Grid Connected Power Electronic Converters Research Articles

On Biased Harmonic Signal Estimation: Application to Electric Power Grid Monitoring

Parametric estimation of a biased harmonic signal is a significant technical challenge for many engineering applications. Such a problem is particularly important for electric utility grid-connected power electronic converters. This article utilizes a linear regression model of the signal to solve this interesting practical problem. A continuous-time dynamic regressor extension and mixing (DREM) based approach is then applied for parameter estimation. For practical implementation, continuous-time estimators are discretized using implicit and explicit Euler methods. We then prove that the implicit discretization can achieve fixed-time convergence for the unknown frequencies estimation. Thanks to the estimated frequencies, another DREM-based linear regression problem is solved for the parameter estimation purpose. The overall order of the proposed technique is the same as the number of unknown parameters, making the estimator suitable for real-time implementation in embedded devices. Theoretical results are validated through extensive comparative experimental studies.

Control of a Three-Phase Power Converter Connected to Unbalanced Power Grid in a Non-Cartesian Oblique Frame

The article presents a new approach to positive and negative sequence current vector control of a grid-connected three-phase three-wire power electronic converter operating under grid voltage imbalance conditions. The concept utilizes the representation of unbalanced converter current in the new coordinates frame in which the current vector components are constant. The nonlinear trigonometric transformation of two-dimensional current vector components from the stationary frame to the new frame is found online depending on the reference current asymmetry. The presented concept of new coordinates utilization allows implementation of proportional-integral terms as current regulators without the use of resonant terms and without the use of the measured current symmetrical sequences decomposition. The article presents the theoretical approach, simulation results, as well as laboratory tests results.

Cyber-Attack Detection and Countermeasure for Distributed Electric Springs for Smart Grid Applications

With increasing installations of grid-connected power electronic converters in the distribution network, there is a new trend of using distributed control in a cyber layer to coordinate the operations of these power converters for improving power system stability. However, cyber-attacks remain a threat to such distributed control. This paper addresses the cyber-attack detection and a countermeasure of distributed electric springs (ESs) that have emerged as a fast demand-response technology. A fully distributed model-based architecture for cyber-attack detection in the communication network is developed. Based on a dynamic model of ES with consensus control, a local state estimator is proposed and practically implemented to monitor the system. The estimator is fully distributed because only local and neighboring information is necessary. A countermeasure for the distributed ESs to ride through the cyber-attack and maintain regulatory services in a microgrid is demonstrated successfully. Experimental results are provided to verify the effectiveness of the proposed cyber-attack detection method and its ride-through capability.

A Power Hardware-in-the-Loop Based Method for FAPR Compliance Testing of the Wind Turbine Converters Control

A task for new power generation technologies, interfaced to the electrical grid by power electronic converters, is to stiffen the rate of change of frequency (RoCoF) at the initial few milliseconds (ms) after any variation of active power balance. This task is defined in this article as fast active power regulation (FAPR), a generic definition of the FAPR is also proposed in this study. Converters equipped with FAPR controls should be tested in laboratory conditions before employment in the actual power system. This paper presents a power hardware-in-the-loop (PHIL) based method for FAPR compliance testing of the wind turbine converter controls. The presented PHIL setup is a generic test setup for the testing of all kinds of control strategies of the grid-connected power electronic converters. Firstly, a generic PHIL testing methodology is presented. Later on, a combined droop- anFd derivative-based FAPR control has been implemented and tested on the proposed PHIL setup for FAPR compliance criteria of the wind turbine converters. The compliance criteria for the FAPR of the wind turbine converter controls have been framed based on the literature survey. Improvement in the RoCoF and and maximum underfrequency deviation (NADIR) has been observed if the wind turbine converter controls abide by the FAPR compliance criteria.

Sensorless Virtual Flux Combined Control of Grid Connected Converters With High Power Quality Under Unbalanced Grid Operation

This paper presents a novel sensorless combined control for grid connected power electronic converters (GCCs) under both balanced and unbalanced grid operation. Under unbalanced grid conditions, output power quality of GCCs would usually be deteriorated by increased current harmonics and power oscillations injected to grids. To overcome the problem, an improved sensorless combined control method with a new switching logic is presented in this paper based on instantaneous control of grid current components. The simplicity and robustness of combined control is fully preserved due to a current reference vector, which is calculated using the virtual flux components in the grid voltage, and virtual flux reference frames without any positive and negative sequence extraction. The proposed method, in comparison with voltage oriented control, avoids problems of tuning current controller, PWM sub-system, and decoupling circuit. Compared to the direct power control, the method lacks active and reactive power calculation and voltage sensors and enjoys improved power ripples and current THD.

On the Ground Potentials and Grounding Circuits of Transformerless Grid-Connected Multilevel Power Electronic Converters

Transformerless grid-connected power electronic converters (PECs) are used in photovoltaic systems, motor drives, and solid-state power transformers. In these applications, transformerless grid-connected multilevel PECs can reduce harmonic distortions, increase power, and voltage ratings. This article aims to develop models for the ground potentials in these PECs, and to design grounding circuits for them. The models for ground potentials are developed using the common-mode voltages across each leg of the multilevel PEC. Grounding circuits for transformerless grid-connected multilevel PECs are designed using frequency selective circuits to limit ground potentials, and block ground currents from flowing through grounding of the host grid. The developed models, and grounding circuits are evaluated for transformerless grid connected, diode clamped, flying capacitor, and cascaded $H$ -bridge multilevel PECs under different operating conditions. Test results demonstrate the significant advantages of the frequency-selective grounding for transformerless grid-connected PECs. Observed advantages include reduced harmonic distortion, minimized ground potentials, and improved efficiency.

