NPC Converter DC Link Capacitor Discharge via Filter Inductors
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Solution Overview
Problem
Existing AC-AC frequency converters require additional hardware for discharging DC link capacitors, increasing production and maintenance costs, and existing methods are inefficient.
Innovation Solution
A method and controller for discharging DC link capacitors in AC-AC converters by interconnecting half-bridges via neutral points and AC outputs with electrical filters, allowing for simultaneous and efficient discharge of capacitors without additional hardware like resistors or relays, using existing converter components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional hardware elements (high voltage relay and discharging resistors) are used for discharging DC link capacitors, then the discharging function is achieved, but production and maintenance costs increase
Solution Approach 1:
The patent makes existing converter components (switches, DC link capacitors, and filter inductors) perform multiple functions. During normal operation, these components perform their primary conversion function. During discharge, the same components create a discharge circuit by switching to a specific configuration where capacitors discharge through the filter inductors, eliminating the need for dedicated discharge resistors and relays
Solution Approach 2:
The patent combines the discharge function with the existing converter structure. The discharge circuit is merged with the converter's filter inductors and switching devices, so that the discharge path shares components with the normal operation circuit, thereby reducing the need for separate discharge hardware
2Reliability
If additional hardware elements (high voltage relay and discharging resistors) are used for discharging DC link capacitors, then the discharging function is achieved, but maintenance costs increase
Solution Approach 1:
The patent makes existing converter components (switches, DC link capacitors, and filter inductors) perform multiple functions. During normal operation, these components perform their primary conversion function. During discharge, the same components create a discharge circuit by switching to a specific configuration where capacitors discharge through the filter inductors, eliminating the need for dedicated discharge resistors and relays
Solution Approach 2:
The patent extracts the discharge function from requiring separate hardware components and integrates it into the existing converter structure. By removing the need for external discharge resistors and high voltage relays, the maintenance burden is reduced to only the existing converter components that are already part of the system
3Reliability
If conventional discharging methods are used, then DC link capacitors are discharged, but the process is inefficient and requires more time
Solution Approach 1:
The patent uses dynamic switching of the converter devices to control the discharge process. By actively switching the semiconductor devices between different states (conducting, blocking, neutral), the system dynamically creates and controls the discharge current flow through the filter inductors, enabling faster and more controllable discharge compared to passive resistor-based methods
Solution Approach 2:
The patent employs periodic switching of the converter devices to achieve discharge. The switching devices are activated in a periodic sequence to repeatedly charge and discharge the filter inductors, which progressively transfers energy from the DC link capacitors, achieving efficient discharge through repeated cyclic action
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
Reduces production and maintenance costs by utilizing existing converter components to efficiently discharge DC link capacitors, ensuring safe and effective capacitor discharge during maintenance.
Implementation Method 1
at least one of the first connection and the second connection comprises an electrical filter... such that a DC link capacitor of the first converter unit and a DC link capacitor of the second converter unit are interconnected oppositely to each other and discharged via the electrical filter
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
An electrical converter (10) comprises at least two converter units (16), each converter unit (16) comprising a neutral point clamped half-bridge (32, 34) with a neutral point (30), an AC output (38, 40), a positive DC output (26) and a negative DC output (28) and each converter unit (16) comprising a DC link (20) with two DC link capacitors (31), which are interconnecting the positive DC output (26) and the negative DC output (28) with the neutral point (30). The half-bridges (32, 34) of the converter units (16) are interconnected via its neutral points (30) via a first connection and via its AC outputs via a second connection and at least one of the first connection and the second connection comprises an electrical filter (52, 54). A method for discharging the DC link capacitors (31) comprises: switching a first half bridge (32, 34) of a first converter unit (16) to a positive state, in which its AC output (38, 40) is connected with its positive DC output (26), and simultaneously switching a second half bridge (32, 34) of a second converter unit (16) to a negative state, in which its AC output (38, 40) is connected with its negative DC output (28), such that a DC link capacitor (31) of the first converter unit and a DC link capacitor (31) of the second converter unit (16) are interconnected oppositely to each other and discharged via the electrical filter (52, 54).