A transformer is an electrical device which, by the principles of electromagnetic induction, transfers electrical energy from one electric circuit to another, without changing the frequency.

The energy transfer usually takes place with a change of voltage and current. Transformers either increases or decreases AC voltage.

Transformers are used to meet a wide variety of needs. Some transformers can be several stories high, like the type found at a generating station or small enough to hold in your hand, which might be used with the charging cradle for a video camera. No matter what the shape or size, a transformers purpose remains the same: transforming electrical power from one type to another.

TRANSFORMER

WORKING PRINCIPLE OF A TRANSFORMER

Transformer works on the principle of mutual induction of two coils. When current in the primary coil is changed the flux linked to the secondary coil also changes. Consequently an EMF is induced in the secondary coil.

Transformers do not generate electrical power; they transfer electrical power from one AC circuit to another using magnetic coupling. The core of the transformer is used to provide a controlled path for the magnetic flux generated in the transformer by the current flowing through the windings, which are also known as coils.

PARTS OF A BASIC TRANSFORMERThere are four primary parts to the basic transformer.

Input Connections - The input side of a transformer is called the primary side (Primary Coil) because the main electrical power to be changed is connected at this point.

Output Connections - The output side or secondary side (Secondary Coil) of the transformer is where the electrical power is sent to the load.

Winding - Transformers have two windings, being the primary winding and the secondary winding. The primary winding is the coil that draws power from the source. The secondary winding is the coil that delivers the energy at the transformed or changed voltage to the load.

Core - The transformer core is used to provide a controlled path for the magnetic flux generated in the transformer. The core is generally not a solid bar of steel, rather a construction of many thin laminated steel sheets or layers. This construction is used to help eliminate and reduce heating.

Types of Transformer

There are several types of transformers, depending on the application, design and constructions.

Transformers generally have one of two types of cores: Core Type and Shell Type. These two types are distinguished from each other by the manner in which the primary and secondary coils are place around the steel core.

Core type - With this type, the windings surround the laminated core. The coils used for this transformer are form-wound and are of cylindrical type.Shell type - In shell-type transformers the core surrounds a considerable portion of the windings. With this type, the windings are surrounded by the laminated core.

Common Types of Transformers

1.The voltage transformer is similar to the more common power transformer. It is also referred to as a potential transformer and is used for metering and protection in high voltage circuits. It is designed in order to achieve an accurate voltage ratio over the range of the load and is often used to step up low voltages or to step down high ones.

2. The power transformer is the most commonly used transformer. One thing to remember is that a transformer does not actually create power, but transfers it from one coil winding to another. The power transformer is actually a type of voltage transformer and it is used in many different power type applications. Some of these transformers are immersed in oil in order to keep them cooler and stop them from overheating.

3. The current transformer is also known as a series transformer. It is often placed in series with a high current circuit. It is used for the measurement of electric circuits and they are often used in metering and protective relays.

4. The impedance transformer was designed to ensure accurate impedance transformation. For example, a transformer may be used to "match" the impedance of an amplifier to a speaker. In fact, they are often used in low-frequency amplifiers.

The Isolation transformer has no direct connection between windings, but they are connected via the magnetic flux in the core. In most cases, the winding ratio is a one to one. In other words, it is neither a step up, nor a step down but a means of isolating the circuit from the power supply.

The auto transformer is unique in its tapped windings. The primary is normal, but the secondary has at least three taps where electrical connections can be made and these different taps result in different voltages. They are often used in applications where it is needed to interconnect systems operating at different voltages.

TRANSFORMER COOLING SYSTEMS

The Transformer is a device used to convert the energy at one voltage level to the energy at another voltage level. During this conversion process, losses occur in the windings and the core of the transformer. These losses appear as heat. The transformer’s output power is less than its input power. The difference is the amount of power converted into heat by core loss and winding losses. The losses and the heat dissipation increases with increase in the capacity of the transformer.

