Sinusoidal Grid Currents Research Articles

A Cascaded Generalized Integral Control for Multiobjective Grid-Connected Solar Energy Transfer System

A cascaded generalized integral control algorithm is presented in this article for a solar energy transfer system (SETS), connected to the grid. The control algorithm introduces a two part fundamental component extraction such as, primary harmonics filter with higher order harmonic reduction and dc-offset rejection, and secondary fine fundamental component tuning with fractional order and lower harmonics rejection property. This SETS transfers electrical energy converted from solar radiant energy, efficiently to the three-phase distribution grid. The two-stage system consists of a boost type dc–dc converter on the primary side. Moreover, the incremental conductance algorithm-based operation of the boost converter ensures maximum power output from the photovoltaic (PV) array. This system is designed to meet load demands and feeds remaining power to the grid. It mitigates the reactive power and harmonics requirements of the load at any irradiance level, and thereby keeps the balanced sinusoidal grid currents, in-phase with the respective grid voltages. As the PV array power and load power are varying, performance of SETS with adverse operating conditions is validated in this work. A prototype is developed and tested under varying irradiance conditions as well as nonlinear unbalanced load. The steady-state and dynamic behaviors of the system during transient operating conditions are expressed to substantiate the acceptability of the control technique for multiobjective grid-connected SETS.

A novel tri‐capacity battery charger topology for low‐voltage DC residential nanogrid

This study communicates a single-stage, 3-ɸ, rooftop solar photo voltaic (PV) and grid compatible tri-capacity battery charger (TBC) topology for a home-based low voltage direct current nanogrid (LVDC NG). The proposed architecture performs power factor correction drawing sinusoidal grid currents at unity power factor, single-stage rectification of 3-ɸ grid voltages and constant current mode battery charging simultaneously. The employed control scheme is experimentally validated on a 120 W scaled-down hardware prototype energising three lead-acid-based battery energy storage system (BESS) of nominal voltages 48/36/24 V and capacities 28/21/14 Ah, respectively. The obtained results affirm the effective operation of the LVDC NG under islanded/grid-to-battery and battery-to-grid modes. Further, an extension to the proposed work where the TBC functions charging a single BESS is put forward and validated adding merit to the proposed work.

Model Predictive Direct Power Control of Grid-connected Converters Considering Unbalanced Filter Inductance and Grid Conditions

The grid-connected converter (GCC) is widely used as the interface between various distributed generations and the utility grid. To achieve precise power control for GCC, this paper presents a model predictive direct power control (MPDPC) with consideration of the unbalanced filter inductance and grid conditions. First, the characteristics of GCC with unbalanced filter inductance are analyzed and a modified voltage control function is derived. On this basis, to compensate for the power oscillation caused by unbalanced filter inductance, a novel power compensation method is proposed for MPDPC to eliminate the DC-side current ripple while maintaining sinusoidal grid current. Besides, to improve the control robustness against mismatched filter inductance, a filter inductance identification scheme is proposed. Through this scheme, the estimated value of filter inductance is updated in each control period and applied in the proposed MPDPC. Finally, simulation results in PSCAD/EMTDC confirm the validity of the proposed MPDPC and the filter inductance identification scheme.

Predictive Direct Power Control for Dual-Active-Bridge Multilevel Inverter Based on Conservative Power Theory

This paper explores the feasibility of multilevel dual-active bridge-inverter (DABMI) applications for grid-connected applications of a modern Model of Predictive Direct Power Control (MPDPC) based on the conservative power theory (CPT). In the case of unbalanced grid voltages, the objective of the study is to promote continued active and reactive energy in MPDPC without reducing efficiency such as transient response and current harmonics. The nature of the instantaneous p-q theory permits only one out of three control targets to be fulfilled. The proposed control approached directly regulates the instantaneous active and reactive power to achieve three particular control objectives namely sinusoidal and symmetrical grid current, cancelling twice of fundamental grid frequency reactive power ripples, and removing twice grid frequency active power ripple. The techniques of complicated Grid part sequence extraction are unnecessary and improved at no extra expense, as is the case with current MPDPC fault-tolerant approaches. The instantaneous power at the next sampling instant is predicted with the newly developed discrete-time model. Each possible switching state will then be evaluated in the cost function defined until the optimal state which lead to the minimum power errors is determined. In MATLAB/Simulink simulation, the proposed CPT-based MPDPC measures reliability and performance at balanced and unbalanced grid voltages then compared with the conventional and existing MPDPC The proposed method manages to achieve all of three control targets which generates sinusoidal grid currents and attenuates active and reactive power ripple of twice the grid frequency exactly at the same time without losing its critical efficiency including transient reaction and current harmonics.

