Control System Design, Analysis, and Simulation of a Photovoltaic Inverter for Unbalanced Load Compensation in a Microgrid

This thesis presents a control scheme for a single-stage three-phase Photovoltaic (PV) converter with negative sequence load current compensation. In this thesis a dual virtual impedance active damping technique for an LCL filter is proposed to address the issue of LCL filter resonance. Both inverter-side current and the capacitor current are used in the feedback loop. Using both signals provides higher DC rejection than using capacitor current alone. The proposed active damping scheme results in a faster transient response and higher damping ratio than can be obtained using inverter-side current alone. The feedback gains can be calculated to achieve a specified damping level. A method of determining the gains of the Proportional and Resonant current controller based on frequency response characteristics is presented. For a specified set of gain and phase margins, the controller gains can be calculated explicitly. Furthermore, a modification is proposed to prevent windup in the resonator. A numerically compensated Half-Cycle Discrete Fourier Transform (HCDFT) method is developed to calculate the negative sequence component of the load current. The numerical compensation allows the HCDFT to accurately estimate the fundamental component of the load current under off-nominal frequency conditions. The proposed HCDFT method is shown to have a quick settling time that is comparable to that obtained with conventional sequence compensation techniques as well as immunity to harmonics in the input signal. The effect of unbalance compensation on the PV power output depending on the irradiance and the operational region on the power-voltage curve is examined. Analysis of the DC link voltage ripple shows the region of operation on the P-V curve affects the amplitude of the DC link voltage ripple during negative sequence compensation. The proposed control scheme is validated by simulation in the Matlab/Simulink® environment. The proposed control scheme is tested in the presence of excessive current imbalance, unbalanced feeder impedances, and non-linear loads. The results have shown that the proposed control scheme can improve power quality in a hybrid PV-diesel microgrid by reducing both voltage and current imbalance while simultaneously converting real power from a PV array

Design and Implementation of Shunt Active Power Line Conditioner using Novel Control Strategies

Shunt Active Power Filter (APF) or Active Power Line Conditioner (APLC) is designed and implemented for power quality improvements in terms of current harmonics and reactive-power compensation. The widespread use of non-linear loads in industrial, commercial and domestic facilities cause harmonic problems. Harmonics induce malfunctions in sensitive equipment, overvoltage by resonance, increase heat in the conductors, harmonic voltage drop across the network impedance and affects other customer loads connected at the Point of Common Coupling (PCC). Active power line conditioner is implemented for compensating the harmonics and reactive-power imultaneously in the distribution system. The performance of the active power line conditioner depends on the design and characteristics of the controller adopted for APLC. The objective of this research is to find a suitable control strategy for reference current extraction as well as PWM-VSI current controller. PI / PID / FLC / PI-FLC, Fryze power theory, proposed instantaneous realpower theory, proposed sinusoidal extraction controller and modified-synchronous reference frame theory methods are utilized for extracting reference current.Furthermore, indirect PWM-current control (triangular-carrier / triangular-periodical current controller, space vector modulation controller, fixed-Hysteresis Current Controller (HCC), adaptive-HCC and adaptive-fuzzy-HCC) approach is applied to generate switching pulses of the PWM-inverter. Each reference current extraction method in conjunction with various PWM-current control techniques (or vice-versa) are simulated and investigated for the active power line conditioner. For experimental validation, the modified-synchronous reference frame with adaptive-fuzzy-HCC technique is adopted. This control algorithm is demonstrated through the TMS320F240 Digital Signal Processor for shunt APLC system

A harmonic compensation strategy in a grid-connected photovoltaic system using zero-sequence control

Mitigation of harmonics for a grid-connected inverter is an important element to stabilize the control and the quality of current injected into the grid. This paper deals with the control method of a three-phase Grid-Connected Inverter (GCI) Photovoltaic (PV) system, which is based on the zero-sequence current adjuster. The proposed method is capable of removing the harmonic current and voltage without using any active and passive filters and without the knowledge of the microgrid topology and also impedances of distribution bands and loading conditions. This concept is adopted for the control of a Distributed Generator (DG) in the form of grid-connected inverter. The proposed control can be applied to the grid connected inverter of the PV. The fast dynamic response, simple design, stability, and fast transient response are the new main features of the proposed design. This paper also analyzes the circuit configuration effects on the grid connected inverter capability. The proposed control is used to demonstrate the improved stability and performance