Power transformers are considered as important equipment of Electrical energy transmission and distribution systems. These power transformers are continuously in operation since their construction and hence their ageing process are going on towards their end of life which cannot be avoided . Due to economic factors , it is necessary to keep the current working transformers in service as long as possible without taking the risk of failure. The lifespan of transformer is determined by the ability of the insulation system’s mechanical resistance to withstand the shorts circuit current forces. However, the continued operation of transformers will lead to ageing and degradation of both liquid and solid insulation materials (Oil and cellulose).
Insulation system of a transformer
The electrical energy is converted from one voltage to another (lower to higher / higher to lower) voltage by means of a transformer. During such conversion heat is developed in the transformer . This temperature must be within limits so that it will not adversely affect the dimensional stability of the materials used in the construction of the transformer. In power transformers, the high electrical stress, the mechanical stress ( stress developed due overloading) and amount of heat developed requires both solid (cellulose paper) and liquid insulation (oil) to withstand the effects caused by them. The most commonly used solid insulation is the Kraft (cellulose paper) and liquid insulation used by the manufacturers was mineral oil which has remained unchanged for reasons of its electric strength and cost. The life of the transformer depends on the strength of its internal insulation system. Therefore the design and selection of these materials must enhance the life expectancy of the assembly as a whole. Due to extreme operating conditions, rapid ageing and wear and tear will occur , thereby affecting the life of the transformer. Most of the items like tap changers, bushings, pumps and fans can be replaced in a timely manner to extend the life of the transformer. But the insulation materials like cellulose paper and the transformer oil cannot be replaced immediately by the time when fault is determined.
The cellulose paper (Kraft paper) is used as insulation material for transformer windings since early 1920s. The cellulose products is commonly used for paper insulation because of the high withstanding resistance it offers to the electromagnetically induced force. Cellulose, which is the main chemical component of cellulose paper, is a unique form of fiber that richly exists in wood or Cotton. The cellulose is composed of polymerized glucose molecules that break up into small chains under thermal stress. The function of transformer cellulose paper is to insulate windings by acting as a dielectric material and to provide mechanical support of the windings thereby protect the windings from possible physical damage. In these ways, the cellulose material provides dielectric and mechanical strength to the transformer windings.
Failure of the solid insulation material
The breakdown of insulation system commonly occur due to thermal stress produced by the flow of high current and voltage (above the rated values). The breakdown of the insulation results in the flash over of winding turns and cause a short circuit. The major reasons for the flow of over voltage and current are.
1 High voltage due to lighting impulse
2. Fault voltages.
The heat generated by the loading of a transformer is also a cause that adds to the degradation of the insulation. In the end, degradation results in the loss of physical strength of the insulation that it fails to serve the purpose for which it is used. If proper maintenance is done at regular intervals the transformer insulation system is capable of providing a reliable service for nearly 25 to 40 years or more. The causes for the premature failure of transformer insulation are
Overloading of the transformer beyond its rated values for an extended time period.
Operating a transformer in an over-excited condition (Over voltage or under frequency).
Operation of the transformer under excessive ambient temperature conditions.
Failure of the transformer cooling system.
In order to avoid the failure it is highly necessary to analyses the causes and effects of degradation. New mathematical models are developed to relate the change in mechanical strength to ageing time and to the degree of polymerization (DP) of the cellulose paper. The Cellulose component degrades slowly but undeniably starts to loss its mechanical properties due to continued deterioration. During this degradation process breakage of inter monomer bonds in the glucose molecules takes place which reduces the chain strength. Failure occurs when the insulation paper losses all its mechanical strength and reduces to the point where it becomes brittle and easily subjected to damage. The damage may result from the load changes or flow of oil over the paper and some other factors can also cause failure to occur early.
The evaluation of the degradation process of solid insulation can be determined by the method of degree of polymerization (DP) with time. The analysis of DP of cellulose paper is a standard method of quantifying the cellulose degradation. The DP value indicates the average length of the polymer chain molecules. It is suggested that the primary loss of strength results from loss of Inter fiber strength. Lower the value of DP of the sample, the greater will be the degradation. Whenever the DP value tends to fall to zero, it is observed that the furan levels in the oil increases. Furans are chemical compounds that are produced in the transformer oil due to the decomposition of the cellulose paper.
Most of the times, the power transformer failure is the reason behind frequent causes of long interruptions in power supplies which seriously affects the power system reliability. Therefore it is indeed essential to closely monitor the equipment in order to detect if any failure was found to appear and can be diagnosed and cleared in its formative stage. This procedure requires the repetitive and periodic testing of equipment insulation referred as diagnostic testing and condition monitoring (DTCM) technique.
Using diagnosis and monitoring systems the condition of the power transformer can be monitored continuously through out its life time. maintenance strategies and the volume of measures can be tailored to minimize the life cycle costs and the risk of long servicing and fall out time. The reinvestment can be delayed for several years and the point of renewal can be appreciated much better. The basis for a condition evaluation of electrical power equipment is the knowledge about the characteristics of the insulation system and their behavior. According to the definition and terminology of maintenance the following measures can be distinguished: inspection, extended inspection, overhaul and servicing. The condition of a transformer is dependent on the operation condition and the number and amount of the maintenance measures and their intervals.
