Single-phase inverter double-closed-loop PI control strategy based on double feedforward

Abstract

The invention belongs to the technical field of inverters, and particularly relates to a single-phase inverter double-closed-loop PI control strategy based on double feedforward, and the strategy comprises a single-phase inverter hardware circuit; a plurality of voltage and current sensors are arranged in a hardware circuit of the single-phase inverter to accurately collect input direct-current bus voltage Udc, single-phase inverter output voltage uo, load current io and inductive current iL, and collected analog signals are digitalized and then sent to the DSP control module and the DSPP control module; the single-phase inverter double-closed-loop PI control strategy based on double feedforward has the advantages that the control strategy can be rapidly adjusted in a complex and dynamic working environment, and the effects of rapid response decision making and high-speed accurate control are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of inverters, and in particular relates to a double closed-loop PI control strategy for a single-phase inverter based on double feedforward. Background Art

[0002] Off-grid voltage source inverter systems can only rely on battery packs or other energy storage devices to provide power. Fluctuations in load or input voltage may cause battery voltage fluctuations, thereby affecting the output voltage of the inverter, and voltage fluctuations or instability may occur. In addition, off-grid voltage source inverter systems usually need to adapt to different types of loads (such as resistive loads, inductive loads, and nonlinear loads). For some high-peak power loads (such as motor starting, etc.), the output voltage of the inverter may produce phase shifts, voltage fluctuations, and current waveform distortion, causing the output result to deviate from the target value. Therefore, off-grid voltage source inverter systems usually need to use control strategies (such as PID control, current control, etc.) to adjust the output voltage and current. The dual closed-loop control scheme of current inner loop and voltage outer loop is one of the current development directions of high-performance inverter power supplies. When the load changes suddenly and the input voltage fluctuates, the current inner loop is responsible for quickly responding to the load change and quickly adjusting the output current to ensure current stability and avoid system instability caused by excessive current or current fluctuations; the voltage outer loop is responsible for adjusting the output voltage to ensure that the output voltage is stable after the current changes and is not affected by load mutations or power supply fluctuations, thereby improving the response speed and stability, which can significantly improve the robustness and adaptability of the power supply system and ensure high-precision control of the output voltage and current.

[0003] The early dual closed-loop control scheme of the current inner loop and the voltage outer loop mainly relied on the effective value of the output voltage as the feedback signal of the voltage outer loop. However, since the effective value is an average measure of the entire waveform, it cannot accurately reflect the instantaneous changes of the waveform, nor can it effectively suppress the harmonic components, resulting in the output voltage waveform may contain a large number of harmonics, causing the waveform to be distorted, which in turn affects the stability and accuracy of the equipment. In addition, when the load changes or the input voltage fluctuates, due to the relatively slow response speed of the effective value control, the output voltage waveform may fluctuate greatly and take a long time to return to a stable state, which will affect the stability and reliability of the system.

[0004] In order to solve the problems of the early solutions, the voltage and current dual closed-loop control scheme gradually turned to instantaneous value control. This control strategy can respond to changes in voltage and current more quickly and improve the dynamic performance of the system. At the same time, by introducing advanced control algorithms such as PI control and PR control, the control accuracy and stability are further improved. For the dual closed-loop PI control system, it can be divided into two dual closed-loop control schemes: the inductor current inner loop output voltage outer loop and the capacitor current inner loop output voltage outer loop. The inductor current inner loop output voltage outer loop dual closed-loop control scheme can quickly respond to load changes and input voltage fluctuations through real-time feedback and adjustment, which helps to reduce the fluctuation of the system in the transient process and improve the reliability and stability of the system. However, since the inductor current is used as the inner loop feedback quantity, its size is greatly affected by the load. When the load changes greatly, the inductor current may produce large fluctuations, affecting the stability of the system. The capacitor current inner loop output voltage outer loop dual closed-loop control scheme can achieve rapid adjustment of the output voltage by adjusting the capacitor current, and shows good dynamic suppression for both linear and nonlinear loads. However, this control scheme increases control complexity and does not have an automatic current limiting function. When the load current is too large, additional protection measures may be required to prevent overcurrent from damaging the device.

[0005] In the current practical application of power electronics, the voltage and current dual closed-loop control scheme of single variable control can only control one variable (such as voltage or current), which cannot fully reflect the system status, has certain limitations, and cannot meet the control requirements of high precision and high stability. The adaptability of the single variable control scheme to different working conditions is also relatively poor, that is, it cannot automatically adjust the control parameters according to different working conditions and load conditions, resulting in insufficient dynamic response performance of the system, and even causing the system to fail to operate normally or performance degradation. When the load changes or other external interference factors enter the system, the single variable control scheme may not be able to respond to these changes quickly, and thus cannot effectively suppress the impact of external interference on the system, resulting in output voltage or current fluctuations or instability, affecting the stability and reliability of the system. In addition, the control algorithm design and implementation of the single variable control scheme may be difficult, requiring a high level of technology and experience. This may lead to problems such as extended development cycle and increased costs.

