Three phase load preference

[hurk27]

Re: Three phase load preference

I guess I should restate what I added to my other post as after thinking about it in the several times I was called out for this problem it was only in three cases that there was a loss of the MGN that caused primary return current through the service grounding that turned out to be the cause. So I guess it wouldn't a main reason.
The other case it was a loss neutral at the transformer. another was a break in the underground neutral on the secondary side after the transformer, and one was even voltage drop in the neutral between the service and the transformer 800' run the POCO ran a triplex 250,250,250,2/0 AL. and the 200 amp service was very unbalanced as well as very over loaded. The meter was tapped to feed the barns as well as the house, and all was using the neutral as the grounding conductor.
We did have one that had the MGN snapped by a falling tree down the road about a ? mile. I guess it went unnoticed until the grounding to Earth got weak and the voltage started to rise. the home owner remembered that that tree fell about 3 months before this, from a storm but didn't knock out the power.

But I don't see where current can pass through a transformer as current can only return to source. unless it is caused by another service down the road fed from the same transformer bank or from the primary side? why would an open 4-wire delta cause voltage to Earth on the neutral on the secondary side of the transformer that a 120/208 Y wouldn't? as the current should return back to source (generator plant) and not be drawing current from Earth?

And what about all those old 240 volt motors that don't like 208? The farmer would have to change them or add boost transformers.

 

[charlie]

Re: Three phase load preference

Wayne, that is why we have to investigate and document what we find. In rural areas, we may have miles of 1?, 2?, and 3? taps extending down roads to serve farmsteads. If we have an open neutral or a bad neutral connection on the primary circuit, the return will be through the earth to the substation even on a 1? line. Some of the Amish in your area have had problems with stray voltage because of that (the Amish do not use electricity).

It is foolish to spend megabucks to correct a problem that doesn't exist. On the other hand, we do investigate stray voltage complaints and find open primary neutrals on our side as well as bad wiring on the customer's premises. I do not believe we have ever changed a bank configuration for a customer because of stray voltage but we have responded to the complaints and cleared up the problems.

 

[jtester]

Re: Three phase load preference

wayne
I was wrong in my original statement about converting from delta to wye. I dug out my file on the incident, and the utility had changed from an open wye-delta to a closed delta bank, not to 120/208.
It seems that removing the neutral on the open wye side of the transformer has a significant effect on n-g voltages on the farm.
My concern should not be for converting to 120/208 as much as closing the delta on open wye open delta banks. I am not convinced that open delta-open delta banks would exhibit the increase in n-g voltages on the farm.
Sorry for the error in my thinking.
Jim T

 

[don_resqcapt19]

Re: Three phase load preference

My understanding that the following three things are the most common causes of stray voltage problems on farms.
1) Multiple grounding to grounded connections on the load side of the service.
2) Open or high resistance connection on the primary neutral.
3) Use of undersized primary neutrals on the MGN distribution system causing more of the neutral current to return via the earth.
Don