Flexible operation of grid-interfacing converters in distribution networks : bottom-up solutions to voltage quality enhancement

Due to the emerging application of distributed generation (DG), large numbers of DG systems are expected to deliver electricity into the distribution network in the near future. For the most part these systems are not ready for riding through grid disturbances and cannot mitigate unwanted influences on the grid. On the one hand, with the increasing use of sensitive and critical equipment by customers, the electricity network is required to serve high voltage quality. On the other hand, more and more unbalanced and nonlinear equipment, including DG units, is negatively affecting the power quality of distribution networks. To adapt to the future distribution network, the tendency for grid-interfacing converters will be to integrate voltage quality enhancement with DG functionality. In this thesis, the flexible operation of grid-interfacing converters in distribution networks is investigated for the purpose of voltage quality enhancement at both the grid and user sides. The research is carried out in a bottom-up fashion, from the low-level power electronics control, through the realization of individual system functionality, finally arriving at system-level concepts and implementation. Being essential to the control of grid-interfacing converters, both stationaryframe techniques for voltage detection and synchronization in disturbed grids, and asymmetrical current regulation are investigated. Firstly, a group of high performance filters for the detection of fundamental symmetrical sequences and harmonics under various grid conditions is proposed. The robustness of the proposed filters to small grid-frequency variation and their adaptability to large frequency change are discussed. Secondly, multiple reference frame current regulation is explored for dealing with unbalanced grid conditions. As a complement to the existing proportional resonant (PR) controllers, sequence-decoupled resonant (SDR) controllers are proposed for regulating individual symmetric sequences. Based on the modeling of a four-leg grid-connected system in different reference frames, three types of controllers, i.e. PI, PR, and proportional plus SDR controllers are compared. Grid-interactive control of distributed power generation, i.e. voltage unbalance compensation, grid-fault ride-through control and flexible power transfer, as well as the modeling of harmonic interaction, are all investigated. The in-depth study and analysis of these grid interactions show the grid-support possibilities and potential negative impact on the grid of inverter-based DG units, beyond their primary goal of power delivery. In order to achieve a co-operative voltage unbalance compensation based on distributed DG systems, two control schemes, namely voltage unbalance factor based control and negative-sequence admittance control, are proposed. The negativesequence voltages at the grid connection point can be compensated and mitigated by regulating the negative-sequence currents flowing between the grid and DG converters. Flexible active and reactive power control during unbalanced voltage dips is proposed that enables DG systems to enhance grid-fault ride-through capability and to adapt to various requirements for grid voltage support. By changing adaptable weighting factors, the compensation of oscillating power and the regulation of grid currents can be easily implemented. Two joint strategies for the simultaneous control of active and reactive power are derived, which maintain the adaptive controllability that can cope with multiple constraints in practical applications. The contribution of zero-sequence currents to active power control is also analyzed as a complement to the proposed control, which is based on positive- and negative-sequence components. Harmonic interaction between DG inverters and the grid is modeled and analyzed with an impedance-based approach. In order to mitigate the harmonic distortion in a polluted grid, it is proposed to specify output impedance limits as a design constraint for DG inverters. Results obtained from modeling, analysis, and simulations of a distribution network with aggregated DG inverters, show that the proposed method is a simple and effective way for estimating harmonic quasi-resonance problems. By integrating these proposed control strategies in a modified conventional series-parallel structure, we arrived at a group of grid-interfacing system topologies that is suitable for DG applications, voltage quality improvement, and flexible power transfer. A concrete laboratory system details the proposed concepts and specifies the practical problems related to control design. The introduction of multi-level control objectives illustrates that the proposed system can ride through voltage disturbances, can enhance the grid locally, and can continue the power transfer to and from the grid while high voltage quality is maintained for the local loads within the system module. A dual-converter laboratory set-up was built, with which the proposed concepts and practical implementation have been fully demonstrated

A STUDY ON COMBINATION OF SERIES ACTIVE POWER FILTER AND SHUNT PASSIVE POWER FILTER FOR HARMONIC COMPENSATION

This project is carried out to study on the performance of combination of series active power filter and shunt passive power filter for harmonic compensation through simulation. Two different control methods which are instantaneous active and reactive power (p-q method) and instantaneous active and reactive current component (id-iq method) are applied to the combined filter and the comparative study of their performance is carried out. The simulation is carried out using MATLAB Simulink- SimPower System block and a model of a complete network with a thyristor converter driven load was developed. The model was simulated in several phases; without any of the filter operating on it, with shunt passive filter acting alone, with a series active filter acting alone and with combination of both filters in order to examine the role of each filter involved. For each of the simulation, the THD of input line current is used as a performance measure. The studies had shown that the series active filter have a small effect in compensating the harmonic current while the shunt passive filter had a more significant role to compensate the harmonic current. The combination of both filters had enhanced significantly the ability of the filter to compensate the harmonic current. The simulation of the combined filter is extended by applying the two different control methods with three different conditions of supply voltage; a balanced and sinusoidal, unbalanced and sinusoidal and balanced and non-sinusoidal voltage. From the analysis of the harmonic content in the input line current (THD), it can be stated that under balanced and sinusoidal supply, p-q method and id-iq method have the same performance in compensating the harmonic. Nevertheless, under unbalanced and non-sinusoidal supply, the id-iq method had given a slightly better performance for harmonic compensation. Despite this slightly better performance of id-iq method, it can be conclude that it does not have much significant impact for the combination of series active filter and shunt passive filter in compensating the harmonic

Digital implementation of modified phase locked loop based harmonic extraction for shunt active filter

This paper presents a digital implementation of modified synchronous reference frame in which the Phase Locked Loop (PLL) is customized to get the angle for the reference frames from the supply voltage by Enhanced Phase Locked Loop (EPLL). The extracted harmonics currents are given to an Artificial Neural Network based Space Vector Pulse Width Modulation (ANNSVPWM) which has better switching control and reduced stress on the switches to cancel the distortions at the Point of Common Coupling (PCC). The algorithm was modelled and simulated by Matlab/Simulink to validate the results. The experimental verification is carried on Field Programmable Gate Array (FPGA) Spartan board to check the effectiveness of the control strategy being implemented and the results conclude that the Total Harmonic Distortion (THD) values are below the required levels of power quality standards