Ultra fast switches - basic elements for future medium voltage switchgear (2001)

The steadily increasing demand on electric energy and the continuing liberalization process of the electricity market raise the utilities’ need for further reduction of operation costs, increased functionality and power quality. One possibility to do so is the use of fault current limiters. Although many solutions for fault current limiting devices at medium and high-voltage levels have been presented in literature, utilization of these limiters is restricted to particular installations due to missing economical feasibility. One specific hybrid system for current limiting using an assembly of three electro dynamically driven opening switches, power semiconductors and a pure wire resistor, was already proposed and successfully tested in this ongoing research project. This solution has a much better potential of being economical than previous ones but some features are still not satisfying. Therefore, in the present work, investigations for medium-voltage fault current limiting devices were decided starting from a fundamental discussion on ultra fast-acting switches. As the basic element of these ultra fast switches, so-called “elementary switches” are defined which are only capable of carrying the nominal current when closed and withstanding the dimensioning voltage when open. The basic idea of the project was, to realize high power switchgear with a combination of such elementary switches using precise control of the operation instants. Elementary switches are not designed to withstand high-current arcing and they do not have facilities for forced arc cooling. Given this definition and using a repulsion drive, the vital characteristics of elementary switches using linear or rotational operation are derived as follows: The moving mass in an elementary switch turns out to be around 1 g/MW dimensioning power, resulting in a drive energy requirement of several 100 J depending on design parameters. Linear operation requires up to 3 times the drive energy at rotational operation, especially if using air as switching medium. A generally applicable equation for calculating the resulting opening velocity as a function of design parameters is given. The results were compared to measured velocities and show good coincidence for linear and rotational operation up to 50 m/s. With the use of several implementations of elementary switches, theoretical and experimental investigations of its switching performance were conducted. Elementary switches can also be closed using a repulsion drive, reviii Ultra Fast Switches – Basic elements for future medium voltage switchgear sulting in an overall closing time down to 0.5 ms. The contact force required to carry the nominal current is provided by the inertia of the moving part and results in a 20 % decrease of opening velocity. Repetitive switching at a repetition frequency above 10 Hz is not feasible duo to bad drive efficiency. Elementary switches are well suited for synchronous interruption of high ac currents at medium voltage levels up to 24 kV. As the energy dissipated by the arc at synchronous interruption is only around 1 J and the fast switch noticeable increases its contact gap during rise of TRV, elementary switches show a dielectric breakdown mechanism. The reignition voltage linearly increases with time from current zero but shows at least minimum breakdown strength according to the instantaneous recovery. The arc voltage of fast opened elementary switches is higher than the arc voltage of free burning arcs but lower than the one of strongly cooled arcs. This can be utilized to commutate high currents on a parallel resistor or capacitance. Both are treated theoretically and experimentally with an eye on applications in a hybrid system. Applications of elementary switches arise either from combinations of several switches in series or in parallel with one common drive or by using the switches in a hybrid system. Combinations of elementary switches in parallel leads to a further reduction of dissipated arc energy in any single switch. A combination of many elementary switches in series leads to a linearly increasing gain of breakdown strength after synchronous interruption. Theoretically, even the ability for transient recovery performance of a standard vacuum switch is obtained by a combination of elementary switches using air at 5 bar, 12 breaks in series and an opening velocity of 50 m/s. A hybrid fault current limiting circuit breaker using three elementary switches or combinations with many breaks in series and one pure-wire resistor is proposed together with a trigger strategy to get low contact wear at load switching. Successful tests in a medium voltage circuit were performed. Feasibility of this system is given up to a rated voltage of 24 kV and a rated current of 2...3 kA. It is concluded from this work, that the given characteristics of ultra fast acting switches have the potential to be the basic elements for future applications in medium voltage systems, such as fault current limiting circuit breakers and dc breakers. Ultra Fast Switches – Basic elements for future medium voltage switchgear ix