Rui Castro

A Comparison Between Chronological and Probabilistic Methods to Estimate Wind Power Capacity Credit

In this paper, the issue of the capacity credit provided by wind energy conversion systems (WECS) is addressed. A chronological method of post-evaluation of the capacity credit is presented and compared with a pre-evaluation probabilistic method. The proposed chronological approach is based on the computation of the WECS capacity factor (ratio between average and total output) over some relevant time period. An appropriate choice of the time interval-for instance, the peak load hours-will lead to a closer approximation of the capacity credit. The comparison is illustrated with two case studies, concerning the Portuguese electric system. The analysis of the theoretical background of both methods and the results obtained allow the conclusion that chronological methods are best designed to assist system operators, whereas probabilistic methods, which are developed within the logic of the public system avoided resources, are a helping tool for system planners.

A wind park reduced-order model using singular perturbations theory

Due to the increasing penetration of wind energy, it is necessary to possess design tools that are able to simulate the impact of these installations on electric utility grids. In order to provide a net contribution to this issue, a detailed wind park model has been developed and is briefly presented. However, the computational costs associated with the performance of such a detailed model in describing the behavior of a wind park composed by a considerable number of units may render its practical application very difficult. To overcome this problem, singular perturbations theory has been applied to reduce the order of the detailed wind park model, and therefore create the conditions for the development of a dynamic equivalent which is able to retain the relevant dynamics with respect to the existing AC power system. Validation results proving the ability of singular perturbations reduced order model to match the results provided by the detailed model are shown and commented upon.

Assessment and Optimization of Wind Energy Integration Into the Power Systems: Application to the Portuguese System

The increasing integration of wind energy in power systems can be responsible for the occurrence of over-generation, especially during the off-peak periods. This paper presents a dedicated methodology to identify and quantify the occurrence of this over-generation and to evaluate some of the solutions that can be adopted to mitigate this problem. The methodology is applied to the Portuguese power system, in which the wind energy is expected to represent more than 25% of the installed capacity in a near future. The results show that the pumped-hydro units will not provide enough energy storage capacity and, therefore, wind curtailments are expected to occur in the Portuguese system. Additional energy storage devices can be implemented to offset the wind energy curtailments. However, the investment analysis performed show that they are not economically viable, due to the present high capital costs involved.

Wavelets pre-filtering in wind speed prediction

Wind power is the fastest growing renewable energy technology and is becoming a significant component of the energy mix. The secure and reliable operation of the power system implies the need for scheduling in advance the energy sources that will produce, so that the power system is balanced. Therefore, the use and importance of the wind power is strictly dependent on the ability to predict the wind in advance. In this paper, ARMA models are used to forecast the wind speed in terms of a medium-term prediction. Furthermore, an investigation on the benefits of pre-filtering the wind speed time series using wavelets is carried out. Some simulations are done with the twofold purpose of evaluating the performance of ARMA models as compared with reference models and investigating whether the wavelet pre-filtering technique leads to an improvement of the forecast results.

Validation of Photovoltaic Electrical Models against Manufacturers Data and Experimental Results

This paper presents two models that describe the electrical behavior of photovoltaic modules. One of them is an easy to compute three parameters model, with a simplified power output calculation algorithm, whereas the other is a more detailed four parameters model. Both models take as input only readily available manufacturers data. They are validated against the catalogue I-V temperature and irradiation curves. Furthermore, a performance comparison with experimental results is presented too. Obtained outcome allows the conclusion that both models simulation results are quite satisfactory, thus providing a way to prove the accuracy of the three parameters model.

Evaluation of energy storage devices for renewable energies integration: Application to a Portuguese wind farm

This paper evaluates the benefits of energy storage systems applied to renewable intermittent sources like wind. With this objective, a 144 MW Portuguese wind farm is used as a case-study. The storage system design and evaluation processes use a computational tool developed in MatLab to simulate the wind farm operation with an energy storage system and computes the remuneration tariff. The energy storage design settles on a system of 5 MW with 30 MWh. The simulation results show a 2.1% gain in wind farm tariff and a improved adjustment on generation to the typical electricity demand curve. Considering the storage system design and an available storage technology survey, one concludes that redox flow batteries are the best suited to application, in spite of their present prices being not the most advantage.

RenH 2 - Stand-Alone Energy System Supported by Totally Renewable Hydrogen Production

This paper describes a Stand-Alone Energy System Supported by Totally Renewable Hydrogen Production. The system was conceived for off-grid operation and is composed by solar panels, a wind turbine, a fuel-cell, an electrolyzer, hydrogen tanks and power electronics converters. The basic control strategy for the overall system considers the pressurized hydrogen gas storage as the energy buffer. The basic logic is that the exceeding renewable energy (solar and wind) is used to accumulate hydrogen, while the fuel cell uses this hydrogen to produce electrical energy when there is insufficient solar/wind energy.

Contribution of energy storage systems for power generation and demand balancing with increasing integration of renewable sources: application to the Portuguese power system

Increasing integration of renewable sources introduce some new issues on the operation of the power system, such as potential unbalancing between generation and demand, especially in off-peak periods with high levels of hydro generation. This scenario can get worse when associated with the installation of a nuclear powerplant, which operates on a must run basis. This paper analyses the contribution of energy storage systems use to avoid future power generation and demand unbalance in the Portuguese power system. Moreover it is also presented an overview of the available energy storage technologies and the role of power electronics on its appropriate operation.

Modeling the stochastic dependencies in a probabilistic load flow including wind generation

Nowadays, the implementation of environmentally-friendly and uncontrollable primary energy sources in the electrical power system production is increasing, introducing abatement in generation dispatchability. The incorporation of high levels of small-scale non-dispatchable distributed generation leads to the transition from the traditional vertical power system structure to a horizontally-operated power system, where the distribution networks contain both stochastic generation and load. This leads to a large number of stochastic system inputs and complex dependencies between them being involved in the analysis. In this paper, a Monte Carlo Simulation approach is used for the power system multivariate uncertainty analysis problem. The Stochastic Bounds Methodology is applied in order to tackle this problem through the definition of extreme dependence structures that can bound all real cases. On the other hand, the Joint Normal Transform models the exact dependence structure between the clusters based on the mutual correlations obtained by data analysis and provide more accurate results.

Power Flow Analysis of HVAC and HVDC Transmission Systems for Offshore Wind Parks

As the onshore wind resource is running shorter, wind power promoters are paying attention to the offshore resources. As in most cases there is no load offshore, wind power must be transmitted to the main land. To do so, two options are available: HVAC and HVDC transmission systems. In this paper, the two options are analysed from a power flow point of view. The influence in the voltage regulation of the onshore connection busbar is investigated in a 57 bus test power system. The simulation results obtained for each one of the above mentioned transmission system configurations allow the conclusion that HVAC solution is limited by the distance to shore and by the wind transmitted power. HVDC options do not show these limitations, but are more expensive and more delicate to deal with, because there is a lack of operational experience, so far.