Cascaded H-Bridge PV Inverter Control for Active Power Backflow
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Solution Overview
Problem
Existing control methods for cascaded H-bridge medium-voltage direct-mounted photovoltaic inverters fail to effectively suppress active power backflow during interphase short-circuit faults in the power grid, particularly under conditions of low output power and high voltage sag depth, leading to instability and potential disconnection from the grid.
Innovation Solution
A control method combining adaptive zero-sequence voltage compensation and maximum-minimum harmonic zero-sequence voltage compensation strategies to reduce active power backflow, involving calculations for desired zero-sequence and harmonic voltages to stabilize the inverter operation during interphase short circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If traditional power frequency transformers are used for medium-voltage photovoltaic grid connection, then the system structure is simple, but the volume and weight are large, and no-load loss is high
Solution Approach 1:
The patent replaces traditional power frequency transformers with a cascaded H-bridge medium-voltage direct-mounted photovoltaic inverter system. This substitution eliminates the need for bulky transformers and reduces no-load loss by using power electronic devices instead of electromagnetic transformers, directly addressing the energy loss problem while maintaining system functionality.
Solution Approach 2:
The patent changes the operating parameters by using multi-level output voltage from the cascaded H-bridge inverter, allowing the system to operate at lower switching frequencies. This parameter change improves efficiency and reduces losses compared to traditional transformer-based systems, while enabling direct medium-voltage grid connection.
2Measurement precision
If LCL filters are used for grid connection, then filtering performance is improved, but inherent resonance affects system stability
Solution Approach 1:
The patent extracts and removes the LCL filter from the system by using the cascaded H-bridge inverter's inherent multi-level output voltage capability. This eliminates the inherent resonance problem of LCL filters while maintaining good current quality through the inverter's natural voltage synthesis capability.
3Reliability
If active current injection strategy is used for LVRT control, then reactive current support is improved, but active power backflow occurs under low sunlight conditions
Solution Approach 1:
The patent introduces zero-sequence voltage as an intermediary control variable to mediate between the conflicting requirements of LVRT performance and active power backflow prevention. By adjusting the zero-sequence voltage component, the system can control the balance between active and reactive current during voltage sags, preventing active power backflow while maintaining LVRT capability.
4Loss of energy
If zero-sequence voltage compensation is applied, then active power backflow is suppressed, but over-modulation occurs during single-phase or interphase short-circuit faults
Solution Approach 1:
The patent implements dynamic adjustment of zero-sequence voltage based on real-time detection of grid fault conditions. The control system dynamically adapts the zero-sequence voltage component, increasing it during normal voltage sags to suppress active power backflow, and reducing or eliminating it during single-phase or interphase short-circuit faults to prevent over-modulation, thereby maintaining system reliability under varying fault conditions.
Data Source
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AI summary
The present invention discloses a control method for a cascaded H-bridge medium-voltage direct-mounted photovoltaic inverter, including the steps of: S1, determining two phases of a power grid where a short circuit occurs; S2, calculating an adaptive zero-sequence voltage of a phase A by means of an adaptive zero-sequence voltage compensation strategy, wherein the phase A refers to a phase of three-phase the power grid where no short circuit occurs; S3, compensating for the adaptive zero-sequence voltage, then calculating an amplitude of a modulation voltage of the phase A, and calculating the modulation voltage of the phase A based on the amplitude of the modulation voltage of the phase A; S4, expanding the modulation voltage of the phase A to symmetrical three-phase voltages based on the modulation voltage of the phase A; S5, calculating a harmonic zero-sequence voltage by means of a maximum-minimum harmonic zero-sequence voltage compensation strategy; and S6, acquiring three-phase modulation voltages based on the adaptive zero-sequence voltage and the harmonic zero-sequence voltage, to suppress active power backflow. The present invention can significantly reduce a region with the active power backflow, and is more applicable to conditions with higher voltage sag depth and lower system output power.