{s, b} = A(b) v _{s , c,-1} +B(b) v _{s, a, 1} to obtain A(b), B(b), based on -v _s,c = A(c) v _{s, a, -1} +B (c) v _{s, b, 1} to obtain A(c), B(c);

In step 3.5, the values v _{c, pa} and v _{c, na} , v _{c, pb} and v _{c, nb} , v _{c, pc} and v _{c, nc} of step 1.1 are sent to three single-way subtractors respectively, and the The output is sent to three single-channel PI regulators, and the output values of the three PI regulators are defined as: c _a , c _b and c _c ;

In step 3.6, substitute the variables generated in steps 3.3, 3.4, and 3.5 into the following relational formula:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; a</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}$

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}$

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}}$

Obtain the AC circulation command of three-phase decoupling.

Step 4, balance control of the DC bus voltage among the various chopper units inside the bridge arm.

同理，将A相下桥臂斩波模块的平均直流侧电压

和A相下桥臂第一个模块的直流侧电压v_an1送入单路减法器，单路减法器的输出送入比例调节器，调节器的输出值与下桥臂电流的反相值-i_n,a相乘，得到A相下桥臂第一个模块交流电压的微调量

沿用同样的思路，得到B、C相上下桥臂第一个模块的交流电压微调量

Step 4.1, detect the actual current i _{p,a of the six bridge arms, i n,a} , i _p,b , i _n,b , i _p,c , in _,c , and switch the phase A upper bridge arm chopper module The average DC link voltage of and the DC side voltage v _ap1 of the first module of the upper bridge arm of phase A are sent to the single-way subtractor, and the output of the single-way subtractor is sent to the proportional regulator, and the output value of the regulator is related to the current i _p,a of the upper bridge arm Multiplied together to get the fine-tuning amount of the AC voltage of the first module of the upper bridge arm of phase A

Similarly, the average DC side voltage of the lower bridge arm chopper module of phase A

And the DC side voltage v _an1 of the first module of the lower bridge arm of phase A is sent to the single-way subtractor, the output of the single-way subtractor is sent to the proportional regulator, and the output value of the regulator is the inverse value of the lower bridge arm current - Multiply _{in and a} to get the fine-tuning value of the AC voltage of the first module of the lower bridge arm of phase A

Following the same idea, get the AC voltage fine-tuning of the first module of the upper and lower bridge arms of the B and C phases

A相下桥臂第二到第N个模块微调电压指令

B相上桥臂第二到第N个模块微调电压指令B相下桥臂第二到第N个模块微调电压指令

C相上桥臂第二到第N个模块微调电压指令

C相下桥臂第二到第N个模块微调电压指令

Step 4.2, respectively calculate the fine-tuning voltage commands of the second to Nth modules of the upper bridge arm of phase A

Fine-tuning voltage command for the second to Nth modules of the lower bridge arm of phase A

Fine-tuning voltage command for the second to Nth modules of the upper bridge arm of phase B Fine-tuning voltage command for the second to Nth modules of the lower bridge arm of phase B

Fine-tuning voltage command for the second to Nth modules of the C-phase upper bridge arm

Fine-tune the voltage command of the second to Nth modules of the C-phase lower bridge arm

Step 5, the chopper module AC voltage PWM modulation wave instruction generation

和模块化多电平变流器环流电流环控制系统把步骤2.2和3.5生成的直流环流指令和交流环流指令加和作为总的环流指令，并对总的环流指令电流和实际环流电流进行闭环跟踪控制，得到三相控制环流的指令电压PWM调制波

和

Step 5.1, the current loop control system of the output current of the modular multilevel converter performs closed-loop tracking control on the command current and the output current, and its output is subjected to dq inverse transformation to obtain a three-phase output command voltage PWM modulation wave

and The circulating current loop control system of the modular multilevel converter takes the sum of the DC circulating current command and the AC circulating current command generated in steps 2.2 and 3.5 as the total circulating current command, and performs closed-loop tracking control on the total circulating current command current and the actual circulating current , to obtain the command voltage PWM modulation wave of the three-phase control circulation

and

与步骤5.1生成的

和按关系式

运算，得到A相上桥臂第一个模块的PWM调制波；把A相下桥臂第一个H桥模块PWM调制波的微调指令

与步骤5.1生成的

和

按关系式

运算，得到A相下桥臂第一个模块的PWM调制波；以此类推，得到A相上下桥臂中剩余模块的最终PWM调制波和

B相上下桥臂中剩余模块的最终PWM调制波和

C相上下桥臂中剩余模块的最终PWM调制波

和 Step 5.2, fine-tune the PWM modulation wave of the first H-bridge module of the upper bridge arm of phase A

generated with step 5.1

and By relation

Calculate the PWM modulation wave of the first module of the upper bridge arm of the A phase; fine-tune the PWM modulation wave of the first H bridge module of the lower bridge arm of the A phase

generated with step 5.1

and

By relation

Operation, get the PWM modulation wave of the first module of the lower bridge arm of phase A; and so on, get the final PWM modulation wave of the remaining modules in the upper and lower bridge arms of phase A and

