Potential Transfomer Selection 3

[mmt019]

How do you calculate if a P.T. is suitable for a meter? I have a three phase P.T. (open delta- 4200:120V) that is 0.3WX, 0.6M, 1.2Y AND 500VA thermal rating @ 30 Degree C. The meter I am connecting it to has a 5M ohm input impedance. I want to connect 5 meters to the same P.T. Any advice guys?

 

[scottf]

Your PT will work fine for this application.

The 0.3WX rating means that the PT will maintain an accuracy of 0.3% (normal revenue metering accuracy) up to an applied burden of 25 VA. 'X' stands for 25 VA and 'W' stands for 12.5 VA.

5 meters with an input impedance of 5Mohm each will be well below 25 VA, assuming a 120 V rated secondary voltage.

 

[busbar]

You may also want to consider the added burden of meter auxiliary power if served from the VT set.

 

[scottf]

Busbar-

Good point. I assumed with 5 meters that this was a station install and that aux. power would be available.

Aux. power for a typical electronic meter is about 10 VA or so on 1 phase.

However, with a 0.3WX rating, might be best to use a seperate aux power source.

 

[jbartos]

Suggestion: Visit

http://www.pge.com/002_biz_svc/gen/pdf/wi_handbook/app_c.pdf

etc. for more info

 

[scottf]

jbartos-

I don't see anything in the link you posted concerning this topic. Am I missing something?

 

[jbartos]

Suggestion: Visit

http://www.netaworld.org/files/ItemFileA336.pdf

etc. for more info
and Reference:
ANSI/IEEE C57.13 IEEE Standard Requirements for Instrument Transformers
The transformer temperature rise should be addressed and included to avoid the transformer thermal damage.

 

[scottf]

Not to be a pest, but the VT's thermal burden rating really has nothing to do with this application.

A VT's thermal burden rating is how much VA the VT can put out at a 30 C ambient without exceeding a 55C temp rise.

In this application, the user is concerned with maintaining the VT's accuracy, to which the thermal burden is not a factor.

 

[jbartos]

Suggestion to the previous posting: If the VT or PT happens to be protected, e.g. by fuses, it just may be a good idea to find out the potential transformers thermal capability, rather than to have them damaged during a short circuit. Of course, this applies if money matters.

 

[twosockets]

Scott,

Also not wanting to be a pest, but your 10 VA for a typical solid state meter power supply is way out of line. It is important to remember that the VA burden imposed by a wide voltage range power supply goes up virtually exponentially once you exceed ~300 volts. In the 120 volt range, which this application represents, the power supply burden will probably not exceed ~2-4 VA except for a short duration.

If the cumulative VA of a number of meter power supplies is a concern, one might consider spreading the meter power supplies out over the various phases to create diversity. However, those that want to maintain the phase relationships throughout the system might object to this technique. In that case, a beefed up VT on the power supply phase may be the only alternative.

 

[scottf]

Twosockets-

I agree that it is normally less than 10 VA, but I was giving a worst-case value, since I don't know what meter is being used. The ION 8300-8500 is listed as a max of 6.6 VA/phase and they use all 3 phases. The MAXsys2510 literature lists less than 10 VA max. on one phase.

 

[mmt019]

The Meter is a Square D PM-850.

 

[scottf]

mmt019-

Are you powering the meter from the VTs or do you have an aux power source? If you're powering from the VT's, I suggest you look-up the power requirements of the meters.

 

[madura]

I am not familiar with American Standards. The VT must be loaded to 25 % of rated burden to make it work within the accuracy limits.This may entail adding dummy burdens. This is the case when electronic devices ( 1- 2VA budens) are used in conjuction with VT of high VA ratings.

 

[jghrist]

madura,

ANSI/IEEE C57.13-1978 calls for routine phase angle and ratio accuracy tests at both zero burden and at maximum standard burden. I don't have immediate access to the latest version, but I doubt that this has changed. I never heard of adding dummy burdens to a VT to improve accuracy. Is this necessary for IEC VT's? If so, it's a good reason to specify ANSI VT's.

 

[scottf]

madura-

The 25%-100% burden range is unique to the IEC60044-2 standard and is not the same in IEEEC57.13. In the IEEE world most vendors test at 0 up to the full burden ratings.

My company lobbied to have the 25%-100% burden clause changed to 0%-100% burden, but we were outvoted by many European/Asian instrument transformer companies, as they would have to redesign many of their units to comply. In my opinion, most voltage transformer's burden ratings are often overkill. In the IEEE world it doesn't matter from an application point of veiw, but in the IEC world it does, as stated above.

 

[twosockets]

scott,

I totally agree with you. This point, along with the differences between accuracy burden rating and thermal burden rating are not well understood by many people.

One of the problems with the 25%-100% burden rating is that the burden imposed by the non-power supply phases of solid state meters is generally equivalent to an ~1 megohm resistor. In other words, essentially 0 VA of burden. For that reason alone, I would never recommend VTs that meet only the IEC standard.

 

[scottf]

twosockets-

Unfortunately, with the exception of North America and a few other markets, most of the world uses IEC rated equipment. I suspect that in reality, most vendors meet the accuracy at 0 burden (I know my company does), but unless it's spelled-out in the customer's specification, you won't know for sure.

To make matters worse, for high-voltage units, it is common to see winding burden ratings in excess of 200 VA, meaning that by the letter of the standard, unless they have the winding loaded to 50 VA, then they're not assured to meet the accuracy class. As stated above, in an HV installation, VT rarely see above 5 VA, and that's with a string of relays and meters connected to them!

 

[madura]

Scottf,

Adding a dummy burdens to bring VTS and CT to work within the accuarcy limits is a common practice in this part of the world, specially in revenue class metering applications.

I do agree 0 -100% accuracy requirements of IEEE would help in Voltage measurements but for current measurements it is not feasible.

 

[scottf]

madura-

It is perfectly feasible to have CT accuracy from 0-100% of rated burden. There is no question about that. Please explain what you mean here.