CN104993716A - Modular multilevel converter and hybrid double-unit sub-module - Google Patents

Abstract

The invention relates to a modular multilevel converter and a hybrid twin sub-module. The modular multilevel converter includes 3-phase 6 bridge arms, each bridge arm is composed of n sub-modules cascaded, at least one sub-module is a hybrid twin sub-module, and the hybrid twin sub-module includes 4 power modules: T1, T2, T3, T4 and 2 capacitors: C1, C2, the anode of T1 is connected to the anode of T4, the cathode of T2 is connected to the cathode of T3, the cathode of T1 is connected to the anode of T2, the cathode of T4 is connected to the anode of T3 through capacitor C2, T1 and T4 A capacitor C1 is connected between the connection point of T2 and T3, the connection point of T1 and T2 is a port of the hybrid twin sub-module, and the connection point of C2 and T4 is another port of the hybrid twin sub-module. A hybrid twin sub-module can replace two half-bridge sub-modules at the same time, and it has the negative voltage characteristics of the full-bridge sub-module. The hybrid twin sub-module can improve the utilization rate of DC voltage and increase the capacity of the system.

Description

A Modular Multilevel Converter and a Hybrid Twin Submodule

technical field

The invention relates to a modular multilevel converter and a hybrid twin sub-module, belonging to the technical fields of flexible direct current transmission and distribution of power systems, power electronics and user power.

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 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 redundant operation capabilities after failure, and there is 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.

DC short-circuit fault is the most common fault in the MMC-HVDC system. The MMC converter based on the half-bridge sub-module cannot cut off the energy feed from the AC system to the DC short-circuit point by blocking the sub-module IGBT when the DC bipolar short-circuit fault occurs. circuit, the AC circuit breaker or DC isolating switch must be quickly tripped to clear the fault current, which not only increases the system cost and raises the technical requirements for the equipment, but also reduces the system operation rate and slows down the fault recovery speed.

At present, most projects use DC cable laying lines, which are difficult to manufacture and high in cost, to reduce the incidence of DC faults, but this cannot fundamentally solve the problem of the failure of the half-bridge MMC converter to handle DC faults. In view of this, it is the most economical and effective method to realize the fault current self-clearing through the control of the converter itself, which also makes it a research trend to find the converter topology with DC fault ride-through capability.

At present, the MMC sub-module topology with DC fault ride-through capability includes full-bridge sub-modules, clamped twin sub-modules, etc., as shown in Figure 1. Figure 1-1 is a half-bridge sub-module, and Figure 1-2 is a full-bridge sub-module. Figure 1-3 shows the clamping twin sub-modules. Among them, in addition to the DC fault ride-through capability, the full-bridge sub-module MMC can also improve the DC voltage utilization due to the negative voltage characteristics of the module, thereby increasing the system capacity; the clamped twin-sub-module MMC has fewer devices than the full-bridge sub-module. The number of pieces is more economical, but the sub-module does not have the ability to increase the system capacity. For example, the Chinese patent application with the application number 201410558336 and the invention title "Hybrid Sub-Modular MMC Converter with DC Fault Ride-through Capability" discloses an MMC converter, including three-phase six bridge arms, each Each bridge arm is composed of m half-bridge sub-modules, n full-bridge sub-modules and one clamping double sub-module cascaded. However, since the existing three sub-modules have certain limitations or defects, it is very necessary to propose an MMC converter including a new type of MMC sub-module, so that the MMC converter has both fewer components and high capacity. With the advantages of DC fault ride-through function.

Contents of the invention

The purpose of the present invention is to provide a modular multi-level converter to solve the problem that the existing MMC converter cannot have the advantages of less components, high capacity and DC fault ride-through function due to the defects of sub-modules The problem.

In order to achieve the above object, the solution of the present invention includes a modular multilevel converter, including 3 phases, each phase is composed of upper and lower bridge arms, and each bridge arm is composed of n sub-modules cascaded, so Among the submodules in the modular multilevel converter, at least one submodule is a hybrid twin submodule, and the hybrid twin submodule includes 4 power modules: T1, T2, T3, T4 and 2 capacitors: C1, C2 , the anode of T1 is connected to the anode of T4, the cathode of T2 is connected to the cathode of T3, the cathode of T1 is connected to the anode of T2, and the cathode of T4 is connected to the T3 through the capacitor C2 Anode, the capacitor C1 is connected between the connection point of T1 and T4 and the connection point of T2 and T3, the connection point of T1 and T2 is a port of the hybrid twin sub-module, and the C2 and T4 The connection point is another port of the hybrid twin sub-module.

