Grid Synchronization Research Articles

GWO-based direct power control with improved LCL filter design for three-phase inverters

This paper presents a direct power control (DPC) technique based on instantaneous power theory with optimised PI controllers using a grey wolf optimiser (GWO) algorithm. The method allows to control active and reactive power and to improve the generated power quality. Double second-order generalised integrator with frequency-locked loop (DSOGI-FLL) is performed to ensure the grid synchronisation. New objective functions were defined and the optimisation process is carried using the grey wolf optimiser. A three-phase inverter based grid-connected system was investigated for the simulation. The superiority of the new method was proven by analysing of the different operating modes and conducting a comparative study.

Phase-Locked Loop Independent Second-Order Generalized Integrator for Single-Phase Grid Synchronization

Second-order generalized integrator (SOGI) requires fast and accurate grid voltage frequency information for quadrature signal generation (QSG) in single-phase systems. Conventionally, synchronous reference frame phase-locked loop (PLL) (SRFPLL) is used in a cascade manner and provides frequency feedback to SOGI. PLL is a non-linear closed-loop control system that requires adequate gain parameter design trade-off to provide accurate grid synchronization. Additionally, it creates interdependent loops while estimating the grid frequency and phase-angle. This results in unwanted frequency disturbances during grid transients, such as voltage sag and phase-angle jumps (PAJs) which are fed back to the SOGI as well. Thus, this article proposes two PLL independent frequency-feeding approaches for SOGI: Type-A and -B. The frequency is estimated using the normalized single-phase grid voltage signal in both types. In Type-A, the normalization is done using cascaded recursive discrete-Fourier transform (RDFT) and inverse RDFT (IRDFT). On the other hand, in Type-B, SOGI outputs are used for normalization. The grid voltage amplitude is estimated using the absolute value of the in-phase and in-quadrature components of the proposed frequency-adaptive SOGI. Furthermore, the third-order polynomial approximation of the arctangent function is implemented for phase-angle estimation in both types. The performance of both the types during transients and steady-state grid voltage disturbances (dc offset and harmonics) are compared with SOGIPLL using both simulations and experiments.

Realization of Cascaded H-Bridge Multilevel Inverter Based Grid Integrated Solar Energy System With Band Stop Generalized Integral Control

The high power grid integrated solar energy system (GISES) prefers multilevel inverters for reduced switching losses and lower harmonics in the injected current. The integration of solar PV array with multilevel inverters (MIs) requires a proper selection of multioutput dc–dc converters. Moreover, the performance of two-stage GISES with isolated multioutput dc–dc converters and dc–ac multilevel inverters are less investigated in the literature. In this article, a cascaded H-bridge multilevel inverter with isolated multiple-output zeta converter is used. It exhibits reduced switching stresses and improved efficiency, concerning to multiple dc–dc converters fed MI. The maximum power point tracking method, based on the firefly algorithm, is implemented on the isolated multiple output zeta converter to ensure maximum power extraction at adverse irradiation conditions. The power quality improvement and active power exchange through a band stop generalized integral control, and the grid synchronization is achieved through an enhanced phase-locked loop method. The proposed system is designed and simulated in MATLAB/Simulink, and the results validate the theoretical claims. The performance of the GISES is verified using an experimental prototype developed with the help of a dSPACE Micro Lab Box and a solar PV simulator.

Improving synchronization stability of grid connected converters by virtual impedance

With the increasing penetration of inverter based distributed generation, recent grid codes do not permit the disconnection of converters as soon as fault happens. Considering the fact that electrical grids are not purely inductive, the grid connected converters face instability issues by fault occurrence. Converters applying Phase Lock Loop (PLL) are not able to synchronize with the weak grid during deep low voltage faults. This paper proposes a novel control strategy based on virtual impedance to maintain the synchronization of grid connected converters during heavy decrease of the grid voltage. Utilizing a virtual impedance and the measured current at the point of common coupling, the inverter can be virtually synchronized to a point which has a stronger connection. The virtual impedance can be a rough estimation of the line impedance or resistance from point of common coupling to the fault point. Furthermore, to avoid the need for impedance estimation, a simple technique is also proposed. Simulation results with MATLAB confirms the competence of the proposed method in improving the synchronization stability of the grid connected converters.

