how is the minimum current for measure in a CT

[gaux]

I have an equipment that measures the PF, it works fine on 100% , 50% load but start to work wrong on 20 % or less, and the manufacturer tells me that this is normal cause of the errors of the CT, with a accuracy class 0,5.. i´ll like to know if there´s some standard that talks about it, and what´s your opinion..... they says that measure the angle between the voltage and current... that´s why the error is so important.... please send me some link to the standard... and if you know the european standard..... i think i´m asking so much.

 

[DanDel]

You may just have a limitation on this instrument and it's CT. I know most recording instruments manufacturers redcommend at least 10-20% load for accurate readings of CTs.

 

[gaux]

The ct is the ct i have used for the measuring, and i look for a standard that justify it or not, why 10-20%??
thanks in advance for your attention

 

[dpc]

What type of error are you seeing, and how do you know it is wrong.

The current transformer requires excitation current. As the primary current decreases, this excitation current becomes a larger part of the total current, increasing the error.

Also, many of these probes have a small shunt resistor and sometimes a diode that is used to provide a voltage output to the measurement device. At very low currents these can be an issue.

If these are clamp-on style current probes, be sure to check the mating surfaces where the two jaws meet. Any gap will definitely cause an error. I like to clean these off with alcohol to make sure that there is good contact between the two halves of the CT.

 

[gaux]

I cheek with the net company, and they measure and i pay for it.... and the equipment i install can´t measure right... in pf the diference is 0,01, in example if i measure 0,99 they measure 0,98....
thanks....

 

[dpc]

If the current low, a difference of 0.01 in power factor may be within the stated accuracy of the measurement equipment.

 

[slavag]

Hi Gaux.
It will possible a long story.
Please give us more info
1. What is voltages
2. What is a your CT, class, connection erc.
3. What is a net company CT, class, etc.
4. Same for VT
5. What is your revenue meter, what is a net company revenue meter.
Difference 0.01 is .....not so big error.
What about kWh and kVarh difference.
Yep, we have some "boost" effect in the low current, <10% of load, but same error for net company.
Best Regards.
Slava

 

[gaux]

voltages are 20 kV
ct 0,5 connetion star and the voltage transformer are phase to ground
they use the same transformers for the net company revenue meter.

 

[gaux]

i have attach the drawing of the installation on the last post

 

[slavag]

Hey Gaux.
Spanish isn't my srong side .
But isn't same CT and VT.
Your side is 0.5S class, cogeneration side is 0.2S
Your side is 10-20/5A, company side is 50-100/5A.
0.01 for PF is nothing in this case.
Maybe your meas on the generator side?
Best Regards.
Slava

 

[ameeraqeel]

Hi Gaux ,


You are right and your readings are correct for load more than 20% , I have the same with my equipments,If you use second CT with half ratio of installed one you will get correct results,this case is explained well by IEC conerning with p.f measuring equipment.


Ameer

 

[gaux]

No, they use the 0,5s class current transformer, cause they mesasure with the same CT that the measuring device used by the net company....
I don´t know so much about currents, but i thought in a fast thinking that if we measure the same current and the same voltage, we´ll have to measure the same pf, but the manufacturer of the new device says that below 20% it´s no possible to measure with acuracy the pf cause of the error in the CT´s,
My other question is that voltage transformer have an error,does this error changes with voltage? I mean if I rise voltage or reduce, does the voltage transformer error change?
In other thinking, they use a CT 5A/5A, to take the signal from the ct (used by the net company) and send this signal with a 6mm^2 wire, with a distance of 30 metres, what do you think about this? could be a diference on the measuring? How much is admisible?

 

[gaux]

where can i find this document of de IEC, concerning to pf measuring....?

thanks in advance to all of you

 

[ameeraqeel]

Hi,

Which country you are from .

Ameer

 

[gaux]

spain