Advertisement
3.3.4
VT Failure Supervision
This function is available for [APPL-VT] = "3PN" and "3PV" settings.
When a fault occurs in the secondary circuit of the voltage transformer (VT), the voltage
dependent measuring elements may operate incorrectly. GRD150 incorporates a VT failure
supervision function (VTFS) as a measure against such incorrect operation. When the VTFS
detects a VT failure, it can alarm and block the following voltage dependent protections by PLC
function.
• Directional overcurrent protection
• Directional earth fault protection
• Directional sensitive earth fault protection
• Directional negative overcurrent protection
A binary input signal (external VTF) to indicate a miniature circuit breaker trip in the VT circuits
is also available for the VTFS.
Scheme logic
Figure 3.3.2 shows the scheme logic for the VTFS. VT failure is detected by the following two
schemes.
VTF1: The residual overcurrent element EFF(EFVF) does not operate (EFF=0), the residual
overvoltage element ZOVF(ZOVVF) operates (ZOVF=1) and the phase current
change detection element OCDF(OCDVF) does not operate (OCDF=0).
VTF2: The phase undervoltage element UVF(UVVF) operates (UVF=1) when the three
phases of the circuit breaker are closed (CB CLOSE=1) and the phase current change
detection element OCDF(OCDVF) does not operate (OCDF=0).
In order to prevent detection of false VT failures due to unequal pole closing of the circuit breaker,
the VTFS is blocked for 200 ms after line energization.
The VTF signal is reset 100 ms after the VT failure condition has reset. When the VTF continues
for 10s or more, an alarm signal "VTF1 alarm" or "VTF2 alarm" is output.
Further, the VT failure is detected when the binary input signal (external VTF) by PLC function is
received.
This function can be enabled or disabled by the scheme switch [VTF1EN] or [VTF2EN] and has a
programmable reset characteristic. For latching operation, set to "ON", and for automatic reset
after the recover, set to "OPT-ON".
⎯
⎯
132
6 F 2 S 0 8 4 2
Advertisement
