Zero-sequence circulating current suppression method for modular neutral-point-clamped three-level inverter

Abstract

The invention discloses a modular neutral-point-clamped three-level inverter zero-sequence circulating current suppression method, and relates to the field of electrical engineering, and the method comprises the steps: building a zero-sequence circulating current mathematical model of a modular neutral-point-clamped three-level inverter parallel system based on the Kirchhoff voltage and current law; on the basis of the mathematical model, combining with the Thevenin equivalent law, simplifying an original circuit, establishing an equivalent zero-sequence circulating current circuit model, and in the zero-sequence circulating current circuit model, providing a low-impedance bypass for the high-frequency zero-sequence circulating current through a filter capacitor branch, and shunting the high-frequency zero-sequence circulating current; an LCL filter structure is optimized through a scheme that modules share a filter capacitor, so that a plurality of inverter modules share one filter capacitor, the consumption of the filter capacitor is reduced, and current ripples on a connecting cable are reduced; a switch ripple phase shift regulation strategy is implemented, phases of switch signals are staggered among different inverter modules, and automatic offset of high-frequency switch ripples on a module shared filter capacitor is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of electrical engineering, and in particular to a method for suppressing zero-sequence circulating current of a modular mid-point clamped three-level inverter. Background Art

[0002] The circulation path and components of the neutral point clamped three-level inverter are more complex than those of the two-level inverter due to the increase in the number of switch tubes and output levels. In addition, due to its inherent neutral point potential imbalance, the circulation suppression of the NPC inverter parallel system is more challenging, and it is necessary to adopt a combination of software and hardware suppression measures to suppress it. While applying various software modulation control measures, a large number of hardware measures are also widely used to suppress zero-sequence circulating current due to economic considerations and controller bandwidth limitations. The most direct and effective methods to eliminate zero-sequence circulating current in existing projects are to add an isolation transformer and independently use a DC source. These two hardware methods cut off the circulating current loops on the AC side and the DC side of the modular inverter parallel system respectively, and can completely suppress the zero-sequence circulating current. However, with the increase in the number of parallel modules, the overall cost and volume of the system where each module uses an isolation transformer and an independent DC source will rise rapidly; in addition, there are some low-cost hardware methods that can suppress zero-sequence circulating current of specific components, such as connecting the midpoints of the DC side of each module to form a shared neutral line, and connecting the midpoints of the AC and DC capacitors to form an improved LCL filter when using a star-connected filter capacitor. These methods are easy to implement but have shortcomings. For example, the current on the connecting cable added in the improved LCL filter is too large, which will introduce additional ripple hazards and lead to poor electromagnetic compatibility performance of the device. The existing technical solutions for suppressing zero-sequence circulating current in the modular three-level inverter parallel system generally make comprehensive use of hardware and software methods. Most of the improvement focuses are based on the research on software control strategies to improve the suppression measures of zero-sequence circulating current, in order to pursue a balance between control effect and algorithm complexity. As for the use of hardware measures, following traditional solutions to suppress corresponding types of zero-sequence circulating currents is a common choice, and there are few technical solutions to improve and optimize traditional hardware measures. Summary of the invention

[0003] The purpose of the present invention is to solve the problem that it is difficult to effectively suppress the zero-sequence circulating current in the existing modular mid-point clamped three-level inverter parallel system.

[0004] The technical solution of the present invention is to provide a method for suppressing zero-sequence circulating current of a modularized midpoint clamped three-level inverter, the method comprising:

[0005] S1. Establish a zero-sequence circulating current mathematical model for a modular midpoint-clamped three-level inverter parallel system. The mathematical model is based on Kirchhoff's voltage law and current law, analyzes the DC side capacitor voltage, inverter output voltage, and filter capacitor current, and describes the generation and impact of zero-sequence circulating current;

[0006] S2, based on the mathematical model established in step S1, combined with the Thevenin equivalent law, simplify the original circuit, and establish an equivalent zero-sequence circulating current circuit model, in which a low-impedance bypass is provided for the high-frequency zero-sequence circulating current through a filter capacitor branch, so as to shunt the high-frequency zero-sequence circulating current;

[0007] S3. Propose a scheme for sharing filter capacitors among modules, and use this scheme to optimize the LCL filter structure so that multiple inverter modules share one filter capacitor, thereby reducing the consumption of filter capacitors and reducing the current ripple on the connecting cables;

[0008] S4. Implement the switching ripple phase shift control strategy to stagger the phase of the switching signal between different inverter modules, so as to achieve automatic cancellation of the high-frequency switching ripple on the shared filter capacitor of the modules, and further reduce the current stress and heat loss of the connecting cables.

