DC Link Capacitor RMS Current Estimation for Inverter Systems
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Solution Overview
Problem
Current methods for calculating the effective current of an intermediate circuit capacitor in systems involving batteries, inverters, and electrical machines are limited to specific control methods and operating points, failing to provide accurate estimates for various control strategies and modes.
Innovation Solution
A method and device that calculate the effective current of an intermediate circuit capacitor using a calculation rule based on switching times, measured phase currents, and battery direct current, allowing for exact estimation regardless of the control method or operating point, and utilizing only N-1 phase current sensors due to symmetry, with optional quadratic and mixed integral calculations for precise results.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If analytical calculation methods are used for RMS current of DC link capacitor, then calculation is simple for specific drive methods, but the method is not valid for arbitrary drive methods and operating points
Solution Approach 1:
The patent replaces traditional analytical calculation methods with a measurement-based approach using current sensors and a microcontroller. Instead of relying on mathematical models that are only valid for specific drive methods, the system directly measures phase currents and switching states to calculate RMS current, making it universally applicable to any drive method and operating point.
2Measurement precision
If N phase current sensors are used to measure all phase currents, then complete current information is obtained, but the device complexity and cost increase
Solution Approach 1:
The patent extracts and utilizes the neutral point current as a separate measurement source. By measuring the neutral point current in addition to N-1 phase currents, the system obtains complete current information for all N phases without requiring N sensors. The neutral point current provides the missing phase current information through current summation principles.
Solution Approach 2:
The neutral point current acts as an intermediary measurement that bridges the gap between N-1 phase current measurements and complete N-phase current information. This intermediary measurement enables the system to derive all necessary phase currents using current conservation principles at the neutral point.
3Productivity
If RMS current is not monitored accurately, then system operation continues without interruption, but damage to the electric drive or inverter may occur
Solution Approach 1:
The patent implements a feedback mechanism where the microcontroller continuously calculates RMS current based on measured phase currents and switching states, compares it against predefined thresholds, and triggers protective actions when limits are exceeded. This closed-loop monitoring ensures both continuous operation under normal conditions and timely protection when damage risks are detected.
Data Source
Figure 1
Figure 2A~2D
Figure 3A~3B
AI summary
The invention provides a method and an apparatus for estimating an RMS current (lc,rms) of an intermediate circuit capacitor (CZWK) for a complete system of a battery (1), an inverter (9) and an N-phase electrical machine (6), wherein, in the complete system, the inverter (9) converts a DC power received from the battery (1) for the purpose of energizing the electrical machine with N phase currents, and wherein the RMS current (lc,rms) of the intermediate circuit capacitor to be estimated is calculated according to a calculation rule which is supplied with predefined switching times (tLow,off, tHigh,off) of the N phases of the electrical machine, measured phase currents (ISu, ISV, ISW) of N-1 phases or N phases of the electrical machine and a predefined battery direct current (lBatt) as input variables. This makes it possible to exactly estimate the RMS current (Ic,rms) of an intermediate circuit capacitor for any desired pulse patterns for controlling the inverter.