Placing Grid-Forming Converters to Enhance Small Signal Stability of PLL-Integrated Power Systems

The modern power grid features the high penetration of power converters, which widely employ a phase-locked loop (PLL) for grid synchronization. However, it has been pointed out that PLL can give rise to small-signal instabilities under weak grid conditions. This problem can be potentially resolved by operating the converters in grid-forming mode, namely, without using a PLL. Nonetheless, it has not been theoretically revealed how the placement of grid-forming converters enhances the small-signal stability of power systems integrated with large-scale PLL-based converters. This paper aims at filling this gap. Based on matrix perturbation theory, we explicitly demonstrate that the placement of grid-forming converters is equivalent to increasing the power grid strength and thus improving the small-signal stability of PLL-based converters. Furthermore, we investigate the optimal locations to place grid-forming converters by increasing the smallest eigenvalue of the weighted and Kron-reduced Laplacian matrix of the power network. The analysis in this paper is validated through high-fidelity simulation studies on a modified two-area test system and a modified 39-bus test system. This paper potentially lays the foundation for understanding the interaction between PLL-based (i.e., grid-following) converters and grid-forming converters, and coordinating their placements in future converter-dominated power systems.

Multi‐objective control and operation of grid‐connected small hydro‐solar PV‐battery energy storage‐based distributed generation

This work deals with the implementation, control and operation of a reconfigurable small hydro-solar photovoltaic (PV)-battery energy storage (BES)-based distributed generation system (DGS) working in isolated mode (IM) and grid connected mode (GCM). The DGS consists of a hydro generation-based on a PMSG integrated with a PV array supported by the BES. It operates in an IM during the grid outage and when the grid is available, it synchronizes and operates in GCM. A frequency adaptive comb filter-frequency locked loop (FLL)-based control is adopted to estimate errorless phase angles of the hydro generator and grid voltages for quick and accurate grid synchronization. A third-order band pass filter-based control technique is used to extract fundamental component of load currents and determine switching pulses to VSC. The overall multi-objective control is used to maintain voltage and frequency of the system at the point of common coupling during IM and GCM. The power quality of grid and generator currents are maintained at non-linear loads. The modified perturb and observe-based control technique is used to extract maximum power from the PV array and it provides drift-free operation during the change in insolation with de-rating capability to protect the BES from overcharging in IM.

Passivity enhancement for LCL‐filtered grid‐connected inverters using the dominant‐admittance‐based controller

LCL filters are widely used for power quality improvement. However, its resonance complicates the design process. Although the single-loop filter-based control reduces the cost, it is not reliable because of the grid impedance variation at the point of common coupling (PCC). This study proposes a dominant-admittance-based controller to improve the passivity of the inverter. Meanwhile, no extra hardware cost increases. It first offers a single-loop damping method using a low-pass filter, which frees inverter from the conventional limitation (fres>fs/6). A graphical procedure is applied to coordinate the current loop design to mitigate the low-pass filter's negative effects on bandwidth. Then it introduces the dominant admittance concept for the inverter with PCC voltage feedforward control, which provides a new starting point for the admittance reshaping of the filter-based damping inverter. Based on this concept, a PCC voltage feedforward control using a second-order low-pass filter is presented. The inverter output admittance is passive up to the Nyquist frequency with the feedforward control. Although the additional control loop is introduced, the controller requires no extra sensors because the PCC voltage measurement has already existed for the grid synchronisation. Experimental results validate the proposed scheme.

Evaluation of Quadrature Signal Generation Methods with Reduced Computational Resources for Grid Synchronization of Single-Phase Power Converters through Phase-Locked Loops

Low-cost single-phase grid connected converters require synchronization with the grid voltage to obtain a better response and protection under diverse conditions, such as frequency perturbations and distortion. Phase-locked loops (PLLs) have been used in this scenario. This paper describes a set of quadrature signal generators for single-phase PLLs; compares the performances by means of simulation tests considering diverse operation conditions of the electrical grid; proposes strategies to reduce the computational burden, considering fixed-point digital implementations; and provides both descriptive and quantitative comparisons of the required mathematical operations and memory units for implementation of the analyzed single-phase PLLs.

