Selective Current Controller for Parallel Power Converters
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Solution Overview
Problem
Existing parallel three-phase voltage source power converters face inefficiencies due to cross-currents, which reduce overall system efficiency and can cause overloads, particularly in hard paralleling configurations where semiconductor switch differences lead to asynchronous switching, and in soft paralleling where low inductance values limit effective current control.
Innovation Solution
A selective current controller using a bank of resonant controls is introduced to target and fully suppress cross-current components at specific harmonic frequencies, allowing for zero steady-state errors and optimized current control gains, even in systems with low inductance paralleling, thereby enhancing the performance of cross-current control loops.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If hard paralleling is used with direct converter output connections, then system complexity is reduced, but cross-currents increase due to asynchronous switching caused by semiconductor parameter variations
Solution Approach 1:
The patent introduces shared inductors as intermediary elements between paralleled converters. These inductors act as mediators that limit the rate of change of cross-currents during asynchronous switching events, thereby reducing the harmful effects of cross-currents while maintaining the simplicity of hard paralleling architecture.
Solution Approach 2:
The control system applies preliminary anti-action by detecting asynchronous switching conditions and actively compensating for cross-currents before they reach harmful levels. The controller monitors converter states and applies corrective control signals to counteract the effects of parameter variations in semiconductor devices.
2Object-generated harmful factors
If soft paralleling with series inductors is used, then cross-currents are limited, but device complexity and inductance requirements increase
Solution Approach 1:
The patent merges the functions of individual converter inductors into shared inductors that serve multiple converters simultaneously. This consolidation reduces the total inductance required in the system while maintaining effective cross-current limitation, thereby reducing device complexity compared to traditional soft paralleling with individual inductors for each converter.
Solution Approach 2:
The patent changes the inductance parameter from individual converter-specific values to shared inductance values that are optimally sized for the parallel system. By carefully selecting the shared inductor parameters, the system achieves cross-current limitation with lower total inductance than traditional soft paralleling approaches.
3Ease of operation
If proportional current control loops are used, then current sharing is achieved, but controller gain is limited by stability constraints to approximately 1/4 to 1/2 of deadbeat gain
Solution Approach 1:
The shared inductors serve as intermediaries that improve the stability characteristics of the current control loops. By providing additional inductance in the current paths, the system can tolerate higher proportional controller gains without compromising stability, thereby overcoming the gain limitations of traditional proportional control.
Solution Approach 2:
The patent replaces the reliance on purely electronic control mechanisms with a hybrid approach that incorporates passive inductive elements. This substitution allows the system to achieve better stability margins and higher allowable controller gains by combining passive energy storage elements with active control.
4Quantity of substance
If low inductance values are used for paralleling, then system size is reduced, but effective current control and cross-current suppression become difficult
Solution Approach 1:
The patent combines the inductance resources of multiple converters into shared inductors, effectively multiplying the available inductance for cross-current suppression. This merging allows the system to use low individual inductance values while maintaining sufficient total inductance for effective current control and cross-current suppression.
Solution Approach 2:
The patent changes the approach from individual inductor parameters to shared inductor parameters, optimizing the inductance value for the parallel system as a whole. The shared inductors are sized to provide adequate cross-current limitation while minimizing total inductance, achieving better control performance with lower overall inductance than traditional approaches.
Data Source
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AI summary
Provided is an approach for active control of cross currents flowing among multiple paralleled converters. Control of cross currents is achieved by using at least one proportional-integral (PI) controller (310, 320) and at least one resonant controller (320) to target several selected dominant harmonics with infinite gains to ensure elimination of targeted harmonic cross currents in steady state. The cross currents are decomposed into (i) common mode and differential mode components or (ii) current phase domain components, and each component is suppressed to a value approximately near zero using the controller (300).