• Energy Research

Abstract This paper focuses on the performance analysis of the corrosion protection system of HV power cables for undersea applications. The aim of the work is to describe the relations between the main electrical parameters affecting the electric potential distribution in the cable layers and in the surrounding medium, when an active cathodic protection (CP) arrangement is used. In particular, the analysis focuses on the design of an active CP for reducing corrosion phenomena of the cable metallic shield. This study is based on a 2D analytical approach supported by a detailed 3D finite element analysis (FEM). It gives evidence to the relation between the electrical variables and the design criteria of the active cathodic protection system.

The modern electric power system operates with restricted stability margins, A new approach to on-line optimal dispatching, considering a global transient stability constraint, is proposed. The transient stability constraint is formulated in a probabilistic frame using the Lyapunov direct method. The approach leads to a constrained optimisation problem and the Chua neural network is used for problem solution. A numerical application to the New England test network is shown in the final part of the paper.

Intra-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.

Harmonic bursts are short-duration waveform distortions which result from the sudden variations in nonlinear loads; they often cause damage to common power system components. A probabilistic model is proposed that describes these disturbances in terms of suitable stochastic processes. The use of this model results in indices that can be used to characterise the system state. The indices are based on appropriate mean values of random functions which measure the quality degradation in terms of the occurrence of disturbance levels higher than a fixed value. They are particularly suited to the characterisation of the risk related to the severe burst occurrences and can allow a suitable prediction of system behaviour over time. A numerical application is presented to illustrate the physical meaning of these indices. The key feature of the proposed model is that it can be used for both burst amplitude and duration characterisation.

The paper deals with a novel method to measure inter-area oscillations, i.e., electromechanical oscillations involving groups of generators geographically distant from one another and ranging within the frequency interval from 0.1 Hz up to 1 Hz. The estimation of the parameters characterizing inter-area oscillations is a crucial objective in order to take the necessary actions to avoid the instability of the transmission electrical system. The proposed approach is a signal-based method, which uses samples of electrical signals acquired by the phasor measurement unit (PMU) and processes them to extract the individual oscillations and, for each of them, determine their characteristic parameters such as frequency and damping. The method is based on Hilbert transformations, but it is optimized through further algorithms aiming at (i) improving the ability to separate different oscillatory components, even at frequencies very close to each other, (ii) enhancing the accuracy associated with the damping estimates of each oscillation, and (iii) increasing the robustness to the noise affecting the acquired signal. Results obtained in the presence of signals involving the composition of two oscillations, whose damping and frequency have been varied, are presented. Tests were conducted with signals either synthesized in simulated experiment or generated and acquired with actual laboratory instrumentation.

The present paper proposes an optimal tuning of Power System Stabilizer (PSS) parameters for synchronous generators. Many approaches have been proposed in the relevant literature which gave significant contribution for the most convenient choice for the parameters of the aforementioned control systems. It is difficult to recognize the superiority of a certain approach above the other ones, but some deficiencies can be observed especially when dealing with very large multi-machine systems. In this case the inadequacy of the methodology developed with reference to the machine connected to the infinite power network emerges. In this paper by using the modified Heffron-Phillips model, an optimal control strategy is adopted by locating the eigenvalues in a convenient circle. The linearized model allows to properly handle the variations of the network parameters, which are described as disturbances. The optimal control strategy is combined with the structural constraints of actual power system stabilizers. The effectiveness of the proposed approach is tested by verifying the damping action in the face of variations in both the amplitude and the phase of the secondary bus voltage of the step-up transformer.

This paper focuses on very-short-term photovoltaic power forecasting based solely on power measurement. The approach proposed in the paper is tailored for real time operation of smart grids and microgrids, that deals with energy management and control of the various resources of the grid and, particularly, on those based on renewable energy sources. These resources are characterized by variable power production, thus constituting a challenge for the effectiveness of strategies and tools of management and control. This paper refers to photovoltaic power production and presents a new real time forecasting method which improves the classical persistence model. The proposed method is based on the idea of conveniently weighting information on past data by imposing continuity of the function, of its first derivative and of the second derivative so obtaining three estimates of the function in the forecast interval which are then opportunely weighted to provide the forecast. The effectiveness of the proposed approach is shown in the numerical application based on actual measured data. The accuracy of the proposed approach is also tested through comparisons with two models based on the persistence and auto regressive moving average models.

The paper proposes an analytical approach to evaluate the effects of series compensation of an overhead transmission line (OHL) concerning the increase of (1) loadability and (2) practical loading of the line. The first part of the paper focuses only on the line characteristics, and proposes an analytical approach to determine the loadability curves of the line as a function of the compensating capacitive reactance. In the second part of the paper, the perspective shifts to examine the power loading of the selected OHL in a meshed electrical network. The target is to evaluate the compensation degree required to achieve a desired power flow increase in the line. To this purpose a fast analytical method, based on the compensation theorem, is presented. Preserving the linearity of the network model, the proposed method allows computation of the needed compensation degree by means of a closed form equation. Numerical applications performed on two simple test networks allow to compare the method with an ad hoc implemented classical optimal power flow. The results validate the method and highlight its effectiveness as a simple and useful tool to support power system planners and designers.