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Wind farm 35kV collector intermittent fault
2

Wind farm 35kV collector intermittent fault

Wind farm 35kV collector intermittent fault

(OP)

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.  
 

RE: Wind farm 35kV collector intermittent fault

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

RE: Wind farm 35kV collector intermittent fault

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.

RE: Wind farm 35kV collector intermittent fault

(OP)

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.  
 

RE: Wind farm 35kV collector intermittent fault

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.

 

RE: Wind farm 35kV collector intermittent fault

(OP)
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.  

RE: Wind farm 35kV collector intermittent fault

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.

RE: Wind farm 35kV collector intermittent fault

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. looking around

RE: Wind farm 35kV collector intermittent fault

(OP)

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.          
 

RE: Wind farm 35kV collector intermittent fault

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

RE: Wind farm 35kV collector intermittent fault

(OP)
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.
 

RE: Wind farm 35kV collector intermittent fault

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.

 

RE: Wind farm 35kV collector intermittent fault

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.

RE: Wind farm 35kV collector intermittent fault

(OP)

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.  
 

RE: Wind farm 35kV collector intermittent fault

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.  

 

RE: Wind farm 35kV collector intermittent fault

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.

RE: Wind farm 35kV collector intermittent fault

(OP)
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.  
 

RE: Wind farm 35kV collector intermittent fault

(OP)
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?
 

RE: Wind farm 35kV collector intermittent fault

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

RE: Wind farm 35kV collector intermittent fault

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.

RE: Wind farm 35kV collector intermittent fault

(OP)
Than you again pwrengrds. Forgot to comment on your previous questions - does the fault level point to a spot on the line that would have that level of fault?

According to the coordination study, the fault level at any of the OH sections is higher than 2500Amp. Minimum is about 4kA. So if the fault magnitude is only 2500-3000Amp, while what event records lists as 4kA is the peak value, then turbine transformers are definitely a suspect.

However, the current SEL351 measures in the substation is lower than the actual short circuit level along the feeder, since there is also contribution from all feeder turbines which might contribute up to 1kA (there is 25 of turbines on that feeder).

The turbine fault contribution is low, they use inverters, and the actual SC contribution may not be more 1.2 pu (don't know the exact number), but turbine converters can push more reactive current than that if they are operating in Low Voltage Ride Through mode during the fault, which is possible.

Secondary breaker in the TX don't exist, but there is one in the turbine, so yes, the cable from the LV cable from TX to turbine is a suspect, but how likley that it would not be permanently damaged since the short circuit level on that cable must be in the range of 15kA or more.

In any case, with 2500-3000A from the substation + up to 1000A the turbines can contribute, the possibility of having a fault at the end of several OH feeder branches still seems to be strong as well.
 

RE: Wind farm 35kV collector intermittent fault

(OP)
davidbeach (Electrical) 10 Mar 12 21:03   No peak values in the SEL reports.  Settings are in RMS, event reports are in RMS.  

Thank you davidbeach, but how do I then interpret the fact that the same fault file shows 4kA in the event record while the charts show magnitudes of 2.5-3kA RMS?  And that happens regardless if I use SEL or Siemens software to open the same COMTRADE file.
 

RE: Wind farm 35kV collector intermittent fault

(OP)
Or, if 4kA in the event record is actual and real fault magnitude, in RMS, then the peak would then be 4kA*sqrt(2) ~ 5.6kA

However, since charts are plotted so that the peak is at 2.5 – 3kA, it then seems that peak is scaled not once but two times for sqrt(2) before it is ploted, i.e, 5.6/ sqrt(2)/ sqrt(2) = ~ 2.8 ?
 

RE: Wind farm 35kV collector intermittent fault

I don't understand the confusion.  At cycle 24 of the first event, phase A has a magnitude of 3688.85, B 4172.43, and C 3887.53.  Exactly what the event summary says.  Those are all RMS values.  I'm not sure why you say they are only 2.5-3kA.

RE: Wind farm 35kV collector intermittent fault

(OP)

OK, I get it, finally :).

Several things confused me that the RMS was only 2500 Amp (and David (pwrengrds) seemed to be under a similar impression too).  That is why I had problem believing that there is no discrepancy.

First, SEL chart scaling down thing to make instantaneous values for 1.41 lower then they are (why this actually is needed?).  

Second, SIGRA was constantly showing a discrepancy 2.5kA RMS in the chart but 4kA in the event log (I attached a sample few posts above).  

Seems Siemens software was "confused" in a similar way as I was, and for the same reason. In theory, one could use any software to analyze COMTRADE files,  in practice, looks like due to this 1.41 rescaling embedded somewhere in the SEL file, SIGRA shows both, RMS currents and RMS voltages 1.41 times lower then they actually are.

