The ITER modular converter system is composed of 20 type A units and 14 type B the first type is rated for 2 kV, 45 kA steady state and the second for 1.5 kV, 45 kA. The basic 12 pulse unit is composed of two subunits, four quadrants operating, connected in parallel by choke inductors. The system is rated for a total installed power of about 2600 MVA the ac power is delivered by the HV Grid via a HV substation and the voltage is reduced to an intermediate level (69 kV) via four step-down transformers.

The design specification of the units requires bypass operation performed through the bridge thyristor, which will be utilized both for protection purposes and for reactive power (Q) reduction. In fact the reference control strategy, proposed by the JCT, defines that for each group of series connected units, only one converter regulates the output voltage and the others supply fully positive or negative voltage or operate in bypass control.

The bypass operation was analyzed in detail to define the time intervals in which the unit can be commanded to bypass and those in which can resume to regulate. In particular it was studied the current unbalance between the two subunits in correspondance of the turning into bypass operation and it was derived an approach to limit it. These analysis were carried out both with analitical calculations and by means of numerical simulations of the unit operation utilizing ATP and PSCAD/EMTDC programs. The results of these analyses provide also the data for the design of the control of the group of series connected units which feed each coil. It will consist of a regulator which will receive the total coil reference voltage and the feedback signal, followed by a block which will provide to distribute the reference voltages to the units and to produce the bypass operation commands. The fully positive or negative values of the reference signal of each unit will be calculated in dependance of the coil current and the medium voltage values.

The results of the analyses and the details of the design of the control system will be described in the paper and the istantaneous waveforms of the output voltage and current of a group of four units obtained by means of computer simulation will be shown.