Method and system for measuring frequency deviation of low frequency signals
AssigneeHydro-Quebec
InventorsKrishnaswamy Srinivasan, Chinh T. Nguyen
Life of this patent
Estimating the term…
Abstract
A method and a system for frequency measuring or monitoring and particularly suitable for power system frequencies in the range of approximately 0-2 KHz is disclosed. The technique is suitable for single-phase or three-phase periodic signals. Synchronism of sampling to power systems waveform and zero crossing detection are not required. The method is particularly attractive when used in conjunction with other on-line digital processing/control tasks since the interface between the power network and the computer may be shared and the fast and accurate detection of frequency variation can be beneficially used by the rapid digital processors.
Claims10
A method of measuring frequency or frequency deviation or time period deviation of a low frequency signal in the range of approximately from 0 to 2 KHz, said method comprising the steps of:
- (i) sampling said periodic signal at precise time intervals to obtain sampled values,
- (ii) converting said sampled values to provide digital value signals,
- (iii) storing a predetermined number of said digital value signals representative of at least one time period of said periodic signal, and
- (iv) deriving said frequency or frequency deviation or time period deviation by linearly interpolating two successive samples of said digital value signals to determine the time period deviation of said periodic signal and in accordance with the formula
- N=(V.sub.i-n -V.sub.i)Sgn(V.sub.i-1 -V.sub.i)
- D=.vertline.V.sub.i-1 -V.sub.i .vertline.+.vertline.V.sub.i-n+1 -V.sub.i-n .vertline.
- 1/2.alpha.=.SIGMA.N/.SIGMA.D
- f=1/(n-.alpha.).delta.
- where V.sub.i, V.sub.i-1, V.sub.i-n and V.sub.i-n+1 are digital value signals and Sgn is the sign + or -, .alpha. the time period deviation and f the frequency V.sub.i-1 and V.sub.i-n+1 are to be replaced by V.sub.i-3 and V.sub.i-n+3 in case of a three-phase multiplexed signal.
A measuring system for measuring power system frequencies in the low frequency range of approximately 0 to 2 KHz, said system comprising an analog to digital converter circuit having an input for receiving a signal to be measured, a real time clock connected to said converter circuit to provide periodic impulses for sampling said periodic signal at precise intervals, said converter circuit providing output digital value signals at said precise intervals and derived from a periodic signal at its input, memory device for storing a predetermined number of said digital values representative of the lapse time of at least a time period of said periodic signal, and operator system means for processing selected samples of said stored digital values to provide output numerator and denominator values expressed by the formula:
- N=(V.sub.i-n -V.sub.i)Sgn(V.sub.i-1 -V.sub.i)
- D=.vertline.V.sub.i-1 -V.sub.i .vertline.+.vertline.V.sub.i-n+1 -V.sub.i-n .vertline.,
- wherein V.sub.i, V.sub.i-1, V.sub.i-n and V.sub.i-n+1 are digital values and Sgn is the sign + or -, and divider to provide a resultant output from said N and D values, said resultant value being representative of the time period deviation, frequency deviation or frequency of said input periodic signal.
Dependencies are inferred from the claim text.