LCL type photovoltaic grid-connected inverter control method and device based on feedforward compensation

Abstract

The application relates to a kind of LCL type photovoltaic grid-connected inverter control method and device based on feedforward compensation, method includes obtaining the grid voltage of inverter, calculating the component of grid voltage in two-phase stationary coordinate system, calculating the difference of grid voltage and voltage instruction value, the reference current signal is obtained by PI regulation and coordinate transformation calculation to difference;Obtain the current on the side of inverter, actual current signal is obtained by coordinate transformation;The difference of reference current signal and actual current signal is calculated, input QPR regulator, obtain reference modulation signal;Obtain the capacitance current of inverter, calculate the difference with reference modulation signal, carry out internal damping feedforward compensation based on PFC after gain, recalculate difference, gain to obtain output voltage instruction value;After coordinate transformation to three-phase stationary coordinate system, input space vector pulse unit to obtain SVPWM waveform, input inverter to control.Compared with prior art, the application has the advantages of simple control means, good control effect and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of inverter grid-connected control, and particularly to a LCL type photovoltaic grid-connected inverter control method and device based on feedforward compensation. BACKGROUND

[0002] The grid-connected inverter, as one of the key interface devices in AC / DC microgrid, has attracted extensive attention, and its performance is directly related to the quality of grid-connected current. In order to meet the harmonic standard of grid-connected, a filter needs to be connected in series between the AC side of the grid-connected inverter and the power grid. The output end of the traditional grid-connected inverter usually needs to select a larger filter inductance, which not only has a large volume and high cost, but also affects the dynamic performance of the system. In order to solve this problem, an LCL filter can be used. Compared with the traditional L-type filter, the LCL filter adds a capacitor branch, and its impedance value to high-frequency ripple is very small, so that the ripple current injected into the power grid will be reduced. However, the LCL filter is a third-order system, and has a resonance peak, which is easy to cause system instability. Therefore, in order to avoid the amplification of harmonic amplitude near the resonance and increase the content of high-order harmonics of the grid-connected current, damping means must be used for suppression. There are usually two methods, passive damping and active damping. Passive damping increases the damping unit in the circuit to achieve resonance peak elimination, but it will cause system loss. At present, the method of active damping is usually used, which realizes damping from the control strategy, and the commonly used methods include virtual impedance, split capacitor, wave trap and other methods, as well as capacitor current, capacitor voltage and other state quantity feedback methods.

[0003] The grid-connected current tracking and harmonic current suppression of the grid-connected inverter are also the focus of its control. The common control means include PI control, PR control, deadbeat control, hysteresis control and some intelligent control methods. For a three-phase grid-connected system, the PI control is simple and easy to implement, but the fundamental frequency gain is limited, and there is a steady-state error and poor anti-interference ability. In addition, in the three-phase grid-connected system, the PI control in the dq coordinate system needs to decouple the current components through multiple coordinate transformations, which is complex in calculation. SUMMARY

[0004] The purpose of the present application is to overcome the defects of the prior art and provide a LCL type photovoltaic grid-connected inverter control method and device based on feedforward compensation.

[0005] The purpose of the present application can be achieved by the following technical solutions:

[0006] A LCL type photovoltaic grid-connected inverter control method based on feedforward compensation, comprising the following steps:

[0007] S1, obtain the grid voltage of the inverter, calculate the component of the grid voltage in the two-phase stationary coordinate system, calculate the difference between the grid voltage and the voltage command value, and obtain the reference current signal through PI regulation and coordinate transformation of the difference;

[0008] S2, obtain the inverter-side current, and obtain the actual current signal through coordinate transformation;

[0009] S3, calculate the difference between the reference current signal and the actual current signal, input the QPR regulator, and obtain the reference modulation signal;

[0010] S4, obtain the capacitor current of the inverter, calculate the difference between the reference modulation signal after coordinate transformation, gain the difference, and perform PFC-based internal damping feedforward compensation, recalculate the difference, gain, and obtain the output voltage command value;

[0011] S5, perform coordinate transformation on the output voltage command value to the three-phase stationary coordinate system, input the space vector pulse unit to obtain the SVPWM waveform, and input the SVPWM to the inverter for control.

