HYBRID GROUNDING IN OFF-SHORE UTILITY PLANT

The electrical power system in offshore oil & gas installation, consists of a large distribution network, generally operating in island mode i.e., without grid support. For a compact utility plate form design, multiple gas turbine-generators without generator transformers, feed directly to 11kV switchgear. Such a configuration however, introduces high capacitive charging current (Ico), which is more than the preferred high resistance grounding of generator neutral through 10A, 10sec resistor, to safeguard the generator core from damage during an earth fault. Therefore, some utility prefers to select low resistance grounding to limit the fault current above Ico; however this can cause severe damage to generator core. Generally, oil & gas installation is a customized design. So, earthing scheme of 11kV generating utility system should be selected judiciously at basic engineering stage to avoid equipment damage and protection mal-operation during operation. Different methods of earthing scheme are available to mitigate the same. One of the method is presented here in which generator neutral is connected to high resistance grounding and 11kV switchgear connected to low resistance grounding though zig-zag transformer, subject to single grounding operation at a time. Prior to synchronization or under complete load throw scenario, generator circuit breaker is opened. So, an earth fault in generator or evacuation system, create over-voltage or ferro-resonance conditions, stressing insulation of generator and associated system. This is mitigated by putting neutral earthing resistor into service at generator neutral. This paper presents the experience learned in designing neutral earthing scheme for off-shore utility plant in view of high capacitive charging current at 11kV voltage level, outlines impact on stator core damage, mitigation and conclusion.