Voltage Transformer Supervision (Vts); Applications; Description Of Operation; General Operation - Siemens 7SG164 Ohmega 400 Series Manual

1.7 Voltage Transformer Supervision (VTS)

1.7.1 Applications.

A protection voltage transformer (V.T.) would normally be connected to the protection relay terminals
via a fuse or a miniature circuit breaker. Operation of these would remove the voltage source for one
or more phases. With load current flowing in the circuit the measured impedance (V/I) would be zero,
thus it would appear to the relay that a fault had occurred, possibly causing a healthy system to be
tripped out. The V.T.S. is used to identify this condition and in some cases prevent tripping by blocking
the operation of one or more of the zones of protection.

1.7.2 Description of Operation.

1.7.2.1. Residual Current and Voltage
The following description applies if the VTS Input Source is set to Residual V and I. In the event that
one or two phases of the VT are lost, a residual voltage will be developed across the relay terminals,
without a corresponding residual current being present. The relay incorporates a zero sequence
overvoltage detector and a zero sequence undercurrent detector. The simultaneous operation of both
of these detectors indicates a fuse failure. This generates a signal that gives an alarm and may be
used to inhibit the distance protection (the relay may be set to ALARM or ALARM & INHIBIT).
If, during this voltage transformer failure, an earth fault occurs, the zero sequence current will increase
which will then remove the VTS blocking and allow the relay to trip (the indication may be incorrect
due to the relay not having all the voltage inputs).
If the VTS condition remains on the system for a time longer than the "VTS Latch PU Delay" time
setting found in the Reylogic Configuration (5 seconds default), then the VTS condition will latch in.
When latched the VTS blocking will not be removed by the presence of zero sequence current and will
only be removed when the voltages are restored..
If a phase fault occurs during the voltage transformer failure there will be no zero sequence current.
Thus, if a phase fault occurs during a VT failure, the zero sequence undercurrent detector will not
reset itself and the relay will remain blocked.
For this reason there are two settings for the VTS mode:
VTS MODE:
VTS PHASE FAULT INHIBIT:
With the relay in ALARM ONLY mode, the relay will raise an alarm, when it detects a VT failure, but
will not prevent the relay from tripping.
With the VTS mode set to ALARM AND INHIBIT the relay will inhibit the earth fault elements from
causing a trip.
If VTS PHASE FAULT INHIBIT is ENABLED, the relay will remain stable during a two phase VT
failure, but will not operate if a phase fault occurs while the VTS is picked-up.
If VTS PHASE FAULT INHIBIT is DISABLED, the relay will trip if the phase fault elements pickup,
regardless of whether the relay is set to ALARM AND INHIBIT or ALARM ONLY. This means that if
two phases of the VT fail, the relay will trip, regardless of whether the relay is set to ALARM & INHIBIT
or ALARM ONLY.
1.7.2.2. NPS Current and Voltage
Alternatively, negative phase sequence current and voltage can be used to detect the loss of a VT
fuse. The operation is similar to that of the residual operation described above except that NPS current
and voltage mismatch is used to detect the operation of a fuse. The main advantage of the NPS
system is that during a phase to phase fault, NPS current is generated which will cause the VTS trip
inhibit to be removed such that once again the relay can trip correctly for the fault. For this reason, if
NPS is selected, the Phase Fault Inhibit should always be set to Enabled.
The NPS settings are scaled such that they are equivalent to the Residual settings i.e. the voltage
setting Vop = 3V0 for Residual or 3V2 for NPS.

1.7.3 General Operation

This arrangement is relatively simple and readily lends itself to application assessment in terms of its
effect, if any, on the earth fault protection coverage. The minimum time response is arranged to be
©2010 Siemens Protection Devices Limited
7SG164 Ohmega 400 Series Impedance Functions
ALARM ONLY / ALARM AND INHIBIT
ENABLED / DISABLED
Chapter 2 Page 17 of 21