Grid-Connected Inverter Mode Switching for Transient Overvoltage
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Solution Overview
Problem
Existing grid-connected inverters struggle to respond quickly enough to transient overvoltages due to the constraints of current-loop control, leading to potential system instability and safety issues.
Innovation Solution
A method and system for a grid-connected inverter that switches between current-source and voltage-source modes based on voltage thresholds, enabling rapid reactive power injection through a transient voltage source characteristic, utilizing a power supply characteristic similar to a synchronous generator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the inverter operates in current-source mode with current-loop control, then the device complexity is reduced and normal operation is maintained, but the response time for transient overvoltage suppression is too slow to meet system safety requirements
Solution Approach 1:
The inverter dynamically switches between current-source mode and voltage-source mode based on grid voltage conditions. During transient overvoltage events, the control mode transitions from current-loop control to voltage-source control with modulation voltage limiting, enabling the system to adapt its response characteristics to match the emergency situation and achieve rapid reactive power injection within the preset duration
Solution Approach 2:
The patent changes the control parameters by introducing modulation voltage limiting during overvoltage conditions. The modulation voltage magnitude is constrained to ensure the inverter operates in voltage-source mode with transient characteristics, fundamentally altering the system's electrical behavior from current-following to voltage-controlling, thereby achieving instantaneous response
2Productivity
If the inverter uses current-loop control for reactive power compensation, then the control structure is simplified, but the response speed cannot satisfy the reactive power demand during transient overvoltage
Solution Approach 1:
The inverter is designed with multi-functionality to operate in both current-source mode for normal reactive power compensation and voltage-source mode for transient overvoltage suppression. The controller integrates both control strategies and automatically selects the appropriate mode based on voltage threshold detection, making the single device capable of handling both steady-state and emergency conditions
3Reliability
If the inverter responds to transient overvoltage with inductive reactive power injection through current-loop response, then the control mechanism is straightforward, but the response time lag causes substantial voltage fluctuation
Solution Approach 1:
The system performs preliminary action by detecting voltage threshold exceedance and pre-switching to voltage-source mode with modulation voltage limiting before the transient overvoltage fully develops. This proactive response prevents the voltage from rising to dangerous levels by injecting reactive power at the earliest possible moment, eliminating the detection and control time lag
Data Source
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AI summary
Provided in the present application are a grid-connected inverter and an overvoltage suppression method therefor. In the overvoltage suppression method, when an alternating-current port voltage of a grid-connected inverter does not exceed a preset threshold value, the grid-connected inverter is controlled to perform grid-connected operation in a current source mode; when the alternating-current port voltage exceeds the preset threshold value, the grid-connected inverter is controlled to perform grid-connected operation in a voltage source mode, such that a response to a reactive current or reactive power, which is outputted by an alternating-current port of the grid-connected inverter, is realized within a preset duration; and the grid-connected inverter is controlled to resume the current source mode until the reactive current or the reactive power is greater than or equal to a corresponding preset reactive instruction value. Therefore, when a temporary overvoltage occurs in an electric power system, transient voltage source characteristics are constructed, and a power supply characteristic of a grid-connected inverter is used to enable the grid-connected inverter to perform spontaneous reactive injection, so as to greatly reduce a reactive response time so that same is synchronous with a rising speed of an alternating-current port voltage, that is, an instantaneous reactive power support can be achieved, and rapid suppression of an instantaneous overvoltage can thus be achieved.