Reasons for unbalanced DC filter capacitor protection action

A DC filter is a type of filter used primarily to block and short-circuit AC signals. Capacitors can be used for filtering to make the output waveform smoother. By connecting capacitors in series to improve voltage resistance and in parallel to increase capacity, the output DC is more stable. DC filter as an important equipment of converter station, installed between DC high voltage bus and neutral bus, is a means of suppressing DC side harmonics of high voltage DC transmission system.

The unbalance protection of the DC filter is used to protect the capacitors from avalanche damage caused by overstressing of the intact capacitor bank by a faulty capacitor unit. High-voltage capacitors are expensive components of DC filters that can withstand all DC voltages and most of the harmonic voltage drops on the DC line, and therefore require high sensitivity for unbalance protection. Capacitors are generally divided into two branches of equal capacity to detect faults. When the capacitor is damaged, the harmonic currents in the two branches become unbalanced. However, the harmonic components in the harmonic currents flowing through the DC filter vary with time due to the diversity of DC system operation modes and the variability of harmonic voltages and currents. The unbalance current varies when the capacitor damage is the same. In addition, under normal conditions of DC filters, the capacitance value changes due to manufacturing errors, aging of capacitor components, temperature, sunlight, and other external environmental parameters, which can also lead to unbalanced currents.