Hello and welcome everybody to today's lecture about protection in electrical power systems. And the focus today is on distance protection and here on the field of distance protection signal comparison. Why do we need that? As you can remember in a normal distance protection grading scheme we have different steps or zones as they are called which act with different tripping times and due to the uncertainty of the accuracy of the measurement at the end of the line we must raise the tripping time for falls at the end of the line or behind it to a higher value than the very quick immediate trip which is say 100 milliseconds. So if we have a fault at the beginning at the middle of the line this will be tripped in quick time but if there's a fault at the end of the line it is in race time but it is still selective and if a fault is behind the next bus bar so in the next section then the relay be in this case green will pick up and trip. If you look now at the oval picture for the tripping times we see that in the first 85% of this line we have a quick trip and in the remaining 15% we have a retarded trip and if you keep in mind that distance protection has normally two ends we have the same picture for the opposite relay and that means that approximately 30% of all faults or lines are not cleared by the appropriate distance protection in quick time so this is a disadvantage and that can be overcome by two solutions. There are two basic solution schemes we start with solution scheme number one the so-called permissive underage transfer trip or PUTT. We have again a fault at the middle of the line and that will be tripped by the distance protection A just as it was before. Now what happens if we have a fault at the end of the line? The end of the line starts with a fault. The relay A1 picks up and normally would wait till the time T2 has expired but in this case it will just store this measurement something like a starter. The next is what does the relay A2 do? It is in blue here and you see it picks up and sees the fault immediately in the close vicinity and it will send a release signal to the other end. This release signal is received there is a logic connection the opposite relay picks up the other one picks up and we have a successful trip in quick time a little bit time must be added for the signal in cell. So the whole picture is due to this comparison of the signals relay A trips in T1 plus a little bit delay so this is now satisfactory. Now according to the philosophy of checking the effectiveness of protection schemes we also have to look at a fault further downstream on the next section that is a fault behind the green bars bar. In this case relay A1 will detect this fault in zone Z2 and will keep this in mind. But relay A2 will not see the fault in its zone which looks towards the left end and will not release the trip so that means we do not have a fault strip and after a little time as you can remember we have a retarded trip in it to the second zone with the increased tripping time like T2. So we have in this case also a fully successful trip this which would be a backup trip by the way. So this is the permissive overreach transfer trip scheme. So there is a second solution to the problem of the remaining 15% especially in the case of weak infeed that means that relay at the far end will not pick up and it will not send a release signal. So in this case the so-called permissive overreach transfer blocking P O B is applied and I will explain this to you. So first we see the basic scheme you should note that the relay A1 this is the one on which we focus has now operating on three zones. So one zone is already known 85 percent under reach zone. Then we have a second zone which is definitely overreaching something like 120, 150 percent of the own line impedance and there is a third zone which later on will act as a backup stage. So the interaction for this overreach blocking is between zone 1 and 2 of both relays. So let's start with a fault at the middle of the line which is very simple. If it is in the middle of the line relay A1 will pick up detected in the first zone and trip. So this is okay. So the fault starts at the far end of the line. It causes relay A1 to see the fault point within the extended zone, this overreaching zone and it also will collect something with a relay A2, the relay at the far end. This relay has to answer one question. Is the fault towards relay A1 or is it in the reverse direction? In this case it will not detect the fault in the reverse direction. It will not initiate the blocking signal and this no blocking signal is inverted by means of an inverter put through an end stage and now with a short time delay it will generate after this time delay has elapsed a trip command. So in total we have reached a trip in a very quick time even for faults at the far end of the line. So now let's check what happens if we have a fault further downstream at the next section which should not be tripped in quick time. So we have a fault. This fault will cause a signal like a 1 sees this fault in the extended overreaching zone and now again the question is towards relay A2. Relay A2 in this time sees yes there is a fault behind me and it will send this blocking signal. This blocking signal again is inverted put through this end connection and now this end connection blocks any trip. So there is no trip but what happens if relay B further downstream fails. So we need something like backup protection and that is initiated by the next step. So the relay A1 has a third zone which detects this fault and will after the elapsed of time t3 trip this fault. So after this we have a successful trip of A1 as a backup function. So this was today's lecture about distance protection and the chapter was signal comparison. Thank you very much.