Research on Control Strategy of Single-phase Photovoltaic Grid-connected
Inverter With LCL Filter
Zhenxiong ZHOU
1,
*
Shuo SHI
1
1
College of Electrical and Information Engineering, Beihua University, Jilin 132021, China
ABSTRACT: In the same filtering effect, the single-phase photovoltaic grid-connected inverter with L-type
filter needs large inductance, and the traditional PI controller of grid-connected inverter can’t obtain zero
steady-state error. A Quasi-PR control strategy by using LCL-type filter with current control outer loop based
on grid current and current control inner loop based on capacitor current is proposed in this paper, which
increases system damping, restrain oscillation and improve the stability of system. On this basis, the control
system is modeled and analyzed, as well as the parameters are designed, the simulation model is designed to
verify the theoretical analysis under MATLAB/Simulink. The results show that the grid current distortion rate
is smaller, the grid current can be tracked with zero steady-state error., and the system output meets the
requirements of grid.
1. INTRODUCTION
In recent years, due to the effects of the energy crisis, the distributed generation system with solar energy,
wind energy and other clean renewable energy mainly has become the research focus. Grid-connected inverter
is used as a core component of grid-connected PV system. Its main function is to convert the direct current of
the PV array into alternating current with the same frequency and phase of the grid. Its control quality is
directly related to the overall performance of the system, thus it has been recognized and studied by many
researchers.
In order to obtain a low THD (total harmonic distortion) of the grid current, the filter for voltage source
inverter have L type, LC type and LCL type. Because the harmonic suppression of LCL type filter is better
than the previous two kinds of filter under the condition of low switching frequency and small inductance, and
it has been widely used
[6]
. LCL, however, is the three-order system without damping and easy to produce
resonance. The introduction of LCL to increase the order of the system, which requires higher requirements on
the control strategy of control system. The grid-connected inverter with LCL filter that use the control strategy
for direct closed-loop current is unstable. Reference [7] proposed that connecting damping resistance to the
capacitor branch to suppress the resonance and avoid the system instability, but it resulted in power losses and
also decreased the degree of attenuation of high frequency components.
The traditional small power single-phase grid-connected inverter commonly use PI controller, but the PI
controller in tracking the sinusoidal current instruction has large steady-state error and poor anti-interference
ability. In order to solve the shortage of the PI controller, reference [5] proposed control strategy for
single-phase grid-connected inverter based on two phase rotating reference frame. Although the system can
achieve unity power factor, but it needed to go through complex coordinate transformation. Reference [8]
proposed quasi proportional-resonant control in two phase static frame, which achieved no steady error
control of three phase grid-connected inverter.
On the basis of these studies, a Quasi-PR control strategy by using LCL-type filter with current control outer
loop based on grid current and current control inner loop based on capacitor current is proposed in this paper.
The grid current loop is controlled by Quasi-PR controller because the traditional PI control of the sinusoidal
reference quantity is difficult to eliminate the steady-state error. The use of high gain at the resonant frequency