Topological Control Method for Bridge Arm Frequency Division and Switching Modular Multilevel Converter

Abstract

The present invention discloses a topology control method for a modular multilevel converter with bridge arm frequency division switching. Each phase unit consists of an upper bridge arm, a lower bridge arm, and a bridge arm switching switch. Each of the two bridge arms contains N cascaded sub-modules. The bridge arm switching switch includes 6 groups of cascaded IGBT clusters, and every two IGBT clusters form a phase switching unit. One IGBT cluster is composed of two groups of IGBTs connected in reverse series, and each group of IGBTs is further composed of multiple IGBTs connected in cascade in the same direction. The three phases a, b, and c on the AC side are led out from the middle of each phase switching unit and connected to a matching transformer or directly connected to the AC main grid. Compared with the standard MMC, on the one hand, the topology of the present invention does not require bridge arm inductors. On the other hand, during the normal operation and fault ride-through process of the MMC, by quickly switching between phase switching units, the charge and discharge times of the MMC sub-module string within a power frequency cycle can be changed, so as to achieve the effect of greatly reducing the sub-module capacitor voltage ripple during normal operation and fault ride-through processes.

Description

Technical Field

[0001] The present invention belongs to the field of flexible DC power transmission, and particularly relates to a topological control method for a modular multilevel converter with arm frequency division switching. Background Art

[0002] Modular multilevel converters (MMCs) have a modular cascaded and arbitrarily expandable topological structure, can obtain good waveform quality at very low switching frequencies, are simple in design and maintenance, and have high reliability. Currently, they have become an ideal solution in the field of flexible DC power transmission.

[0003] The sub-module capacitor of the MMC is the largest device in the sub-module, accounting for a large proportion of the construction cost and floor area of the entire converter station. Although increasing the sub-module capacitor value can reduce the capacitor voltage fluctuation, this method will increase the volume and cost of the capacitor, and the economy is poor. Therefore, researching methods to reduce the capacitor voltage ripple of the MMC sub-module, thereby reducing the requirement for the capacitance value of the sub-module capacitor, realizing the lightweight design of the converter, and reducing the floor area and construction cost of the converter station has important engineering significance.

[0004] In addition, in the case of an AC main network fault of the MMC, due to the asymmetry inside the MMC, the ripple of the sub-module capacitor voltage increases significantly. On the one hand, this will affect the fault ride-through performance of the MMC and reduce its fault ride-through ability; on the other hand, the large voltage ripple will increase the collector-emitter voltage of the IGBT, leading to IGBT breakdown and seriously affecting the safety of the IGBT.

[0005] To address such problems, the commonly used method is the double-frequency circulating current injection method. However, this method not only increases the loss of the MMC and reduces the system efficiency, but also causes over-modulation phenomena in the MMC under certain conditions, affecting the normal operation of the MMC. Summary of the Invention

[0006] The purpose of the present invention is to provide a topological control method for a modular multilevel converter with arm frequency division switching to meet the requirements of lightweight of the MMC converter station and enhance its fault ride-through ability.

[0007] For this purpose, the present invention adopts the following technical solutions:

[0008] A topological control method for a modular multilevel converter with arm frequency division switching, characterized in that the topology includes an MMC model composed of three phase units A, B, and C; each phase unit consists of an upper arm, a lower arm, and an arm switching switch. Each of the two arms contains N cascaded sub-modules, and the arm switching switch is composed of 6 cascaded IGBT clusters; every two IGBT clusters form a phase switching unit; an IGBT cluster is composed of two groups of IGBTs connected in reverse series, and each group of IGBTs is composed of multiple identical IGBTs connected in series in the same direction;

