DC-Eliminating Current Controller for Grid-Connected Inverters
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Solution Overview
Problem
Grid-connected DC/AC converters face challenges in eliminating DC current injection into the utility grid due to the limitations of Proportional-Resonant (PR) controllers, which cannot provide high enough feedback gains for DC signals, violating strict regulatory standards like IEEE 1547.
Innovation Solution
A nonlinear adaptive filter is used in conjunction with an integrator to estimate and remove the DC component of the grid current by adjusting the duty cycle of the inverter, employing a compensation block for high gains at fundamental and harmonic frequencies, and a linear state-feedback for stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If PR-controllers are used to control the output current of the DC/AC inverter, then the control system can provide feedback gain for AC signals, but the controller cannot eliminate DC current from being injected into the grid
Solution Approach 1:
The control system is segmented into separate functional blocks: a PR-controller for AC current control and a dedicated DC-elimination block with an adaptive filter for DC component detection and removal. This segmentation allows each block to specialize in its specific function, with the adaptive filter specifically targeting DC components while the PR-controller handles AC components.
Solution Approach 2:
An adaptive filter acts as an intermediary component between the PR-controller and the inverter output. This filter specifically targets and extracts the DC component from the output current, allowing the system to maintain AC control through the PR-controller while independently managing DC elimination through the adaptive filter's estimation and compensation mechanism.
2Object-generated harmful factors
If high feedback gain is applied to eliminate DC current, then DC current injection is reduced, but the system may become unstable at fundamental and harmonic frequencies
Solution Approach 1:
The compensation block provides different gain characteristics at different frequency regions: high gain specifically at DC frequencies for effective DC elimination, while maintaining appropriate gain levels at fundamental and harmonic frequencies to preserve system stability. This localized frequency-dependent gain optimization allows DC elimination without compromising overall system stability.
Solution Approach 2:
The system dynamically adjusts the duty cycle parameter of the inverter based on the estimated DC component from the adaptive filter. By changing the duty cycle in response to detected DC offsets, the system actively compensates for DC current injection while maintaining stable operation through the feedback mechanism.
3Manufacturing precision
If a third-order LCL-filter is used at the output, then output current quality is improved, but resonance is created that requires additional control complexity
Solution Approach 1:
The adaptive filter performs preliminary estimation of the DC component before the current is injected into the grid. By detecting and compensating for DC offsets in advance through duty cycle adjustment, the system prevents DC current injection without requiring complex post-processing or additional hardware, thus maintaining current quality while managing control complexity.
Data Source
AI summary
Systems, methods, and devices which eliminate the DC current from the output of grid-connected inverters. A current controller is provided which interfaces with a grid-connected DC/AC inverter. The current controller uses a nonlinear adaptive filter which receives, as input, the output current of the inverter along with grid current frequency. The nonlinear adaptive filter estimates the DC value of the grid current and, in conjunction with an integrator, removes this DC current component. This is done by adjusting the duty cycle of the grid-connected inverter.