A three-phase grid-connected inverter with unbalanced single-cycle control and its control method

Abstract

A three-phase grid-connected inverter with unbalanced single-cycle control and a control method thereof, including a three-phase inverter power circuit and an unbalanced single-cycle control system, including a three-phase grid negative sequence voltage feedforward control unit, a grid feedback current correction control unit, and a basic single-cycle control unit composed of a DC voltage controller PI, a timed reset integrator, three-phase corresponding comparators and an RS trigger; the three-phase corresponding comparator signal output ends are respectively connected to the corresponding RS triggers to generate a drive signal to drive the switch device in the three-phase inverter power circuit to perform a switching action. Under the condition of unbalanced voltage in the three-phase grid, the present invention introduces grid negative sequence voltage feedforward control to suppress grid current distortion and stabilize grid active power and control the DC side voltage stability of the inverter, thereby improving the grid-connected power transmission quality, operation efficiency and stability of the grid-connected inverter under the condition of unbalanced grid voltage.

Description

Technical Field

[0001] The invention relates to the technical field of grid-connected power generation inverter control, and in particular to a three-phase grid-connected inverter with unbalanced single-cycle control. Background Art

[0002] Renewable energy power generation is one of the important ways to solve the energy crisis and environmental pollution. Renewable energy power generation represented by wind power and photovoltaics has been rapidly developed at home and abroad, playing an important role in promoting and achieving carbon peak and carbon neutrality, and has become an important global energy industry with broad application prospects. With the substantial increase in the installed capacity of new energy power generation and the increasing proportion of its access to the power grid, the mutual influence between the new energy grid-connected power generation system and the power system cannot be ignored. Medium and large capacity new energy power generation systems mainly use three-phase grid-connected inverters as interfaces to transmit electrical energy to the power grid. As a key equipment for new energy grid-connected power generation, the grid-connected inverter has a grid-connected operation performance, especially the operation characteristics under unbalanced grid fault conditions, which directly affects the safe operation of the new energy power generation system itself and the power system.

[0003] During the actual operation of the power grid, three-phase asymmetry and single-phase voltage drop faults often occur. Relevant theoretical research and actual engineering practice show that the DC side voltage of the three-phase grid-connected inverter using various balanced control structures will fluctuate and the grid-connected current will be non-sinusoidally distorted, which will in turn cause the inverter grid-connected power fluctuation, reduce the efficiency of new energy grid-connected power generation, increase the harmonic loss of the inverter, and even cause the inverter power control system to be unstable and burn the inverter, resulting in the collapse of the operation of the new energy grid-connected system; the distortion of the grid-connected current will directly affect the power supply quality of the new energy power generation grid-connected system, pollute the power grid, aggravate the instability of the power grid, and endanger the safe operation of the power grid. In order to maintain the stability of the power grid, it can be foreseen that in the future, new energy grid-connected equipment that does not have the ability to overcome the grid voltage imbalance fault will not be allowed to access the power grid. Therefore, the new energy grid-connected power generation system must be designed and produced using an unbalanced control structure.

[0004] In summary, in view of the grid voltage unbalance faults that often occur during the operation of the power grid, the three-phase grid-connected inverter urgently needs to have the operating performance to overcome the grid voltage unbalance faults, so as to improve the transmission quality of its grid-connected power generation and maintain the stable operation of the power system. Therefore, the research and innovation of the inverter topology and control structure under the condition of grid voltage asymmetry has positive significance for stabilizing the DC side voltage of the grid-connected inverter, suppressing the grid current harmonics, stabilizing the grid-connected power, improving the inverter conversion efficiency, and simplifying the inverter unbalanced control circuit topology, and has high theoretical research value and broad application prospects. Summary of the invention

[0005] The purpose of the present invention is to solve the problem of the operation stability of the new energy grid-connected power generation system under the unbalanced working condition of the three-phase grid, and propose an unbalanced single-cycle control structure of a three-phase grid-connected inverter and its physical implementation method. For the unbalanced working condition of the grid voltage, on the basis of the traditional inverter single-cycle control system, the grid negative sequence voltage feedforward control is introduced to suppress the grid current distortion and stabilize the grid active power and control the DC side voltage stability of the inverter, so as to improve the grid-connected power transmission quality, operation efficiency and stability of the grid-connected inverter under the unbalanced working condition of the grid voltage.

