Analysis of Short-Circuit Transients in the LHC Main Dipole Circuit and Development of an Automated Analysis Algorithm

The Large Hadron Collider (LHC) spreads over a total distance of 26.7 km and comprises 8 sectors. In each sector there is one main dipole circuit, where 154 superconducting dipole magnets are connected in series. Since 2007, there have been 19 occurrences of short-to-ground faults in the superconducting LHC main dipole circuits, making their analysis and understanding necessary for the efficient operation of the accelerator. In the case where a short to ground event occurs, the short current that flows through the resistor to ground also flows through a fuse that is present in the grounding subcircuit. After the occurrence and detection of a short to ground, a fast power abort is triggered and the current in the circuit starts decaying semi-exponentially from a maximum value of 11.85 km to zero, with a time constant of about 103 s. During such an event, the current also flows through the fuse that is present in the grounding subcircuit. Depending on the value of its thermal load, the fuse first enters a pre-arcing region where it starts intermittently blowing up, until the blow-up threshold is reached, after which the fuse stays definitively blown. A simulation scheme utilising a common interface between PSpice and MATLAB is proposed in order to simulate the blow-up behaviour of the fuse and to increase the accuracy of the circuit model. A parametric analysis of the short to ground parameters is performed and a better understanding of the circuit's behaviour under different conditions is obtained. The worst-case values of the voltage to ground in the LHC main dipole circuit are identified for the case where the intermittent behaviour of the fuse is included in the model and a comparison is given with the values obtained when the behaviour of the fuse is not modelled. The appearance of a fault in the circuit requires the immediate switch-off of the machine, so that experts can visit the site and resolve it. Due to the large circumference of the LHC, searching for the fault's position without any prior knowledge requires a large amount of time, increasing the need of a more automated solution able to provide information regarding the short circuit. With a better understanding of the circuit behaviour after the occurrence of a short to ground event stemming from the first part of the thesis, an equivalent circuit of the LHC main dipole circuit for short transients, that can be solved analytically, is derived. An algorithm is proposed to take advantage of the reduced time needed to solve the system analytically, when compared to a numerical approach. The algorithm is able to provide information regarding the short location, as well as identify the range of values in which the short resistance belongs. This greatly reduces the time needed by an expert to analyse a short-circuit event in the LHC main dipole circuit. The algorithm is tested using measured data from a real short-circuit event. Due to the fact that the simplified circuit model consists only of inductive and resistive elements, the algorithm is flexible and can also be applied to different accelerator magnet circuits.