Frequency Converter Current Estimation via DC-Link Voltage
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Solution Overview
Problem
Frequency converters with passive rectification stages face challenges in obtaining current information for rectifier-side and grid-side currents without using current sensors, which are costly and space-intensive, while existing methods require current sensors to assess harmonic performance and thermal conditions.
Innovation Solution
A method that calculates rectifier-side and grid-side currents using voltage values and characteristics of the rectification stage components, such as DC-link inductors and capacitors, to derive corrected currents, eliminating the need for current sensors by combining measured currents or fractions thereof with calculated values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current sensors are used to measure rectifier-side and grid-side currents, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces physical current sensors with a calculation-based method that uses voltage measurements and circuit characteristics to determine current values. This substitutes direct mechanical/electrical sensing with a computational approach, eliminating the need for additional current sensors while maintaining measurement capability.
Solution Approach 2:
The patent introduces voltage measurements as an intermediary to indirectly obtain current information. Instead of directly measuring current, the system measures voltage across known circuit elements (resistors, inductors, capacitors) and calculates current from these voltage measurements using circuit laws, serving as an indirect measurement path.
2Loss of information
If current sensors are installed at grid-side and rectifier stage, then current information is obtained, but manufacturing cost increases
Solution Approach 1:
The patent uses inexpensive voltage dividers and voltage measurement circuits instead of expensive current sensors. These voltage measurement components are significantly cheaper than precision current sensors, reducing the overall manufacturing cost while providing the necessary current information through calculation.
Solution Approach 2:
The patent substitutes expensive current sensing hardware with a combination of simple voltage measurements and computational processing. This replacement dramatically reduces component costs while maintaining the ability to obtain accurate current information for both rectifier-side and grid-side measurements.
3Loss of information
If current sensors are added to the frequency converter, then current measurement capability is improved, but device space requirements increase
Solution Approach 1:
The patent extracts the current measurement function from dedicated current sensor hardware and relocates it to the control unit's computational domain. By taking out the sensing function from physical sensors and implementing it through calculation based on voltage measurements, the system eliminates the need for additional sensor mounting space.
Solution Approach 2:
The patent replaces physical current sensors that occupy board space with a virtual sensing approach using voltage measurements and mathematical calculation. This substitution eliminates the need for additional sensor components and their associated mounting space within the frequency converter.
4Device complexity
If voltage measurements and circuit characteristics are used to calculate current, then device complexity is reduced, but measurement precision may deteriorate
Solution Approach 1:
The patent incorporates feedback mechanisms where the control unit continuously monitors voltage measurements and adjusts calculations based on measured current values from existing sensors. This feedback loop allows the system to refine current estimates and compensate for calculation errors, maintaining precision while using the simplified measurement approach.
Solution Approach 2:
The patent uses a hybrid approach where voltage-based calculation is combined with partial use of existing current sensor measurements. By incorporating some actual current measurements into the calculation process, the system achieves higher precision than pure calculation while still reducing overall sensor requirements compared to full sensor deployment.
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
Enables accurate estimation of currents without additional sensors, reducing costs and space requirements while providing necessary information for harmonic analysis and thermal protection, effectively addressing the limitations of existing methods.
Implementation Method 1
a passive rectifier, an inverter, a DC-Link with a DC-link inductor and a DC-link capacitor
Implementation Method 2
a passive rectifier, an inverter, a DC-Link with a DC-link inductor and a DC-link capacitor
Implementation Method 3
Currents and voltages supplied from the grid-side are rectified by the rectification stage
Data Source
AI summary
A method for determining rectifier-stage output current and/or grid-side currents (iu, iv, iw) of a frequency converter (1) having a passive rectifier (3), an inverter (4), a DC-link with a DC-link inductor (Ldc) and a DC-link capacitor (Cdc) between the rectification stage (3) and the inverter stage (4) is described. In a frequency converter the current information for the grid-side currents (iu, iv, iw) should be obtained without a current sensor at the grid-side (2). To this end the method comprises the step of calculating a current in the DC-link (5) by using at least a voltage value (Urec) and characteristics of the rectifier (3) in the DC-link (5) and/or grid side currents to form a corrected current using the calculated current and a measured current or currents or a fraction of a measured current or currents, or a fraction of a measured current or currents.