Digital Filter for CVT Transient Error Compensation
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Solution Overview
Problem
Capacitive Voltage Transformers (CVTs) introduce significant transients into voltage signals, leading to distorted measurements that can underestimate fault magnitudes, compromising the accuracy of protective relays in power systems, especially in high voltage and extra high voltage systems where Source Impedance Ratio is large.
Innovation Solution
A digital filter is designed based on an equivalent circuit diagram of the CVT, using a three-parameter model to correct for transient errors, with self-tuning mechanisms to adjust filter coefficients in response to system disturbances, ensuring accurate voltage signal reproduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a CVT is used to reduce voltage in high voltage and extra high voltage power systems, then the cost is reduced compared to magnetic voltage transformers, but the CVT introduces significant transients that distort voltage signals and compromise measurement accuracy
Solution Approach 1:
The patent introduces a digital filter as an intermediary component between the CVT and the protective relay. This filter processes the distorted voltage signal from the CVT, removing transient components and reconstructuring the signal to accurately represent the primary system voltage, thereby eliminating the measurement accuracy problem while preserving the cost advantage of using CVTs
Solution Approach 2:
The patent changes the parameters of the voltage signal by applying digital filtering operations. The filter modifies the signal's frequency spectrum, attenuating transient components and preserving the fundamental frequency components, thus transforming the distorted CVT output into an accurate voltage representation for protective relaying
2Measurement precision
If filter coefficients are fixed based on CVT parameters, then the filter can remove distortion effectively, but the filter cannot adapt to changes in CVT parameters or system conditions
Solution Approach 1:
The patent implements a feedback mechanism where the digital filter continuously monitors the voltage signal and adjusts its coefficients in real-time based on detected signal characteristics and system conditions. This feedback loop enables the filter to maintain optimal performance despite changes in CVT parameters, system impedance, or operating conditions, resolving the contradiction between fixed coefficients and adaptability
3Ease of operation
If the Fourier algorithm is used for voltage measurement, then the measurement process is simple and widely applicable, but the magnitude is significantly underestimated due to CVT transients
Solution Approach 1:
The patent applies preliminary action by placing the digital filter before the Fourier algorithm in the signal processing chain. The filter pre-processes the CVT output signal, removing transient components that would otherwise distort the Fourier analysis. This preliminary filtering enables the subsequent Fourier algorithm to operate on clean signal data, maintaining measurement simplicity while achieving accurate fault magnitude determination
Data Source
AI summary
A method of digitally correcting the raw output voltage from a Capacitive Voltage Transformer (CVT) with the intent to remove transient components impacting on transient accuracy of protection function. A typical CVT is represented using three parameters in the linear CVT model. A digital filter designed based on the three parameters and incorporating a dedicated mechanism to ensure numerical stability of the former. A method of self-adjusting the said filter based on system events and performed after the method has been deployed in the field and supplied from a specific CVT.