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Wind energy conversion systems (WECSs), based on permanent magnet synchronous generators (PMSGs), are becoming common sources in dc grids. However, in previous dc grids integration studies, turbine-generator mechanical dynamics are represented by a single-mass model. A practical direct-drive connection in a PMSG-WECS yields lightly-damped torsional speed oscillations because of the double-mass mechanical nature of the generator and the wind turbine. Active damping strategies are usually employed to suppress the mechanical oscillations in a full back-to-back converter interfacing PMSG-WECSs into ac grids; nevertheless, the active damper performance in dc grids is unknown, particularly under dc grid uncertainties and, more importantly, the presence of dynamic and constant power loads commonly used in dc grids. To fill out this gap, this paper presents a detailed modeling and comprehensive stability assessment of a dc grid with a high penetration level of wind power generation. Moreover, stability enhancement strategies are proposed to increase the damping of the entire system, considering different operating and installation scenarios that might face a system integrator/designer. Time-domain simulation studies, based on nonlinear models, are conducted to validate the analytical results. Furthermore, hardware-in-loop real-time simulation studies demonstrate the feasibility of hardware implementation.

Nowadays, grid-connected active dc distribution systems are gaining widespread acceptance due to the remarkable development of the dc technology, high penetration levels of dc loads, and the market availability of dc-based distributed generators. One of the main features of dc systems is the elimination of multiple conversion stages required for variable frequency ac loads, such as variable-speed drive applications; therefore, dc distribution systems are considered as an efficient and cost-effective choice for supplying such dynamic loads. Induction motors (IMs) equipped with open-loop constant voltage/frequency $(V/f)$ variable speed drives are considered as the workforce for many industrial loads due to their simplicity and satisfactory dynamic performance. However, $V/f$ IM drives exhibit poor stability dynamics, particularly, at low-speed operation, which might negatively interact with the dc distribution system, leading to further stability degradation. Therefore, this paper investigates the interaction dynamics and the performance of a grid-connected dc distribution system with a high penetration level of dynamic loads. A detailed small-signal model of the entire system is developed to characterize the overall system stability margins with the help of the eigenvalues and impedance-based analysis. Moreover, the uncertainties affecting the marginal stability such as motor operating speed, dc feeder length, and bus capacitance, are thoroughly addressed. It has been found that the dynamic loads in grid-connected dc distribution systems would exhibit instability issues due to various dynamic interactions; therefore, two different stabilizing compensation methods are proposed to mitigate the associating instability issues and enhance the system damping capability. Detailed time-domain non-linear simulations and experimental results are presented to validate the analytical results. [ABSTRACT FROM AUTHOR]

Active dc distribution systems are gaining widespread acceptance in modern power distribution grids. Islanding detection is very crucial for safety and protection purposes in active distribution systems; therefore, distributed generators (DGs) are usually equipped with active islanding detection methods to detect grid disconnection conditions. The high penetration level of tightly regulated converters to interface both DGs and loads and the poorly damped LC networks structured by the filtering inductors, feeder impedances, and bus capacitors can cause severe stability problems. This paper presents an impedance-based analysis of a grid-connected dc active distribution system, where DGs equipped with active positive feedback islanding detection schemes and a high penetration level of constant power loads (CPLs) are considered. The output impedance of a DG equipped with active islanding detection schemes is derived, and the interactions of the system impedances are discussed to characterize the dynamics of the dc distribution system. Moreover, the performance of multiple DG systems with the islanding detection schemes is investigated and thoroughly addressed. A simple, yet effective, stabilization method is also developed. Detailed time-domain nonlinear simulations and experimental results validate the analytical results. [ABSTRACT FROM AUTHOR]

The direct current (DC) technology has gained significant momentum in modern distribution systems due to the high penetration levels of dc loads and dc-based distributed generators (DGs). Unlike conventional ac distribution systems, dc distribution systems have the following distinct features that challenge the system stability. 1) The high penetration level of tightly regulated converters used to interface both DGs and loads yields a destabilizing constant power load (CPL) effect in a considerable range of frequencies. 2) The filtering inductors and capacitors form poorly damped LC networks that interact negatively with the CPLs leading to further deterioration of the system stability. 3) Because islanding in a dc system can be hardly detected with passive methods due the absence of the frequency and reactive power terms, DGs are usually equipped with active islanding detection methods to detect the grid disconnection state; however, the islanding detection schemes could negatively impact the distribution system stability. The analysis and mitigation of undesirable interaction dynamics in a dc distribution system considering the aforementioned practical characteristics are not reported in the current literature. In this paper, the interaction dynamics of a dc distribution system characterized by a high penetration level of CPLs, and DGs equipped with positive feedback islanding detection scheme are investigated. The factors affecting the system stability with a single and multiple DGs are thoroughly addressed. Further, a stabilizing compensation loop is proposed to mitigate the stability problems and poor damping capability. Detailed time-domain nonlinear simulations and experimental results validate the analytical results. [ABSTRACT FROM PUBLISHER]

Due to the high penetration level of dc-based distributed generators (DGs) and dc loads, dc distribution systems are gaining widespread acceptance in modern power grids. Therefore, dc distribution systems are expected to operate parallel to the existing ac ones. However, the techniques of islanding detection in dc grids have not been fully studied in the current literature. This paper presents a detailed analysis, performance comparison, and design guidelines of four different positive feedback islanding detection methods in dc distribution systems. In each method, the range of control parameters that guarantee system stability is analytically obtained. The effects of system parameters, such as the dc system resistance and inductance, DG filter capacitance, and local load resistance, on each islanding detection method, are thoroughly addressed. Furthermore, the interactions between DGs connected at different locations of the distribution feeder and equipped with positive feedback islanding detection methods are studied and characterized. Detailed time-domain nonlinear simulations and experimental results validate the analytical results. [ABSTRACT FROM AUTHOR]