CN105429495A - A Modular Multilevel Converter Using Polymorphic Submodules - Google Patents

Abstract

The invention discloses a modular multilevel converter using a multi-state submodule. A basic structure of a submodule is characterized in that based on a half-bridge module topology formed by two insulated gate bipolar transistors S1 and S2 with anti-parallel diodes and a capacitor C, a parallel side positive port P3 and a parallel side negative port P4 are parallelly led out from a positive polarity end of the capacitor C; a bridge side possesses a bridge side positive port P1 and a bridge side negative port P2; two metal-oxide semiconductor field effect switch tubes S3 and S4 with the anti-paralleled diodes are reversely connected in series between the positive polarity end of the capacitor and the parallel side positive port P3 to form a bidirectional switch. The multi-state submodule topology can be operated under three kinds of modes which are investment, removing and parallel connection and real-time series-parallel switching of the adjacent modules is realized so that purposes of reaching module capacitance voltage sharing and increasing a bridge-arm equivalent capacitance are achieved. Through whole bridge arm module investment operation and series-parallel selection control, overall switch losses of the converter can be greatly reduced, and the excellent module capacitance voltage sharing and a pulsation inhibition effect are obtained.

Description

A Modular Multilevel Converter Using Polymorphic Submodules

technical field

The invention belongs to power electronic equipment, in particular to a modular multilevel converter using multi-state sub-modules

Background technique

Since the modular multilevel converter was proposed, due to its highly modular structure and easy expansion, good redundancy characteristics and flexible configuration methods, it can effectively realize power conversion at high voltage levels, and is mainly used in static Reactive power compensation, high-voltage direct current transmission and high-voltage high-power motor speed regulation fields.

However, because the modules of the modular multilevel converter are independent of each other and there are differences in electrical parameters, sub-module capacitor voltage fluctuations and imbalances are prone to occur. The capacitor voltage fluctuations are proportional to the current amplitude of the AC side and are proportional to The frequency is inversely proportional to the module capacitance. If appropriate control measures are not taken, it will lose stability at higher AC side current or lower AC side frequency, and even further cause device damage. This greatly limits the application of modular multilevel converters in the field of variable speed drives for medium and high voltage motors.

The sub-module topologies of existing modular multilevel converters mainly include half-bridge topology and full-bridge topology. It can work in two quadrants, so the direction of the current passing through the capacitor cannot be changed, and the pulsation suppression ability is weak. The full-bridge topology can operate in four quadrants, and its structure determines that it has the ability to switch between capacitor charging and discharging and DC short-circuit blocking, which can greatly enhance the ability to suppress the voltage ripple of the module capacitor. The cost is increased, and at the same time, the on-state loss and switching loss are doubled, so the use of a full-bridge topology will double the device cost and operating cost of the converter.

The modulation methods of the existing modular multilevel converters mainly include ladder wave modulation and carrier phase shift modulation, etc. The main way to suppress the voltage ripple of the sub-module is to select the switching sub-module according to the switching of the bridge arm current direction, so that To ensure that the capacitor voltage offset of the sub-module is within the allowable range, the control ability of the voltage pulsation in the traditional way is guaranteed by increasing the switching frequency, which greatly increases the switching loss of the module. Not only that, based on the carrier phase-shift PWM modulation method, each sub-module corresponds to a carrier, and its control calculation amount increases significantly as the number of levels increases, and the control accuracy of the phase difference will also decrease, so it is difficult to increase the output voltage. Step wave modulation has a small amount of calculation, low switching frequency, and is easy to implement, but at the same time it brings a decrease in the ability to suppress voltage fluctuations, and the module voltage equalization and sorting algorithm in the bridge arm will bring a great burden to the controller. This is especially noticeable for converters with a large number of levels.

The main problem of the existing technology is to increase the loss (including on-state loss and switching loss) in exchange for limited ripple suppression ability, and at the same time limit the number of levels of the multi-level converter, the controller has a large amount of calculation, which greatly limits the Application of modular multilevel converters in medium and high voltage fields.

