Study For UHVDC Transmission Line Protection Based On Wavelet Transform

In China, energy such as solar, coal is mainly distributed in the north and west.But load center is mainly distributed in the east and south. The imbalance shows that the development of HVDC is an important part of power grid construction in China. HVDC project has got rapid development recently because of its advantages on power transmission. Remarkable achievements have been made on theory research and equipment manufacturing since 1980s when China began to design HVDC project. The operation statistical results of the present HVDC projects in China show that transmission line faults are the main reason to HVDC system outage. It also shows that the present HVDC transmission line protection has low accuracy and insufficient sensitivity under high transient resistance fault.±660 kV and±800kV HVDC project models are built by the PSCAD/EMTDC electromagnetic transient simulation software, on the basis of parameters of Ningdong HVDC Transmission System. Typical faults at AC area, DC area are simulated in this paper. Wavelet transform tools are used to extract fault characteristics. Different transient resistances are also taken into consideration. Current polarities extract by wavelet transform tools can be used to distinguish different fault positions. Simulations show that this method has high reliability on fault location.Most of the existing HVDC systems are bipolar systems. For parallel DC transmission lines, because of the affect of electromagnetic induction between two lines, faults on a line will cause transient voltage and current on the healthy one. Extensive simulations have been carried out to find out the general regularity of voltage variations. Ratio of voltage variation between fault line and healthy line are used to separate fault line from the healthy one. By using ratio of voltage variation, faults at different positions can be easily detected. Combined with the characteristic of current polarities, a new protection criterion for bipolar HVDC transmission system is proposed.Different conditions are considered in the verification of the proposed protection criterions, including short circuit faults on AC-side bus, lightning stroke and short circuit faults on dc transmission lines with different positions and transition resistances. Simulations results show the reliability of the protection principle...