six pulse bridge rectifier

Hello,

I am having problems measuring the voltage of a 600 kW six pulse bridge rectifier with a simple multimeter .... it is runing through 3 pairs of thyristors (1800 V, 1500 A 200 =ECs typical time) the AC burs after a Current transformer (???) attached to each one passes current to thyristors in pair When I power ON at around 250 kW , it is an induction furnace solid state inverter at 600 kW - yet I am reaching just the 500 kW for the moment ... and I measure (multimeter in AC V scale) the AC bars in one end and the other to DC (upper thyristors blocks) I get respectively say 1-2 ~ 650 V 1-3 ~ 1000 V(!) and 2-3 1350 V (!!!), to the lower thyristor block from AC bars again , low voltage like

120-220 Volts DC+DC- around 1000 Volts when power of the generator is OFF , normal 400 V AC is coming through the 3 AC bars .... measuring the AC burs at the exit of the main switch from my board to the cabinet I get 500, 730, 840 Volts respectively the AC bars leading to the thyristor pairs usualy overheats, I suspect it is a thyristor triggering related problem but cant measure them correct , though in 200 Ohms none is showing short circuit

thyristors are like ...

1 3 5 2 6 2

and bus bars 1 2 3

I would apreciate if someone could enlighten me on how to measure the voltages and/or thyristors thanks !