Grid Current Distortion Research Articles

A New SVPWM Strategy for Three-Phase Isolated Converter with Current Ripple Reduction

Three-phase isolated matrix converters enable bidirectional power conversion and galvanic isolation, and they are suitable for widespread applications in industry. However, excessive DC-link current ripple not only increases the inductor loss and switching loss but also causes more electromagnetic interference and grid current distortion. Traditionally, increasing DC-link inductance or switching frequency can reduce the current ripple to a certain extent, but it is not cost-effective due to the bulky size of the inductor and higher switching losses. To address the above issue, optimizing the modulation control strategy is more attractive. This paper proposes a new SVPWM strategy to reduce the current ripple. First, the inherent limitation of the conventional modulation scheme is revealed. Then, the new optimal modulation scheme is proposed for the isolated matrix converters to reduce the current ripple without increasing the DC-link inductor or switching frequency. Moreover, the power density of the system is effectively increased. Finally, simulation in a MATLAB environment and a laboratory prototype of the isolated matrix converter have been built to verify the effectiveness of the proposed strategy.

A lyapunov‐based adaptive voltage controller for a grid connected PV system

A new Lyapunov-based adaptive controller (LBAC) for a single stage three-phase grid-connected PV system (GCPVS) is proposed here. The uncertainties in solar irradiation affects the performance of the GCPVS. Further, ageing in the DC-link capacitor affects the PV voltage tracking. To achieve an improved performance of the GCPVS under both high and low irradiation, and under an aged DC-link capacitor, a new LBAC scheme has been proposed to control the PV voltage. In this LBAC, the gains of the controller are adapted online by a Lyapunov-based adaptation mechanism to provide the nominal tracking response of PV voltage. Further, a maximum power point tracking algorithm and grid current controllers are designed to transfer the maximum PV power to the grid, ensuring good power quality. As a result, improved performances of the GCPVS are achieved in terms of low PV voltage ripple, enhancement of delivered grid energy, and low grid current total harmonics distortion (THDs). Computer simulation studies are carried out in MATLAB/Simulink environment to verify the effectiveness of the proposed LBAC. Further, experiments are performed in a 2.5 kW GCPVS prototype for real time implementation of the proposed controller. The total harmonics distortion (THD) of grid currents is found to be within IEEE-1547 standards.

Current-Control Inverter Schemes for a Grid-Connected PV Generator

Photovoltaic Generators (PVGs) are one of the popular renewable energy sources (RESs), which achieve 47% of RES in microgrids. The main components of the proposed system are: (i) a PVG considering extremum operating conditions, the irradiance change is 400-1000 W/m2 and the temperature change is 25-60 °C; (ii) a DC-DC boost converter used for regulating the variable DC-link voltage via an inner voltage-loop controller (iii) a single-phase H-bridge inverter controlled by an outer current-loop controller for implementing the maximum power point tracking (MPPT) algorithm at grid side; (iv) an inductive power filter for improving the total harmonic distortion (THD) of the grid current. In this paper, the grid current is controlled to follow the maximum power point (MPP) current considering the losses when transferring the DC power to the grid. Two current-loop controllers are implemented, which are: (i) hysteresis current controller (HCC) and PI current controller (PICC). The proposed design is validated in MATLAB/SIMULINK and the simulation results show significant improvement in power quality, Total Harmonic Distortion (THD) of the output grid current is less than 2% and the power factor is close to unity.

Analysis and Control of Battery Energy Storage System Based on Hybrid Active Third-Harmonic Current Injection Converter

This paper applies the emerging hybrid active third-harmonic current injection converter (H3C) to the battery energy storage system (BESS), forming a novel H3C-BESS structure. Compared with the commonly used two-stage VSC-BESS, the proposed H3C-BESS has the capability to reduce the passive components and switching losses. The operation principle of the H3C-BESS is analyzed and the mathematical model is derived. The closed-loop control strategy and controller design are proposed for different operation modes of the system, which include the battery current/voltage control and the injected harmonic current control. In particular, active damping control is realized through the grid current control, which could suppress the LC-filter resonance without the need of passive damping resistors. Simulation results show that the proposed topology and its control strategy have fast dynamic response, with a setup time of less than 4 ms. In addition, the total harmonic distortions of battery current and grid currents are only 2.54% and 3.15%, respectively. The amplitude of the injected harmonic current is only half of the grid current, indicating that the current injection circuit generates low losses. Experimental results are also provided to verify the validity of the proposed solution.

