Angela Russo

Distributed generation siting and sizing under uncertainty

The necessity for flexible electric systems, changing regulatory and economic scenarios, energy savings and environmental impact are providing impetus to the development of distributed generation, which is predicted to play an increasing role in the future electric power system; with so much new distributed generation (DG) being installed, it is critical that the power system impacts be assessed accurately so that DG can be applied in a manner that avoids causing degradation of power quality, reliability and control of the utility system. Considering that uncertainties on DG power production are very relevant and different scenarios have to be taken into consideration, traditional deterministic planning techniques should not be used to take the right decisions. In this paper a three step procedure, based on genetic algorithms and decision theory, is applied to establish the best distributed generation siting and sizing on an MV distribution network, considering technical constraints, like feeder capacity limits, feeder voltage profile and three-phase short circuit currents in the network nodes. In the last part of the paper the procedure is applied to an actual distribution network.

Risk Assessment of Malicious Attacks Against Power Systems

The new scenarios of malicious attack prompt for their deeper consideration and mainly when critical systems are at stake. In this framework, infrastructural systems, including power systems, represent a possible target due to the huge impact they can have on society. Malicious attacks are different in their nature from other more traditional cause of threats to power system, since they embed a strategic interaction between the attacker and the defender (characteristics that cannot be found in natural events or systemic failures). This difference has not been systematically analyzed by the existent literature. In this respect, new approaches and tools are needed. This paper presents a mixed-strategy game-theory model able to capture the strategic interactions between malicious agents that may be willing to attack power systems and the system operators, with its related bodies, that are in charge of defending them. At the game equilibrium, the different strategies of the two players, in terms of attacking/protecting the critical elements of the systems, can be obtained. The information about the attack probability to various elements can be used to assess the risk associated with each of them, and the efficiency of defense resource allocation is evidenced in terms of the corresponding risk. Reference defense plans related to the online defense action and the defense action with a time delay can be obtained according to their respective various time constraints. Moreover, risk sensitivity to the defense/attack-resource variation is also analyzed. The model is applied to a standard IEEE RTS-96 test system for illustrative purpose and, on the basis of that system, some peculiar aspects of the malicious attacks are pointed out.

Chaos-based modeling of DC arc furnaces for power quality issues

DC arc furnaces are more and more applied in large industrial systems and represent one of the major sources of perturbations for the feeding system. This paper deals with the problem of the dc arc modeling using three well-known chaotic attractors (Ro/spl uml/ssler, Chua, and Lorenz attractors). A new tuning procedure is adopted to determine the most adequate parameters of the attractors to model the dc arc furnace. Waveform distortions and voltage fluctuations indexes are calculated from the simulation results of a whole existing plant and compared with measured data. The proposed models of dc arc furnace can be used to assess the impact in terms of power quality of the dc arc furnace when planning new plants or evaluating the performances of compensating devices.

Analytical modeling for harmonic analysis of line current of VSI-fed drives

The ac/dc/ac double-stage converter, frequently used nowadays to provide ac adjustable frequency and magnitude voltages in the adjustable-speed drives, consists of a voltage source inverter (VSI) fed by a three-phase diode bridge rectifier; this rectifier generates high line current harmonics in the supply system. This paper presents a complete modelling of the VSI-fed drives, when energized by nonsinusoidal supply voltages, based on the analytical form of the current harmonics injected in the supply system; the equations of the current harmonics are obtained for both the continuous and discontinuous current modes and are derived by Fourier analysis of steady-state current time evolution. The high accuracy of the proposed models is demonstrated by comparison with the results of time-domain simulation. The modelling is useful to study the effects of supply voltage distortions on current harmonics injected in the supply system, both in continuous and discontinuous mode, with a very short computing time.

Site and System Indices for Power-Quality Characterization of Distribution Networks With Distributed Generation

The problem of developing a definition that adequately assesses power-quality (PQ) levels in the presence of distributed generation (DG) is addressed by using proper probabilistic indices for distribution networks. In the planning of new DG installations, these indices take the variation of PQ level into account and use weighting functions properly. They are useful in quantifying the impact of the installation of DG units because they can consider several PQ disturbances simultaneously. Also, they indicate how each PQ disturbance may affect the decision concerning the installation of DG units. Several tests on real distribution networks were performed and discussed in order to show the usefulness of these indices in evidencing the impact of DG on PQ levels. The analysis of the electrical distribution systems on the basis of the considered indices is valuable also to help decide the best allocation and size of distributed generators.

An integrated probabilistic harmonic index

Both amplitude and argument of voltage harmonics can affect the electrical component behavior. In the paper an integrated index taking into account both of them is proposed for inclusion in harmonic standards. This index, which refers to the well known peak value of distorted voltage, allows taking into account the main effect of distorted voltages on component life. The time varying nature of harmonics is also considered and, coherently, limits to be assigned to this index are proposed also in probabilistic scenarios.

The effects of voltage waveform factors on cable life estimation using measured distorted voltages

This work investigates expected life reduction of power cables under harmonic pollution, by processing the results of voltage harmonics measurements. Life estimates come from a life model developed by the authors in former studies, used here in its complete form, with all voltage waveform factors, i.e. peak, shape and rms factor. The application regards MV XLPE-insulated cables. The whole day duty cycle is split into rush hours and non-rush hours, each characterized by a different level of harmonic distortion. The results show that life is significantly reduced with respect to rated sinusoidal life, and that both shape and peak factors play a major role, shape factor during rush hours, peak factor during non-rush hours

The Inherent Structure Theory of Network for power quality issues

The Inherent Structure Theory of Network is a valuable tool to solve power quality problems. In fact, in the frame of this theory, the direct or inverse sequence impedance matrix at the fundamental frequency or the harmonic impedance matrices can be formulated in terms of their eigenvalues and eigenvectors; these expansions are useful to create a framework in which the power system response to power quality disturbances can be better understood. In this paper, after briefly outlining the theoretical background, the above theory is applied to solve some power quality problems, evidencing when the proposed tool can be more useful and what are the benefits deriving from its application. Numerical applications on a test system are presented and discussed to give evidence of the significance and potentiality of this new tool.

Comparison of some active devices for the compensation of DC arc furnaces

DC arc furnaces are more and more applied in large industrial systems. They represent a source of perturbations for the feeding system, in dependence on the available short-circuit power at the point of common coupling and on the arc furnace rating. This work deals with the problem of the compensation of perturbations caused by the DC arc furnaces operation; in particular, a comparison among several active compensation devices is made. The devices under consideration are active filters, STATCOMs and decoupled compensators. The study is performed with reference to a typical DC arc furnace and by using computer simulations developed by means of power system blockset. Waveform distortions and voltage fluctuations indices are assessed in absence and in presence of the compensation devices and compared each other. Finally, the results obtained with the active devices are compared with the ones obtained with a passive filtering system.

Decision theory criteria for capacitor placement in unbalanced distribution systems

In this paper, decision theory criteria are applied to select the optimal allocation and the sizing of capacitors in unbalanced systems with the presence of harmonic sources, also taking into account the uncertainties due to the presence of unbalanced loads. The decision theory is a valuable tool for distribution engineers that have to deal with financial costs and with uncertainty. The financial costs must include, in the most general case, at least initial investment cost, Joule losses costs, quality costs; the uncertainties should refer at least to non linear and linear load scenario, also including issues due to deregulation. A case study referred to the IEEE 13-node test feeder is presented and discussed