Search

Hybrid microgrids (HMGs) are more flexible and require fewer power converters than ac and dc microgrids (MGs); however, the number of wires/buses in HMGs is still considerable, especially when there are multiple voltage levels in the dc subgrid. There are five wires/buses in conventional single-phase ac/bipolar dc (BDC) HMGs, two (phase and neutral) in the ac subgrid, and three (positive, zero, and negative) in the BDC subgrid. This article presents a new four-wire architecture with a common zero and neutral bus for a single-phase ac/BDC HMG that features a new bidirectional interlinking converter (IC). Using the proposed structure, the number of wires/buses and total costs are reduced without increasing the number of IC switches or complicating its control method. Besides, the common zero and neutral bus in the proposed structure provides proper grounding for both subgrids, reduces leakage current (≈0 mA), and improves fault tolerability in dc fault conditions. For this purpose, the dc-side fault ride-through (FRT) capabilities of the conventional and proposed ICs are compared by driving their mathematical models under dc fault conditions. The most severe currents experienced by conventional IC switches in a resistive grounded system are 760 A under positive and 1300 A under negative and zero terminal-to-ground (ZG) fault conditions. However, the proposed IC does not experience any overcurrent in ZG fault conditions, and its switches’ current stresses are reduced to 170 A for positive and 320 A for negative terminal-to-ground fault conditions. To validate the performance of the proposed IC and its modulation technique, simulation results in PSCAD/EMTDC environment and experimental results based on a scaled-down prototype are presented.

A hybrid microgrid (HMG) is comprised of both ac and dc subgrids interconnected via an interlinking converter (IC). Conventional single-phase ac/low voltage dc (LVdc) HMGs require four wires or buses, two buses for the ac subgrid as phase and neutral, and two buses for the dc subgrid as positive and negative. In this article, a new three-wire topology is proposed for single-phase ac/LVdc HMGs with a common neutral and negative bus. The proposed HMG topology benefits from two properly grounded subgrids and has some features, which help to reduce wiring costs, suppress leakage current, and improve dc and ac-side fault ride-through (FRT) by using a common ground strategy. For this purpose, a new three-wire single-phase ac/LVdc IC and its modulation method are also proposed. The proposed IC dc and ac-side FRT is investigated and compared with the conventional IC dc and ac-side FRT. To this aim, suitable models for the conventional single-phase and proposed ICs are derived. The models reveal the advantages of the proposed IC in the case of switches transients in fault conditions. The performance of the proposed IC and its control method in an HMG is proved through simulations in MATLAB/SIMULINK environment and by constructing an experimental setup.

This paper develops a distributed control framework to coordinate large-scale wind turbines for fairly sharing the power reserve request on a wind farm. A cooperative output regulation problem is formulated in a multi-agent framework, where the synchronization of turbine system outputs are established regarding the heterogeneity of turbine models. For this purpose, a homogeneous virtual system is created first and embedded into each turbine to generate the agreed trajectories across the wind farm in a distributed manner. The local power tracking of individual turbines is then enforced by the synthesized output regulator, whose control gains are determined by a variant of the Sylvester equation. The proposed approach issues the generator torque and blade pitch commands in a unified manner and is resistant to wind disturbances. The case studies are carried out through simulations and in a real laboratory-scale distributed communication network. The simulation results demonstrate the effectiveness of coordinated actions of wind turbines in response to the changing demands, under both constant and turbulent wind scenarios with the plug-and-play capability.

This article proposes a coordinated control scheme of a utility-scale photovoltaic (PV) power plant and a wind power plant (WPP) to support voltage stability and provide fast recovery after the grid faults in a power system with high penetration renewable energy sources (RES). The operation of PV and wind plants are time-shifted by approximately twelve hours. PV generates during the daytime, and wind plants usually produce peak power at night. Seasonally, the PV plants generate peak power on summer days, and wind plants produce maximum power during winter nights. These characteristics present an opportunity to use both types of renewables to complement each other in producing power and transforming energy, and also in providing active damping to safeguard the power system. To achieve these goals, two separate power oscillation damping (POD) control loops have been included for active and reactive power modulation in both PV and WPP. In both PV and WPP, an active power damping function without any power curtailment and a reactive power damping function modulating the reactive current (IQ) on the voltage control loop have been included. This article uses the nearest synchronous generator's local bus angle and speed as feedback for each of these loops. The performance of the proposed coordinated scheme based on the time of the day has been verified in a two-area system using the PSCAD simulator. The effectiveness of the control schemes has been investigated during the different timings of the day by observing the efficiency and the response time of the post-fault oscillations mitigation and the voltage restoration.

Presents the introductory editorial for this issue of the publication.

