Basler BE1-59NC, Instruction Manual

9279400990 Rev B 

BE1-59NC General Information 

1-1 

SECTION 1 

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 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: 

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  Unit dielectric failure 

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  Capacitor bank insulator failure 

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 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.