Synchronization Method For Distributed Grid-connected System In Non-ideal Grid Conditions

As more and more distributed power generation systems(DPGS)are connected to the public grid,the power quality requirements for DPGS grid-connected become higher and higher,which promotes the rapid development of grid-side converters.As an important part of the grid-side converter,the grid synchronous signal extraction scheme ensures that the grid-side converter can quickly and accurately extract the synchronous information of voltage amplitude,phase and frequency in various non-ideal grid conditions.In this paper,various open loop and closed loop synchronization schemes represented by zero crossing detection method and single phase locked loop(PLL)based on multipliers are described at first,and their shortcomings are pointed out.On the basis of synchronous reference system phase locked loop(SRF-PLL),the basic principle and characteristics of phase detector,loop filter and voltage controlled oscillator are analyzed,and the mathematical model of phase locked loop is established.Furthermore,the realization principle of double second order generalized integrator phase-locked loop(DSOGI-PLL)is derived and analyzed theoretically.In this paper,an improved orthogonal signal generator(TOGI)structure is proposed to suppress dc offset,because the second-order generalized integrator in DSOGI-PLL can not effectively suppress dc offset.The orthogonal signal generator is realized by integrating a first-order high-pass filter into a second-order generalized integrator(SOGI).the second order generalized integrator suppresses harmonic distortion and the first order high pass filter enables the system to eliminate dc offset.In this paper,the improved orthogonal signal generator model is derived mathematically in the complex frequency domain,and the correctness and robustness of the improved synchronization scheme are verified by simulation experiments.On the basis of the improved structure,this paper further proposes a fast synchronous phase-locked scheme(DTOGI-FPLL),which uses the characteristics of sliding mode control structure to change on demand to realize the rapid extraction of power grid voltage information such as phase,frequency and amplitude.Finally,the simulation and experiment of DTOGI-FPLL in various non-ideal power grid conditions were carried out by Matlab and DSP experimental board,and the feasibility and advantages of DTOGI-FPLL were verified by comparing the rapidity and robust performance of various existing synchronous signal extraction schemes.