Control of Three Phase Grid Connected Photovoltaic System

Abstract

To counter environmental pollution arising out of fossil fuel power generation,the world is gradually shifting towards renewable sources of energy, with solar energy leading at the front. In a grid-connected solar PV system, the energy generation system and utility grid are interfaced by using power electronics converters which are commonly based on a VSI (voltage source inverter) connected to the mains supply network, which is used to regulate power flow and ensuring operation with unity power factor. The grid requirements are improved power quality, Good transient response during fault conditions, grid synchronization and independent control of active & reactive power and etc.

To achieve this requirement of the grid, here we designed a 10kW PV system which is to be interfaced to a utility grid to be performed. This project represents athree-phase, single-stage grid integrated solar PV system. The proposed system serves for power factor correction,the maximum power point tracking (MPPT), the grid 5 currents balancing and harmonics mitigation of loads connected at the PCC (point5 of common coupling). The solar photovoltaic system uses a three- phase VSC (voltage source converter) to perform these functions. This system extracts the solar energy from the photovoltaic system and feeds to the grid.MPPT algorithm is put to use for extracting maximum power from Photovoltaic system. There are many types of MPPT algorithms are there, but especially which are widely used is perturb and observer and incremental conductance algorithm. Here, the proposed system used the incremental conductance which is more accurate and more efficient. The reference DC link voltage of voltage source converter is also decided by the MPPT algorithm.

In a simplified way, the control algorithm of a solar photovoltaic system consists of two parts, in the first part the MPPT decides the reference D.C. link voltage and secondly the VSC is controlled by the use of control functions. The PV system voltage is maintained at the reference which is controlled by a PI(proportional integral)controller using an MPPT algorithm. For controlling the VSC, an Improved linear sinusoidal tracer (ILST) and Damped-second order generalized integrator (Damped-SOGI) is proposed. The main characters of the Improved linear sinusoidal tracer control algorithm are fast convergence, control simplicity while the Damped-SOGI is having the higher converging state, good noise immunity response and improved steady-state performance. The fundamental component of the load current is extracted by using an ILST and a Damped-SOGI based control algorithm, zero crossing detectors and sample and hold log are used to extract the active power component of load currents. PI controller is used to estimate the loss component of VSC and reference grid currents are estimated by combining all these components. The sensed grid currents and the reference grid currents are compared and a current controller is used for producing the switching pulses for the grid VSC interfaced. The unity power factor (UPF),as well as the sinusoidal balancing of grid currents, are controlled by the VSC. The currents injected into the grid are synchronized using unit templates derived from PCC voltages. Only active power is exchanged through the grid because the grid currents are at UPF. The total active power in the system must be balanced among the grid, the loads and the SPV. There is a comparison between the Damped SOGI and ILST algorithm on the basis of THD and other waveforms. The simulation results are obtained by using MATLAB/Simulink for controlling the active current between the PV-grid systems.