WO2016029831A1

WO2016029831A1 - Direct current voltage compensation method for parallel mixed-type multi-level converter

Info

Publication number: WO2016029831A1
Application number: PCT/CN2015/087936
Authority: WO
Inventors: 杨杰; 贺之渊; 李强; 马巍巍; 周扬
Original assignee: 国家电网公司; 国网智能电网研究院; 中电普瑞电力工程有限公司; 国网浙江省电力公司
Priority date: 2014-08-25
Filing date: 2015-08-24
Publication date: 2016-03-03
Also published as: CN105375793A; CN105375793B

Abstract

A direct current voltage compensation method for a parallel mixed-type multi-level converter. The converter is a three-phase power transmission structure, the three-phase power transmission structure consisting of a parallel-type single-phase structure series connection. A parallel-type single-phase structure consists of an H bridge full-control device structure (2) and a sub-module cascade structure (1) connected in parallel, the H bridge full-control device structure consisting of a cascade structure having four full-control devices (S1-S4), the sub-module cascade structure consisting of a half-bridge sub-module series connection. The compensation method comprises: an isolation time is added during a converter normal operating mode, the sub-module cascade structures are freely controlled in the isolation time, a required compensation voltage is thereby outputted, compensation of a direct current voltage shortfall is implemented, and a compensation ratio in the three-phase power transmission structure adjusts a three-phase distribution ratio according to requirements. The compensation method effectively implements direct voltage fluctuation compensation of a parallel-type mixed new topology, and is particularly suitable for voltage compensation in the case of an alternating current system fault.

Description

DC voltage compensation method for parallel hybrid multilevel converter

Technical field

The invention relates to a DC voltage compensation method, in particular to a DC voltage compensation method for a parallel hybrid multilevel converter.

Background technique

The flexible DC transmission system is a DC transmission system based on a voltage source converter with a fully controlled device (IGBT) as its core. It has broad applications in wind power access, grid interconnection, urban power supply, and island power supply. Prospects, since the development of flexible DC technology, have experienced two main technical routes, namely two-level technology and modular multi-level technology, the latter has low switching frequency and low loss, and has now become the main development of flexible DC transmission technology. trend.

The converter of the flexible DC transmission system is the core component of the whole system, which is used to realize the conversion of AC and DC electric quantities. The current modular multi-level topology consists of 6 bridge arms, each of which consists of multiple sub-modules ( Submodule, SM) are connected in series. The submodule is a half bridge (or full bridge) structure composed of two (or four) IGBTs (Insulated Gate Bipolar Transistors) and capacitors, as shown in Figure 1. The working principle is that the IGBT device is turned on and off, the capacitor is put into the circuit or the circuit is exited, and the input and exit of the plurality of sub-modules are controlled reasonably, so that a stable voltage can be formed on the AC/DC side, thereby forming a stable system working point. Perform power transfer.

However, the existing modular multi-level technology uses multiple sub-modules to be superimposed, which requires a large number of capacitors and IGBT devices. If a full-bridge topology is used, the number of devices required is more, the price is very expensive, and the footprint is due to the large-capacity capacitor. The existence of this has increased dramatically. This is unfavorable for the compact design of offshore platforms that are very important in offshore wind power access applications. It has become a very important research direction to adopt new topologies to reduce system cost.

In 2010, Alstom proposed a new hybrid multilevel converter topology. The parallel single-phase topology is shown in Figure 2. The topology is cascaded by a sub-module cascaded by half-bridge submodules. The switching action forms the absolute value of the sinusoidal voltage |Uc|, and the |Uc| is led to the AC side to form the required AC voltage through the switching action of the H-bridge full control device structure 2.

The structure shown in Figure 3 is a three-phase power transmission structure, which is formed by connecting the above single-phase structures in series, respectively. In order to effectively realize power transmission, the new topology is greatly optimized, the number of devices and capacitors required is small, and the economic advantage is very large. In the article "A New Hybrid Voltage-Sourced Converter for HVDC Power Transmission" disclosed by DRTRAINER et al. in 2010, a new topology was proposed, and the parallel hybrid new topology proposed a new way for flexible DC to develop in various fields. method. The new topology has a relatively low price and low loss, enabling high-capacity applications and broad application prospects.

However, there is a serious coupling between the AC and DC voltages of the new topology, which causes the DC voltage of the flexible DC system to fluctuate with different operating conditions. Especially in the case of AC system failure, the DC voltage of the converter drops sharply. , can not operate normally, which is contrary to the operational requirements of the flexible DC system, that is, the DC voltage is stable and the AC fault ride-through capability is inconsistent.

