Microgrids control: AC or DC, that is not the question

The P-V Curve, Q-V Curve, or P-Q-V Curve have been widely used to analyze power system behaviors under varying loading conditions. These curves have been generated under the condition that tire constant P-Q load component of a bus (or a collection of buses) varies, with the constant current load and constant impedance load being kept indeed As such, the physical meaning of these curves can be easily explained. Motivated by the facts that load models have profound impacts on power system behaviors and that the nonlinear load model ZIP-model is popular in modeling nonlinear behaviors of loads, this paper proposes a new class of curves, called ZIP-V curves, to better trace power system steady-state stationary behavior due to load and generation variations. The ZIP-V curves encompass the traditional P-V, Q-V, P-Q-V curves (constant P-Q load), I-V curve (constant current load), Z-V curve (constant impedance load), or generalized curves such as IP-V (constant current and constant power load), ZP-V (constant impedance and constant power load) or IZ-V (constant current and constant impedance load) curve when the values of corresponding components are kept constant. A tool based on the continuation power flow (CPFLOW) method useful for generating the ZIP-V curves is developed and its application to generate ZIP-V curves of a 4561-bus interconnected power system is illustrated.

The paradigm shift in electrical power grids and the increased interest towards decentralisation has opened a new window in the design, control and theoretical analysis of small scale power systems, i.e. microgrids, which aim at the integration and utilisation of renewable energy sources, energy storage systems and responsive loads at a local scale. Given their DC nature, DC microgrids have attracted significant interest as they provide a natural interface to the main grid by avoiding additional conversion steps. The aim of this thesis is to design and analyse novel hierarchical control schemes, both at the primary and secondary control level, that guarantee tight voltage regulation, accurate power sharing, current or voltage limitation, and present a straightforward approach towards deriving stability conditions for DC microgrids that incorporate nonlinear loads. As the microgrid configuration is paramount in the theoretical analysis, a rigorous method of computing the admittance matrix is developed that facilitates the stability analysis of DC microgrid systems supplying a constant impedance (Z), constant current (I) or constant power (P) load. This method is particularly useful as it permits the factorisation of the admittance matrix, while separating the singular matrices. In this way, the closed-loop asymptotic stability proof can be more easily approached by isolating the singularities and then, employing straightforward linear algebra tools, such as quadratic eigenvalue problem (QEP) theory, to derive the stability conditions. As stated in the technical specifications of every source, a crucial issue is represented by the ability to protect itself and its interface device (power converter) during faults, transients and unrealistic power demands. That is why current-limiting control is often combined with the primary control, i.e. often droop control, in DC microgrids to ensure the desired unit protection. In this thesis, novel current-limiting droop controllers areformulated for DC microgrids consisting initially of multiple unidirectional DC/DC boost converters and, later on, bidirectional DC/DC boost and three-phase AC/DC converters, that integrate different distributed generation units with the local grid. In contrast to the traditional approaches that use small-signal modelling, here, the accurate nonlinear models of the converter units are taken into account to prove the boundedness and the current-limiting property, by employing Lyapunov methods and the ultimate boundedness theory. Exponential stability at the desired equilibrium point is mathematically guaranteed, and further analysed from a graphical perspective, providing insights of the load’s effect onto the system performance and stability. To address the shortcomings that conventional droop control approaches introduce, i.e. inaccurate power sharing and significant load voltage drop, a DC microgrid architecture that takes into account the power converter dynamics of distributed generation units is deployed under decentralised primary and distributed secondary control scheme, in a hierarchical control framework. At the primary control layer, a novel current-limiting droop control scheme is implemented in a decentralised manner, whereas at the secondary control layer a fully distributed controller that performs a voltage restoration and improves the power sharing is deployed using a nearest-neighbour coupling communication network. By investigating for the first time both the dynamics of the converters with the nonlinear load and the two-layer control, singular perturbation theory is applied to analytically guarantee the stability of the entire DC microgrid. Finally, apart from the desired overcurrent protection, since DC capacitors are customary used at the output of each converter unit to stabilise the output voltage, they also introduce a maximum voltage limit. Hence, as the need for protection against overvoltages has emerged, droop controllers with inherent overvoltage protection are also proposed for parallel and meshed configuration networks. The upper limit of the voltage of each source is rigorously proven using ultimate boundedness theory. Asymptotic stability to the desired equilibrium for the closed-loop system is analytically guaranteed, and detailed conditions are derived to guide the control design. In order to validate the effectiveness of the different control methodologies developed in this thesis, both simulation and experimental testing are performed for each one of the methods, and are also compared with the conventional approaches to highlight their superiority.

