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Removal of moisture from transformers in service

Professor Dennis J Allan FREng, FIEE, FIMechE, FIEEE

Introduction

The design life of power transformers is usually 30 to 35 years. In fact thetypical Time to Failure of a large generator transformer (working at constantfull load) is 18 to 24 years and a transmission or distribution transformer(working at half load or less) can be 40 to 60 years. The actual life of atransformer is determined by ageing of the cellulosic insulation in the form ofpaper on conductors and leads, and of pressboard used for inter-turn orintersection spacers. The insulation life is determined by three factors:

Operating temperature

Access to oxygen

Water absorbed in the paper.

As cellulose ages the length of the glucose chain slowly reduces due to chainschism from 1200 molecules to about 200 molecules when it no longer hassufficient mechanical strength to be viable.

Operating temperature

Unless redundancy has been built into the transformer rating at the

specification stage, a transformer is always likely to operate at or near therated value. For every 6.5 C increase in operating temperature the insulationlife will halve, based on a moisture content in the paper of 0.3% by weight.The remnant life of the transformer will be reduced whenever it is operating athigh temperature. The rate of ageing will be higher (up to 50 times faster atmoisture contents of 5% in the paper), and the remnant life will be muchreduced.

Oxygen

Ageing is due to chemical reactions between the long-chain glucose molecule

of cellulose and oxygen. With no oxygen present there can be no ageing ofthe paper, but oxygen is always present either as air dissolved in the oil or aswater in the oil or paper. Attempts to replace air in the oil by nitrogen or anelectro-negative gas have always failed, but a rubber-based membrane hasbeen used with success to prevent direct contact between the oil and theatmosphere. Unfortunately the material of the membrane has a lifetime ofonly 10 to 15 years before is begins to allow air and water vapour to diffusethrough it. In addition, water at up to 10% of the weight of cellulose is formedby the chemical process of ageing. Fitting a membrane will trap water formedduring this process and moisture is locked into the system where it is acatalyst causing more rapid ageing of the insulation.

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Water

Under normal conditions oil will dissolve 60 ppm water before it saturates.Cellulose saturates at 10% water content when it is dry and between 12 and18% water content when it is oil-impregnated. The presence of moisture as a

contamination in the oil-paper insulation system will compromise the dielectricstrength of the paper and will act as a catalyst for rapid ageing of theinsulation system. Moisture can enter the oil-paper system in several ways:

It can remain absorbed in the insulation if the factory drying processhas been inadequate.

It can enter the transformer during service if the air drying system hasnot been properly maintained, diffuse through gasket material or enterthrough cracks in the tank (welding defects).

It can enter through openings in the tank if the internal insulation hasnot been correctly protected during site erection operations or duringservice outages.

In addition, water is generated internally in the transformer as the paperand pressboard materials age in service. The rate of ageing in serviceis accelerated by operation at high temperature, by the presence ofoxygen as air dissolved in the oil or as water in the paper.

Mitigation to reduce ageing

The traditional means of protecting the insulation system of a transformer fromthe ingress of water is to fit a silica gel breather. These breathers need to be

maintained as often as every two weeks and do not remove moisturegenerated inside the transformer by ageing of the insulation.

Refrigerated breathers based on Peltier devices are widely used in the UK tocontinuously remove water from oil in the conservator. These devices willslowly remove water from the cellulose insulation but are ineffective whenwater in the paper exceeds 2.5% by weight.

Oil filtration plant based on heat and vacuum operations are effective in dryingthe oil, but as more than 99% of the moisture is absorbed in the paper, itquickly migrates into the oil and the oil remains wet.

Molecular sieve drying devices are connected to circulate the main tank oilover a charge of molecular sieve material; this is a naturally occurring zeoliteselected with a 4 Angstrom pore size to match the size of a water molecule.Water is trapped at up to 40% by weight of molecular sieve material throughchemical bonding, and can only be removed by a heat and vacuum process tobreak the energy bonds holding water in the material.

Molecular sieve drying devices can be used to slowly remove moisture fromthe cellulose insulation by removing water from the oil. The movement ofwater from cellulose to oil takes place at the same rate as the movement of

water from oil into the molecular sieve material. High levels of moisture havebeen removed from the insulation of transformers over a period of weeks and

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months to reduce the risk of the transformers failing due to electrical surges orthrough mechanical faults associated with high through-fault currents.

An alternative water management scenario is to fit molecular sieve devices tonew transformers, in combination with refrigerated breathers or diaphragm

seals. The molecular sieve device absorbs water dissolved in the oil andremoves water produced by degradation of the cellulose as it is formed. Thiscombination of devices is effective in maintaining high integrity of thetransformer insulation by eliminating the main catalyst for ageing and avoidinga reduction in the dielectric performance of the insulation structure.

A typical 10 litre capacity molecular sieve installation

Professor Dennis Allan is an engineering consultant working in the area ofelectrical power systems technology. Prior to 1997, after 47 years in the transformer industry,he held the appointment of Director and Technical Manager for GEC Alsthom Transformers atStafford, England.

Professor Allan was a Royal Academy of Engineering Visiting Professor inPrinciples of Engineering Design at UMIST, and a Visiting Professor in Power DistributionEngineering at the University of Manchester. He is a Fellow of the Royal Academy ofEngineering, the IEE and the IEEE. He has been Chairman of CIGRE Study Committee 12(Transformers) and IEC Technical Committee 14 (Transformers). He is Chairman of theManagement Board of AFAQ-EAQA, the International Certification and Accreditation Bodyserving Industry.

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Possible figures to support text

Figure x Piper curves showing equilibrium between water in oil and in

paper

l

Figure xx Increased rate of ageing of cellulose with water content