Virtual Synchronous Inverter Control for Weak Grid Stability
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Solution Overview
Problem
The increasing penetration of wind power into grids has led to instability due to fluctuations in grid voltage and frequency, affecting the performance of asynchronous machines and grid-following control methods, which can result in grid collapse, especially in weak grids with reduced synchronous machine stability margins.
Innovation Solution
Operating inverter-based resources with asynchronous machines as virtual synchronous machines, receiving frequency or voltage reference commands to generate current vectors and control voltage and frequency at the point of interconnection, enabling grid-forming control that actively participates in stabilizing the grid through closed-loop feedback and droop characteristic functions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If wind turbines operate in grid-following mode with current source control, then they can inject specified current into the grid based on fundamental voltage waveforms, but grid voltage and frequency variations adversely affect the performance and stability of the PLL and wind turbine current control
Solution Approach 1:
The patent inverts the conventional grid-following approach by implementing grid-forming control where the inverter-based resource becomes the voltage and frequency reference source. Instead of tracking grid voltage waveforms using PLL, the system generates its own voltage reference and controls current injection to match grid demands, thereby eliminating PLL instability issues while maintaining current injection capability
Solution Approach 2:
The patent changes the control parameters from current-source-based (grid-following) to voltage-source-based (grid-forming). By switching from controlling current injection based on grid voltage to controlling voltage output and deriving current from grid impedance characteristics, the system maintains productivity while improving reliability under voltage and frequency variations
2Adaptability or versatility
If the proportion of synchronous machines is reduced in favor of asynchronous machines, then more inverter-based resources can be integrated into the grid, but stability margins decrease leading to grid collapse under voltage and frequency disturbances
Solution Approach 1:
The patent applies virtual synchronous machine control to inverter-based resources, creating a control algorithm that copies the voltage-source and frequency-regulating characteristics of synchronous machines. This allows asynchronous machines to emulate synchronous machine behavior, maintaining grid stability margins while enabling higher integration of inverter-based resources
Solution Approach 2:
The patent introduces virtual synchronous machine control algorithms as an intermediary layer between the inverter and the grid. This control layer provides the missing inertia and voltage-stability characteristics that synchronous machines naturally provide, acting as a mediator that enables high inverter penetration without sacrificing grid stability
3Device complexity
If inverter-based resources operate without grid-forming capability, then device complexity is reduced, but they cannot actively support or stabilize grid voltage and frequency
Solution Approach 1:
The patent implements a universal control framework where the inverter-based resource can operate in multiple modes (grid-following and grid-forming) using a unified voltage-source control architecture. This multi-functionality allows the system to provide both current injection and active grid support, enhancing reliability without proportionally increasing complexity
Data Source
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AI summary
A method for operating an asynchronous inverter-based resource connected to a power grid as a virtual synchronous machine to provide grid-forming control thereof includes receiving a frequency reference command and/or a voltage reference command. The method also includes determining at least one power reference signal for the inverter-based resource based on the frequency reference command and/or the voltage reference command. Further, the method includes generating at least one current vector using the power reference signal(s). Moreover, the method includes determining one or more voltage control commands for the inverter-based resource using the at least one current vector. In addition, the method includes controlling the inverter-based resource based on the one or more voltage control commands such that the inverter-based resource actively participates in controlling at least one of voltage and frequency at a point of interconnection between the inverter-based resource and the power grid in a closed loop manner.