×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

CT for metering revenue

CT for metering revenue

CT for metering revenue

(OP)
- 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?
 

RE: CT for metering revenue

Less burden means better accuracy performance in general.

Also, by the standard for 0.3 class:

100%Inom -> rating factor = guaranteed 0.3% performance

10%Inom -> 100%Inom = guaranteed 0.6% performance

If you have the factory test results, you should have accuracy performance at 0 and full burden at 10%, 100%, and RF*Inom. Since accuracy moves linearly/proportionally with burden, you can plot the actual accuracy performance with 10VA burden at 10%Inom.

I would bet you'll find it will perform within 0.3% performance limits at that point and with that burden.

RE: CT for metering revenue

Adding burden resistors to circutis is bad news, it increases the risk of an open circuit.

If you've got the test certificate you should have the ratio and phase errors at a minimum of two different points (usually 25% and 100% rated burden on the ones I see in the UK).  As Scottf said the change in accuracy with burden is pretty much linear so you can calculate the error at the normal working point.

The tariff meters I use can have CT and VT errors programmed into them so it's no so important to get the CT operating at the lowest error point.  I use the normal operating point (rated volts and full load current for a generator) to determine the CT and VT errors.

RE: CT for metering revenue

(OP)
scottf (Electrical)
for metering CT I guess by IEC standards is:
10%Inom < Ict < 100%Inom -> rating factor = guaranteed 0.3% performance.
Ict < 10%Inom  = nao guaranteed 0.3% performance.

LSpark
Can you send information of tariff meters with CT and VT errors programmed ?










 

RE: CT for metering revenue

Odlanor-

First, the 0.3 accuracy class is not an IEC class, it's an IEEE/CSA (and a few others) accuracy class. So you need to check to which standard the unit was manufactured to, it does make a difference in performance guarantees.

With that said, I seriously doubt the unit you have which is marked as 0.3-50VA is guaranteed/tested to have 0.3% performance at 10% rated current. The ranges I listed above are per IEEEC57.13.

Attached are the standard IEC metering accuracy classes. You'll notice that as the current is below 100% Inom, the guaranteed accuracy performance drops off. For IEC, the accuracy listed must be met from 25%-100% rated burden. My company happens to always design to meet accuracy at 0VA wherever possible.

Lspark-

Most meters now contain instrument transformer correction tools, however, very few utilities that know actually use them. Just curious, do you calculate/measure the actual connected burden in order to figure out what correction to use? Also, I've been privy to many situation where correction is just made at the expected nominal operating current and end up making the install LESS accurate across the entire operating range based on incorrect correction factors. For example, many folks using correction factors use the data from the test reports at the full burden rating. In reality, most metering installs have much less than the rated burden connected to the CT. Since CT error is almost always negative and becomes more negative as the burden increases, correcting at the full rated burden can incorrectly cause higher Kwh billed than is actually consumed.


Some meters do offer CT correct factors at a number of different current points and then do linear extrapolation between the various points.


 

RE: CT for metering revenue

Odlanor - all the high accuracy meters approved for use in the UK can have single point CT and VT errors programmed in.  I use CEWE prometer R meters but there are other meters out there.  I know Landis and Gyr have one approved, the other manufactures names evade me - I haven't had my morning coffee yet!  One of my contractors raves (in a good way) about a Canadian manufactured meter.

Scottf - I do calculate and/or measure the CT and VT burdens, these days they're normally not much above 0VA unless it's a long run between the CTs and meter panel.  Most of the circuits I look after sit between 50 and 75% meter rating when they're on so the corrections do improve the accuracy.  The newer sites seem to have CTs with fairly stable errors at a particular burden, the %current has a much smaller effect on error.  I look after generation sites mainly.

Most of my fellow metering engineers look slightly blank when I mention CT and VT error correction and totally blank when I mention power transformer loss correction.  They prefer to use complex models to produce P and P*P factors rather than simple error correction in the meter.

 

RE: CT for metering revenue

Adding resistance (additional burden) in the metering CT circuit in order to bring the connected burden to >25% rated for measurement accuracy reasons is done some times.
This also helps in improving the instrument safety by lowering the effective Fs.
For a new installation, it is better to specify CT with low burden rating as the present day digital meters hardly impose any burden.

RE: CT for metering revenue

(OP)
raghun,
do you have any reference about that?

RE: CT for metering revenue

(OP)
We have a hydraulic power plant with 2*150MVA-13,8kV generator sending energy in 500kV at a substation 120km distance.
energy sending => 68,871796 MWh
energy receiving => 69,172963 MWh
How can you receive more than you sent?

RE: CT for metering revenue

Are we seeing Ferranti effect here! Receiving end voltage is supposed to be higher (than the sending end one) when the EHV lines are lightly loaded. The line generates VARs but how can it show up as active energy!!
The discrepancy in the records is most likely due to voltage drop in the VT secondary wiring (and hence lower voltage available at the meter terminals) at power plant end.

