Conversion of Ungrounded Systems to High-Resistance Grounding Systems

An innovative method is proposed where charging current can be measured and grounding resistors sized without staging a fault. Overvoltages have caused damage to equipment and voltage transformer fuse blowing an ungrounded 4.8 kV delta distribution system. Three causes are suspected: single-line-to-ground faults, multiple restrikes and ferroresonance. The first is the most likely. The existence of ferroresonance can be determined by taking recordings of transient waveforms. The solution evaluated here is conversion to a high-resistance grounded system using banks of distribution transformers with broken-delta secondaries. Evaluation of system parameters shows that the grounding resistance should be in the range of 0.5 to 1.0 times the per phase capacitive reactance. This method will require that all equipment in the 4.8 kV distribution systems be rated for full line-to-line voltage. Ground fault current will be limited to less than 10 A. Simulation of multiple restrikes shows that overvoltages will be limited to 2.5 per unit. Properly sized high-resistance grounding can eliminate ferroresonance.