AC Device Fault Detection Using RMS and Average Quotient
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Solution Overview
Problem
Type A residual current circuit breakers in AC networks are unable to detect direct current components in fault currents, which can impair their protective function and lead to reduced availability and safety of electrical devices like frequency converters and inverters.
Innovation Solution
Calculating a quotient from the time mean value and effective value of sensor data from current sensors to determine a criterion for disconnecting the device from the AC network, using a quotient value between 0.6 and 0.8 as an upper limit to ensure balanced availability and safety, and implementing this calculation within a microcontroller.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Type A residual current circuit breakers are used to detect fault currents in AC networks, then sinusoidal alternating fault currents can be detected, but direct current components in fault currents cannot be detected and may impair the protective function
Solution Approach 1:
The fault current detection is segmented into two independent evaluation methods: one for AC components (using RMS value comparison) and one for DC components (using time average value comparison). Each method independently evaluates its respective component type, allowing the system to detect both AC and DC fault currents without interference between the two detection mechanisms.
Solution Approach 2:
The invention changes the evaluation parameter from a single RMS-based metric to a dual-parameter system: RMS value for AC component detection and time average value for DC component detection. This parameter differentiation enables the residual current device to distinguish and evaluate different types of fault currents using appropriate metrics for each.
2Productivity
If DC components are present in fault currents, then device availability can be maintained, but the protective function of residual current devices is impaired
Solution Approach 1:
The protective function is segmented into AC component protection (using RMS evaluation) and DC component protection (using time average evaluation). This segmentation allows the system to maintain device availability for DC-containing operations while providing dedicated protective evaluation for DC fault currents through the time average comparison method.
Solution Approach 2:
The time average value calculation acts as an intermediary mechanism that specifically targets DC components without being affected by AC components. This intermediary evaluation method bridges the gap between maintaining device availability and providing DC fault protection, as it isolates DC detection from the AC-based RMS evaluation.
3Device complexity
If a single RMS-based evaluation method is used, then the device structure remains simple, but both AC and DC fault current components cannot be reliably distinguished
Solution Approach 1:
The evaluation method is segmented into two independent comparison operations: RMS value comparison for AC components and time average value comparison for DC components. This segmentation achieves precise fault current detection without requiring complex additional hardware, as the segmentation is implemented through software-based evaluation of existing sensor signals.
Solution Approach 2:
The invention introduces a parameter change from single RMS evaluation to dual-parameter evaluation (RMS and time average). This parameter expansion enables distinction between AC and DC fault components while maintaining structural simplicity, as the additional time average calculation uses the same sensor inputs without requiring new hardware.
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 method allows the continued operation of electrical devices in AC networks by compensating for the negative influence of direct current components, enhancing device availability and safety without impairing the protective function of type A residual current circuit breakers.
Implementation Method 1
fault and/or leakage currents on the connection line are continuously detected as a sensor value by means of a current sensor assigned to the connection line within the device
Data Source
AI summary
A method for operating an electrical device in an AC power network, wherein the device is connected to the AC power network via a single-phase or multi-phase connection line, is characterized according to the invention in that fault and/or leakage currents on the connection line are continuously detected as sensor values by means of a current sensor assigned to the connection line within the device, that a time average and an RMS value are formed from the detected sensor values, that a criterion for the safe operation of the device is formed from the quotient of the average value and the RMS value, and that the device is disconnected from the AC power network when the criterion is met.