[0006] The technical solution of the present invention is:

[0007] A three-phase grid-connected inverter with unbalanced single-cycle control, comprising:

[0008] Three-phase inverter power circuit,

[0009] and an unbalanced single-cycle control system, including a three-phase grid negative-sequence voltage feedforward control unit, a grid feedback current correction control unit, and a basic single-cycle control unit consisting of a DC voltage controller PI, a timed reset integrator, a three-phase comparator, and an RS trigger;

[0010] Wherein: the input end of the grid negative sequence voltage feedforward control unit is connected to the output end of the inverter DC voltage sensor, the output end of the inverter DC voltage controller, the output end of the three-phase AC voltage sensor, and the output end of the timing reset integrator; the in-phase comparison signal output end of the grid negative sequence voltage feedforward control unit is connected to the in-phase comparison input end of the corresponding phase comparator of the basic single cycle control unit; the current correction signal output end of the grid negative sequence voltage feedforward control unit is connected to the current correction input end of the grid feedback current correction control unit;

[0011] The current input terminal of the power grid feedback current correction control unit is connected to the output terminal of the power grid AC current sensor, the current correction input terminal of the power grid feedback current correction control unit is connected to the current correction output terminal of the power grid negative sequence voltage feedforward control unit, and the inverting comparison signal output terminal of the power grid feedback current correction control unit is connected to the inverting comparison input terminal of the corresponding phase comparator of the basic single cycle control unit;

[0012] The three corresponding comparator signal output terminals are respectively connected to corresponding RS triggers to generate drive signals to drive the switch devices in the three-phase inverter power circuit to perform switching actions.

[0013] Furthermore, the power grid negative sequence voltage feedforward control unit includes an analog-to-digital converter AD, a digital-to-analog converter DA, an embedded microprocessor MPU and an operational amplifier, the clock signal output end of the MPU is connected to the reset end of the timing reset integrator and the set end of the RS trigger, AD is connected to the output end of the inverter DC voltage sensor, the output end of the inverter DC voltage controller, and the output end of the three-phase AC voltage sensor as the input end of the power grid negative sequence voltage feedforward control unit, AD and DA are connected to the MPU in a serial or parallel data bus manner, the analog output port of DA serves as the in-phase comparison signal output end and the current correction signal output end of the power grid negative sequence voltage feedforward control unit, its in-phase comparison output end is connected to the in-phase comparison input end of the corresponding phase comparator in the basic single cycle control unit, and its current correction output end is connected to the current correction input end of the power grid feedback current correction control unit.

[0014] Furthermore, the grid feedback current correction control unit includes an operational amplifier, the current input end of the grid feedback current correction control unit is connected to the output end of the grid AC current sensor, the current correction input end of the grid feedback current correction control unit is connected to the current correction output end of the grid negative sequence voltage feedforward control unit, and the output port of the operational amplifier is connected to the inverting comparison input end of the corresponding phase comparator in the basic single cycle control unit as the inverting comparison signal output end of the grid feedback current correction control unit.

[0015] Furthermore, the grid negative sequence voltage feedforward control unit and the grid current feedback control unit constitute an unbalanced single cycle control structure, wherein a digital signal processing method is used for the voltage signal, and an analog signal processing method is used for the current signal.

[0016] Furthermore, the clock input signal of the timing reset integrator is generated by the MPU using its internal timer, and the clock signal is output to the set end of the RS trigger in the basic single-cycle control unit. The signal input end of the timing reset integrator is connected to the output end of the inverter DC voltage controller, and the output end of the timing reset integrator is connected to the non-inverting input end of the operational amplifier in the grid negative sequence voltage feedforward control unit.

