Study On Traveling-wave Based Fault Location For Extra High Voltage Transmission Lines And Research On Medium-Voltage Inverter

With the development of China power grid, transmission lines with voltage of 500kV are gradually becoming the backbones in all the power groups of China. Because EHV lines are always across mountains, have long distances and are hard to find the fault locations, it is important for EHV transmission lines to quickly and accurately locate faults with aspects to their safe and economical operation. Recently, traveling-wave based fault location (TWFL) progresses rapidly, compared with conventional fault location based on fundamental components, traveling-wave based fault location, TWFL is more accurate and not apt to be affected by transition resistor, fault type, bus configuration and CT saturation, etc. In practice, TWFL adopting currents is easy to be blurred by electromagnetic noise and other interferences, while TWFL using voltages not and suits for the construction fault location system based on the total transmission network to implement information fusion and redundancy for accurate fault location. Therefore, the solution of directly extract voltage traveling-wave from existing CVTs will be valuable in field of TWFL application for no need of specially designed voltage traveling-wave transducer and change of primary connection. In the proposed high-frequency equivalent circuit of CVTs, stray capacitance between primary and secondary side of Step-Down-Transformer is considered to meet the actually fact of traveling-wave transform of CVT. A method based on wavelet transform modulus sum (WTMS) is proposed to detect the inceptive voltage traveling-waves from secondary voltages and verified by simulation and recorded fault data, then fault location scheme based on CVT output voltage is proposed. Influence of line traps on fault location based on two line ends is less enough, and scheme using both the specially designed traveling-wave transducer and CVT output voltage for fault location system is carried out.Also, traveling-wave based protection using CVT output voltage is discussed in this paper. In our study, polarity of inceptive voltage traveling-wave can be found accurately on the wavelet decomposition scales corresponding to frequency bands covering several tens kHz in both the simulation and recorded data cases by method of wavelet transform modulus maxima. The studied directional protection relaying based on polarity comparison of traveling-wave can operate correctly in both simulation and recorded data cases and shows the feasible protection scheme. Also, on the basis of study of influences of line traps, bus types and bus capacitances on inceptive traveling-waves, a fast relaying protection scheme using fault generated inceptive traveling-waves's transient energy ratio within different frequency bands of line traps is proposed considering transient energy of different frequency bands are not the same in the case of in-section and out-of-section faults at singular point of inceptive traveling-waves.From point view of practical application, new comprehensive fault location system and traveling-wave based protection component are discussed, including specially designed traveling-wave transducer, hardware polarity detection principle, high-capacity and high-speed data acquisition system, accurate GPS time clock.In recent years, along with the development of power electronics, medium-voltage inverters have attracted much attention of universities, research institute for reasons of policy induce and increases of market requirements. The second part of this paper concerns on cascaded multi-level inverters with respects to control strategy, suppression of common-mode voltage, dead-time analysis an compensation, harmonics analysis and implementation scheme, etc.Simulation system for cascaded multi-level inverters is discussed in this part, including isolated input transform, power stack, and control systems. And a non-inphase power cell modulation scheme is proposed to reduced the common-mode voltage created by PWM along with adopting 3rd harmonics injection method. Also, input and output harmonics...