The final PWM modulation wave of the remaining modules in the upper and lower bridge arms of phase B and

The final PWM modulation wave of the remaining modules in the upper and lower bridge arms of phase C

and

In step 5.3, the modulation wave of each module of each phase is compared with the phase-shifted triangular carrier wave in sequence to generate switching signals of each module.

The present invention is characterized by four control loops in the above steps, wherein step 1 is the first control loop, the purpose is to control the amount of active power absorbed by the entire modular multilevel converter from the grid, so as to offset the power loss of the entire converter . Step 2 achieves the balance of the DC bus voltage between the three phases by automatically adjusting the DC circulating current required to generate the modular multilevel converter. The purpose of step 3 is to generate mutually decoupled AC circulation commands, redistribute the active power between the upper and lower bridge arms of each phase, and balance the DC voltage between the upper and lower bridge arms of each phase. The purpose of step 4 is to fine-tune the command voltage of each module in each bridge arm, reconfigure the active power absorbed by each module, so that the actual active power absorbed by the module can just offset the loss of the module itself, so that the actual DC side of each module The voltage is equal to the rated value.

Compared with the prior art, the present invention has the following beneficial effects: the present invention provides a balanced control method between upper and lower bridge arms of a modular multilevel converter based on circulating current decoupling, and a balanced control method among various modules. In order to verify the feasibility of the control method, the inventor built a small experimental prototype with a capacity of 5kVA and two chopper modules connected in series for each bridge arm in the laboratory. It can be seen from the experimental waveform that this control method can well realize the balanced control of the DC bus voltage between the upper and lower bridge arms of each phase. Cooperating with the other three levels of voltage control loops, the DC side voltage of each module is well balanced and stabilized at a given value. The control strategy has a good performance under various working conditions and even under the fault state of the power grid. The experimental results have proved the balanced control between the three phases and the phase-to-phase modules. This method is correct and reliable, and provides a good reference value for engineering applications.

Description of drawings

Figure 1 is a structural diagram of the main circuit of a modular multilevel converter;

Figure 2 is a block diagram of the DC side capacitor voltage control system of a modular multilevel converter; Figure 2(a) is the total DC bus voltage control; Figure 2(b) is the balance control between the three phases, k=a,b, c; Figure 2(c) shows the DC bus voltage control between the upper and lower bridge arms of each phase, where k-1=c,a,b;k=a,b,c;k+1=b,c,a ; Figure 2(d) shows the DC bus voltage control between modules inside each bridge arm, where k=a,b,c;j=1,2...N.

Figure 3 is the block diagram of the current control system of the modular multilevel converter; Figure 3(a) is the output current control based on dq; Figure 3(b) is the circulating current control based on the proportional regulator, k=a,b,c ;The total AC side output voltage command of each module is: $v_{p,\mathrm{kj}}^{*}=\frac{\mathrm{E.}}{2N}-\frac{v_k^{*}}{N}-\frac{v_{z,k}^{*}}{2N}+\mathrm{\&Delta;}{v}_{p,\mathrm{kj}}^{*};$ $v_{\mathrm{no},\mathrm{kj}}^{*}=\frac{\mathrm{E.}}{2N}-\frac{v_k^{*}}{N}-\frac{v_{z,k}^{*}}{2N}+\mathrm{\&Delta;}{v}_{\mathrm{no},\mathrm{kj}}^{*}.$

Figure 4 is a PWM modulation effect diagram. The waveforms of the four channels are, CH1: total AC side voltage of the upper arm of phase A; CH2: upper arm current; CH3: lower arm current; CH4: total output current.

Figure 5 is the experimental verification diagram of the total DC side voltage control. The waveforms of the four channels are as follows: CH1: grid voltage of phase A; CH2: output current of phase A; CH3: total output voltage of the upper bridge arm of phase A; CH4: DC side voltage of the first module of the upper bridge arm of phase A.

