Ultra-fast electromechanical commutation switch for hybrid breakers
Time: Thu 2021-06-10 14.00
Subject area: Electrical Engineering
Doctoral student: Mrunal Parekh , Elektroteknisk teori och konstruktion, Applied physics in Electrotechnology
Opponent: Prof. Dr. Christian Franck, ETH, Zurich
Supervisor: Associate Professor Marley Becerra Garcia, Elektroteknisk teori och konstruktion
In a power system with the ability to integrate geographically spread diverse renewable energy sources, there is a need for efficient control of the power flow and effective power transmission. Power electronics can provide this controllability, at the expense of conductive losses in the used components. These losses can be reduced substantially by the use of ultra-fast electromechanical switches in parallel with power semiconductor devices, known as hybrid breakers. There are three major requirements for such breakers. They should i) have higher mechanical and electrical operations, ii) have sufficient and rapid arc voltage build up to facilitate successful current commutation into parallel power semiconductor and iii) be able to withstand the transient interruption voltage.
An ultra-fast electromechanical switch is operated with a Thomson coil based electromagnetic actuator. This actuator can generate a sufficient force and swiftly open electrical contacts in a couple of milliseconds. Since contacts are opening with high velocities, there is a need for timely and controlled damping to secure a long lifetime of the device. Different eddy current based damping actuator concepts are presented in this thesis. Modelling using the finite element method is accompanied by experimental results to study the effect of load mass attached to the actuator and incoming velocities. It was observed that the magnetic field component perpendicular to the direction of the contact movement is key in achieving high damping forces. Higher damping forces are achieved if the magnitude of this magnetic field is high and has a uniform distribution.
The success of current commutation in hybrid breakers depends on the arc voltage formed across the contacts in an ultra-fast switch. Towards that, the behaviour of rapidly elongating arcs generated during the opening of an ultra-fast mechanical switch in the air is also investigated in the thesis. The voltage-current characteristics of the arc generated between the contacts of a model ultra-fast commutation switch are obtained for different contact opening velocities. It is found that at a given current and contact separation, the arc voltage increases with the contact opening velocity. It is also shown that stationary, zero-contact-velocity characteristics can not be used to accurately quantify the voltage build-up in fast elongating arcs in hybrid breakers. Furthermore, the obtained arc characteristics are used as input to simulate the current commutation process in a medium voltage hybrid DC circuit breaker case study. Different failure scenarios of current commutation related to the arc voltage build up are identified. It is shown that these failure scenarios can be avoided by increasing the contact opening velocity. Finally, a 2D axisymmetric magneto-hydrodynamic model of a rapidly elongated arc plasma in the air is presented in the thesis. It is observed that convective cooling dominates over other cooling mechanisms. The magnitude of the convection cooling is higher for higher contact opening velocity, immediately after the contact opening.