PV String I-V Curve Measurement with Fixed-Ratio DC/DC Converters
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Solution Overview
Problem
Existing methods for determining the characteristic curve of a photovoltaic (PV) string with DC/DC converters as power optimizers distort the curve, making it difficult to detect real performance issues like voltage-induced degradation, and current methods require regular time intervals for measurement.
Innovation Solution
A method that allows for the determination of the characteristic curve of a PV string with DC/DC converters operating in a second mode with a constant conversion ratio, using a current signature generated by the inverter to reflect the curve of an identical string without converters, enabling detection at varying times and using existing measurement units within the inverter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If DC/DC converters operate with MPP tracking method, then yield loss of shaded PV modules is limited, but the characteristic curve of the PV string is distorted
Solution Approach 1:
The DC/DC converter dynamically switches between two operating modes: MPP tracking mode for normal operation and measurement mode for characteristic curve determination. This dynamic switching allows the system to optimize energy harvest during normal operation while enabling accurate measurements when needed, resolving the contradiction between energy efficiency and measurement accuracy.
Solution Approach 2:
The controller acts as an intermediary that coordinates between the MPP tracking function and the measurement function. It temporarily suspends MPP tracking to enable accurate characteristic curve measurement, then resumes MPP tracking afterward. This intermediary control mechanism allows both functions to coexist without permanently compromising either energy harvest or measurement accuracy.
2Loss of energy
If DC/DC converters are integrated into PV string, then loss of yield is limited to shaded modules only, but detection of real characteristic curve becomes difficult
Solution Approach 1:
The measurement function is extracted from the normal MPP tracking operation. During characteristic curve determination, the DC/DC converter is temporarily taken out of MPP tracking mode and operated in a dedicated measurement mode with constant conversion ratio, allowing the characteristic curve to be measured without distortion while maintaining the benefits of MPP tracking during normal operation.
Solution Approach 2:
Before performing characteristic curve measurement, the system preliminarily switches the DC/DC converter from MPP tracking mode to measurement mode. This preliminary action ensures that the converter is properly configured for accurate measurement before the actual measurement begins, preventing distortion of the characteristic curve while allowing MPP tracking to resume afterward.
3Productivity
If measurement is performed at regular time intervals, then characteristic curve can be determined periodically, but flexibility and responsiveness to performance issues are reduced
Solution Approach 1:
The system supports periodic measurement at regular time intervals for routine monitoring while also enabling on-demand measurements triggered by specific events or conditions. This combination allows the system to maintain regular measurement schedules while providing the flexibility to perform additional measurements when performance issues are suspected or when diagnostic information is needed.
Solution Approach 2:
The measurement system incorporates feedback mechanisms that allow characteristic curve determination to be triggered by specific conditions, such as detected anomalies, user requests, or system events. This feedback-driven approach provides flexibility in measurement timing while maintaining the ability to perform regular periodic measurements, enabling the system to adapt to different operational needs.
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 simple and cost-effective determination of the characteristic curve at any time, allowing for targeted analysis and detection of performance issues without additional components, facilitating the evaluation of PV string performance.
Implementation Method 1
with a DC/DC converter that is operated at a constant conversion ratio, a current-voltage characteristic curve of a DC source connected to the DC/DC converter on the input side is transferred to an output of the DC/DC converter
Data Source
AI summary
The disclosure relates to a method for determining a characteristic curve for a photovoltaic (PV) string of a photovoltaic system having an inverter which is connected to the photovoltaic string and to a power supply network. The photovoltaic string includes a series connection of a plurality of photovoltaic modules, in which series connection at least one of the photovoltaic modules is integrated into the series connection of the photovoltaic modules via a DC/DC converter. The at least one DC/DC converter operates the photovoltaic module assigned thereto in a first operating mode M1 at a maximum power point by varying, over time, a conversion ratio of output voltage (UOut) to input voltage (UIn), and operates the photovoltaic module in a second operating mode M2 with a conversion ratio of output voltage (UOut) to input voltage (UIn) that is constant over time. The method includes operating the at least one DC/DC converter in the second operating mode M2 in response to a current signature for the current (IStr) through the photovoltaic string, determining the characteristic curve by varying the current IStr or the voltage UStr of the photovoltaic string by the inverter, and detecting values assigned to one another for current IStr and voltage UStr of the photovoltaic string in the second operating mode M2 of the DC/DC converter.