Degradation due to environment
The environmental factors that influence the degradation of insulation material are moisture, chemicals, dirt, oils, acids and alkalies. Moisture is conductive in nature because of its impurity content. When the insulation material is continuously exposed to moisture it absorbs the humidity present in it and tends to decrease the insulation resistance. The moisture penetrates through the cracks and pores of the insulation, especially older insulation, and provides low resistance paths for leakage currents and potential sources that accounts for the dielectric failure of insulation material . Chemical fumes such as acids and alkalies often found in the industrial environment directly attack insulation and permanently lower its insulation resistance. Similarly, oil films that occurs through the surrounding environment or through the internal leakage will get sediment over the internal surfaces of the insulation material of transformer. It will tend to lower the insulation resistance, thus affecting the ability to dissipate heat to outer space and promote thermal stress and unexpected failure. Therefore the life of equipment may be longer if the insulation is suitably protected than if it were freely exposed to the industrial atmosphere.
Transformer oil deterioration
Oil is used as insulation medium for high voltage transformers as it reduces the heat by acting as a coolant. Generally mineral oil is used in oil filled transformers. It is necessary to agree with that the quality of insulating oil gradually deteriorates under the impact of thermal and chemical stresses Power Transformers are highly expensive, critical, and essential equipment of the electricity generation and distribution network. In these expensive equipments, oil insulating materials are considered for their electrical insulating property and to dissipate the heat generated by the windings by acting as a coolant.
Hence the ability of the insulating liquid to serve as an effective insulator and coolant is an important factor in determining the current condition of the transformer. While in service, both the liquid and solid insulation of windings undergo a slow but steady decay process under the impact of electrical, thermal, mechanical and chemical stresses. The application of the operating voltage combines with the stresses cause here a gradual deterioration and premature ageing of the insulation. Therefore, analyzing the Condition of transformer oil turns to be important for determining the serviceability of the transformer. By studying the variations in the mineral oil ( transformer oil) parameters during the thermal and electrical ageing the transformer’s insulation health can be assessed.
Oil ageing: Once put into operation, the insulating oils may exposed to miscellaneous conditions such as contact with air, atmospheric moisture, high operating temperature which in turn will promote deterioration of transformer oil molecules regardless of the oil’s composition of hydrocarbon blend and degree of purity. Among all the factors influencing oil decomposition, contact with oxygen or oxidation is regarded to be the primary oil ageing mechanism considering the large population of free-breathing transformers.
Transformer oil gas analysis
The detection and clearing of incipient faults in transformers as early as possible is extremely cost-effective by reducing unplanned outages. Hence to ensure reliable operation of the transformer, analysis of the levels and ratios of dissolved combustible gases in transformer insulating oils is of great importance. The dissolved gas analysis (DGA) technique have grown as one of the most significant technique available to diagnose the probable transformer incipient faults. Insulating oils under abnormal electrical and thermal stresses breakdown to liberate small quantities of fault gases.
Internal faults in the transformer usually develop by decomposition of the insulating oils which in turn produces gases such as hydrogen, methane acetylene, ethane, ethylene. The degradation of cellulose paper produces methane, hydrogen, carbon monoxide and carbon dioxide. Carbon monoxide and carbon dioxide reveal paper degradation related faults. Ethylene and ethane are decisive in indicating the rise in oil temperature. By means of DGA technique, the internal faults that occurs in the transformer can be classified as partial discharge (corona), Overheating (pyrolysis) and arcing. The amount and types of gases found in the oil are typical expressiveness of the severity and type of fault occurring in the transformer
It is found to be a fault of low-level energy and is caused by ionic bombardment of the oil molecules. It usually occurs in gas-filled voids surrounded by oil-impregnated materials. The major gas produced by partial discharge is hydrogen and a small amount of methane.
2. Thermal faults
A small amount of decomposition occurs at normal operating temperatures. As the fault temperature increases, the formation of degradation gases change from methane to ethane to ethylene. Higher the temperature, greater will be the production of ethylene.
It is a fault caused by a high energy discharge. The major gas produced during arcing is acetylene. Power arcing can cause temperatures of over 30000-degree celsius.
Transformer oil Moisture content
It is a generally known fact that most transformers operate with open systems and the paper-oil insulation system “breathes” with air. Different physical and chemical processes that takes place in the transformer cause the absorption of the air humidity by the transformer insulation system, that leads to the deterioration of the mineral oil and the paper. Water molecules that comes in contact with oil and paper catalyze the depolymerization of paper and also makes drastic changes in the quality of mineral oil which finally affects the electric strength parameters (breakdown voltage etc.). For this reason, the water content in oil which is important parameter needs to be analysed can be determined by chemical analysis or dielectric measurements.
The chemical analysis is done by the Karl-Fischer titration (KFT) method. It is the mostly recommend method for the moisture determination of oil and is a standard test in chemical oil analysis. By removing the moisture and acids from transformer oil, the ageing process can be made gradual and thus lifetime of the insulation system can be slightly extended . These removal processes are preferred to be carried onsite rather shifting the transformer to some other site for maintenance as it requires considerable amounts of time and money . Therefore, onsite drying and online oil reclamation are two processes that will extend the remaining lifetime of the insulation system and also not affect the cost factors.
By diagnosing and measuring out the gases found in transformer oil tie to time , the condition of the transformer can be monitored consistently. If faults are found to be occurring, appropriate outages can be planned and the fault can be rectified before major damage can occur. Thus condition assessment of insulation materials provide admissible information to support information based decisions needed to implement condition-based maintenance and reduce repair time by better planning the restoring tasks.