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Transmission Line Riddle No.3 - Permissive Schemes
What is POTT and PUTT and where these schemes are used i.e. short and long transmission lines.
Author : Muhammad Faheem - From: Pakistan
Sun, April 7th, 2013 - 19:22
Refer to ALSTOM protective relay handbook for useful descriptions about PUP & POP scheme design and applications.

Permissive Under-reach Transfer Tripping (PUP) Scheme
The direct under-reach transfer tripping scheme is made more secure by supervising the received signal with the operation of the Zone 2 relay element before allowing an instantaneous trip, as shown in Figure below. The scheme is then known as a 'permissive under-reach transfer tripping scheme' (sometimes abbreviated as PUP Z2 scheme) or ‘permissive under-reach distance protection’, as both relays must detect a fault before the remote end relay is permitted to trip in Zone 1 time.

Time delayed resetting of the 'signal received' element is required to ensure that the relays at both ends of a single-end fed faulted line of a parallel feeder circuit have time to trip when the fault is close to one end. Consider a fault F in a double circuit line, as shown in Figure below. The fault is close to end A, so there is negligible infeed from end B when the fault at F occurs.
The protection at B detects a Zone 2 fault only after the breaker at end A has tripped. It is possible for the Zone 1 element at A to reset, thus removing the permissive signal to B and causing the 'signal received' element at B to reset before the Zone 2 unit at end B operates. It is therefore necessary to delay the resetting of the 'signal received' element to ensure high speed tripping at end B.

Permissive Over-Reach Transfer Tripping (POP) Scheme
In this scheme, a distance relay element set to reach beyond the remote end of the protected line is used to send an intertripping signal to the remote end. However, it is essential that the receive relay contact is monitored by a directional relay contact to ensure that tripping does not take place unless the fault is within the protected section; see Figure below. The instantaneous contacts of the Zone 2 unit are arranged to send the signal, and the received signal, supervised by Zone 2 operation, is used to energise the trip circuit. The scheme is then known as a 'permissive over-reach transfer tripping scheme' (sometimes abbreviated to ‘POP’), 'directional comparison scheme', or ‘permissive overreach distance protection scheme’.

Since the signalling channel is keyed by over-reaching Zone 2 elements, the scheme requires duplex communication channels - one frequency for each direction of signalling.
If distance relays with mho characteristics are used, the scheme may be more advantageous than the permissive under-reaching scheme for protecting short lines, because the resistive coverage of the Zone 2 unit may be greater than that of Zone 1.
To prevent operation under current reversal conditions in a parallel feeder circuit, it is necessary to use a current reversal guard timer to inhibit the tripping of the forward
Zone 2 elements.
Author : Hamid - From: Iran
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