Solar PV Sustained Quasi Z-Source Network-Based Unified Power Quality Conditioner for Enhancement of Power Quality

In this article, a Quasi Z-source inverter (QZSI)-based unified power quality conditioner (UPQC) backed by the solar photovoltaic (SPV) is presented in order to enhance power quality. The UPQC consists of converters connected in parallel and series. The active power filters (APFs) connected in parallel and series is one of the versatile custom power circuitries to reduce current and voltage instabilities. The main functions of QZSI can increase the variable direct current (DC) voltage to any given alternating current (AC) output voltage, reduce the necessary elements, and alleviate harmonic content. The UPQC’s compensation function primarily relies on the control system used for the generation of reference current and voltage. The enhanced second order generalized integrator (ESOGI) is used in this proposed system to extract the reference current of QZSI-UPQC. The proposed UPQC uses the SPV system, which has an energy storage unit to offset long-term current and voltage disruptions and fulfill the active power demands of the grid. The experimental results confirm that the proposed SPV-supported QZSI-UPQC generates sinusoidal grid currents of about 1.2% of total harmonic distortion (THD), thus increasing the power efficiency of the interconnected SPV power distribution network.

Single‐phase current source converter with high reliability and high power density

For single-phase current source converter (SCSC), at least one upper device and one lower device have to be gated on and maintained at any time. Otherwise, an open-circuit of the DC-link current would occur and destroy the devices. In addition, a bulky DC-link inductor is usually required to obtain a smooth DC-link current. To address these issues, this study proposes a novel SCSC with high reliability and high-power density. In the proposed circuit, even all the switches are turned on or turned off simultaneously, neither short-circuit nor open-circuit will happen. The inductance in DC-side is reduced significantly byadopting the active power decoupling technique. First, the operation principle of the proposed converter is introduced in detail. Then, a closed-loop control method is presented to regulate the DC-link current, maintain the DC component of the decoupling capacitor voltage, and achieve sinusoidal grid current. Besides, the parameter design of the circuit and controller is investigated. Moreover, the efficiency comparison between the proposed and other SCSCs is carried out. Finally, the effectiveness of the proposed topology is verified by the experimental results.

Closed-Loop Control and Performance Evaluation of Reduced Part Count Multilevel Inverter Interfacing Grid-Connected PV System

Multilevel inverters (MLIs) have drawn tremendous attention in the power sector. Application of MLI has grown extensively to improve the power quality and efficiency of the photovoltaic (PV) system. For an MLI interfacing PV system, the size, cost and voltage stress are the key constraints of the MLI that need to be minimized. This paper presents a novel reduced part count MLI interfacing single-stage grid-tied PV system along with a closed-loop control strategy. The proposed MLI consists of n repeating units and a level boosting circuit (LBC) that assists in generating 4n + 7 voltage levels instead of 2n + 3 levels. Three different algorithms are proposed for a proper selection of dc-link voltages to enhance the levels further. A comparative analysis is carried out to confirm the superiority of the developed MLI. The workability of the proposed MLI is investigated with a 1.3 kW PV system. The closed-loop control strategy ensures the maximum power tracking, dc-link voltage balancing, satisfactory operation of the MLI and injection of clean sinusoidal grid current under any dynamic changes. Comprehensive simulation analysis is carried out considering a 15-level MLI structure. Experimental tests further confirm the practicality of the topological advancement for a PV system under different dynamic conditions.

