Research On Control Strategy Of AC Excitation For Flexible Power Conditioner Under Symmetrical Voltage Dips

As fast development of electric power industry of our country, power grid is acomplex grid now. Moreover, the day-by-day increasing grid synchronization of powerdistribution system creates problems for the grid, like grid instability and even outage.Flexible power conditioner (FPC) which employed flywheel energy storage not only canperform multi-function in four-quadrant when used in power system, but also is easy to set.It possibly makes FPC to be one set of PSS. In this paper, it takes the model of the doublyfed induction generator (DFIG) as the background, furthermore, dynamic behavior of DFIGunder three-phase symmetrical voltage dips and control strategies of ac excitation for FPCunder three-phase symmetrical voltage dips are studied in detail.DFIG is an important part of FPC, so it is also important to establish a precise modelof DFIG. In the paper, the DFIG model in synchronously rotating reference frame isintroduced. Then, the vector control strategies are studied to control DFIG rotor side powerconverter unit in normal voltage condition.Both FPC and doubly fed wind generator system (DFWGS) are systems basing DFIG.Besides, there are similar in topological structure. Therefore, it is useful to compare twosystems. In the paper, the statuses of DFIG in the FPC and in the doubly fed windgeneration system are compared. The results show that the statuses of DFIG in the doublyfed wind generation are part of statuses of DFIG working in FPC.It is a channel to find solution understanding dynamic behavior of DFIG underthree-phase symmetrical voltage dips. In the paper, the dynamic behavior of DFIG in twomodes, stand-by mode and generator mode, during three-phase balanced voltage dips wereanalyzed theoretically and simulated in detail. The results help us to understanding thecause of the problem during symmetrical voltage dips.Basing on analysis of dynamic behavior of DFIG under three-phase symmetricalvoltage dips, the physics meaning of one method of low voltage ride-through (LVRT)technologies basing fictitious inductance was explained. And, the method was modified andused as one method of FPC low ride-through technologies. Then, two methods were introduced as FPC control strategies during three-phase symmetrical voltage dips.Simulation results are shown to demonstrate the viability of the methods. Finally, scope ofapplication of the methods was studied.