Power System Frequency Detection During Phase Jumps
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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.
Innovation Solution
A system frequency detector that includes an orthogonal coordinate signal generator and a frequency calculator. The orthogonal coordinate signal generator converts three-phase voltage signals into orthogonal two-phase voltage signals, calculates a moving average, and performs an inverse transformation. The frequency calculator calculates the angular frequency using proportional-integral control and includes a rate limiter to limit frequency changes, a prediction calculator to predict angular frequency changes, and a switching circuit to manage input states based on phase jumps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a filter with a long time constant (not less than 200 msec) is used to acquire resistance to system disturbances, then the reliability of frequency detection is improved, but the speed of tracking system frequency changes deteriorates (delay not more than 40 msec cannot be achieved)
Solution Approach 1:
The patent applies dynamics by making the filter time constant adjustable rather than fixed. The time constant is dynamically changed based on the operating state: set to a longer value (200 msec or more) during normal operation for stability, and shortened during frequency changes for rapid tracking. This resolves the contradiction by allowing the system to adapt between reliability and speed requirements depending on conditions.
Solution Approach 2:
The patent changes the parameter of filter time constant based on system state. By detecting whether the system is in a normal state or a frequency change state, the controller adjusts the time constant parameter accordingly - using a longer time constant for disturbance resistance during normal operation, and a shorter time constant for rapid tracking during frequency transitions.
2Speed
If PLL with DQ transformation is used for frequency calculation, then the tracking speed of system frequency changes is improved, but the reliability deteriorates (great oscillation occurs when phase jump occurs in system voltage)
Solution Approach 1:
The patent introduces an intermediary mechanism - a phase jump detection unit and switching control - that monitors system conditions and determines when to use which frequency detection method. When phase jumps are detected, the system switches to the zero-crossing detection method which is more reliable under such conditions, thereby resolving the reliability issue while maintaining the speed advantages of PLL during normal operation.
Solution Approach 2:
The patent dynamically switches between two frequency detection methods (PLL and zero-crossing detection) based on system conditions. During normal operation, PLL provides fast tracking. When phase jumps occur, the system dynamically transitions to zero-crossing detection for reliable measurement, thus resolving the contradiction between speed and reliability.
3Reliability
If zero crossing point detection is used for frequency calculation, then the reliability is improved (resistance to system disturbances), but the tracking speed of system frequency changes deteriorates (rapid output tracking with delay not more than 40 msec is difficult)
Solution Approach 1:
The patent dynamically adjusts the filter time constant based on system state. During normal operation, a longer time constant (200 msec or more) provides disturbance resistance. When frequency changes are detected, the time constant is shortened to enable rapid tracking with delay not exceeding 40 msec, thus resolving the time loss issue while maintaining reliability when needed.
Solution Approach 2:
The patent employs periodic switching between different filter settings based on detected system conditions. The filter time constant is periodically adjusted - using longer settings for stability during normal operation and shorter settings for rapid response during frequency transitions, thereby balancing reliability and tracking speed.
Data Source
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Figure 4A~4G
AI summary
According to one embodiment, a system frequency detector is provided. The system frequency detector 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; 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 includes a prediction calculator calculating a predicted value of the angular frequency after a prescribed period of time has elapsed based on the angular frequency and a differential of the angular frequency. In a state in which the phase jump of the power system is not detected, the frequency calculator calculates the system frequency based on the angular frequency. When the phase jump of the power system is detected, the frequency calculator calculates the system frequency based on the predicted value for a constant amount of time. Accordingly, 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.