P.3.6
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Theory of Operation
The frequency tracking algorithm is common for the local signals, protection
functions, and the 87L. Frequency tracking improves accuracy of the 87L
function, particularly the sequence differential elements 87LQ and 87LG.
The relay uses the tried-and-true full-cycle cosine filter algorithm to pass
frequency-tracked 16-samples-per-cycle 87L currents for phasor estimation.
The relay then derives symmetrical components to facilitate the 87LQ and
87LG functions. A number of other signal processing operations also take
place to enable external fault detection and other built-in elements of the 87L
scheme.
In addition, relays measure and communicate magnitudes of the 87L zone
currents. Such measurement and communication of this information, part of
the generalized Alpha Plane algorithm, enhances security during external fault
conditions. As Figure 3.2 illustrates, each relay adds the magnitudes of all
local currents and provides the result to all other relays in the scheme. In this
way, each relay develops a representative value of the through current for the
87L zone. The sum of all such current magnitudes from all line terminals
produces the true restraining current for the complete 87L zone (see
Figure 3.2).
Equation 3.2
We can therefore refer to the transmitted current magnitudes (I
RST LOC
) as
“partial restraining” currents.
The relay applies the restraining current in the generalized Alpha Plane
algorithm; it does not use this current value directly for tripping. Each 87L
data packet includes five magnitudes (A, B, C, Q, and G). The relay uses these
for security of the 87LP, 87LQ, and 87LG elements, accordingly. Each packet
includes a total of 3 (phases) • 4 (samples) = 12 current samples and a total of
5 restraining quantities. In this way, the relay optimizes 87L communications
further to provide maximum fidelity to the operating signal and sufficient
fidelity to the auxiliary restraining quantities.
In summary, the relay signal-processing scheme allows consolidation of all
87L zone currents with good fidelity toward the operating signal and adequate
fidelity for other auxiliary portions of the 87L algorithm. As a result of data
acquisition, transmission, reception, alignment, and frequency tracking, each
relay working in the master mode can access differential current as samples,
phase, and symmetrical phasors; other signal features from samples at the
receiving end; and the total magnitude of the 87L zone through current. These
operating and restraining quantities feed into the generalized Alpha Plane
algorithm, as explained in Generalized Alpha Plane.
Generalized Alpha
Plane
The Alpha Plane line current differential protection principle can be applied
naturally to zones bounded by two currents, such as for two terminal lines
connected in a single-breaker arrangement (see Figure 3.3). For this case, we
measure and compare a complex ratio of the two currents (k = I
L
/I
R
) with the
operating characteristic.
I
RST LOC
I
RST REM1
I
RST REM2
++ I
CT1 MAG
... I
CT6 MAG
++ I
RST
==
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