[0006] Aiming at the problems existing in the voltage and current dual closed-loop control scheme of single variable control, the present invention proposes a dual closed-loop PI control strategy for a single-phase inverter based on dual feedforward. Summary of the invention

[0007] The purpose of the present invention is to provide a dual closed-loop PI control strategy for a single-phase inverter based on dual feedforward to solve the technical problems of insufficient dynamic response performance, poor adaptability and weak anti-interference ability, so as to achieve the purpose of enabling the system to quickly adjust the control strategy in a complex and dynamic working environment, and realize rapid response decision-making and high-speed and precise control.

[0008] In order to solve the above technical problems, the present invention provides a double closed-loop PI control strategy for a single-phase inverter based on double feedforward, comprising: a single-phase inverter hardware circuit;

[0009] The single-phase inverter hardware circuit has multiple voltage and current sensors built in to accurately collect the input DC bus voltage U dc , single-phase inverter output voltage u o , load current i o and the inductor current i L , the collected analog signal is digitized and sent to the DSP control module, and the DSPP control module;

[0010] The following steps are also included:

[0011] Step 1: Calculate the reference sine wave u ref The instantaneous sampling value u of the single-phase inverter output voltage o The difference is sent to the voltage outer loop PI regulator, and the adjusted error signal is used as the current reference value i ref For use in the inner current loop;

[0012] Step 2: Set the current reference value i ref and the instantaneous sampling value of the load current i o Add and then subtract the instantaneous sampling value of the inductor current i L The result is sent to the current inner loop PI regulator, and the output signal after current loop regulation is marked as i on ;

[0013] Step 3: Transform the signal i on The instantaneous sampling value u of the single-phase inverter output voltage o After decoupling, the signal is multiplied by the corresponding modulation ratio to generate an accurate duty cycle signal, which can then be converted into a corresponding SPWM switch control signal.

[0014] Step 4: The SPWM switch control signal is filtered out through an LC filter circuit to remove high-order harmonics and output a high-quality sine wave that meets specific frequency and amplitude.

[0015] Furthermore, both the voltage outer loop and the current inner loop adopt PI regulators.

[0016] Furthermore, the current inner loop adopts a closed-loop control strategy of output voltage feedforward and inductor current negative feedback.

[0017] Furthermore, the load current of the inverter is fed back by the current sensor to the PI controller in the current inner loop, which o With the current reference value i ref The error between them adjusts the switching state of the inverter.

[0018] For the current inner loop, the output voltage is an external disturbance. It is introduced into the loop as a feedforward quantity for decoupling. As long as a suitable feedforward decoupling coefficient β is set, the output voltage can be completely decoupled and the cross-feedback effect of the output voltage on the current inner loop can be completely eliminated.

[0019] Furthermore, the voltage outer loop adopts a closed-loop control strategy of load current feedforward and output voltage negative feedback.

[0020] Furthermore, the PI controller in the voltage outer loop is based on the output voltage u of the single-phase inverter. o With the reference sine wave u ref The error between them adjusts the switching state of the inverter.

[0021] For the voltage outer loop, the load current is an external disturbance. It is introduced into the loop as a feedforward quantity for decoupling. As long as a suitable feedforward decoupling coefficient α is set, the load current can be completely decoupled and its disturbance effect on the voltage outer loop can be completely eliminated.

[0022] The beneficial effects of the present invention are:

[0023] 1. The dual closed-loop PI control loop of the single-phase inverter based on dual feedforward of the present application generates an accurate duty cycle signal and converts it into a corresponding SPWM switch control signal. The signal is then filtered out of high-order harmonic components through an LC filter circuit, and finally outputs a high-quality sine wave that meets the specific frequency and amplitude.

[0024] 2. The load current and output voltage are selected for coordinated control to more comprehensively reflect the system status. This synergy solves the previously existing problems of limited control accuracy and stability. Secondly, the present invention also introduces dual feedforward to improve the feedback mechanism. By configuring a reasonable feedback gain, the system can effectively predict and suppress disturbances in the system when facing sudden load changes or changes in internal parameters, and take compensation measures in advance to achieve faster response speed, better tracking performance and smaller harmonic distortion (THD), solving the previously existing problems of insufficient dynamic response performance, poor adaptability and weak anti-interference ability.

[0025] 2. The DSP (digital signal processor) control module is selected as the hardware support, so that the system can quickly adjust the control strategy in a complex and dynamic working environment, achieve rapid response decision-making and high-speed and precise control, and solve the previous problems of high complexity of control algorithms, slow computing speed, poor flexibility and real-time performance.

[0026] 3. It has higher control accuracy and stability, can control the output current and voltage of the system more accurately, and reduce the difference in output changes and system response time; secondly, compensation measures can be taken in advance under load changes or other external interferences, effectively predict and suppress disturbances in the system, and greatly reduce network losses and harmonic distortion while maintaining good tracking performance, thereby improving the output voltage quality of the single-phase inverter as a whole, and improving the dynamic responsiveness and stability of the system; then, it also enhances the protection capability of the system, can effectively limit the current of the switching device, and extend the service life of the equipment; in addition, the present invention uses a DSP control module as hardware support, which more intuitively reflects the dynamic characteristics of the system, significantly improves the system's computing speed and flexibility, and thus improves the overall work efficiency.

