1.method of Cooling
transformer cooling system is Increasing the cooling rate of a transformer will increase its capability. Cooling ways should not solely maintain a sufficiently low average temperature however should stop associate degree excessive temperature rise in any portion of the transformer. For this reason operating elements of enormous transformer are typically submerged in best insulating oil.
This oil should be unbroken as free as attainable from wet and dissolved flammable gases, and particulates. Ducts are organized to supply free circulation of oil through the core and coils; hotter and lighter oil rises to the highest of the tank, cooler and heavier oil settles to all-time low. Many ways are developed for removing heat that’s transmitted to the transformer oil from the core and windings (figure).
1.1. Oil-Filled – Self-Cooled Transformers
In small- and medium-sized transformers, cooling takes place by direct radiation from the tank to close air. In oil-filled, self-cooled varieties, tank surfaces are also furrowed to supply a larger diverging surface. Oil in grips with the core and windings rises because it absorbs heat and flows outward and downward on tank walls, wherever it’s cooled by diverging heat to the encircling air. These transformers may additionally have external radiators connected to the tank to supply larger expanse for cooling. 2
1.2. Forced-Air and Forced-Oil-Cooled
Transformers Forced-air-cooled transformers have fan-cooled radiators through that the transformer oil circulates by gravity, as shown in figure. Fans force air through radiators, cooling the oil.
Forced-air/oil/water-cooled transformers have a self-cooled (kVA or MVA) rating and one or a lot of forced cooling ratings (higher kVA or MVA). Higher ratings are because of forced cooling in increasing amounts. As temperature will increase a lot of fans or a lot of oil pumps ar turned on mechanically.
The forced-cooling principle is predicated on a trade-off between additional cooling and producing prices. Transformers with forced-cooling have less weight and bulk than self-cooled transformers with identical ratings. In larger-sized transformers, it’s a lot of economical to feature forced cooling, even if the electricity required to control fans and pumps will increase the operating expense.
1.3. transformers Oil
In addition to dissipating heat because of losses in a very transformer, insulating oil provides a medium with high stuff strength within which the coils and core are submerged. This enables the transformers to be a lot of compact that reduces prices.
Insulating oil in good shape can stand up to much more voltage across connections within the transformer tank than can air. Associate degree arc would jump across identical spacing of internal energized elements at a way lower voltage if the tank had solely air. Additionally, oil conducts heat off from energized elements far better than air.
Over time, oil degrades from traditional operations, because of heat and contaminants. Oil cannot retain high stuff strength once exposed to air or wet. Stuff strength declines with absorption of wet and O. These contaminants conjointly deteriorate the paper insulation.
For this reason, efforts are created to forestall insulating oil from contacting air, particularly on larger power transformers.
employing a tightly sealed electrical device tank is impractical, because of pressure variations ensuing from thermal growth and contraction of insulating oil.
Common systems of protection oil-filled transformers are the conservator with a versatile diaphragm or bladder or a positive pressure inert-gas (nitrogen) system. Reclamation GSU transformers are typically purchased with conservators, whereas smaller station service transformers have a controlled chemical element blanket on prime of oil. Some station service transformers are dry-type, self-cooled or forced air cooled.
1.4. Conservator System
A conservator is connected by piping to the most transformer tank that’s fully full of oil. The conservator is also full of oil associate degree contains an expandable bladder or diaphragm between the oil and air to forestall air from contacting the oil. Figure twenty three may be a schematic Figure
Representation of a conservator system (figure one is associate degree actual photograph of a conservator). Air enters and exits the area on top of the bladder/ diaphragm because the oil level within the main tank goes up and down with temperature. Air usually enters and exits through a sedative-type air drier that has got to have the desiccant replaced sporadically.
The most elements of the system are the growth tank, bladder or diaphragm, breather, vent valves, liquid-level gauge and alarm switch. Vent valves are wont to vent air from the system once filling the unit with oil. A liquid-level gauge indicates the requirement for adding or removing transformer oil to take care of the right oil level and allow flexing of the diaphragm
1.5. Oil-Filled, Inert-Gas System
A positive seal of the transformer oil is also provided by inert-gas system. Here, the tank is slightly controlled by element like Nitrogen.
The main tank gas house on top of the oil is given a gauge (figure 24). Since the whole system is intended to exclude air, it should operate with a positive pressure within the gas house on top of the oil; otherwise, air are going to be admitted within the event of a leak. Smaller station service units don’t have N tanks hooked up to mechanically add gas, and it’s common observe to feature N yearly every fall because the tank starts to draw partial vacuum, thanks to cooler weather. The surplus gas is expelled every summer as masses and temperatures increase.
Some systems square measure designed to feature Nitrogen mechanically (figure 24) from controlled tanks once the pressure drops below a collection level. A positive pressure of roughly 0.5 to 5} pounds per square inch (psi) is maintained within the gas house on top of the oil to forestall ingress of air. this technique includes a Nitrogen gas cylinder; three-stage, pressure-reducing valve; high-and nonaggressive gauges; high- nonaggressive alarm switch; oil/condensate sump drain valve; an automatic pressure-relief valve; and necessary piping.
The operate of the three-stage, automatic pressure-reducing valves is to cut back the pressure of the Nitrogen cylinder to provide the house on top of the oil at a maintained pressure of 0.5 to 5psi.
The hard-hitting gauge ordinarily includes a vary of 0 to 4,000 psi and indicates Nitrogen cylinder pressure. The nonaggressive gauge ordinarily includes a vary of regarding -5 to +10 psi and indicates Nitrogen pressure on top of the transformer oil.
In some systems, the gauge is supplied with high- nonaggressive alarm switches to alarm once pressure level reaches abnormal value; the hard-hitting gauge is also equipped with a pressure switch to sound an alarm once the provision cylinder pressure is running low.
A sump and drain valve offers a method for collection and removing atmospheric phenomenon and oil from the gas. A pressure-relief valve opens and closes to unharnessed the gas from the transformer and, thus, limit the pressure within the transformer to a secure most price. As temperature of a transformer rises, oil expands, and internal pressure will increase, which ought to be mitigated. Once temperature drops, pressure drops, and Nitrogen ought to be additional, looking on the extent of the action and pressure limits of the system.