Where does moisture come from in a transformer?

Moisture can ingress the transformer in several ways. The most common is through the breather. As the transformer oil volume changes everyday with load & temperature variations, some air has to go in and out of the transformer through the breather. Even though the air goes through a compartment of silica gel before entering the tank, it cannot be dried down to 0% humidity. Hence a little bit of moisture enters the tank every day. Over a period of 40 years, this would make a lot of moisture. This effect can be avoided with sealed transformers or with a rubber bag inside the breather.

Furthermore, the insulation paper inside the transformer generates its own moisture over time. The degradation of the cellulose material generates molecules of water. This chemical reaction is accelerated at higher temperature or in the presence of oxygen or water. This means that the more water builds up in a transformer the more the transformer generates water. This effect cannot be avoided.

Finally water could also ingress a transformer if there is a leak in the gasket or during an open tank repair.

Where is moisture located inside the transformer?

The moisture stays in the insulation material of the transformer. Even though moisture is only checked in the oil, there is a very small amount of water in the tank. Even in large transformers the volume of water is generally lower than a glass full compare to the thousands of litres of oil in the tank. We can conclude that there is less than 2% of the total water volume in the oil and more than 98% in the insulation material.

What are the effects of moisture on the transformer characteristics?

Moisture inside the oil and insulation paper can have a negative effect on the breakdown voltage. The higher the concentration of water, the lower is the breakdown voltage. This negative effect is particularly visible during the changing phases of the transformer (load increase or decrease). During this time there are exchanges of water molecules between the insulation and the oil which decrease drastically the breakdown voltage value.

Also, moisture increases the degradation speed of the insulation paper. The more the paper contains water the faster it degrades. Each time the water content in the paper is increased by a percent, the life expectancy of the transformer is divided by two.

What is the best solution to dry a power transformer?

The most efficient way to dry a power transformer is to use a vacuum based technic (low frequency heating or other). It can extract several tens of litres of water within a few days. However these technics require the transformer to be shut down for about a week. Also there is a risk of damage as a vacuum is applied inside the tank. This maintenance can be quite costly and requires a trained team and specific material.

Otherwise it is possible to dry the paper by drying the oil while the transformer is still in operation. But this process should be done on a long period of time. Circulating and drying the oil for a few days would be inefficient as only 2% of the water is contained in the oil. A couple of month after the maintenance the moisture returns in the oil. However drying the oil for a long period of time will maintain a dry oil hence a high breakdown voltage and will extract moisture step by step from paper hence extending the life expectancy of the asset. This can be done online without any operator.

Is it possible to define how wet is the insulation paper inside the transformer?

Yes it is possible. The direct way is to stop the transformer, open it and then take a sample of paper for laboratory analysis. It is very accurate but very difficult to apply also.

The indirect way is to check the amount or the concentration of water in the oil. Equilibrium curves linking the water concentration in oil, the oil temperature and the water content in the paper. This method can be quite inaccurate as it requires the transformer to be in equilibrium phase which almost never happens on a running transformer. However combining this method with a continous monitoring of water concentration and oil temperature allows to have pretty accurate trends and therefore to define well the current water content in insulation paper.

What is the acceptable level of water content in the insulation paper?

A new transformer will have a water content in paper lower than 1% and ideally closer to 0.5%. Overtime the transformer gets more and more wet. All standards (CIGRE, IEC, IEEE) agree that the water content in a power transformer should be maintained below 2%. Above this level the transformer can be considered as wet and some sort of drying maintenance should be engaged.

Can the PPM of water in the oil alone be considered when evaluating the water situation of a transformer?

No. The PPM of water in oil is not sufficient to understand the water situation in a transformer. First of all, the PPM changes constantly depending of the load and the current status of the transformer. Also the PPM value should always be linked to the temperature of the oil. It is completely normal that a warm oil has more water inside as the water solubility in oil is increased at high temperature. Therefore a high PPM does not necessarily mean a wet transformer and on the other hand a low PPM does not necessarily mean a dry transformer. Ideally we would advise to monitor these values constantly with sensors but also to consider the relative saturation (%RS) of water in the oil which is more accurate than the PPM.

What should be the maximum relative value of saturated water in the oil?

It is strongly advised to keep the relative saturation (%RS) below 20% at all time. Above this value the breakdown voltage of the oil drastically dropes. For example, at 30%RS the breakdown voltage can be only about 70% of its initial value and at 50%RS about 35% of its initial value.

How long can it take for TRANSEC to dry a power transformer?

This depends alot on the conditions of the transformer and the amount of water available. In this case the speed of extraction for TRANSEC is high. When 20 PPM enters the unit only 1 PPM goes out but if the water PPM in oil is low then the overall speed of extraction will be slower. Each cylinders can extract a minimum of 3 litres of water. If the transformer is very wet and the oil fairly warm there will be a high level of water PPM in the oil. Within 6 months the TRANSEC unit is able to extract 10 litres then. However the same unit mounted on a new transformer will not reach saturation for a few years as there is less water available to extract.

Can the TRANSEC overdry the insulation paper?

No it cannot. TRANSEC is a “passive”, maintenance only which dries the oil continuously. TRANSEC is not able to force the extraction of water from the paper as no vacuum is applied. Nonetheless STREAMER added a possibility to interrupt the drying from TRANSEC when a certain value of water content is reached. Thanks to this, the user can control the drying level of its transformer.

Can TRANSEC be installed and maintained online?

Yes absolutely. And it does not require any operator once it is installed and running.

Does TRANSEC follow any electrical standard?

Yes, TRANSEC units are compliant with IEEE Std C57.140-2006. The monitoring cabinet follows Low Voltage and EMC standards. The TRANSEC unit also is compliant with CE requirements. Additionally internal tests (pressure, vibration, climatic) are performed in order to control leakage within the unit.

Can I move the TRANSEC from one transformer to another?

Yes, it is possible. However, it has two drawbacks. First, it is imperative to replace the filtering cylinders with new cylinders before applying the TRANSEC unit to another transformer. Indeed, the used cylinders will contain about 40 litres of the first transformer oil which should not contaminate the second transformer oil. The second drawback is that as soon as the TRANSEC unit is removed from transformer 1, the moisture production and concentration in this transformer will start rising again. Unless the water content in paper has been decreased to a low level (below 1.8%), it is not advised to remove the TRANSEC unit from this transformer.

What maintenance procedures are necessary for TRANSEC?

TRANSEC is very low maintenance. TRANSEC does not require any personnel on site during its operation. The only moving part of the unit is the pump which could fail eventually after more than 10 years of use and can be easily replaced. Otherwise, the cylinders and particle filters should be changed once saturated. This operation can be completed while the transformer is online.

Can particles of filtering material enter the transformer?

No. A particle filter is always placed before the outlet to avoid any ingression to the transformer.

Is it possible to use a TRANSEC on a transformer which is already equipped with a DGA unit?

Of course. TRANSEC does not alter the concentration of the 9 dissolved gases. It only extracts water molecules. Therefore, the DGA reading will not be affected. If the transformer valves are limited it is even possible to connect the DGA unit and TRANSEC to the same bottom and top valves.

Can TRANSEC function on transformer with natural ester oil?

Yes. The type of oil should be mentioned stated in the request in order for Streamer to plan the adequate oil inside the unit and also to adapt the power of the pump, if required.

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