Harmonics Mitigation and Non-Ideal Voltage Compensation Utilizing Active Power Filter Based On Predictive Current Control

It is well-known that the presence of non-linear loads in the distribution system can impair the power quality. The problem becomes worse in microgrids and power electronic-based power systems as the increasing penetration of single-phase distributed generation may result in a more unbalanced grid voltage. Shunt active power filters (SAPFs) are used for improving the power quality and compensating for the unbalance grid voltage. This study presents a modification of the classical control structure based on the finite control set model predictive control (FCS-MPC). The proposed control structure can retain all the advantages of FCS-MPC, while improving the input current quality. Furthermore, a computationally efficient cost function based on only a single objective is introduced, and its effect on reducing the current ripple is demonstrated. The presented solution provides a fast response to the transients as well as compensates for the unbalanced grid voltage conditions. A straightforward single loop controller is compared to the conventional way of realising the active power filters, which is based on space vector pulse width modulation. The simulation results have been obtained from MATLAB/SIMULINK environment, while the obtained experimental results, utilising a 15 kVA power converter, highlight the effective performance of the proposed control scheme and verifies the introduced MPC-based method as a viable control solution for SAPFs

Mitigation of power quality issues due to high penetration of renewable energy sources in electric grid systems using three-phase APF/STATCOM technologies: a review.

This study summarizes an analytical review on the comparison of three-phase static compensator (STATCOM) and active power filter (APF) inverter topologies and their control schemes using industrial standards and advanced high-power configurations. Transformerless and reduced switch count topologies are the leading technologies in power electronics that aim to reduce system cost and offer the additional benefits of small volumetric size, lightweight and compact structure, and high reliability. A detailed comparison of the topologies, control strategies and implementation structures of grid-connected high-power converters is presented. However, reducing the number of power semiconductor devices, sensors, and control circuits requires complex control strategies. This study focuses on different topological devices, namely, passive filters, shunt and hybrid filters, and STATCOMs, which are typically used for power quality improvement. Additionally, appropriate control schemes, such as sinusoidal pulse width modulation (SPWM) and space vector PWM techniques, are selected. According to recent developments in shunt APF/STATCOM inverters, simulation and experimental results prove the effectiveness of APF/STATCOM systems for harmonic mitigation based on the defined limit in IEEE-519

Power Quality Improvement of Distributed Generation Integrated Network with Unified Power Quality Conditioner.

With the increased penetration of small scale renewable energy sources in the electrical distribution network, maintenance or improvement of power quality has become more critical than ever where the level of voltage and current harmonics or disturbances can vary widely. For this reason, Custom Power Devices (CPDs) such as the Unified Power Quality Conditioner (UPQC) can be the most appropriate solution for enhancing the dynamic performance of the distribution network, where accurate prior knowledge may not be available. Therefore, the main objective of the present research is to investigate the (i) placement (ii) integration (iii) capacity enhancement and (iv) real time control of the Unified Power Quality Conditioner (UPQC) to improve the power quality (PQ) of a distributed generation (DG) network connected to the grid or microgrid

Cooperation of voltage controlled active power filter with grid-connected DGs in microgrid

Due to the excessive use of nonlinear loads and inverter interfaced distributed generators, harmonic issues have been regarded as a major concern in power distribution systems. Therefore, harmonic compensation in microgrids is a subject of current interest. Consequently, a novel direct harmonic voltage-controlled mode (VCM) active power filter (APF) is proposed to mitigate the harmonics in a cooperative manner and provide a better harmonic compensation performance of less than 5%. Due to the dispersive characteristics of renewable energy resources, voltage feedback based on a harmonic compensation control loop is implemented for the first time. This system can be smoothly combined with the current control loop. Our method proposes a better performance while mitigating the harmonics in comparison with conventional resistive active power filters (R-APF). Based on direct voltage detection at the point of common coupling (PCC), the proposed VCM-APF can therefore be seamlessly incorporated with multiple grid-connected generators (DGs) to enhance their harmonic compensation capabilities. The advantage of this scheme is that it avoids the need for designing and tuning the resistance, which was required in earlier conventional control schemes of R-APF for voltage unbalance compensation. Additionally, our scheme does not require the grid and load current measurements since these can be carried out at the PCC voltage, which further reduces the implementation cost of the system. Furthermore, the simulation results show the significance of proposed method

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

Enhanced decoupling current scheme with selective harmonic elimination pulse width modulation for cascaded multilevel inverter based static synchronous compensator

This dissertation is dedicated to a comprehensive study and performance analysis of the transformer-less Multilevel Cascaded H-bridge Inverter (MCHI) based STATic synchronous COMpensator (STATCOM). Among the shunt-connected Flexible AC Transmission System (FACTS) controllers, STATCOM has shown extensive feasibility and effectiveness in solving a wide range of power quality problems. By referring to the literature reviews, MCHI with separated DC capacitors is certainly the most versatile power inverter topology for STATCOM applications. However, due to the ill-defined transfer functions, complex control schemes and formulations were emerged to achieve a low-switching frequency high-bandwidth power control. As a result, adequate controller parameters were generally obtained by using trial and error method, which were practically ineffective and time-consuming. In this dissertation, the STATCOM is controlled to provide reactive power (VAR) compensation at the Point of Common Coupling (PCC) under different loading conditions. The goal of this work is to enhance the performance of the STATCOM with the associated proposed control scheme in achieving high dynamic response, improving transient performance, and producing high-quality output voltage waveform. To evaluate the superiority of the proposed control scheme, intensive simulation studies and numerous experiments are conducted accordingly, where a very good match between the simulation results and the experimental results is achieved in all cases and documented in this dissertation