[0009] In any of the above technical solutions, further, the mathematical model established in step S1 is expressed as:

[0010]

[0011] Where U Nj is the capacitor voltage on the DC side; u kj is the output PWM voltage of the k-phase bridge arm of inverter j to the DC side midpoint oj; i zj is the zero-sequence circulating current; i oj is the current on the AC side filter capacitor; u ON It is the voltage difference between the neutral point of the grid or load and the negative bus N.

[0012] In any of the above technical solutions, further, the solution of module shared filter capacitor in step S3 includes: the bridge arm output of each inverter module is connected to the same group of AC filter capacitors after passing through the inverter inductor, and the AC filter capacitor provides filtering function for multiple modules at the same time, thereby reducing the number of filter capacitors in the system and effectively suppressing zero-sequence circulating current.

[0013] In any of the above technical solutions, further, in step S4, the switching ripple phase shift control strategy is implemented by adding a delay between the switching carrier signals of each inverter module, so that the high-frequency ripple of the odd switching frequency is transferred to the common capacitor and automatically offset.

[0014] The beneficial effects of the present invention are:

[0015] The present invention establishes a mathematical model of zero-sequence circulating current and optimizes the circuit structure, so that the high-frequency zero-sequence circulating current component is bypassed through a low-impedance capacitor branch, thereby greatly reducing the zero-sequence circulating current in the system; compared with traditional methods, the present invention can effectively improve the suppression effect of zero-sequence circulating current, especially has significant advantages in suppressing zero-sequence circulating current in the high-frequency band.

[0016] The present invention adopts an improved circuit structure of a module-shared filter capacitor, combined with a switching ripple phase-shift control strategy, so that the high-frequency ripples of multiple inverter modules are automatically offset, significantly reducing the current ripple on the connecting cable; compared with the traditional method, the high-frequency ripple on the connecting cable is reduced by nearly 50%, which effectively reduces the current stress of components.

[0017] In a preferred implementation of the present invention, the high-frequency current in the system is reduced, the electromagnetic compatibility of the system is improved, and the electromagnetic interference to the external environment is reduced through phase-shifting control of the switching ripple. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The advantages of the above and additional aspects of the present invention will become apparent and easily understood in the description of the embodiments in conjunction with the following drawings, in which:

[0019] Figure 1 1 is an improved circuit model diagram of a method for suppressing zero-sequence circulating current of a modularized neutral point clamped three-level inverter according to an embodiment of the present invention;

[0020] Figure 2 is a conventional circuit model diagram of a zero-sequence circulating current suppression method for a modularized midpoint clamped three-level inverter according to an embodiment of the present invention;

[0021] Figure 3 is an equivalent circuit model diagram describing the zero-sequence circulating current behavior of a method for suppressing zero-sequence circulating current of a modularized midpoint clamped three-level inverter according to an embodiment of the present invention;

[0022] Figure 4 It is a schematic diagram of a ripple phase shift modulation mode when a module shares a capacitor structure in a method for suppressing zero-sequence circulating current of a modularized midpoint clamped three-level inverter according to an embodiment of the present invention;

[0023] Figure 5 It is a waveform diagram of the internal circulating current flowing through the connecting cable in a conventional circuit of a method for suppressing zero-sequence circulating current of a modularized midpoint clamped three-level inverter according to an embodiment of the present invention;

[0024] Figure 6 It is a waveform diagram of the internal circulating current flowing through the connecting cable when the modules share the capacitor in the method for suppressing zero-sequence circulating current of a modular midpoint clamped three-level inverter according to an embodiment of the present invention;

[0025] Figure 7 is an FFT analysis spectrum diagram of the internal circulating current flowing through the connecting cable in a conventional circuit of a method for suppressing zero-sequence circulating current of a modularized midpoint clamped three-level inverter according to an embodiment of the present invention;

[0026] Figure 8 It is a FFT analysis spectrum diagram of the internal circulating current flowing through the connecting cable when the modules share the capacitor in the method for suppressing zero-sequence circulating current of a modular midpoint clamped three-level inverter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0027] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0028] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0029] This embodiment provides a method for suppressing zero-sequence circulating current of a modular midpoint clamped three-level inverter. Figure 1 The figure shows the wiring mode of the module shared filter capacitor in the improved midpoint clamped three-level inverter parallel system of the present invention. The suppression method provided by this embodiment includes:

[0030] In the modular midpoint clamped three-level inverter parallel system, there is a zero-sequence circulating current problem. This circulating current will affect the efficiency and stability of the inverter, so it is necessary to adopt Figure 2 A modular mid-point clamped three-level inverter parallel system with a conventional improved LCL filter is shown to establish a mathematical model to analyze and quantify the problem.

