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MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
ELECTRICAL INSTALLATION
CHAPTER 2: INSTALLATION
CAUTION:
Polarity of the phase CTs is critical for negative-sequence unbalance calculation,
power measurement, and residual ground current detection (if used).
Two CT configuration
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 CT1. 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 CT3. This summed current flows through
the interposing CT marked CT2 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.
Only one ground connection should be made as shown. If two ground connections are
made, a parallel path for current has been created.
In the two CT configuration, the currents will sum vectorially at the common point of the
two CTs. The diagram illustrates the two possible configurations. If one phase is reading
high by a factor of 1.73 on a system that is known to be balanced, simply reverse the
polarity of the leads at one of the two phase CTs (taking care that the CTs are still tied to
ground at some point). Polarity is important.
NOTE
NOTE:
Change CT wiring only if the system is de-energized!
Figure 2-18: 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 2-19: 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
853715A1.CDR
1.73
1
1
1
1
60°
60°
60°
808701A1.CDR
1
C
A
B
1.73
1
B
A
C
Two-phase CT currents
Two-phase CT currents,
180° out-of-phase
Содержание Multilin MM300
Страница 20: ...1 14 MM300 MOTOR MANAGEMENT SYSTEM INSTRUCTION MANUAL SPECIFICATIONS CHAPTER 1 INTRODUCTION ...
Страница 104: ...3 44 MM300 MOTOR MANAGEMENT SYSTEM INSTRUCTION MANUAL SOFTWARE CHAPTER 3 INTERFACING WITH THE MM300 ...
Страница 184: ...6 8 MM300 MOTOR MANAGEMENT SYSTEM INSTRUCTION MANUAL COMMANDS CHAPTER 6 DIAGNOSTICS ...
Страница 192: ...8 2 MM300 MOTOR MANAGEMENT SYSTEM INSTRUCTION MANUAL COMMUNICATIONS INTERFACES CHAPTER 8 COMMUNICATIONS ...
Страница 198: ...A 6 MM300 MOTOR MANAGEMENT SYSTEM INSTRUCTION MANUAL REPAIRS CHAPTER A APPENDIX A ...