Grounding of distrubution grids : High impedancegrounding compared to solid grounding with Fault Current Limiter

Sammanfattning: Today cables replace overhead lines in distribution systems and cause higher system capacitance and higher capacative fault currents. This fault current, in the fault location, is limited by a Petersén coil and a resistance in parallel when a fault occurs in so called “high impedance grounded systems” which are commonly used in Europe. The high impedance however has the disadvantage that it needs to be adjusted to the capacitance in the system in order to optimize the limitation of the fault current. Another option is to use a solidly grounded system with a so called “Fault Current Limiter” (FCL) instead, on the outgoing terminals of the transformer feeding the system. The FCL interrupts the outgoing current on the load side of the transformer that feeds the system, in case of a fault, by forcing the current to a zero with a counter voltage. The FCL in this master thesis triggers on 1.5 times higher current than nominal current. After fault clearing the FCL is reclosed. The FCL has the advantage that it does not have to be adapted to the capacitance in the system. It should be noted that the current in all of the phases (even the healthy phases) are interrupted downstream the FCL. It also has the ability to interrupt the single line-to-ground fault before it develops into a three phase fault. This thesis presents a comparison between the high impedance grounding and the FCL. The characteristics of the two grounding principles are investigated in a distribution system from Vattenfall. The assessment is done via simulations in the program PSCAD with three types of grounding of the transformer; high impedance grounding, solid grounding (without FCL) and solid grounding with FCL. The system is simulated with the faults “single line-to-ground” and “three phase short circuit” respectively, even if a line-to-line can occur. The results from the simulations show that the FCL gives a short fault duration time and a possibility to limit both single line-to-ground faults and three phase faults. The high impedance grounding on the other hand is able to limit single line-to-ground faults whereas it is generally known that not able to limit three phase short circuits.