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Transcript

14th International Conference on Optimization of Electrical and Electronic Equipment OPTIM 2014 May 22-24, 2014, Brasov, Romania Calculation methods for lightning impulse voltage distribution in power transformers M. A. Boruz, M. P. Mircea, M. Ciontu, I. Mircea University of Craiova, Romania Abstract This paper aims to present and develop a methodology for calculating the stress to which the internal insulation of power transformer is subjected during lightning impulse test. The transformer’s insulation must withstand the voltage testing levels prescribed by the standards. One of the methods for determining the internal stress of various parts of the power transformer’s insulation is by using mathematical models. Determination of stress produced by lightning impulse regarding power transformer’s insulation must be as accurate as possible and the following two aspects should be taken in consideration: The internal insulation must withstand the stress that appears during service with a proper safety coefficient; Insulation sizing should lead to an economic product. There are two known theories regarding the analysis of the transient phenomena when testing power transformers with lightning impulse voltage: the theory of traveling wave and the theory of standing wave (see figures bellow). Fig. 2. Transformer’s winding equivalent circuit diagram according to standing wave Fig. 1. Transformer’s winding equivalent circuit diagram according to traveling wave It is considered that on the column of a transformer there are placed “s” concentric coils. In order to determine the initial voltage distribution every winding of the transformer is expressed as a finite number of elements. Every element has a serial capacity and more capacities to ground or to the nearby winding (see figure 3). 4,0 3,5 3,0 2,5 Minim position 2,0 Nominal position 1,5 Maxim position 1,0 0,5 0,0 10 Fig. 3. Equivalent capacitive diagram of a transformer having “s” concentric coils placed on a column 12 14 16 18 20 Fig.4. Comparison between voltage distribution for different position of the tap changer This method of splitting the entire transformer in a series of capacities offers the possibility to calculate the initial voltage repartition for any kind of connection between transformer’s coils and it also takes in consideration the different types of tap changer [2]. This calculation method can also be applied for any kind of windings: continuous, interlaced buckets, buckets with or without reinforced insulation etc. The mathematical algorithm presented can be used by the transformer designer to determine the stress to which the internal insulation of power transformer is subjected during lightning impulse test. The designer can also check if the internal transformers distances were chosen correctly. It can be possible to avoid the situations when the transformer isolation is oversized with a high and unjustified safety coefficient or the situations when safety factor is too small or sometimes missing. References [1] IEC International Standard, IEC 60076-11, Power Transformers – Part 11: Dry-type Transformers, International Electrotechnical Commission, Geneve, Switzerland, 2004. [2] I. Răducanu, Contribution at the transient phenomena study which take place in power transformers when testing them with lightning impulse voltage - PhD theses, Bucharest 1977, pp 14-17. [3] C. Ambrozie, Measurement and calculation of the capacities inside continuous windings in power transformers - PhD theses, Iasi 1972. 23