LOAD BEHAVIOUR DURING VOLTAGE DIPS: A VOLTAGE QUALITY STUDY IN LOW VOLTAGE DISTRIBUTION SYSTEM I. Rendroyoko R.E. Morrison Peter K.C. Wong* Department of Electrical & Computer Science Monash University, PO BOX 35, CLAYTON, VICTORIA 3800 Phone: 61-03-99053465, Fax: 61-03-99053454 Ignatius.rendroyoko@eng.monash.edu.au * United Energy Ltd., PO BOX 1185, Moorabbin, VIC 3189 Abstract Over the last ten years, many studies have been performed on voltage dip characteristics in industrial, commercial and residential systems. The characteristic of each voltage dip is unique and particular to each electrical system. This paper presents a study of load behaviour during voltage dips on a low voltage distribution system, consisting of light commercial and residential customers. The recorded voltage dip measurements were used to analyze the system voltage dip characteristic. The Power System Blockset (PSB) from MATLAB was used in modeling the distribution system components and simulating the voltage dips caused by faults. The influence of loads on voltage dips during faults, especially during the winter and summer season, is presented. It is concluded that the load may influence the voltage dip characterization and that load effects must be accounted for to achieve high modeling accuracy. 1. INTRODUCTION Voltage dips have become a major concern in power quality in the past decade. The cost of economical loses and inconveniences caused by voltage dips have triggered some studies and research activities. Many experts have tried to characterise voltage dips [1,2,3]. The existing standard on voltage dip characterises the voltage dips in terms of magnitude and duration. The characterisation of the standard is based on the assumption that faults will cause rectangular voltage dips. It is also assumed that the voltage drops to a certain low value immediately and when the fault is cleared, the voltage recovers back to normal immediately. The assumption of rectangular voltage dips, however, is not correct in a realistic system, which largely consists of rotating machines or motors. When a fault occurs, all the rotating machines in the system slow down and after the fault is cleared, the motors will accelerate to the normal condition. During acceleration, motor will draw high current from the system and thus prolong the voltage dip. This paper will discuss load behaviour on a system during and after a voltage dip. For the purpose of this paper, one sub-system in south-eastern Victoria was selected. This sub-system has significant differences in load characteristics between the summer and winter season. In the summer season, there is an increase of energy consumption, which is mostly due to the operation of air conditioners. Therefore, part of the load consists of electrical rotating machines. 2. SYSTEM COMPONENTS The distribution system under study is presented in fig. 1. A main 66kV bus bar supplies the 22kV distribution system trough 2 66/22kV 30-MVA transformers and the sub-system is supplied at 415 V from a 22/0.415kV 400-kVA transformer. The transmission and distribution systems supply electric power to a south-east area of Melbourne in the geographical distribution area of United Energy Ltd. (UE). The sub-system supplies mostly commercial customers, and a few residential and light industrial customers.