BECKWITH ELECTRIC M-3425, Instruction Book

Page: 50 / 225

Application – 2
2–13
Figure 2-6 illustrates a composite graph of generator
and transformer limits, a chosen inverse time curve
and pickup, and a definite time pickup and delay.
Be aware that all V/Hz inverse timing curves saturate
at 150%, above which timing will be constant. If
definite time element #1 or #2 is to be used for fixed
time trip operation in conjunction with the inverse
time element, definite time pickup must be
programmed above the inverse time pickup. Also,
the time delay at this pickup should be less than the
inverse operating time at that same level to be
effective.
After any V/Hz excursion, cooling time must also
be taken into account. If the unit should again be
subjected to high V/Hz before it has cooled to
normal operating levels, damage could be caused
before the V/Hz trip point is reached. For this
reason, a linear reset characteristic, adjustable to
take into account the cooling rate of the unit, is
provided. If a subsequent V/Hz excursion occurs
before the reset characteristic has timed out, the
time delay will pick up from the equivalent point (as
a %) on the curve. The Reset Rate setting entered
should be time needed for the unit to cool to normal
operating temperature if the V/Hz excursion time
was just under the trip point.
Percentage Volts/Hz
100 105 110 115 120 125 130 135 140 145 150
0.01
0.1
1
10
100
1000
T
ime in Min
u
tes
Inverse Time Pickup
Generator Limit Curve
Transformer Limit Curve
on Generator Base
Inverse Square Curve
K = 5
Combined Protection
Curve
Definite Time Pickup
Definite Time Delay
A2
A1
Figure 2-6 Example of Capability and Protection Curves (24)