A low common-mode voltage modulation method for a three-phase inverter

Abstract

This invention discloses a low common-mode voltage modulation method for three-phase inverters. The key feature of this method is that it detects the direction of the current to flip the modulation voltage of the corresponding phase under any power factor condition. Analysis of different voltage and current sectors reveals that the proposed modulation method reduces the common-mode voltage amplitude across the entire power factor range, effectively reducing leakage current caused by common-mode voltage. Simulations and experiments show that after implementing the proposed low common-mode voltage modulation method for three-phase inverters, the system common-mode voltage amplitude is reduced to 0.167. V dc .

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, specifically to a control method for a three-phase inverter, and more particularly to a modulation method for reducing the common-mode voltage of a three-phase inverter system, an inverter using the method, and a computer-readable storage medium. Background Technology

[0002] With the continuous increase in grid-connected capacity of renewable energy sources such as photovoltaic and wind power generation, the performance and safety requirements for three-phase inverters, as the core equipment for energy conversion, are becoming increasingly stringent. In inverters employing pulse width modulation (PWM) technology, the high-speed switching of power switching devices generates significant common-mode voltage. Common-mode voltage refers to the potential difference between the three-phase output terminals and the reference ground or system neutral point.

[0003] High-frequency, high-amplitude common-mode voltages can couple into common-mode currents through the system's parasitic capacitance, leading to electromagnetic interference and affecting the normal operation of surrounding electronic equipment. Simultaneously, common-mode currents increase system leakage current, threatening personal safety and potentially causing malfunctions in protection devices. Long-term common-mode voltage stress can also accelerate the aging of motor windings and cable insulation, reducing equipment lifespan.

[0004] Current measures to suppress common-mode voltage can be categorized into four main types: improved modulation strategies, special topologies, the addition of common-mode filters, and optimized grounding and wiring design. Using special topologies such as three-level or multi-level inverters can naturally reduce the common-mode voltage amplitude and improve waveform quality, but this increases system complexity, cost, and control difficulty. While common-mode filters (such as common-mode inductors and Y-capacitors) can effectively attenuate common-mode current and electromagnetic interference, and are suitable for back-end management of existing systems, they cannot eliminate the common-mode voltage itself and have problems such as large size, increased leakage current, and potential resonance risks. Furthermore, while reasonable grounding methods and wiring optimization are low-cost, easy to implement, and contribute to improved overall electromagnetic compatibility, their suppression effect is limited and they are insufficient to handle high-amplitude common-mode voltages. Among these, improved modulation strategies (such as low common-mode voltage SVPWM or dual-carrier PWM) can significantly reduce common-mode voltage at its source by avoiding the use of switching states that generate high common-mode levels. They have the advantages of requiring no additional hardware and being easy to implement, but often come at the cost of sacrificing output voltage utilization and increasing harmonics or switching losses. Summary of the Invention

[0005] To address the issue of high common-mode voltage amplitude in inverters, this invention proposes a low common-mode voltage modulation method suitable for three-phase inverters. It identifies the sector containing the current by accurately detecting the direction of the load phase current, and uses the current direction to align the switching states and flip the modulation wave, thereby avoiding the use of voltage vectors with high common-mode voltage and ultimately suppressing the system's common-mode voltage amplitude.

[0006] According to one aspect of the present invention, a low common-mode voltage modulation method for a three-phase inverter is provided, comprising: Obtain voltage control commands, wherein the voltage control commands are voltage components in a two-phase stationary coordinate system; According to the voltage control command, a three-phase initial modulation voltage signal is generated using a space vector pulse width modulation algorithm; Detect the three-phase load current at the output of the three-phase inverter and extract the current direction information of each phase load current; Based on the current direction information of the load current of each phase, the three-phase initial modulation voltage signal is adjusted to generate the three-phase actual modulation voltage signal; wherein, for the phase with positive current direction, its actual modulation voltage signal is equal to the initial modulation voltage signal of that phase; for the phase with negative current direction, its actual modulation voltage signal is equal to the carrier amplitude minus the initial modulation voltage signal of that phase. The actual three-phase modulated voltage signal is compared with the carrier signal to generate a PWM control signal for controlling the on / off state of each power switch in the three-phase inverter.

[0007] As a further technical solution, the comparison result between the actual three-phase modulated voltage signal and the carrier signal ensures that, within one switching cycle, the switching state sequence output by the three-phase inverter does not contain a zero voltage vector that is simultaneously 000 or simultaneously 111.

