Oil Tan Delta Tester

1.What is Oil Tan Delta Tester?

The transformer oil dielectric loss tester (Oil Dielectric Loss Tester / Oil Tan Delta Tester) is a high-precision, fully automatic specialised instrument designed primarily to measure the dielectric loss factor (tanδ) of transformer oil. This enables the assessment of whether the oil has deteriorated, become damp or been contaminated, making it a crucial piece of testing equipment for ensuring the safe operation of power transformers.

 

This instrument primarily measures two key parameters:

• Dielectric loss tangent (tanδ): This is the most critical parameter. It can be understood as a measure of the degree of ‘energy loss’ in insulating oil. If the oil is pure, the loss is minimal; if the oil contains water, impurities or ageing by-products, the leakage current increases and the tanδ value rises significantly. This is a sensitive indicator for determining whether insulating oil has deteriorated.
• Volume resistivity (ρ): This measures the insulation resistance of the oil. The better the oil, the higher the resistivity (typically, >1×10¹³Ω*m )the resistivity drops significantly once the oil becomes damp or contaminated).

Why Use it?

In power systems, high-voltage equipment such as transformers, current transformers and bushings all rely on insulating oil for insulation and heat dissipation. If the insulating properties of the oil deteriorate, this may lead to internal breakdowns and short circuits in the equipment, or even cause power outages.

An oil dielectric loss tester is the tool used by power maintenance personnel to carry out regular ‘health checks’ on the oil. Through regular testing, it is possible to:

• Determine whether new oil meets the required standards.
• Monitor the ageing trends of oil in service.
• Determine whether oil filtration, regeneration treatment or replacement is required.

1. Basic Physical Principles

Transformer oil acts as an insulating medium; under an alternating electric field, two types of current are generated:

• Capacitive current: Normal charging and discharging current that does not result in energy loss
• Loss current: Leakage current caused by poor insulation, the presence of water or impurities, which generates heat and results in energy loss

The core measurement performed by the instrument is:

• Dielectric loss factor tanδ = Loss current / Capacitive current
• The higher the tanδ value, the poorer the oil quality and the poorer the insulation.

2. Measurement Principle (Cyril Bridge / Variable-Frequency Dielectric Loss Principle)

Current mainstream instruments utilise a fully automatic, variable-frequency, anti-interference dielectric loss measurement principle:

A stable high voltage is generated internally (typically 2 kV at power frequency or variable frequency)

which is applied to the oil sample in a three-electrode standard oil cup

A high-precision sampling circuit simultaneously measures:

• The applied voltage
• The total current flowing through the oil sample
• Digital algorithms are used to separate the capacitive and loss components
• Directly calculating:
• Dielectric loss tanδ
• Volume resistivity ρv
• Relative permittivity εr

Measurement of volume resistivity (DC method)

• Apply a DC voltage to the oil cup (typically 500V or 1000V).
• Measure the leakage current I flowing through the oil cup.
• Calculate the resistivity using Ohm’s law and the oil cup’s electrical constant.

ρ=U/I_{S *} K (where K is the oil cup’s electrical constant)

The lower the resistivity, the greater the amount of conductive impurities in the oil and the poorer the insulation performance.

3. Principle of Temperature Control

Since tanδ is highly sensitive to temperature, a constant temperature must be maintained:

• The instrument incorporates a built-in heating and temperature control system
• The oil sample is typically stabilised at 90°C (the standard test temperature)
• Temperature control accuracy is usually ±0.1°C to ±0.5°C
• This ensures that results obtained at different times and on different instruments are comparable.

Key Functions

1. Measurement of the dielectric loss tangent (tanδ)

Core function, providing a direct indication of the degree of moisture ingress, ageing and contamination in insulating oil.

2. Measurement of sample capacitance (Cx)

Simultaneous display of the oil sample’s capacitance value, aiding in the assessment of changes in oil quality.

3. High-precision temperature control

Automatic heating and temperature maintenance; standard operating temperature of 90°C, meeting national standard test requirements.

