Sag mitigation by an electronic tapchanger: specifications and comparisons with other custom power tools J. Faiz and B. Siahkolah Abstract: Recently, the electronic tapchanger has received increasing attention because of its quick response, better performance and cheaper maintenance compared to the mechanical tapchanger. The capability of a distribution transformer equipped with an electronic tapchanger for sag mitigation is presented. To this end, the analytical computation for determining the compensating of sag limit of the electronic tapchanger is given. The system is then simulated to show its capabilities for sag mitigation and power quality enhancement. The performance and economic aspects of the electronic tapchanger is also compared to that of other custom power tools. 1 Introduction The expansion of applications of new electronic equipment, such as computers, advanced telecommunications devices, microprocessor systems, automatic controllers, robots and adjustable speed drives, and their high sensitivity to the quality of the input supply, leads to one of the very important aspects of power delivery, i.e. the power quality problem. Among several phenomena of power quality, short-term voltage drop (sag) and also temporary interruptions are regarded as the most important problems for industrial and commercial customers [1] . These problems result in large-scale damage, such as cessation and loss of products, restarting the product line and the reduction of quality of the products. For instance, a sag in typical solid-state industries produces losses between $100 000 and $1 000 000 [2] . It is noted that, although a temporary interruption is more destructive than a voltage sag, the number of occurrences of the sag is usually much more than that of a temporary interruption. Therefore, in total the damage due to a sag is larger than that of an interruption in the supply. It has been shown that 92% of power quality accidents result from voltage sags, up to 40–50% of which last less than 2 s [3] . The main factor producing the short-term voltage drops include short-circuit faults in the system, overloads and also the starting of large motors. Short-circuit faults are, however, more common and dangerous [4] . The following methods can be used to prevent damage due to short-term voltage drop: (a) Preventing the occurrence of short-circuit faults in the system by improving the system structure and the application of more advanced protective tools. (b) Preventing the short-term voltage drop for customers by the application of compensating devices in the distribu- tion network. (c) Designing the user’s devices such that the short-term voltage drop does not lead to an undesirable effect on performance. Method (a) is a convenient one, but it is costly and insufficient. Method (c) is practicable, but cannot totally solve the problem. This paper concentrates on method (b). Various item of equipment have thus far been recom- mended for the compensation of voltage sags. These include a motor–generator set, UPS, DVR, ferroresonance trans- formers and transformers with solid-state tapchangers. Precise realisation of the capabilities and limitations of each tool is necessary for proper choice and use. This paper introduces the tapped-transformer with solid- state tapchanger and then gives the capabilities and limitations of such a system in compensating for the sag. The required information for its comparison with other tools is provided by the realisation of the capabilities of the system. This comparison is carried out quantitatively by a given example. 2 Electronic tapchanger One of the most common techniques for regulating and controlling the voltage is by inserting several taps in the primary or secondary of the transformer using an on-load tapchanger [5]. Nowadays, this is extensively used. The common tapchanger system is a rather complicated movable mechanical system that makes it possible to change the tap of an on-load transformer. These tapchan- gers have the following major limitations and disadvan- tages: (a) Due to a multi-stage mechanical mechanism, it is slow (changing one tap takes several seconds). (b) It is only possible to change from one tap to a higher or lower one, it is not possible to skip several intermediate taps. The authors are with the Department of Electrical and Computer Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran E-mail: jfaiz@ut.ac.ir r IEE, 2005 IEE Proceedings online no. 20045073 doi:10.1049/ip-gtd:20045073 Paper received 16th June 2004 IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 5, September 2005 697