Contents of the invention

The purpose of the present invention is to provide a converter topology with more flexible control and lower loss. The multi-state sub-module used has three working states: input, cut-out, and parallel connection, which can adapt to more flexible control Compared with the modular multi-level converter composed of traditional half-bridge modules, this converter has a stronger ability to suppress voltage fluctuations of sub-module capacitors, and by using parallel mode during operation, the switching of the converter can be greatly reduced. Loss, especially suitable as a motor drive power supply with speed regulation requirements.

The present invention is for realizing the purpose of the invention, and the technical scheme adopted is:

A modular multi-level converter using a multi-state sub-module is characterized in that the converter uses a four-terminal multi-state sub-module, and the sub-module is basically constructed as follows: two insulators with anti-parallel diodes On the basis of the half-bridge module topology composed of gate bipolar transistors S1 and S2 and capacitor C, the positive port P3 on the parallel side and the negative port P4 on the parallel side are drawn out in parallel at the positive terminal of the capacitor C; among them, the bridge side has a positive port P1 on the bridge side And bridge side negative port P2; two metal-oxide semiconductor field effect switch tubes S3 and S4 with anti-parallel diodes are connected in reverse series between the positive polarity end of the capacitor and the parallel side positive port P3 to form a bidirectional switch; the parallel side negative port P4 is directly led out from the positive terminal of the capacitor; the multi-state sub-module topology can be operated in three modes of input, disconnection, and parallel, realizing real-time series-parallel switching of adjacent modules;

When the above polymorphic sub-modules constitute a modular multilevel converter, the negative port P2 on the bridge side of the previous module is connected to the positive port P1 on the bridge side of the subsequent module, and the negative port P4 on the parallel side of the previous module is connected to the The positive port P3 on the parallel side of the first-level module is connected, and the specific method of module cascading is as follows:

1) In addition to the first and last modules of the upper and lower bridge arms, the bridge-side negative port P2 of the previous-stage module is connected to the bridge-side positive port P1 of the subsequent-stage module, and the parallel-side negative port P4 of the previous-stage module is connected to the subsequent-stage module The positive port P3 of the parallel side is connected;

2) For the first module of the upper bridge arm, the positive port P1 on the bridge side is connected to the positive terminal on the DC side of the converter, the negative port P2 on the bridge side is connected to the positive port P1 on the bridge side of the next-level module, and the parallel side The positive port P3 is not connected, and the negative port P4 on the parallel side is connected to the positive port P3 on the parallel side of the next module;

3) For the last module of the upper bridge arm, the positive port P1 on the bridge side is connected to the negative port P2 on the bridge side of the previous module, and the negative port P2 on the bridge side is connected in parallel to the positive port P1 on the bridge side of the first module of the lower bridge arm The positive port P3 on the side is connected to the negative port P4 on the parallel side of the previous module, and the negative port P4 on the parallel side is not connected;

4) For the first module of the lower bridge arm, the positive port P1 on the bridge side is connected to the negative port P2 on the bridge side of the end module of the upper bridge arm, and the negative port P2 on the bridge side is connected to the positive port P1 on the bridge side of the next module. connection, the positive port P3 on the parallel side is not connected, and the negative port P4 on the parallel side is connected to the positive port P3 on the parallel side of the next module;

5) For the last module of the lower bridge arm, the positive port P1 on the bridge side is connected to the negative port P2 on the bridge side of the previous module, and the negative port P2 on the bridge side is connected to the negative terminal of the DC side of the converter. The positive port P3 is connected to the negative port P4 on the parallel side of the preceding module, and the negative port P4 on the parallel side is not connected.

