Transformer Converter Zero-Crossing Control to Cut Grid Current THD
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Solution Overview
Problem
Existing converters experience mode switching errors and oscillations near zero-crossing points of grid voltage due to voltage distortion and low sampling accuracy, leading to total harmonic distortion (THD) and potential short circuits.
Innovation Solution
Implement a method to control the converter by determining if the grid voltage is within a predetermined range of zero-crossing, switching to normal or predetermined operation modes, and using strategies like open-loop control, deadbands, and hysteresis loops to minimize frequent mode changes near zero-crossing points.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If closed-loop control with mode switching at zero-crossing point is used, then the converter achieves single-stage grid-connection, but the grid voltage distortion and low sampling accuracy cause polarity misjudgment and frequent mode switching leading to current oscillation and THD increase
Solution Approach 1:
The patent applies preliminary action by detecting the zero-crossing point in advance and pre-switching to open-loop control mode before the actual zero-crossing occurs. This anticipatory approach avoids the instability caused by real-time mode switching at the zero-crossing point, thereby preventing polarity misjudgment and current oscillation while maintaining reliable grid-connection capability.
2Speed
If frequent mode switching is performed at zero-crossing point, then the control responds to voltage changes, but the sampling accuracy is insufficient leading to oscillation and waveform distortion
Solution Approach 1:
The patent changes the control parameter from closed-loop mode switching to open-loop control with a predetermined phase-shift angle when approaching the zero-crossing point. This parameter change bypasses the measurement precision limitation by using a fixed, pre-determined control strategy instead of relying on real-time voltage sampling accuracy, thus eliminating oscillation while maintaining responsive control.
3Productivity
If normal operation mode is used during positive and negative half-cycles, then the converter operates efficiently, but the mode switching back and forth at zero-crossing introduces oscillation and influences current waveform
Solution Approach 1:
The patent segments the control strategy into two distinct parts: closed-loop control for normal operation during positive and negative half-cycles to maintain efficiency, and open-loop control with predetermined phase-shift angle for the zero-crossing transition period to eliminate oscillation. This segmentation allows the system to enjoy the benefits of both control modes without the harmful effects of frequent mode switching.
Solution Approach 2:
The patent extracts the problematic mode switching operation from the zero-crossing region and replaces it with open-loop control. By taking out the closed-loop mode switching mechanism from the zero-crossing point, the patent eliminates the source of oscillation and THD while maintaining efficient operation during stable voltage periods.
Data Source
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AI summary
Provided in the present application are a converter and a zero crossing point control method therefor. The zero crossing point control method comprises: first, determining whether the current power grid voltage is within a preset range of a zero crossing point; if the current power grid voltage is not within the preset range of the zero crossing point, controlling a primary-side circuit and a secondary-side circuit, which are connected to a transformer in a converter, to operate in a normal working mode; and if the current power grid voltage is within the preset range of the zero crossing point, controlling the primary-side circuit and the secondary-side circuit to operate in a preset working mode. That is, in the present application, a transition area is added near the zero crossing point, thereby reducing the frequent switching between the working modes of the primary-side circuit and the secondary-side circuit at the zero crossing point of the power grid voltage, reducing the current oscillation of the converter, such as a miniature inverter, at the zero crossing point, and reducing the THD of a grid-connected current.