CT for metering revenue 1

[odlanor]

- CT data
ratio: 8000 / 5 A - class: 0.3% burden: - 50VA

- Calculations:
Irated: 6350 A - total load of CT circuit: 10.13 VA

standard:
With 10% of 8000 there is no guarantee of accuracy of 0.3% to 50VA burden.

Questions:
1)For a circuit load of 10VA error CT will be greater?
How much?

2) Increasing the load circuit by adding resistance, will be improvement?

 

[odlanor]

scottf (Electrical),
...Are you sure the connected burden on each CT
-it doesnt make difference;Value is very low.


...Is your "problem" just that the recorded MWh differ at each
-you're right!

... After that, it seems you might want to look at using a
-bad suggestion! CTs are installed.

LSpark (Electrical),
...You need to consider the maximum operational current
-it is operational current!

...In the UK the metering CTs and VTs should be located at
-this is an industrial generation producer selling to National Grid;
I am not sure if Regulatory Agency permit this kind of correction.

 

[LSpark]

If it is the max operational current then the remote end CT is very badly specified and is always operating at below 10% of rating.

Can you get hold of the relevant metering code of practice (or whatever it's called where you are) to clarify what is expected and allowed of a tariff metering installation?

In the UK we'd need class 0.2 VTs and class 0.2s CTs & meters with an overall system accuracy of 0.5% or better with documentary evidence to prove the overall accuracy for a generator over 100MVA. The codes also state that correction for CT and VT errors is allowed and under what circumstances power transformer and line loss factors can be used.

 

[odlanor]

LSpark
You´re right! old times, hydro power plant projects spent 2 year of studies and 3,4 years for execution. Nowadays ,spent 4 month of studies and 2 years for execution.

http://extranet.ons.org.br/operacao/prdocme.nsf/frmApresentaIO?OpenForm&NumeroIO=12.2

 

[scottf]

Olander-

LSpark asked you what the MAXIMUM current could be for that CT.

If changing the CT isn't an option and the normal current varies very little, than you could look at correct the meter at the 500 kV side for the CT error. You'll probably need to contact the manufacturer of the CT to find out what the expected error for those units would be at that current level and at the exact connected burden you have.

I'm a little surprised the connected burden is only 10 VA for a 500 kV CT. That means the meter must be around 150Ft/45m from the CTs?

 

[odlanor]

scotff,
sorry can not answer you. This issue is relevant to the project substation which I did not participate.
My intervention is associated with powerPlant design review. I can not influence the measurement of billing adjustments.
My intervention would add resistance at secondary of CT, but unfortunately this alternative was not addressed in this forum.

 

[stevenal]

See 14 Sep 11 4:49 post of raghun.

 

[scottf]

Adding burden to the CT will likely make things worse and not better. More burden means more negative error, in general.

I really doubt your problem is not having enough burden on the CT.

You really need to look at the factory test results. Everything else is just speculation.

 

[RRaghunath]

Odlanor,
I wasn't accessing the site for the last couple of days.
The voltage drop in VT secondary is an issue in many of the substations where the distance between the VT in the switchyard and the meter in the control room is large. This can affect even if connected burden happens to be low.
One of the stations I had worked, we replaced the 2.5sqmm wire on VT sec with 10sqmm wire.

 

[odlanor]

raghun,
We always use 12sqmm wire.No problem!

 

[slavag]

My last calculation for revenue meter on the 500kV
CT: 0.2FS5 300/1A, 5VA, cable about 100m 4mm^2, Rct<2Ohm
VT 0.2 10VA. cable 2.5mm^2