Using relaying CT's for measuring loads.
Using relaying CT's for measuring loads.
(OP)
What it the typical accuracy of relaying CT's at typical load values of 0.1 to 1.0? I am asking because I just tried to add up the MW & MVAR at one our buses with c400 & c800 CT's feeding panel meters. The net MVA at the bus was about 7% of the total flowing through all the lines. What suprised me was variation in the total for each phase was much less than the unbalanced currents between the phases. I guess it is really two questions.
1. For a set of identical CT's, will they normally read the same value within say, 1%.
2. In comparing two different sets of relaying CT's, how much error should I expect?
I know IEEE C57.13 defines worst case accuracy as +/-10% between 1.0 and 20 times full load, but it doesn't address less than 1.0 PU loading, nor would it address typical nominal values.
Thanks
1. For a set of identical CT's, will they normally read the same value within say, 1%.
2. In comparing two different sets of relaying CT's, how much error should I expect?
I know IEEE C57.13 defines worst case accuracy as +/-10% between 1.0 and 20 times full load, but it doesn't address less than 1.0 PU loading, nor would it address typical nominal values.
Thanks






RE: Using relaying CT's for measuring loads.
High voltage bushing CTs may not be quite as good as that.
Maybe Scottf will be along to enlighten us both.
RE: Using relaying CT's for measuring loads.
However, they aren't tested at those levels for metering accuracies.
The above is assuming the full winding is being used. If one of the taps is being used then it will be worse and potentially much worse, depending on the construction of the CT (i.e. if each winding segment is equally distributed around the core).
In general, you shouldn't see too much variation between like CTs of the same vintage and manufacturer. If they're different types of CTs, then you may see some noticeable difference, but I would think within .5% at rated current. Unless you're using taps, then all bets are off.
RE: Using relaying CT's for measuring loads.
Exchange of current transformers of defferent class
thread238-206308: Exchange of current transformers of defferent class
For other issues.
respectfully
RE: Using relaying CT's for measuring loads.
RE: Using relaying CT's for measuring loads.
RE: Using relaying CT's for measuring loads.
For a given core size, the fewer the secondary turns, the worse the accuracy.
The example dbc gives is valid for CTs with equally distributed secondary windings, meaning each winding segment is equally distributed around the core. This is not a requirement of the standard and can't be assumed to be true unless specified that way. With that said, most manufactures try to supply distributed windings when possible, i.e. if it doesn't add too much cost.
RE: Using relaying CT's for measuring loads.
I wouldn't get too concerned unless my end user was pursuing the mythological "revenue accuracy".
old field guy