DETERMINING OF FILTER CAPACITANCES

A method of determining condition of filter capacitance of a filter connected to the phases of a converter and adapted to be connected to an alternating voltage source (12) and a converter. The converter comprising parallel converter legs having upper (T1, T2, T3) and lower semiconductor switches (T4, T5, T6) with antiparallel connected diodes (D1, D2, D3, D4, D5, D6) connected in series between positive and negative buses (DC+, DC-), wherein the points between the upper and lower semiconductor switches form the phases (a, b, c) of the converter. The filter comprising inductive components (L c,a , L c,b , L c,c ) and capacitors (C ab , C bc , C ca ), the inductive components being connected to the phases of the converter and the capacitors connecting the other ends of the inductive components. The method comprises charging the capacitors of the filter, controlling the semiconductor switches of the converter for short circuiting the capacitors through the inductive components while the filter is disconnected from the alternating voltage source, measuring a current or a voltage in the circuit, determining from the oscillation of the measured current or voltage the condition of the capacitors of the filter.