The power module is an IGBT module, the anode of the power module is the collector of the IGBT module, and the cathode of the power module is the emitter of the IGBT module.

Each of the power modules is connected with a diode in antiparallel.

A hybrid twin module, the hybrid twin module includes 4 power modules: T1, T2, T3, T4 and 2 capacitors: C1, C2, the anode of T1 is connected to the anode of T4, and the cathode of T2 Connect the cathode of T3, the cathode of T1 is connected to the anode of T2, the cathode of T4 is connected to the anode of T3 through the capacitor C2, the connection point of T1 and T4 is connected to the connection of T2 and T3 The capacitor C1 is connected between the points, the connection point of T1 and T2 is a port of the hybrid dual sub-module, and the connection point of C2 and T4 is the other port of the hybrid dual sub-module.

The power module is an IGBT module, the anode of the power module is the collector of the IGBT module, and the cathode of the power module is the emitter of the IGBT module.

Each of the power modules is connected with a diode in antiparallel.

The novel hybrid twin sub-module provided by the present invention is composed of 4 power modules and 2 module capacitors. The hybrid twin sub-module has two working modes: normal operation mode and blocking mode. In normal operation mode, the hybrid twin sub-module can output 4 There are two kinds of voltages, which are double capacitance voltage, capacitance voltage, zero voltage and negative capacitance voltage. Since one hybrid twin sub-module can output twice the capacitance voltage, one hybrid twin sub-module is equivalent to two half-bridge sub-modules, and can replace two half-bridge sub-modules at the same time. Moreover, since the hybrid twin sub-module can output negative voltage, which has the negative voltage characteristics of the full-bridge sub-module, the hybrid twin sub-module can improve the utilization rate of DC voltage and increase the capacity of the system.

A hybrid dual sub-module can realize all the functions of the full-bridge sub-module and the half-bridge sub-module at the same time, but the hybrid dual sub-module only contains 4 power modules and 2 module capacitors, while a full-bridge sub-module and a half-bridge sub-module The components in the module add up to 6 power modules and 2 module capacitors. Therefore, the hybrid dual sub-module realizes the same function with fewer components and saves costs.

Therefore, the MMC based on the hybrid twin sub-module has the advantages of less components, high capacity and DC fault ride-through function.

The two capacitors inside the hybrid twin sub-module can be reasonably configured to have different voltages as required. In this way, the application range of the MMC can be effectively expanded, for example, the voltages of the two capacitors in the sub-module can be reasonably configured to improve the modulation degree of the MMC while having the STATCOM fault ride-through capability.

In addition, the submodules in the MMC can all be the hybrid twin submodules provided by the present invention, and it can also be the following situation: the bridge arm in the MMC is formed by cascading the hybrid twin submodules with other one or several existing submodules, Then, the MMC is a hybrid MMC converter. The hybrid MMC converter has a wide range of applications when it is expanded according to the actual situation, such as improving the modulation rate, having the fault ride-through capability of STATCOM operation, and saving system hardware costs.

Description of drawings

Figure 1-1 is a structural diagram of an existing half-bridge MMC sub-module;

Figure 1-2 is a structural diagram of an existing full-bridge MMC sub-module;

Figure 1-3 is a structural diagram of an existing clamping twin sub-module;

Fig. 2 is the structural representation of the hybrid twin module provided by the present invention and the MMC of composition thereof;

Fig. 3 is the approximate composition connection diagram of hybrid twin sub-module;

Figure 4-1 is a schematic diagram of the first working state of the hybrid sub-module in normal working mode;

Figure 4-2 is a schematic diagram of the second working state in the normal working mode of the hybrid sub-module;

Figure 4-3 is a schematic diagram of the third working state in the normal working mode of the hybrid sub-module;

Figure 4-4 is a schematic diagram of the fourth working state in the normal working mode of the hybrid sub-module;

Figure 5-1 is a schematic diagram of one of the working states in the hybrid sub-module locking mode;

Figure 5-2 is a schematic diagram of another working state in the mixed sub-module locking mode;

Fig. 6 is a block schematic diagram when the DC fault of the converter occurs.

