PV Inverter Grid-Forming Control Without PLL Synchronization
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Solution Overview
Problem
Conventional PV systems lack grid-forming capacity and rely on phase-locked loops, which hinder independent black-start operations and are sensitive to solar power fluctuations, affecting grid stability and requiring complex model-based control schemes.
Innovation Solution
A model-free control scheme for PV systems that includes an active power-frequency droop controller, proportional integral controller, and a control mode switch, enabling grid-forming capabilities in islanded and grid-connected modes without relying on phase-locked loops, with a smoothing circuit for mode transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional current source control with phase-locked loop is used, then the PV system can operate as a current source connected to the grid, but the system lacks grid-forming capacity and cannot perform independent black-start operations
Solution Approach 1:
The patent replaces the conventional phase-locked loop (PLL) based current source control mechanism with a voltage source control mechanism. The inverter is controlled to output voltage instead of current, enabling grid-forming capability. The control system uses voltage references and modulation techniques to make the inverter behave as a voltage source, fundamentally changing the control paradigm from current-based to voltage-based control.
Solution Approach 2:
The patent changes the control parameter from current injection (conventional mode) to voltage output (grid-forming mode). By switching the inverter control from current source behavior to voltage source behavior, the system gains the ability to establish voltage and frequency, enabling independent black-start operations and grid-forming capabilities without requiring external grid support.
2Measurement precision
If model-based grid-forming control schemes are used, then real-time maximum power point estimation can be achieved, but the computation process becomes complex and time-consuming
Solution Approach 1:
The patent extracts the complex model-based computation and maximum power point tracking algorithms from the control system. Instead of relying on real-time model calculations, the system uses a simplified control approach that achieves grid-forming operation without complex iterative computations, significantly reducing processing time and computational burden while maintaining effective power control.
Solution Approach 2:
The patent replaces complex, computationally intensive model-based control with a simpler, more efficient control scheme. The simplified controller achieves the necessary control functions without requiring heavy computational resources or complex algorithms, making the system faster and more suitable for real-time operation in grid-forming mode.
3Adaptability or versatility
If PV systems are connected to weak grids, then solar power fluctuations can occur, but this leads to increased magnitude and frequency variations in grid voltage
Solution Approach 1:
The patent implements feedback control mechanisms where the inverter continuously monitors grid conditions and adjusts its voltage output accordingly. The control system uses feedback from voltage and frequency measurements to regulate the inverter output, maintaining stable operation even when connected to weak grids with varying load conditions and solar power fluctuations.
Solution Approach 2:
The patent employs dynamic control strategies that allow the inverter to adapt its output characteristics in real-time based on grid conditions. The control system dynamically adjusts voltage magnitude and frequency to compensate for fluctuations, enabling stable operation in weak grid conditions where conventional fixed-frequency, fixed-voltage control would fail.
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AI summary
A grid-forming photovoltaic (PV) system and method for both islanded connection and grid-connected mode is provided. An inverter converts PV array voltage to a voltage usable as a power source to an electric power system load. Active power-frequency droop controller regulates a modulator that generates drive signals to drive the inverter. Proportional integral controller generates a frequency shift value that adjusts output of the active power-frequency droop controller to yield a phase angle control for modulation of the inverter drive signals. A control mode switch selects among a plurality of control modes for operation of the proportional integral controller. A model-free control algorithm controls the control mode switch, including a control mode (221b) for synchronizing the PV system with the grid in which proportional integral controller (a) detects offset between inverter output voltage and grid output voltage and (b) generates the frequency shift value.