Boundary Protection Of HVDC Transmission Lines Based On Generalized S-transform

Since HVDC owns the characteristics of large transmission capacity,narrow transmission corridor and good control performance,it has turned as an important means for long-distance and large-capability electricity transmission,urban power grid expansion,asynchronous grid interconnection,isolate islands and new energy power generation connection to grids.It occupies an increasingly important position in modern electric power system.Reliable and safe operations of HVDC is essential for the reliability of whole power grid.Due to long transmission line distance,various crossing climatic environment and bad operating conditions,the failure rate of transmission lines is high,which is about 50% of the DC system failures and more than 90% of the line transient failures.Therefore,a high-performance HVDC transmission line protection is important for improving the safe and reliable operation level of DC system and its associated AC system.Smoothing waves reactor and DC Filters are the filters of DC transmission system,which constitute the physical boundary of DC transmission system.In theory,12-pulse converter can only generate 12 n voltage harmonics.Therefore,the boundary element parameter should be adapted to those harmonics for filtering them and getting more ideal dc waveform.So,after filtering,the energy of 600 Hz,1200Hz and 1800 Hz components is small at the location of relay on DC transmission lines when external fault occurs.By contrast,600 Hz,1200Hz and 1800 Hz components is great because the signal is not filtered by boundary elements when internal fault occurs.The principle of boundary protection just depends on difference among those frequency components to distinguish internal faults and external faults.This paper studies a new principle of boundary protection for HVDC transmission lines based on generalized S-transform.Firstly,the principle and characteristics of S-transform and S-transform are analyzed.And then,according to specific signal input and the desired frequency extraction,Generalized S-transform can freely adapt Gaussian window function to obtain the optimal time-frequency resolution and the characteristics of different frequency components at different time slots by one calculation and extract 12/24/36 th characteristic harmonic at different time.After analyzing the normal operation of HVDC system and the external fault of line,the maximum value of characteristic harmonics that may appear at the outlet of DC converter is established,which lays the theoretical foundation for the setting calculation of protection.This paper studied a voltage characteristic harmonic based boundary protection principle for HVDC transmission lines.The operating criterion was proposed,the setting principle was given and the operating scheme was designed.Finally,the protection scheme is verified by simulation.The results from theoretical analysis and simulation demonstrated that the proposed protection scheme can correctly distinguish fault from inside or outside and select the fault pole,which is suitable for the backup protection of HVDC transmission lines.