Date Code 20011026 Applications 5-5
SEL-321/321-1 Instruction Manual
Zone 2 element settings with a reach of 120% of protected line impedance account for the effects
of infeed. This point must be verified using a fault study to calculate the apparent ohms at the
local terminal for a fault at the remote end of the transmission line. In the example system, 120%
is selected for the Zone 2 elements with assurance that all faults in line 2 are detectable, even
with infeed from the remote terminals.
Assuming the Zone 1 reach for the line protection at Breakers 5 and 11 is set for 80% of the line
impedances of Lines 3 and 6 respectively, verify that a Zone 2 reach of 120% for Breaker 3 does
not overreach the Zone 1 elements at Breakers 5 and 11.
Line 2 Impedance + [0.8 ⋅ Line 3 Impedance] = 78.03 Ω + [0.8 • 39.12 Ω]
= 109.33 Ω primary
When Zone 2 at Breaker 3 has a set reach of 120%, the effective reach is:
Zone 2 at Bkr. 3 = 1.20 ⋅ 78.03 Ω = 93.64 Ω primary ∠84°
Since 93.64 Ω < 109.33 Ω, the Zone 2 setting of 120% at Breaker 3 does not overreach the
Zone 1 protection of Lines 3 and 6. Any effect of infeed tends to increase the apparent ohms
seen at Breaker 3 for faults on Lines 3 and 6.
The Zone 2 quadrilateral element setting is based upon the same requirement established for the
Zone 2 mho elements. The reactive reach is set the same as the mho distance setting. The
resistive reach determines sensitivity for high resistance faults on a radial basis. For this
example, the Zone 2 resistive reach is set for 50 Ω primary.
Zone 3 Setting
This example uses a Permissive Overreaching Transfer Trip (POTT) communications scheme.
The current reversal block, echo and weak-infeed logic require reversal of the Zone 3 elements.
The Zone 3 element reach at Breaker 3 must be selected to detect all out-of-section faults also
detected by the overreaching elements at Breaker 4. At a minimum, the Zone 3 reach setting
must equal the impedance the overreaching element at Breaker 4 sees beyond Breaker 3.
Assuming the Zone 2 reach for the line protection at Breaker 4 is set for 120% of the line
impedance of Line 2, the Zone 3 reach at Breaker 3 is set at 120% of the Zone 2 overreach from
Breaker 4.
When Zone 2 at Breaker 4 has a set reach of 120%, the effective reach is:
Zone 2 at Bkr. 4 = 1.20 ⋅ 78.03 Ω = 93.64 Ω ∠84°
The Breaker 4 overreach is equal to the Zone 2 setting minus the line impedance:
93.64 - 78.03 Ω = 15.61 Ω primary
Zone 3 Reach = 1.2 ⋅ 15.61 Ω primary ∠84° = 18.73 Ω primary ∠84°
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