SFC Output Transformer Differential Protection Against Frequency Variation
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Solution Overview
Problem
Conventional differential protection methods for output transformers in static frequency converter (SFC) systems are ineffective due to frequency variations, leading to low sensitivity in fault detection and reliance on instantaneous overcurrent protection, which is harmful to the protected device.
Innovation Solution
A frequency-conversion differential protection method that calculates amplitude values of differential and restraint currents using a generator start-up and shutdown algorithm, unaffected by frequency, and implements biased differential characteristics to adapt to large-scale frequency variations, improving fault detection sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional differential protection algorithms based on power frequency current are used, then the protection function can be applied to conventional transformers, but it cannot be applied to the output transformer of SFC system due to variable frequencies
Solution Approach 1:
The patent changes the fundamental parameter basis from power frequency current to frequency-independent current amplitude calculation. By using the zero crossing point integral algorithm and current peak value determining method, the protection algorithm calculates current amplitudes without relying on fixed frequency assumptions, enabling it to adapt to the variable frequency conditions of SFC output transformers while maintaining reliable protection functionality.
2Measurement precision
If instantaneous overcurrent protection is used as primary quick protection, then the protection system is simple to implement, but the sensitivity is low and it is harmful to the protected device
Solution Approach 1:
The patent replaces the crude instantaneous overcurrent protection mechanism with a sophisticated differential protection system that uses biased differential characteristics. This substitution enables precise fault detection through differential current calculation while the biasing mechanism prevents false operations during external faults, thereby improving sensitivity without causing harm to the protected transformer.
Solution Approach 2:
The patent introduces restraint current as an intermediary element that mediates between the differential current and the protection operation. The biased differential characteristic uses the restraint current to modulate the protection response, allowing high sensitivity for internal faults while maintaining security against external faults, thus avoiding harmful false tripping.
3Measurement precision
If differential protection is implemented for output transformer, then the sensitivity of fault detection is improved, but the complexity of protection algorithm increases due to frequency variation
Solution Approach 1:
The patent extracts the frequency-dependent components from the protection algorithm by using frequency-independent calculation methods. The zero crossing point integral algorithm and current peak value determining method separate the amplitude calculation from frequency variations, simplifying the overall algorithm complexity while maintaining high fault detection sensitivity.
Data Source
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AI summary
In a frequency-conversion differential protection method for an output transformer of a static frequency converter (SFC) system, a protection device measures the three-phase current on each side of the output transformer of the SFC system; according to the connection manner of the output transformer, triangle side is used as a reference to perform phase correction on the star side; at the same time, in consideration of the secondary rated current on each side of the transformer are different, a balance coefficient on each side is adjusted, to calculate sampling values of correcting current on each side and the differential current; a generatorstart-up and shutdown protection algorithm which unaffected by frequency is used to calculate amplitude values of the correcting current, the differential current, and the restraint current; and frequency-conversion differential protection for the output transformer is implemented by using biased differential characteristic and according to magnitudes of the differential current and the restraint current. The protection method adapts to a large-scale frequency change, and compared with instantaneous overcurrent protection, greatly improves the sensitivity of detection on an internal fault of the output transformer.