I hate to spoil a good argument, but skin effect at 60 Hz is pretty insignificant, and has nothing to do with the choice between solid or stranded wiring for control circuits in substation. (especially the dc circuits )
One of the biggest issues with control wiring in substations is that it often lands on the doors, which means the wires will be flexed relatively frequently. If possible, the permanently installed control wiring should first land on terminal blocks to avoid this.
Any control wiring installed entirely within a switchboard, that lands on the switchboard door and which crosses the door hinge should be specified to be type SIS, as defined in Article 310.13 as approved for use as "switchboard wiring only". This is a very flexible conductor which is made specifically to withstand the abuse of flexing across a door hinge.
peebee's comments are consistent with IEEE C37.20, .21 verbiage. Having to install any significant quantity of switchgear/control wiring that is not of K/H stranding would be insanity.
My understanding from the power electronics end of the world is as follows:
Skin effect is present to similar level in solid and stranded cable. The effect can be minimised by using special types of stranded cable in which individual strands are insulated from each other. This is known as 'Litz wire', and is used for high-frequency work in RF and in power electronics applications. At 50 / 60Hz skin effect is negligible except in exceptionally large conductors such as are found in generator bushings and gen xfmr LV bushings. The unequal distribution of current within insulated strands of a bundle is due to proximity effect, which is similar to but quite distinct from skin effect.
Suggestion: It depends. As NEC indicates the limit for larger conductors to be stranded, this is matter flexibility. However, smaller conductors, e.g. up to #10AWG would be wiring inconvenience for many devices, e.g. receptacles, that have snap in hole for solid conductors. Also, many screw terminals of smaller size conductors fit better to solid conductors since special pliers produce a suitable hook or eye on the solid conductor, which fits properly under the screw head.
Just my 2 cents worth!
Being an Emergency Generator Tech I can say this after some thirty years in the field, solid wire tends to fracture do to the constant vibration after a while particularly at the terminal.
To 144X:
I agree with most of the arguments here, stranded wire is easy to install, you don't have to worry about the bends where you can break a solid wire and its easy in your fingers when you install it.
But all of that is not the point...When you're checking for a faulty wiring in an old substation where they used solid wire, pray that you don't pull a current transformer wire too hard because it will slip from the terminal and that'll be your last day...I don't know the relation between pulling stranded wire or solid wire and disconnecting it, what I do know is that we had many substations with both of them and we had a lot of problems with the ones with solid wire so we forbade the engineer department to approve the use of solid wire in any new substation due to personal safety.
I hope this usefull to you.
Suggestion: It appears that the stranded conductors may absorb more of corrosive agents among strands, which may lead to faster dilapidation of the stranded conductors.
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