Abstract: To expand the power output capacity of a power plant comprising a first and a second rectifier connected in parallel to a varying load, the output voltage of the first rectifier being essentially constant within its load current operating range, said second rectifier is controlled to supply, at said first rectifier output voltage (U1), a constant load current (0.8×I3max) that is lower than its maximum current (I3max) to shift the operating range of said first rectifier correspondingly to higher current values. Said second rectifier is controlled to supply, at a predetermined lower output (U3″) voltage, load currents between said constant load current (0.8×I3max) and said maximum current (I3max) in response to maximum current (I1max) from said first rectifier. In response to load currents that are lower than said constant load current (0.8×I3max), the output voltage is limited to a predetermined higher voltage (U3′).