CN103269172A - A Control Method for Bridge Arm Asymmetry of Modular Multilevel Converter - Google Patents

Abstract

The invention discloses a modular multi-level inverter bridge arm asymmetric control method which achieves good control over three-phase circulation flow asymmetry and direct current fluctuation caused by asymmetry of modular multi-level inverter bridge arms. The method includes a first step of determining the number of bridge arm rated submodules, rated capacitance voltage instruction values and rated average energy according to system capacity and voltage classes, a second step of detecting the number of the submodules, normally working actually, of each bridge arm and calculating an actual instruction value of capacitance voltage of each bridge arm according to the relation that the average energy of the bridge arms is equal to the rated average energy, and a third step of enabling the instruction value of the voltage of each bridge arm to divide the actual instruction value of the capacitance voltage of each bridge arm, and obtaining the number of the submodules, needing to be inputted actually, of each bridge arm. The modular multi-level inverter bridge arm asymmetric control method effectively restrains the three-phase circulation flow asymmetry and direct current fluctuation caused by asymmetry of the bridge arms.

Description

A Control Method for Bridge Arm Asymmetry of Modular Multilevel Converter

technical field

The invention relates to the fields of power electronics and direct current transmission, in particular to a control method in the case of asymmetric bridge arms of a modular multilevel converter.

technical background

With the development of renewable energy such as wind power and solar energy, the research on the access of these distributed energy sources to the power grid has become more and more in-depth. These power generation methods have the characteristics of strong randomness and being far away from the main power grid. The flexible DC transmission system has the advantages of the ability of fast decoupling control of active and reactive power, and the ability to limit the short-circuit current of the AC system. It is a good way for renewable energy to be connected to the grid.

Modular multilevel converter is a novel converter suitable for flexible DC transmission that has appeared in recent years. The modular multilevel converter is composed of 6 bridge arms, and each bridge arm is composed of a certain number of sub-modules and bridge arm reactors in series. By controlling the number of input and removal sub-modules of each bridge arm, the AC The output voltage on the DC side is close to the AC sinusoidal voltage, and the output voltage on the DC side is close to the DC voltage, so as to realize the stable operation of the system.

When the modular multilevel converter is in normal operation, the number of sub-modules of the six bridge arms is equal, which is called the symmetrical state of the bridge arm, and the number of the sub-modules of the six bridge arms is not completely equal, which is called the asymmetrical state of the bridge arm. The asymmetric state of the bridge arm is generally due to the failure of some sub-modules of a certain (some) bridge arm and it is bypassed, resulting in fewer sub-modules than other bridge arms. The asymmetry of the bridge arm will lead to the asymmetry of the three-phase circulating current and the fluctuation of the DC current in the modular multilevel converter.

Contents of the invention

The purpose of the present invention is to address the deficiencies in the prior art, and propose a method for controlling the bridge arm asymmetry of a modular multilevel converter.

A method for controlling the asymmetry of bridge arms of a modular multilevel converter, wherein the modular multilevel converter is composed of 6 bridge arms, and each bridge arm is composed of sub-modules and bridge arm reactors in series, By controlling the number of input and removal sub-modules of each bridge arm, the output voltage on the AC side is close to the AC sinusoidal voltage, and the output voltage on the DC side is close to the DC voltage; when the number of sub-modules of the six bridge arms is not exactly equal, the control Proceed as follows:

1) Assume that the capacitance value of any bridge arm sub-module in the 6 bridge arms is C, the number of bridge arm rated sub-modules is N _rated , the bridge arm sub-module capacitor voltage rated command value U _Crated , and the bridge arm rated average energy is W _rated : $W_{\mathrm{rated}}={0.5\mathrm{CN}}_{\mathrm{rated}}{u}_{\mathrm{Crated}}^2;$

2) Assume that the actual number of sub-modules of any one of the six bridge arms is N, the actual command value of the capacitor voltage U _C , and the actual average energy of the bridge arm is W:

3) According to the fact that the actual average energy of the bridge arm is equal to the rated average energy, the actual command value U _C of the capacitor voltage can be calculated as follows: $u_C=\sqrt{\frac{N_{\mathrm{rated}}}{N}}{u}_{\mathrm{Crated}};$

4) Assuming that the voltage command value of the bridge arm is U _arm , the actual number of sub-modules N _arm to be put into the bridge arm is: N _arm = U _arm U _C .

