Inverter Current Measurement Using Parallel Capacitor Branches
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Solution Overview
Problem
Current methods for measuring inverter currents in high-power applications face challenges, particularly in detecting overcurrent situations efficiently without increasing impedance in the intermediate circuit, especially when high currents need to be measured accurately and quickly.
Innovation Solution
The method involves using at least two parallel capacitors in the intermediate voltage source, with current measurement taken from one capacitor branch, allowing for direct indication of overcurrent and calculation of output phase currents using known capacitance ratios, and employing a shunt resistor and operational amplifier or current transformer for precise measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a shunt resistor is added to measure DC current in the intermediate circuit, then current measurement is enabled, but the impedance of the intermediate circuit increases
Solution Approach 1:
The patent introduces a capacitor connected in parallel with the intermediate circuit capacitor as an intermediary measurement element. By measuring the current through this capacitor using a shunt resistor, the system obtains current measurement capability without requiring a shunt resistor in the main intermediate circuit path, thus avoiding impedance increase in the power circuit.
Solution Approach 2:
The patent replaces direct electrical measurement (which would require inserting a shunt resistor in the current path and increase impedance) with a capacitive coupling measurement approach. The measurement is performed indirectly through the capacitor current, substituting a high-impedance measurement path with a low-impedance power path.
2Reliability
If voltage measurement is used for overcurrent protection, then protection is provided, but the measurement only gives voltages over lower switches and requires subtraction to obtain upper switch voltages
Solution Approach 1:
The patent uses the capacitor current as an intermediary quantity that directly reflects the total inverter output current. By measuring the current through the capacitor connected in parallel with the intermediate circuit capacitor, the system obtains direct information about the total current without needing to measure individual switch voltages and perform subtractions.
Solution Approach 2:
The capacitor current measurement serves multiple functions simultaneously: it provides overcurrent protection capability and enables determination of individual phase currents. This single measurement approach replaces the need for separate voltage measurements across multiple switches and complex calculation procedures.
3Measurement precision
If direct phase output current measurement is performed, then accurate phase current measurement is achieved, but at least two measurements are required in a three-phase system
Solution Approach 1:
The patent makes the capacitor current measurement serve multiple purposes: it simultaneously provides the total inverter current for overcurrent protection and enables calculation of individual phase currents. This single measurement approach replaces the need for multiple direct phase current measurements, reducing device complexity while maintaining measurement precision.
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 enables efficient detection of overcurrent situations with minimal component size and impedance addition, allowing for low-inductance design maintenance and accurate phase current measurement without the need for additional voltage measurements or synchronization.
Implementation Method 1
Measurement is typically carried out by using a shunt resistor and by measuring the voltage drop in the resistor due to the current
Implementation Method 2
employing a shunt resistor and operational amplifier or current transformer for precise measurement
Data Source
Figure 1
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Figure 4~5
AI summary
A method and an arrangement of measuring inverter current, where the inverter is connected to and supplied by a DC intermediate circuit having two or more parallel capacitor branches (C1, C2; Ca, Cb) connected between the positive and negative rail (Udc+, Udc-) of the DC intermediate circuit, and the capacitance of the capacitor branches being known. The method comprises the steps of measuring the current (IC1, Icb) of one of the parallel capacitor branches, and determining from the measured current the magnitude of the inverter current.