Grid Interface Research Articles

A Novel Control Strategy in Grid-Integrated Photovoltaic System for Power Quality Enhancement

The integration of solar photovoltaic (PV) systems and utility grids has gradually gained significant interest in improving the sustainability of clean power supply for society. However, power quality remains a challenge due to partial shading conditions and harmonics. To overcome these drawbacks, a flexible radial movement optimization based on a dynamic safety perimeter maximum power point tracking algorithm is employed to track maximum power out of a PV system and to ensure that the optimum voltage level at the common DC bus is obtained under partial shading conditions using fixed-tilt installation configuration. Furthermore, a novel inverter control loop system with a double second order generalized integrator phase-locked loop (DSOGI-PLL) is also proposed to mitigate harmonics and improve the power quality of the grid interfacing PV system using MATLAB SIMULINK software. The proposed system has several merits, such as better harmonic suppression capability, control adaptivity, rapid tracking speed, low computational burden and phase and grid synchronization.

Review: Ludified, edited by Marko Ciciliani, Barbara Lüneburg, and Andreas Pirchner

Review: <i>Ludified</i>, edited by Marko Ciciliani, Barbara Lüneburg, and Andreas Pirchner

Grid interface of solar PV transformer-less inverters using enhanced phase locked loop

The era of Renewable Energy sources has overtaken the usage of conventional energy sources like coal, fossil fuels etc. This paper presents grid interface of single phase transformerless inverter system using enhanced phase locked loop. To connect solar PV with grid it is required to estimate phase angle, voltage amplitude and phase angle measurement of grid and the output of inverter. It is required to track exact utility voltage vector for reliable control of power to the grid. This paper compares the frequency measurement and phase angle and voltage amplitude using simple phase locked loop system along with enhanced phase locked loop so that the enhanced phase locked loop (EPLL) is proved to be giving superior performance over simple phase locked loop. The simulation of both the techniques are pursued and the results are compared.

Power electronics‐interfaced cyber‐physical power systems: A review on modeling, simulation, and cybersecurity

AbstractWe present the review of two interlinked challenges in modern electric power systems: the transformation to a cyber‐physical system, and the integration of power electronics‐interfaced renewables. Electric power systems are being modernized with the integration of power electronics‐interfaced devices (PEID) and communication‐enabled cyber‐applications. This paper reviews the concepts, studies, and testbeds for cyber‐physical power systems (CPPS), as well as the modeling of power electronics‐based devices for physical power system stability simulations. The CPPS concept is introduced in the National Institute of Standard Technology framework for cyber‐physical systems, with an emphasis on CPPS subsystems. For the physical subsystem, PEID components are generalized into the primary source and the grid interface, while controllers are generalized as a reference generator and a reference tracker. Next, the cybersecurity research objectives are summarized, followed by a categorization of CPPS studies. Further, testbed techniques for integrating communication networks with power system simulation are reviewed. Also, challenges and future directions in the area of CPPS are discussed.This article is categorized under: Energy Infrastructure &gt; Systems and Infrastructure

Harmonic Voltage Control in Distributed Generation Systems Using Optimal Switching Vector Strategy

With increased penetration of renewable power and the nonlinear loads in the distributed generation (DG) systems, increased power quality concerns are exhibited in the active distribution networks, especially the challenges associated with the current and voltage harmonics in the system. Various conventional harmonic compensation techniques are developed for voltage-controlled DG inverters in past, majority involve either multiple proportional-integral (PI) or proportional-resonant (PR) controllers in eliminating grid current harmonics. The current-controlled inverters, on the other hand, are not preferred in industrial applications, accounting to their wide variations in the switching frequency. A novel and adaptive harmonic voltage control is developed here, for voltage-controlled DG inverters, which neither uses any PI regulators nor imposes stability issues associated with nonideal implementation of infinite gains of PR controllers. Interestingly, the developed control logic can be used for DG inverters, both in grid-connected and off-grid operational modes. Furthermore, this strategy allows a network operator to use this as an additional supplement that can be enabled/disabled as per the network requirement. The control logic exploits the property of an optimal-switching-vector controller, i.e., accurate output voltage tracking. Simulations results demonstrate the effectiveness of the controller, to suppress grid current harmonics and load voltage harmonics in grid-interfaced (GI) and off-grid modes, respectively, ultimately satisfying the mandatory IEEE standard-1547. Experimental results verify the viability of the controller for practical applications.

