Jose C. M. Vieira

Comparative analysis between synchronous and induction machines for distributed generation applications

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.

Performance of frequency relays for distributed generation protection

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.

Efficient coordination of ROCOF and frequency relays for distributed generation protection by using the application region

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

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.

Influence of excitation system control modes on the allowable penetration level of distributed synchronous generators

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.

Fault Location in Distribution Systems Based on Smart Feeder Meters

This paper proposes a fault-location method based on smart feeder meters with voltage sag monitoring capability. The main idea is to explore voltage measurements from monitors placed in different buses of distribution systems to estimate the fault location. The estimation is achieved by relating the voltage deviation measured by each meter to the fault current calculated based on the bus impedance matrix, considering the fault in different points. In order to improve the method accuracy, the loads are represented by constant impedance models and included into the bus impedance matrix. The performance of the proposed method is demonstrated by using a real distribution system. Sensitivity studies results show that the method is robust since it has good performance for different values of fault resistance, quantity, and location of the smart meters.

Performance curves of voltage relays for islanding detection of distributed generators

This letter proposes a method for systematically evaluating the voltage-based relays 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.

Utility Harmonic Impedance Measurement Based on Data Selection

Determination of the utility harmonic impedance based on measurements is a significant task for utility power-quality improvement and management. Compared to those well-established, accurate invasive methods, the noninvasive methods are more desirable since they work with natural variations of the loads connected to the point of common coupling (PCC), so that no intentional disturbance is needed. However, the accuracy of these methods has to be improved. In this context, this paper first points out that the critical problem of the noninvasive methods is how to select the measurements that can be used with confidence for utility harmonic impedance calculation. Then, this paper presents a new measurement technique which is based on the complex data-based least-square regression, combined with two techniques of data selection. Simulation and field test results show that the proposed noninvasive method is practical and robust so that it can be used with confidence to determine the utility harmonic impedances.

Investigation on Voltage Sags Caused by DG Anti-Islanding Protection

This paper investigates the voltage sag caused by distributed generation anti-islanding protection. This is a new power-quality concern associated with distributed generation expansion. Anti-islanding protection aims to disconnect distributed generators immediately after the opening of a recloser. As a consequence, after the reclosing operation, voltage sag will occur in some parts of the distribution system, since the loads are energized immediately but the generators are no longer present. In this context, this work presents the findings on our investigations about this problem. The results show that the resulting voltage sag level can violate power-quality limits. Thus, a set of indices has been introduced to characterize the severity of the voltage sag, and a load flow-based method has been proposed to predict the phenomenon. Sensitivity studies are also conducted in order to identify some key factors that influence this type of voltage depression.

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.