overhead line capacity

[HamidEle]

Currently we are working on a overhead line project. The client specification limits the maximum delivered load for 34.5KV below 40MVA. I am trying to understand the reason behind it. We could use multiple conductor per phase at 34.5KV to increase the overline capacity similar to cables. But our client insists on changing to 72.5KV level, which would cause a lot of design changes. Any suggestions?

 

[waross]

One limit on transmission line capacity is a combination of the voltage drop and the ability of the OLTC to correct the voltage drop.
Reactance may be a significant portion of the impedance. Parallel conductors will lower the resistance but not the reactance unless a second line is constructed.

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

 

[HamidEle]

You meant to regulate the bus which is remote from OLTC? If that is the case, we almost hit 12.5% voltage regulation limit. So we probably have to go with high voltage levels.

 

[davidbeach]

Actually, the use of bundled conductors does reduce the reactance.

There's probably a good reason behind a 40MVA limit even if it isn't readily apparent.

 

[Marmite]

Hamid, you could always ask the client.
However, if their specification limits it to 40MVA and they as client are insisting on using 72.5kV it doesn't really matter what the reason is, if you want the job.
Regards
Marmite

 

[HamidEle]

Waross was right to the point. Overhead line has much higher reactance than cables. Cables can deliver more power than overhead lines.

 

[davidbeach]

No, for a given conductor size, you can transmit far more more using open conductors (overhead) than you can using cable. Open conductors can both dissipate heat much better than cables and run hotter than cables.

 

[HamidEle]

But reactance is always the limiting factor. If you compare the cable impdance with the overhead conductor impedance, you will notice the reactance of the same size of overhead conductor is much higher than the cable impedance.

 

[jghrist]

The limit may be stability based rather than thermal capacity based.

 

[magoo2]

One thing not mentioned is the fact that many if not most distribution circuits are standardized to a maximum load current of 600 amps. At 34.5 kV, this works out to 36 MVA. At 12.5 kV, it is 13 MVA.

This standardization permits the use of readily available components like reclosers which typically have 600 A limits. On the distribution side of things, equipment is used in such high volumes that standardization offers great economies.

If you design it more like a transmission line, then you're more likely to customize it. In this case, you can consider 72 kV or 115 kV.

We have quite a bit of 35 kV distribution. On only a handful of situations have we paralleled conductors for higher capacity.

One drawback of introducing 72 kV - if you don't have any other 72 kV on your sytem - you pay a price for the complement of spare equipment. Of course, if you have other 72 kV stations, it's not an issue.

 

[bacon4life]

Have they specified the the maximum allowable losses? Losses, stability and per unit voltage drop all improve based on the square of the voltage.

What size wire are you putting in parallel? 670A doesn't seem like a particularly high ampacity, easily reached with 556 MCM conductor. Do you already have hardware in mind? Although parallel conductors at above 200 kV is common, I have not run across overhead distribution class equipment designed for it before.

 

[HamidEle]

They haven't specified the power losses. We are considering Drake 795MCM condctor for 35KV lines ( 5km) . The issue is, the voltage drop at the end of the line is about 4% with the voltage regulation at the upstream main substation.
The availability of the autorecloser is another issue. This might be the reason our client specified 40MVA as 35KV Overhead line limit.