4) The bridge arm voltage command value U _arm is the bridge arm voltage command value generated by the DSP or NI Compactrio controller; 4) The N _arm mentioned in the above is the actual input of the bridge arm generated by the DSP or NI Compactrio controller The number of submodules.

Beneficial effects of the present invention:

Modular multi-level converters contain a large number of sub-modules, and the probability of sub-module failure is high, so modular multi-level converters are prone to bridge arm asymmetry (that is, the sub-modules of one or several bridge arms The number of modules is not equal to other bridge arms). When the bridge arm is asymmetrical, the DC current of the modular multilevel converter will generate a fluctuating component, resulting in an increase in DC side loss and harmonics. The patent of the invention can effectively suppress the modular multilevel converter bridge The DC current fluctuations caused by arm asymmetry reduce losses and harmonics.

Description of drawings

Fig. 1 is a schematic structural diagram of a modular multilevel converter.

FIG. 2 is a schematic structural diagram of a bridge arm sub-module.

Fig. 3 is a control block diagram of a control method for bridge arm asymmetry of a modular multilevel converter.

Fig. 4 is a schematic diagram of the effect of the control method for the bridge arm asymmetry of the modular multilevel converter.

Detailed ways

The purpose and effects of the present invention will become more apparent by describing the present invention in detail below with reference to the accompanying drawings.

A method for controlling the asymmetry of bridge arms of a modular multilevel converter, wherein the modular multilevel converter is composed of 6 bridge arms, and each bridge arm is composed of sub-modules and bridge arm reactors in series, As shown in Figure 1. Figure 2 shows the structure of the sub-module of the bridge arm. The sub-module is composed of two IGBTs with anti-parallel diodes connected in series, and then connected in parallel with a DC capacitor. The connection point of the two IGBTs and the cathode of the DC capacitor are the output terminals of the sub-module. By controlling the number of input and removal sub-modules of each bridge arm, the output voltage on the AC side is close to the AC sinusoidal voltage, and the output voltage on the DC side is close to the DC voltage; when the number of sub-modules of the six bridge arms is not exactly equal, the control The steps are as follows (as shown in Figure 3):

其中，1）中所述的电容容值C为图2所示的子模块电容的容值；1）中所述的额定子模块数N_rated为图1所示每个桥臂额定子模块的个数；1）中所述的额定电容电压U_Crated为图2所示的子模块额定电容电压；1）中所述的额定平均能量W_rated为图1所示桥臂额定子模块的平均能量。1) Assume that the capacitance value of any bridge arm sub-module in the 6 bridge arms is C, the rated number of bridge arm sub-modules is N _rated , the rated command value of the bridge arm sub-module capacitor voltage is U _Crated , and the rated average energy of the bridge arm is W _rated :

Among them, the capacitor value C mentioned in 1) is the capacitance value of the sub-module capacitor shown in Figure 2; the rated number of sub-modules N _{rated in 1) is the rated} sub-module number of each bridge arm shown in Figure 1 Number; 1) The rated capacitor voltage U _Crated is the rated capacitor voltage of the sub-module shown in Figure 2; 1) The rated average energy W _rated is the average energy of the rated sub-module of the bridge arm shown in Figure 1 .

2）中所述的桥臂实际子模块数N为图1所示每个桥臂实际子模块的个数；2）中所述的电容电压实际指令值U_C为图2所示的子模块的实际电容电压指令值；2）中所述的桥臂实际平均能量W为图1所示每个桥臂子模块实际平均能量。2) Assume that the actual number of sub-modules of any one of the six bridge arms is N, the actual command value of the capacitor voltage U _C , and the actual average energy of the bridge arm is W:

2) The actual number of sub-modules of the bridge arm N is the number of actual sub-modules of each bridge arm shown in Figure 1; the actual command value U _C of the capacitor voltage described in 2) is the sub-module shown in Figure 2 2) The actual average energy W of the bridge arm is the actual average energy of each bridge arm sub-module shown in Figure 1.

