CN104796025B - A kind of Modularized multi-level converter sub-module topological structure - Google Patents

Abstract

The present invention proposes an improved structure of a modular multilevel converter sub-module, including a first switch module and a second switch module connected in series, the negative pole of the first switch module is connected to the positive pole of the second switch module; the first capacitor The positive pole of the first capacitor is connected to the positive pole of the first switching module, the negative pole of the first capacitor is connected to the negative pole of the second switching module; the negative pole of the third switching module is connected to the negative pole of the second switching module, and the third switching module is composed of a current limiting device and a switching device Composed in parallel, the switching device is composed of insulated gate bipolar transistors and diodes connected in antiparallel, and the current limiting device is composed of capacitors and diodes connected in series. The invention is mainly applied to high-voltage and large-capacity modular multilevel converters. When the DC side fails, it can effectively block the DC fault current, better realize fault isolation, and does not need AC circuit breaker action at the same time. . The invention has a simple structure and has fast fault recovery capability after the fault is cleared.

Description

A Modular Multilevel Converter Sub-module Topology

technical field

The invention relates to the technical field of electric power transmission and distribution, and relates to a novel sub-module topology of a modular multilevel converter.

Background technique

Flexible DC transmission technology provides new means and technical solutions for solving the grid-connected bottleneck of renewable energy such as large-scale wind farms, and for the expansion and transformation of urban high-voltage power grids, grid interconnection and island power supply. The strategic choice of the power grid development pattern has been included in the "National Energy Science and Technology "Twelfth Five-Year" Plan" key equipment for high-performance power transmission and transformation, and is a major technical equipment that requires key technological breakthroughs.

Since flexible direct current transmission is developed on the basis of conventional direct current transmission, in addition to the advantages of conventional direct current transmission technology, flexible direct current transmission also has a compact and modular design compared with conventional direct current transmission, which is easy to move, install and debug And maintenance, easy to expand and realize the advantages of multi-terminal DC transmission.

MMC-HVDC (Modular Multilevel Converter Based DC Transmission Technology) is a new structure in VSC-HVDC (Voltage Source Converter Type High Voltage DC Transmission). Compared with cascaded H-bridge converters, MMC retains a highly modular structure, and at the same time has a high-voltage DC bus, which can realize four-quadrant operation of output voltage and output current. Compared with the traditional two-level and three-level converters, MMC does not have the problem of voltage equalization and current equalization of switching tubes connected in series and parallel. Each sub-module of the MMC has a relatively simple structure, is easy to control, and can be expanded infinitely, which is especially suitable for the HVDC field. Since the MMC increases the two-level or three-level VSC to tens of levels or even hundreds of levels, the system voltage level is increased on the basis of maintaining the withstand voltage level, and the switching frequency of the switching device is greatly reduced, thereby reducing the Small switching loss, the output voltage waveform is closer to a sine wave to further reduce the harmonic content. At the same time, MMC adopts the same modular technology. Its modular structure makes it highly scalable and easy to realize redundant control. It has important advantages in research and development, manufacturing, dynamic and static pressure equalization and reduction of circulation.

In terms of engineering construction, the Nanhui Wind Farm Flexible Transmission Demonstration Project, as my country's first MMC project, completed the project acceptance in July 2011 and became my country's first MMC-based HVDC power transmission project officially put into commercial use. At the same time, the large-scale wind farm flexible DC transmission project of China Southern Grid and the five-terminal flexible DC project of State Grid Zhoushan are under construction.

Traditional MMC-HVDC projects usually use DC cables for transmission. DC cables have better safety and stability, but their prices are higher, and the cost of long-distance transmission is higher. In large-capacity transmission, overhead line transmission is more commonly used, which is cheaper and has better cooling effect. In overhead line transmission, the traditional half-bridge structure essentially lacks DC fault isolation capability: when a fault occurs on the DC side, the anti-parallel freewheeling diode of the fully-controlled switching device easily constitutes an energy feed directly connected to the AC system at the fault point The circuit must be cut off by jumping off the AC circuit breaker. The disadvantage is that the mechanical response is slow (2 to 3 cycles are required at the fastest), which affects the normal transmission of power, and may cause overcurrent and overcurrent of the converter valve device. overvoltage. Auxiliary measures such as increasing the rated parameters of the equipment and configuring high-speed bypass switches are often required. Therefore, this topology is not suitable for power transmission on overhead lines that are prone to temporary faults such as flashovers, and it is necessary to lay cables that are expensive and have a low failure rate. line.

