Method and arrangement for balancing voltages of series connection of storage units for electrical energy

A frequency converter, which comprises

• at least two energy storage units connected in series,
• at least two balancing resistor units, wherein each balancing resistor unit is connected in parallel with one of the energy storage units;
• means for determining a voltage (Utot) over all of the series-connected energy storage units;
• and
• means for determining energy storage unit voltages (Ucap) between the poles of the energy storage units, wherein the at least two balancing resistor units comprise:
• a series connection of a base resistor unit and an control resistor unit;
• and
• a switching device connected in parallel with the control resistor unit,
• wherein the frequency converter comprises:
• means for determining reference voltages (Uref) for the energy storage units based on the voltage (Utot) over all of the series-connected energy storage units;
• and
• means for controlling the switching devices to control the resistance(s) of one or more of the balancing resistor units so that the energy storage unit voltage (Ucap) of each energy storage unit is maintained within a set range of the reference voltage (Uref) for each energy storage unit.

A method of controlling the voltages of two or more energy storage units in an arrangement, which comprises

• at least two energy storage units connected in series;
• and
• at least two balancing resistor units, wherein each balancing resistor unit is connected in parallel with one of the energy storage units;
• means for determining a voltage (Utot) over all of the series-connected energy storage units;
• and
• means for determining the energy storage unit voltages (Ucap) between the poles of the energy storage units, wherein the method comprises:
• determining reference voltages (Uref) for the capacitors based on the voltage (Utot) over all series-connected energy storage units;
• and
• controlling the energy storage unit voltage (Ucap) of each energy storage unit to stay within a set range of the reference voltage (Uref) for said energy storage unit by controlling the resistance(s) of one or more of the at least two balancing resistor units comprising a base resistor unit and an control resistor unit connected in series and a switching device connected in parallel with the control resistor units, wherein controlling the resistance of the at least two balancing resistor units is done by controlling the switching devices.