Improvement of the current control loop of the single-phase multifunctional grid-tied inverter of photovoltaic system

Alexandr Shavelkin,Jasim Mohmed Jasim Jasim,Iryna Shvedchykova

doi:10.15587/1729-4061.2019.185391

Abstract

Improvement of the current control loop of the multifunctional grid-tied inverter of the local object electrical power system is considered. The purpose is ensuring compliance with the quality standards of the grid current in the whole range of its values when connecting the load to the AC distribution grid and the converter unit of the photovoltaic system. The use of relay current controller with combining of unipolar and bipolar modulation in combination with a modified algorithm of switching the inverter switches is proposed. It is shown that at a non-linear load, the jump-like change of the derivative of the inverter reference current leads to a sharp change in the switching frequency of the switches. This leads to the appearance of “splashes” in the grid current, which worsens its harmonic composition at small current values.In the nonlinear reactor with an increase in the inverter current, the “splash” is also caused by changes in the switching frequency of the switches. This takes place in the areas of current growth (fall) due to a change in the reactor inductance. The dependence of the given deviation for the relay controller is established. In the case of a linear reactor, this provides a practically constant switching frequency for the inverter switches. It is proposed to take into account the derivative of the inverter current in the deviation reference of the relay controller. This eliminates an abrupt change in switches switching frequency. The necessity of taking into account the non-sinusoidal grid voltage during the determination of the voltage value at the inverter input is shown. It is proposed to reduce the capacitance of the filter capacitor at the point of common coupling and use of capacitor current coupling. This will improve the quality of the grid current at the non-sinusoidal voltage of the grid. The structure of the current control loop with a relay current controller with the combined modulation and regulation of the reference value of the controller deviation is proposed. Regulation is carried out in accordance with the reference value of the amplitude and the derivative of the inverter current. A mathematical model of the system “grid – grid inverter – load” with a block of determination of power losses in the switches and a nonlinear reactor is developed

Highlights

In combined electric power systems (EPS) of local objects with renewable energy sources (RES), the load of the object is connected to the point of common coupling (PCC) of 0.4 kV AC distributed grid (DG) and RES converter unit (CU)
A promising direction is the use of a multifunctional grid voltage source inverter (VSI) that combines the functions of an active power filter (APF) [2]
This contributes to increasing the utilization factor of CU due to its round-the-clock use [2,3,4,5] to maintain the maximum power factor at the PCC to DG

Summary

Introduction

In combined electric power systems (EPS) of local objects with renewable energy sources (RES), the load of the object is connected to the point of common coupling (PCC) of 0.4 kV AC distributed grid (DG) and RES converter unit (CU). A promising direction is the use of a multifunctional grid voltage source inverter (VSI) that combines the functions of an active power filter (APF) [2]. This contributes to increasing the utilization factor of CU due to its round-the-clock use [2,3,4,5] to maintain the maximum (close to one) power factor at the PCC to DG. Improving the indicators of CU for systems with RES is an urgent task and contributes to the further development of energy with distributed sources of electricity

Literature review and problem statement

The aim and objectives of the study

Findings

Conclusions