Electrical arc fault. More precisely a series arc fault taking place on what looks like a 69,000 or 138,000 volt disconnect switch inside an electrical substation. High resistance at the contacts of the switch can cause this, where electricity is literally forced to travel through the air in order to complete the circuit.
If left to continue (system operators don't open the breakers around said arcing) the intense heat will melt through metal conductors or cause the porcelain insulators underneath to give way. Either of which will result in a violent short circuit which will (hopefully) trigger protective relaying to open breakers de-energizing it. If breakers fail to open, the arcing from the resulting short circuit will light up the sky for miles like it did at Astoria Substation in Queens.
Series arcs are tame in comparison limited by the impedance of the load, while arcing taking place phase to phase or phase to ground is limited by the system's short circuit current which is usually obscene- often in excess of 60,000 amps.
The blue/white light comes from the nitrogen in the air (think how neon lights work- the gas insides determines their color) while other colors like red, orange, green ect can be attributed to the metal the electricity is arcing off of.
