469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL A–1
469 Motor Management Relay
Appendix
Digital Energy
Multilin
Appendix
A.1 Two-Phase CT Configuration
A.1.1 Description
This appendix illustrates how two CTs may be used to sense three phase currents.
The proper configuration for the use of two CTs rather than three to detect phase current is
shown. Each of the two CTs acts as a current source. The current that comes out of the CT
on phase A flows into the interposing CT on the relay marked A. From there, the current
sums with the current that is flowing from the CT on phase C which has just passed
through the interposing CT on the relay marked C. This ‘summed’ current flows through the
interposing CT marked B and from there, the current splits up to return to its respective
source (CT). Polarity is very important since the value of phase B must be the negative
equivalent of A + C in order for the sum of all the vectors to equate to zero. Note that
there is only one ground connection as shown. If two ground connections are made, a
parallel path for current has been created.
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