Design method of low-pass filter for digital protection of smart substation

Abstract

A digital low-pass filter design method based on Tukey window function is used to limit the original signal frequency band in the digital protection of smart substations to prevent frequency aliasing in the interpolation and extraction process. The extraction algorithm cooperates with the protection algorithm that directly uses the sampled value criterion. First determine the time domain width of the Tukey window function 2Tc, and then determine the cut-off frequency fc=1 / Tc; fix fc unchanged, and then according to the sampling frequency fs, the maximum length of the low-pass filter can be determined as nmax=2fs / fc, whichever is lower The length of the pass filter is N, and N≤nmax is satisfied, and finally the coefficient of the designed filter is: The filter designed according to the method of the present invention is superior to the filter designed by the traditional window function method in both group delay and transient delay performance.

Description

technical field [0001] The invention belongs to the field of electric power system relay protection, and in particular relates to a digital low-pass filter design method suitable for intelligent substation relay protection. Background technique [0002] The information collection of the smart substation is fully digitalized. The merging unit is used as the data source. The sampling value sequence provided to the digital protection is generally fixed at a sampling rate of 4 kHz. The protection device needs to down-sample it to the required sampling rate, so a down-sampling rate link. The down-sampling process includes interpolation and decimation operations. In order to prevent frequency aliasing in the interpolation and decimation process, before the sample value sequence enters the down-sampling process, a digital low-pass filter is needed to limit the original signal frequency band. [0003] In addition, the phasor fast extraction algorithm based on the sinusoidal signal ...

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Problems solved by technology

[0017] In order to obtain ideal amplitude-frequency characteristics and filtering effects, when designing filters with the window function method, it is often necessary to select a larger N , which will result in τ g =( M -1) / 2 The filter group delay determined is too large, and when a fault occurs or the electrical quantity changes suddenly, the transient time delay of the filter is too long, which affects the quick action of the relay protection

when N When a smaller value is selected, although the group delay can be reduced, the ability to filter out higher harmonics will decrease accordingly

It can be seen that it is difficult to coordinate the contradiction between filtering effect and response speed in designing low-pass filter by traditional window function method.

[0018] In addition, when designing a filter with the traditional window function method, the filter length is first given M , and it is fixed during the design process, which is not suitable for digital protection that needs to flexibly adjust the filter length according to specific filtering requirements

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