Therefore, it is necessary to design a new sub-module structure, which can effectively isolate the DC fault.

Contents of the invention

Aiming at the above-mentioned problems and application requirements in high-voltage and high-power occasions, the present invention proposes a modular multilevel converter sub-module topology on the basis of the original half-bridge structure, which consists of a series half-bridge structure and a current-limiting Module composition, the new improved structure does not change the structure, control strategy and capacitor voltage equalization mode of the original sub-module, and the improved series part can realize independent control. The new improved structure improves the fault ride-through capability of the system, and can effectively isolate the fault current when the DC side is faulty.

The technical solution of the present invention: a sub-module topology structure of a modular multilevel converter, which is composed of a half-bridge structure and a current limiting module in series; the half-bridge structure includes a first switch module, a second switch module and a The first capacitor; the negative pole of the first switch module is connected to the positive pole of the second switch module; the positive pole of the first capacitor is connected to the positive pole of the first switch module, and the negative pole of the first capacitor is connected to the negative pole of the second switch module ;

The current limiting module is composed of a second capacitor, a diode and a third switch module; the negative pole of the diode is connected to the positive pole of the second capacitor, the positive pole of the diode is connected to the positive pole of the third switch module, and the negative pole of the third switch module connected to the negative pole of the second capacitor, and the negative pole of the second capacitor of the current limiting module is connected to the negative pole of the second switch module;

The node between the negative pole of the first switch module of the half-bridge structure and the positive pole of the second switch module is the input terminal of the topology structure, and the node between the positive pole of the third switch module of the current limiting module and the positive pole of the diode is output of the topology.

Further, the first switch module, the second switch module and the third switch module are all composed of an insulated gate bipolar transistor and a diode connected in antiparallel.

Further, under normal working conditions, pulses are always applied to the IGBT of the third switch module in the current limiting module, so that it is always in a conduction state, so that the second capacitor and the diode are in a short-circuit state, which has no effect on the output influences.

A modular multilevel converter based on the sub-module topology of the modular multilevel converter, including three phase units, each phase unit is divided into upper and lower bridge arms, and each bridge arm includes several series-connected Sub-module topology structure, the number of sub-module topology structures connected in series with the upper and lower bridge arms of each phase is the same; the upper and lower bridge arms are respectively connected in series with current-limiting reactors, and each phase from top to bottom is: all sub-modules of the upper bridge arm, upper bridge arm reactors, The reactor of the lower bridge arm and all sub-modules of the lower bridge arm; and the connection of the upper and lower bridge arms of each phase is externally connected with a three-phase AC voltage, the input terminal of the top sub-module topology of the upper bridge arm is connected to the DC positive pole, and the bottom terminal of the lower bridge arm The output terminal of the module is connected to the DC negative pole.

Further, in the DC power transmission system, when a bipolar short-circuit fault occurs on the DC side, the fault current is detected first, and then all the trigger signals of the first switch module and the second switch module are turned off, and at the same time, the third switch module in the current limiting module is turned off. Switching module signal, after the current enters the topology from the output terminal of the topology, it flows out from the input terminal of the topology through the diode and the second capacitor of the current limiting module, instead of passing through the third switching module and the first capacitor of the current limiting module . This structure can reduce the fault current through the capacitance to achieve the purpose of eliminating the fault. The capacitor connected in parallel in the half-bridge structure does not flow current and is protected so that it can recover quickly after a fault.

Further, when the fault is a permanent DC fault, the specific process is: turn off the trigger pulses of the IGBTs in the first switch module, the second switch module and the third switch module, and then disconnect the AC The side circuit breaker is inspected and repaired. After the fault is repaired, the switch is reclosed, and then the first switch module, the second switch module and the third switch module are turned on.

