Hi;
I happened to be walking past a water intake facility next to a small river today.
There is a small building which houses the pumps and probably filtering elements.

As I passed beneath the electric utility lines feeding the facility, there was first a loud "whirring" like a motor running, then a very loud "clack" which sounded like a large relay operating.
My first thought before I heard the clack was that a camera mounted on a pole was moving, but then I heard the clack and for a brief moment, thought something was about to explode on the pole.
In reality, the whirring noise was way too loud to have been the motor of a movable camera.

Looking up at the pole, I saw a bank of capacitors which are used to correct power factor.
I believe what I heard was the relay which controls the switching in and out of the capacitor bank, which is most likely operated by a timer also on the pole.

My guess is that the first noise I heard (the whirring) is directly related to the relay itself. Perhaps a motor winds a large spring, then releases the relay's armature causing a much faster switch than would occur if a simple coil and spring were used.
Are there any such relays used on power lines which would require this kind of action?
If not, my guess was just a hunch. I have never worked in the power industry, nor have I ever seen such a relay.

Can anyone suggest any other cause of the whirring sound before the relay switching?

Thanks

FW

... Can anyone suggest any other cause of the whirring sound before the relay switching?
All I can add here is that capacitors are used to balance the influence on inductive loads, caused by motors and ballast in the connected service load.
Inductive loads can be many times higher than the effective load, and capacitors are used for their property to have the opposite effect of inductive loads. When capacitors are added to a circuit the load becomes more resistive (resistive means here less phase difference between Voltage and Current).
In offices with many tube-light armatures, every second armature must have a capacitor added to the circuitry for that purpose.
I never have seen the system you describe, but your description of what is installed may be well spot-on. Most probably it is either operated manually from remote, or automatically by some measuring device.
As I indicated that capacitors have the opposite load effect to inductive loads, connecting and disconnecting them will switch very high currents. That may be the reason for the spring coil activated change-over system you described. The faster that happens, the less firework is produced.
Excellent topic. Good perception of what was happening there! Congrats!
;)

Actually the whirring sound is the capacitors charging very quickly and the cracking sound is that of the caps discharging to a definite safe point. If you ever have the opportunity to stand near a power distribution grid, they are not at all quiet. Electricity effects the materials that all of these componets are made of and the noise can be deafening at times.