A power apparatus for a high voltage electrical power system, that includes a voltage source converter and a high voltage dc power source including one or more strings having a plurality of dc power source members connected in series, and switches configured to connect and disconnect the strings, where the switches are solid-state switches, each string is subdivided into a plurality of dc power source units each dc power source unit including a plurality of dc power source members connected in series and each dc power source unit is provided with one of the solid-state switches configured to connect and disconnect the dc power source unit, and that all solid-state switches in the string are arranged so that they are turned on and off simultaneously.