CN102931863A - Method for setting up modularized multi-level converter composite structure model - Google Patents

Abstract

The invention discloses a method for setting up a modularized multi-level converter composite structure model in the field of power transmission and distribution technology. Firstly, sub-modules on each bridge arm of a half-bridge converter HBMMC are divided into groups, and the sub-modules in an upper group are changed into full-bridge sub-module FBSM structures, and the sub-modules in a lower group are changed into half-bridge sub-module HBSM structures. Secondly, the upper group and the lower group of every bridge arm are respectively subjected to sequencing gating, and the group in full-bridge sub-module HBSM structures is switched on through adoption of an alternative breakover control method. Finally, six bridge arm electric reactors on a modularized multi-level converter are respectively connected with a discharging access in parallel mode. The method has the advantages that the modularized multi-level converter composite structure model is strong in direct current fault ride-through capability, and simultaneously does not need so many semiconductors like a full-bridge modularized multi-level converter FBMMC, and achieves good uniformity in technology and cost.

Description

A Method for Establishing Hybrid Structure Model of Modular Multilevel Converter

technical field

The invention belongs to the technical field of power transmission and distribution, and in particular relates to a method for establishing a hybrid structure model of a modular multilevel converter. the

Background technique

Modular multilevel converters (MMCs) have been successfully used in high-power converters, mainly in the field of high voltage direct current (HVDC) transmission. Compared with the traditional two-level and three-level voltage source converter based HVDC (Voltage Source Converter based HVDC, VSC-HVDC), the modular multilevel converter based HVDC (MMC- HVDC) has many advantages: the AC side and the DC side can be fully controlled, the DC bus does not need to install capacitors, the power electronic equipment has the ability to operate redundantly after a fault, no need to install AC filters, etc. Due to the unique advantages of MMC, MMC-HVDC has become the development trend in the field of HVDC in the future. In 2010, the first commercial MMC-HVDC project "Trans Bay Cable Project (TBC)" was put into operation in the United States, with a maximum operating DC voltage of ±200kV and a maximum transmission capacity of 400MW. In addition, more than 4 MMC-HVDC projects around the world will be put into operation in 2013. The Shanghai Nanhui flexible DC project that has been completed and put into operation in China, as well as the Zhoushan multi-terminal flexible DC project and the Dalian flexible DC project that are about to start construction all use half-bridge MMC (Half-Bridge MMC, HBMMC) as the converter topology. , almost all MMC-HVDC projects use the Half-Bridge Sub-Module (HBSM) topology. the

When a bipolar DC short-circuit fault occurs in the flexible DC transmission system, the freewheeling effect of the anti-parallel diode of the insulated gate bipolar transistor (Insulated Gate Bipolar Transistor, IGBT) in the HBMMC-HVDC and the two- and three-level VSC-HVDC makes the AC The system has a three-phase short-circuit at the short-circuit point, and the inverter itself cannot cut off the short-circuit current, which seriously endangers the safe operation of the system. At the same time, due to the immature manufacturing process of high-voltage and large-capacity DC circuit breakers, the existing multi-terminal flexible DC projects must require DC cables to have extremely high reliability, that is, require extremely low DC fault occurrence probability, which limits to a certain extent The development and application of flexible direct current to multi-terminal transmission field. the

In order to solve this problem, Professor R. Marquardt, the founder of the HBMMC topology, introduced a full-bridge MMC (Full-BridgeMMC, FBMMC) structure in his thesis that can traverse severe DC faults. Each full-bridge sub-module (Full-BridgeMMC) Bridge Sub-module, FBSM) can output three levels, which can interrupt the bridge arm current in any direction, and can change the polarity of the DC bus voltage of the MMC, but it requires double the number of semiconductor devices. Since FBMMC requires twice as many semiconductor devices as HBMMC, this severely restricts the engineering application of FBMMC, which is why the current MMC-HVDC project adopts a half-bridge structure.

