Maximum continuous operating voltage and equipment selection 1

[qman5]

Hello all,

I'm working for a Canadian engineering company, and I have encountered a number of times where the maximum system voltage is 253 kV (or 1.1 x nominal voltage [230 kV]) and we always seem to get into a discussion if the equipment, which is usually built and tested for 242 kV and 245 kV (IEC), is suitable for use at 253 kV. I've had cases where we need to provide cables, sealing ends, and disconnect switches rated to 300 kV (next IEC class). The same topic has come up with lower rated gear, but this goes into the realm of type testing, which most companies have not done for this voltage or won't do it without considerable cost!

What would be the general approach to this? I've had instances also where a certain GIS cable sealing end vendor certifies their GIS sealing ends to 253 kV, but it is tested formally to 245 kV, and they certify the max voltage via a signed letterhead with statements providing the standard hi-pot test voltages. But I've also had cases where the same vendor provided confirmation that their cable is rated to 253 kV when using ICEA standards , but the same cable would need to be upsized to a nominal voltage class of 300 kV when using IEC standards. And one other thing, the GIS manufacturer has tested to 253 kV, but the gear is rated at 230 kV and has a nominal MCOV of 245 kV!

This has considerable cost implications for various projects and it seems the unharmonized standards are a culprit here.

Care to share your thoughts and experiences?

 

[sambapati]

We have the same problem qman5. In our utility, our existing nominal rating is 220kV which is OK with the 242 kV max design voltage of IEEE. Even if our transmission grid operator is supplying us with a nominal voltage of 230kV, the 10kV difference between our 220kV primary transformer rating is still OK because we allotted the 10kV difference in the 230kV xmission line voltage drop. But when the grid operator required us to revise our primary nominal voltage to 230kV, we requested clarification since the design voltage will now be referred to the next voltage class. We just complied with the requirement but we have not yet revised our specs and still maintains the 242kV voltage class.

 

[bacon4life]

We have run into this as FERC/NERC requires clearer definition for, and monitoring of System Operating Limits. IEEE equipment standards were written around measureable quantities related to physical failure modes. For equipment without magnetic saturation, the physical limits are not typically due to operation at system nominal voltage. Instead the physical failure occurs due to some sort of overvoltage factor times the nominal voltage. In some cases, you may be able to operate the equipment at the higher voltage if a lower overvoltage factor is applicable to your system.