Figure 143: VTS logic
The IED may r
espond as follows, on operation of any VTS element:
● VTS set to provide alarm indication only
● Optional blocking of voltage-dependent protection elements
● Optional conversion of directional overcurrent elements to non-directional protection (by setting the
relevant current protection status cells to Enabled VTS. In this case, the directional setting cells are
automatically set to non-directional.)
The VTS I> Inhibit or VTS I2> Inhibit elements are used to override a VTS block if a fault occurs that could trigger
the VTS logic. However, once the VTS block is set, subsequent system faults must not override the block. Therefore
the VTS block is latched after a settable time delay (VTS Time Delay). Once the signal has latched, there are two
methods of resetting. The first is manually using the front panel HMI, or remote communications (if the VTS
condition has been removed). The second is in Auto mode, by restoring the 3 phase voltages above the phase level
detector settings mentioned previously.
VTS Status can be set to Disabled, Blocking or Indication. If VTS Status is set to Blocking, a VTS
condition will block operation of the relevant protection elements. In this case, a VTS indication is given after the
VTS Time Delay has expired. If it is set to Indication, their is a risk of maloperation because protection
elements are not blocked. In this case the VTS indication is given before the VTS Time Delay expires, if a trip signal
is given (in this case a signal from the VTS acceleration logic is used as an input).
This scheme also operates correctly under very low load or even no load conditions. To achieve this, it uses a
combination of time delayed signals derived from the DDB signals VTS Fast Block and All Poles Dead, to generate
the distance blocking DDB signal called VTS Blk Distance.
Note:
All non-distance voltage-dependent elements are blocked by the VTS Fast Block DDB.
If a miniature circuit breaker (MCB) is used to protect the voltage transformer output circuits, MCB auxiliary
contacts can be used to indicate a thr
ee-phase output disconnection. It is possible for the VTS logic to operate
correctly without this input, but this facility has been provided to maintain compatibility with some practises.
Energising an opto-isolated input assigned to the MCB/VTS provides the necessary block.
The VTS function is inhibited if:
● An All Poles Dead DDB signal is present
● Any phase overcurrent condition exists
● A Negative Phase Sequence current exists
● If the phase current changes over the period of 1 cycle
Chapter 13 - Supervision P54A/B/C/E
288 P54xMED-TM-EN-1
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