Partial Core Transformer for Energisation of High Voltage Arc-Signs K. Lynch 1 , P. S. Bodger 1* , W. G. Enright 1 and S. C. Bell 1 1 Electrical and Computer Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand *Email: pat.bodger@canterbury.ac.nz Abstract : A high voltage partial core resonating transformer has been designed and constructed such that its magnetising current reactance is matched to the reactive current drawn by the capacitance of an arc-sign. The supply only provides the real power losses of the transformer plus any reactive power mismatch between the magnetizing reactance and the capacitance of the arc-sign. A mathematical model of the transformer is developed using a reverse design modelling technique. The model is then used to design a 50Hz, 8kVA, 230V/80kV, partial core transformer to meet the required electrical demand of the load. The transformer was constructed and tested. The transformer successfully resonated with the load and provided 68VAr of compensation when operating at 10kV while being supplied from a domestic 230V, 10A, power outlet. The completed transformer has a finished weight of 69kg and has been successfully used for powering an arc-sign at an exhibition of electric sculptures. 1 INTRODUCTION A partial core transformer has a central core with the primary and secondary windings wrapped around it [1[. The limbs and yoke of a conventional full core transformer are absent in a partial core transformer. Partial core transformers are designed using a Reverse Design Transformer Modelling Technique [2, 3[. A partial core resonating transformer has a characteristically low magnetizing reactance. Through careful design, the reactance of the transformer can be matched to a capacitive load. This has seen effective industrial use in the testing of generator stators [4, 5[. A parallel resonant circuit is formed where the reactive current drawn by the load is provided by the magnetisation of the transformer. The supply only has to provide the real power losses of the transformer plus any mismatch between the magnetizing reactance and the capacitance of the load. Lightning arc drawings or “arc-signs” are presently being developed in the department. These arc-signs require a very high voltage to obtain their arcing effect. A power source capable of very high voltage is required. This paper describes the design, fabrication and testing of a partial core resonating transformer which is capable of supplying the very high voltages required by arc-signs whilst being able to be operated from a single phase, 10A, 230V, domestic power socket. 2 PARTIAL CORE TRANSFORMER ARRANGEMENT The usual transformer winding arrangement is to have the LV winding on the inside to reduce its length and hence the copper losses due to its relatively high current. The transformer designed for the arc-sign application has the LV winding wrapped around the HV winding. The LV winding shields the HV winding from electric field coupling to grounds external to the transformer and reduces corona from the windings [4[. A cross sectional view of a partial core resonating transformer is presented in Fig. 1. Laminated Core LV Winding HV Winding LV Winding HV Winding Fig. 1: Cross section and end elevation of a partial core resonating transformer. By design, partial core transformers are generally long and thin in shape to obtain the required value of magnetising current reactance as given by (1). core core rT m l A N X μ μ ϖ 0 2 1 = (1) where: 㲐 is the supply frequency in rads/sec N 1 is the number of primary turns µ 0 is the permeability of free space µ rT is the effective permeability of magnetic flux path through the laminated core material and the air return path A core is the cross-sectional area of the core l core is the length of the core The fundamental advantage of a partial core transformer it that there is a significant reduction in size and weight when compared to the conventional full core equivalent. This creates some design challenges but generally they are easy and inexpensive to manufacture. XV International Symposium on High Voltage Engineering th University of Ljubljana, Elektroinštitut Milan Vidmar, Ljubljana, Slovenia, August 27-31, 2007 T3-304.pdf 1