Current Limiting Reactors
 
  • When a capacitor bank is energized, the bank and the network are subjected to transientís voltage and current. The severity of the effect is determined by the size of the capacitor and the network impendence.
  • The worst case occurs when a capacitor bank is energized close to a bank that is already connected. The inrush current into the newly connected bank is determined by the size of capacitor bank and the inductance between two banks. The larger the banks, and smaller the inductance between banks, the higher will be the inrush current.
  • The frequency of the inrush current is determined by the inrush current is determined by the ratio of capacitor bank reactance and the impedance between the banks. The smaller the impedance, the higher will be the frequency.
  • In most of the installations, the inductance between the banks will be only few micro-Henrys, a peak current of more than 150 times nominal current; at a frequency of more than 8 kHz can be expected.
  • Capacitor standards such as IEC 60871 state that capacitors should be able to withstand inrush currents up to 100 times nominal. The standards suggest a lower value if banks are switched frequently.
  • Large and high frequency inrush current can damage capacitors, circuit breakers and contactors. All connected equipments are subject to voltage transients and may result in sporadic malfunction or failure.
  • To avoid this problem, it is common practice to insert inrush limiting reactors in series with the capacitors.
 
 
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