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 It is a single-phase full-bridge inverter control circuit diagram;

[0030] Figure 2 is the transfer function block diagram of the equivalent single-phase full-bridge inverter;

[0031] Figure 3 This is the principle diagram of the unipolar frequency-multiplied SPWM modulation method;

[0032] Figure 4 It is the current flow diagram of the single-phase full-bridge inverter control circuit during the period t0 to t1;

[0033] Figure 5 It is the current flow diagram of the single-phase full-bridge inverter control circuit during the period t1 to t2;

[0034] Figure 6It is the current flow diagram of the single-phase full-bridge inverter control circuit during the period t2 to t3;

[0035] Figure 7 It is the current flow diagram of the single-phase full-bridge inverter control circuit during the period of t3 to t4;

[0036] Figure 8 It is a double closed-loop control block diagram of a single-phase full-bridge inverter;

[0037] Fig. 9 It is the block diagram of the inner loop control of the inductor current of a single-phase full-bridge inverter;

[0038] Fig.10 It is the block diagram of the output voltage outer loop control of a single-phase full-bridge inverter;

[0039] Fig.11 This is the simulation diagram of the single-phase full-bridge inverter in MATLAB / Simulink;

[0040] FIG12( a ) is an output voltage waveform of a double closed-loop PI control system of a single-phase inverter based on double feedforward under no-load conditions;

[0041] FIG12( b ) is the output voltage waveform of a common double closed-loop control system under no-load conditions;

[0042] FIG13( a ) is a total harmonic distortion analysis of a double closed-loop PI control system of a single-phase inverter with double feedforward under no-load conditions;

[0043] Figure 13(b) is the total harmonic distortion analysis of a common dual closed-loop control system under no-load conditions;

[0044] FIG14( a ) is an output voltage waveform of a double closed-loop PI control system of a double-feedforward single-phase inverter when the inverter suddenly changes from no-load to heavy-load at 0.1 s;

[0045] Figure 14(b) is the output voltage waveform of the common dual closed-loop control system when it changes from no-load to heavy-load at 0.1s;

[0046] FIG15( a ) is a total harmonic distortion analysis of a double-feedforward single-phase inverter double closed-loop PI control system when the system changes suddenly from no-load to heavy-load at 0.1 s;

[0047] Figure 15(b) is the total harmonic distortion analysis of a common dual closed-loop control system when the system changes suddenly from no-load to heavy-load at 0.1s. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] Example:

[0050] like Figure 1 to Figure 1 5, a double closed-loop PI control strategy for a single-phase inverter based on double feedforward includes: a single-phase inverter hardware circuit; a single-phase inverter hardware circuit has multiple voltage and current sensors built in to accurately collect the input DC bus voltage U dc , single-phase inverter output voltage u o , load current i o and the inductor current i L , the collected analog signal is digitized and sent to the DSP control module, and then to the DSPP control module.

[0051] The following steps are also included:

[0052] Step 1: Calculate the reference sine wave u ref The instantaneous sampling value u of the single-phase inverter output voltage o The difference is sent to the voltage outer loop PI regulator, and the adjusted error signal is used as the current reference value i ref For use in the inner current loop;

[0053] Step 2: Set the current reference value i ref and the instantaneous sampling value of the load current i o Add and then subtract the instantaneous sampling value of the inductor current i L The result is sent to the current inner loop PI regulator, and the output signal after current loop regulation is marked as i on ;

[0054] Step 3: Transform the signal i on The instantaneous sampling value u of the single-phase inverter output voltage o After decoupling, the signal is multiplied by the corresponding modulation ratio to generate an accurate duty cycle signal, which can then be converted into a corresponding SPWM switch control signal.

[0055] Step 4: The SPWM switch control signal is filtered out through an LC filter circuit to remove high-order harmonics and output a high-quality sine wave that meets specific frequency and amplitude.

[0056] Among them, both the voltage outer loop and the current inner loop use PI regulators.

[0057] Among them, the current inner loop adopts a closed-loop control strategy of output voltage feedforward and inductor current negative feedback.

[0058] The load current of the inverter is fed back by the current sensor to the PI controller in the current inner loop, which controls the load current i o With the current reference value i ref The error between them adjusts the switching state of the inverter.

[0059] Among them, the voltage outer loop adopts a closed-loop control strategy of load current feedforward and output voltage negative feedback.

[0060] Among them, the PI controller in the voltage outer loop is based on the single-phase inverter output voltage u o With the reference sine wave u ref The error between them adjusts the switching state of the inverter.

[0061] In summary:

[0062] 1. The dual closed-loop PI control loop of the single-phase inverter based on dual feedforward of the present application generates an accurate duty cycle signal and converts it into a corresponding SPWM switch control signal. The signal is then filtered out of high-order harmonic components through an LC filter circuit, and finally outputs a high-quality sine wave that meets the specific frequency and amplitude.

