Study Of Grid Interconnection Based On Variable Frequency Transformer

Back-to-back high voltage direct current(HVDC) converter is a typical method to interconnect two grids. HVDC uses power electronics devices to control power flow between two power systems, and it has advantages of quick response and isolating fault. However, it also has some disadvantages, such as complex devices, harmonic interference, the risk of commutation failure and being difficult to adjust power smoothly.Because of a series of disadvantages of the HVDC, the GE Energy developed a new alternative device for interconnecting two grids: the variable frequency transformer(VFT). As a kind of mechanical and electrical equipment, the structure of VFT is solid and durable. Despite of its being easily controlled, changing the power flow smoothly, no harmonics, it can also isolate fault while it occurs on one side of the grid. Compared with the existing products, VFT has a promising future.Firstly, the mathematical model of VFT on global Cartesian is given, then an easier mathematical model—the mathematical model on dq axis is given after introducing the coordinate transformation. Then the basic structure and operating principle of VFT are detailed introduced; the mathematical function of power and torque is also derived.After introducing the control system design of DC motor, the model of the VFT interconnecting two grids is developed with MATLAB/SIMULINK. Both the feasibility of interconnecting asynchronous and synchronous grid is simulated, including the case that the power flow is changed. The results show that the VFT can interconnect two synchronous or asynchronous grid, even when the power is changed. Then, the paper simulates the behavior of the VFT in case of change in frequency in the grid to which the VFT is connected, and the results show that the VFT has a very good performance under frequency disturbances. The impact on both sides is also simulated when short-circuit fault occurs on one side, the results show that the VFT can isolate fault; At last, an experimental platform is set up to verify VFT’s ability to interconnect two grids in cases of normal operating, under frequency disturbance and power flow changing.Because of the good performance of high frequency injection at low speed, the paper introduces the principle of the high frequency injection and uses it to estimate the speed of the VFT, the model is developed with the MATLAB/SIMULINK. The speed is estimated and fed back to the DC motor control system, including the four cases: normal operating condition, frequency disturbances on one side of VFT, change in power and short-circuit to ground fault. The results show that the high frequency can be used to estimate the speed of VFT. Last, an experimental platform is built to verify the feasibility of the application of high frequency injection to VFT.Last,the paper introduces the MRAS(Model Reference Adaptive System) and a way based on stator flux-oriented to estimate the speed, then the way is used to estimate the speed of VFT in the MATLAB/SIMULINK model, also including the four cases: normal operating condition, frequency disturbances on the rotor side of VFT, change in power order and short-circuit to ground fault. The results show that the MRAS can be used to estimate the speed of VFT.