ПОВЫШЕНИЕ ЭНЕРГОЭФФЕКТИВНОСТИ ПРОИЗВОДСТВА ЗА СЧЕТ ПРИМЕНЕНИЯ СИЛОВЫХ ФИЛЬТРОВ ВЫСШИХ ГАРМОНИК

Актуальность. В настоящее время все большее количество технологических процессов и производств в нефтегазовой отрасли реализуется на базе высокопроизводительного оборудования с полупроводниковыми преобразователями электрической энергии, в число которых, например, входят частотно-регулируемые электроприводы переменного тока или бесперебойные источники питания, которые относятся к классу нелинейных нагрузок и могут вызывать большое количество негативных явлений в работе распределительных сетей, включающих в себя несимметрию фазных напряжений и токов, резонансные процессы, тепловые потери в нейтральном проводе, перегрев двигателей и трансформаторов и т. д. Для улучшения электромагнитной совместимости различного рода электроприемников в составе промышленных систем электроснабжения довольно часто используют пассивные конденсаторные батареи для коррекции коэффициента мощности, которые малоэффективны в случае нелинейных процессов. Кроме этого, данный тип компенсационных устройств продолжает потреблять реактивную энергию при отсутствии каких-либо нагрузок и не удовлетворяет современным требованиям в области энергосбережения. Описанные выше недостатки в работе трехфазных систем переменного тока требуют разработки новых схемотехнических решений, методов анализа энергетических процессов и синтеза алгоритмов силовой фильтрации, позволяющих обеспечить нормированные показатели качества электрической энергии в распределительных сетях низкого и среднего классов напряжения вне зависимости от конкретного вида электрических цепей нагрузки. Цель исследования заключается в комплексном анализе существующих технических решений, направленных на повышение электромагнитной совместимости распределительных сетей, а также в описании основных ограничений в работе силовых фильтрокомпенсирующих устройств применительно к промышленным объектам минерально-сырьевого комплекса; в разработке базовых структур активных силовых фильтров с пониженными требованиями по производительности, объему памяти и быстродействию программно-аппаратной части системы управления, практическое применение которых обеспечивает нормированное качество электрической энергии при изменении режимов работы технологического оборудования или конфигурации питающей линии. Объекты: автономные или децентрализованные электроэнергетические системы переменного тока с преобразовательными устройствами силовой электроники и частотно-регулируемыми электроприводами, а также элементами силовой цепи с нелинейными характеристиками, например, реакторами или трансформаторами с насыщенными сердечниками, в которых присутствуют значительные искажения в мгновенной форме трехфазных сигналов; пассивные и активные силовые фильтры высших гармоник. Методы: некоммутативная алгебра кватернионов; четырехмерное гиперкомплексное пространство; методы спектрального анализа и разложения в ряд Фурье. Результат: краткий обзор различных подходов и технических средств к повышению качества электрической энергии в системах электроснабжения промышленных объектов минерально-сырьевого сектора, а также системы управления,рамках которых достигается синусоидальный закон изменения во времени сетевых токов с нулевым или опережающим/отстающим угловым сдвигом при одновременном соблюдении условия симметрии по мгновенным значениям.

Direct Power Control of PWM Rectifiers With Online Inductance Identification Under Unbalanced and Distorted Network Conditions

In the conventional direct power control (DPC) methods, the grid currents are highly distorted, and substantial ripple is present in the dc-bus voltage under unbalanced and distorted grid conditions. Various improved DPC methods have been proposed to address these problems, but these methods are generally complicated and not easy to use because of the complex controlling structures, need for considerable tuning, and so on. This paper proposes an improved DPC method obtained by simply adding a compensation term to the original power references. The compensation power is obtained by analyzing the power ripple in the input line inductance based on extended power theory. The reference voltage vector is subsequently calculated from the new power reference and synthesized by space vector modulation. To improve the robustness of the proposed DPC against inductance variations, an online inductance identification technique based on the gradient correction method is proposed and combined with the proposed DPC. The presented simulation and experimental results demonstrate that the proposed method can achieve sinusoidal grid currents and significantly reduce the dc-voltage ripple even with mismatched inductance under unbalanced and distorted grid conditions.