[0063] 2. The load current and output voltage are selected for coordinated control to more comprehensively reflect the system status. This synergy solves the previously existing problems of limited control accuracy and stability. Secondly, the present invention also introduces dual feedforward to improve the feedback mechanism. By configuring a reasonable feedback gain, the system can effectively predict and suppress disturbances in the system when facing sudden load changes or changes in internal parameters, and take compensation measures in advance to achieve faster response speed, better tracking performance and smaller harmonic distortion (THD), solving the previously existing problems of insufficient dynamic response performance, poor adaptability and weak anti-interference ability.

[0064] 2. The DSP (digital signal processor) control module is selected as the hardware support, so that the system can quickly adjust the control strategy in a complex and dynamic working environment, achieve rapid response decision-making and high-speed and precise control, and solve the previous problems of high complexity of control algorithms, slow computing speed, poor flexibility and real-time performance.

[0065] 3. It has higher control accuracy and stability, can control the output current and voltage of the system more accurately, and reduce the difference in output changes and system response time; secondly, compensation measures can be taken in advance under load changes or other external interferences, effectively predict and suppress disturbances in the system, and greatly reduce network losses and harmonic distortion while maintaining good tracking performance, thereby improving the output voltage quality of the single-phase inverter as a whole, and improving the dynamic responsiveness and stability of the system; then, it also enhances the protection capability of the system, can effectively limit the current of the switching device, and extend the service life of the equipment; in addition, the present invention uses a DSP control module as hardware support, which more intuitively reflects the dynamic characteristics of the system, significantly improves the system's computing speed and flexibility, and thus improves the overall work efficiency.

[0066] like Figure 1 As shown, the mathematical modeling of a single-phase full-bridge inverter based on a double feedforward double closed-loop PI control strategy is as follows:

[0067] Among them U dc is the input DC bus voltage; i dc is the input DC bus current; VT1, VT2, VT3, VT4 are ideal SiC MOSFET power switch tubes; VD1, VD2, VD3, VD4 are freewheeling diodes; u in is the output voltage of the full-bridge inverter circuit; L is the output filter inductance; i L is the current flowing through the output filter inductor, referred to as the inductor current; r is the equivalent series resistance of the filter inductor, and its value is generally small; C is the output filter capacitor; i C is the current flowing through the output filter capacitor, referred to as capacitor current; u o is the output voltage of the full-bridge SPWM inverter circuit; i o It is the current flowing through the load, referred to as load current.

[0068] The inverter is essentially a nonlinear system, so the state space averaging method is usually used in engineering applications. Since the frequency (50Hz) of the inverter circuit system in the present invention is much smaller than the cutoff frequency (2.4kHz) of the switching frequency, the instantaneous change value of the system in the switching cycle can be replaced by the corresponding average value, so that the original nonlinear system can be converted into a linear system, and the system model can be analyzed using linear theory.

[0069] The small signal model of the present invention can be derived by combining the graph with Kirchhoff's voltage law and Kirchhoff's current law:

[0070] The output voltage u of the full-bridge inverter circuit in is the system input, and the inverter output voltage u oand the output filter inductor current i L is the system state variable, the load current i o is the system disturbance variable, then the continuous-time state equation (continuous model) of the present invention is:

[0071]

[0072] in is the inverter output voltage u o The derivative of is the output filter inductor current i L The derivative of .

[0073] Then the equivalent state space average model of the present invention is shown in the figure.

[0074] The formula (2) is written as:

[0075]

[0076] Where A is the state transfer matrix, B is the input matrix, C is the disturbance input matrix, and D is the output matrix. The expressions of each matrix are:

[0077]

[0078] If the reference sine wave (modulation wave) signal is set to u ref =U r ·sinωt, the triangular carrier is set to u c , then we can deduce the inverter output u in It can be expressed as:

[0079]

[0080] make Then formula (4) can be simplified as:

[0081] u in =K PWM ·u ref #(6)

[0082] Then the original equivalent state space average model can be simplified to an input u ref , the output is u in , the proportionality coefficient is K PWM The linear system, supplemented by the LC filter circuit, can obtain the equivalent single-phase full-bridge inverter transfer function block diagram, as shown in Figure 2 shown.

[0083] 2. Modulation strategy of single-phase full-bridge inverter

[0084] (1) Single-stage frequency-doubling SPWM modulation principle

[0085] The present invention adopts the unipolar frequency multiplication SPWM modulation method, and its principle is as follows: Figure 3 The present invention uses two triangle waves u with opposite polarities. c and -u c and a reference sinusoidal modulation wave u s .u c and u s The intersection generates two complementary SPWM pulse control signals u g1 and u g2 , in order to control the on and off of the power switches VT1 and VT2 in the first arm of the full-bridge circuit. That is, when u c >u s When VT1 is turned off, VT2 is turned on; when u c <u s When VT1 is turned on, VT2 is turned off. c and u s The intersection generates two complementary SPWM pulse control signals u g3 and u g4 , in order to control the on and off of the power switches VT3 and VT4 in the second bridge arm of the full-bridge circuit. c >u s When VT3 is turned on and VT4 is turned off; when -u c <u s , VT3 is turned off and VT4 is turned on.