Figure 6 is the experimental verification diagram of the three-phase balanced control. The waveforms of the four channels are as follows: CH1: grid voltage of phase A; CH2: DC side voltage of the first module of the upper bridge arm of phase A; CH3: DC side voltage of the first module of the upper bridge arm of phase B; CH4: upper phase C The DC side voltage of the first module of the bridge arm.

Figure 7 is an experimental verification diagram of the balance control between the upper and lower bridge arms inside each phase. The waveforms of the four channels are as follows: CH1: Phase A grid voltage; CH2: Phase A output current; CH3: DC side voltage of the first module of the upper bridge arm of Phase A; CH4: DC side of the first module of the lower bridge arm of Phase A Voltage.

Figure 8 is an experimental verification diagram of the balance control among the modules inside the bridge arm. The waveforms of the four channels are as follows: CH1: Phase A grid voltage; CH2: Phase A output current; CH3: DC side voltage of the first module of the upper bridge arm of Phase A; CH4: DC side of the second module of the upper bridge arm of Phase A Voltage.

Fig. 9 is an experimental verification diagram of a modular multilevel converter compensating a symmetrical reactive load. The waveforms of the four channels are as follows: CH1: grid voltage of phase A; CH2: grid side current of phase A; CH3: output current of phase A; CH4: DC side voltage of the first module of the upper bridge arm of phase A.

Fig. 10 is an experimental verification diagram of a modular multilevel converter compensating an asymmetric nonlinear load. Figure 10(a) is the three-phase asymmetric grid voltage; Figure 10(b) is the three-phase nonlinear load current; Figure 10(c) is the three-phase output current; Figure 10(d) is the grid voltage and DC side capacitor voltage .

Detailed ways

Referring to Fig. 1, a modular multilevel converter is connected between the three-phase power supply system and the three-phase load. The main circuit structure of the hybrid multilevel converter consists of six bridge arms connected in series with six connected reactors respectively, and then forming a double star connection. Each bridge arm is composed of two chopper modules connected in series, the DC side of the module is connected in parallel with an electrolytic capacitor, and the switching device adopts a high-power full-control device such as IGBT or GTO. The selection of the parameters of the connected reactor mainly depends on the switching frequency of the H-bridge module.

There is no upper limit to the number of modules connected in series in each bridge arm, and the value depends on the voltage level of the power supply system. For the convenience of description, in the present invention, two modules are connected in series as an example for detailed description. The three-phase voltage of the power grid is recorded as u _s , namely: u _sa , _usb , us _sc ; the three-phase current of the power supply is recorded as is _s , namely: _isa , _isb , and i _sc ; the DC side voltage of 12 units of the chopper module in series Recorded as v _ap1 , v _ap2 , v _an1 , v _an2 , v _bp1 , v _bp2 , v _bn1 , v _bn2 , v _cp1 , v _cp2 , v _cn1 , v _cn2 ; H-bridge multi-level grid-connected inverter in series The output three-phase compensation current is recorded as i _c , namely: i _ca , i _cb , i _cc ; the three-phase load current is recorded as i _L , namely: i _ka , i _lb , i _lc .

Referring to Fig. 2, the DC bus voltage control method of the modular multilevel converter in the present invention includes four control loops, wherein step 1 is the first control loop, that is, the total AC/DC energy exchange, and step 2 is The second control loop, step 3 is the third control loop, step 4 is the fourth control loop, the specific steps are as follows:

Step 1, three-phase total DC bus voltage control

用此方法依次求出A相下桥臂及B、C两相各桥臂的总直流侧电压 $v_{c,\mathrm{na}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{ani}},$ $v_{c,\mathrm{pb}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{bpi}},$ $v_{c,\mathrm{nb}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{bni}},$ $v_{c,\mathrm{pc}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{cpi}},$ $v_{c,\mathrm{nc}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{cni}},$ 求出各相上下两个桥臂的总直流侧电压v_ph，a=v_c，pa+v_c，na，v_ph，b=v_c，pb+v_c，nb，v_ph，c=v_c，pc+v_c，nc及三相平均电压

最后求出各桥臂的模块平均直流侧电压 $\stackrel{\&OverBar;}{V_{\mathrm{cel},\mathrm{pa}}}=v_{c,\mathrm{pa}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cel},\mathrm{na}}}=v_{c,\mathrm{na}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cel},\mathrm{pb}}}=v_{c,\mathrm{pb}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cel},\mathrm{nb}}}=v_{c,\mathrm{nb}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cel},\mathrm{pc}}}=v_{c,\mathrm{pc}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cel},\mathrm{nc}}}=v_{c,\mathrm{nc}}/N.$ Step 1.1, detect the DC side voltages v _api , _vani , v _bpi , v _bni , v _cpi , v _cni of all three-phase chopper units of the modular multilevel converter; where i=1,2...N, where N is Natural number; calculate the DC side voltage and

