The dielectric strength of transformer oil is known as BDV or break-down voltage. A Transformer Oil BDV test is performed to find the dielectric strength of transformer oil.
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BDV is the minimum voltage to withstand the capacity of transformer oil without breakdown.
Transformer Oil BDV testing is necessary to determine the percentage of contamination and moisture content present in transformer oil to ensure its safe operation and reliability.
Therefore, transformer oil BDV test must be carried out periodically to ensure the transformer oil’s healthiness.
The oil used in the transformer is hygroscopic that is it absorbs moisture when it comes in contact with moist air.
If oil comes in contact with moist air it absorbs moisture content from the air and its dielectric properties decrease hence BDV value also deteriorates.
The breathing air inside the transformer tank is passed through the breather, which is connected to the conservator tank. The breather is filled with silica gel crystal to absorb the moisture from the air and supply moisture-free air for the breathing of the transformer.
As per IS 6792:1992, the minimum BDV value of transformer oil shall be as follows.
1) As per IEC, the minimum BDV value of transformer oil should be 30KV at. a 2.5mm gap.
2) The Breakdown voltage after oil filtration treatment should be a minimum 70KV at a 2.5mm gap.
Understanding Transformer Oil
The BDV test of transformer oil is very important because transformer oil’s main function is to provide cooling and insulation between the transformer winding and the transformer body.
It functions by insulating components within the transformer, which prevents electrical arcs and discharges. This is crucial for the reliable function of transformers, as even minor issues can lead to significant operational disruptions.
There are two main types of transformer oil:
- Paraffin-based oil
- Naphtha-based oil
Each type has traits that affect how efficiently they oxidize and cool. Naphtha-based oil oxidizes quickly, yet its produced sludge is usually soluble, maintaining smooth oil circulation. In contrast, paraffin oil undergoes slower oxidation, forming insoluble sludge that could clog the transformer’s system.
Breakdown Voltage (BDV) Test Principles
The Breakdown Voltage (BDV) test evaluates the dielectric strength of transformer oil. This test involves applying a steadily increasing voltage to an oil sample until it fails to insulate effectively, allowing electrical current to pass between the electrodes immersed within it. The voltage at which this breakdown occurs assesses the oil’s insulation capability.
The BDV test is conducted using a breakdown voltage tester. An oil sample is placed into a test cell, typically a cylindrical vessel, with a pair of submerged electrodes positioned at a specified gap, often 2.5 mm. The voltage is increased incrementally until an electrical arc forms, marking the oil’s dielectric breakdown point.
This test offers insights into the state of the transformer oil, particularly focusing on impurities and degradation. A high breakdown voltage indicates excellent insulating properties, suggesting that the oil is free from excessive moisture, dirt, and gas contamination. Conversely, a low BDV may point to a compromised oil condition, often due to moisture infiltration, particulate matter, or the presence of dissolved gases.
Why do we perform the Transformer Oil BDV Test?
Purpose of Transformer Oil BDV Test
- To predict the Residual life of the transformer
- Increase operation safety
- Increase transformer reliability
BDV Testing Procedure
The process involves:
- Preparation:
- Clean and dry all equipment thoroughly.
- Clean the sample valve of the transformer before taking the oil sample into the sample bottle then flush the sample bottle using transformer oil to avoid contamination of the oil
- Collect the oil sample from the bottom sample valve of the transformer into the sample bottle
- Cool the oil sample to approximately 27°C.
- Setup:
- To perform a Transformer Oil BDV Test, a BDV testing kit is used which contains a vessel that contains two electrodes mounted and separated by a 2.5mm or in some kits 4mm gap
- Set the electrode gap in the oil cup at 2.5 mm using a GO or NO GO Gauge.
- Pour the oil into the sample vessel, ensuring the level remains at least 10 mm above the electrodes.
- Allow the oil to settle for about 15 minutes to dissipate air bubbles.
- Testing:
- Power on the BDV tester, which increases voltage across the electrodes at a controlled rate (typically 3-5 kV per second).
- Continue elevating the voltage until a breakdown occurs, marked by an electrical arc forming between the electrodes.
- Post-test:
- Remove carbon residue from the electrodes using a non-corrosive glass stirring rod.
- Repeat the test six times, disregarding the first reading.
- Average the remaining five results to achieve an accurate BDV value.
- Interpretation:
- A BDV reading exceeding 30 kV is generally deemed satisfactory for transformer oils.
- Lower readings may indicate the presence of contaminants or moisture, necessitating corrective measures.
Maintaining high testing standards and rigorously following this protocol ensures accurate assessment of transformer oil, enabling proactive maintenance and efficient operation of transformer assets.
Factors Affecting BDV
Several internal and external factors influence the Breakdown Voltage (BDV) of transformer oil:
- Moisture content: Even trace amounts pose significant risks by introducing a conductive path within the oil.
- Contaminant intrusion: Particles like dust, dirt, and metallic deposits form conductive channels and instigate a breakdown in insulating properties.
- Operational stress: High operational temperatures and periods of overload hasten oil degradation. Under thermal stress, oil can oxidize, leading to the formation of acids and sludge.
- Dissolved gases: By-products of thermal and electrical stress, these gases can form bubbles when the oil is subjected to high electrical fields, decreasing its dielectric capacity.
- Ambient conditions: Fluctuations in ambient temperature can impact oil viscosity, affecting convective flow and cooling efficiency.
- Maintenance practices: Poor routines can lead to undetected contaminants and sub-optimal oil conditions.
Improving Transformer Oil BDV
To enhance the Breakdown Voltage (BDV) of transformer oil, several methods can be employed:
- Filtration: Pass the oil through high-efficiency filters to remove suspended particles and impurities that contribute to dielectric breakdown.
- Dehydration: Remove moisture content through vacuum dehydration or specialized dehydration plants. This process significantly improves BDV values by eliminating water, a primary conductor.
- Comprehensive oil management: Implement regular BDV testing and dissolved gas analysis to guide maintenance schedules and preemptive interventions.
By integrating these enhancement strategies, utilities can prolong the operational life of transformers and minimize the risk of unexpected outages. Maintaining high BDV values through effective oil conditioning not only protects electrical assets but also aligns operational practices with the evolving demands of modern energy systems.
In conclusion, maintaining the quality of transformer oil through regular testing and appropriate interventions is key to safeguarding transformer efficiency. By focusing on enhancing its dielectric strength, we can significantly extend the operational life of transformers, ensuring they continue to function reliably within our power systems.
