CN103066805B - Capacitor voltage balance control method based on bundling type switching of multiple sub modules - Google Patents

Abstract

The present invention provides a capacitor voltage balance control method based on bundling switching of multiple sub-modules. By bundling, the number of sub-modules of a modular multi-level converter valve can be reduced by one level, which is beneficial to the calculation amount of capacitor voltage balance control. The reduction reduces the requirements for control equipment. After the sub-modules are bundled into a module group, they are switched on and off as a whole, and the voltage balance of the sub-module group is realized by using the capacitor voltage balance algorithm. The self-adaptive voltage equalization inside the sub-module group can realize the overall voltage equalization of the sub-modules. The capacitive voltage balance control method based on the bundled switching of multiple sub-modules in the present invention can effectively solve the problem of large-capacity flexible direct current transmission converter valve control based on the modular multi-level converter mode, and can realize a large-scale number of converters Flow valve operation control and monitoring.

Description

A capacitor voltage balance control method based on bundled switching of multiple sub-modules

technical field

The invention belongs to the technical field of flexible alternating current transmission, and in particular relates to a capacitive voltage balance control method based on bundled switching of multiple sub-modules.

Background technique

At present, various power electronic circuits based on the fully-controlled power electronic device IGBT have been increasingly used in power systems, locomotive traction, aerospace and other fields. With the development of power electronics technology, materials, and manufacturing processes, the current flow capacity of IGBT devices has become stronger and stronger, making it an important space to play in the field of DC power transmission, which directly promotes the birth and development of flexible DC power transmission technology. Different from the traditional high-voltage direct current transmission technology, the flexible direct current transmission converter replaces the thyristor series converter valve with a high-voltage converter valve composed of IGBT series, forming a voltage source type flexible direct current converter. Flexible DC transmission can realize power supply to long-distance small and medium-sized isolated and weak loads; it can carry out independent, accurate and flexible active/reactive power control, and improve the economy and stability of power flow transmission in the system; The polarity of the voltage remains unchanged, which is convenient to form a multi-terminal DC transmission system; when the connected system is short-circuited, the short-circuit capacity of the system is not increased, which is beneficial to limit the short-circuit current and prevent the spread of system faults; it can provide reactive power support and frequency control, and is used for wind power Grid connection of renewable energy such as field and distributed power generation has special advantages; it can provide black start power after the failure of the connected grid, and speed up the rapid recovery ability after the grid failure; the area occupied by the converter station is greatly reduced compared with ordinary DC Small.

The rich performance advantages of flexible DC transmission technology have attracted many scientific researchers and technicians to invest in related research and practical work. Its flexible control performance also makes the control and protection methods and control protection devices of flexible DC a research hotspot of flexible DC technology. In the flexible DC control based on the modular multilevel converter topology, the internal control and protection of the converter sub-module is a very important link in the entire control and protection system.

Contents of the invention

In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a capacitive voltage balance control method based on multiple sub-module bundled switching, which can effectively solve the problem of large-capacity flexible DC transmission based on modular multi-level converters. The problem of valve control can realize the operation control and monitoring of a large number of converter valves; through the bundling method, the number of sub-modules of modular multi-level converter valves can be reduced by one level, which is conducive to the reduction of the calculation amount of capacitor voltage balance control. Reduced, reducing the requirements for control equipment.

In order to realize the above-mentioned purpose of the invention, the present invention takes the following technical solutions:

A capacitor voltage balance control method based on bundled switching of multiple sub-modules is provided, the method includes the following steps:

Step 1: Upload and summarize the submodule status;

Step 2: Group and bundle the M sub-modules included in the bridge arm to obtain m module groups, and sort the voltages of the m module groups, and the module groups are input or cut out as a whole;

Step 3: Determine the total number of sub-modules that need to be invested at present, and then obtain the number m' of module groups that need to be invested at this time;

Step 4: Select the number of switching module groups and the specific module group according to the current direction of the bridge arm and the sorting results of the module capacitor voltage.

In the step 1, the bridge arm segmentation control unit uploads the submodule status to the bridge arm summary control unit, and the bridge arm summary control unit summarizes the received submodule status.

In the step 2, the bridge arm aggregation control unit bundles the M sub-modules into groups of n to obtain module groups, where m=M/n.

The bridge arm summary control unit sorts the voltages of the m module groups in descending order.