In the present invention, the basic structure of the multi-state sub-module is derived from the traditional half-bridge module. The supervisors S1 and S2 use insulated gate bipolar transistors (IGBTs), and two metal-oxide semiconductor field effect transistors in reverse series are added. Transistors (MOSFETs) S3 and S4 are used as bidirectional switches for parallel connection. The on-off mode of switches S1 and S2 is the same as that of traditional modules. Switches S3 and S4 use the same on-off signal to start the parallel operation state and increase the lead-out parallel connection The side ports make the module four ports. When the parallel switch is not used, the basic operation mode of the module is the same as that of the half-bridge module, as shown in Figure 1.

When the module is in the non-input state, the added bidirectional switch is turned on, then the module and the previous module can form a parallel pair, that is, run in parallel mode. Since the capacitance value is doubled after the capacitor is connected in parallel, the parallel pair has a stronger external At the same time, because the capacitors with voltage difference are connected in parallel, energy will flow, so that the energy will be distributed evenly, so the imbalance of the capacitor voltage of the two modules connected in parallel will also be eliminated, because the parallel state uses a pair of converters Therefore, not only does not affect the normal operation of the module, but also makes full use of the modules that have been cut out. At the same time, parallel operation will not increase the steady-state loss of the converter.

The structure of the modular multilevel converter composed of multi-state modules is shown in Figure 2. Except for the first module units of the upper and lower bridge arms, the other modules can be connected in parallel with the previous stage. When the second module enters the parallel state When , the first module is connected in parallel with the second module, therefore, each module of the full bridge arm can be connected in parallel with the adjacent modules. , which is essentially the number of modules that have been put into operation. Therefore, all the modules cut off in the traditional operation mode can be changed to a parallel state, that is, the modules in the bridge arm only have two operating states: input and parallel connection. Therefore, all modules are connected in parallel or in series. In the bridge arm, it is the full module operation mode.

The full module operation mode can significantly improve the ability to suppress the module capacitor voltage ripple during operation, because, whether it is the influence of the AC side current amplitude on the capacitor voltage ripple, or the influence of the AC side frequency on the capacitor voltage ripple, essentially , all because the energy injection (output) in the upper (lower) bridge arm per unit time is too large, causing the capacitor voltage ripple to exceed the allowable range, high-order common-mode voltage injection and other methods are all within the unit time, by changing the energy flow direction, thereby suppressing capacitor voltage ripple. In other words, increasing the load capacity of the bridge arm to energy injection (output), that is, the equivalent bridge arm capacitance, can also achieve the effect of suppressing capacitor voltage fluctuations. The multi-state module has a parallel state, which means it has the ability to transfer energy between modules in the bridge arm. At the same time, through parallel connection, the total bridge arm capacitance is increased, especially when the required level is less than half of the bridge arm module. For a while, at this time, it can be guaranteed that each input module has at least one module connected in parallel with it, and the total bridge arm capacitance becomes more than twice that of the traditional method, which greatly improves the load capacity of the bridge arm's energy injection (output) per unit time , thereby greatly improving the low-frequency characteristics.

On the other hand, the use of the parallel state can greatly reduce the overall switching loss of the converter. Taking the upper bridge arm of the five-level converter as an example, if the number of input units at this moment is 3, and the number of input units at the next time is 2, Assuming that the current direction remains unchanged in the previous control cycle and is sufficient to cause a rise in the capacitor voltage of a single module, the switch state changes are shown in Figure 3 (the shaded area represents the switching tube in action). In general, only need to change the second unit from the input state to the parallel state, it can ensure that the capacitance of the bridge arm is multiplied and the voltage difference is eliminated at the same time. The current has changed the order of the voltage levels of each module. Therefore, the most likely switch state change is that among the three modules that have been switched on at this time, the two with the highest voltage are cut off, and the modules that are not switched on at this time are switched on at the next time. . This situation is especially obvious when the load current is large, and the parallel module topology can largely transfer the switching action required by the supervisor (IGBT) to the It is used on the auxiliary tube (MOSFET) connected in parallel, so that the switching loss can be greatly reduced.

