Grid-connected AC Research Articles

Distributed Event-Triggered Control for Power Sharing in Grid-Connected AC Microgrid

The traditional communication method will bring a heavy burden to the grid system in the distributed collaborative control of microgrids. In grid-connected ac microgrid, this paper adopts an event-triggered control method to resolve the power sharing problem and reduce the communication cost. To begin, the problem is stated as the control problem of a leader-follower multiagent system and uses a distributed hierarchical control strategy to solve it. The primary controller uses the internal model principle to control the voltage output. To optimize the secondary controller, this paper not only adopts an event-triggered control strategy for the microgrid based on the Lyapunov principle to control the output of the compensator state, but also carries out relevant theoretical verification and analysis to ensure that each agent can exclude Zeno behavior. Finally, simulation instances are used to verify the usefulness of the studied control algorithm.

Model predictive control–based robust-control strategy of distribution control for a grid-connected AC microgrid

The challenge of ensuring the reliable running of power systems has gotten more difficult in recent years due to the rising complexity of power system networks. The decreasing accessibility of fossil fuels has necessitated a greater dependence on renewable energy sources, such as solar systems, wind power, and hydroelectric power, by the international community. As a result, there is an increasing demand for AC microgrids to offer an effective approach for distributing power. The power system networks that consist of microgrids frequently have a significant number of failures, surpassing 80%. These failures occur because microgrids are susceptible to unexpected changes in different distributed generating sources. The variations greatly impair the operating efficiency of the microgrid and have negative consequences for the distribution system. The microgrid consists of numerous dispersed generation units and local loads. The load in a microgrid exhibits parametric uncertainty, which adds to the fluctuation observed in its performance. The formulated control strategy is Model Predictive Control, which aims to achieve robust performance even in the presence of unaccounted-for loads, dynamic loads, harmonic loads, and both balanced and unbalanced loads. The authors of this paper have developed a control approach that utilizes model predictive control (MPC) and is characterized by its robustness and optimality. MPC has the ability to predict the future behavior of a certain system. The controller successfully mitigates and reduces any disruptions that may occur within the power distribution system by taking into account its healthy characteristics. The model is implemented in the MATLAB Simulink environment, where it produces an accurate and appropriate total harmonic distortion value. The model was compared to previous efforts and significantly improved by increasing some crucial parameters by up to 90%. The value functions as a measure of the controller's performance quality and the improved efficiency of the microgrid system.

A Basic Design Tool for Grid-Connected AC–DC Converters Using Silcon Carbide MOSFETs

The design and optimization of power converters is a key factor in the growth and development of the power electronics field. However, the process of designing a power converter is not straightforward, and engineers often rely on experience and intuition, sometimes requiring time-consuming computer simulations. This paper presents a tool for the basic design of grid-connected AC–DC converters. The design tool takes specifications and operating conditions for two-level and three-level NPC converter topologies and derives a draft design. The tool calculates the input filter’s electrical parameters, the converter’s losses, the temperature rise of the power semiconductor devices, and the ripple current and voltage of the DC-link capacitor. In order to validate the proposed design tool, four AC–DC converters using SiC MOSFETs were designed. Based on the design results, simulation models and prototypes were fabricated to verify the performance and confirm that the proposed design tool can be used in the basic design process of converters.

Voltage-Oriented Control-Based Three-Phase, Three-Leg Bidirectional AC–DC Converter with Improved Power Quality for Microgrids

Renewable energy sources (RESs) and energy storage schemes (ESSs) integrated into a microgrid (MG) system have been widely used in power generation and distribution to provide a constant supply of electricity. The power electronics converters, particularly the bidirectional power converters (BPCs), are promising interfaces for MG infrastructure because they control the power management of the whole MG system. The controller of BPCs can be designed using several different control strategies. However, all the existing controllers have system stability, dynamics, and power quality issues. Therefore, this study demonstrates the development of an LCL-filtered grid-connected bidirectional AC–DC converter’s (BADC) control strategy based on voltage-oriented control (VOC) to overcome these issues. The proposed VOC-based inner current control loop (ICCL) is implemented in synchronous dq-coordinate with the help of proportional-integral (PI) controllers. An observer-based active damping (AD) is also developed in order to estimate the filter capacitor current from the capacitor voltage instead of directly measuring it. This developed AD system helps to damp the resonance effect of the LCL filter, improves system stability, and also eliminates the practical challenges of measuring capacitor current. The proposed controller with AD is able to realize bidirectional power transfer (BPT) with reduced power losses due to the elimination of passive damping and improved power quality, system dynamics, and stability. The mathematical modeling of the suggested system was developed, and the structure of the system model was established in the MATLAB/Simulink environment. The performance of the proposed system was validated with real-time software-in-the-loop (RT-SIL) simulation using the OPAL-RT simulator for a 16 kVA converter system. The real-time (RT) simulation results show that the BADC with the proposed control scheme can provide better dynamic performance and operate with tolerable total harmonic distortion (THD) of 2.62% and 2.71% for inverter and rectifier modes of operation, respectively.