In the step 3, the minimum approximation method is used to determine the total number of sub-modules that need to be invested according to the instructions issued by the upper-level DC control and protection system, and then the number m' of module groups that need to be invested at this time is obtained.

Described step 4 comprises the following steps:

Step 3-1: Take the positive busbar flowing to the negative busbar as the positive direction, if the current direction is positive at this time, select the voltage and the smallest m ₁ ′ sub-module group to put in, and the remaining mm ₁ ′ sub-modules are cut out;

Step 3-2: Take the negative direction of the negative bus to the positive bus as the negative direction. If the current direction is negative at this time, select the voltage and the largest m ₂ ′ sub-module groups to put in, and the remaining mm ₂ ′ sub-module groups are cut out.

The bridge arm segmentation control unit, the bridge arm summary control unit, the optical CT merging and interface unit, the circulation control unit and the MMC valve monitoring unit form the valve base control equipment of the modular multilevel converter control and protection system; The bridge arm segmentation control unit uploads the status of the submodules to the bridge arm aggregation control unit for summarization; the circulation control unit sends modulation amount information, and through the modulation amount information, summarizes and calculates to determine the switching of the submodules; The MMC valve monitoring unit monitors the status of the MMC valve and uploads the monitoring information to the host computer.

Compared with prior art, the beneficial effect of the present invention is:

1. It can effectively solve the problem of large-capacity flexible DC transmission converter valve control based on the modular multi-level converter method, and can realize the operation control and monitoring of a large number of converter valves;

2. This patent reduces the number of sub-modules of the modular multi-level converter valve by one level through bundling, which is conducive to reducing the calculation amount of capacitor voltage balance control and reducing the requirements for control equipment;

3. After the sub-modules are bundled into a module group, they are switched on and off as a whole, and the voltage balance of the sub-module group is realized by using the capacitor voltage balance algorithm. The self-adaptive voltage equalization inside the sub-module group can realize the overall voltage equalization of the sub-modules.

Description of drawings

Figure 1 is a topology diagram of capacitor voltage balance control based on bundled switching of multiple sub-modules.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

Modular multilevel converter (Module Multilevel Convertor, MMC converter for short) is composed of multiple sub-modules in series, and it is necessary to perform different real-time switching controls on individual sub-modules according to the capacitor voltage and the working status of the sub-modules. The more the number is, the shorter the real-time control period is, and the more difficult it is to realize the capacitor voltage balance control. The high-voltage and large-capacity flexible DC transmission converter valve based on the modular multi-level converter MMC technology requires hundreds or even thousands of sub-modules to be connected in series. Each sub-module must be controlled independently, and the valve-based control equipment VBC controls each sub-module separately by adopting a layered and segmented processing method. As the intermediate unit of the valve-base control device—the bridge arm summary unit, it collects the sub-module information of the entire bridge arm of multiple bridge arm segment units. Summarize, collate and summarize the bridge arm voltage balance strategy processing and the intermediate unit for the implementation of the bridge arm overall protection strategy; as the bottom control unit of the valve base control device—the bridge arm segment unit, it is to summarize the submodule information of each section of the bridge arm The underlying control unit that organizes and realizes the bridge arm segment voltage balance strategy processing and bridge arm segment protection strategy implementation.

As shown in Fig. 1, the modular multilevel converter is composed of 3 phases and 6 bridge arms. The balance control involved in the present invention is based on the capacitor voltage balancing strategy of the valves of the modular multilevel converter. The capacitor voltage balance strategy is jointly completed by the bridge arm segment control unit and the bridge arm summary control unit controller, in which the bridge arm segment control unit is responsible for completing the communication with the converter valve and collecting the capacitor voltage and operating status of the sub-modules. Send sub-module input and exit and other protection controls to the sub-modules. Each bridge arm segment controller is responsible for monitoring the status of the sub-modules. The bridge arm summary control unit is responsible for summarizing the status uploaded by each bridge arm and completing the overall sub-module of the bridge arm. Balance control of capacitor voltage.

The bridge arm summary control unit is responsible for summarizing the status of the sub-modules uploaded by the bridge arm segment control unit, and grouping the sub-modules. A bridge arm includes 540 modules, and every 9 modules are bundled into a module group. The bridge arm has a total of 540 modules. It will include 60 module groups, and the bridge arm summary control unit will switch the 60 module groups as a whole. The entire bridge arm summary control unit only needs to sort the 60 module groups and perform capacitor voltage balance on the 60 module group voltages. Due to the self-adaptive voltage balance of the sub-module voltages in the module group, the module voltages will remain consistent. According to the above description, the number of each module group is N, and the voltage of the module group is the voltage sum of n modules in the group. The bridge arm segments add the fixed voltages in each group, and then upload the voltage sum to the bridge arm Summary unit.