[0031] S1. Using Kirchhoff's voltage-current law to describe the voltage and current at different nodes, the modular midpoint clamped three-level inverter parallel system of the traditional improved LCL filter is analyzed and processed. According to the circuit structure of the traditional improved LCL filter, the basic equations of voltage and current are listed, and the mathematical model of the system zero-sequence circulating current is established:

[0032]

[0033] Where U Nj is the capacitor voltage on the DC side; u kj is the output PWM voltage of the k-phase bridge arm of inverter j to the DC side midpoint oj; i zj is the zero-sequence circulating current; i ojis the current on the AC side filter capacitor; u ON It is the voltage difference between the neutral point of the grid or load and the negative bus N.

[0034] S2. Based on the mathematical model established in step S1 and combined with the Thevenin equivalent theorem, an equivalent circuit model of the inverter parallel system is simplified and approximated. The equivalent circuit model is as follows: Figure 3 As shown, the equivalent circuit is an equivalent circuit that can describe the zero-sequence circulating current behavior.

[0035] In this equivalent circuit model, by adding a capacitor branch, a low-impedance bypass is provided for the high-frequency zero-sequence circulating current, thereby effectively reducing the impact of the circulating current in the system. A part of the original zero-sequence circulating current on the output side is shunted to this capacitor branch, i.e. oj , so that the zero-sequence circulating current i zj The output side zero-sequence circulating current can be reduced and suppressed. However, this approach also brings additional high-frequency current ripple problems, so further optimization measures are needed.

[0036] S3, improve the wiring method and structure of the traditional improved LCL filter hardware measures, such as Figure 1 As shown, two or more modules share a group of filter capacitors. The specific operation is: the bridge arm output of each module passes through the inverter inductor and is connected to the same group of filter capacitors, thereby forming a shared capacitor structure between modules.

[0037] The original modules are changed from "each module has a group of AC filter capacitors" to "modules share a group of capacitors". This connection method can save the consumption of capacitor banks, and the parallel connection point of the two converter modules is still after the second inductor of each module. This connection method can ensure that the original converter working principle remains unchanged, and at the same time, it can maintain a larger circulating loop impedance. At this time, since the filter capacitor on the AC side is only shared but still exists, part of the zero-sequence circulating current on the output side i oj The current is still shunted as in step S2, the output current quality is still improved, and the zero-sequence circulating current suppression effect still exists.

[0038] S4. Based on the optimized structure provided in step S3, a control strategy based on inter-module switch ripple phase shift is adopted to reduce the current stress on the shared capacitor bank and the connecting cable. The control method of the switch ripple phase shift is specifically as follows:

[0039] Add a time delay between the switching carrier signals of each inverter module so that the phases of the switching carrier signals between each module are actively staggered by a certain value, thereby obtaining a driving signal with the same staggered phase, such as Figure 4As shown, under this characteristic driving signal, the direction of the switching ripple at a certain number of times will be opposite. Due to the proposed improved connection method, this type of ripple will be transferred and injected into the shared capacitor of the module and automatically offset, so that Figure 3 The proposed shared structure can retain the original circulating current suppression effect and obtain additional advantages under the control of ripple phase shift.

[0040] When this technical solution is adopted, the number of filter capacitors can be saved, the overall high-frequency ripple on the connecting cable is reduced by nearly 50%, the current stress and heat loss of the component circuit are reduced, and the power density and electromagnetic performance of the prototype are improved.

[0041] In another embodiment of the present invention, the zero-sequence circulating current suppression effect of the improved neutral point clamped three-level inverter of the present invention is verified. Figure 5 and Figure 6 As shown, the internal circulating current waveforms flowing through the connecting cable of the conventional circuit and the improved circuit provided by the present invention are measured respectively, and it can be clearly seen that Figure 6 The internal circulation of Figure 7 and Figure 8 As shown, at 1.5×10 4 At Hz, the current amplitude measured by the solution provided by the present invention is significantly reduced, indicating that the zero-sequence circulating current suppression effect is obvious.

