Wind farm 35kV collector intermittent fault 2

[ters]

System:

The facility is a larger wind farm with two 35kV collector feeders each having about twenty 1.5MW turbines. There is one step up 230/35kV transformer in the substation serving both feeders, connected as WYE/DELTA (230/35).

The turbine step up transforms use DELTA on the 35kV side, so since there is a DELTA at both ends of the collector, both feeders employ separate grounding transformers in the substation.

Both feeders are equipped with SEL351 protections. From the 230/35kV substation, both feeders use 35kV UG cables for some distance (about 1km) and then the cables connect to two overhead circuits. The OH circuits use the same poles for few km and then diverge. Each feeder also has several OH branches along which turbines connect via UG cable and one pole mounted aerial switch. Each such switch location also includes surge arresters.

The number of turbines connecting to the OH line via UG cable and the aerial switch varies, in some cases it is only one, in other cases two or max three turbines are daisy chained by UG cables before they connect to the OH line.

Turbine transformers are outdoor and use current limiting + expulsion fuses in series with a disconnect switch in-between.

Problem:

Occasionally, at high outputs, one of the feeder 51 protection trips on the phase-phase fault. The fault starts between B and C phase but it does occasionally progress to phase A too. The fault circuit current is fairly consistent, about 4kA which is about 40% of the (bolted) fault level at the substation. The SEl351 51 element is set to clear the fault within about 400ms (including breaker time). The voltage, as seen by SEL351 in the substation, does not collapse dramatically suggesting that the fault is not very close to the substation.

The nature of the problem is rather random, so sometimes on a windy day it may trip twice, but the other time on even a windier day may not trip at all. There is never a fault on restoring the grid power to the feeder. Sometimes there may not be any trips for couple weeks.

Line inspections were conducted several times and revealed nothing – there are no suspicious places at all.

Some problem with SEL351 relay is also eliminated since the fault is also seen by another feeder protection, which does pick up but does not trip (the current goes to 200% or higher).

The OH lines do employee some self resettable fault indicators for different sections and branches. One of these indicators was tripping suggesting that the fault is on a particular OH branch serving 4 turbines each connecting to the OH line individually. The indicators are single phase and are those which are installed on the conductor by a hot stick, but the make and model unknown. It does change the color from yellow to black when tripped.

However, after the last trip, that particular indicator never reset itself to normal state (to display yellow) suggesting that it is possibly defective, so any conclusion based on its operation is not particularly reliable - if it was too sensitive before it finally failed, maybe it was tripping on faults elsewhere along the feeder rather than on faults on its own branch.

Question:

Where to go from here? Any idea how to narrow down the possible location of the problem?

I sort of exclude the possibility that the problem is underground, since cables cannot behave that way, when they fail they are unlikely to repair themselves after the trip, so suspects are OH structures or possibly (but less likely) turbine transformer termination compartments, where 35kV cables connect.

Thank you for reading.

 

[DTR2011]

Where is the windfarm located? I would be looking at different things in Minnesota, South Dakota vs. Texas at this time of the year.

 

[cranky108]

Have you tried setting the fault location reporting in the 351 relay? Because of the different configurations, calculate the total line impedance (to the end), and set the line length to 100 (%). This will give you some idea. You can also model the system and run test faults to attempt to find model impedance that matches the recorded fault impedances.

The fault could be a varity of things. Depending on the location of the relay and grounding transformer, it maybe a true phase to phase or a phase to ground appearing as a phase to phase.

 

[ters]

Thank you for replies.

The wind farm is in Ontario. Due to laziness to type 34.5kV each time, I call the collector 35kV one.

The modeling has already been done for the purpose of coordination and arc flush studies for various locations along the affected feeder, and if these numbers are correct, what the SEL351 relay sees during the fault is matching calculated SC values more towards the end(s) of the feeder.

Re setting SEL351 to fault location reporting, that has not been done yet, but I'm not sure how much it could possibly help – the feeder configuration is rather complex, but in a simplified form, it can be described as a radial feeder which splits into main three branches going different directions and that at the end of each branch the calculated short circuit level is similar to what SEL351 is measuring during the fault. However these three approximate locations may be something like up to 10km apart, so how do I know which branch is having the problem?

The more likely is still the one where the line fault indicator was tripping as well, but it is hard to rely on it since it remained defective after last trip.

One way to narrow down the location would be to use some voltage recording devices at one of the turbines at the end of each branch. In which case, one would hope that the measured voltage during the fault would be the lowest at the turbine closest to the fault.

 

[Chiprme]

What if the wind was blowing strongly, and one or more OH line segments were too long, too slack, too close, etc. where phases could "slap" together ? If the wind direction were mostly parallel to the conductors, it's less likely to blow phases together.

You might be able to see burned conductors from touching near mid-span, but it sounds like a lot of distance to patrol on a hunch.

 

[ters]

Yes, that possibility is the one mentioned the most so far - that phases “slap” together somewhere mid span. And it is also true that there is tens of km to patrol, but the good news is that all OH lines are just along the public roads. So spotting a place which looks suspicious should still be possible and since this trip occured about 10 times so far, it would be logical that after so much arcing (each time the fault is lasting about 400ms), it did leave some visible signs. But so far none was identified.

