12.

Life and Ageing

Life management of power transformer is very important aspect both for utilities and manufacturer as well. Utility engineers have greater expectations from this important piece of equipment. They want to draw maximum output with optimum efficiency from this equipment in its complete life span. The life of a transformer may be introduced as the change of its condition with time under impact of thermal, electric, chemical, electromagnetic and electro-dynamic stresses, as well as under the impact of various contaminations and ageing processes. Technical life of a transformer may be thought as of several components: Dielectric life, Thermal life, Mechanical life and life of accessories. In fact Dielectric life is shorter than Thermal life due to critical effect of oil ageing products resulting in reduction of dielectric withstands strength of oil and degradation of surface strength. The life of a transformer depends initially on the design and quality of manufacture and later on service conditions. Estimation of life of the transformer can be assessed by: The design stresses and safety margins, the materials used to operate this equipment, power system parameters and periodic checking/testing/ maintenance. The Life of a transformer is the life of the insulation. When insulation fails the transformers life ends. The life is a controllable factor. The Estimated life of a transformer as per seventh schedule in Indian Electricity Supply Act, 1948 for 100 kVA and above is 35 years and in respect of lower capacities is 25 years. The service life of a dielectric subject to heating is therefore also dependant on temperature, the higher the temperature the shorter the service life. This means that the life of a given insulation will be determined by its thermal and electrical loading conditions during service and is closely related to the heat resistance of the insulating materials concerned. The heat resistance is defined to be the limiting temperature at which a given class of insulation can operate for a period of life of the equipment in which it is used i.e transformer. Ageing is a concept with several possible meaning, with passage of time or grow old or even a negative change in the property. Ageing is a natural phenomenon in transformers and influenced by several factors. It is influenced by over loads, intensity of short circuits, incidence of lightning surges and internal faults. Transformer ageing would be irreversible, deleterious changes to the serviceability of the transformers. Further, ageing of a material is an irreversible negative change in its pertinent property. Transformer insulation system paper-oil is by far the best known system and is adopted in power transformers even for EHV/UHV transformers. The three materials used paper, press board and oil. Paper is hygroscopic and does not have good

mechanical properties while oil shows very marked deterioration of its electrical properties when contaminated with water even to a small degree and in presence of fibrous material. This behaviour of insulating material results in greater impact in very high voltages. Transformer insulation is subjected to lightning impulses, switching impulses, low frequency, and temporary over voltages and the normal operating voltages during its life in service and the unit can be expected to work without insulation failure for its expected life. However, insulation ageing cannot be prevented but can be retarded in oil-insulation system. Degradation mechanism of an oil-paper insulation falls into P.D of low intensity with gaseous voids, P.D in oil or impregnated paper or arcing between conductors and hot-spots in oil-impregnated paper. Weakness of the insulation whether they were there originally in the insulation or resulted from gradual ageing of insulation causes P.D at certain voltages. Transformer oil is adversely affected by, oxygen, moisture (as a catalyst) and heat (as an accelerator). Unstable hydrocarbons (in the oil) + Oxygen + Catalyst + Accelerators = Oxidation by-products. The effect of moisture in oil is two-fold. It reduces the resistivity and thus causes higher leakage currents and accelerates ageing of oil. Ageing of oil results in higher acidity, higher viscosity and sludge formation, These products of decomposition of oil in turn causes corrosion of metallic parts, inefficient cooling and blocking of cooling passes through windings leading to accelerated decomposition of oil. Ultimate result of all these is shortening of life of transformer in service. Transformers Do not die of Old age, but they are Killed. The primary factors that contribute to eventual failure consist of, operating incidents, over loading and poor maintenance. The end-of-life can be defined in three different ways; Strategic endof-life, Economic end-oflife and Technical end-oflife. It is of paramount importance that to understand the behavour of transformer under varying conditions of ambient air temperature, load and winding temperatures and how they affect the life of the transformer, so that it would help in conserving the vital materials, energy and capital resources by maximizing their use without unduly sacrificing the safety, reliability and normal expected life. 1 What is the life of a machine/equipment / system? Life of a machine / equipment / system is a function of the integrated or cumulative effects of time and temperature. Secondly, there is always an interaction between different materials in given equipment and behaviour of a specific material depends upon the function of the materials in the system. 2What is the criterion for evaluating the life of an asset?

