Maximum Voltage Ratings - Substation

[rterickson]

The majority of our substation equipment was procured under ANSI standards in effect at the time of construction. Accordingly, our older 115 kV (nominal) disconnect switches, PT’s, circuit breakers, etc. are nameplated at 121 kV Rated Maximum Voltage. Since efforts in the mid-90’s to normalize ANSI standards with IEC, newer equipment is now marked 123 kV Maximum on the nameplate.

Since we typically operate at or below 120 kV, this is rarely an issue, but we have one distant switching station built in the 1950s that does see 122 kV under certain light load conditions. The Compliance folks are concerned about operating outside of ratings during these times.

The circuit breakers at this station were changed out in 2001, and we are adding new CVT’s later this year (planned end-of-life replacements). However the disconnects are still “good”, and likely run cooler than what we could purchase today.

It does not seem like a wise use of resources to replace all our switches to get new nameplates based on what seems like an arbitrary change in standards. Has anyone faced a situation like this? Is there any conceivable basis for rerating the switches?

 

[waross]

You may want to check the over voltage tolerances on the old switches. The old switches just may be within tolerance limits. It's a long shot but you may get lucky.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

Your CVTs are likely to be fine at 122 or 123kV. Frankly, the ratings for CVTs are pretty screwed up under ANSI C93.1.

For instance, the units you have are probably rated 115,000GY/69,0000V with a 600/1000:1:1 ratio and an over-voltage factor of 1.1-continuous. That 1.1 continuous applies to the 69,000V rated voltage. If you convert that to a line-to-line system voltage, you get 69kV * sqrt3 * 1.1 = 131kV. However, in the same standard, the max system voltage for that class is listed as 121kV or 123kV (depending on the year). Bottom line is that the CVTs you have likely are indeed "rated" to be operated at 123kV....they just don't appear that way at first glance.

 

[bacon4life]

Take a look at footnote 2 of C37.32-2002: "... It is not necessary to retest switches previously rated at 121 kV, 169 kV and 242 kV."

The NERC standards just require you to follow your rating methodology. Your methodology is allowed include engineering analysis or operating history as a basis for the rating. The IEEE standards were not written with NERC compliance in mind. In some cases, the voltage listed is simply on of several variables to calculated the shape of the test transients; the actual maximum could be higher (or lower) if the other system parameters are different that the nominal conditions the IEEE standard assumes.

As a point of reference, one of the large utilities in the northwest typically allows a few percent over 245 kV for breakers and switches name-plated at 242 kV.

 

[rterickson]

Thank you all for your responses.

Bill, I don't seem to have any documentation for these switches from a long-gone manufacturer.

Scottf, how did you know I'm running OCTF123's?

Bacon, I saw the footnote, but completely failed to recognize its meaning. I think your interpretation is spot-on.

Thank you again, I appreciate the help.

Randy