[0086] Output voltage o The positive half cycle is given by u g1 and u g4 The output voltage is determined by the AND logic, and its output voltage is only U dc and 0. That is, when u g1 and u g4 When both are high level, VT1 and VT4 are turned on, and the output voltage u o =U dc ; when u g1 and u g4 When one of them is at a low level, VT1, VT3 or VT2, VT4 is turned on, and the output voltage u o =0. And in the positive half cycle, u g1 The high level pulse width is always greater than u g4 If the low level pulse width is wide, VT2 and VT3 will not be turned on at the same time, so the output voltage is only U dc and 0. Similarly, the output voltage u o The negative half cycle is caused by u g2 and u g3 The output voltage is determined by the AND logic. dcand 0 levels.

[0087] (2) Application of single-stage frequency-doubled SPWM modulation

[0088] The single-phase full-bridge inverter circuit of the present invention comprises two bridge arms, wherein VT1 and VT2 form a group of bridge arms, VT3 and VT4 form a group of bridge arms, and VD1, VD2, VD3 and VD4 are freewheeling diodes. In the present invention, a group of pulse control signals T2-V1 and T2-V2 with opposite phases are added to the bases of VT1 and VT2, and a group of pulse control signals T1-V1 and T1-V2 with a phase lagging behind T2-V1 and T2-V2 by 180 degrees are added to the bases of VT3 and VT4. The working process of the single-phase full-bridge inverter circuit of the present invention is as follows:

[0089] During the period of t0 to t1, T2-V1 and T1-V2 both input high-level pulse control voltage, then VT1 and VT4 are both turned on, and the current in the circuit flows as follows: Figure 4 As shown, the current flow direction is: U dc +→VT1→A→L→B→VT4→U dc -. Since point A is connected to the input DC bus voltage U through VT1 dc The positive voltage end, point B is connected to the input DC bus voltage U through VT4. dc The negative voltage end, so the voltage across A and B, that is, the output voltage u of the full-bridge inverter circuit o =U dc , and the polarity is A positive and B negative. At this time, the circuit is in active output mode, the voltage is positive, and the current is positive.

[0090] During the period from t1 to t2, T2-V1 inputs a high-level pulse control voltage, and T1-V2 inputs a low-level pulse control voltage, then VT1 is turned on and VT4 is turned off. At this time, the current i flowing through the output filter inductor L is L will suddenly decrease, and the inductor L will immediately generate an electromotive force (positive on the right and negative on the left), which will form a current loop through the freewheeling diode VD3. Figure 5 As shown, the current flows as follows: L+→B→VD3→VT1→A→L-. Since point A is connected to the input DC bus voltage U through VT1 dc The positive voltage end, point B is connected to the input DC bus voltage U through VD3 dc The positive voltage end of A and B, that is, the output voltage u of the full-bridge inverter circuit o = 0. At this time, the circuit is in freewheeling mode, the voltage is 0, and the current is positive.

[0091] During the period of t2 to t3, both T2-V2 and T1-V1 input high-level pulse control voltages. However, since there is still energy in the output filter inductor L that has not been fully released in the previous period, and the base of VT1 is cut off due to the input of low-level pulse control voltage, the electromotive force on the inductor L needs to pass through VD3 and VD2 to charge the output filter capacitor C, that is, the current flow during charging is: L+→B→VD3→C→VD2→A→L-. Since VD3 and VD2 are turned on, VT2 and VT3 are turned off due to the equal voltages of their respective c and e poles. At this time, point A is connected to the input DC bus voltage U through VD2. dc The negative voltage end of point B is connected to the input DC bus voltage U through VD3. dc The positive voltage end of A and B, that is, the output voltage u of the full-bridge inverter circuit o =U dc , and the polarity is A negative and B positive. When the electromotive force on the output filter inductor drops to the same level as the input DC bus voltage U dc When the inductor is equal to the output filter capacitor, VD2 and VD3 are cut off, VT2 and VT3 are turned on, and the current in the circuit flows as follows: Figure 6 As shown, the current flow direction is: U dc +→VT3→B→L→A→VT2→U dc -. At this time, the circuit is in active output mode, the voltage is negative, and the current is negative.

[0092] During the period of t3 to t4, T2-V2 inputs a high-level pulse control voltage, and T1-V1 inputs a low-level pulse control voltage, then VT2 is turned on and VT3 is turned off. At this time, the current i flowing through the output filter inductor L is L will suddenly decrease, and the inductor L will immediately generate an electromotive force (positive on the left and negative on the right), which will form a current loop through the freewheeling diode VD4. Figure 7 As shown, the current flows as follows: L+→A→VT2→B→VD4→L-. Since point A is connected to the input DC bus voltage U through VT2 dc The negative voltage end, point B is connected to the input DC bus voltage U through VD4. dc The negative voltage end, so the voltage across A and B, that is, the output voltage u of the full-bridge inverter circuit o = 0. At this time, the circuit is in freewheeling mode, the voltage is 0, and the current is negative.

