Por Schemes For Teed Feeders; Pur Schemes For Teed Feeders - GE MiCOM P40 Technical Manual

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Carrier aided schemes can also be used in conjunction with distance elements to protect teed feeders. Although
Permissive Overreaching and Permissive Underreaching schemes may be used, they suffer some limitations.
Blocking schemes are generally considered to be the most suitable.
7.8.1

POR SCHEMES FOR TEED FEEDERS

A Permissive Overreach (POR) scheme requires the use of two signalling channels between each pair of terminals.
On a teed feeder, a permissive trip is issued only if local Zone 2 operation is accompanied by receipt of signals from
both remote terminals. The 'AND' function of received signals can either be implemented using external logic, or
within the internal Programmable Scheme Logic (PSL).
To ensure operation for internal faults in a POR scheme, the protection at each of the three terminals should be
able to see a fault anywhere on the protected feeder. This may demand very large Zone 2 reach settings to
address the apparent impedances seen by the Distance elements.
Although POR schemes are feasible for teed feeders, the signalling requirements and the very large Zone 2
settings can make its use unattractive.
7.8.2

PUR SCHEMES FOR TEED FEEDERS

For a Permissive Underrreaching (PUR) scheme, the signalling channel is only keyed for internal faults. The channel
is keyed by Zone 1 operation. Aided tripping will occur at a receiving terminal if its overreaching Zone 2 setting
requirements have been met.
On teed feeder applications, a permissive trip is issued at a terminal if that terminal's Zone 2 operation is
accompanied by receipt of a signal from EITHER remote terminal. This makes the signalling channel requirements
for a PUR scheme less demanding than for a Permissive Overreach (POR) scheme. Either a common power line
carrier (PLC) signalling channel or a triangulated signalling arrangement can be used, making a PUR scheme
generally more attractive than a POR scheme for protecting for a teed feeder.
It must be recognised, however, that there are cases where instantaneous tripping will not occur with PUR
schemes. The following figure illustrates three cases for which delayed tripping will occur:
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Chapter 8 - Carrier Aided Schemes
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