A coordinated control method for mid-point voltage balance and circulating current suppression in NPC grid-connected inverter parallel systems

Abstract

The present invention discloses a coordinated control method for midpoint voltage balancing and circulating current suppression in an NPC grid-connected inverter parallel system. Based on an NPC grid-connected inverter parallel system comprising multiple three-phase NPC grid-connected inverters with independent midpoints, the system eliminates the need for complex control and interconnected neutral lines. While balancing the midpoint voltages of the NPC grid-connected inverters, the method can suppress zero-sequence circulating currents during the parallel operation of the inverters, thereby ensuring safe, reliable, and efficient operation of the parallel system and avoiding equipment failures.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics technology and relates to DC-AC conversion and inverter parallel systems. Specifically, it relates to a coordinated control method for midpoint voltage balance and zero-sequence circulating current suppression in a multi-level NPC grid-connected inverter parallel system, which is used to achieve stable and efficient operation of the NPC grid-connected inverter parallel system. Background Art

[0002] Faced with the depletion of traditional fossil energy and increasingly severe environmental challenges, new energy sources, thanks to their renewability and environmental friendliness, demonstrate tremendous development potential. Against this backdrop, solar photovoltaic power generation, as a leading new energy source, holds particularly broad prospects. In solar photovoltaic power generation systems, inverters play a central role in energy conversion. With the growing demand for high-voltage, high-capacity applications, multilevel inverter technology has become a widely adopted and important solution.

[0003] NPC inverters, a type of multilevel inverter, have gained widespread use. Compared to two-level converters, multilevel converters offer lower voltage ratings for switching transistors at the same voltage level, resulting in a smaller voltage variation across the switching devices and less electromagnetic interference. However, NPC inverters also face challenges, including complex control strategies and a large number of components. In particular, imbalance in the DC midpoint potential can lead to increased low-order harmonics, impacting the quality of the output voltage and current waveforms. Prolonged imbalance can also cause voltage waveform distortion and even damage components. Therefore, a midpoint balancing control strategy is essential to ensure stable system operation.

[0004] To increase the power and capacity of NPC inverters, a parallel system can be used. Increasing the number of inverters improves system redundancy, allowing the system to remain operational even if some inverters fail. Parallel systems also increase power levels and allow for flexible selection of the number of inverters based on demand, thus controlling costs. However, parallel NPC inverter systems present circulating current issues. Due to differences in PWM control algorithms, device parameters, and carrier synchronization among inverters, zero-sequence circulating currents can occur. Excessive zero-sequence circulating currents can affect output current quality, induce electromagnetic interference, and lead to uneven inverter current stress. Furthermore, circulating currents increase device losses, reduce system efficiency, and even damage power switching devices.

[0005] Therefore, when designing and operating a parallel NPC inverter system, effective measures must be taken to suppress circulating current problems and ensure stable and efficient system operation. How to reliably achieve stable and efficient operation of a parallel NPC inverter system is a concern for those skilled in the art. Summary of the Invention

[0006] In response to the shortcomings of the existing technology and the need for improvement, the present invention provides a coordinated control method for midpoint voltage balancing and zero-sequence circulating current suppression in a parallel system of NPC grid-connected inverters. The purpose is to balance the midpoint voltage of the NPC grid-connected inverters while suppressing the zero-sequence circulating current in the parallel operation of the inverters, thereby ensuring the safe, reliable and efficient operation of the parallel system and avoiding equipment failures.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a coordinated control method for midpoint voltage balance and circulating current suppression in an NPC grid-connected inverter parallel system, based on an NPC grid-connected inverter parallel system comprising multiple three-phase NPC grid-connected inverters with independent midpoints, comprising the following steps:

[0008] S1 collects the inductor currents i of phases A, B, and C respectively according to the sampling period. Laz 、i Lbz 、i Lcz , and after 3s / 2s transformation, the inductor current i in the αβ coordinate system is obtained α 、i β , using PR controller and combined with grid voltage feedforward, according to i α 、i β Get the output voltage given value d of the αβ coordinate system αβ , use 2s / 3s transformation to get the output voltage duty cycle d of phases A, B, and C a d b d c ;

