DC Bus Capacitor Pre-Charge via SCR Rectifier Control
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Solution Overview
Problem
In power conversion systems, the initial application of three phase AC power to an uncharged DC bus capacitor can cause excessive currents, potentially damaging inverter switches and loads, necessitating pre-charging of the DC bus capacitor before full rectification.
Innovation Solution
A rectifier bridge with three controllable switching devices, such as silicon controlled rectifiers (SCRs), and a pre-charging circuit that uses high frequency pulse generation to charge the DC bus capacitor during positive half cycles, avoiding large startup currents and allowing normal rectifier activity once the target voltage is reached.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a parallel resistor and relay are used to pre-charge the DC bus, then the DC bus can be pre-charged before full rectification, but the cost increases due to the relay and resistor components
Solution Approach 1:
The patent extracts the pre-charge function from the main rectifier circuit by using a dedicated pre-charge circuit with resistors that are only active during startup. This separates the pre-charge operation from normal rectification, eliminating the need for relays to switch between modes while maintaining the protective pre-charge function.
Solution Approach 2:
The rectifier circuit is designed to perform both pre-charge and normal rectification functions using the same components. The controllable switches can operate in different modes (pre-charge mode with resistors, normal mode directly to DC bus) without requiring additional switching devices like relays, making the circuit multi-functional and reducing component count.
2Reliability
If silicon controlled rectifiers (SCRs) are used to control turn on times, then the DC bus can be charged in a controlled manner, but the cost increases due to SCR switches and switch controller
Solution Approach 1:
The pre-charge circuit uses the inherent properties of the circuit components and the natural progression of DC bus voltage to control the pre-charge process. As the DC bus voltage rises during pre-charge, the voltage difference across the pre-charge resistors automatically reduces, limiting the current without requiring external control. The controllable switches in the main rectifier circuit automatically transition to full operation when the DC bus reaches operating voltage, providing self-controlled pre-charge functionality.
3Speed
If full rectification is applied immediately to an uncharged DC bus capacitor, then the system can start up quickly, but excessive currents can damage inverter switches and loads
Solution Approach 1:
The patent implements a pre-charge circuit that activates before full rectification to charge the DC bus capacitor to a safe voltage level. This preliminary action prevents excessive inrush current by limiting the charging current through pre-charge resistors, protecting the inverter switches and loads from damage while enabling subsequent full-power operation.
Solution Approach 2:
The pre-charge resistors act as intermediary elements between the AC input and the DC bus capacitor during startup. These resistors limit the current flow to safe levels while the DC bus is being charged, serving as a protective mediator that prevents direct high-current connection. Once the DC bus is charged, the main rectifier switches take over and the pre-charge resistors are effectively bypassed.
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 effectively pre-charges the DC bus capacitor without requiring expensive relays or phase angle tracking, reducing the risk of excessive currents and enabling safe startup of power conversion systems.
Implementation Method 1
the rectifier, as the label implies, rectifies the three phase AC voltages to generate a DC voltage across the DC bus
Implementation Method 2
The DC bus typically includes a DC bus capacitor that charges as the rectifier operates
Data Source
AI summary
A method and apparatus for converting three phase AC voltages on first, second and third input lines to DC voltage across positive and negative DC buses, the apparatus comprising a rectifier including first, second and third rectifier legs, each leg including a switch and a diode wherein the switch is linked between the positive DC bus and an cathode of the diode, an anode of the diode is linked to the negative DC rail and the first, second and third diode cathodes are linked to the first, second and third input lines, respectively, a DC bus voltage sensor linked to the positive DC bus and measuring the DC bus voltage to generate a measured DC bus voltage and a rectifier controller that receives the measured DC bus voltage, a reference voltage value and the three phase AC voltages wherein, when the measured DC bus voltage is at least equal to the reference voltage value, the controller turns on the switches in the first, second and third rectifier legs when the voltages on the first, second and third input lines are positive, respectively.