OPERATE
Neutral
admittance
calculation
BLOCK
START
IRES
Io
Uo
RELEASE
t
Timer
URES
Blocking
logic
Operation
charasteristics
FAULT_DIR
Figure 279: Functional module diagram
Neutral admittance calculation
When the residual voltage exceeds the set threshold
Voltage start value
, an earth
fault is detected and the neutral admittance calculation is released.
To ensure a sufficient accuracy for the Io and Uo measurements, it is required that
the residual voltage exceeds the value set by
Min operate voltage
. If the admittance
calculation mode is "Delta", the minimum change in the residual voltage due to a
fault must be 0.01 × Un to enable the operation. Similarly, the residual current must
exceed the value set by
Min operate current
.
The polarity of the polarizing quantity Uo can be changed, that is,
rotated by 180 degrees, by setting the
Pol reversal
parameter to "True" or
by switching the polarity of the residual voltage measurement wires.
As an alternative for the internal residual overvoltage-based start condition, the
neutral admittance protection can also be externally released by utilizing the
RELEASE input.
When
Admittance Clc mode
is set to "Delta", the external logic used must be able
to give RELEASE in less than 0.1 s from fault initiation. Otherwise the collected
pre-fault values are overwritten with fault time values. If it is slower,
Admittance Clc
mode
must be set to “Normal”.
Neutral admittance is calculated as the quotient between the residual current and
residual voltage (polarity reversed) fundamental frequency phasors. The
Admittance
Clc mode
setting defines the calculation mode.
Admittance Clc mode
= "Normal"
(Equation 57)
Admittance Clc mode
= "Delta"
Yo
Io Io
Uo Uo
Io
Uo
fault prefault
fault prefault
=
−
− −
=
−( )
∆
∆
(Equation 58)
Protection functions
1MRS759142 F
492 REX640
Technical Manual
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