Multi-Mode Active Inertia Support Strategy for MMC-HVDC Systems Considering the Constraint of DC Voltage Fluctuations

The Modular Multilevel Converter based High Voltage Direct Current Transmission (MMC-HVDC) systems can provide active inertia support for the connected ac grid for fast frequency recovery. However, the submodule (SM) capacitance voltages will be affected by the transient power flow and there exist dc voltage fluctuation issues during the active inertia support process. This paper proposes a multi-mode active inertia support strategy for MMC-HVDC systems considering the constraint of dc voltage fluctuations. The analytical relationship of dc voltage fluctuations and ac frequency variations when MMC adopts the virtual inertia control is firstly established, and then different inertia support operation modes are divided according to the allowable dc voltage fluctuation ranges. Next, the multi-mode active inertia support strategy for MMC-HVDC is proposed, which can provide flexible active inertia support on the premise of reasonable dc voltage fluctuations according to different frequency variations and recovery requirements. Besides, the inertia support process of MMC is equivalent to the zero-input response of first-order circuit to calculate the maximum sustainable supporting time, which ensures that the dc voltage fluctuation constraint can be well satisfied in different operation modes. Finally, several simulation results verify the effectiveness of the proposed control strategy.

Modelling and control of a grid-connected AC microgrid with the integration of an electric vehicle

Abstract The purpose of this paper is to propose an efficient model and a robust control that ensures good power quality for the AC microgrid (MG) connected to the utility grid with the integration of an electric vehicle (EV). The MG consists of two renewable energy sources: a photovoltaic system (PVS) and a wind turbine system (WTS) based on a permanent magnet synchronous generator (PMSG), with the integration of an EV. These sources are used to supply active and reactive power to the AC bus and the utility grid. Maximum power point tracking (MPPT) based on the perturb-and-observe (PO) method is used to increase the efficiency of the photovoltaic modules and improve overall performance. The MG system includes a 2-MW WTS, a 100-kW PVS and 12 kW provided by the EV. To validate the performance of the proposed system, a series of simulations were conducted using the MATLAB®/Simulink® environment. The results demonstrate that the proposed system ensures high performance in terms of power quality, system stability, power tracking and safe integration of the EV.

Controllability analysis of renewable embedded AC microgrid with multi-band stabilizers

The depletion of conventional energy sources and the increasing demand for electricity due to industrialization and rising global living standards has led to the need for a transformation of the traditional power grid. The microgrid concept has emerged as a solution that integrates renewable energy sources into the main grid. This paper examines an AC microgrid consisting of solar, wind, hydro, and diesel power stations, and investigates the effect of various power system stabilisers (PSSs) on the silent-pole synchronous generator of the diesel and hydro power plant. The effectiveness of different PSSs, including - PSS, ΔPa-PSS, and Multi-band PSS, is compared using a grid-connected AC microgrid. The simulation results show that Multiband PSS performs better than conventional PSSs in eliminating electromechanical oscillation, even in the presence of Gaussian and colored noise. PSSs are useful in managing the power flow of various DERs and improving power system stability. The simulation is conducted using the MATLAB/SIMULINK software.

Stability enhancement of AC microgrid using multi-band stabilizers

In this study, the functionality of a grid-connected AC microgrid with four distribution generators (DG) units is examined. The AC microgrid must respond rapidly during an islanding operation to preserve overall system stability. A gridconnected AC microgrid is used to examine the overall performance of the PSSs in minimizing electromechanical oscillation, including —PSS, Pa-PSS, and Multi-band PSS (MB-PSS4B and MB-PSS2B). Multi-band PSS provides better dampening performance than other PSSs when Gaussian and colored noise are measured in the system. MATLAB/SIMULINK software is used to simulate an AC Microgrid that is connected to the grid and to track how well different PSS are working. The simulation results show that Multi-band PSSs outperform traditional PSSs (CPSS) in a variety of situations.

