Transform more Percentage Impedance and Losses. This topic covers percentage impedance and losses. Vector groups, capitalization of transformer losses and transformer redundancy philosophy. These are some typical values for percentage impedance. Further transform more up to 630 kV. The recommended percentage impedance shall be four percent. For transformers of rating 631 kVA to 1250 kVA the percentage impedance shall be five percent, and so on. The values of short circuit impedance are subjected to allowable tolerances. For more details, please refer IEC 60076 or IS 2026. The losses consists of the following components. No load, that is iron loss, load, that is copper loss, and auxiliary loss. The loss figures are designed considering the economical requirement. The vector group depends on the type of winding adopted for the primary and secondary of the transformers. The Dyn11 or Dyn1 are used in primary distribution transformer. For example, 33 by 11 kV. Dyn11 or Dyn1 are used for LV power distribution transformer. For example, 11 by 0.433 kV. Will now see how losses need to be evaluated from commercial aspects. Remember, each type of loss will cost money. Have a look at this analogy to understand this concept. When you buy a bike of same capacity, say 100 cc from two different companies, the mileage offered by each bike will be different, which basically decides running cost. Hence, to compare both bikes on same platform, you need to consider capital cost and running cost. The objective of capitalization to minimize the overall cost of a transformer, which is a function of initial cost, running cost, and maintenance cost. The operating cost is a direct function of the losses. The losses are dependent upon the operating condition of the transformer. The end-user shall carefully formulate the formula for capitalization of losses, so that the life cycle cost of the transformer is minimized. Since the transformer is energized always. Iron loss will be present 24 by 7, 365 days. Copper losses depend on the percentage loading of the transformer. Accordingly, a weightage should be assigned to the copper losses. Based on the distribution companies, tariff rate is fixed, the manufacturer will be asked to guarantee the losses, else penalty will be levied. Important features in the sizing of transformers are generally the utility transformer is sized based on the load requirements, such as residential loads, commercial loads, industrial loads, both present loads and future expected load growth. The load consumption pattern differs for different set of loads. Hence, they'll always be a variance in the pattern of load during the day and night time. This will also vary according to the seasonal requirement. Load flow study needs to be done to determine the rating of the transformer. A margin of 10 percent on the total capacity is generally considered to provide general contingency. The transformer configuration can be of these three types. One into 100 percent, two into 200 percent, three into 50 percent, one into 100 percent configuration. Under this philosophy, only a single transform of rated capacity with some 10 percent margin or the maximum demand is provided. In the event of a transformer outage, there is no Standby to feed the loads. There is a constraint for maintenance scheduling of the equipment. Two in 200 percent configuration two transformers which are capable of meeting the full load is provided. Under normal operation, each transformer shares 50 percent of the total load. When there is an outage of van transformer, the other one feeds the entire load. There is provision for ease of maintenance scheduling for one transformer at a time, three into 50 per cent configuration with only two out of three transformers in service. Each transformer has to cater 50 percent of the total load. The third transformer is a hot standby so that it takes over the load of 1 of the two transformers under conditions of outage or maintenance scheduling. Here is a quick recap of what you have learned thus far. The transformer losses consist of no load, that is iron loss, load that is copper loss, and auxiliary loss. Standard typical percentage impedance values are recommended in International standards. The manufacturers are required to guarantee the loss figures and percentage impedance subject to allowable tolerances. The end user may formulate the formula to put a cap on the losses based on the life cycle cost of the transformer or economical considerations. The life cycle cost of the transformer consists of initial capital cost, running and maintenance costs. Proper balancing is needed to optimize the overall life cycle cost. Utility transformer is sized based on the load requirements like residential, commercial and industrial loads and its load patterns. The number of transformers maybe in configuration of one into 100 percent, two into 200 percent, three into 50 percent, depending on the level of security required in maintaining power supply from the substation.