In electrical equipment, perhaps the most important factor is the maximum hot-spot temperature to which the insulation is subjected. That is where the conditions prescribed in the standard specifications for evaluating rating become so important. If the plant is operated within the limitations prescribed, the equipment may be expected to last its life term, provided of course that it is operated and maintained well. The life of no equipment ceases abruptly it starts giving more and more trouble as age advances and a point comes, when the cost of repair becomes excessive or its dependability becomes reduced that it has to be scrapped. It could of course be kept going on indefinitely by replacing part by part and by patch repair work, but ceases to be effectively useful. From the economic point of view life should be deemed as having ended when the probability of failure becomes too high or the cost of repairs becomes excessive compared to the service it renders. Quite often however machines used for bulk production of parts become obsolescent and out of date much before the end of their useful life, because of new inventions or improvements in the engineering art, leading to greater efficiency and higher production rate. In such cases it may be worthwhile replacing the machine by a new improved unit at considerable additional cost if the resulting savings are sufficiently high. The equipment thrown out of use, which may be serviceable, would fetch a high credit or salvage value from someone whose requirement is more modest. Assuming that the equipment (transformer) receives reasonable care in operation and maintenance, the average life in years, which may be expected for transformers below 100 kVA and for above 100 kVA indicated 25 and 35 years respectively (as per 7th schedule of Indian Electricity Supply Act, 1948). 3What are the factors upon which the life of insulating materials depends? The useful life of a transformer is wholly dependant upon the ability of its insulation to withstand deteriorating effects of heat and moisture and the physical and dielectric stresses at which it is subjected to under operating conditions. Proper insulation therefore is the most critical feature of transformer construction. Insulation is perhaps the most important part of a transformer since the performance of transformer is poor in other respects it will still perform its major function of transforming power, but insulation failure at any point is disastrous. The life of insulating materials used in transformers depends upon the temperatures to which they are subjected to and duration of such temperature. For transformers, since the actual temperature and the temperature rise is apparent that ambient temperature is determines the load, which can reasonably be carried in service. Other factors upon which the life of insulating materials depends on: -Electric stress and associated effects; -Vibration or varying mechanical stresses;

-Repeated expansions and contractions; -Expose to air, moisture, etc,. 4When an insulating material ceases to be an insulator and becomes a conductor? When there is an e.m.f. between two conductors that are separated by an insulating material, there are electrical forces exerted upon the electrons in the atoms of the insulating material. These electric forces are trying to pull the electrons out of the atoms. If these forces succeed in pulling the electrons out of the atoms, the material ceases to be an insulator and becomes a conductor. When this occurs, it would be said that the insulation had broken down. When the voltage between the conductors is small, these forces exerted in the atoms are not great enough to pull the electrons out of the atoms. As the voltage between the conductors is increased, the forces on the electrons are increased. If the voltage continues to be increased, the forces will eventually become great enough to pull the electrons out of the atoms and the insulation will breakdown. The voltage at which the material ceases to be an insulator is called the breakdown voltage. Any insulating material can be broken down and cease to be an insulator if the voltage impressed across it is raised high enough. The breakdown voltage of any material depends upon thickness, condition of the surface, the homogeneity of the material. Q What is the relation between electrical stress and the life of insulation in a transformer (EHV)? There are two approaches to obtain the co-relation between electric stress and life of insulation system. Those are through 1. 2. through Electrical stress (E)- Partial Discharge (P) and life of insulation (L) and through E-L path

Present technology is to follow E-L path and avoid partial discharge totally in case of EHV/UHV transformers. The E-P-L diagram for the relation among E-P-L is shown: 6What are the important characteristics determine the life and performance of the oil in the transformer? In developing a oil for insulation purposes, information related to most of the characteristics should be obtained so that potential users may see the scope for possible application and use. Certain characteristics are inherent to the type of fluid and this should therefore only be applied to those fields. The important characteristics from the specification point of view are those which affect the fluid life and performance. These parameters must be limited in order that the material will fulfill its intended purpose. Therefore, a standard can be divided into various