Simplified Model Predictive Control for AC/DC Matrix Converters With Active Damping Function Under Unbalanced Grid Voltage

Model predictive control (MPC) is applied to ac/dc matrix converters (MCs) to achieve fast dynamic response and simple implementation. However, the large number of calculations required for a conventional MPC (C-MPC) is an obstacle for its wide application. Moreover, active damping is used to mitigate LC resonance on the input side, which increases the computational burden of the C-MPC. The control performance of the C-MPC also deteriorates under unbalanced grid voltage conditions. To overcome these problems, this paper presents a simplified MPC (S-MPC) for ac/dc MCs to reduce the computational burden. A novel method is proposed to realize the active damping function, which involves a damping function in the S-MPC without increasing the calculation time. Thus, the system performance is easily improved by shortening the sampling period. Furthermore, the grid current reference is generated based on a quantitative analysis of the power flow to achieve constant output current, sinusoidal grid current, and an input power factor near unity under unbalanced grid voltage. Simulation and experiment were done to verify the effectiveness of the proposed control method.

Direct Power Control of PWM Rectifier With Feedforward Compensation of DC-Bus Voltage Ripple Under Unbalanced Grid Conditions

Three-phase voltage source pulsewidth modulation (PWM) rectifiers can achieve constant dc voltage and sinusoidal grid currents under ideal grid voltages. However, under unbalanced grid voltage conditions, there are ripples at twice the grid frequency in the dc voltage, and the grid currents become highly distorted. To address the aforementioned problems, control methods developed under ideal grid voltage conditions should be modified. Various techniques have been proposed in the literature to calculate the new current reference or power reference. However, most methods use conventional pq theory and only consider the grid-side power control. Hence, the dc voltage ripples are not completely eliminated. Furthermore, the derived mathematical expressions for reference calculations are usually very complicated. In contrast to current methods, this paper proposes a very simple yet effective direct power control (DPC) method based on extended pq theory. By analyzing the instantaneous extended power of the line inductance, a simple and concise power compensation is analytically derived and added to the original power references. As a result, the dc voltage ripples are significantly reduced, and the grid currents remain sinusoidal even under unbalanced grid voltage conditions. The proposed method is compared to conventional DPC methods, and its effectiveness is confirmed by the presented experimental results.

An Enhanced Control Strategy for AC–DC Matrix Converters Under Unbalanced Grid Voltage

Under unbalanced grid voltage conditions, an ac-dc matrix converter (MC) induces ripple on the dc side, as well as severe grid current harmonics. To overcome these problems, an enhanced control strategy for an ac-dc MC based on an independent control scheme for the active and reactive powers is proposed in this paper. The generated current reference is directly synthesized from instantaneous power analysis in a stationary frame, and a constant dc voltage and current and sinusoidal grid current are obtained. Moreover, the input power factor (IPF) becomes almost unity due to the reactive power control, in spite of unbalanced grid voltage conditions. There is no need to extract grid voltage components such as the positive and negative sequences, and a controller for the grid current is not required, in contrast to previous methods. Thus, the proposed control method can be implemented easily without a large storage requirement. Simulation and experimental results are presented to verify the effectiveness of the proposed control strategy.

Weak Grid Intertie WEGS With Hybrid Generalized Integrator for Power Quality Improvement

The wind energy generating system (WEGS) in this paper, aims to provide the required active peak power without high frequency fluctuations even under variable wind speed conditions. The generator speed is adjusted according to the intermittent wind speeds through two voltage source converters (VSCs) connected back to back, namely, machine-side VSC (MSVSC) and utility grid side VSC (UGVSC) across the dc-link capacitor. In this paper, the hybrid generalized integrator control is proposed for switching UGVSC and providing dc offset rejection and immunity against oscillatory errors due to subharmonics, thereby improving the power quality (PQ). Fuzzy logic controller (FLC) is implemented for the speed control of the salient pole synchronous generator (SG) driven by the wind turbine. The FLC provides the tracking of the reference speed under high overshoot transient conditions and narrow bandwidth. The switching of MSVSC is obtained by field oriented control. The dynamic performance is improved by the wind feed-forward term, which reduces the oscillation, ensuring balanced and sinusoidal grid currents. The generated power from the WEGS is fed to the grid. The weak grid conditions, namely, grid voltage unbalance, voltage sag, voltage swell, and grid voltage distortion, are considered. The performance of the system is tested on a laboratory prototype. Test results provide the effectiveness of the system with increased wind penetration and performance under weak grid conditions. Moreover, improving the PQ, the grid current total harmonic distortion meets the requirement of IEEE-519 standard, and is found to be less than 5%.

