Cycloconverter Zero-Crossing Control to Prevent Bridge-Arm Short Circuits
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Solution Overview
Problem
Due to distortion of the zero crossing of grid voltage, sampling and filtering errors, and control delays, controllers struggle to accurately determine the zero crossing time of grid voltage, leading to a short-circuit risk in the bridge arm of cycloconverters.
Innovation Solution
A DC/AC conversion circuit and control method that includes an inverter unit, cycloconverters with sets of switching elements, and a controller. The controller complementsarily turns on corresponding sets of switching elements when the grid voltage is within a threshold range, which includes the zero crossing, to prevent short circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the controller uses conventional sampling and filtering methods to detect grid voltage zero crossing, then the circuit structure remains simple, but the measurement precision of zero crossing time deteriorates leading to short-circuit risk
Solution Approach 1:
The controller proactively detects when grid voltage approaches the threshold range (before actual zero crossing) and preemptively adjusts switching element states. By anticipating the zero crossing event and preparing switching actions in advance, the system ensures safe commutation without waiting for precise zero crossing detection, thereby eliminating short-circuit risk while maintaining simple circuit structure.
Solution Approach 2:
A threshold range is introduced as an intermediary zone around the zero crossing point. Instead of directly detecting the exact zero crossing moment, the system uses this threshold range as a mediator to trigger preliminary control actions. The threshold range acts as a buffer that allows the controller to prepare switching operations before the actual zero crossing occurs, improving reliability without compromising measurement precision.
2Reliability
If the controller extends the threshold range to improve zero crossing detection robustness, then the reliability improves, but the loss of time increases due to delayed switching actions
Solution Approach 1:
The controller dynamically adjusts switching frequencies based on the grid voltage state. When grid voltage is within the threshold range, the system employs high-frequency complementary switching to ensure reliable commutation. Outside the threshold range, normal switching frequencies are used. This dynamic adaptation allows the system to maintain reliability during critical zero crossing periods while minimizing time loss during normal operation.
Solution Approach 2:
The controller implements periodic high-frequency switching actions specifically during the threshold range period around zero crossing. This periodic intensive switching ensures that switching elements are fully commutated before the actual zero crossing, preventing short circuits. The periodic nature of this action is confined to the critical threshold period, minimizing overall time loss while maximizing reliability during vulnerable periods.
Data Source
AI summary
A DC/AC conversion circuit and a control method therefor, and a modulation method for a cycloconverter are provided. The circuit includes: an inverter unit, which is configured for converting a direct current into an alternating current; at least one cycloconverter, which includes a plurality of sets of switching elements, the plurality of sets of switching elements are connected to an output port of the inverter unit respectively and are configured for performing AC-to-AC conversion, and an output port of the at least one cycloconverter is configured to connect to a power grid and provide an alternating current output; and a controller, which is connected to the inverter unit and the at least one cycloconverter respectively and is configured for controlling, when a grid voltage is within a threshold range, corresponding two sets of switching elements among the plurality of sets of switching elements to complementarily turn on.