Control And Realization Of Voltage Sag Generator (VSG) Based On Transformer Form

With the wide application of wind power in power grid, in order to improve the stability of grid, grid companies in many countries have established corresponding grid codes for wind power system connecting to grid, while one vital rule is the ability of Low Voltage Ride Through(LVRT). For the sake of simulating the capacity of LVRT, it's necessary to propose a special device called Voltage Sag Generator(VSG) to simulate the voltage fault bug, which provide a key guarantee for LVRT researching.This paper first introduced three common VSG topologies based on impedance, power electronics and transformer forms, then compared the advantages and disadvantages of three forms and confirming the transformer form as research program considering realities. Then introduced the working principle of transformer form device, and analyses of whole process of voltage failure and recovery have been presented.Research and analysis for commutated control of IGBT switches has been paid highly attention. There would be voltage or current sudden changes when IGBT was in the process of switching so that a surge voltage would appear to damage the IGBT, in order to prevent the occurrence of the above harmful phenomenon, suitable commutation control methods should be taken. Two methods were commonly used: one is by changing the control strategy to commutate, this paper has discussed the four-step commutation; the other one is by adding buffer circuit to protect it, this paper has analyzed RC buffer circuit.Simulations were made in MATLAB/Simulink software, the values of time interval for four-step commutation and R, C parameters of RC buffer circuit have been analyzed and calculated. the designing VSG of this paper could be sorted as six situation of voltage fault: 120%,100%,80%,50%,20%,0,while 100% is for normal mode, others are for fault mode. The buffer circuit would be necessary for over voltage suppression in the situation of above 80% default, because it's very difficult to solve only by the control strategy.Finally, based on the theories analyzed above, the VSG experimental system was designed, including parameters calculation, main circuit, experimental system and control system. The hardware was installed in the cabinet in a reasonable way, and the VSG experimental prototype was accomplished. Experimental results showed that the designed VSG could simulate the LVRT, reached the design requirements and could be applied in double-fed wind power converter.