VLF Cable Tester With Integrated Fault Pre-Location
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Solution Overview
Problem
Existing fault location methods for high- or medium-voltage cables are limited by inaccurate current measurement, high energy discharges that can damage the cable, and the need for breakdown detection, which can be unreliable and require complex evaluation techniques.
Innovation Solution
A compact and cost-effective VLF test device with integrated fault pre-location capabilities, utilizing a circuit arrangement with a test voltage generation unit, current collection point, and evaluation electronics to generate both low-frequency diagnostic signals and high-frequency pre-location signals without the need for external breakdown detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external coupling elements (capacitive voltage divider or inductance) are used for fault location measurement, then fault distance can be determined, but the current measurement becomes inaccurate and device complexity increases
Solution Approach 1:
The patent combines the fault location measurement function with the existing VLF test generator by utilizing the internal current measurement capability already present at the output. Instead of using external coupling elements, the invention merges the diagnostic function into the core device, thereby maintaining measurement accuracy while avoiding additional complexity from external components.
Solution Approach 2:
The VLF test generator is designed to perform multiple functions: it generates the test voltage for insulation testing and simultaneously measures the current for fault location determination. This multi-functionality eliminates the need for separate external measurement devices, resolving the contradiction between measurement precision and device complexity.
2Measurement precision
If high-voltage breakdown method is used for fault location, then fault distance can be determined, but high energy discharges can damage the cable
Solution Approach 1:
The patent performs fault location measurement before applying high-voltage breakdown. By using the VLF test voltage to generate diagnostic signals and determine fault distance in advance, the system identifies the fault location without causing damage, thereby eliminating the harmful effects of high-energy discharges while maintaining measurement precision.
Solution Approach 2:
The invention converts the low-energy VLF test voltage, which would normally only be sufficient for insulation testing, into a useful diagnostic tool for fault location. By utilizing the existing test voltage to generate measurable diagnostic signals, the system achieves accurate fault location without the harmful high-energy discharges associated with traditional breakdown methods.
3Measurement precision
If breakdown detection is used for fault location, then fault distance can be determined, but reliable detection is difficult and requires complex evaluation techniques
Solution Approach 1:
Instead of waiting for complete breakdown to occur, the patent uses partial diagnostic signals generated during the VLF testing phase. By measuring the current and voltage relationship at the existing test voltage level, the system determines fault distance without requiring full breakdown, thereby simplifying the evaluation process while maintaining measurement precision.
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 precise and efficient fault pre-location during cable testing, reducing the risk of secondary damage, eliminating the need for high-energy discharges, and providing continuous fault distance evaluation without reliance on breakdown detection.
Implementation Method 1
mobile VLF test devices enable testing of a device under test using a test voltage, e.g., in the range of 20 kVpeak to 120 kVpeak (generally not limited), which is generated with a highly precise voltage waveform at a frequency in the range of 0.01 Hz to 1 Hz, the so-called Very Low Frequency (VLF)
Implementation Method 2
a connecting conductor (39) which electrically connects the low-voltage earthing input (7A) to the protective earth connection (35), so that the connecting conductor (39) represents a current collection point through which a measuring current flows
Implementation Method 3
a low-frequency signal tap at the current collection point, at which a low-frequency diagnostic signal is generated based on the measuring current for a VLF phase rotation measurement
Implementation Method 4
a high-frequency signal tap at the current collection point, at which a high-frequency pre-location signal for measuring a transient high-frequency oscillation is generated based on the measuring current
Implementation Method 5
evaluation electronics which are connected to the low-frequency signal tap for receiving the low-frequency diagnostic signal and are designed for a VLF phase rotation measurement and which are connected to the high-frequency signal tap for receiving the high-frequency pre-location signal and are designed for a fault distance evaluation
Data Source
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AI summary
A test device (1) for testing a high- or medium-voltage cable (3) comprises a circuit arrangement (5) with a test voltage generation unit (7) comprising a low-voltage earth input (7A) and a high-voltage output (7B) configured to provide a variable test voltage, a test-device terminal (33) for connecting the high-voltage output (7B) to a conductor (3A) of the high- or medium-voltage cable (3), a protective earth terminal (35) for connecting to protective earth (19), a connecting conductor (39) electrically connecting the low-voltage earth input (7A) to the protective earth terminal (35), such that the connecting conductor (39) constitutes a current collection point through which a test current flows during testing, and a high-frequency signal tap (43) at the current collection point.The test device (1) generates a high-frequency pre-localization signal based on the measuring current for measuring a transient high-frequency oscillation. Furthermore, the test device (1) includes evaluation electronics (9) connected to the high-frequency signal tap (43) for receiving the high-frequency pre-localization signal and configured for fault distance evaluation. The test device (1) thus enables, in addition to VLF phase rotation measurement, a pre-localization measurement with respect to a defect.