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Research on Control Strategy of New Type Solar Grid-connected Inverter

Nov 13, 2020

Research on Control Strategy of New Type Solar Grid-connected Inverter

In the 21st century, solar grid-connected inverter technology has been continuously developed. This article introduces a new type of inverter, which is different from traditional inverters and can improve the quality of inverter output current.


1. System composition


1.1 Main circuit structure


It is a voltage-type active inverter with output current control. The system adopts a full bridge structure composed of intelligent power modules. It can be seen that the energy output by the solar array is first passed through a full-bridge inverter and inductive filtering, and then merged into the grid in the form of a controlled current source. The control process is: compare the reference current given value of the same frequency and phase with the grid voltage with the actual instantaneous feedback value of grid-connected current. After the difference is processed by the PI regulator, it is compared with the actual instantaneous feedback value of the grid voltage. After triangular wave modulation, a sine wave pulse width modulation signal is output, which is amplified by the drive circuit to drive the power switching device, thereby generating a sine wave current with the same frequency and phase as the grid voltage.


1.2 Mathematical model of system inverter link


Take the current iL flowing through the filter inductor L as the state variable. Available:


uab=unet+L(di/dt)+ir


From equation (1) after Laplas transformation, i(s) can be solved:


i(s)=[1/(sL+r)][uab(s)-unet(s)]


Where: uab is the unfiltered inverter output voltage; r is the equivalent resistance of the line.


When the switching frequency of the inverter is high, ignoring the effects of switching devices and dead zone characteristics, the bridge inverter under the SPWM control mode can be approximated as an equivalent amplification factor of K, namely:

G(s)=K (3)


2 System control strategy


In order to make the inverter output a good grid-connected current waveform, the output grid-connected current of the inverter must be closed-loop controlled. The existence of dead zone, asymmetry factors inside the inverter, DC side voltage and grid disturbances will cause the grid-connected current waveform output by the inverter to be distorted. However, when the traditional PI control is used to track a given sinusoidal signal, the system's static-free tracking cannot be achieved.


According to the theory of automatic control, when the actual system has both input signal function and disturbance, in order to reduce or eliminate the steady-state error of the system under the simultaneous action of input and disturbance, the following methods can be adopted: before increasing the disturbance action point The forward channel gain can reduce the steady-state error, but for high-order systems, too large gain will degrade the dynamic performance of the system; introducing an integral link in the forward channel before the disturbance point can eliminate the system’s Steady-state error, but the introduction of the integral link is detrimental to the stability of the system; the feedforward compensation method is adopted. This method can not only make the system have higher steady-state accuracy, but also have good dynamic performance. Therefore, in the process of tracking control of grid-connected current, in order to offset the influence of grid voltage and its disturbance, the system adopts grid voltage feedforward control. In order to improve the steady-state performance of the system, suppress the periodic disturbance of the grid-connected output current from the grid side and the load side, and reduce the THD value of the grid-connected output current, the system introduces repetitive control technology on the basis of PI control.

Where:

g(x)=[0 0 I/Lt]


Combining the above-mentioned control strategy, this paper carries out the experimental research of the solar grid-connected inverter. The experimental parameters are as follows: the output power is 1 kW, the switching frequency is 20 kHz, the filter inductance is 0.5 mH, and the control chip uses TI’s TMS320LF2407A. Grid-connected current inet and grid voltage unet experimental waveforms. It can be seen from the experimental results that when the above control strategy is adopted, the grid-connected current waveform is better, which can fully meet the requirements of THD.


3 Conclusion


In order to reduce the harmonic pollution of grid-connected current to the grid, this paper improves the control strategy of solar grid-connected inverters, and proposes a new type of solar grid-connected inverter control strategy based on direct current tracking control. This paper analyzes the control algorithm theoretically and conducts related experiments. The experimental results show that the new control strategy can effectively improve the grid-connected current waveform, reduce the THD value of the grid-connected output current, and reduce the harmonic pollution of the grid-connected current to the grid. .