Transformer oil : Answers Every Electrician Should Know

Heat kills electrical transformers. So one of the most important maintenance tasks for transformers is to check oil level in the oil tank. But what would happen if the oil level becomes too low? What if the transformer tank become empty? In this article I will answer important questions about transformer oil. Let’s get started.

Why is Transformer Oil Important?

Transformer oil increases the isolation between transformer winding and core, protects metallic parts from oxidation, transfers heat from core and winding to the outside atmosphere, helps in decreasing transformer size and gives us a good analysis view in case of transformer internal faults.

One of the main functions of oil in a transformer is to keep the transformer temperature stable. As oil is placed around the windings inside the transformer core. So, oil acts as an insulator in windings, as a result, a reduction in copper losses reduces heating. In short, Transformer oil acts as a cooling agent. Oil provides an arc quenching medium. It reduces different types of discharging effects and improves insulation. Another function of oil in a transformer is to increase the lifetime of the transformer.

So, we can summaries the answer of the important question, What is the importance of transformer oil? as follows:

  • Increases isolation between windings and core. (Transformer oil has a dielectric strength of 80 KV/CM).
  • Transfers transformer temperature from windings and core to the outside atmosphere with the help of cooling methods. Current passing through a coil or a conductor produces heat. And oil takes that heat and transfers it to the outside environment.
  • Protects metallic parts inside the transformer from oxidation which can affect conductivity.
  • A good oil analysis gives us information about transformer internal faults.
  • Helps in decreasing the overall size of the transformer, As the oil dielectric strength of 80 KV/CM while this of air is 30KV/CM, This means that the same voltage level transformer will be smaller for oil immersed transformer than dry type transformer.

As mentioned above, the oil has an important part to make the transformer working properly. But, Does the oil remain forever? When should I replace it?

When Should I Change The Transformer Oil?

You should change transformer oil based on tests and analysis, You need to change it in the following cases:

  • If  the transformer gets very wet. And the water contaminate in the oil exceeds the accepted levels.
  • When the transformer has an internal fault, like short circuit, in this case the oil properties will change, and it will no longer good for insulation.
  • When the viscosity of the oil becomes very high or very low.
  • If the dielectric test shows a very low breakdown voltage.

The quality of the oil degrades with the passage of time and usage of the transformer. Therefore, it is important to check transformer oil during a specific period of time. The oil can last for a long period of time, if the transformer has no internal faults and the oil is protected from the moisture and environmental pollution.

The oil should do what it should do, which is isolation and cooling. This requires the oil properties not to change. Some of these properties are :

  • Dielectric strength. We perform this test to ensure that the oil break down voltage is acceptable. A good oil should has break down voltage of 30KV per 2.5 mm for the oil used in 11KV transformers. And 55KV per 2.5 mm for oil used in 66KV transformers.
  • Oil viscosity is so important for oil movement inside the transformer and of course for cooling the transformer. Oil viscosity should be within accepted limits, because the lower the viscosity the bad cooling, and the higher the viscosity the higher the oil evaporation process. (note that oil vapor is flammable). Oil viscosity should be around the value of 37 centi-stokes or mm2/sec.
  • Oil should be impurities free.
  • Moisture in oil, Moisture of 0.06% in the oil decreases its dielectric properties 50%. I have written a detailed article about oil moisture you can read it here.

However, it is worth mentioning that different range and rating transformer needs different time duration to change the oil.  A transformer of 132 KV should be checked for Gas Dissolve Analysis (GDA) after a time gap of two years. Some transformer yearly, Half-yearly, or even monthly check-up for an oil change. Some other factors we need to keep in mind and that require to do oil change are also discussed here. 

Another thing to consider for oil changing is to check the color of the oil. If the color of transformer oil is too dark or black. Then it is necessary to change it. As it is a sign of danger and the oil change is necessary.  Similarly, If you hear any noise inside the transformer, you should check for an oil change. 

