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. "
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
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
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
(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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