High-Voltage Air Disconnect - arc horn or no arc horn?

[rand1234]

Hi Everyone,

Hope you all are doing well and thank you ahead of time for taking the time to read this post.

I wanted to get some feedback regarding arc horn versus non-arc horn design for high voltage air disconnects. Is there any industry standard/norm that dictates when arc horns should be installed for an air disconnect? All I have seen from one disconnect manufacturer is that if "very low levels" of resistive/magnetizing and capacitive currents exist during the operation of the switch, than arc horns are not required. This is pretty vague to me. Does this mean 1 amp, 2 amps, etc.?

The reason why I am asking is because vertical air disconnects equipped with arc horns require quite a bit more clearance phase to phase and phase to ground per IEEE 36.30.1. For example, taking the scenario I am looking into, recommended clearances for the 245kV max system voltage level @ 900kV BIL is 192 inches phase to phase and 152 inches phase to ground. This is in comparison to non-arc horn air disconnects which have a recommended clearance of 132 inches phase to phase and 80 inches phase to ground for the same system voltage.

I have actually measured the no load current in the location where I am evaluating the feasibility of installing a vertical air disconnect which is ~1.5 amps. Resistive/magnetization current is estimated at ~.37 amps based on transformer FAT reports and the rest is assumed to be capacitive. Due to clearance issues in my specific application, there is no way an air disconnect equipped with arc horns can be installed. Due to this, I am trying to determine whether a non-arc horn design would can be installed safely. I understand the reason why arc horns are installed and their benefit of diverting the arc from the main current carrying contacts. I wanted to get your guys' opinion based on your industry experience.

I appreciate any feedback anyone can provide. Thank you.

 

[bacon4life]

Sometimes switches are rated for distance of line drop rather than amps. A current of 1.5 amps sounds like you are dropping a section of line rather than just buswork inside the substation. It may be worthwhile to calculated the expected capacitive current and compare the calculations to your measurements. Current transformers can under-read at very low loads.

I suspect the additional clearance is due to the likelihood of arcing rather than just the presence of arcing horns.

When I needed a 230 kV disconnect capable of dropping a transformer several years ago, I came across some cautions against using air break switches to drop EHV transformers due to the possibility of restrike transients while interrupting highly inductive excitation currents. So even though I purchased a switch rated to drop the transformer excitation current, the operating practice is to use a circuit breaker to energize/de-energize if at all possible.

 

[rand1234]

Appreciate the response, bacon! Apologies, I should've specified the application. The disconnect would go on the high side of a main bank transformer for isolation purposes; the power plant wants a demarcation point with the switchyard for boundary-related reasons. My main concern is also the capacitive portion of the no load current due to a portion of the 230kV line being underground (~450 feet) before it transitions aboveground and hits its' position in the rack (e.g. capacitance portion is dominant due to the highly capacitive nature of underground cable versus their aboveground counterpart). And as you stated, the chances of re-striking and longer arcs across the disconnect gap is much more likely to occur with the breaking of capacitive currents. I have brought up interlocking the disconnect with the main 230kV circuit breaker in the rack, but that conversation didn't go anywhere.

Out of curiosity, did your disconnect include arc horns or were there no need to install them since you open the circuit breaker first? And sorry this may be a dumb question, but I have been thinking about this recently. When calculating capacitive currents for a certain line, where do you stop? Meaning, let's say that a 1000 feet of 230kV above ground cable connects your main bank to the position in the rack. Do you just take that portion into account? Or is it more complicated than that since you have transmission lines, buses, etc. connected to that position indirectly?

 

[bacon4life]

I ended up purchasing a switch than included high speed whips in case the switch is ever used to de-energize the transformer.

For calculating current through the switch, you would need to account for all equipment the switch would de-energize. Having isolation switches that can only be operated de-energized is somewhat common for HV equipment. Having a local, visible isolation point certainly makes lockout/tagout easier.

 

[cuky2000]

Check If there is an opportunity to consider a 230 kV Gas Insulated switch that may meet the clearance constraints in your project. GIS switches are available with position indicators, side glass, and cameras to verify the status of switch position.

If an air-insulated switch is installed without arcing horn, the small arcing generated during the switch operation could damage the contact and might require test and replacement of the part often. It also recognized that the switch operation is not a frequent event.