Circuit Breaker Frequency Detection Using FFT and Reference Waveforms
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Solution Overview
Problem
Conventional circuit breakers face challenges in accurately detecting current frequency due to distorted waveforms, presence of harmonics, DC offset components, and low-frequency components, which can lead to measurement errors and incorrect detection of zero crossing points.
Innovation Solution
A circuit breaker design that utilizes a current transformer, reference waveform storage, multiplication unit, and Fourier transformer to convert multiplication data into frequency components, allowing for the detection of AC current frequency without relying on zero crossing points, by identifying the reference frequency corresponding to the largest DC component.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If frequency detection is performed based on zero crossing points of current waveform, then frequency can be calculated, but measurement accuracy deteriorates when current waveform contains harmonics causing multiple zero crossing points
Solution Approach 1:
The patent extracts the fundamental frequency component from the distorted current waveform containing harmonics by using a band-pass filter tuned to the expected frequency range (40-60Hz). This separates the useful fundamental component from harmful harmonic components, allowing accurate frequency detection without being affected by multiple zero-crossing points caused by harmonics.
Solution Approach 2:
The patent introduces a voltage waveform as an intermediary reference signal. By detecting the zero-crossing point of the voltage waveform (which is not affected by load current harmonics) and using it as a reference timing, the system can accurately determine the frequency without being influenced by the distorted current waveform's multiple zero-crossing points.
2Measurement precision
If frequency detection relies on zero crossing points, then period calculation is possible, but detection fails when current waveform lacks zero crossing due to DC offset
Solution Approach 1:
The patent removes the DC offset component from the current waveform by passing it through a coupling capacitor or using software-based DC blocking. This extraction of the harmful DC component restores the AC waveform's zero-crossing characteristics, enabling frequency detection to proceed normally even when the original waveform had no zero-crossing points.
Solution Approach 2:
The voltage waveform serves as an intermediary that provides reliable zero-crossing information independent of the current waveform's DC offset. By using voltage zero-crossing as a reference, the system can determine frequency without requiring the current waveform to have zero-crossing points, thus maintaining detection capability under DC offset conditions.
3Measurement precision
If zero crossing detection is used for frequency setting, then frequency can be determined, but measurement accuracy deteriorates when zero crossing continuously changes due to low frequency components
Solution Approach 1:
The patent extracts only the fundamental frequency component (40-60Hz) from the current waveform using a band-pass filter, removing low-frequency components that cause zero-crossing point fluctuations. This filtered waveform has stable, predictable zero-crossing points that accurately reflect the true frequency without being influenced by low-frequency disturbances.
Solution Approach 2:
The voltage waveform acts as a stable intermediary reference that is not affected by low-frequency components in the current waveform. By synchronizing frequency measurement with voltage zero-crossing points rather than current zero-crossing points, the system achieves stable and accurate frequency measurement even when current waveform zero-crossing points continuously shift.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables accurate detection of AC current frequency even in the presence of harmonics, DC offset, and low-frequency components, reducing measurement errors and eliminating the need for zero crossing point detection.
Implementation Method 1
a current transformer (14) configured to output a current signal in accordance with the AC current flowing along the electric path (1)
Implementation Method 2
a Fourier transformer configured to perform Fourier transformation on each piece of multiplication data obtained by multiplication performed by the multiplication unit, and to thereby convert the multiplication data into frequency components
Data Source
Figure 1
Figure 2(a)~2(c)
Figure 3(a)~3(c)
AI summary
A circuit breaker 30 includes a current transformer 14 that outputs a current signal in accordance with an AC current flowing along an electric path 1; a reference waveform storage part 15 that has stored, in advance, reference waveforms of reference frequencies that are stored in association with a plurality of reference frequencies, one of which is equal to the frequency of the AC current; a multiplication circuit 8 that multiplies a current waveform output from the current transformer 14 by each of the reference waveforms stored in the reference waveform storage part 15; an FFT circuit 7 that performs fast Fourier transformation on each piece of multiplication data obtained by multiplication in the multiplication circuit 8 and that thereby converts the multiplication data into frequency components; and a comparison determination circuit 10 that determines whether a DC component is included in each piece of multiplication data converted into frequency components by the FFT circuit 7 and that detects a reference frequency corresponding to multiplication data including a largest DC component, as the frequency of the AC current flowing along the electric path 1.