Damping Effect of Virtual Synchronous Machines Provided by a Dynamical Virtual Impedance

Virtual synchronous machines (VSMs) emulate the swing equation for grid synchronization and inertia provision. Usually, a large damping coefficient is necessary in VSMs to provide equivalent damper winding effect, which is, however, not aligned with the fact that the physical swing equation of synchronous generators (SGs) contains only a small mechanical friction factor. This large damping coefficient could result in undesired droop characteristics. In this letter, we use damping torque analysis to reveal the damping mechanism of SGs, and show that the damping effect actually comes from the dynamics of the transient/subtransient reactances aggregated as a dynamical impedance. Based on this finding, we elaborate on how to get rid of the large damping coefficient in VSMs by using a dynamical virtual impedance (DVI) to emulate SG's damping mechanism.

A Comprehensive Motivation of Multilayer Control Levels for Microgrids: Synchronization, Voltage and Frequency Restoration Perspective

The current paradigm in integrating intermittent renewable energy sources into microgrids presents various technical challenges in terms of reliable operation and control. This paper performs a comprehensive justification of microgrid trends in dominant control strategies. It covers multilayer hierarchical control schemes, which are able to integrate seamlessly with coordinated control strategies. A general overview of the hierarchical control family that includes primary, secondary, tertiary controls is presented. For power sharing accuracy and capability, droop and non-droop-based controllers are comprehensively studied to address further development. The voltage and frequency restoration techniques are discussed thoroughly based on centralized and decentralized method in order to highlights the differences for better comprehend. The comprehensive studies of grid synchronization strategies also overviewed and analyzed under balanced and unbalanced grid conditions. The details studies for each control level are displayed to highlight the benefits and shortcomings of each control method. A future prediction from the authors’ point of view is also provided to acknowledge which control is adequate to be adopted in proportion to their products applications and a possibility technique for self-synchronization is given in this paper.

Model-Based Predictive Rotor Current Control Strategy for Indirect Power Control of a DFIM Driven by an Indirect Matrix Converter

This paper presents a new control strategy using model-based predictive current control (MB-PCC) for a doubly fed induction machine (DFIM) driven by an indirect matrix converter (IMC). This strategy proposes the control of rotor currents, whose references are calculated from active and reactive stator power set points and the dynamic model of the DFIM. The control strategy works well in the four P-Q operating regions of the DFIM. The grid synchronization process is carried out by setting the P-Q power set points to zero. The results include the DFIM synchronization procedure as well as the active and reactive power control at variable shaft speed to validate the feasibility of the proposed strategy.

Presynchronous Grid-Connection Strategy of Virtual Synchronous Generator Based on Virtual Impedance

The virtual synchronous generator (VSG) technology of inverter is widely used to provide the inertia and damping support for power system. However, an additional measurement device PLL (phase-locked loop) is required in the virtual synchronous generator grid connection to track the voltage phase, amplitude, and frequency, which restricts the flexible output of the distributed power generation system. To tackle this challenge, a method for grid-connected control of virtual synchronous generator based on virtual impedance is proposed. It is assumed that there is a virtual power exchange between the synchronous machine and the power grid when the virtual synchronous generator is off-grid, the virtual impedance is developed to calculate the virtual current, and when the virtual current is zero, the output voltage of the VSG can be synchronized with the voltage of the power grid, thereby seamlessly switching between off-grid and grid-connected VSG. A semiphysical simulation platform is built based on RT-LAB; simulation and experimental results show that the proposed grid synchronization control strategy of the VSG can achieve seamless transform between different VSG modes, which is simpler than the conventional synchronization control, while having a good active and reactive power tracing performance.

A Simple Sinusoidal Buck Converter Working as A Single-Phase Grid-Connected Inverter

The need for a simple grid-connected inverter is increasing. The integration of a simple buck converter and a push-pull converter to be a grid-connected inverter is presented in this paper. The buck converter is controlled by a sinusoidal duty ratio to produce a series positive half-cycle of a sine wave. Then, the push-pull converter solves it to generate the complete sinusoidal output voltage and current. Moreover, grid synchronization works properly by sensing the grid voltage, which is then used as a reference voltage to connect the inverter to the grid successfully. The grid sees the inverter as a resistive load with a unity power factor. The total harmonic distortion (THD) of the grid current is 4.87% for the THD grid voltage of 2.07%.