[0017] A control method for a three-phase grid-connected inverter with unbalanced single-cycle control, wherein the three-phase grid-connected inverter operates under a three-phase grid voltage unbalanced condition, and the grid negative-sequence voltage feedforward control unit and the grid feedback current correction control unit correct the input signals of the same-phase comparison terminal and the reverse-phase comparison terminal of the corresponding phase comparator in the three-phase basic single-cycle control unit to form a negative-sequence voltage feedforward closed-loop control, so as to suppress the distortion of the grid-connected current of the three-phase inverter under the three-phase grid voltage unbalanced condition and control the DC side voltage of the inverter to be stable.

[0018] Furthermore, the grid negative sequence voltage feedforward control unit performs calculation of the ratio of the grid negative sequence voltage to the DC voltage, and calculation of the current correction value;

[0019] Based on the negative sequence voltage and the DC voltage of the power grid, a proportional calculation is performed, and the above-mentioned proportion and the output signal of the timing reset integrator are subtracted by a subtraction circuit composed of an operational amplifier to generate an in-phase comparison signal, and the in-phase comparison signal is transmitted to the in-phase comparison input terminal of the comparator in the basic single-cycle control unit;

[0020] Proportion = DC voltage controller output value × (1 + 2 × negative sequence voltage ÷ DC voltage);

[0021] Calculating a current correction value based on the grid voltage and the negative sequence voltage, and transmitting the current correction value to a current correction input terminal of the grid feedback current correction control unit;

[0022] Current correction value = 0.5 × grid-connected current limiting coefficient × (grid voltage – 2 × negative sequence voltage).

[0023] Furthermore, the grid feedback current correction control unit performs the inverted comparison signal value calculation:

[0024] Based on the real-time current of the power grid and the current correction value from the power grid negative sequence voltage feedforward control unit, a subtraction circuit composed of an operational amplifier is used to perform subtraction calculation to generate an inverted comparison signal, and the inverted comparison signal is transmitted to the inverted comparison input terminal of the comparator in the basic single-cycle control unit;

[0025] The inverted comparison signal value = current correction value - current sampling resistance × grid current.

[0026] Beneficial effects of the present invention:

[0027] The present invention relates to a three-phase grid-connected inverter with unbalanced single-cycle control. Under the condition of unbalanced voltage in the three-phase grid, grid-connected current distortion is suppressed by introducing grid negative sequence voltage feedforward control, thereby realizing sinusoidal control of grid-connected current and improving grid-connected power transmission quality.

[0028] The present invention relates to a three-phase grid-connected inverter with unbalanced single-cycle control. Under the condition of unbalanced voltage in the three-phase grid, the grid-connected power fluctuation is suppressed to stabilize the grid-connected power, the operation stability of the grid-connected inverter under the condition of unbalanced grid voltage is improved, and the necessary support is provided for the grid to recover from voltage unbalance fault.

[0029] The present invention relates to a three-phase grid-connected inverter with unbalanced single-cycle control. Under the condition of unbalanced voltage of the three-phase grid, the DC side voltage of the grid-connected inverter is controlled to be stable, so as to effectively reduce the operation loss of the inverter and improve the operation efficiency of the new energy grid-connected inverter.

[0030] The present invention relates to an unbalanced single-cycle control structure composed of a grid negative-sequence voltage feedforward control unit and a grid current feedback control unit, wherein a voltage signal adopts a digital signal processing method, which has high data processing accuracy and flexibility, and a current signal adopts an analog signal processing method, which has high dynamic response characteristics and can form reliable overcurrent protection for inverter switch devices.

[0031] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention with reference to the accompanying drawings.

