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In part 1 of this series we’re going to do something a little bit differently this time and, in this series, we are going to look at a particular article by Schweitzer engineering labs. The author is David Costello and the article is called event analysis tutorial and we’ll be going over this particular tutorial.
There’s part 1 and in part 1 it lays out the problem statements which has several examples of fault analysis and then part 2 which is this part right here it essentially gives us the key so we are going to be going over both parts and they are publicly downloadable so you can search for event analysis tutorial and then search for this author and you should be able to get this PDF.
So in the first part, we are going to be analyzing this particular waveform and the introduction for this particular waveform is that it’s a distribution feeder fault and in here it says the event occurred on a distribution collector at a wind farm and for practical purposes fault on a collector behaved like fault on a radial feeder fed from a DY1 transformer. A dy1 transformer is our standard Delta Wye Transformer connection where the low-side reference quantity lags the high side reference quantity by 30 degrees so it’s typical Delta Wye transformer connection in the United States.
The wind turbine does not contribute any significant fault current. The location and the connection of the potential transformers PTs are unknown at the time of the publication. So, a lightning arrester, one per phase, are positioned on the top of the steel support structure. Each arrester is connected by a jumper to the phase conductor.
A bird caused a fault near on lightning arrester, which caused its jumper to blow loose and contact other phases. So a bird came in the line which causes a fault and that causes a jumper to blow and blow looses and make contact with other phases so that makes that kind of the situation that’s in hand and what we end up getting through the relays in this particular fault and the question that the author asks is before the fall in what direction the power is flowing, what is the power system phase rotation, what type of fault occurred, what protection element within the relay caused the trip, how long did it take for the relays to operate, how long did the breaker take to clear the fault, did the relay in protection system operate correctly as designed.
So, let’s go through and let’s analyze this event and answer those questions one by one.
So, the first question of course is before the fall In what direction is power flowing and so the easiest way to answer this question is to look at the events before the fault had occurred and the inception of the fault is essentially at this particular point here and we can see that the waveforms are positioned like this and we are looking at the voltage waveforms here. So, this is the voltage waveform, this right here are our current waveforms and we’ll talk about it in a little bit more detail. These right here are our sequence component magnitudes and these guys right here are our digitals. And so, from the waveforms itself it peers like power is flowing into the bus or in the reverse direction.
The second question is what is a system phase rotation and we could actually zoom in and let’s zoom in into the voltage waveforms, what we find is that the voltage waveform essentially starts with the purple and then it goes into this greenish color and then it’s this teal color so if we look at our map the purple represents phase A voltage in KV, the teal color which is I mean the greenish color represents the phase B voltage in KV and then the teal color represents phase C voltage in KV. So, here we know exactly that this right here is A then it’s B then it’s C. So, the phase rotation is essentially an ABC phase rotation and we look at it by, we look at the voltage waveforms before the fault actually occurred and the inception of the fault is essentially this dotted blue line right here.
So, let’s look at the third question is what type of fault occurred so there’s a lot going on in here and we would really have to look at this holistically. So, initially of course we want to start from before the fault and make sure things are good, make sure that we understand exactly what’s going on here. So, we have a good balanced system before the fault occurred. Phase A, B and C voltage quantities are pretty well-balanced meaning that there are equal magnitude and there are a hundred and twenty degrees apart from each other and even though we won’t be able to see it here but there are you know there should be some load current on the on our phase current quantities or current waveforms.
So, in the pre fault we have good balances voltages and we have some load current that we can see and visually verify so at the inception of the fault which is this blue dashed line, what we see is that we see a huge presence of this particular waveform on the current magnitude, and it’s like greater than 10000 amps of phase current and by the color of this waveform we see that it’s the green color so it must be phase A current right so we see a huge presence of phase A current whereas the other phases phase B and phase C current at this point are pretty damn small so we could immediately tell that this appears to be a phase A to ground fault and another way to confirm that is by looking at the voltages so what we see when we have a phase A to ground fault we should see a huge depression on the faulted phase.
So, over here we see immediately that the voltage, the purple, which is phase A voltage quantity, the purple waveform actually is depressed as we can see so we kind of confirmed that the phase A current as it has a high magnitude, phase A voltage is certainly depressed and so this certainly is a signature; a classical signature of a phase A to ground faults and even on the voltages we can see that the other the remaining phase B and C are relatively intact, they are definitely depressed but the phase rotation are relatively intact meaning it’s purple then green then teal so it’s purple, purple is depressed then green then teal right so relatively intact but then at this particular point here something happens right and this is where the fault is actually transformed into a different type of fault.
And what we see is there’s another phase current that is prominent and that is your blue current so it’s phase B current that becomes prominent and it may be a two line or it may be a line to ground but the line fault with phase A and B current that are prominent or it could be a line A to B to ground fault meaning A, B and ground fault now one of the signatures for a line to line fault is that when you are looking at when we are looking at the current quantities we should expect the phase A current and phase B current to be equal magnitudes but a 180 degrees apart and we don’t see that here they may be close to equal magnitude but they are certainly not a hundred and eighty degrees apart meaning if there were 180 degrees apart then this peak for phase A current would match another peak down here for phase B current and we don’t see that so what this appears to be is A to B to ground fault and we could look at the voltage quantities to also confirm that.
So, at this particular point when the fault changes we see a depression on both the purple which is the voltage waveform and as well as the green which is this voltage waveform here so both the purple and the green there is a voltage depression there and the teal is the phase C voltage so that is depressed but not as much as purple and green which is phase A and B voltages so we can confirm right here that this is a two line to ground Fault and then as we get closer towards the very end we can see that all three phases are involved which means that this is more like a three phase fault meaning phase A, B and C fault and you can see in the voltage waveform again all three waveforms are depressed quite a bit so this is definitely towards the very end it becomes a three phase fault and towards a very last piece right here what happens is that the current waveforms goes away right and the voltage waveforms become balanced again.