A Novel Modified Control Scheme in Grid-Tied Photovoltaic System for Power Quality Enhancement

In this article, a modified reduced order generalized integrator-based frequency-locked loop (MROGI-FLL) is proposed for controlling the interfacing inverter of the grid-tied photovoltaic system to mitigate the harmonics. Additionally, a fuzzy tuned PID controller is integrated in the proposed scheme to minimize the steady-state error, which results the voltage level is maintained constant at the common dc-link bus terminals. The MROGI-FLL is designed to evaluate the three-phase reference currents by extracting the fundamental constituents from the load currents and grid voltages. The proposed MROGI-FLL has several advantages such as better harmonic mitigation capability, control adaptivity, adaptive frequency and phase, grid synchronization, and low computation burden. The proposed reference current generator is simulated on MATLAB/Simulink platform under steady state and various transient conditions.The comparative study of proposed scheme is compared with the existing and adaptive control techniques, which assures the potency of the proposed control algorithm in-terms of harmonic mitigation, dc-offset rejection, frequency variation, and computation burden. Finally, the experimental results are verified through an experimental platform by using the dSPACE evaluation kit and both the simulation and hardware characteristics are well justified to the IEEE-519 standard limits.

Trinary Hybrid Cascaded H-Bridge Multilevel Inverter-Based Grid-Connected Solar Power Transfer System Supporting Critical Load

Higher power rating solar photovoltaic systems are emerging in the power system owing to the reduction in the cost of photovoltaic arrays and environmental concerns of the conventional power generation systems. Multilevel inverters are reported at high power levels due to reduced switching frequency and increased efficiency. Cascaded H-bridge (CHB) multilevel inverters have been reported in the literature due to its redundant structure compared to other multilevel topologies. When CHBs with dc sources are in the ratio of 1:3, it is named as a trinary hybrid multilevel inverter, and it generates a maximum number of equal output voltage levels. In this article, a trinary CHB multilevel inverter-based grid-connected solar power transfer system using modified second-order generalized integral control is proposed. A two-stage solar PV system consists of a single-input-multiple-output single-ended primary inductance converter and two CHB structures per phase used to verify the proposed system. A modified second-order generalized integral control is presented for active power control and grid synchronization. The controller is designed to eliminate dc-offset from the sensed load currents and to ensure balanced, sinusoidal, and unity power factor grid currents. Moreover, the shunt-connected system mitigates the reactive power and harmonic demands of the local load, thus maintaining the grid current power quality within the standard prescribed by IEEE-519. The performance of the proposed system is validated with an experimental prototype, and the test results validate the theoretical claims.

Power quality enhancement in solar power with grid connected system using UPQC

The need to generate pollution free energy has triggered the effect towards the usage of solar energy interconnection with the grid. Consequently, the Photovoltaic (PV) panel interfaced with the grid causes the power quality problems such as a voltage harmonics and voltage distortion etc., Active power filters are the powerful tool for mitigation of harmonics. This work involves the use of single-phase Unified Power Quality Conditioner (UPQC) based on a unit vector template control algorithm for its functions with grid integration of photovoltaic, such as voltage sags/ swell, unit power factor correction, voltage and current harmonic cancelation. The unit vector template control algorithm includes a phase-locked loop (PLL) mechanism that is responsible for avoiding multiple zero crossings during highly distorted grid voltage detection. A unit vector template control with a PLL-based control algorithm is applied to the shunt and series inverters of PV grid connected UPQC. In addition to normalizing voltage and current disturbances, the proposed controller has the functions of phase detection and perfect grid synchronization. It is proposed that the system performance is fully verified by MATLAB simulation with the response of load variation, transient response, THD, voltage swell and sag. The Total Harmonic Distortions (THDs) of proposed grid integration of photovoltaic systems through single-phase unified power quality conditioner (UPQC) obtain the range of IEEE standard.

