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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. Index Terms--Distributed generation (DG), frequency relay, islanding, nondetection zones, synchronous machines, voltage relay.

This paper proposes a method for determining the coordination of the rate of change of frequency (ROCOF) and under/overfrequency relays for distributed generation protection considering islanding detection and frequency-tripping requirements. The method is based on the concept of application region, which defines a region in the trigger time versus active power imbalance space where frequency-based relays can be adjusted to satisfy the anti-islanding and frequency-tripping requirements simultaneously. This method can be used to optimally determine the instantaneous and time-delay settings of different frequency-based relays in a unified manner. The application region can be also employed to evaluate how much an existing frequency-based protection scheme violates one of the above-mentioned requirements. In addition, it can be used to set different combinations of frequency-based relays in order to make the distributed generator frequency-based protection system as efficient as possible, since it respects the region that satisfies both requirements. Index Terms--Distributed generation, dynamic simulation, frequency relays, islanding detection, rate of change of frequency relays, synchronous generators.

This paper investigates the efficiency of under/over frequency relays for protection of distributed synchronous generators considering islanding detection and frequency-tripping requirements. Comparison between frequency and vector surge relays, which are islanding detection-dedicated devices, shows that the former can be as effective as the latter. This feature supports the idea that frequency relays can replace vector surge relays for islanding detection purpose. In this case, frequency relays must also meet the generator manufacturer recommendations and the utility frequency-tripping requirements. In order to investigate the relay capability to meet both necessities, it is proposed the concept of application region, which defines a region in the detection time versus active power imbalance space where frequency-based relays can be adjusted to satisfy the anti-islanding and frequency-tripping requirements simultaneously. The paper also presents a set of formulas to determine directly the application region. Index Terms--Distributed generation, dynamic simulation, frequency relay, islanding detection, synchronous generator.

This paper presents a detailed comparative analysis between synchronous and induction machines for distributed generation applications. The impacts of these generators on the distribution network performance are determined and compared by using computational simulations. The technical factors analyzed are steady-state voltage profile, electrical power losses, voltage stability, transient stability, voltage sags during unbalanced faults, and short-circuit currents. The results showed that the best technical choice depends on the network characteristics, i.e., the main factors that may limit the penetration level of distributed generation. Index Terms--Distributed generation, induction generator, short-circuit currents, steady-state voltage profile, synchronous generator, transient stability, voltage sag, voltage stability.

This paper investigates the impacts of different excitation system control modes on potential factors that can limit the maximum allowable number of synchronous generators connected to power distribution systems. Excitation systems acting as either a voltage regulator or a power factor regulator are examined. The impacts on steady-state voltage, angle stability, short-circuit currents and voltage stability are determined. Simulation results show that the usage of the excitation system as a voltage regulator can increase the maximum penetration level of synchronous generators in distribution systems. Index Terms--Distributed generation, excitation system, short-circuit, steady-state voltage, synchronous generator, transient stability, voltage stability.

This letter proposes a method for systematically evaluating the voltage-based relay's capability to detect islanding of distributed generators connected to distribution systems. The method is based on a set of detection time versus reactive power imbalance curves, which can be obtained through repeated dynamic simulations. With the curves, one can determine the characteristics of voltage-based anti-islanding protection systems. Index Terms--Distributed generator, islanding detection, synchronous generator, voltage relay.

Byline: Walmir Freitas, Ahmed Faheem Zobaa, Jose C.M. Vieira, James S. McConnach There is growing interest in the development of more sustainable electricity systems employing renewable, low-emission resources. In this context, the number of wind power generators installed in the world is increasing, and there are strong indicators that such growth should continue in the next decades. The intensity of wind power expansion depends on different factors related to technical, economic, environmental, governmental, and regulatory issues. This paper presents an overview on various issues related to wind energy conversion systems.