NPC Converter Voltage Balancing via SVPWM Current Steering
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Solution Overview
Problem
Neutral point (NP) voltage balancing in neutral point clamped (NPC) multilevel converters leads to uneven thermal stress among power semiconductor devices, requiring overrating or limiting the converter's operating range to manage thermal constraints, and existing solutions are computationally complex.
Innovation Solution
An additional current regulator controls the balancing current, and an additional voltage regulator manages the differential voltage across the DC Link capacitors, using a space vector pulse width modulated (SVPWM) signal to adjust switch conditions based on sensed currents and voltage differences, allowing for improved voltage balance control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If common mode voltage injection is used for NP voltage balancing, then voltage balance is improved, but uneven thermal stress among power semiconductor devices increases
Solution Approach 1:
The patent applies local quality by selectively controlling the duration of short vectors in different sectors based on the specific voltage imbalance condition of the neutral point. Instead of uniform voltage injection, the control strategy adjusts the duty cycle of short vectors locally in each sector to achieve precise voltage balancing while distributing thermal stress more evenly across all semiconductor devices.
2Reliability
If semiconductor devices are overrated to overcome thermal stresses, then reliability is improved, but device complexity and cost increase
Solution Approach 1:
The patent implements feedback control by continuously monitoring the neutral point voltage and adjusting the short vector duration in real-time. The control system measures the voltage difference across the DC link capacitors and dynamically modifies the PWM signals to correct imbalances, ensuring that semiconductor devices operate within their rated capabilities while maintaining reliable voltage balance.
3Temperature
If converter operating range is limited to manage thermal constraints, then thermal stress is reduced, but productivity and versatility decrease
Solution Approach 1:
The patent applies dynamics by making the converter adaptable to different operating conditions through dynamic adjustment of short vector duration. The control strategy dynamically modifies the duty cycle based on real-time voltage measurements and operating sector, allowing the converter to maintain optimal thermal performance across the full operating range without limiting productivity or versatility.
4Reliability
If carrier offset or bias injection techniques are used for voltage balancing, then voltage balance is improved, but computational complexity increases
Solution Approach 1:
The patent simplifies the control approach by changing the control parameter from complex carrier offset or bias injection to direct manipulation of short vector duration in SVPWM. This parameter change reduces computational complexity by working directly with the PWM duty cycle values that are already being generated, rather than requiring additional complex modulation techniques or harmonic evaluation.
Data Source
AI summary
An NPC converter is controlled by estimating current flow during the short vector periods of a space vector modulation drive signal. The current estimation is used to close a first order inner current regulator loop that steers current to either the upper capacitor or the lower capacitor of a DC link. An outer voltage regulator measures the voltage imbalance across the capacitors and drives the inner current loop. By managing the associated duty cycle of each vector a bias current is formed that balances the capacitor voltages.