Numerical Simulation of Optimized Placement of Distibuted Generators in Standard Radial Distribution System Using Improved Computations

The need for energy globally has increased recently. Additionally, the idea of distributed generation is propelled by the absence of suitable transmission capacity, exaggerated transmission and distribution disasters, and the release of power advertising. Numerous benefits of distributed generation (DG) include a decrease in energy loss during force transfer and a reduction in the length and width of electrical cables. By lowering power quality and other problems, the use of the DGs with the current force distribution arrangements may enhance the intensity standard. The impacts of distributed generators (DGs) are discussed in this article in relation to various operating situations and the voltage measurement output of the regulator, which is typically accessible in grid interface. The investigation's goals are to locate the best DG connection point in the DS, reduce the power and voltage profile, and improve compliance with the power efficiency limitations. The procedure has been simulated in the MATLAB Simulink system using the Institute of Electricity's test system (IEEE), and the results are displayed using numerical simulation. As a result, the system voltage profile was improved.

Designing UPFC with Controller Driving Dual Converter Topology in a Grid Integrated PV System

Renewable energy source (RES)-based electricity generating plants are expensive and complicated, requiring controls to deal with the energy input's intermittent renewables and fluctuation. If the grid interface of these devices is not monitored and maintained within the norms, it presents a problem of providing electricity to the utilities. Thus researchers have discusses several grid interface and control strategies for RES-based generators. This paper discusses the technique of improving power quality by using UPFC (Unified Power Flow Control) having PI controller.

The influence of non-conformal grid interfaces on the results of large eddy simulation of centrifugal blood pumps.

Computational fluid dynamics has been widely used to assist the design and evaluation of blood pumps. Discretization errors associated with computational grid may influence the credibility of numerical simulations. Non-conformal grid interfaces commonly exist in rotary machines, including rotary blood pumps. Should grid size across the interface differ greatly, large errors may occur. This study explored the effects of non-conformal grid interface on the prediction of the flow field and hemolysis in blood pumps using large eddy simulation (LES). Two benchmarks, a nozzle model and a centrifugal blood pump were chosen as test cases. This study found that non-conformal grid interfaces with considerable change of grid sizes led to discontinuities of flow variables and brought errors to metrics such as pressure head (7%) and hemolysis (up to 14%). The results on the full unstructured grid are more accurate with negligible changes of flow variables across the non-conformal grid interface. A full unstructured grid should be employed for centrifugal blood pumps to minimize the influence of non-conformal grid interfaces for LES simulations.

Control Design of a 99% Efficiency Transformerless EV Charger Providing Standardized Grid Services

A nonisolated DC charger is proposed for electric vehicle (EV) battery system. The leakage current can be reduced by the novel charging system without bulky transformer. A zero sequence voltage control method is developed to stabilize the common-mode voltage, thus, reduce the leakage current. The proposed fast charger includes two energy conversion stages: DC–DC converter for battery side control; DC–AC converter for grid interface and common-mode voltage control. The parameters of the system with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter are analyzed and optimized for a better performance of grid interface. Grid service functions are designed for the EV charger to provide grid-voltage/frequency compensations. High power (22 kW) and high efficiency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 99%) are achieved with low-leakage current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt;$</tex-math></inline-formula> 20 mA). The experiments are implemented to verify the proposed EV charger system.

Measurements of magnetic characteristics of laminated Fe-Si steel filter inductors in grid interface converters

The knowledge of the magnetic characteristic of an inductor is a key factor for the design of filters of power converter systems for electric grid interfaces. The accurate and reliable definition and measurement of the parameters such as the differential inductance, the saturation level and the linearity range is a difficult task, in particular for high current inductors. In this paper a comparison between different methods of measure of the magnetic characteristic of a gapped filtering inductor versus current, in the range from zero magnetization to the saturation, is presented and discussed. It is shown that the different measurement methods provide appreciable differences in the evaluation of the magnetic characteristic of a benchmark inductor. The differences found are also analyzed by suitable numerical simulations.

