Line Loss Calculator

[Rob Beckers]

I've attached a small spreadsheet that calculates line losses for 3-phase AC type wind generators. It was written for the Scirocco wind turbine, but will actually work for any, by simply filling in the voltage and power of the alternator. I'm hoping one of the wind gurus on this list can review this, to see if it works correctly. While I'm an electrical engineer, my field is more in electronics than power electrics, so it would be good to have someone review it. Please also check if the notes I've added make sense. Possibly there are more things that need to be in there, useful tips, safety related items etc. Once I'm sufficiently sure it's working as it should I'll put it up on my Scirocco related Web site.

Besides having this reviewed, I thought it might be useful for others as well. Sizing the hookup wires for a wind genny is something that is frequently needed. My own rule-of-thumb is that a wire size that will give around 5% or less power loss should be fine.

By the way, the spreadsheet calculates power loss in the lines at full power (or whatever power number you fill in). I realize that actual power loss is less than this, as the bulk of the energy produced by a wind generator happens at less than rated power (usually much less). To fully calculate actual power losses one would have to incorporate the genny's power curve, and use the site's average wind speed together with a Weibull distribution (yet another approximation) to get to the line losses. This spreadsheet is intended as a quick tool to get to a realistic wire size, not a "precision instrument".

Thank you for any comments/corrections!

Here is the link to the wire gauge calculator. (http://www.solacity.com/Docs/Scirocco%20Wire%20Gauge.xls)

-Rob-

[Rob Beckers]

I've updated the attached wire gauge calculator in the initial post. This was after consultation with Eoltec. The original calculator was not wrong as such, but specifically for the Scirocco the requirement is that line losses do not exceed 3%. The reason is that the MPPT algorithm in the inverter will load up the turbine so it gets 6kW on its output (when it senses the turbine is spinning at what should be full power). That means the current delivered by the alternator will have to go up in case the line losses go up. For losses of more than 3% (in power) in the lines that can lead to overloading and overheating of the alternator.

As mentioned, this wire gauge calculator will work equally well for other 3-phase wind turbines. Just change the voltage and power numbers. Other turbines (particularly when used with non-MPPT inverters/controllers, which is the vast majority) will not require 3% line losses, using 5% as a target number is likely just fine.

-Rob-

[Mark Parsons]

Thanks for the line loss spreadsheet Rob.:)

Saves me creating one. I may modify yours to handle both 3 phase outputs and direct DC so comparison of extra wire cost can be determined.

The resistance per 1000 foot column data is for DC current and copper at 25degreeC? Not 60Hz AC at 10degreeC (buried)? Skin effect also does add a little impedance at 60Hz.

[Rob Beckers]

Hi Mark,

Skin thickness for copper at 60 Hz is around 9 mm, so you'd need conductors of at least 15 mm diameter or more before that starts to begin to be a factor. For all intends and purposes I believe it can be ignored here.

I believe those resistivities are for 20C. The temperature coefficients for copper is such that at 40C the error in resistance would be 8%, at 90C (the limit of most cables) it would be around 25%. Loss is linear with resistance, so if one shoots for a wire gauge resulting in 4% loss, the maximum temperature error would cause one to be off be about 1 percent, and that would be at the temperature extreme. With all the other variables one can safely ignore this.

Of course, for colder than standard temperatures resistance goes down for copper, and so would line losses.

In short, IMO it's very safe to ignore frequency and temperature for line loss calculations.

-Rob-

[Paul Bailey]

Thanks Rob. Here's a couple as well..Paul