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SECTION 4 - Description of the CR192 Micro-starting
and Protection Module
a phasor diagram depicting the relationship of voltage and
current for various power factors.
The CR192 automatically suppresses power factor until
the programmed set-point "FCX" times out. The CR192 can be
programmed to suppress power factor trip action if the line
current is less than 50 percent activate the PF Protection. A
value of "2" in Parameter 10 gives the above feature with the
"Ride Thru" mode.
A
value of "3" in Parameter 1 0 provides the
above described feature with "Resync" mode operation. The
normal "Ride Thru" modes (0 in Parameter 1 0) and "Resync"
mode (1 in Parameter 1 0) are described in Section 4.1.3.2.
Sequence
V.-V,V,
V:,=v;-V,
V.----~~
V;
V,
I, ___
l
~~-:
is same
_
~hase
as V
I
V,
1-
j
Unity
P F
V, V
2
V, -
PHASE-TO-NEUT.
VOLTAGES.
,
13 -
PHASE 3
V
2
CURRENT
_V}
- V .
I
,
,
V.
~
I
I] LAGS V,
V
1
-~
J
/
I
J
~/
90' Lag
I;
t~~EADS
V,
V,
\
--.l
90' Lead
Figure 17. Phasor diagram description of power factor sensing
from voltage across two phases and current in third
phase
4.1.3.2 Controller action during pull-out
If excessive mechanical load is applied to the motor shaft
during normal running of the motor in synchronism, the resulting
lagging power factor and/or line current surge will be detected
by the IlSPM. Two forms of pull-out protection are available.
They are as follows.
1. Resync mode. Resync mode operation will cause the
IlSPM Field Application Relay, FAR, to act to remove the motor-
field excitation. Action will occur from either lagging power factor
below the programmed set-point, or a line current surge above
approximately four times motor full-load current. See Figure 20A.
Relay FCX drops out at the same time as FAR. Load is
removed if an automatic loader is connected.
The motor will continue to run with field removed for the
programmed power factor delay time, and if resynchronization
does not occur within this time, the TRIP relay will operate and
the motor will stop.
The display will indicate "PF TRIP."
2. Ride-thru Mode.
If
the alternate "ride-thru" mode is
selected, the field is not removed immediately as in the resync
mode.
Instead, if the power factor dips below the trip point and
persists for the PF time delay, the TRIP relay will operate and
the motor will stop. Also, a line current surge greater than
approximately four times motor full load will cause TRIP
operation if the PF time delay is exceeded. Power factor trips
are indicated by "PF TRIP" in the display. Line current
surges greater than four times rated line current are indicated
by "PULL-OUT TRIP"
4.1 .3.3 Effect of voltage dips on motor power
factor
Solid-state excitation systems have an effect on the way
motor power factor responds to line voltage dips. The effect may
be to cause a power-factor relay to operate inadvertently. This
causes the motor to trip on lagging power factor caused by a
transient condition which is not an actual pull-out condition.
A solid-state exciter differs from a rotating exciter in the way
it responds to voltage dips. The rotating inertia of a Motor-
Generator set may maintain excitation voltage relatively
constant for several seconds, but a solid-state exciter has
practically no built-in delay in the way it responds to line voltage.
Therefore, any delay in change of motor-rotor flux following an
excitation voltage change is determined by the time constant of
the rotor field poles themselves. This is usually 0.5 to 1.0
seconds.
The sequence of events transpiring during a voltage dip
with a solid-state exciter is depicted in Figure 18. Assuming the
condition of a line voltage decrease of 15 percent with the motor
initially at unity power factor, the power factor will swing leading
momentarily because the generated EMF does not change until
the rotor flux decreases (determined by field time constant) and
the motor will tend to maintain constant horsepower by slightly
increasing line current. As the field flux decreases, generated
EMF also decreases, and the power factor will move back
towards unity, and there will be a load angle increase to permit
motor torque to be restored to that required to drive the load.
During both of these sequences the motor power factor has not
become significantly lagging, so the power-factor relay does not
operate.
Finally, when line voltage comes back to normal, the power
factor will momentarily swing over to lagging and the power
factor protection relay will trip because the rotor flux does not
respond as rapidly to change as the stator, and generated EMF
15
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