Alstom MFAC - Metrosil Non-linear Resistors and MFAC Relay Application

4
V
p
= 2 2 V
K
(V
r
- V
K
)
I
(rms) = 0.52 (
Vs(rms) x 2
)
4
C
V
SA
=
B
I
r
R
r
=
B
I
r
2
R
ST
=
V
S
- R
r
I
r
required stability voltage setting (V
S
)
and the relay current setting (I
r
).
Note: the MCAG14/34 is a fixed
burden relay, therefore the ohmic
impedance of the relay will vary
with setting. The ohmic impedance
(R
r
) of the MCAG14/34 can be
calculated using the relay VA
burden at current setting (B) and the
relay current setting (I
r
);
The ohmic impedance (R
r
) of the
MCTI14/34 over the whole setting
range is less than 0.25Ω for 1A
relays and less than 0.02Ω for 5A
relays i.e. independent of current.
The stabilising resistor supplied is
continuously adjustable up to its
maximum declared resistance. In
some applications, such as
generator winding differential
protection, the through fault current
is low which results in a low
stability voltage setting. In many
such cases, a negative stabilising
resistor value can be obtained from
the above formula. This negative
result indicates that the relay will be
more than stable without a
stabilising resistor. When a
stabilising resistor is not required,
the setting voltage(V
SA
) can be
calculated using the following
formula and the current transformer
kneepoint voltage should be at least
twice this value.
Use of Metrosil non-
linear resistors -
MCAG14/34 & MCTI14/
34
When the maximum through fault
current is limited by the protected
circuit impedance, such as in the
case of generator differential and
power transformer restricted earth
fault protection, it is generally found
unnecessary to use non-linear
voltage limiting resistors (metrosils).
However, when the maximum
through fault current is high, such
as in busbar protection, it is always
advisable to use a non-linear resistor
(metrosil) across the relay circuit
(relay and stabilising resistor).
Metrosils are used to limit the peak
voltage developed by the current
transformers under internal fault
conditions, to a value below the
insulation level of the current
transformers, relay and
interconnecting leads, which are
normally able to withstand 3000V
peak.
The following formulae should be
used to estimate the peak transient
voltage that could be produced for
an internal fault. The peak voltage
produced during an internal fault
will be a function of the current
transformer kneepoint voltage and
the prospective voltage that would
be produced for an internal fault if
current transformer saturation did
not occur. This prospective voltage
will be a function of maximum
internal fault secondary current, the
current transformer ratio, the current
transformer secondary winding
resistance, the current transformer
lead resistance to the common
point, the relay lead resistance, the
stabilising resistor value and the
relay VA burden at relay operating
current.
where V
p
= peak voltage
developed by the CT
under internal fault
conditions.
V
k
= current transformer
knee-point voltage.
V
f
= maximum voltage that
wouldbe produced if
CT saturation did not
occur.
I
f
= maximum internal
secondary fault
current.
R
ct
= current transformer
secondary winding
resistance.
RL = maximum lead burden
from current
transformer to relay.
R
ST
= relay stabilising resistor.
R
r
= Relay ohmic impedance
at setting.
When the value given by the
formulae is greater than 3000V
peak, non-linear resistors (metrosils)
should be applied. These non-linear
resistors (metrosils) are effectively
connected across the relay circuit,
or phase to neutral of the ac
buswires, and serve the purpose of
shunting the secondary current
output of the current transformer
from the relay in order to prevent
very high secondary voltages.
These non-linear resistors (metrosils)
are externally mounted and take the
form of annular discs, of 152mm
diameter and approximately 10mm
thickness. Their operating
characteristics follow the expression:
V = CI
0.25
where V = Instantaneous voltage
applied to the non-linear
resistor (metrosil)
C= constant of the non-
linear resistor (metrosi)
I = instantaneous current
through the non-linear
resistor (metrosil).
With a sinusoidal voltage applied
across the metrosil, the RMS current
would be approximately 0.52x the
peak current. This current value can
be calculated as follows;
where Vs(rms) = rms value of the
sinusoidal voltage applied
across the metrosil.
This is due to the fact that the
current waveform through the non-
linear resistor (metrosil) is not
sinusoidal but appreciably
distorted.
For satisfactory application of a
non-linear resistor (metrosil), it’s
characteristic should be such that it
complies with the following
requirements:
At the relay voltage setting, the non-
linear resistor (metrosil) current
should be as low as possible, but
no greater than approximately
30mA rms for 1A current
transformers and approximately
100mA rms for 5A current
transformers.
V
f
= I
f
(R
CT
+ 2R
L
+ R
ST
+ R
r
)