Transformer ratings. The rating of transformer depends on the type of oil cooling adopted. Some kinds of cooling medium are mineral oil designated as O, and air designated as A. Some kinds of circulation are natural, designated as N, forced, designated as F, and forced oil direct designated as D. The types of transformer cooling are, ONAN, that is oil, natural, air, natural; ONAF, oil, natural, air, forced; OFAF, oil, forced, air, forced; and ODAF, oil, directed, air, forced. In oil natural type of cooling, oil circulation is affected due to natural currents caused by the temperature difference between the top and bottom oil. In air forced type of cooling, the cooling is affected through cooler fans. The cooling fans are provided in each of the radiator banks. In oil forced type of cooling, the flow of oil is increased through electrically operated pumps. Fans and pumps are affected automatically based on the temperature of the oil. Spare fans and pumps are provided for backup in case of failure of the primary ones. In OFAF, oil is circulated within the closed loop of the transformer tank using pumps that are available in the oil circulation path in the radiator banks. The same is improved in ODAF, where the oil is made to flow through pre-determined paths between conductors to ensure a faster rate of heat transfer. The rated capacity as per IEC 60076-1, and IS 2026-1, is the value of apparent power input to the transformer, that is sum of power output and losses. IEEE C57.12.00 implies that the rated kVA of the transformer shall be the output that can be delivered. Rated power is defined at rated, opens circuit secondary voltage, and the rated frequency without exceeding allowable temperature rise. Applicable standards, prescribed testing methodology. A transformer can deliver its rated capacity under the following conditions. Design ambient air temperature, 50 degrees Celsius. This is as per IS. For IEC, the ambient temperature is 40 degrees Celsius. Accordingly, temperature rise needs to be considered. The monthly average temperature of the hottest month, 40 degrees Celsius, for overload application as per national and international standards. Maximum yearly weighted average air temperature, 32 degrees Celsius for aging application as per national and international standards. Maximum altitude of operation, 1,000 meters above mean sea level. Selection of open circuit voltage. Open circuit voltage is the no load voltage between the terminals of the secondary winding. In the case of transformers with OCTC or OLTC, the principal tap should be considered. Open circuit voltage is selected to account for the regulation of the transformer on full load and the percentage impedance of the transformer. For example, primary distribution transformer, say 10MVA capacity with percentage impedance of eight percent. Voltage ratio is chosen as 33 by 11.8 kV. 11.8 kV is the secondary open circuit voltage, so that when the transformer is fully loaded, the secondary voltage will be regulated to 11 kV. Maximum temperature rise as per IS standard under rated operating condition. Considering the transformer is operating at extreme tap position, incurring extra copper losses. If ambient temperature is 50 degrees Celsius, the rated oil temperature rise should not exceed 50 degrees Celsius, , over the ambient temperature. If ambient temperature is 50 degrees Celsius, the rated winding temperature rise should not exceed 55 degrees Celsius over the ambient temperature. Overloading. The application of load in excess of the name plate rating involves a degree of risk and accelerated aging. Considered a transformer of capacity 10 MVA with the connected load of 10 MVA, including losses at it's LV side. Under this condition, the transformer operates safely. Now, if the load is increase to say 12 MVA, this condition is overloading the transformer and it increases the temperature of the windings, inducing stress. The rating of a transformer is defined by the type of cooling and the associated loading capacity of the transformer. For example, a 125 MVA power transformer can be operated with different cooling systems, such as Oil Natural Air Natural cooling, Oil Natural Air Forced cooling, and Oil Forced Air Forced cooling. Stage one; the transformer can be loaded up to 75 MVA without fans and pumps. Here, the cooling system employed to the transformer is ONAN. Stage two; the same transformer can be loaded up to 100 MVA with the help of cooling fans. Now, the cooling system employed to the transformer is ONAF. The transformer can be loaded up to 125 MVA by operating the cooling fans and oil pumps. This is Oil Forced Air Forced cooling. Winding temperature alum and trip settings are done based on the allowable temperature rise. There is a correlation between load on transformer and the temperature of transformer. The transformer can be overloaded within its permissible limits of temperature rise. Types of loading beyond name plate are; Normal cyclic loading, longtime emergency loading, and short-time emergency loading. Limiting values are top oil temperature and winding hot spot temperature. The current and temperature limits applicable to loading beyond name plate rating as per IEC 60076 are given on the screen. Let us consider for a distribution transformer, normal cyclic loading, where the winding hot spot temperature should not exceed 120 degrees Celsius. The top oil temperature should be limited to 105 degrees Celsius and the current per unit should not exceed 1.5. With this condition, you can operate the transformer. Similar analogy can be drawn for the other conditions also. As discussed in basic substation design parameters, for a transformer manufactured for operation in planes up to an altitude of 1000 meters, if it is installed in areas of altitude of more than 1000 meters, the allowable temperature rise shall be decreased in accordance with relevant national or international standards. Here is a quick recap of what you have learned thus far. The transformer rating is based on the cooling methods. The types of cooling commonly adopted are ONAN, that is Oil Natural Air Natural, ONAF, Oil Natural Air Forced, OFAF, Oil Forced Air Forced, and ODAF, Oil Directed Air Forced. In ONAN, the oil circulation is affected due to natural currents caused by the temperature difference between the top and bottom oil. In ONAF, cooling is provided by the forced air through cooler fans. The fans are switched on when the winding temperature exceeds the set value. In OFAF, the fans and pumps are controlled automatically based on the temperature of the winding. In ODAF, oil is directed to flow through predetermined paths between conductors or windings to ensure a faster rate of heat transfer. Transformer power can be rated based on IEEE or IEC standard. As for IEEE, the name plate rating of the transformer is it's rated output power. As per IEC, it is equal to its rated apparent power input, which includes both output power and its losses. Overloading of transformer is loading of transformer beyond its rated capacity. There are three types of overloading namely; normal cyclic loading, short-term overloading, and long-term overloading. Overloading is permitted within the permissible limits provided by IEC standard.
