PURPOSE: To improve the reliability of an apparatus by providing each power transducer with a first circuit including a first voltage-limiting nonlinear element and a resistor and with a second circuit including a second voltage-limiting nonlinear element.

CONSTITUTION: A power converter connects four thyristors 11∼14 in series. Said thyristors 11∼14 are provided with surge-absorbing resistors 31∼34, surge- absorbing capacitors 41∼44, and balancer resistors 21∼24 to make the applied voltage to said thyristors 11∼14 constant. In this case, one end of a first circuit in which a voltage-limiting nonlinear element (TNR) 51 and a by-pass resistor 61 are connected in series between the anode A and gate G of said thyristor 11 is connected with said anode A of the thyristor 11. A diode 71 is connected between the junction of the TNR 51 and resistor 61 and a gate terminal G. Other thyristors 12∼14 are constituted in like manner. Thus, a minute current flowing through the TNRs 51∼54 normally flows through said resistors 61∼64 and not through the gate terminal.