With the development of semiconductor devices, the idea of an energy Internet is being realized in FREEDM system center. The use of Solid State Transformer (SST) which makes flexible energy exchange between AC
and DC possible is an essential part of this grand plan. However, semiconductor devices like SST will bring distorted current into grid for their non-linear characters. To solve this problem, people have designed various methods to reach unit power factor and keep the grid current in a sinusoidal form. Both conventional synchronous frame PI controller and a relatively new stationary frame proportional-resonant (PR) controller have been simulated in this research project and proved effective to reach the goal if used on the rectifier part of SST. Both method reach unit power factor in approximately one line-frequency period (0.02s) and steady DC voltage state in less than 0.06s, which is three line-frequency periods
. The quasi direct power control method used in PR controller also has its potential in var generator control. Additionally, for other non-linear loads in the system, a hybrid controller with harmonics suppressing
as an active power filter is also simulated. This largely reduces the 5th and 7th harmonic current on the AC side thus improve power quality and system stability.