[0012] Further, the expression of the reference modulation signal generated by the QPR regulator is as follows:

[0013]

[0014] where s represents the error value between the reference current signal and the actual current signal, K R represents the amplitude gain at the fundamental frequency, K P represents the amplitude gain at low and high frequencies, and ω c represents the bandwidth of the system, and ω0 represents the resonant angular frequency of the system.

[0015] Further, the compensation signal expression of the PFC-based internal damping feedforward compensation is as follows:

[0016]

[0017]

[0018] where P represents the proportional gain, K PFC represents the damping constant, K PWM represents the equivalent gain of the inverter, G d (s) represents calculation, sampling, and PWM delay, G ic (s) represents the uncompensated LCL filter transfer function, and L1, L2, and C represent the inductance and capacitance of the LCL filter.

[0019] Further, the component calculation expression of the grid voltage in the two-phase stationary coordinate system is as follows:

[0020]

[0021] wherein U a , U b and U c are grid voltage values of the grid voltage in a three-phase stationary coordinate system, U α and U β represent values of the grid voltage in a two-phase stationary coordinate system.

[0022] Further, the gain of the difference value in step S4 is a proportional gain.

[0023] A control device of an LCL type photovoltaic grid-connected inverter based on feedforward compensation, comprising a memory and a processor; the memory is used for storing a computer program; the processor is used for realizing the following method when executing the computer program:

[0024] S1, acquiring the grid voltage of the inverter, calculating the components of the grid voltage in a two-phase stationary coordinate system, calculating the difference value of the grid voltage and the voltage instruction value, and obtaining a reference current signal through PI regulation and coordinate transformation calculation of the difference value;

[0025] S2, acquiring the inverter side current, and obtaining an actual current signal through coordinate transformation;

[0026] S3, calculating the difference value of the reference current signal and the actual current signal, inputting a QPR regulator to obtain a reference modulation signal;

[0027] S4, acquiring the capacitor current of the inverter, calculating the difference value with the reference modulation signal after coordinate transformation, gain processing the difference value, and then performing internal damping feedforward compensation based on PFC, recalculating the difference value, and obtaining an output voltage instruction value after gain processing;

[0028] S5, inputting the output voltage instruction value into a space vector pulse unit through coordinate transformation into a three-phase stationary coordinate system to obtain an SVPWM waveform, and inputting the SVPWM into the inverter for control.

[0029] Compared with the prior art, the present application has the following advantages:

[0030] 1. The present application converts the calculated parameters through coordinate conversion from a three-phase coordinate system to a two-phase coordinate system, simplifying the calculation process in the control process; and obtains a reference modulation signal through a QPR regulator, compares the capacitor current, and modifies the value of the control signal through internal damping feedforward compensation, thereby completing the control of the inverter by only setting a compensator in the control circuit, and adjusting based on the parameters of the inverter itself, which has good control effect and low cost.

[0031] 2、The proportional gain is introduced in the feedforward compensation of the application, which ensures the stable margin of the control signal and further improves the control effect on the inverter. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The circuit diagram of the LCL type photovoltaic grid-connected inverter related to the application.

[0033] Figure 2 The flowchart of the application.

[0034] Figure 3 The coordinate conversion schematic diagram of the application.

[0035] Figure 4 The LCL filter model block diagram of the application in the two-phase coordinate system.

[0036] Figure 5 The simplified equivalent diagram of the LCL grid-connected inverter related to the application.

[0037] Figure 6 The equivalent control block diagram of the internal damping feedforward compensation of the application.

[0038] Figure 7 The overall control topology diagram of the application.

[0039] Figure 8 The voltage and current simulation diagram of the application.

