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Date Code 20011026
Applications
5-11
SEL-321/321-1 Instruction Manual
Negative-Sequence Directional Element Settings
The SEL-321 Relay uses a unique negative-sequence directional element. The directional
element calculates the negative-sequence impedance at the relaying point and determines the
fault direction from the magnitude and sign of the calculated negative-sequence impedance.
Setting the negative-sequence directional element requires careful fault study analysis. Evaluate
unbalanced faults (phase-to-phase and phase-to-ground) for the protected line to determine the
strongest negative-sequence source at the relay location. Then calculate the negative-sequence
impedance seen at the relay location.
Set the forward directional element based upon the strongest negative-sequence source (smallest
Z2) behind the relay for faults in the forward direction. For our example, the strongest negative-
sequence source for forward faults is with all lines and sources in service. The negative-sequence
source impedance is calculated by applying a phase-to-ground fault at Bus B. Determine the
total negative-sequence current contribution from all lines except the protected line and the
negative-sequence voltage at Bus B.
The reverse directional element is set based upon the strongest negative-sequence source in front
of the relay for faults in the reverse direction. For our example, the strongest negative-sequence
source for reverse faults is with Line 5 removed from service. The negative-sequence impedance
is calculated by applying a phase-to-ground fault at Bus B. Determine the total negative-
sequence current contribution from the protected line and the negative-sequence voltage at Bus
B.
The negative-sequence directional element forward (Z2F) and reverse (Z2R) impedance settings
are calculated from the Z2F and Z2R impedance values defined above. Sum the magnitudes
calculated for Z2F and Z2R, divide by three, and subtract this number from Z2R and add this
number to Z2F. The resultant Z2F and Z2R are the forward and reverse setting values. This
methodology works for most applications, consult the factory for other setting guidelines for
unique applications.
Z2F must be less than Z2R to avoid any overlap where measured Z2 satisfies both forward and
reverse conditions. For security reasons, Z2F and Z2R thresholds must be separated by at least
0.1
Ω
secondary.
The negative-sequence overcurrent elements, 50QF and 50QR, are set at the minimum setting to
provide the maximum sensitivity.
The a2 factor is used to compensate for highly unbalanced systems. This is typical of systems
that have many untransposed lines. The a2 compensation is a ratio of the negative-sequence
current to the positive-sequence current.
The a2 compensation is typically set based upon the sensitivity requirements of the 50QF and
50QR elements. The a2 factor must be set so that any load derived positive-sequence current
does not prevent operation of the directional element at the minimum negative-sequence current
value.
Calculation: PTR = 2000:1
CTR = 200:1
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