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

Abstract

The invention discloses a double-M3C converter parallel control method for a flexible low-frequency power transmission system, the flexible low-frequency power transmission system comprises two parallel M3C converter stations, the two converters adopt master-slave control, one M3C converter is set as a master converter, the other M3C converter is set as a slave converter, the power frequency side is connected with a three-phase power supply, and the two M3C converter stations are connected in parallel. A low-frequency side receives new energy equivalent to a controlled current source. The method comprises the following steps: determining a master-slave control strategy according to parallel interaction characteristics of the two M3Cs; a grid-connected control method is adopted at the power frequency side of each M3C converter, an inner ring is controlled by alternating current, and an outer ring is controlled by stator module average capacitance voltage and fixed reactive power; the M3C low-frequency side of the master station adopts an alternating-current voltage control strategy; and the M3C low-frequency side of the slave station adopts a current tracking control strategy. According to the invention, high-precision and fast-dynamic-response current-sharing control can be effectively realized, and the parallel operation performance and reliability of the flexible low-frequency power transmission system in scenes of new energy collection and transmission and the like can be remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of power transmission, specifically relating to a method for parallel control of two M3C converters in a flexible low-frequency power transmission system. Background Technology

[0002] As the global energy structure shifts towards cleaner and lower-carbon energy sources, the development and utilization of large-scale new energy sources such as offshore wind power and deep-sea renewable energy are increasingly becoming a focus of power system development. However, traditional industrial frequency AC transmission (50Hz) faces challenges such as large capacitive charging current and limited transmission capacity in long-distance cable transmission, restricting its application in medium- and long-distance new energy aggregation and transmission scenarios. To address this issue, low-frequency AC transmission (LFAC) technology has emerged. Compared to flexible direct current transmission (VSC-HVDC), LFAC technology not only possesses the inherent voltage regulation and fault protection capabilities of AC systems but also avoids the use of expensive DC submarine cables, reducing the overall system cost. In particular, in recent years, with the development of Modular Multilevel Matrix Converter (M3C) technology, flexible low-frequency transmission systems have achieved significant breakthroughs in controllability, reliability, and economy. M3C, as a direct AC-AC converter topology, can achieve efficient conversion between power frequency and low frequency without the need for an intermediate DC link. It has advantages such as low harmonic content, fast dynamic response, and high modularity, and has been successfully verified in projects such as the 35kV / 50MVA demonstration project in Taizhou, Zhejiang.

[0003] However, with the continuous expansion of the capacity of new energy power plants such as offshore wind power, the transmission capacity of a single M3C converter is no longer sufficient to meet the demand for high-power transmission. To improve system capacity and reliability, parallel operation of multiple M3C converters has become an inevitable choice. However, while parallel systems increase capacity, they also bring key technical challenges such as circulating current suppression, power sharing, and dynamic response coordination. When two M3Cs are connected in parallel to the same bus, circulating currents can easily occur between the systems due to differences in control parameters, inconsistent line impedances, or sudden load changes. Circulating currents not only increase the losses of switching devices and capacitors, affecting system efficiency, but may also cause local overloads, equipment damage, or even system instability. Currently, research on parallel M3C systems is still in its early stages. Although there has been some accumulation in the parallel operation of conventional modular multilevel converters (MMCs), the parallel control of M3Cs is more complex due to their unique matrix structure and AC-AC direct conversion characteristics. Especially in low-frequency operating environments, traditional control strategies based on power frequency design are difficult to apply directly, and the circulating current characteristics, power distribution mechanism, and fault cooperative ride-through capability all need to be studied in depth.

[0004] Therefore, developing a parallel control method that can effectively suppress circulating current, achieve precise power distribution, and possess high reliability and dynamic stability, tailored to the operational characteristics of a dual-M3C parallel system, has become crucial for the engineering and large-scale application of flexible low-frequency transmission technology. Based on previous in-depth research on single-M3C modeling, control, and fault ride-through strategies, this paper further explores the interaction characteristics and collaborative mechanisms of a dual-M3C parallel system. A hybrid control strategy combining the advantages of master-slave and droop control is proposed, aiming to improve the system's current sharing performance and operating efficiency, providing technical support for the efficient collection and transmission of large-capacity renewable energy. Summary of the Invention

[0005] This invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a method for parallel control of two M3C converters in a flexible low-frequency power transmission system.