[0040] Figure 9 The voltage and current simulation diagram of one phase of the application.

[0041] Figure 10 The voltage and current simulation diagram of the application when the voltage suddenly drops.

[0042] Figure 11 The Bode diagram comparison of the control method of the application and the control method under the open loop condition.

[0043] Figure 12 The Bode diagram comparison of the control method of the application and the control method under the closed loop condition. DETAILED DESCRIPTION

[0044] The application will be described in detail below in combination with the drawings and specific embodiments. The embodiments are implemented on the premise of the technical scheme of the application, and detailed implementation modes and specific operation processes are given, but the protection scope of the application is not limited to the following embodiments.

[0045] The three-phase grid-connected inverter of the LCL type filter is shown in Figure 1 The front end is a T-type three-level inverter, the DC bus side voltage is U dc , and the current is i dc; DC side voltage is U d1 and C d2 , and the switching devices are considered as ideal devices. The grid-connected inverter AC side uses L1, C, L2 to represent the LCL type grid-connected filter. The inverter output voltage is U 1a , U 1b , U 1c The output current is i 1a , i 1b and i 1c ; the current flowing through the filter capacitor C is i ca , i cb and i cc ; the grid-connected current is i 2a , i 2b and i 2c ; the three-phase grid voltage is U ga , U gb and U gc , and the inductance resistance and the parasitic resistance of the filter capacitor are ignored.

[0046] The embodiment provides a control method of an LCL type photovoltaic grid-connected inverter based on feedforward compensation, as shown in Figure 2 , comprising the following steps:

[0047] Step S1, sampling the grid voltage U abc of the grid-connected inverter through a grid voltage sensor, the state variables of the mathematical model established in the three-phase stationary coordinate system are time-varying and high-order. In order to reduce the coupling variables of the system and simplify the mathematical model, the Clark coordinate transformation is used to convert the LCL type grid-connected inverter model established in the three-phase stationary coordinate system into the two-phase stationary coordinate system, and the conversion diagram of the coordinate system is as shown in Figure 3 , and the expression of the coordinate transformation formula is as follows:

[0048]

[0049] The LCL filter model block diagram after coordinate conversion and the simplified equivalent diagram of the inverter are as shown in Figure 4 and Figure 5 .

[0050] The component of the grid voltage vector in the stationary coordinate system can be obtained through the coordinate transformation formula from the three-phase stationary abc coordinate system to the two-phase stationary αβ coordinate system, and the expression of the component is as follows:

[0051]

[0052] The grid voltage amplitude and phase angle information are extracted through the phase-locked loop PLL. The DC side voltage U dc and the command value U dc*Comparative PI regulation reference current signal i 1α *, 1β *.

[0053] Step S2, sampling to inverter side current i 1abc , actual current signal i 1α and i 1β .

[0054] Step S3, inputting error value between reference current signal i 1α *, 1β *and actual current signal i 1α , i 1β to QPR regulator to obtain reference modulation signal. Based on QPR regulation mode, not only amplitude gain is maximum at resonance frequency point, given quantity zero error tracking can be realized without complex coordinate transformation, but also line current harmonic can be effectively reduced when power grid frequency deviates. Reference modulation signal G QPR (s) output by QPR regulator is expressed as follows:

[0055]

[0056] Wherein, s represents error value between reference current signal i 1α *, 1β *and actual current signal i 1α , i 1β , parameter K R represents amplitude gain at fundamental frequency, parameter K P represents amplitude gain at low frequency and high frequency, ω c represents bandwidth of system, and ω0represents resonance angular frequency of system.

[0057] Step S4, sampling to capacitor current i c through filter capacitor current sensor, and obtaining capacitor current signal i cα , i cβ in two-phase static αβcoordinate system through coordinate transformation. After difference between capacitor current signal and reference modulation signal, P gain, internal damping feedforward compensation, output voltage instruction value is obtained.