Detailed ways

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

Embodiment of Modular Multilevel Converter

The modular multilevel converter shown in Figure 2 includes 3 phases, each phase is composed of upper and lower bridge arms, each bridge arm is composed of n sub-modules cascaded, in this modular multilevel Among the sub-modules in the converter, at least one sub-module is a hybrid twin sub-module, which includes 4 IGBT modules: T1, T2, T3, T4 and 2 capacitors: C1, C2, and the collector of T1 is connected to T4 The collector of T2, the emitter of T2 is connected to the emitter of T3, the emitter of T1 is connected to the collector of T2, the emitter of T4 is connected to the collector of T3 through capacitor C2, the connection point of T1 and T4 is connected with the connection point of T2 and T3 The connection point between the capacitors C1, T1 and T2 is one port of the hybrid twin sub-module, and the connection point of C2 and T4 is the other port of the hybrid twin sub-module. All the IGBTs (T1, T2, T3, T4) in the hybrid twin sub-module are anti-parallel connected with freewheeling diodes, and the bases of T1, T2, T3, T4 respectively receive control signals provided by external devices.

As shown in Figure 3, the hybrid twin sub-module can be approximately considered to be composed of two half-bridge sub-modules, and the positive port (that is, the two IGBT connection points) of the second half-bridge sub-module is disconnected into two ports ( The upper tube port and the lower tube port) are cross-connected to both ends of the capacitor C1 in the first half-bridge sub-module (that is, the upper tube port is connected to the negative terminal of the capacitor C1, and the lower tube port is connected to the positive terminal of the capacitor C1) .

The hybrid twin sub-module has two modes of operation, normal operation mode and latch-off mode. In normal operation mode, at most one IGBT can be turned on between T1 and T2. In order to prevent short circuit of capacitor C1, both T1 and T2 cannot be turned on at the same time; only one IGBT can be turned on between T3 and T4.

The hybrid twin module has 4 working states in the normal working mode, and the 4 working states in the normal running mode are shown in Figure 4-1 to 4-4, (1) is the current flow direction when T1 and T3 are turned on , (2) is the current flow direction when T1 and T4 are turned on, (3) is the current flow direction when T2 and T3 are turned on, and (4) is the current flow direction when T2 and T4 are turned on. As shown in Table 1, when T1 and T3 are turned on, the port output voltage is the sum of the two capacitor voltages; when T1 and T4 are turned on, the port output voltage is zero; when T2 and T3 are turned on, the port output voltage is Capacitor C2 voltage; when T2 and T4 are turned on, the port output voltage is the reverse voltage of capacitor C1, which is the output negative voltage. The direction of current flow does not affect the port output voltage. In Table 1, Usm represents the output voltage of the sub-module port.

Table 1

From the normal working mode of the hybrid twin sub-module, it can be seen that the sub-module can output 4 kinds of voltages, which are double capacitance voltage, capacitance voltage, zero voltage and negative capacitance voltage. It shows that the sub-module can replace the two half-bridge sub-modules to output twice the capacitor voltage, and at the same time, it has the negative voltage characteristics of the full-bridge sub-module, which can improve the utilization rate of DC voltage and increase the system capacity.

The hybrid twin module has two working states in the blocking mode, and the two operating states in the blocking mode are shown in Figure 5-1 and 5-2. In the blocking state, all IGBTs are in the off state. When a positive current flows (the current direction is from A to B), the port output voltage is the sum of the two capacitor voltages; when a negative current flows, the port output voltage is the negative voltage of capacitor C1, which is opposite to the current voltage.

There are two ways for the MMC to perform DC fault ride-through: fault ride-through by converter blocking and fault ride-through by converter STATCOM operation.

Converter blocking fault ride-through: After the system detects a DC fault, it immediately blocks the converter valve. At this time, the hybrid dual sub-module in the bridge arm can output an electromotive force opposite to the fault current at the port, forcing the fault current to drop rapidly, thereby realizing DC Fault ride-through capability, as shown in Figure 6.

