Controlled Switching - Solution For Reducing The Overvoltages At Commutation Of The Transport And Distribution Lines

Recently, worldwide, the controlled switch became a technical solution for reducing the switching stress. After 90es, the number of equipment which uses the controlled switch fast increased, mainly due to the achieved performances. The controlled switching can be applied to any type of commutations. Now there are dedicated controllers which are used more often for the switching of the transport and distribution lines, of the small inductive loads, of the capacitors batteries, or the energizing the no load power transformers. The paper deals with the simulation of the commutation of a transportation line by using the distributed parameters model. The controlled switch of the line is compared with the uncontrolled one, highlighting the major differences in what concerns the stress. TRANSPORT AND DISTRIBUTION LINES SWITCH When the long lines are energized, undesired over voltages can be generated in the transport and distribution networks. These are caused by the propagation, reflection and refractions of electromagnetic waves, both in ramifications points and end of the lines. Such phenomenon both occur when the lines are quick re-energized after a disconnection. The cause of the over voltages in this case are the residual charges on the line. As will be seen, both the over voltages and their slopes determine important stress of the equipment insulation. When connected, at the end of the long lines over voltages can occur due to more causes. For lines shorter than 1500 km, the input impedance has capacitive character and the voltage drop on the equivalent inductive reactance of the system has the same sign with the supplying fem. Consequently, this voltage drop is added to the fem. Thus, the resulted overvoltage highly depends by the line length and by the short circuit power of the system. Another cause of the over voltages resides in the oscillations which occur when the initial repartition of the voltage across the line is replaced by the new repartition corresponding to the new steady state operation, after its connection. The stress of the insulation depends both by the amplitude of the overvoltage and by its shape, characterized by two parameters: rising duration and falling duration. (Gusa 2002). The mathematical models used to determine the over voltages can be more or less complex, depending on the characteristics of the studied commutation phenomenon. The long lines can be modeled by circuits with either uniform distributed parameters or concentrated parameters. The models can be single phase or three phase ones. The over voltages when a line is energized can be analytically estimated if two hypotheses are considered: the lines have no losses and all the three phases of the breaker are simultaneously connected. For the analytically evaluation of the over voltages at the line energizing with no residual charge on the line, the simplified diagram in Figure 1 can be considered. The equation specific to the equivalent electric system is: 1 1 s s di e u L dt = + ⋅ . (1) The equations specific to the uniform distributed long line model are: u i r i L x t ∂ ∂ − = ⋅ + ⋅ ∂ ∂ , (2) i u g u C x t ∂ ∂ − = ⋅ + ⋅ ∂ ∂ , (3) where: ⋅ es – the instantaneous value of the fem of the system; ⋅ u1 – instantaneous voltage at the front end of the long line; Proceedings 23rd European Conference on Modelling and Simulation ©ECMS Javier Otamendi, Andrzej Bargiela, José Luis Montes, Luis Miguel Doncel Pedrera (Editors) ISBN: 978-0-9553018-8-9 / ISBN: 978-0-9553018-9-6 (CD) Figure 1: Simplified Equivalent Diagram of a No Load Line when is Connected ⋅ i1 – the instantaneous value of the current at the front end of the long line; ⋅ Ls – the inductance of the supplying electric system; ⋅ u, i the voltage and the current in a point of the line; ⋅ r, L, g, C – the parameters of the line per length unit. By solving the equations (1-3), the voltage at the end of the line results: