Cascaded Photovoltaic Inverter Over-Modulation Control
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Solution Overview
Problem
Cascaded photovoltaic grid-connected inverters face issues with over-modulation, leading to a power factor in the system being less than the minimum permitted value, which is currently addressed by injecting reactive power into the grid, but this lowers the power factor and is not always effective.
Innovation Solution
A control method and device for the cascaded photovoltaic grid-connected inverter that determines if an inverter unit is over-modulated, calculates the maximum and minimum reactive power to be injected, and simulates the operation state after injecting reactive power to ensure the power factor remains above the minimum threshold, while controlling the inverter to exit MPPT control mode if requirements cannot be met.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If reactive power is injected to the power grid to prevent inverter over-modulation, then over-modulation is prevented, but the power factor of the system decreases below the minimum permitted value
Solution Approach 1:
The patent dynamically adjusts the reactive power injection amount based on real-time system state simulation. By calculating the minimum reactive power needed to prevent over-modulation and comparing it with the maximum reactive power that maintains power factor compliance, the system optimizes the injection amount to satisfy both constraints simultaneously
Solution Approach 2:
The system implements a closed-loop control mechanism where the actual system state is continuously monitored, simulated under different reactive power injection scenarios, and used to adjust the injection amount. This feedback loop ensures both over-modulation prevention and power factor compliance are maintained
2Productivity
If independent MPPT control is performed on each photovoltaic panel to improve solar energy utilization, then energy utilization ratio increases, but inverter units may become over-modulated due to unbalanced active power output
Solution Approach 1:
The patent introduces a reactive power injection mechanism as an intermediary between the independent MPPT control of each panel and the overall inverter operation. This intermediary allows each panel to operate independently for maximum energy harvest while the reactive power injection balances the system to prevent over-modulation
Solution Approach 2:
The system dynamically adjusts the reactive power injection amount based on real-time conditions including the active power output of each inverter unit. This dynamic adjustment allows the system to maintain both independent MPPT operation and modulation stability under varying operating conditions
Data Source
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AI summary
Provided are a cascaded photovoltaic grid-connected inverter and a control method and control device thereof. The method includes: determining whether at least one of inverter units of the inverter is over-modulated; if yes, calculating a maximum reactive power and a minimum reactive power permitted to be injected to a power grid in a situation that a power factor of a system is not less than a minimum permitted value; simulating an operation state of the inverter after the maximum reactive power is injected to the power grid and determining whether each of the inverter units in the operation state is in a modulating region; if yes, injecting a suitable reactive power to the power grid; and if no, controlling the inverter to exit an operation mode of independently performing maximum power point tracking control on each photovoltaic panel.