115 kV Line Tap - Disconnect Sw Requirements 1

[dpc]

We've been involved with several cogeneration projects that involved interconnection to a nearby 115 kV transmission line. In addition to the circuit switcher and disconnect switch at the substation at the cogen site, a second disconnect switch is always provided at the actual line tap, which could be some distance from the site.

Utilities sometimes install two additional disconnects on each side of the line at the tap point - three disconnects at the tap point.

I'm interested in some feedback from some of the utility engineers regarding the need for the two disconnect switches in the line - do you require these, and if so, what are the arguments for the necessity of these two additional switches?

Thanks,

Dave

 

[QBplanner]

Dave:
I experienced once a time with a pure load supplying line tap (50km) on a 1xxkV transmission line. We did not pay attention to the tap configuration at the very beginning since it is a small part of the whole project- cost aspect.

Later on, when the time came, people asked me to tell how many disconnect switches( vacuum interrupters)on the tap point are required.

The major concern I can recall was related to real time operation. How to isolate the fault after line tripped (Main 1XX transmission line and tap line)and operate the rest of the system. Finally, after considering all possible system scenarios together with the CB configurations on other ends, we decided to only install two disconnect switches at TAP position.

I would believe how many disconnect switchs at TAP point will be depend on case by case - your system configuration, operating orders and the way to operate the system.




How many VI will depends on teh EMTP people to tell drop the light loaded capacitive line concerns.

 

[dpc]

Thanks QBplanner. These have (almost) always been manually-operated disconnects on our past projects. The line relaying and/or transfer trip dumps the generation at the first sign of trouble via the circuit switcher or low side breaker at the plant. The disconnects at the tap were more for maintenance/safety concerns, but the logic is somewhat escaping me right now.

 

[davidbeach]

If the line is damaged, or needs maintenance, tree trimming, etc., the disconnects allow the generation to be connected to one end of the line while the other end of the line is out of service. Without the disconnects the generation is subject to all line outages, with the switches there will be few line outages that keep the generation off-line.

 

[bacon4life]

We usually have the two additional disconnects in line for the reasons David mentioned for our 115 kV/12.5 kV distribution stations. The switches are not always right at the tap point, so sometimes we only have one switch between two tap points.

 

[dpc]

I guess the continuity of service is the most compelling argument, but if the IPP says they'll take the risk because they don't want to pay for the additional disconnects, would you still insist on the switches.

In this particular case,one side of the 115 kV line runs a couple of miles to a small distribution sub and stops so they could never feed power that way if the other side of the 115 kv line was open.

 

[bacon4life]

In this case I would only put them on the distribution side and the generator sides of the tap. Having one on the system side of the tap wouldn't do anything.

More generally, it would depend on how easy it would be to schedule maintenance outages with the IPP.