US2012/0069610A1 discloses the topology, but does not provide a corresponding DC voltage compensation mechanism. In the article "A Low Loss Modular Multilevel Voltage Source Converter for HVDC Power Transmission and Reactive Power Compensation", a three-harmonic based The voltage compensation method of wave voltage injection, but this method cannot be directed to system failure, especially voltage drop under asymmetric fault.

Summary of the invention

In view of the deficiencies of the prior art, the object of the present invention is to provide a DC voltage compensation method for a parallel hybrid multilevel converter, by which the DC voltage fluctuation compensation of the parallel hybrid new topology can be effectively realized, especially applicable. Voltage compensation in the event of an AC system failure.

The object of the present invention is achieved by the following technical solutions:

The invention provides a DC voltage compensation method for a parallel hybrid type multi-level converter, wherein the converter is a three-phase power transmission structure, and the three-phase power transmission structure is composed of a parallel type single-phase structure connected in series; The single-phase structure is composed of a parallel H-bridge full control device structure and a sub-module cascade structure, and the H-bridge full-control device structure is composed of a cascade structure of four full-control devices; the sub-module cascade structure is composed of a half bridge Modules are connected in series;

The improvement is that the compensation method includes the following working mode: adding an isolation time during the normal operation mode of the converter, freely controlling the cascade structure of the sub-module during the isolation time, and outputting the required compensation voltage to realize the DC voltage. For the compensation of the deficiency, the compensation ratio in the three-phase power transmission structure adjusts the three-phase distribution ratio according to the demand.

Further, the method turns off one of the bridge arms in the normal opening of the H-bridge full control device structure in advance during the isolation time, and opens the bridge arm in the opposite direction to form the upper two bridge arms simultaneously open or the lower two bridge arms At the same time, the working conditions are opened, forming an AC short circuit and isolating the AC and DC side.

Further, the isolation time is a fixed length or an unfixed length; the starting position and the ending position are freely determined as needed.

Further, the turn-off of the bridge arm in the normally open state and the turn-on of the bridge arm in the reverse direction are simultaneous or different.

Further, in the case of a three-phase power transmission structure, the total output DC voltage is superposed by three parallel single-phase structures, and the total DC voltage deficiency is allocated to three parallel single phases according to the same or different ratios according to requirements. Structurally, the compensation of the total DC voltage is achieved.

Further, in the DC voltage compensation process, when the original DC voltage is higher than the rated value, the output compensation voltage is lower than the normal output value or the output zero voltage; when the original DC voltage is low relative to the rated value, the output is The compensation voltage is higher than the normal output value.

Compared with the prior art, the beneficial effects achieved by the present invention are:

1. The DC voltage compensation method provided by the present invention compensates the DC voltage during the time by adding a small isolation time, and solves the DC voltage fluctuation problem of the parallel hybrid topology;

2. The DC voltage compensation method provided by the present invention has only a small distortion of the output waveform of the AC voltage, and has little effect on the AC output voltage quality;

3. The method is also applicable to DC voltage deficiency compensation under AC system failure, effectively implementing system AC fault traversal.

DRAWINGS

Figure 1 (a) is a structural diagram of a half bridge submodule;

Figure 1 (b) is a structural diagram of the full bridge submodule;

Figure 2 is a parallel type single-phase structure diagram;

3 is a structural diagram of a three-phase power transmission provided by the present invention;

4 is a schematic diagram of an operation mode of the compensation method provided by the present invention.

detailed description

The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

The invention provides a DC voltage compensation method for a parallel hybrid type multi-level converter, wherein the converter is a three-phase power transmission structure, and the three-phase power transmission structure is composed of a parallel type single-phase structure connected in series; The single-phase structure is composed of a parallel H-bridge full control device structure and a sub-module cascade structure, and the H-bridge full-control device structure is composed of a cascade structure of four full-control devices, and the sub-module cascade structure is composed of a half bridge The submodules are connected in series. The specific implementation of the method is as follows:

As shown in Figure 3, in normal operation mode, when S1 and S4 are turned on, the output voltage of module 1 is led to the AC side to form an AC positive half wave; when S2 and S3 are turned on, the output voltage of module 1 is reversed. Lead to the AC side, forming an AC negative half-wave, S1, S4 and S2, S3 form a complementary pair, exchange the switch state at the voltage zero-crossing point, thus completing the power conversion.

