TECHNOCRAT Vol. 3 2008 Current Trends in the Design of On-Load Tap Changers Ashraf Yahya, PNEC, NUST ayahya@pnec.edu.pk Abstract Regulating transformers with on-load tap changers are extensively used in distribution networks or with loads which are sensitive to voltage fluctuations. Voltage sags of below 85% can cause supply disruption and increased down time which may prove very costly for a production industry. In power system, regulating transformer aided by the tap changers quickly restore the system voltage to avoid outages as a result of under-voltage operation. There are two new approaches to accomplish on-load tap changers: Thyristor assisted and fully static solid state tap changers. This paper will review the two approaches discussing the related merits and demerits of each of these approaches. Keywords Tap changing Transformers, LTC, solid state tap changer, sags, vacuum switch. Introduction On load tap changers (LTCs) are vital components of any power system. They are used in inductive apparatus such as AC voltage regulating transformers, HVDC rectifiers, inverter transformers and phase angle regulators to adjust the turns ratio or phase angle of the secondary while it is conducting [1]. LTCs allow restoration of secondary voltage of the regulating transformer within a specified range when the voltage at power grid changes or simply voltage varies with load changes. The tap changer automatically connects the desired tap of a voltage regulating transformer and brings back the secondary voltage to permissible range by readjusting the turns ratio of the transformer. This on-load transfer from one tap to other results in severe arcing at the contacts of the tap changer. The arc is extinguished by insulating oil present in the chamber of the tap changer contact assembly. The result is, however, slower response, contact wear condition, increased maintenance of tap changer and deterioration of insulating oil [2]. In order to minimize the maintenance duty and fast response to voltage sags, these conventional tap changers are being replaced by the newer designs which employ either the conventional method of tap changing assisted by solid state switches or fully static solid state tap changers. Power quality is now a major concern of power utilities. Deregulation in power market has also heightened the awareness of this aspect. It is incumbent upon power utilities to maintain the customers’ supplies within specified parameters for varying condition of the load. One of the parameters to judge power quality is the voltage sag for which extensive studies have been done in U.S.A and in Europe. The studies show that 70% of the voltage sags are within the range of 65 to 85% of the nominal voltage [3]. Fig 1 shows how a voltage dip can look like; these dips were measured in Sweden in 1996 by the power utility V.B.Elbat. Fig.1: voltage sag condition The voltage on the load side can be restored to nominal value by regulating transformers assisted by the tap changer. These tap changers are composed of oil immersed switches driven by a complicated gear mechanism. Slow response due to mechanical assembly and sequential travel of tap selector, arcing due to current break and resulting maintenance requirements have all contributed towards thinking of alternative arrangements. In large power system, where the regulating transformers are usually large in number, high level of maintenance requirements is undesirable. Therefore in recent years, alternative designs for coping with all or some of the aforementioned problems have been proposed. In designing a Tap changer, it is necessary to consider that optimum operating point of tap changer depends on a number of factors [4]. These are, 1) the power system voltage level 2). Loading 3). power factor ,and , 4). for parallel operation of transformers, the magnitude of any inter-transformer circulating current. The conventional design (Fig2) comprises a selector mechanism (2) and a diverter(12). For successful on load tap changing, the flow of load current should not be interrupted. The selector mechanism has switches which