R.1.40
SEL-421 Relay Reference Manual Date Code 20111215
Protection Functions
CVT Transient Detection
67P3TC, 67P4TC, 51S1TC, 51S2TC, 51S3TC) can be used to achieve
directional control, as shown in the 230 kV Overhead Transmission Line
Example on page A.1.2.
.
CVT Transient Detection
The SEL-421 detects CVT (Capacitor Voltage Transformer) transients that
can cause Zone 1 distance elements to overreach during external faults. If
CVT transient blocking is enabled and the relay detects a high SIR (source-to-
impedance ratio) when a Zone 1 distance element is picked up, the relay
delays tripping for as long as 1.5 cycles to allow the CVT transients to
stabilize.
You do not need to enter settings. The relay adapts automatically to different
system SIR conditions by monitoring the measured voltage and current.
If the distance calculation does not change significantly (i.e., is smooth), the
SEL-421 unblocks CVT transient blocking resulting from low voltage and low
current during close-in faults driven by a source with a high SIR. Therefore,
Zone 1 distance elements operate without significant delay for close-in faults.
Consider using CVT transient detection logic when you have either of the
following two conditions:
➤ SIR greater than or equal to five
➤ CVTs with AFSC (active ferroresonance-suppression circuits)
The following conditions can aggravate CVT transients:
➤ CVT secondary with a mostly inductive burden
➤ A low C value CVT, as defined by the manufacturer
Table 1.34 Zone Directional Settings
Setting Description Range Default
DIR3 Zone/Level 3 Directional Control F, R R
DIR4 Zone/Level 4 Directional Control F, R F
DIR5 Zone/Level 5 Directional Control F, R F
Tabl e 1 . 35 CV T Tran sie nt Detection Logic Setting
Name Description Range Default
ECVT CVT Transient Detection Y, N N
Table 1.36 CVT Transient Detection Logic Relay Word Bit
Name Description
CVTBL CVT transient blocking active
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