Transmission line conductor current rating vis-a-vis the wind velocity 4

[RRaghunath]

Current rating of a OHL conductor is a function of allowable temperature rise and the wind velocity for a given location.

Many specifications indicate what should be the refenerce ambient temperature for current rating calculations and the maximum operating temperature rise is generally limited to 85degC (AAC/ACSR conductor).

How about the wind velocity, is considering 0.5m/sec more common or should I consider Still air conditions (om/sec) that happens to be the worst condition from the point of convection heat loss?

Any suggestions!!

Thanks in anticipation.

 

[Electic]

Power companies will give two ratings to conductors, one for still air and the other for 'wind'; and then operate according to conditions.

If you have no control over operations, or are designing for worst case, it would seem appropriate to design for still air.

 

[cuky2000]

Do to the random variation on wind speed and other environment design parameters, the conventional engineering design procedure use conservative values to determine a safe steady state rating of the transmission line. This uncertainty may overdesign the line to guaranty sag, clearances, permanent deformation, etc.

One viable alternative is using the dynamic rating techniques sometimes called "real-time rating". Weather stations or other mean of monitoring and predict weather and sag along the line corridor is required to re-evaluate the line capability at any specific time.

A few options of dynamic rating systems are described in the following links

http://www.pserc.wisc.edu/ecow/get/publicatio/2001public/heydt-sydney01-4.pdf

http://www.epriweb.com/public/000000000001012135.pdf

 

[berkg]

The inclination (angle) between the wind and conductor is important as well. Refer to IEEE 738-1999 for the most detailed formulation. Some utilities take 2 ft/sec perpendicular wind. In Ontario the minimum wind condition is 4 km/hr at 20 degree inclination with the conductor.

For planning purposes these are OK but in actual operations I'd suggest a more conservative approach depending on age and condition of the line, conductor, hardware (splices clamps etc.) since these are the first to fail due to thermal runaway.

 

[MikeDB]

Most operating systems (e.g. MAIN, ECAR, SWPP)in the US have developed standards that should be used in their are, so that all utilities are operating on the same page.

The 2 feet per second speed is often considered a minimum generated by natural convection around the conductor due to the temperature difference between the conductor and the surrounding air. Larger wind speeds should be used with caution, even if you are monitoring local conditions for "real time" rating.

 

[bacon4life]

Mike, do you have links for the any of the NERC region recommendations?

Looking at the equations in IEEE 738 and using Drake conductor, I got that the crossover between natural and forced convection was more on the order of 0.4 ft/s, but would of course depend on the temperature differential.

 

[MikeDB]

No. I have seen the one for MO, but do not have a copy of it. Perhaps some of the system planning folks will have some info to share.

 

[RRaghunath]

Thanks everyone for the responses.

Is there no codal guideline as to the minimum wind speed to be considered, say perpendicular to the conductor!!

Thanks in anticipation.

 

[transmissiontowers]

There is a Ratekit program that is used to calculate the current flow given the parameters of wind and sun and each utility has their own guidelines. We run our ACSS up to 200C and 250C in an emergency. PLSCADD also has some conductor rating functions. For ACAR and AAC you have to watch Elevated Temperature Creep and loss of life if you run the aluminum alloy too hot and anneal it.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[xabproject]

Dear RRN,

Free convection air current is taken as 0.2-0.5ft/sec. All formulae for Convectively dissipated Watts are applicable only above "these" air current values (- look for specific formula for application). Standstill air cannot dissipate convective heat, by logic! Your Wc will be zero, if you use air current of zero m/sec!!; standstill air can still be achieved in lab conditions though.
Average wind velocity of a location or the region is normally available in the GENERAL DESIGN CONDITIONS of any specification; a good one will have min and max values. While the 'max' value is used for calculating Wind Loading of structures, the 'min' value is used for Heat Dissipation calculations!

Codal requirements can be looked into if you could tell whether it is for IEC or for ANSI/IEEE world.

Best regards,

 

[xabproject]

Dear RRN,
One particular Client's specification calls for the use of 0.609m/sec!
Let us know what was finally accepted in your case.
best regards,