Split DC-Link Precharge Circuit for Low-Difference Grid Switch Turn-On
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Solution Overview
Problem
The sticking risk during turn-on of grid-connected switches in power conversion systems occurs when the voltage difference between the two ends is large, preventing successful connection to the grid, and existing soft-start circuits cannot maintain sufficient charging voltage during inversion.
Innovation Solution
A power conversion apparatus with a precharge circuit that connects the bus midpoint to the alternating current end when the bus voltage is less than twice the grid voltage, and controls the precharge circuit to charge either the positive or negative bus capacitor, ensuring the grid-connected switch meets the turn-on condition at a small voltage difference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a soft-start circuit is used to charge bus capacitors, then the PCS can obtain power from the alternating current side, but the charging voltage drops during inversion and the grid-connected switch cannot meet the turn-on condition
Solution Approach 1:
The precharge circuit performs preliminary charging of the bus capacitors before the grid-connected switch is turned on. By connecting the bus midpoint to the alternating current end and charging one of the bus capacitors in advance, the system ensures that the DC bus voltage reaches at least twice the grid voltage amplitude before the switch operation, preventing voltage drop during inversion and ensuring reliable switch turn-on.
2Reliability
If the bus voltage is raised to twice the grid voltage amplitude, then the grid-connected switch meets the turn-on condition, but the circuit complexity increases
Solution Approach 1:
The precharge circuit utilizes existing components in the power conversion system, particularly the bus midpoint and alternating current end connections that are already part of the split DC link topology. By repurposing these existing structural elements for the precharge function, the circuit avoids adding significant complexity while achieving the required voltage boosting capability.
3Productivity
If the grid-connected switch is turned on with a large voltage difference, then the connection can be established, but the switch has a sticking risk
Solution Approach 1:
The system performs preliminary voltage establishment before the grid-connected switch is turned on. By ensuring the DC bus voltage reaches at least twice the grid voltage amplitude through the precharge circuit, the voltage difference across the switch at turn-on is minimized, eliminating the sticking risk while enabling reliable grid connection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively raises the bus voltage to at least twice the grid voltage amplitude, preventing switch sticking and ensuring safe grid connection by meeting the turn-on condition, while reducing circuit costs and complexity.
Implementation Method 1
control the precharge circuit to electrically connect the positive direct current bus to the first alternating current end of the power conversion apparatus, so that the power grid charges the positive bus capacitor
Data Source
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AI summary
This application provides a power conversion apparatus with a grid-connected switch turn-on function at a small voltage difference. The power conversion apparatus includes a direct current end, a positive direct current bus, a negative direct current bus, a positive bus capacitor, a negative bus capacitor, a power conversion circuit, a precharge circuit, a grid-connected switch, a controller, a first alternating current end, and a second alternating current end. One end of the positive bus capacitor is connected to the positive direct current bus, and the other end of the positive bus capacitor is connected to the negative direct current bus through the negative bus capacitor. The controller is configured to: when a bus voltage between the positive direct current bus and the negative direct current bus is less than twice a voltage amplitude of a power grid, electrically connect a bus midpoint to the second alternating current end; and control, based on a value relationship between a voltage amplitude of the first alternating current end and a voltage amplitude of the second alternating current end, the precharge circuit to electrically connect the positive direct current bus or the negative direct current bus to the first alternating current end. In this way, the grid-connected switch can meet a turn-on condition at a small voltage difference, to avoid a sticking risk.