In dc microgrids, load power sharing and bus voltage regulation are two common control objectives. In this article, a consensus-based algorithm is presented to achieve proportional power sharing and regulation of weighted geometric mean of bus voltages in dc microgrids with ZIP (constant impedance, constant current, and constant power) loads simultaneously. By using the virtue of the Laplacian matrices of undirected connected graphs, a lemma is derived to assist the stability analysis of the dc microgrids. Thus, a sufficient condition that stabilizes the system with ZIP loads is established. In addition, with the help of a distributed voltage regulation error estimator, the tuning of the bus voltages can be realized without the requirement on initial voltage conditions. Through the Lyapunov synthesis, the large-signal stability of the closed-loop system is theoretically ensured for a wide range of load conditions. Finally, simulation studies are performed to validate the merits of the proposed consensus-based algorithm.

In this article, we propose a novel four-stage distributed controller for a dc microgrid that achieves proportional power sharing and average voltage regulation for the voltages at actuated and unactuated buses. The controller is presented for a dc microgrid comprising multiple distributed generation units (DGUs) with time-varying actuation states, dynamic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$RLC$</tex-math> </inline-formula> lines, nonlinear constant impedance, current, and power (ZIP) loads, and a time-varying network topology. The controller comprising a nonlinear gain, proportional–integral (PI) controllers, and two dynamic distributed averaging stages is designed for asymptotic stability. This constitutes deriving passivity properties for the dc microgrid, along with each of the controller subsystems. Thereafter, design parameters are found through a passivity-based optimization using the worst-case subsystem properties. The resulting closed loop is robust against DGU actuation changes, network topology changes, and microgrid parameter changes. The stability and robustness of the proposed control are verified via simulations.

This paper studies the effect of Power System Stabilizers in damping of low frequency oscillations in a multi-machine power system with different load combinations. Various load models considered are constant impedance load, mixture of constant impedance and constant current load and mixture of constant impedance and constant power load. For this, a linearized model of multi-machine power system was developed. After forming state matrix, Eigen value analysis was carried out to find out low frequency local and inter-area modes of oscillations. For the optimal placement of PSS, participation factor analysis was carried out. The generator with highest participation factor is considered best candidate for the placement of PSSs. This analysis is carried out for two area (4-machines, 14 bus system) and multi area (16 machines, 68 bus, NY-NETS) system. The performance of PSSs in damping both local and inter-area modes of oscillations was found to be satisfactory.

Direct Current (DC) microgrids are expected to become larger due to the rapid growth of DC energy sources and power loads. As the scale of the system expends, the importance of voltage control will be increased to operate power systems stably. Many studies have been performed on voltage control methods in a DC microgrid, but most of them focused only on a small scale microgrid, such as a building microgrid. Therefore, a new control method is needed for a middle or large scale DC microgrid. This paper analyzes voltage drop problems in a large DC microgrid and proposes a cooperative voltage control scheme with a distributed generator (DG) and a grid connected converter (GCC). For the voltage control with DGs, their location and capacity should be considered for economic operation in the systems. Accordingly, an optimal DG allocation algorithm is proposed to minimize the capacity of a DG for voltage control in DC microgrids. The proposed methods are verified with typical load types by a simulation using MATLAB and PSCAD/EMTDC.

Energy management in DC microgrid with an efficient voltage compensation mechanism

10 - DC microgrids for electric vehicle wireless charging

Over the past decade, the high penetration of renewable-based distributed generation (DG) units has witnessed a considerable rise in electrical networks. In this context, direct current (DC) microgrids based on DGs are being preferred due to having less complexity for the establishment and control. At the same time, they offer higher efficiency and reliability compared to their alternating current (AC) counterparts. This paper proposes a new model predictive control (MPC)-trained artificial neural network (ANN) control strategy being an ANN-MPC instead of conventional cascaded-proportional-integral (PI)-trained ANN control for dynamic damping of photovoltaic (PV)-battery-based grid-connected DC microgrids. Unlike traditional controllers, the proposed control approach more rapidly attains generation-load power balancing under variable climate input (meteorological sensor data) and output (load demand), hence achieving quick DC-bus voltage damping. The proposed ANN-MPC scheme is examined under different operating conditions, and the results are compared with the ANN-based conventional PI controller. The results show the proposed control strategy's efficacy to lessen the instability issues and achieve effective attenuation of oscillations in DC microgrids.

