It is of great significance for the grid-connected converter control to extract the grid fundamental positive

negative sequence component and each harmonic component quickly and accurately under non-ideal conditions.In this regard

a positive and negative sequence observer technology is proposed.Based on the modern control theory methods of status observation

the observer is implemented in two-phase stationary reference frame and includes the fundamental and harmonic sub-observer.It can distinguish positive and negative sequence signal with the same angular frequency.It does not require real-time trigonometric functions and is easy to realize.The effectiveness of the proposed observer is demonstrated by the comparative experimental results which show the accurate extraction for the fundamental positive

negative components and harmonic components and good dynamic performance under the unbalance and harmonic interference grid.