sections to cover physical, chemical and electrical properties of the oil. These properties can be classified as; Functional: Properties of oil, which have on its function as an insulating and cooling liquid. Refining and stability: Properties of oil that are influenced by quality and type of refining and additives. Performance: Properties that are related to long-term behavior of oil in service and or its reaction to high electrical stress and temperature. Health, Safety and Environment: Oil properties related to safe handling and environment protection. Under certain situations there is a possibility of overlap among the above properties. 7What is the influencing factor for bubble evolution? Moisture content is the most important influencing factor for bubble formation. The temperature at which bubbles evolve decreases exponentially as the moisture content in the cellulose insulation increases. Increasing content of other gases also significantly influences bubbles evolution when high moisture content exists. Data show that in a dry transformer (less than 0.5% moisture by dry weight) bubble evolution from overload may not occur below 200 C. A service aged transformer with 2.0% moisture may evolve bubbles even at 140 C or less (Oommen et al,. 1995). 8What is the aim of studying the life assessment of transformers? The key concept for life assessment is serviceability, i.e. the ability to function as intended. The serviceability is endangered when the transformer malfunctions or when it is unusable from some other point of view. (Too small, too expensive). The life assessment studies aim at comparing and ranking of transformers with respect to their suitability for use in a relative evaluation procedure of several transformers. It could also be used for an absolute determination of the serviceability (in the remaining years). The serviceability is determined by actions taken before the transformer was pressed into operation and from the history and events occurring in service. Just as an introduction one should examine the risk factors. These factors describe what may jeopardize the serviceability. serviceability Detailed evaluation Non-technical suitability. Technical suitability

General evaluation (Structured acc, to possible risks) Auxiliaries Electrical Mechanical Thermal Experience Global view Strategy Economy Environment. Advanced evaluation Normal evaluation Events Handling PF Oil DGA Temp Time Experience Size Application Economy Safety/ Environment Screening evaluation 19What is meant by life of a transformer? As per IEEE C 57.71-1955 Guide for loading mineral oil filled transformer is defined as: For a given temperature of the transformer insulation the total time between the initial state for which the insulation is considered new and the final state for which dielectric stress, short circuit stress or mechanical movement, which would occur in normal service, and would cause an electrical failure. 20What are the factors influencing the life of transformer? The useful life of a transformer is wholly dependant upon the ability of insulation to withstand the deteriorating effects of heat and moisture and the physical and dielectric stresses at which it is subjected to under operating conditions. Paper insulation therefore constitutes the most critical feature of transformer construction. The life of insulating materials commonly used in apparatus depends very largely upon the temperatures to which they are subjected to and the duration of such temperatures. For transformers, since the actual temperature is the sum of the ambient temperature and temperature rise. It is apparent that the ambient temperature very largely determines the load, which can reasonably be carried in service. It is well established that the major stresses on power transformer windings due to various conditions which ages the insulation either individually or in conjunction are mechanical, thermal, dielectric, magnetic and chemical degradation in electrical insulating paper in service. Other factors upon which the life of insulating materials depends are: -Electric stress and associated effects;

-Vibrations or varying mechanical stresses; -Repeated expansions and contractions; -Exposure to air, moisture, etc,. Experience has shown that moisture and temperature are the main parameters influencing the ongoing characteristics of oilpaper insulation and thereby life expectancy. Additional transformer life influencing factors by utilities are: -Changes in load profile; -Effectiveness of oil preservation system; - Effective control of oil temperature. -Operating practices. Major factors affecting the transformer life are: Effect of moisture: Transformer oil absorbs readily moisture from the air. It is also known that the effect of water in solution in the oil is to decrease the dielectric strength of insulation paper and oil. Insulating paper has higher affinity of water than oil. Therefore it is essential to take preventive steps to guard against moisture penetration to the inside of the transformer. This will include blocking of all openings for free access to air in storage and frequent reactivation of breathers in service. Effect of oxygen: Oxygen is always present inside the transformer due to air in the oil, air pockets trapped in the windings, etc. The oxygen react with the insulation and decomposition products lead to the formation of organic acids soluble in oil and sludge blocks the circulation of oil. The adverse effect of oxygen, which may also be aggravated by catalytic action between hot oil and bare copper, increase the operating temperature. Effect of solid impurities: The dielectric strength of oil is diminishing appreciably by minute particles of solid impurities present in the oil. New transformers may contain particles of insulating materials and other solid impurities. It is, therefore good practice to filter the oil after it has been in service for a short time, especially for the units of higher voltage class. Effect of varnishes: Oxidizing type varnishes readily in reaction with transformer oil and precipitate sludge on the windings. This should be carefully noted particularly while rewinding and replacing the coils.

