Surge Arrester Placement 1

[hidalgoe]

Hello:

A decision was made by my management to install surge arresters at every other pole of a 69kV line thats 3200 feet long between the utility metering point and the main substation 69kV incoming switch. The distance between poles is 200 ft. The line has a guard wire.

Also the surge arresters were chosen by asking the utility technicians for their opinion of what type should be obtained and installed.

I had recommended that an insulation coordination study be done by a competent professional (electrical PE). I was told that that doing a study was to "theoretical".

Are my management folks correct?

Respectfully,

hidalgoe, PE

 

[odlanor]

empirical methods exist for making calculations of maximum distance between transformer and arrester as a
result of the first reflection of a traveling wave.
See Electrical-Transmission-and-Distribution-Reference-Book , chapter 18.
Those methods are very conservative.
Probabily a competent professional using EMTP can find distance greater than 3200ft.
You have to find a reference why 3200ft.

 

[wareagle]

Hidalgoe
What is on the end of the 69kv ckt?

 

[LionelHutz]

It doesn't sound very ideal to me. I'm picturing wooden poles with the arrestor ground wire running down the pole. That ground wire will have such a high voltage drop during a fault that the arrestor won't protect anything.

It seems to me that the arrestors should be installed at the ends of the line. The cable should pass by the arrestor terminal. By this I mean no T tap to the arrestor but rather the cable goes to the arrestor and then continues on to the next component. Then, you have to make sure the ground side of the arrestors is well connected to the ground of the component you are trying to protect, likely by mounting the arrestors and the component to be protected onto a common steel structure. The arrestor has to be installed directly across the component to be protected as much as possible.

 

[potteryshard]

Installing arrestor stations on every other pole (resulting in ~450' spacing?) is probably overkill, but should protect the line. With this many arrestors installed, type and rating will be relatively unimportant; sheer numbers will ensure that surges are snubbed.

Yes, the long ground lead down the pole creates an issue with voltage drop, but two factors reduce the impact; the ground or shield wire is tied to earth ground at every pole and so is essentially a ground bus carried at line level, and secondly, the arrestors don't need to control line to earth voltage, they need to control line to ground wire voltage.

Note that this type of installation is intended to protect the insulators on the line itself... It does not protect equipment at the end of the line; that requires heavier station-class arrestors since line to earth voltage control now becomes central.

Where the shield wire is carried above the phase conductors, that shield wire typically takes the strike, and in-line arrestor stations pull up the line voltage to limit across the insulator voltages. Where the grounded neutral wire is beneath the phases, the line typically takes the hit, and the arrestors stations restrain the strike voltage on the line to limit insulator voltage.

If the arrestors were indeed installed to protect the line itself, this brute force approach should do the job. Arranging to have an insulation coordination study might offer a less expensive route to provide equivalent protection. (These efforts are not going to reduce the actual number of strikes... They will reduce the damage resulting from the strikes.) If the arrestors are intended to protect equipment at the end of the line; wrong application.