In order to complete the converter blocking fault ride-through, the back electromotive force 2NU _c1 provided by the module capacitor needs to be greater than the peak value of the valve side line voltage to force the fault current to drop rapidly, that is, Among them, N is the number of hybrid twin sub-modules (if the bridge arm contains other modules that can provide back electromotive force, then N is the sum of the number of modules of this type), U _c1 is the blocking voltage of the hybrid twin sub-module under negative current, U _m is the peak value of the phase voltage on the valve side of the converter.

Converter STATCOM operation fault ride-through: The converter does not need to be blocked, and only needs to use the negative voltage output characteristics of the hybrid dual sub-module to reverse the total output voltage of the sub-modules of the upper and lower bridge arms of the same phase, and control the DC voltage to Zero, the AC voltage is the instantaneous voltage value on the valve side of this phase required for STATCOM operation. The basic control method is basically the same as normal operation.

In order to complete the fault ride-through of STATCOM operation, the reverse voltage output by the sub-modules in the bridge arm should be able to provide the peak value of the phase voltage on the valve side of the converter, that is, the system needs to meet NU _c1 ≥ U _m .

The following are four situations in which STATCOM runs through faults.

1. The system does not need to increase the utilization rate of DC voltage, but needs STATCOM to run fault ride-through.

In this case, the MMC topology consisting of hybrid twin sub-modules with equal internal capacitor voltages can fulfill this system requirement. In the MMC topology, each bridge arm is composed of N hybrid twin sub-modules, and the voltage of the module capacitors is Uc.

The system does not need to increase the DC voltage utilization rate, and the system modulation degree can be considered as 1 (when the modulation degree is less than 1, the system can meet the requirements). In this case, the degree of modulation When overmodulation is not required, the bridge arm voltage can be designed as a DC voltage to maintain the normal operation of the system, and the AC side voltage is half of the bridge arm voltage. That is: U _dc =U _bridge =2NU _c and U _m =U _bridge /2=NU _c .

Since the voltages of the capacitors in the modules are the same, the hybrid twin-submodule MMC system can just satisfy NU _c1 = NU _c = U _m , and the reverse voltage output by the sub-modules in the bridge arm can provide the peak value of the phase voltage on the valve side of the converter, that is, the STATCOM operation can be completed Fault ride through.

2. The system needs to improve the DC voltage utilization rate, the system modulation degree is 2, and STATCOM is required to run fault ride-through.

Due to the increased modulation of the system, the system in the first case above cannot meet the requirements. Therefore, the system can be improved, such as setting the voltage value of the capacitor C1 in the sub-module to twice the voltage of the capacitor C2, that is, 2NU _c2 =NU _c1 .

System modulation That is, U _dc =U _m . At this time, the rated voltage of the bridge arm needs to reach 1.5U _m , so that the over-modulation operation can be realized in the configuration where the upper and lower bridge arms are respectively 1.5U _m and -0.5U _m . At this time, N hybrid sub-modules are set according to a single bridge arm, then U _bridge =N(U _c1 +U _c2 )=3NU _c2 .

In order to complete the fault ride-through of STATCOM operation, the reverse voltage output by the sub-modules in the bridge arm can provide the peak phase voltage of the valve side of the converter, that is, the system meets the requirement of NU _c1 ≥ U _m .

3. The system needs to improve the DC voltage utilization rate, the system modulation degree is 2, and STATCOM is required to run fault ride-through.

In this case, a hybrid MMC system with hybrid twin sub-modules can be used to fulfill the system requirements. The MMC system includes not only hybrid twin sub-modules, but also other existing sub-modules. For example, the same number (each N) of full-bridge sub-modules and hybrid twin sub-modules are connected in series in each bridge arm of the system. The capacitor voltage is the same as that of the full-bridge sub-module capacitor (ie, U _c ).

System modulation That is, U _dc =U _m . At this time, the rated voltage of the bridge arm needs to reach 1.5U _m , so that the over-modulation operation can be realized in the configuration where the upper and lower bridge arms are respectively 1.5U _m and -0.5U _m . At this time, U _bridge =N(2U _c +U _c )=3NU _c .

In order to complete the fault ride-through of STATCOM operation, the reverse voltage output by the sub-modules in the bridge arm is 2NU _c (including the reverse voltage output by the full-bridge sub-module and the hybrid dual sub-module each NU _c ), which is enough to provide the peak value of the phase voltage on the valve side of the converter. .