Keeping the oil in a good condition and protecting it from environmental conditions, doesn’t mean that the oil is safe. One of the most common issues that I face in my work, is Oil leakage. Some times, we find the oil level too low. This is an issue that we take seriously. Let’s discuss the oil leak in more details.

What Are The Transformer Oil Leaks Reasons?

Transformer oil leaks can occur due to rubber sealant bad condition, bad sampling valve, transportation or mechanical damage of the transformer tank. The most common leakage causes I have ever seen are bad sampling valve and bad rubber sealant of the transformer tank and bushings.

Oil is one of the most important factors for a transformer. Sometimes oil leaks from the transformer tank, it is very dangerous and causing adverse effect to both distribution system, transformer as well as environment. Let’s discuss different reasons for transformer oil leaks in details.

  • Sealant Quality and condition

One of the main causes for transformer oil leaks is the cracking or aging of the transformer sealant. Sometime repairing firms or even manufacturing companies use low-quality material in the sealant, which causes aging to the sealant. And thus, leaks occur.

  • Oil sampling Valve

Some transformers have poor-quality butterfly valves. This means they consist of a single layer and have a rough and thin surface. And it is another main reason for oil leakage. The continuous usage of the valve causes it to leak. The leak from the valve is usually in shape of drops.

You can discover it easily, as the oil will be just under the valve on the ground, the body of the transformer will not be highly affected with the spillage.

  • Transformer Installation

Another reason for transformer oil leakage is the improper installation of a transformer. Like if the transformer is unstable or in even condition. Or the flange connection is dislocated or deformed.

  • Rubber seals

Some transformer have rubber seals to prevent oil leakage from transformer tank and bushings. Sometimes the rubber get damaged causing oil leakage from the tank.

Exposure to environmental conditions such as temperature rise, cold and sun as well as aging of the rubber causes it to get damaged causing oil leakage. In this case the oil will be clear on the transformer body and on the ground.

  • Miscarriage and transportation

Sometimes transformer sealant or the tank is damaged due to transportation from one place to another. As a result, there is a chance of oil leak from the transformer.

Many cases the oil leakage due to the transportation is temporary and will stop as well as the transportation process ends. 

I highly recommend to clean the oil off the transformer tank, let it 48 hours and check it again for any leakage. If the leakage stops, you will just need to check the oil level inside the transformer. I also highly recommend to transfer the transformer empty of oil. And refilling it on the location to avoid any leakage.

  • Tank body mechanical damage

One of the transformer oil leakage causes is a mechanical damage in the tank body. This damage may be a manufacturing defect or a hit in the body during the transportation or installation. The most common part I have ever seen affected by a mechanical damage is the transformer radiator.

What Happens When A Transformer Leaks Oil?

Oil leaks cause serious issues to the transformer and environment and can reach to a complete transformer damage due to internal short circuit due to overheated transformer windings and core. Below are oil leaks effects.

  • The windings will get hot if the oil level in the tank is very low. It will cause a short circuit.
  • Moisture and oxygen will enter the transformer and it will damage the insulation as well as decreasing the transformer life span. For more information about moisture causes and solutions read my article here.
  • Oil leak oil on the ground causes different environmental hazards.
  • If oil leaks occur from the top parts of the transformer, it will allow the nitrogen blanket to escape (in Nitrogen positive pressure transformers).
  • If the oil level becomes lower than the Buchholz relay, the relay will act as if there is internal fault inside the transformer.

What happens if the transformer oil level is very low?

If the oil level falls below the standard level in the transformer tank. It will cause the windings to overheat, windings insulation to decrease and an internal short circuit can occur causing transformer complete damage.

Let me explain it a little bit. The oil acts as a coolant in the transformer. As windings are covered with oil. In case the level goes down, the windings will be exposed and it will overheat with continuous operation. Sometimes there may be a chance of short circuit and as a whole transformer failure. To check the oil level in the conservator (MOG) Magnetic Oil level Gauge is used. Or a side glass pipe indicator shows the oil level.

How to Repair Transformer Oil Leak?

Welding, using sealants, replacing rubber sealant, re-tighten tank bolts and replacing sampling valve are all possible actions to fix the oil leak of transformer. depending on the leakage location and type, different repairing methods are in use to repair transformer oil leaks. Some common methods are discussed below.

  • Classic Repair technique:

In this method, the transformer is disconnected for maintenance and shifted to the repair workshop. During this situation, all existing oil is drained out. And tank is checked for leaks. This is one of the costly methods because the transformer will be out of service for longer time.

This method of transformer oil leak repair should be limited to transformers that have other standby transformer or transformers with lower priority loads. 

  • Drill and inject leak seal technique

In this method, the sealant compound is injected into the transformer gasket using drilling technique. This method is suitable for radiator flange, gas, and pump flange.

This method repairs the leak of the flange without any need to disassembly the transformer or drain the oil to reach the gasket and replace it. It cost effective method.

  • Fabricated clamps techniques

In this method fabricated molten aluminum is poured into the core box. This creates a high-quality sealant with top-class power factor and sealant qualities. This repair method is low cost and reduces the downtown of apparatus.

  • Other methods

Some other methods that are common in use for transformer oil leaks are welds and threaded fittings. Flapper valve packing, cover plates, & tap changers. However, these methods are not common to use

Cooling the transformer oil

As mentioned above the oil transfers heat out side the transformer. This is done by cooling methods as below.

  • ONAN: Cooling: this is the basic cooling method. In this method oil and air are moved naturally. ONAN transformer cooling stands for Oil Nature, Air Nature. Oil moves inside the transformer by the nature flow as hot oil goes up and cold oil takes its place. Out side the body of the transformer the nature air flows and cool down it.
  • ONAF : Oil Nature Air Forced. Using fans mounted on the body of the transformer heat is dissipated. While oil is natural flow.
  • OFAF: Oil forced Air Forced. In this transformer cooling method oil is drawn by a pump, and air is forced by fans.
  • OFWF: Oil is Forced by pump while Water is forced on the transformer body.

I have written a detailed article about transformer cooling methods, You can find it here.

Types of oil used in transformer

There are two types of oil used in transformer. Mineral & Synthetic oil. We will discuss each type in detail.

  1. Mineral oil

This type of oil is a petroleum product, different types of mineral oil are Paraffinic based and Naphthenic based. Talking about the Naphthenic based oils. This type of oil is the wax-free and better flowing feature. However proper heat distribution is one of its main disadvantages.

These type of oils is used for low-temperature. Similarly, the Naphthenic based transformer oil used in the method from paraffinic crude oil. Due to good viscosity, oxidation, and thermal durability feature, it is used for high temperatures also. The oxidation rate of Paraffin oil is lower than Naphtha oil, the sludge is an insoluble product it stays at the bottom of the tank. And as a result, it causes a problem for the cooling system of the transformer.

  1. Synthetic Oil

This type of oil was more popular during the ’70s. The silicon-based oil was fire-prone. The disadvantages of this type of oil were moisture absorption and low heat dissipation. The use of this type was later on limited due to its cost.

Alternative to the transformers oil

We discussed different types of transformer oil. Each type has its pros and cons. However, there is always a substitute available in every field.The petroleum-based oil was used for a long time, but due to the high inflammability and less environment-friendly synthetic oil was introduced. Synthetic oil is fire-prone and is used where there is the risk of fire. However, now the use of the synthetic type oil is also limited. Dry-type transformers, synthetic esters, and Nano particles are used nowadays.

Transformer oil tests

To determine the quality of transformer oil different tests are conducted on regular basis. These types of tests are listed below and we will discuss each in our upcoming topic. Dielectric Strength, Dielectric Dissipation Factor, Acidity,Oxidation Stability, Pour Point, Sludge Test, Flash Point, and many more types of tests are performed.