So, what this is telling us is that the fault is eliminated through a breaker so the breaker opens up and isolates the fault and we get a restoration of the voltage goes back to normal so that’s what this is telling us right.
So, we can look at the answers but let’s continue with the questions what type of fault occurred we kind of answered that. What the protection element within the relay caused the trip? So, let’s look at these guys right here, now before we look at the digitals I do want to comment on the sequence components part of it right So again the sequence components are the green and the red so grey is the zero sequence current magnitude.
The green color is the positive sequence current magnitude and the red is the negative sequence current magnitude when it’s a line to ground fault we can see that both all three positive negative and zero sequence currents are moving together and they’re relatively equal that is what makes sense when there’s a ground when there’s a one line to ground fault, the positive negative and zero sequence current should be equal in magnitude and then towards when the fault became a two line to ground fault then we see a difference in positive negative and zero sequence currents meaning that there are non-conforming values there are three different magnitude quantities for positive negative and zero sequence current right and they continue to be.
We know that this occurs for a two line to ground fault and towards the end right here when it becomes a three phase fault we see that both grey and the red drops out meaning the zero sequence current and the negative sequence current kind of drops out when it becomes a three phase fault but we see a continuing rise of the green which is the positive sequence current magnitude we see a rise in that so what this is telling us is that it becomes a three phase fault in which the phase currents are relatively balanced which means we will get a drop off the zero sequence current and the negative sequence current because they are measuring the unbalance in our phase sequence and we see a continuing rise of positive sequence current right because it becomes a balanced circuit and with a three balanced phase fault we would normally see a high positive sequence current magnitude.
So, from a sequence components perspective this makes complete sense right and is let’s look at the digitals now and the question was what protective element it tripped on so this is a trip signal right here and so what this is telling us the first one the trip element did not assert until the more of a bold line right here so this right at this point right here this is what is telling us that the trip signal within the relay asserted right and it kept on asserting until the end of the waveform. And then this 52A contact tells us the breaker position whether or not the physical breakers are actually in the open position or the closed position. The contacts are so what this is telling us is that the breaker is closed meaning it’s bolded line and the breaker continues to be closed until this particular point and then the 52A contact drops off which means it becomes zero and breaker is actually open so the breaker opens at this particular point here.
The 67 p2t elements if we look at the relay manual which is described within this particular article the 67 p2t, the 67 element is a directional overcurrent element and then p2 “p” means it’s a phase element and this “2” means it’s a second element there’s 67 p1 and there’s 67 p2 so p2 just means there’s the second element second set and this “T” right here means that is actually times so it’s actually you know timed and tripped. So, what this is telling us this line right here is that it didn’t pick up meaning it’s zero and continued to be zero until this particular point and then the 67 p2t actually asserted and it became one. And so this right here is what I believe the tripping signal for the relay so the 67 p2 elements is actually the element that tripped the breaker right and then the 50 p2 “50” is an instantaneous element “P” is the phase so this as you can see is the asserted and then it gets asserted here and then it continues to be asserted until right here when all of the currents are actually dropped off so this is probably some sort of indication that you have a fault above a certain or the current is above a certain threshold so the 67 p2 is the element that actually tripped that caused the tripped.
And then how long did it take for the relays to operate and how long did the breaker takes to clear the fault? The blue line indicates that the fault is triggered right, the fault triggers and we know it’s the 50 p2 element is picked up right and so when the relay actually sensed that it was a fault it was at this particular point right here and that’s the element that tripped it so from here to here is actually 15 cycles so it took 15 cycles for the relay to detect and operate one of this protection element and told the breaker to actually open up and how long did the breaker take to open up well it would be after this signal here and between this and the 52A contact dropping off which is this right here so the difference between these two points is approximately, I would say, three and a half cycles so it takes though it took the breaker three and a half cycle to actually clear the fault which makes sense and it’s a common value. So how long did it take for the relay to operate? Approximately 15 cycles. And how long did the breaker take to clear the fault? And this was approximately three and a half cycles.
Did the relay and production system operate correctly as expected? So I would say yes the relays and the protection system operated correctly as expected for this particular design so I hope this was useful and an enlightening so let’s look at the key , the answer key which is part two of this tutorial and see what it’s telling us.
So, for question A but it’s said before the fall in what direction is the power flowing? Power is flowing into the bus or in the reverse direction. For B, what is the system phase rotation? And we said it was ABC and it is actually ABC and we verified by looking at the pre fault voltage waveform. What type of fault occurred? This is an evolving fault on the feeder forward direction. It started as a AG fault which is phase a to ground fault evolves into an ABG fault which is phase A to B to ground fault and then finally evolves into a three phase fault which is in this particular direction in this particular area here so this all is correct. What type of protection element within they caused the trip? 67 p2t a non-directional phase over current definite time delay element so I m not sure what this means non-directional 67 if you look up what the ANSI device number 467 is. It’s actually a directional element so I’m not sure whether or not this is a typo but regardless it’s a phase over current definite time delay element that actually operates and it’s interesting to note that this is a definite time delay right and definite time delay is a element that has to wait a certain amount of time before tripping action could occur and it appears from how long the relay actually waited that the time delay is 15 cycles so the relay detected the fault but because it was a 15 cycles delay it waited that long before it could issue the trip this. How long did it take for the relay to operate 15 cycles and how long did the breaker take to clear the fault? And we said about three and a half cycles which is what this is also saying so. And did the relays protection system operated correctly and that was yes.
So, I hope this was useful and this was enlightening so we will continue to go through this particular article in this particular tutorial bus rise engineering laboratories and David Costello is the author. Thankyou.