Siemens 7SC80 Manual page 443

Function Description
A 1- or 2-phase voltage-transformer fault causes an unbalance of the measured voltage level. As a conse-
quence, the side 2 voltage levels of the negative sequence system V2S2 increase. In case of 1- or 2-phase
voltage-transformer faults, values up to 0.3 times the rated voltage are reached. In contrast to a real failure,
however, the current levels of the negative sequence system I2 do not increase. Thus, the 7SC80 can differen-
tiate between the voltage-transformer fault and a failure.
Table 2-30
Voltage of side 2 V2S2 negative
sequence system
> setting value
> setting value
After a voltage-transformer fault, the current values of the negative sequence system are only dependent on
the load unbalance and reach maximum values of approx. 0.1 x In.
Load unbalances may cause problems in the analysis of the negative sequence system. As load unbalances
also create relatively high negative sequence system currents, there is a risk that voltage-transformer faults of
side 2 are not detected. To prevent that, the setting value for the current threshold I2 is to be set above the
current value caused by load unbalances. At the same time, the setting value must, of course, remain below
the current value expected in the event of a failure.
I2 load unbalance < setting value I2 < I2 failure
In case of a 3-phase voltage-transformer fault, the side 2 voltage levels of the negative sequence system V2S2
and also of the zero system V0S2 will hardly change. The side 2 voltage levels of the positive sequence system
V1S2, however, will clearly fall below the minimum measured values to be expected for the normal operation.
As this can also happen in the event of a local fault, the current of the positive sequence system I1 must be
used additionally for the analysis. A 3-phase voltage-transformer fault of side 2 must be assumed if I1 is above
the minimum load current and below the minimum fault current. However, I1 being above the minimum fault
current can occur in the event of a failure.
Table 2-31
Voltage of side 2 V1S2 positive
sequence system
< setting value
< setting value
Designed as a monitoring function, the voltage-transformer monitoring function of side 2 does not result in a
device pickup or a device TRIP command.
If you set parameter 5537 BLOCK PROT. S2 to YES while the voltage-transformer monitoring is activated
and a voltage-transformer fault is detected, the following functions will be blocked and thus malfunctions of
these protection elements prevented:
•
Undervoltage protection of side 2
The voltage-monitoring function of side 2 works with the fundamental components of current and voltage.
The negative, positive, and zero sequence voltages are derived from the line voltages; a suitable voltage-trans-
former connection-type needs to be selected.
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
Voltage-transformer fault detection of side 2 (1- or 2-phase, use of negative sequence system)
Current I2 negative sequence
Voltage-transformer fault detection of side 2 (3-phase)
Current I1 positive sequence
2.30 Additional Applications with 6 Voltage Inputs
system
> setting value
< setting value
system
> min. fault current
Min. load current <
AND
< min. fault current
Functions
Decision
Failure
Voltage-transformer fault
Decision
Failure
Voltage transformer fault
443