Welcome to this module on transformers, classification of transformers. The on-screen video shows a transformer bay, starting from the high level bus bar and terminating at the transformers. The visual shows a 360 degree view of a transformer bay. The transformer is mounted on a foundation grid. The visual gives you a 360 degree view of a transformer mounted in an outdoor switch yard. See the HV, LV bushings, radiators, and conservator. This module will familiarize you with the major components of a transformer. This topic covers the overview, various types, purposes and functions of oil-filled, hermetically sealed and dry type transformers. Transformers are static equipment utilized for transferring electric power from one voltage level to another. Transformers are classified into power transformers and auxiliary or distribution transformers. Power transformers are large size transformers for transfer of bulk power from one voltage to the other. For example, in power generation stations 250, 500, 660, or 800 megawatt, the power that is generated at 11, 15 or 21 kilovolt level is transferred to 110, 132, 220, 400, or 765 kilo volt for transmission over long distances. Distribution transformers as small-sized transformers used for feeding the auxiliaries, such as fans, pumps, blowers, and lighting. The rating of these transformers is generally in the range of 250 kilovolts ampere to one mega volt ampere at a voltage of 415 volt. These transformers generally have a primary voltage of 11 kilo volt, or 6.6 kilo volt. Distribution transformers help utilities distribute the power supply to consumers like residences, public places, industry, etc. Large sized transformers are outdoor oil-filled type. Oil is the insulating and cooling medium. Transformers dissipate heat due to iron loss and copper loss. Oil-filled transformers are provided with a cylindrical vessel called conservator on top of the transformer to accommodate the volumetric expansion of the oil in the transformer due to load and ambient temperature variations. Conservator reduces the area of interface between oil and atmospheric air so that oxidation of oil and the dissolved oxygen in oil is reduced. Oxidation tends to shorten the insulation life of the oil. Conservator holds five percent of the total quantity of oil in the transformer, and is provided prismatic oil level indicator or Magnetic Oil Level gauge, that is MOG. The conservator is connected to a device called breather to extract moisture and dirt from the air entering the transformer during the varying load cycle. It is also usually called a silica gel breather, as silica gel crystals are used in the breather. Silica gel, which is blue in color, turns pink when it has fully absorbed moisture and is due for replacement. In very large transformers, the contact between oil and atmospheric air is eliminated by deploying air cell or rubber bellow on the top surface of the oil in the conservator. Look at the animation and observe how the air cell operates. The conservator is provided with filling, draining, and, sampling valves. We'll see all the other accessories a little later in this module. A conservator is provided with partition for catering to oil DC chamber. Sealed transformers are non-breathing type as there is no interchange between its contents and external atmosphere. In this type of construction, tank cover is welded or bolted to the curb or curb of the tank. Spaces provided above the core coil assembly where an inert gas cushion system accommodates the oil expansion under variable pressure. The transformer tank shall be of adequate strength to withstand positive and negative pressure buildup when the transformer is in operation. These transformers are available with off circuit tap changers. Sealed transformers generally find application up to 3.15 mega volt ampere, that is, as distribution transformers. Accessories for hermetically sealed transformers are, HV bushing, LV bushing, oil level gauge, drain cum sampling valve, oil temperature indicator and winding temperature indicator, pressure relief valve or rupture diaphragm, lifting hooks, cooling fins, and wheels. The applications of hermetically sealed transformers are, pole mounted distribution transformer, chemical plants, pharmaceutical plants, cement plants, and refineries. A transformer in which mineral oil or any other liquid is not employed for insulation or cooling purposes is dry-type transformer. Cooling is by natural air circulation or by forced air cooling. There are two types in dry-type transformers, cast resin type and vacuum pressure impregnated type. Cast resin type. The transformers consists of HV and LV windings made of copper conductors or foils of either aluminum or copper that are impregnated and cast under vacuum in epoxy resin. These transformers are fitted with PT100 thermistors embedded in coils to sense temperature rise for alarm and tripping. These transformers are suitable for indoor applications as there is no fire risk. They're supplied with enclosures up to IP-40 degree of protection. Vacuum pressure impregnated type. The transformer coils are impregnated with varnish under vacuum pressure. These transformers offer excellent mechanical and short circuit strength, less weight compared to cast resin transformers. Dry-type transformers are offered in class F and H installations. The temperature rise is limited to 90 degrees Celsius and 115 degrees Celsius, respectively. Temperature measurement is direct, unlike in oilfield transformer wherein both oil and winding temperatures are measured. Adequate safety measure shall be in place to prevent heart patients with pacemaker implant to go anywhere near the dry-type transformer within 10 feet. Here is a quick recap of what you have learned thus far. Transformers are static equipment utilized for transferring electric power from one voltage level to another. Power transformers are large sized transformers for transfer of bulk power from one voltage to the other. Distribution transformers are small size transformers used for feeding the auxiliaries such as fans, pumps, blowers, and lighting. Oil field transformers are provided with conservator on top of the transformer to accommodate the volumetric expansion of the oil in the tank during operation. Expansion bellow or air cell is provided in the conservator to prevent external air entering the oil. Breather with silica gel is provided to extract moisture and dirt from the air entering the transformer during the varying load cycle for oil field transformer. Sealed transformers are non breathing type. There are two types in dry type transformers, cast resin type and vacuum pressure impregnated type.
