Permissive signalling is used to achieve selective clearing of faults without time delay over the entire length of the line to be protected. Where permissive signalling is not present in the scheme, if a fault is present within zone 1, an instantaneous trip will be initiated by the relay, but if the fault is present in zone 2, the relay initiates a trip only after an intentional time-delay has elapsed. This delay is not ideal, since the fault will remain on the protected transmission line for a period of time, causing possible damage to plant and equipment; however, as explained earlier, the delay is necessary to allow coordination with the zone and elements of other lines’
protection schemes, because of the over-reaching nature of the zone 2 element. Nevertheless,
with additional supervision logic, it is possible to bypass the zone 2 time delay under certain conditions without compromising the security of the protection scheme. This is the so-called permissive signalling - i.e. zone 2 can be given permission to trip without waiting for the zone 2 time delay if certain supervisory logic conditions can be satisfied.
A scheme that employs permissive signalling requires a communication link between the two relays that comprise the protection scheme. This communication link can be pilot wire, power- line carrier, microwave or fibre optic. As discussed in Section 2.6, the under-reaching zone will generally cause the breaker to trip instantaneously and the over-reaching zone will cause the breaker to trip after an intentional time delay. In the case of a permissive signalling scheme the relays on either end of the protected zone will communicate, via the communication link, with each other to provide near-instantaneous tripping. If certain supervisory logic conditions are satisfied, these communicated signals can be used to block, trip or transfer a trip signal, dependent on the protection engineer and the application he/she requires.
Permissive signalling schemes can be [10]:
Direct under-reaching transfer trip (DUTT)
Permissive under-reaching transfer trip (PUTT) with starter (fault detector)
Permissive under-reaching transfer trip (PUTT) with zone extension
Permissive over-reaching transfer trip (POTT)
The POTT scheme will be discussed and studied in this thesis since it is the main method of permissive signalling used for distance protection schemes in South Africa. The permissive over-reaching scheme derives its name from the fact that the over-reaching zone governs the permissive signalling. The POTT is the most popular distance scheme used in protecting transmission lines due to the greater protective coverage and resistive coverage offered by the zone 2 elements [11]. Consider the diagram in Figure 2.13, which shows a line between busbars A and B, protected by a permissive over-reaching protection scheme. This trip scheme (POTT) is known to initiate a near-instantaneous trip when the relays at both busbar A and B detect a fault in the over-reaching zone.
Consider a transmission line to be protected with relays at busbar A and busbar B with both relays having their zone 1 and zone 2 elements activated.
Figure 2.13: Permissive over-reaching transfer trip scheme
Consider a fault F1 in Figure 2.13, the relays at busbars A and B will see the fault in their respective zone 1. Both relays will initiate an instantaneous trip.
For faults F2 and F3, one relay will see the fault in zone 1 and the other relay will see the fault in zone 2. For F3, the relay at A will see the fault in both its zone 1 and 2 and the relay at B will see the fault in its zone 2. For F2, the relay at B will see the fault in both its zone 1 and 2, while and the relay at A will see the fault in its zone 2. It is these cases where the POTT scheme becomes important. For a fault such as that at F2 that is actually on the protected line and is seen by the relay A only in its zone 2, the relay B will see the fault in both zone 1 and zone 2.
For a fault that is actually on the protected line, the zone 2 element of relay B then can be used to initiate a permissive signal to send to relay A. Relay B would trip the breaker B instantaneously. With the implementation of permissive over-reaching signalling relay B would trip instantaneously and concurrently send a permissive signal to the relay A. The relay A will now see the fault in zone 2 and additionally will receive the permissive signal sent by relay B.
This received permissive signal will cause the breaker A to trip without further delay. Hence both breaker A and breaker B will trip at high speed. Alternatively if relay A only sees the fault in its zone 2 but relay B does not see the fault in its zone 2, this means that the fault is behind relay B (fault F4 in Figure 2.13) and it is not secure for relay A’s zone 2 to trip faster that its zone 2 time delay. In this case no permissive signal is sent, since relay B does not see the fault in its zone 2, so the relay at A will wait until its delay is timed out before initiating a trip to the breaker A.
The principle depicted above can be illustrated by the logic circuit below:
Figure 2.14: Logic circuit for POTT scheme