Power quality and low voltage ride-through capability of induction generator-based wind power generating system
Date of Issue2014
School of Electrical and Electronic Engineering
Energy Research Group
Wind energy has become one of the most important clean energy sources all over the world. As compared to fixed speed based wind power generators, the variable speed generators obtain a much higher efficiency. Among the variable speed generators, permanent magnet synchronous generator (PMSG) is one of the commonly used wind power generator but it requires a fully rated back-to-back converter connected to the grid. The doubly-fed induction generator (DFIG) usually obtains the advantages from economic point of view because the converters are equalized to handle 20-30% of the rated power. As a result, DFIG has become one of the most widely used wind power generator nowadays. However, smaller power rating of the converters also means that the DFIG system has a smaller tolerance to voltage disturbances. When an external fault occurs, the DFIG is required to keep connected to the grid and generate reactive power. Thus, low voltage ride through (LVRT) capability for DFIG is required. However, most of the LVRT methods either loose control of the generator or significantly increase the cost of the DFIG system. Mode switch method is proposed in order to improve the LVRT performance of DFIG. The mode switch DFIG (MSDFIG) switches from the normal operation expand (DF mode) to induction generator mode (IG mode) while a grid fault is detected. In IG mode, the stator side is isolated from the grid and in that case, the transient large current caused by the sudden grid voltage drop can be avoided. Meanwhile, the rotor-side is kept connected to the grid through a back-to-back converter by which the generator is still under control and reactive power could be delivered to the grid. In order to achieve a smooth mode switching, the transient phenomenon of the generator switches from DF mode to IG mode is analyzed and a stator-side crowbar is proposed in order to contain the transient current. The resynchronization control of the generator switches from IG mode back to DF mode when the grid voltage is recovered is also developed. Analysis shows that the proposed MSDFIG can smoothly ride through the complete low-voltage and voltage recovery stages. Effectiveness of the scheme is demonstrated through simulation and experiment studies. The second contribution of this thesis is the harmonic and unbalanced problems analysis and elimination for DFIG. In the stand-alone DFIG system, harmonics and unbalanced components would appear in the voltage at PCC due to nonlinear and unbalanced loads. The distorted voltage would not only reduce the power quality at the PCC, but also be harmful for the generator. On the other hand, while the DFIG is connected in the large grid, the stator current of the generator would be distorted by the nonlinear or unbalanced loads in the grid. In order to improve the power quality and avoid harmful effects on the generator, harmonic and unbalanced components should be eliminated in stand-alone and grid connected DFIGs. Active power filter (APF) is one of the most commonly used method for harmonic elimination but it increases the cost of the DFIG system and also needs an individual controller for the APF. Thus, the compensation control from the rotor-side converter is encouraged recently. The PI controller, PI-resonant (PIR) controller and PI based repetitive controller (PIRC) for harmonic and unbalanced components elimination are analyzed. The effectiveness of PIRC is discussed in great detail and approved by simulation and experimental results. Unlike conventional power systems, nowadays some of the microgrids adopt DC distribution because of the availability of increasing number of DC output type sources such as photovoltaics and fuel cell and also loads such as laptops, computers, LED, lightening etc. In the conventional connection between the DFIG and DC microgrid (DCMG) requires a fully rated converter to transfer the AC power generated from the generator into the DCMG. In that case, the cost of DFIG would significantly increase and lose its benefits from economic point of view. A new scheme of DFIG is proposed in which the stator windings are connected to the DCMG through a three phase diode rectifier and the rotor windings are still connected to a rotor-side converter. Compared to the conventional scheme, the proposed configuration of DFIG saves the fully rated converter. But the stator-side rectifier would introduce distorted stator voltages which are uncontrollable and as a result, harmonic components would appear in the stator current. The current injected into the DCMG would also be distorted. In order to eliminate the harmonics in the stator current, PIRC is applied in the controller of rotor-side converter. For the DC current, the harmonic components are proposed to be eliminated by a harmonic compensator which is controlled by PIRC. Simulation and experimental results verify that the PIRC could effectively reduce the harmonics in both the stator currents and DC current. Meanwhile, in order to improve the efficiency of the system, the DFIG is controlled by maximum power point tracking (MPPT) and a battery energy storage system (BESS) is proposed to smooth the power flow and better load sharing under droop control.
DRNTU::Engineering::Electrical and electronic engineering::Power electronics