Surge Arrestor - 3 Wire Medium Voltage System

[mvsubstation]

How should a surge arrestor be rated on an overhead 12kV, 4 wire grounded neutral system that drops the neutral halfway along the circuit and becomes 3 wire. Would it still be considered solidly grounded system at the end of the 3 wire line, or sized like it's ungrounded?

 

[Gragas]

There are several standards for surge arresters that should get you where you need to go.

 

[Mbrooke]

Once the neutral is dropped I would begin to consider treating it the way you would with an ungrounded/high impedance grounded system (phase rated insulators and suppressors) since soil resistance can get to be quite high.

If soil resistance is guaranteed to always be low and ground rods always good, you can calculate the voltage rise of a fault using the ground resistance and select a lower level suppressor based on the maximum voltage rise that can occur. If low enough you can get away with standard insulation.

 

[Marmite]

The method of grounding is determined at the source. If the star point of the source transformer is solidly grounded that doesn't change if you drop the neutral half way along the line.
Regards
Marmite

 

[Mbrooke]

@ Marmite, I disagree. In my area when ever a line relies entirely on the earth for fault clearing/detection insulation values increase on everything regardless of source substation grounding. The reasoning behind it is, for example, if a pole with a very high soil resistance has a single phase come in contact with the cross arm the voltage may rise up to full phase potential on the 2 remaining insulators and suppressors given ground resistance is high enough. The impedance is to high to cause a voltage sag equal to or less than the phase neutral voltage.

On a line with all loads being Delta connected there is a good chance line devices would trip, since ground pickup can be set at a very low value. (few amps) However,In a system where the the equipment grounding conductor is also used as a load neutral (multi-grounded neutral system) (which may be the case with the OP), the breaker/reclosers are set often with a very high ground pick up value. In the USA usually 1/3 to 2/3 of the phase pickup value. That combined with high soil resistance may result in over voltage values for an extended period of time without load showing it (ie, voltage drop, flickering)or line equipment clearing.

 

[Mbrooke]

Just forgot to add above, When the grounding neutral is run with the lines, it guarantees a low impedance path for fault current to travel regardless of ground rod resistance. However when relying on ground rods alone, its a gamble, soil resistance governs how much current will flow during a fault. It may be a lot, or a little. In the little case voltage will rise around a faulted pole/transformer ect on the 2 remaining phases.

 

[Marmite]

In the situation Mbrooke describes, the protection clearance time could be long. In that case the arresters should have a minimum continuous operating voltage of at least the system voltage. Normally a safety margin of plus 10% would be added on top. As a rule, in medium voltage systems the withstand
voltage values of the insulation are rather high in relation to the system voltage.
Regards
Marmite

 

[waross]

You can't rely on the neutral to carry the ground fault current, especially on a distribution line. A line to ground fault may not be the same as a ground fault. A line down will probably fault to ground with little or no involvement of the neutral conductor.
And a line to neutral fault far out on a distribution circuit may exhibit some of the characteristics of a line to ground fault due to the impedance of the neutral system. (Neutral system; the combined impedance of the neutral conductor and parallel paths through the multiple grounding points that are common on distribution circuit neutral conductors.)

Bill
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"Why not the best?"
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