[0008] As a further technical solution, the current space is divided into six consecutive current sectors according to the direction of the three-phase load current, and the voltage space is divided into six consecutive voltage sectors according to the magnitude of the initial modulation voltage signal of the three phases; the switching state sequence is uniquely determined by the combination of the current sector and the voltage sector.

[0009] As a further technical solution, the current sector is divided based on the zero-crossing point of the three-phase load current. Each current sector corresponds to a current distribution state where one phase current is positive and two phase currents are negative, or one phase current is negative and two phase currents are positive.

[0010] As a further technical solution, the voltage sectors are divided based on the amplitude relationship of the three-phase initial modulation voltage signals, and each voltage sector corresponds to the amplitude sorting relationship of the three-phase modulation voltage signals.

[0011] As a further technical solution, the switching state sequence consists of three or four consecutive switching states, and the first switching state in the switching state sequence is determined by the current sector.

[0012] As a further technical solution, the carrier signal is a sawtooth wave.

[0013] According to one aspect of the present invention, a three-phase inverter is provided, comprising: The main power circuit includes multiple power switching transistors; The current detection unit is used to detect the three-phase load current at the output of the three-phase inverter. A controller configured to perform the low common-mode voltage modulation method for a three-phase inverter.

[0014] As a further technical solution, the main power circuit is a two-level voltage source inverter topology, and its DC side includes a first DC side capacitor and a second DC side capacitor. The connection point of the first DC side capacitor and the second DC side capacitor is connected to the neutral point of the three-phase load and grounded.

[0015] According to one aspect of the present invention, a computer-readable storage medium is provided having a computer program stored thereon that, when executed by a processor, implements the aforementioned low common-mode voltage modulation method for a three-phase inverter.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Effective suppression of common-mode voltage across the entire power factor range: By detecting the current direction and flipping the modulation wave, the switching state sequence output by the inverter within one switching cycle does not contain the two zero voltage vectors (000) and (111), thereby suppressing the maximum common-mode voltage amplitude of the system to one-sixth of the DC side voltage under any power factor, effectively reducing common-mode leakage current and electromagnetic interference.

[0017] 2. High output power quality: While significantly reducing common-mode voltage, it maintains good output current waveform quality, and the current harmonic characteristics meet application requirements without affecting the normal operation performance of the inverter.

[0018] 3. Simple to implement and widely applicable: This method only requires adding current direction detection and modulation wave reversal to the existing SVPWM algorithm. It does not require modification of the main circuit hardware, is easy to implement on existing digital control platforms, and can be widely applied to various three-phase two-level inverter topologies.

[0019] 4. Comprehensive protection: This invention protects the method, device and storage medium simultaneously, forming a multi-layered protection system that can effectively prevent different forms of infringement. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a topology diagram of the three-phase inverter in an embodiment of the present invention; Figure 2 This is a diagram showing the switching vector distribution of a three-phase inverter; Figure 3 This is a logic block diagram of the low common-mode voltage modulation method provided in the embodiments of the present invention; Figure 4 This is a schematic diagram illustrating the generation principle of the PWM control signal in an embodiment of the present invention; Figure 5 This is a schematic diagram of the load phase current waveform and current sector division of a three-phase inverter; Figure 6 This is a schematic diagram of the voltage modulation waveform and voltage sector division of a three-phase inverter under traditional SVPWM modulation; Figure 7 This is the voltage modulation waveform after flipping under a low common-mode voltage modulation method applicable to three-phase inverters proposed in this embodiment of the invention; Figure 8 This is a schematic diagram of the simulation results of the output current and system common-mode voltage of a three-phase inverter under traditional SVPWM modulation; Figure 9 This is a schematic diagram showing the simulation results of the output current and system common-mode voltage of a three-phase inverter under the implementation of a low common-mode voltage modulation strategy for three-phase inverters proposed in this invention. Detailed Implementation

[0022] The terms “comprising” and “having”, and any variations thereof, in the specification, claims, and accompanying drawings of this invention are intended to cover a non-exclusive inclusion, such as a process, method, system, product, or apparatus that includes a series of steps or units, not necessarily limited to those explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. In addition, the technical features of the various embodiments or individual embodiments provided by the present invention can be arbitrarily combined to form new technical solutions. Such combinations are not bound by the order of steps and / or structural composition patterns, but must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0024] This invention provides a low common-mode voltage modulation method suitable for three-phase inverters. Unlike traditional control methods, this method detects the direction of the current and flips the modulation voltage of the corresponding phase under any power factor condition, thereby reducing the common-mode voltage amplitude across the entire power factor range and effectively reducing leakage current problems caused by common-mode voltage.

