It is a core diagnostic tool used to measure the tangent of the dielectric loss angle (tan delta) of insulating materials in high-voltage electrical equipment.
It is used to determine whether the insulation performance of electrical equipment (such as transformers, instrument transformers, bushings, arresters, capacitors, etc.) is damp, aged, or defective.
1. Core Measurement Indicators
Tangent of the dielectric loss angle (tan delta)
Measure the actual capacitance value of the equipment.
By comparing with factory values or historical data, if the capacitance changes significantly (e.g., an increase may indicate moisture or short circuit, a decrease may indicate open circuit or oil shortage), it can assist in diagnosing internal structural faults.
2. Working Principle
Modern dielectric loss testers mainly use variable frequency anti-interference technology and digital signal processing (DSP) technology:
Variable frequency output: The instrument internally generates a high-precision sinusoidal voltage with a frequency adjustable between 45Hz and 65Hz (to avoid on-site 50Hz power frequency interference and its harmonics).
Signal acquisition: Simultaneously acquires the voltage signal applied to the test object and the current signal flowing through it.
Digital filtering: Uses Fourier transform (FFT) or correlation algorithms to accurately extract the fundamental component from signals containing strong power frequency interference, calculating the phase difference between voltage and current.
Calculation result: Calculates tan delta based on the phase difference.
3. Main Wiring Modes
To adapt to different test objects and field conditions, dielectric loss testers typically support multiple wiring methods:
Positive Connection:
Applicable: When both poles of the test object are insulated from ground (e.g., coupling capacitors, independent bushings).
Features: High measurement accuracy, less affected by external interference, and is the preferred method.
Reverse Connection:
Applicable: When one end of the test object is grounded (e.g., transformer windings, circuit breakers, most equipment in operation).
Features: The instrument is at high potential, requiring special safety precautions, but there is no need to remove the equipment's grounding wire, making on-site operation convenient.
Self-Excitation:
Applicable: Specifically for testing electromagnetic voltage transformers (PTs).
Principle: Utilizes the PT's own electromagnetic induction characteristics, applying voltage from the secondary side to induce high voltage on the primary side for measurement, without the need for an external high-voltage source.
CVT Method (Capacitive Voltage Divider Method):
Applicable: Capacitive voltage transformers (CVTs).
Features: Can measure the tan delta and capacitance of the main capacitor (C1) and the intermediate voltage transformer (C2) in one test.
External Excitation / Different Frequency Method: Used with an external high-voltage test transformer for testing large-capacity or ultra-high-voltage equipment.
4. Key Technical Features
Strong anti-interference capability: This is the most important indicator of modern dielectric loss testers. It can accurately measure even under strong electromagnetic interference at substation sites (even when the interference current is greater than the test current).
High precision: Tan delta measurement accuracy can reach 0.05% or even higher.
Safety: Equipped with overvoltage, overcurrent, short-circuit, and poor grounding protection functions; comprehensive shielding and isolation measures are provided for reverse connection.
Intelligence: Automatically identifies wiring mode, automatically boosts voltage, automatically switches ranges, stores data, prints reports, and communicates with a host computer.
5. Typical Application Scenarios
Transformers: Measure the tan delta of windings and bushings to assess insulation moisture.
Instrument transformers: Detect the main insulation and end-screen insulation of current transformers (CTs) and voltage transformers (PTs).
High-voltage bushings: Monitor the insulation condition of capacitive bushings to prevent explosion accidents.
Arresters: Measure the resistive current or tan delta under total current of metal oxide arresters (MOAs) (some specialized instruments).
Capacitors: Detect insulation degradation of coupling capacitors and circuit breaker grading capacitors.
Cables: Partial discharge measurement of long-distance high-voltage cables (requires specialized low-frequency or very low-frequency equipment, but the principle is similar).
6. Brief Operation Procedure
Preparation: Power off the test object, fully discharge, and attach grounding wires; clean the surface of the test object.
Wiring: Choose positive, reverse, or self-excitation method based on the type of test object, and connect the high-voltage line, measurement line, and ground wire. Ensure reliable grounding.
Parameter setting: Select the test mode on the instrument, set the test voltage (usually 10kV, but can be customized), and choose the frequency (e.g., 47.5Hz for different frequency).
Start test: Start the instrument, which automatically boosts voltage, samples, and calculates.
Read data: Record the tan delta and capacitance values.
Discharge and disconnect: After the test, the instrument automatically reduces voltage and discharges. Confirm no residual voltage before disconnecting wires.
7. Result Analysis and Judgment
Longitudinal comparison: Compare with factory values, commissioning test values, or historical preventive test data. If tan delta significantly increases (e.g., an increase of more than 30% or absolute value exceeds the standard), there may be insulation hazards.
Horizontal comparison: Compare data of three-phase equipment of the same model, batch, and environment. If one phase's data is significantly larger, that phase may have defects.
Temperature conversion: Tan delta is greatly affected by temperature; it is usually necessary to convert the measured value to a standard temperature (e.g., 20°C) for comparison.
Note: Surface contamination and high humidity can also cause tan delta to increase. Before testing, the influence of surface leakage should be eliminated (e.g., using a shielding ring).
Summary
The dielectric loss tester is the "microscope" for insulation supervision of power equipment. By capturing subtle changes in energy loss, it can sensitively detect early insulation hazards such as moisture and aging before equipment breakdown accidents occur. Its anti-interference capability and measurement accuracy are key indicators for evaluating the performance of a dielectric loss tester, playing an irreplaceable role in ensuring the safe and stable operation of the power grid.
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