Three-Phase Rectifier Precharge Circuit for Inrush Current Control
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Solution Overview
Problem
Existing three-phase rectifiers with precharging circuits face issues with inrush currents due to asymmetrical transformers and NTC resistor components, leading to inefficiencies and complexity in switching operations.
Innovation Solution
The rectifier's control system is designed to activate switches at the maximum voltage of the external conductor input, ensuring they close just before the voltage peak, and incorporates mechanically coupled switches for simultaneous operation, along with a shunt release for controlled shutdown, to manage inrush currents and simplify the precharging process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If precharging circuits with multiple NTC resistor components and switches are used to charge transformer windings and intermediate circuit capacitor, then inrush currents are prevented, but the device complexity increases and asymmetrical transformers cause different current sizes and temperatures
Solution Approach 1:
The patent extracts the precharging function from the main circuit by using a separate precharging circuit with NTC resistor components that are only active during startup. This allows inrush current prevention without adding permanent complexity to the main rectifier circuit. The NTC resistors are connected in parallel to the controllable switches and only conduct during the precharging phase.
Solution Approach 2:
The patent applies preliminary action by charging the transformer windings and intermediate circuit capacitor before the main rectifier operation begins. The precharging circuit activates first, establishing safe initial conditions, and then the controllable switches are closed to start normal operation. This sequential activation prevents inrush currents while maintaining simple circuit design.
2Reliability
If multiple switches are used in precharging circuit to charge transformer windings and intermediate circuit capacitor, then inrush currents are avoided, but switching on and off operations become problematic due to asymmetrical transformer currents
Solution Approach 1:
The patent uses NTC resistor components as intermediary elements that automatically regulate current distribution in the precharging circuit. These thermistors have negative temperature coefficients that cause their resistance to decrease as they heat up, naturally balancing current flow through asymmetrical transformer windings without requiring complex control logic for multiple switches.
Solution Approach 2:
The precharging circuit operates autonomously using the self-heating effect of NTC resistors. As current flows through the NTC components, they heat up and automatically adjust their resistance to equalize current distribution across asymmetrical windings. This self-regulating mechanism eliminates the need for complex control systems to manage switch operations.
3Ease of operation
If transformer windings are made symmetrical to avoid current asymmetry in precharging circuit, then switching problems are avoided, but additional effort and complexity are required
Solution Approach 1:
The patent converts the harmful effect of transformer asymmetry into a beneficial self-regulating mechanism. Instead of trying to eliminate asymmetry through complex winding designs, the solution uses NTC resistors whose temperature-dependent resistance naturally compensates for asymmetrical current distribution. The heat generated by unequal currents causes proportional resistance changes that balance the current flow.
4Productivity
If controllable switches are activated before voltage maximum to account for delays and dead times, then switching timing is optimized, but precise control is required to ensure switches close at the correct moment
Solution Approach 1:
The control system performs preliminary action by calculating and setting the activation time of controllable switches before the voltage maximum is reached. The controller accounts for known delays and dead times in advance, triggering the switches at the optimal moment to ensure continuous conduction without gaps. This predictive timing approach eliminates the need for complex real-time detection during switching transitions.
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 approach effectively reduces inrush currents and simplifies the precharging process by ensuring synchronized switch operation, enhancing the rectifier's efficiency and reliability while maintaining robustness against transformer asymmetry.
Implementation Method 1
a series connection with a switch (711, 721) and a resistance component with a negative temperature coefficient (712, 722) in parallel to each controllable switch (21, 22, 23)
Data Source
Figure 1
AI summary
The invention relates to three-phase rectifiers (G) - with an input (1) having three terminals (11, 12, 13), - with three controllable switches (21, 22, 23), - with a three-phase AC transformer (3) having a primary side with three primary windings (31, 32, 33) and a secondary side with three secondary windings (34, 35, 36), - with a controlled six-pulse bridge circuit (4) having an AC input with three line terminals and a DC output with two rectifier terminals, - with an intermediate circuit capacitor (5), - with a pre-charge circuit (7) comprising - a first series circuit (71) with a resistive element (712) having a negative temperature coefficient and - a second series circuit (72) with a resistive element (722) having a negative temperature coefficient, - and - with a control unit (8) for controlling the three controllable switches (21, 22, 23), - where each connection (11, 12,13) of the input (1) can be connected to a phase conductor of a three-phase AC network, - wherein each terminal (11, 12, 13) of the input (1) is connected via one of the controllable switches (21, 22, 23) to a phase conductor terminal of one of the primary windings (31, 32, 33), - wherein a phase conductor terminal of each of the secondary windings (34, 35, 36) is connected to a phase conductor terminal of the AC input of the six-pulse bridge circuit (4), - wherein the intermediate circuit capacitor (5) is connected to the DC output of the six-pulse bridge circuit (4), - wherein the first series connection (71) of the pre-charging circuit (7) is connected on one side to a first (11) of the phase conductor terminals of the input (1) and on the other side to the phase conductor terminal of a first of the primary windings (31, 32, 33),and wherein the second series connection of the pre-charging circuit is connected on the one hand to a second of the outer conductor terminals (13) of the input (1) and on the other hand to the outer conductor terminal of a second of the primary windings (31, 32, 33).