Use Of MOV Arresters In The Reduction Of Momentary Outages

[opmgr1]

Recently the local regulating body decided that they were going to introduce MAIFI and the utility will be penalized if MAIFI exceeds certain values as stipulated by the regulator. In the quest to reduce momentary interruptions, lightning arresters are installed on the worst performing distribution feeders.
This solution comes with its sets of problems as when these arresters fail there is no way of detecting them if the failure is not explosive and they create a maintenance nightmare. I have consulted with various other utilities that indicated that they use simulations/ fault locater to locate these arrester related faults. I was thinking and came up with an idea to install a cut-out switch in series with the arrester and to fuse it using type D fuse links. The thinking behind the idea is that these surge durable fuse will pass surge to the arrester, however in the event of a permanent fault created by the arrester failure the fuse will melt isolating the arrester from the circuit.
Any ideas? any significant drawback to this or is there anyway that it can be improved?

 

[jghrist]

The ground lead disconnector is supposed to perform the function that your fuse link would perform. How about an inexpensive radio transmitter that connects to the ground lead disconnector in some manner to send a signal when it blows?

 

[opmgr1]

I guess this could work. My main aim was to isolate the arrester from the circuit to prevent any tracking that may occur due to the defective arrester remaining on the circuit for an extensive period. In addition to this there are instances when the disconnecter fails for whatever the reason so the cut-out is there as a sort of backup. One practice in the utility especially for arrester banks is to use a common lead to interconnect the arrester base and as such if the middle one fails then it is not disconnected properly (disconnect not visible). The cut-out however have a visible disconnect. I will research the transmitter to see how best it could fit in the scheme of things.

Thanks.

 

[burnt2x]

Not to be off-tangent, we had lots of problems related to arresters failing in my previous employment and it's really a maintenance nightmare. As pointed out, you will have no visual indication as the arresters look good. I think your idea will work, with additional costs of course.
To achieve a long term fix in avoiding penalties, what we did was to make a replacement schedule of all arresters, year-round; the replacements are the ones being tested/ replaced from previous arrester replacement.
The scheme was hatched based on the principle that arresters will survive a number of hits but will fail ultimately. Then we extended the replacement/inspection period as failures tapered off. A lot of failed arresters were discovered (open) and we didn't exceed the specified length of interruptions due to shorted MOV's.
If you have the luxury of a double-circuit arrangement, you could schedule replacements during low-demand hours.
Hope this helps.

 

[MxGxK]

Is lightning causing so many momentary outages that you need to install additional arresters? I worked for a gulf coast utility (high incidence of lightning) who installed riser pole (ground disconnector) arresters at the first OVHD pole outside the sub and at all cap banks and pole mounted transformers. I don't remember lightning every being an issue for shielded (35kV) or unshielded (12kV) lines. Just curious.

 

[cranky108]

In a former job, we used shield wires on lines that extended outside of the town areas. It worked well.
Then some arrestor salesman tried to convence us that adding an arrester every third pole was just as effective. We continued to use the shield wires.

At one time we had one manufacturer that had a very high failure rate. We tried to evaluate arresters on the thermal runaway basis, but still no one could explain the bad failure rate. We finally quit using that manufacturer's arresters.

But along with the arrestors, you need to be up on your tree program for over head lines.

More recently it has come to light with my present employer that adding arrestors on the end of underground is important in reducing cable failures.

 

[davidbeach]

Arresters are available with disconnecting devices that blow away from the arrester when it fails. The disconnecting device hanging open isn't quite a big a flag as the fuse hanging out of a cut out but is certainly visible.

 

[apowerengr]

You only need to use D links in sizes under 5 amps to coordinate with the arrester time-current curve. Instead of stocking another fuse you could use a standard fuse link greater than 5 amps which I assume will coordinate with your feeder level overcurrent protection (assumes you don't use fast curves on your feeders). D links were invented to handle the problem of distribution transformers with tank-mounted arresters on MV systems (> 10kV l-g) with small power fuses (< 5 amps, non D-link fuses) in cutouts blowing multiple cutout fuses for a lightning stroke. It took hours to refuse all those small distribution transformers for one lightning stroke.

Cutouts are an expensive addition for arrester failures. At the feeder level I would expect it to be a rare event to have an arrester hang up through multiple feeder-level recloses without clearing the disconnector. You are looking at at least 4 sets/mile of feeder to be relatively effective so that's another 12 cutouts/fuses/installation labor, etc.

If the arrester hangs up it won't be a MAIFI event anyhow

 

[opmgr1]

apowerengr,
You are right. If the standard way of installation is followed then the cost/ labour can be pretty big. Just to back up a bit though, you mentioned that the D links were invented to handle transformers with tank mounted arresters, and this means that it has to be constructed to withstand surge current. These D-links are already a part of our stock. It is this characteristics that is utilised in placing the arrester in series with the switch (the down dropper from the switch bottom is connected to the line side of the arrester).
The method that we used is to disable the fuse saving methodology resulting in the recloser going through 3 time delayed operations before it locked out. What this does is that the lateral fuses (K-Links) operated on transients and points us to where the arresters should be installed.
In response to MxGxK these feeders are long rural feeders and traverse very hilly terrain. This is in the tropics and the lines are exposed to severe lightning activity especially during the summer months. Vegetation control is not a very big contributor to the various reliability indices and a recent study indicated that 62% of all momentary outages was due to lightning. A point to note is that fuse saving methodology is implemented on all the feeders emanating from the various substations.

 

[MxGxK]

opmgr1, interesting problem. Just because I haven't seen the problem doesn't mean it doesn't exist. I have to concur will several others, the riser pole arrester is equipped with a disconnector on the ground terminal and will not require fuses/cutouts. Internal failure of the arrester can easily be identified by the hanging ground lead. I don't think you'll find a less expensive arrester and you'll only need a stirrup and hot line clamp for the line side. Good luck.

 

[apowerengr]

Removing the fuse savings curves from the feeder protection does next to nothing to help with arrester placment (lightning does not strike the same place twice concept). To make your D link concept work you'd need to coordinate your D-link for your arrester fusing with the feeder TCC curves, otherwise you'll be tripping feeders and blowing arrester fuses. I agree, the presence of a ground-disconnector on an arrester does not make for a full-proof system.

 

[jghrist]

Removing the fuse savings curves from the feeder protection does next to nothing to help with arrester placment (lightning does not strike the same place twice concept). To make your D link concept work you'd need to coordinate your D-link for your arrester fusing with the feeder TCC curves, otherwise you'll be tripping feeders and blowing arrester fuses.

Removing the fuse saving is what is needed to coordinate the D-link with the feeder TCC.

 

[opmgr1]

apowerengr,
Removing the instantaneous operation from the recloser to determine (prioritize) arrester placement in no way implies that lightning strike the same place twice. You would however agree that lightning will cause the same protective device to operate more than once. This of course is related to the circuit elevation and the GFD of the area the line traverse. See Eriksson, 1987 equations.
The concept behind disabling the instantaneos relay is to identify poorly performing sections of the circuit. (with respect to lightning) The blown branch fuses provide a convient fault location method. These fuse (K-links) will operate as the disabling of the instantaneous relay will allow transient faults to remain on the line-section long enough for it to be cleared by the branch fuse. We will then prioritize our placement in this regard, giving priority to the most active lateral.