Inverter Phase Current Measurement Using Stray Current Compensation
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Solution Overview
Problem
Voltage source inverters face challenges in accurately measuring load currents due to capacitive stray currents, especially in low-powered loads with long cables, where the magnitude of stray currents is significant compared to load currents, leading to errors in current measurement.
Innovation Solution
The method involves measuring charge and discharge currents before inverter use and using this information to accurately determine load current values by shifting the sampling instant relative to voltage pulses and removing DC current components from measured samples to isolate stray currents, which are then subtracted from total current measurements to obtain accurate load current values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If DC current measurement is used to measure load current, then measurement simplicity and cost-effectiveness are improved, but measurement precision deteriorates due to capacitive stray currents
Solution Approach 1:
The patent applies preliminary action by measuring the total current in the DC bus at multiple time instants before actual load operation begins. These preliminary measurements capture the capacitive stray current characteristics when no load current is present. The measured stray currents are then stored and subtracted from subsequent total current measurements during normal operation, thereby eliminating the harmful capacitive current component while maintaining the simplicity of DC bus measurement.
Solution Approach 2:
The patent extracts the harmful capacitive stray current component from the total current measurement by performing measurements at specific time instants when load current is zero. The extracted stray current values are then used to correct subsequent measurements, separating the useful load current information from the harmful capacitive current interference in the DC bus measurement.
2Measurement precision
If current measurement is timed to avoid capacitive stray currents, then measurement precision is improved, but measurement timing complexity increases
Solution Approach 1:
Instead of continuously adjusting measurement timing during operation, the patent performs all necessary timing-sensitive measurements during the commissioning phase before load operation begins. This preliminary action captures the stray current characteristics under controlled conditions, eliminating the need for complex real-time timing adjustments during normal operation.
Solution Approach 2:
The patent uses feedback by measuring the total current at multiple time instants during commissioning and storing these values for later use. The measured stray current values are fed back into the measurement correction process, where they are subtracted from subsequent total current measurements, automatically compensating for capacitive current effects without requiring complex timing control.
3Measurement precision
If multiple current transducers are used to measure each phase current separately, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent applies universality by using a single current transducer on the DC bus to measure total current that represents all phase currents combined. The DC bus current measurement serves multiple purposes: it captures load current information and, when combined with timing information, can identify which phases are active. This single measurement replaces what would traditionally require multiple phase-specific current transducers, reducing device complexity while maintaining measurement capability.
Solution Approach 2:
The patent creates a virtual copy of phase current information by deriving it from the DC bus total current measurement combined with switch state information. Instead of directly measuring each phase current with separate transducers, the system reconstructs phase current data from the combined DC bus measurement and switching state signals, achieving equivalent information with simpler hardware.
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 allows for precise determination of load current values, essential for motor control and parameter estimation, reducing measurement errors and improving control accuracy in voltage source inverter systems.
Implementation Method 1
Cables that are connected from the output of an inverter to the load are somewhat capacitive against the earth and against the cables of other phases, and this capacitance is charged when the output voltage of the inverter changes abruptly
Data Source
AI summary
A method and arrangement for measuring output phase currents of a voltage source inverter when the inverter is connected to a load, the method comprising, during commissioning of a drive, the steps of forming consecutive voltage pulses to the load by using the inverter, measuring current of a DC bus for each consecutive pulse such that for each consecutive pulse, a time difference between a current measurement instant and a start of a voltage pulse is changed, removing a value of DC current from measured current samples to obtain sample values for stray current, storing the sample values of the stray current and their time instants with respect to the start of respective voltage pulses, the method further comprising during the use of the inverter the following steps of, measuring DC-bus current, determining an output phase current of the inverter from the measured DC-bus current and from the stored stray current samples.