Two most important equipment found on the power distribution grid are transformers and circuit breakers. Their reliability and performance is crucial to power distribution. Any breakdown of such equipment entails loss of revenue for both suppliers as well as consumers. Therefore it is of upmost importance to keep them operational throughout their existence. Maintenance is the only way to keep them fit for duty. Minor degradation in transformers and circuit breakers should not be neglected because sooner or later it becomes the weak point in the equipment. Detecting these weaknesses in advance helps in many ways to minimize maintenance cost, prevent unforeseen outages and provide a smooth power distribution on the network. Continuous monitoring of these equipment need to be carried out permanently in order to detect any degradation and failures using proper instruments based on manufacturers data. Using the data collected, the condition assessment of each device can be build up.
Condition assessment history helps to schedule the right type of maintenance in time which will guaranty the lifetime of equipment. It also helps manufacturers and developers to bring improvements to their products which at the end will be beneficial for end users. It can also be used by the planning team to assess the loading capabilities as time passes.
Condition assessment of equipment should therefore form part along with the production line .It should be scientifically carried out to get the maximum of the equipment.
THE POWER TRANSFORMER
http://www.o-digital.com/uploads/2179/2225-1/S11_M_30~1600kVA_Oil_Immersed_Power_Transformer_913.jpgFIG 1.
SOURCE http://www.mhprofessional.com/downloads/products/0071494626/CroftCh5.pdf
A transformer is an equipment for transforming electrical energy in an ac circuit from one voltage or current into electrical energy at a higher or lower voltage or current.
A constant-voltage transformer consists essentially of three parts:
1. Primary coil acting as the input from generating plant,
2. A magnetic core producing an alternating magnetic field ,
3. The secondary coil connected to the load.
The high-voltage winding consists of many turns of insulated copper wire compared to the low-tension winding composed of lesser turns of thick copper wire carrying high current at a low applied potential.
Different types of transformers that are used in the power distribution grid for both industrial and domestic uses. Power Transformers, Distribution Transformers of step up and step down types are widely used. Transformers play an important part in the power distribution network and its failure leads to many problems both economically and socially for example in transportation and hospitals. Our prime objective is to keep the mean time between failures of these equipment as low as possible or ideally no down time.
Recently there has been studies in transformer life evaluation and monitoring because there are large number of the transformers in the world getting old ,that is nearing end-of-life and the need increases for better methods to see whether the transformers are still fit for use or need to be upgraded or replaced. Transformer condition assessment as well as on-site repair solution as means to increase both the reliability and availability of transformers has been developed. The advanced diagnosis of the transformer condition is used to identify defects. Failure of such important equipment implies loss of revenue for industries which have to reduce their production by relying on in house generators and many inconveniences for domestic appliances.
Table 1.0 shows causes of transformers failure due to both external and internal factors.
CAUSES OF FAILURES OF POWER TRANSFORMERS.
http://www.dieselserviceandsupply.com/Causes_of_Power_Failures.aspx
EXTERNAL/ Natural Causes - Weather Related
INTERNAL
Lightning
OVERLOAD/DIMENSIONING
SOLAR STORMS
AGEING
AMBIENT TEMPERATURE
INSULATION DEGRADATION
MOISTURE
NO ROUTINE CHECK
Table 1.0
Of all the factors listed, it is understood that some of them are beyond our control and little can be done to minimize their effects. These are solar eruption, heavy rain and humidity and lightning. Surge protectors, switch breakers and arrestors are used to save transformer from total loss in certain cases. As for climatic effects, high quality materials are employed for transformer casings which provide both protections against corrosion and at the same time acts as a good heat dissipating material.
A direct lightning strike on power lines causes extremely high voltage pulses at the strike point, which travel from the point of strike to TRANSFORMER.http://electrical-engineering-portal.com/wp-content/uploads/effect-lightning-strike-electrical-lines.jpg
http://t0.gstatic.com/images?q=tbn:ANd9GcQVBnCSllL3zHtdJLKaZLlu2rAHxCiLlgFyWoDSBZtaGKfq2ihXlgp6MGNe
Condition assessment of a power transformer is based on regular routine checks of basic things with the transformers .Examination of the cellulose paper insulation, performing load tests, testing oil for contaminations, checking the winding resistances, impedance and the phase angle. Appropriate tests gears are readily available for technicians to carry out the set of tests.
OIL CONDITION TEST.
(source)www.transformerworld.co.uk/oxygen.htm.
Oxidation of oil.
