Given:
o Generator: 1.6MW, 480Y/277V, 60Hz.
o GSU Connection: YG-YG (solidly grounded).
o Assumption: Industrial application interconnected with a power utility in US.
[blue]COMMENT: YG-YG connection may not the most desirable configuration because during ground fault event, the current is divided in some undetermined ratio between the ground legs on utility and customer side of the transformer. This imprecision makes it difficult to determine the setting on the ground fault relay and to coordinate it with other relay in the network. On the other hand, harmonic may be a concern since small and mid size generators are prone to generate high level of third harmonics while the YG-YG transformers do not suppress the third harmonic as the D-YG does. The consequences are additional cost for filters schemes or nuisance tripping since the OC relay may see the high level of harmonic as a ground fault. [/blue]
Referring to grounding the generator neutral:
- What is the best method of grounding and minimum protection (relaying) necessary for the generator? The protection of the generator will be greatly imposed by the transformer winding configuration (YG-YG) and the utility requirement for IPP interconnection at the common point of delivery . Since appears the transformer connection is alredy set, the other compliance should be with your local utility. NOTE: Request information for the interconnection requirement for small power generation and consult the IEEE 1547 Standard for Interconnecting Distributed Resources with Electric Power System.
a- Is a floating neutral (ungrounded) possible (at the generator)? Yes, it is possible to have ungrounded neutral. Although this limits the phase-to-ground fault current, generators are seldom ungrounded because of the risk to produce high transient overvoltages during fault and difficulties associated with fast fault location.
b- If the neutral will be grounded, what are the options? The methods most commonly used for generator grounding are high-impedance, low-resistance, reactance and grounding transformer. Check the ANSI/IEEE STD C37.101 & 102 “Guide for generator ground protection”. NOTE: Solidly grounded generator neutral is not recommended in mid and large size unit since this practice can result in high mechanical stresses and excessive fault damage.
o Is reactance grounding better than resistance grounding? The industrial practice in the US prefers resistance grounded over reactance grounded because reduce significantly the fault current without risk of overvoltage. Reactance grounded, on the other hand, have typical limiting fault current values of 25% to 100% of the generator three-phase fault and much higher overvoltage than resistance grounded.
o What if utility company requires limiting the zero sequence current to say 25 Amps? High resistance grounded could meet this requirement. The most economical choice appears to be connecting directly the resistor to the generator grounding.