GE MM3, Instruction Manual, Page: 211 / 230

CHAPTER 10: MISCELLANEOUS CT ISOLATION
MM3 MOTOR MANAGER 3 – INSTRUCTION MANUAL 10–5
The curve below represents a typical excitation curve for a CT. The Y-axis represents
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 10-4: Excitation Curves
For a protection class CT with a 5A 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 below. The Y-axis represents secondary exciting voltage; the X-axis
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 kneepoint (A or B on the graph (ANSI or
IEC respectively) or just slightly above it, staying within 10% CT ratio error correction 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 secondary 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:
(EQ 10.4)
Secondary exciting amps
0.001
0.01
0.1 1
10
100
Secondary exciting v
olts
0.1
1
10
100
1000
10 000
100 V
20 5 A
×
---------------------
1 Ω
=