[0025] The modulation method described in this invention is applicable to various three-phase two-level inverter topologies, one typical circuit being as follows: Figure 1 As shown, the positive direction of a physical quantity is indicated by an arrow. Figure 1 A typical circuit includes a DC-side power supply. V dc First DC-side capacitor C dc1 Second DC-side capacitor C dc2 First main switch transistor S a_P Second main switch transistor S a_N Third main switch tube S b_P Fourth main switch transistor S b_N Fifth main switch tube S c_P The sixth main switch tube S c_N A three-phase filter and a three-phase Y-type load. The first DC-side capacitor... C dc1 Second DC side capacitor C dc2 satisfy C dc1 = C dc2 = Cdc The midpoint of the capacitor is grounded to the neutral point of the three-phase Y-connected load.

[0026] To simplify the analysis, the following assumptions are made: all components operate under ideal conditions. Termination resistance and line parasitic resistance are ignored. The DC bus capacitor has a sufficiently large value to ensure minimal fluctuations in the DC bus midpoint voltage.

[0027] For a three-phase inverter, the bridge arm state is recorded as "1" when the upper switch is turned on and as "0" when the lower switch is turned on. Therefore, a three-phase inverter contains 8 bridge arm switch states, such as... Figure 2 As shown, they are (000), (001), (010), (011), (100), (101), (110), and (111), respectively. In a typical three-phase system, the common-mode voltage is defined as shown in the formula, where... V CM Indicates the common-mode voltage of the system. V Sa , V Sb , V Sc This represents the three-phase bridge arm voltage, where the bridge arm voltage is when the bridge arm state is "1". V dc / 2; When the bridge arm state is "0", the bridge arm voltage is - V dc / 2.

[0028] .

[0029] Therefore, the eight bridge arm states of the three-phase inverter can generate four different common-mode voltage amplitudes, among which (000) generates an amplitude of - V dc / 2 common-mode voltage; (001), (010), (100) generate an amplitude of - V dc / 6 common-mode voltage; (011), (101), (110) generate amplitudes of V dc / 6 common-mode voltage; (111) generates an amplitude of V dc / 2 common-mode voltage.

[0030] In summary, to suppress the common-mode voltage amplitude in a three-phase inverter system, it is necessary to avoid using the zero voltage vectors (000) and (111).

[0031] The low common-mode voltage modulation strategy for three-phase inverters proposed in this invention is based on the fundamental principle that the three-phase load current must have two different directions at the same time. Its specific implementation logic is as follows: Figure 3 As shown, where u α , u β This represents the control signal output by the controller; u ma , u mb , u mc This represents the modulated wave calculated under traditional SVPWM modulation. t PWM Indicates the amplitude of the triangular modulated wave; This represents the voltage modulation wave that actually has an effect. s a_P and s a_N This represents the drive signals for the upper and lower transistors of phase a bridge arm. Compared to traditional SVPWM modulation, the low common-mode voltage modulation strategy proposed in this invention modulates the modulation waveform under the SVPWM modulation strategy only by sampling the direction information of the load current.

[0032] like Figure 3 As shown, the method of this embodiment of the invention includes the following steps: Step 1: Obtain voltage control commands.

[0033] A two-phase stationary coordinate system is obtained through a closed-loop control system (such as DQ decoupling control of output voltage or output current). α-β Voltage control command u α , u β Among them, voltage control commands u α , u β As input for subsequent SVPWM or other modulation methods.

[0034] Step 2: Generate the three-phase initial modulation voltage signal.

[0035] In space vector pulse width modulation (SVPWM), two-phase stationary coordinate systems are used. α-β Voltage vector under u α , u β Reconstructing three-phase modulation voltage u ma , u mb , u mcThe process can generally be calculated using the following formula: .

[0036] Step 3: Detect the direction of the load current.

[0037] The direction of the load phase current is detected by a current sensor, and the direction information is extracted. Sign ( x ),in x for i a , i b , i c .

[0038] , When the current direction is defined as positive Sign ( x When )=1, it is negative. Sign ( x )=0.

[0039] Step 4: Modulation wave flip.

[0040] Based on the voltage modulation signals obtained in steps 2 and 3, and the current direction information, the final voltage modulation signal acting on the converter is obtained, and its mathematical expression is: , in, t PWM This represents the amplitude of the carrier signal.

[0041] The adjustment rules are as follows: For a phase with a positive current direction, the actual modulation voltage signal is equal to the initial modulation voltage signal of that phase; For a phase with a negative current direction, the actual modulated voltage signal is equal to the carrier amplitude minus the initial modulated voltage signal of that phase.

