Modified cosine filters
AssigneeABB Power T&D Company Inc.
InventorsDavid G. Hart, Damir Novosel, Robert A. Smith
Life of this patent
Estimating the term…
Abstract
A high sample rate cosine filter eliminates DC components by summing them such that they sum to zero. A non-orthogonal cosine filter is also provided. When the cosine filter is applied for N=4 samples per cycle, the samples are separated by 90 degrees. However, at higher sampling rates, it is not necessary to wait for 90 degrees to estimate the phasor value. Non-orthogonal components are used to estimate the phasor value. The time delay associated with the cosine filter is reduced in the process.
Claims19
A method for deriving a phasor representation of a current or voltage waveform on a power line, comprising the steps of:
- (a) measuring time-domain samples s.sub.k of said waveform;
- (b) employing a cosine filter to obtain a first cosine term C1 on the basis of the sample s.sub.k, k=0 to N-1, wherein N is a predetermined number;
- (c) measuring an additional sample s.sub.k for k=N, wherein said additional sample is obtained less than a quarter-cycle after s.sub.N-1 is obtained;
- (d) employing the cosine filter to obtain a second cosine term C2 based on at least said additional sample;
- (e) computing a phasor representation of said waveform on the basis of C1 and C2;
- and
- (f) performing a prescribed power system function using said phasor.
A method for deriving a phasor representation of a current or voltage waveform on a power line, comprising the steps of:
- (a) measuring time-domain samples sk of said waveform;
- (b) employing a cosine filter to obtain a first cosine term C1 on the basis of the sample sk, k=0 to N-1, wherein N is a predetermined number and wherein said first cosine term C1 is of the form, ##EQU11## (c) measuring an additional sample sk for k=N, wherein said additional sample is obtained less than a quarter-cycle after sN-1 is obtained;
- (d) employing the cosine filter to obtain a second cosine term C2 based on at least said additional sample;
- (e) computing a phasor representation of said waveform on the basis of C1 and C2;
- and
- (f) performing a prescribed power system function using said phasor.
A protective relaying system for use in connection with a power system in deriving a phasor representation of a current or voltage waveform on a power line, comprising:
- (a) means for measuring time-domain samples s.sub.k of said waveform;
- (b) means for employing a cosine filter to obtain a first cosine term C1 on the basis of the sample s.sub.k, k=0 to N-1, wherein N is a predetermined number;
- (c) means for measuring an additional sample s.sub.k for k=N, wherein said additional sample is obtained less than a quarter-cycle after s.sub.N-1 is obtained;
- and
- (d) means for employing the cosine filter to obtain a second cosine term C2 based on at least said additional sample;
- and
- (e) means for computing a phasor representation of said waveform on the basis of C1 and C2.
A protective relaying system for use in connection with a power system in deriving a phasor representation of a current or voltage waveform on a power line, comprising:
- (a) means for measuring time-domain samples s.sub.k of said waveform;
- (b) means for employing a cosine filter to obtain a first cosine term C1 on the basis of the sample s.sub.k, k=0 to N-1, wherein N is a predetermined number and wherein said first cosine term C1 is of the form, ##EQU17## wherein N is the number of samples per cycle, .psi. is the angle between samples and .delta. is a predetermined offset angle;
- (c) means for measuring an additional sample s.sub.k for k=N, wherein said additional sample is obtained less than a quarter-cycle after S.sub.N-1 is obtained;
- and
- (d) means for employing the cosine filter to obtain a second cosine term C2 based on at least said additional sample;
- and
- (e) means for computing a phasor representation of said waveform on the basis of C1 and C2 as follows:
- ##EQU18##
Dependencies are inferred from the claim text.