A Novel Control Technique for Enhancing the Operation of MTDC Grids

This paper develops a novel control approach for the droop-controlled Voltage Source Converters (VSC) of Multi-Terminal High Voltage Direct Current (MTDC) systems. The frequency consensus controller is shown to assist in damping the inter-area oscillations and providing enhanced mutual frequency support. Such features, however, might be achieved at the expense of overloading some of the VSCs interfaced to the synchronously connected ac grids. Thus, a new power-sharing control loop, based on the proposed power deviation ratio (PDR) index, is developed to enhance the distribution of active power mismatches between those VSCs. The developed PDR loop, which regulates the ratios of the mismatched power-sharing by considering both the scheduled power injections and the available capacities of the VSCs, enhances the mutual frequency support capability between ac areas of the MTDC system. Furthermore, a newly proposed equidistant voltage control (EVC) loop of the proposed controller regulates the dc system’s voltages such that they are equally far from upper and lower voltage limits. This technique increases the safety margin in voltage regulation during events that cause dc system’s voltage profile variation. The comparative advantage of the proposed controller is verified through modal and participation factor analysis and through comprehensive time-domain simulations.

Simple and Seamless PWM Scheme of Isolated Bidirectional AC–DC Converter for Reducing Voltage Spike

This article proposes an improved modulation scheme based on unipolar sinusoidal pulsewidth modulation for a single-phase single-stage isolated bidirectional ac–dc converter. A direct ac–ac conversion system connected with a high-frequency transformer typically suffers from high-voltage spikes because of the leakage inductors of the high-frequency transformer and the grid filter inductors, resulting in reduced reliability and large power losses. The proposed modulation scheme can eliminate the high-voltage spikes in bidirectional power flow without the use of additional circuits while realizing zero-voltage switching. In addition, because the proposed method does not require individual gate control of the bidirectional power switches, the generation of gate signals can be simplified using the proposed modulation scheme. A simulation model and a 1.2 kW grid-connected ac–dc converter based on silicon-carbide metal–oxide–semiconductor field-effect transistors are implemented to validate the effectiveness of the proposed modulation scheme.

A New Discrete Four-Quadrant Control Technique for Grid-Connected Full-Bridge AC–DC Converters

This paper presents a new control technique for grid-connected full-bridge AC–DC converters. The proposed control scheme is based on one-cycle control approach and enables the converter to process power in all four quadrants. In the proposed method, switching pulses are generated using a discrete control law with a superimposed fictitious reactive current term. This term enables seamless four-quadrant operation of the converter. Implementation of the discrete controller includes estimation of the current ripple based on measured values of the input current and voltages, sampled at the beginning of each switching cycle. The estimated current ripple is then used for a carrier-less implementation of the proposed control technique. A detailed controller stability analysis using Lyapunov theory is also presented. Theoretical analysis, simulation results, and experimental results show fast dynamic response for the grid current.

A modified droop control structure for simultaneous power‐sharing and DC voltage oscillations damping in MT‐HVDC grids

With increasing energy demand, the use of renewable energy resources is increasing. Renewable energy sources require power electronic converters to get integrated into power grids. Multi-terminal high voltage direct current (MT-HVDC) systems are a promising solution for their grid integration. Using droop-based controller strategies in power converter stations of MT-HVDC grid, in addition, to providing power-sharing, can support the frequency of the connected AC grids. However, power changes lead to direct voltage deviations, thereby disrupting the stability of the MT-HVDC system. Here, a new control structure for the droop controller of the voltage source converter (VSC) controller is proposed which is named modified droop controller (M-Droop). This method, in addition to power-sharing, uses an appropriate control action to provide the required voltage stability. Here, analytical studies of the modified control strategy are reported. Moreover, through a developed MATLAB Simulink platform, its performance is verified in the events of transients, consisting of fault, variable load, and change in power generation.

A Bidirectional Grid-Connected DC–AC Converter for Autonomous and Intelligent Electricity Storage in the Residential Sector

