This paper focuses on the islanding transition of an AC microgrid equipped with a doubly-fed induction generator wind turbine, a photovoltaic unit, a battery energy storage system (BESS) and local loads. It is well known that the islanding transition put at risk system stability, especially for the systems with low inertia. This paper has explored the combined contribution of virtual inertia support schemes applied to the BESS and to the DC-link capacitor of the back-to-back inverter of the wind power generator in order to ensure a smooth islanding transition by preserving the microgrid stability. For this purpose, we analyse the frequency/inertia support provided by two virtual inertia control schemes: i) a virtual synchronous generator technique on the BESS and ii) a DC-link capacitor control. The analysis is carried out by using time domain simulations in the DIgSILENT PowerFactory environment.

This paper deals with the protection against earth faults in medium voltage meshed networks with unearthed neutral. A communication-supported protection system is proposed and assessed through numerical simulations. The protection system is based on the use of directional relays installed at both ends of each line of the network. Each relay communicates by sending permissive signals following the so-called directional comparison overreaching transfer trip scheme. Each directional relay implements a new transient fault detection algorithm based on the angle between the zero-sequence voltage and current phasors. The phasors are estimated at the dominant frequency of the network transient-response within the first milliseconds after the fault, which range can be evaluated by means of a simplified circuit equivalent representation. The performances of the protection system and the detection algorithm are evaluated for different network configurations. By using a specifically developed co-simulation tool that interfaces EMTP-RV with Matlab, the influence of the communication latency is also analyzed. A Monte Carlo method is applied to analyze the effects of fault resistance, incidence angle and fault location variations. Results of a real-time simulation obtained in the Opal-RT environment further support the applicability of the algorithm.

The state estimation of distribution networks has long been considered a challenging task for the reduced availability of real-time measures with respect to the transmission network case. This issue is expected to be improved by the deployment of modern smart meters that can be polled at relatively short time intervals. On the other hand, the management of the information coming from many heterogeneous meters still poses major issues. If low-voltage distribution systems are of interest, a three-phase formulation should be employed for the state estimation due to the typical load imbalance. Moreover, smart meter data may not be perfectly synchronized. This paper presents the implementation of a three-phase state estimation algorithm of a real portion of a low-voltage distribution network with distributed generation equipped with smart meters. The paper compares the typical state estimation algorithm that implements the weighted least squares method with an algorithm based on an iterated Kalman filter. The influence of nonsynchronicity of measurements and of delays in communication and processing is analyzed for both approaches.

Virtual Power Plants (VPPs) represent an aggregation of distributed generators and loads that are capable of following power references sent by grid operators. To guarantee system stability, grid operators need to verify the impact of VPPs on the small-signal stability of electrical transmission systems. Since dynamic models of VPPs have a large number of states, the application of model order reduction (MOR) techniques should be considered for stability analyses. In this paper, the influence of VPPs on the small-signal stability of electrical transmission systems is evaluated. For that purpose, VPPs are modelled in detail, including the dynamics of renewable sources such as wind and solar. Then, the models are reduced by using two MOR techniques, namely state relevance and balanced truncation. The VPPs are connected to the benchmark two-area power system model and their impact on the small-signal stability assessed. The results show that renewable energy-based VPPs have a significant impact on the system eigenvalues and that they provide additional damping to the system. It is shown that the low-frequency dynamic properties can be preserved using low-order reduced models, while higher-order ones are needed for the medium-frequency dynamics. Eigenvalue analysis is used to validate the theoretical developments.

To address the decreasing power system inertia challenges due to the increasing penetration of Renewable Energy Sources (RESs), this paper proposes a Fast Frequency Response (FFR) technique for wind turbines equipped with Doubly Fed Induction Generators (DFIG). The DC-link of a DFIG unit can be controlled to partially exchange energy in case of frequency transients, thus providing FFR that in turn helps maintain grid stability. This control is achieved by levelling up or down the DC-link voltage. Supercapacitor units connected to the DC-link provide the required energy for FFR services. We perform a detailed analysis to achieve the expressions of the minimum DC-link voltage required for the proper operation of the grid side converter and the factors affecting its value. We then propose a procedure to estimate the optimal value for FFR controller gain and a DC-link protection scheme that decouples the FFR controller design from the grid characteristics. The method can be applied to any RES using converter with sufficient energy buffer in order to provide FFR service.