Development and Investigation of a Proposed Voltage Sag Index

Introduction

In modern power systems, electricity is produced at generating stations, transmitted through an interconnected high voltage network, and finally distributed to consumers. Electricity, being a form of energy itself, needs to be produced with the use of some energy source; that being usually fuel or some form of renewable energy. Due to the rapid increase in power demand, electric power systems have developed extensively during the 20^th century, resulting in today’s power industry probably being the largest and most complex industry in the world. Electricity is one of the key elements of any economy, industrialized society or country. People do not often realize the importance of electric energy and its complexity. A modern power system should provide reliable and uninterrupted services to its customers at a rated voltage and frequency within a constrained variation limit. If the supply suffers a reduction outside those constrained limits, sensitive equipment might trip, and motors connected on the system might stall. The electrical system should not only be able to provide cheap, safe and secure energy to the consumer, but also to compensate for the continually changing load demand. During that process the quality of power could be distorted by faults on the system, or by the switching of heavy loads within the customers facilities. In the early days of power systems, distortion did not pose severe problems towards end-users or utilities. Engineers first raised the issue in the late 1980s when they discovered that the majority of equipment interruptions were due to power quality disturbances (Yalcinkaya, Bollen, & Crossley, 1998). The reliability of power systems has improved due to the growth of interconnections between utilities. As a result the complexity of the system has increased. In the modern industrial world, many electronic and electrical control devices are part of automated processes in order to increase energy efficiency and productivity. However, these control devices are characterized by an extreme sensitivity on power quality variations, which has led to growing concern over the quality of the power supplied to the customer. That is due to the high interest developed on voltage quality issues and the fact that Voltage Sag is the most important power quality issue of our time. Some researchers use the term ‘voltage quality’ and others use ‘quality of supply’ to refer to the same issue of power quality

Voltage Sag is defined as a short reduction in voltage magnitude for a duration of time. The characterization of voltage sags in terms of duration and magnitude vary according to the authority (Bergeron, 1998; Bollen & Styvaktakis, 2000; Sabin, 2000; Thallam, 2000; Thallam & Heydt, 2000). The IEEE STD 1159-1995 defines voltage sag as a decrease of RMS voltage from 0.1 to 0.9 p.u, for a duration of 0.5 cycle to 1 minute. Voltage sag is caused by faults on the system, transformer energizing, or heavy load switching.

To date, total prevention of voltage sag does not exist. A fault cannot be predicted to occur within a radius of hundreds of kilometres of the facility under investigation. Prediction of voltage sag occurrence probability requires knowledge of voltage sag characteristics, statistical information obtained through measurements describing the history of voltage sag occurrence, and information describing the sensitivity of important loads within the facility.

Generally, there are two types of methods used for voltage sag analysis (Bollen, 2000, 2001; Polycarpou, Nouri, Davies, & Ciric, 2004). Statistical analysis methods observe the behavior of the system and calculate averages from the observations. They require a number of monitors to observe the system for a time period. To get accurate results a large number of monitors and a long observation period are required. The major disadvantage of this type of method is that the results have limited application for other locations and for future prediction. Stochastic methods use a model of the system to predict the stochastic properties of the system, like voltage sag frequency. Site indices can be calculated for all locations within the system and for any period in the past or in the future. The disadvantage is that the results are as accurate or inaccurate as the model used. Emphasis in the literature is given to statistical methods. Stochastic methods have not yet received the attention they require.