Controlling the cost of electricity consumption remains a major concern, particularly in the residential sector. Smart home electricity management systems (HEMS) are becoming increasingly popular for providing uninterrupted power and improved power quality, as well as for reducing the cost of electricity consumption. When power transfer is required between a storage system and the AC grid, and vice versa, these HEMS require the use of a bidirectional DC–AC converter. This paper emphasizes the potential value of an almost unexplored topology, the design of which was based on the generation of sinusoidal signals from sinusoidal half waves. A DC–DC stage, which behaved as a configurable voltage source, was in series with a DC–AC stage, i.e., an H-bridge, to achieve an architecture that could operate in both grid and off-grid configurations. Wide bandgap power switches (silicon carbide metal-oxide-semiconductor field-effect transistors [MOSFETs]), combined with appropriate control strategies, were the keys to increasing compactness of the converter while ensuring good performance, especially in terms of efficiency. The converter was configured to automatically change the operating mode, i.e., inverter or rectifier in power factor correction mode, according to an instruction issued by the HEMS; the latter being integrated in the control circuit with automatic duty cycle management. Therefore, the HEMS set the amount of energy to be injected into the grid or to be stored. The experimental results validate the operating modes of the proposed converter and demonstrate the relevance of such a topology when combined with an HEMS, especially in the case of an AC grid connection. The efficiency measurements of the bidirectional DC–AC converter, performed in grid-connected inverter mode, show that we exceeded the efficiency target of 95% over the entire output power range studied, i.e., from 100 W to 1.5 kW.

Extended Nodal Admittance Matrix Based Stability Analysis of HVDC Connected AC Grids

Motivated by emerging oscillation issues of high-voltage direct-current (HVDC) transmission system resulted from the interaction with AC grids, impedance-based stability analysis becomes an attractive approach in the de-risking studies due to the capability of black-box modelling and clear physical meaning. However, impedance-based stability assessment of HVDC is typically conducted either from the AC side or from the DC side, without fully considering both AC and DC grid dynamics, thus impairs evaluation accuracy. In this paper, the nodal admittance matrix based resonance mode analysis method is extended to include both AC and DC dynamics through formulating a hybrid AC-DC admittance matrix. To achieve this goal, HVDC interfacing-converters are modelled as three-port admittance networks with one DC port and two AC ports (in dq-frame) integrating both the AC side frequency coupling and the AC-DC dynamic coupling. In contrast to the widely adopted Nyquist-based methods, the proposed method does not requires knowing the number of right half-plane poles of system minor-loop gain, thus simplifies stability analysis, and moreover, the participation factors of both AC and DC grid nodes in critical resonance modes can be identified through eigenvalue-decomposition analysis, which is in favor of the design of mitigation measures. The effectiveness of the proposed method is validated by EMT-simulations in MATLAB/Simulink.

Design and Simulation of Selective Short Circuit Protection System in AC Microgrid with Grid-connected Mode

The fast detection and accurate location on short-circuit faults has always been the research direction of AC microgrid fault protection. A short-circuit fault protection system for grid-connected AC microgrid is designed based on dsPIC33FJ32GP204 in this paper, which realizes the fast detection and regional location. The short-circuit fault simulation model is built on PSCAD, and all kinds of fault waveforms generated by simulation are imported into Proteus. The designed protection system is used to transform the wavelet energy spectrum of all kinds of fault waveforms generated by simulation, and then output the corresponding control signal to circuit breakers. Simulation results show that the control system can quickly perceive and accurately locate the microgrid short-circuit fault area and realize the output of selective protection signal.

Photovoltaic Power Plant Collection and Connection to HVDC Grid with High Voltage DC/DC Converter

Photovoltaic (PV) power plant collection and connection to a high voltage direct current (HVDC) grid has many advantages. Compared with the traditional AC collection and grid-connection scheme, it can reduce the power conversion links and improve the system efficiency. As one of the most important devices in the application of a PV HVDC collection and grid-connection system, a high voltage, large capacity, high step-up ratio DC/DC converter is the critical technology. A DC/DC converter scheme based on a boost full bridge isolated power module cascaded in input parallel output series (IPOS) structure is proposed to meet the technical requirements of PV power conversion with high voltage, large capacity, and high step-up ratio. The operation mode of the power module is analyzed, the soft switching method is proposed, and the constraint to realize the soft switching of the power module is deduced. Aiming to resolve the problem of multi-module voltage and current equalization in a cascaded DC/DC converter, a distributed module equalization control strategy is proposed to realize the reliable operation of a power module and converter. A 5 kV/80 kW standard power module is developed, the DC/DC converter experimental platform is built, and the proposed system scheme and control strategy are verified by experiments. Based on a 14-power module input parallel output series connection, a ±30 kV/1 MW PV DC/DC converter is developed, a ±30 kV PV HVDC collection and grid-connection demonstration system is established, and the experimental test is completed to realize the stable operation of the system.