Cooling of transformer is the process of dissipation of heat developed in the transformer to the surroundings. The losses occurring in the transformer are converted into heat which increases the temperature of the windings and the core.

How to cool the transformer?There are two ways of cooling the transformer:First, the coolant circulating inside the transformer transfers the heat from the windings and the core entirely to the tank walls and then it is dissipated to the surrounding mediumSecond, along with the first technique the heat can also be transferred by coolants inside the transformer.The coolants used in the transformer are air and oil. In dry type transformer air coolant is used and in oil immersed one, oil is use. In the first said, the heat generated is conducted across the core and windings and is dissipated from the outer surface of the core and windings to the surrounding air. In the next, heat is transferred to the oil surrounding the core and windings and it is conducted to the walls of the transformer tank

METHODS OF COOLING OF TRANSFORMER

Based on the coolant used the cooling methods can be classified into:

Air cooling:In this method, the heat generated is conducted across the core and windings and is dissipated from the outer surface of the core and windings to the surrounding air.

Oil and Air cooling:Heat is transferred to the oil surrounding the core and windings and it is conducted to the walls of the transformer tank. Finally the heat is transferred to the surround air by radiation and convection.Oil coolant has two distinct advantages over the air coolants.•It provides better conduction than the air•High coefficient of conduction which results in the natural circulation of the oil

Oil and Water cooling:In this method along with oil cooling, water is circulated through copper tubes which enhance the cooling of transformer. This method is normally adopted in transformers with capacities in the order of several MVA.

TRANSFORMER COOLING DESIGNATIONS

1. Air cooling (Dry type transformers)Air Natural(AN)This method uses the ambient air as the cooling medium. The natural circulation of the air is used for dissipation of heat generated by natural convection. The core and the windings are protected from mechanical damage by providing a metal enclosure. This method is suitable for transformers of rating up to 1.5MVA. This method is adopted in the places where fire is a great hazard.Air Blast (AB)In this method, the transformer is cooled by circulating continuous blast of cool air through the core and the windings. For this external fans are used. The air supply must be filtered to prevent accumulation of dust particles in the ventilating ducts.

2. Oil cooling (Oil immersed transformers)Oil Natural Air Natural (ONAN)The transformer is immersed in oil and the heat generated in the cores and the windings is passed on to oil by conduction. Oil in contact with the surface of windings and core gets heated up and moves towards the top and is replaced by the cool oil from the bottom. The heated oil transfers its heat to the transformer tank through convection and which in turn transfers the heat to the surrounding air by convection and radiation.Oil Natural Air Forced (ONAF)In this method, the heated oil transfers its heat to the transformer tank. The tank is made hollow and air is blown to cool the transformer. This increases the cooling of transformer tank to five to six time its natural means. Normally this method is adopted by externally connecting elliptical tubes or radiator separated from the transformer tank and cooling it by air blast produced by fans.Oil Forced Air Natural (OFAN)In this method, copper cooling coils are mounted above the transformer core. The copper coils will be fully immersed in the oil. The tank is made hollow and air is blown to cool the transformer. This increases the cooling of transformer tank to rapidly above its natural cooling level.Oil Forced Air Forced (OFAF) OR (ODAF)In this method the oil is cooled in the cooling plant using air blast produced by the fans. These fans need not be used all the time. During low loads fans are turned off. Hence the system will be similar to that of Oil Natural Air natural (ONAN). At higher loads the pumps and fans are switched on and the system changes to Oil Forced Air Forced (OFAF).

3. Oil and Water cooling (For capacity more than 30MVA)Oil Natural Water Forced (ONWF)In this method, copper cooling coils are mounted above the transformer core. The copper coils will be fully immersed in the oil. Along with the oil natural cooling the heat from the core passes to the copper coils and the circulating water inside the copper coil takes away the heat. The disadvantage in this method is that since water enters inside the transformer any kind of leakage will contaminate the transformer oil.

