Fault-tolerant grid frequency measurement algorithm during transients
A system determines the frequency of grid signals corresponding to an electrical grid in real time. The system includes a transient detector that monitors a grid signal from a voltage meter or a current meter connected to the electrical grid. The system produces, in real time and at a sampling rate, a deviation signal indicative of a periodicity of the monitored grid signal. The system determines, over one or more cycles of the monitored grid signal, a measurement signal corresponding to the deviation signal. The system determines a frequency signal that corresponds a frequency estimation of the monitored signal by applying a frequency estimation when values of the measurement signal are less than a deviation threshold and maintaining the frequency signal at a constant value when values of the measured signal equal or exceeds the deviation threshold.
1. An apparatus that determines in real time frequency of grid signals x(t) corresponding to an electrical grid comprising:
a transient detector configured to:
monitor a signal indicative of a grid signal x(k) from a voltage meter or current meter connected to the electrical grid, where the grid signal is expected to include a periodic signal with a nominal frequency f o and a sampling rate f s ;
produce, in real time and at the sampling rate of the monitored signal, a deviation signal X dev indicative of whether or not the monitored signal includes the periodic signal; and
determine, over one or more cycles, a measurement signal corresponding to the deviation signal; and
a frequency estimator communicatively linked with the transient detector, the frequency estimator configured to:
determine a frequency signal fe that corresponds to a frequency estimation of the monitored signal while values of the measurement signal are less than a deviation threshold; and
maintain the frequency signal at a constant value while values of the measured signal equal or exceed the deviation threshold;
where the transient detector is configured to transmit a flag to the frequency estimator indicating that the monitored signal:
comprises a periodic signal when values of the measurement signal are less than a deviation threshold expressed as γ<thr, where γ comprises a measurement of the deviation signal and thr comprises the deviation threshold; and
exhibits a transient when values of the measurement signal equal or exceed the deviation threshold expressed as γ≥thr.
2. The apparatus of claim 1 , where the transient detector is configured to determine the measurement signal by integrating the deviation signal over one or more monitored signal cycles.
3. The apparatus of claim 2 , where integrating the deviation signal over one or more cycles is executed according to:
γ
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N
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where γ comprises the measurement of the deviation signal, k is a sample index, and N is a number of sampled values in one or more cycles that varies with estimated grid frequency.
4. The apparatus of claim 1 , where the flag is transmitted in response to a transient event comprising a phase angle jump and/or a magnitude jump.
5. The apparatus of claim 1 , where the deviation signal comprises x dev (k)=x(k+n)−x(k), where k is a sample index, and N is a number of sampled values in one or more cycles that varies with estimated grid frequency.
6. The apparatus of claim 1 , where the frequency estimation that determines the frequency signal while values of the measurement signal are less than the deviation threshold comprises one of at least:
a discrete Fourier transform-based frequency-measurement algorithm; or
a model-based frequency-measurement algorithm.
7. The apparatus of claim 1 , where the grid signal comprises a single-phase grid signal or a three-phase grid signal.
8. The apparatus of claim 1 , where the transient detector and the frequency estimator comprises a unitary part of:
a protection relay,
a distributed energy resources (DER) device,
a phasor measurement unit (PMU),
a smart meter,
a digital fault detector, or
a power quality analyzer.
9. The apparatus of claim 1 , where the transient detector and the frequency estimator comprises a unitary part of:
a solar inverter;
a power generator;
a wind turbine; or
a flexible alternating current transmission system.
10. The apparatus of claim 1 , where the transient detector and the frequency estimator comprise a plurality of software instructions stored on a non- transitory computer-readable medium executed by a processor.