Research On Detection And Analysis Algorithms Of Microgrid Electrical Signals

With the global energy crisis and environmental issues being exacerbated,renewable energy has been widely concerned and rapidly developed.Connecting the distributed power supplies to the grid in the form of micro-grid is generally considered to be an efficient way of using renewable energy.However,the stable operation of the micro-grid depends on the real-time control of the central controller,which has high requirements on the real-time performance and precision of the signal acquisition and analysis.Meanwhile,the distributed power supplies in the micro-grid are connected to the micro-grid through power electronics,which brings serious power quality prob-lems and increases the difficulty of signal acquisition and analysis.In this paper,the acquisition and analysis of electrical signals in the micro-grid is studied,and the following problems are solved:The existing windowed interpolation algorithms are numerous and inconsistent,and cannot meet the real-time control requirements;The existing frequency domain algorithms have low estimate accuracies for single-phase signals in short records;The existing frequency domain algorithms fail to estimate three-phase signals with multi components overlapped;Methods based on Kalman fil-tering have difficulty in providing fundamental frequency estimation directly.The details can be divided into the following four parts:1)Among the frequency domain methods for harmonic analysis,the Hann-windowed interpolation algorithms are widely applied.Some of the implementations have similar structures,with merely different interpolation point numbers.By using the relationship between Hann and Rectangular windows,this paper transforms two known Hann-windowed algorithms into their equivalent un-windowed forms,discovers their common features and develops a generalized un-windowed p-point algorithm.Combined with the improved DFT recursive formula,this paper obtains a recursive version of the un-windowed p-point algorithm.The computational complexity of the algorithm is only linearly related to the interpolation point p,so it can reduce the computational complexity and improve the real-time performance.2)The commonly used frequency domain analysis algorithm will not have an accurate estimation result for single-phase signals in short records with multi components overlapped.This is because the fundamental component of the single-phase signal contains positive and negative frequency parts with the same amplitudes,therefore the leakage of the negative frequency part on the posi-tive one cannot be ignored even if the signal is windowed.Especially for single-phase signals in short records with DC component,the positive and negative frequency parts of the fundamental component and the DC component overlap each other,making the estimation a difficult task.For the above reasons,this paper presents a precise analysis algorithm for single-phase signals in short records,which takes the fundamental component’s positive and negative frequency parts and the DC component into consideration and improves the parameter estimation accuracies of single-phase signals in short records.3)In three-phase signals with multi components overlapped,the frequencies of overlapping com-ponents are non-zero and independent of each other,and some components have non-zero damping coefficients.These characteristics fail to meet that of single-phase signals in short records and make the estimation accuracies of the above algorithm and commonly used frequency domain algorithms dissatisfied.To solve this problem,this paper proposes an algorithm which applies the Prony’s method in frequency domain to estimate overlapping components of damped multi-frequency sig-nals.Based on the property of Hann window’s DTFT,the estimation of complex frequencies can be transformed to an equation group of higher degree with two unknowns,and can be further trans-formed to the estimation of the coefficients of a difference equation according to Prony’s method.Rewriting the difference equation into the matrix form,the number of overlapping components is obtained by singular value decomposition of the data matrix.According to the number of overlap-ping components,different algorithm coefficients are provided to estimate the complex frequency of each component,which improves the estimate accuracy for signals with multi components over-lapped.4)When estimating parameters of micro-grid electrical signals with large and fast fluctuations,the commonly used frequency domain algorithms have large computational delays,and the traditional Kalman filtering based algorithms only establish a Kalman filter for the amplitude and phase of the harmonics while using other algorithms to estimate the fundamental frequency.In this paper,a dual Kalman filter based method is proposed for the time domain analysis of three-phase grid signals with high precisions.The method sets up two Kalman filters,one of which works for the estimation of harmonics’ amplitude and phase,and the other works for the estimation of fundamental frequency.The relationships between these two Kalman filters are described.The state estimate of the former filter can be used to calculate the measurement of the latter one,and the state estimate of the latter filter provides the model frequency for the former one.The proposed method is effective for signals with abrupt changes of amplitude,phase and frequency,with continuous changes of amplitude and frequency,and under unbalanced conditions.In this paper,simulations and experiments are carried out in Zhoushan Island micro-grid demonstration project and Zhejiang University micro-grid laboratory experiment platform.Some of the results are applied to monitoring terminals of electric power equipments in the island micro-grid.