[0006] The technical solution of this invention is: a parallel control method for two M3C converters in a flexible low-frequency power transmission system, comprising the following steps: A. Determine the master-slave control strategy based on the parallel interaction characteristics of the two M3C units; B. Both M3C converters on the power frequency side adopt grid-connected control methods. The inner loop is AC current control, and the outer loop is stator module average capacitor voltage control and constant reactive power control. C. The low-frequency side of the M3C in the main station adopts an AC voltage control strategy; D. The low-frequency side of the M3C at the slave station adopts a current tracking control strategy.

[0007] Furthermore, in step A, the two M3C converters operate according to a preset power ratio. The preset power of the two converters is the same. Under the same voltage, measures need to be taken to ensure that the power flowing through the two converters remains the same. Based on the current sharing characteristics of the two M3C converters in parallel, master-slave control is adopted for the two M3C converters.

[0008] Furthermore, step A determines the master-slave control strategy based on the parallel interaction characteristics of the two M3C units. The specific process is as follows: First, a converter is taken as the main unit and operates normally according to the system's preset state. It is defined as the master station, and the converter station should have a certain amount of spare regulation capacity. Then, another converter is used as a slave unit, defined as a slave station, and the internal control quantities are adjusted in real time according to the operating status of the master station converter. Finally, different control methods are adopted for the low-frequency side of the two converters at the master station and slave station.

[0009] Furthermore, the master station adopts an AC voltage control strategy, while the slave station adopts a current tracking control strategy.

[0010] Furthermore, one of the goals of the intermediate frequency side control strategy design in step B is to determine the command values ​​of the control variables. , Make the output variable , Track its instruction value , .

[0011] Furthermore, determine the command value of the control variable. , Make the output variable , Track its instruction value , The specific process is as follows: First, the current and voltage signals on the power frequency side are acquired, and the phase angle is obtained through a phase-locked loop. The collected power frequency side current and voltage signals are obtained through dq transformation. , ; Then, generated through the total capacitor voltage control loop. , ; Finally, , , , The input is fed into the inner loop current controller to obtain the reference value of the three-phase internal potential on the power frequency side of the M3C converter. .

[0012] Furthermore, in step C, the AC voltage control strategy adopted on the low-frequency side of the M3C converter in the main station is mainly achieved through two parts: closed-loop control and open-loop control.

[0013] Furthermore, the specific processes of the closed-loop control and open-loop control are as follows: First, when there is no new energy source connected to the M3C, there is no grid connection; it is merely a grid connection, and the reference voltage is directly provided through open-loop control. ; Then, when new energy sources are connected, the actual power will fluctuate, and there will be a voltage drop on the line. Therefore, closed-loop control is added to generate additional compensation voltage. , and open-loop control generated Adding them together, we obtain the low-frequency side bus reference voltage. .

[0014] Furthermore, step D employs a current tracking control strategy on the low-frequency side of the M3C at the substation, as detailed below: First, the low-frequency current of the host and slave devices is collected separately; Then, the three-phase currents of the master and slave M3C converters were collected and transformed to the αβ0 stationary coordinate system using Clark transformation to obtain the αβ axis current components of the master station M3C converter. , and the αβ axis current components from the M3C converter ; Then, an additional compensation voltage is output through the PI controller. ; Finally, the open-loop control generated by the host computer Adding them together, we obtain the low-frequency side bus reference voltage. .

[0015] Furthermore, the current tracking control strategy enables zero steady-state error tracking of the slave low-frequency side current to the master low-frequency side current, thereby achieving the current sharing objective required by the two M3C converters.

[0016] The beneficial effects of this invention are as follows: The key technical feature of this invention lies in constructing a high-precision master-slave control architecture based on current tracking to achieve high-performance current sharing in parallel systems. In this architecture, one M3C converter is designated as the master, employing a low-frequency AC voltage control strategy to establish and stabilize the low-frequency bus voltage; the other M3C converter acts as the slave, employing a current tracking control strategy. By acquiring the low-frequency currents of both the master and slave converters, a PIR controller is used to perform zero steady-state error tracking of the αβ current component of the master converter in the αβ0 coordinate system. This invention effectively suppresses inter-system circulating currents caused by differences in control parameters and uneven line impedance through direct control of the current loop, achieving precise power sharing between the two M3C converters on the low-frequency side.

[0017] This invention achieves effective suppression of circulating current and precise power distribution between parallel systems through high-precision current tracking control, significantly reducing additional losses caused by circulating current and uneven current, and improving the overall operating efficiency of the system.