Oxygen has a strong affinity for electrons. Oxidation of oil arises in a chemical reaction of oil with oxygen to produce an oxide. Oil and paper degrade due to oxidation. With the ageing of oils, there is a decrease in the amount of antioxidants present and the formation of acids and eventually sludge. If the oil is not renewed, the contaminants attack the cellulose in the insulating material and the acid react with the copper coils. Once the insulation is destroyed, the transformer is of no use as the windings are short circuited.
http://www.tradeindia.com/fp154878/Transformer-Oil-Testing-Machine.html
Transformer Oil Testing MachineTransformer Oil Testing Machine
Transformer Oil Testing Machine checklists
1. Specific Resistance (Resistivity).
2. Dielectric Dissipation Factor (Tan Delta).
3. (Acidity).
4. Sediment (Sludge).
5. Flash Point.
6. Water Contents (PPM).
7. Inter facial Tension.
8. Dielectric Strength (BDV).
9. Dissolved Gas Analysis.
10. Appearance.
11. Density.
12. Kinematic Viscosity.
13. PNA Analysis (Paraffinic, Naphthanic & Aromatic content) .
14. Pour Point.
15. Corrosive Sulphur.
16. Oxidation Stability.
17. Aging Characteristic.
18. Presence of oxidation Inhibitor.
Insulation testing.
Oxidation of Cellulose (present in insulating material).
Cellulose oxidizes because it contains oxygen within its molecular structure. During oxidation process generates water, carbon dioxide, which is speed up by high temperature and increase in oil acidity.
Megger Delta-4310 Power Factor Test Set
Megger DELTA4000 Series 12 kV Insulation Diagnostic System
http://www.atecorp.com/Equipment/megger/delta-4000-series.asp
Applications:
Power transformers.
Distribution transformers.
Oil insulation.
Bushings.
Cables.
Circuit breakers.
Surge arrestors.
Transformer Winding resistance tester.
http://www.sivanandaelectronics.com/images/TWRM-10A.jpg
http://www.irispower.com/PDF/WRT-100_english.PDF
Winding resistance measurement in a transformer is very important for the following purposes:-
1. Calculations of power loss in conductor .
2. Winding temperature calculation at the end of a temperature test cycle.
3. to assess how much its value has deviated from the original valer.
Transformer are tested above rated voltage to ensure that there is no excessive leakage
between the windings ( low voltage windings to high voltage windings ) and to ground.
LOAD TEST
indian institute of technology madras
Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao
Performing load test allows us to calculate the total loss taking place in the windings of the transformer at different current and voltage values.
Load test is used mainly
1. to determine the current load and the temperature rise.
2. to determine the efficiency of the transformer.
Rated current load is calculated by supplying the transformer continuously with current and recording the temperature rise. The losses that are generated inside the transformer
on load appear as heat.
MOISTURE
Dehydrating Breathers.
http://www.silicagelmanufacturer.com/silica-gel-breather.htm
Breathing Process of Transformer:
Breathing is the process where the transformer takes in and out the air from its casing due to thermal contraction & expansion of the oil present inside. When the transformer is operating the oil temperature inside the transformer may rises or falls depending upon the loads the windings are carrying. We say that the transformer is breathing. The air movement in and out contains r dust particles and humidity that alters the dielectric properties of the oil. For proper functioning of transformer the dielectric strength of the oil should remain as far as possible to the same value when manufactured. Hence, the air entering the transformer need to be freed from any dust and water vapour.
Silica Gel Breather is widely used in the manufacturing of transformer because of their efficiency in the controlling of the level the moisture, entering in the Transformer. Silica Gel Breather is made from the high quality Blue Silica Gel crystal. The Silica Gel, which is filled in the breather, is deep blue crystals at start and has considerable absorption power for moisture. When Silica Gel absorbs moisture its color changes depending upon the saturation level with water.
Color as follows.
Deep Blue Silica Gel completely dry.
Light Blue Silica Gel partly humid. (Absorbed water for about 15% of its weight).
Pink Silica Gel saturated with moisture. (Absorbed water for about 30 - 40% of its weight).
Dehydrator unit attached to transformerhttp://www.reinhausen.com/en/Portaldata/1/Resources/tc/news/2005-11-02/scaled/2_m.jpg http://t2.gstatic.com/images?q=tbn:ANd9GcTkNMNjWS9bDt1wV31idABkU9g8kHzzEHFPgxBKskBP-JNrXRMsNg
MAINTENANCE MATRIX.
Having gone through the different causes of failures due climatic and in-service conditions, we can extend the life of the transformers by performing scheduled routine test and carry out the required overhauling ( servicing ).A maintenance matrix can be set up which will be executed by both the indoor and outdoor maintenance division .All testing gears shall be supplied to the mobile team and with the concerned checklists including results records.
This matrix can be used on a periodically .This will enable ,through the check list ,to determine the current states of the transformers both region wise as well as at national level. Unnecessary outage will not be encountered, thus running a healthy power distribution grid. By building statistics we can predict and forecast, when certain transformers need to be definitely replaced, thus having plenty of time for budgeting and ordering of new ones.
