Any device that is capable of continuous electro-mechanical energy conversion is known as an Electrical Machine. Therefore, transformer is not an electrical machine. It is simply a Static Device which transmits power from one circuit to another at different voltage levels and same frequency. In case, the voltage level also remains the same, it is known as an Isolation Transformer. A transformer consists of a core which basically has two functions; to provide mechanical support and to provide low reluctance path to the core flux. The core is normally made up of Cold Rolled Grain Oriented Steel, in order to reduce hysteresis losses (Iron Loss) and to reduce eddy current losses (Iron Loss); the core is laminated and each lamination is being insulated from each other by Impregnated Cellulose Paper Insulations. Also necessary amount of precautions are taken so that each lamination is kept as thin as possible, since; the eddy current losses are directly proportional to the square of the thickness.
A sinusoidal voltage source is applied at the primary side of the transformer. This leads to the development of sinusoidal flux in the core. This core flux remains constant no matter what happens. Now, as per Faraday’s law of Electro-Magnetic Induction, statically induced electro-motive force (emf) is produced in primary as well as secondary winding. The direction of induced emf is given by Lenz’s law and which terminal of the winding is chosen as positive. According to Lenz’s law, the direction of induced emf is such that if it is allowed to cause a current by short-circuiting the coil, the current so produced has an effect that opposes the cause. Remember, the induced field only opposes the change in field of the supply not the field itself.
Under No-Load Condition, a non-zero highly lagging current is drawn from the input. This no-load current is known as No-Load Current or Exciting Current. It accounts for setting up of flux in the core and losses in the core. However, when a load is applied at the secondary terminals, current starts flowing in the secondary & tends to reduce the already existing flux in the core by opposing it. However, as soon as that tends to happen, equivalent amount of current is drawn from the primary to nullify the opposing effect & thereby ensuring constant flux in the core of the transformer. This way, corresponding to the load applied, a primary current is drawn, in addition to the no-load current.
Components & Practical Aspects of a Power Transformer
Leakage Reactance & Resistance: The leakage reactance accounts for leakage flux i.e. flux which is not able to link with both primary & secondary winding whereas resistance accounts for copper losses occurring in the windings. Since resistance is already kept at an optimal value due to efficiency considerations, leakage reactance should be kept to a minimum value in order to have good voltage regulation. This can be achieved by reducing the leakage flux which is possible if the two windings are kept physically close together. In core type transformer, physical proximity is obtained using concentric cylindrical windings whereas in shell type transformer, it is obtained by using sandwich winding (also known as pancake winding or interleaved winding). The leakage reactance in a shell type transformer may be graded by using different size & combinations of HV & LV layers.
In a core type transformer, copper surrounds iron whereas in a shell type transformer, iron surrounds copper. Hence, shell has greater mechanical strength. Therefore it is used for high current low voltage applications (because repulsion forces will be large if the current is high, hence high mechanical strength is needed to withstand it; low voltage because shell has sandwich winding, layers of LV & HV are there, HV touches iron, but we are least bothered because we are using it for low voltage applications).In India, all the transformers are of Core Type.
Conservator Tank: Its capacity is around 12-16 % of main oil tank. When the load changes, the oil expands/contracts and the function of the conservator is to take up that expansion/contraction. If the main tank is sealed, it will explode when expansion takes place. The conservator also avoids the contact of the oil to the atmospheric air as it may contaminate the oil.
Breather: During expansion, the vapours go outside. But, when there is any contraction, the moisture present in atmospheric air may enter the transformer and eventually contaminate the transformer oil and in order to avoid this, a bottle containing Silica Gel Crystals which are impregnated with Cobalt Chloride is used. It is Blue in colour and absorbs moisture then it turns into Pink colour when it becomes moist. It is then reheated to remove all the moisture and it regains its original colour.
