Luiz C. P. da Silva

A comparative analysis of FBD, PQ and CPT current decompositions — Part I: Three-phase, three-wire systems

This paper investigates the main similarities and discrepancies among three important current decompositions proposed for the consideration of unbalanced and/or non linear three-phase three-wire power circuits. The considered approaches were the so called FBD Theory, the pq-Theory and the Conservative Power Theory (CPT), recently presented by Tenti et al. Such decompositions and related definitions may influence the power measurement techniques, revenue metering, instrumentation technology and also power conditioning strategies. The three methods have been summarized, discussed and compared by means of computational simulation. Although the three methods are based on different concepts, the results obtained under ideal conditions are very similar. The main differences appear in the presence of unbalanced and non linear load conditions. Under linear unbalanced conditions, both FBD and pq-Theory suggest that the some current components contain a third-order harmonic. Besides, neither pq-Theory nor FBD method are able to provide accurate information for reactive current under unbalanced and distorted conditions, what can be done by means of the CPT-Theory. The paper tries to explain the causes of these differences in terms of the decompositions foundations and the resulting waveforms and spectra.

Evaluation of the impact of distributed generation on power losses by using a sensitivity-based method

The connection of generators on distribution feeders may cause significant impact on the steady-state performance of the network. Quantification of the impacts on power losses, in a systematic way, is a difficult task due to the complexity of network operation since generators can operate with different lead and lag power factors and, occasionally, can inject variable active power on the network. This paper evaluates the impact of distributed generators on the active and reactive power losses of the system by using a sensitivity-based method. From one base case power flow solution it is possible to estimate the active and reactive power losses for a new generator installed at any bus of the system, for any combination of active power injection, and also for any operating power factor. The effects of varying the location, generation level and operating mode of the generators can be easily assessed by using the analytical method. Moreover, a numerical index to quantify the impact of multi-distributed generators on power losses is also proposed. The method is applied to a 70-bus distribution network. The simulations results are compared with those obtained by the repetitive power flow solutions in order to validate the results obtained by the sensitivity-based method.

Three-phase four-wire circuits interpretation by means of different power theories

In order to contribute to the discussion of defining a generalized power theory, valid for unbalanced and non linear circuits, this paper discusses the relationship and discrepancies among four modern power theories. Three-phase four-wire circuits, under different conditions, have been analyzed, since the most conflicting and intriguing interpretations take place in case of return conductor occurrence. Simulation results of different load, power supply and line conditions will be discussed in order to elucidate the authors conclusions and to provoke the readers for additional discussions.

Alternative parameters for the continuation power flow method

Abstract The conventional Newtons method has been considered inadequate to obtain the maximum loading point (MLP) of power systems. It is due to the Jacobian matrix singularity at this point. However, the MLP can be efficiently computed through parameterization techniques of continuation methods. This paper presents and tests new parameterization schemes, namely the total power losses (real and reactive), the power at the slack bus (real or reactive), the reactive power at generation buses, the reactive power at shunts (capacitor or reactor), the transmission lines power losses (real and reactive), and transmission lines power (real and reactive). Besides their clear physical meaning, which makes easier the development and application of continuation methods for power systems analysis, the main advantage of some of the proposed parameters is that its not necessary to change the parameter in the vicinity of the MLP. Studies on the new parameterization schemes performed on the IEEE 118 buses system show that the ill-conditioning problems at and near the MLP are eliminated. So, the characteristics of the conventional Newtons method are not only preserved but also improved.

Towards a real-time Energy Management System for a Microgrid using a multi-objective genetic algorithm

This paper proposes a real-time Energy Management System (EMS) for a low voltage (LV) Microgrid (MG). The system operation consists in solving the Unit Commitment (UC) and Economic Load Dispatch (ELD) simultaneously for 24 hours ahead at every 15-minute period. This operation is formulated as a multi-objective optimization problem where the minimization of operational cost, total emissions and power losses is simultaneously pursued using the Non-dominated Sorting Genetic Algorithm II (NSGA-II). In this algorithm, crossover and mutation operators were improved with respect to existing approaches to achieve an adequate characterization of the energy management problem and a good algorithm performance. Simulation studies have outlined that, in fact, the NSGA-II can be used as a real-time optimization tool providing a good-quality Pareto front to operate optimally the MG in a limited time of 15 minutes.

