Protection

Protection of capacitor banks has always been difficult. It is especially difficult to

sense failures inside the capacitor banks because of the configuration.

Experience indicates that most capacitor bank faults involve one or more insulator

failures with arcing across groups and/or phase-to-phase inside the bank. In most

cases, these types of faults are not seen by the bus differential or other protection

unless the arcing spills over to the area between the fuses and the circuit switcher.

A fault across an insulator usually means that one or more groups of parallel units

are shorted. This will cause a neutral shift and unbalanced phase currents.

Unbalanced phase current magnitudes are determined by the number of series

connected groups. For full phase-to-neutral flashover, the maximum phase current

is three times normal capacitor bank load in the faulted phase.

One main protection concern is overvoltage cascading. A capacitor bank is unique

in that cascading of units may take place after a predetermined number of unit

fuses have operated. Normally after a fuse has blown in any other type of equipment,

the faulted apparatus is disconnected and usually does not affect any remaining

equipment that is in service. That is not so with a capacitor bank. Each fuse that

blows to isolate the faulted unit sets up an increased voltage stress on the remaining

 units (Figure 1-1). Sometime later, the next weakest unit in that group fails.

As each successive fuse blows, the voltage increases another step and rapidly causes

the next unit to fail. Cascading takes place and results in serious damage to the

capacitor bank and possible hazards to personnel. While the capacitor bank is failing,

the station in minimally affected. The voltage is nearly normal, the current flow is almost

unaffected, and station relay protection is not taking any action until the failure has

developed into a phase-to-phase or phase-to-ground fault.

A solution was to develop a protective scheme for the capacitor bank with the main

emphasis on preventing overvoltage cascading. To do this, a ground fault relay or neutral

shift device had to be developed that was sensitive enough to detect blown fuses for both

alarming and tripping purposes. The best place to obtain the sensing information is

between the neutral of the capacitor bank and ground.

Voltage differentials between the normal capacitor bank status and that of one blown

fuse are very small.

However, BE1-59NC relays are sensitive enough to differentiate between these conditions

and act decisively.