6.128
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
Out-of-Step Logic Application Examples
You can increase the adaptive UBOSBD timer calculation in multiples of setting
UBOSBF. If UBOSBF is a multiplier of one, the relay calculates the expected
time to traverse across the inner blinders based on the rate at which the swing
moved from Zone 7 to Zone 6. Similarly, if UBOSBF is a multiplier of four, the
relay multiplies UBOSBD by four.
Out-of-Step Unblocking During Unbalanced Faults
The SEL-421 treats Zone 1 phase and ground-distance elements differently than
phase-distance elements of other zones.
Operation of either of two negative-sequence directional elements, 67QUBF (for-
ward-looking), or 67QUBR (reverse-looking), defeats the OSB logic and
unblocks the phase-distance elements (except the Zone 1 elements) when an
unbalanced fault occurs following a power swing. Therefore, the phase-distance
protection operates in the POTT scheme and high-speed clears the unbalanced
fault if it is an internal one. The time-delayed elements of associated zones also
start timing to initiate backup protection functions. The 67QUBF element
unblocks forward-looking zones and 67QUBR unblocks reverse-looking zones.
The relay supervises the 67QUBF and 67QUBR elements with negative-
sequence pickup setting 50QUBP. When you set the 50QUBP pickup level to
other than OFF, the level of negative-sequence current exceeds the 50QUBP set-
ting threshold, and the relay has made a valid directional decision (32 elements),
the relay asserts either the 67QUBF or the 67QUBR directional element after
time delay setting UBD. In this manner the relay removes out-of-step blocking
for phase-distance elements other than Zone 1 elements during unbalanced faults.
The 50QUBP setting is an advanced setting and must be coordinated with the dis-
tance protection for the protected line. Setting UBD is also an advanced setting;
set the UBD timer to coordinate clearing times with protection external to the
protected line.
For out-of-step unblocking on unbalanced faults you must do the following:
Step 1. Set EADVS := Y to enable advanced settings.
Step 2. Set the negative sequence unblocking element pickup with setting
50QUBP (Negative-Sequence Current Supervision).
Coordinate with line distance protection.
Step 3. Set the unblock delay timer UBD (Negative-Sequence Current
Unblock Delay).
Coordinate clearing times with other protection.
If a power swing center is on the line under protection, the Zone 1 distance ele-
ments at one or both terminals may operate if the OSB is removed. For example,
during an unstable swing, if an external A-Phase ground fault occurs beyond the
remote terminal R in Figure 6.28, the A-Phase ground-distance elements at both
terminals operate correctly; that is, A-Phase distance element picks up in Zone 2
at the S terminal, and in reverse Zone 3 at the R terminal. However, all Zone 1
phase and ground-distance elements at both terminals may also operate if the
swing center is within the Zone 1 reach and a negative-sequence overcurrent ele-
ment removes the OSB. The undesirable operations of Zone 1 elements may trip
all three phases at both terminals for an external A-Phase fault.
The SEL-421 uses a directional negative-sequence element (67Q1T) to supervise
the out-of-step blocking of Zone 1 distance elements. 67Q1T is independent from
67QUBF and 67QUBR, which are used to defeat the OSB for distance elements
other than Zone 1 elements. This separation gives you a choice to control the
Zone 1 element operations during an unstable swing situation.
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