[0093] After the t4 period, the circuit will repeat the above four working processes.

[0094] 3. Control strategy of single-phase full-bridge inverter

[0095] (1) Dual closed-loop control scheme

[0096] For the inverter system, the present invention selects a dual closed-loop control strategy. Compared with a single-loop control system, the dual closed-loop system can more effectively adjust the output of the inverter through feedback control of voltage and current, reduce the fluctuation and harmonic distortion of the output waveform, and thus improve the quality, stability and reliability of the output waveform. The closure of the current inner loop can quickly respond to external disturbances and load changes, and ensure rapid adjustment of the output current by real-time monitoring and adjusting control parameters, thereby improving the system's anti-disturbance ability; the closure of the voltage inner loop can ensure the stability of the output voltage. This system can adapt to different working environments and load conditions, especially under dynamic load conditions, it is more flexible and more adjustable.

[0097] The control quantity of the current inner loop is generally divided into the inductor current and the capacitor current. The current inner loop based on the capacitor current uses the voltage feedback of the capacitor to control the current. The current of the capacitor is proportional to its voltage change rate, so the voltage change measured by the capacitor can be converted into the control current. Although this design is simple and can play a buffering role in the case of voltage fluctuations, its response speed is slow, its anti-interference ability is weak, its efficiency is low, and its dynamic performance is not as good as the inductor current control. It is only suitable for applications that require output stability and energy storage. The current inner loop based on the inductor current uses the current feedback of the inductor for control. In the inductive load, the rate of change of the current is proportional to the voltage of the inductor, so the current measured by the inductor can be used to control the output of the system. This design has good dynamic response capability when the current changes, can quickly track the changes of the input signal, and has relatively high efficiency. In addition, the inductor has a certain suppression ability for high-frequency noise and electromagnetic interference, and can provide good short-circuit protection and overload protection, and can maintain stability in a noisy environment. Therefore, the present invention selects a double closed-loop control structure based on the inductor current inner loop and the output voltage outer loop.

[0098] The present invention adds output voltage feedforward decoupling and load current feedforward decoupling to the dual closed-loop control structure of the general inductor current inner loop and output voltage outer loop, and adds corresponding voltage and current sampling gain and decoupling coefficient in the loop to fit the debugging of the actual circuit. The control block diagram is as follows Figure 8 shown.

[0099] Among them G v (s) is the output voltage outer loop regulator, G i (s) is the inductor current inner loop regulator, is the output voltage sampling gain, is the output current sampling gain, is the inductor current sampling gain, K PWM is the amplitude modulation ratio of single-stage frequency-doubled SPWM modulation, α is the load current feedforward decoupling coefficient, and β is the output voltage feedforward decoupling coefficient.

[0100] The control scheme principle of the present invention is specifically explained as follows: First, the reference sine wave u is calculated ref The instantaneous sampling value u of the single-phase inverter output voltage o The difference is sent to the voltage outer loop PI regulator, and the adjusted error signal is used as the current reference value i ref For use in the current inner loop; then the current reference value i ref and the instantaneous sampling value of the load current i o Add and then subtract the instantaneous sampling value of the inductor current i L The result is sent to the current inner loop PI regulator, and the output signal after current loop regulation is marked as i on ; Then the signal i on The instantaneous sampling value u of the single-phase inverter output voltage o After decoupling, the signal is multiplied by the corresponding modulation ratio to generate an accurate duty cycle signal, which can then be converted into a corresponding SPWM switch control signal. Finally, the SPWM switch control signal is filtered out of high-order harmonics through an LC filter circuit to output a high-quality sine wave that meets the specific frequency and amplitude.

[0101] (2) Obtain the inductor current inner loop transfer function

[0102] The inductor current inner loop of the present invention is as follows Fig. 9 As shown, a negative feedback and feedforward control method is adopted, that is, a negative feedback of the inductor current and a feedforward control method of the output voltage.

[0103] For the inductor current inner loop, the output current of the inverter is also the load current i o The current sensor is used to feed back the PI controller, which then calculates the load current i o With the current reference value i ref The error between is used to adjust the switching state of the inverter. At this time, the output voltage u o For the inner current loop, it is an external disturbance, so its disturbance to the inner current loop cannot be ignored.

[0104] The present invention is to eliminate the output voltage u o The disturbance is selected to add the output voltage decoupling amount u after the PI controller. ob , the output of the current loop is:

[0105] u on =G i (s)·(i ref -i La )+u ob #(7)

[0106] As long as the output voltage feedforward decoupling coefficient When this happens, the output voltage can be completely decoupled, completely eliminating its cross-feedback effect on the current inner loop, and achieving flexible, stable, fast and accurate response of dual closed-loop control.

