Water kills electrical transformers, This is why we use moisture protection like transformer breather and Nitrogen blanket.
In this article, let’s go in deep details to discover every thing about moisture in transformer oil.
I made a deep searching about this topic, And I will answer the important question that I found.
Can Transformer Get wet, and Why?
Yes, transformers can get wet, One of the largest transformers in my work was too hot, By making deep analysis and investigating, we discovered that the oil has water, But how did this water get into the oil tank? Moisture can enter to the transformer tank from the outside environment as follows:
- Through the breather, during the breathing in process, if the silica gel is not fresh.
- Also, if the transformer has any other holes or openings, then moisture can enter through.
- During oil replacement, if the new oil has moisture contaminates.
While the amount of moisture in transformer depends on many factors, and can only be detected through oil testing and analysis.
Moisture isn’t always in the oil, it also can hide well in the other insulation between coils and the core. Where exactly? Let’s find out that.
Where Does Exactly Moisture Stay?
A transformer has two insulation types, one is solid and the other is liquid. For solid insulation, plant’s cellulose is used which originally contains around 5 percent of water, and undergoes certain drying process to bring this level down to less than 1 percent.
For liquid insulation, which surrounds the solid insulating material, oil is used because of its better cooling and insulating properties.
The water content in the insulation keeps changing in both these insulators throughout the transformer’s lifetime, based on the temperature and external moisture.
In normal cases, most of the water remains in the solid insulation, but when temperature rises from its base value, water starts flowing from cellulose to oil.
The state of solubility of water in oil largely depends on temperature. Data suggests that at 10 degree Celsius, water solubility remains 36ppmm, which could go up to 600ppm at 90 degree Celsius.
Moisture Types in Transformers
Based on its state, there are three types of moisture present in the oil.
- Oil consist of hydrocarbons, and there is a possibility that some of the water in high temperature gets dissolved due to bonding of hydrogen and hydrocarbons inside the oil.
- It could be present in emulsified form, which means the water is saturated in such a way that it can’t be separated without proper procedure.
- It could be in free form, easily separated from oil in the form of droplets.
How Moisture Reduces The Life of Transformer?
Moisture in transformer oil is one of the most serious enemies of transformer life time. Moisture do decrease the oil break down voltage and can lead to temperature rise of windings and then a short circuit can occur.
An oil with 0.06% moisture has a lower insulation properties of 50% compared to other free of moisture oil.
It worth to mention that transformer life time depends on the insulation paper between the coils and windings, the moisture, Oxygen and temperature in transformer are the killers of the insulation paper, and makes its degradation faster and so decreases the life time of the transformer.
Moisture keeps accumulating in the transformer’s insulation with the passage of time over the years. Very little portion (1 or 2 percent) gets diffused in the oil, while remaining (98 percent) actually gets absorbed by the solid insulation made of cellulose (insulation paper).
If regular maintenance and filtration/drying is done, the situation remains under control. However, in case of no-maintenance, this moisture content keeps increasing and the amount could become around 3 to 5 percent of the insulation paper weight.
At this stage, transformer actually becomes wet and potential serious damage can occur leading to dehydration.
Additionally, in the presence of moisture, paper used in insulation becomes prone to various electrical phenomenon such as flashover, tearing off the surface, drastic increase of temperature and formation of free water at top or bottom of the surface of the oil.
Another disadvantage is the decreased mechanical strength of the insulation which can make it deteriorate further.
Real moisture numbers:
In transformers of 66KV at 60°C a moisture of 35ppm is acceptable, while in a 220KV transformer at the same temperature only 12ppm moisture is acceptable.
Transformer oil plays a critical part in the transformer, so its essential to protect it from leak and, of course, moisture which decreases its break down properties and cause internal short circuit between its windings. What are the protection methods? Are they the same for small and large size transformers? Let’s discuss that.
How to Protect Transformer Oil From Moisture?
Transformer manufacturers make it easy to protect oil for small distribution transformers. While for larger ones, the protection gets more complicated.
For smaller transformers:
The only protection against moisture is sealing off the tank. No other method is needed for these transformers. I have seen transformers of 25KVA and 160KVA without any conservator or silica gel breather.
For larger transformers:
- In this case the conservator tank and silica gel breather are used to prevent any moisture from entering the oil tank. The silica gel absorbs any water parts from the air entering the tank. 300KVA transformers with silica gel breather are so familiar in my work.
- An other protection type is making a sealing Nitrogen blanket above the transformer oil. This pressurized Nitrogen blanket works as isolation between oil and the outside environment.
In this case, its important to check the positive pressure of the Nitrogen periodically in the Preventive maintenance program. Some larger size transformers have a Nitrogen cylinder with automatic pressure regulator to keep the pressure steady inside the tank. In my work We have transformers of 2500KVA with Nitrogen positive pressure.
Methods to Discover Water in Transformer Oil
Initially, the water gets absorbed by the cellulose insulation, and with the increase in temperature the moisture continues to flow in the oil. Therefore, regular testing and monitoring is a way to protect transformer oil from moisture. For measuring the water content in transformer, either of the two methods can be used i.e. drew point analysis and water in-oil analysis.