Use this method to sequentially calculate the total DC side voltage of the lower bridge arm of phase A and the bridge arms of the two phases B and C $v_{c,\mathrm{na}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{ani}},$ $v_{c,\mathrm{pb}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{bpi}},$ $v_{c,\mathrm{nb}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{bni}},$ $v_{c,\mathrm{pc}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{cpi}},$ $v_{c,\mathrm{nc}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{cni}},$ Find the total DC side voltage v _{ph, a} =v _{c, pa} +v _{c, na} , v _{ph, b = v c, pb} +v _{c, nb} , v _{ph, c} = v of the upper and lower two bridge arms of each phase _{c, pc} +v _{c, nc} and three-phase average voltage

Finally, calculate the average DC side voltage of each bridge arm module $\stackrel{\&OverBar;}{V_{\mathrm{cell},\mathrm{pa}}}=v_{c,\mathrm{pa}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cell},\mathrm{na}}}=v_{c,\mathrm{na}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cell},\mathrm{pb}}}=v_{c,\mathrm{pb}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cell},\mathrm{nb}}}=v_{c,\mathrm{nb}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cell},\mathrm{pc}}}=v_{c,\mathrm{pc}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{cell},\mathrm{nc}}}=v_{c,\mathrm{nc}}/N.$

与直流侧电压给定值

送入单路减法器进行运算，运算结果送入单路比例积分调节器进行调整，其输出值作为模块化多电平变流器与交流电网交换的有功电流分量作为附加量注入到基于dq解耦控制的电流内环d轴上来控制整个模块化多电平变流器从电网吸收的有功功率。Step 1.2, the average value of the three-phase DC bus voltage

and DC side voltage given value

Send it to the single-way subtractor for calculation, and the calculation result is sent to the single-way proportional-integral regulator for adjustment, and its output value is used as the active current component exchanged between the modular multi-level converter and the AC grid As an additional amount injected into the d-axis of the current inner loop based on dq decoupling control to control the active power absorbed by the whole modular multilevel converter from the grid.

Step 2, synthesize the instruction of the DC component in the circulating current

分别与v_ph,a，v_ph,b，v_ph,c做差，送入三个单路PI调节器分别生成

Step 2.1, using the amount detected in step 1.1

Make difference with v _{ph, a} , v _{ph, b} , v _{ph, c} respectively, and send them to three single-channel PI regulators to generate

作为环流中直流分量的指令。Step 2.2, generated by the previous operation

As a command for the DC component in the circulating current.

Step 3, synthesize the instruction of the AC component in the circulation

Step 3.1, detecting three-phase grid voltages v _s,a , v _s,b , v _s,c .

In step 3.2, the three-phase grid voltage is transformed from the three-phase stationary coordinate system to the two-phase rotating coordinate system, and the value of the transformed d-axis is sent to a single-channel low-pass filter for filtering, and the output of the filter is recorded as V _pd ; Then exchange the detected B and C two-phase grid voltages, and then carry out the transformation operation from the three-phase stationary coordinate system to the two-phase rotating coordinate system. The filter performs filtering, and the values of the d-axis and the q-axis after filtering are denoted as V _nd and V _nq respectively. The transformation matrix in this step is:

${\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; abc</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; dq</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; 3</span>}}\left[\begin{array}{ccc}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>\\ \mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>\end{array}\right]$

In step 3.3, substitute the V _pd , V _nd and V _nq generated in step 3.2 into the following relational formula to generate the inductive and capacitive reference directions of the grid side voltage of each phase:

v _s,a,1 ＝V _pd cosωt+V _nd cosωt+V _nq sinωt

v _s,a,-1 =-V _pd cosωt-V _nd cosωt-V _nq sinωt

${\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nq</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}$

${\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nq</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}$

${\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nq</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}$

${\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; pd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}<span\; class="notranslate">\; -</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; nq</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&omega;t</span>}$

Step 3.4, the generation of v _s,b,-1 and v _s,c,1 generated in step 3.3 is the following relationship

-v _{s, a} =A(a) v _s,b,-1 +B(a) v _s,c,1 The coefficients of sinωt and cosωt on the left and right sides of the equation are respectively equal, and the coefficient A(a) is obtained _and B(a); similarly _, A(b) _, B(b ), calculate A(c) _{, B(c) according to -v s, c =} A(c) v _{s , a, -1} +B(c) v s, b, 1;

In step 3.5, the values v _{c, pa} and v _{c, na} , v _{c, pb} and v _{c, nb} , v _{c, pc} and v _{c, nc} of step 1.1 are sent to three single-way subtractors respectively, and the The output is sent to three single-channel PI regulators, and the output values of the three PI regulators are defined as: c _a , c _b and c _c .