Oil Forced Water Forced (OFWF) OR (ODWF)In this method hot oil is passed though a water heat exchanger. The pressure of the oil is kept higher than that of the water therefore there will be leakage from oil to the water alone and the vise versa is avoided. This method of cooling is employed in the cooling of transformers with very larger capacity in the order of hundreds of MVA. This method is suitable for banks of transformers. Maximum of three transformers can be connected in a single pump circuit. Advantages of this method over ONWF are that the transformer size is smaller and the water does not enter into the transformer. This method is widely used for the transformers designed for hydro electric plants.

Treatment of Dielectric Oil of Electrical Transformer

Why need treat Transformer oil? The majority of power transformers in operation today are filled with mineral oil. The primary function of the oil is to provide a high dielectric insulating material and an efficient coolant. The effectiveness of the oil as an insulating material is reduced as the moisture level increases, while cooling is reduced as the oil oxidizes. Paper insulation will also absorb moisture from the oil, thus increasing power factor readings. The oxidation of transformer oil begins as soon as the transformer is energized. A chemical reaction occurs when the oil is exposed to a combination of heat, oxygen, and core and coil components. As the process of oxidation progresses, acids and polar compounds are formed and in turn become sludge. This sludge will then coat heat transfer surfaces on the core/coil and the tank/radiators, reducing the heat transfer capacity of the system. The operational temperatures are increased, thus accelerating the degradation of the oil.

OXIDIZING TRANSFORMER INSULATION OILS · Oil Which Is In The Initial Stages Of Oxidization, Forming Acids And Polar Compounds. Some sludge deposits will be found in a small percentage of oils in this initial stage of oxidization (Acidity levels <.20mg KOH/g oil). · Oil Which Has Advanced In The Oxidization Process To The Point Where Sludge Deposits Have Been Formed. This precipitating sludge coats all surfaces of the transformers tank and radiator walls, as well as the core and coil oil ways. In so doing, heat transfer is reduced causing the transformer to operate at higher than normal temperatures, which in turn speeds up the oxidation process (Acidity levels of .20mg KOH/g oil or greater).

MOISTURE INTRODUCED INTO TRANSFORMER OILS

· Through Absorption From The Atmosphere Above The Oil Level. Many transformer tanks are designed to seal the transformer from the outside atmosphere; however, top side leaks may develop that allow normal temperature changes to cause breathing . With each new inhalation comes more moisture to be potentially dissolved in the oil. Units designed as free breathing also can experience a build-up of dissolved moisture. In extreme cases, top cover leaks may be present which can allow rain to enter into the unit directly.

· Condensation Inside Transformers. The moisture is introduced by exposure to the atmosphere above the oil level. Sudden temperature changes can condense the moisture allowing it to run down the tank walls into the oil. There it will dissolve slowly. · Oxidation Of Oil And Paper Insulation. Since oil and paper are organic compounds containing hydrogen, gradual oxidation will allow the formation of moisture. This can account for a major portion of the moisture in badly deteriorated oils.

LEVELS AT WHICH TREATMENT IS RECOMMENDED TSI has found that production of moisture can become a problem if oil is allowed to deteriorate beyond an acidity level of .05mg KOH/g oil; therefore, we recommend treatment of oils that have reached this level. In cases where acidity levels do not require treatment, I.F.T.'s of less than 24 dynes/cm, dielectrics of less than 25Kv, and moisture contents above 30ppm signal the need for hot oil treatment. Units with primary voltages above 15Kv should have dielectric readings of 30Kv or above and moisture contents below 25ppm.

OIL TREATMENT METHODS

Correcting the problems of oil oxidation can be accomplished in several ways with varying degrees of success.

Changing the oil – This will result in clean oil, but will do little to remove sludge adhering to the radiators, tank walls, and core and coil. Within a year of changing the oil, oxidation products not removed will be redissolved into the new oil resulting in acidity and polar compound levels appreciably above those of new oil. Subsequent oil changes may be required to remove these redissolved products of oxidation. Each time this is done, the transformer must be de-energized.

