5
I
op
= (CT ratio) x (
I
r
+ n
I
e
+
I
SH
)
I
SH
= V
S
R
SH
At the maximum secondary current,
the non-linear resistor (metrosil)
should limit the voltage to 1500V
rms or 2120V peak for 0.25
second. At higher relay voltage
settings, it is not always possible to
limit the fault voltage to 1500V rms,
so higher fault voltages may have to
be tolerated.
Applying the MFAC14/34
As the MFAC14/34 is a voltage
calibrated relay with setting ranges
of 25-175V in 25V steps, 25-325V
in 50V steps and 15-185V in 5V
steps, it is inherently a high
impedance relay requiring no
external resistors. Due to the relay
circuit impedance always being
relatively high, significant voltages
can be produced across the current
transformers and secondary wiring
during an internal fault. To limit the
voltage to a value below the
insulation level of the current
transformers, relay and
interconnecting leads, a non-linear
resistor (metrosil) is always required
and should always be used by
connecting in parallel with the relay.
Refer to metrosil publication for
selection chart).
The operating current is virtually
fixed at around 20mA, but there is
some slight variation with relay
voltage setting as a result of
variation in the current drawn by the
non-linear resistor (metrosil). The
operating current, including the non-
linear resistor (metrosil) current, for
the various voltage settings is stated
to the right:
The relay effective current setting
can be calculated in the same
manner as described for the
MCAG14/34. For busbar
protection, it is considered good
practice by some utilities to set the
minimum primary operating current
in excess of the rated load. Thus, if
one of the current transformers
becomes open circuit the MFAC14/
34 does not maloperate. The
MVTP11/31 (busbar supervision
relay) should give an alarm for open
circuit conditions but will not stop a
maloperation if the relay is set
below rated load. Thus, if the
resultant value of Iop is too low, it
may be increased by the addition of
a shunt resistor (RSH) to give a
current of:
The increased primary operating
current with the shunt resistor
connected is:
Figure 2:
Phase and earth fault differential protection for generators, motors or reactors
Figures 2 to 8 show how high
impedance relays can be applied
in a number of different situations.
Figure 3:
Restricted earth fault protection for a 3 phase, 3 wire system-applicable to star
connected generators or power transformer windings
Setting range: 25-175V
Setting Voltage 25 50 75 100 125 150 175
I
r
(mA) 19 19 20 23 27 36 53
Setting range: 25-325V
Setting Voltage 25 75 125 175 225 275 325
I
r
(mA) 19 19 20 22 24 31 44
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