In step 3.6, substitute the variables generated in steps 3.3, 3.4, and 3.5 into the following relational formula:

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; a</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}$

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}$

${\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; c</span>}}$

Obtain the AC circulation command of three-phase decoupling.

Step 4, balance control of the DC bus voltage among the various chopper units inside the bridge arm.

和A相上桥臂第一个模块的直流侧电压v_ap1送入单路减法器，单路减法器的输出送入比例调节器，比例调节器的输出值与上桥臂的电流i_p,a相乘，得到A相上桥臂第一个模块交流电压的微调量

同理，将A相下桥臂斩波模块的平均直流侧电压

和A相下桥臂第一个模块的直流侧电压v_an1送入单路减法器，单路减法器的输出送入比例调节器，比例调节器的输出值与下桥臂电流的反相值-i_n,a相乘，得到A相下桥臂第一个模块交流电压的微调量沿用同样的思路，得到B、C相上下桥臂第一个模块的交流电压微调量

Step 4.1, detect the actual current i _{p,a of the six bridge arms, i n,a} , i _p,b , i _n,b , i _p,c , in _,c , and switch the phase A upper bridge arm chopper module The average DC link voltage of

and the DC side voltage v _ap1 of the first module of the upper bridge arm of phase A are sent to the single-way subtractor, and the output of the single-way subtractor is sent to the proportional regulator, and the output value of the proportional regulator is related to the current i _{p of the upper bridge arm,} Multiply by _a to get the fine-tuning amount of the AC voltage of the first module of the upper bridge arm of phase A

Similarly, the average DC side voltage of the lower bridge arm chopper module of phase A

And the DC side voltage v _an1 of the first module of the lower bridge arm of phase A is sent to the single-way subtractor, and the output of the single-way subtractor is sent to the proportional regulator, and the output value of the proportional regulator is the inverse value of the lower bridge arm current -i _{n, multiplied by a} to get the fine-tuning value of the AC voltage of the first module of the lower bridge arm of phase A Following the same idea, get the AC voltage fine-tuning of the first module of the upper and lower bridge arms of the B and C phases

A相下桥臂第二到第N个模块微调电压指令

B相上桥臂第二到第N个模块微调电压指令B相下桥臂第二到第N个模块微调电压指令

C相上桥臂第二到第N个模块微调电压指令

C相下桥臂第二到第N个模块微调电压指令

Step 4.2, respectively calculate the fine-tuning voltage commands of the second to Nth modules of the upper bridge arm of phase A

Fine-tuning voltage command for the second to Nth modules of the lower bridge arm of phase A

Fine-tuning voltage command for the second to Nth modules of the upper bridge arm of phase B Fine-tuning voltage command for the second to Nth modules of the lower bridge arm of phase B

Fine-tuning voltage command for the second to Nth modules of the C-phase upper bridge arm

Fine-tune the voltage command of the second to Nth modules of the C-phase lower bridge arm

Step 5, the chopper module generates an AC voltage PWM modulation wave command.

模块化多电平变流器环流电流环控制系统把步骤2.2和3.5生成的直流环流指令和交流环流指令加和作为总的环流指令，并对总的环流指令电流和实际环流电流进行闭环跟踪控制，得到三相控制环流的指令电压PWM调制波

和

Step 5.1, the current loop control system of the output current of the modular multilevel converter performs closed-loop tracking control on the command current and the output current, and its output is subjected to dq inverse transformation to obtain a three-phase output command voltage PWM modulation wave and

The circulating current loop control system of the modular multilevel converter takes the sum of the DC circulating current command and the AC circulating current command generated in steps 2.2 and 3.5 as the total circulating current command, and performs closed-loop tracking control on the total circulating current command current and the actual circulating current , to obtain the command voltage PWM modulation wave of the three-phase control circulation

and

与步骤5.1生成的

和

按关系式

运算，得到A相上桥臂第一个模块的PWM调制波；把A相下桥臂第一个H桥模块PWM调制波的微调指令与步骤5.1生成的和

按关系式

运算，得到A相下桥臂第一个模块的PWM调制波。以此类推，得到A相上下桥臂中剩余模块的最终PWM调制波

和B相上下桥臂中剩余模块的最终PWM调制波

和

C相上下桥臂中剩余模块的最终PWM调制波

和

Step 5.2, fine-tune the PWM modulation wave of the first H-bridge module of the upper bridge arm of phase A

generated with step 5.1

and

By relation

Calculate the PWM modulation wave of the first module of the upper bridge arm of the A phase; fine-tune the PWM modulation wave of the first H bridge module of the lower bridge arm of the A phase generated with step 5.1 and

