Basler Electric BE1-59NC - Manual (Page 9)

SECTION 1
• GENERAL INFORMATION
DESCRIPTION
BE1-59NC Neutral Overvoltage Relays provide sensitive protection for capacitor banks. There are three
common types of capacitor bank failures that BE1-59NC Neutral Overvoltage Relays recognize. They are:
• Unit dielectric failure
• Capacitor bank insulator failure
• Blown fuses
BE1-59NC Neutral Overvoltage relays protect for overvoltage due to internal voltage shifts that occur as a
result of these types of failures.
APPLICATION
Capacitor banks are widely used by utilities to maintain specified system voltage. Addition of capacitive
loads at appropriate points on the system compensate for heavy inductive loading that normally tends to
reduce voltage. This adding of leading megavars to compensate for the lagging megavar component of
electric loads is frequently referred to as power factor correction. Capacitor banks must be switched in
response to actual load conditions in order to obtain maximum power factor correction benefits.
Capacitor Bank Switching
One of the common methods of maximizing capacitor bank benefits is by evaluating the bus voltage. A
bandwidth surrounding the desired bus voltage level is established. When the bus voltage falls below the
bandwidth level, the capacitor bank is switched into the circuit. When the bus voltage rises above the
bandwidth level, the capacitor bank is switched out.
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 Neutral Overvoltage relays are sensitive enough to differentiate between these
conditions and act decisively.
9279400990 Rev D BE1-59NC General Information 1-1