Power quality improvement using three‐phase transformerless grid interfaced SECS enabling leakage current suppression

This paper presents a harmonic compensation strategy for a 3-ϕ grid-tied solar energy conversion system with leakage current attenuation feature. As parasitic capacitance exists between the solar photovoltaic panels with the ground, the deviation in common-mode voltage compels the probable safety hazards such as electromagnetic interference and harmonics injection in the grid currents. Additionally, the power quality indices of the grid, are exacerbated with the coupling of the non-linear load. Owing to these issues, a generalized integrator based approach is introduced to ensure the harmonics suppression, power factor correction, leakage current alleviation. Additionally, the grid current distortion limits are ensured within limits in order to comply with the IEEE std. 519. The presented algorithm ensures a robust operation with respect to various anomalies in the grid voltages. Simulation results show the satisfactory response of the harmonics controller even under abnormal operating scenarios including the symmetrical/unsymmetrical faults in the grid side network. In comparison with the traditional algorithms, the harmonic controller suppresses the leakage current and harmonics within restricted limits despite the abnormal conditions. Real-time simulator results demonstrate the performance of the controller under distinct operating conditions such as load unbalancing, distorted grid voltages, etc.

Iterative learning control of dispatchable grid-connected distributed energy resources for compensation of grid current harmonic distortions

This paper proposes a power regulation scheme for dispatchable grid-connected distributed energy resource (DER) units based on the iterative learning control (ILC) strategy. The proposed control provides fast and stable control of the real and reactive powers that the DER unit delivers to the grid. In addition, it compensates the harmonic distortion of the grid current caused by the local nonlinear loads and delivers a high quality power to the grid. To this end, the paper presents the mathematical model on which the proposed control is based, as well as the control design methodology. A new MIMO iterative learning control with perfect power tracking and harmonic rejection performance as well as minor measurement requirements is proposed. The performance of the proposed ILC strategy is demonstrated through time-domain simulation of a grid-connected DER unit under various operating scenarios, and it is compared with the performance of the conventional control based on the instantaneous power theory (IPT).

Direct control of active and reactive power for a grid-connected single-phase photovoltaic inverter

This paper presents a single-phase grid-connected photovoltaic system with direct control of active and reactive power through a power management system of a Photovoltaic inverter. The proposed control algorithm is designed to allow maximum utilization of the inverter’s available KVA capacity while maintaining grid power factor and current total harmonic distortion (THD) requirements within the grid standards. To reduce the complexity and improve the efficiency of the system, two independent PI controllers are implemented to control single-phase unipolar PWM voltage source inverter. One controller is used to control the power angle, and hence the active power flow, while the other controller is used to control the reactive power, and consequently the power factor by adjusting the voltage modulation index of the inverter. The proposed system is modelled and simulated using MATLAB/Simulink. The PV inverter has been examined while being simultaneously connected to grid and local load. Results obtained showed the ability of the PV inverter to manage the active and reactive power flow at, and below rated levels of solar irradiances; resulting in an increased inverter utilization factor, and enhanced power quality. The proposed system, was capable of operating at power factors in the range of 0.9 lead or lag for reactive power compensation purposes and delivered its power at a wide range of solar irradiance variations.

Dual Two-Level Voltage Source Inverter Virtual Inertia Emulation: A Comparative Study

In this paper, the virtual synchronous generator (VSG) concept is utilized in the controller of the grid-connected dual two-level voltage source inverter (DTL VSI). First, the topology of the VSG and the DTL VSI are presented. Then, the state-space equations of the DTL VSI and the grid-connected two-level voltage source inverter (TL VSI), regarding the presence of the phase-locked loop (PLL) and the VSG, are given. Next, the small-signal modeling of the DTL VSI and the TL VSI is realized. Eventually, the stability enhancement in the DTL VSI compared with the TL VSI is demonstrated. In the TL VSI, large values of virtual inertia could result in oscillations in the power system. However, the ability of the DTL VSI in damping oscillations is deduced. Furthermore, in the presence of nonlinear loads, the potentiality of the DTL VSI in reducing grid current Total Harmonic Distortion (THD) is evaluated. Finally, by using a proper reference current command signal, the abilities of the DTL VSI and the TL VSI in supplying nonlinear loads and providing virtual inertia are assessed simultaneously. The simulation results prove the advantages of the DTL VSI compared with the TL VSI in virtual inertia emulation and oscillation damping, which are realized by small-signal analysis.