The arm summary control unit calculates the number m' of module groups that need to be put into operation according to the voltage requirements. Then sort the sum of the voltages of the 60 module groups, and judge the direction of the bridge arm current. Then, according to the current direction and the voltage and order of the module group, select the module group to be put into operation. If the current direction is positive at this time (positive from the positive bus to the negative bus), select the voltage and the smallest m ₁ ′ sub-module group to put in, and cut out the remaining 60-m ₁ ′ sub-modules. If the current direction is negative at this time (the flow from the negative bus to the positive bus is negative), select the voltage and the largest m ₂ ′ sub-module groups to be put in, and the remaining 60-m ₂ ′ sub-module groups to be cut out.

The sub-modules in the module group are switched on or off at the same time, maintaining the same switching status. Each sub-module capacitor is connected in parallel with a voltage-balancing resistor, and the sub-module capacitor is discharged through the voltage-balancing resistor. When there is voltage unevenness between the sub-modules in the module group, the sub-module with higher capacitor voltage will have a larger current through the grading resistor and discharge faster; the sub-module with lower capacitor voltage will have a smaller current through the grading resistor and discharge. slower. Therefore, when the voltage of the sub-modules in the module group is not uniform due to the difference of the sub-modules, the voltage equalizing resistor can play a role in adjusting the voltage balance in the module group.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the present invention can still be Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims (5)

1. A capacitor voltage balance control method based on a plurality of submodule bundled switching, characterized in that: the method comprises the following steps: Step 1: Upload and summarize the status of multiple sub-modules in the modular multilevel converter; Step 2: Group and bundle the M sub-modules included in the bridge arm to obtain m module groups, and sort the voltages of the m module groups, and the module groups are input or cut out as a whole; Step 3: Determine the total number of sub-modules that need to be invested at present, and then obtain the number m' of module groups that need to be invested at this time; Step 4: Select the number of switching module groups and specific module groups according to the current direction of the bridge arm and the sorting results of the module capacitor voltage; In said step 3, the minimum approximation method is used to determine the total number of sub-modules that need to be invested according to the instructions issued by the upper-level DC control and protection system, and then the number m' of the module groups that need to be invested at this time is obtained; Described step 4 comprises the following steps: Step 4-1: Take the positive busbar flowing to the negative busbar as the positive direction, if the current direction is positive at this time, select the voltage and the smallest m ₁ ′ sub-module group to put in, and the remaining m-m ₁ ′ sub-modules are cut out; Step 4-2: Take the negative busbar flowing to the positive busbar as the negative direction. If the current direction is negative at this time, select the voltage and the largest m ₂ ′ sub-module groups to put in, and the remaining m-m ₂ ′ sub-module groups are cut out. 2. The capacitive voltage balance control method based on multiple sub-module bundled switching according to claim 1, characterized in that: in the step 1, the bridge arm segmentation control unit uploads the status of the sub-modules to the bridge arm summary control unit, the bridge arm summary control unit summarizes the received sub-module states. 3. The capacitive voltage balance control method based on multiple sub-module bundled switching according to claim 1, characterized in that: in the step 2, the bridge arm summary control unit groups the M sub-modules according to n groups Bind to get module groups, where m=M/n. 4. The capacitive voltage balance control method based on a plurality of sub-module bundled switching according to claim 3, characterized in that: the bridge arm summary control unit conducts the voltage of the m module groups according to the principle from large to small Sort. 5. The capacitive voltage balance control method based on multiple sub-module bundled switching according to claim 2, characterized in that: the bridge arm segmentation control unit, the bridge arm summary control unit, the optical CT combination and interface unit, The circulation control unit and the MMC valve monitoring unit constitute the valve base control equipment of the modular multilevel converter control and protection system; the bridge arm segmentation control unit uploads the status of the sub-modules to the bridge arm summary control unit for summary; The circulation control unit sends the modulation quantity information, through which the modulation quantity information is summarized and calculated to determine the switching of the sub-modules; the MMC valve monitoring unit monitors the status of the MMC valve and uploads the monitoring information to the host computer.