Low Ze, high PFC in industrial application

[davesparks]

The second problem - is there a reliable means of measuring such low Zes, or is it a fool's errand and you're best just calculating it?

Yes, a high resolution loop impedance tester would be more suitable, Megger make a reliable one.

I have known of people introducing a known resistance into the test circuit to move the reading into a testers band of greater accuracy.

Note that it's never given a higher reading, which is partly why I think the Ze on the EIC is suspect.

Why do you think it's suspect? A change of 0.01 ohms in 20 years doesn't seem like the most unlikely thing in the world.

 

[alexcarnes]

Why do you think it's suspect? A change of 0.01 ohms in 20 years doesn't seem like the most unlikely thing in the world.

I just meant I believe it to be lower. If I really make every effort to zero the test leads till I get a consistent resistance measurement for them, give the instrument 20 minutes to stabilise, give it a good couple of hours to equilibrate to room temperature, and average out a few separate measurements, it actually comes to about 0.02 ohms.

I don't believe it's that low. The 50mm2 cable from the transformer to the origin calculates to a smidgen over 0.02 ohms (assuming the cable is laid 'as the crow flies') and then there's whatever impedance the transformer secondaries have...

It's still extremely low though!

 

[pc1966]

I have known of people introducing a known resistance into the test circuit to move the reading into a testers band of greater accuracy.

I don't think that works. Typically most meters are specified as something like +/-X% + N digits, your X percentage is the same on the total, once you remove the added R and its known range you get an even wider percentage of the result. Similarly once your R is subtracted from the reading, you still have the N least digits to consider.

Sadly none of the MFT I have ever seen even have the option for Kelvin cables, even as extra-cost option, which would get rid of a bit of the low R uncertainty at relatively low cost.

 

[pc1966]

I just meant I believe it to be lower. If I really make every effort to zero the test leads till I get a consistent resistance measurement for them, give the instrument 20 minutes to stabilise, give it a good couple of hours to equilibrate to room temperature, and average out a few separate measurements, it actually comes to about 0.02 ohms.

I don't believe it's that low. The 50mm2 cable from the transformer to the origin calculates to a smidgen over 0.02 ohms (assuming the cable is laid 'as the crow flies') and then there's whatever impedance the transformer secondaries have...

It's still extremely low though!

Have you got the transformer specification?

0.02 seems very low, my lowest was 0.04 at the busbar chamber off a 500kVA transformer, and 10 (+/-2 or so) kA fault current. Close to the 11kA the DNO gave me.

 

[alexcarnes]

Have you got the transformer specification?

0.02 seems very low, my lowest was 0.04 at the busbar chamber off a 500kVA transformer, and 10 (+/-2 or so) kA fault current. Close to the 11kA the DNO gave me.

Not to hand but I should be able to get it. It's on my to-do list!

I found the original cable calcs for the installation in the commissioning manual and they actually used a Ze of 0.026 Ohms for the Zs calcs! I can't remember the exact figures, but I couldn't help noticing that that's pretty much the value one gets for the ~65m loop of 50mm2 cable from the transformer to the MCC...

 

[davesparks]

I don't think that works. Typically most meters are specified as something like +/-X% + N digits, your X percentage is the same on the total, once you remove the added R and its known range you get an even wider percentage of the result. Similarly once your R is subtracted from the reading, you still have the N least digits to consider.

Sadly none of the MFT I have ever seen even have the option for Kelvin cables, even as extra-cost option, which would get rid of a bit of the low R uncertainty at relatively low cost.

Yeah I have never been too certain of the method either but it was championed by the late Engineer54, you could probably find his posts on the subject with a bit of searching.

 

[Marvo]

I've also heard of adding a known resistance to bring the measurement more into the accurate band of the instrument but I've never seen the advantages of this where as PC1966 point out you're basically robbing Peter to pay Paul. Personally once you get much below 0.05 ohms I'd be derriving the PFC by calculation referencing the transformer published specs and known supply cable impedances unless I had access to a high res loop tester.

 

[loz2754]

I noticed that the new Megger MFT X1 has a higher Ze resolution:

The MFT-X1 has extended the low end of the loop impedance range from 0.01 ohms to 0.001 ohms resolution and 50kA current calculation.

Even so, I don't think I'll be getting one at that price.

 

[alexcarnes]

I noticed that the new Megger MFT X1 has a higher Ze resolution:

The MFT-X1 has extended the low end of the loop impedance range from 0.01 ohms to 0.001 ohms resolution and 50kA current calculation.

Even so, I don't think I'll be getting one at that price.

Yes I noticed that too, although the Fluke 1664 that I use claims the same precision (it has a milliohms mode in the high current loop impedance test settings). I think it's a bit of a gimmick to be honest - the accuracy is poor at very low impedances so you just get the wrong answer to another decimal place, basically. At higher impedances it's not really significant.

I wondered whether the MFT-X1 would work any better than a tool like the LTW425, and accordingly emailed Megger to ask, but they didn't even dignify it with a response.