Grid Independent Control Unit Using Load Current Estimation
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Solution Overview
Problem
In grid independent operation, existing control units for distributed power supplies face challenges in maintaining stable output voltage without load current sensors, leading to potential overcurrent issues and system shutdowns during overload states, especially when DC bus voltage fluctuates.
Innovation Solution
The proposed grid independent operation control unit estimates load current using output current and voltage, normalizes these values with the DC bus voltage, and employs feedback control to regulate the inverter's duty ratio, eliminating the need for load current sensors and preventing overcurrent by limiting duty ratio and load current values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If load current sensors are not provided to reduce device complexity and cost, then the control unit cannot directly measure load current for precise voltage regulation, but this leads to overcurrent issues and system shutdowns during overload states
Solution Approach 1:
The patent introduces an intermediary estimation mechanism that uses readily available measurements (output current iinv and output voltage esd) to infer load current indirectly through mathematical relationships. This mediator approach avoids direct sensor installation while maintaining control reliability by calculating load current from the relationship: load current = output current - capacitor branch current, where capacitor branch current is derived from output voltage and capacitor parameters.
Solution Approach 2:
The patent replaces the physical mechanical/electrical measurement system (load current sensors) with a computational/mathematical system. Instead of directly measuring load current through hardware sensors, the system uses mathematical modeling and calculation based on measurable quantities (output current and voltage) to determine load current, thereby substituting physical measurement with computational analysis.
2Stability of the object's composition
If duty ratio is increased to maintain output voltage during DC bus voltage fluctuations, then output voltage stability is improved, but this causes overcurrent in the inverter
Solution Approach 1:
The patent implements a feedback control mechanism that continuously monitors output voltage and adjusts the duty ratio accordingly. The feedback loop calculates the required duty ratio based on the relationship between output voltage, DC bus voltage, and estimated load current, then applies appropriate limiting to prevent excessive duty ratio values that would cause inverter overcurrent. This closed-loop feedback ensures voltage stability while preventing harmful overcurrent conditions.
Solution Approach 2:
The patent dynamically adjusts the duty ratio parameter based on real-time system conditions (DC bus voltage fluctuations, load changes). By changing the duty ratio parameter adaptively rather than using a fixed value, the system maintains output voltage stability during DC bus voltage fluctuations while preventing duty ratio excursions that would lead to inverter overcurrent through implemented limiting mechanisms.
3Measurement precision
If normalization of load current and voltage values with DC bus voltage is implemented to improve control accuracy, then feedback control precision is enhanced, but this increases computational complexity
Solution Approach 1:
The patent transforms the control variables by normalizing load current and voltage values with respect to DC bus voltage. This parameter transformation improves measurement precision and feedback control accuracy by accounting for DC bus voltage fluctuations. The normalization creates dimensionless or scaled parameters that remain stable despite voltage variations, enhancing control precision without requiring complex computational algorithms.
Data Source
AI summary
A grid independent operation control unit includes a load current estimator to estimate a load current supplied to stand-alone power system in accordance with an output current of the inverter and an output voltage, and a feedback controller configured to PWM control the inverter at a duty ratio feedback calculated to cause the inverter to output an output voltage command value in accordance with the output voltage and the load current. The feedback controller is configured to PWM control the inverter at a duty ratio feedback calculated for output of a normalized output voltage command value obtained by normalizing the output voltage command value with the DC bus voltage in accordance with normalized output voltage obtained by normalizing the output voltage with the DC bus voltage and normalized load current obtained by normalizing the load current with the DC bus voltage.