Multi-Objective Solar Power Conversion System With MGI Control for Grid Integration at Adverse Operating Conditions

This article presents a new control for the multi-functional operation of a grid-connected solar power conversion system (SPCS). It is a combination of several generalized integrals for grid synchronization as well as fundamental load current component extraction. This multiple generalized integral (MGI) control is designed to overcome the demerits of conventional integral controllers with inadequate operation at adverse grid conditions such as grid frequency variation, phase jumps, DC offsets, noise, harmonics and other waveform distortions. The control is designed to separate the load power accurately into two-parts, i.e. active fundamental part (AFP) and non-active fundamental part (NAFP). This control extracts the AFP of the load current even during the disturbed grid and unbalanced load conditions. According to the comparison with the load current AFP and the photovoltaic (PV) available power ( $P_{pv}$ ), three operating modes are used such as bright mode, dim mode and dark mode. During the bright mode, the total load power is being supported by the system and the remaining power is injected to the grid, the load power is shared by the system and the grid during the dim mode, and the load current AFP is supported by the grid during the dark mode. In every mode of operation, the SPCS mitigates the total load demanding NFAP and thereby keeping the grid currents as sinusoidal and in-phase with the fundamental grid phase voltages and maintaining the IEEE-519 grid power quality standard. This system is modelled and the performance is simulated in MATLAB/Simulink platform, and simulated results are presented. A prototype of the system, is developed in the laboratory and test results at various operating conditions are reported for validation of the system.

Virtual charge‐based synchronisation and feedback linearisation‐based current control of a three‐phase grid‐connected inverter without grid voltage sensors

A conventional synchronous (rotating) reference frame control requires small-signal models and introduces cross-coupling terms. In this study, a virtual charge-based scheme for grid synchronisation with only current measurements has been proposed for a three-phase grid-connected inverter with a first-order filter. The methodology described in this study curbs the dependency of the inverter on the information of the grid voltage to generate unit vectors from the phase-locked loop. This kind of estimation is especially important during grid voltage sensor failures or a distant point common coupling or weak grid conditions. In the proposed estimation scheme, no noisy differentiation is involved and only the nominal values of the interfacing filter parameters are utilised, making the implementation easier in digital microcontrollers. Also, a current control architecture based on feedback linearisation concept has been explored and embedded with the proposed grid voltage estimation technique, and the corresponding performance has been evaluated via simulation and experimental studies.

The Control Strategy for the Grid-Connected Inverter Through Impedance Reshaping in q-Axis and its Stability Analysis Under a Weak Grid

The grid-connected inverter is the vital energy conversion device in renewable energy power generation. With the increasing installed capacity of renewable energy, the grid presents characteristics of weak grids with large grid impedance. In general, the inverter often obtains grid synchronization information by the phase-locked loop (PLL) and to suppress the background harmonic and amplitude disturbance of grid voltage, the grid voltage feedforward (GVF) control is also needed. However, previous researches revealed that the PLL and the GVF would shape the quadrature-axis ( <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</b> -axis) output impedance of the inverter into a negative resistance in the low-frequency band, and this would be responsible for the instability of the inverter under a weak grid. To resolve this instability issue, this article proposes an impedance controller for reshaping the <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</b> -axis impedance into a positive resistance in the low-frequency band, which eliminates the negative effect introduced by the PLL and the GVF; therefore, the inverter will operate under very weak grids. Moreover, the proposed control strategy is only for <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</b> -axis impedance reshaping, and it does not affect the output characteristics of <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</b> -axis, thus ensuring the ability of the inverter to suppress background harmonics and amplitude disturbance of the grid voltage. The conclusions are verified by experimental results.

Investigation of Photovoltaic Grid System under Non-Uniform Irradiance Conditions

Photovoltaic (PV) systems have recently been recognized as a leading way in the production of renewable electricity. Due to the unpredictable changes in environmental patterns, the amount of solar irradiation and cell operating temperature affect the power generated by the PV system. This paper, therefore, discusses the grid-integrated PV system to extract maximum power from the PV array to supply load requirements and the supply surplus power to the AC grid. The primary design is to have maximum power point tracking (MPPT) of the non-uniformly irradiated PV array, conversion efficiency maximization, and grid synchronization. This paper investigates various MPPT control algorithms using incremental conductance method, which effectively increased the performance and reduced error, hence helped to extract solar array’s power more efficiently. Additionally, other issues of PV grid-connected system such as network stability, power quality, and grid synchronization functions were implemented. The control of the voltage source converter is designed in such a way that PV power generated is synchronous to the grid. This paper also includes a comparative analysis of two MPPT techniques such as incremental conductance (INC) and perturb-and-observe (P&amp;O). Extensive simulation of various controllers has been conducted to achieve enhanced efficient power extraction, grid synchronization and minimal performance loss due to dynamic tracking errors, particularly under fast-changing irradiation in Matlab/Simulink. The overall results favour INC algorithm and meet the required standards.