Transmission lines capacity

[21121956]

Hello everybody:

As you can see, I am Mechanical Engineer and, because of my current job, I need to learn/to know about the fundamentals of transmission lines.

I am working as part of a multidisciplinary group planning mini and small hydro power plants, and I would like to have access (in addition obviously to your comments) to books, papers or any kind of information related with this matter.

As an example, I would like to know the answers to questions like these:

1. If a hydropower plant generates 5 MW at 13,8 kV at the generator outlet, is it possible to transfer this power 10 km through a transmission line of 13,8 kV? or it is required a line of 34,5 kV?

2. How about if the power is 10 MW an the conditions for the question #1 remain the same?

Maybe it would not be necessary to say it but, any way, I am not trying to usurp the field of action of an Electrical Engineer, my only purpose is to get to understand about this matter by myself.

Thanks in advance

 

[davidbeach]

faq238-1287.

Start with the Power System Analysis selections.

The answer to both questions, absent a tremendous amount of additional information, is "It Depends". Any amount of power can be transmitted any distance at any voltage, but the economics of the situation will typically eliminate a lower range of voltages as requiring too much conductor and an upper range of voltages as requiring equipment that is too expensive. What's left in the middle would be workable, but the specific selection would come down to other factors.

 

[Overvoltage]

As davidbeach said, there are a lot of factors which would go into how you would design the system. Now, with that said, you could, in theory, transfer the power from that unit to the grid at 13.8kV for either of those situations based only on the information given. Of course, for the higher flow you would need larger conductor to handle the higher currents, so that may become a factor in the decision to move to a higher voltage. There are also many other factors involved that may make it infeasible or impossible that weren't mentioned by you in your post.

There are a lot of "it depends" as was previously mentioned, and really the best way to find out the optimum solution is to involve a EE. I can understand and appreciate your wanting to understand what's going on, but it may be best to just ask the EE to explain it to you instead of trying to figure it out on your own. Definitely should try to find out what's going on, though, if it interests you. It can be interesting how interwoven some of our disciplines are when you get down to it.

 

[waross]

Step one; Determine the impedance of the proposed transmission line. This depends partly on the configuration and spacing of the conductors. It depends, you have to do this or get an EE to help.
Step two; How much voltage control or adjustment do you have?
This may be a transformer with an On Load Tap Changer at either end, generator voltage control or both. Note; Generator voltage control is hard on station auxiliaries unless they can be fed from a separate source.
Step three: Use the impedance of the and the voltage adjustment range to determine the maximum current the line voltage adjustment can compensate for.
Step four; Divide the KVA to be transferred by the maximum allowable current to determine the minimum voltage that may be used.
This is a simplified solution to give you an idea what you are asking. The power factor of the transfer should also be considered.
Perhaps one of our power transmission or distribution experts will suggest the spacing and impedance per mile of a typical 13 kV distribution line.

Another way of saying this is that there should be an adequate range of voltage adjustment to compensate for the voltage drop of the line at full load.
A higher voltage means less current and less voltage drop, and an even smaller percentage voltage drop.


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

 

[cranky108]

A good part of the voltage selected is the loss factor, and cost of wire size and structure.

As you should know your loss is I^2Z, so if you reduce Z, you have a lowering in losses at the cost of bigger wire and/or structures. But if you can reduce I, such as rising the voltage you make a much bigger impcct on reducing your losses, at the cost of larger insulators, and substation equipment.

Another limit is the sag of the wires. Taller structures allow more sag in the wire, which allows more capacity at cost. On the other hand, larger wire requires a beefer structure which also has a cost. Where a higher voltage keeps the wire smaller (Not always applied that way)which reduces the cost of the structure.

 

[Electic]

5 MW, 6 miles at 13.8kV could be served by 40' poles, 9' crossarms on approx. 300' spacing with 1/0 ACSR, if voltage drop can be managed. 10MW same distance might be handled by 336.4KCMIL ACSR, once again if voltage drop can be managed.

Voltage drop @ 10MW on 336.4KCMIL Merlin would be approximately 720V or approx 9% of phase to ground voltage, within the range of a typical regulator.

These are simple 'rule of thumbs' to be supported by calculations as others have indicated.

 

[waross]

A large part of the voltage drop may be a reactive drop and will not be helped by larger conductors.

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

 

[cranky108]

But spacing the conductrs further apart would reduce the inductive reactance. Or just add capacitors.

 

[Electic]

Inductive reactance in my answer was figured at .462 ohms per mile based on 1' phase spacing. It does not change much for other conductor sizes. Resistance was figured at .307 ohms per mile, it does change much with conductor size.

Shunt capacitance for this configuration would be .1055 mega-ohms per mile, not much hope for corrective compensation, but the inductance was already figured into the voltage drop.

This is a 10k (6.21 mile line); not a 300 mile line. 7 mile long, 300A feeders (built to 600A construction) are routine for power companies. It could also be routine to install an underground feeder for these distances.

 

[21121956]

Hello everybody:

I am really thankful for the support I have received from all of you on this subject. Thanks.

 

[tlrols]

Some rules of thumb that help guide you--these are rough and as said by previous posters it depends.

1 kV per mile for transmitting energy. In other words, if you want to send energy 30 miles, use at least a voltage level of 34.5 kV, don't use 13.8 kV.

Surge Impedance Loading...kV^2/400 for lines less than ~345 kV... the resultant is the loading at which the line's capacitive nature is canceled out by its inductive nature...you can assume that you can load the line 3 to 10 times beyond the SIL if you can support the voltage. In your case these rules of thumb say you can bring in 5 MW...barely.

You really should perform a power flow analysis. The problem you describe is trivally easy to set up and study. A P-V analysis will tell you everything you wanted to know.

As pointed out by others your project is "doable" at 13.8 kV...just however. Presumably you aren't planning on expanding the generation in the future...