Transferred Potential for HV Transformers

[McN41R]

The Project is very large. it has a primary substation at each end 66kV/11kV which will feed a series of secondary substation at 11kV/400V. We will distribute at 11kV and install the above 11kV/400V transformers. In additional we will install LV, ELV and Mechanical systems associated with the infrastructure network.

The 11kV System, the 400V system, 230V lighting and power, District Cooling systems, UPS Generator Systems, Ventilation systems will all be installed in networked service tunnels (big enough to drive trucks through) underneath the project.

My concerns are, generally the risk of any any transferred potential via delibrate or non delibrate earth paths if the transformer faults 11kV to earth (ie fault to the transformer tank).

1. Is there a risk of transfered potential arrising on the LV System and any other earthed systems in the the service tunnels?
2. Do 11kV/400V transformers require any special protection to limit the duration of earth fault or the magnitude of earth fault when installed in shuch a specialist area?
3. Do I need to provide any type of surge, transient overvoltage protection to ELV systems such as telecoms?
4. Any advice on the best method and frequency to provide bonding (equalisation ring) of all services within the service tunnels.

Thank you for any ideas.
Regards

 

[jghrist]

Transferred potentials are more likely to be a problem for a 66 kV ground fault. 66 kV ground fault current will return partially through the ground grid and the earth, causing a Ground Potential Rise (GPR) in the ground grid and anything connected to the ground grid. This could cause touch-voltage concerns and concerns with voltage between telecom equipment bonded to the local ground and telecom cable shields grounded remotely.

If there are metallic ground connections between the 11kV/400V transformers and the 66kV/11kV transformers, the different ground systems are tied together, and the distances are not too great, then the voltage rise for a fault at the 11kV/400V transformer will not be that large because not much current will flow through the earth.

Protection of the transformers to limit fault duration is more for the protection of the transformers than for voltage rise considerations.