IEEE NESCIR559-2009 pdf download Grounding methods for electric supply and communications facilities
Question: The opinion of engineers in the industry is divided on whether the“four- grounds each mile” stated under Rule 96C is applicable to medium voltage underground cable systems for utility scale wind farms. These wind farm medium voltage cable systems typically consist of three underground single phase cables with Jacketed Concentric Neutrals (JCN). Each wind turbine would connect to the medium voltage system via LV/34.5 kV step-up transformer.
The 34.5 kV underground cable system would be terminated at a 34.5 kV/HV substation before the wind farm is interconnected to the grid. A typical one-line diagram of a wind farm is shown below. The 34.5 underground cable system could have a Delta connection at the turbine step-up transformers (LV/34.5 kV – Wye/Delta) and a Wye connection at the main power transformer at the substation. The cable concentric neutrals would be effectively grounded and bonded to the main power transformer neutral at the substation. The concentric neutrals will also be bonded to the low voltage secondary neutral of the turbine step-up transformers.
For the underground cable system described above, is the “four-grounds each mile” requirement stated in Rule 96C applicable? Additional information on different opinions in the industry is provided below. The additional details furnished are for information only and intended to further clarify the main question for which a code interpretation is sought.
Discussion: The 35 kV direct-buried cable concentric neutrals (JCN) of a wind farm collector system are typically grounded to meet four ground points each mile to meet the NESC Rule 96C requirement. Questions have been raised regarding the applicability of Rule 96C when the wind-turbine step-up transformers (LV/34.5 kV) have an ungrounded Delta connection on the high side (34.5 kV side). The main rationale presented for not applying the “4 grounds per mile” is as follows.