Sel 411L - Figure 3.6 DC Saturation Path of the External Fault Detector (Simplified)

P.3.12
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Theory of Operation
detection results to ensure that all relays are informed about possible CT
saturation even if relays have no direct access to individual currents and their
dc component measurements (see Figure 3.5).
Figure 3.6 DC Saturation Path of the External Fault Detector (Simplified)
Because the 87L function of the relay incorporates the ac saturation path,
requirements for CTs can be relaxed. It is sufficient for security that all CTs of
the differential zone perform reasonably well under high-current external
faults for about a quarter power cycle. This allows reliable detection of
external faults and increases security of the relay. Subsequent saturation of
one or more CTs does not impact the external fault detection algorithm.
Incorporation of the dc saturation path into the 87L function of the relay
allows relaxing CT requirements with respect to large X/R ratio or decaying
dc components. All CTs of the differential zone should perform reasonably
well under magnetizing inrush and other heavily offset current waveforms for
about three power cycles. This allows reliable detection of slowly decaying dc
component and increases security of the relay. Subsequent saturation of one or
more CTs does not impact the external fault detection algorithm.
Upon asserting the EFD Relay Word bit locally or remotely via either the ac
saturation or dc saturation path of the algorithm, the 87L function engages
extra security as follows.
➤ The function switches blocking angle and radius of the Alpha
Plane operating characteristic from normal security values to
extended security values. This applies to all three differential
elements (87LP, 87LQ, and 87LG). Refer to 87LP Phase
Differential Elements for more information about the normal
and extended security 87L settings and logic.
➤ The function adds harmonics in the phase differential currents
to the phase restraint terms on a per-phase basis, with factory
pre-selected multipliers. As the spurious differential signal is
typically considerably distorted, the restraint term increases
significantly. This increase in the restraining term moves the
operating point on the generalized Alpha Plane toward the
ideal blocking point (1–180°), increasing security of the
87LP element. Refer to 87LP Phase Differential Elements for
more information about the 87L logic.
➤ The function adds a factory pre-selected portion of the
maximum phase restraint term to the restraint terms of the
87LQ and 87LG elements. This accounts for phase-to phase
external faults that do not produce any zero-sequence restraint
for the 87LG element and for three-phase balanced faults that
do not produce any natural restraint for either the 87LG or
87LQ elements. In addition, the added term already receives
harmonics from the differential current, magnifying the
EFD
DC
3 CYC
1 CYC
I
DIF MAG
I
RST
kRD
I
(DC)
I
(AC)
kDC
Other local 87L currents
(identical logic)
I
(DC)
PKP