Functionality; Operation Principle - ABB 615 series Technical Manual

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4.2.4.3
4.2.4.4
615 series ANSI
Technical Manual

Functionality

The admittance-based ground-fault protection function 21YN provides a selective
ground-fault protection function for high-resistance grounded, ungrounded and
compensated networks. It can be applied for the protection of overhead lines as well as
with underground cables. It can be used as an alternative solution to traditional residual
current-based ground-fault protection functions, such as the IoCos mode in 67/51N,
67/50N. Main advantages of 21YN include a versatile applicability, good sensitivity and
easy setting principles.
21YN is based on evaluating the neutral admittance of the network, that is, the quotient:
Yo IG / VG
= −
GUID-2243DAE1-B639-48A9-9A19-AA4159413026 V1 EN
The measured admittance is compared to the admittance characteristic boundaries in the
admittance plane. The supported characteristics include overadmittance,
oversusceptance, overconductance or any combination of the three. The directionality of
the oversusceptance and overconductance criteria can be defined as forward, reverse or
non-directional, and the boundary lines can be tilted if required by the application. This
allows the optimization of the shape of the admittance characteristics for any given
application.
The function supports two calculation algorithms for admittance. The admittance
calculation can be set to include or exclude the prefault zero-sequence values of IG and
VG. Furthermore, the calculated admittance is recorded at the time of the trip and it can
be monitored for post-fault analysis purposes.
To ensure the security of the protection, the admittance calculation is supervised by a
residual overvoltage condition which releases the admittance protection during a fault
condition. Alternatively, the release signal can be provided by an external binary signal.
The function contains a blocking functionality. It is possible to block function outputs,
timers or the function itself, if desired.

Operation principle

The function can be enabled and disabled with the Operation setting. The corresponding
parameter values are "Enable" and "Disable".
The operation of 21YN can be described using a module diagram. All the modules in the
diagram are explained in the next sections.
Section 4
Protection functions
(Equation 23)
371