1Improved control strategy of turbine driven doubly-fed induction generators under harmonic grid voltage conditions

Abstract: Recently, wind power industry has got rapid development with widespread concern at home and abroad. In China, it plays an important role to solve the problem of power supply in rural and remote areas without electricity. As the key to connect wind turbines with the grid, the converter's output current will directly affect the quality of power. In actual operation, because the grid voltage contains a certain low-order harmonic components, IEEE519-1992 and ERG5/4-1 predetermines that grid voltage allows no more than 5% of the 5th order and 7th order harmonic components. And with the development of wind industry, more power electronic converter devices are incorporated into the grid, which leads to the increasement of harmonic in the grid. These harmonic components may bring more current and voltage harmonics to the Point of Common Coupling (PCC) of the system. The DFIG(doubly fed induction generator)'s stator current will distort and electromagnetic torque will pulse under low order grid harmonics voltage. Therefore, a useful way to suppress the current harmonics, which comes from the DFIG under lower order harmonics grid voltage is very important. Because of the harmful of these harmonics, this paper analyzes a control method to suppress the current harmonic and power ripple. First, This paper establish mathematical model of DFIG under the 5th and 7th order harmonics grid voltage and analyze the stator current and electromagnetic torque pulsation problems caused by the harmonics distortion. Then this paper compared rotor current loop based on proportional-integral (PI) and proportional-integral-resonant (PIR) strategies of harmonic suppression of system. Based on above analysis, stator current closed loop based on resonance(R) regulator and rotor current loop based on PIR regulator is designed to suppress stator current harmonics and electromagnetic torque pulsation. This strategy did not need sub-sequence grid voltage and notch filter to extract different order harmonics and did not depend on motor parameters. The operating performance of DFIG at low order harmonics grid voltage has also improved. Finally, the control strategies was proposed to reduce the grid current harmonic components and power ripple is verified by simulation and the experiment. In experiment, when stator output at 0.5pu,THD of stator current has reduced 5.8%,5th order harmonic of stator current has reduced 81% , 7th order harmonic of stator current has reduced 27% and electromagnetic torque pulsation has reduced 50%.