Virtual synchronous machine-controlled grid-connected power electronic converter as a ROCOF control device for power system applications

Rate of change of frequency (ROCOF) is an indicator of frequency stability of a power system network. Maintaining the ROCOF value within the acceptable limit is a major challenge with increasing penetration of converter-based renewable energy generation technologies. A virtual synchronous machine (VSM) is proposed in this paper as a ROCOF control device with a novel battery power management algorithm to extend the lifetime of the batteries. The VSM unit consists of a DC storage connected through a three-phase voltage source converter with a harmonic filter to the grid and the corresponding local control system. The Kundur two-area system is used as a test case to analyse the performance of the VSM. A hardware prototype of the VSM with 1 kW, 200 V and 50 Hz rating is developed in the laboratory environment. The modified Kundur two-area system and the control strategies are implemented in OPAL-RT real-time simulator. The effect of VSM on the modified Kundur two-area system is analysed using the power hardware-in-the-loop concept. The experimental results indicate that the VSM is effective in reducing the ROCOF values in power system networks during rapid fluctuations in frequency caused due to the load switching events. The hardware implementation of a VSM interfaced with the modified Kundur two-area system, the novel battery power management algorithm and the application of VSM as a ROCOF control device for power system applications are the unique contributions in this paper.

Development of an Adaptive Controller for Gridconnected Power Electronic Converter

Power electronics converter (PECs) has become an attractive replacement for the conventional inverter in grid-tied photovoltaic (PV) applications. This is due to improved the transient performance and provided better power quality. In this paper, an improved controller and a novel topology for a PEC is proposed. In this paper, an improved controller and a novel topology for a PEC is proposed. The proposed PEC topology is based on the three-level converter and adaptive PI with a fuzzy logic controller (API-FLC). The PEC with the novel controller is designed to achieve constant DC link voltages and sinusoidal current and voltage with near unity power factor when operating in different conditions. In order to evaluate the validity of the proposed PEC, the model was simulated by MATLAB/Simulink Toolbox. The results obtained with the proposed converter gives a better dynamic performance and improved power quality compared to the conventional converter.

Digital Control Method for Grid-Connected Converters Supplied With Nonideal Voltage

When employed in systems with higher frequency variability and nonideal supply voltage, the synchronization method used in grid-connected power electronic converter control systems must be considered. A method is presented here that achieves synchronization implicitly through the use of a discrete Fourier transform in combination with the computation of an effective admittance. It is shown that the method is suitable for systems with high frequency variability as well as for systems with nonideal supply voltage including heavy voltage asymmetry.

A Unified Control Strategy for Electronically Interfaced Distributed Energy Resources

This paper proposes a unified control strategy that enables islanded and grid-connected operations of three-phase electronically interfaced distributed energy resources (DERs), with no need for knowing the prevailing mode of operation or switching between two corresponding control architectures. The proposed strategy benefits from both active feedback compensation and the droop method. It allows the employment of the same power circuit and control architecture for the islanded operation as those established and optimized for grid-connected power-electronic converter systems. The proposed strategy can be directly adopted for dispatchable systems (e.g., battery energy storage systems) or, alternatively, it can be embedded in a nested control loop for non-dispatchable systems. This paper presents the mathematical model on which the proposed strategy is based. Further, the effectiveness of the proposed strategy is demonstrated through time-domain simulation of a two-unit test microgrid in the PSCAD/EMTDC software environment.

Grid synchronization and symmetrical components extraction with PLL algorithm for grid connected power electronic converters - a review

Grid synchronization and symmetrical components extraction with PLL algorithm for grid connected power electronic converters - a reviewIn this paper, a review of Phase Locked Loop (PLL) algorithms and symmetrical component extraction methods intended for grid-connected power electronic converters are presented. Proposed classification is based on voltage representation in three coordinates: natural (abc), stationary (αβ) and rotating coordinates (dq). The three selected algorithms are described in details: Dual Second Order Generalized Integrator (DSOGI-PLL), Dual Virtual Flux - both in stationary coordinates. The third one, in rotating dq coordinates, is Dual Synchronous Reference Frame PLL (DSRF-PLL). A comparison of PLL algorithms is presented. Also, selected experimental results are given to verify practical application of discussed algorithms.

Effects of virtual resistor in power electronic converter on subsynchronous oscillation damping

AbstractVirtual impedance concept provides the impedance effects without need to put real physically impedance in the circuit. In power system, oscillation will not occur as long as the impedance seen by turbine‐generator set is ‘passive’. In order to ensure the impedance is seen as ‘passive’ by the turbine‐generator set, a series resistor can be placed in the system. This paper proposes implementation of virtual resistor concept in grid connected power electronic converter to replaced the aforementioned series resistor, then analyze its impact on subsynchronous oscillation phenomena. It can be shown that virtual resistor can improve the negative region of converter input admittance. Moreover, it also gives us better damping in system involving grid connected inverter. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.