148
Accurate CT Calibration for the Model 1133A
reference CT and M turns through the CT under test, ratios of (1000
×
M
÷
N
) : 5, (800
×
M
÷
N
) : 5,
can be provided. This allows a wide range of test CT ratios to be calibrated with a single accurate
reference artifact.
15.4
Excitation Current Source
For a high current supply required to perform the calibration, consider the Model 935A, Current
Source. This will work on the principle of a “CT in reverse,” that is, a “donut” having a multi-turn
primary excited at a reasonable current level (5 to 10 amps maximum). This device will allow a
heavy-gauge winding to be placed through its center, exciting it at the high currents required (up
to 2000 ampere-turns).
While it is possible, and perhaps even desirable on some accounts, to provide a regulated,
solid-state source to drive the excitation coil, this would be very expensive and heavy considering
the powers involved (up to 1000 VA). Due to the accurate ratio-metric measurement technique to
be proposed in the next section, it is not necessary to have the excitation current be particularly
accurate or even stable. Therefore, we can use a much simpler brute-force design based on a multi-
tap autotransformer and Variac to set the current. The design contemplated will offer settings for
input voltage range (100, 120, 200, 220 and 240 Vrms) and output range (100, 50, 20, 10 and 5%
of full-scale), and continuous adjustment from zero to the selected output range. The device would
be powered on site from the ac–mains power.
15.5
Comparison Technique
The most direct method of comparison involves making a measurement using the reference CT,
then using the unknown, and then calculating the ratio correction factor and phase angle from
the data (Figure 15.1). This has the drawbacks that the current source and measurement device
must be substantially more accurate and stable than the desired measurement result, typically by
a factor of 4 or 5. If this were possible, it would greatly increase the cost and size of the equipment.
Figure 15.1: Comparison Technique A
A better technique is called “nulling” (Figure 15.2). In this method, the outputs of the two
CT’s, reference and test, are subtracted electrically by using superposition (Kirchhoff’s law), and
the difference (error) measured directly. This subtraction is exact, and introduces no error. Using
a second channel of the same measuring instrument, the actual secondary current can also be
measured, and the ratio correction factor and phase angle are then determined mathematically.
Best of all, if the measuring instrument can make the two measurements simultaneously, then
