denote the conductors at the output sides reference numerals 40a and 40b denote a three phase power source fig1 shows the outer appearance of the first phase four stage thyristor valve comprising the units 50a 52a 54a and 56a shown in fig8 fig1 shows a two stage water cooled thyristor conversion device having modules 10 arranged in the same manner as shown in fig4 except that a cooling water pipeline network 31 is provided in this case the cooling water supplied from the cooler 20 is raised to a level between the unit 12 of the high voltage side and the unit 14 of the low voltage side through a pipe 64 from this level the cooling water is branched into the high and low voltage sides and supplied to the units 12 and 14 after uniformly cooling all the modules 10 the cooling water returns to the suction side of the pump 18 as shown in the figure the cooling water is not branched from the beginning the cooling water is first raised to high level and supplied to the units 12 and 14 therefore the cooling water is substantially uniformly supplied to the units 12 and 14 the diameter of pipes 66 68 70 and 72 extending near the conductor p of the high voltage side can be small the leakage current flowing through the conductors p m and n is smaller than that in the conventional device shown in fig1 fig1 shows a flow path of the pipeline network shown in fig1 the mode of operation of the circuit diagram in fig1 is substantially the same as that of the circuit diagram in fig5 and a detailed description thereof will be omitted.