saturation on a current transformer 2

[mottt]

If i got a current transformer rated in example for 200 A and I put through it 250 A, what would be the sinuidal wave, is it possible to get some error in the angle?.
This question is cause, my company manufacture a device that works with the power factor, and we mount in a factory, it works properly,for months but when they change installation, and work on the 100% aprox. of its capacity, i got diferences in my measured power factor and the power factor they use for the bill. I just wondering if i could got a saturation problem and a deformation of the sine wave....
installaction is two generators with 900 kw each one and a 2500 Kva transformers, and the main problem is when they work with the generators in the subsexcited area... 0,93 power factor.
thanks and sorry for my badly english i hope you´ll understand me.

 

[slavag]

Hi Motorxplosion.
Before two days was thread about it.
Please see ttached with Scottf's explanation.
thread238-227076
250A for the 200A CT is only overheating issue, you are very far from the saturation region. "Problem" isn't CT, "problem" is new installation.
For example: Generators supplied most of real power to the factory and reactive power supplied from the grid, this is reason of low power factor.
Regards.
Slava

 

[mottt]

slava, thanks a lot for your reply.... the situation is the next.... we have two facturation points...the generation point... and two, the factory point.... and the problem is that in the generation counter, we mesure in example 0,96 subexcite, and in our manufactured device we measure 0,93... with our equipment, we try to mantain the power factor in the point at customer desire... we use the same signal they use for the generation point, voltage and current... to know the angle... the thing, is that this work for long time... but in that time, we got 1400 kw pf 0,93 and now, we try to work with 1900 kw pf 0,93... and start the problems.....

thanks a lot

 

[slavag]

Hi Motorxplosion.
What is subexcite term? I guess is underexcitation region of generator operation curve.
Please send us single line diagram of your factory with power flow.
It's so difficult recommend something.
Regards.
Slava

 

[mottt]

hi, slavag, i attached the diagram of the factory, and the underexcitation is the capacitive side of the generator.... as you see in this metering we only read the pf of the generator with the step on transformer... I see we have a 22 kv voltage transformer and a 50 A current transformer, If you calculate. 1900 KVA... but i calculate that i generate with a pf 0,93 capacitive (lead) 2000 KVA,
i was wondering that if I could get some distortion with the sine wave of the CT.
thanks

 

[slavag]

Motorxplosion. Lets go step by step.
You are have two gen and one step-up transformer.
After, you are have Medida Cogeneracion 1 with 50A CT class 0.2S.
After the line and next trafo and Medida de Fabrico with 10A CT of class 0.2S.
Please explain again, where is problem? What point of meas?
As I understand, in the point Medida Cogeneration-1.
Are you equl. this value with meas on the generator?
In this case , you forgot about trafo losses.
Best Regards.
Slava

 

[slavag]

BTW, capacity of generators is only 1860kW , PF=1.

 

[mottt]

the problem is: i measure in the medida cogeneracion 1 in example pf 0,96, but i read on my device 0,93...and i was wondering if this problem is cause some saturation or deformation of the sine wave coming from the CT. Yes, capacity of the generators is 1860 kw but if you use them on 0,93 pf you´ll get more than 2000 kva..and if you multiply 50 A per 22 kv and square 3 you´ll get less power... for this the ct have more current and in the output give us more than 5 A.

 

[scottf]

What is the rating factor of the CTs?

 

[slavag]

Motorxplosion.
What is your device? On the generator?
Please write down all parameters and will send us:
1. Current on the generators
2. Voltage on the generators
3. P on the generators ( with +/-)
4. Q on the generators (with +/-).
5. PF on the generators
6. S on the generators (with +/-)
Same values for the medida 1.
Are you work all time in the underexcit region or only in the night?
When last time you test all meas. devices?
Best Regards.
Slava

 

[kssschsekhar]

Metering class CT (200/5) working 250 Amps will have

1. Ratio error
2. Phase angle error.

Working at more than rated current of CT always have more errors. That means it will work above its class. Hence try to use within rated capacity of CT
or if not possible go for replacement of CT
Or
reduce current to within CT rated current limits (Not economical and not advisable)

you can ask your CT manufacturer about CT specifications and test result.

you can test CT with osciloscope and get exact error and verify sine wave of CT secondary.

 

[waross]

Something that I came across a long time ago and don't remember where. Maybe someone else will be able to provide a reference.
A metering CT will maintain good accuracy up to 200% of its rated current.
Protection CTs have less accuracy but will work over a much broader range without saturating.
But that's in NEMA land. I don't know thr standards for CTs in IEC land. Do you have standard CTs all araound or did someone find non-standard transformers with marginal performance.
Back to your problem.
You say that your generators are "two generators with 900 kw each one ". That's 1800 kW. You are trying to extract 1900 kW. your prime movers may be limiting you to 1800 kW when you think that you have 1900 kw.
Possibly part of the instrumentation package rather than the CTs is not able to handle the extra current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

waross-

There are metering CTs that are accurate well above 400%...it just depends on the rating factor of the CT.

In the US, it's common to use Class 20 meters, which mean they're built to handle 20A inputs, i.e. 5A rated secondary of CT and a rating factor of 4.0.

Protect CTs often maintain as good are better accuracy at rated current when compared to metering CTs. Metering CT cores are generally much smaller (thus cheaper) and are really only made to work (accurate) during steady state situations. Protection CT cores are much larger (in cross sectional area) and are designed to ride transients up to some higher level, depending on rating. Because of the larger cross sectional area of the core, they are often very accurate at rated current. However, they tend to drop off more sharply in accuracy as the current drops below rated current.

 

[waross]

Thanks Scott.
I think that you have made my point.
Whether the CTs are protection class or metering class, 250 A on a 200 A CT should not present a problem with standard CTs.
However, your instruments may develop errors if the input currents are above 5 amps. It may be well to review the design and circuits of the unit that is not responding properly to verify that the design will function on over currents.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Yuma]

Hi Motorxplosion;

As I understand it, you have a device that controls the power factor of both generators, and this device must be connected to some voltage transformers and current transformers that are not represented in the single line diagram, but surely they exist. Please tell us the rating of those transformers.

Regards

 

[rcw retired EE]

Is the problem the power factor measurement at the genearators is different than the power factor measured at the load? Maybe the KVAR losses in the gnerator step up transformer are not being considered.

If the generators produce 0.93 power factor, it is possible that load power factor will be 0.96. The kW at both points is the same, but the kVAR's at the load are less due to losses in the transformer.

 

[electricpete]

I don't know if it was mentioned. The error depends not just on amps but also on the burden. Product of high amps and high burden gets you in trouble. At low burden you can push more amps with less much error than at high burden.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.