Drag Factor

Formula? Wind tunnel tests would be a more fruitful source of information.

this is a case with body section made of angular profiles in electrical power transmission lattice towers...!!

Wind tunnel models. Definitely. There must already be standards for that sort of thing, though, as there are many hundreds of thousands of the darn things all over the globe.

I assume this is to calculate wind loads? If you take pressure = 0.5*ρ*V² you won't go far wrong. Usually it's multiplied by a factor < 1 for most sections so should be safe. Then you need to work out the forces and moments to finalise the design.

Elaborating on what Codemaster already wrote, the simple formula for aerodynamic drag is 1/2 x rho x v2 x area x Cd (coefficient of drag). However, this does not take into account the interaction of forms found in a tower of many elements. For area you could double the projected area seen in a side or front view, to allow for the condition when the wind is slightly off axis (by say 20 degrees) which has negligible effect on Cd but a large effect on exposed area (with the elements "back" no longer in the wind shadow of those in "front").

To calculate toppling moments, etc, the tower would have to be divided into sections heightwise to allow for differences in area and wind speed gradients. The effect of the area of the transmission wires would need to be added in.

However, something the size and aerodynamic complexity of a transmission tower should be designed to a higher standard than a simple calculation. At least, model it in a good 3D CFD program, or put a model in a wind tunnel. Even things as aerodynamically simple as a car or airplane are routinely modeled or wind tunnel tested.

Unless this is a revolutionary new tower design, I'd think your tower supplier would have this information on file.

Are they not trustworthy?

There are codes which specify member coefficients as well as structure coefficients taking void ratios and down stream trusses into account (considering the tower to consist of three or four vertical trusses with common chords). These codes usually require that you consider diagonal wind in addition to the orthogonal axes.