[0107] At this point, the open-loop transfer function G of the inner loop of the inductor current can be easily obtained: i1 (s) is:

[0108]

[0109] Then the closed-loop transfer function of the inductor current inner loop is G i2 (s) is:

[0110]

[0111] In the present invention, a PI (proportional integral) regulator is used for the current inner loop. Its proportional link generates a control output according to the current error (the difference between the set value and the actual value). An appropriate proportional gain can reduce the steady-state error of the system and quickly respond to the change of the error, so that the tracking performance of the system is improved. Such a controller has a simple structure and is relatively easy to implement and adjust. Its integral link integrates the current error, accumulates past errors, and generates a control output, which can eliminate the steady-state error of the system and improve the stability of the system in an environment with a large external disturbance, which is particularly important for high-precision equipment.

[0112] You can get:

[0113]

[0114] Substituting formula (10) into formula (8) and formula (9) respectively, we can obtain:

[0115]

[0116]

[0117] Convert equations (11) and (12) into the complex frequency domain and define ω1 = 2πf cI ,ω2=2πf bI :

[0118]

[0119]

[0120] The present invention obtains the parameters of the current loop PI controller by combining constraints, so that the current inner loop can obtain faster current following performance. The first constraint is the open-loop transfer function G of the current loop. i1 The crossover frequency f of (jω1) cI=0dB, the second constraint is the closed-loop transfer function of the current loop G i2 The cut-off frequency f of (jω2) bI =-3dB. That is:

[0121]

[0122] In addition, the present invention sets the crossover frequency of the current loop to be 1 / 100 of the switching frequency. Pick up The cut-off frequency of the current loop is the switching frequency Pick up The parameters of the current loop can be preliminarily determined by combining the basic hardware parameters.

[0123] (3) Obtain the output voltage outer loop transfer function

[0124] The output voltage outer loop of the present invention is as follows Fig.10 As shown, a negative feedback and feedforward control method is adopted, that is, an output voltage negative feedback and a load current feedforward control method.

[0125] For the output voltage outer loop, the output voltage u of the inverter o The voltage sensor is used to feed back the PI controller, which calculates the instantaneous output voltage u of the single-phase inverter. o With the reference sine wave u ref The error between the load current and the load current is used to adjust the switching state of the inverter. o For the voltage outer loop, it is an external disturbance, so its disturbance to the voltage outer loop cannot be ignored. o The disturbance is selected to add the load current decoupling amount i after the PI controller. oa , the output of the current loop is:

[0126] i on =G v (s)·(u ref -u oa )+u oa #(16)

[0127] As long as the load current feedforward decoupling coefficient When the load current changes, the load current can be completely decoupled and its disturbance to the voltage outer loop can be completely eliminated. In addition, when the load changes suddenly, it can also suppress the output waveform to a certain extent, improve the overall output accuracy of the system, and thus achieve flexible, stable, fast and accurate response of dual closed-loop control.

[0128] At this point, the open-loop transfer function G of the output voltage outer loop can be easily obtained: v1 (s) is:

[0129]

[0130] Substituting formula (10) into formula (17), we can obtain:

[0131]

[0132] Then the closed-loop transfer function of the output voltage outer loop is G v2 (s) is:

[0133]

[0134] Substituting formula (10) into formula (19) yields:

[0135]

[0136] Convert equations (18) and (20) into the complex frequency domain and define ω1 = 2πf cV ,ω2=2πf bV :

[0137]

[0138]

[0139] The present invention obtains the parameters of the voltage loop PI controller by combining constraints, so that the voltage outer loop can obtain faster voltage following performance. The first constraint is the open-loop transfer function G of the voltage loop. v1 The crossover frequency f of (jω1) cv =0dB, the second constraint is the closed-loop transfer function of the current loop G v2 Cut-off of (jω2) The cut-off frequency of the outer pressure loop is Pick up The parameters of the voltage outer loop can be preliminarily determined by combining the basic hardware parameters.

[0140] 4. Single-phase full-bridge inverter dual closed-loop control strategy simulation

[0141] Furthermore, the examples proposed in the present invention are simulated in MATLAB / Simulink. As shown in Table 1 below, the system specifications of the simulation are listed, and the proposed control system in Table 1 is simulated and evaluated experimentally. Fig.11 shown.

[0142] Table 1 Simulation system parameter list

[0143] variable name Numeric <![CDATA[U dc ]]> Input DC bus voltage 630V <![CDATA[U o ]]> Rated output AC voltage 350V(effective value) <![CDATA[P N ]]> Rated Power 5kW <![CDATA[f r ]]> Rated frequency 50Hz <![CDATA[f c ]]> Switching frequency 100kHz L Filter Inductor 3.3mH r Filter inductor equivalent series resistance 0.1Ω C Filter capacitor 1μF

[0144] The simulation and experimental evaluation are as follows:

[0145] Figure 12(a) shows the output voltage waveform of the double closed-loop PI control system of the single-phase inverter based on double feedforward under no-load condition, and Figure 12(b) shows the output voltage waveform of the ordinary double closed-loop control system under no-load condition. Figure 13(a) shows the total harmonic distortion analysis of the double closed-loop PI control system of the single-phase inverter based on double feedforward under no-load condition, and its output voltage effective value is 498.1V, and the total harmonic distortion rate THD is 0.12%. Figure 13(b) shows the total harmonic distortion analysis of the ordinary double closed-loop control system under no-load condition, and its output voltage effective value is 491.9V, and the total harmonic distortion rate THD is 0.66%.

