CHAPTER 7: PROTECTION CONTROL ELEMENTS
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL 321
communications equipment fails. This loss-of-guard output connects to a contact input of
the D90
Plus
. The power line carrier should also provide an output contact when the
permissive frequency is received. Wire this output to any other contact input of the D90
Plus
.
The directional comparison unblocking scheme allows a window of opportunity for fast
tripping of the breaker if the permissive signal gets attenuated by the fault to a level less
than the receiver's threshold. The guard signal is monitored and relay allowed to trip for a
loss-of-guard without receipt of the permissive signal if the D90
Plus
sees the fault in the
forward and not reverse direction by means of the forward looking distance zone 2 and
ground directional forward function (if configured) and reverse looking distance zone 4 and
ground directional reverse function (if configured).
The directional comparison unblocking scheme is active and ready for operation when the
Block setting is “Off” and the Scheme Function setting is “Enabled.” The user can disable or
block the scheme by assigning any FlexLogic operand to the Block setting. This operand
can consist of any pre-defined logic equation, including a pilot cutout switch connected to
any contact input of the D90
Plus
. As such, the scheme can be disabled with this pilot cutout
switch and the pre-defined logic state on and assigned to the Block setting. The directional
comparison unblocking scheme supervises the operation and keying of all scheme
functions. The lockout feature also supervises all scheme functions. All timers are fully
adjustable, with typical values shown as defaults.
Lockout of the scheme is started for a time duration specified by the
Loss of Guard Trip
Window setting if all of the following conditions hold:
• The directional comparison unblocking scheme is enabled and not blocked (that is, the
scheme is active)
• No permissive RX signal is received
• Any loss-of-guard signal is received
Lockout is sealed-in through an AND gate with the directional comparison unblocking
scheme active and any loss-of-guard through the
Guard Healthy Reset Delay timer. The
scheme remains blocked during longer loss-of-guard or channel conditions for any of the
configured channels. The time specified by the
Guard Healthy Reset Delay timer ensures
that the scheme remains locked out slightly longer after the loss-of-guard signal is reset.
This allows for all channel stabilizing and contact denouncing. This timer determines how
long the logic waits before enabling the directional comparison unblocking scheme when
the communications channel is restored.
Operating of the directional comparison unblocking scheme consists of two parts: normal
operation and loss-of-guard operation.
Normal operation picks up if all of the following conditions hold:
• The directional comparison unblocking scheme is enabled and not blocked (that is, the
scheme is active)
• Any permissive signal is received with its associated loss-of-guard signal (for example,
DCUB RX1 and DCUB LOG1)
• A fault is seen in the forward zone 2 distance elements or ground directional forward
function (if configured)
• The reverse zone 4 distance elements or ground directional reverse function (if
configured) did not pickup to set the transient blocking
• The directional comparison unblocking scheme is not locked out
The trip table is run if all of these conditions are met. This operates the scheme and asserts
the DCUB OP FlexLogic operand. The trip table, with the aid of the local phase selector and
received RX signals, determine which tripping operands operate. During normal operation,
the loss-of-guard signal is seen momentarily before the permissive DCUB RX signal is
received for each channel being operated, as the channel transitions from guard to
permissive frequency, but the scheme operates instantaneously after any DCUB RX signal is
received.
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