Contents of the invention

The technical problem to be solved by the present invention is that MMC can realize better DC fault ride-through capability with as few semiconductor devices as possible, and an improved hybrid structure model of modular multilevel converter is proposed. the

A method for establishing a hybrid structure model of a modular multilevel converter, characterized in that the method specifically includes the following steps:

Step 1: Group the sub-modules on each bridge arm of the half-bridge converter HBMMC into two groups of the same number, the upper and lower groups, and assign the same number of sub-modules to each group, and change all the sub-modules in the upper group to Full-bridge sub-module FBSM/half-bridge sub-module HBSM structure, the sub-modules in the lower group are all half-bridge sub-module HBSM/full-bridge sub-module FBSM structure;

Step 2: The upper and lower groups of each bridge arm are respectively sorted and strobed, and the group using the full bridge sub-module FBSM structure is turned on by means of a rotation conduction control method;

Step 3: Connect a discharge path in parallel on each bridge arm reactor of the modular multilevel converter MMC structure, and when the converter MMC is blocked, the path is turned on and discharged. the

The discharge path includes bidirectionally conducting parallel bidirectional thyristors and series resistors. the

The beneficial effect of the present invention is that the hybrid structure model of the proposed modular multilevel converter has strong DC fault ride-through capability, and at the same time does not require as many semiconductors as the full bridge modular multilevel converter FBMMC Devices, which have achieved better unification in technology and cost. the

Description of drawings

Fig. 1 is the topological structure diagram of general modular multilevel converter MMC provided by the present invention;

Fig. 2 is the submodule topology structure of the half-bridge structure MMC provided by the present invention and the current path during its fault locking; Wherein, (a) is the submodule topology structure of the half-bridge structure MMC; (b) is the structure of the half-bridge structure MMC The current path when the sub-module is faulty and blocked;

Fig. 3 is the submodule topology structure of the full bridge structure MMC provided by the present invention and the current path when its fault is blocked; Wherein, (a) is the submodule topology structure of the full bridge structure MMC; (b) is the submodule topology structure of the full bridge structure MMC The current path when the sub-module is faulty and blocked;

Fig. 4 is an explanatory diagram of the MMC structure model of the hybrid modular multilevel converter provided by the present invention. the

Detailed ways

The preferred embodiments will be described in detail below in conjunction with the accompanying drawings. It should be emphasized that the following description is only exemplary and not intended to limit the scope of the invention and its application. the

Fig. 1 is a topological structure diagram of a general modular multilevel converter MMC provided by the present invention. In Fig. 1, A, B and C represent the three phases of the AC side of the MMC converter respectively; SM1, SM2, ..., SMn, represent the first sub-module, the second sub-module, ..., the nth sub-module in a certain bridge arm of the MMC module; L represents the bridge arm reactor; Udc represents the voltage difference between the positive and negative DC bus bars of the MMC. the

Fig. 2 is the sub-module topology of the half-bridge structure MMC provided by the present invention and its current path when it is fault-locked. In Figure 2, T ₁ and T ₂ represent the upper and lower IGBTs in the half-bridge sub-module; D ₁ and D ₂ represent the anti-parallel diodes of the corresponding IGBTs; C0 represents the capacitor in the half-bridge sub-module; Uc Indicates the capacitor voltage of the sub-module; u _sm indicates the output voltage of the sub-module port.

Fig. 3 is the sub-module topology of the full-bridge structure MMC provided by the present invention and its current path during fault latching. In Figure 3, T ₁ , T ₂ , T ₃ and T ₄ represent the four insulated gate bipolar transistors IGBTs in the full bridge sub-module respectively, and D ₁ , D ₂ , D ₃ , D ₄ represent the corresponding insulated gate bipolar transistors respectively IGBT anti-parallel diode; C ₀ represents the capacitor in the full bridge sub-module; Uc represents the capacitor voltage of the sub-module; u _sm represents the output voltage of the sub-module port.

Fig. 4 is an explanatory diagram of the MMC structure model of the hybrid modular multilevel converter provided by the present invention. In Figure 4, HBSM represents the half-bridge sub-module, FBSM represents the full-bridge sub-module, and U _dc represents the voltage difference between the positive and negative DC bus bars of the MMC.

