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3-44
Protection Functions
Zero sequence voltage
protection U
> (59N)
0
857-UM001A-EN-P – July 2009
The zero sequence voltage protection is used as unselective backup
for earth faults and also for selective earth fault protections for
motors having a unit transformer between the motor and the busbar.
This function is sensitive to the fundamental frequency component of
the zero sequence voltage. The attenuation of the third harmonic is
more than 60 dB. This is essential, because 3n harmonics exist
between the neutral point and earth also when there is no earth fault.
Whenever the measured value exceeds the user's pick-up setting of a
particular stage, this stage picks up and a start signal is issued. If the
fault situation remains on longer than the user's operation time delay
setting, a trip signal is issued.
Measuring the residual voltage
The zero sequence voltage is either measured with three voltage
transformers (e.g. broken delta connection), one voltage transformer
between the motor's neutral point and earth or calculated from the
measured phase-to-neutral voltages according to the selected voltage
measurement mode (see "Voltage Measurement Mode" in Chapter 5
•
Phase: the zero sequence voltage is calculated from the phase
voltages and therefore a separate zero sequence voltage
transformer is not needed. The setting values are relative to the
configured voltage transformer (VT) voltage/√3.
•
Line+U
: The zero sequence voltage is measured with voltage
0
transformer(s) for example using a broken delta connection. The
setting values are relative to the VT0 secondary voltage defined
in configuration.
NOTE: The U
signal must be connected according the connection
0
diagram (Figure 9.14 in Chapter 9) in order to get a correct
polarization. Please note that actually the negative U
connected to the device.
Two independent stages
There are two separately adjustable stages: U
stages can be configured for definite time (DT) operation
characteristic.
The zero sequence voltage function comprises two separately adjust-
table zero sequence voltage stages (stage U
Setting groups
There are two settings groups available for both stages. Switching
between setting groups can be controlled by digital inputs, virtual
inputs (mimic display, communication, logic) and manually.
, –U
, is to be
0
0
> and U
>>. Both
0
0
> and U
>>).
0
0
):
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