Mario Russo

Modeling Guidelines and a Benchmark for Power System Simulation Studies of Three-Phase Single-Stage Photovoltaic Systems

This paper presents modeling guidelines and a benchmark system for power system simulation studies of grid-connected, three-phase, single-stage Photovoltaic (PV) systems that employ a voltage-sourced converter (VSC) as the power processor. The objective of this work is to introduce the main components, operation/protection modes, and control layers/schemes of medium- and high-power PV systems, to assist power engineers in developing circuit-based simulation models for impact assessment studies, analysis, and identification of potential issues with respect to the grid integration of PV systems. Parameter selection, control tuning, and design guidelines are also briefly discussed. The usefulness of the benchmark system is demonstrated through a fairly comprehensive set of test cases, conducted in the PSCAD/EMTDC software environment. However, the models and techniques presented in this paper are independent of any specific circuit simulation software package. Also, they may not fully conform to the methods exercised by all manufacturers, due to the proprietary nature of the industry.

Constrained least squares methods for parameter tracking of power system steady-state equivalent circuits

The paper deals with the problem of the online parameter identification of power system harmonic equivalent circuits as seen from a bus at which a power electronic device is connected. Recursive constrained least squares methods are considered and models of different order are compared one to another. The proposed online identification procedure is able to identify both equivalent impedances and ambient voltage harmonics and to track their changes during normal operating conditions. Numerical applications evidence the performance of the proposed methods in terms of both steady-state accuracy and responsiveness to changes of the parameter values.

Dispersed generation modeling for object-oriented distribution load flow

The interest in dispersed generation (DG) is world-widely increasing. The connection of small generating units affects the operation of distribution systems and classical modeling and analyzing techniques must be revised. Recently, a novel approach to the distribution load-flow problem has been proposed: the object-oriented (OO) paradigm has been applied both to the system modeling and to the Newton-Raphson solving algorithm in the cases of radial and weakly meshed distribution systems. In this paper, the OO load-flow modeling and algorithm are extended to account for the inclusion of DG. The OO paradigm allows easily introducing accurate models of DG interfacing to the network by various electric devices (synchronous generators, induction machines, power electronics converters). Numerical applications are presented to evidence the features of the algorithm evaluating the effects of DG on the operation of two test systems.

Decentralized Control of Distributed Generation for Voltage Profile Optimization in Smart Feeders

Distributed generation, which is installed to exploit renewable energy sources, can also be used as a reactive power resource and contribute to tackle the voltage regulation problem in smart distribution grids. Adopting a decentralized approach with off-line coordination, the paper proposes an optimal set-point design for the voltage/reactive power control scheme of distributed generation. The objective is to improve the voltage profile along the feeders of a distribution system. Using only local measurements, the actual operating conditions of the feeder are firstly estimated; then, the set-point is evaluated by solving an optimal voltage profile problem. The off-line coordination avoids any modification to the architecture of existing control systems and requires communication only in the case of significant changes of the distribution system topology. The results of numerical simulations are presented for MV feeders with wind and photovoltaic generations. A comparison with standard control schemes is reported, as well as considerations about the off-line coordination among the distributed generations and the tap-changer of the HV/MV transformer.

Object-oriented load flow for radial and weakly meshed distribution networks

Object-oriented load flow modeling is presented for both radial and weakly meshed distribution systems. An OO algorithm based on the Newton-Raphson technique is proposed. In the object oriented formulation, some approximations to the full Jacobian matrix are introduced. Consequently, a detailed study of the convergence characteristics of the proposed object oriented algorithm is presented and some sufficient conditions for convergence are derived. Particular attention is paid to the relationship between the electrical parameters of the distribution system and the mathematical parameters that influence the convergence properties of the algorithm. The numerical results obtained in the case of some test systems give evidence of the features of the algorithm.

Simulation and Evaluation of Optimization Problem Solutions in Distributed Energy Management Systems

Deregulation in electricity markets requires fast and robust optimization tools for a secure and efficient operation of the electric power system. In addition, there is the need for integrating and coordinating operational decisions taken by different utilities acting in the same market. Distributed energy management systems (DEMS) may help to fulfill these requirements. The design of DEMS requires detailed simulation results for the evaluation of its performance. To simulate the operation of DEMS from the optimization standpoint, a general purpose distributed optimization software tool, DistOpt, is used, and its capabilities are extended to handle power system problems. The application to the optimal power flow problem is presented.

Adaptive Voltage Regulator Design for Synchronous Generator

This paper presents the adaptive design of an automatic voltage regulator (AVR) control scheme for synchronous generators that is capable of providing satisfactory voltage control performance in the presence of unknown variations of the power system operating conditions. The design is articulated in four steps: voltage and current phasor estimation, estimation of the parameters of the Thevenin equivalent circuit adopted to represent the power system response using a steady-state model, determination of the sampled-data transfer function of the system, and AVR design developed according to discrete-time techniques. A suitable procedure is described to derive the sampled-data transfer function of the system starting from models of the power system, synchronous generator, and exciter. The obtained transfer function depends on the estimated parameters yielded by a recursive least-squares algorithm subject to constraints deriving from the Thevenin circuit. The AVR design is based on the pole-assignment technique while the phasors estimation is performed by two Kalman filters. Finally, the results of accurate numerical simulations conducted for a test network are reported, comparing the performance of the proposed adaptive control scheme to the one of a PID controller with fixed parameters.

Self-tuning regulator design for nodal voltage waveform control in electrical power systems

The paper presents a systematic approach to self-tuning regulator design for nodal voltage waveform control in electrical power systems by means of shunt power electronic devices (SPEDs). The proposed approach is structured into three main tasks. In the first one, starting from the sampled measurements of the nodal voltage and of the current, which is injected by the SPED into the power system, a Kalman filtering technique is adopted to estimate the voltage and current phasors at fundamental and harmonic frequencies. These estimates are then used by the second task for the on-line parameter identification of the Thevenin equivalent circuits that represent the electrical power system at each phasor frequency. Finally, in the third task, the closed-loop voltage regulator is adapted so as to satisfy the design requirements expressed in terms of desired closed-loop pole locations. After illustrating the algorithms and the design criteria related to each of the mentioned tasks, the proposed approach has been applied to the design of a self-tuning regulator for the control of an active filter (the SPED) in an IEEE-test industrial electrical system. The results obtained by accurate numerical simulations confirm the validity of the proposed approach.

DistOpt: a software framework for modeling and evaluating optimization problem solutions in distributed environments

We present a flexible software environment, named DistOpt, which is useful in building coarse-grain algorithms for solving optimization problems and simulating the solution of the resulting subproblems in multicomputer systems. It is based on a decomposition-coordination approach, by which large optimization problems can be split into subproblems, which are then easier to solve and can be solved in parallel. The object-oriented methodology on which DistOpt is based, and its graphical user interface, are well suited to modify, interface, and extend existing software modules with minimal impact. DistOpt is a flexible and innovative software environment that can be customized by the user in a short development time.

A nonlinear control of synchronous generator excitation for voltage regulation in power systems

This paper presents the excitation control design of a synchronous generator aimed at voltage regulation in power systems. The design is based on the use of the direct feedback linearization technique. With the help of this technique, a nonlinear output variable is set up which makes the nonlinear model linear versus a new input. Subsequently this new input, which represents the output of a feedback controller, is subsequently used in the nonlinear compensating law which is valid in the whole operating region of the synchronous generator. Numerical simulation shows that the voltage regulation objective is accomplished applying the proposed design.