The specific steps to establish a hybrid structure model are:

Step 1: Changes in the structure of the MMC bridge arm sub-module of the hybrid modular multilevel converter:

As shown in Figure 1, the general topology of the modular multilevel converter MMC is composed of three-phase and six bridge arms, and its sub-module structure is generally in the form of a half-bridge (as shown in Figure 2), which is the HBMMC model. Its DC fault ride-through capability is poor. Figure 3 is the full-bridge sub-module topology (FBSM) and its current path diagram during fault blocking. Obviously, it has a relatively good DC fault ride-through capability and can effectively suppress the DC fault current, but it requires double Semiconductor devices have high engineering costs. In order to achieve better DC fault ride-through capability with as few semiconductor devices as possible, the sub-modules in the lower bridge arm (or upper bridge arm) of the original HBMMC converter are replaced, and the original HBMMC converter The sub-modules on each bridge arm of the bridge arm are grouped and divided into two groups of the same number of upper and lower groups, and each group is allocated an equal number of sub-modules, but the sub-modules of the upper group (or lower group) are all changed to FBSM structure, and the lower group ( or the above group) sub-modules are all HBSM structures, as shown in Figure 4, so that the unification of technology and cost can be achieved. the

Step 2: The upper and lower groups of each bridge arm are respectively sorted and selected, and the group with the FBSM structure of the full bridge sub-module is added with a rotation conduction control strategy:

In step 1, the bridge arm sub-modules of the half-bridge modular multilevel converter HBMMC are grouped, and the hybrid modular multilevel converter MMC is constructed, and the corresponding controllers need to perform corresponding Change and add the control strategy of group sorting, so that its sub-modules can be effectively controlled. At the same time, since each bridge arm has a group of sub-modules that adopt the full-bridge sub-module FBSM structure, as shown in Figures 3 and 4, the full-bridge sub-module FBSM has 4 IGBTs, and the triggering of the pulses must be taken Alternate conduction strategy to control. the

Since FBSM has four IGBTs, 1) T ₁ and T ₄ are turned on, and the output voltage is the capacitor voltage Uc; 2) T ₁ and T ₂ are turned on or T ₃ and T ₄ are turned on, and the output voltage is 0; 3) T ₂ and T ₃ open, the output voltage is -Uc

Since T ₁ and T ₂ are turned on or T ₃ and T ₄ are turned on, the output voltage is 0, so in order to balance the switch, we need to switch on when outputting 0 potential, and turn on T ₁ and T when outputting 0 this time. ₂ , turn on T ₃ and T ₄ when the next output is 0.

Step 3: Bridge arm reactor parallel discharge path:

The hybrid structure model has the ability to ride through DC faults. After the DC fault occurs, the sub-modules of the hybrid MMC discharge rapidly before the converter is blocked, and the DC voltage decreases accordingly; after the blockage, the sub-modules stop discharging and still retain some power. When the energy stored in the arm reactor is released, the capacitor of the sub-module will be charged. In order to prevent the capacitor voltage from overcharging and make the capacitor voltage exceed the voltage value that the capacitor can withstand, it is necessary to connect the discharge path in parallel on the six arm reactors, such as Figure 4 shows. That is, on each bridge arm reactor, a discharge path of a parallel bidirectional thyristor and a series resistor is connected in parallel, and when blocked, the path is turned on and discharged to avoid overcharging of the capacitor. the

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed in the present invention can easily think of changes or Replacement should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims. the

Claims (2)

1. A method for setting up a hybrid structure model of a modular multilevel converter, characterized in that, the method specifically comprises the following steps: Step 1: Group the sub-modules on each bridge arm of the half-bridge converter HBMMC into two groups of the same number, the upper and lower groups, and assign the same number of sub-modules to each group, and change all the sub-modules in the upper group to Full-bridge sub-module FBSM structure/half-bridge sub-module HBSM, the sub-modules in the lower group are all half-bridge sub-module HBSM/full-bridge sub-module FBSM structure; Step 2: The upper and lower groups of each bridge arm are respectively sorted and selected, and the group using the full-bridge sub-module FBSM structure is conducted by means of a rotation conduction control method; Step 3: Connect a discharge path in parallel on each bridge arm reactor of the modular multilevel converter MMC structure, and make the path conductive and discharge when the converter MMC is locked. 2 . A method for establishing a hybrid structure model of a modular multilevel converter according to claim 1 , wherein the discharge path includes a bidirectional parallel bidirectional thyristor and a series resistor. 3 .

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