[0058] Wherein, equivalent control block diagram of internal damping feedforward compensation is shown in Figure 6 , and expression of internal damping is as follows:

[0059]

[0060]

[0061] where P represents a proportional gain, K PFC represents a damping constant, K PWM represents an equivalent gain of the inverter, G d (s) represents calculation, sampling and PWM delay, G ic (s) represents an uncompensated LCL filter transfer function.

[0062] Step S5, coordinate transformation is performed on the output voltage instruction value, from two-phase static alpha-beta coordinate system to three-phase static abc coordinate system, sent into a space vector pulse unit to generate SVPWM waveform, finally sent into an inverter unit to drive the on-off of the switching tube, control the operation of the inverter. The overall control block diagram is shown in Figure 7 .

[0063] Simulation test is performed, Figure 8 is a voltage and current simulation diagram, Figure 9 is a voltage and current simulation diagram of one phase, Figure 10 is a voltage and current simulation diagram when the voltage suddenly drops, from Figures 8 to 10 The waveform in the figure shows that under the control of the embodiment, the grid-connected current can achieve stable frequency and amplitude, and can achieve fast and stable tracking when the current instruction changes.

[0064] Figure 11 is a comparison of the Bode diagram of the control method of the embodiment and the control method without control under the open-loop condition, Figure 12 is a comparison of the Bode diagram of the control method of the embodiment and the control method without control under the closed-loop condition. From Figures 11 to 12 the change curve, it can be seen that the method (PFC) widens the effective damping interval. It has faster dynamic response, good tracking performance and resonance suppression ability.

[0065] At the same time, from the closed-loop frequency response, it can be seen that the method has better phase characteristics and higher gain in the low frequency band, which shows that it has good low-order harmonic suppression ability under the condition of grid voltage distortion.

[0066] The embodiment also provides an LCL type photovoltaic grid-connected inverter control device based on feedforward compensation, comprising a memory and a processor; the memory is used for storing a computer program; the processor is used for realizing the above-mentioned LCL type photovoltaic grid-connected inverter control method based on feedforward compensation when the computer program is executed.

[0067] The embodiment also provides a computer readable storage medium, which stores a computer program. The computer program is executed by a processor to implement the feed-forward compensation based LCL type photovoltaic grid-connected inverter control method mentioned in the above embodiment. Any combination of one or more computer readable media can be used. The computer readable medium can be a computer readable signal medium or a computer readable storage medium. The computer readable storage medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, device or apparatus, or any suitable combination of the above. More specific examples (a non-exhaustive list) of the computer readable storage medium include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the above. In this document, the computer readable storage medium can be any tangible medium that contains or stores a program that can be used by or in connection with an instruction execution system, apparatus or device.

[0068] The preferred embodiments of the present application have been described in detail. It should be understood that those skilled in the art can make many modifications and variations without departing from the concept of the present application. Therefore, any technical solution obtained by logical analysis, reasoning or limited experiments based on the concept of the present application and the prior art should be within the protection scope defined by the claims.

Claims

1. A control method for an LCL-type photovoltaic grid-connected inverter based on feedforward compensation, characterized in that, Includes the following steps: S1. Obtain the grid voltage of the inverter, calculate the component of the grid voltage in the two-phase stationary coordinate system, calculate the difference between the grid voltage and the voltage command value, and calculate the reference current signal by PI regulation and coordinate transformation of the difference. S2. Obtain the inverter-side current and transform it to get the actual current signal. S3. Calculate the difference between the reference current signal and the actual current signal, input it into the QPR regulator, and obtain the reference modulation signal; S4. Obtain the inverter's capacitor current, calculate the difference between it and the reference modulation signal after coordinate transformation, gain the difference, perform PFC-based internal damping feedforward compensation, recalculate the difference, and obtain the output voltage command value after gain. The compensation signal expression for PFC-based internal damping feedforward compensation is as follows: Where P represents the proportional gain, K PFC K represents the damping constant. PWM This represents the equivalent gain of the inverter. G d ( s () represents the calculation, sampling, and PWM delay. G ic ( s () represents the transfer function of the uncompensated LCL filter. L 1. L 2 and C Indicates the inductance and capacitance of an LCL filter; S5. The output voltage command value is transformed into a three-phase stationary coordinate system and input to the space vector pulse unit to obtain the SVPWM waveform. The SVPWM is then input to the inverter for control.

