Transient analysis of multi-circuit overhead transmission lines with two voltage levels
Department of Theory of Electrical Engineering at University
Department of Theory of Electrical Engineering at University of West Bohemia
The growing demands of electric energy needs require to intensify an energy transmission and to build multi-circuit overhead lines with four or six three-phase systems. In this case the design of a transposition tower is very difficult and the lines are usually carried-out as non-transposed. The conductor’s layout causes non-symmetry of lines parameters and inductive and capacitive couplings between three-phase systems play an important role. If the systems with various voltage level are placed on the same pylon then many negative phenomena as consequence of these mutual coupling can arise.
In this paper a method for a common transient analysis of the mutual coupling influences in such systems is presented. It is based on two-port approach, where the phase conductors and earth wires are modelled by inductive and capacitive coupled cascades of two-ports. The basic element involves passive parameters R, L, C a G and two dependent sources. One of them respects induced voltages caused by inductive couplings among all conductors and the second one respects the capacitive couplings. The mutual couplings depend on the given geometric conductor´s layout and can be evaluated by matrices of inductances and capacitances. For a considered network model a system of ordinary differential equations has been formulated and it has been solved in MATLAB using Runge-Kutta method of 4th order. It results in the time distribution of currents and voltages in all conductors and it allows to appreciate the mutual influences in those particular systems.
A system with four three-phase system of two voltage levels (EHV and HV) was analysed. At first the switching on/off phenomena were observed. According to our expectation the influence of EHV on HV system is stronger. The switching on/off is joined with the origin of the oscillating voltage and current peaks. Secondly, some non-standard operations such a single phase short circuit or an automatic reclose cycle were studied and very interesting results were obtained. Based on them it is possible to formulate some recommendations for the tower design and on setting of a protection system.