connecting a delta-wye transformer to a 12.47kV distribution system: pro's and cons

[meternerd]

Much food for thought. It is my understanding that we ground every third pole to a ground rod or wrapped pole bottom to try to minimize the distance a fault path has from the downed conductor through the earth and up to the neutral. Agreed that down wire can remain energized due to low fault current, especially on snow or in rocky soil. But we have to try to minimize the time a conductor is down, because many areas up here are sparsely populated (second homeowners, etc.) so we don't usually know about outages until a customer calls. Been this way since long before I got here, so I guess that's what the engineers decided was the best solution. Smart metering will help with outage notification, but still a real problem. As far as recloser tripping, on three phase circuits we try to keep the loads balanced between phases. Trip settings are such that a single phase condition will hopefully cause enough imbalance to trip on a "ground" fault. Fuses are still the best on single phase taps. Summer settings are non-reclose with one fast trip. Better to have an outage than spark a forest fire. I think I'm gonna back out of this thread....probably getting into areas way beyond Code. Thanks for the "fun"! Have a gidday.

 

[kwired]

Much food for thought. It is my understanding that we ground every third pole to a ground rod or wrapped pole bottom to try to minimize the distance a fault path has from the downed conductor through the earth and up to the neutral. Agreed that down wire can remain energized due to low fault current, especially on snow or in rocky soil. But we have to try to minimize the time a conductor is down, because many areas up here are sparsely populated (second homeowners, etc.) so we don't usually know about outages until a customer calls. Been this way since long before I got here, so I guess that's what the engineers decided was the best solution. Smart metering will help with outage notification, but still a real problem. As far as recloser tripping, on three phase circuits we try to keep the loads balanced between phases. Trip settings are such that a single phase condition will hopefully cause enough imbalance to trip on a "ground" fault. Fuses are still the best on single phase taps. Summer settings are non-reclose with one fast trip. Better to have an outage than spark a forest fire. I think I'm gonna back out of this thread....probably getting into areas way beyond Code. Thanks for the "fun"! Have a gidday.

Every pole around here has a grounding electrode installed. About only time you may find one without an electrode may be a pole that only has secondary conductors attached to it, and that is typically only if the pole is a support only and has no other equipment on it. Even transmission lines with no neutral still have a shield wire that is grounded at every pole.

Amount of lightning seen in different areas maybe has some impact on these kinds of design decisions.

 

[mbrooke]

Every pole around here has a grounding electrode installed. About only time you may find one without an electrode may be a pole that only has secondary conductors attached to it, and that is typically only if the pole is a support only and has no other equipment on it. Even transmission lines with no neutral still have a shield wire that is grounded at every pole.

Amount of lightning seen in different areas maybe has some impact on these kinds of design decisions.

Lightning does have a major impact in design. Florida utilites pound ground rods and use heavy duty surge arrestors every where and I mean almost every other pole.

 

[mbrooke]

Much food for thought. It is my understanding that we ground every third pole to a ground rod or wrapped pole bottom to try to minimize the distance a fault path has from the downed conductor through the earth and up to the neutral. Agreed that down wire can remain energized due to low fault current, especially on snow or in rocky soil. But we have to try to minimize the time a conductor is down, because many areas up here are sparsely populated (second homeowners, etc.) so we don't usually know about outages until a customer calls. Been this way since long before I got here, so I guess that's what the engineers decided was the best solution. Smart metering will help with outage notification, but still a real problem. As far as recloser tripping, on three phase circuits we try to keep the loads balanced between phases. Trip settings are such that a single phase condition will hopefully cause enough imbalance to trip on a "ground" fault. Fuses are still the best on single phase taps. Summer settings are non-reclose with one fast trip. Better to have an outage than spark a forest fire. I think I'm gonna back out of this thread....probably getting into areas way beyond Code. Thanks for the "fun"! Have a gidday.

:lol: I think I went overboard with info on that one:ashamed1:.


You are correct. Having a ground rod and wire on the pole reduces the distance the fault has to travel to get to the ground wire. In all truth when the soil is dry or frozen many times over a ground wire becomes the best path. Hence why its always nice to have one.


If it helps the rules that apply to LV also hold true for MV.


Hope you stay, your info is interesting

 

[george89]

@GoldDigger: Hi, I want to know little more about the solution of circulating current as you mentioned we can use wye to delta but common point of input is neither connected to neutral or ground and what will happen if we will connect the common point of input to neutral?

 

[GoldDigger]

@GoldDigger: Hi, I want to know little more about the solution of circulating current as you mentioned we can use wye to delta but common point of input is neither connected to neutral or ground and what will happen if we will connect the common point of input to neutral?

It is very simple. The wye point of the upstream source is connected to ground, so you will not leave the source ungrounded if you do not connect a local ground at the primary.
If you connect the source neutral to the primary wye point you are rigidly fixing the three primary winding voltages. If they are, for any reason, not balanced (not producing a zero volt path around the delta) currents will flow in both primary and secondary until the IR voltage drops in the windings make the voltages balance.
This can cause far greater than full load currents on the primary neutral which may not trip over current protection, with very unfortunate results. If you leave the wye point open it can shift its voltage until a balance is reached instead.

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