Virtual Synchronous Generator Control for Fault Current Limiting
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Solution Overview
Problem
Existing power conversion devices struggle to accurately suppress overcurrent when simulating the behavior of a synchronous generator, particularly during events like short-circuit faults in the power grid.
Innovation Solution
A power conversion device with a control device that includes a phase synchronizing unit, characteristics simulating unit, and voltage command generating unit to accurately control the power converter, ensuring the output current remains below a threshold by simulating synchronous generator characteristics, using a phase-locked loop and feedback control mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the internal impedance of the virtual synchronous generator is changed to limit output current, then the output current does not exceed the current limit value during short-circuit faults, but the overcurrent suppression accuracy is insufficient because the output current is indirectly controlled
Solution Approach 1:
The patent changes the control parameter from indirect impedance adjustment to direct voltage command control. By generating an AC voltage command value based on DC voltage command, grid voltage command, and reference phase, the system directly controls the output current magnitude, achieving accurate overcurrent suppression during short-circuit faults while maintaining virtual synchronous generator characteristics.
Solution Approach 2:
The patent implements feedback control by using the reference phase (obtained through phase-locked loop synchronization with the power grid) and grid voltage command to dynamically adjust the AC voltage command value. This feedback mechanism ensures the output current responds accurately to grid conditions while maintaining the current limit, resolving the contradiction between suppression accuracy and control complexity.
2Reliability
If virtual synchronous machine control is used to simulate synchronous generator behavior, then the inertia energy in the power grid is made up for, but the output current control during faults becomes indirect and less accurate
Solution Approach 1:
The patent performs preliminary action by pre-calculating the AC voltage command value using the DC voltage command, grid voltage command, and reference phase before the short-circuit fault occurs. This preliminary preparation of control parameters enables the system to respond immediately and accurately when faults occur, maintaining both grid stability and current control precision without the indirect control delay.
3Adaptability or versatility
If the power converter is controlled to simulate synchronous generator characteristics, then the behavior equivalent to synchronous generator is achieved, but the output current may exceed the current limit value during short-circuit faults
Solution Approach 1:
The patent applies dynamics by making the AC voltage command value adaptive to changing grid conditions. The control system dynamically adjusts the voltage command based on real-time reference phase and grid voltage command while incorporating current limiting logic, enabling the power converter to simulate synchronous generator behavior during normal operation while automatically complying with current limits during short-circuit faults.
Data Source
AI summary
A power conversion device includes a power converter and a control device. The power converter converts a DC power output from a power storage element into an AC power. The control device includes a phase synchronizing unit to estimate a reference phase of an output voltage of the power converter synchronized with a phase of an AC voltage at an interconnection point, a characteristics simulating unit to generate an active power command value based on a phase difference between a reference phase and the phase of the AC voltage at the interconnection point, a DC voltage command generating unit to generate a DC voltage command value, a voltage command generating unit to generate an AC voltage command value so that an output current of the power converter becomes less than a threshold, and a signal generating unit to generate a control signal based on the AC voltage command value.