P.3.4
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Theory of Operation
a per phase basis and in the time domain. Therefore, the compensation is
accurate under balanced and unbalanced conditions and for line pickup with
uneven breaker pole operation, internal and external faults, etc.
The 87L function follows the tried-and-true Alpha Plane operating
characteristics. The relay applies a generalized Alpha Plane algorithm to allow
applications with multiple currents of the differential zone (multiple- and
dual-breaker line terminals), applications with harmonic restraining for in-line
transformers, and applications with line charging current compensation. The
generalized Alpha Plane principle is similar to the two-terminal version used
by the SEL-311L. However, the SEL-311L and SEL-411L are two completely
independent hardware and firmware platforms. They are not compatible to be
applied together in a line current differential scheme.
The following subsections elaborate on the 87L algorithm, providing greater
understanding of how the 87L function works, its inherent advantages, and
implications of design choices for application and testing. This manual
provides more detailed description of internal logic, settings, Relay Word bits
and analog quantities in respective sections.
Signal Processing
Through the use of instantaneous values, the 87L function in the relay
provides the following:
Calculation of harmonics in the differential current for in-line transformer
applications with no need to send harmonics of the terminal currents over the
87L channel.
➤ External fault detection based upon the sequence of rise in the
differential and through currents.
➤ Independent sampling and subsequent resampling when relays
align data, with no need to synchronize relay analog-to-digital
converters (ADCs).
These and other advantages of the sample-based 87L implementation simplify
relay design and yield a more robust product.
The relay samples its ac input current and voltages at a fixed sampling rate of
8 kHz for digital fault recording functionality. The relay then downsamples
and tracks the frequency of this stream of samples to feed local functions such
as voltage and current magnitude calculations.
The relay performs a separate downsampling process to a fixed rate of 1 kHz
to produce current samples for transmission to the remote relays in the 87L
scheme. The relay applies an appropriate anti-aliasing filter when
downsampling and scales current signals prior to transmission, to account for
differences in the CT (ratios and nominal secondary currents) and in-line
transformer ratios (if applicable).
The relay adds all local currents that bound the 87L differential zone prior to
transmission (see Figure 3.1) to optimize communications bandwidth
requirements by sending only one current, even in dual-breaker applications.
Effectively, the transmitted instantaneous current represents the current that
flows into the protected line at the local terminal.
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