Comprehensive Analysis of Transformer Oil

Transformer oil is a kind of petroleum liquid, which has the possibility of combustion and has shortcomings in environmental protection. However, due to the excellent insulation and thermal conductivity of transformer oil and its low price, therefore, most power transformers still use transformer oil as insulation and cooling medium.

1. Composition of Transformer Oil

Transformer oil, a mineral oil refined from petroleum, contains alkanes, cycloalkanes, aromatic hydrocarbons and other components. In low temperature environment, it will be solidified, while in high temperature environment, it may decompose and burn. It is worth mentioning that when transformer oil decomposes at high temperature, it will generate methane, ethane, ethylene, acetylene, hydrogen, water and other substances. In addition, after a transformer accident, we can determine the temperature of the accident point by measuring the proportion of various gases through chromatographic analysis of the transformer oil.

2. Label of Transformer Oil

The name of transformer oil is based on its freezing point and is usually presented in the form of DB-XX. Wherein, "D" represents "oil used for electric power", "B" represents "transformer oil", and XX represents the specific value of the freezing point. For example, DB-10 transformer oil means that its freezing point is -10 ℃, while DB-25 transformer oil has a freezing point of -25 ℃, and so on.

In general, there are three types of transformer oil in China's transformer oil standards, i.e., 10#, 25#, and 45#, which correspond to different freezing point ranges. This standard was formulated in the 1960s with reference to the GOST standard (ΓΟСТ 10121) of the former Soviet Union and IEC 296 standard.

When the freezing point of transformer oil reaches below -7 ℃, the transformer oil is classified as 10#. Similarly, if its freezing point reaches below -22 ℃, the transformer oil is determined as 25#. When the freezing point is further reduced below -45 ℃, the transformer oil is classified as 45#.

3. Function of Transformer Oil

1. Insulation

Transformer oil can effectively isolate the live parts with different potentials to prevent short circuit risk.

2. Cooling

Transformer oil can absorb the heat generated inside the iron core and winding, and dissipate the heat through circulation to ensure the safety of the equipment.

3. Arc Extinguishing

On the oil circuit breaker and the on load tap changer of the transformer, the arc generated during contact switching will cause the decomposition of transformer oil and generate a large amount of gas and pressure, so as to enhance the arc extinguishing ability of the medium and make the arc extinguish rapidly.

4. Basic Requirements for Transformer Oil

Electrical equipment puts forward a number of key requirements for transformer oil, including:

1. It must have high dielectric strength to adapt to various working voltage conditions.

2. Its viscosity should be kept at a low level, which helps to improve the effect of circulating convection and heat transfer.

3. It shall also meet the high flash point temperature to ensure fire safety; It shall have good low temperature performance to withstand the low temperature environment that the equipment may encounter.

4. Its antioxidant capacity can not be ignored, because it will directly determine the service life of oil products.

5. Quality Standard of Transformer Oil

Electrical equipment has strict standards for the quality of transformer oil.

1. The dielectric strength of transformer oil must be high enough to cope with different working voltages.

2. The viscosity of transformer oil should be moderate, neither too high nor too low, to ensure good circulating convection and heat transfer effect.

3. The flash point temperature is also a key indicator, which is directly related to fire safety.

4. The transformer oil also needs to have good low-temperature fluidity to adapt to the low-temperature environment that the equipment may face.

5. The oxidation resistance of transformer oil is very important, because it will determine the service life and overall performance of oil products.

6. Expansion Characteristics of Transformer Oil

The transformer oil has the characteristics of thermal expansion and cold contraction, and its volume change rate is about 7/10000. This means that when the temperature changes by one degree, the volume of oil will correspondingly increase or decrease by seven tenths of a million. During the operation of the transformer, the top oil temperature is allowed to rise by 55 ℃. Based on this data, we can calculate that the volume change of transformer oil is about 3.85%. In order to cope with this volume change, 5%~8% of the space needs to be reserved inside the oil immersed transformer. These spaces are usually realized through the design of oil conservator, corrugated oil tank and corrugated oil conservator, so as to ensure the free thermal expansion and contraction of transformer oil.

7. Physical Properties of Transformer Oil

The transformer oil not only has the characteristics of thermal expansion and cold contraction, but also shows other important physical properties. These properties are very important for the normal operation and protection of transformer.

1. Appearance and Transparency

(1) Color Change: new transformer oil is usually light yellow, but with oxidation, its color will gradually change to dark red. During operation, the rapid darkening of oil color often means that the oil quality has deteriorated.

(2) Odor Identification: high quality transformer oil shall be odorless or only slightly kerosene smelling. If there is a burning smell, it may indicate that the oil is overheated when it is dry; If it is sour, it indicates that the oil has been seriously aged; The appearance of acetylene odor indicates that there may be an over-current phenomenon in the oil.

(3) Transparency Observation: the new oil in the glass bottle should be transparent with blue purple fluorescence. If the fluorescence and transparency disappear, it usually means that water, mechanical impurities or free carbon are mixed in the oil.

2. Flash Point of Transformer Oil

During the heating process of oil, the combustibles formed by the mixture of its steam and air will produce instant flash fire when they contact the flame. The temperature at this time is called the flash point of oil. This index reflects the evaporation degree of oil, that is, the lower the flash point, the higher the volatility of oil. In order to ensure the quality of transformer oil, its flash point should be at least 135 ℃.

3. Density of Transformer Oil

The density of transformer oil is about 895kg/m³, which has certain reference significance in evaluating the quality of oil.

4. Moisture in Transformer Oil

Moisture is the key factor affecting the electrical performance of transformer oil. Excessive moisture will lead to a significant decline in insulation performance. Therefore, it is very important to keep the moisture content in transformer oil as low as possible. Generally, the water content of new oil shall not exceed 20mg/l.

8. Chemical Properties of Transformer Oil

1. PH Value of Water Soluble Acid

The PH value of new oil shall be ≥ 5.4, while that of oil in operation shall be ≥ 4.2. This is an important index to measure the acid content in transformer oil.

2. Acid Value

The acid value of transformer oil indicates the content of organic acids in the oil, while the neutralization acidity reflects the acidity of the oil after oxidation test. Acidity is usually measured by the mass of KOH required for neutralization (mg), in mgkoh/g. Oil with high acidity will cause corrosion to metal equipment, especially when water is mixed in the oil, its corrosiveness is stronger, and the acid value will gradually increase, which is also an important sign of oil deterioration. The acid value standard of new oil is ≤ 0.03mgkoh/g, while that of oil in operation is ≤ 0.1mgkoh/g.

9. Electrical Properties of Transformer Oil

Insulation strength is a key electrical characteristic of transformer oil under high voltage electric fields. When transformer oil breaks down in a standard oil cup (electrode distance 2.5mm), the voltage at this time is called the "breakdown voltage", and the electric field strength at this time is defined as the "insulation strength". When evaluating the quality of transformer oil, the average of six breakdown voltages of a standard oil cup is usually taken as a reference. According to the national standard, the breakdown voltage of transformer oil in operation should reach a certain standard.

Impurities and moisture can significantly reduce the insulation performance of transformer oil. The dielectric loss angle tan δ, which measures the ratio of active power to reactive power flowing through insulation materials under AC voltage, is closely related to the performance of insulation materials and is not affected by size and volume. For new oil, its dielectric loss angle tan δ should be controlled below 0.5%. At the same time, the volume resistivity of transformer oil at 90 ℃ should reach or exceed 6 × 1010 (Ω· m) to ensure its good insulation performance.