4. The system does not need to increase the DC voltage utilization rate, and only needs to complete the DC fault ride-through in the blocking mode. However, it is necessary to reduce the hardware cost of the main circuit of the system as much as possible.

Due to the low system function requirements, the hybrid MMC system with hybrid twin sub-modules and half-bridge sub-modules can be used to save the hardware cost of the main circuit.

In each bridge arm of the system, N1 hybrid twin sub-modules and N2 half-bridge sub-modules are connected in series, and the internal capacitor voltage of the hybrid twin sub-module is the same as that of the half-bridge sub-module.

The system does not need to increase the DC voltage utilization rate, and the system modulation degree can be considered as 1 (when the modulation degree is less than 1, the system can meet the requirements). In this case, the degree of modulation When overmodulation is not required, the bridge arm voltage can be designed as a DC voltage to maintain the normal operation of the system, and the AC side voltage is half of the bridge arm voltage. That is: U _dc =U _bridge =(2N ₁ +N ₂ )U _c and U _m =U _bridge /2=(2N ₁ +N ₂ )U _c /2.

In order to complete the converter blocking fault ride-through, the back electromotive force provided by the module capacitor needs to meet $2N_1{u}_c\&Greater\; Equal;\sqrt{3}{u}_m.$

Therefore, the distribution of the number of hybrid twin sub-modules and half-bridge sub-modules in the system is as follows:

$N_1\&Greater\; Equal;\frac{\sqrt{3}{u}_m}{2u_c}$ and $N_2=\frac{u_{dc}}{u_c}-2N_1.$

In the above embodiments, the power module is an IGBT. As other embodiments, the power module may also be other fully-controlled devices.

Hybrid Twin Module Embodiment

The hybrid twin modules have been described in detail in the above embodiments, and will not be repeated here.

Specific embodiments have been given above, but the present invention is not limited to the described embodiments. The basic idea of the present invention lies in the above-mentioned basic scheme. For those of ordinary skill in the art, according to the teaching of the present invention, it does not need to spend creative labor to design various deformation models, formulas, and parameters. Changes, modifications, substitutions and variations to the implementations without departing from the principle and spirit of the present invention still fall within the protection scope of the present invention.

Claims (6)

1. A modular multi-level converter, comprising 3 phases, each phase is composed of upper and lower bridge arms, each bridge arm is formed by cascading n sub-modules, it is characterized in that the modular multi-level Among the sub-modules in the level converter, at least one sub-module is a hybrid twin sub-module, and the hybrid twin sub-module includes 4 power modules: T1, T2, T3, T4 and 2 capacitors: C1, C2, the T1 The anode of T4 is connected to the anode of T4, the cathode of T2 is connected to the cathode of T3, the cathode of T1 is connected to the anode of T2, the cathode of T4 is connected to the anode of T3 through the capacitor C2, and the The capacitor C1 is connected between the connection point of T1 and T4 and the connection point of T2 and T3, the connection point of T1 and T2 is a port of the hybrid twin sub-module, and the connection point of C2 and T4 is Another port of the hybrid twin sub-module. 2. The modular multilevel converter according to claim 1, wherein the power module is an IGBT module, the anode of the power module is the collector of the IGBT module, and the cathode of the power module is The emitter of the IGBT module. 3. The modular multilevel converter according to claim 1 or 2, wherein each of the power modules is connected with a diode in antiparallel. 4. A hybrid twin sub-module, characterized in that the hybrid twin sub-module includes 4 power modules: T1, T2, T3, T4 and 2 capacitors: C1, C2, the anode of T1 is connected to the anode of T4 , the cathode of T2 is connected to the cathode of T3, the cathode of T1 is connected to the anode of T2, the cathode of T4 is connected to the anode of T3 through the capacitor C2, the connection point of T1 and T4 is connected to the The capacitor C1 is connected between the connection point of T2 and T3, the connection point of T1 and T2 is a port of the hybrid twin sub-module, and the connection point of C2 and T4 is another port of the hybrid twin sub-module port. 5. The hybrid twin module according to claim 4, wherein the power module is an IGBT module, the anode of the power module is the collector of the IGBT module, and the cathode of the power module is the emitter of the IGBT module . 6. The hybrid twin sub-module according to claim 4 or 5, wherein each of the power modules is connected with a diode in reverse parallel.