In our little island system, we ran the neutral high between population centers for lightning protection. In built-up areas we ran the neutral low to avoid possible elevated ground potentials despite multiple grounds in the event of a lightning strike. It was somewhat of a judgement call to decide whether to run the neutral high or low.
Personally I would not run the neutral high in the vicinity of service transformers. I want to see a couple of ground connections away from any transformer before transitioning to a high neutral.
Hope this helps.
Bill
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"Why not the best?"
Jimmy Carter
Thanks Davidbeach & Waross for your inputs.So it its possible not use overhead static wire by neutral high method, but aside from possible elevated ground potentials is there any other parameters must be considered using neutral high? we're considering to switch to netral high due to reports of neutral wire thefts in our system.
Be aware that a lightning strike on a neutral close to a service transformer may result in the ground connection failing and high voltages being impressed on the neutral throughout the customer's premises.
An anecdote:
A friend of mine was the investigating engineer after several fires were started.
This was in a city in Central America. The safety grounds were open or in bad order.
The distribution system was 13.8/7.8 kV.
Very light gauge PVC tubing is commonly used as raceway in buildings.
The event started when the ties fastening a line conductor to an insulator failed and the conductor fell onto the cross arm. There was not enough current to ground to trip the protection.
The 7.8 kV tracked across the cross arm to the system neutral conductor. Due to the poor condition of the multiple grounds there was still not enough current to trip the protection.
The "Arkin' and the Sparkin'" continued until the condition was reported and the circuit was cut off at the sub station.
By that time there were fires in at least two buildings.
The 7.8 kV impressed on the neutral had caused several insulation failures and the resulting arcs had started fires in the buildings.
It is incredible that the buildings were saved with minor damage and there were no deaths or injuries.
With this kind of damage from 7.8 kV I would be very cautious with any arrangement that may allow lightning potentials to propagate on customer neutrals.
The first question many will ask and I would ask myself in North America is how could the protection not trip with a line to neutral fault?
David has heard me rant against the dreaded four-wire-wye:delta connection many times. That connection may have contributed to this event.
The event was in an area where the four-wire-wye:delta connection is common.
Due to the frequency of blown primary fuses with the four-wire-wye:delta connection it is common for service crews to progressively increase the size of fuses and increase protection settings. #12 AWG wire in place of fuses in fused cutouts is common and substation protection settings may be increased to ridiculous levels in an attempt to avoid "nuisance" tripping.
Bill
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"Why not the best?"
Jimmy Carter
Dang. I hit submit when I meant to enter preview.
Further to the above;
The investigating engineer told me that he was sure that the scene was "Sanitized" before he arrived. Any high settings and/or blocked trips and excessive fusing would have been returned to normal before he arrived.
Bill
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"Why not the best?"
Jimmy Carter
