GE MiCOM P40 Agile, Technical Manual, Page: 207 / 962

This feature is called permissive trip reinforcement. It is designed to ensure that synchronous tripping occurs at all
protected terminals.
4.2.2 PERMISSIVE OVERREACH WEAK INFEED FEATURES
Special weak infeed logic (WI) can be used with the POR schemes. The Weak Infeed setting can be found in the
SCHEME LOGIC column. Weak Infeed can be set to
Echo
,
Echo and Trip
, or
Disabled
.
Weak Infeed Echo
For a POR scheme, a signal is normally sent only if the appropriate signal associated with the pick up of the
sending zone detects a fault. However, under some circumstances, the fault current infeed at one line end may be
so low that it is insufficient to operate any of the distance zones. In another scenario, if a circuit breaker at one of
the terminals is in an open state, the current infeed will be zero. These scenarios are termed Weak Infeed (WI)
conditions. Without special attention they could result in slow fault clearance at the terminal that is feeding the
fault current (called the ‘strong infeed’ terminal). This would typically result in tripping after the Zone 2 time delay
(tZ2). To avoid Zone 2 delayed tripping, a device subject to Weak Infeed conditions can be set to echo signals
received on the Channel Aided link back to the initiating IED without delay. This allows the devices at points of
strong infeed to permissively trip instantaneously.
The Weak Infeed Echo Send logic is:
● No Distance Zone Operation has been detected, but a POR Aided signal has been received.
Weak Infeed Echo and Trip
The Weak infeed echo logic will produce an aided trip at a strong infeed terminal, but it does not produce a trip at
the weak infeed. Setting Weak Infeed to
Echo and Trip
allows tripping of the weak infeed circuit breaker of a
faulted line. Three undervoltage elements, Va<, Vb< and Vc< are used to detect the line fault at the weak infeed
terminal. This voltage check prevents tripping during spurious operations of the channel or during channel testing.
The Weak Infeed Echo and Trip logic is:
●
No Distance Zone Operation has been detected, but a POR signal has been received, AND a V< condition
exists.
Weak infeed tripping is time delayed according to the value set in the WI Trip Delay settingin the SCHEME LOGIC
column. Due to the use of phase segregated undervoltage elements, single-phase tripping can be enabled for WI
trips if required. If single-phase tripping is disabled, under Weak Infeed conditions, three-phase tripping will occur
after the WI Trip Delay time has expired.
4.3 PERMISSIVE SCHEME LOSS OF GUARD
This scheme is intended for use with frequency shift keyed (FSK) power line carrier (PLC) communications.
When the protected line is healthy, a guard frequency is sent between line ends to verify the channel is in service.
However, when a line fault occurs and a permissive trip signal must be sent over the line, the power line carrier
frequency is shifted to a different (trip) frequency. Therefore the distance function should receive either the guard
frequency or the trip frequency, but not both together. For certain fault types, the line fault can attenuate the PLC
signals so the permissive signal is lost and not received at the other line end. To overcome this problem, when the
guard is lost and no trip frequency is received, the protection opens a window of time during which the permissive
scheme logic acts as though a trip signal had been received. Two opto-isolated inputs need to be assigned: One is
for Channel Receive; The second is designated Loss of Guard (the inverse function to guard received).
The Loss of Guard logic is described in the table below:
System condition
Permissive channel
received
Loss of guard
Permissive
trip allowed
Alarm
generated
Healthy Line No No No No
Internal Line Fault Yes Yes Yes No
P446SV Chapter 8 - Carrier Aided Schemes
P446SV-TM-EN-1 175