[0009] A further improvement of the present invention lies in that the sub-module string numbers of the upper arm of the A-phase unit are sequentially 1 to N from top to bottom, and these N sub-modules are connected in series with each other; similarly, the sub-module string numbers of the lower arm are sequentially N + 1 to 2N from top to bottom, and these N sub-modules are connected in series with each other; the midpoint of the IGBT module of the first sub-module of the upper arm is connected to the positive pole of the DC bus; the negative electrode of the capacitor of the Nth sub-module of the upper arm is simultaneously connected to the collectors of three IGBTs, and these three IGBTs are each connected in series with M - 1 same IGBTs in the same direction to form three positive upper IGBT strings; the IGBT numbers of these three positive upper IGBT strings are sequentially 1 to M from top to bottom; the emitters of the Mth IGBTs of these three positive upper IGBT strings are each connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 same IGBTs in the same direction to form three negative upper IGBT strings; the IGBT numbers of these three negative upper IGBT strings are sequentially M + 1 to 2M from top to bottom; the collectors of the 2Mth IGBTs of these three negative upper IGBT strings are respectively connected to the a-phase, b-phase, and c-phase of the matching transformer or the AC main network; the collectors of the 2Mth IGBTs of these three negative upper IGBT strings are each connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 same IGBTs in the same direction to form three positive lower IGBT strings; the IGBT numbers of these three positive lower IGBT strings are sequentially 2M + 1 to 3M from top to bottom; the emitters of the 3Mth IGBTs of these three positive lower IGBT strings are each connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 same IGBTs in the same direction to form three negative lower IGBT strings; the IGBT numbers of these three negative lower IGBT strings are sequentially 3M + 1 to 4M from top to bottom; the collectors of the 4Mth IGBTs of these three negative lower IGBT strings are interconnected and simultaneously connected to the midpoint of the IGBT module of the (N + 1)th sub-module string of the lower arm; the midpoint of the IGBT module of the 2Nth sub-module of the lower arm is connected to the negative pole of the DC bus. The connection methods of the B-phase unit and the C-phase unit are the same as that of the A-phase unit.

[0010] The trigger signals of all IGBTs in each phase switching unit are the same, that is, all IGBTs in each phase switching unit act simultaneously. This reduces the requirements for the controller. Especially in a high-voltage and large-capacity converter station, the more cascaded IGBTs there are, the more prominent this advantage is;

[0011] A further improvement of the present invention lies in dividing a power frequency period of the modulation voltage into several equal parts. In the first interval, the first phase switching unit of the A-phase unit is turned on, so that the a-phase is connected to the A-phase unit; the second phase switching unit of the B-phase unit is turned on, so that the b-phase is connected to the B-phase unit; the third phase switching unit of the C-phase unit is turned on, so that the c-phase is connected to the C-phase unit; in the second interval, the second phase switching unit of the A-phase unit is turned on, so that the b-phase is connected to the A-phase unit; the third phase switching unit of the B-phase unit is turned on, so that the c-phase is connected to the B-phase unit; the first phase switching unit of the C-phase unit is turned on, so that the a-phase is connected to the C-phase unit; in the third interval, the third phase switching unit of the A-phase unit is turned on, so that the c-phase is connected to the A-phase unit; the first phase switching unit of the B-phase unit is turned on, so that the a-phase is connected to the B-phase unit; the second phase switching unit of the C-phase unit is turned on, so that the b-phase is connected to the C-phase unit; and so on, the fourth interval is the same as the first interval, the fifth interval is the same as the second interval, the sixth interval is the same as the third interval...

[0012] Under normal conditions and in the case of an AC side main network fault, through high-frequency switching of the bridge arm switching unit within a power frequency period, the current flowing through the sub-modules in each phase unit is changed to increase the charge and discharge times of the sub-module capacitors within a power frequency period, so as to reduce the magnitude of the sub-module capacitor voltage ripple, so that the required capacitance value of the sub-module capacitor can be greatly reduced under normal conditions, and the voltage fluctuation of the sub-module capacitor can be reduced in the case of an AC main network fault, enhancing the fault ride-through ability of the MMC.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] Through high-frequency switching of the bridge arm switching unit within a power frequency period, the current flowing through the sub-modules in each phase unit is quickly changed, increasing the charge and discharge times of the sub-modules within a power frequency period, reducing the magnitude of the sub-module capacitor voltage ripple, so that the required capacitance value of the sub-module capacitor can be greatly reduced under normal conditions, and the voltage fluctuation of the sub-module capacitor can be reduced in the case of an AC main network fault, enhancing the fault ride-through ability of the MMC.

