3-Level ANPC Converter Self-Test Using LC Filter Capacitor Voltage
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Solution Overview
Problem
Inverters with ANPC topologies are prone to damage due to switch failures, as the full input voltage can inadvertently be applied to intact switches, leading to subsequent damage and potential destruction of components.
Innovation Solution
A method for testing an inverter's bridge circuit by applying a link circuit voltage and isolating the bridge output, using short pulses to clock specific switches while monitoring the voltage across a filter capacitor to identify faults within a predetermined voltage window.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the inverter is started up without testing, then the startup process is quick and simple, but switch failures may cause full input voltage to be applied to intact switches leading to component destruction
Solution Approach 1:
The patent applies preliminary action by performing a self-test of the bridge switches before the inverter is connected to the grid and started up. The control unit tests each switch by applying test voltages and measuring currents in advance, so that potential failures are detected before they can cause damage during normal operation. This resolves the contradiction by ensuring reliability through pre-testing without adding complex external testing equipment.
Solution Approach 2:
The inverter performs self-testing of its own bridge switches using its existing control unit and circuitry. The control unit independently tests each switch by controlling other switches and measuring electrical parameters, eliminating the need for external testing devices. This self-service approach maintains simplicity while ensuring reliability before startup.
2Reliability
If additional bridge switches are added for protection, then switch failure protection is improved, but the cost and complexity of the inverter increase
Solution Approach 1:
The patent implements feedback by having the control unit continuously monitor the electrical parameters (currents and voltages) during switch operation and compare them against expected values. When a deviation indicating switch failure is detected, the control unit immediately responds by adjusting the switching states to prevent damage. This feedback mechanism provides protection without requiring additional physical protection components.
Solution Approach 2:
The control unit acts as an intermediary that mediates between the bridge switches and the potential damage from failures. Instead of adding physical protection components, the control unit uses intelligent control logic to detect failures and prevent their harmful effects by coordinating the switching states of all switches. This software-based mediation avoids increasing hardware complexity.
3Measurement precision
If the full input voltage is applied during testing, then comprehensive switch testing is achieved, but defective switches can cause subsequent damage to intact switches
Solution Approach 1:
The patent applies partial action by using reduced voltage levels for testing compared to the full input voltage. The control unit tests each switch by applying test voltages that are sufficient to detect failures but limited in magnitude to prevent damage even if a switch is defective. This allows comprehensive testing while maintaining safety margins.
Solution Approach 2:
The testing method incorporates beforehand cushioning by designing the test procedure to inherently limit the stress on switches. The control unit monitors all electrical parameters during testing and is prepared to immediately stop or modify the test if abnormal conditions are detected. This pre-prepared protective response cushions against potential damage from defective switches during the testing process.
Data Source
AI summary
A method for testing an inverter having a bridge comprising a first switch (T1) arranged between a positive connection (DC+) of a divided link circuit, having a center point (M), and a positive inner connection (PI), a second switch (T2) arranged between the positive inner connection (PI) and a bridge output (BR), a third switch (T3) arranged between the bridge output (BR) and a negative inner connection (NI), a fourth switch (T4) arranged between the negative inner connection (NI) and a negative connection (DC−) of the divided link circuit, a fifth switch (T5) arranged between the center point (M) and the positive inner connection (PI), and a sixth switch (T6) arranged between the center point (M) and the negative inner connection (NI) is disclosed. A grid filter having a filter inductor (LF) and a filter capacitor (CF) is connected to the bridge output (BR). The method comprises applying a link circuit voltage to the divided link circuit, while the bridge output (BR) is isolated from a connected grid using the connected grid filter, fully discharging the filter capacitor (CF), closing the first switch (T1) and the sixth switch (T6), while the fourth switch (T4) and the fifth switch (T5) are open, subsequently clocking the second switch (T2) using a plurality of short pulses, wherein the duty cycle of the short pulses is predetermined between 1% and 5%, subsequently to the clocking determining a voltage dropped across the filter capacitor (CF) and identifying a fault state of the bridge when the voltage dropped is outside of a voltage window with an upper window limit and a lower window limit. An inverter is also disclosed, which has a control system designed and set up to execute the method according to one of the preceding claims and to connect the inverter to a connected grid only if a fault state is not identified.