Now having understood the overall power distribution philosophy, loads and their operational requirements, we will move into methodology for sizing of station auxiliary transformer. Before that, we need to decide which standard is to be used. There are two standards. One is IS:2026, Indian standard being used for domestic projects, and another is IEC 60076 standard for international projects. However, the approach wise, the Indian standard IS:2026, adopts the international standards, IEC 60076. Further, to have the idea about the substation configuration, the SLD shall be referred. Now, let us see how to carry out the assessment in detailed manner. The first step is preparation of load lists. Step 2 determines the maximum demand for each load. Step 3 is for computation of the maximum demand for the substation and the selection of transformer based on steady-state condition. Fourth step is to identify the largest induction motor in the group and it's type of starting. In Step 5, determine the base load and the transient load on the transformer. In Step 6, select the transformer rating satisfying both steady-state and transient state. In Step 7, declare the final rating of the transformer. Now we will discuss about the preparation of load list through Step 1. The typical proposed load list is shown in the table. Therein, you can observe that both non-essential loads and essential loads are listed down. Now against each type of load indicator, the number of loads and its rating in kilowatt will be provided. These loads are to be obtained from the manufacturer. Just add all these loads and you will get the total connected load. Once connected load is computed, now, we will determine the maximum demand of each load. First, we need to understand what is maximum demand? Let us take an example of motor for a pump. In the industry, they keep on frame ratings as standardized as far as possible. To put it in an understandable manner, the rated motor capacity is selected higher than the actual mechanical power output of the pump due to standardization of frames. Therefore, during actual operating conditions, the power consumption by the motor will be lower than its motor name plate rating. You can note that the ratio of the actual power consumed by the motor to its rated nameplate power rating is called demand factor. Similarly, we can consider another example of our home. A typical home consist of, hall, bedrooms, kitchen, toilet, electrical appliances like, water geyser, light fittings, fans, induction stove, washing machine, refrigerator, TV, iron box, etc. You can observe that all these loads will not operate simultaneously at any point in time due to the nature of demand. Therefore, the maximum demand of the power will be lower than the total connected load. Same philosophy can be applied for substation as well. Let us see its key aspects. This table indicates the demand factor. For power transformer cooling fan, consider the maximum demand as unity, as it needs to be operated at anytime. The same logic is applied for transformer OLTC also. Now coming to the circuit breaker spring charging motor. All the breakers spring charging motor will not be operated simultaneously, as switching can be done one after the other. Therefore, we can consider around 30 percent of the spring charging motors will be on at a time as worst-case, that is demand factor of 0.3, or at least two numbers to be considered, whichever is higher. Similarly, for space heater and the cubicle illumination distribution board, the demand factor of one can be considered. If the substation consists of two battery chargers, say two into 100 percent configuration, then we can assume one battery is under boost charging mode and therefore, the demand factor can be considered as 0.5. The demand factor for normal lighting, emergency lighting can be considered as 0.9 and the unity respectively, as during night hours most of the light fittings will be on. While the air condition and the ventilation loads for a substation can be considered as unity, as they need to be operated continuously. The demand factor for water pump can be considered as 0.9. The transformer oil filtration can be considered as unity, as in a two into 100 percent substation configuration. One transformer can be taken up for maintenance activity at anytime. However, welding socket is a miscellaneous load and the demand can be considered as 0.25 as it is seldom used. Let us talk about the computation of maximum demand for a substation. Now once Step 1 and Step 2 are completed, just add individual loads demand. Additionally, consider the design margin of 10 percent to take care of any variation in load particulars. The result, what you have obtained is sum of all individual load demand. We need to consider diversity factor to calculate the maximum demand for complete substation as a group load. You can also note that diversity factor is expressed as a ratio of sum of all individual demand to the maximum demand of the group. This is generally considered as greater than unity for a substation therefore, we can consider a factor of 1.1. At the end of this exercise, you will find that the maximum demand of the substation under steady-state condition. Now how to select the transformer capacity? Is there any method available? Yes. Just look for the standard rating of transformer available in the industry and select the nearest one, which is higher than the able computer maximum demand.