[0015] The present invention does not inject multiple-frequency harmonics into the bridge arm and will not cause over-modulation of the MMC. In addition, the present invention only increases the loss of some bridge arm switching switches and does not increase the on-state loss of the MMC sub-module string. Therefore, in terms of loss, the increase of the present invention is small. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Other features, objects and advantages of the present invention will become more apparent by reading the description with reference to the following drawings:

[0017] Figure 1Schematic diagram of the topology structure of the arm frequency-division switching type modular multilevel converter of the present invention.

[0018] Figure 2 Schematic diagram of the conduction sequence of the phase switching unit of the present invention.

[0019] Figure 3 Steady-state waveform of the capacitor voltage of the sub-module of the traditional MMC under normal conditions;

[0020] Figure 4 Steady-state waveform of the capacitor voltage of the sub-module of the topology of the present invention under normal conditions.

[0021] Figure 5 Waveform of the capacitor voltage of the sub-module of the traditional MMC under fault conditions.

[0022] Figure 6 Waveform of the capacitor voltage of the sub-module of the topology of the present invention under fault conditions. Specific implementation manner

[0023] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several changes and improvements can still be made. These all belong to the protection scope of the present invention.

[0024] Reference Figure 1 , the topology of the arm frequency-division switching type modular multilevel converter includes an MMC model composed of three phase units A, B, and C; each phase unit is composed of an upper arm, a lower arm, and an arm switching switch. Each of the two arms contains N cascaded sub-modules. The arm switching switch is composed of 6 cascaded IGBT clusters; every two IGBT clusters form a phase switching unit; an IGBT cluster is composed of two groups of IGBTs connected in reverse series, and each group of IGBTs is composed of multiple identical IGBTs connected in series in the same direction.

[0025] The sub-module string numbers of the upper arm of the A-phase unit are 1 to N from top to bottom, and these N sub-modules are connected in series with each other; similarly, the sub-module string numbers of the lower arm are N + 1 to 2N from top to bottom, and these N sub-modules are connected in series with each other; the midpoint of the IGBT module of the first sub-module of the upper arm is connected to the positive pole of the DC bus; the negative electrode of the capacitor of the Nth sub-module of the upper arm is connected to the collectors of three IGBTs at the same time, and these three IGBTs are each connected in series with M - 1 identical IGBTs in the same direction to form three positive upper IGBT strings; the IGBT numbers of these three positive upper IGBT strings are all 1 to M from top to bottom; the emitters of the Mth IGBTs of these three positive upper IGBT strings are each connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 identical IGBTs in the same direction to form three negative upper IGBT strings; the IGBT numbers of these three negative upper IGBT strings are all M + 1 to 2M from top to bottom; the collectors of the 2Mth IGBTs of these three negative upper IGBT strings are respectively connected to the a-phase, b-phase, and c-phase of the matching transformer or the AC main network; the collectors of the 2Mth IGBTs of these three negative upper IGBT strings are each connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 identical IGBTs in the same direction to form three positive lower IGBT strings; the IGBT numbers of these three positive lower IGBT strings are 2M + 1 to 3M from top to bottom; the emitters of the 3Mth IGBTs of these three positive lower IGBT strings are each connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 identical IGBTs in the same direction to form three negative lower IGBT strings; the IGBT numbers of these three negative lower IGBT strings are 3M + 1 to 4M from top to bottom; the collectors of the 4Mth IGBTs of these three negative lower IGBT strings are interconnected and at the same time connected to the midpoint of the IGBT module of the (N + 1)th sub-module string of the lower arm; the midpoint of the IGBT module of the 2Nth sub-module of the lower arm is connected to the negative pole of the DC bus. The connection methods of the B-phase unit and the C-phase unit are the same as that of the A-phase unit.

[0026] M satisfies the condition:

[0027]

[0028] Among them, U l represents the peak value of the AC side line voltage, and U c represents the rated collector-emitter voltage of the IGBT.

[0029] Reference Figure 1 , the trigger signals of all IGBTs in each phase switching unit are the same, that is, all IGBTs in each phase switching unit act simultaneously.