Direct Power Control of Pulsewidth Modulated Rectifiers Without DC Voltage Oscillations Under Unbalanced Grid Conditions

Direct power control with space vector modulation (DPC-SVM) features simple structure, fast dynamic performance, and little tuning work. However, the conventional DPC-SVM cannot achieve accurate power control under unbalanced grid conditions. A modified DPC-SVM is thus proposed for accurate power control under both ideal and unbalanced grid conditions. Though the power control accuracy is improved when compared with the conventional DPC-SVM, it still suffers highly distorted grid current and dc voltage oscillations with an unbalanced network. Therefore, a power compensation method is subsequently derived aiming at the following targets: eliminating dc voltage oscillations, achieving sinusoidal grid current, and obtaining unity power factor. To that end, average grid-side reactive power and oscillations in converter-side active power are controlled as zero by simply adding a compensation to original power reference. Additionally, the proposed method does not require extraction of a positive sequence or negative sequence component of grid voltage. Compared with the conventional DPC-SVM in an ideal grid, only additional compensation of power reference is required. As a result, control performance can be significantly improved without substantial increase in complexity. The superiority of the proposed method over the prior DPC-SVM is validated by both simulation and experimental results obtained on a two-level pulsewidth modulation voltage source rectifier.

Implementation of PV System Integrated DSTATCOM with Modified UVT Control Scheme for Harmonic and Neutral Current Elimination

Abstract The interconnection of renewable energy sources into the utility grid gives lot of issues. The important issues faced by the systems are synchronized with the utility grid, power quality, and others. The distribution static compensator (DSTATCOM) is the efficient custom power device that can resolve the power quality problems in the distribution system and interconnect the Photovoltaic (PV) system into the utility grid. The Second Order Generalized Integrator (SOGI) Phase Locked Loop (PLL)–based Unit Vector Template (UVT) control scheme is employed for extraction of reference current to generate gating signals of DSTATCOM. The proposed control algorithm is employed for compensation of current harmonics, reactive power, and voltage interruption. The dc link storage capacitor of the DSTATCOM is utilized to interconnect the PV system into the utility grid, and it is controlled by the UVT-based controller. The PV-supported DSTATCOM provides uninterrupted active and reactive power compensation when disturbances occur in the distribution system. The simulation and experimental results confirm that the proposed DSTATCOM results in purely sinusoidal grid currents with the Total Harmonic Distortion level about 1.28 %, which enhances the power quality of the interconnected PV system.

Control and energy management of a large scale grid-connected PV system for power quality improvement

Power quality is highlighted as an important parameter in modern power systems. Moreover, grid-connected photovoltaic power plants are increasing significantly in size and capacity. Elsewhere, due to the progressive integration of nonlinear loads in the grid, the principal role of a Solar Energy Conversion System (SECS) is not only to capture the maximum power from solar but, also to ensure some ancillary services and improve the quality of power. This paper presents a novel strategy dedicated to improve the management of active power generation, reactive power compensation and power quality of a SECS, while guaranteeing the possibility of exploiting the full capacity of the Power Conditioning System (PCS) and the PhotoVoltaic System (PVS). The proposed control algorithm is applied to a large scale PVS connected to the grid through a cascade of a DC-DC converter and a PWM inverter. This control strategy manages the SECS function’s priorities, between main active power generation, reactive power compensation and active filtering in such a way to guarantee a smooth and stable DC voltage and ensure a sinusoidal grid current. Top priority is given to the active power production over power quality improvement. Then, priority is given to reactive power compensation over mitigation of current harmonics absorbed by the non-linear load connected to the Point of Common Coupling (PCC). Moreover, the whole system upper limits of active and reactive powers have been determined in the (PQ) power plane on the basis of PVS available power, converters rated power and DC bus voltage smoothness and stability. Finally, a control procedure dedicated to the calculation of the inverter current commands is proposed in order to exploit the full capacity of the SECS and respect the determined power limits. Simulation results confirm the effectiveness and the performance of this control strategy and prove that the SECS can operate at its full power whilst the power quality can be improved by reactive power compensation and active filtering.