Control scheme based on capacitor voltage second-order complex-vector feedforward for LCL-filtered inverter

For an inverter with LCL filter, voltage at the point of common coupling (PCC) is equivalent to the filter capacitor voltage in practical applications. However, the PCC voltage background harmonics will cause the distortion of grid current. Some problems exist in the traditional methods for harmonics suppression, such as insufficient phase margin, increasing design complexity and computation burden. Thus, a scheme of second-order complex-vector feedforward (SOCVF) of capacitor voltage is proposed in this article. First, based on the vectorial principle, the active unit vectors of capacitor voltage were calculated. Then, its fundamental component was obtained by the SOCVF-function. Furthermore, the references of the grid current were obtained with the instantaneous power theory. Hence, the proposed scheme helps effectively dampen the LCL resonance to make the system stable, and works well at suppressing the injected grid current distortion simultaneously. Simulation and experimental results validated the effectiveness of the proposed scheme.

Design and Control of Grid-Connected PWM Rectifiers by Optimizing Fractional Order PI Controller Using Water Cycle Algorithm

In this paper water cycle algorithm-based fractional order PI controller (FOPI) is proposed for virtual flux-oriented control of a three-phase grid-connected PWM rectifier. FOPI controller makes the PWM rectifier control more robust due to the fractional behavior. Fractional-order controllers have an additional degree of freedom, so a wider range of parameters is available to provide better control and robustness in the plant. The optimization and design of the FOPI controller are done using the water cycle algorithm (WCA). WCA is an optimization method inspired by monitoring the water cycle operation and flow of water bodies like streams and rivers toward the sea. The performance of the FOPI controller is compared with the classical integer order PI controller. The parameters of PI and FOPI controllers are optimized and designed using the WCA technique, leading to WCA-PI and WCA-FOPI controllers. The system is tested using MATLAB/Simulink. The simulation results verify the better performance of WCA-FOPI in terms of settling time, rise time, peak overshoot, and Total Harmonic Distortion (THD) of grid current. A robustness measurement with line filter parametric variations and non-ideal supply voltage (unbalance and distorted supply voltage) is carried out. The WCA-FOPI demonstrates more robustness as compared to WCA-PI. Simulation findings validate the WCA-FOPI controller outcomes as compared to WCA-PI in terms of control effect and robustness.

Harmonic Current Distortion Using the Linear Quadratic Regulator for a Grid-Connected Photovoltaic System

This paper presents a comparison between a proportional-integral controller, low pass filters, and the linear quadratic regulator in dealing with the task of eliminating harmonic currents in the grid-connected photovoltaic system. A brief review of the existing methods applied to mitigate harmonic currents is presented. The Perturb & Observe technique was employed for maximum power point tracking. The PI control, low pass filters, and the linear quadratic regulator are discussed in detail in terms of their control strategies. The grid current was analyzed in the system with all three of the controllers applied to control the voltage source inverter of the solar photovoltaic system connected to the grid through an L filter and LCL filter and simulated in MATLAB/SIMULINK. The simulation results obtained have proven the robustness of the linear quadratic regulator over other methods. The technique lowers the grid current total harmonic distortion from 7.85% to 2.13%.

Control Strategy of Single-Phase UPQC for Suppressing the Influences of Low-Frequency DC-Link Voltage Ripple

The single-phase unified power quality conditioner (UPQC) is widely used to improve power quality for sensitive end-users. However, the inherent instantaneous power difference between parallel and series converter will generate significant low-frequency dc-link voltage ripple and degrade the compensation performance of UPQC. The mechanism of dc-link voltage ripple generation and its influences on compensation voltage and current are analyzed in this article. To suppress the influence, a control strategy is proposed for the single-phase UPQC. For parallel converter, a notch filter is introduced in the outer voltage loop to prevent the voltage ripple entering the control loop, the specific order harmonics compensation is proposed in the inner-current loop to reduce the grid current harmonics. For the series converter, the dc-link voltage feedback is adopted to suppress the influence of voltage ripple on the compensation performance. Simulation and experimental results show that the grid current total harmonic distortion (THD) can be reduced effectively with the fixed dc-link capacitance compared with the traditional control strategy. More than half of the load voltage THD are also decreased by the dc-link voltage feedback control. Moreover, with the smaller dc-link capacitance, single-phase UPQC can maintain the better performance under the proposed control strategy.

