CL 0.2 Voltage Transformers used for protection? 1

[Power0020]

An supplication with a dual class VT, 0.2 & 3P.

The utility wants to used separate VT winding for each protection set. To save money, they seem to be used to go with metering winding for protection looping,

I don't have much experience dealing with VTs but I think it is ok to use a more accurate VT for protection application.

Any other opinion?

 

[scottf]

Yes, a VT metering accuracy winding can be used for protection.

Technically, the protection class has some different requirements than the metering class in IEC, specifically in regards to accuracy at lower %Vrated.

Metering accuracy is specified from 80% to 120% Vrated and protection accuracy from 5% Vrated up to what ever the over-voltage factor rating is. However, from a practical perspective, a winding rated for metering will behave the same at the lower voltages.

 

[submonkey]

Hi Scottf,

What is the basis for your assumption that metering class
VTs will behave appropriately at the lower end of the voltage
range?

I would have thought that some VTs might perform poorly -
especially CVTs, where the electromagnetic parts need to
be matched to the capacitive divider.

I wouldn't use a metering VT for distance protection unless
the manufacturer was willing to assign a 3P class, or state
that the construction is identical to a unit with a 3P class.

Thanks,
Submonkey

 

[scottf]

First, the post said VT, so I'm assuming inductive VT and not CVT for my comments.

For VTs (and CVTs for that matter), the secondary windings share the same core. So if you have a winding rated for metering and a winding rated for protection, in reality, the 2 windings are likely identical, (especially for HV units)and are wound about the same core. If the windings are not identical, the difference would be that the winding rated just for protection would be made with smaller diameter wire. Either way, the performance/accuracy change, as a function of rated voltage, would be the same for each winding. In all of my years working for instrument transformer companies, I've never seen a VT with 2 secondary windings where the winding rated for metering wouldn't meet the protection accuracy class as well.

For CVTs, it's essentially the same argument, however, the phase-angle change as the series reactor and capacitor divider "fall out of tuning" plays a part. But same issue as above, in that the metering winding and the protection winding share the same primary winding, core, and series reactor. I don't believe it would really be possible to make a CVT where there is 1 winding for protection and 1 for metering and the metering winding not meet the protection class (at the rated burden of the metering class that is).

 

[submonkey]

Thanks Scottf - interesting comments.

 

[Power0020]

Many thanks Scottf and submonkey for the discussion.

Just to conclude that, the 3P class has an error greater than 0.2 metering, I can see the cost saving here is to use a smaller winding CSA (i.e. higher resistance) for 3P winding.

Other than that, why do manufacturer bother producing 3P winding if the higher accuracy (0.2) is not difficult to achieve? (compared with current transformers, their accuracy means much more the magnetic properties of core material)

 

[scottf]

It's not that manufactures "choose" to build one rating versus another, it's more of a response to how the units are specified by the customer. The customer specifies a protection class if they want the accuracy tested down to 5% Vrated (or whatever depending on the standard) and metering if they want it confirmed for metering accuracy.

Note that IEEE and CSA don't have distinguished protection and metering classes for inductive VTs. The classes are 0.15/0.3/0.6/1.2 and all are just variations in ratio and phase angle and not variation on %Vrated. The requirement is 90-110% Vrated for all classes.

For CCVTs, there is an "R" class (like 1.2R) available in ANSI, but it's rarely, if ever, used.