Walmir Freitas

An Investigation on the Nondetection Zones of Synchronous Distributed Generation Anti-Islanding Protection

Anti-islanding protection is an important technical requirement when interconnecting distributed generators. Unfortunately, most anti-islanding protection schemes cannot detect islanding situations under certain system operating conditions - there are nondetection zones within which anti-islanding protection schemes are ineffective. This paper investigates these nondetection zones associated with the common anti-islanding protection schemes of synchronous distributed generators: frequency and voltage-based relays. Moreover, the characteristics of the nondetection zones and the key factors that influence them are analyzed. The results are useful for utility companies and distributed generators owners to design and to evaluate the effectiveness of anti-islanding protection schemes proposed for various distributed generation interconnection projects.

A Monte Carlo Simulation Platform for Studying Low Voltage Residential Networks

The smart grid vision has resulted in many demand side innovations such as nonintrusive load monitoring techniques, residential micro-grids, and demand response programs. Many of these techniques need a detailed residential network model for their research, evaluation, and validation. In response to such a need, this paper presents a sequential Monte Carlo (SMC) simulation platform for modeling and simulating low voltage residential networks. This platform targets the simulation of the quasi-steady-state network condition over an extended period such as 24 h. It consists of two main components. The first is a multiphase network model with power flow, harmonic, and motor starting study capabilities. The second is a load/generation behavior model that establishes the operating characteristics of various loads and generators based on time-of-use probability curves. These two components are combined together through an SMC simulation scheme. Four case studies are presented to demonstrate the applications of the proposed platform.

Comparative analysis between SVC and DSTATCOM devices for improvement of induction generator stability

This work presents a comparative analysis between a SVC (static VAr compensator) and a DSTATCOM (distribution static synchronous compensator) for improvement of induction generator stability. Although both approaches of dynamic reactive power compensation can improve the stability performance of induction generators, simulations results show that the utilization of a DSTATCOM with the same power capability of a SVC leads to a greater gain of the dynamic performance of induction generators.

Planned islanding for Brazilian system reliability

This paper presents an impact study case of a new operational strategy regarding planned islanding operation of distributed generation. Traditionally, interconnection standards avoid islanding operation of distributed generation due to the concerns of equipment failure and safety issues. However, allowing the islanded operation of distributed generation may enhance reliability to final consumers and decrease outage cost by providing an alternative power source when there is an interruption in the upstream network. This paper performs dynamic studies in a Brazilian system which operates in the islanded mode during fault to supply critical loads. The analysis presented in this paper includes the behavior of the islanded system under load following and load rejection, faults, the load-frequency control and varying short-circuit level. The results show that this operational strategy can potentially bring many benefits to the distributed generator owner and customers.

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.

Analysis of the harmonic distortion impact of photovoltaic generation in Brazilian residential networks

In Brazil, in spite of the outstanding solar irradiance levels, the installed amount of photovoltaic (PV) generation was practically inexistent until two years ago. This scenario started to change with the publication of resolution #482/2012 by the Brazilian Electricity Regulatory Agency, which establishes the main technical and commercial rules for residential power generation. This paper presents an overview of the current and near future scenarios of residential PV generation in Brazil, including regulatory aspects. The largest Brazilian field project on PV integration (about to start in 2014) is described. Since PV arrays use power electronic inverters, the imminence of a high PV penetration has raised utilities attention to its potential impacts on system harmonic distortion levels. In such context, this paper also identifies, through extensive simulations, the key factors influencing PV impacts on harmonic distortion level. It is found that the most important factors are: network loading level; PV connection point; and PV power factor.

Control systems analysis of industrial plants with synchronous generators during islanded operation

This work presents a methodology that permits to show, in a graph, the maximum allowable time for changing the distributed synchronous generator excitation system and speed governor control modes after the occurrence of an islanding, for a wide range of values of power unbalances between load and generation. The visualization of this parameter becomes interesting when the conflict between the synchronous generator control modes for grid connection operation (parallel) and for islanding operation is taken into account. At the point of view of practical implementation, this time is desirable to be sufficiently high, so that the decision of altering the control mode does not result in a false detection of an islanding situation. From another perspective, changing the generator control mode needs to be sufficiently fast in order to avoid the operation of voltage and frequency relays used in the generator protection.

Dynamic improvement of induction generators connected to distribution systems using a DSTATCOM

The usage of distributed generation and devices based on power electronics have significantly increased in electric power distribution systems. In this context, induction generators have received more attention. However, it is known that such machines draw very large reactive currents during fault occurrence, which depresses the network voltage further and can lead to voltage instability. A solution for this problem is to employ local dynamic reactive power compensation. Therefore, in this work the behavior of a DSTATCOM to improve the voltage stability performance of distribution systems with induction generators is investigated based on three-phase non-linear dynamic simulations. Two control strategies for a DSTATCOM are analyzed: voltage and power factor control. In such studies, a DSTATCOM is simulated through a model based on controllable three-phase voltage sources, which has shown to be suitable for stability studies. Test results have indicated that a DSTATCOM with voltage control mode can improve the voltage stability margins.

Dynamic analysis of wind turbines considering new grid code requirements

E.ON Netz new grid code requires the ability from the wind turbines to remain connected to the network when terminal voltage level drops to zero for at least 150 ms. Moreover, wind turbines must also contribute with voltage regulation during and after grid faults. A comparative study analyzing the dynamic behavior of conventional versions of wind turbines based on doubly-fed induction generator and on permanent magnet synchronous generator is presented in this paper. Computational simulations have showed that wind turbine based on PMSG can contribute with more injection of reactive power to the network. The converters nominal power and the strategy of its current controllers can impact the dynamic behavior of wind turbines.

Long-term voltage stability of distribution systems with induction generators

This paper presents an investigation about the longterm or small-disturbance voltage stability of distribution systems with induction generators by using time-domain nonlinear dynamic simulations. Results show that the presence of induction generators may decrease the system voltage stability margin. It was verified that in the maximum loading point, if the system loading is increased even more, then the induction generator accelerates to a high speed, becoming unstable and leading the system to a voltage collapse.