By relation

Calculate and obtain the PWM modulation wave of the first module of the lower bridge arm of phase A. By analogy, the final PWM modulation wave of the remaining modules in the upper and lower bridge arms of phase A is obtained

and The final PWM modulation wave of the remaining modules in the upper and lower bridge arms of phase B

and

The final PWM modulation wave of the remaining modules in the upper and lower bridge arms of phase C

and

In step 5.3, the modulation wave of each module of each phase is compared with the phase-shifted triangular carrier wave in sequence to generate switching signals of each module.

Step 1 is the first control loop, the purpose is to control the entire modular multilevel converter to absorb active power from the grid to offset the loss generated by the entire grid-connected inverter. Step 2 synthesizes the DC component command in the circulating current through calculation to adjust the balance of the DC bus voltage among the three phases. Step 3 realizes the DC bus voltage balance between the upper and lower bridge arms of each phase by calculating and synthesizing the AC component commands in the mutually decoupled circulating currents. Step 4. By fine-tuning the output voltage command of each module inside the bridge arm, redistribute the active power absorbed by each module, so that the actual active power absorbed by the module can just offset the loss of the module itself, so that each module is at the rated command voltage stable operation at the value. Step 5 synthesizes the final output voltage commands of each module for PWM modulation.

In order to verify the feasibility of the control method, the inventor built a small experimental prototype with a capacity of 5kVA and two chopper modules connected in series for each bridge arm in the laboratory. Fig. 4, 5, 6, 7, 8, 9, 10 have provided and adopted the experimental wave form of control method in the present invention, are PWM modulation effect respectively, total direct-current side voltage, balanced control between three phases, each phase interior two up and down DC bus voltage balance control between bridge arms, DC bus voltage balance control between modules inside each bridge arm, compensation effect when compensating three-phase symmetrical loads, and compensation effect when compensating three-phase nonlinear loads in the case of grid failure . It can be seen from the experimental waveform that this control method can well realize the balanced control of the DC bus voltage between the upper and lower bridge arms of each phase. Cooperating with the other three levels of voltage control loops, the DC side voltage of each module is well balanced and stabilized at a given value. The control strategy has a good performance under various working conditions and even under the fault state of the power grid.

Claims (6)

1. modular multilevel current transformer dc capacitor voltage control method based on the circulation decoupling zero; It is characterized in that; The total DC bus-bar voltage of active current control that utilizes the modular multilevel current transformer to absorb from electrical network; Make dc-voltage balance between the three-phase with the flip-flop in the circulation; With the balance of two brachium pontis dc buss about the every phase of first-harmonic Composition Control of the decoupling zero in the circulation, realize the equilibrium of dc capacitor voltage between inner each module of brachium pontis at last along the output voltage of each module of brachium pontis sense of current fine setting.

2. the modular multilevel current transformer dc capacitor voltage control method based on the circulation decoupling zero is characterized in that, may further comprise the steps:

Step 1, the DC bus-bar voltage control that three-phase is total

Step 1.1, all copped wave unit dc voltage v of many level current transformers of detection moduleization three-phase _Api, v _Ani, v _Bpi, v _Bni, v _Cpi, v _CniI=1 wherein, 2 ... N, N are natural number; Obtain A go up mutually the brachium pontis dc voltage with

A descends brachium pontis and B, C two total dc voltage of each brachium pontis mutually mutually $v_{c,\mathrm{Pb}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{Bpi}},$ $v_{c,\mathrm{Nb}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{Bni}},$ $v_{c,\mathrm{Pc}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{Cpi}},$ $v_{c,\mathrm{Nc}}=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{v}_{\mathrm{Cni}},$ Obtain each phase total dc voltage v of two brachium pontis up and down _{Ph, a}=v _{C, pa}+ v _{C, na}, v _{Ph, b}=v _{C, pb}+ v _{C, nb}, v _{Ph, c}=v _{C, pc}+ v _{C, nc}And three-phase average voltage

