Transmission line voltage level 4

[sberbece]

Hello!

I am working on a feasibility study, where I have to define a concept design and estimate de cost of the project.

In this project, the process plant load will be supplied through a transmission line from a hydro power plant. The transmission line should be double circuit, each circuit rated for 600 MVA. Plant load 600 MVA (50 Hz).
Transmission line length, up to 300 km. Plant operating time per year, 8000 hours. Plant life time 20 years.

Question. What voltage level would be recommended for the transmission line, 400 kV or 525 kV?

Thank you in advance!

Regards,
Stefan

 

[marks1080]

Whats the distance of transmission? What would the losses be at each level vs the cost per km of different the different voltages. I would also recommend talking with the utility in your area as well. I would consider other voltage levels besides the two you mentioned in your study. Is there any reason you would limit yourself to just 400 or 525? You also need to consider the availability of equipment (ie: transformers).

 

[rmw]

I think the OP stated 300 km.

rmw

 

[dpc]

I think that determination is part of your study. There is a lot of information available on economic loading versus conductor size. There is a cost tradeoff between higher voltage and smaller conductor size that must be analyzed.

Higher voltage will increase costs for circuit breakers, insulators, towers/poles and probably transformers.

David Castor

http://www.cvoes.com

 

[m3ntosan]

A 400 kV OHL has the SIL close to your needs.

May you grow up to be righteous, may you grow up to be true...

 

[sberbece]

m3ntosan,
What means SIL ?

 

[davidbeach]

I think this sounds like an academic exercise, not real work. As such it doesn't belong.

 

[burnt2x]

Please tell us David is wrong Stefan? We're bound by posting policies here.

 

[m3ntosan]

Stefan,
In Romania you call it natural load? P0= V2/Zch and it gives you an idea about the optimal operation point for a transmission path and tells you if you need to take into consideration reactive compensation equipments. Roughly for a 400 kV OHL it will be something like 500 – 550 MW, and when you’re energizing the OHL from one end you’ll get like 50 – 55 Mvar/ 100 km. If you’re functioning above the OHL natural load, the OHL will ‘eat’ Mvar, if you’re below your OHL will ‘produce’ Mvar.
Hope this helps.


May you grow up to be righteous, may you grow up to be true...

 

[sberbece]

In terms of OHL efficiency (i.e. Preceiving/Psending [MW]) what is an acceptable value 5%, 8%, 10 %?

Thank you m3ntosan! (Multumesc mult!)

Thank you all for your help!

 

[m3ntosan]

Please expand, are you talking about transmission losses?

May you grow up to be righteous, may you grow up to be true...

 

[sberbece]

Yes,
I'd like to know what is (are) typical / acceptable transmission losses for EHV lines, particularly for 400 kV and 500 kV.

Thank you !

 

[kh2]

The following information is about the surge impedance loading or SIL (in MW). The Surge Impedance Loading (SIL) of a transmission line is the MW loading of a transmission line at which a natural reactive power balance occurs.

The concept of SIL is explained as follows:
1) Transmission lines produce reactive power (Mvar) due to their natural capacitance. The amount of the Mvar produced is dependent on the transmission line's capacitive reactance (Xc) and the voltage (kV) at which the line is energized. Therefore Mvar produced = (kV^2)/Xc.
2) Transmission lines also utilize reactive power to support their magnetic fields. The magnetic field strength is dependent on the magnitude of the current flow in the line and the line's natural inductance (XL). It follows then that the amount of Mvar used by a transmission line is a function of the current flow and inductive reactance. Mvar used = (I^2)XL.
3) A transmission line's surge impedance loading or SIL is the MW loading (at a unity power factor) at which the line's Mvar usage is equal to the line's Mvar production.(I^2)XL = (V^2)/Xc, or XLXc = (V^2)/(I^2).
4) Rearrange the variables yields to V/I = Impedance = Square Root L/C = Surge Impedance.
5) The surge impedance loading or SIL (in MW) is equal to the voltage (line to line) squared (in kV) divided by the surge impedance (in ohms). SIL (in MW) = (kV^2)/Surge Impedance. Note that the SIL (in MW) is dependent only on the kV and the line's surge impedance. The line length is not a factor.

If a transmission line is loaded below its SIL, a line supplies lagging reactive power to the system, tending to raise system voltages. Loaded above it, the line absorbs reactive power tending to depress the voltage.

Attached is an example of the SIL loading.

The best of luck.

Kh2

 

[m3ntosan]

Must be hard to quantify transmission losses for OHL since some of the losses are load related (Joule losses - I2R) and some are non-load related (corona i.e. weather, number of conductors per phase/diameter related).

May you grow up to be righteous, may you grow up to be true...

 

[sberbece]

M3ntosan,

I’ve dusted a bit my HV OHL theory.
Based on a two port network:

1) Vs = AVr + BIr
2) Is = CVr + DIr
Where:
Vs = V @ sending end and Vr = V @ receiving end.
A, B, C and D are OHL parameters.

Pr is known, Ps is calculated from expressions 1. & 2. above.

3.) Ps - Pr = Plosses. Theoretical OHL loss. I’ll call it theoretical loss since the losses in dielectric and corona are neglected, when calculating the OHL parameters.

Then OHL efficiency [%]is EFF =Pr*100/Ps

What is the acceptable value, during the design of an OHL, for OHL efficiency?


Thank you in advance!

Regards,
Stefan

 

[tlrols]

Google Kelvin's Law, from Wikipedia:

An optimization rule called Kelvin's Law states that the optimum size of conductor for a line is found when the cost of the energy wasted in the conductor is equal to the annual interest paid on that portion of the line construction cost due to the size of the conductors. The optimization problem is made more complex due to additional factors such as varying annual load, varying cost of installation, and by the fact that only definite discrete sizes of cable are commonly made.

Kelvin's Law is fine in theory, but in practice utilities try to build transmission lines using a very small set of possible conductors for a variety of reasons, not the least in that it makes it simpler to stock parts, etc. Generally speaking utilities have settled on certain conductors for certain voltages through years of "practice."

In your case either voltage of 400 kV or 525 kV would be more than adequate for a 600 MVA flow. In general a 400 kV circuit is consider adequate for something around 800 to 1000 MW and a 525 kV circuit is good for around 1500 MW or so. It really depends on what is in parallel with the circuit. Note that you seldom approach the thermal limit of a EHV line in practice...