Why higher substation earthing resistance is allowed for MV and LV systems?

[Power0020]

I feel a bit confused about the requirement. Some standards ask for 0.5 to 1 ohm for EHV substations which is justified by the need for lower GPR and also fast acting protection.

However, for MV installations in IEC world, most of zone substations have an earthing resistance sometimes in the range of 5 ohms, the fault current is limited by NERs usually so that the resultant GPR will not be excessive.

What if the MV neutral point is connected directly to ground? I couldn't figure out any standard recommendation to reduce the resistance. or should it be needed?

 

[krisys]

Power0020,

Generally medium voltage system earthing is to limit the earth fault current. The NER (or NGR) resistance is derived from the desired earth fault current in the system.

As I know, the selection of desired earth fault current is not to control the GPR, it depends on the system insulation level, earth fault protection philosophy, the utilisation equipments connected to that voltage level, mainly medium voltage motors.

The lower the system earth fault current, less damage to the iron and slots of the rotating machines. If it is too low, then the earth fault detection becomes the issue. The phase to earth insulation requirement increase. Even there can be arcing due to voltage re-strikes.

 

[Power0020]

Well, the faults in a HV system can be quite high with no NER, if an NER is introduced, over-voltages on healthy phases will require additional insulation so that these are limited to MV range.

The system earthing resistance, not NER only, has to do with substation GPR.

 

[krisys]

Power0020 (Electrical),

GPR has no bearing on the equipment insulation level. When the entire ground is at higher voltage level due to high GPR, only the reference voltage level will be increased. However, the voltage between the conductors and the conductor to reference point would remain the same.
Hence in my view there should not be any risk for the equipment.

 

[7anoter4]

BS 7430:2011 Code of practice for protective earthing of electrical installations
Annex A [informative] 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 HV equipment 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.”
That means, in my opinion, it will be close to IEEE 80/2013.