Grid-Forming Inverter Stabilizes Weak AC Grids
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Solution Overview
Problem
Feeding high electric power from a photovoltaic system into an AC grid with low short-circuit power is challenging due to high grid impedances and voltage fluctuations, leading to instability in inverters and potential emergency shutdowns.
Innovation Solution
A method where at least one inverter operates as a current source connected to the photovoltaic generator and a second inverter operates as a voltage source or grid former, stabilizing the AC voltage by increasing the short-circuit power at the grid connection point, using a second inverter with enhanced capacitance and potentially bidirectional configuration to manage both active and reactive power.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If electric power is fed into an AC grid with low short-circuit power through long connection lines, then decentralized renewable energy integration is achieved, but high grid impedances cause significant AC voltage increase and fluctuations at the grid connection point
Solution Approach 1:
A second inverter operated as a voltage source (grid former) is introduced as an intermediary device between the photovoltaic system and the weak AC grid. This grid-forming inverter stabilizes the AC voltage at the grid connection point by providing virtual short-circuit power, enabling other current-source inverters to operate reliably despite the weak grid conditions.
Solution Approach 2:
The system changes the operational parameters of inverters by distinguishing between grid-forming inverters (voltage source mode) and grid-following inverters (current source mode). The grid-forming inverter adjusts its output characteristics to provide stable voltage reference and synthetic short-circuit power, transforming the system's ability to interface with weak grids.
2Power
If high electric power is fed into an AC grid with low short-circuit power, then power transfer capability is increased, but inverter stability deteriorates due to voltage fluctuations and high grid impedances
Solution Approach 1:
The grid-forming inverter acts as an intermediary that decouples the power transfer function from the voltage stability function. It provides a stable voltage reference and synthetic impedance that isolates current-source inverters from the weak grid's voltage fluctuations, enabling high power transfer without compromising inverter stability.
Solution Approach 2:
The inverter system is segmented into two functional groups: grid-forming inverters that provide voltage stability and synthetic short-circuit power, and grid-following inverters that handle power transfer. This functional segmentation allows each type to operate optimally without interfering with the other's stability requirements.
3Reliability
If STATCOMs, capacitor banks, or synchronous capacitors are used to stabilize AC voltage at the grid connection point, then voltage stability is improved, but system complexity increases
Solution Approach 1:
The grid-forming inverter performs multiple functions simultaneously: it stabilizes AC voltage, provides synthetic short-circuit power, enables power transfer, and synchronizes with the grid. This multi-functionality replaces the need for separate dedicated devices like STATCOMs or capacitor banks, reducing overall system complexity while achieving the same stability goals.
Data Source
AI summary
A method and associated apparatus for feeding electric power from a photovoltaic system via a grid connection point into an AC grid having a low short-circuit power is disclosed. The method includes connecting a DC voltage side of at least one first inverter of the photovoltaic system to a photovoltaic generator and an AC voltage side of the at least one first inverter to the grid connection point, wherein the at least one first inverter is operated as a current source, and connecting an AC voltage side of a second inverter of the photovoltaic system to the grid connection point, wherein the second inverter is operated as a voltage source based on measurement values of an AC voltage measured in the region of the photovoltaic system and a predefined characteristic curve. For a first total short-circuit power of all first inverters operated as a current source, and a second total short-circuit power of the AC grid and of the second inverter operated as a voltage source, a ratio of the second total short-circuit power to the first total short-circuit power is greater than or equal to 2.