 

[cranky108]

Sort of dumb question, but how long to they take to patrol the circuit? How many km is the circuit? Patrol speed=circuit length/patrol time. You should find that at what ever speed it is to fast to see much.

The intent of using the 351 for fault location is to verify that the faults are in the same location. Not several locations at different times.

Agreed the fault indicators would help. However as pointed above, the direction of the line with the wind will also make a difference.

 

[dpc]

The fact that the fault always starts between the same two phases strongly suggests a physical issue with the line - conductors slapping together, or getting close enough to arc over.

If you are bringing voltages into the SEL-351, can you determine the power factor of the fault? Anything interesting in the fault waveform?

Brute force approach is to add fault detectors to "half-split" the location and keep narrowing it down by moving the detectors. You can determine if this is indeed occurring in the same location every time. If you can narrow it down to a few spans, you might come up with some ideas. Of course if the fault is occurring in exactly the same spot each time, eventually it will probably burn through a conductor and that will make it fairly easy to locate.

 

[ters]

Thank you again very much for replies.

The actual main feeder length is about 2km before it starts splitting into 3 main branches, lets call them Branch 1 ~ 4km, Branch 2 ~ 6km and Branch 3 ~ 8km. A section of branch 3 monitored by that particular indicator which was tripping is about 5km long (and has two sub-branches).

So, yes, maybe inspections have not been performed very carefully and slowly enough everywhere, but a particular attention was paid to that section with tripped indicator and nothing suspicious was noticed yet.

The first 2km is a very thick conductor and phase are arranged vertically so probability of a fault occurring there is lower due to physical arrangements and also the fault level there is about 2 times higher than what the relay registers.

Then, when the main circuit braches off, the configuration of conductors changes from vertical to a triangle, with the middle phase, B, sitting on vertical insulator on top of the pole, while phases A and C are at lower left and right being attached to two horizontal insulators at the same elevation.

Obviously, the places where conductors branch off, cross, change configuration, poles are supported by guy wires which are close to conductors, etc, are all suspicious but there is nothing obvious.

Looking from below, things are sometimes a bit deceiving as poles are high, so perhaps finding a way to look at wires from a close and higher perspective would help.

So, in a this triangle configuration, if phases B – C are slapping together, that would mean that the B phase conductor which is sitting elevated for the insulator length on the top of the pole is touching the C phase which is on the side. Probability of this occurring due to high wind seems to be lower than phases A and C hitting each other (as those are in the same horizontal plane), but it is possible that due to increased load the mid phase sags too much and then arcs to C as the wind moves it close to it.

The other possibility is that phases A and C are actually hitting each other but the relay sees it as B and C due to improper CT wiring (perhaps two phases are swapped).

As the fault current tends to be consistent, I believe we do not deal with multiple locations but only one.

The fault wave form shows that the current quickly reaches the maximum (within half cycle) and just stays there more or less flat until the breaker opens. The Vb and Vc phase to ground voltages collapse to about 60%.

The phasor diagram shows that the angle Vb –Vc is about 90DEG during the fault while the angled between Ib - Ic is about 180DEG. Angled Vb-Ib is ~ 30DEG while Vc-Ic is ~ 150DEG. Va and Ia remain similar to the pre-fault expect in case when the fault progresses to phase A too. Ig current remains very low (as SEL351 sees it).

DPC,I like your brute force method, seems that we just need enough indicators to place around. Still may take weeks to narrow it down, depending on the winds.

 

[pwrengrds]

What does the SEL 351 download of the fault look like? I would be interested in seeing it, that would give you a better indication of the nature of the fault.

Have you sampled the transformer oil in the pad mounts? It is probable that the fault is inside one of transformers and sampling the oil would tell you which one it was, due to the high arcing during the fault.

If it is wind slap due to sag, due to heated wiring, which is possible, then an investment in some more fault indicators would be in order. SEL sells them for under $100 each that are able to be put on live with a hot stick, and for a couple of grand you should be able to isolate the problem.

David

 

[ters]

Thank you for the reply David. Two fault records charts are attached (for two faults). This is how they are seen by SEL Analytic Assistant. I also used Siemens SIGRA to open the same COMTRADE files, but things look about the same except SIGRA offers a button for a quick switch between instantaneous and RMS values which does not seem to exist in case of Analytic Assistant, or maybe I don’t know where it is. Also, SIGRA seems to be confused when opening SEL relay created files on regular basis as the amplitude of the fault is different than what either the relay itself or Analytic Assistant see.

One of the charts shows the fault which also progressed to phase A, while the other one remained B-C only until tripped.

I placed IG together with IA just to indicate that the ground current remains within few Amps.

No, the transformer oil has not been sampled yet, and I was also thinking of one of the turbine transformers as a possibility. But, by now, after a number of faults, one would think that the transformer would finally fail, however the frequency and randomness of the fault is about the same as two months ago. Also, the transformer is protected by two fuses in series and according to coordination study, the fuse clearing time for currents above 1kA is 0.2 sec, while SEL351 element 51 is set to trip after 0.35 sec, so one would hope that the fuse would blow first, but for some reason that is not occurring if the transformer is really a problem.

