Analysis Of Electromagnetic Coupling Effects For VSC HVDC With Paralleled AC Transmission System And Its Mitigation Measures

Siting of new transmission corridors has become increasingly difficult due to economical, environmental and ecological reasons. As the development of high power electronic devices and the availability of Voltage Sourced Converters (VSC) at higher voltage and power ratings, converting one alternating current (ac) line to direct current (dc) line for two circuits transmission lines, can satisfy the increasing load growth and improve the overall system reliability using an existing right of way, also can make full use of the ac network advantage. However, many issues of ac/dc line conversion need to be addressed and investigated carefully. One such aspect is that with such conversions, there is a high likelihood of ac and dc lines sharing a common right of way. This dissertation gives comprehensively insight into the induced voltage and current for interactions resulting from ac and dc transmission lines in the same corridor.1) Frequency Dependent Parameters of per-unit-length and Two Port Theory for ac/dc linesThe frequency dependent per-unit-length (p.u.l) parameter matrices of resistance, inductance, conductance and capacitance are determined by many factors, such as the ground resistivity, lighting protection angle, the number of sub-conductors, the parameters of conductors, separation distance, paralleled length between the ac and dc circuits and so on. Solving the set of distributed transmission line equations with boundary conditions can give an insight analysis for the relationship between the electromagnetic coupling and the transmission line parameters.2) Boundary Conditions Analysis of Paralleled Transmission LinesUsing the approach of switching function and modulation theory, the equivalent admittance circuit of the VSC-HVDC converter as viewed from the dc side is deduced. Solving the set of distributed transmission line equations with the equivalent circuit equations, the electromagnetic coupling between ac/dc lines can be determined precisely. Though comparing the differences between paralleled HVDC-AC systems with VSC based and line commutated converters (LCC), a useful conclusion is given for the ac/dc conversion.3) Parametric Studies for Dc Bias of Paralleled ac/dc SystemThrough solving the set of distributed transmission line equations with VSC-HVDC equivalent circuit equations, the parametric studies for dc bias of paralleled ac/dc system are conducted, including the influence of parameters of transmission lines, the VSC converters and the ac system. Results show that the separation distance and transposition are the two main factors of dc bias. Also the modulation index of VSC-HVDC converter has a surprising influence to the dc bias.4) Mitigation Measures of dc Bias for Paralleled ac/dc SystemUsing the controllability and flexibility afforded by VSC-HVDC converters, control strategies are developed to reduce the influence of ac/dc coupling. Theory analysis indicates that the magnitude modulation and the single frequency angle modulation methods are the same. Both of them are using dc and harmonic modulation control strategies. The electromagnetic transient simulation software PSCAD/EMTDC shows the effectiveness of the control strategies for two different scheme systems which infeed into ac system with transformers and infeed into load without transformers.