A novel single-phase shunt active power filter based on a current-source converter with reduced Dc-Link

Nowadays, the majority of electronic equipment behave as nonlinear loads, introducing Power Quality (PQ) problems into the Power Grid (PG), namely, current harmonics and low power factor. These PQ problems contribute to the reduction of the efficiency of the transmission and distribution PG, as well as induce the malfunctioning of sensitive loads connected to the PG. Therefore, the development of equipment able to mitigate these PQ problems is extremely important. In this context, this paper presents a novel single-phase Shunt Active Power Filter (SAPF) based on a current-source converter, where the key differencing factor, when compared with the conventional approach, is the reduced dc-link. As the proposed topology requires a reduced dc-link, it represents a relevant advantage, since a typical current-source converter needs an inductor with a high inductance in dc-link, which results in higher losses, costs and component sizing. The proposed SAPF with reduced dc-link is introduced in detail along the paper and a comprehensive comparison with the conventional SAPF is established based on computer simulations. Besides, an experimental validation was carried-out with a developed laboratory prototype, validating the main advantages of the proposed SAPF with reduced dc-link.This work has been supported by FCT – Fundação para a Ciência e Tecnologia with-in the Project Scope: UID/CEC/00319/2019. This work has been supported by the FCT Project QUALITY4POWER PTDC/EEI-EEE/28813/2017, and by the FCT Project newERA4GRIDs PTDC/EEI-EEE/30283/2017

Developing a voltage-source shunt active power filter for improving power quality

Active filters are controlled current or voltage sources that can be used, for example, to compensate current harmonics, interharmonics and reactive power. They offer a wide and/or selectable filtering bandwidth and they are small in size. In addition, active filters can solve almost all the problems that exist with conventional passive filters. This thesis is concerned with developing a digitally controlled three-phase voltage-source shunt active power filter. First, the current compensation characteristic of the active power filter is studied and methods to improve this by compensating and minimizing effects caused by control system delays are investigated and proposed. Computational and prediction-based delay compensation methods are presented. Also, two methods in which the effect of the processing delay is eliminated by applying current-sensorless control and modified main circuit structure are proposed. Both the theoretical study and the experimental results presented show that all the studied methods provide effective compensation characteristics. The use of the LCL-type supply filter in an active power filter is studied by comparing an active and a passive resonance damping method and by assessing the suitability of each for the active power filter application. The results presented show that both of the damping methods provide the fast dynamic responses required in using the active power filter as well as efficient current ripple attenuation. In addition, the results obtained show that the passive damping method increases the power losses only slightly. In contrast, the active damping requires several current sensors and more complicated control than the passively damped system. The power loss profile of the active filter is determined and the effect of replacing the antiparallel silicon diodes in the IGBT bridge with their silicon carbide (SiC) counterparts is studied. The calculation and measurement results show that SiC diodes provide a reduction in the semiconductor power losses of the active filter. The reduction is important, since this would make it possible to reduce the cooling or to increase the switching frequency. The higher switching frequency would enable the use of smaller filter chokes. A comparison of the digitally controlled and vector-modulated voltage-source and currentsource active power filters is presented. The main circuit configurations and space-vector modulation techniques used are discussed as well as the load current detection -based control systems. In addition, the filtering characteristics, power loss distributions, and efficiencies of both systems are studied and compared in various operating points. Finally, a case study in which a combined active and passive compensator is applied to mitigate the voltage flicker problem caused by a resistance spot welding process is presented. The compensation characteristic of the solution is considered comprehensively, using simulations and practical measurements. Furthermore, the resulting flicker severity indices are assessed. The results show that the compensator offers a great reduction of the voltage drops causing the flicker

Real-time grid impedance estimation using a converter

This paper presents two real-time methods for estimating the grid inductance and resistance using a three-phase converter. In the methods, a single-frequency rotating voltage signal with a small magnitude is injected to the converter voltage reference. The converter measures the currents and voltages at the point of common coupling (PCC). The grid impedance is estimated from the measured data using two alternative methods: an estimator based on the sliding discrete Fourier transform (SDFT) and an adaptive grid observer. Dynamic performance of these methods is compared by means of simulations and experiments in the case of a changing grid impedance.Peer reviewe

Estimation of an unbalanced grid impedance using a three-phase power converter

This paper proposes a real-time method for estimating an unbalanced grid impedance using a three-phase converter. In the method, a periodic single-frequency or multi-frequency excitation signal is added to the converter voltage reference. The converter measures currents and voltages at the point of common coupling. The impedance estimate is obtained from the measurements using sliding discrete Fourier transform (SDFT). The method is experimentally validated.Peer reviewe

A voltage-sensorless controller for grid converters

Grid converters need information about the point-of-common-coupling voltage for their control. The voltage is typically measured for current and ac-bus voltage control, and for synchronization. In order to increase reliability by decreasing dependence on this measurement, this paper proposes a multifunctional grid-voltage sensorless controller that can operate either in current- or voltage-control mode. The control system is designed to be robust against variations in grid strength, covering strong and very weak grid conditions. The controller is experimentally tested and its robustness is verified in these conditions. The results show that sensorless ac-bus voltage control can provide grid voltage support.Peer reviewe

Reference-feedforward power-synchronization control

In this letter, an enhancement of power-synchronization control is proposed, whereby pole-zero cancellation in the closed-loop system is achieved. An effect thereof is that step-response ringing and overshoot are eliminated. For strong grids, the closed-loop bandwidth increases, allowing a shorter step-response rise time.Peer reviewe

A dynamic model for saturated induction machines with closed rotor slots and deep bars

This paper deals with a dynamic model for three-phase induction machines equipped with closed rotor slots and deep rotor bars. The thin bridges closing the rotor slots saturate highly as a function of the rotor current. The impedance of the rotor bars also varies much as a function of the rotor current frequency. An extended dynamic model, which takes into account the slot-bridge saturation and the deep-bar effect, is developed. The model extensions can be plugged into a standard machine model and parametrized easily. The proposed model can be applied to time-domain simulations, real-time control, and identification. The model is validated by means of finite-element analysis and experiments using a four-pole 5.6-kW induction machine. The results show that the accuracy of the proposed model is superior to the standard model, particularly under transient excitation typically used in standstill self-commissioning tests of induction motor drives.Peer reviewe

An Approach Utilizing Converters for Locating Faults in LV Distribution Grids

This paper proposes an approach for locating faults in a distribution grid by utilizing data measured and gathered by distributed converters. The data, comprising grid voltages and impedances from multiple locations, is processed using multinomial logistic regression, a machine learning algorithm, to classify a fault location in the grid. The algorithm is first trained with simulation data, followed by evaluation of its predictive performance using a set of test data previously unseen by the algorithm. The fault location accuracy of the proposed approach is found resonable and encourages further studies of the unused potential in the converters.Peer reviewe