Multivariable Model Predictive Control for a Virtual Synchronous Generation-Based Current Source Inverter

Three-phase current-source inverters are an alternative solution for interfacing photovoltaic modules to the utility thanks to its voltage boosting ability. This paper presents a virtual synchronous generator strategy for a three-phase current-source inverter using a multivariable model predictive control. The proposed method can ensure operations in both grid-connected and islanded modes while achieving virtual inertia features to stabilize the grid frequency and active damping to reduce grid current distortions caused by an output CL filter included on the grid side of the system. The obtained simulation results in the PSCAD/EMTDC environment software verify the effectiveness and the excellent performance of the proposed method.

High performance current control of single‐phase grid‐connected converter with harmonic mitigation, power extraction and frequency adaptation capabilities

Grid-connected converters in distributed generation systems are required to operate under highly distorted voltage with a wide frequency range, and to provide active/reactive power information for ancillary services and power control. This paper presents multiple unbalanced synchronous reference frame control for regulating and mitigating the grid current distortion of a voltage source converter caused by grid voltage distortion and converter deadtime. The proposed control technique has intrinsic power extraction and frequency adaptation capabilities. The multiple unbalanced synchronous reference frame control is compared with the proportional multi-resonant control. A new controller which combines the unbalanced synchronous reference frame control for regulation of the fundamental component current and the multi-resonant control for harmonic compensation is presented. The proposed control techniques were verified with a 1.5-kVA LCL-filtered grid connected inverter. The multiple unbalanced synchronous reference frame control with odd harmonic orders 3rd to 13th exhibited excellent harmonic rejection with a total harmonic distortion (THD) less than 0.60% under a highly distorted voltage. The proposed combined control technique provided slightly inferior performance with the grid current THD less than 0.65%. The combined control scheme retains functionalities as the multiple unbalanced synchronous reference frame control with a significant reduction of computational effort.

Model predictive flexible power control for grid‐connected inverter under unbalanced network

Model predictive power control (MPPC) is considered as a promising algorithm utilised in grid-connected inverter due to its fast dynamic response, simple control structure and multi-objective optimisation. However, under unbalanced network, the grid current with excessive distortion troubles the application of MPPC. Based on the conventional MPPC, this study proposes a model predictive flexible power control (MPFPC) to reduce the current total harmonic distortion and achieve three flexible targets, including the cancellation of the active- and reactive-power oscillation and the negative-sequence grid current. In this algorithm, the power references are divided into two parts, which are used as reference values for the instantaneous active power control (IAPC) and the instantaneous reactive power control (IRPC), respectively. The three flexible targets are achieved by regulating the ratio of the two parts of power reference. Meanwhile, the grid current of MPFPC is the sum of IAPC and IRPC; therefore, the grid current distortion can be suppressed effectively. Simulation and experimental platform constructed by a three-level inverter are established to validate the effectiveness of MPFPC, and the results exhibit that the MPFPC meets the above expectations.

Grid integration of Photovoltaic System Interfaced with Artificial Intelligence based Modified Universal Power Quality conditioning system

Unified Power Quality Compensators (UPQC) in load side utility system is not able to fully suppress harmonies in power grid and distortion in the interface system. To enhance the operation of the UPQC, a Fuzzy based Modified Universal power quality conditioning system (Fuzzy-MUPQCS) is developed. Modified Universal power quality conditioning system (MUPQCS) is a combination of two active filters in series and one active filter in shunt. A 3φ voltage source inverter is also used to maintain a common DC link capacitor voltage through Photo voltaic (PV) system connected to a utility grid. Highest Power Point Tracking (HPPT) algorithm is mainly used to pull out the maximum amount of power in the PV system. In this paper, Proportional integral controller based MUPQCS (PI- MUPQCS) and Fuzzy-MUPQCS are used to analyse the load voltage and load current distortion, THD analysis, power factor measurement, grid voltage and grid current distortion in the three phase system under nonlinear load conditions. The execution of the MUPQCS based 3φ system and the controllers are simulated using Matlab /Simulink model.

