Having understood about type of loads and sources, we are now going to see how it is practically distributed. First, we'll start with the AC supply distribution. Right now, you are looking into the figure showing the singular diagram for power distribution. The main source fed by utility is feeding a panel. This is called the main source board. The main source also feeds all the non-essential loads. It also feeds essentially loads through a change of switch. On the right-hand side, emergency source is shown, which feeds the supply to the essential loads through the change of switch. Now, you can closely observe that change of switch receives supply from main source as well as the emergency source. But you can keep it in mind that change of switch can get connected to only one source at a time. For easy understanding, let us name position 1 as main source and position 2 as emergency source. Now look at this figure. When the main source is healthy, it will feed both essential and non-essential loads. You can keenly notice that change of switch is in position number 1. Under this condition, the emergency power dataset will be in off condition. In case the main source fails, both nonessential and essentially loads will stop working. Now let us know about AMF feature. AMF feature is called Auto Means Failure. It will continuously monitor the main source healthiness. The moment the main source fails, it will trip the main source circuit breaker at the start the DG SET. Once the alternator-rated output voltage is ramped up, the output voltage is made available at position number 2. The switch will be automatically changed to position number 2 and now the essential loads will receive supply from emergency source. Under this situation, you can clearly observe that non-essential loads will remain in off condition. Having understood the overall schematic for AC power distribution, now we are going to move to the next step for having a detailed discussion on how your typical load in substation appears and how each load is categorized. Now we'll look at this table which indicates various loads in substation. Let us discuss this one by one. In point number 1, power transformer, cooler fan for ONAF configuration is shown. This indicates that if transformer operates under ONAF mode, the cooler fans must run. It comes under the essential load category. Point number 2 shows the Power Transformer OLTC. This OLTC must be operated anytime as per power system requirement. Therefore, it comes under essential load category. In point number 3, circuit breaker spring charging motor is shown. The spring charging motor activities important, one for closing the circuit breaker. This is generally provided as a universal motor so that it can accept both AC and DC. If it needs to be energized from AC supply, then it is considered under essential load category. Circuit breaker. Closing and tripping coil are shown in point number 4. This is a very critical one as at any moment, the tripping operation is needed by production. That is why it is considered under critical load. Point number 5, isolator-motor operating mechanism is indicated. The isolator needs to be operated at any time depending upon the requirement. It is considered under essential loads. From point number 6-8, control and protection panel RTCC panel, RTC system loads are shown. These are all critical loads with respect to monitoring, protection, and control of substation. In point 9 and 11, UPS AC supply load and station battery charger load are shown. This is considered under essential loads so that maximum availability of charge is ensured. Point number 10 particularly indicates the space heater loads. As you know, the space heaters are provided in the LV cubicle of main switch gear, control and protection panel, AC and DC switch gear, transformer marshaling box. These are required to prevent the condensation inside the cubicle when the external weather changes like humidity. Since changes in weather will influence all equipment, heater with the humidity control is to be switched on when it is demanded. If the condensation is not prevented, it will lead to major complication like short-circuit, flashover, etc. Therefore, it is considered under essential loads. While point number 12-14 are dealing with lighting and small power loads. The station lighting is divided into three parts, namely, normal lighting, AC emergency lighting, and DC emergency lighting. Only AC emergency lighting and DC emergency lighting will be connected to AC emergency power and station battery respectively. Ventilation load and the air conditioning loads are essential for satisfactory operation of equipment and maintaining safe environment inside the battery room, hence these loads considered under essential loads. Potable water pump load is a non-essential load because water can be stored in tank, loss of power will not affect the water availability. Considering transformer oil filtration load. If the substation consists of two transformers and one may be taken under shutdown, in such a condition, transformer oil filtration activity may be needed. If there is a power interruption, it will not affect the function of substation. Therefore, it is considered under non-essential load. Now consider the welding circuit load. This is for miscellaneous works in the substation during maintenance. The actual load may vary depending upon the requirement, hence, any interruption in supply will not affect the function of substation, and so it is considered under non-essential loads. Also, you can observe from the table that non-essential loads are fed by main source. Essential loads are fed by emergency source and vital loads are fed by station battery. In this table, you can find that the essential loads have facility to get supply from both normal power and emergency power.
