Utility Transformer

[ACE88]

Hi,

I have a couple questions regard the transformer and how they connect in different configuration.

1. what the difference between single bushing primary and double bushing primary and why are we using one vs. the other.

2. Why for some instance, the first bushing would be connected to the Oh primary and we would connect the second bushing to the neutral?
Aren't we provide a path to ground for the primary voltage (since the neutral is ground at every other pole or certain amount of distance).
If that's the case, wouldn't all the voltage or majority of them take that path instead of keep going down the feeder line?

OH Primary -> Fuse -> Double Bushing transformers with arrestors mounted but one of the bushing is connect to neutral which is path to ground.

Thank you,

 

[don_resqcapt19]

The number of bushings that are required is based on the transformer use. You only need one bushing for transformers that are connected line to neutral, but you need 2 for transformers that are connected line to line. It is really a cost issue.

As far as the current going to the neutral, it is no different than any other line to neutral load. The only current that flows is the current that load requires.

 

[wirenut1980]

Single primary bushing transformer has the primary neutral (H2) internally bonded to the ground lug, which is then bonded to the pole ground/system neutral. Two primary bushing transformers has the primary neutral connection exposed on top.

Single primary bushing overhead transformers are typically used for single phase (split phase) services, and 3 phase solidly grounded wye-wye transformers banked together. Two primary bushing transformers can be used on these same services, but are required to be used on grounded wye-delta transformers banked together. The reason two primary bushing transformers are required on grounded wye-delta transformer banks is because the primary neutral is required to be open, which physically cannot safely be done with a single primary bushing transformer. If the primary neutral of the transformer bank is not open, the transformer bank will feed fault current into faults onto the medium voltage system, and attempt to balance voltage on the medium voltage.

Regarding why there isn't a ground fault when connecting a transformer between phase and ground, the answer has to do with transformer theory, and the fact there is self inductance, which limits current flow between phase and neutral. I am not great at explaining the theory.

 

[Sahib]

Hi,

Why for some instance, the first bushing would be connected to the Oh primary and we would connect the second bushing to the neutral?

To keep neutral to ground resistance as low as possible and to reduce the effect of direct lightning strike.

Aren't we provide a path to ground for the primary voltage (since the neutral is ground at every other pole or certain amount of distance).
If that's the case, wouldn't all the voltage or majority of them take that path instead of keep going down the feeder line?
Thank you,

A significant percentage of load neutral current returns to source via due to multiple grounding of POCO neutral. This is the main reason behind stray voltages all over the land of US.

 

[wirenut1980]

A significant percentage of load neutral current returns to source via due to multiple grounding of POCO neutral. This is the main reason behind stray voltages all over the land of US.

I disagree with the term significant. Almost all current returns on the utility neutral unless there is a bad connection on the utility neutral. You are correct that there is stray voltage, however cases of this affecting people or animals are rare, and in almost all cases, limited to issues with pools and on farms. With investigation, the affects of stray voltage can be overcome with proper bonding around pools and in barns, or through the use of a neutral blocking device such as a Ronk blocker.

And in many cases, the stray voltage is caused by a fault that is not cleared on building power distribution systems.

 

[kwired]

Earth has a pretty constant potential, it is when you bring an otherwise grounded conductor with some voltage drop on it near something that is at earth potential that you see the so called "stray voltages"

 

[mivey]

Almost all current returns on the utility neutral unless there is a bad connection on the utility neutral.

The zero sequence current is roughly half on the neutral and half in the Earth.

 

[wirenut1980]

The zero sequence current is roughly half on the neutral and half in the Earth.

Interesting, is there a case study where this has been proven that you can point me to? And is neutral return current considered all zero sequence, or some portion? I suppose it is possible with hundreds of parallel paths from utility pole ground to earth. But the many field measurements I have taken do not agree with what you stated.

I have measured phase and neutral current on many utility distribution circuits (3 phase and single phase), and the amount of neutral current I would expect to see (compared to calculations) has almost always been very close, and nowhere near half. In the rare case where the neutral current was much less than expected, there was a cause found such as an open, or corroded connection in the neutral.

 

[domnic]

transformer bushing

transformer bushing

What is a transformer bushing and how are they used ?

 

[wirenut1980]

What is a transformer bushing and how are they used ?

A bushing is where the connection is made between (in this case) a transformer and a conductor (primary or secondary). On overhead utility transformers, the primary bushings are typically made of porcelain and have "skirts." Their function is to provide insulation between the conductor and the grounded case of the transformer. Generally, the bigger the bushing, the higher the voltage is between phase and ground.

 

[mivey]

Interesting, is there a case study where this has been proven that you can point me to? And is neutral return current considered all zero sequence, or some portion? I suppose it is possible with hundreds of parallel paths from utility pole ground to earth. But the many field measurements I have taken do not agree with what you stated.

There is case info and you can calculate it as well. I used zero sequence to avoid using "neutral" term as a qualifier for both conductor and current but yes, zero sequence current is the unbalanced or return current that would flow in the non-phase conductors.

I have measured phase and neutral current on many utility distribution circuits (3 phase and single phase), and the amount of neutral current I would expect to see (compared to calculations) has almost always been very close, and nowhere near half. In the rare case where the neutral current was much less than expected, there was a cause found such as an open, or corroded connection in the neutral.

It can depend on where you are in the circuit, the single-phase load location in the circuit, and the circuit length. The most dramatic will be with a long single-phase feeder with a concentrated load at the far end. Calc that and you will see significant Earth current.

I'll try to remember to find you some references when I get back to the office if you can't locate any. I did not try Google but that may turn them up quicker than me digging in my stuff.

 

[mivey]

FWIW, the Earth current can be higher than neutral conductor current. It would not be unheard of to have a 60/40 split of Earth/Neutral conductor current.

 

[mivey]

I'll try to remember to find you some references when I get back to the office if you can't locate any. I did not try Google but that may turn them up quicker than me digging in my stuff.

I am not at office but two good reference books come to mind:

Power System Grounding and Transients: An Introduction - A.P. Sakis Meliopoulis

Distribution System Modeling and Analysis - William H. Kersting


You can also search for Carson's Equations as it is the basis for these calcs. Either read the original paper, Edith Clarke's work with them, or other published works referencing them (like the two above).

I made some calcs on a 6 mile single-phase line with light loading and got an average of a 54/46 N/E split with good Earth connections and low resistivity to an average 66/34 N/E split with poor Earth connections and high resistivity. Near mid-feeder it was about a 38/62 N/E split with good connections and 57/43 N/E split with poor connections. Near the load it was almost all N current of course.

The Earth connection quality was more a factor than soil resistivity.

 

[GoldDigger]

I

The Earth connection quality was more a factor than soil resistivity.

Theory tells us that the quality of the ground electrode to earth connection (which depends in part on local soil resistivity) plays a far more important role than the resistivity of the earth between the two connection points as long as the distance is large compared to the sphere of influence of the two electrodes.

For all practical purposes, once you get past what we lump into the electrode to earth resistance the resistance of the intervening earth is zero.

 

[mivey]

For all practical purposes, once you get past what we lump into the electrode to earth resistance the resistance of the intervening earth is zero.

I think higher resistivity just makes the effective return depth deeper. I need to look to be sure about that and any loss as it has been too long to recall exactly from memory.