[0146] According to the result diagram, when running in no-load state, the output voltage effective value of the double closed-loop PI control system of the single-phase inverter with double feedforward is the same as the reference sine wave v ref The difference is smaller, and the system signal distortion is also greatly reduced compared with the ordinary double closed-loop control system. It can be seen that the double closed-loop control system of the present invention has a higher degree of restoration of the input signal, higher quality of output power, more stable equipment operation, and improved overall system efficiency.

[0147] Figure 14(a) shows the output voltage waveform of the double closed-loop PI control system of the single-phase inverter based on double feedforward when it suddenly changes from no-load to heavy load at 0.1s, and Figure 14(b) shows the output voltage waveform of the ordinary double closed-loop control system when it suddenly changes from no-load to heavy load at 0.1s. Figure 15(a) shows the total harmonic distortion analysis of the double closed-loop PI control system of the single-phase inverter based on double feedforward when it suddenly changes from no-load to heavy load at 0.1s, and its output voltage effective value is 494.1V, and its total harmonic distortion rate THD is 0.11%. Figure 15(b) shows the total harmonic distortion analysis of the ordinary double closed-loop control system when it suddenly changes from no-load to heavy load at 0.1s, and its output voltage effective value is 491.5V, and its total harmonic distortion rate THD is 0.66%.

[0148] According to the result diagram, when the system changes suddenly from no-load to heavy-load, the double-closed-loop PI control system of the single-phase inverter with double feedforward can adjust the output voltage waveform more quickly and smoothly, and the output voltage effective value is consistent with the reference sine wave v ref The difference is only 0.87V, and the system signal distortion is only 0.11%. It can be seen that the dual closed-loop control system of the present invention can ensure that the load demand is met, and more effectively convert the input DC power into high-quality AC power, thereby reducing ineffective energy loss and improving the overall efficiency of the system, especially maintaining stable and efficient output when the load changes suddenly.

[0149] The various devices selected in this application are all universal standard parts or parts known to those skilled in the art, and their structures and principles can be known to those skilled in the art through technical manuals or conventional experimental methods.

[0150] In the description of the embodiments of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0151] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0152] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A double closed-loop PI control strategy for a single-phase inverter based on double feedforward, characterized in that: include: Single-phase inverter hardware circuit; The single-phase inverter hardware circuit has multiple voltage and current sensors built in to accurately collect the input DC bus voltage U dc , single-phase inverter output voltage u o , load current i o and the inductor current i L , the collected analog signal is digitized and sent to the DSP control module, and the DSPP control module; The following steps are also included: Step 1: Calculate the reference sine wave u ref The instantaneous sampling value u of the single-phase inverter output voltage o The difference is sent to the voltage outer loop PI regulator, and the adjusted error signal is used as the current reference value i ref For use in the inner current loop; Step 2: Set the current reference value i ref and the instantaneous sampling value of the load current i o Add and then subtract the instantaneous sampling value of the inductor current i L The result is sent to the current inner loop PI regulator, and the output signal after current loop regulation is marked as i on ; Step 3: Transform the signal i on The instantaneous sampling value u of the single-phase inverter output voltage o After decoupling, the signal is multiplied by the corresponding modulation ratio to generate an accurate duty cycle signal, which can then be converted into a corresponding SPWM switch control signal. Step 4: The SPWM switch control signal is filtered out through an LC filter circuit to remove high-order harmonics and output a high-quality sine wave that meets specific frequency and amplitude.

2. A double closed-loop PI control strategy for a single-phase inverter based on double feedforward as claimed in claim 1, characterized in that: Both the voltage outer loop and the current inner loop use PI regulators.

3. A double closed-loop PI control strategy for a single-phase inverter based on double feedforward as claimed in claim 2, characterized in that: The current inner loop adopts a closed-loop control strategy of output voltage feedforward and inductor current negative feedback.

4. A double closed-loop PI control strategy for a single-phase inverter based on double feedforward as claimed in claim 3, characterized in that: The load current of the inverter is fed back by the current sensor to the PI controller in the current inner loop, which o With the current reference value i ref The error between them adjusts the switching state of the inverter.

5. A double closed-loop PI control strategy for a single-phase inverter based on double feedforward as claimed in claim 4, characterized in that: The voltage outer loop adopts a closed-loop control strategy of load current feedforward and output voltage negative feedback.

6. A double closed-loop PI control strategy for a single-phase inverter based on double feedforward as claimed in claim 5, characterized in that: The PI controller in the voltage outer loop is based on the output voltage u of the single-phase inverter. o With the reference sine wave u ref The error between them adjusts the switching state of the inverter.

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