[0009] S2, the A, B, C phase inductor current i Laz 、i Lbz 、i Lcz Add up to get the zero sequence current i z , the required injected zero-sequence voltage d is obtained through the first PI controller z , and the duty cycle d of the output voltage of phases A, B, and C a d b d c Add up to get the duty cycle value d az d bz d cz , sent to the PWM modulation module;

[0010] S3, after every N switching cycles, the midpoint voltage of the NPC grid-connected inverter is compared with the given value U DC / 2 is used as the difference and the second PI controller is used to obtain the midpoint voltage charge Q required for injection. NP , according to the inductor current value of a specific phase, Q NP The absolute value of is divided by the absolute value of the inductor current to calculate the additional pulse duty cycle d0 required for this specific phase;

[0011] S4, set the duty cycle value d of each phase az d bz d cz The pulse duty d0 that increases every N switching cycles is sent to the PWM modulation module to generate PWM pulses.

[0012] Furthermore, the principle for selecting the specific phase in step S3 is: when the output Q of the second PI controller is NP When it is greater than 0, the phase with the largest current among the three-phase currents is selected as the specific phase, otherwise the phase with the smallest current among the three-phase currents is selected as the specific phase.

[0013] Furthermore, the additional pulse duty cycle d0 in step S3 is: d0 = |Q NP | / |i Lkz |, where k = a, b, c.

[0014] Furthermore, N in step S3 can be set arbitrarily, and is preferably set to 3.

[0015] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0016] 1. For the three-phase NPC grid-connected inverter parallel system, the traditional control method requires the addition of a DC bus. The coordinated control method proposed in this patent eliminates the interconnection of the DC bus and simplifies the system structure.

[0017] 2. Each three-phase NPC grid-connected inverter has the same structure, does not require complex communication, supports hot plugging, and further simplifies the system structure;

[0018] 3. Grid-connected current control, zero-sequence current suppression, and midpoint voltage control are independent of each other, and linear control analysis methods can be used to analyze loop stability and design controller parameters, simplifying the design process;

[0019] 4. Adding only one switching state in multiple switching cycles can effectively reduce additional losses and improve system efficiency;

[0020] 5. While balancing the neutral point voltage of the NPC grid-connected inverter, it can suppress the zero-sequence circulating current in the parallel operation of the inverter, ensuring the safety of system operation;

[0021] 6. The control method is simple, effective and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a flow chart of the coordinated control method of the present invention;

[0023] Figure 2 This is a structural diagram of the three-phase NPC inverter of the present invention;

[0024] Figure 3 The parallel system structure diagram of two three-phase NPC grid-connected inverters;

[0025] Figure 4 The block diagram of the coordinated control method of the mid-point voltage and circulating current in the NPC grid-connected inverter parallel system;

[0026] Figure 5 Schematic diagram of the inductor current area;

[0027] Figure 6 Schematic diagram of the output voltage of each phase bridge arm when no additional pulse duty cycle d0 is applied to phase A;

[0028] Figure 7 Schematic diagram of the output voltage of each phase bridge arm after applying an additional pulse duty cycle d0 to phase A;

[0029] Figure 8 It is the transfer function block diagram of the zero-sequence current suppression control system;

[0030] Figure 9 It is the transfer function block diagram of the midpoint voltage control system;

[0031] Figure 10 When two three-phase NPC grid-connected inverters are connected in parallel, the output voltage waveform of the three-phase bridge arm is when the first inverter adds one pulse every three switching cycles;

[0032] Figure 11 This is a process diagram before and after implementing the coordinated control method for the mid-point voltage and circulating current of the NPC grid-connected inverter parallel system when two three-phase NPC grid-connected inverters are connected in parallel. DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0034] In the present invention, the terms "first", "second", etc. (if any) in the present invention and the drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0035] Figure 1 Flowchart of the coordinated control method of the mid-point voltage and circulating current in the NPC grid-connected inverter parallel system according to the embodiment of the present invention (hereinafter referred to as the coordinated control method). Figure 1 , combined with Figures 2 to 6, the coordinated control method in this embodiment is described in detail.