Filter press the oil - The only thing accomplished by filter pressing is the removal of solid particles that have been in suspension and free water. This process does not significantly change the acid or polar compound levels, or remove dissolved water. Oxidation and sludge formation will continue as soon as filtering is stopped. Very little is gained from this method.

Un-tank a unit, flush the tank, radiators, and core and coil with solvents, then refill the unit with new oil - This method can result in a successful stabilization of the oil, but there are several major drawbacks. The units must be de-energized and sent to a service shop. This means days or weeks without the use of the unit, plus expensive handling, transportation, and service charges.

HOT OIL TREATMENT - ENERGIZED EQUIPMENT The treatment of oil in the initial stages of oxidization is called HOT OIL TREATMENT. Within this category there are two exposure time periods. · Oils With Acid Levels Below .10mg KOH/g oil have an exposure time based on six (6) passes at a flow rate of 600 to 900 GPH. · Oils With Acid Levels From .10 To .19mg KOH/g oil have an exposure time based on ten (10) passes at a flow rate of 600 to 900 GPH. D-SLUDGING - ENERGIZED EQUIPMENT The treatment of oil in the advanced stages of oxidation is called D-SLUDGING, which is a two step treatment process. Oils with acid levels greater than .20mg KOH/g oil are exposed to 10 passes for Step 1, and six (6) to ten (10) passes for Step 2, at an average flow rate of 600 to 900 GPH. A time interval of at least six (6) months occurs between Steps 1 and 2. This time interval is referred to as the D-SLUDGING period. The clean oil from Step 1 redissolves decay products into the oil which are removed from the oil in Step 2. After treatment the oils will meet or exceed the following specifications: I.F.T. (Interfacial Tension Test)- 34.0 dynes/cm MINACIDITY - .03mg KOH/g oil MAXDIELECTRIC - 35Kv MINMOISTURE - <15Kv - 30ppm >15Kv - 20ppmDEHYDRATION/DEGASSING Hot oil treatment through the vacuum degasser will remove moisture and gasses from the oil. Drying of the solid insulation is a multi pass process. The number of passes is a function of the actual moisture and gas content. Processing from ten to fifty passes is the average range. The higher passes are needed when the goal is to dry the solid (paper) insulation that has absorbed moisture from the oil.

Open circuit and Short circuit Test on transformer

These two transformer tests are performed to find the parameters of equivalent circuit of transformer and losses of the transformer Open circuit test and short circuit test on transformer are very economical and convenient because they are performed without actually loading of the transformer.Open circuit or No load test on TransformerOpen circuit test or no load test on a transformer is performed to determine 'no load loss (core loss)' and 'no load current I0'. The circuit diagram for open circuit test is shown in the figure below. Usually high voltage (HV) winding is kept open and the low voltage (LV) winding is connected to its normal supply. A wattmeter (W), ammeter (A) and voltmeter (V) are connected to the LV winding as shown in the figure. Now, applied voltage is slowly increased from zero to normal rated value of the LV side with the help of a variac. When the applied voltage reaches to the rated value of the LV winding, readings from all the three instruments are taken.

Short circuit or Full Load Impedance test on Transformer

The connection diagram for short circuit test or impedance test on transformer is as shown in the figure below. The LV side of transformer is short circuited and wattmeter (W), voltmetre (V) and ammeter (A) are connected on the HV side of the transformer. Voltage is applied to the HV side and increased from the zero until the ammeter reading equals the rated current. All the readings are taken at this rated current.

The ammeter reading gives primary equivalent of full load current (Isc).The voltage applied for full load current is very small as compared to rated voltage. Hence, core loss due to small applied voltage can be neglected. Thus, the wattmeter reading can be taken as copper loss in the transformer.