[0033] Figure 1 The overall structure diagram of the three-phase grid-connected inverter with unbalanced single-cycle control of the present invention;

[0034] Figure 2 It is a structural diagram of the unbalanced single cycle control system of the present invention;

[0035] Figure 3 It is a structural diagram of a power grid negative sequence voltage feedforward control unit of the present invention;

[0036] Figure 4 This is a structural diagram of the grid feedback current correction control unit of the present invention. DETAILED DESCRIPTION

[0037] The following will refer to the attached Figure 1-4 The preferred embodiments of the present invention are described in more detail. Although the preferred embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. The following focuses on the implementation of the unbalanced single-cycle control unit of the three-phase grid-connected inverter composed of the grid negative sequence voltage feedforward control unit and the grid feedback current correction control unit. The remaining parts do not fall within the scope of the claims of this article and are not described in detail.

[0038] The present invention provides a three-phase grid-connected inverter with unbalanced single-cycle control, comprising: a traditional inverter single-cycle control unit composed of an RS trigger, a comparator, and a resettable integrator; an unbalanced single-cycle control unit composed of a grid negative sequence voltage feedforward control unit and a grid feedback current correction control unit; an inverter power unit composed of a capacitor C, a filter inductor Lf, and a converter;

[0039] The grid negative sequence voltage feedforward control unit is composed of AD, DA, MPU and operational amplifier. At the same time, MPU outputs a timing clock signal through an internal timer to reset the timing reset integrator and set the RS trigger. AD collects the DC side voltage value of the inverter, the output signal of the DC voltage controller, and the voltage value of the three-phase grid through the analog input port and performs analog-to-digital conversion. MPU reads the AD conversion value through the digital interface and calculates according to the above calculation rules, and outputs the digital amount of the calculation result to the DA converter through the digital interface. DA outputs a current correction signal and a grid negative sequence voltage and DC voltage proportional signal at its analog output port. The current correction signal is output to the current correction input port of the grid feedback current correction control unit. The grid negative sequence voltage and DC voltage proportional signal is output to a subtraction circuit composed of an operational amplifier and the output signal of the timing reset integrator for subtraction calculation. The output of the subtraction circuit composed of the operational amplifier in the grid negative sequence voltage feedforward control unit is a common-phase comparison signal, and this common-phase comparison signal is output to the common-phase comparison input end of the comparator in the basic single-cycle control unit.

[0040] The grid feedback current correction control unit is a subtraction circuit composed of a high-speed operational amplifier.

[0041] The non-inverting input port of the operational amplifier serves as the input end of the grid current signal, and collects the output signal from the three-phase grid current sensor. The inverting input port of the operational amplifier serves as the input end of the grid negative sequence voltage and DC voltage proportional signal, and collects the current correction signal from the grid negative sequence voltage feedforward control unit. The operational amplifier outputs an inverted comparison signal and transmits it to the inverting comparison input end of the comparator in the basic single-cycle control unit.

[0042] An unbalanced single-cycle control method for a three-phase grid-connected inverter with unbalanced single-cycle control, comprising:

[0043] The calculation rule of the ratio of the negative sequence voltage to the DC voltage of the power grid implemented in the MPU is: DC voltage controller output × (1 + 2 × negative sequence voltage ÷ DC voltage);

[0044] The calculation rule of the current correction calculation module implemented by software in the MPU is: 0.5×grid-connected current limit coefficient×(grid voltage–2×negative sequence voltage);

[0045] The grid feedback current correction control unit is a subtraction circuit composed of a high-speed operational amplifier, and the calculation rule is: current correction signal - current sampling resistance × grid current.

[0046] Example:

[0047] like Figure 1 , 2 As shown, Figure 1The invention discloses an overall structure of a three-phase grid-connected inverter with unbalanced single-cycle control, which includes an unbalanced single-cycle control system and a three-phase inverter power circuit.

