Inverter Current Limiting for Independent Active and Reactive Power
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Solution Overview
Problem
Existing power conversion devices struggle to independently limit active and reactive power at different ratios, leading to instability in grid frequency and voltage due to deviations between internal and grid phases.
Innovation Solution
A power conversion device with an inverter controller that includes a dq transformation unit, internal phase derivation, AVR unit, and current command value correction unit, allowing for the transformation and independent limiting of d-axis and q-axis current command values based on grid phase, enabling separate control of active and reactive power.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If active power and reactive power are limited at the same ratio using a dq absolute value limit, then the current output from the power conversion device is prevented from becoming an overcurrent, but it becomes impossible to independently limit active power and reactive power at different ratios
Solution Approach 1:
The patent segments the single current limiting function into separate d-axis and q-axis current limiting functions. The d-axis current limit value and q-axis current limit value are independently set, allowing independent control of active power and reactive power limits. This segmentation enables different limiting ratios for active and reactive power while still preventing overcurrent conditions.
2Adaptability or versatility
If the internal phase and grid phase are shifted, then the virtual synchronous generator control can operate with phase deviation, but the active power and reactive power cannot be accurately limited at desired magnitudes
Solution Approach 1:
The patent introduces a current command value correction unit as an intermediary between the current command value derivation unit and the switching controller. This correction unit calculates correction values based on the phase difference between internal and grid phases, and applies these corrections to the d-axis and q-axis current command values. This intermediary mechanism enables accurate power limiting despite phase deviations.
3Device complexity
If a single current limit value is applied to both d-axis and q-axis current command values, then the control structure remains simple, but independent control of active and reactive power limiting is lost
Solution Approach 1:
The patent applies segmentation by dividing the current limiting operation into separate d-axis and q-axis operations with independent limit values. The d-axis current limit value controls active power output, while the q-axis current limit value controls reactive power output. This segmentation maintains relative control structure simplicity while enabling independent power control.
Solution Approach 2:
The patent implements local quality by allowing different limit characteristics for different axes. The d-axis and q-axis have different current limit values that can be independently adjusted according to specific operational requirements. This enables tailored limiting strategies for active and reactive power while maintaining overall system coordination.
4Device complexity
If active power and reactive power are limited using the same limiting mechanism, then the control logic remains unified, but the ability to apply different limiting ratios is lost
Solution Approach 1:
The patent segments the power limiting mechanism into separate d-axis and q-axis limiting operations. Each axis has its own current limit value and limiting logic, enabling different limiting ratios for active and reactive power. The segmentation is implemented in the current command value correction unit, which independently processes d-axis and q-axis current commands with their respective limit values.
Data Source
AI summary
To provide a power conversion device capable of limiting active power and reactive power at different rations respectively. A current command value correction unit includes a command value transformation unit that transforms a d-axis current command value and a q-axis current command value having an internal phase into a first d-axis current command value and a first q-axis current command value having a grid phase, respectively, in consideration of a phase difference between the internal phase and a grid phase, a current limiting unit that is limited to at least one of the first d-axis current command value and the first q-axis current command value, and a command value inverse transformation unit that inversely transforms a third d-axis current command value and a third q-axis current command value having the grid phase after the limitation into a corrected d-axis current command value and the corrected q-axis current command value having the internal phase, respectively.