[0042] In summary, the present invention proposes a method for suppressing zero-sequence circulating current of a modularized neutral-point clamped three-level inverter, comprising:

[0043] S1. A zero-sequence circulating current mathematical model of a modular mid-point clamped three-level inverter parallel system is established. The mathematical model is based on Kirchhoff's voltage law and current law, analyzes the DC side capacitor voltage, inverter output voltage and filter capacitor current, and describes the generation and influence of zero-sequence circulating current.

[0044] S2. Based on the mathematical model established in step S1, combined with the Thevenin equivalent law, the original circuit is simplified to establish an equivalent zero-sequence circulating current circuit model. In the zero-sequence circulating current circuit model, a low-impedance bypass is provided for the high-frequency zero-sequence circulating current through a filter capacitor branch to shunt the high-frequency zero-sequence circulating current.

[0045] S3. A scheme for modules to share filter capacitors is proposed. This scheme is used to optimize the LCL filter structure so that multiple inverter modules can share one filter capacitor, thereby reducing the consumption of filter capacitors and reducing the current ripple on the connecting cables.

[0046] S4. Implement the switching ripple phase shift control strategy to stagger the phase of the switching signal between different inverter modules, so as to achieve automatic cancellation of the high-frequency switching ripple on the shared filter capacitor of the modules, and further reduce the current stress and heat loss of the connecting cables.

[0047] The steps in the present invention can be adjusted in order, combined or deleted according to actual needs.

[0048] The units in the device of the present invention can be combined, divided and deleted according to actual needs.

[0049] Although the present invention is disclosed in detail with reference to the accompanying drawings, it should be understood that these descriptions are merely exemplary and are not intended to limit the application of the present invention. The scope of protection of the present invention is defined by the appended claims and may include various modifications, alterations and equivalents made to the invention without departing from the scope and spirit of the present invention.

Claims

1. A method for suppressing zero-sequence circulating current in a modular midpoint clamped three-level inverter. It is characterized in that The method comprises: S1. Establish a zero-sequence circulating current mathematical model of a modular mid-point clamped three-level inverter parallel system. The mathematical model is based on Kirchhoff's voltage law and current law, analyzes the DC side capacitor voltage, inverter output voltage and filter capacitor current, and describes the generation and influence of zero-sequence circulating current; S2, based on the mathematical model established in step S1, combined with the Thevenin equivalent law, simplify the original circuit, and establish an equivalent zero-sequence circulating current circuit model, in which a low-impedance bypass is provided for the high-frequency zero-sequence circulating current through a filter capacitor branch, so as to shunt the high-frequency zero-sequence circulating current; S3. Propose a scheme for sharing filter capacitors among modules, and use this scheme to optimize the LCL filter structure so that multiple inverter modules share one filter capacitor, thereby reducing the consumption of filter capacitors and reducing the current ripple on the connecting cables; S4. Implement the switching ripple phase shift control strategy to stagger the phase of the switching signal between different inverter modules, so as to achieve automatic cancellation of the high-frequency switching ripple on the shared filter capacitor of the modules, and further reduce the current stress and heat loss of the connecting cables.

2. The method for suppressing zero-sequence circulating current of a modularized midpoint clamped three-level inverter according to claim 1, It is characterized in that The mathematical model established in step S1 is expressed as: Where U Nj is the capacitor voltage on the DC side; u kj is the output PWM voltage of the k-phase bridge arm of inverter j to the DC side midpoint oj; i zj is the zero-sequence circulating current; i oj is the current on the AC side filter capacitor; u ON It is the voltage difference between the neutral point of the grid or load and the negative bus N.

3. The method for suppressing zero-sequence circulating current of a modularized midpoint clamped three-level inverter according to claim 1, It is characterized in that The scheme of the module shared filter capacitor in step S3 includes: the bridge arm output of each inverter module is connected to the same group of AC filter capacitors after passing through the inverter inductor, and the AC filter capacitor provides filtering function for multiple modules at the same time, thereby reducing the number of filter capacitors in the system and effectively suppressing zero-sequence circulating current.

4. The method for suppressing zero-sequence circulating current of a modularized midpoint clamped three-level inverter according to claim 1, It is characterized in that The switching ripple phase shift control strategy in step S4 is implemented by adding a time delay between the switching carrier signals of each inverter module, so that the high-frequency ripple of the odd-numbered switching frequency is transferred to the common capacitor and automatically offset.

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