Low Frequency Oscillation Of Wind-Integrated Power Systems

Nowadays, the traditional fossil energy is being gradually dried up. The development and utilization of renewable energy has become one of the global energy development direction in the future. Wind power is one of the most important renewable energy, which has been developed widely around the world. But with large-scale wind power connected to the grid, the small signal stability of power system will aslo be affected. On the one hand, the random fluctant characteristics of wind speed may result in forced power oscillation of DFIG(Doubly Fed Induction Generator), which will further leads to forced power oscillation of the system; on the other hand, the introduction of the DFIG will change the system damping characteristics, which also depends on operation modes, access site and permeability of DFIG. Therefore, research on forced oscillation frequency and negative damped oscillation of wind-integrated power systems is very necessary, which can provide beneficial theoretical basis for prevention and control of wind-integrated power systems’ low frequency oscillation in the future.Based on the per-unit value system, this paper established a 7-order model of DFIG which contains wind turbine model, two-mass shaft system model, pitch angle control system model, induction machine model, simplified rotor converter control system model and analyzed the small signal linear mathematical model of One Machine Infinity Bus containing wind farms under various wind speed, calculated the characteristic root and participation factor, concluded that there are two forced low oscillation modes separately of high and low frequency in the system which are caused by shaft system and pitch angle control system respectively. What’s more, it’s also found the damping of the former is relatively large and that of the latter is relatively small.Next, based on Matlab/Simulink simulation, this article first compared the similarities and differences between DFIG phasor model and discrete model then found that discrete model can reflect the dynamic characteri stics of DFIG more accurately. In the following, the time domain simulation of DFIG single machine infinite system forced oscillation and negative damped oscillation was realized under various wind speed. What’ more, the result is consistent with the previous theoretical analysis.Finally, a linear state equation of multi-machine systems containing DFIG is established. By eigenvalue analysis and time domain simulation, it is found that DFIG will introduced oscillation modes which are strongly associated wit h itself and have favorable damping in the system. For forced oscillation caused by the fluctuations of wind speed, the oscillation frequency is the same everywhere in the system, but the impact of DFIG on the original system oscillation modes varies with the changes of gust frequency. For negative damped oscillation, when DIFG replaced the synchronous machine for power supply of the system, the oscillation modes strongly related to the replaced synchronous machine is almost disappeared, but the oscillation modes strongly associated with other units is almost unchanged; when DFIG is bundled with synchronous machine or feeds directly into the load nodes, it also has little effect on the oscillation modes of the original system, but can make some improvement weakly. The research results of this article can provide theoretical basis for DFIG control parameters optimization as well as planning, operation and control of wind-integrated power systems.