[0048] The control structure of the unbalanced single-cycle control system is as follows: Figure 2 As shown in the figure, the unbalanced single-cycle control system mainly includes a DC voltage controller, a grid negative sequence voltage feedforward control unit, a grid feedback current correction control unit, and a timed reset integrator, a comparator, and an RS trigger of a traditional basic single-cycle control unit. Here, the DC voltage controller adopts a proportional integral controller (PI). The unbalanced single-cycle control system collects the DC side voltage of the inverter, the AC voltage and AC current of the three-phase grid. After calculation and processing by the unbalanced single-cycle control system, the three sets of complementary output terminals of the RS trigger generate driving signals to drive the six switching devices of the three-phase grid-connected inverter to perform switching actions.

[0049] like Figure 3 As shown, the power grid negative sequence voltage feedforward control unit is designed with MPU, AD, DA and high-speed operational amplifier. MPU can use domestic microprocessor GD32F407, AD can use parallel analog-to-digital converter AD7656, DA can use parallel digital-to-analog converter AD5725, high-speed operational amplifier can use TL074 and TL072, AD and DA can be connected to MPU parallel data and address bus by parallel digital interface, the operational amplifier in the power grid negative sequence voltage feedforward control unit is designed as a subtractor according to the analog operation requirements of the power grid negative sequence voltage feedforward control unit, the calculation steps follow the above calculation rules, and the physical meaning of the circuit parameters is as follows Figure 3 Marked, Figure 3 Among them, R1=R2, R3=R4.

[0050] like Figure 4 As shown, the grid feedback current correction control unit is designed with a high-speed operational amplifier. The operational amplifier is designed as a subtractor according to the analog operation requirements of the grid feedback current correction control unit. The calculation steps follow the above calculation rules. The physical meaning of the circuit parameters is as follows Figure 3 Marked, Figure 3 In the figure, R1 = Rs, Rs is the equivalent current sampling resistor, R2 = 0.5*R3, and R3 = R4.

[0051] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A three-phase grid-connected inverter with unbalanced single-cycle control, Features include: Three-phase inverter power circuit, and an unbalanced single-cycle control system, including a three-phase grid negative-sequence voltage feedforward control unit, a grid feedback current correction control unit, and a basic single-cycle control unit consisting of a DC voltage controller PI, a timed reset integrator, a three-phase comparator, and an RS trigger; Wherein: the input end of the grid negative sequence voltage feedforward control unit is connected to the output end of the inverter DC voltage sensor, the output end of the inverter DC voltage controller, the output end of the three-phase AC voltage sensor, and the output end of the timing reset integrator; the in-phase comparison signal output end of the grid negative sequence voltage feedforward control unit is connected to the in-phase comparison input end of the corresponding phase comparator of the basic single cycle control unit; the current correction signal output end of the grid negative sequence voltage feedforward control unit is connected to the current correction input end of the grid feedback current correction control unit; The current input terminal of the power grid feedback current correction control unit is connected to the output terminal of the power grid AC current sensor, the current correction input terminal of the power grid feedback current correction control unit is connected to the current correction output terminal of the power grid negative sequence voltage feedforward control unit, and the inverting comparison signal output terminal of the power grid feedback current correction control unit is connected to the inverting comparison input terminal of the corresponding phase comparator of the basic single cycle control unit; The three corresponding comparator signal output terminals are respectively connected to corresponding RS triggers to generate drive signals to drive the switch devices in the three-phase inverter power circuit to perform switching actions.

2. The unbalanced single-cycle controlled three-phase grid-connected inverter according to claim 1, Features: The power grid negative sequence voltage feedforward control unit comprises an analog-to-digital converter AD, a digital-to-analog converter DA, an embedded microprocessor MPU and an operational amplifier. The clock signal output end of the MPU is connected to the reset end of the timing reset integrator and the set end of the RS trigger. AD is connected to the output end of the inverter DC voltage sensor, the output end of the inverter DC voltage controller and the output end of the three-phase AC voltage sensor as the input end of the power grid negative sequence voltage feedforward control unit. AD and DA are connected to the MPU in a serial or parallel data bus mode. The analog output port of DA serves as the in-phase comparison signal output end and the current correction signal output end of the power grid negative sequence voltage feedforward control unit. Its in-phase comparison output end is connected to the in-phase comparison input end of the corresponding phase comparator in the basic single cycle control unit, and its current correction output end is connected to the current correction input end of the power grid feedback current correction control unit.

