Substation Ground Grid Resistance Limit 1

[cuky2000]

If step and touch potential are safe,Why many utility ask for low grid resistance 1 Ω or less for transmission substation?
Please let us know if any comments on:
1) Concern with transfer potential on telephone line in substation. (Today there is fiber optic and better surge protector for isolation).
2) Some engineers are concerned with large GPR=I_ground x R_subimposed imposed on low insulation for auxiliary AC & DC cable and control wires inside the substation.
3) Some engineers were mentioned concern with the zero sequence resistance for relay protection and control. (Not sure if this is an issue with digital relays).
4) Some countries such as Canada limite the GPR for <5 kV. In the USA the IEEE Std 80 have indicative value of 1 Ohm or less indirectly control GPR.

 

[7anoter4]

BS 7430:2011 Annex A (informative) Guidance on typical HV/LV interfaces.A.2 Legislation:
The Electricity Supply Regulations (replaced by The Electricity Safety, Quality and Continuity Regulations in 2002) required that where in a substation the HVequipment earth and the LV neutral earth were common, that the resistance to earth has to not exceed 1 Ω. For most substations this value was provided by the un-insulated protective sheaths of the older types of cables in use and was normally sufficient to lower the impedance of these cable sheaths to ensure sufficiently low earth potential rise (EPR) for general combination of HV and LV earth systems even with very high earth-fault current. However this simple requirement is no longer adequate. The current advice of the Health and Safety Executive is that touch voltages should not exceed curve the recommendations of BS EN 50522, National Annex NA.2.

 

[Power0020]

Some utilities require 220 kV substations and above to have a grounding grid resistance less than or equal to 0.5 ohm. I think it has to do with GPR as well as network stability, since higher resistance will cause lower ground fault currents and slower relaying means longer faults and accordingly, more rotor angle fluctuation.

This is the classical understanding I think, based mostly on slow electromechanical relaying and copper communication cables.

 

[7anoter4]

Years ago the trend was 0.5 ohm maximum for lightning protection purpose.
Now 10 ohm it is recommended. See:
IEC 62305-3 5.4 Earth-termination system 5.4.1 General :
When dealing with the dispersion of the lightning current (high frequency behaviour) into the ground, whilst minimizing any potentially dangerous overvoltages, the shape and dimensions of the earth-termination system are the important criteria. In general, a low earthing resistance (if possible lower than 10 when measured at low frequency) is recommended.