A generalized switching function-based SVM algorithm of single-phase three-leg converter with active power decoupling

In this paper, a generalized switching function-based space vector modulation (SVM) algorithm is presented and evaluated to minimize the dc voltage utilization and the ac utility grid current total harmonic distortion. This paper explores the control and modulation techniques of a single-phase three-leg converter with an active power decoupling method, where a generalized SVM algorithm is proposed and evaluated for easy implementation in a digital control platform. The active power decoupling method with the proposed converter can be achieved via dependent control and modulation techniques. The control method is separated into the ac active power control part and the dc power ripple control part, which can maintain a unity power factor at the ac utility grid and reduced the double-frequency ripple power effect on the dc-side. Simulation results validate the performance of the modulation algorithm and its control and demonstrate the feasibility of the proposed power converter, as well as the two mentioned operation modes of the power converter.

QNBP NN-based I cos ϕ algorithm for PV systems integrated with LV/MV grid

In this paper, a robust control technique is designed presented for a grid-integrated solar photovoltaic system. The proposed system consists of a solar photovoltaic array, converter along with linear/nonlinear loads. The main aim of this controller is to obtain maximum power from solar photovoltaic array and control the DC link voltage and injected grid current under different scenarios. The performance analysis of the proposed controller is demonstrated under different scenarios and meteorological conditions. The proposed system performs very well under transient conditions and gives very fast response. Moreover, various power quality indices were monitored and analyzed at unity power factor using hybrid control technique based on I cos ϕ approach with quasi-newton backpropagation learning. The grid current distortion is kept within the prescribed limits given by IEEE-519 and the IEEE-1547 standard. Experimental results illustrate the capabilities of proposed system under abnormal and unbalanced conditions.

Improved model predictive control scheme for AC/DC matrix converters by considering input filter power ripple under imbalanced grid voltage conditions

The power ripple in the input filter is normally ignored in the conventional model predictive control scheme (MPC) of an AC/DC matrix converter under imbalanced grid voltage conditions. Unfortunately, this power ripple causes output power and current ripples. Some methods compensate this power ripple to obtain a ripple-free output current. However, these methods are generally complicated due to the increased computational and control burden or the use of a digital filter to estimate the power ripple. Moreover, the compensation of power ripple results in current distortion on the grid side, which has yet to be fully addressed. This paper presents an improved MPC scheme to simultaneously compensate input filter power ripple and reduce grid current distortion under imbalanced grid voltage conditions. The power ripple is calculated based on the grid voltage and its 90 electrical degrees delay signal, which makes the implementation simple without grid voltage components extraction or digital filter design. Furthermore, a closed-loop current controller is proposed to reduce the harmonic distortion of the grid current. The feasibility of the proposed MPC scheme is confirmed by both simulation and experimental results.

Capacitor Voltage Full Feedback Scheme for LCL-Type Grid-Connected Inverter to Suppress Current Distortion Due to Grid Voltage Harmonics

For the LCL -type grid-connected inverter, grid voltage full feedforward scheme is an effective method to improve the quality of the injected grid current. However, in the applications where a step-up transformer is adopted and its leakage inductance serves as the grid-side inductor, it is difficult to directly measure the grid voltage. For this case, the capacitor voltage full feedback scheme is proposed in this article to suppress the injected grid current distortion due to the grid voltage harmonics, and the full feedback function includes proportional, derivative, and second-derivative components. It is found that the derivative component counteracts the capacitor current feedback active damping, and both of them can be removed. Thus, the current sensor for sensing the capacitor current is saved. Instead, the LCL filter resonance is damped by the proportional and second-derivative feedback components, and a low-pass filter is added to the second-derivative component for ensuring a positive equivalent resistance within the full controllable frequency range. Experimental results from a 6-kW single-phase grid-connected inverter are provided to verify the effectiveness of the proposed method.