Digital Thermal Model Compensates CT Error in Solid-State Overload Relays
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Solution Overview
Problem
Existing overload relays struggle to compensate for variations in current transformer (CT) error across the range of currents protected by a trip curve, particularly failing to accurately adjust for changes between the ultimate trip current and the locked rotor current, which limits their ability to meet calibration points and is resource-intensive in terms of processing power and component usage.
Innovation Solution
The use of two independent compensation parameters, a gain compensation parameter (K2) for the ultimate trip current and a filter update period parameter (tau) for the trip time at the locked rotor current, allows for independent adjustment of calibration points without significant computational overhead, implemented in a digital thermal model that compensates for CT error across the trip curve current axis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If digital error compensation is implemented, then CT error can be corrected, but processing power and current consumption increase
Solution Approach 1:
The patent replaces complex digital error compensation algorithms with a simplified mathematical model that uses only two adjustable parameters (K1 and K2). This substitution reduces the computational burden and current consumption while maintaining effective CT error compensation across the trip curve.
Solution Approach 2:
The patent changes the approach from full digital error compensation to a parameter-based compensation model with only two adjustable parameters. This parameter reduction significantly decreases processing power requirements and current consumption while still achieving accurate compensation at the two critical calibration points.
2Power
If analog circuits are used for error compensation, then processing power is reduced, but component cost and physical size increase
Solution Approach 1:
The patent replaces analog compensation circuits with a digital implementation using a simplified mathematical model. This substitution reduces device complexity and component count while maintaining low processing power requirements, achieving a balance between digital and analog approaches.
Data Source
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AI summary
A digital thermal model for compensating for error in a current transformer used in a solid-state overload relay. The thermal model implements a difference equation that determines using a low-pass filter two parameters corresponding to calibration points along an overload trip curve. The trip curve is adjusted at an ultimate trip current (one calibration point) independently of a trip time at a locked rotor current (another calibration point) of a motor protected by the overload relay. The ultimate trip current and trip time can be adjusted based on a motor full load current set by a user. Large CT error will cause the thermal model to adjust the trip time at the locked rotor current, increasing the range of acceptable CT error, allowing the overload relay to have a wider adjustment range.