Oscar Cuaresma Zevallos

Instability issues arising from the interaction of Phase-Locked Loops (PLLs) and weak grids have become relevant research topics due to the increasing penetration of inverter-based resources in distribution systems. Several mitigation strategies have already been proposed, yet they generally focus on inverters employing Synchronous Reference Frame (SRF) current control. In contrast, for three-phase Mixed-Domain Controlled (MDC) converter applications, which is the combination of SRF-PLL and stationary-frame current control, the literature remains incipient regarding effective stabilization approaches. This paper addresses this gap by proposing a PLL-Current Controller Stabilizer (PCCS) method to improve the stability of MDC grid-following inverters connected to weak grids. In short, the oscillating component of the PLL-tracked frequency, extracted through a washout filter, is used as part of a feedforward term in the inverter current reference, which enhances system stability. A thorough investigation of the instability effects caused by PLL on MDC converters is also conducted. Offline simulations and real-time Controller-Hardware-in-the-Loop (C-HIL) results are presented to validate the proposed method.

Currently, the global use of electric vehicles is still low; however, a significant increase is expected in the coming years. Determining the optimal location of charging stations in distribution systems can influence the increased adoption of this technology in transportation, as it contributes to the proper functioning of distribution networks. There are several optimization methods, which can be classified into exact, heuristic, and metaheuristic methods, each with different characteristics and applications. This article presents a literature review of the main optimization methods currently used to determine the location of charging stations in distribution systems. It concludes that metaheuristic optimization methods are the most widely used. In addition, the review identifies current research gaps, particularly the limited use of real EV demand data and the lack of stochastic approaches to represent demand variability. The main contribution of this work lies in emphasizing the importance of incorporating stochastic methods to adequately address the uncertainty of EV demand in distribution networks.

Power systems are complex and often subject to faults, requiring accurate mathematical models for a thorough analysis. Traditional time-domain models are employed to evaluate the dynamic response of power system elements during transmission system faults. However, only the positive sequence components are considered for unbalanced faults, so the small-signal stability analysis is no longer accurate when assuming balanced conditions for asymmetrical faults. The dynamic phasor approach extends traditional models by representing synchronous machines with harmonic sequence components, making it suitable for an unbalanced condition analysis and revealing dynamic couplings not evident in conventional methods. By modeling electrical and mechanical equations with harmonic sequence components, the study implements an eigenvalue sensitivity analysis and participation factor analysis to identify the variable with significant participation in the critical modes and consequently in the dynamic response of synchronous machines during asymmetric faults, thereby control strategies can be proposed to improve system stability. The article validates the dynamic phasor model through simulations of a single-phase short circuit, demonstrating its accuracy and effectiveness in representing the transient and dynamic behavior of synchronous machines, and correctly identifies the harmonic sequence component with significant participation in the critical modes identified by the eigenvalue sensitivity to the rotor angular velocity and rotor angle.

In recent years, microgrid (MG) deployment has significantly increased, utilizing various technologies. MGs are essential for integrating distributed generation into electric power systems. These systems’ economic dispatch (ED) aims to minimize generation costs within a specific time interval while meeting power generation constraints. By employing ED in electric MGs, the utilization of distributed energy resources becomes more flexible, enhancing energy system efficiency. Additionally, it enables the anticipation and proper utilization of operational limitations and encourages the active involvement of prosumers in the electricity market. However, implementing controllers and algorithms for optimizing ED requires the independent handling of constraints. Numerous algorithms and solutions have been proposed for the ED of MGs. These contributions suggest utilizing techniques such as particle swarm optimization (PSO), mixed-integer linear programming (MILP), CPLEX, and MATLAB. This paper presents an investigation of the use of model predictive control (MPC) as an optimal management tool for MGs. MPC has proven effective in ED by allowing the prediction of environmental or dynamic models within the system. This study aims to review MGs’ management strategies, specifically focusing on MPC techniques. It analyzes how MPC has been applied to optimize ED while considering MGs’ unique characteristics and requirements. This review aims to enhance the understanding of MPC’s role in efficient MG management, guiding future research and applications in this field.

