Inverter Fault Current Detection Using Adaptive Reference Functions
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Solution Overview
Problem
Existing methods fail to accurately determine the residual current component in differential currents measured at inverters, particularly in multi-phase systems, leading to incorrect identification of fault currents due to increasing leakage currents and capacitances, which reduces sensitivity in fault current detection.
Innovation Solution
A method that involves detecting the AC voltage at the inverter's AC output, generating a periodic reference function with a phase offset and frequency that matches the anticipated potential fluctuations on the DC side, and averaging the product of the differential current with this reference function to separate the AC residual current component from capacitive leakage currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If leakage current compensation is performed using evaluation software to separate slowly changing currents, then leakage currents can be compensated, but jumps in leakage current are incorrectly interpreted as fault currents
Solution Approach 1:
The patent applies dynamics by making the reference function adaptive to different inverter operating states. The system dynamically adjusts the reference function's characteristics (frequency, phase offset) based on the detected operating state, allowing accurate separation of leakage current components across varying conditions while maintaining reliable fault detection
Solution Approach 2:
The patent changes parameters of the reference function based on operating state. By detecting the operating state and selecting appropriate reference function parameters (frequency, phase offset), the system accurately tracks leakage current variations without misinterpreting them as faults, resolving the contradiction between measurement precision and reliability
2Power
If transformerless inverters with increasing power are developed, then inverter power and system capacity increase, but capacitances to earth and leakage currents increase, reducing fault detection sensitivity
Solution Approach 1:
The patent extracts the leakage current component from the total differential current by comparing it with a dynamically adapted reference function. This separation allows the system to isolate and compensate for large leakage currents associated with high-power transformerless inverters, maintaining sensitivity for detecting actual fault currents
Solution Approach 2:
The system dynamically adapts the reference function to match the operating state-dependent characteristics of leakage currents in high-power inverters. This dynamic adaptation enables accurate leakage current separation even as inverter power and associated capacitances increase
3Measurement precision
If the reference function frequency is increased to an integer multiple of the AC voltage frequency to match potential fluctuations, then accurate separation of residual current components is achieved, but the complexity of determining the correct frequency increases
Solution Approach 1:
The patent uses feedback by detecting the actual operating state and using this information to automatically determine the appropriate reference function frequency. The system monitors operating state and adjusts the reference function parameters accordingly, eliminating the need for manual configuration and ensuring accurate separation without increasing operational complexity
4Measurement precision
If a phase offset is applied to the reference function to match inverter operating states, then accurate tracking of potential fluctuations is achieved, but the complexity of determining the correct phase offset increases
Solution Approach 1:
The patent applies feedback by detecting the operating state and using this detection to automatically set the appropriate phase offset for the reference function. This eliminates manual configuration complexity while maintaining accurate tracking of potential fluctuations across different inverter operating states
Data Source
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Figure 3
AI summary
When determining a fault current portion (IF) in a differential current (idiff(t)) measured at an inverter (1), an AC voltage (uAC(t)) applied to an AC output of the inverter (1) is measured and a periodic reference function (y(t)) of changing sign is generated on the basis of the measured AC voltage (uAC(t)) in order to determine an AC fault current portion (IFAC) in the differential current (idiff(t)). The differential current (idiff(t)) is multiplied by the periodic reference function (y(t)), and the product of the differential current (idiff(t)) and the reference function (y(t)) is averaged over an integral number of periods (T) of the reference function (y(t)). In this case, the reference function (y(t)) is generated, at least for one operating state of the inverter (1), with a predefined phase offset with respect to the measured AC voltage (uAC(t)) and/or with a frequency which is an integer multiple of the frequency of the measured AC voltage (uAC(t)).