PV Micro-Inverter Off-Grid Control Using a Lie Group Controller
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Solution Overview
Problem
Existing control systems for PV micro-inverters face challenges in off-grid operation due to intermittency of solar power, sensitivity to output impedance, and instability under transient conditions, particularly in systems without significant energy storage, leading to increased costs and limited functionality.
Innovation Solution
A control system for DC/AC inverters using a geometric Lie Group controller for off-grid operation, which generates a frequency reference signal through a Lie Group controller and integrates it to produce a phase angle reference, allowing robust regulation of AC voltage and current without the need for external communication or smart switches, enabling parallel operation of multiple inverters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If droop control techniques are used for parallel operation of inverters, then system reliability and ease of operation are improved, but sensitivity to output impedance and instability under transient conditions worsen
Solution Approach 1:
The patent transforms the control approach by changing the mathematical parameters and control law structure. Instead of traditional droop control with fixed impedance sensitivity, the invention uses a nonlinear control law with adaptive parameters that automatically adjust to system conditions, eliminating the trade-off between reliability and stability.
Solution Approach 2:
The patent replaces the mechanical/electrical droop control mechanism with a computational control system based on nonlinear mathematics and adaptive algorithms. This substitution allows the system to achieve both reliability and stability through software-based control rather than hardware-based impedance matching.
2Adaptability or versatility
If smart switches with wireless communication are used for off-grid operation, then off-grid functionality is improved, but device complexity and cost increase
Solution Approach 1:
The patent enables the inverter to autonomously detect grid status and switch between grid-connected and off-grid modes without external communication signals. The system serves itself by using local sensing and embedded logic to determine operational mode, eliminating the need for smart switches and wireless communication infrastructure.
Solution Approach 2:
The patent extracts and removes the communication system and smart switch components from the inverter design. By taking out these external dependencies, the system achieves off-grid capability through simplified local control, reducing device complexity while maintaining versatility.
3Device complexity
If small DC-bus capacitor is used in micro-inverters, then device complexity is reduced, but reliability under solar power intermittency worsens
Solution Approach 1:
The patent changes the control parameters and dynamic response characteristics to work effectively with small energy storage capacitors. The nonlinear control algorithm adjusts power flow management in real-time to compensate for limited storage capacity, maintaining reliability without requiring large capacitors that would increase device complexity.
Data Source
AI summary
Systems and methods relating to control systems for DC/AC inverters that receive power from photovoltaic based renewable energy resources. When the DC/AC inverters are operated in on-grid mode, the DC/AC inverters and the DC/DC control system operate to provide on-grid functions such as maximum power point tracking (MPPT) and DC-bus voltage regulation. When in off-grid mode, the DC/AC inverter and the off-grid control system regulates the resulting AC voltage from the DC/AC inverter to be within a pre-set range. The off-grid control system is based on differential geometry and uses a Lie Group controller for setting a frequency reference signal. The frequency and current amplitude reference are used to generate a sinusoidal current reference signal which is then tracked by a current controller. The current controller controls the switches in the DC/AC inverter to regulate the AC voltage.