P.3.79
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
➤ LOP condition at Terminal 1. Assume normal operational
conditions, followed by an LOP condition at Terminal 1. Relay 1
stops compensating. Relays 2 and 3 respond by switching their
multipliers from 1/3 to 1/2. The total charging current still
receives proper compensation, including at Relay 1. The
accuracy is slightly degraded, as the stub bus scenario explains
(the effective voltage is the average of two terminal voltages,
not three). All three relays assert the 87CCD Relay Word bit,
signaling degraded compensation.
➤ Loss of communications between Terminals 1 and 2. Assume
normal operational conditions, followed by a loss of channel
between Relays 1 and 2. Relays 1 and 2 become slaves (87SLV
asserted) and stop compensating. Relay 3 remains a master
(87MTR asserted). Upon determining that it is the only relay
providing compensation, Relay 3 switches its multiplier from
1/3 to 1. The total charging current still receives compensation,
but accuracy is degraded, as the stub bus scenario explains (in
this case, the effective voltage in use is the voltage at
Terminal 3). All three relays assert the 87CCD Relay Word bit,
signaling degraded compensation. Relay 3 is the only master
that can trip, and it works with differential current
compensated for the line charging current.
➤ Open-pole condition at Terminal 2. Assume normal operational
conditions followed by an open-pole condition at Terminal 2.
Relay 2 stops compensating because it cannot rely upon the
bus-side voltage to represent the line voltage when the breaker
is opened. Relays 1 and 3 respond by switching their multipliers
from 1/3 to 1/2. The total charging current still receives proper
compensation, including at Relay 2. Accuracy is slightly
degraded, as stub bus scenario explains (the effective voltage
is the average of two terminal voltages, not three). All three
relays assert the 87CCD Relay Word bit, signaling degraded
compensation.
➤ Open-pole condition at Terminal 2 and an LOP condition at
Terminal 3. Assume normal operational conditions followed by
an open pole condition at Terminal 2 and an LOP condition at
Terminal 3. Relays 2 and 3 stop compensating because their
voltages have become unreliable. Relay 1 responds by switching
its multiplier from 1/3 to 1. The total charging current still
receives proper compensation, including at Relays 2 and 3.
Accuracy is degraded, as the stub bus scenario explains (in this
case, the effective voltage in use is the voltage at Terminal 1,
which does not reflect voltage changes along the line
segments). All three relays assert their 87CCD Relay Word bits,
signaling degraded compensation.
➤ Open-pole condition at Terminal 2, and LOP conditions at
Terminals 1 and 3. Assume further that Terminal 1 now suffers
an LOP condition. At this point, no relay compensates. The
87CCU (charging current compensation unavailable) Relay
Word bit asserts, resulting in the relays switching to extended
security Alpha Plane settings (87LnPS, 87LnRS, and 87LnAS
assert, where n = P, Q, G). You selected the extended security
settings to ensure security in the absence of charging current
compensation, so the 87L scheme remains secure.
This example shows that compensation exists for the entire 87L scheme
despite problems with voltage sources, so long as the scheme can access at
least one valid voltage source. The scheme provides a fallback response when
the last relay stops compensating.
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