Relaying Accuracy CT's 1

[anggap]

Hello,
In my plant there is a 34.5 kV feeder that supply very far but light load (current parameters at supply feeder: V = 34 kV, I= 21 A , PF= 0.06). The feeder use C100 CT's ( multi-tap CT 100/5) and ION 7300 to measure the load.
I found out that whenever the load was disconnected, PF reading was changing in range of 0.02 - 0.07. While the amperage and voltage is relatively same. I've conform the load side that there was no load changing during that time.

I am suspecting the CT error due to Relaying Accuracy CT that used to measure very low PF load, but I can't find any reference to prove my point.
Is there any standard or reference about C100 accuracy to measure low PF load?

I found this graphic at IEEE Std C57.13-1993, but I think it for Metering accuracy CT

Thanks in advance,
Angga

 

[waross]

The charging current of the line will be at a very low PF.

Bill
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"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

I am lost why you care that the PF changes of the current changes a few hundredths when the load disconnects. You have charging current but why for relaying would you care about the PF to that degree?

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If you can't explain it to a six year old, you don't understand it yourself.

 

[scottf]

Are you saying you are using the 100:5A tap of a multi-ratio CT? If so, what is the full ratio (highest tap)?

 

[cranky108]

I understand that you are using a relay accuracy CT's for metering, but not revenue metering. If so why do you care if it is accurate at low levels?

In many places they use a meter on relay accuracy CT's to verify if the load is present or has been removed. It is important when someone needs to open the line switches, as some switches will not break load current.

100/5 is much too low for relaying as a saturation concern, but for general metering, it is fine.

I assume this circuit might be for some water wells for the plant (not that it matters).

 

[dpc]

To Scott's point, if you have a C100 CT with a full ratio of 600/5 but tap it at 100/5, the effective relaying accuracy of the CT is much less than C100. Something like C16. If the concern is metering, there will be significant error at very low primary current and very low pf.

As waross indicated, the charging current could be a significant part of what you are seeing if the line is long.

 

[anggap]

To HamburgerHelper, This feeder supply power to another company plant, and KWh measurement at this feeder being calculated for the billing.
PF reading changes then KWh reading changes as well but there is no load changes at the other plant.
Note: we are an oil company, so we are not used to sell Power to another company.

To Scott, the highest Tap is 800.

 

[bacon4life]

Do you have the kW and kVAR readings? Perhaps a trend showing your concern I generally find PF readings often have hidden assumptions inside the metering software.

It seems like your load is approximately 100 kW + 1200 kVAR. At this extremely low power factor, a small change in kW will have a relatively large change in PF.

 

[scottf]

100:5A tap of a 800:5A MR CT rated C100 is not going to have very good accuracy and/or a very high burden rating. If the meter is any distance from the CT, the error at around 20% rated current (100:5A tap) could be pretty bad.

Unless I'm missing something, the pf of the load should not impact the accuracy of the CT itself. It might impact the accuracy of the meter, but probably not.

 

[LionelHutz]

The CT can cause an error, but it won't be the direct cause the fluctuation.

My guess would be that low signal levels going into an electronic meter that is trying to detect the exact zero crossings is what causes the power factor flutter. So, why are you using 800:5 tapped relaying CT's with a meter that needs to accurately measure 21A?

 

[stevenal]

Sounds like a misapplication if relaying CTs are being used for billing.

 

[anggap]

@bacon4life
before load disconnected → V = 33.7 kV, I= 21 A , P= 37 kW.
after load disconnected → V = 34 kV, I= 21 A , P= 76 kW.

@lionelHutz
Originally this feeder was a spare feeder, the MRCT was already installed before.

 

[mvcjr]

If I were that other plant customer, I would request for billing metering to be installed near my facility so I would not absorb the losses in the long line and most probably eradicate the erratic registrations. Basing from your statement that the long line was once a spare feeder, I would presume you own that line.