Grounding of Surge Arresters on a Transformer Secondary

[constantlylearning]

I think it's a common rule that you usually want to install a surge arrester as close as possible to the terminals of the equipment you're trying to protect. One of the primary reasons is that increased lead length between the transformer terminals and the arrester terminals increases voltage drop along the leads and reduces the protective margins. So, having a very short grounding lead between the arrester base and a grounding pad on the tank wall decreases (as much as possible) the total voltage drop between the transformer tank (ground) and the transformer terminal during a lightning arrester discharge.

My question is:

In addition to the 1 to 2 foot copper lead between the arrester base and the tank wall, would it not also be advantageous to include a copper lead from the arrester base down to the ground grid pigtail. My reasoning is that the transformer tank wall is the ground reference for voltage transient protection, so you definitely want the arrester grounded directly to the tank wall. However, you might not want the tank wall to be your primary fault path for 60 Hz current during a lightning arrester failure. In truth, I don't know how the 60 Hz impedance for a 4/0 copper lead vs. a transformer tank compares. It could be that the tank is a lower imepdance path to 60 Hz current as well as fast transients like lightning and the 4/0 lead would conduct very little 60 Hz fault current. Does anyone know what the standard practice is? I did not find a reference to this in C62.22.

Please see the linked GIF file. The red down lead is the one in question.

 

[dpc]

Most installations I have seen run a copper conductor from the arresters straight down to a ground grid connection. This conductor should be as straight and as short as possible.

The concern is not the 60 Hz impedance - the concern is the surge impedance for the traveling wave going through the ground system during a transient event.

 

[stevenal]

We run the copper conductor as dpc said. A continuous length jumpers from arrester to arrester to arrester to the tank and to the ground grid.

 

[jghrist]

Unless specifically requested otherwise, the arrester base will be grounded to the tank by virtue of an uninsulated base. Normally a ground connection is also run from a ground connector on the base to the transformer grounding pad and then to the ground grid. The lowest impedance path at lightning surge frequencies is probably through the tank.

Since the surge is entering at the top of the transformer, the voltage at the top of the tank is more important than the voltage at the bottom. I don't think that voltage drop through the tank is important, so the important connection is at the top. It probably doesn't make any difference whether or not a ground lead is connected to the arrester as long as the arrester base is bolted to the tank without insulation. It doesn't hurt having another ground lead, however.

 

[magoo2]

You'll find some pretty good info on arresters and lead length considerations on the attached link. Jonathan Woodworth was formerly an engineering manager and later a marketing manager at Cooper's arrester plant.

Jon's been active in IEEE and is well respected.

 

[constantlylearning]

There are 2 things I'm certain of.

1.) Simply bolting the arrester base to the tank is not necessarily good enough. You really want a good metalurgical/corrosion-resistant connection beween the arrester base and the tank wall. The arrester mounting bracket is usually a painted bracket. So, you would be relying on the bolts for a good connection. Not a good idea. Instead, you probably should have a copper conductor between the arrester base and the tank grounding pad.

2.) Ideally, you want the arrester phase lead and the arrester grounding lead to be as short as possible. They both contribute to voltage drops that eat away at your protective margins during a surge event such as lightning. The important thing is to minimize the surge voltage between the transformer terminals and the tank (the transformer's ground reference).

What I'm trying to find out is, if you've got an arrester grounding lead running a very short distance to the tank wall and you actually have a faulted arrester, is it sufficient to rely on the path (through the tank wall) between the top grounding pad and the bottom grounding pad? (See my NEW attachment.) I do understand that for high frequency events, the tank wall is the lower impedance path to the ground grid, even though you really don't care what the voltage drop between the tank wall and the ground grid is. I want to know if the tank wall is an adequate path for 60 Hz fault current or is it advisable to add the lead shown in red on the attachment.

I realize I did not ask my question in the clearest possible manner. It was late last night when I typed it. If I'm still not making any sense, please let me know.

 

[jghrist]

1. What is the BIL of the paint coating on the arrester bracket? I can't imagine that this would be significant during a lightning event. Nevertheless, I do think a separate ground wire to the bottom ground pad is a good idea.
2. There is no ANSI requirement for a ground pad at the top of the transformer tank, although it could be specified.
3. I think that the tank wall is adequate to carry fault current from a failed arrester.
4. If this is a delta-grd wye power transformer, then the X0 bushing will have a connection to ground also. We have specified transformers with a ground bar from X0 to the ground pad and connected the secondary side arresters to this ground bar. This eliminates the need for a separate ground wire for the secondary arresters.

 

[constantlylearning]

Certainly, the paint won't stand up to any appreciable voltage. My point was that you need have as good a connection as possible. A layer of paint is just not a good the to have between 2 mated electrical conductors.

The reasoning behind the ground pad at the top of the tank, which we already have on the transformer in question, is to minimize voltage drop between the arrester and the tank wall.

In the case of this transformer, the X0 is resistance grounded, so your scenario doesn't work in our case.

Thank you all for your responses. Great, great feedback!

 

[stevenal]

I can state from experience that a tank wall will carry sufficient 60 Hz fault current for protection to operate. Once was not proof enough, the transformer was re-fused so we could go two for two. The fault was not in the arrester, but internal to the transformer about half-way down.

 

[electricpete]

Sometimes the arrestor and grounding lead are intentionally insulated from the tank in order to provide the ability to measure leakage current. Is that a bad practice?

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[jghrist]

Sometimes the arrestor and grounding lead are intentionally insulated from the tank in order to provide the ability to measure leakage current. Is that a bad practice?

It increases the lead length and can reduce the transformer protective margin. See thread238-210659

 

[vima]

Most installations I have seen run a copper conductor from the arresters straight down to a ground grid connection. This conductor should be as straight and as short as possible.