Finally, the SEL chart scale was crude too, and even when I ploted Im on the top of instantaneous curve, it still sort of looked to be closer to 2.5kA. But it does not when I zoom in one phase, sample a attached, which does show the Im being about 3.7kA (this is for file No 3).

So looks like my measured fault level is consistent and about 3.7 – 4.2 kA, depending if it does progress to phase A too or nor, and when I add say at least 600Amp more (that would be 25 turbines contributing at only 120% In), the actual fault current is probably about 4.5kA average.        

Thank you again for your help.

RE: Wind farm 35kV collector intermittent fault

Maybe I'm just used to it, but I find the SEL ossilography much easier to work with the way it is.  Actually the viewer doesn't scale anything, that's the way the numbers come out of the relay.  If I want to compare a waveform against an instantaneous setting, or the pickup of a time overcurrent element, I don't want to have to through in a computation involving sqrt(2).  If the Siemens software doesn't give exactly the same results as the SEL software, that means that the Siemens software is doing scaling all on its own, and that shouldn't be done; ever.  It should just present the numbers to the engineer exactly as they are supplied by the relay.  Otherwise there is no telling how to compare results against settings.

If you click on a trace in the SEL viewer, you can see the values in the upper left corner of the window, no need to guess what the values are.  

RE: Wind farm 35kV collector intermittent fault

(OP)
Siemens software does not do anything strange when opening Siemens or for example ABB or GE files.

However, when opening SEL files, it seems to be equally confused as a live person is when looking at SEL instantaneous charts – when it looks at the SEL records, it sees sinusoidal form as the SEL device records it, with, in this case, peaks ploted at about 4kA. Based on this, Siemens software will plot its own instantaneous wave form which looks exactly as SEL form does.

But, when you "ask" Siemens tool to open a SEL files and show RMS forms (there are two handy buttons to toggle between instantaneous and RMS values) it does something which in my mind is logical – it does not plot a line which sits on the top of something which looks as sine form peaks and calls that line RMS (what SEL for some reason seems to be doing), but it rather calculates the RMS with the assumption that the real sine peaks are at 4000A resulting in RMS being ~ 2800Amp (4000/1.41).

And it does exactly the same for voltage – for a 34.5kV system, healthy L-G voltage is about 20kV (RMS), sine form peaks being at 28kV. However, SEL elects to show those peaks at 20kV instead,  so when opening SEL files Siemens tool sees that actual peaks being at 20kV  (does not "think" that real peaks are higher at 20*1.41 = 28kV), therefore it decides to show RMS as 14kV (20/1.41) instead 20kV.

It may be possible to somehow "explain" to Siemens tool that SEL peaks are actually not peaks, and that real sine form peaks are for a 1.41 factor higher, but I don't know how to do it.

RE: Wind farm 35kV collector intermittent fault

OK, but why even try?  Just use the SEL software with SEL event reports.

RE: Wind farm 35kV collector intermittent fault

Also consider that unless you request a RAW event from the SEL relay, what you get is filtered data (fundamental only).  The relay responds to filtered data, so this is often the useful quantity to view.  If the raw data has a lot of distortion, this could point to particular causes (maybe inverter problems) that get hidden when viewing filtered data.
 

RE: Wind farm 35kV collector intermittent fault

On further reflection I think that the fault current is to high for the fault to be on the secondary side(fault current would be limited to less than 1000 amps), plus the fuses make that less likely.  The lack of ground fault current does not mean anything on an ungrounded system. The fault is less likely to be in the Tx because of the healing nature of the fault.  It doesn't seem to be wind related due to one of the faults is with the current is just magnetizing current (20 amps which is typical for the capacitance of the cables).  This leaves something like flash over on an insulator as a better guess.  I think that you will want to invest in some better fault detectors to find it.    

RE: Wind farm 35kV collector intermittent fault

Do you have arrester cabinets for the 35 kV cable?  If so, I would check the terminations in that cabinet.

RE: Wind farm 35kV collector intermittent fault

(OP)
Arresters are installed at all places where UG cables going to turbines connect to OH lines and also inside the termination cabinet of each turbine transformer which is the last one in a daisy chain (there is 1 - 3 turbines per each UG daisy chain).  

But if arresters are a suspect, that means that at least two of them are failing or flashing over at exactly the same time, which is still possible, but can something like that come with such a long lasting and random intermittency without having a permanent fault ?  

Since there are two low impedance grounding transformers in the substation (one for each feeder), relatively good ground fault detection does seem to exist and having two undetected ground faults which always occur at exactly the same time and with both fault current gradients being the same (B and C), is sort of unlikely to happen through insulators or arresters, but possible.
 

