OPERATE
Neutral
admittance
calculation
BLOCK
START
Io
Uo
RELEASE
t
Timer
Operation
characteristics
Blocking
logic
Figure 213: Functional module diagram
Neutral admittance calculation
The residual current can be selected from the
Io signal Sel setting. The setting
options are "Measured Io" and "Calculated Io". If "Measured Io" is selected, the
current ratio for Io-channel is given in Configuration > Analog inputs > Current
(Io,CT). If "Calculated Io" is selected, the current ratio is obtained from phase-
current channels given in Configuration > Analog inputs > Current (3I,CT).
Respectively, the residual voltage can be selected from the
Uo signal Sel setting.
The setting options are "Measured Uo" and "Calculated Uo". If "Measured Uo"
is selected, the voltage ratio for Uo-channel is given in Configuration > Analog
inputs > Voltage (Uo,VT). If "Calculated Uo" is selected, the voltage ratio is
obtained from phase-voltage channels given in Configuration > Analog inputs >
Voltage (3U,VT).
Example 1: Uo is measured from open-delta connected VTs (20/sqrt(3) kV : 100/
sqrt(3) V:100/3 V). In this case, "Measured Uo" is selected. The nominal values for
residual voltage is obtained from the VT ratios entered in Residual voltage Uo :
Configuration > Analog inputs > Voltage (Uo,VT): 11.547 kV : 100 V. The residual
voltage start value of 1.0 × Un corresponds to 1.0 × 11.547 kV = 11.547 kV in the
primary.
Example 2: Uo is calculated from phase quantities. The phase VT-ratio is 20/sqrt(3)
kV : 100/sqrt(3) V. In this case, "Calculated Uo" is selected. The nominal value for
residual voltage is obtained from the VT ratios entered in Residual voltage Uo :
Configuration > Analog inputs > Voltage (3U,VT) : 20.000kV : 100V. The residual
voltage start value of 1.0 × Un corresponds to 1.0 × 20.000 kV = 20.000 kV in the
primary.
In case, if "Calculated Uo" is selected, the residual voltage nominal value
is always phase-to-phase voltage. Thus, the valid maximum setting for
residual voltage start value is 0.577 × Un. The calculated Uo requires that
all three phase-to-earth voltages are connected to the protection relay.
Uo cannot be calculated from the phase-to-phase voltages.
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.
Protection functions
1MRS757644 H
430
620 series
Technical Manual
