3-Level Converter Neutral Point Potential Stabilization
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Solution Overview
Problem
In power conversion devices with multiple-connected 3-level converters, the reduction of switching losses and maintenance of control responsiveness is hindered by unstable neutral point potential variations due to increased current flowing through the neutral point, which complicates DC voltage balance control, especially under no load or light load conditions.
Innovation Solution
A power conversion device with a control system that includes a calculation unit for generating a compensation amount to balance capacitor voltages, a carrier signal generation unit that synchronizes with the output voltage command, and a pulse width modulation control unit that adjusts the carrier signal phase to stabilize the neutral point potential by delaying the carrier signal phase based on voltage differences across capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the number of switching times in one period is reduced to minimize switching loss, then switching loss decreases, but neutral point potential variations increase due to increased current through the neutral point
Solution Approach 1:
The patent implements feedback control by continuously monitoring the voltage difference between the two DC capacitors and using this information to adjust the switching patterns of the 3-level converters. The control device calculates the voltage difference and modifies the switching commands to balance the capacitor voltages, thereby stabilizing the neutral point potential while maintaining reduced switching frequency.
Solution Approach 2:
The patent changes the switching parameters of the 3-level converters dynamically based on the detected voltage difference. By adjusting the switching patterns and timing according to the capacitor voltage imbalance, the system optimizes the current distribution through the neutral point, stabilizing its potential while keeping switching losses low.
2Stability of the object's composition
If DC voltage balance control is implemented to suppress neutral point potential variations, then neutral point stability improves, but control becomes unstable under no load or light load conditions due to current pulsation
Solution Approach 1:
The patent applies preliminary action by anticipating the instability issue under light load conditions and implementing a detection mechanism that identifies when the system is operating in this regime. The control device detects current pulsation characteristics and proactively adjusts the control strategy before instability occurs, ensuring continuous stable operation across all load conditions.
Solution Approach 2:
The patent implements dynamic control by adapting the DC voltage balance control strategy based on the operating conditions. The control device dynamically adjusts the control parameters and switching patterns according to the detected load level and current pulsation characteristics, maintaining stability across varying operating conditions rather than using a fixed control approach.
3Power
If multiple 3-level converters are connected in series to achieve high voltage and large capacity, then voltage and capacity increase, but neutral point current increases causing excessive potential variation
Solution Approach 1:
The patent merges the control of multiple 3-level converters into a unified control system that manages all converters simultaneously. The control device coordinates the switching patterns across all converters to balance the cumulative current through the neutral point, leveraging the combined capacity of multiple converters while maintaining neutral point potential stability through integrated control.
Data Source
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AI summary
A power conversion device includes: a plurality of 3-level converters (31 to 35) that are multiple-connected in series to an AC power supply; and a control device (10) controlling operations of the plurality of 3-level converters (31 to 35). The control device (10) includes: a calculation unit calculating an output voltage command for the plurality of 3-level converters (31 to 35); a carrier signal generation unit generating a carrier signal; a correction unit correcting a phase of the carrier signal based on a potential variation on a DC neutral point bus (7); and a pulse width modulation control unit delaying the phase by a prescribed amount based on the carrier signal having the phase corrected by the correction unit as a reference phase, to generate a plurality of carrier signals, and comparing the output voltage command with each of the plurality of carrier signals to generate a control command for each of the plurality of 3-level converters (31 to 35).