As deployment of power electronic coupled generation such as photovoltaic (PV) systems increases, grid operators have shown increasing interest in calling on inverter-coupled generation to help mitigate frequency contingency events by rapidly surging active power into the grid. When responding to contingency events, the faster the active power is provided, the more effective it may be for arresting the frequency event. This paper proposes a predictive PV inverter control method for very fast and accurate control of active power. This rapid active power control (RAPC) method will increase the effectiveness of various higher-level controls designed to mitigate grid frequency contingency events, including fast power-frequency droop, inertia emulation, and fast frequency response, without the need for energy storage. The RAPC method, coupled with a maximum power point estimation method, is implemented in a prototype PV inverter connected to a PV array. The prototype inverter's response to various frequency events is experimentally confirmed to be fast (beginning within 2 line cycles and completing within 4.5 line cycles of a severe test event) and accurate (below 2% steady-state error).

Calculation of eigen-solutions plays an important role in the small signal stability analysis of power systems. In this paper, a novel approach based on matrix perturbation theory is proposed for the calculation of eigen-solutions in a perturbed system. Rigorous theoretical analysis is conducted on the solution of distinct, multiple, and close eigen-solutions, respectively, under perturbations of parameters. The computational flowchart of the unified solution of eigen-solutions is then proposed, aimed toward obtaining eigen-solutions of a perturbed system directly with algebraic formulas without solving an eigenvalue problem repeatedly. Finally, the effectiveness of the matrix perturbation based approach for eigen-solutions’ calculation in power systems is verified by numerical examples on a two-area four-machine system.

The alternating current machines in a power system have the ability to remain synchronized following a severe disturbance such as loss of generations, line switching, or fault. This is described as power system transient stability. During system transients, the machines will accelerate or decelerate because of the mismatch between electrical torque and mechanical torque. Their power angles will travel and finally settle down to a new equilibrium, if the system has enough stored energy to absorb the disturbance, and rest the system at another steady state. In case of system instability, some machines will have aperiodic angular separation from the rest of the system and finally lose synchronization. Therefore, the power system transient stability is also called angle stability. The total system inertia is an essential force to rest the system transient. The inertias stored in all rotating masses that are connected to a power system, such as synchronous generators and induction motors, typically respond to disturbances voluntarily, without any control actions; however, several types of renewable generation, particularly those with power electronic interfaces, have an inertial response governed by a control function. To ensure bulk power system stability, there is a need to estimate the equivalent inertia available from a renewable generation plant. An equivalent voluntary inertia constant analogous to that of conventional rotating machines can be used to provide a readily understandable metric, such as the angle instabilities detections, because one of the most difficult obstacles for angle instability detection is the knowledge of the real-time generator inertias. This paper explores a method that utilizes synchrophasor measurements to estimate the equivalent inertia of a power source such as synchronous generators or wind turbine generators. This paper also investigates the angle instability detection method for a system with high wind power penetration using the synchrophasor measurements.

Failure of the drivetrain components is currently listed among the most problematic failures during the operational lifetime of a wind turbine. Guaranteeing robust and reliable drivetrain designs is important to minimize the wind turbine downtime as well as to meet demand in both power quantity and quality. While aeroelastic codes are often used in the design of wind turbine controllers, the drivetrain model in such codes is limited to a few (mostly two) degrees of freedom, resulting in a restricted detail in describing its dynamic behavior and assessing the effectiveness of controllers on attenuating the drivetrain load. In the previous work, the capability of the well-known FAST aeroelastic tool for wind turbine has been enhanced through integration of a dynamic model of a drivetrain. The drivetrain model, built using the Simscape in the MATLAB/Simulink environment, is applied in this paper. The model is used to develop a power-electronics-based controller to prevent excessive drivetrain load. The controller temporarily shifts the closed-loop eigenfrequency of the drivetrain through the addition of virtual inertia, thus avoiding the resonance. Simulation results demonstrating the fidelity of the expanded drivetrain model as well as the effectiveness of the virtual inertia controller are presented.

Utilization of wind energy in modern power systems creates many technical and economical challenges that need to be addressed for successful large scale wind energy integration. Variations in wind velocity result in variations of output power produced by wind turbines. Variable power output becomes a challenge as the share of wind energy in power systems increases. Large power variations cause voltage and frequency deviations from nominal values that may lead to activation of protective relay equipment, which may result in disconnection of the wind turbines from the grid. Particularly community wind power systems, where only one or few wind turbines supply loads through a weak grid such as distribution network, are sensitive to supply disturbances. Energy storage integrated with wind turbines can address this challenge. In this paper, Li-ion capacitors are investigated as a potential solution for filtering power variations at the scale of tens of seconds. A novel topology and control technique has been introduced to integrate capacitors and power conversion circuitry. Modeling and scaled-down experimental results are provided to verify the theoretical analyses.

Presents the introductory editorial for this issue of the publication.

Discusses energy storage implementation options.