Modeling And Operating Simulation Of Variable Frequency Transformer

Traditional method of interconnecting two asynchronous power systems is to use a kind of high voltage direct current tran smission (HVDC) with back-to-back AC-DC converter station. In that case, the power exchange between two systems can be easily controlled, and one system could i solate fr om another when a fault occurs. The disadvantage is com plex, such as harm onic interferen ce, the risk of commutatio n failure and difficulty to achieve a smooth adjustment.VFT can control the quantity and direc tion of power transmission by adjusting the DC drive m otor torque. Com pared with back to back HVDC connection, VF T performs be tter (easily c ontrolled, sm oothly power re gulation, no harmonics and ability to feed passive network ) and costs less as well.This paper describes the basic structure and operation principle of VFT. Study in the rota ting transf ormer is also include d. Firstly , it is necessa ry to build the steady-state mathematical model of VFT, which is reduced by using the coordinate transformation m ethod .VFT m athematical model in d-q-0 coordination m akes it possible to solve the VFT voltage equa tions sim ultaneously, and derived the expression of driving torque to maintain sy stem stability by ana lysis the ac tive component and reactive com ponents of th e power transm ission respectively. VFT power transm ission is controlled by torque , when either side of grid frequency changes, it is neces sary to change its ro tation speed .Finally we do simulations in PSCAD/EMTDC to analyze the impact of motor torque o n the power transm ission and voltage of both sides, in which we use three-phase winding induction m otor model to represent VFT . Further analysis of VFT para meters variation when the short-circuit fault occurs on either side s of interconnected power system is done. The results show that the VFT has a superior characteristic of fault isolation.