Effect of slackness of windings: Due to repeated movement of coils during operation and also at momentary short-circuits slackness of windings may cause failure due to wear of the insulation, at some places can lead to an inter-turn failure, electric and magnetic unbalance and even greater displacement. It is therefore, a good practice to lift the core and coils of transformer and take-up any slackness, which may have developed, by tightening the tie-rods or pressure screws where provided for this purpose. In all cases, the instruction given by the manufacturers should be followed carefully. 21What is the consideration for fixing the transformer service life of 25-35 years? This estimate is based on the continuous operation at rated load and service conditions with an average yearly ambient temperature of 32 C and the winding hottest temperature of 98 C as per standards (IS 2026). The characteristics of a transformer (losses, short-circuit, voltage drop) depend on the rated values of the power, current and voltage at which the heating of the different parts must not exceed the limits set by standard specifications. These limits have been set after years of service experience to ensure that transformer may be operated permanently at their rated power and in the predicted ambient conditions for a normal service life. Paper impregnated with insulating oil, which is expected to last the life of the transformer for minimum of 25 years at operating top temperature of 65 - 95 C. Temperature and heating limits sets by the standards ensure an estimated normal life of 25 - 35 years 22What is meant by Remnant Life Assessment of transformer and what are the methods? Transformer life is its life of insulation-system. Failure is the condition in which a transformer is unable to perform satisfactorily. During operation due to undesirable and unavoidable losses, results in heating of the insulation which degrades their dielectric characteristics. Normally customer must certainly likely to know, how long his aged transformer will render service, so that he can make proactive decisions for refurbishment or replacement of the transformer. There are two methods of performing RLA. The first method is the analysis of furan content in the liquid insulation and the second is the estimation of DP of the insulation paper. The insulation material used in transformer consists of cellulose paper, pressboard, wood and impregnating mineral oil. When the paper insulation is overstressed by high temperatures or electrical discharges, the chemical bonds in it molecules break creating new ones. These irreversible reactions cause ageing of the transformer, with decrease in the electrical and mechanical strengths of its insulation. These

reactions are further catalyzed by the presence of moisture leading to a chain reaction inside the paper. The byproducts, so generated are called Furans which get dissolved in the transformer oil. These Furan- compounds keep on accumulating in the oil as age passes and therefore by determining the concentration of furans in oil, it is possible to find out the remnant life of the transformer. During ageing, along with furans, another phenomenon that happens is the rupture of large cellulose chains or polymers into smaller ones with less number of glucose molecules in them. The number of glucose units in a cellulose polymer of the paper insulation, is called the Degree of Polymerization (DP) of the paper. Fresh paper insulation may have a DP value between 1000- 1500, whereas a aged paper insulation may have a DP value of 200-250. Therefore DP analysis predicts remnant life of the transformer by counting the numbers of glucose units in the cellulose paper samples directly, unlike furan analysis. Practical values ascertained that the credibility of DP as a promising remnant life estimation tool. 24Differentiate among strategic / economic / technical END-OF-LIFE of transformer? -When the transformer is judged unsuitable for use in its present location, it has reached the end of its useful life. Taking a transformer out of service always involves fundamental cost estimation. It is evident that not only technical but also strategic, economic and other factors should be taken into account for these estimations. Thus, end-of-life can be defined in (at least) three different ways; Strategic end-of-life: It occurs when the transformer has become too small in some way for its present location and should be replaced. Possible causes to be considered include: -Load -ability, -Short-circuit capability, -Service voltage, -Risk evaluation. Economic endof-life: The economical life of a transformer is the time span beyond which the losses and maintenance costs become a liability. That means in some cases, economic reasons have an impact on the decision, for instance losses, maintenance costs, spare-parts value Technical End-of-life: Technical life of a transformer is the time span beyond which it is deemed not suitable on technical reasons like failure of windings, explosion, fire accident and damages that cannot be repaired. Technical End-of-life occurs only

applicable on purely technical reasons. Possible causes include true failures and preventive scrapping. 25What are the various components of Technical Life? The life of a transformer may be introduced as the change of its condition with time under impact of thermal, electric, chemical, electromagnetic and electrodynamics stresses, as well as under the impact of various contamination and ageing processes. Technical end of transformer life comes when the unit is deemed unsuitable or unusable for purely technical reasons. Technical life of a transformer is the time span beyond which it is deemed not suitable on technical reasons like failure of windings, explosion, due to fire and damages that cannot be repaired. Technical Life of a transformer may be thought as of several components: Dielectric Life: Life span up to critical reduction of dielectric margin of insulation. Thermal Life: time up to critical decomposition of winding conductors insulation, e.g. DP