Mario Paolone

Short-Term Scheduling and Control of Active Distribution Systems With High Penetration of Renewable Resources

Among the innovative contributions to electric distribution systems, one of the most promising and qualified is the possibility to manage and control distributed generation. Therefore, the latest distribution management systems tend to incorporate optimization functions for the short-term scheduling of the various energy and control resources available in the network (e.g., embedded generators, reactive power compensators and transformers equipped with on-load tap changers). The short-term scheduling procedure adopted in the paper is composed by two stages: a day-ahead scheduler for the optimization of distributed resources production during the following day, an intra-day scheduler that every 15 min adjusts the scheduling in order to take into account the operation requirements and constraints of the distribution network. The intra-day scheduler solves a non-linear multi-objective optimization problem by iteratively applying a mixed-integer linear programming (MILP) algorithm. The linearization of the optimization function and the constraints is achieved by the use of sensitivity coefficients obtained from the results of a three-phase power flow calculation. The paper shows the application of the proposed approach to a medium-voltage 120 buses network with five wind plants, one photovoltaic field, ten dispatchable generators, and two transformers equipped with on-load tap changers.

Continuous-Wavelet Transform for Fault Location in Distribution Power Networks: Definition of Mother Wavelets Inferred From Fault Originated Transients

The paper presents a fault location procedure for distribution networks based on the wavelet analysis of the fault-generated traveling waves. In particular, the proposed procedure implements the continuous wavelet analysis applied to the voltage waveforms recorded during the fault in correspondence of a network bus. In order to improve the wavelet analysis, an algorithm is proposed to build specific mother wavelets inferred from the fault-originated transient waveforms. The performance of the proposed algorithm are analyzed for the case of the IEEE 34-bus test distribution network and compared with those achieved by using the more traditional Morlet mother wavelet.

Mitigation of lightning-induced overvoltages in medium Voltage distribution lines by means of periodical grounding of shielding wires and of surge arresters: modeling and experimental validation

In this paper, we investigate the effect of periodically-grounded shielding wires and surge arresters on the attenuation of lightning-induced voltages. We discuss the adequacy of the commonly made simplification of assuming the shielding wire at ground potential, instead of being treated as one of the conductors of the multiconductor system. We also compare then the mitigation effect of shielding wires with that achievable by the insertion of surge arresters along the line. The computation results are first validated by means of calculations obtained by other authors referring to a simple line configuration, and then by means of experimental results obtained using a reduced-scale line model illuminated by a nuclear electromagnetic pulse (NEMP) simulator. One of the main conclusions is that the effectiveness of shielding wires and surge arresters depends mostly on the spacing between two adjacent grounding points or surge arresters.

Optimal Allocation of Dispersed Energy Storage Systems in Active Distribution Networks for Energy Balance and Grid Support

Dispersed storage systems (DSSs) can represent an important near-term solution for supporting the operation and control of active distribution networks (ADNs). Indeed, they have the capability to support ADNs by providing ancillary services in addition to energy balance capabilities. Within this context, this paper focuses on the optimal allocation of DSSs in ADNs by defining a multi-objective optimization problem aiming at finding the optimal trade-off between technical and economical goals. In particular, the proposed procedure accounts for: network voltage deviations; feeders/lines congestions; network losses; cost of supplying loads (from external grid or local producers) together with the cost of DSS investment/maintenance; load curtailment; and stochasticity of loads and renewables productions. The DSSs are suitably modeled to consider their ability to support the network by both active and reactive powers. A convex formulation of ac optimal power flow problem is used to define a mixed integer second-order cone programming problem to optimally site and size the DSSs in the network. A test case referring to IEEE 34 bus distribution test feeder is used to demonstrate and discuss the effectiveness of the proposed methodology.

An Improved Procedure for the Assessment of Overhead Line Indirect Lightning Performance and Its Comparison with the IEEE Std. 1410 Method

This paper deals with the assessment of the lightning performance of distribution lines, namely the estimation of the annual number of lightning-induced flashovers versus the critical flashover voltage of the line insulators. The procedure proposed by the authors is compared with the one described in IEEE Std. 1410-2004 Guide for improving the lightning performance of electric power overhead distribution lines. The two methods differ: 1) for the models adopted to evaluate the induced voltages and 2) for the adopted statistical approach. The reasons for differences in the results predicted by the two methods are discussed and the parameters playing the major role in the achievement of the results are identified. The proposed method represents an improvement compared to IEEE Std. 1410 because it takes into account more realistic line configurations and the effect of the finite ground conductivity

Enhanced Interpolated-DFT for Synchrophasor Estimation in FPGAs: Theory, Implementation, and Validation of a PMU Prototype

The literature on the subject of synchrophasor estimation (SE) algorithms has discussed the use of interpolated discrete Fourier transform (IpDFT) as an approach capable to find an optimal tradeoff between SE accuracy, response time, and computational complexity. Within this category of algorithms, this paper proposes three contributions: the formulation of an enhanced-IpDFT (e-IpDFT) algorithm that iteratively compensates the effects of the spectral interference produced by the negative image of the main spectrum tone; the assessment of the influence of the e-IpDFT parameters on the SE accuracy; and the discussion of the deployment of IpDFT-based SE algorithms into field programmable gate arrays, with particular reference to the compensation of the error introduced by the free-running clock of A/D converters with respect to the global positioning system (GPS) time reference. The paper finally presents the experimental validation of the proposed approach where the e-IpDFT performances are compared with those of a classical IpDFT approach and to the accuracy requirements of both P and M-class phasor measurement units defined in the IEEE Std. C37.118-2011.

