Research On Design Of PLC Noise Filters In HVDC Systems

HVDC system provides a technically reliable and economically feasible method for long distance bulk power transmission and power grids interconnection. With the rapid development of power systems and the fulfillment of West-East electricity transmission projects, the HVDC technology has been widely used in China, and China has become the country with the most prosperous future for HVDC application in the world. Some special research must be done about certain characteristics of HVDC projects which are not shared with the AC systems, such as the High-Frequency (HF) interference with Open-Wire Carrier System caused by the operation of HVDC converter stations, which is unique in the HVDC systems. Under such background, the design of PLC High-Frequency interference filters is studies deeply in this paper which is organized as follows.Firstly, the HF interference noise source model which induces interference with PLC is studied in this paper. Based on the deep analysis of the basic principles on which the HF noise in PLC band is produced during the commutation process, the equivalent model of HVDC valve with snubber circuit is applied to the simulation of the GUI-GUANG HVDC project in PSCAD/EMTDC. Then spectra plots of the noise source in the frequency domain are obtained by FFT analysis of the voltage waveforms gained from the former simulation. By analyzing the spectrum characteristics of the noise source in various operating modes, the main factors influencing the level of PLC noise are found. A model from experience used to calculate the level of the noise source is modified and verified with particular explanation of the physical quantities used in the model. Based on the GUI-GUANG HVDC project, the spectra of the noise source gained from experiential model and simulation are compared with the real spectrum given in reference to draw the conclusion that the experiential model can meet the requirement of the PLC filters.Secondly, the PLC HF interference noise conducted channel in HVDC converter station is studied deeply. Based on the traditional noise source model , voltage-divide model of 12-pulse bridge is put forward with 3-pulse bridge as the basic unit of high-frequency noise generation. The voltage waveforms of some positions are simulated in PSCAD/EMTDC and the noise spectra are obtained by FFT analysis to verify the validity of the voltage-divide model. The voltage-divide model proves to be able to simulate the distribution of the noise voltage inside 12-pulse bridge. Take GUI-GUANG project in China as an example, interference noise using the two models are calculated at AC side of the HVDC project. It proves that the result from voltage-divider model is accurate through comparison between reference noise curve and calculated noise curve.Thirdly, the HF models of a series of HVDC equipments in converter stations are studied, and the converter transformer is the key research direction. Two models of the converter transformer for the HF interference calculation in the converter station are proposed. Based on the analysis of the traditional simple model, a modified model is brought out to simulate the HF and relative low-frequency performances of converter transformer at the band 1-500 kHz, considering the capacitance between windings and the earth capacitance of low-voltage winding. Above 20kHz a simplified pure capacitance model can be used to replace the modified model regardless of the leakage inductance. Besides, a method to determine the parameters of the two models is presented. Then the models of converter transformers are applied to GUI-GUANG project, and the result shows that the modified model and its simplified pure capacitance one can meet the requirement of HF interference calculation.Fourthly, the idea of the HF signal filter is introduced to design the PLC HF interference filter. Based on the HF interference noise calculation circuit, four PLC HF interference filter configurations are presented considering the impedance mismatch problem. The idea of the filer's cut-off frequency and insertion loss is introduced, and the former four filters' cut-off frequencies are deduced. A method based on the idea of the filters' insertion loss is presented to determine the main parameters of PLC HF interference filters. Meanwhile, a design method derived from the AC line trap is introduce to calculate the series tuned parameters. The design process and parameter calculation method of PLC HF interference filters are expatiated based on GUI-GUANG project. It proves the cancel of filters' shunt tuned device is viable and reasonable through the analysis of simulation results and measured data.