Further, when the fault is a temporary DC fault, the specific process is: turn off the trigger pulses of the IGBTs in the first switch module, the second switch module and the third switch module, and wait for the DC side fault After the current is zero, the IGBTs of the first switch module, the second switch module and the third switch module are retriggered to establish a DC side voltage and wait for the system to enter a steady state operation.

The technical solution of the present invention: the present invention mainly aims at bipolar short-circuit faults on the DC side, and studies the fault current shutdown mechanism and suppression measures under bipolar short-circuit faults.

The advantages of the present invention are: 1) During the high-voltage and large-capacity transmission process, the fault current on the DC side can be effectively suppressed without the action of the AC circuit breaker; 2) The circuit can effectively protect the capacitance of the atomic module, and can Realize fast recovery; 3) Compared with other topological structures, components are saved.

Description of drawings

Fig. 1 is a schematic topology diagram of the modularized multi-level improved sub-module of the present invention.

Fig. 2 is a schematic diagram of a three-phase modular multilevel converter with a new sub-module structure of the present invention.

Fig. 3 is a schematic diagram of the flow direction of the current flowing from the positive pole (1 terminal) under the DC fault of the present invention.

Fig. 4 is a schematic diagram of the flow direction of the current flowing from the negative pole (2 terminals) under the DC fault of the present invention.

Fig. 5 is a schematic diagram of current flow under a bipolar short-circuit DC fault according to the present invention.

detailed description

Below in conjunction with embodiment the present invention will be further described.

Fig. 1 is the topology of the improved modular multilevel converter sub-model proposed by the present invention. Among them, two terminals are connected in series, and a module is connected in parallel with the IGBT by the capacitor diode in series. When this structure is working normally, the IGBT connected in series at the two terminals is always on, and the capacitor is short-circuited. The original half-bridge sub-module structure is controlled according to the original control method, without changing the original modulation method, and the output of the sub-module is also controlled. Voltage has no effect. When a fault occurs on the DC side, the IGBTs connected in series at 2 terminals are turned off.

Figure 2 is a three-phase modular multilevel converter structure, each sub-module is composed of improved sub-modules shown in Figure 1.

Figure 3 shows that when a fault occurs on the DC side, the current flows in from terminal 1, the current flows through the atomic module capacitor C1, and then flows out from terminal 3 through D3 (does not flow through the capacitor connected in parallel).

Figure 4 shows that when a fault occurs on the DC side, the current flows in from terminal 2, the current flows through the resistors and capacitors connected in parallel, and then flows out from terminal 1 through D2. This structure can reduce the fault current through the capacitance to achieve the purpose of eliminating the fault. The capacitor connected in parallel in the half-bridge structure does not flow current and is protected so that it can recover quickly after a fault.

According to the drawings, when running in normal state, the IGBT (S3) of the third switch in the current limiting module is in the normally open state, and the capacitor diode in the current limiting module has no current passing through. When a bipolar short-circuit fault occurs in the system, the third switch S3 in the current limiting module is turned off, and the fault current passes through the upper branch capacitor of the current limiting module to charge the capacitor C2 of the current limiting module.

Figure 5 is a diagram of the current flow when a bipolar short-circuit fault occurs. The figure takes two phases A and B as an example (each phase is simplified and replaced by a sub-module). When there is a fault, the current flows from one phase of the power grid to the fault. The current passes through one phase bridge arm, then passes through the fault point, and then passes through another phase bridge arm, and then flows into another phase. As shown in FIG. 5 , the current flow path is D _B4 -C _B2 -D _B2 -D _A4 -C _A2 -D _A2 . The current passes through 2N sub-modules in total, and the voltage of each sub-module is U _m , and the grid connection point is the line voltage U _AB .

U _dc = NU _m

U _AB ＝2NU _m +2N×(U _D2 +U _D4 )

Available:

And because: m<1

U _D2 +U _D4 <0

Among them: U _dc is the DC side voltage under normal operation; U _A and U _B are the phase voltages of phase A and phase B of the three-phase AC side output respectively; m is the modulation ratio; U _AB is the line voltage of AB phase; U _D2 and U _D4 are the voltages across diodes D2 and D4 in the sub _- module respectively _.

