11
2.2
Calculation of relay setting
The minimum setting voltage to ensure stability is:
V
S
≥
(R
CT
+ 2R
L
)
I
F
N
The minimum knee point voltage (V
K
) of the CT must be:
V
K
≥
2V
S
The relay is current calibrated and has settings chosen from the range given in 2.1.1.
It has a burden of approximately 1VA at setting.
Having calculated V
S
a suitable relay tap must be chosen to give the required
primary operating current
I
P
.
I
P
= N (
I
R
+ n
I
E
)
When the value of
I
R
has been selected the value of stabilising resistor R
ST
must be
calculated.
R
ST
=
V
S
I
R
VA
I
R
2
–
2.3
External Metrosils
When the maximum voltage which appears across the relay circuit on internal faults
exceeds 3000 volts peak a metrosil non-linear resistor must be fitted across the relay
and the stabilizing resistor.
The peak voltage can be calculated from the formula:
V
P
= 2 2V
K
(V
F
– V
K
)
The metrosil should be chosen so that the current at the voltage setting of the relay is
less than 30mA for a 1 amp and 100mA for 5 amp current transformers.
At the maximum secondary internal fault current the metrosil cut-off point should not
be greater than 1500 volts rms or 2120 volts peak.
2.4
Commissioning preliminaries
2.4.1
Inspection.
Carefully examine the module and case to see that no damage has occurred during
transit. Check that the relay serial number on the module, case and cover are
identical, and that the model number and rating information are correct.
Carefully remove any elastic bands/packing fitted for transportation purposes.
Carefully actuate the armature of each unit in turn with a small screwdriver/probe.
Note that immediately after the point where any normally open contacts just make,
there is a small further movement of the armature. This ensures that contact follow
through and wiping action is present. On units fitted with hand reset flag indicators,
check the flag is free to fall before, or just as, any normally open contacts touch.
Check that the external wiring is correct to the relevant relay diagram or scheme
diagram. The relay diagram number appears inside the case.
Particular attention should be paid to the correct wiring and value of any external
resistors indicated on the wiring diagram/relay rating information.
