AC-DC Converter Control Angle Adjustment for Open-Line Test
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Solution Overview
Problem
In direct current power transmission systems, open-line tests face challenges with excessive current generation and load on AC-DC conversion devices due to voltage increases, leading to undesired losses in snubber circuits during the open-line test, where one end of the direct current circuit is open and parasitic capacitance is charged.
Innovation Solution
The AC-DC conversion device employs a control mechanism that reduces the control angle α from 180° to 90° at a controlled rate during the open-line test, transitioning to a discontinuous conduction state to suppress excessive current and load on switch portions and snubber circuits, thereby optimizing voltage application and minimizing losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If AVR control is used to increase direct current voltage to the prescribed voltage command value during open-line test, then the voltage value is improved, but excessive current flows and excessive load is applied to switch portions
Solution Approach 1:
The control angle α is dynamically adjusted from 180° toward 90° during the open-line test, transitioning the converter from continuous conduction state to discontinuous conduction state. This dynamic control limits the charging current to the direct current circuit capacitance while still achieving the prescribed voltage command value, thereby resolving the contradiction between achieving high voltage and limiting excessive current/load on switch portions
Solution Approach 2:
The invention changes the control parameter (control angle α) from its normal operation value of 180° to a reduced value approaching 90° during open-line test. This parameter change fundamentally alters the conduction state and current characteristics, enabling voltage buildup without excessive current flow, thus resolving the technical contradiction
2Stress or pressure
If voltage applied when switch portions are triggered is excessive, then the voltage value is improved, but excessive loss occurs in snubber circuits
Solution Approach 1:
By dynamically controlling the converter to operate in discontinuous conduction state during open-line test, the voltage application to switch portions is better synchronized with the AC voltage waveform. This dynamic operation mode prevents excessive voltage stress on switch portions at triggering moments, thereby reducing snubber circuit losses while still achieving the required voltage level
Data Source
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AI summary
According to an embodiment of the invention, an AC-DC conversion device that includes a major circuit portion and a control circuit is provided. The major circuit portion includes a converter in which multiple switch portions in a bridge connection include separately-excited switching elements and snubber circuits connected in parallel with the switching elements; and the major circuit portion is connected to an alternating current power supply and a direct current circuit and applies, to the direct current circuit, an alternating current voltage applied from the alternating current power supply by an ON of the multiple switch portions. The control circuit controls the voltage applied to the direct current circuit by controlling the ON timing of the multiple switch portions by inputting a control pulse to each of the multiple switch portions. In addition to the function of operating using a control method of a normal operation, the control circuit includes the function of performing an operation of gradually reducing the control angle of the control pulse input to each of the multiple switch portions in a range between 90° and 180° phase voltage equivalent when performing an open-line test of increasing the direct current voltage of the direct current circuit to the voltage command value by charging a capacitance component including a parasitic capacitance included in a power transmission line or the like of the direct current circuit. Thereby, an AC-DC conversion device and a method for controlling the AC-DC conversion device are provided that the occurrence of an excessive load in the switch portions and/or the snubber circuits of the AC-DC conversion device is avoided.