[0018] The control strategy of this invention responds rapidly during dynamic processes, ensuring that the output current of the two converters remains highly consistent under conditions such as power ramp-up and load surges, thus avoiding power oscillations and voltage instability and enhancing the dynamic stability and reliability of the system.

[0019] The parallel control method of this invention provides an effective solution for the large-capacity expansion of flexible low-frequency power transmission systems. Through reliable operation of dual-machine parallel connection, it significantly improves the system capacity and reliability of new energy collection and transmission, and has important engineering application value. Attached Figure Description

[0020] Figure 1 This is a flowchart of the method of the present invention; Figure 2 This is a schematic diagram of the M3C topology of the present invention; Figure 3 This is a topology diagram of the dual M3C parallel system of the present invention; Figure 4 This is the overall control architecture diagram of the dual M3C parallel system of the present invention; Figure 5 This is a block diagram of the master-slave control of dual M3C converters in a flexible low-frequency power transmission system according to the present invention, wherein (a) is the master control and (b) is the slave control; Figure 6 This is a simulation result diagram verifying the effectiveness of the master-slave control implementation of this invention. Detailed Implementation

[0021] The present invention will now be described in detail with reference to the accompanying drawings and embodiments: like Figures 1 to 6 As shown, the parallel control method for two M3C converters in a flexible low-frequency transmission system includes the following steps: A. Determine the master-slave control strategy based on the parallel interaction characteristics of the two M3C units; B. Both M3C converters on the power frequency side adopt grid-connected control methods. The inner loop is AC current control, and the outer loop is stator module average capacitor voltage control and constant reactive power control. C. The low-frequency side of the M3C in the main station adopts an AC voltage control strategy; D. The low-frequency side of the M3C at the slave station adopts a current tracking control strategy.

[0022] In step A, the two M3C converters operate according to the preset power ratio. The preset power of the two converters is the same. Under the same voltage, measures need to be taken to ensure that the power flowing through the two converters remains the same. Based on the current sharing characteristics of the two M3C converters in parallel, master-slave control is adopted for the two M3C converters.

[0023] Step A determines the master-slave control strategy based on the parallel interaction characteristics of the two M3C units. The specific process is as follows: First, a converter is taken as the main unit and operates normally according to the system's preset state. It is defined as the master station, and the converter station should have a certain amount of spare regulation capacity. Then, another converter is used as a slave unit, defined as a slave station, and the internal control quantities are adjusted in real time according to the operating status of the master station converter. Finally, different control methods are adopted for the low-frequency side of the two converters at the master station and slave station.

[0024] The master station adopts an AC voltage control strategy, and the slave station adopts a current tracking control strategy.

[0025] One of the goals of the intermediate frequency side control strategy design in step B is to determine the command values ​​of the control variables. , Make the output variable , Track its instruction value , .

[0026] Determine the command value of the control variable , Make the output variable , Track its instruction value , The specific process is as follows: First, the current and voltage signals on the power frequency side are acquired, and the phase angle is obtained through a phase-locked loop. The collected power frequency side current and voltage signals are obtained through dq transformation. , ; Then, generated through the total capacitor voltage control loop. , ; Finally, , , , The input is fed into the inner loop current controller to obtain the reference value of the three-phase internal potential on the power frequency side of the M3C converter. .

[0027] In step C, the AC voltage control strategy adopted by the low-frequency side of the M3C converter in the main station is mainly achieved through two parts: closed-loop control and open-loop control.

[0028] The specific processes of closed-loop control and open-loop control are as follows: First, when there is no new energy source connected to the M3C, there is no grid connection; it is merely a grid connection, and the reference voltage is directly provided through open-loop control. ; Then, when new energy sources are connected, the actual power will fluctuate, and there will be a voltage drop on the line. Therefore, closed-loop control is added to generate additional compensation voltage. , and open-loop control generated Adding them together, we obtain the low-frequency side bus reference voltage. .

[0029] Step D employs a current tracking control strategy on the low-frequency side of the M3C at the slave station. The specific process is as follows: First, the low-frequency current of the host and slave devices is collected separately; Then, the three-phase currents of the master and slave M3C converters were collected and transformed to the αβ0 stationary coordinate system using Clark transformation to obtain the αβ axis current components of the master station M3C converter. , and the αβ axis current components from the M3C converter ; Then, an additional compensation voltage is output through the PI controller. ; Finally, the open-loop control generated by the host computer Adding them together, we obtain the low-frequency side bus reference voltage. .