Obtain the average dc voltage of module of each brachium pontis at last $\stackrel{\&OverBar;}{V_{\mathrm{Cel},\mathrm{Pa}}}=v_{c,\mathrm{Pa}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{Cel},\mathrm{Na}}}=v_{c,\mathrm{Na}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{Cel},\mathrm{Pb}}}=v_{c,\mathrm{Pb}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{Cel},\mathrm{Nb}}}=v_{c,\mathrm{Nb}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{Cel},\mathrm{Pc}}}=v_{c,\mathrm{Pc}}/N,$ $\stackrel{\&OverBar;}{V_{\mathrm{Cel},\mathrm{Nc}}}=v_{c,\mathrm{Nc}}/N;$

Step 1.2; Three-phase dc busbar voltage mean value

and dc voltage set-point

are sent into the single channel subtracter carry out computing; Operation result is sent into the single channel proportional and integral controller and is adjusted, and its output valve is injected into as additional amount as the active current component

of modular multilevel current transformer and AC network exchange and controls the active power that whole modular multilevel current transformer absorbs from electrical network on the current inner loop d axle based on dq decoupling zero control.

3. a kind of modular multilevel current transformer dc capacitor voltage control method based on the circulation decoupling zero according to claim 2 is characterized in that said control method is further comprising the steps of:

Step 2, the instruction of DC component in the synthetic circulation

Step 2.1 is utilized step 1.1 detection limit

Respectively with v _{Ph, a}, v _{Ph, b}, v _{Ph, c}It is poor to do, and sends into three single channel pi regulators and generates respectively

Step 2.2, on go on foot

that computing generates instruction as DC component in the circulation.

4. a kind of modular multilevel current transformer dc capacitor voltage control method based on the circulation decoupling zero according to claim 3 is characterized in that said control method is further comprising the steps of:

Step 3, the instruction of alternating current component in the synthetic circulation

Step 3.1 detects three phase network voltage v _{S, a}, v _{S, b}, v _{S, c}

Step 3.2 is carried out the transform operation that the three phase static coordinate is tied to two cordic phase rotators system with three phase network voltage, the numerical value of d axle after the conversion is sent into the single channel low pass filter carry out filtering, and the output of filter is designated as V _PdThen with detected B, C two phase line voltage switches; Carry out the three phase static coordinate again and be tied to the transform operation that two cordic phase rotators are; D axle and q axis values are sent into two single channel low pass filters respectively and are carried out filtering after the computing, and the numerical value of d axle and q axle is designated as V respectively after the filtering _NdAnd V _NqTransformation matrix in this step is:

$T_{\mathrm{abc}-\mathrm{dq}}=\frac{2}{3}\left[\begin{array}{ccc}\sin \left(\mathrm{\&omega;t}\right)& \sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3)& \sin (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3)\\ \cos \left(\mathrm{\&omega;t}\right)& \cos (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3)& \cos (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3)\end{array}\right]$

Step 3.3 is with the V of step 3.2 generation _Pd, V _NdAnd V _NqFor people's following relationship formula, generate the perception and the capacitive reference direction of each phase voltage on line side:

v _s，a,1＝V _pd?cosωt+V _nd?cosωt+V _nq?sinωt

v _s，a,-1＝-V _pd?cosωt-V _nd?cosωt-V _nq?sinωt

$v_{s,b,1}=\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\&omega;t}-\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\&omega;t}-\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\&omega;t}-\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\&omega;t}$

$v_{s,b,-1}=-\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\&omega;t}+\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\&omega;t}+\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\&omega;t}+\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\&omega;t}$

$v_{s,c,1}=-\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\&omega;t}-\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\&omega;t}+\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\&omega;t}+\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\&omega;t}$

$v_{s,c,-1}=\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\&omega;t}+\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\&omega;t}-\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\&omega;t}-\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\&omega;t}$

Step 3.4 is with the v of step 3.3 generation _{S, b ,-1}And v _{S, c, 1}For people's following relationship formula-v _{S, a}=A (a) v _{S, b ,-1}+ B (a) v _{S, c, 1}Coefficient according to equality the right and left sin ω t and cos ω t equates respectively, asks for coefficient A (a) and B (a); Similarly, foundation-v _{S, b}=A (b) v _{S, c ,-1}+ B (b) v _{S, a, 1}Ask for A (b), B (b), foundation-v _{S, c}=A (c) v _{S, a ,-1}+ B (c) v _{S, b, 1}Ask for A (c), B (c);

Step 3.5 is with the numerical value v of step 1.1 _{C, pa}And v _{C, na}, v _{C, pb}And v _{C, nb}, v _{C, pc}And v _{C, nc}Send into three single channel subtracters respectively, three single channel pi regulators are sent in the output of subtracter again, and the output valve of three pi regulators is defined as respectively: c _a, c _bAnd c _c