Fractional-Order Sliding Mode Control for D-STATCOM Connected Wind Farm Based DFIG Under Voltage Unbalanced

This paper studies the use of distribution static synchronous compensator (D-STATCOM) integrated with wind farm based on doubly fed induction generators. The short-term energy storage system based on a supercapacitor is introduced in this study to improve the operation of D-STATCOM. The dynamic performances of the grid-connected wind farm with D-STATCOM are enhanced by the proposed control scheme, which is based on fractional-order sliding mode control theory with dual-sequence decomposition technique. Some results of simulation show that, under an unbalanced grid voltage conditions, the proposed control strategy for the D-STATCOM not only eliminates effectively the oscillations of the active and reactive powers exchanged between the wind generator and the grid but also achieves the symmetrical and sinusoidal grid currents with less harmonics.

Modeling and Sliding Mode Control for Three-Phase Active Power Filters Using the Vector Operation Technique

Traditionally, the vector operation technique (VOT) has been used to control three-phase converters using one cycle control. In this paper, a three-phase active power filter large-signal model with a VOT in a new coordinate system is presented. By using the VOT, only two phase-legs are switching at high frequency, thus reducing the switching losses. This paper not only covers the literature gap about the modeling of three-phase converters using the vector operation, but also presents a sliding mode control for this converter. The control scheme consists of a nonlinear matrix transformation in order to obtain the voltages and currents in a new two-dimensional frame, $\gamma \theta$ -frame, a sliding mode controller designed in these coordinates, and a modulator to obtain the control signals in a natural frame. The sliding mode control is designed with the help of the presented large-signal model assuring sinusoidal grid currents in phase with the grid voltages. This controller provides a fast transient response against sudden load changes with a good current tracking capability and a reduction of the switching losses. A stability analysis is performed in order to validate the control parameters. Experimental results are provided using a fully digital control system in order to validate the performances of the proposed controller.

A direct power control strategy for three level neutral-point-clamped rectifier under unbalanced grid voltage

Direct power control (DPC) strategy has attracted wide attention due to its advantages of simple structure, quick response, strong robustness, and elimination of current regulation loops/PWM blocks. Unfortunately, under unbalanced grid voltage, the conventional DPC (CDPC) scheme with the conventional definitions of active and reactive power cannot work well. In order to solve this problem, a new definition of the active power instead of the conventional one is proposed, discussed and used in this paper. As a result, good performance of the system is achieved and neither complicated calculation of a power compensation term nor positive/negative sequence extraction of grid voltages/currents are required. Then, a switching table based DPC strategy is designed based on the new definition of active power and conventional definition of reactive power. The corresponding switching table is suitable to achieve constant active power, constant reactive power and sinusoidal grid currents with very low total harmonic distortions (THDs). Simulation results are presented to confirm the theoretical study and the effectiveness of the proposed DPC with the new definition of active power (DPC-NP). The performance of the proposed DPC-NP is compared with that of the CDPC and that of the DPC with a new definition of reactive power (DPC-NQ).

A Modified Active Power Control Scheme for Enhanced Operation of PMSG-Based WGs

This paper emphasizes the development of a simplified active power control scheme for enhanced operation of grid integrated permanent magnet synchronous generator based wind-driven generators (WGs). An active power reference generation scheme is proposed for the machine side converter (MSC) to inject active power into the grid even under grid disturbances, without violating system components rating. In this scheme, the controller employed for MSC adjusts the active power captured proportionate to the drop in the grid voltage upon considering wind speed and rotor speed. Furthermore, unlike dual vector control scheme, the grid side converter controller is implemented in a positive synchronous frame with the proposed current oscillation cancellation scheme to suppress the oscillations in dc-link voltage, active and reactive power of the grid and to obtain symmetrical sinusoidal grid current. Extensive analytical simulation has been carried out in PSCAD/ EMTDC to validate the superiority of proposed control scheme over the conventional schemes when WG is subjected to various grid disturbances. The reduced percentage of oscillation in the system parameters such as dc-link voltage and grid active power confirms the efficacy of the proposed method when compared with the conventional control techniques.