New Modulation Technique in Smart Grid Interfaced Multilevel UPQC-PV Controlled via Fuzzy Logic Controller

The use of sensitive electronic devices has increased recently; hence, power quality has become an important factor in electrical power systems. The various disturbances occurring in the electrical network (harmonics, voltage swells, sags, etc.) may lead to technical-economic damage that negatively impacts the quality of the supplied power. Therefore, the use of the Unified Power Quality Conditioner (UPQC) is a promising solution to limit such damage. In this paper, a nine-level structure of the proposed standardized power quality conditioner is analyzed. The conditioner is connected between a photovoltaic system (UPQC-PV) and a smart grid. Then, a new approach to generate switches for the switches of both the series and parallel converters is proposed. This modulation technique relies on an adaptive hysteresis band (AHB) that is determined via a fuzzy logic controller in order to obtain the required modified output voltage with minimum distortion. Simulation results, using MATLAB/Simulink, of different disturbance scenarios that may occur in the network are presented to verify the accuracy of the proposed fuzzy-logic-based AHB control system. Compared with the conventional SPWM, it is found that the proposed AHB modulation technique significantly improves the power quality in terms of producing less total harmonic distortion in both the voltage and current waveforms.

A Unified Topology for the Integration of Electric Vehicle, Renewable Energy Source, and Active Filtering for the Power Quality Improvement of the Electrical Power Grid: An Experimental Validation

Electrical power grids are facing challenges concerning new linked technologies and associated contributions of power electronics, both regarding innovative topologies of power converters and advanced power management algorithms. Additionally, technologies related to renewables and electric mobility have several points in common, especially about the interface with the power grid, which allows to foresee a convergence for unified solutions in the power grid interface, without jeopardizing the functionalities and added values of each technology. Encompassing this purpose, this paper presents a unified topology, based on a three-phase structure, which, in addition to a collaborative operation with the power grid targeting the compensation of power quality problems, also enables the integration of a renewable energy source and an electric vehicle. The main contribution of this paper resides in the fact that only an interface with the power grid is necessary to involve three central features of smart grids: renewables, electric mobility, and power quality. Overall, the unified topology presents a four-quadrant structure, both in the perspective of AC and DC interfaces, offering multiple functionalities, mainly to the power grid. In the AC interface, the structure operates in interleaved mode, while in the DC interface, the structure operates in multilevel mode. The global control algorithm is presented, covering the interconnection between the mentioned technologies, as well as the details of implementation of the individual control algorithms regarding each interface. A laboratory prototype, connected to a three-phase 400 V-50 Hz power grid, was used to obtain an experimental validation for a maximum operating power of 12.5 kW, corroborating the essential advantage characteristics and the correct functioning of the presented unified topology.

Optimal Management for Megawatt Level Electric Vehicle Charging Stations With a Grid Interface Based on Modular Multilevel Converter

This paper proposes a strategy to manage an electric vehicle charging station (EVCSs) with a grid-side interface based on a Modular Multilevel Converter (MMC). The MMC topology is studied due to its potential for reducing the footprint and the use of active material in the internal distribution system by allowing for transformer-less connection to the medium voltage distribution grid. However, heterogeneous charging demands and arrival-departure profiles of the electric vehicles (EVs) could lead to significant loading unbalances among the MMC arms and among the modules of a single arm. Nevertheless, the current in the grid interface must be kept balanced and sinusoidal. Furthermore, the voltages of the modules of an arm must be balanced. This work combines a load management (LM) algorithm with a power flow management (PFM) algorithm to achieve the required characteristics of grid current and module voltages under the heterogeneity of the charging demand in MMC-based EVCSs. The PFM algorithm controls the circulating currents to compensate the phase-to-phase, arm-to-arm and intra-arm unbalances of the given loading. To minimize the additional losses resulting from active balancing by the PFM, the LM optimizes the charging schedules and allocations of incoming EVs into charging units in order to minimize phase-to-phase and arm-to-arm unbalances in the system. The performance of the proposed optimization-based LM is compared with a rule-based benchmark LM by simulating the daily operation of an example shopping mall parking with MMC-based grid interface. In scenarios with pronounced unbalance limitations, the optimization-based LM increases the supplied energy significantly. Real-time (RT) simulations demonstrate a balanced and sinusoidal grid current profile and balanced module voltages in MMC arms over the daily scenarios. These results indicate that the proposed strategy combining LM and PFM is applicable for real-world deployments.

