Sel 411L - Figure 3.4 AC Saturation Path of the External Fault Detector (Simplified)

P.3.10
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Theory of Operation
the differential current. Increase in the restraining current shifts
the operating point of the Alpha Plane toward the ideal
blocking point of 1–180°.
➤ An external fault detection algorithm, upon detecting an
external fault and in anticipation of possible CT saturation, can
increase the level of restraining signal for the 87LQ and 87LG
elements with a portion of the maximum phase restraint current
to secure these functions in cases where they have no natural
restraint. Increase in the restraining current shifts the operating
point of the Alpha Plane toward the ideal blocking point of
1–180°.
In summary, the presence of an intermediate layer of differential and
restraining signals before transitioning into the Alpha Plane allows the relay to
apply tried-and-true protection concepts and maximize advantages of both
traditional restraint and the Alpha Plane.
External Fault
Detection
The 87L function of the relay incorporates an external fault detection
algorithm. The algorithm analyzes certain features of the 87L zone currents to
declare an external event such as a fault, a load pickup under exceptionally
high X/R ratio, or a transformer inrush current that could jeopardize security
of the 87L as possible CT saturation may follow. Assertion of the algorithm
occurs before and regardless of CT saturation, bringing proper security to the
87L scheme, particularly to the 87LQ and 87LG elements.
The external fault detection algorithm consists of two paths. The ac saturation
path guards against potentially fast and severe CT saturation resulting from
very high current magnitude such as during close-in external faults; the dc
saturation path guards against typically slower and less severe saturation that
can result from relatively large and long-lasting dc component in the current
signals such as during transformer inrush or remote faults with large X/R
ratios that cleared slowly.
Figure 3.4 shows a simplified logic diagram of the ac saturation path of the
algorithm. The principle of operation is based on the observation that all CTs
of the differential zone perform adequately for a short time after the fault
inception. If so, the differential current does not develop immediately on
external faults, but the restraining signal increases immediately. This external
fault pattern differs from the internal fault pattern, in which both the
differential and restraining currents develop simultaneously. The algorithm
monitors the difference by responding to changes in the instantaneous
differential current and the instantaneous restraining currents. If the algorithm
detects sufficient increase in the restraining signal with no corresponding
increase in the differential signal, and the situation persists for a pre-
determined portion of a power cycle, it declares an external fault. When both
signals develop simultaneously, the EFD
AC
logic does not assert.
Figure 3.4 AC Saturation Path of the External Fault Detector (Simplified)
Note that the algorithm needs no user settings, and the parameters of the
scheme have been selected properly as factory constants.
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