Hello, learners. We have discussed about first three steps, so far covering selection of transformer based on steady-state condition. Now, we will see a case study through which we will calculate and select the transformer based on steady-state condition. Once we complete this, we will then move into other steps, that is Step 4-Step 7, dealing with a transient aspects. Now, let us have a look at the case study. In this case study, we're going to see gear 33/11 kV substation. This is an indoor substation consisting of 33 kV and 11 kV indoor air insulated switch gears, power transformer with RTCC Panel, AC and DC auxiliary system, lighting system, ventilation and air conditioning system, potable water pump for the substation, transformer oil filter machine, and welding socket for maintenance. The substations SLD is shown in this figure. This indicates the number of feeders , transformer configuration, etc. In addition to that, AC auxiliary single-end diagram and space heater SLD are also shown. Now let us review about the loads. You can observe that there are seven numbers of 33 kV feeders and 11 numbers of 11kV feeders. The circuit breaker in switchgear is provided with spring charging motor with a load rating of 300W. Each indoor feeder is provided with three numbers of 60W space heater, two numbers of 20W cubicle illumination lamp, and one number of 5A socket outlet for maintenance purpose. You can note that the transformer is of 12 W/16 MVA capacity with ONAN and ONAF configuration. Each transformer is prevented with 10 numbers of cooler fans, and each cooler fan load is 0.5 kW. The transformer is also provided with On Load Tap Changer, and it's load rating is 1kW. Transformer marshaling box, and OLTC mechanism box, each provided with one space heater, one cubicle illumination lamp, and 5A circuit. Let us assume AC panel consists of 4 verticals, and DC parallel consists of 2 verticals. Each vertical consists of one number of space heater with a 60W rating, and the cubicle illumination lamp with a 20W rating and 5A circuit outlet for its maintenance. Here, the battery charger is provided with 200 percent configuration. The chargers is of float cum boost charge, and when it works on boost mode, it will consume around 30kW. It is also provided with space heater 60W, cubicle illumination lamp, a and socket outlet. Remember, the substation needs lighting, and it is provided with three categories, namely outdoor lighting with a low rating of 6kW, indoor lighting of 4kW, AC emergency lighting of 5kW load rating. The substation air conditioning system load rating is 15kW, and in which 5kW is pertaining to fan load. This fan motor would be started through DOL mode. The water requirement is taken care by water pump of rating 10kW, which would be started based on DOL starter method. The substation transformer needs periodical maintenance, which is taken care by transformer oil filter machine. The total power consumption is 50kW, and in which 45kW of heater load and the remaining 5kW of inlet and outlet pumps. You can note that the various loads power factor can be considered as 0.80 lagging. We know our objective is to find out the auxiliary transfer capacity, and let us do it systematically by carrying out a few important steps. The first step is to compile the load list, and in the second step, demand factor needs to be applied for each load. In the third step, compute the maximum demand for a substation and select the terms of our capacity based on steady-state condition. Let us start with preparation of load list. Now let us focus on the table. You will have to list down the loads and classify it as essential and non-essential loads as per our earlier discussion. Then provide the quantity against each item as given in the case study, and enter the power consumption of load as against respective heater. Regarding the space heater and cubicle illumination load, take a look at this table and enter the loads as per case study. All the space heater loads can be combinely analyzed here. The next step talks about assessment of demand factor. As we have already discussed, take a look at this table that talks about demand factor for each load as explained during our discussion. Now look at this table, which talks about demand factor for space heater and cubicle illumination load, and enter the load as part of case study. As you can see, the space heater can be energized automatically at any time due to weather condition. Therefore, the demand factor is considered as unity. Since the cubicle illumination and a socket outlet or used based on occasional requirements, therefore, the demand factor is 0.1. The third step is the determination of maximum demand for substation. Now, looking at this table, add the individual demand and it is around 178.24 kVA. Consider your design merging, say 10 percent on the calculated maximum demand. Now the calculated maximum demand with the design margin is 196 kVA. Recall that all the loads will load to consume its maximum demand simultaneously. Therefore, a play an overall diversity factor of 1.1. Considering the diversity factor, the computed maximum demand for a substation is 178.24 kVA. Now, how will we select the right transformer? We'll look for the nearest standard rating of transformer, which works out to be 200 kVA. Therefore, the terms overrating selected based on steady-state condition is 20 kVA.