Flexible parameterizable grid-forming converter control by separated fast synchronization and slow inertia response control loops of Direct Voltage Control

As power systems, transit to a state of high renewable penetration with predominant converter-based grid interface instead of synchronous generators applied by conventional power plants, the dynamics of the grid significantly change, causing new challenges for transmission system operations and arising new opportunities, as converter-based generation is highly controllable in faster timescales. This paper discusses the extension of the direct voltage control as ‘a decoupled grid forming converter controller’, which effectively relies on an accurate and fast d/q-reference frame determination to control the frequency as well as its rate of change at the common coupling point with the grid. The proposed controller separates the task of synchronizing the converter to the grid and providing virtual inertia into two distinct control functions, which enables a specific tuning of both functions for their task avoiding the necessity to find any kind of trade-off between quality of synchronization and inertia provision. The functionality of the proposed control is explained and demonstrated via EMT-Type simulations in different study cases for system-spilt, islanding, and re-synchronization scenarios.

Embedded Switched Z-Source Multilevel Inverter for Grid Interfaced Photovoltaic Systems

Embedded Switched Z-Source Multilevel Inverter for Grid Interfaced Photovoltaic Systems

High Voltage Gain Switched-Z-Source Bidirectional DC-DC Converter

The DC-DC converters are the essential modules in electric vehicles, grid interface of renewable energy sources and DC power supplies. This paper presents a novel high gain bidirectional switched Z-source DC-DC converter. The proposed bidirectional converter utilizes switched-capacitor concept with inductors for high voltage gain. The proposed bidirectional converter presents wide operating range of operation in both boost and buck mode of operation. The proposed converter has common ground between input and output, and uses only 5 active switches and 4 passive elements with reduce switch voltage stress. The current drawn from the low voltage side input source is continuous with reduced ripple. The operating principle, circuit analysis, design, mathematical model and closed loop operation of the proposed converter are presented. The proposed converter is verified through computer simulation and laboratory experiments. The simulation and experimental results are presented.

A resonant damping control and analysis for LCL-type grid-connected inverter

The inverter becomes an essential part in the distributed energy units, where an inductor–capacitor–inductor (LCL) filter is an up-to-date adoption for grid interfacing. However, the challenging issue of resonance related to the LCL-filter worsens the dynamic control characteristics and produces stability threat for the voltage source inverter system. The proper design of inverter control plays a substantial part in ensuring a steady state operation and a high quality of grid injected current according to grid connection codes. This paper offers a different design method of inverter control which alters the inner damping loop structure to enhance the damping and stability features of the inverter system. In the proposed solution, a modified compensator is employed through the LCL filter network and filter capacitor current feedback loop, the resultant augmented plant’s output is then sent back at the reference point of a grid-connected inverter system to damp the unwanted resonance spike. This particular arrangement to form a damping loop is termed as parallel feedforward compensation. The damping loop is implemented by calculating the filter capacitor current, and a first order high-pass filter is employed as a damper in the suggested arrangement. Moreover, for comparative studies and analysis, the conventional filter capacitor current feedback active damping method is selected. In addition, the stability margins and control performance of the open current loop are examined under different filter parameters values. The substantial results of the suggested structure are quicker dynamic response, comparatively improved resonance damping, better tracking performance and higher delay compensation capability. A laboratory prototype is established to confirm the effectiveness of the suggested scheme on the bases of control performance and stability margins.

