System Frequency Detector With Dynamic Filter Time Constant
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Solution Overview
Problem
Existing system frequency detectors face challenges in quickly tracking changes in system frequency and suppressing erroneous detections, especially during system disturbances, due to limitations in filtering and phase jumps.
Innovation Solution
A system frequency detector is designed with an orthogonal coordinate signal generator and a frequency calculator that includes a moving average filter and a rate limiter, which generates orthogonal two-phase voltage signals and calculates the system frequency using PLL, while limiting frequency changes beyond a prescribed rate to prevent abrupt fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a filter of relatively long time constant (200 msec or more) is provided to acquire resistance to system disturbances, then reliability is improved, but response speed deteriorates making rapid output tracking difficult
Solution Approach 1:
The patent applies dynamics by making the filter time constant variable rather than fixed. The time constant is dynamically adjusted based on the detected system frequency: when frequency deviation exceeds a threshold, the time constant is reduced to improve response speed; when frequency is stable, the time constant is increased to maintain disturbance resistance. This resolves the contradiction by allowing the system to adapt its filtering characteristics to current operating conditions.
Solution Approach 2:
The patent changes the parameter of filter time constant based on system frequency conditions. By monitoring frequency deviation and adjusting the time constant accordingly, the system transitions between two operational states: one optimized for rapid response during frequency changes, and another optimized for disturbance rejection during stable operation. This parameter adaptation resolves the trade-off between reliability and speed.
2Speed
If PLL with DQ transformation is used to track system frequency quickly, then response speed is improved, but measurement precision deteriorates when phase jump occurs causing frequency oscillation
Solution Approach 1:
The patent introduces an intermediary mechanism - a frequency deviation detection unit and conditional control logic - that mediates between the PLL's fast tracking capability and the need for accurate frequency measurement. When phase jumps are detected (indicating potential measurement errors), the system intervenes by adjusting the filter time constant to suppress oscillations, thereby maintaining measurement precision while preserving the PLL's inherent speed advantage.
Solution Approach 2:
The patent implements feedback by continuously monitoring system frequency and phase conditions, and using this information to dynamically adjust the filter time constant. When frequency deviation exceeds thresholds or phase jumps are detected, the feedback mechanism triggers time constant adjustment to prevent erroneous frequency detection. This closed-loop control maintains measurement precision while allowing fast tracking during normal operation.
3Measurement precision
If moving average filter is applied to voltage signal in rotating coordinate system, then measurement precision is improved by suppressing disturbances, but response speed deteriorates
Solution Approach 1:
The patent makes the moving average filter's time constant dynamic rather than fixed. The filter adapts its smoothing strength based on system conditions: during frequency transitions, the time constant is reduced for faster tracking; during stable operation, the time constant is increased for better noise suppression. This dynamic adjustment resolves the contradiction between measurement precision and response speed.
Solution Approach 2:
The patent changes the filter parameter (time constant) based on detected frequency conditions. By monitoring whether the system is in a transient or steady state, the patent adjusts the moving average filter's characteristics accordingly, allowing it to provide appropriate levels of smoothing without permanently sacrificing response speed. This parameter adaptation enables the filter to serve both precision and speed requirements under different operating conditions.
Data Source
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AI summary
According to an embodiment of the invention, a system frequency detector is provided and includes an orthogonal coordinate signal generator generating an orthogonal two-phase voltage signal from a three-phase voltage signal of three-phase alternating current power of a power system by converting the three-phase voltage signal into a two-phase voltage signal orthogonal to the three-phase voltage signal, converting the two-phase voltage signal into a voltage signal of a rotating coordinate system, calculating a moving average of the voltage signal of the rotating coordinate system, and performing an inverse transformation of the voltage signal of the rotating coordinate system after calculating the moving average; and a frequency calculator including an angular frequency calculator calculating an angular frequency of the power system based on the two-phase voltage signal, and an arithmetic unit calculating a system frequency of the power system from the angular frequency, the frequency calculator further including a rate limiter provided in series with the arithmetic unit, the rate limiter limiting a change of the system frequency equal to or greater than a prescribed change rate. Thus, a system frequency detector is provided in which the change of the system frequency can be quickly tracked and an erroneous detection of the system frequency can be suppressed even when a system disturbance occurs.