F. Torelli

A Coherency Recognition Based on Structural Decomposition Procedure

The aim of this paper is to present an algorithm for coherency recognition. In the proposed decomposition model, the system structure is explored by analyzing and quantifying the strength of connection among generating units by means of suitable coupling factors. These factors take into account interconnection constraints among dynamic components of the system. In particular, coherency indicators are based on the influence that state variables of generators have on the voltages of all other generators. Test results demonstrated that the coherency recognition obtained with this procedure is independent of fault location and of size of disturbance, since it is based on the very structural characteristics of coupling factors. The practicality of the proposed procedure is demonstrated by numerical examples involving the IEEE 118-bus test system.

The Global Grounding System: Definitions and guidelines

The present paper presents the preliminary results of the ongoing Italian METERGLOB project on the contribution given by the exposed conductive parts to a Global Grounding System. One of the expected results of METERGLOB is to carry out guidelines for the identification of a Global Grounding System. These guidelines must be defined on the basis of the definitions and methods present in the current international standards on grounding and safety. In the paper some definitions and elements to be taken into account for the identification of a Global Grounding System are given.

Current and voltage behaviour during a fault in a HV/MV system: Methods and measurements

When a single line to ground fault happens on the MV side of a HV/MV system, only a small portion of the fault current is injected into the ground by the ground-grid of the faulty substation. In fact the fault current is distributed between grounding electrodes and MV cables sheaths. In systems with isolated neutral or with resonant earthing this may be sufficient to provide safety from electric shock. Experimental measurements were performed on a real MV distribution network: a real single line to ground fault was made and fault currents were measured in the faulty substation and in four neighbouring substations. In this paper the problem of fault current distribution is introduced, the test system is described and the measurements results are presented.

A practical method to test the safety of HV/MV substation Grounding Systems

The adequacy of a Grounding System (GS) to the safety conditions has to be periodically tested by measurements. The test methods and techniques used to verify the electrical characteristics of the GS include the measurements of step and touch voltages. The goal of the test is to verify that touch voltage and step voltage remain below a safe value in all the zones of the installation. The measurements can present some operational difficulties. The purpose of this paper is to present the procedure, step-by-step, of a practical method of measuring touch/step voltages in grounding systems located in urban or industrial areas with reduced accessibility. The suggested method uses auxiliary current electrodes located at short distances. This paper demonstrates by test measurements done in a real case that the method provides conservative results.

Influence of LV neutral grounding on global Earthing Systems

International Standards define a Global Earthing System as an earthing net created interconnecting local Earthing Systems (generally through the shield of MV cables and/or bare buried conductors). In Italy, the regulatory authority for electricity and gas requires distributors to guarantee the electrical continuity of LV neutral conductor. This requirement has led to the standard practice of realizing “reinforcement groundings” along the LV neutral conductor path and at users’ delivery cabinet. Moreover, in urban high-load scenarios (prime candidates to be part of a Global Earthing System), it is common that LV distribution scheme creates, through neutral conductors, an effective connection between grounding systems of MV/LV substations, modifying Global Earthing System consistency. The aim of this paper is to evaluate the effect, in terms of electrical safety, of the aforementioned LV neutral distribution scheme when an MV-side fault to ground occurs. For this purpose, simulations are carried out on a realistic urban test case and suitable evaluation indexes are proposed.

A generalized computing paradigm based on artificial dynamic models for mathematical programming

In this paper a novel computing paradigm aimed at solving non linear systems of equations and finding feasible solutions and local optima to scalar and multi objective optimizations problems is conceptualized. The underlying principle is to formulate a generic programming problem by a proper set of ordinary differential equations, whose equilibrium points correspond to the problem solutions. Starting from the Lyapunov theory, we will demonstrate that this artificial dynamic system could be designed to be stable with an exponential asymptotic convergence to equilibrium points. This important feature allows the analyst to overcome some of the inherent limitations of the traditional iterative solution algorithms that can fail to converge due to the highly nonlinearities of the first-order conditions. Besides we will demonstrate as the proposed paradigm could be applied to solve non linear equations systems, scalar and multi-objective optimization problems. Extensive numerical studies aimed at assessing the effectiveness of the proposed computing paradigm are presented and discussed.

An extension of feedback-feedforward technique to input-output feedback linearization

Large scale systems, such as interconnected power systems, need to be described in a more accurate form than in the past allowing the overexploitation of system resources. Not negligible nonlinear phenomena increase computational complexity especially for what control system design concerns. Classical control design, based on linearization techniques has become no more suitable to take into account strong nonlinear effects and unmodeled dynamics, while input-output feedback linearization shows computational difficulties as the order of the system to be linearized increases. In this paper the classical feedback-feedforward technique for linear systems has been applied to a particular cascade connection of an input-output linearized system feeded by a linear system. The proposed approach achieves perfect tracking for the linearized model and an overall reduction in computational effort.

An optimal strategy for switching devices allocation in radial distribution network

Modern life strongly depends on the continuous supply of electrical energy, increasing interests on electrical operators and customers to power quality and service reliability issues. Emergency planning techniques need to be developed in order to satisfy European guidelines and standards on power quality levels. Restoring power supply is a complicated problem to solve especially when faults on radial distribution systems occur and, consequently, long interruptions take place. Automatic switching devices greatly improve service reliability in electrical systems where an optimal device placement problem has been solved. An optimal device allocation strategy has been implemented in order to obtain a trade-off solution between technical performances and economical investments. Test results are provided for an actual distribution system demonstrating the methodology effectiveness.

Estimation of power system dominant modes

The aim of this paper is to propose an approach to estimate the nature (damping and frequency) of a dominant mode. The suggested method, based on Prony analysis, aims to directly evaluate dominant modes of electromechanical oscillations. The basic idea is to fit the given oscillation with a fitting function having one mode only whose parameters need to be identified by any identification process. When the transient of the given signal vanishes, modes having high damping attenuate reaching a quasi linear behavior and revealing the dominant mode only. In this sense, the fitting function gains characteristic parameters (damping and frequency) of the dominant mode. For the identification process we suggest a nonlinear analysis approach based on the Lyapunov method applied to the Sensitivity theory. The proposed methodology is tested on the New England 39-bus system.

Nodal unbundling of transmission losses

This paper presents a new methodology to evaluate and charge system losses shares among market participants on a nodal basis. The approach is based on the assumption that all power injections take place simultaneously, giving rise to loss responsibilities on an a posteriori basis. This requirement enables the method to be used in a clearing house for losses. The formulation utilizes a static power system representation expressed in terms of a nodal admittance matrix in conjunction with a classic power flow. With this formulation, it is possible to derive system losses, evaluated at the slack bus, as a sum of partial contributions due to each injection. Results are reported for two test systems proving that the methodology is able to share system losses in an attractive way thanks to its simplicity and its transparent and equitable basis.