Research On Key Technologies Of Full Current Compensation Flexible Grounding Device

In the new urban power grid,with the expansion of the construction scale and the advancement of the progress of the overhead line into the ground,the power system’s impedance to the ground continues to decrease.The neutral point ungrounded system and the arc suppression coil grounded system widely used in the power system gradually reduce the arc suppression capability for single-phase grounding faults,cannot effectively reduce the fault current,and affect the normal operation of the power grid.With the advancement of power electronic technology,the flexible grounding compensation device can reduce the voltage of the grounding point and eliminate the single-phase short-circuit arc by actively injecting compensation current into the power grid.It realizes the function of single-phase short-circuit arc suppression and achieves the goal of coping with different operating fault states,suppressing ground faults,and ensuring the reliable operation of the power grid.The existing research results expound on the basic principle of the flexible grounding compensation system and realize the basic functions of flexible grounding.Still,the quantitative analysis of the flexible grounding compensation’s transient process and harmonic compensation problems has not been perfected.In this thesis,the full current compensation of the flexible grounding device is realized based on the research on the transient process of flexible grounding compensation and the research on harmonic compensation.The main research contents are as follows:1.In terms of the grounding system model,a quantitative analysis of the flexible grounding system is carried out,and a grounding system model including a full-current compensation flexible grounding device is established.A distribution network transfer function model,including harmonics,is established.The quantitative analysis results show that the grounded power grid system is a large phase lag and strong nonlinear system,and the stability of the flexible grounding system is greatly affected by the grounding resistance.If the flexible grounding compensation device adopts single-loop control for full current compensation,losing stability in the harmonic sub-frequency band is easy.In order to achieve stable full current compensation,it is necessary to adopt a double closed-loop system with inner loop neutral point current control and outer loop neutral point voltage control.The results of the quantitative analysis show that the harmonic current at the ground fault point is dominated by the 5th,7th,and 11 th harmonic current.When designing the full current compensation controller,it is necessary to realize the harmonic current compensation of these three frequencies.2.In terms of line parameter estimation and fault phase line selection,a power flow-based line parameter estimation method and a faulty phase line selection based on the grey correlation method are proposed for the requirement of segmented accurate compensation and the purpose of rapid ground fault diagnosis.According to the design control strategy of the fault compensation structure,the correlation simulation analysis of parameter estimation and fault compensation is carried out.By establishing the power flow model of the distribution network and substituting it into the power conversion formula,this thesis transforms the conventional power flow equation into a power flow equation without phase angle to realize the distribution line’s parameter estimation.Combining the advantages of the wavelet packet transform method and the similarity comparison method,an accurate and fast line selection method is proposed.Through the wavelet packet to achieve the location of the fault moment.Using the timeliness of the wavelet packet to quickly low-pass filter the signal,remove high-frequency interference,and amplify the characteristics of the signal,to quickly and accurately extract the transient feature in the fault waveform.Combined with the grey correlation coefficient,which represents the similarity between the dynamic change trends of feeders,it can effectively reflect the correlation between the dynamic change trends of overhead lines and cables.The similarity of the relative change trend of each feeder waveform is analyzed to realize the line selection.It also reduces the misjudgment caused by the difference in the capacitance value of cables and overhead lines to the ground.3.In the aspect of a full-current compensation flexible grounding device,a flexible grounding compensation device based on a three-level T-converter is designed and a segmented parameter setting method is proposed for the problems of harmonic compensation and ground fault variability.The hardware and software design of the full current compensation flexible grounding device is realized.In software,the fault line selection method,the current inner loop proportional control and the voltage outer loop multi-frequency proportional resonance control are implemented.A segmented parameter tuning method based on global stability,harmonic suppression capability and converter output voltage capability is also proposed.The simulation and experimental results both verify the effectiveness and rapidity of the designed full-current flexible grounding compensation device.