In direct current (DC) microgrids, traditional droop control methods facilitate power sharing and stabilize bus voltage. However, this communication‐less approach is constrained by challenges including insufficient voltage restoration and imbalances in battery state of charge (SOC). This paper introduces a novel communication‐less control strategy designed to address these limitations in photovoltaic energy storage system (PV‐ESS) DC microgrids. This method assigns the energy storage unit (ESU) closest to the critical load as the ‘master unit’ setting its output voltage to nominal levels. This master unit injects an alternating current (AC) signal into the DC bus, with the signal frequency modulated by its SOC and output current. Additional ESUs, referred to as ‘slave units’ adjust their output currents based on the frequency of the AC signal and their local SOC. Furthermore, the master unit utilizes a logical signal based on its SOC to switch the photovoltaic module between droop control mode and maximum power point tracking (MPPT) mode. Stability analysis verifies the robustness of the proposed method, and simulation results demonstrate that it surpasses conventional droop control approaches. Specifically, the proposed method enhances load‐sharing precision, minimizes voltage deviation, and ensures SOC equalization across the DC microgrid without the need for communication links. © 2026 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

DC microgrid small-signal stability and control: Sufficient stability criterion and stabilizer design

Nowadays, cyber-attacks are immoral ways to disturb power grids and make them devitalized or unstable. To avoid such issues, detection and estimation of cyber-attacks are essential in modern power utilities. This paper investigates a new nonlinear Kalman filter, called Spherical Simplex Radial Cubature Kalman Filter (SSRCKF), as a reliable and accurate estimator to detect cyber-attacks in nonlinear Direct Current (DC) microgrids (MGs) with Constant Power Loads (CPLs). The developed nonlinear Kalman filter offers a high resilient State Estimation (SE) performance in presence of noise as well as desired accuracy, and low computational burden. The SSRCKF is modified for the attack detection issue by augmenting the occurring attacks to the original nonlinear dynamics. The developed SSRCKF approach is deployed to estimate the DC MG states in the presence of Denial-of-Service (DoS) and data integrity attacks. Simulation results show that the presented filter estimates the DC MGs states of the system immediately after the attack.

Constant power loads may form a major proportion of the system loads in automotive power systems. Without proper control the presence of constant power loads in a multi-converter environment can cause system instability. In this paper, the concept of autonomous control of a dc microgrid is extended to automotive power systems. An autonomous control law is derived for an automotive power system which consists of system of parallel connected boost converters supplying a constant power load. The control law derived using the passivity based approach consists of two stages. The first, primary stage involves a non-linear droop of the microgrid voltage with respect to the current supplied by each of the converters thereby enabling current sharing. The primary controller also damps the oscillations in the dc microgrid caused by the presence of the constant power load. The secondary controller adjusts for the voltage deviations due to the primary controller without affecting the current sharing. The conditions to ensure the asymptotic stability of the equilibrium points are derived. Experimental results are presented to verify the proposed control law.

Static load which is known for three different characteristics i.e. constant power, constant current and constant impedance shows a different voltage profile because both of their real power and reactive power vary differently as the voltage varies except for constant power. This paper will analyze the impact of each load characteristic individually and also as combination on voltage stability of an IEEE 34 Bus Distribution System by increasing the load demand, and also transient analysis i.e. creating a single phase-to-ground fault i.e. on phase a and balanced three phase fault. The most suitable load to study the voltage stability is also proposed. It was found that, with constant power loads, the reactive power demand increases significantly when load real power and reactive power increases by 25%. Constant current loads and combination loads, on the other hand, the reactive power demand increases but only slightly and the demand is even lesser when with constant impedance loads. However, as the load real power and reactive power increases by 50%, constant power loads causes voltage collapse whilst constant current loads and combination loads turn out to be the next in the order followed by constant impedance loads. In transient analysis, during the single phase fault, with combination loads, phase b voltage increases drastically but only small increment observed in phase c voltage. During the three phase fault, phase b voltage is slightly higher than phase c voltage followed by phase a voltage. As comparison to combination loads, constant power loads causes lesser three phase voltages. After the fault was cleared, a smooth voltage waveform obtained in the period of retaining back to the same pre-fault voltage, however, distortions in the voltage recovering waveform seen when dynamic load included in the system. Constant power load is found to be the suitable load to study the voltage stability.

Direct Current (dc) microgrids due to their efficiency and energy savings are being deployed to provide power for servers in Internet of Things (IoT) data centers, in more electric aircrafts (MEA), electric ships and in rail systems round the word. In this paper, Takagi-Sugeno fuzzy inference method is used to establish a Lyapunov stability candidate for a 380 V ring bus dc microgrid modeled with Matlab. To determine suitability of using powerline communication (PLC) to monitor stability condition on the 380 V dc microgrid, impact of distortion caused by microgrid constant power loads (CPL) on signals transmitted over the dc microgrid PLC channel is examined. It is shown in this paper that while Lyapunov asymptotic stability is maintained on the dc bus, increasing CPL on the microgrid causes the dc microgrid PLC channel to experience growing signal distortion.