[0036] To verify the effectiveness of the present invention, this embodiment is based on Figure 2 The three-phase NPC inverter structure shown in the figure is constructed based on this Figure 3 The two three-phase NPC inverters are connected in parallel in a system. This parallel system serves as a typical application scenario for verifying the present invention and will be used to further illustrate the technical features and advantages of the present invention.

[0037] See Figure 4 Describe the execution process of each operation.

[0038] In operation S1, the control system collects the inductor currents i of phases A, B, and C respectively according to the sampling period. Laz 、i Lbz 、i Lcz , and after 3s / 2s transformation, the inductor current i in the αβ coordinate system is obtained α 、i β , using PR controller and combined with grid voltage feedforward, according to i α 、i β Get the output voltage given value d of the αβ coordinate system αβ , use 2s / 3s transformation to get the output voltage duty cycle d of phases A, B, and C a d b d c .

[0039] In operation S2, the control system converts the A, B, and C phase inductor currents i Laz 、i Lbz 、i Lcz Add up to get the zero sequence current i z , the required injected zero-sequence voltage d is obtained through the first PI controller z , and the duty cycle d of the output voltage of phases A, B, and C a d b d c Add them together to get the duty cycle value d sent to the PWM modulation module az d bz d cz .

[0040] In operation S3, after every N switching cycles, the control system compares the midpoint voltage of the NPC grid-connected inverter with the given value U DC / 2 is used as the difference and the second PI controller is used to obtain the midpoint voltage charge Q required for injection. NP , according to the inductor current value of a specific phase, Q NPThe absolute value of the inductor current is divided by the absolute value of the inductor current to calculate the additional pulse duty cycle d0 required for this specific phase. Specifically, the principle for selecting the specific phase is: when the output Q of the second PI controller is NP When it is greater than 0, the phase with the largest current among the three-phase currents is selected as the specific phase, otherwise the phase with the smallest current among the three-phase currents is selected as the specific phase.

[0041] See Figure 5 Assuming that the inductor current is a three-phase standard sine wave, according to the principle of selecting a specific phase, when the output Q of the second PI controller is NP When it is greater than 0, in the range of 1 to 6, the specific phases are A phase, B phase, B phase, C phase, C phase, and A phase respectively; when the output Q of the second PI controller is NP When it is less than 0, in the range of 1 to 6, the specific phases are phase C, phase C, phase A, phase A, phase B, and phase B, respectively.

[0042] In operation S4, the control system sets the duty cycle value d of each phase az d bz d cz The pulse duty cycle d0 that increases every N switching cycles is sent to the PWM modulation module to generate PWM pulses.

[0043] See Figure 6 When no additional pulse duty cycle d0 is applied, the output voltage of each phase bridge arm is only DC / 2,0 or 0,-U DC / 2. Figure 7 , after applying an extra pulse duty cycle d0 to a specific phase, the output voltage of the bridge arm of this phase will be DC / 2,0,-U DC The additional pulses will cause the current flowing through the midpoint to become:

[0044] i NP =(1-|d az |-2d0)i Laz +(1-|d bz |)i Lbz +(1-|d cz |)i Lcz

[0045] =(3i z -|d az |i Laz -|d bz |i Lbz -|d cz |i Lcz )-2d0i Laz

[0046] =I NPe -INPc

[0047] Where I NPe is the inherent component in the parallel inverter system, and I NPc The control component introduced by the additional pulse is d0. Therefore, changing the additional pulse duty cycle d0 can control the midpoint voltage of each NPC inverter in the parallel system.

