High-Voltage Circuit Assembly for Low-Capacitance Dissipation Factor Testing
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Solution Overview
Problem
Current test devices for measuring the dissipation factor in high- and medium-voltage cables face limitations due to interference-induced measurement inaccuracies and restricted measuring ranges, particularly when dealing with low capacitance values, as a result of unwanted harmonics generated by control interventions on switched-mode power supplies.
Innovation Solution
A circuit arrangement where high-voltage sources are controlled independently by a clock signal, synchronized to generate precise, sinusoidal AC voltages without regulation interference, minimizing harmonics in the test voltage and current, thus enhancing measurement accuracy and expanding the measuring range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If control interventions are applied to switched-mode power supplies to regulate test voltage, then the test voltage can be adjusted to match load requirements, but unwanted harmonics are generated that reduce measurement accuracy
Solution Approach 1:
The system is divided into two independent control loops: a voltage regulation loop that adjusts the amplitude of the test voltage to match load requirements, and a measurement loop that determines the dissipation factor. By segmenting the control functions, the voltage regulation can operate without introducing measurement errors, while the measurement loop independently captures accurate phase information despite voltage variations.
Solution Approach 2:
An intermediary measurement circuit is introduced that measures the actual test voltage and uses this information to compensate for variations in the dissipation factor calculation. This intermediary measurement allows the system to maintain accurate measurements even when the test voltage amplitude is dynamically adjusted by the control interventions.
2Reliability
If control interventions are applied to switched-mode power supplies, then voltage regulation is achieved, but measurement inaccuracies occur due to interference from control signals
Solution Approach 1:
The measurement function is extracted from the voltage control function. The system separates the voltage regulation task (performed by control interventions on the switched-mode power supply) from the measurement task (performed by an independent measurement circuit that captures voltage and current phase information). This extraction allows reliable voltage regulation while obtaining accurate phase measurements free from control signal interference.
Solution Approach 2:
The measurement circuit performs preliminary measurement of the test voltage and current before the control interventions modify the voltage waveform. By capturing the phase relationship at the point of injection, the system obtains accurate measurement data before harmonics and control signal distortions affect the waveform, ensuring measurement precision while maintaining voltage regulation stability.
3Adaptability or versatility
If the measuring range is extended to include low capacitance values, then more cable types can be tested, but measurement accuracy deteriorates due to low test current amplitudes
Solution Approach 1:
The system replaces direct measurement of small test current amplitudes with an indirect measurement approach. Instead of relying on measuring very small currents directly (which suffers from low signal-to-noise ratio), the system measures the voltage across a known reference impedance and calculates the current from this voltage measurement. This substitution allows accurate measurement of phase angles even when the actual test current amplitude is very low, enabling extended measuring range while maintaining 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
This approach significantly reduces harmonic distortion in test voltages and currents, allowing for precise determination of the dissipation factor across a broader range of capacitances, including those below 15 nF, with improved accuracy and reduced noise, enabling the measurement of longer high-voltage cables with minimal interference.
Implementation Method 1
a high-voltage transformer, and a rectifier circuit (23, 24)
Implementation Method 2
a switched-mode power supply, a high-voltage transformer, and a rectifier circuit (23, 24)
Data Source
Figure 1
Figure 2
Figure 3a~3b
AI summary
The invention relates to a circuit assembly (1) for producing a test voltage for testing a test object (2), comprising two high-voltage sources (3, 4) for producing a positive and negative high voltage of variable amplitude at respective outputs (5, 6) thereof and a high-voltage switch assembly (7), which is arranged between the outputs (5, 6) of the two high-voltage sources (3, 4) and the test object (2) and which can be switched suitably in order to successively charge and discharge the test object (2), wherein furthermore a closed-loop controller (8) is provided, which measures the present test voltage on the test object (2) and acts on the high-voltage switch assembly (7) in order to charge and discharge the test object (2) in a defined manner in dependence on the measured test voltage. According to the invention, the closed-loop controller (8) does not act on the two high-voltage sources (3, 4), and a separate open-loop controller (14) is provided for the two high-voltage sources (3, 4), wherein the open-loop controller (14) produces a clock signal (T) that is independent of the voltage on the test object (2), such that a synchronized, predefined high voltage (U1, U2) not influenced by the closed-loop controller (8) is provided by the high-voltage sources (3, 4).