Three-Level NPC Excitation Converter for Overcurrent Continuity
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Solution Overview
Problem
Secondary magnetic excitation power converters face challenges in maintaining operational continuity and voltage balance during overcurrent conditions, particularly due to the degradation of coil insulation caused by high voltage rate of change in two-level converters, which limits their application in high-capacity systems like variable-speed pumped storage power plants and offshore wind turbines.
Innovation Solution
A three-level NPC power converter is employed, allowing for a higher AC output voltage with reduced voltage rate of change and incorporating a circuit configuration with two units of two-level converters connected to DC capacitors to maintain voltage balance, ensuring operational continuity during overcurrent events.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a two-level converter is used for secondary magnetic excitation, then the circuit complexity is reduced, but the voltage rate of change increases causing coil insulation degradation
Solution Approach 1:
The patent divides the two-level converter into two separate one-level converters, each handling a different voltage level. This segmentation reduces the voltage rate of change for each converter while maintaining the overall functionality, thereby preventing coil insulation degradation without significantly increasing circuit complexity.
Solution Approach 2:
The patent introduces a new dimension by adding a neutral point to the converter structure, transforming the conventional two-level topology into a three-level NPC (Neutral Point Clamped) converter. This dimensional change enables reduced voltage stress on the excitation winding while maintaining system performance.
2Power
If the AC output voltage is increased for high-capacity systems, then the power capacity is improved, but the voltage rate of change increases causing insulation degradation
Solution Approach 1:
The patent transitions from a two-level to a three-level NPC converter structure, adding a neutral point dimension. This enables the system to deliver high power capacity while reducing the voltage rate of change applied to the excitation winding, thereby preventing insulation degradation even in high-capacity applications.
Solution Approach 2:
The patent applies different voltage levels to different parts of the converter output. The three-level structure provides localized voltage control, allowing high output voltage for power delivery while maintaining lower voltage rate of change at the excitation winding connection point, thus protecting against insulation degradation.
3Reliability
If a bypass circuit is used during overcurrent conditions, then the converter is protected from damage, but the degree of freedom of current is reduced affecting system stability
Solution Approach 1:
The patent implements dynamic control of the NPC converter during overcurrent conditions by selectively adjusting the conduction states of switching elements based on current direction. This dynamic control maintains the degree of freedom of current flow while providing overcurrent protection, thereby preserving system stability without sacrificing converter reliability.
Solution Approach 2:
The patent uses feedback control to monitor current conditions and adjust the converter operation accordingly. During overcurrent events, the control system detects the anomaly and dynamically modifies the switching patterns to protect the converter while maintaining system stability through continuous feedback adjustment.
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
The solution enhances system stability and operational continuity by minimizing torque fluctuations and promptly resuming reactive-power supply, even during ground faults or other anomalies, while reducing the risk of coil insulation degradation.
Implementation Method 1
A three-level NPC power converter, viewed from an AC side, is connected to the rotor-side excitation winding 6
Implementation Method 2
incorporating a circuit configuration with two units of two-level converters connected to DC capacitors to maintain voltage balance
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
A secondary magnetic excitation generator-motor device that inputs a first ignition pulse command to a three-level NPC power converter such that a detected excitation current value corresponds with an excitation current command value, the secondary magnetic excitation generator-motor device having a function of identifying a first phase, a second phase, or a third phase in descending order of a current absolute value, the secondary magnetic excitation generator-motor device including a pulse command device to fix an ignition pulse in either a first-phase P group or a first-phase N group to an on-side, while fixing an ignition pulse in the other group to an off-side in a direction in which a first DC capacitor and a second DC capacitor are charged, to fix an ignition pulse in a second-phase P group to the same side as the first-phase N group, while fixing an ignition pulse in a second-phase N group to the same side as the first-phase P group, and to fix an ignition pulse in a third-phase first group to the on-side, while fixing an ignition pulse in a third-phase second group to the off-side to output the ignition pulses as a second ignition pulse command, wherein a pulse switch to output an ignition pulse command to the three-level NPC power converter switches the ignition pulse command to the second ignition pulse command when a current absolute value exceeds a set overcurrent level 1, and switches the ignition pulse command to the first ignition pulse command when current absolute values for three phases are all equal to or smaller than a set overcurrent level 2.