Step 3.6, with step 3.3,3.4,3.5 variablees that generate are for people's following relationship formula:

$i_{\mathrm{ac},a}^{*}=c_a\&CenterDot;v_{s,a}+c_b\&CenterDot;B\left(b\right)\&CenterDot;v_{s,a,1}+c_c\&CenterDot;A\left(c\right)\&CenterDot;v_{s,a,-1}$

$i_{\mathrm{ac},b}^{*}=c_a\&CenterDot;A\left(a\right)\&CenterDot;v_{s,b,-1}+c_b\&CenterDot;v_{s,b}+c_c\&CenterDot;B\left(c\right)\&CenterDot;v_{s,b,1}$

$i_{\mathrm{ac},c}^{*}=c_a\&CenterDot;B\left(a\right)\&CenterDot;v_{s,c,1}+c_b\&CenterDot;A\left(b\right)\&CenterDot;v_{s,c,-1}+c_c\&CenterDot;v_{s,c}$

Obtain the interchange circulation instruction of three-phase decoupling zero.

5. a kind of modular multilevel current transformer dc capacitor voltage control method based on the circulation decoupling zero according to claim 4 is characterized in that said control method is further comprising the steps of:

Step 4, the equilibrium control of DC bus-bar voltage between inner each copped wave unit of brachium pontis

Step 4.1, the actual current i of six brachium pontis of detection _{P, a}, i _{N, a}, i _{P, b}, i _{N, b}, i _{P, c}, i _{N, c}, A is gone up mutually the average dc voltage of brachium pontis copped wave module

Go up the dc voltage v of first module of brachium pontis mutually with A _Ap1Send into the single channel subtracter, proportional controller is sent in the output of single channel subtracter, the output valve of proportional controller and the current i of last brachium pontis _{P, a}Multiply each other, obtain the amount trimmed that A goes up first module alternating voltage of brachium pontis mutually

A is descended mutually the average dc voltage of brachium pontis copped wave module

Descend the dc voltage v of first module of brachium pontis mutually with A _An1Send into the single channel subtracter, proportional controller is sent in the output of single channel subtracter, the inverse value-i of the output valve of proportional controller and following brachium pontis electric current _{N, a}Multiply each other, obtain the amount trimmed that A descends first module alternating voltage of brachium pontis mutually

Along using the same method, obtain the alternating voltage amount trimmed of B, first module of C phase upper and lower bridge arm

Step 4.2, obtain respectively A go up mutually brachium pontis second to N module trim voltage instruct A descend mutually brachium pontis second to N module trim voltage instruct

B go up mutually brachium pontis second to N module trim voltage instruct

B descend mutually brachium pontis second to N module trim voltage instruct

C go up mutually brachium pontis second to N module trim voltage instruct C mutually the brachium pontis of Xiaing second to N Ge module trim voltage Zhi Linged

6. a kind of modular multilevel current transformer dc capacitor voltage control method based on the circulation decoupling zero according to claim 5 is characterized in that said control method is further comprising the steps of:

Step 5, the instruction of copped wave module alternating voltage PWM modulating wave generates

Step 5.1; The current loop control system of modular multilevel current transformer output current carries out the closed loop tracking Control to instruction current and output current; Its output through the dc loop-current instruction that obtains three-phase output order voltage pwm modulating wave

and

modular multilevel current transformer loop current ring control system after the dq inverse transformation and generate step 2.2 and 3.5 with exchange circulation and instruct and add and instruct as total circulation; And total circulation instruction current and actual loop current carried out the closed loop tracking Control, obtain command voltage PWM modulating wave

and

of three-phase control circulation

Step 5.2; Go up A mutually the fine setting instruction of first H bridge module of brachium pontis PWM modulating wave and press relational expression

computing, obtain the PWM modulating wave that A goes up first module of brachium pontis mutually with

and

that step 5.1 generates; Descend A mutually the fine setting instruction

of first H bridge module of brachium pontis PWM modulating wave to press relational expression

computing, obtain the PWM modulating wave that A descends first module of brachium pontis mutually with

and that step 5.1 generates; By that analogy, the final PWM modulating wave that obtains in the A phase upper and lower bridge arm residue module and

B mutually in the upper and lower bridge arm the final PWM modulating wave

of residue module and

C remain final PWM modulating wave

and

of module mutually in the upper and lower bridge arm

Step 5.3 is compared with the triangular carrier of phase shift successively with the modulating wave of every each module of phase, generates the switching signal of each module.

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