SERVICE MANUAL R5888B
QUADRAMHO Chapter 5
Addendum 4
Page 1 of 2
USE OF DOBLE TEST TO COMMISSION QUADRAMHO
There are no problems with the Doble F3 test set; however, a problem occurs when
finding phase to phase reach using the teed configuration of voltage from the test
set.
In the teed configuration, two of the voltages are set 180° apart, these voltages
are then varied to find the phase to phase reach of that particular element. By
examining Figure 39, Chapter 2 and using an applied B-C fault as an example, it
can be seen that the V
B
and V
C
voltages will sum to zero at the –ve input of
IC1.
Since the voltage level detectors will have dropped off due to the low input voltage
from the Doble the V
MA
signal will be replaced by a small percentage of –IZ. The
squaring action of
IC1 will therefore produce a voltage V
KA
which is almost totally
all –
IZ. When summed with V
A
the resulting V
QA
will have the effect of restraining
the relay.
It can therefore be seen that if B and C are not totally opposite in magnitude and
phase, their relationship varies slightly, the relay may go from an operate to a
restrain condition resulting in chattering of the comparators.
This problem may be cured by using the more conventional wye type of voltage
connection and tolerating the extra calculations involved.
An example, of setting up the Doble for finding a BC reach is given below.
V
A
(reference)
R
CA
V
B-C
I
B-C
V
B
V
C
V
B-C
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