Cool Time Constants; Selection Of Cool Time Constants - GE 469 Instruction Manual

469
Motor Management Relay
Selection of Cool Time
Constants
8–2
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.
To illustrate the point further, the following diagram shows how the current in
phases A and C sum up to create phase 'B'.
1
2-PHASE CT 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.

Cool Time Constants

Thermal limits are not a black and white science and there is some art to setting a
protective relay thermal model. The definition of thermal limits mean different
things to different manufacturers, and information is often not available. Therefore,
it is important to remember the goal of the motor protection thermal modeling: to
thermally protect the motor (rotor and stator) without impeding the normal and
expected operating conditions of the motor.
The 469 thermal model provides integrated rotor and stator heating protection. If
supplied with the motor, the cooling time constants recommended by the
manufacturer should be used. Since rotor and stator heating and cooling is
integrated into a single model, use of the longest cooling time constants (rotor or
stator) is recommended.
http://www.GEindustrial.com/multilin
60°
1.73
1
60° 60°
1 1
808702A1.CDR
1.73
1
C
A
B
2-PHASE CT CURRENTS
180° OUT-OF-PHASE
Cool Time Constants
1
B
A
C
808701A1.CDR
GE Multilin