Modular Multilevel Converter Precharge Control for Capacitor Overcharging
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Solution Overview
Problem
In high voltage direct current (HVDC) systems with modular multi-level converters, capacitors are prone to overcharging and sudden current flows due to differential charging voltages across sub-modules, leading to potential failures and reduced lifespan.
Innovation Solution
An initial charging device and method that includes a sub-module control unit to manage charging, discharging, and bypassing of capacitors within sub-modules, allowing for passive, active, and normal operation modes to prevent overcharging and control current flows, using diodes and IGBTs to regulate voltage and bypass inputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If charging is performed according to a predetermined charging order per sub-module arm with different charging voltages, then the initial charging of capacitors can be completed, but the duration (life-span) of capacitors is reduced due to overcharging
Solution Approach 1:
The patent implements dynamic charging voltage adjustment by switching between two charging voltages (first charging voltage and second charging voltage) based on the real-time charging state of capacitors in different sub-module arms. The controller dynamically selects which arms receive the first charging voltage and which receive the second charging voltage, preventing overcharging and extending capacitor lifespan while ensuring all capacitors are properly charged.
2Productivity
If multiple capacitors are charged simultaneously with high voltage, then charging speed is improved, but sudden current flows occur causing potential failures
Solution Approach 1:
The patent employs periodic action by implementing multiple charging stages with different charging voltages. Instead of charging all capacitors simultaneously at maximum voltage, the system charges different sub-module arms in stages using alternating first and second charging voltages. This staged approach maintains charging speed while preventing sudden current flows that could cause system failures.
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 enables independent charging and discharging of capacitors, preventing overcharging and sudden current flows, thereby extending capacitor lifespan and ensuring stable operation of HVDC systems.
Implementation Method 1
a capacitor that is used in a direct current link is mostly used for the purpose of voltage connection and voltage smoothing of DC energy, and for the purpose of full-charging of charging/discharging energy
Implementation Method 2
a first diode providing a charging path to the capacitor
Implementation Method 3
an IGBT1 providing a discharging path of the capacitor by the control of the sub-module control unit
Data Source
AI summary
The present invention relates to a first-cycle charging device and method of a modular multi-level converter, in which, in order to prevent the overcharging of a capacitor in a submodule in a HVDC system, charging and discharging of the capacitor in the submodule are independently carried out, and charging of the submodule in the HVDC system is enabled to be gradually carried out so as to prevent sudden flow of an instantaneous current. The first-cycle charging device of the modular multi-level converter of the present invention comprises: a submodule provided with a capacitor so as to carry out alternating current power charging and discharging; and a submodule control unit for controlling the charging, discharging, and bypassing of the submodule.