Integrated Use of Time-Frequency Wavelet Decompositions for Fault Location in Distribution Networks: Theory and Experimental Validation

The paper presents a procedure for fault location in distribution networks, based on the use of the integrated time-frequency wavelet decompositions of the voltage transients associated with the fault-originated travelling waves. The proposed analysis of time-frequency wavelet decompositions has been found to improve the identification accuracy of the frequencies associated to the characteristic patterns of a fault location with respect to a sole frequency-domain wavelet analysis. Several laboratory fault tests, carried out by means of a reduced-scale model of a distribution feeder, are used to illustrate the characteristics and assess the performances of the proposed improved procedure. The paper also illustrates the application of the proposed procedure to a transient, originated by a permanent phase-to-phase fault, measured in a real distribution network in which a post-test analysis has identified the faulted branch.

Synchronized Phasors Monitoring During the Islanding Maneuver of an Active Distribution Network

The paper describes the performances of a phasor measurement unit (PMU) prototype based on a synchrophasor estimation algorithm conceived for the monitoring of active distribution networks, as well as its experimental application during some intentional islanding and reconnection tests of an urban medium voltage power network. With respect to typical applications in transmission networks, the use of PMUs in distribution networks requires very low values of total vector error (TVE), which involves particular low values of phase errors of the synchrophasor estimates. These requirements are met by a specifically developed PMU, the characteristics and experimental characterization of which are illustrated in the paper. Three of these PMUs have been then used to monitor experimental tests carried out to assess the capability of a urban distribution network to operate autonomously when fed by a local 80 MW combined-cycle power plant. The information provided by the installed PMUs significantly facilitate the operator maneuvers and appear to be useful for the development of an improved control and management system of the active distribution network.

Lightning induced disturbances in buried Cables-part I: theory

In this paper, we present a review of theoretical methods to compute lightning induced currents and voltages on buried cables. The evaluation of such induced disturbances requires the calculation of the electric field produced by lightning along the cable path. We show that the Coorays simplified formula is capable of predicting accurately the horizontal electric field penetrating the ground, at distances as close as 100 m. Regarding the parameters of the buried cable, a comparison of several approximations of the ground impedance is presented. We show that the Pollaczek expression corresponds to the Sunde general expression, when the displacement current is neglected. The analysis shows also that all the proposed approximations provide very similar results for the considered range of frequencies (up to 30 MHz). Most of the approximate formulas neglect the contribution of the displacement current and, therefore, predict values for the ground impedance which tend to infinity at higher frequencies. This corresponds in the time domain to a singularity of the ground transient resistance at t=0. By analogy to the Sunde approximation for the ground impedance of overhead lines, we propose a logarithmic approximation for the ground impedance of a buried cable. In addition, unlike most of the considered approximations, the proposed formula has an asymptotic behavior at high frequencies; therefore, the corresponding transient ground resistance in the time domain has no singularity at t=0. It is also demonstrated that within the frequency range of interest, the wire impedance can be neglected, due to its small contribution to the overall longitudinal impedance of the line. The ground admittance, however, can play an important role at high frequencies (1 MHz or so) especially in the case of poor ground conductivity. The ground admittance needs to be taken into account in the calculation of lightning induced currents and voltages on buried cables. This is in contrast with the case of overhead lines in which its contribution is generally negligible, even in the MHz range. We also investigate the time-domain representation of field-to-transmission line coupling equations. The coupling model includes the effect of ground admittance which appears in terms of an additional convolution integral. An analytical expression for the ground transient resistance in the time domain is also proposed which is shown to be sufficiently accurate and nonsingular. Finally, we present a time domain solution of field-to-buried cable coupling equations using the point-centered finite difference time domain (FDTD) method, and a frequency domain solution using Greens functions. In our companion paper (Part II), we compare both solutions to experimental waveforms obtained using triggered lightning.

A Microcontroller-Based Power Management System for Standalone Microgrids With Hybrid Power Supply

The paper presents a microcontroller-based power management system (PMS) designed for the online operation of an experimental low voltage microgrid equipped with a battery storage system and two power supplies: a kilowatt (kW)-class proton exchange membrane (PEM) fuel cell (FC) and a photo-voltaic (PV) module emulator, both connected to a low voltage ac node. The connections of the energy sources to the common ac bus make use of power inverters with specific functionalities. The ac node feeds electric active and reactive load emulators able to reproduce programmable profiles. The automatic PMS provides the microgrid monitoring and the FC power scheduling in both grid-connected and islanded operating conditions. The paper describes the structure and functionalities of the PMS as well as a specific experimental investigation aimed at assessing the dynamic performance of the microgrid in islanded conditions.