Processor in the Loop Verification of Fault Tolerant Control for a Three Phase Inverter in Grid Connected PV System

ABSTRACT The techno-economic feasibility of photovoltaic (PV) solar generation systems is greatly dependent on its operating conditions. However, significant penetration of PV sources into alternating current (AC) grids may cause lower system inertia, decreased system damping, and higher frequency fluctuations when grids are subjected to fault conditions. In this paper, a two-stage three-phase grid-connected PV-based inverter system is analyzed under grid fault conditions using a robust fault-tolerant super twisting sliding mode control scheme. Moreover, closed control system is formulated using Lyapunov theorem and its global stability is ensured. The proposed controller is tested in processor in the loop experimentation using a Texas Instrument (TI) Launchpad (TMS 320F28379D). The control performance and resilience of the proposed control system is tested under three-phase faults introduced between the inverter–grid interface buses. Under the above test conditions, a comparative analysis is performed for the proposed adaptive super twisting sliding mode control (ASTSMC) and fixed gain sliding mode control (SMC) control method. Under fault conditions, the inverter power has a settling time of 50 m-s with ASTMC control while with fixed gain STSMC, the settling time is longer and the observed settling time is 115 m-s. Moreover under faulty conditions, a voltage dip of 400 V is recorded with ASTSMC while with fixed gain STSMC controller the observed dip is 600 V. With the proposed ASTSMC controller, THD settles down to under 3% in 50 m-s while the fixed gain STSMC takes 115 m-s.

Bidirectional Buck-Boost and Series LC-Based Power Balancing Units for Photovoltaic DC Collection System

The input-independent and output-series (IIOS) structure is often used in the high step-up ratio converters required for medium-voltage dc (MVdc) grid interface of photovoltaic (PV) systems, which achieves high voltage gain and ensures high efficiency. However, due to the characteristics of IIOS structure, when some PV arrays are shadowed or malfunction, the generated power of these modules will be mismatched. Mismatched power will result in output capacitor voltages deviating from the rated value, which is not conducive to the standardized multimodule design. Besides, voltages of some PV arrays will deviate from their maximum power point (MPP) value, leading to a decrease in the total generated power. In order to balance the module output voltages of the IIOS structure, this article proposes bidirectional buck-boost and series <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC-</i> based power balancing units (LC-PBUs) for PV dc collection systems. Compared with a traditional solution, nearly half of the switches are saved and half of the inductors are displaced by small inductors. Power balance can be achieved through the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -PBUs by controlling the duty ratio and the phase of the pulse-width modulation (PWM) signal. Simulations and experiments are carried out to verify the effectiveness of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -PBU under power sags.

Two-Stage Control for Small-Signal Modeling and Power Conditioning of Grid-Connected Quasi-Z-Source Inverter with LCL Filter for Photovoltaic Generation

Grid-connected inverter-based photovoltaic (PV) systems play an important role in Distributed Power Generation (DPG). For this application, quasi impedance source inverter is very suitable due to its ability to increase or decrease the output voltage of the inverter in a single-stage and high reliable condition. Conventionally, to remove the harmonics, which are yielded by switching the grid-connected inverter, LCL filters are utilized at the inverter output. These filters can cause some problems at the Point of Common Coupling (PCC). The aim of this paper is to improve the quality of the injected power of the photovoltaic array, which is connected to the low voltage grid by quasi-Z-source inverter (QZSI). For this purpose, a two-stage control procedure containing DC and AC stages is performed. In the DC stage, the dynamic characteristics of the quasi-Z-source network are investigated by small-signal analysis. Using the transfer functions obtained from the dynamic model, the capacitor voltage of the quasi-Z-source network is suitably controlled to generate the appropriate voltage to the grid interface inverter. In the AC stage, in order to inject high-quality current into the grid as well as eliminating the resonance peak caused by the LCL filter, a systematic procedure is used to design the PR controller parameters and active damping coefficient. Simulation of the overall system includes solar panels, maximum power point tracking algorithm, quasi-Z-source inverter, and LCL filter to model the grid-tied PV system with the possible details. Simulations are carried out in MATLAB/Simulink environment, and results depict suitable performance of the studied power conditioning system with designed parameters.