CN-121984081-A - Parallel control method for double M3C converters of flexible low-frequency power transmission system

CN121984081ACN 121984081 ACN121984081 ACN 121984081ACN-121984081-A

Abstract

The invention discloses a parallel control method of two M3C converters for a flexible low-frequency power transmission system, which comprises two M3C converter stations connected in parallel, wherein the two converters adopt master-slave control, one of the two converters is set as a master converter, the other is a slave converter, a power frequency side three-phase power supply is connected with a controlled current source in a side-to-side mode, and the other is a new energy source equivalent to a controlled current source in a side-to-side mode. The invention can effectively realize high-precision and fast dynamic response current sharing control and remarkably improve the parallel operation performance and reliability of the flexible low-frequency power transmission system in the scenes of new energy collection and delivery and the like.

Inventors

LI YU
SONG PENG
HUAI QING
JI YIRUN
YUAN QIAN
YUAN WENQIAN
JIE ZIYAO
ZHENG LINZI
HAO ZHEN

Assignees

国网冀北电力有限公司电力科学研究院

Dates

Publication Date: 20260505
Application Date: 20251215

Claims (10)

1. The parallel control method for the double M3C converters of the flexible low-frequency power transmission system is characterized by comprising the following steps of: A. determining a master-slave control strategy according to the parallel interaction characteristics of the two M3C; B. the power frequency sides of the two M3C converters are all controlled by adopting a grid-connected control method, wherein the inner ring is an alternating current control, and the outer ring is a stator module average capacitor voltage control and a fixed reactive power control; C. an alternating current voltage control strategy is adopted on the M3C low-frequency side of the master station; D. The M3C low frequency side of the slave station adopts a current tracking control strategy.
2. The parallel control method of the two M3C converters for the flexible low-frequency power transmission system according to claim 1, wherein in the step A, the two M3C converters operate according to a preset power proportion, the preset powers of the two converters are the same, measures are required to be taken to ensure that the power flowing through the two converters is kept the same under the same voltage, and master-slave control is adopted for the two M3C converters according to the parallel current sharing characteristic of the two M3C converters.
3. The parallel control method for the double M3C converters of the flexible low frequency power transmission system according to claim 2, wherein the step A is characterized in that a master-slave control strategy is determined according to the parallel interaction characteristics of the double M3C converters, and the specific process is as follows: firstly, taking a converter as a main unit, and normally operating according to a preset state of a system, defining the converter as a main station, wherein the converter station is required to have certain margin regulation capacity; then, another converter is used as a slave unit, a slave station is defined, and the internal control quantity is adjusted in real time according to the running state of the master station converter; And finally, the low frequency sides of the two converters of the master station and the slave station respectively adopt different control modes.
4. The method for parallel control of two M3C converters for a flexible low frequency power transmission system as in claim 3, wherein said master station employs an AC voltage control strategy and said slave station employs a current tracking control strategy.
5. The parallel control method of a dual M3C converter for a flexible low frequency power transmission system as defined in claim 1, wherein one of the targets of the intermediate frequency side control strategy design in step B is to determine a control variable command value , To make the output variable , Tracking its instruction value , 。
6. The method for parallel control of two M3C converters in a flexible low frequency power transmission system as in claim 5, wherein said method comprises determining a control variable command value , To make the output variable , Tracking its instruction value , The specific process is as follows: Firstly, collecting the signals of current and voltage at the power frequency side, and obtaining the phase angle through a phase-locked loop And the acquired power frequency side current and voltage signals are obtained through dq conversion , ; Then, generating by the total capacitance voltage control loop , ; Finally, will , , , Input into an inner loop current controller to obtain a three-phase inner potential reference value of the power frequency side of the M3C converter 。
7. The parallel control method of the two M3C converters for the flexible low-frequency power transmission system of claim 1, wherein the low-frequency side of the M3C converter of the main station in the step C adopts an alternating voltage control strategy, and the alternating voltage control of the low-frequency side of the M3C converter is mainly realized by two parts of closed loop control and open loop control.
8. The parallel control method for the double M3C converters of the flexible low frequency power transmission system as set forth in claim 7, wherein said closed loop control and open loop control are as follows: Firstly, when M3C has no new energy access, no grid connection is performed at the moment, only grid connection is performed, and reference voltage is directly given through open loop control ; Then, when new energy is connected, the actual power fluctuates and the voltage drop occurs on the circuit, so that the closed-loop control is added to generate additional compensation voltage , And generated by open loop control Adding to obtain low-frequency side bus reference voltage 。
9. The parallel control method of the double M3C converters for the flexible low frequency power transmission system of claim 1, wherein the M3C low frequency side of the slave station in the step D adopts a current tracking control strategy, and the specific process is as follows: firstly, collecting low-frequency side currents of a master machine and a slave machine respectively; then, the collected three-phase currents of the master-slave M3C converter are respectively converted into an alpha beta 0 static coordinate system through Clark conversion to obtain an alpha beta axis current component of the master-station M3C converter , And an alpha beta-axis current component from an M3C converter ; Then, the PI controller outputs additional compensation voltage ; Finally, the control is generated by open loop with the host Adding to obtain low-frequency side bus reference voltage 。
10. The parallel control method for the double M3C converters of the flexible low-frequency power transmission system of claim 1, wherein the current tracking control strategy is used for realizing no static difference tracking of slave low-frequency side current to host low-frequency side current, so that the purpose that the double M3C converters need to be subjected to current sharing is achieved.

