DFIG-Based Power Generation System With UPS Function for Variable-Speed Applications

Abstract

The power generation system with a doubly fed induction generator (DFIG), which can be used as an autonomous power system after the loss of mains in a distributed generation network, is described. After the mains outage, a fixed frequency and an amplitude of the output voltage are obtained, despite the variable rotor speed. For this reason, it can be successfully applied in the variable-speed wind turbines, adjustable speed water plants, or diesel engines. Moreover, the stand-alone operation of DFIG is useful in a flywheel-based high-energy rotary uninterruptible power supply system. An output voltage is controlled directly by the synchronization of an actual voltage vector with the reference vector represented in a synchronously rotating polar frame. The rotor current angular speed is obtained as a result of vectorial phase-locked loop operation. Any sensors or estimators of the rotor speed or position are unnecessary. Both amplitude and angle control loops are linear. The use of stand-alone operation in grid-connected systems requires mains outage detection. Also, the grid voltage recovery requires a method of synchronization and soft connection of a generator to the grid. The proposed methods of output voltage control, synchronization, and detection of mains loss were tested in a laboratory system.

Fault Current Characteristics of the DFIG under Asymmetrical Fault Conditions

“…） is the rotor slip in the positive dq frame, and 2 2 ω ω ω r r   （ ） . The subscript "+"and "−" indicate positive-sequence and negative-sequence components, respectively.…”

“…Figure 1 depicts the equivalent circuit model of the DFIG. (a) Equivalent circuit model of the DFIG in the positive dq reference frame rotating at the synchronous speed 1 ω ; and (b) equivalent circuit model of the DFIG in the negative dq reference frame rotating at the synchronous speed 2 ω .…”

“…It can be seen from the Figure 2, the open-loop transfer function of the control system without time delay can be presented as: 2 2…”

“…Wind power generation based on the doubly-fed induction generator (DFIG) has gained increasing popularity due to several advantages, including smaller converter ratings around 30% of the generator rating, variable speed and four-quadrant active and reactive power operation capabilities, lower converter cost, and power losses compared with the fixed-speed induction generators or synchronous generators with full-sized converters [1,2]. As the power penetration from the DFIG-based wind turbines into the grid has increased steadily in recent years, the grid codes require that the wind power generators should not be tripped from the grid but stay connected during the fault conditions [3], which means that the wind turbines have the capability of low voltage ride through (LVRT) [4][5][6].…”

Characteristics on Stand-alone Operation of a Doubly-fed Induction Generator Applied to Adjustable Speed Gas Engine Cogeneration System

“…To operate during blackout as an emergency power supply is one of its important roles for the gas engine cogeneration system. A control scheme of the DFIG as an uninterruptible power supply function is studied in reference [2]. However, the DFIG requires initial excitation source for startup during blackout.…”