Zero-Sequence Current Compensation in Multilevel Power Converters
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Solution Overview
Problem
Three-phase, three-level power converters in uninterruptible power supply (UPS) systems face issues with common-mode voltage generation due to unbalanced distribution lines and loads, leading to excessive power losses and ground current fluctuations, which are not effectively mitigated by existing solutions.
Innovation Solution
A control system and method for a multilevel power converter that includes a processor coupled with a memory device to generate a reference current command based on DC link voltage differences and determine zero-sequence current components, which are then subtracted from the reference command to reduce ground current injection by the rectifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the midpoint of the DC-link is connected to ground to contain fault currents, then circuit protection is improved, but ground current injection increases due to non-linearities in the power converter
Solution Approach 1:
The patent extracts and separates the zero-sequence current component from the total reference current command. By identifying and removing this harmful component specifically, the solution addresses ground current injection without compromising the grounding protection function. The zero-sequence current is calculated and subtracted from the reference current before being applied to the rectifier controls.
Solution Approach 2:
The patent modifies the reference current command parameter by subtracting the zero-sequence current component. This parameter change transforms the current command to eliminate the ground current injection while maintaining the necessary protection functions. The adjustment is made in the control domain rather than physically altering the grounding connection.
2Reliability
If standard grounding connections are used in the power converter, then system grounding is maintained, but power losses increase due to zero-sequence current components
Solution Approach 1:
The patent implements a feedback mechanism where the zero-sequence current component is continuously calculated from the measured input current and used to adjust the reference current command. This closed-loop approach dynamically compensates for zero-sequence currents, reducing power losses in the N-line while preserving system grounding. The feedback ensures that grounding is maintained while minimizing harmful current components.
3Productivity
If the rectifier operates with non-linear characteristics to meet UPS standards, then power conversion efficiency is improved, but ground current injection increases
Solution Approach 1:
The patent converts the harmful effect of non-linear operation by explicitly calculating the zero-sequence current component that arises from non-linearities and using this information to compensate. Rather than trying to eliminate non-linear operation (which would reduce efficiency), the solution uses the knowledge of resulting zero-sequence currents to cancel their harmful effects through control adjustment.
Data Source
AI summary
A control system and methods for a multilevel power converter are provided. The converter includes a rectifier coupled to a DC link having a midpoint coupled to electrical ground that divides the DC link into two halves. The control system is configured to generate a reference current command for controlling an output of the rectifier, the reference current command generated based on a difference between a desired and measured DC link voltage value. The system is also configured to determine a zero-sequence current component of input current supplied to the multilevel power converter, wherein the zero-sequence current component is associated with non-linearities in said multilevel power converter that cause ground current injection into the input current by the rectifier. The system is further configured to subtract the determined zero-sequence current component from the reference current command to reduce the ground current injected into the input current by the rectifier.