Control of a single-phase inverter in grid-connected distributed generation system

Abstract :

This thesis proposes a new control strategy to control single-phase grid-connected DG [1] distributed generation system based on Lyapunov stability theory. The main idea in this strategy is that a Lyapunov function similar to energy is defined by considering different states of the distributed generation system and then the control law that makes the time derivative of the Lyapunov function always negative for all states is determined. It is shown that a stable control is possible by defining a time-varying reference function for the DC voltage of the capacitor. It should be noted that this strategy does not help to estimate or measure the harmonic ripple components of the capacitor voltage. Therefore, a modified control will be presented by ignoring the harmonic ripple components. The distributed generation current reference is obtained from the difference of the measured load current with the generated power supply current reference. The reference range of the supply current can be adjusted using a proportional-integral controller (PI) that adjusts the DC voltage of the capacitor.

Simulation results obtained for both linear and non-linear and for steady-state operation and step changes applied to the loads are presented in order to check the correct operation of the control method.

Also, non-linear control strategies such as Adaptive, feedback input-output linearization and passivity controller will be investigated in this thesis. And the hysteresis control method is used for a three-phase distributed generation system to compensate the unbalanced loads connected to the network.

1-1- Introduction

In recent years, the control of distributed generation (DG) systems based on inverters, whose main purpose, in addition to producing active power, is to produce reactive power similar to an active filter, whether in the mode without connection to the grid [2] or in the mode of connection to the grid, In order to help the power networks in order to maintain and increase the quality and reliability, as well as providing far and far away loads from the network, it has been given a lot of attention [1]. This increasing interest is caused by preserving the environment and reducing air pollution due to the lack of traditional fuels and the use of renewable sources such as wind, photovoltaics, etc. In DG systems based on the inverter, DC power is the input of the inverter. Although the performance of the distributed generation system without connection to the grid is more sensitive to various factors such as linear and nonlinear control methods, linear and nonlinear loads, unbalanced loads, transient and dynamic changes of loads, etc., but the quick adjustment of the voltage of the common connection point (PCC) [3] and the effective reduction of the rapid disturbances of this voltage in DG systems connected to the grid are also of interest. In this part, we will briefly examine the distributed generation systems based on single-phase inverter in the states without connection to the grid and connected to the grid and review its control objectives] 3-2

Distributed generation system (DG) based on single-phase inverter without connection to the grid is shown in figures (1-1) - (1-3). As it is clear from these figures, the input DC source of the single-phase inverter is renewable energy sources such as photovoltaic, wind energy, geothermal energy and so on. And they can also be batteries. The point where both the inverter and the load are connected is called the common connection point or PCC.

[(images are available in the main file)

These figures show how to connect linear and non-linear loads to the single-phase inverter separately and together. Linear loads are resistance and inductor. The active power resistor and the inductor consume reactive power, which must be fully supplied by the single-phase inverter. Any load that consumes harmonic and non-sinusoidal current can be considered as a non-linear load. The most common nonlinear load is a diode rectifier that consumes a harmonic current as shown in figure (1-4).  The total of inverter and renewable energy sources is called distributed generation system or DG.The goal in distributed generation systems that are directly connected to linear and non-linear loads is to sinusoid the common connection point voltage and bring its THD to an acceptable limit and IEEE standard.