This paper presents a new operational strategy for a small scale wind farm which is composed of both fixed and variable speed wind turbine generator systems (WTGS). Fixed speed wind generators suffer greatly from meeting the requirements of new wind farm grid code, because they are largely dependent on reactive power. Integration of flexible ac transmission systems (FACTS) devices is a solution to overcome that problem, though it definitely increases the overall cost. Therefore, in this paper, we focuses on a new wind farm topology, where series or parallel connected fixed speed WTGSs are installed with variable speed wind turbine (VSWT) driven permanent magnet synchronous generators (PMSG). VSWT-PMSG uses a fully controlled frequency converter for grid interfacing and it has abilities to control its reactive power as well as to provide maximum power to the grid. Suitable control strategy is developed in this paper for the multilevel frequency converter of VSWT-PMSG. A real grid code defined in the power system is considered to analyze the low voltage ride through (LVRT) characteristic of both fixed and variable speed WTGSs. Moreover, dynamic performance of the system is also evaluated using real wind speed data. Simulation results clearly show that the proposed topology can be a cost effective solution to augment the LVRT requirement as well as to minimize voltage fluctuation of both fixed and variable speed WTGSs.Index Terms-Frequency converter, low voltage ride through (LVRT), multilevel converter/inverter, permanent magnet synchronous generator (PMSG), voltage source converter (VSC), wind generator.
A power packet dispatching system is expected to be one of the advanced power distribution systems for controlling electric power, providing energy on demand, and reducing wasted energy consumption. In this paper, power packet routers are designed and experimentally verified for realizing a networked power packet distribution system. While the previously developed router directly forwards the power packet to a load, the new router forwards the packet to the other router with an information tag reattached to the power payload. In addition, the new router can adjust the starting time for forwarding the received power packet to the other site, thus utilizing storage capacity integrated into the router. The results successfully clarify the feasibility of the power packet distribution network.Index Terms-Power distribution system, power packet, power routing.
This paper proposes a control strategy of doubly fed induction generators (DFIGs) with new protection schemes for enhancing fault ride through capability of wind farms composed of DFIGs and induction generators (IGs). Since the DFIGs will be stressed or overloaded in the process of stabilizing the wind farm during a grid fault, it is paramount to consider a protection scheme for the DFIG, in order to protect its power converters. Two schemes, the DC-link chopper-controlled braking resistor with the supplementary rotor current (SRC) control of the rotor side converter of the DFIG and series dynamic braking resistor (SDBR) connected to the stator of the DFIG, are proposed and compared. Merits and drawbacks of both schemes are highlighted as well. The simulation results in PSCAD/EMTDC show that the two proposed schemes can eliminate the need for an expensive crowbar switch in the rotor circuit, because both could limit the rotor current of the DFIG within its nominal value during a grid fault. Finally, considering the overall system performance, the latter is recommended.
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