C I R E D 18 th International Conference on Electricity Distribution Turin, 6-9 June 2005 A NEW METHOD FOR TESTING AND CALIBRATION OF HIGH-RESISTANCE GROUNDING SYSTEMS Peter SUTHERLAND Arshad MANSOOR EPRI Solutions, Inc. - USA peter.sutherland@ieee.org INTRODUCTION 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. GOUNDING SYSTEMS Distribution primary circuits typically have one of the following wire configurations: three-wire ungrounded, three- wire unigrounded, four-wire unigrounded neutral system or four-wire multigrounded neutral system. The ungrounded circuit historically has been selected for those systems where service continuity is of primary concern. One of the issues with an ungrounded system is transient overvoltages due to restriking or intermittent ground faults. These type of faults can and do develop substantial overvoltages on ungrounded electrical systems with respect to ground. There have been many documented cases of measured line-to-ground voltages of 3 pu or higher resulting in equipment damage. In all instances, the cause has been traced to a low-level intermittent arcing ground fault on an ungrounded system. One possible solution is to convert the ungrounded system to a high resistance grounded system by deriving a neutral point through grounding transformers. [1] [2] Ungrounded systems An ungrounded feeder in a power system might look like Fig. 1. The source would be a distribution substation transformer with an ungrounded delta or wye secondary. Three-phase loads would also be ungrounded delta or wye connected. Single-phase loads would be connected phase to phase. The transformers and feeder conductors will contain distributed capacitances, which form the unintentional grounding system (Fig. 2). In the transformer, these are mostly from windings to the grounded core. In cables, these are mostly from phase conductors to grounded shields. In overhead lines, these are both phase to phase and phase to ground. Y-gnd ∆ φ-φ 1-φ Y-gnd ∆ Source 3-φ Load 1-φ Load Fig. 1. Ungrounded three phase feeder Transformer windings to ground Phase to phase Phase to Ground Lines or cables Phase to Ground Fault Fig. 2. Distributed capacitances in ungrounded three-phase feeder. The sequence network derived from this feeder configuration results in the distributed capacitances being shunted across the positive, negative and zero-sequence networks, as shown in Fig. 3. The capacitances have little effect on the positive and negative sequence networks, as they are shunted by the source and transformer inductance. However, capacitance is the dominant component of the zero-sequence network, as the zero-sequence capacitive reactance is usually much larger than the conductor impedance. CIRED2005 Session No 3