Current And Voltage Transformer Monitoring - Siemens 7SC80 Manual

Functions
2.12 Monitoring Functions
2.12.1.7

Current and Voltage Transformer Monitoring

General
In practice, voltage-transformer faults occur rather rarely. More frequently, it is the fuses connected in series
to the voltage transformers which fail. Such a failure is detected by the voltage transformer monitoring func-
tion and signaled with a warning indication. 1-, 2-, and 3-phase faults are detected.
The 7SC80 additionally provides a current-transformer monitoring function. The occurrence of a 1- or 2-phase
fault is detected and signaled with a warning indication. The case that all 3 current transformers fail is not
relevant in practice. Current transformers are independent components. The simultaneous failure of all three
current transformers is therefore so unlikely that there is no need to consider this case.
The limit values and delay times for both monitoring functions can be parameterized.
Voltage Transformer Monitoring
A 1- or 2-phase voltage transformer fault causes an unbalance of the measured voltage level. As a conse-
quence, the voltage levels of the negative sequence system V2 increase. In case of 1- or 2-phase voltagetrans-
former 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 differentiate between
the voltage transformer fault and a failure.
Table 2-12
Voltage V2 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
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 voltage levels of the negative sequence system V2 and also
of the zero system V0 will hardly change. The voltage levels of the positive sequence system V1, however, will
clearly fall below the minimum measured values to be expected for 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 must be assumed if I1 is above the minimum load current, but
above the minimum fault current that can occur in the event of a failure.
Table 2-13
Voltage V1 positive sequence
system
< setting value
< setting value
Designed as monitoring function, the voltage transformer monitoring function does not result in device
pickup or a device TRIP command.
If you set parameter 5310 BLOCK PROT. to YES while 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:
234
Voltage transformer fault detection (1- or 2-phase, use of negative sequence system)
Current I2 negative sequence
Voltage transformer fault detection (3-phase)
Current I1 positive sequence
system
> setting value
< setting value
system
> min. fault current
Min. load current <
AND
< min. fault current
Decision
Failure
Voltage transformer fault
Decision
Failure
Voltage transformer fault
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017