This is mention in a book that I don't quite understand how it happens
"There are several disadvantages to mechanically switched capacitors (here they are talking about shunt caps). ... If voltage collapse results in a systems in a system breakup, the stable parts of the system may experience damaging overvoltages immediately following separation. "
Cheers,
Caps put 'across' the line (i.e. not in series with the load but in parallel) are often used to help control voltage levels when the line has to support a significant reactive load.
Now, the amount of capacitance needed varies with the amount of reactive load being supplied (i.e. more inductive load, more capacitance to compensate).
So far, so good? Now, if/when a significant portion of the load suddenly trips off, you now have too much capacitance on the line. The 'mechanical switch' will sense this and start disconnecting caps until things are back to the right voltage range. BUT, the *mechanical* switch does not react very fast.
They were basically designed to switch caps in/out as the load varied throughout a normal day of operation. So a sudden *large* drop in load means the 'voltage support' being delivered by the caps will be too much and not cut out by the switch fast enough. Until the mechanical switch 'catches up' to the change in load, too many caps are on the line and you get overvoltages.
daestrom (to understand how caps on the load end of a transmission line cause overvoltage is a different discussion, see 'four-terminal networks' or 'transmission line theory')
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