The system operation mode added to the compensation scheme is shown in Figure 4. The scheme works as follows:

(1) When S1, S4 are turned on, before the zero crossing point, S4 (or S1) is turned off in advance, and S2 (or S3) is turned on, so that the AC system is short-circuited by the upper two bridge arms or the lower two bridge arms. Cause AC and DC isolation time;

(2) During the isolation time, the DC side sub-module cascade structure compensates for the DC voltage deficiency by outputting a certain voltage. When the original DC voltage is higher than the rated value, the output voltage of the sub-module cascade structure is low during the isolation time. The normal value or the output zero voltage; when the original DC voltage is lower than the rated value, the output voltage of the sub-module cascade structure in the isolation time is higher than the normal value;

(3) When the DC voltage is compensated, turn off S1 (or S4) and turn on S3 (or S2). At this time, S2 and S3 are turned on, and the output voltage of the sub-module cascade structure returns to the normal output voltage, and the AC side continues to output. Normal negative half-wave AC waveform.

The same reason:

1 When S2 and S3 are turned on, before the zero crossing point, S2 (or S3) is turned off in advance, and S4 (or S1) is turned on, so that the AC system forms a short circuit through the next two bridge arms or the upper two bridge arms, resulting in the intersection. DC isolation time;

2 During the isolation time, the DC side sub-module cascade structure 1 compensates for the DC voltage deficiency by outputting a certain voltage. When the original DC voltage is higher than the rated value, the output voltage of the sub-module cascade structure is lower than the isolation time. Normal value or output zero voltage; when the original DC voltage is lower than the rated value, the output voltage of the sub-module cascade structure in the isolation time is higher than the normal value;

3 When the DC voltage is compensated, turn off S3 (or S2) and turn on S1 (or S4). At this time, S1 and S4 are turned on, and the output voltage of the sub-module cascade structure returns to the normal output voltage, and the AC side continues to output normal. Positive half wave AC waveform.

The isolation time is a fixed length or an unfixed length; its starting position and ending position are freely determined as needed.

The closing of the bridge arm in the normal opening and the opening of the bridge arm in the opposite direction are simultaneous or different.

The three-phase distribution ratio is adjusted according to different requirements, so that the three-phase power transmission structure is symmetrically distributed or different distribution coefficients are used. In the case of a three-phase structure, the total output DC voltage is superposed by three single phases, and the total DC voltage deficiency can be distributed to three single-phase structures in the same or different proportions according to requirements, thereby realizing the total DC voltage. make up.

Finally, it should be noted that the above embodiments are only for explaining the technical solutions of the present invention and are not limited thereto, although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be The invention is to be construed as being limited by the scope of the appended claims.

Claims

A DC voltage compensation method for a parallel hybrid type multi-level converter, wherein the converter is a three-phase power transmission structure, and the three-phase power transmission structure is composed of a parallel type single-phase structure in series; a parallel type single-phase structure The parallel H-bridge full control device structure and the sub-module cascade structure, the H-bridge full-control device structure is composed of a cascade structure of four full-control devices; the sub-module cascade structure is composed of a half-bridge sub-module connected in series ;

The compensation method comprises the following working mode: adding an isolation time in a normal operation mode of the converter, freely controlling the cascade structure of the sub-module in the isolation time, and outputting a required compensation voltage to realize a DC voltage deficiency. Compensation, the compensation ratio in the three-phase power transmission structure adjusts the three-phase distribution ratio according to demand.
The DC voltage compensation method according to claim 1, wherein the method forms a bridge arm in the normal opening of the H-bridge full control device structure in advance during the isolation time, and opens the bridge arm in the opposite direction to form a bridge arm. The upper two bridge arms are opened at the same time or the lower two bridge arms are simultaneously opened, forming an AC short circuit and isolating the AC and DC sides.
The DC voltage compensation method according to claim 2, wherein the isolation time is a fixed length or an unfixed length; and a start position and an end position are freely determined as needed.
The DC voltage compensation method according to claim 2, wherein the switching of the normally open middle bridge arm and the opening of the reverse direction bridge arm are simultaneous or different.
The DC voltage compensation method according to claim 1, wherein in the case of the three-phase power transmission structure, the total output DC voltage is superposed by three parallel single-phase structures, and the total DC voltage deficiency is according to requirements. The same or different proportions are distributed to three parallel-type single-phase structures to compensate for the total DC voltage.
The DC voltage compensation method according to claim 1, wherein in the DC voltage compensation process, when the original DC voltage is higher than the rated value, the output compensation voltage is lower than the normal output value or the output zero voltage; When the DC voltage is low relative to the rated value, the output compensation voltage is higher than the normal output value.