Description

Parallel control method for double M3C converters of flexible low-frequency power transmission system Technical Field The invention belongs to the field of parallel control methods of two M3C converters for a flexible low-frequency power transmission system. Background Along with the transformation of global energy structures to clean and low-carbonization, the development and utilization of large-scale new energy sources such as offshore wind power, deep open sea renewable energy sources and the like are increasingly important in the development of an electric power system. However, the conventional power frequency alternating current transmission (50 Hz) faces the problems of large capacitive charging current, limited transmission capacity and the like in long-distance cable transmission, and limits the application of the system in a medium-distance and long-distance new energy collection and transmission scene. To solve this problem, low frequency transmission techniques (Low-Frequency AC Transmission, LFAC) have been developed. Compared with flexible direct current transmission (VSC-HVDC), the low frequency transmission technology not only has the inherent voltage regulation and fault protection capabilities of an alternating current system, but also can avoid using expensive direct current sea cables, and reduces the overall cost of the system. Particularly, in recent years, with the development of the technology of a modularized multi-level matrix converter (Modular Multilevel Matrix Converter, M3C), a flexible low-frequency power transmission system has made a significant breakthrough in terms of controllability, reliability and economy. M3C is used as a topological structure of direct AC-AC conversion, can realize high-efficiency conversion between power frequency and low frequency without an intermediate DC link, has the advantages of low harmonic content, quick dynamic response, high modularization degree and the like, and has been successfully verified in projects such as Zhejiang Taizhou kV/50MVA demonstration projects. However, with the continuous expansion of the capacity of new energy stations such as offshore wind power, the transmission capacity of a single M3C has been difficult to meet the high-power delivery demand. To improve the system capacity and reliability, the parallel operation of a plurality of M3C converters becomes a necessary choice. But the parallel system improves the capacity and simultaneously brings key technical challenges such as loop current inhibition, power sharing, dynamic response coordination and the like. When two M3C are connected in parallel to the same bus, loop current between systems is easy to generate due to factors such as control parameter difference, inconsistent line impedance or abrupt load change. The circulation not only can increase the loss of a switching device and a capacitor and influence the system efficiency, but also can cause the problems of local overload, equipment damage, even system instability and the like. At present, research on an M3C parallel system is still in a starting stage. Although there has been a certain accumulation in the parallel connection of conventional Modular Multilevel Converters (MMCs), M3C is more complicated to control in parallel due to its unique matrix structure and ac-dc conversion characteristics. Particularly in a low-frequency operation environment, the control strategy based on the power frequency design is difficult to directly apply, and the circulation characteristic, the power distribution mechanism and the fault collaborative ride-through capability are required to be studied deeply. Therefore, aiming at the operation characteristics of the double M3C parallel system, a parallel control method which can effectively inhibit circulation, realize accurate power distribution and have high reliability and dynamic stability is developed, and the parallel control method becomes a key point for the flexible low-frequency power transmission technology to be applied to engineering and large scale. Based on the advanced study of modeling, control and fault ride-through strategies of a single M3C, the interactive characteristics and the cooperative mechanism of a double M3C parallel system are further explored, a hybrid control strategy combining advantages of master-slave and droop control is provided, current sharing performance and operation efficiency of the system are improved, and technical support is provided for efficient collection and delivery of high-capacity new energy. Disclosure of Invention The invention provides a parallel control method for two M3C converters of a flexible low-frequency power transmission system, which aims to solve the problems existing in the prior art. The technical scheme of the invention is that the parallel control method of the double M3C converters for the flexible low-frequency power transmission system comprises the following steps: A. determining a master-slave cont