It can be seen that the diode withstands the reverse voltage, shuts off the fault, and reliably shuts off the fault current.

The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations can be carried out to the technical solutions of the present invention. These equivalent transformations All belong to the protection scope of the present invention.

Claims (7)

1. A kind of 1. Modularized multi-level converter sub-module topological structure, it is characterised in that：By half-bridge structure and current limliting module string Connection composition；The half-bridge structure includes first switch module, second switch module and the first electric capacity；The first switch module Negative pole is connected with the positive pole of second switch module；The positive pole of first electric capacity is connected with the positive pole of first switch module, and first The negative pole of electric capacity is connected with the negative pole of second switch module；

   The current limliting module is made up of the second electric capacity, diode and the 3rd switch module；The negative level of the diode and the second electricity The positive pole of appearance is connected, and the positive pole of diode is connected with the positive pole of the 3rd switch module, the negative pole of the 3rd switch module and the second electricity The negative level held is connected, and the second electric capacity of current limliting module is born level and is connected with the negative level of second switch module；

   Node between the negative pole of the first switch module of the half-bridge structure and the positive pole of second switch module is the topology The input of structure, the node between the positive pole and diode cathode of the 3rd switch module of current limliting module are the topological structure Output end.
2. A kind of 2. topological structure of Modularized multi-level converter sub-module according to claim 1, it is characterised in that：Institute First switch module, second switch module and the 3rd switch module are stated by an insulated gate bipolar transistor and two poles Pipe inverse parallel forms.
3. A kind of 3. topological structure of Modularized multi-level converter sub-module according to claim 1, it is characterised in that：Just Under normal working condition, the insulated gate bipolar transistor of the 3rd switch module in current limliting module applies pulse always, makes one It is straight in the conduction state, so that the second electric capacity and diode are in short-circuit condition, on exporting without influence.
4. 4. a kind of modularization multi-level converter, including three facies units, each facies unit divide upper and lower bridge arm, its feature exists In：Each bridge arm includes the submodule topological structure as described in claim 1-3 is any of several series connection, per phase upper and lower bridge arm The submodule topological structure quantity of series connection is identical；Upper and lower bridge arm is connected current-limiting reactor respectively, per being from top to bottom mutually：Upper bridge arm All submodules, upper bridge arm reactor, lower bridge arm reactor, all submodules of lower bridge arm；And per the junction of phase upper and lower bridge arm External three-phase alternating voltage, the input of upper bridge arm the top submodule topological structure are connected with direct-flow positive pole, and lower bridge arm is most lower Terminal module output end is connected with direct current negative pole.
5. A kind of 5. modularization multi-level converter according to claim 4, it is characterised in that：In DC transmission system, When the bipolar short trouble of DC side generation, fault current is first detected, is then shut off all first switch modules and second switch The trigger signal of module, simultaneously turn off the 3rd switch module signal in current limliting module, output of the fault current from topological structure After end enters topological structure, flowed out by the diode and the second electric capacity of current limliting module from the input of topological structure, and no longer Pass through the 3rd switch module of current limliting module and the first electric capacity.
6. A kind of 6. modularization multi-level converter according to claim 5, it is characterised in that：When described failure is direct current During permanent fault, detailed process is：Insulated gate is double in shut-off first switch module, second switch module and the 3rd switch module The trigger pulse of bipolar transistor, AC breaker is then turned off, is overhauled, after fault restoration, carry out reclosing, then open Open first switch module, second switch module and the 3rd switch module.
7. A kind of 7. modularization multi-level converter according to claim 5, it is characterised in that：When described failure is direct current During temporary fault, detailed process is：Insulated gate is double in shut-off first switch module, second switch module and the 3rd switch module The trigger pulse of bipolar transistor, it is zero to wait DC side fault current, after Failure elimination, retriggered first switch module, The insulated gate bipolar transistor of second switch module and the 3rd switch module, establishes DC voltage, and waiting system enters steady State is run.