3. The unbalanced single-cycle controlled three-phase grid-connected inverter according to claim 1, Features: The grid feedback current correction control unit includes an operational amplifier, the current input end of the grid feedback current correction control unit is connected to the output end of the grid AC current sensor, the current correction input end of the grid feedback current correction control unit is connected to the current correction output end of the grid negative sequence voltage feedforward control unit, and the output port of the operational amplifier is connected to the inverting comparison input end of the corresponding phase comparator in the basic single cycle control unit as the inverting comparison signal output end of the grid feedback current correction control unit.

4. The unbalanced single-cycle controlled three-phase grid-connected inverter according to claim 1, Features: The unbalanced single-cycle control structure formed by the grid negative-sequence voltage feedforward control unit and the grid current feedback control unit adopts a digital signal processing method for the voltage signal and an analog signal processing method for the current signal.

5. The unbalanced single-cycle controlled three-phase grid-connected inverter according to claim 1, Features: The clock input signal of the timing reset integrator is generated by the MPU using its internal timer, and the clock signal is output to the set end of the RS trigger in the basic single-cycle control unit. The signal input end of the timing reset integrator is connected to the output end of the inverter DC voltage controller, and the output end of the timing reset integrator is connected to the non-inverting input end of the operational amplifier in the grid negative sequence voltage feedforward control unit.

6. A control method for an unbalanced single-cycle controlled three-phase grid-connected inverter according to any one of claims 1 to 5, Features The three-phase grid-connected inverter operates under the condition of unbalanced three-phase grid voltage. The grid negative-sequence voltage feedforward control unit and the grid feedback current correction control unit correct the input signals of the same-phase comparison terminal and the reverse-phase comparison terminal of the corresponding phase comparator in the three-phase basic single-cycle control unit to form a negative-sequence voltage feedforward closed-loop control, so as to suppress the distortion of the grid-connected current of the three-phase inverter under the condition of unbalanced three-phase grid voltage and control the stability of the DC side voltage of the inverter.

7. A control method for a three-phase grid-connected inverter with unbalanced single-cycle control according to claim 6, Features The grid negative sequence voltage feedforward control unit performs calculation of the ratio of the grid negative sequence voltage to the DC voltage, and calculation of the current correction value; Based on the negative sequence voltage and the DC voltage of the power grid, a proportional calculation is performed, and the above-mentioned proportion and the output signal of the timing reset integrator are subtracted by a subtraction circuit composed of an operational amplifier to generate an in-phase comparison signal, and the in-phase comparison signal is transmitted to the in-phase comparison input terminal of the comparator in the basic single-cycle control unit; Proportion = DC voltage controller output value × (1 + 2 × negative sequence voltage ÷ DC voltage); Calculating a current correction value based on the grid voltage and the negative sequence voltage, and transmitting the current correction value to a current correction input terminal of the grid feedback current correction control unit; Current correction value = 0.5 × grid-connected current limiting coefficient × (grid voltage – 2 × negative sequence voltage).

8. The method for controlling a three-phase grid-connected inverter with unbalanced single-cycle control according to claim 6, Features The grid feedback current correction control unit performs the calculation of the inverted comparison signal value: Based on the real-time current of the power grid and the current correction value from the power grid negative sequence voltage feedforward control unit, a subtraction circuit composed of an operational amplifier is used to perform subtraction calculation to generate an inverted comparison signal, and the inverted comparison signal is transmitted to the inverted comparison input terminal of the comparator in the basic single-cycle control unit; The inverted comparison signal value = current correction value - current sampling resistance × grid current.

Images