- Drew Point analysis: This method measures the saturation point of the air and assesses the dew point temperature. The dew point temperature actually determines the threshold and air needs to cooled at this point under constant level of pressure. By achieving this point, the air will no longer be able to absorb any more water in the form of gas.
- Water-in-oil analysis: This method actually detects the amount of water in oil by various methods. “Karl Fischer Coulometric Titration” method is the most popular one and measure the amount of iodine required for titration of the oil sample. Another method is “Visual crackle test”, which is very subjective and can’t determine the actual quantity of water.
Once detection of water is done, any of the two methods i.e. Hot Oil flush and Hot oil spray can be used to dry the transformer.In Hot Oil Flush treatment, hot oil is pressed through a filter made of paper elements. That filter is sprayed on the transformer coil, which absorbs any kind of moisture from the coil assembly.
In comparison to that, hot oil spray method involves spraying hot oil on insulation and sucking up the air surrounding the insulation. Heat present in hot oil causes water to evaporate, which is collected by the suction of vacuum.While Hot oil spray method is more popular, Hot oil flush method is easy and can remove a great amount of moisture from the transformer.
What is transformer oil filtration?
Transformer oil filtration is a process which involves removing impurities in the form of undissolved particles, dirt and water from the oil for increasing the overall insulating properties of oil.
The first step of oil filtration is to raise the temperature of the oil to up to 65 degree Celsius. This is done to provide latent heat, which assists in separating the undissolved moisture and gases from the oil. Additionally, the heating process also decreases the viscosity of the oil, helping the smooth filtration during the process.
Second step is to remove the separated particles from the oil and usually involves either of the following two methods:
- Using Filter: This is a modern process and requires a filtration machine which incorporates a series of different filters i.e. press filter, cartridge filter. Initially, oil is filtered out using press filter, which primarily removes any kind of sludge from the oil. The impurities filter out in this phase are generally bigger than 50 micros. In the next phase, oil is passed through cartridge filter with 1-micron rating. These filters remove dust particles and has a great capacity to hold dust particles inside. However, these filters need to be constantly replaced for better output.
- Centrifuging action: It involves typical centrifuging method for separating dirt particles from the oil. This is a cost-effective method, as it eliminates the condition of continuous changing of filters.
In third step, oil is dehydrated by degasification for removing any kind of moisture and unwanted gases. A degassing chamber is required at this stage to ensure the safety of environment.
Read Also: Transformers Types
Why Oil Filtration is Required?
Transformer oil acts as a liquid insulator and performs various critical functions. For example, oil used in transformers has great stability at high temperatures, which is essential for smooth insulating performance.
In such condition,winding and core transfer its heat to the oil, which makes transformer oil as a cooling agent. As per the study, almost 75 percent of the transformer failures are due to presence of contamination of deteriorated state of transformer oil. If transformer is oil is not regularly cleaned, it could create issues such as arcing, overheating of transformer windings, corona losses and reduced insulating of transformer oil.
Hence, for smooth and efficient performance, a transformer requires cleaned and dried oil, free from any kind of impurity. However, during its operation and continuous usage, transformer oil absorbs dust particles, solid insulator particles and moisture from the environment and cellulose.
Moreover, even without usage, transformer oil can get dirty due to the contamination it absorbs from the walls of storage tank. As a result, the efficiency of transformers gets significantly down with the increased in moisture level. Therefore, filtration is required to enhance and improve the insulation properties of oil.
What is The Transformer Oil filtration Frequency?
There is no specific schedule for oil filtration of transformers. There are various factors such as importance of transformer, its design and durability which plays their role in scheduling a filtration. However, there are recommended timelines for conducting various kind of tests on transformer for assessing its condition and mitigating any danger.
Physical inspection is recommended annually while leakage of oil and reading of Magnetic Oil Gage (MOG) is recommended to be done on daily basis. Testing of di-electric strength and detection of moisture content is done annually, whereas dissolve gas analysis tests is recommended annually for 132 KV transformer, and twice in a year for below 132 KV transformers.
On-line Filtration of Transformer Oil?
On-line oil filtration is performed within the transformer oil tank, which means the overall operation is quite easy and performed with more safety and good performance. It is also a cost-effective method compared to off-line filtration because it doesn’t involve draining, flushing and refilling phases.
The procedure for online filtration requires a high-quality depth media filter. This filter is placed and oil is drawn from bottom of the reservoir, an area where most of the contamination lies. It then gets through the filter and returns back to the top of reservoir, forming a loop.
There are certain precautionary measures which needs to be taken for on-line filtration. Since most of these plants are operating in remote areas, the filter should be installed in such a way that minimum possible amount of oil could spill in case of a breakdown in suction line. Furthermore, high- and low-pressure alarms, Supervisory control and data acquisition equipment (SCADA) and spill should be installed to warn about any system failure.
Is Transformer Oil Filtration a Dangerous Process?
The dangerous are mostly associated to high risk areas such a nuclear power plant. In those areas, there is a danger of explosion and therefore explosion proof equipment is use tie guarantee safety.
This vulnerability to explosion could be due to the various reasons such as use of flammable oil, presence of hydrogen in cooling systems, short circuit in power cables, oil coming in close contact with hot plates or any other human error.
