Inverter Load Impedance Estimation for Stability
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Solution Overview
Problem
Conventional inverter control methods, such as dead-beat control, sliding mode control, and repetitive control, face challenges in eliminating harmonic distortion with non-linear loads and require complex mathematical derivations, and are not adaptable to changes in load impedance, leading to system instability and unsuitable feedback compensation.
Innovation Solution
A load impedance estimation and repetitive control method that predicts next-period switching duty cycles for a three-phase four-wire inverter by sampling phase voltages, calculating voltage compensations, and incorporating inductance variations into the load impedance estimation matrix equation, allowing for steady output voltage even with changes in connected loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional DQ transformation method is used for inverter control, then the control design is simplified, but the system becomes inadequate when inductance varies with output current, leading to system divergence
Solution Approach 1:
The patent transforms the control approach by changing the mathematical parameters from conventional DQ transformation to a new transformation method based on actual inductance values. The state equations are derived considering variable inductance L(i) that changes with output current, and a new transformation matrix is constructed to accommodate this parameter variation, thereby maintaining system stability without excessive complexity
Solution Approach 2:
The patent introduces dynamic adaptation by making the transformation matrix dependent on the actual inductance values that vary with operating conditions. The controller continuously adapts to changing inductance parameters through the derived state equations, allowing the system to maintain stability across different operating points rather than relying on fixed parameter assumptions
2Reliability
If feedback compensation is designed for a specific load, then the controller works well for that load, but it becomes unsuitable when a different load is connected
Solution Approach 1:
The patent creates a universal control method that can handle different load types by deriving state equations that explicitly account for load impedance characteristics. The transformation matrix and control algorithm are designed to accommodate variable load conditions through load impedance estimation, making the controller adaptable to different load scenarios rather than being optimized for a single specific load
Solution Approach 2:
The patent incorporates feedback mechanisms through load impedance estimation that continuously monitors and adapts to changing load conditions. By estimating load impedance from measured voltages and currents and feeding this information back into the control algorithm, the system automatically adjusts to different load types, maintaining performance across varying operating conditions
3Device complexity
If Park Transformation is used to transform RST or RSTN coordinates to DQ or αβγ coordinates, then the transformation is mathematically straightforward, but harmonic compensator or predictor must be added to compensate for inadequate transformation of signals containing harmonic components
Solution Approach 1:
The patent extracts and separately handles the harmonic components by deriving state equations that explicitly account for their presence. Rather than relying on Park Transformation that assumes fundamental sinusoidal functions, the new transformation method separates and processes harmonic content through the derived state equations, eliminating the need for additional harmonic compensators or predictors
Data Source
AI summary
The present invention provides a load impedance estimation and repetitive control method capable of allowing inductance variation for an inverter, wherein the method is applied for predicting corresponding next-period switching duty cycles for four switching member sets of the inverter by way of sampling three phase voltages and calculating next-period voltage compensations based on the previous line-period voltage compensations. Moreover, during the calculation and prediction, the method also involves the inductance variations of the output inductors of the inverter into the load impedance estimation matrix equation. Therefore, the three phases four wires inverter with the presented load impedance estimation and repetitive control method can provide a steady output voltage to the loads even if the originally-connected loads are replaced with other different loads. Thus, this load impedance estimation and repetitive control method can indeed improve the drawbacks of the inverter controller based on conventional DQ transformation method.