GE 269Plus Instruction Manual page 208

CT SIZE AND SATURATION H CURRENT TRANSFORMERS
The curve in Figure H–1: TYPICAL CT EXCITATION CURVE represents a typical excitation curve for a CT.
The y-axis represent secondary exciting voltage; the x-axis represents the secondary exciting current. When
the CT secondary exciting voltage level is picked off the graph, the corresponding secondary exciting current is
the amount of current required to excite the core of the CT. With respect to the ideal CT that conforms perfectly
to its ratio, the exciting current could be considered loss.
Figure H–1: TYPICAL CT EXCITATION CURVE
For a Protection Class CT with a 5 A secondary and maximum 10% ratio error correction, it is probable that the
design point for 20 times rated secondary will be at or slightly lower than the 10 A secondary exciting current
point (10% of 20 × 5 A). To design such that the 20 times rated secondary current is in the linear region would
be more expensive.
In order to determine how much current CTs can output, the secondary resistance of the CTs is required. This
resistance will be part of the equation as far as limiting the current flow. This is determined by the maximum
voltage that may be developed by the CT secondary divided by the entire secondary resistance, CT secondary
resistance included.
The easiest method of evaluating a CT is by the Excitation Curves Method, as illustrated by the curves shown
in Figure H–2: EXCITATION CURVES METHOD. The Y-axis represents secondary exciting voltage; the x-axis
H
represents the secondary exciting current. These curves may be obtained from the CT manufacturer, or by
experimentation (see ANSI/IEEE C57.13.1 for procedures). The curves illustrate the values of secondary volts
for which the output of the CT will be linear. The desired operating secondary voltage is below the knee point
(A or B on the graph (ANSI or IEC respectively) or just slightly above it, staying within 10% CT ratio error cor-
rection at 20 times rating. Using this information, it is important to recognize that the secondary exciting voltage
is the total voltage that the CT can develop at the secondary. In this case, that voltage will drop across the sec-
ondary winding resistance as well as any load that is applied to the unit. Therefore, the secondary winding
resistance must always be included with the excitation curves, or the information is incomplete. A curve with a
knee at 100 V for example could drive a total burden of 100 V / (20 × 5 A) or 1 Ω.
Evaluation of CT performance is best determined from the excitation curves. They present the complete story
and eliminate any guess work. Most CT manufacturers will provide excitation curves upon request.
H- 269Plus Motor Management Relay GE Power Management
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