RE: CT for metering revenue

(OP)
raghun,
how to minimize this problem?

RE: CT for metering revenue

(OP)
ragun,
...The discrepancy in the records is most likely due to voltage drop in the VT secondary wiring (and hence lower voltage available at the meter terminals) at power plant end...

Do you mean the powerPlant-meter is registering less than it should register?
  



 

RE: CT for metering revenue

Quote:

energy sending => 68,871796 MWh
energy receiving => 69,172963 MWh
energy sending + 0.3% error => 69,078,411 MWh
energy receiving - 0.3% error => 68,965,444 MWh
 

RE: CT for metering revenue

Voltage drop in the secondary wiring should only be a problem if you've got an appreciable amount of burden connected.  Modern meters with a seperate power supply are very low burden, so it is much less of a problem on modern installations (in the UK anyway).  

I've still got some older meters and voltage imbalance relays that are powered from the VT so I include the voltage drop in the VT error that I program into the meter.  Some circuits have additional equipment connected so this can increase the burden too.

RE: CT for metering revenue

(OP)
jghrist,
what about loss of stepup transformer 13,8/500kV and loss of 500kV LT?
 

RE: CT for metering revenue

Quote:

what about loss of stepup transformer 13,8/500kV and loss of 500kV LT?
I haven't tried to account for any losses, I just wanted to point out that because of metering and CT errors, you can't determine losses with any degree of accuracy by measuring load in vs load out, and might get negative numbers.  Is it possible that one of the energy meters includes loss compensation for the transformer?  This would not be uncommon.  If so, there might be an error in the loss compensation settings.
 

RE: CT for metering revenue

The chance for difference in MWh readings is much likely the result of CT errors/difference than VT secondary voltage drop (which is almost never an issue for modern applications as mentioned above).

What are the accuracy ratings and ratios of the CTs on either side. It's quite common to have CTs sized incorrectly (ratio too high) and the higher in voltage rating, the worse the sizing seems to get.

The story I normally tell to illustrate this is I once sold 30 pcs of 765 kV CTs to a US utility. The metering core was rated 5000:5A, 0.3B1.8, RF1.0. I pleaded with the utility to at least consider 2500:5A RF2.0, but they wouldn't budget. The meters were located about 800 feet away from the CTs, so they were using every bit of the burden rating using #10AWG. This was a major interchange billing point and millions of $'s were being billed across these CT every month. They paid >$100,000 for each CT and were basically probably getting actual CT errors of roughly 0.5% or so at the nominal current level. 0.5% on $10MM per month is $50k per month lost in just the CTs being sized wrong.

RE: CT for metering revenue

(OP)
CT at 13,8kV side:
ratio: 8000 / 5 A - class: 0.3%  burden:  50VA  
Calculations:
Iusual: 5600 A  - total load of CT circuit: 10.13 VA

CT at 500kV side (remote):
ratio: 2200 / 5 A - class: 0.3%  burden: - 50VA
Calculations:
Iusual: 156 A  - total load of CT circuit: 10.13 VA

scottf ,raghun
How do you suggest to minimize this problem now?

 

RE: CT for metering revenue

odlanor-

Are you sure the connected burden on each CT is exactly the same? Hard to believe the meter is located exactly the same distance from the CT for each installation.

Is your "problem" just that the recorded MWh differ at each end by about 1%?

If that's your problem, then you need to look at your transformer and line losses first to make sure those readings aren't indeed accurate.

After that, it seems you might want to look at using a lower ratio CT on the 500 kV side and/or buy a CT with a higher accuracy/wider range, something like a 0.15 or 0.l5S class (IEEE) or 0.2S class (IEC).

RE: CT for metering revenue

You need to consider the maximum operational current (not fault current) that flows through the circuit not the usual current when sizing a metering CT.  So whilst from the given info it looks like the remote end CT is badly sized that may not be the case if additional circuits sometimes flow through that meter point.

In the UK the metering CTs and VTs should be located at the commercial boundary for all new installations (i'd have thought this was the same the world over) but this isn't possible for some of the older power stations that were build prior to privatisation.  In this case we apply a correction for power transformer loss (load and no-load seperately) in the meter.  The load loss usually has to be adjusted for the meter full load current rather than the transformer current and is put in as a percent rather than kW.

In your case I'd want to look at the programming of the meter to see if this has been programmed and if so what the values are.  I'd also want to see copies of the CT and VT test certificates and any compensation figures for those in teh meters.

Scottf - on the last new build I had to get the metering CT ratios changed (multi ratio CTs fortunately) as they'd used the same ratio that they'd used for the protection.

RE: CT for metering revenue

(OP)
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.
  
 

RE: CT for metering revenue

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.

RE: CT for metering revenue

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?  

RE: CT for metering revenue

(OP)
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.
  

RE: CT for metering revenue

See 14 Sep 11 4:49 post of raghun.

RE: CT for metering revenue

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.

 

RE: CT for metering revenue

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.  

RE: CT for metering revenue

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

RE: CT for metering revenue

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources