What is Transformer Oil Level Indicator?

The Transformer oil level indicator is specially designed to measure and indicate the oil level in transformer tanks, oil conservators and on-load tap-changers. Transformer Oil Level Indicator – Also known as Liquid Level Indicator or OLI for short. Glass tube oil level gauge and pointer oil level gauge, these two are traditional Transformer oil level indicators.

Transformer Oil Level Indicator

About Transformer Oil

Transformer oil is a fractionation product of petroleum. Its main components are alkanes and naphthenes. It is a light yellow transparent liquid. It is a kind of insulating oil. In oil-immersed transformers, transformer oil mainly plays the role of insulation and cooling.

Why is Transformer Oil Level Indicator so important?

The abnormal oil level of the transformer is mainly due to the oil level being too high and too low.

Oil level too high

When the transformer oil level is higher than the maximum oil level line. Depending on the cause, different methods should be used to deal with it.

If it is caused by overload, the load should be reduced.

If the three-phase current is seriously unbalanced and the current of a certain phase exceeds the rated value. The load should be adjusted to achieve a basic balance.

If the cooler is abnormal, the temperature of the transformer oil rises. Oil expands when heated. cause the oil level to rise. Then check whether the cooler is blocked by dust accumulation, and whether the upper and lower valves of the oil pipe are open. Whether the fan and submersible pump are running normally. Whether the temperature of the cooling medium is appropriate, etc. Prompt treatment returns the cooler to normal operation.

If the oil level is too high due to excessive oiling of the transformer, the oil should be drained to an appropriate height.

Oil level too low

When the transformer oil level is significantly lower than the oil level corresponding to the oil temperature at that time. The oil level is too low when the oil level is below the minimum oil level line or is not visible.

When the transformer leaks oil. The oil was not replenished in time after the oil was drained due to work, or the original oil level of the transformer was not high. When the transformer load suddenly drops or the external ambient temperature drops significantly, the oil level will be too low.

Too low oil level will cause gas protection action. When there is a severe shortage of oil, the transformer core and windings will be exposed to the air. It is easy to get wet and reduce the insulation ability, and may even cause insulation breakdown.

Therefore, when the transformer oil level is found to be too low. The reason should be found and dealt with.

If the oil level is low and no oil leaks are found. The operator should report and replenish qualified transformer oil of the same type as soon as possible.

If it is because of an oil leak, the oil level has dropped significantly. Measures should be taken to eliminate the oil spill and refuel immediately to bring the oil level back to normal.

If a lot of oil leaks. When the oil level drops too much, it endangers the safe operation of the transformer. A dispatch application should be reported to deactivate the transformer.

Transformers for large-scale strong oil circulating water cooling. If the oil level is found to be low. Check the water for oily flowers. To prevent water seepage in the oil from endangering the transformer insulation. Replenish the oil after finding out the cause.

On the other hand, the transformer oil level rises or falls due to seasonal changes. is a normal phenomenon.

If the oil level is too high in summer, try to drain the oil. When the oil level is too low in winter, try to refuel. In order to maintain the normal oil level and ensure the safe operation of the transformer.

Therefore, the normal oil level should be maintained when the transformer is running. In order to monitor the oil level of the transformer, the oil pillow of the transformer is equipped with a glass tube oil level gauge or a magnetic needle oil level gauge.

When the transformer is running, under normal circumstances, the oil level of the transformer changes with the change of the transformer oil temperature.

When checking the oil level, pay attention to whether it is a fake oil level.

When the oil pillow respirator is blocked, the vent of the explosion-proof pipe is blocked, the oil gauge pipe is blocked or the oil level gauge is damaged, the false oil level will appear.

At this time, although the changes of transformer oil temperature, ambient temperature, load, etc. are normal. But the oil level does not change or changes abnormally.

Extended reading: Ultrasonic Oil Level Sensor-External Paste-Truck Fuel Tank

Oil level indicator types for transformers

transformer oil level sight glass

An oil pillow is an oil storage device used in transformers.

Its function is that when the transformer increases the load, the oil temperature rises, and the oil in the oil tank expands, and then too much oil will flow into the oil pillow.

Conversely, when the temperature drops, the oil in the oil pillow will flow into the oil tank again. It can automatically adjust the oil level. That is, the oil pillow plays the role of oil storage and oil replenishment. Make sure the tank is full of oil.

At the same time due to the equipped oil pillow. Reduce the contact area between the transformer and the air. And the moisture, dust and oxidized grease absorbed from the air are deposited in the depositor at the bottom of the oil pillow. Thereby greatly slowing down the deterioration rate of transformer oil.

There is a glass oil level gauge (or oil level gauge) on the side of the oil pillow. The change of the oil level in the oil pillow can be observed at any time.

Pointer Type Oil Level Gauge

Pointer Type Oil Level Gauge

In power systems, oil-immersed transformers above 35 kV usually adopt a hermetic design. And equipped with oil conservator and oil level gauge with capsule bag.