Analysis of Stationary- and Synchronous-Reference Frames for Three-Phase Three-Wire Grid-Connected Converter AC Current Regulators

The current state of the art shows that unbalance and distortion on the voltage waveforms at the terminals of a grid-connected inverter disturb its output currents. This paper compares AC linear current regulators for three-phase three-wire voltage source converters with three different reference frames, namely: (1) natural (abc), (2) orthogonal stationary (αβ), and (3) orthogonal synchronous (dq). The quantitative comparison analysis is based on mathematical models of grid disturbances using the impedance-based analysis, the computational effort assessment, as well as the steady-state and transient performance evaluation based on experimental results. The control scheme devised in the dq-frame has the highest computational effort and inferior performance under negative-sequence voltage disturbances, whereas it shows superior performance under positive-sequence voltages among the reference frames evaluated. In contrast, the stationary natural frame abc has the lowest computational effort due to its straightforward implementation, with similar results in terms of steady-state and transient behavior. The αβ-frame is an intermediate solution in terms of computational cost.

Coordinated stochastic optimal energy management of grid-connected microgrids considering demand response, plug-in hybrid electric vehicles, and smart transformers

A microgrid comprises renewable and non-renewable power generating sources, controllable loads, energy storage devices and works as a single controllable entity. A gradual shift from conventional internal combustion engine-based vehicles to electric/hybrid electric vehicles has also led to a new load type in the power system. Moreover, demand-side management measures like demand response programs have become popular. This work deals with the optimal coordinated operation of a grid-connected AC microgrid consisting of controllable and uncontrollable power sources, battery storage units, considering plug-in hybrid electric vehicles and demand response programs. Stochastic models of renewable power sources, electric load demand, loads of hybrid electric vehicles (with battery charging characteristic), and grid power price are fed into “Hong’s 2 m point estimate method” embedded optimal operating strategy. The objective is to minimize the cost of operation subject to the satisfaction of technical constraints. A nested stochastic optimization algorithm is implemented to find optimal generation schedule, battery dispatch strategy, and the best incentive value for an incentive-based demand response program. Different charging strategies of hybrid electric vehicles are studied, and their impacts on system operation are investigated. The optimal coordination between a voltage control scheme using a smart transformer with the energy management scheme is also investigated. Simulation studies on a thirty-three bus test system prove the efficacy of the proposed algorithm. The proposed coordinated optimal operating strategy reduces the operating cost by 17.53%∼17.74%. The system loss also reduces by 29.49%∼31.36%.

A flexible analytical model for operational investigation of solar hydrogen plants

Hydrogen will become a dominant energy carrier in the future and the efficiency and lifetime cost of its production through water electrolysis is a major research focus. Alongside efforts to offer optimum solutions through plant design and sizing, it is also necessary to develop a flexible virtualised replica of renewable hydrogen plants, that not only models compatibility with the “plug-and-play” nature of many facilities, but that also identifies key elements for optimisation of system operation. This study presents a model for a renewable hydrogen production plant based on real-time historical and present-day datasets of PV connected to a virtualised grid-connected AC microgrid comprising different technologies of batteries, electrolysers, and fuel cells. Mathematical models for each technology were developed from chemical and physical metrics of the plant. The virtualised replica is the first step toward the implementation of a digital twin of the system, and accurate validation of the system behaviour when updated with real-time data. As a case study, a solar hydrogen pilot plant consisting of a 60 kW Solar PV, a 40 kW PEM electrolyser, a 15 kW LIB battery and a 5 kW PEM fuel cell were simulated and analysed. Two effective operational factors on the plant's performance are defined: (i) electrolyser power settings to determine appropriate hydrogen production over twilight periods and/or overnight and (ii) a user-defined minimum threshold for battery state of charge to prevent charge depletion overnight if the electrolyser load is higher than its capacity. The objective of this modelling is to maximise hydrogen yield while both loss of power supply probability (LPSP) and microgrid excess power are minimised. This analysis determined: (i) a hydrogen yield of 38–39% from solar DC energy to hydrogen energy produced, (ii) an LPSP <2.6 × 10−4 and (iii) < 2% renewable energy lost to the grid as excess electricity for the case study.

Early Identification and Location of Short-Circuit Fault in Grid-Connected AC Microgrid

With the rapid development of microgrid and large-scale grid-connected operation, the detection and location of short-circuit faults in microgrid has become a bottleneck. In this paper, a simulation model of short-circuit fault in low-voltage AC microgrid is built on PSCAD/EMTDC. The characteristics of current wavelet energy spectrum under various short-circuit fault states are studied and the early detection of faults is realized. Based on the relationship between the first peak value of fault current wavelet energy spectrum and the location of short-circuit points, a method for early detection and area location of short-circuit faults in microgrid is proposed. The simulation and experimental results show that this method can detect and locate the short-circuit fault area of microgrid quickly and accurately.