RE: Wind farm 35kV collector intermittent fault

Although I mentioned arrester cabinets, it's the terminations in those cabinets that I would be checking.

RE: Wind farm 35kV collector intermittent fault

ters,

Why do you have two earthing transformers at the substation?

Why not just one earthing transformer connected to the delta of the larger 230/35kV transformer which is utilised by both collector circuits?

 

RE: Wind farm 35kV collector intermittent fault

A grounding transformer per circuit means that the circuit continues to have a ground source even with the collector breaker open.  Worst case, breakers is tripped open for a ground fault on the collector circuit.  Before the generation all goes away, assuming grounding transformer on the system side of the collector breaker, the voltage on the unfaulted phases jumps to 34.5kV to ground.  Not pleasant for arrestors rated 20kV or anything else on the circuit.  Don't plan on using 34.5kV equipment on an ungrounded system, it isn't really rated for that use.

Even with wye-delta-wye 230-34.5kV transformers, collector circuits we have with delta high-side on the tower transformers all have grounding transformers at the switchyard on the collector side of the breakers.

RE: Wind farm 35kV collector intermittent fault

Thanks for your thoughts davidbeach.

My understanding is that all normal equipment should be factory tested for power frequency withstand voltages in excess of the phase to phase voltage.

For example, 70kV for 60 seconds is applied to all equipment with a 33kV voltage rating plate in IEC land as a part of factory acceptance tests.

So, although the stress on the equipment and cable insulation is greater than that which would be seen with an earthing transformer for each collector circuit, some may argue that the equipment IS rated for the short overvoltage seen after the fault is disconnected by the collector circuit breaker and before the turbine converters disconnect.

Surge arrestors could be specified to have a turn on voltage in excess of the phase to phase voltage such that the insulation co-ordination of the system is correct and the surge arrestors do not operate.

Grounding transformers are expensive.

RE: Wind farm 35kV collector intermittent fault

(OP)
Use of grounding transformers is required on delta-delta systems by the power utility but I don't know if it was them imposing one per feeder (I was not involved in the design). In any case, as davidbeach explained, they are there to also suppress voltage on healthy phases in case one phase is faulted to ground.  

35kV equipment is designed for maximum operating voltage of 38kV, equipment is factory hipot tested at 80kV 60Hz withstand and BIL 150kV (or more in some cases).

Re arresters, the way how they are rated seems to be rather different in IEC and ANSI worlds. I have recently worked on an unusual case where I had to choose IEC arresters for a 27.6kV system in Ontario, since wind turbines came from Europe with a 36kV GIS IEC spec switchgear requiring IEC cable elbows (which are not interchangeable with similar ANSI 35kV elbows) and hence IEC arresters which plug piggyback to elbows were required.  

So while searching for those arresters, I concluded that the IEC does not offer more than one MCOV arrester rating within the same equipment voltage class.  Seems to be as simple as this: equipment voltage class rating L-L = arrester MCOV rating L-G.  For example, for 36kV class L-L, the only available arrester is 36kV MCOV (obviously L-G). For 24kV class, the only arrester is 24kV MCOV, etc.
 
However, the IEEE world offers several arrester MCOV ratings for the same equipment voltage class what allows for a bit better insulation coordination. For example, 35kV IEEE equipment class offers several arresters with different MCOV to choose from,  such as 22kV, 24kV, 27kV and 29kV. This luxury to choose appear not to exist in the IEC world.

This difference is probably closely related to the fact that IEEE/ANSI/CSA allow for about 17 different system voltages in the medium voltage category (anything from 2.4kV to 46kV in steps of 2-3kV), while Europe has just few, seem to be not more than about 5, systems voltages in the MV category.  
 

RE: Wind farm 35kV collector intermittent fault

(OP)

Would you gents agree with me that the CT phasing is reversed here?
Please look at the attached phasor diagram This is a pre-fault condition, some 3 cycles before the fault, with 3 symmetrical currents RMS 650Amp, which represents normal operating conditions at full load (which is ~ 37MW).

The voltage measurement is via three PTs L-G.

Current vs voltage angle (IA vs VB) is ~ 200 DEG and is consistent for all phase.  This cannot be right for normal operating conditions, so I suspect that most likely CT wiring is reversed on the SEL351 device so A current is actually C, C is actually B and B is actually A.   

This would make much more logical sense, the angle between voltage and current would be much more "normal",  about 45 DEG, which is still a bit wide, but rather possible depending what the PF correction system at the substation was doing at that time.  
 

