Math H. J. Bollen

Power quality following deregulation

Utility deregulation will have tangible and intangible effects on power quality requiring industry-wide action to maintain adequate standards. These effects are discussed in the first part of the paper. The increasing trend towards more extensive use of power electronic control at the generation, transmission and utilization systems following deregulation has power quality implications that will affect the standards, system simulation and monitoring tools. The paper reviews the present methods available in these areas to achieve specified levels of power quality in the deregulated environment.

Characterisation of voltage sags experienced by three-phase adjustable-speed drives

It is shown in this paper that voltage sags experienced by three-phase loads, such as adjustable-speed drives, can be classified into four types. Each sag can further be characterized by a magnitude and a phase-angle jump. Relations between fault type, sag type and load connection are presented. The transfer of sags through transformers is discussed. The magnitude and phase-angle jump of sags are directly related to the voltage in the faulted phase, or between the faulted phases, at the point-of-common coupling between the load and the fault.

IEEE Richard Harold Kaufmann Award Call for Nominations

This letter further discusses the difference between different definitions of voltage unbalance. Contrary to an earlier letter (see P. Pillay et al., ibid., vol.5, p.50-1, 2001), it is concluded that different definitions may give significantly different results. The two IEEE definitions that were not discussed in the earlier letter give different results and both deviate significantly from the true value (ratio of negative, and positive-sequence voltage) when a zero-sequence component is present.

Characterization of voltage sags in industrial distribution systems

This paper describes the various characteristics of voltage sags experienced by customers within industrial distribution systems. Special emphasis is paid to the influence of the induction motor load on the characterization of voltage sags. During a fault, an induction motor operates as a generator for a short period of time and causes an increase in sag magnitude. Their re-acceleration after the fault clearance results in an extended post-fault voltage sag. The influence of the induction motor on the imbalanced sags caused by single line-to-ground and line-to-line faults has been analysed in detail. For an imbalanced fault, the induction motor current contains only positive and negative-sequence components. Induction motors create a low impedance path for the negative sequence voltage due to an imbalanced fault. This causes a small sustained nonzero voltage with large phase angle-jump in the faulted phase and a voltage drop in the nonfaulted phases with a small phase angle-jump. The symmetrical components of the induction motor during the imbalanced sags have been studied. The results show that induction motor behaviour is determined by positive and negative sequence voltages during the imbalanced sag.

The influence of motor re-acceleration on voltage sags

The assumption that a voltage sag is rectangular is not correct in a power system with large induction motor loads. The motors decelerate during the short circuit. After fault-clearing, they will accelerate again, drawing a high reactive current from the supply, causing a prolonged postfault voltage sag. This is aggravated by the removal of branches by the protection. The resulting shape of some voltage sags in an example power system is shown and discussed. For the example power system, a stochastic voltage sag table is determined. This table gives the expected number of sags of different depth and duration. The influence of faster protection and of reduced transformer impedance on the table is presented. A simple motor model is implemented in a method for including interruptions due to voltage sags in the reliability analysis of power systems. This model is presented briefly and used to show the influence of motor parameters on the number of sags that lead to an interruption of plant operation. >

Stochastic prediction of voltage sags in a large transmission system

In this paper we discuss two stochastic assessment methods for voltage sags and apply them to a 98-bus model of the 400 kV National Grid of England and Wales. The method of fault positions is most suitable for implementation in a software tool. It has been used to get exposed areas and sag frequencies for each bus. The results are resented in different ways, including a so-called voltage sag map showing the variation of the sag frequency through the network. The method of critical distances is more suitable for hand calculations, as both the amount of data and the complexity of the calculations are very limited. It has been used to obtain sag frequencies for a number of buses. A comparison with the results obtained by using the method of fault positions, shows that the method of critical distances is an acceptable alternative were software or system data are not available for a more accurate analysis.

Fast assessment methods for voltage sags in distribution systems

Based on a simple voltage divider model, a relation is derived between the load sensitivity to voltage sags (expressed as a critical voltage) and the vulnerable area (expressed as a critical distance). The critical distance increases roughly linear with voltage level. The relation found is similar to relations found in power quality surveys. An equivalent expression is found for the critical distance as a function of the critical phase-angle jump. Realizing that faults downstream of a transformer do not significantly influence the expected number of sags, it is possible to estimate the number of sags at a specific load location. The method is extended to some nonradial systems: simple subtransmission loops; local generation; feeding from two substations; and operating with a normally open breaker.

Support Vector Machine for Classification of Voltage Disturbances

The support vector machine (SVM) is a powerful method for statistical classification of data used in a number of different applications. However, the usefulness of the method in a commercial available system is very much dependent on whether the SVM classifier can be pretrained from a factory since it is not realistic that the SVM classifier must be trained by the customers themselves before it can be used. This paper proposes a novel SVM classification system for voltage disturbances. The performance of the proposed SVM classifier is investigated when the voltage disturbance data used for training and testing originated from different sources. The data used in the experiments were obtained from both real disturbances recorded in two different power networks and from synthetic data. The experimental results shown high accuracy in classification with training data from one power network and unseen testing data from another. High accuracy was also achieved when the SVM classifier was trained on data from a real power network and test data originated from synthetic data. A lower accuracy resulted when the SVM classifier was trained on synthetic data and test data originated from the power network.

Analysis of voltage tolerance of AC adjustable-speed drives for three-phase balanced and unbalanced sags

Adjustable-speed drives are the type of equipment most sensitive to voltage sags. This paper analyses the behavior of three-phase AC adjustable-speed drives during balanced and unbalanced sags. Emphasis is placed on the DC bus voltage and on the drop in speed, both assuming that the drive will not trip. By using a previously-introduced classification of three-phase unbalanced sags, voltage tolerance curves are obtained for AC adjustable-speed drives. The conclusion from the analysis is that voltage sags due to three-phase faults are a serious problem for adjustable-speed drives. However, single-phase and phase-to-phase faults, causing the majority of voltage sags, can be tolerated by adding a relatively small amount of DC bus capacitance.

Mitigation of unbalanced voltage dips using static series compensator

The static series compensator (SSC) is suited to protect sensitive loads against voltage dips. Because most of the power system faults are single- or double-phase, the control algorithms of the SSC should be adapted for unbalanced dips. This paper proposes two control strategies to improve the dynamic performance of the SSC. The first strategy uses a fast technique for separating positive and negative sequence components of the supply voltage, which are then controlled separately. Thus, two controllers are implemented for the two sequences, each based on vector control. The second strategy is based on using only a positive sequence controller and increasing the switching frequency. Consequently, the negative sequence due to the unbalanced dip is transformed into variations in the positive sequence. As the switching frequency increases, the ability of the controller to follow those variations improves. The validity of the proposed strategies is demonstrated through PSCAD/EMTDC simulation and experimental measurements carried out on a 10-kV SSC, when the grid is subjected to unbalanced voltage dips.