Digital Simulation of a Complete Sybsynchronous Convertor Cascade with 6/12-Pulse Feedback System

The subsynchronous convertor cascade is today the most favourably priced method of low-loss speed variation of three-phase electrical drives with power ratings between 0.5 and 20 MW. Having regard to the repercussions on the supply network and torque swings, the use of 12-pulse convertor arrangements has been found preferable for feeding the slip energy back to the supply network. Steady-state operation and the dynamic response of the entire drive system can be calculated in advance with high accuracy, using a completely digital simulation. The paper describes the mathematical model for the sub-systems induction motor, static frequency changer, feedback transformer and their interconnection to form the complete system as a basis for the digital computer program. Simulation calculations that have been carried out for steady-state operation with a subsynchronous convertor cascade of medium capacity are compared with corresponding measurements and exhibit very good results indeed. As an example for the calculation of dynamic phenomena in the event of a fault, the simulation of a short circuit in the network supplying a high-power subsynchronous convertor cascade is demonstrated.