Voltage Balancing in VSC Clamp Capacitors via Dynamic Load
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Solution Overview
Problem
Voltage imbalance across the switch elements and associated clamp capacitors in voltage source converters, particularly in Alternate-Arm-Converters (AACs), leads to inefficiencies and potential component damage due to the constant power load characteristic of the floating power supply, exacerbating voltage differences between director switch units.
Innovation Solution
Implementing a voltage balancing mode where the power demand from the clamp capacitor varies based on its voltage, mimicking a resistive load characteristic, by adjusting the power or current demand based on voltage thresholds and using a crowbar circuit to provide a controlled resistance, ensuring that clamp capacitors with higher voltages are discharged more rapidly than those with lower voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a constant power load characteristic is used for the floating power supply, then the power supply can maintain stable operation, but voltage imbalance across clamp capacitors is exacerbated
Solution Approach 1:
The patent applies dynamics by transitioning the power supply load characteristic from static (constant power) to dynamic (variable power based on voltage). The control system continuously adjusts the power drawn from each clamp capacitor based on real-time voltage measurements, creating a dynamic response that actively compensates for voltage imbalances while maintaining stable operation.
2Adaptability or versatility
If equal power is drawn from all clamp capacitors, then the power distribution is uniform, but voltage differences between director switch units increase
Solution Approach 1:
The patent applies local quality by making the power draw from each clamp capacitor location-specific rather than uniform. Each director switch unit's floating power supply draws power according to its local clamp capacitor voltage level, with higher power drawn from capacitors with higher voltages and lower power from those with lower voltages, creating a localized balancing effect at each position in the circuit.
3Ease of operation
If higher power is drawn from clamp capacitors with higher voltages, then voltage balancing is achieved, but additional control complexity is introduced
Solution Approach 1:
The patent applies feedback by implementing a closed-loop control system where the voltage across each clamp capacitor is continuously monitored and fed back to the control circuit. This feedback signal is used to adjust the power draw from each capacitor, creating a self-regulating mechanism that automatically balances voltages without requiring complex external control intervention.
Data Source
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AI summary
This application describes methods and apparatus for achieving voltage balancing of clamp capacitors of director switch units of a voltage source converter (VSC). An arm or director switch (104) may be formed from a number of director switch units (200) connected in series, each director switch unit having a semiconductor switching element (110) such as an IGBT. Typically a clamp capacitor (202) may be connected across the IGBT. In some arrangements a floating power supply (208) may draw power from the clamp capacitor (202) to provide power for the director switch unit (200). In the present application the director switch unit (200) is operable in a voltage balancing mode such that the power drawn from the clamp capacitor varies based on the voltage across the clamp capacitor. In this way the power demand for power drawn from the clamp capacitor may have the characteristic of a resistive load.