1,721,091 research outputs found
Probabilistic Transient Stability Margins Assessment based upon quadratic stability region approximation
No full text
DFIG Capability under Weak Grid Connection and Different Reactive Power Control Modes
No full textDoubly Fed Induction Generators (DFIG) used in Wind Energy Conversion Systems (WECS) differ significantly from Synchronous Generators (SG). In fact, the machine flux rating, the current rating, and the PWM saturation constraints of the grid and rotor-side converters affect the DFIG operation. Furthermore, WECS are frequently installed in remote areas with weak grid connection, so a strong coupling exists between the DFIG voltage and active and reactive power injected into the grid. Therefore, a direct extension of the SG capability curves to DFIGs can lead to unstable or unfeasible operation. This paper presents a comprehensive analysis of the DFIG capability under weak grid connection and for different reactive power control (RPC) and current priority modes. A constrained optimization problem based on the mathematical modeling of the RPC modes, the machine and converters constraints, and the grid limits is formulated and solved under different wind conditions. Numerical analyses for a case study show the superiority of the Voltage Control RPC mode with q-axis current priority for weak grid connections. The proposed analysis can be useful for the design of control strategies that account for the actual limits of the system
Design and control of coupled inductor DC–DC converters for MVDC ship power systems
No full textThis paper deals with the design and control aspects of modern ship power systems within the paradigm of an all-electric ship. The widespread use of power electronic converters is central in this context due to the technological advances in automation systems and the integration of the electrical propulsion systems and other components, such as electrical energy storage systems and renewable energy sources. The issue to address in this scenario is related to the request of increased performances in dynamic operation while pursuing advantages in terms of energy savings and overall system security. In addition, the presence of large load changes requires providing robustness of the control in terms of system stability. This paper is focused on medium voltage direct current (MVDC) ship power systems and the design and control of coupled inductor DC-DC converters. The load is handled in terms of a constant power model, which generally is considered the most critical case for testing the stability of the system. The robustness of the design procedure, which is verified numerically against large and rapid load variations, allowed us to confirm the feasibility and the attractiveness of the design and the control proposal
Caputo derivative applied to very short time photovoltaic power forecasting
No full textIntra-hour photovoltaic power forecasting provides essential information for real time optimal control of microgrids. At this purpose, a critical issue is the selection of the forecasting method. The choice of a forecasting method depends on many factors such as the availability of historical data, the time horizon, the lag period, and the time available for the forecast. Persistence based methods are particularly tailored for real time forecasting which require fast information and are typically a good trade-off choice when dealing with real time operation of microgrids. Their accuracy, however, could be not satisfactory in some cases such as when it appears critical to consider the trend of the power output in the last few minutes rather than only the last measured value. Derivatives help reach this goal, but fractional derivatives seem to be a more accurate choice in order to take into account the history of the variable to be forecasted as they are a promising tool for describing memory phenomena. In this paper, a novel intra-hour forecasting method is proposed based on the Caputo derivative. Numerical applications are carried out to show the efficacy of the proposed approach. Also, the accuracy of the proposed approach is tested through comparison with three models namely, persistence, derivative-persistence and auto regressive moving average models. The strength of the proposed forecasting tool is strictly related to its low computational burden without compromising accuracy. This makes of it an interesting means for real time grid operation strategies and can be of interest for the grid operators especially in vision of the changes distribution grids are witnessing with the transition to the smart grid paradigm
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
