DC Bus Voltage Control via 90-Degree Phase Shift
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Solution Overview
Problem
Grid-connected converters experience significant second-order harmonic oscillations in DC bus voltage due to unbalanced grid voltages and impedance imbalances, leading to increased filter costs and reduced system reliability.
Innovation Solution
The method generates q-axis and d-axis reference currents with a 90-degree phase difference, using a voltage regulator and inner-loop current controls to mitigate second-order harmonics in the DC bus voltage, thereby canceling out these harmonics in the q-axis and d-axis currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional DC bus voltage control is used, then the control system is simple, but second-order harmonic oscillations occur due to unbalanced grid voltages and impedance imbalances
Solution Approach 1:
The patent segments the current control into two independent components: d-axis current control and q-axis current control. The d-axis controller handles active power and second-order harmonic mitigation, while the q-axis controller handles reactive power. This segmentation allows targeted suppression of second-order harmonics without complicating the overall control structure, thereby improving system reliability while maintaining manageable complexity.
Solution Approach 2:
The patent changes the control parameters by introducing separate d-axis and q-axis current references with specific harmonic compensation terms. The d-axis current reference includes a second-order harmonic component that is 90 degrees out of phase with the q-axis harmonic, creating a counterbalancing effect that mitigates voltage oscillations without requiring fundamental changes to the control architecture.
2Reliability
If filters are used to reduce second-order harmonics, then harmonic oscillations are mitigated, but filter cost and maintenance increase
Solution Approach 1:
The patent converts the harmful second-order harmonic oscillations into a beneficial control signal by generating compensating harmonic currents through the d-axis and q-axis controllers. Instead of passively filtering harmonics with expensive capacitors, the system actively generates counter-harmonics that cancel the oscillations, transforming a parasitic effect into a useful control mechanism that reduces both harmonics and filter requirements.
Solution Approach 2:
The patent replaces the mechanical/electrical filter system with a control-based solution. Instead of using physical filter components (capacitors, inductors) to attenuate second-order harmonics, the system uses electronic control to generate compensating currents that actively cancel the harmonics, substituting a control algorithm for physical filtering hardware and thereby reducing filter cost and maintenance.
3Reliability
If d-axis and q-axis currents are controlled with 90-degree phase difference, then second-order harmonics are mitigated, but control complexity increases
Solution Approach 1:
The patent segments the control into independent d-axis and q-axis channels, each with its own PI controller and harmonic compensation logic. This segmentation allows the complex harmonic mitigation to be handled in a modular fashion, where each axis can be tuned independently, reducing the overall control complexity while achieving the desired 90-degree phase difference for effective harmonic cancellation.
Data Source
AI summary
For Direct Current (DC) bus voltage control, a method generates a q-axis reference current from a DC voltage error that includes a DC voltage input modified by a DC bus voltage in a closed outer loop. The method further generates a d-axis reference current from the DC voltage error, wherein the second-order harmonic in the d-axis reference current is delayed from that in q-axis reference current by 90 degrees. The method generates a q-axis current from the q-axis reference current. The method generates a d-axis current from the d-axis reference current. The second-order harmonic in d-axis current is offset from the second-order harmonic in q-axis current by 90 degrees. The method controls the DC bus voltage of a voltage control plant to mitigate a second-order harmonic in the DC bus voltage with the second-order harmonics in the q-axis current and the d-axis current.