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.

 

[ters]

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?

 

[pwrengrds]

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.

 

[ters]

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.

 

[davidbeach]

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

 

[jghrist]

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.

 

[ters]

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).

 

[pwrengrds]

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.