The increasing rate of renewable energy penetration in modern power grids has prompted updates to the regulations, standards, and grid codes requiring ancillary services provided by photovoltaic-generating units similar to those applied to conventional generating units. In this work, a comprehensive survey presents a comparison of requirements related to voltage ride through reactive current injection/absorption; active power restoration; frequency stability regulation and active power control; voltage regulation and reactive power control; and the energy quality requisites included in the standards and grid codes of countries around the globe. The survey can be used to observe the differences between the requirements established in the grid codes depending on the power system operating characteristics, development of technology, and renewable energy penetration level. Many of these factors determine the parameters used to establish requisites for different grid codes, making a global standardization of the renewable energy interconnection requirements much harder.

The goal of achieving decarbonized operation of power systems in response to climate change has led to an increase in the number of sources connected to electrical grids through inverters, of which photovoltaic systems are an example. The operating characteristics of these systems in steady-state or transient operation are different from those of synchronous machines, giving rise to issues related to power system stability. The vast majority of grid codes stipulate that fault ride-through operating requirements should be enabled during severe contingencies in the transmission network, prioritizing delivery of reactive power to the power system to support voltage stability. However, this control action may contribute to rapid collapse of the system voltage under adverse operating conditions, such as when the critical voltage in the power vs. voltage curve is near the reliable voltage operating bound. Supporting power system stability under these circumstances represents a new challenge for inverter control schemes. This paper adapts a recently published fault ride-through control scheme for PV inverters for use in a multimachine power system and shows that the scheme can successfully maintain transient and voltage stability even under adverse voltage operating conditions.

The increasing number of megawatt-scale photovoltaic (PV) power plants and other large inverter-based power stations that are being added to the power system are leading to changes in the way the power grid is operated. In response to these changes, new grid code requirements establish that inverter-based power stations should not only remain connected to the grid during faulty conditions but, also provide dynamic support. This feature is referred in the literature to as momentary cessation operation. The few published studies about momentary cessation operation for PV power plants have not shed much light on the impact of these systems on the overall power system stability problem. As an attempt to address this issue, this paper proposes a control scheme for PV inverters that improves the transient stability of a synchronous generator connected to the grid. It is shown through the paper that the proposed control scheme makes the PV inverter's dc link capacitors absorb some of the kinetic energy stored in the synchronous machine during momentary cessation. Besides that, the proposed solution is also able to improve voltage stability through the injection of reactive power. Experimental and simulation results are presented in order to demonstrate the effectiveness of the proposed control scheme.

The increase in electricity generation from wind and solar sources brings important changes in the operational and dynamic characteristics of the power system when it is subjected to major disturbances. One important technical problem is the risk of losing transient stability by progressively replacing of conventional generation with energy sources connected to the system through electronic converters, reducing the overall system inertia. However, the converters could potentially bring new opportunities of fast control to give support to synchronous generators connected to the power system in response to a disturbance. In this paper, a control strategy for the inverter is proposed based on the injection of the current identified as having the major participation in the variation of the synchronous machine rotor angle. The identification is based on the eigenvalue sensitivity analysis with respect to the rotor angle. The proposed method is tested in a hybrid system with a synchronous generator connected in parallel with an inverter, both connected to an infinite bus through two transmission lines. The dynamic response of the rotor angle is assessed simulating a two phase to ground fault. The results show that the inverter must absorb active power, releasing the kinetic energy stored in the rotational masses of the synchronous machine, and effectively reducing the rotor angle excursions.

The problem of voltage control inadequacy in voltage control devices in a power system is revealed, when an inverse relationship between controlling and controlled variables exists. The voltage control inverse relationship could adversely affect the voltage profile in a power system, thus, it is necessary to determine the relationship between controlling variables and controlled voltages, which can be accomplished through the analysis of the elements in the voltage controlled sensitivity matrix. The analysis of these elements, besides determining the adequacy of voltage control actions, allows determining possible conflicts between controls which, if not considered, could limit the adequate adjustment of the voltage profile. The identification of voltage control inadequacy of an operating point allows assessing the impact of the voltage control inadequacy in voltage stability conditions through voltage stability indexes. The proposed strategy for voltage stability assessment is tested on a 5-bus power system. From this analysis, the impact on voltage stability conditions is non-dependent on the reverse relationship between controlling variables and controlled voltages, as long as the best strategy for control actions is established, when voltage control inadequacy is determined.