I was also thinking of adding more fault indicators and still need to determine what is the make and model of the existing ones and possibly use another kind. The fuzziness related to existing indicators is substantial – according to the O&M staff, two of them at the same location (two phases) were occasionally tripping but never at the same time. One would think that, if we are having B-C fault, which seems to be the case, two indicators would trip (the same current goes through both?), which looks like never happened yet. Unless, one of them elects to trip only in cases when the fault progresses to all three phases, in which case their setting (if settable) or selection (if not settable) seems to be inadequate.

Also, as I mentioned above, one of the indicators which was tripping more frequently remained tripped permanently, which makes me to believe that making conclusions on operations of the existing indicators may be unreliable until they are replaced with possibly another make.

I’m an outsider to this facility, have not been involved in either design, studies, procurement, construction or maintenance, nor I’m close to the facility, and for now I sort of have to believe that previous SC and coordination studies are correct.

 

[pwrengrds]

The fault level is consistent at 2500 amps, that would indicate that the fault is near the end of one of the lines. It's really a fairly low level fault. Does the fault level point to a spot on the line that would have that level of fault?

Is the pre fault power levels always high? I couldn't tell what the running current was but is seemed low. I am thinking to see if it's related to high power or the oil temperature in the TX. I would not rely on the fuses to protect the transformers. Are the fuses before or after the switch in the transformer? What brand are the TX?

I would recommend that you contact SEL about the fault indicators if your going that way, if you sent them the fault data they would be happy to recommend a detector. For the price and level of service I think they are a good buy.

 

[davidbeach]

It is always best to post the actual files, rather than pictures of the files.

SEL scales the data to a fundamental RMS value, so when you plot the magnitude along with the the wave form the magnitude rides on the crests of the waveforms.

 

[ters]

Thank you David and Davidbeach again.

Here is also an actual event file which combines 4 events. Two events are trips, while the other two show contribution of another feeder to the fault (another relay, pickup but not trip).

If you open this file with SEL software, a small window will pop up asking which event you want to see. One of the trip events is BC fault which progressed to A too, while the other one is BC only.

I’m sort of confused how to read the actual RMS fault level. It does seem about 2500Amp from the charts, but then Analytical Assistant the summary window for the first fault included with the attached file shows:

Currents: A B C N G Q 3688 4172 3887 1 5 749 which indicates that fault currents are about 4kA, and the very same numbers are also displayed on the relay display when events are browsed manually.

 

[dpc]

I'm at home and can't check, but those current magnitudes may be peak values and not rms?

The way SEL handles the fault current magnitude in the event reports can be confusing.

 

[davidbeach]

On the first event the summary currents happen at cycle 24. Add IAMag, IBMag, ICMag to the plot. The wave forms are plotted at 1/sqrt(2) times the actual instantaneous value.

 

[ters]

pwrengrds , forgot to answer your questions, I cannot positive say that pre-fault levels are always high, I do not have all the fault records but they say that the fault only happens when it is windy, which means also higher output.

The transformer comes from Mexico, but don’t know the manufacturer, while the configuration is fuse-switch-expulsion fuse.

Here is how SIGRA sees the fault. I’m sort of still confused what the actual fault level is. The first text summary page of the attached file indicates that the fault level is about 4kA. But the RMS charts on the next page show about 2.5-3kA.

The voltage level, as understood by SIGRA, is always wrong with SEL files – normal L-G voltage should be 20kV but SIGRA shows only 15kV, seems that it sees voltage measurents as being scaled down for root 2 for some reason.

 

[ters]

Davidbeach, when I add Im(s) here is what I get.

I will try to summarize what seems to be the way to read SEL charts and event reports:

-The instantaneous values, as shown on the SEL charts, are scaled down and plotted at 1/sqrt(2) times the actual instantaneus value.

-The actual RMS values are actually equal to the peak (maximum) of the scaled down instantaneous values shown on the charts.

-Various even reports (tex part) or relay display itself, when they refers to values around 4kA in this case, are (as DPC indicated) actually refereeing to peak values and not RMS?

If the above is correct, what would be SEL's rational for this confusion and fuzziness?

 

[pwrengrds]

I know who made the padmounts, have toured that factory, they are actually fairly robust. Do you have secondary breakers in the TX? I bet not.

I am thinking that you have a secondary fault (575v) in one of the TX, likely in the cables to the tower. That would explain the low fault current. Also explains why you have a bolted fault and no ground current.

Recommend that you have a visual on the cables on the secondary of the TX. Look for overheated cables or damaged cables. Also look the temperature gauge in the TX that may provide indications. An oil sample will likely tell you which TX has the thermal overheating.

David

 

[davidbeach]

No peak values in the SEL reports. Settings are in RMS, event reports are in RMS. Much easier to compare system values in RMS to settings in RMS than to compare system values in instantaneous magnitude against settings in RMS.