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Xu T.,Zhejiang University | Weng H.,Zhejiang University | Xu Z.,Zhejiang University | Huang Y.,China Southern Power Grid Research Center | Li X.,China Southern Power Grid Research Center
Zhongguo Dianji Gongcheng Xuebao/Proceedings of the Chinese Society of Electrical Engineering | Year: 2011

In this paper, the difficulty of calculating the DC side harmonic currents in a double circuit on the same tower HVDC system was analyzed. Since the symmetrical component method was very difficult to apply in the double circuit on the same tower HVDC transmission lines, the universally applicative multi-phase coupled transmission line model and its calculating method was adopted, and this method was improved by changing the series expansion form of the transmission line admittance matrix, which avoided the increase in the series terms at high frequency. As a result, the improved method not only reduces the calculation burden but also ensures the calculation accuracy. Furthermore, in order to guarantee the convergence of the admittance matrix series in the frequency range of interest, the studied transmission line was divided into some small sections. Under the same conditions, the calculating result of the admittance matrix of a line section was compared with the result by Matlab, and the line harmonic current result was compared with the result by PSCAD/EMTDC, both show a good consistency, and this verifies the validity of the improved method. © 2011 Chinese Society for Electrical Engineering. Source


Li Z.,South China University of Technology | Wang G.,South China University of Technology | Li H.,South China University of Technology | Li X.,China Southern Power Grid Research Center | Fu C.,China Southern Power Grid Research Center
Dianli Xitong Zidonghua/Automation of Electric Power Systems | Year: 2010

Taking into account the variation of switching instants, unbalanced commutation overlaps and the nonlinear commutation currents caused by asymmetric faults, a detailed analysis of the dynamic process in the HVDC converter is given. Based on the superposition of basic component, modification component and commutation component, a three-phase voltage and current switching function model and sequence component switching function model of HVDC converter are established. Based on the model, an analysis and calculation method of harmonics in AC/DC system is proposed, in which the effects of harmonic interactions between the AC and DC system and the asymmetric operating conditions of the converter are studied. The mechanism of the harmonic interaction between AC and DC system is revealed by algebraic analytic expression. The method is used to the harmonic analysis and calculation of CIGRE HVDC system and Guiguang II HVDC system. The results are compared with those obtained by simulation using PSCAD/EMTDC software. It is shown that the proposed method is accurate and effective, and provides the analysis basis of harmonic mitigation, filter configuration and protection analysis in AC/DC system. ©2010 State Grid Electric Power Research Institute Press. Source


Xue Y.,Zhejiang University | Li P.,Zhejiang University | Xu Z.,Zhejiang University | Huang Y.,China Southern Power Grid Research Center | Li X.,China Southern Power Grid Research Center
Dianli Xitong Zidonghua/Automation of Electric Power Systems | Year: 2010

About double circuit transmission high voltage direct current (DCT-HVDC) project, there is no actual operation and design experience in the world. The previous alternating current side harmonic calculation and filter design methods are only suitable for the traditional single circuit HVDC project, and can not be used for the harmonic calculation and filter design of the DCT-HVDC project. In this paper, the technical difficulties of DCT-HVDC are analyzed, and some solutions are proposed. First, based on the characteristics of the HVDC system, the harmonic voltages on the entrance of the DC line are assumed to be zero. It decouples the harmonic current calculation process of the rectifier side and inverter side, and avoids the modeling of the DC line in the harmonic current calculation. The DCT-HVDC project can thus be reduced as two traditional single circuit HVDC projects. Second, two operation modes of the single circuit HVDC projects are synthesized as a new DCT-HVDC operation mode. In this mode, the maximal harmonic combination is picked up among different power combinations as the harmonic current sources for the performance and rating calculation, which eliminates the adverse effect of different power combinations on harmonic calculation and greatly reduces the computation burden. Third, the algorithm and basic principle of filter switching strategy are studied. At last, the validity of the proposed method is demonstrated by the engineering calculation and preliminary design of the Xiluodu-Guangdong ±500 kV DCT-HVDC project. © 2010 State Grid Electric Power Research Institute Press. Source


Xu Z.,Zhejiang University | Qiu P.,Zhejiang University | Huang Y.,China Southern Power Grid Research Center | Li X.,China Southern Power Grid Research Center
2010 International Conference on Power System Technology: Technological Innovations Making Power Grid Smarter, POWERCON2010 | Year: 2010

In HVDC systems, power converters are well known the generators of harmonics on both ac and dc side. Because of the interaction between ac and dc systems, some kinds of disturbances in the ac or dc side may cause the fundamental or second harmonic current on the dc side. When the dc loop of the HVDC system has natural resonant frequencies on the fundamental or the second harmonic frequencies, the low-order harmonic current oscillation will arise, which will damage the station equipment. In order to move the natural resonant frequencies at DC side far away from the fundamental and the second harmonic frequencies, the characteristics of the system harmonic impedance should be studied through varying the parameters of some elements of the HVDC system. In this paper, the DC loop impedance at any frequency is defined, and an approach to calculate the DC loop impedance is proposed, which is based on the test signal method and time domain simulation. Through a typical ±500 kV HVDC sample system, the impacts of some important factors on the DC loop resonant characteristics are studied. Those important factors include the short circuit ratio of the AC systems, the capacity and type of the AC filters, the short circuit voltage drop of the converter transformers, the firing angle of the converters, the transmitted power of the HVDC system, the inductance of the smoothing reactor, the number of the DC filters, the length of the DC transmission line, as well as the operation mode of the HVDC system. Furthermore, based on the proposed approach, the resonant characteristics of the DC loop of the Nuozhadu-Guangdong ±800kV UHVDC project is analyzed. The results are very useful when the DC loop resonant characteristics needs to be changed through varying the parameters of some elements of the HVDC system. ©2010 IEEE. Source


Wang G.,South China University of Technology | Li Z.-K.,South China University of Technology | Li H.-F.,South China University of Technology | Li X.-L.,China Southern Power Grid Research Center | Fu C.,China Southern Power Grid Research Center
Zhongguo Dianji Gongcheng Xuebao/Proceedings of the Chinese Society of Electrical Engineering | Year: 2010

Effective converter modeling is one of the key problems for analyzing the dynamic characteristic of the large-scale AC/DC hybrid power system. The dynamic processes of the HVDC converter is analyzed under the condition of the asymmetric AC system, involving the variation of switching instants and unbalanced commutation overlaps. Then, an advanced dynamic phasor model for the HVDC converter, which is suitable for the normal operating and fault conditions of the system, is established. Based on the Guiguang II HVDC system and the CIGRE HVDC system, the model has been tested widely for the dynamic characteristic analysis. Compared with those obtained from the PSCAD/EMTDC simulation, the results show that the model can overcome the application limitation of the traditional dynamic phasor model of the HVDC converter and has high precision and validity. © 2010 Chin. Soc. for Elec. Eng. Source

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