[0042] Step 5: Generate PWM control signal.

[0043] like Figure 4 As shown, after obtaining the voltage modulation signal in step 4, it is combined with the triangular carrier signal. c The comparison generates the main switch control signal for the three-phase bridge arm of the inverter. s a_P , s a_N , s b_P , s b_N , s c_P , sc_N Its expression is: , in f This indicates the switching frequency of the inverter.

[0044] Furthermore, analyzing the magnitude of the common-mode voltage amplitude of the three-phase system under the low common-mode voltage modulation strategy for three-phase inverters proposed in this invention, the load phase current of the three-phase inverter can be divided into six sectors according to the current direction, such as... Figure 5 As shown; the three-phase modulated wave can be divided into six sectors according to its amplitude, as follows: Figure 6 As shown, by analyzing the switching vector types of the six current sectors under different six voltage sectors, the maximum common-mode voltage amplitude of the proposed low common-mode voltage modulation strategy applicable to three-phase inverters under full-load power factor can be determined.

[0045] Taking current sector 2 and voltage sector 1 as an example, the current in phase a is positive, and the currents in phases b and c are negative. At the very beginning of a switching cycle, the inverter's state is (100). Under current sector 2, the voltage modulation waveform after the flip is as follows: Figure 7 As shown. Meanwhile, by Figure 4 Therefore, under sawtooth wave modulation, the amplitude of the modulating wave represents the duration of the initial switching state of the corresponding phase. When the voltage sector is 1, the magnitude relationship of the voltage modulation wave acting on the converter is as follows: .

[0046] Therefore, the inverter's switching state changes for the first time. c The phase switch state flips, and the switch state switches to (101); the second state occurs a The phase state flips, and the switch state changes to (001); finally b The phase switch state flips, and the switch state switches to (011). At this time, the bridge arm switch state will not have the effect of zero vector, and the system common-mode voltage amplitude is [value missing]. V dc / 6.

[0047] Meanwhile, the inverter was analyzed in all voltage and current sectors, and the switching states are listed in Tables 1 to 6: Table 1 Current sector is 1 .

[0048] Table 2 shows that the current sector is 2. .

[0049] Table 3 shows that the current sector is 3. .

[0050] Table 4 shows that the current sector is 4. .

[0051] Table 5 shows that the current sector is 5. .

[0052] Table 6 shows that the current sector is 6. .

[0053] It can be seen that the inverter does not have a zero vector effect at any power factor, meaning that the maximum common-mode voltage amplitude of the system at any power factor is... V dc / 6.

[0054] To verify the effectiveness of the low common-mode voltage modulation method for three-phase inverters proposed in this invention, a simulation model was constructed. The inverter simulation parameters are shown in Table 7.

[0055] Table 7 Inverter Simulation Parameters .

[0056] in Figure 8 Simulation results of the output current waveform and system common-mode voltage under the traditional SVPWM modulation strategy are presented. The results show that, due to the unavoidable use of zero-voltage vectors (000) and (111) under traditional SVPWM modulation, the maximum common-mode voltage amplitude of the system is... For V dc / 2, i.e., 400V. After applying the low common-mode voltage modulation strategy for three-phase inverters proposed in this invention, the simulation results are as follows: Figure 9 As shown, the results indicate that the common-mode voltage amplitude of the system decreased to 133.33V, which means... V dc / 6, effectively reducing the impact of common-mode voltage on the system. On the other hand, compared to Figure 8 and Figure 9 As can be seen from the output current waveform shown, after applying the low common-mode voltage modulation strategy for three-phase inverters proposed in this invention, the output current harmonic characteristics remain good and meet all the requirements.

[0057] Based on the same inventive concept as the foregoing embodiments, this embodiment of the invention also provides a three-phase inverter, the structure of which is as follows: Figure 1 As shown, it includes a main power circuit, a current detection unit, and a controller.

[0058] The main power circuit is a two-level voltage source inverter topology, including a DC-side power supply. Vdc First DC side capacitor C dc1 Second DC side capacitor C dc2 and six power switching transistors (S a_P S a_N S b_P S b_N S c_P S c_N First DC-side capacitor C dc1 Second DC side capacitor C dc2 satisfy C dc1= C dc2= C dc The midpoint is connected to the neutral point of the three-phase Y-type load for grounding.

[0059] The current detection unit is used to detect the three-phase load current at the output of the three-phase inverter. i a , i b , i c .

