Micom P721, Техническое руководство

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 13/52
2.6.1 Use of non-linear resistors
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 unnecessary to use non-linear voltage limiting resistors. However,
when the maximum through fault current is high, such as in busbar protection, it is more
common to use a non-linear resistor across the high impedance path (relay and stabilizing
resistor). Non-linear resistors 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 typically able to withstand
approximately 3000 V peak.
The following formulae should be used to estimate the peak transient voltage that could be
produced for an internal fault. This voltage is 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.
The internal fault level,
I
’
F
, can be significantly higher than the external fault level,
I
F
, on
generators where current can be fed from the supply system and the generator. Similarly
the internal fault level on motors and shunt reactors will be considerably higher than the
values used in the stability calculations (motor start/locked rotor and inrush currents
respectively).
( )
k F k P
V V V 2 2 V
− × =
( )
r ST L CT
'
F F
R R R 2 R I V
+ + + =
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 would be produced if CT saturation did not occur
I’
F
= maximum internal secondary fault current
R
CT
= current transformer secondary winding resistance
R
L
= maximum lead burden from CT to relay
R
ST
= relay stabilizing resistor
R
r
= relay ohmic impedance at setting
When the value of V
P
is greater than 3000 V peak, non-linear resistors should be applied.
They are effectively connected across the relay circuit, or phase to neutral of the ac
buswires, and shunt the secondary current output of the current transformer from the relay
circuit to prevent very high secondary voltages.
Traditionally, Schneider Electric recommends the use of non-linear resistors manufactured
by Metrosil. They are externally mounted and take the form of annular discs, of 152 mm
diameter and approximately 10 mm thickness. Their operating characteristics follow the
expression:
25 . 0
CI V
=
Where:
V = instantaneous voltage applied to the non-linear resistor (Metrosil)
C = constant of the non-linear resistor (Metrosil)
I = instantaneous current through the non-linear resistor (Metrosil)