GE Multilin MM300, Быстрый старт

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CHAPTER 2: INSTALLATION ELECTRICAL INSTALLATION
MM300 MOTOR MANAGEMENT SYSTEM – QUICKSTART GUIDE 11
The MM300 has three channels for phase current inputs, each with an isolating
transformer. The phase CTs should be chosen so the FLA is not less than 50% of the rated
phase CT primary. Ideally, the phase CT primary should be chosen such that the FLA is
100% of the phase CT primary or slightly less, never more. This will ensure maximum
accuracy for the current measurements. The maximum phase CT primary current is 1000
A.
The ground CT connection can either be a zero sequence (core balance) installation or a
residual connection. Note that only 1 A and 5 A secondary CTs may be used for the
residual connection. The zero sequence connection is shown in the diagram above, and
this is recommended. Unequal saturation of CTs, CT mismatch, size and location of motor,
resistance of the power system, motor core saturation density, etc. may cause false
readings in the residually connected ground fault circuit. Refer to
section 2.2.2.1 for CBCT
connection.
Note that if residual ground connection is selected, the CT primary setpoint used by the
Phase CT's will also be used by the residual ground calculations.
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.
A
C
B
Power flow
– +
D1 D2 D4 D3
Phase current inputs
MM300
Motor Management System
To switchgear
ground bus
853744A2.CDR
D5 D6 D7
CT1 CT2 CT3
Control power
L N D8
G/F
Contactor
R
I
CBCT
/
50
:
0.025
Core Balance (Zero Sequence)