2. The control method for an LCL-type photovoltaic grid-connected inverter based on feedforward compensation according to claim 1, characterized in that, The expression for the reference modulation signal generated by the QPR regulator is as follows: in, s This represents the error value between the reference current signal and the actual current signal; the parameter... K R The parameter represents the amplitude gain at the fundamental frequency. K P This indicates the amplitude gain at low and high frequencies. ω c Indicates the system bandwidth. ω 0 represents the system's resonant angular frequency.

3. The control method for an LCL-type photovoltaic grid-connected inverter based on feedforward compensation according to claim 1, characterized in that, The expression for calculating the components of the grid voltage in a two-phase stationary coordinate system is as follows: in, , and This represents the grid voltage value in a three-phase stationary coordinate system. and This represents the value of the grid voltage in a two-phase stationary coordinate system.

4. The control method for an LCL-type photovoltaic grid-connected inverter based on feedforward compensation according to claim 1, characterized in that, In step S4, the gain method for the difference is proportional gain.

5. A control device for an LCL-type photovoltaic grid-connected inverter based on feedforward compensation, characterized in that, It includes a memory and a processor; the memory is used to store a computer program; the processor is used to implement the following methods when the computer program is executed: S1. Obtain the grid voltage of the inverter, calculate the component of the grid voltage in the two-phase stationary coordinate system, calculate the difference between the grid voltage and the voltage command value, and calculate the reference current signal by PI regulation and coordinate transformation of the difference. S2. Obtain the inverter-side current and transform it to get the actual current signal. S3. Calculate the difference between the reference current signal and the actual current signal, input it into the QPR regulator, and obtain the reference modulation signal; S4. Obtain the inverter's capacitor current, calculate the difference between it and the reference modulation signal after coordinate transformation, gain the difference, perform PFC-based internal damping feedforward compensation, recalculate the difference, and obtain the output voltage command value after gain. The compensation signal expression for PFC-based internal damping feedforward compensation is as follows: Where P represents the proportional gain, K PFC K represents the damping constant. PWM This represents the equivalent gain of the inverter. G d ( s () represents the calculation, sampling, and PWM delay. G ic ( s () represents the transfer function of the uncompensated LCL filter. L 1. L 2 and C Indicates the inductance and capacitance of an LCL filter; S5. The output voltage command value is transformed into a three-phase stationary coordinate system and input to the space vector pulse unit to obtain the SVPWM waveform. The SVPWM is then input to the inverter for control.

6. The control device for an LCL-type photovoltaic grid-connected inverter based on feedforward compensation according to claim 5, characterized in that, The expression for the reference modulation signal generated by the QPR regulator is as follows: in, s The parameter K represents the error value between the reference current signal and the actual current signal. R The amplitude gain at the fundamental frequency is represented by parameter K. P This indicates the amplitude gain at low and high frequencies. ω c Indicates the system bandwidth. ω 0 represents the system's resonant angular frequency.

7. The control device for an LCL-type photovoltaic grid-connected inverter based on feedforward compensation according to claim 5, characterized in that, The expression for calculating the components of the grid voltage in a two-phase stationary coordinate system is as follows: in, , and This represents the grid voltage value in a three-phase stationary coordinate system. and This represents the value of the grid voltage in a two-phase stationary coordinate system.

8. The control device for an LCL-type photovoltaic grid-connected inverter based on feedforward compensation according to claim 5, characterized in that, In step S4, the gain method for the difference is proportional gain.

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