Distribution MOVs

[mbrooke]

Can properly selected and placed MOVs prevent trip and reclose for lightning strikes? What about blowing latteral fuses?

 

[paulengr]

Let’s be clear what’s happening. If we have no surge arresters and no shielding and we get a strike or back strike we get a conductive path of super heated air (plasma). The electrons on the air molecules are either weakly bound or outright just floating around (plasma) which happens at around 6000 degrees K. At this point the air is a conductor, If voltage is high enough (medium voltage) it has enough voltage and current to continue the arc indefinitely until interrupted. Once current is removed the air cools back down and goes from a conductor to an insulator again, which is the magic of reclosers. If you have fuses you can set the recloser more sensitive and open before the fuse trips. Then try reclosing again. Then if that doesn’t work on the third cycle crank up the curves and let the fuses do their job (hopefully) before opening again and locking out. A lot of utilities use crazy values like several minutes between reclosing. This is historical. Back in the data of oil interrupters if you realized to soon the hydrogen released when the arc quenched in oil would ignite and the revloser exploded. You only need a few seconds for the air to cool and with the fuse saver strategy it still causes interruptions but they are very short,

Ok so the next approach is shielding. This is where you simply run the static line at the top of the pole and place the phase conductors low enough so that they are shielded. There are simple calculations for this. The static line is tied to the pole ground wire. A simple wrap (8 wraps) on the butt with staples makes a decent poke ground, Make sure the cross arms are long enough so that you don’t get a back strike (jumps from pole ground to phase conductor indirectly).This avoids the vast majority of strikes

So moving on to surge arresters. The goal here is to knock down any strikes or switching surges below the BIL. It does not and should not stop a trip because at this point we are conducting ALL current straight to ground including the intended phase voltage. So you should expect a trip. So at this point nothing you can do with a surge arrester for the most part can stop it in a nearby strike. If you are several surge arresters away they can quench the arc and keep the overvoltage under control within reason. But at the highly optimistic and basically wrong NESC recommendation if one arrester every 5 poles you might need to be 25-50 poles away. At that distance only a distance relay provides effective protection anyway,

Make your MCOV just above normal surge voltage because remember it’s a voltage variable resistor, not a dead short. Voltage will be higher than MCOV. Also installation is critical. Voltage is at a minimum only at the arrester and rises the farther away you get. It is not dimply “upstream” protection like you get with radial distribution contrary to popular belief. And line lengths to ground are very, very critical to the resulting overvoltage. See arresterworks.com for some decent tutorials on the subject or dive into the more recent IEEE standards on the subject of insulation coordination which is what you are doing.

 

[mbrooke]

This- is the best I've seen to date and should be a sticky.

One question- well two anyway:

The goal here is to knock down any strikes or switching surges below the BIL. It does not and should not stop a trip because at this point we are conducting ALL current straight to ground including the intended phase voltage. So you should expect a trip.

Since we are dealing with an MOV instead of a spark gap, wouldn't the MOV stop conducting after the over voltage is taken to ground? And as such no trip and re-close will take place?

Second- I've heard of MOVs being able to actually stop trip and recloses on medium and high voltage circuits during a direct lighting hit on the shield wire.

 

[mbrooke]

Here is a transmission line example. I'm thinking if its physically possible to do the same for 15kv distribution lines with "off the shelf" equipment:

FWIW- In the real world I see these applied to the bottom most phase(s) only.

 

[PaulMmn]

How will you test the MOVs to see when they've been 'used up?'

 

[paulengr]

This- is the best I've seen to date and should be a sticky.

One question- well two anyway:




Since we are dealing with an MOV instead of a spark gap, wouldn't the MOV stop conducting after the over voltage is taken to ground? And as such no trip and re-close will take place?

Second- I've heard of MOVs being able to actually stop trip and recloses on medium and high voltage circuits during a direct lighting hit on the shield wire.

A spark gap is a type of surge arrester. The earliest surge arresters (1930s) we’re silicon carbide. They work well but they are SLOW so they are combined with either spark gaps (fast but can’t self-clear) and/or surge capacitors to blunt the surge that gets through before they trigger. SiC is essentially a reverse biased diode which has an avalanche point where it stops blocking and switches on, so it requires two back to back. MOVs didn’t show up until the 1970s-1980s. These are actually voltage variable resistors so no turn on delay so no gaps or surge caps needed. So assuming we can keep the voltage below the insulation coordination sure it doesn’t arc and we don’t need to trip, if we wait long enough for the fault to clear via the surge arresters but with essentially a dead short this depends on how fast your recloser settings are and whether or not the strike exceeds that time limit or not. But normally whether or not it trips is incidental to avoiding strikes due to trips. You’re looking at 100 microsecond surges roughly a couple hundred microseconds apart for about a half dozen events. That’s plenty to reach thermal limits and the surge arrester survives. The reality is that maintaining BIL either via line construction or surge arresters is for handling nearby strikes and switching surges. It can’t take direct strikes. That’s where shielding is the only answer for lower voltages.

That being said transmission lines (115 kV plus) are pretty much naturally lightning immune. The Insulators and huge clearances did a great job. V strings are good for hundreds to thousands of kV, and the equipment is already designed around considerable switching surges during normal switching activity. I have seen a 230 kV pole destroyed by a strike during a hurricane once so it can happen but it’s rare. At 69 kV it’s a little more common. Down below medium voltage survival though is pretty much unrealistic and shielding is the only way to go. In between all you have to do is stop putting the static line below the phase conductors. Overbuilt is no more expensive than under built. It adds a few feet to the pole is all but expensive to fix after the fact. Davit arms aren’t long enough to avoid backstrikes. Need roughly 8-10 foot cross arms to get the conductors far enough apart.

You get ground potential rise causing your second issue. Remember we have a huge dump of electrons all trying to get to ground. This is where the surge impedance (length) of the cabling matters. If the surge impedance is too high you get other paths you don’t want. Similarly if the soil isn’t very conductive you get huge ground potential rise issues that are simply not solvable like in mountains and areas in the Southwest.

 

[mbrooke]

Can Spacer cable take a direct hit? What type of damage does a direct hit cause on distribution system?

Any idea why is the minimum size lateral fuse that can survive MOV conduction during a nearby hit?

 

[paulengr]

Can Spacer cable take a direct hit? What type of damage does a direct hit cause on distribution system?

Any idea why is the minimum size lateral fuse that can survive MOV conduction during a nearby hit?

You get little burn marks on the cable. Sometimes insulators explode. Sometimes this catches a pole on fire. On a big enough hit it can obliterate any structure. As I said a rare strike took out a 230 kv transmission line pole in North Carolina during hurricane Irene about 5 or 6 years ago.

And one of the biggest victims is surge arresters. They are made of sintered metal (mostly zinc) oxides. When it is used up it shorts then blows molten metal out as it blows out. Utilities often use small size jumpers like #1 or 1/0 so they act like fuse links.

Spacer cable is just an vulnerable,

Too many variables to suggest any fuse will survive. It’s all a probability thing.

 

[mbrooke]

As I said a rare strike took out a 230 kv transmission line pole in North Carolina during hurricane Irene about 5 or 6 years ago.

What was this pole made out of?


I know utilties that will use lighting mode, opening the recloser as fast as possible to preserve the lateral fuse.

 

[mbrooke]

Too many variables to suggest any fuse will survive. It’s all a probability thing.

I'm bascially debating whether or not to use fuse blowing vs fuse saving for lightning, and making sure the odds of line trips from lightning go down. Tree wire and spacer cable covers you tree contact, wind, animals, ect that would otherwise trip a recloser.