CT/VT ratio 3

[KnicksJets]

Hello
We are considering replacing the current revenue meters (CT/VT) for our 230 kV line (secondary of the GSU). The accuracy and burden and most other specs match.
However, we need current ratio of 1000:5 and the unit that the vendor is offering has multi-tapped ratios: 200/300/400 X 400/600/800 X 800/1200/1600: 5 (Rating factor =4/3/2). Our present CT has rated primary current 1kA. Vendors say that we can use the 800:5 tap on the highest ratio bundle (800/1200/1600) and get the same accuracy(0.15%) from 4A to 3200A.
I want to confirm from the experts here that we can do this. Shouldn’t we need 1000:5 for 1000:5 equivalency? Can 800:5 cut it?
Please let me know if I am missing any other details that can help make the call.

 

[dpc]

If the meter has the thermal rating to handle your max current, and the manufacturer certifies its accuracy at over the expected range of current, I would be OK with the 800/5. It's common to size meterng CTs lower than relaying CTs.

The newer extended range CTs offer a truly amazing accuracy over a very wide range of current. Make sure to request certified factory test reports and re-test in the field after installation if you want to be really sure.

Of course, you will need to adjust your meter multipliers, etc if the CT ratio is changed.

 

[scottf]

What is the maximum continuous current at this install. For metering, one should use the lowest ratio possible. Looks like the vendor is proposing an extended range unit, which is good.

It is common to use a class 20 meter for instrument rated metering, meaning maximum 20A continuous rating into the meter.

 

[KnicksJets]

Dpc,
Thermal current and Mechanical current for the new unit and old units match (50 kA and 125 kA respectively). I am assuming that Dynamic short time current and Mechanical current are pretty much same thing.
Where would you adjust the meter multiplier that you are suggesting?

Scottf,
I will try to get the max continuous current in the system, but the old unit had these specs:
Rated primary current: 1000A
Rated continuous thermal current factor: 1.5 In

You said class 20 meter, are you suggesting the new meters are class 20? I thought they go by accuracy class (1.5). I am CT/VT novice here, so please bear with me.

 

[scottf]

You of course need to check everything, but in the IEEE market, almost all instrument rated meters are class 20.

Dynamic and mechanical short-time current, but dynamic current is an IEC term and is expressed as kAp and Mechnical current is an IEEE/CSA term and is expressed as kArms. Many manufacturers list Mechanical Short-Time Current as a kAp value since expressing this value as an rms value makes little sense. One of the many things in IEEEC57.13 that need fixing.

 

[inpran]

Sdshah,
Its ok to use the 800/5 ratio if the current in your feeder is around the same value. If its more, the Amplitude Limiting Factor of the metering core will have to be checked and matched your metering equipments.