ABB RELION 620 Series Technical Manual page 597

1MAC504801-IB E
6.3.5
620 series ANSI
Technical Manual
Timer
The timer is activated with the FAIL signal. The ALARM output is activated after a fixed
200 ms delay. FAIL needs to be active during the delay.
When the internal blocking is activated, the FAIL output is deactivated immediately. The
ALARM output is deactivated after a fixed 3 s delay, and the FAIL is deactivated.
The deactivation happens only when the highest phase current is more
than 5 percent of the nominal current (0.05 xIn).
When the line is de-energized, the deactivation of the ALARM output is prevented.
The activation of the BLOCK input deactivates the ALARM output.
Application
Open or short-circuited current transformer cores can cause unwanted operation in many
protection functions such as differential, ground-fault current and negative-sequence
current functions. When currents from two independent three-phase sets of CTs or CT
cores measuring the same primary currents are available, reliable current circuit
supervision can be arranged by comparing the currents from the two sets. When an error
in any CT circuit is detected, the protection functions concerned can be blocked and an
alarm given.
In case of high currents, the unequal transient saturation of CT cores with a different
remanence or saturation factor can result in differences in the secondary currents from the
two CT cores. An unwanted blocking of protection functions during the transient stage
must then be avoided.
The supervision function must be sensitive and have a short trip time to prevent unwanted
tripping from fast-acting, sensitive numerical protections in case of faulty CT secondary
circuits.
Open CT circuits create extremely high voltages in the circuits, which
may damage the insulation and cause further problems. This must be
taken into consideration especially when the protection functions are
blocked.
When the reference current is not connected to the protection relay, the
function should be turned off. Otherwise, the FAIL output is activated
Section 6
Supervision functions
591