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

1. Simplify the start-up charging of the converter. The upper and lower bridge arms are respectively paired with two modules, which can charge the capacitors of all modules to the rated operating value at the same time, and the charging process will automatically stop. When the rated value is reached, the AC side is at zero level, which can simplify the start-up charging process;

2. Flexible pressure equalization ability and pulsation suppression ability. Under parallel operation, the parallel connection doubles the external capacitance and improves the ability to suppress the voltage fluctuation of the capacitor. At the same time, it can play a role of natural voltage equalization for the two modules whose parallel voltage is higher than the rated voltage and lower than the rated voltage;

3. The energy flow in the bridge arm is flexible. The energy flow control in the traditional topological form mainly relies on the control of the circulation between the bridge arms and the selection of switching modules, but the energy between the modules cannot flow directly, so there are few means of energy balance control between the modules, and the control period is long. The parallel state directly realizes the energy flow between the modules independent of circulation, and can realize the rapid energy balance in the bridge arm;

Fourth, the switching loss is reduced. MOSFET tubes are selected for the parallel switches. Through parallel switching control, most of the switching actions are transferred from IGBTs to MOSFETs, which can greatly reduce the total switching loss. As for the steady-state loss, since the large AC side current will aggravate the voltage ripple of the module capacitor, the voltage equalization method of the traditional modular multilevel converter is relatively simple and cannot cope with the large current. If the full-bridge module is used, although the increase Capacitor voltage control ability is improved, but the steady-state loss is at least doubled, while the steady-state loss of the parallel switch of the multi-state module is very small, because the parallel switch will only have charge and discharge current flow when there is a voltage difference between the two capacitors, and The time is short, and the bridge arm current does not pass through the parallel switch, so the steady-state loss is significantly lower than that of the full bridge.

Description of drawings

Fig. 1 is the working principle and structural diagram of the multi-state sub-module topology proposed by the present invention (the bold part indicates the conduction path). (a) Input state diagram, (b) Cutout state diagram, (c) Parallel state diagram.

Fig. 2 is a schematic diagram of a modular multilevel converter composed of multi-state modules according to the present invention.

Fig. 3 is a schematic diagram of the switching action of the full module operation mode proposed by the present invention, and a schematic diagram of the switching action comparison (the shaded area is the action switch) (a) topology (b) traditional topology.

detailed description

The specific embodiment of the present invention is:

Above-mentioned accompanying drawing has clearly explained structure of the present invention, further introduces essence and advantage of the present invention from application angle below:

1. The converter starts the pre-charging mode

Due to the use of modules that can be connected in parallel, the pre-charging mode of the traditional topology can be simplified. Only two adjacent modules need to be connected in parallel. The DC side will charge all the modules of the entire bridge arm at the same time, and the capacitor voltage of the modules will be balanced at the operating rating. The process will stop automatically, and the converter can start running from the state of zero level on the AC side, without real-time measurement and control of the voltage of each module capacitor.

2. Operation mode of the whole module of the converter

Using the nearest level modulation method, firstly, according to the modulation reference wave, divide by the rated capacitor voltage Uc of the module, which is the unit level, and then round up to get the output level. The input module is reduced by one. Therefore, it is only necessary to determine the number of modules to be transferred to the parallel state according to the required input number R and the number of bridge arm modules N obtained under the latest level modulation. The only thing required is Select which modules to transfer to the parallel state, the principles are as follows:

(1) Prioritize the maximum equivalent capacitance of the bridge arm; according to the principle of equivalent capacitor series, the equivalent value of the capacitor series is mainly determined by the smallest capacitance value, so the optimal situation of the full bridge arm is that each N/R unit junction is A parallel pair, when the divisibility is not satisfied, the number of units in each pair is obtained by rounding up and down with N/R.

(2) Prioritize parallel connection of modules with serious voltage offset. Prioritize parallel connection of modules with serious offset. On the one hand, the voltage offset can be eliminated to a large extent. On the other hand, the parallel connection is equivalent to increasing the capacitance, which can suppress the further aggravation of the offset. Because the module also has the ability to work in half-bridge mode, the traditional control modulation method can still be used. Therefore, the control method can be adjusted according to the working conditions, which has great flexibility.

