Differential Current Detection in Transformerless PV Inverters
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Solution Overview
Problem
Monitoring critical fault currents in photovoltaic inverters is challenging due to high capacitive leakage currents, making it difficult to detect small short-term increases in resistive fault currents, especially in large photovoltaic systems without galvanic isolation, which can lead to unsafe and uncontrolled fault currents.
Innovation Solution
The method involves measuring differential currents across multiple pairs of input lines in the inverter, using multiple residual current sensors to reduce capacitive leakage current components and increase sensitivity to resistive fault current detection, allowing for direct monitoring of small increases without disconnection, and comparing these with predefined limit values to ensure safety and fire protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single differential current measurement is performed across all photovoltaic generators, then the total differential current can be monitored, but the capacitive leakage current component becomes too large to detect small resistive fault current increases
Solution Approach 1:
The patent divides the photovoltaic generator system into multiple groups, with each group monitored by a separate differential current sensor. This segmentation reduces the capacitive leakage current component measured by each sensor, enabling detection of small resistive fault current increases that would be obscured in a single aggregate measurement.
2Measurement precision
If multiple differential current sensors are used to reduce capacitive leakage current components, then sensitivity to resistive fault current detection increases, but the device complexity increases
Solution Approach 1:
The system is segmented into multiple monitorable groups of photovoltaic generators, where each group is assigned to a separate differential current sensor. This segmentation achieves the dual benefit of reducing capacitive leakage current per sensor while maintaining manageable system complexity through structured grouping.
Solution Approach 2:
Each differential current sensor serves multiple functions: monitoring its assigned group for both capacitive leakage current and resistive fault current, and contributing to overall system safety through coordinated evaluation with other sensors. This multi-functionality reduces the need for additional specialized components.
3Reliability
If the total differential current limit is applied to the sum of all currents, then fire protection and system protection are ensured, but small short-term increases in individual strings may be missed
Solution Approach 1:
The monitoring system evaluates both individual group currents and their sum, applying different limit criteria to each level. This hierarchical segmentation enables detection of small short-term increases in individual groups while simultaneously ensuring overall system protection through the aggregate limit evaluation.
Solution Approach 2:
The system uses feedback from multiple differential current sensors to continuously monitor both individual group currents and their sum. This feedback mechanism enables real-time detection of small increases in any group while maintaining overall system safety through coordinated limit evaluation at both individual and aggregate levels.
4Productivity
If transformerless inverters are used to increase power output, then system efficiency improves, but ground faults cause dangerous resistive fault currents without galvanic isolation
Solution Approach 1:
The patent segments the photovoltaic generator system into multiple monitorable groups, each with dedicated differential current sensing. This segmentation enables precise detection of ground faults in transformerless inverters, allowing the system to maintain high power output while quickly identifying and responding to dangerous resistive fault currents.
Solution Approach 2:
The monitoring system provides continuous feedback on differential currents in each group and their sum, enabling real-time detection of ground faults in transformerless inverters. This feedback mechanism allows the system to maintain high productivity while ensuring safety by immediately identifying dangerous fault conditions.
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 effectively detects small short-term increases in resistive fault currents, ensuring personal and system protection by accurately distinguishing between capacitive and resistive components, thereby preventing unsafe conditions and unnecessary system shutdowns.
Implementation Method 1
The high leakage current component is due to the large capacitance of photovoltaic generators relative to ground. This capacitance directly manifests as a high leakage current as soon as the input leads of an inverter are subjected to potential shifts relative to ground during operation.
Implementation Method 2
a very high capacitive leakage current component often occurs alongside the resistive fault current component of interest
Data Source
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AI summary
The invention relates to the monitoring of an inverter (1) having isolated input-side connections (2) for several direct-current generators (3) for the occurrence of a critical fault current, differential currents (Idiff) across at least two pairs of input lines (14, 15) that conduct the currents fed in at different input-side connections (2) are measured separately in the inverter, wherein all pairs of input lines (14, 15), in the totality thereof, conduct all currents fed in at the connections (2). The differential currents (Idiff) are separately compared with a limit value for each pair of input lines (14, 15), wherein a fault is detected if the limit value is exceeded. In addition, a sum of simultaneously occurring differential currents (Idiff) across all pairs of input lines (14, 15) is determined and compared with an additional limit value, wherein a fault is also detected if the additional limit value is exceeded.