Dry type Transformers

[peetey]

I have a question that I have posted at other forums but have gotten no useful answers. I am an electrical Meterman by trade. Occasionally we go to test a meter that is on a building under construction and generally the service is 277\480 4 wire wye with a stepdown dry type transformer that is loaded but generally only a construction load. Sometimes when I'm talking phase angles with my Arbiter, and tonging the load and taking a rev check on the meter my loads don't always match sometimes the rev of the meter and the measured values are half what they should be. Some times I get really weird phase andgles that correct when there is a heavy load applied. I don't know what the heck this is? Is it harmonics, is it a phase shift due to a lightly loaded transformer, is there some other phenomenon at work here like alchemy or the other black arts? Help

--peetey

 

[DRWeig]

Hiya Peetey,

A little clarification might spark an idea:

1) Exactly which Arbiter model are you using?
2) At how low of loading are you seeing your problem? 10% of transformer rating? 5%? Other?

Let us know,

Old Dave

 

[peetey]

It is an arbiter 918b and I generally don't know what the Kva rating of these are, I just know they're here. It also appears that the phase angles that the electric meter shows are sometimes different from the ones the arbiter shows. Peetey

 

[RalphChristie]

Peetey

I can't answer your question, but something I noted on the 918B-datasheet:

http://www.arbiter.com/catalog/frames/specs/power/918b/918b.php

"The Model 918B measures phase angle and frequency using zero crossings. The calculated parameters are based on linear models, assuming low distortion. Because of this, measurement errors for harmonic-rich signals may exceed these specifications"

Ralph

 

[peetey]

Since it relies on zero crossings do you think the problem is a distorted waveform making measurement inaccurate? I never thought of that, I guess I need a measurement tool that also does Harmonics. Peetey

 

[RalphChristie]

Peetey

Maybe harmonics are not the problem, but if there are harmonics present, you'll find some incorrect meausurements. Are there any harmonic sources feeding from the transformer? (Power supplies, PC's etc.)

Regards
Ralph

 

[busbar]

 
peetey, any way you can get your mitts on a 929/930/931 and see if there is indeed a harmonic problem at the site? It’s not that hard to check clip-on CTs for accuracy before you use them in the field with a 931.

It’s certainly not unreasonable check your 918 side-by-side in the shop and in the field with a more sophisticated instrument if you have one available.

 

[peetey]

I am thinking the same thing as you, maybe we need to get a higher end arbiter. I'm looking at the 928A as it looks portable and handheld and seems to do all the Harmonic stuff we need. I didn't know the 918B used zero crossings to do its checks. I've seen some posts on TheMeterGuy.com that deals with harmonics and it seems to be a zero crossing issue with regard to measurement.

--peetey