DC Link Rectifier Control for Low-THD Pulsed Power Supply
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Solution Overview
Problem
Conventional power supply devices with grid-connected inverters experience undesirable total harmonic distortion (THD) due to periodic load fluctuations, especially when these fluctuations occur at frequencies matching the mains frequency or its harmonics, leading to unpredictable and unwanted current distortions in ozone generators.
Innovation Solution
A power supply device with a DC link converter and controlled rectifier that maintains a constant charging current from the mains, charging a DC link storage until a predetermined voltage is reached, allowing energy pulses to the load only when the DC link voltage meets a trigger voltage, thereby decoupling load-side pulse frequency from grid-side current flow and reducing THD.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a conventional VSI is used to generate constant voltage in DC link capacitors, then the inverter is well suited for loads with very low or non-recurring load fluctuations, but load-side fluctuations become directly noticeable in relative fluctuation of power consumption from the grid, leading to total harmonic distortion when fluctuations occur at mains frequency or harmonics
Solution Approach 1:
The patent segments the power supply system into two distinct functional parts: a controlled rectifier on the grid side that manages power input, and an inverter on the load side that delivers energy pulses. The DC link capacitor is divided into multiple series-connected capacitors, further segmenting the voltage management. This segmentation allows each part to operate independently with optimized control strategies, preventing harmonic distortion from propagating from load to grid.
Solution Approach 2:
The DC link capacitor acts as an intermediary energy storage element between the controlled rectifier and the inverter load. It decouples the grid side from the load side, allowing the controlled rectifier to maintain constant charging current while the inverter delivers pulsed energy to the load. This intermediary function prevents direct transmission of load fluctuations to the grid, eliminating total harmonic distortion.
2Power
If the pulse frequency is adjusted to control total power output, then the desired power level is achieved, but the current peaks overlap unpredictably with mains frequency, causing strong distortions of the supply current
Solution Approach 1:
The control device continuously monitors the actual DC link voltage and uses this feedback to regulate the controlled rectifier's charging current. When the DC link voltage drops below a reference level, the control device increases the charging current to replenish energy. This feedback mechanism ensures that the rectifier adapts to load variations while maintaining stable grid current, preventing distortion regardless of pulse frequency adjustments.
Solution Approach 2:
The patent changes the control parameter from constant voltage (conventional VSI) to constant current (controlled rectifier). By controlling the rectifier to deliver constant charging current rather than maintaining constant DC link voltage, the system allows the DC link voltage to fluctuate naturally with load demands. This parameter change enables the inverter pulse frequency to be adjusted for power control without causing current distortion, as the rectifier continuously adapts its current to match load requirements.
3Stability of the object's composition
If a constant voltage is maintained in the DC link, then the VSI operates in conventional mode, but the charging current from the mains fluctuates to compensate for energy loss after each load pulse
Solution Approach 1:
The patent inverts the conventional control approach by switching from constant voltage control to constant current control at the rectifier input. Instead of maintaining constant DC link voltage and allowing current to fluctuate, the system maintains constant charging current and allows voltage to vary. This inversion eliminates the need for compensatory current fluctuations after each load pulse, as the constant current naturally matches the average power requirements.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution ensures a constant current is drawn from the mains to recharge the DC link storage after each energy pulse, eliminating the influence of load-side pulse frequency on grid-side current shape and preventing undesirable harmonic distortions, ensuring stable and efficient power delivery.
Implementation Method 1
a controlled rectifier (4) connected to a single-phase or multi-phase mains (3) and controlled in such a way that it takes energy from the mains (3) with a constant charging current
Implementation Method 2
a DC link storage device (6) in the form of a capacitor
Data Source
Figure 1
Figure 2
Figure 3a~3b
AI summary
Power supply device (1) for a load (2), having a DC link converter with a controlled rectifier (4), as well as a DC link storage device (6), the controlled rectifier (4) being connected to a single-phase or multi-phase network (3) and being controlled in such a way that it takes energy from the mains (3) with a constant charging current (I_grid ) and charges the intermediate circuit storage (6) until a predetermined intermediate circuit voltage (VDC_nom ) is reached, wherein a control device monitors the actual intermediate circuit voltage (VDC_act ) and always grants the load (2) an energy withdrawal from the intermediate circuit storage (6) only when the actual intermediate circuit voltage (VDC_act ) is equal to a predetermined trigger voltage (VDC_trig).