3) According to the fact that the actual average energy of the bridge arm is equal to the rated average energy, the actual command value U _C of the capacitor voltage can be calculated as follows: $u_C=\sqrt{\frac{N_{\mathrm{rated}}}{N}}{u}_{\mathrm{Crated}};$

4) Assuming that the voltage command value of the bridge arm is U _arm , the actual number of sub-modules N _arm to be put into the bridge arm is: N _arm = U _arm U _C .

4) The bridge arm voltage command value U _arm is the bridge arm voltage command value generated by the DSP or NI Compactrio controller; 4) The N _arm mentioned in the above is the actual input of the bridge arm generated by the DSP or NI Compactrio controller The number of submodules.

Example

The modular multilevel converter works as an inverter, the DC side voltage is ±30kV, the effective value of the AC side line voltage is 35kV, the number of bridge arm rated sub-modules N _rated is 70, and the bridge arm sub-module rated capacitor voltage command value U _Crated is 1kV, sub-module capacitance C is 8000uF, when the bridge arms of the modular multi-level converter are symmetrical, the actual number of sub-modules N of the bridge arm is 70. From 0s to 0.3s, the bridge of the modular multi-level converter The arms are symmetrical; after 0.3s, the actual number of sub-modules of the upper bridge arm of phase A becomes 62, and the number of sub-modules of the remaining 5 bridge arms is still 70. During the period from 0.3s to 0.5s, the method of the present invention is not used, and after 0.5s using the method of the invention. The resulting rendering is shown in Figure 4. Figure 4 shows the DC current in the embodiment. Before 0.3s, the bridge arms of the modular multilevel converter are symmetrical, and the DC current is a constant DC amount; during 0.3s to 0.5s, the bridge arms of the modular multilevel converter Asymmetry, but the control method of the present invention is not used, and the conventional control method is still used, and the DC current fluctuates; after 0.5s, the DC current fluctuation is well suppressed by using the control method proposed by the present invention.

The above description is only a specific embodiment of the present invention, and does not constitute any limitation to the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (2)

1. asymmetric control method of modularization multi-level converter brachium pontis, it is characterized in that, described modularization multi-level converter is made up of 6 brachium pontis, each brachium pontis is composed in series by submodule and brachium pontis reactor, by controlling the number that each brachium pontis drops into, excises submodule, make the output voltage that exchanges side approach AC sinusoidal voltage, the output voltage of DC side approaches direct voltage; When 6 brachium pontis submodule numbers not exclusively equated, the control step was as follows:

1) arbitrary brachium pontis submodule capacitor's capacity of establishing in 6 brachium pontis is C, and the specified submodule number of brachium pontis is N _Rated, the specified command value of brachium pontis submodule capacitance voltage is U _Crated, the specified average energy of brachium pontis is W _Rated: $W_{\mathrm{rated}}={0.5\mathrm{CN}}_{\mathrm{rated}}{U}_{\mathrm{Crated}}^2;$

2) the actual submodule of establishing in 6 brachium pontis of arbitrary brachium pontis is counted N, capacitance voltage actual instruction value U _C, brachium pontis actual average energy is W:

3) equal specified average energy according to brachium pontis actual average energy, capacitance voltage actual instruction value U _CCalculating can get: $U_C=\sqrt{\frac{N_{\mathrm{rated}}}{N}}{U}_{\mathrm{Crated}};$

4) establishing the bridge arm voltage command value is U _Arm, then the brachium pontis actual submodule that need drop into is counted N _Arm: N _Arm=U _ArmU _C

2. the asymmetric control method of modularization multi-level converter brachium pontis according to claim 1 is characterized in that 4) described in bridge arm voltage command value U _ArmBridge arm voltage command value for DSP or the generation of NI Compactrio controller; 4) N described in _ArmThe submodule number that the brachium pontis actual needs that produces for DSP or NI Compactrio controller drops into.

Images