RE: Wind farm 35kV collector intermittent fault

No, I would not agree, its just back feeding and near unity power factor.  The towers are near unity PF, the var load of the padmounts TX make it look lagging (needs some capacitance to achieve unity).   The feeder is set up to look out at the towers and the current is feeding toward the Main TX so it looks backwards.  

RE: Wind farm 35kV collector intermittent fault

(OP)
Thank you David, I overlooked that, I was confused with a recent similar application of SEL351 which had to look the other way (to also detect faults on the line), where when generating the angle was not 200 DEG but more like 10 DEG. In this case it does not matter, they do not use any directional elements, but still at low outputs that angle is nowhere near as per the attached chart – if the first one is near unity , then this PF is sort of near zero which is again puzzling  but possible given the collector length and the fact that turbine reactive capability is such at low kW it can deliver or absorb very little kVARs.  

RE: Wind farm 35kV collector intermittent fault

(OP)
Or, maybe the last chart does not show the generation condition at all but the feeder works as a load, feeding mostly reactive current to the long collector with PF close to zero?

RE: Wind farm 35kV collector intermittent fault

I have attached the adobe with my comments.  When at power the towers likely produce unity power factor.  This become slightly lagging due to the inductance of the pad mounts transformers at the towers.  This shows up as 650 amps and 20 degrees phase angle (going backwards).  During low wind when the towers are idling the cable make it capacitive, adding about 5-10 MVAR's to the farm and it shows up as a 20 amps and angle of 75 degrees on the first fault.  Hope this helps.

RE: Wind farm 35kV collector intermittent fault

(OP)

Thank you very much David.

On a separate note, it was discovered that interpreting operations of fault indicators was misread to date, so the one which was believed to operate during the fault was likely defective, and was randomly changing its state, while all other indicators (and there is about 30 of them of on this feeder) were actually tripped. When tripped, they display yellow color, when normal they are black. Not being familiar with this model, when O&M confirmed that yellow color was the normal state, I had no reason to initially suspect otherwise. The indicators also turned out not to be self resettable, and given both discoveries, who knows when all of them tripped and why.  

But I did discover a place with some arcing signs, although I'm not particularly hopeful this is the place of the present problem since the damage is minimal for a fault ~ 4kA which occurred at least ten times. A picture attached, arcing marks inside red boxes.

This is a place where somebody decided to guy a pole in a very strange way, uses even 3 guy assemblies to guy the pole at each level of conductors separately !(conductors are arranged vertically) and two of the guy assemblies have some rings which go around live conductors - one goes around bottom phase only, while to other one goes around bottom and mid phases.

I was closely inspecting this location during the site visit, as it looked rather awkward and suspicious. But no sings of arcing were visible at all by naked eye looking from the ground. It was only after I watched a HD video (which I took at zoom 25X) for the second time that I spotted those arcing marks, and the attahed picture is a frame extracted from the video.

However, this probably does not hold the water, not only because the damage appears to be insufficient for the magnitude and total duration of the faults, but aslo the guy ring goes around phases A and C, while relay sees the fault as B and C... The ring does however move slightly - swings around it axis a bit when windy.
 

RE: Wind farm 35kV collector intermittent fault

The one event report I looked at was only phase-phase, no ground involved.

RE: Wind farm 35kV collector intermittent fault

I can't picture in my mind what the guying situation is.  Maybe a photo that show more of the pole would explain.  Is it possible for the guys or guy rings to cause a Ø-Ø fault without ground?  Are there guy insulators installed?

The difference between A-C and B-C could be because of relay wiring as you noted in your March 13 post.  This could explain the possible guy contact as well as the possibility of A-C contact on triangular contruction.

 

RE: Wind farm 35kV collector intermittent fault

(OP)
It is always phase to phase, the ground is never involved.

Here is another photo jghrist, which should provide some clarity what guying is doing - I expected this question as this is really an atypical case, at the best.

And if the guy assemblies are a problem, that would still fit in the two phase fault – please see the attached picture.  The upper guy metal ring goes around two bottom phases and at the same time the guy wire is insulted before ground, you can see the insulator.

But, as I mentioned, since the arcing marks are minor, this is not likely the place causing this problem.

And also as you can see the upper guying ring goes around two bottom phases, while the relay says that the fault always starts between the top and mid phase, which is unlikely - the arc cannot start on the top phase, it can only spread there.

Finally, the calculated (bolted) short circuit near this location is about two times higher than what the relay sees,  but nobody knows what the arc impedance is if this is the fault location.   

When windy, those two guying rings tend to wobble a bit around their axis (being guy wire which goes to ground).  

RE: Wind farm 35kV collector intermittent fault

I would bet that's your problem and somewhere your phasing is swapped  A-C.  It's most probable that there is only one fault and you found one that fits your fault.  Fix it and see if it goes away.

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