ABB Relion 670 Series Applications Manual page 238

Section 10
Unbalance protection
parallel to each other. These elements are connected to fuses and the element groups are
shunted by an internal discharging device (resistor). The capacitor units are designed based
on the following specifications:
• Rated reactive power of the bank
• Rated voltage
• Rated frequency
C
C
C
C
C
C
IEC19000195 V1 EN-US
Figure 108: Typical capacitor unit
The main reasons for internal element failure are:
• Transients in the system (high voltage surges or high-frequency currents)
• Poor manufacturing of any capacitor unit
• Harmonics due to non-linear loads
• Excessive operating temperature
As a result of an element failure, the dielectric material in the healthy elements experiences
excessive voltage stress. This can lead to violation of the following limits of the unit as
mentioned in IEEE standard C37.99-2000:
• Exceeding the terminal voltage of the C
harmonic components.
• Exceeding the current through C
If any of the above mentioned limits of the unit is violated, the capacitor bank must be
removed from service. Otherwise, this can lead to cascading failure of capacitor units and
severe damage of the entire bank resulting in explosion or fire.
232
C C
C C
C C
C
C
C C
C C
A capacitor unit (C )
unit
unit
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Element
Branch
A capacitor group (C
above 110% of rated RMS voltage, including
unit
above 135% of the nominal RMS current.
1MRK 511 401-UEN Rev. K
)
group
IEC19000195-1-en-us.vsdx
Bay control REC670
Application manual