Virtual Synchronous Generator Current Limiting via Virtual Impedance
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Solution Overview
Problem
Power generating units configured with virtual synchronous machines may experience damaging overcurrents during abnormal grid conditions, such as sudden grid voltage drops due to faults, which can lead to tripping, damage, or wear of electronic components.
Innovation Solution
A method for controlling power generating units, including determining a virtual impedance value based on a current difference and an overcurrent threshold, and using this virtual impedance to reduce the acceleration of the virtual synchronous generator, thereby limiting the increase in current delivered by the power converter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the power converter is configured to exhibit virtual synchronous generator response, then grid-forming properties are improved, but the risk of damaging overcurrents during grid faults increases
Solution Approach 1:
The patent introduces a virtual impedance element as an intermediary between the power converter and the grid. This virtual impedance acts as a mediator that limits the current flow during grid faults while maintaining the virtual synchronous generator's grid-forming capabilities. The virtual impedance is calculated based on the difference between the current magnitude and an overcurrent threshold, effectively blocking harmful currents without affecting normal operation.
Solution Approach 2:
The patent dynamically changes the impedance parameters of the virtual synchronous generator based on grid conditions. By calculating the virtual impedance value from the current difference and updating the virtual synchronous machine model parameters in real-time, the system adapts its electrical characteristics to prevent overcurrents during faults while maintaining optimal performance during normal operation.
2Power
If the virtual synchronous generator allows high current flow during grid faults, then power delivery capability is improved, but the thermal capacity is exceeded and components are damaged
Solution Approach 1:
The patent applies preliminary anti-action by pre-calculating the virtual impedance value based on the overcurrent threshold before the damaging effects occur. The control system continuously monitors the current magnitude and prepares the virtual impedance parameter to counteract potential overcurrents, preventing the power converter from delivering excessive current that would exceed thermal capacity and cause component damage.
3Reliability
If the current limitation is implemented using virtual impedance, then overcurrent protection is improved, but the control algorithm complexity increases
Solution Approach 1:
The patent implements feedback control by continuously monitoring the current magnitude, calculating the difference from the overcurrent threshold, and using this feedback to determine the virtual impedance value. The control algorithm reads the current feedback, processes it through the virtual impedance calculation, and adjusts the virtual synchronous machine parameters accordingly, creating a closed-loop protection system that is relatively simple to implement.
Data Source
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Figure 3A~3B
AI summary
The invention relates to a method for controlling a power generating unit. The method includes determining a virtual impedance value (Zvir), determining a virtual grid power (Pvsm) based on the virtual resistance value (Rvir) and the grid current (Igrid), determining a virtual synchronous machine rotational speed (ωVSM) and/or a synchronous machine angle (θVSM) of a virtual synchronous generator, and determining a voltage reference (Vabc) for controlling a line side converter to generate the desired reactive power (Qgrid) based on the virtual synchronous machine rotational speed or angle (ωVSM, θVSM), a virtual voltage (ΔVαβ, ΔVdq) and the voltage magnitude reference (Vqref).