Inverter Grid Resynchronization for Seamless Mode Switching
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Solution Overview
Problem
Power converters used in distributed power supply systems often experience interruptions when switching from isolated to interconnected operation due to phase differences between inverter output voltage and grid voltage, leading to power outages upon grid restoration.
Innovation Solution
A power converter system with a controller that detects frequency and phase differences between inverter output and grid power, calculates an output frequency pattern to synchronize the inverter with the grid, and controls the inverter frequency to smoothly transition from isolated to interconnected operation, ensuring continuous power supply.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the PCS halts isolated operation when power is restored in the power grid to synchronize phase, then phase synchronization can be achieved, but power supply to the electric load is interrupted
Solution Approach 1:
The system performs preliminary phase detection and frequency difference calculation during isolated operation, before grid restoration occurs. The controller prepares synchronization parameters in advance, so that when the grid is restored, the inverter can immediately adjust its output phase and frequency to match the grid, avoiding power supply interruption.
Solution Approach 2:
The system dynamically adjusts the inverter output frequency and phase based on real-time detection of grid frequency and phase differences. The controller continuously monitors the frequency difference and calculates the required frequency pattern to synchronize the inverter output with the grid, enabling smooth transition without interruption.
2Loss of time
If the PCS switches directly from isolated operation to interconnected operation without synchronization, then power supply continuity is maintained, but overvoltage or overcurrent occurs due to phase difference
Solution Approach 1:
The system uses feedback control by continuously detecting the phase difference and frequency difference between the inverter output and the grid. Based on this feedback, the controller calculates the appropriate frequency pattern and adjusts the inverter output in real-time, ensuring that synchronization is achieved safely without causing overvoltage or overcurrent conditions.
3Reliability
If a fixed correction value is added to the phase command for synchronization, then phase alignment is achieved, but the method does not account for frequency differences
Solution Approach 1:
The system changes the control parameter from a fixed phase correction value to a dynamic frequency pattern calculation that considers both phase difference and frequency difference. The controller calculates the frequency pattern based on the detected frequency difference and initial phase difference, allowing the inverter to adaptively adjust its output frequency over time to achieve synchronization, thereby improving versatility.
Data Source
AI summary
According to one embodiment, a power converter includes an inverter and a controller. The inverter operates in an interconnected operation mode in which a grid is connected to an electric load and an isolated operation mode in which the inverter is connected to the electric load, for generating power from a local power supply. The controller detects a frequency difference and a phase difference between an inverter output from the inverter and grid power. The controller calculates, based on the frequency difference and the phase difference, an output frequency pattern to be used for synchronizing the inverter output with the grid power. The controller controls, when switching from the isolated operation mode to the interconnected operation mode, a frequency of the inverter output based on the output frequency pattern.