GE MM200 - Page 33

CHAPTER 2: INSTALLATION ELECTRICAL INSTALLATION
MM200 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL 2–15
Figure 2-17: Two CT connection vector diagram
To illustrate the point further, the following diagram shows how the current in phases A
and C sum up to create phase "B".
Figure 2-18: Two CT connection currents
Once again, if the polarity of one of the phases is out by 180°, the magnitude of the
resulting vector on a balanced system will be out by a factor of 1.73.
On a three-wire supply, this configuration will always work and unbalance will be detected
properly. In the event of a single phase, there will always be a large unbalance present at
the interposing CTs of the relay. If for example phase A was lost, phase A would read zero
while phase B and C would both read the magnitude of phase C. If on the other hand,
phase B was lost, at the supply, phase A would be 180° out-of-phase with phase C and the
vector addition would equal zero at phase B.
888715A1.CDR
1.73
11
11
60°
60° 60°
888716A1.CDR
1
C
A
B
1.73
1
B
A
C
Two-phase CT currents
Two-phase CT currents,
180° out-of-phase