[0030] The current tracking control strategy enables zero steady-state error tracking of the slave low-frequency side current to the master low-frequency side current, thereby achieving the current sharing objective required by the two M3C converters. Example

[0031] The system topology of the M3C converter is illustrated in the accompanying diagram. Figure 2 As shown, the entire system comprises a primary three-phase power supply, a secondary three-phase power supply, and nine bridge arms, connecting two three-phase AC systems of different frequencies. Each bridge arm consists of one bridge arm inductor L and several full-bridge submodules (FBSMs) connected in series. The internal resistance of the bridge arm inductors is relatively small and can generally be ignored. The primary frequency is... The three-phase voltages are respectively The three-phase currents are respectively The secondary side frequency is The three-phase voltages are respectively The three-phase currents are respectively The bridge arm voltage is expressed as... The bridge arm current is expressed as . This represents the common-mode voltage on both sides. In the M3C system topology, three bridge arms connected to the same phase input / output power supply can be considered as one sub-converter.

[0032] Specifically, the converter control method of the dual M3C parallel system adopts a similar control method as that of the single-unit system. In order to achieve stable and efficient operation of the parallel system, under steady-state conditions, it is necessary to ensure that the active and reactive power outputs of each converter are distributed in a predetermined ratio through an effective control strategy. In this context, the master-slave control architecture shows significant necessity and superiority.

[0033] Specifically, such as Figure 3 As shown, the power frequency side of this system is connected to the 220kV / 50Hz power grid via a transformer, while the low frequency side has a frequency of 20Hz and is connected to the renewable energy power station in parallel. The system includes two M3C converters, one as the master and the other as the slave.

[0034] Both M3C units employ traditional grid-connected control methods on their power frequency side. The inner loop uses AC current control, while the outer loop uses stator module average capacitor voltage control and constant reactive power control. The overall control architecture is as follows: Figure 5 As shown. One of the goals of designing a power frequency side AC current controller is to determine the command values ​​of the control variables. , Make the output variable , Track its instruction value , .

[0035] First, the M3C acquires the main circuit voltage and current signals as feedback signals for the control system. The phase-locked loop then tracks the voltage phase based on the acquired voltage signal. ; Then, based on the phase-locked loop... Parker transformation is performed on the voltage and current acquisition signals to obtain the dq-axis components of the voltage and current. The total capacitor voltage controller is responsible for maintaining a stable total capacitor voltage, ensuring that the sum of the capacitor voltages of all submodules remains at the set value, preventing voltage fluctuations from affecting system performance, and generating a current reference value. The inner-loop controller uses a proportional-integral (PI) controller to achieve closed-loop tracking of AC current and outputs the reference value of the three-phase internal potential on the M3C power frequency side. ; Finally, the reference value of the three-phase internal potential on the M3C power frequency side was used. Reference value of three-phase internal potential of the MMC circulating current equivalent circuit output by the circulating current suppression controller. and low-frequency side bus voltage reference value Generate bridge arm voltage reference values.

[0036] Specifically, the two M3C units use different low-frequency side control methods, namely master control and slave control. The core of the master M3C's control is low-frequency side voltage control, which combines open-loop setting and closed-loop compensation to generate a stable voltage reference.

[0037] First, when no new energy sources are connected, the M3C is not grid-connected, but merely forming a grid. Therefore, it directly provides the reference voltage through open-loop control. ; Secondly, considering that the actual power will fluctuate and there will be voltage drop on the line after the connection of new energy sources, closed-loop control is added to transform both the reference value and the actual value of the low-frequency side bus voltage to the αβ coordinate system. Then, the low-frequency side bus voltage is tracked in a closed loop to its reference value, and an additional compensation voltage is generated. A 20Hz resonant controller is used to achieve closed-loop zero-steady-state-error regulation of the correction current; Finally, After inverse transformation of αβ back to the abc coordinate system, and generated by open-loop control Adding them together, we obtain the low-frequency side bus reference voltage. This is used to adjust the low-frequency bus voltage and stabilize it at the target value.

[0038] The core of M3C's control is current tracking, which involves separately acquiring the low-frequency current of the master and slave devices, transforming it to the αβ0 coordinate system, and obtaining the αβ component of the master device's low-frequency side. , and slave low-frequency side αβ component Then, the αβ components are tracked without steady-state error by the PIR controller, thereby realizing the current sharing control of the dual M3C low-frequency transmission system.

[0039] To demonstrate the feasibility and effectiveness of the coordinated control strategy of this system, this invention builds a system based on Matlab / Simulink, as follows: Figure 4 The diagram shows a parallel system model with two M3C units. The mains frequency side is powered by a 220kV mains frequency power supply, and the low-frequency side is connected to a new energy module. The M3C units are connected to the power line via transformers on both sides. The system simulation parameters are shown in Table 1.

