Myo Thu Aung

The influence of fault distribution on stochastic prediction of voltage sags

This paper analyzes the influence of modeling of fault distribution along transmission line on the assessment of number and characteristics of voltage sags. The generic distribution network was used in all simulations. Different types of transformer winding connections were modeled and different (symmetrical and asymmetrical) types of faults were simulated. A line was selected from the previously identified area of vulnerability for a given bus and different faults having different distributions along the line were simulated. It was shown that depending on the fault distribution (uniform, normal, exponential) along the line, different numbers and characteristics of voltage sags could be expected at the selected bus.

Propagation of asymmetrical sags and the influence of boundary crossing lines on voltage sag prediction

This paper analyzes the influence of modeling of fault distribution along the boundary crossing lines of the area of vulnerability on the stochastic prediction of voltage sags and their characteristics. The study was performed on a generic distribution network, and all types of faults were simulated throughout the network. Two buses from the 11-kV distribution system were randomly selected and the areas of vulnerability due to each type of fault for specific sensitive equipment at the selected buses were determined. These areas were determined by assessing voltage sag performance at each phase separately. Finally, the influence of different fault distributions along the vulnerability area boundary crossing lines was considered. It was shown that the fault distribution along those lines influences strongly the number and characteristics of voltage sags at the customer connection point.

Stochastic prediction of voltage sags by considering the probability of the failure of the protection system

This paper presents a comprehensive method for stochastic prediction of the number and characteristics of voltage sags in large distribution networks. The novelty of the proposed approach is in probabilistic modeling of the failure of the protection system. The probability of the failure of the protection system is determined using a straightforward, decision tree based method. The paper highlights the effects of the protection system failure on the number and characteristics of voltage sags. The method proposed and its capabilities are demonstrated on the realistic size generic distribution system. The results presented in the paper show that the proposed method is superior to the traditional techniques used for the assessment of voltage sags as it results in higher accuracy of the prediction of the number and characteristics of voltage sags.

The influence of transformer winding connections on the propagation of voltage sags

This paper presents the influence of different types of transformer winding connections on the propagation of voltage sags caused by symmetrical and asymmetrical faults in the power system. Single and multiple transformers are considered. A particular propagation of voltage sags from customer buses to industry facilities through the transformers employed at the entrance of the facilities is highlighted in the study. Different winding connections, dominantly used in the U.K. distribution networks, were modeled at the service transformers. The study was performed on a generic distribution network, and a bus from the 11-kV distribution network was randomly selected. It was found that the performance of voltage sags inside the industry facility is a function of transformer connections used at the service transformer.

The Influence of Induction Motors on Voltage Sag Propagation—Part I: Accounting for the Change in Sag Characteristics

This paper presents an analytical approach to the analysis of the interaction between induction motors (IMs) and voltage sags. The presented methodology enables quick assessment of the influence of IM on sag characteristics and avoids time-consuming transient simulations. The accuracy of the derived analytical formulae is verified through classical transient simulations using the PSCAD/EMTDC package. This paper further proposes a simple, yet efficient transformation of resulting nonrectangular voltage sags, based on the loss of voltage index, into equivalent rectangular sags suitable for conventional voltage sag performance benchmarking.

The influence of process equipment composition on financial losses due to voltage sags

Power quality (PQ) disturbances can have significant economic consequences for different type of customer faculties. Voltage sags are the most common power quality disturbances causing disruption of consumer equipment. The paper therefore, focuses on voltage sags and discusses economic assessment of power quality. By combining the probabilistic assessment of voltage sags with the voltage tolerance curves of electrical equipment an estimate can be made of the sag events responsible for tripping of the equipment over a specified period of time. If the tripping costs per event of the equipment are available, the assessment of customer financial losses due to the inadequate quality of supply can be made. The effect of sensitive load composition, i.e., type of the industrial process on the customer financial losses (damage costs) due to expected voltage sags at a bus has been investigated in the paper. This approach can be used as a tool to select the most favorable location in the network to connect an industrial plant having particular composition of sensitive equipment.

The Influence of Induction Motors on Voltage Sag Propagation—Part II: Accounting for the Change in Sag Performance at LV Buses

This paper describes the influence of induction motors (IMs) on voltage sag characteristics at low-voltage (LV) buses. This influence is particularly pronounced in case of low magnitude and long duration voltage sags. Even though the conventional magnitude duration representation of voltage sags in the presence of IMs is not completely accurate, it is very convenient as it can be directly linked to a standard way of recording/reporting voltage sags. The equivalent rectangular sags developed in this study take into account the effect of IMs and establish voltage sag performance more accurately at LV buses in the distribution network. The methodology developed is not limited to simulation-based voltage sag studies. It can be equally well applied as a ldquopostprocessorrdquo to finely tune the results of monitoring distribution systems as its inputs are ldquoclassicalrdquo voltage sag magnitude duration tables. The automated procedure for counting and characterization of voltage sags at LV distribution buses is illustrated on a 295-bus generic distribution network.

Analytical assessment of the effects of voltage sags on induction motor dynamic responses

This paper presents an analytical approach for the assessment of the effects of voltage sags on operation of induction motors. The results obtained by using this relatively simple method fully agree with those published in the past. The method enables studies of dynamic interaction between induction motors and voltage sags without having to perform transient simulations. It is ideally suited for fast assessment of the influence of induction motors on voltage sag characteristics at low voltage buses.

Automated comprehensive assessment and visualization of voltage sag performance

This paper describes modular software for the automated assessment and visualization of voltage sag performance. The software allows in-depth analysis of voltage sag performance of the individual buses, the performance of the entire network at different voltage levels and inside the industry facility. The prediction and characterization of voltage sag, identifying the area of vulnerability and the area affected by the fault, and the propagation of voltage sags can be done automatically taking into account different fault statistics for symmetrical and asymmetrical faults, and different fault distributions. The module also considers the protection system and effects of its failure on the duration of voltage sags. The software capabilities are demonstrated on a generic distribution network and the results of the module are presented in the graphical and tabular form using specially developed graphical user interface (GUI).