Study of alternative schemes for the parameterization step of the continuation power flow method based on physical parameters, part I: Mathematical modeling

The conventional power flow method is considered to be inadequate to obtain the maximum loading point because of the singularity of Jacobian matrix. Continuation methods are efficient tools for solving this kind of problem since different parameterization schemes can be used to avoid such ill-conditioning problems. This paper presents the details of new schemes for the parameterization step of the continuation power flow method. The new parameterization options are based on physical parameters, namely, the total power losses (real and reactive), the power at the slack bus (real or reactive), the reactive power at generation buses, and transmission line power losses (real and reactive). The simulation results obtained with the new approach for the IEEE test systems (14, 30, 57, and 118 buses) are presented and discussed in the companion paper. The results show that the characteristics of the conventional method are not only preserved but also improved.

A Comparative Analysis of AC and DC Incremental Methods for Transmission Loss Allocation

This paper intends to analyze the accuracy of incremental DC methods for loss allocation in electricity markets. The model under implementation at the Brazilian electricity market is one of this kind and has motivated this investigation. The model to be used in Brazil is based on incremental methods and defines loss factors for generators and loads. This technique uses a DC load flow to calculate the loss factor, and so it does not consider the complete power flow equations. Consequently, this method may provide unfair loss allocation depending on the system operation condition. Hence a method is developed to calculate the loss factors, based on incremental methods but using an AC load flow that takes into account the system nonlinearity. Both methods are applied to a small system for conceptual discussions and to a version of the Brazilian North-Northwest system for the validation of the results by using a real system. The results demonstrate the limitations of the DC method for loss allocation, justifying the use of an AC load flow to calculate the total system losses and the loss factors. The loss factors calculated by both methods depend on the choice of the slack bus. In order to overcome this limitation, the concept of distributed slack bus is included into the AC power flow, and new slack-bus independent results are obtained.

Impacts of dynamic reactive power compensation devices on the performance of wind power generators

This paper investigates the main impacts of dynamic reactive power compensation devices on the performance of induction machine-based wind power generators. The dynamic reactive power compensation devices analysed are the SVC (Static Var Compensator) and the DSTATCOM (Distributed Static Synchronous Compensator). The usage of these devices as a power factor regulator or a voltage regulator is investigated. The technical factors analysed are small-signal voltage stability, transient stability and interactions with the anti-islanding protection system. The analyses are carried out by using a wind farm composed of 30 units of 1 MW induction generators. Such wind farm is connected to a 60 Hz, 33 kV distribution system. The results are a useful guideline to evaluate which control strategy and device are suitable for a determined application as well as to understand the dynamic interactions that can occur.

H 2 Norm-Oriented STATCOM Tuning for Damping Power System Oscillations

This work is mainly concerned with the effects of the static synchronous compensator (STATCOM), a flexible AC transmission system (FACTS) controller, on small-signal power system angle stability. This investigation is carried out for both inter-area and local oscillations mode. The study is based on the investigation of the eigenvalues of the linearized power system models in the framework of dynamic bifurcation theory. The tuning method of the STATCOM gains aiming the H 2 norm minimization is briefly explained, and the presented simulation results are sufficient to provide a comprehensive analysis of the effects of this controller for damping power systems low frequency electromechanical oscillations.

Análise comparativa entre geradores síncronos e geradores de indução com rotor tipo gaiola de esquilo para aplicação em geração distribuída

This paper presents a detailed comparative analysis between synchronous and induction machines for distributed generation applications. The impacts of these generators on the performance of distribution networks are determined and compared by using different computational simulation tools and models.The technical factors analyzed are steady state voltage profile, electrical losses, voltage stability, transient stability, voltage sags due to unbalanced faults and short-circuit currents. The results show that the most suitable choice depends on the network technical characteristics, i.e. what are the main operating restrictions related to distributed generation.