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Grounding System Design

Practical Grounding for a Large Renewable Energy Installation

Scope of Work

Designing an adequate grounding system can be especially challenging when the footprint of the subject substation is relatively small and / or the soil resistivity is high. Higher fault levels, due to the addition of new generating sources to the network, can make it even more difficult to coordinate safe step and touch potentials within the allowable limits.

Therefore, the design of most recently-installed grounding systems in Ontario includes a counterpoise to lower the overall ground grid impedance. However, adding a counterpoise will transfer the ground potential rise during fault conditions to points hundreds of metres outside the station, increasing the zone of influence.

Kinectrics was retained by the client, a group of private wind farm developers, to review the grounding systems of more than ten local wind farms. The scope of the work included the turbines, collection, and switching stations. The project objective was to optimize the grounding system design, and minimize the use of counterpoise therein.

Project Results
Determining the portion of the zero sequence fault current that goes into the grounding system during fault conditions is one of the most important aspects of grounding system design.
Transformer configuration also plays a key role in calculating the zero sequence fault current returning to the remote sources through the soil and, in identifying the circulating current back to the transformer. Although ignoring the zero sequence analysis at the transformer stations is a conservative assumption, this conservative assumption can result in a design with more than a 50% safety margin in some cases. Most of the designs reviewed by Kinectrics did not consider this calculation, which resulted in overly conservative designs, including long runs of counterpoise to coordinate step and touch potentials.
Moreover, the collection cables of the feeders connecting the turbines to the substation usually have concentric neutrals that are grounded periodically. To calculate the driving point impedance of the collection feeders, the series impedance of the concentric neutrals must be known. The mutual impedance between the concentric neutral and the faulted phase comes into play when calculating the portion of the fault current that inductively returns through the concentric neutrals.
Proper modelling of this external connection to the ground grid involves modelling the cross section of the cable installation to derive the impedance matrix considering Carson’s earth return path through the soil. Most of the designs that were reviewed by Kinectrics had ignored modelling of the concentric neutrals, resulting in an overly conservative design for the grounding grid of the turbines and the collection station.


Client Benefits

Accurate modelling of the grounding system of the wind farm substations and turbines by Kinectricsconsidering a realistic safety margin of 5%-25%resulted in a significant reduction in the total required length of counterpoise. For example, in the case of a 60 MW wind farm project with almost 40 turbines, a total of 40 km of 2/0 AWG counterpoise was removed from the original design of the grounding system.

The subsequent savings were not limited only to the cost of the conductors, but also greatly reduced the larger costs of installation and coordination with nearby utilities.