The commonly used oil level gauges for transformers are divided into pointer oil level gauges and tube oil level gauges.

The working principle of the pointer oil level gauge is when the oil level of the oil conservator changes. Cause the change of the diaphragm, causing the linkage rod to move up and down. Through the action of magnetic torque, the driven magnet pointer is driven to indicate the oil level.

The working principle of the pointer oil level gauge is when the oil level of the oil conservator changes. Drive the oil float up and down, and use the pressure relief valve to adjust the oil level to indicate the oil level.

Monitoring Transformer Oil Conservator Oil Levels with Pressure Sensors

Pressure sensors are widely used in liquid height measurement. The liquid height can be obtained by measuring and converting the liquid pressure value. In the transformer conservator, an oil drain is usually connected to the bottom of the conservator for oil draining.

To eliminate the harsh requirements of mechanical transmission and fully enclosed conditions. At the same time, it can meet the precise measurement of the oil level of the oil conservator. By installing a pressure sensor at the end of the oil drain pipe. The oil level height of the oil conservator is calculated by the two-point pressure difference algorithm.

In liquid height measurement, it can be calculated by pressure, and the calculation formula of liquid pressure is:

Oil level detection device

P = ρgh

ρ is the density of the liquid;
g is the acceleration of gravity, which is 9.8 m/s2;
h is the liquid level height.

Through the two pressure sensors installed in the oil level detection device, two pressure values P1 and P2 can be obtained,
P1 = ρ1 gh1
P2 = ρ2 gh2
h3 = h1 – h2

For small objects in the transformer oil conservator, the oil temperature difference between the two points of the oil drain pipe is negligible, so it can be considered that ρ1 = ρ2, h1 can be obtained by calculation, and then the oil level can be calculated.

h1 = P1 h3 P1 – P2

P1 and P2 are the pressure values of two points;
h3 is the height difference between two points (this value is a definite value).

By this method, the influence of the change of the insulation temperature of the oil conservator on the measurement results can be eliminated.

Online Oil Level Monitor for Non-contact Transformer Conservator

The transformer oil pillows are all closed. The maintenance and replacement of the existing oil level gauges require power outages, openings, and oil draining, which is very complicated. Therefore, a non-contact transformer oil pillow oil level is installed in
Line monitor is very necessary.

Sonar ranging principle:

The probe is adsorbed on the bottom of the oil pillow by a magnet. The sonar wave (mechanical wave) emitted by the probe can penetrate the steel wall and propagate upward in the oil. It will be reflected back when it encounters the oil-gas interface, and detected by the probe.

The oil level height h.h=V×t/2 is calculated by detecting the time difference t between the transmitted wave and the returning wave combined with the sound velocity value V. of the oil.

The principle of sonar distance measurement is the best solution for non-contact measurement of oil level in the oil pillow.

It only needs to attach the probe to the outer wall of the bottom of the oil pillow through the magnet to measure the internal oil level height. The probe does not touch the internal medium. Real non-contact measurement is realized.

There is no need to open holes on the oil pillow, which is very convenient for installation and maintenance.

Extended reading: RF Admittance Level Sensor

Featured oil level indicator types

  • Manual Oil Tank Level Measurement
  • Magnetic flip level gauge for Tank Level Sensing
  • Buoyancy Method for Oil Level Measurement
  • Hydrostatic level gauge for Oil Level Measurement
  • Ultrasonic method for Oil Level Measurement
  • Electrical method for Oil Level Measurement
  • Radar Level Sensors for Oil Level Measurement
  • Magnetoelectric method for Tank Level Sensing


The Oil Level Indicator (OLI) was developed in response to the need of maintenance professionals to gauge the oil level inside large tanks, gearboxes and reservoirs.

Extended reading: Magnetostrictive Level Transmitter Working Principle

A tank level sensor is a level sensor used to measure the tank level. The liquid level is the level of the liquid in a sealed container (such as a water tank) or an open container (water tank). The instrument that measures the liquid level is called a liquid level sensor, a liquid level gauge, or a liquid level transmitter. The liquid level sensor is a kind of level instrument. Hydrostatic, Ultrasonic, Magnetostrictive, Radar, Differential Pressure are common choices for tank level measurement. Liquid level sensors have been around for decades for leak detection or level measurement. Commonly measured mediums are Water, Fuel, Diesel, Gasoline, diesel, liquefied gas, liquid ammonia, etc.

Extended reading: Amazing Solutions for Continuous Liquid Level Measurement

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Sino-Inst offers over 10 Solutions for Transformer Oil Level Indicator. About 50% of these are float liquid level meters, 40% are the level switches.

A wide variety of Solutions for Transformer Oil Level Indicator options are available to you, such as free samples, paid samples.

Sino-Inst is a globally recognized supplier and manufacturer of Solutions for Transformer Oil Level Indicator instrumentation, located in China.

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