[0030] Reference Figure 2, a power frequency period of the modulation voltage is divided into p equal parts, denoted as the interval from 1 to p. In the first interval, the first phase switching unit of the A-phase unit is turned on, and the other two phase units are turned off (S1 = 1, S2 = 0, S3 = 0), so that the a-phase is connected to the A-phase unit; the second phase switching unit of the B-phase unit is turned on, and the other two phase units are turned off (S4 = 0, S5 = 1, S6 = 0), so that the b-phase is connected to the B-phase unit; the third phase switching unit of the C-phase unit is turned on, and the other two phase units are turned off (S7 = 0, S8 = 0, S9 = 1), so that the c-phase is connected to the C-phase unit; in the second interval, the second phase switching unit of the A-phase unit is turned on, and the other two phase units are turned off (S1 = 0, S2 = 1, S3 = 0), so that the b-phase is connected to the A-phase unit; the third phase switching unit of the B-phase unit is turned on, and the other two phase units are turned off (S4 = 0, S5 = 0, S6 = 1), so that the c-phase is connected to the B-phase unit; the first phase switching unit of the C-phase unit is turned on, and the other two phase units are turned off (S7 = 1, S8 = 0, S9 = 0), so that the a-phase is connected to the C-phase unit; in the third interval, the third phase switching unit of the A-phase unit is turned on, and the other two phase units are turned off (S1 = 0, S2 = 0, S3 = 1), so that the c-phase is connected to the A-phase unit; the first phase switching unit of the B-phase unit is turned on, and the other two phase units are turned off (S4 = 1, S5 = 0, S6 = 0), so that the a-phase is connected to the B-phase unit; the second phase switching unit of the C-phase unit is turned on, and the other two phase units are turned off (S7 = 0, S8 = 1, S9 = 0), so that the b-phase is connected to the C-phase unit; and so on, the fourth interval is the same as the first interval, the fifth interval is the same as the second interval, and the sixth interval is the same as the third interval.

[0031] As can be seen from the above specific description, in the proposed topology, through the high-frequency switching of the arm switching units, the current flowing through the sub-module capacitors in each phase unit is changed multiple times within a power frequency period, increasing the charge and discharge times of the sub-module capacitors within a power frequency period, reducing the magnitude of the voltage ripple of the sub-module capacitors, enabling a significant reduction in the required capacitance value of the sub-module capacitors under normal conditions, reducing the voltage fluctuation of the sub-module capacitors in the case of an AC main network fault, and enhancing the fault ride-through ability of the MMC.

[0032] Embodiment:

[0033] According to the description of the present invention, in the simulation example, the arm frequency-divided switching type modular multilevel converter topology is applied as Figure 1 shown, and the simulation parameters are shown in Table 1.

[0034] Table 1 Electrical parameters of the MMC system

[0035] Parameter Value Rated apparent power / MVA 1680 Rated active power / MW 1500 Rated reactive power / MVar 750 Rated DC voltage / kV 500 Rated AC voltage / kV 230 / 260 Number of sub - modules in the bridge arm 250 Rated voltage of sub - module / kV 2 Capacitance of sub - module / mF 18 Resistance of bridge arm / Ω 0.4 Equivalent inductance on the AC side / mH 16 Equivalent resistance on the AC side / Ω 0.3

[0036] Reference Figure 3And Figure 4 When there is no fault, the peak-to-peak ripple of the capacitor voltage of the traditional MMC sub-module reaches 0.35 kV, while the peak-to-peak ripple of the capacitor voltage of the new MMC topology proposed by the present invention is only 0.09 kV, a reduction of 74.3% compared with the traditional one. This also means that under the same capacitor voltage ripple of the sub-module, the capacitance value of the topology proposed by the present invention can be reduced by 74.3%.

[0037] Reference Figure 5 And Figure 6 After an asymmetric fault occurs on the AC side, the peak-to-peak ripple of the capacitor voltage of the traditional MMC sub-module reaches 0.8 kV, while the peak-to-peak ripple of the capacitor voltage of the new MMC topology proposed by the present invention is only 0.4 kV, a reduction of 50% compared with the traditional one. The smaller capacitor ripple means that on the one hand, its fault ride-through ability is greatly improved, and the impact on the AC side performance of the MMC is smaller; on the other hand, it means that the collector-emitter voltage of the IGBT is reduced, increasing the voltage margin of the IGBT and protecting the safety of the IGBT.

