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 11)
Admittance Clc mode
= "Delta"
Yo
Io Io
Uo Uo
Io
Uo
fault prefault
fault prefault
=
−
− −
=
−( )
∆
∆
(Equation 12)
Yo Calculated neutral admittance [Siemens]
Io
fault
Residual current during the fault [Amperes]
Uo
fault
Residual voltage during the fault [Volts]
Io
prefault
Prefault residual current [Amperes]
Uo
prefault
Prefault residual voltage [Volts]
Δ Io Change in the residual current due to fault [Amperes]
Δ Uo Change in the residual voltage due to fault [Volts]
Traditionally, admittance calculation is done with the calculation mode "Normal",
that is, with the current and voltage values directly measured during the fault.
As an alternative, by selecting the calculation mode "Delta", the prefault zero-
sequence asymmetry of the network can be removed from the admittance
calculation. Theoretically, this makes the admittance calculation totally immune to
fault resistance, that is, the estimated admittance value is not affected by fault
resistance. Utilization of the change in Uo and Io due to a fault in the admittance
calculation also mitigates the effects of the VT and CT measurement errors,
thus improving the measuring accuracy, the sensitivity and the selectivity of the
protection.
Protection functions
1MRS758755 C
344 REC615 & RER615
Technical Manual
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