Hello learners. I'll be discussing with you about sizing of station auxiliary power supply. This is divided into two major topics. First one deals with the AC auxiliary power sizing. Second one deals with the DC auxiliary power sizing. Coming to the AC auxiliary power, we will be discussing with you about station transformer sizing and emergency power DG set sizing. Similarly, with respect to DC auxiliary power sizing, we'll be discussing about battery and battery charger sizing. Let us have a brief introduction about station auxiliary power supply. First, you have been given exposure on substation overview and its associated equipment like transformer, circuit breaker, switchgear, protection and control systems, station auxiliary power supply system, et cetera. We have also discussed about functionalities of various equipment in detailed manner, particularly the role of AC auxiliary power supply, types of power supply and their important characteristics. Let us consider some of the key aspects; the importance of the auxiliary power and its impact of failure. As you know, the auxiliary power supply is highly essential for satisfactory operation of various equipment in substation. It provides safety to the operation of equipment and a working personnel through protection, control and safety interlocking systems. Also, this provides working environment through elimination, ventilation and air conditioning system. Having seen its importance, let us discuss about its impact or failure of auxiliary power supply or it is not made available in the substation. We can examine few practical cases. As you know, power transformer has got different mode of operations like ONAN and ONAF. When the transformer operates in ONAF, the cooler fans must run to remove the heat from the transformer. What will happen if the auxiliary power supply to the fan fails? The cooler fans will stop working and the heat dissipation cannot take place due to which the temperature rise will increase, which in turn leads to degrade the insulation. Bear it in mind, it will directly affect the main equipment. Let us examine the scenario with protection and control system. In the absence of auxiliary power supply, the protection relays will not be able to detect the fault and hence, it cannot issue trip command to the circuit breaker to clear the fault. The fault current will continue to persist. As you know, the main equipment are designed to withstand the fault current only for a short duration, which is in the order of either one second or three seconds. Then what will happen? The actual fault current will exceed the permissible limit and it will directly damage the equipment. Let us see another important scenario with respect to safety interlocking system. In the Control and Interlocking chapter, you have understood about the importance of control and interlocking system for safe operation of substation. Let us take an example of the interlocking between circuit breaker and isolator. What is the interlocking requirement to be provided between circuit breaker and isolator? Yes, the isolator shall not be operated when the breaker is feeding the load. In the absence of auxiliary supply only to the interlocking circuits the interlocks are the feeder. An isolator can be operated due to which heavy arching will take place and thereby it can cause injuries to the operating personnel and damage to the equipment. Hence, it can be viewed that safety of operation of substation is questionable. Further, you know that in the absence of auxiliary supply, the substation cannot be eliminated through lighting fixtures, which in turn leads to unsafe environment and safety hazard to working personnel due to loss of visibility. Similarly, the ventilation system will not work for battery room and the air conditioning system will not work for control and the protection panel. This will lead to impact the working environment. More important is that each substation equipment must be periodically maintained by carrying out preventive maintenance activity to maintain the operating life of the equipment. In the absence of auxiliary supply, it will not be possible to carry out preventive maintenance activity. For example, in case of transformer, the transformer oil needs to be periodically filtered and circulated. Which will not be possible, due to which the breakdown voltage of the oil will get reduced and that way it leads to major failure in transformer like flashover. The life of the equipment is directly impacted. Hence, considering all the above said facts, it is paramount to have these auxiliary power supply for satisfactory operation and maintenance of the substation. Having seen the importance and impact of auxiliary power supply in substation, now let us see other aspects, like how the auxiliary power supply is properly distributed. For distributing the power supply, we need to understand about the loads and sources for feeding these loads. We will now examine about the load aspects. Substation loads are divided into three categories. That is; vital loads, essential loads and non-essential loads. Coming to the vital loads. Vital loads are those loads in the absence of power supply could result in injury or loss of life or assets. Vital loads shall receive power from the uninterrupted power supplies. Vital loads are also termed as critical loads. For example, if the power supplied to the protection and control system fails, it will not function and would lead to dangerous situation. Now, coming to the essential loads. Essential loads are those loads in the absence of power supply could result in loss of revenue or production or impact on the quality of the product. Essential loads shall receive power supply from the relevant sources so that if the power supply from one source fails, the other source will takeover. Momentary loss, or even few minutes of loss of power supply during changeovers may be tolerated without any major adverse impact. For example, if the force applied to the transformer cooler fan fails for a short time, it will not affect as thermal time constant of the cooling medium, that is, oil, will not change abruptly. That is, the temperature will not increase suddenly. Now coming to the non-essential loads. Non-essential loads are such loads which do not fall under the category of both vital and essential loads. Non-essential loads usefully received power supply from only one source. Interruption of non-essential power supply for longer duration is generally tolerated without any adverse impact on the substation operation. For example, if the supply to the potable water pump is lost, it will not affect the performance of the substation. Having understood about the load classification, let us look into the sources feeding these loads. The sources are derived as below. All substations derive auxiliary load through station and auxiliary transformer. Suitably sized to fit all the identified auxiliary loads. The auxiliary load comprise essential loads, second station auxiliary transformer shall be provided to ensure a redundancy in case of non-availability of first transformer due to fault or planned maintenance. Both transformers shall be operated in two in two 100 percent configuration. The second power source may also be provided by a standby diesel engine-driven generator, that is DG set. However, there will be a brief interruption of power, then main power supply is lost as diesel engine is required to be started and after it ramped up to its rated speed, voltage build-up or the alternate terminal is required to be adjusted to its rated voltages prior to switching out the circuit breaker. Whole process of sensing, loss of mains power supply until CB closure can be made automatic and such a scheme is called AMF. Auto mains failure feature of the standby DG set. Auxiliary power supply can also be provided by tapping a fraction of power from the main transformer. The auxiliary AC supply is three-phase 415 volt with the acceptable voltage variation of plus or minus 10 percent. Auxiliary ports supply is distributed to LVAC DB, which is categorized based on its service duty as normal, LVAC DB, emergency LVAC DB, all essential loads connected to the emergency LVAC DB, which is redundant sources, the AC UPS or DC UPS, to feed the critical loads is derived from the station battery.