[0060] The controller is configured to execute the low common-mode voltage modulation method described in the foregoing embodiments, specifically including: acquiring voltage control commands, generating three-phase initial modulation voltage signals, extracting current direction information, adjusting the modulation wave signal, and generating PWM control signals. The controller can be a digital signal processor (DSP), a microcontroller (MCU), a field-programmable gate array (FPGA), or other processing chips with computing capabilities.

[0061] Based on the same inventive concept as the foregoing embodiments, this embodiment of the invention also provides a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the low common-mode voltage modulation method described in the foregoing embodiments. The storage medium can be any medium capable of storing program code, such as a USB flash drive, portable hard drive, read-only memory (ROM), random access memory (RAM), magnetic disk, or optical disk.

[0062] In summary, this invention discloses a low common-mode voltage modulation method suitable for three-phase inverters. A key feature of this method is that it detects the direction of the current to flip the modulation voltage of the corresponding phase under any power factor condition. Analysis of different voltage and current sectors reveals that the proposed modulation method reduces the common-mode voltage amplitude across the entire power factor range, effectively reducing leakage current problems caused by common-mode voltage. Simulations and experiments show that after implementing the proposed low common-mode voltage modulation method for three-phase inverters, the system common-mode voltage amplitude is reduced to 0.167. V dc .

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the technical solutions of the embodiments of the present invention.

Claims

1. A low common-mode voltage modulation method for a three-phase inverter, characterized in that, include: Obtain voltage control commands, wherein the voltage control commands are voltage components in a two-phase stationary coordinate system; According to the voltage control command, a three-phase initial modulation voltage signal is generated using a space vector pulse width modulation algorithm; Detect the three-phase load current at the output of the three-phase inverter and extract the current direction information of each phase load current; Based on the current direction information of the load current of each phase, the three-phase initial modulation voltage signal is adjusted to generate the three-phase actual modulation voltage signal; wherein, for the phase with positive current direction, its actual modulation voltage signal is equal to the initial modulation voltage signal of that phase; for the phase with negative current direction, its actual modulation voltage signal is equal to the carrier amplitude minus the initial modulation voltage signal of that phase. The actual three-phase modulated voltage signal is compared with the carrier signal to generate a PWM control signal for controlling the on / off state of each power switch in the three-phase inverter.

2. The low common-mode voltage modulation method for a three-phase inverter according to claim 1, characterized in that, The comparison result between the actual three-phase modulated voltage signal and the carrier signal ensures that, within one switching cycle, the switching state sequence output by the three-phase inverter does not contain a zero voltage vector that is simultaneously 000 or simultaneously 111.

3. The low common-mode voltage modulation method for a three-phase inverter according to claim 2, characterized in that, The current space is divided into six consecutive current sectors according to the direction of the three-phase load current, and the voltage space is divided into six consecutive voltage sectors according to the magnitude of the initial modulation voltage signal of the three phases; the switching state sequence is uniquely determined by the combination of the current sector and the voltage sector.

4. The low common-mode voltage modulation method for a three-phase inverter according to claim 3, characterized in that, The current sectors are divided based on the zero-crossing points of the three-phase load current. Each current sector corresponds to a current distribution state where one phase current is positive and two phase currents are negative, or one phase current is negative and two phase currents are positive.

5. A low common-mode voltage modulation method for a three-phase inverter according to claim 3, characterized in that, The voltage sectors are divided based on the amplitude relationship of the three-phase initial modulation voltage signals, and each voltage sector corresponds to the amplitude sorting relationship of the three-phase modulation voltage signals.

6. A low common-mode voltage modulation method for a three-phase inverter according to claim 3, characterized in that, The switching state sequence consists of three or four consecutive switching states, and the first switching state in the switching state sequence is determined by the current sector.

7. The low common-mode voltage modulation method for a three-phase inverter according to claim 1, characterized in that, The carrier signal is a sawtooth wave.

8. A three-phase inverter, characterized in that, include: The main power circuit includes multiple power switching transistors; The current detection unit is used to detect the three-phase load current at the output of the three-phase inverter. A controller configured to perform a low common-mode voltage modulation method for a three-phase inverter as described in any one of claims 1 to 7.

9. A three-phase inverter according to claim 8, characterized in that, The main power circuit is a two-level voltage source inverter topology, and its DC side includes a first DC side capacitor and a second DC side capacitor. The connection point of the first DC side capacitor and the second DC side capacitor is connected to the neutral point of the three-phase load and grounded.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the program implements a low common-mode voltage modulation method for a three-phase inverter as described in any one of claims 1 to 7.

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