[0048] Figure 8 The transfer function block diagram of the zero-sequence current suppression control system is shown, and the midpoint voltage controller is the first PI controller. Figure 9 The transfer function block diagram of the midpoint voltage control system is shown. The zero-sequence circulating current controller is the second PI controller. The method provided by this invention makes zero-sequence current suppression and midpoint voltage control independent of each other. Linear control analysis methods can be used to analyze loop stability and design controller parameters, facilitating the design of controller parameters and ensuring stable system operation.

[0049] In order to verify the practicability of the control method in this embodiment, based on Figure 3 The two inverters are connected in parallel in the system shown in FIG. 4 , and a prototype based on the control method is established, and experimental verification is completed at a rated power of 3 kW.

[0050] Figure 10 The figure shows the output voltage waveforms of the three-phase bridge legs when two three-phase NPC grid-connected inverters are connected in parallel, with N set to 3. The first inverter adds a pulse every three switching cycles. These added pulses appear in phases C, C, C, and A, respectively.

[0051] Figure 11 The figure shows the process of balancing the midpoint voltage and suppressing the zero-sequence circulating current after implementing the control strategy. Before the control strategy, the midpoint voltage offset was 15V, and the zero-sequence circulating current fluctuated within ±4A. After the control strategy was implemented, the midpoint voltage reached equilibrium within 400ms, and the zero-sequence circulating current was suppressed to less than 0.5A.

[0052] The experimental results show that based on the principle prototype constructed by the present invention, this method can achieve coordinated control of the mid-point voltage and circulating current in the NPC grid-connected inverter parallel system, confirming the effectiveness of the control method proposed in this patent.

[0053] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A coordinated control method for midpoint voltage balancing and circulating current suppression in a parallel NPC grid-connected inverter system, based on a parallel NPC grid-connected inverter system comprising multiple three-phase NPC grid-connected inverters with independent midpoints, characterized by: Includes the following steps S1, collects the inductor current of phases A, B, and C respectively according to the sampling period i Laz 、 i Lbz 、 i Lcz , and after 3s / 2s transformation, the inductor current in the αβ coordinate system is obtained i α 、 i β , using PR controller and combined with grid voltage feedforward, according to i α 、 i β Get the output voltage given value of the αβ coordinate system d αβ , use 2s / 3s transformation to get the duty cycle of the output voltage of phases A, B, and C d a 、 d b 、 d c ; S2, the A, B, and C phase inductor currents i Laz 、 i Lbz 、 i Lcz Add up to get the zero sequence current i z , the zero-sequence voltage required to be injected is obtained through the first PI controller d z , and the duty cycle d a 、 d b 、 d c Add up to get the duty cycle value d az 、 d bz 、 d cz , sent to the PWM modulation module; S3, each N After the first switching cycle, the midpoint voltage of the NPC grid-connected inverter is equal to the given value. U DC / 2 is used as the difference and then the second PI controller is used to obtain the midpoint voltage charge required to be injected. Q NP , according to the inductor current value of a specific phase, Q NP The absolute value of the inductor current is divided by the absolute value of the inductor current to calculate the additional pulse duty cycle required for this specific phase. d 0; The principle of selecting the specific phase is: when the output of the second PI controller Q NP When it is greater than 0, the phase with the largest current among the three-phase currents is selected as the specific phase, otherwise the phase with the smallest current among the three-phase currents is selected as the specific phase; S4, set the duty cycle value of each phase d az 、 d bz 、 d cz and every N The pulse duty cycle increases d 0 is sent to the PWM modulation module and PWM pulses are generated.

2. The coordinated control method for mid-point voltage balance and circulating current suppression in a NPC grid-connected inverter parallel system according to claim 1, characterized in that: The additional pulse duty cycle in step S3 d 0=| Q NP | / | i Lkz |, where k = a , b , c .

3. The coordinated control method for mid-point voltage balance and circulating current suppression in a NPC grid-connected inverter parallel system according to claim 2, characterized in that: The N is set to 3.

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