Measurement And Identification On The Impedance Frequency Characteristics Of Power Grid

With the rapid development of power electronic technology and the wide use of high power electronic devices, the harmonic pollution in power system is worsening and becomes a serious threat to the safety of power system and electrical equipment. So we need to make a reasonable assessment of the system and the user’s harmonic emission level of the public connection point, and the evaluation is mainly focus on system impedance frequency characteristics of both system side and user side. At the same time, with the widely used of LCL train in electrified railway, the harmonic resonance caused lots of damage to the electrical railway equipment, and even affect the safe operation of system. Therefore, to avoid adverse effects, it is necessary to determine the frequency characteristics of the system and master the resonance law, so that we can assess and control the system harmonic better,. This paper focuses on the system impedance frequency characteristics of power system, and the main contents are as follows.First, several existing harmonic methods are studied, including fluctuation method, regression method, switch element method, current injection method and the user-side parallel impedance method. And sum up their respective advantages, disadvantages and application scope, as well as problems that may occur in the practical application. By comparison, the harmonic current injection and regression method is selected for further analysis and optimize, because they are high accuracy, low cost and easy to implement.The scheme of the inter-harmonic current injection method is designed. Five level inverter cascade based on deadbeat control is used as an inter-harmonic generation source, and it can generate arbitrary amplitude or frequency current that needed. On the selection of disturbance source, one option is proposed, select the inter-harmonics that more closed to the fundamental and harmonic in frequency, so as to accurately calculate the harmonic impedance and find the system series-parallel resonant point. On harmonic extraction, use the simple and practical DFT analysis. Finally, the accuracy and feasibility of the scheme are verified by simulation analysis.A rank order regression method based on the median initial value is proposed. Rank regression method has good robustness to the Y direction of the data, and it improves the disadvantage of the traditional two element linear regression method. And the improved rank regression method make up the rank regression method’s shortcomings of using the least squares solution as the iterative initial, further reduce the influence of outliers on the regression calculation. By comparing the two linear regression method, the rank regression method and the improved rank regression method, the superiority of the improved rank regression method is proved. At the same time, the method is further analyzed by the measured data of projects. Results show that the calculation of improved rank regression method and the volatility method are nearly, and the Robustness of improved rank regression method is even better, which verify the practicability of the algorithm.