4. Fully Automatic Test Procedure

One-button operation: oil filling → heating → pressurisation → measurement → calculation → printing.

5. Data storage and printing

Built-in memory for storing historical data; supports on-site report printing.

6. Volume resistivity testing (high-specification models)

Simultaneous measurement of ρv for a more comprehensive evaluation of insulation performance.

7. Measurement of relative permittivity εr

Some models can automatically calculate and display this value.

8. Multiple safety protections

Automatic protection against overvoltage, overcurrent, overheating, short circuits and incorrect operation.

Main Features

1.High measurement accuracy

tanδ resolution up to 1×10⁻⁵, suitable for precision testing of low-loss insulating oil.

2.Strong interference resistance

Utilises frequency-modulated measurement technology to ensure stable and accurate results even in strong electromagnetic environments on-site.

3.Precise and stable temperature control

Temperature control accuracy of ±0.1℃ to ±0.5℃, with rapid heating and uniform temperature distribution.

4.Simple and Convenient Operation

Large Chinese display screen with a straightforward menu; no complex wiring required.

5.Fast Testing Speed

Rapid heating and short measurement times ensure high efficiency for single oil sample testing.

6.Compact and Portable Design

Integrated design combining the oil cup and main unit, suitable for both on-site and laboratory use.

7.Safe and Reliable

Automatic high-voltage reset and power-off protection upon lid opening ensure safer operation.

8.Compliant with national and industry standards

Meets the requirements of power industry standards such as GB/T 5654 and DL/T.

I. Preparations Before Testing

1.1 Instrument Preparation

• Place the instrument on a stable surface, connect it to the mains supply and ensure it is earthed.
• Power on the instrument and perform a self-test to confirm that the display, heating system and high-voltage system are functioning correctly.

1.2 Oil Sample Preparation

• Take a sample of transformer oil of at least 100 mL, ensuring it is free from bubbles and impurities.
• Prevent the oil sample from becoming damp or contaminated; operate in a dry environment where possible.

1.3 Cleaning and Drying the Oil Cup

• First, clean the three-electrode oil cup with anhydrous ethanol or petroleum ether.
• Dry using hot air or in an oven to ensure there is no residual oil or moisture between the electrodes.

II. Instrument Operating Procedures

1.Filling with Oil

• Slowly pour the oil sample into the oil cup to avoid creating bubbles.
• Once the liquid level reaches the standard mark, secure the oil cup lid.

2.Placing the Oil Cup

• Place the oil cup into the instrument’s heating chamber, ensuring good contact and central positioning.
• Close the chamber door securely (some models feature door-lock protection).

3.Parameter Settings

• Set the test temperature: 90°C (commonly used in national standards).
• Set the test voltage: generally 2000 V (1000 V or 1500 V may also be selected according to standards).
• Select test parameters: tanδ, Cx, resistivity (if supported).

4.Automatic heating

• Start heating; the instrument will automatically heat up and maintain a constant temperature.
• Once the temperature has stabilised, the system will prompt you to proceed with the test.

5.Start testing

• Press the ‘Start/Test’ button; the instrument will automatically apply voltage and take measurements.
• The process requires no manual intervention and is completed automatically:
• Apply voltage → Sampling → Calculation → De-energise → Display results.

6.Record Results

• The instrument automatically displays:
• Dielectric loss tangent tanδ
• Capacitance Cx
• Test conditions such as temperature and voltage
• Data can be saved and reports printed.

7.End of Test

• The instrument automatically reduces the voltage and resets, ensuring the high voltage is completely discharged.
• Once the oil cup has cooled, remove it, dispose of the oil sample and clean the oil cup.

III. Interpretation of Results

• Very low tanδ (<0.001): Good oil quality, excellent insulation.
• Significant increase in tanδ: The oil is damp, aged or contaminated, and insulation performance has deteriorated.
• Specific pass/fail criteria shall be determined in accordance with relevant standards such as GB/T 5654 and DL/T.

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