DC Link Voltage Control for Medium-Voltage Inverters
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Solution Overview
Problem
Medium-voltage inverters with cascaded H-bridge topology face issues with frequent unnecessary trips during fast acceleration/deceleration or regenerative loads due to inadequate DC link voltage control, require expensive phase-shift transformers, and have reliability and cost concerns due to a large number of power cells.
Innovation Solution
A control apparatus for medium-voltage inverter systems that includes a first control unit for controlling the average DC link voltage using positive sequence voltage and current, and a second control unit for interphase DC link voltage control using negative and positive sequence voltages and currents, allowing for reduced power cell count and replacing expensive transformers with ordinary ones.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a diode front end configuration is used in power cells, then the configuration is simple and easy to control, but unnecessary trips occur frequently during fast acceleration/deceleration or regenerative loads due to inadequate DC link voltage control
Solution Approach 1:
The patent implements dynamic DC link voltage control by enabling bidirectional power flow through active switches (IGBTs) in the converter stage. The control system dynamically adjusts the converter operation mode (rectifier/inverter) based on load conditions, allowing the system to handle fast acceleration/deceleration and regenerative loads without unnecessary trips, while maintaining the simplified single-phase I/O power cell structure
2Device complexity
If a diode-type rectifier circuit is applied to the power cells, then the configuration is simple, but an expensive phase-shift transformer is required to satisfy current THD requirements
Solution Approach 1:
The patent replaces the mechanical phase-shift transformer with an electronic control system. The converter stage with active switches and the control unit generate appropriate switching signals to synthesize the required phase shifts electronically, eliminating the need for expensive phase-shift transformers while maintaining low THD and simple power cell configuration
Solution Approach 2:
The patent changes the operating parameters of the converter stage by using active switches instead of passive diodes, enabling controlled rectification and inversion. This parameter change allows the system to achieve THD compliance through electronic control rather than requiring expensive phase-shifting transformers
3Power
If up to eighteen power cells are used to output 6,600 V, then the voltage requirement is met, but the reliability of the medium-voltage inverter is degraded and cost and size increase
Solution Approach 1:
The patent segments the voltage generation function across three-phase systems, where each single-phase I/O power cell generates one phase of the three-phase output. This segmentation allows achieving 6,600 V output with only three power cells (one per phase) instead of eighteen, improving reliability while meeting voltage requirements through three-phase power synthesis
Solution Approach 2:
The patent makes each power cell universal by enabling it to handle multiple functions: voltage generation, phase shifting, and bidirectional power flow control. The single-phase I/O power cell with active switches can operate in both rectifier and inverter modes, allowing three cells to perform the work of eighteen cells in traditional CHB topologies
Data Source
AI summary
Disclosed herein is an apparatus for controlling DC link voltage in power cells of a medium-voltage inverter. The apparatus controls DC link voltage in single-phase I/O power cells in the medium-voltage inverter, by way of using a first average power based on a positive sequence voltage and a positive sequence current to output a first DC link voltage reference for controlling the average value of the DC link voltages, and using a second average power based on a negative sequence voltage and a positive sequence current or a positive sequence voltage and a negative sequence current to output a second DC link voltage reference for controlling an interphase DC link voltage.