[0040]

[0041] Figure 6 The diagram shows the low-frequency current waveform of a dual M3C parallel system under master-slave control. It can be seen that when the parallel system is fully operational, the current curve of M3C2 closely follows the waveform of M3C1, and the two current curves almost completely overlap, effectively achieving power distribution and current sharing, and effectively suppressing circulating current between systems. During power ramp-up, both curves maintain a stable upward trend without drastic fluctuations, and the response speed meets the system requirements.

[0042] Thus, the task of proposing a parallel control method for dual M3C converters in a flexible low-frequency power transmission system, as proposed in this invention, has been fully completed.

Claims

1. A parallel control method for two M3C converters in a flexible low-frequency transmission system, characterized in that: Includes the following steps: A. Determine the master-slave control strategy based on the parallel interaction characteristics of the two M3C units; B. Both M3C converters on the power frequency side adopt grid-connected control methods. The inner loop is AC current control, and the outer loop is stator module average capacitor voltage control and constant reactive power control. C. The low-frequency side of the M3C in the main station adopts an AC voltage control strategy; D. The low-frequency side of the M3C at the slave station adopts a current tracking control strategy.

2. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 1, characterized in that: In step A, the two M3C converters operate according to the preset power ratio. The preset power of the two converters is the same. Under the same voltage, measures need to be taken to ensure that the power flowing through the two converters remains the same. Based on the current sharing characteristics of the two M3C converters in parallel, master-slave control is adopted for the two M3C converters.

3. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 2, characterized in that: Step A determines the master-slave control strategy based on the parallel interaction characteristics of the two M3C units. The specific process is as follows: First, a converter is taken as the main unit and operates normally according to the system's preset state. It is defined as the master station, and the converter station should have a certain amount of spare regulation capacity. Then, another converter is used as a slave unit, defined as a slave station, and the internal control quantities are adjusted in real time according to the operating status of the master station converter. Finally, different control methods are adopted for the low-frequency side of the two converters at the master station and slave station.

4. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 3, characterized in that: The master station adopts an AC voltage control strategy, and the slave station adopts a current tracking control strategy.

5. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 1, characterized in that: One of the goals of the intermediate frequency side control strategy design in step B is to determine the command values ​​of the control variables. , Make the output variable , Track its instruction value , .

6. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 5, characterized in that: Determine the command value of the control variable , Make the output variable , Track its instruction value , The specific process is as follows: First, the current and voltage signals on the power frequency side are acquired, and the phase angle is obtained through a phase-locked loop. The collected power frequency side current and voltage signals are obtained through dq transformation. , ; Then, generated through the total capacitor voltage control loop. , ; Finally, , , , The input is fed into the inner loop current controller to obtain the reference value of the three-phase internal potential on the power frequency side of the M3C converter. .

7. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 1, characterized in that: In step C, the AC voltage control strategy adopted by the low-frequency side of the M3C converter in the main station is mainly achieved through two parts: closed-loop control and open-loop control.

8. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 7, characterized in that: The specific processes of closed-loop control and open-loop control are as follows: First, when there is no new energy source connected to the M3C, there is no grid connection; it is merely a grid connection, and the reference voltage is directly provided through open-loop control. ; Then, when new energy sources are connected, the actual power will fluctuate, and there will be a voltage drop on the line. Therefore, closed-loop control is added to generate additional compensation voltage. , and open-loop control generated Adding them together, we obtain the low-frequency side bus reference voltage. .

9. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 1, characterized in that: Step D employs a current tracking control strategy on the low-frequency side of the M3C at the slave station. The specific process is as follows: First, the low-frequency current of the host and slave devices is collected separately; Then, the three-phase currents of the master and slave M3C converters were collected and transformed to the αβ0 stationary coordinate system using Clark transformation to obtain the αβ axis current components of the master station M3C converter. , and the αβ axis current components from the M3C converter ; Then, an additional compensation voltage is output through the PI controller. ; Finally, the open-loop control generated by the host computer Adding them together, we obtain the low-frequency side bus reference voltage. .

10. The parallel control method for dual M3C converters in a flexible low-frequency transmission system according to claim 1, characterized in that: The current tracking control strategy enables zero steady-state error tracking of the slave low-frequency side current to the master low-frequency side current, thereby achieving the current sharing objective required by the two M3C converters.

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