PURPOSE: To improve reliability, by providing the tie-ins of cooling water for a thyristor converter each at upper and lower sections of this converter, and by making it a tie-in of feedwater and a tie-in for drain respectively.

CONSTITUTION: The cooling water discharged from a pump 16 is distributed to each module 8 from the floor of a valve hall 20, passing through an inlet distributing water pipe 12A, a main allocation pipe 10A and a distributing water pipe 11A for each thyristor module. The cooling water which cooled each module 8 is collected from a distribututing water pipe 11B for thyristor module to a main distributing water pipe 10B and led to a storage tank 13 provided on the roof of a control room 21 through an outlet distributing water pipe 12B to the ceiling of the valve hall 20. The cooling water is then discharged to a thyristor converter through a heat exchanger 14, an ion exchanger 15 and a pump 16. In this way, the quantity of flow of the cooling water communicating each module 8 is equally distributed, so that the increase of capacity of the pump, etc. can be dispensed with.