[0038] The above description of the embodiments is to enable those of ordinary skill in the art to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A topological control method for a modular multilevel converter with bridge arm frequency division switching, characterized in that, The topology includes an MMC model composed of three phase units A, B, and C; each phase unit consists of an upper arm, a lower arm, and an arm switching switch. Each of the two arms contains N cascaded sub-modules, and the arm switching switch is composed of 6 groups of cascaded IGBT clusters; every two IGBT clusters form a phase switching unit; an IGBT cluster is composed of two groups of IGBTs in reverse series, and each group of IGBTs is composed of multiple identical IGBTs in the same direction in cascade. For each phase unit, the midpoint of the IGBT module of the first sub-module of the upper arm is connected to the positive pole of the DC bus; the negative electrode of the capacitor of the Nth sub-module of the upper arm is simultaneously connected to the collectors of three IGBTs, and these three IGBTs are each connected in series with M - 1 identical IGBTs in the same direction to form three groups of positive upper IGBT strings; the emitters of the Mth IGBTs of these three groups of positive upper IGBT strings are respectively connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 identical IGBTs in the same direction to form three groups of negative upper IGBT strings; the collectors of the Mth IGBTs of these three groups of negative upper IGBT strings are respectively connected to the a-phase, b-phase, and c-phase of the matching transformer or the AC main grid; the collectors of the Mth IGBTs of these three groups of negative upper IGBT strings are respectively connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 identical IGBTs in the same direction to form three groups of positive lower IGBT strings; the emitters of the Mth IGBTs of these three groups of positive lower IGBT strings are respectively connected to the emitters of another three IGBTs, and these three IGBTs are each connected in series with M - 1 identical IGBTs in the same direction to form three groups of negative lower IGBT strings; the collectors of the Mth IGBTs of these three groups of negative lower IGBT strings are interconnected and simultaneously connected to the midpoint of the IGBT module of the first sub-module string of the lower arm; the midpoint of the IGBT module of the Nth sub-module of the lower arm is connected to the negative pole of the DC bus. The trigger signals of all IGBTs within each phase switching unit are the same, that is, all IGBTs within each phase switching unit act simultaneously; one power frequency period of the modulation voltage is divided into p equal parts, denoted as the 1-p interval; within the first interval, the first phase switching unit of the A-phase unit conducts, connecting the a-phase to the A-phase unit; the second phase switching unit of the B-phase unit conducts, connecting the b-phase to the B-phase unit; the third phase switching unit of the C-phase unit conducts, connecting the c-phase to the C-phase unit; within the second interval, the second phase switching unit of the A-phase unit conducts, connecting the b-phase to the A-phase unit; the third phase switching unit of the B-phase unit conducts, connecting the c-phase to the B-phase unit; the first phase switching unit of the C-phase unit conducts, connecting the a-phase to the C-phase unit; within the third interval, the third phase switching unit of the A-phase unit conducts, connecting the c-phase to the A-phase unit; the first phase switching unit of the B-phase unit conducts, connecting the a-phase to the B-phase unit; the second phase switching unit of the C-phase unit conducts, connecting the b-phase to the C-phase unit; and so on, the fourth interval is the same as the first interval, the fifth interval is the same as the second interval, and the sixth interval is the same as the third interval.

2. The topological control method for a modular multilevel converter with bridge arm frequency division switching according to claim 1, characterized in that, Under normal conditions and in the case of AC side main network faults, through high-frequency switching of the bridge arm switching unit within one power frequency period, the current flowing through the sub-module in each phase unit is changed to increase the charge and discharge times of the sub-module capacitor within one power frequency period, so as to reduce the magnitude of the sub-module capacitor voltage ripple, enabling a significant reduction in the sub-module capacitor capacitance requirement under normal conditions, and reducing the fluctuation of the sub-module capacitor voltage in the case of AC main network faults, enhancing the fault ride-through ability of the MMC.

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