Claims (4)

1. A modular multilevel converter using a multi-state sub-module is characterized in that the converter uses a four-terminal multi-state sub-module, and the basic structure of the sub-module is as follows: two bands with anti-parallel diodes Based on the half-bridge module topology composed of insulated gate bipolar transistors S1 and S2 and capacitor C, the positive port P3 on the parallel side and the negative port P4 on the parallel side are drawn out in parallel at the positive terminal of capacitor C; Port P1 and bridge-side negative port P2; two metal-oxide-semiconductor field-effect switch tubes S3 and S4 with anti-parallel diodes are connected in reverse series to form a bidirectional switch between the positive polarity end of the capacitor and the positive port P3 on the parallel side; the parallel side The negative port P4 is directly led out from the positive terminal of the capacitor; the multi-state sub-module topology can be operated in three modes of input, disconnection and parallel connection respectively, realizing real-time series-parallel switching of adjacent modules: When multi-state sub-modules constitute a modular multilevel converter, the bridge-side negative port P2 of the previous module is connected to the bridge-side positive port P1 of the subsequent module, and the parallel-side negative port P4 of the previous module is connected to the subsequent module. The positive port P3 on the parallel side of the first-level module is connected, and the specific method of module cascade connection is as follows: 1) In addition to the first and last modules of the upper and lower bridge arms, the bridge-side negative port P2 of the previous-stage module is connected to the bridge-side positive port P1 of the subsequent-stage module, and the parallel-side negative port P4 of the previous-stage module is connected to the subsequent-stage module The positive port P3 of the parallel side is connected; 2) For the first module of the upper bridge arm, the positive port P1 on the bridge side is connected to the positive terminal on the DC side of the converter, the negative port P2 on the bridge side is connected to the positive port P1 on the bridge side of the next-level module, and the parallel side The positive port P3 is not connected, and the negative port P4 on the parallel side is connected to the positive port P3 on the parallel side of the next module; 3) For the last module of the upper bridge arm, the positive port P1 on the bridge side is connected to the negative port P2 on the bridge side of the previous module, and the negative port P2 on the bridge side is connected in parallel to the positive port P1 on the bridge side of the first module of the lower bridge arm The positive port P3 on the side is connected to the negative port P4 on the parallel side of the previous module, and the negative port P4 on the parallel side is not connected; 4) For the first module of the lower bridge arm, the positive port P1 on the bridge side is connected to the negative port P2 on the bridge side of the end module of the upper bridge arm, and the negative port P2 on the bridge side is connected to the positive port P1 on the bridge side of the next module. connection, the positive port P3 on the parallel side is not connected, and the negative port P4 on the parallel side is connected to the positive port P3 on the parallel side of the next module; 5) For the last module of the lower bridge arm, the positive port P1 on the bridge side is connected to the negative port P2 on the bridge side of the previous module, and the negative port P2 on the bridge side is connected to the negative terminal of the DC side of the converter. The positive port P3 is connected to the negative port P4 on the parallel side of the preceding module, and the negative port P4 on the parallel side is not connected. 2. The modularized multilevel converter using multi-state sub-module according to claim 1, characterized in that: the multi-state module forming the converter passes through the path containing the bidirectional switch drawn from the capacitor end to realize the connection with The capacitors in the previous module are connected in parallel. 3. The modular multilevel converter of multi-state sub-module according to claim 1, characterized in that: the modular multi-level converter using multi-state sub-module implements full-bridge arm module into operation, That is to say, the modules are not cut off when the level changes, but are selectively allocated in parallel, and the modules in the bridge arm are only in the parallel state or the input state. 4. The modular multi-level converter of multi-state sub-module according to claim 1, characterized in that: the modular multi-level converter using multi-state sub-module realizes modular multi-level converter by connecting pairs in parallel. Start-up pre-charging of the leveling converter.