Grid-Side Inverter PLL Misalignment Current Injection Control
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Solution Overview
Problem
Grid-connected power inverters face issues with synchronism loss during severe grid faults, leading to unpredictable operation and inappropriate power fluctuations, which can cause undesired power exchange or tripping of the grid inverter.
Innovation Solution
A grid side inverter system with a phase locked loop controller, current reference decision unit, and current controller that reduces active and reactive current references when the phase difference between the grid voltage vector and the dq-frame exceeds a threshold, allowing the inverter to regain synchronism and prevent power oscillations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the phase locked loop controller continues to operate with misaligned phase during grid faults, then the inverter maintains its power output, but inappropriate power fluctuations and undesired power exchange with the grid occur
Solution Approach 1:
The control system applies preliminary anti-action by detecting phase misalignment before it causes severe power fluctuations. When the electrical parameter indicates phase desynchronization, the system proactively reduces current references to prevent inappropriate power exchange, rather than waiting for the problem to manifest as actual power instability.
Solution Approach 2:
The system dynamically adjusts current references based on real-time phase alignment conditions. The current reference decision unit continuously monitors the electrical parameter and adapts the active and reactive current references accordingly, transitioning from normal operation to reduced current mode when misalignment is detected, and back when synchronization is restored.
2Productivity
If the inverter system operates without reducing current references during phase misalignment, then normal power production is maintained, but the system may trip or operate unpredictably
Solution Approach 1:
The current reference decision unit acts as an intermediary between the phase locked loop controller and the current controller. It monitors the electrical parameter indicating phase alignment and mediates the current references accordingly, reducing them when misalignment occurs to prevent power oscillations, and restoring them when synchronization is achieved.
Solution Approach 2:
The system implements feedback control by continuously monitoring the electrical parameter that indicates phase alignment between the grid voltage vector and the dq-frame. This feedback information is used by the current reference decision unit to adjust current references in real-time, creating a closed-loop control system that responds to synchronization conditions.
3Reliability
If the system reduces active and reactive current references when phase misalignment is detected, then inappropriate power exchange is reduced, but power production capability is temporarily limited
Solution Approach 1:
The system applies beforehand cushioning by reducing current references in advance when phase misalignment is detected, before severe power fluctuations can occur. This preventive reduction cushions against the harmful effects of desynchronization, and the system restores full power production capability once synchronization is reestablished.
Data Source
AI summary
A grid side inverter system includes a grid side inverter, a phase locked loop controller arranged to determine a phase of a dq-frame dependent on a monitored grid voltage vector in the dq-frame and a reference grid voltage vector in the dq-frame, a current reference decision unit arranged to generate a reduction of at least one of an active and a reactive current reference, wherein a decision to generate the reduction is based on an electrical parameter relating to the phase difference of the monitored grid voltage vector and the PLL vector, a current controller arranged to determine control signals for the grid side inverter for controlling generation of active and reactive power to the grid dependent on the active and reactive current references and monitored active and a reactive grid currents in the dq-frame.