Explosive Vent/Diaphragm: Above the transformer, a small glass sheet or a thin metal sheet is present, known as Explosion Diaphragm. Say a major fault occurs inside the tank of transformer. This will lead to massive gas formation due to oil decomposition and at that moment the explosion diaphragm will explode first. This acts like a safety valve. In modern design we are having PRVs (Pressure Regulating Valves), which releases the extra pressure and saves it from explosion.
Tap changer: The main function of the tap changer is to control the voltage on the secondary side. Taps are provided on the HV side. There are two reasons to provide taps on HV side instead of LV side; one is that the current is less on HV side, therefore arcing will be lesser and the second one is fine control is available on HV side. Also, the tapings are given on the central part of HV side. The reason for this is that the axial forces get balanced. The HV side windings are also easily accessible as they are on the outer periphery of the winding arrangement.
Cooling System: For smaller capacity oil filled transformers, the oil is cooled by natural circulation of oil through the Radiators. This is known as Oil Natural Air Natural (ONAN) Cooling, but for large transformers, forced cooling is used known as Oil Natural Air Forced (ONAF) cooling and for even more larger transformers, air quantum is not sufficient hence, oil is directed through a channel inside and that is cooled by Forced Air, known as Oil Directed Air Forced(ODAF). Depending upon the capacity and heat generation various cooling arrangements are employed. Below mentioned are the cooling systems employed in an ascending order of the capacity.
Oil: The mineral oil used in the transformer has basically three functions;
- To provide better insulation
- Acts as a cooling medium for heat dissipation.
- Preserves the cellulose insulation from getting in direct contact with the air.
In large transformers, periodic testing of oil is necessary since it may undergo certain chemical changes. Also, in case of power transformers, external radiators are present through which the oil circulates by natural convection process.
Properties of Transformer Oil:
- Break Down Voltage (BDV): It is the voltage at which the oil will lose its insulation property and arc will be initiated. For transformer having capacity of 170 MVA or 200 MVA it should be above 60 kV(approx.)
- Viscosity: It should be as low as possible for better circulation of oil ,but as per the IS 335/1982,At 27°C it should be less than 27 cst and At 40°C it should be less than 9 cst.
- Acidity: if the acidity is more then it may lead to corrosion and solid concentration in the oil will increase. It should be Max. 0.03 mg KOH/gm (Denoted as KOH required neutralizing).
- Moisture: Lesser the moisture content in the oil, better the dielectric strength of the Oil. It should be around 20 PPM after filtration.
Buchholz relay: It is a Gas Actuated Relay used for the protection of transformer from incipient faults and major faults. Although it is the most efficient way for identification of internal faults, the time of operation is more & hence it is associated with percentage differential relay for complete protection of transformer. It comes under the category of unit protection. When any fault takes place, non-flammable gases are released due to sparking in transformer oil. These gases set off an alarm in case of a minor fault & trip the circuit in case of a severe fault. By studying the composition of the gas, fault location can be easily identified, which is identified by the DGA (Dissolved Gas Analysis). It consists of Mercury Floats which actuate during gas deposition due to faults in transformer.
Location of fault Chemical composition of gas
Construction parts C2H2 , H2
Winding C2H2 , H2 , CH4
Core joints C2H2 , H2 , CH4 , CO2
Bushings: The transformer has two windings (primary & secondary). Proper connections/passage is needed for these windings to get connected to outside for further use. This is where bushings come into play. They are like insulators which provide electrical conductors a safe passage from the conducting parts of the transformer which are earthed. They are made up of porcelain material and are of wavy shape. The wave shape of the bushing is to maximize surface length. If continuous shape is given, then there will be more chances of leakage current.
If, let us say that the linear dimensions of a transformer are x, y & z. If these dimensions are done k times, following relations hold:
- KVA α k4
- Surface area α k2
- Losses α k3
Magnetostriction Phenomenon: If continuously alternating flux keeps encountering laminations, they undergo certain physical changes in the order of 0.00012% at one Tesla. Although it is negligible, but still, it is there. This is also the reason of humming sound from transformers.Transformers are kept on rubber mats or elastic foundation to avoid vibrations.