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33kV system earthing

33kV system earthing

33kV system earthing

(OP)
Can you tell me the type of system earthing (solid or low resistance earthing) practiced at 33kV level in your country!
Also, I would like to have your view on the best suited earthing type for 33kV system that includes overhead line distribution on wooden poles without any shield/earth wire and through harsh soil (desert) conditions.
Thanks in anticipation.

RE: 33kV system earthing

First we don't have 33kV level, the closest are 27.6 and 44.
The method of system grounding depends on what type of application your 33 kV system services for. If it is more for distribution which means there are mixed 3-phase and single phase customers then it can be an effectively grounded system. If it is most like a sub-transmission then it could be impedance grounded, or even a 3-wire ungrounded system.

RE: 33kV system earthing

(OP)
Thanks pwtran for sharing.
In our case, 33kV system extends power supply to the oil wells etc. through 33kV overhead lines and individual step-down transformers. The wells sre deep in desert and the 33kV OHLs are long. What do you think is the appropriate type of system earthing - solid type or low resistance type? Do you have similar systems in your company / country!
Thanks.

RE: 33kV system earthing

raghun, if the operation of the oil wells cannot tolerate single line to ground fault which is about 90% of the total faults, then an ungrounded system is a valid option. You can calculate the capacitive fault current using:
Ic=3*ωC*U
you can convert ωC value in the above equation to per unit value multiplied by the length of the OHL. If the capacitive current is more than 10A you may consider to install a Petersen coil as 7anoter4 pointed out to extinguish the arc.

RE: 33kV system earthing

My present company is mostly urban so we do not have a shield, and is only grounded at the source. However because of under build there is frequently a neutral although all our loads are three phase.

My old company had significent rual applications of 34.5 kV, but again they grounded at the source only. However they frequently did carry a shield above the phases. It maybe a streach to only call it a shield because 80 MCM Grouse has a significent amount of aluminum as well as steel. So they could use the shield for single phase loads, as well as lightning protection.
They also had several platform step down stations in areas that were almost towns, where the platform had the three transformer, and another platform had a set of regulators.

One note is if all your single phase loads are 25 kVA or less than a 100 amp ground setting will coordinate above the transformer fuse.

RE: 33kV system earthing

Our distribution system is mostly 25kV with an overhead neutral and is grounded every 3 kilometers. Our requirement is to ensure that the neutral to ground resistance not exceed 6 ohms. With a low to non-existing water table what sort of restivity are you presently seeing?

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