Mathias Bollen

Expert System for Classification and Analysis of Power System Events

This paper presents an expert system that is able to classify different types of power system events to the underlying causes (i.e., events) and offer useful information in terms of power quality. The expert system uses the voltage waveforms and distinguishes the different types of voltage dips (fault-induced, transformer saturation, induction motor starting), as well as interruptions (nonfault, fault induced). A method for event-based classification is used, where a segmentation algorithm is first applied to divide waveforms into several possible events. The expert system is tested using real measurements and the results show that the system enables fast and accurate analysis of data from power quality monitors.

What is power quality

Abstract This paper introduces the terminology and various issues related to ‘power quality’. The interest in power quality is explained in the context of a number of much wider developments in power engineering: deregulation of the electricity industry, increased customer-demands, and the integration of renewable energy sources. After an introduction of the different terminology two power quality disturbances are discussed in detail: voltage dips and harmonic distortion. For each of these two disturbances, a number of other issues are briefly discussed, which are characterisation, origin, mitigation, and the need for future research.

Tuning software phase-locked loop for series-connected converters

Accurate phase information is crucial for most of the modern power-electronics apparatus such as the static series compensator (SSC). A software phase-locked loop (SPLL) has been proposed in literature to obtain phase and frequency information of the grid voltage. Either a lead/lag filter or a proportional-plus-integral (PI) controller is employed to control the performance of the SPLL. In this paper, a criterion to tune the SPLL is discussed and the gains of the PI-controller are determined to obtain the desired performance. The proposed criterion is based on the fact that a phase shift of the grid voltage is sensed as a frequency deviation by the load. If the deviation of the grid frequency is kept within the range /spl plusmn/1 Hz, most of the loads function properly. Hence and by the SSC, the response of the phase angle of the load voltage is designed to follow the grid voltage angle while satisfying the frequency range at all times. Consequently, the gains of the PI-regulator of the SPLL are dependent on the maximum phase shift of the grid voltage. Unbalanced grid voltages are separated into positive- and negative-sequence components and the SPLL is locked to the positive sequence. The response of the SPLL has been evaluated by using PSCAD/EMTDC simulation package.

Different methods for classification of three-phase unbalanced voltage dips due to faults

Abstract This paper presents a systematic and an intuitive approach to the analysis of voltage dips due to faults in three-phase systems. The systematic approach (symmetrical component classification) is based on the decomposition of the three phase voltages in symmetrical components. From positive and negative-sequence voltage a generalised magnitude for the event is defined as well as a second characteristic that quantifies, among others, the load effect on the voltages. Next to the systematic approach a more intuitive approach is introduced. This so-called ABC classification uses only the generalised magnitude as a characteristic. This method is more appropriate as a basis for testing of equipment and for interpretation of the propagation of dips through the system. The basics behind the symmetrical-component classification are explained from a systematic analysis of non-symmetrical faults. The two methods are compared and the effect of dynamic load on the characteristics is discussed.

Automatic classification of power system events using RMS voltage measurements

Power quality monitors in the occasion of a disturbance can either save the actual voltage waveform that contains the event or the corresponding RMS. The latter option reduces significantly the memory that is needed for saving the event. This paper shows that even with this type of monitoring, analysis of the measurements can be in depth. The paper proposes a method for automatic classification of power system events using RMS voltage measurements. The system is tested with measurements from a distribution network and the results show that classification is possible for the considered types of events. Finally, the limitations of this type of monitoring are shown.

Voltage dip detection and power system transients

Recently developed power quality mitigation equipment, like the static transfer switch, needs methods for fast and reliable detection of voltage dips. Such a detection scheme must be able to detect a voltage dip as fast as possible and be immune to other types of disturbances. In this paper, the authors address the problem of voltage dip detection regarding Kalman filtering, the characteristics of fault-induced voltage dips and other power system disturbances. They investigate how the voltage dip characteristics influence the speed of detection and show that disturbances other than fault-induced dips could trigger a detection scheme. Special attention is given to trans former-related events. Their characteristics are presented using measurements. Kalman filtering modelling issues are discussed. Statistics on the characteristics of fault-induced dips and transformer events are presented from medium voltage networks.

Classification of power system events: voltage dips

The extensive monitoring programs that are run by power utilities enable new insights of the power system operation and new characterisation methods must be used for the classification and analysis of the recordings. This paper focuses on events that cause a temporary decrease in the fundamental frequency voltage magnitude (voltage dip). The analysis of the recordings from surveys in medium and low voltage networks shows that new classes of voltage dips should be introduced in order to characterise the events that are captured by the power quality monitors. The aim of the paper is to show the distinctive characteristics of each class and give the guidelines for the automatic processing of the recordings. Finally, a large number of recordings from a medium voltage network are classified using these characteristics and the results are presented.

Expert system for voltage dip classification and analysis

The increasing amount of data obtained by power quality monitors and the need for better understanding of power system disturbances require new analysis tools. This paper presents an expert system that is able to classify different types of voltage dips according to the underlying causes (i.e. events) and offer useful information in terms of power quality. The expert system uses the voltage waveforms and distinguishes the different types of voltage dips (fault-induced, transformer saturation, induction motor starting), explains the changes in the voltage dip magnitude (change in the system, change in the fault type, transformer saturation, motor load influence) and separates interruptions into non-fault and fault-induced. A method is proposed for event-based classification, where a segmentation algorithm is first applied to divide waveforms into several possible events. Kalman filtering is employed to model the waveforms and the residuals of the model are used for segmentation. The expert system is tested using real measurements and the results show that the system enables fast and accurate analysis of data from power quality monitors.

Positive and negative sequence estimation for unbalanced voltage dips

This paper proposes the use of a complex Kalman filter for the estimation of positive and negative sequences from three phase voltages. A complex voltage is obtained by applying the /spl alpha//spl beta/-transform followed by the dq-transform using a rotational operator. The algorithm for three phase voltages containing K harmonics is also given. In the conventional method, estimation of positive and negative sequences is performed through two steps: the magnitude and phase-angle in each individual phase of the voltages are first estimated and the symmetrical component transformation is then applied. The proposed method offers a direct estimation of the positive and negative sequences that may reduce the estimation errors. In addition, the proposed method has a reduced computational cost since the number of state variables is reduced to 2/3 as compared to that in the conventional method. An experiment was performed on measured three phase voltage data. Results have shown that the proposed method offers a good estimation.

A fault location technique using high frequency fault clearing transients

This letter suggests that a voltage recorder, placed next to a circuit breaker not as usual on the side of the substation, but on the side of the transmission line, may reveal the location of permanent faults. Two different approaches to estimating fault location are presented here: spectrum estimation and wavelet analysis. The methods are tested and compared with simulations of typical transmission systems using the Electromagnetic Transients Program (EMTP).