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Passive filters have been a very effective solution for power system harmonic mitigation. These filters have several topologies that give different frequency response characteristics. The current industry practice is to combine filters of different topologies to achieve a certain harmonic filtering goal. However, there is a lack of information on how to select different filter topologies. This decision is based on the experience of present filter designers. The goal of this paper is to investigate the filter topology selection issue. It presents our research results on the effectiveness and costs of various filter topologies for harmonic mitigation. The research results show that the association of three single-tuned filters is a very appropriate solution for most typical harmonic problems. Index Terms--Passive filters, power system harmonics.

The positive feedback-based anti-islanding scheme has become a common anti-islanding protection technique for inverter-based distributed generators (DGs). This paper investigates the scheme's impact on the small-signal stability of multiple inverter-based DGs installed in power distribution systems during grid-connected operation. The interactions among multiple-DGs due to the anti-islanding scheme are also addressed. An index is proposed to quantify the destabilizing effect of the aggregated positive feedback activities of multiple DGs. This paper further introduces a DG power transfer limit versus islanding detection time curve to reveal the conflicting requirements of anti-islanding protection speed and the maximum power transfer of multi-DG systems. Index Terms--Distributed generator, inverter, islanding, positive feedback, small-signal model.

This paper investigates the impact of positive-feedback anti-islanding methods on the small-signal stability of grid-connected inverter-based distributed generation. The maximum power transfer capability of a distributed generator (DG) is analyzed. Sensitivity studies are conducted for DGs equipped with the Sandia frequency shift anti-islanding scheme. Factors such as positive-feedback gain, initial chopping fraction, local load level, and network line impedance are investigated. The maximum power transfer limit versus positive-feedback gain curve is proposed as an index for the stability analysis. The results show that the positive-feedback anti-islanding scheme does have the potential to destabilize the grid-connected DG system when the grid is weak or the DG size is large. A curve that relates the maximum stable DG power transfer level versus the islanding detection time is proposed to quantify the destabilizing effect of the positive-feedback-based anti-islanding schemes. Index Terms--Distributed generation, inverter-based generator, islanding detection, positive-feedback anti-islanding protection, small-signal stability.

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 presents an analytical method for analysis of large-disturbance stability of squirrel-cage rotor induction generators. The proposed analytical method is based on the steady-state equivalent circuit of the induction generator and on the concepts of stable and unstable electrical-mechanical equilibrium points, which are calculated in the electrical and mechanical torque versus rotor speed space. This analytical approach sheds light on theoretical issues related to the stability of induction generators. Moreover, this method can be used as a guide to significantly reduce the simulation efforts necessary to assess the large-disturbance stability of induction generators. The method has been validated by comparing the results obtained by the analytical method with those determined by extensive dynamic simulation results through a sensitivity study. In the dynamic simulations, the induction generators are represented by a fourth-order algebraic-differential model. In addition, a simple approach is proposed for assessing the fault ride-through capability of induction generators. Index Terms--Critical fault clearance time, distributed generation, distribution system, induction generator, large-disturbance stability.

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 paper presents a simple and reliable method for predicting the islanding detection performance of vector surge relays. The relay performance is characterized by a tripping-time versus power-imbalance curve. With the curve, one can determine the time taken by a vector surge relay to detect islanding for any generation-load mismatch level. The main contribution of this paper is the development of analytical formulas for directly determining the behavior of vector surge relays. As a result, efforts needed to asses the relay performance for a given distributed generation scheme can be simplified significantly. The accuracy of the formulas has been verified by extensive simulation study results. Index Terms--Distributed generation, islanding detection, synchronous generators, vector shift relays, vector surge relays.