Sources of auxiliary power. This topic explains a station auxiliary transformer, standby diesel generator set, DC UPS, AC UPS, typical LVAC SLD and its elements. We have identified the types of loads and the characteristics of a power sources which can be chosen to feed these loads. Let us look at the power sources needed to feed various types of loads. On substations, derive auxiliary power through a step-down station auxiliary transformer, or SAT. Suitably sized to feed all identified auxiliary loads, where auxiliary loads comprise essentially loads. Second station auxiliary transformer shall be provided to ensure redundancy in case of non-availability of a first transformer due to fault or planned maintenance. Good transformers shall be operated in 2 into 100 percent configuration. Auxiliary power supply for the substitution can also be provided by tapping a fraction of power from the power or main transformer. Typically, the supply is 415 volts, three phase. The acceptable variation in AC auxiliary voltage is in the range of a plus or minus 10 percentage. The auxiliary power supply is distributed through LVAC distribution boards. LVAC DBs may also be categorized based on its service duty as normal LVAC DB, and emergency LVAC DB. All essential loads are connected to emergency LVAC DB, which has a redundant power supply sources. Sizing of a station auxiliary transformer. Tabulate the connected kVA of all substation AC loads. Apply demand factor to each. Consider low diversity factor where necessary. Demand factor is equal to maximum demand divided by total connected load. Example, 5.5 kilowatts rated motor can handle a mechanical load equivalent to 5.5 kW on full-scale load. However, actual mechanical load connected maybe less than 5.5 kW. The ratio of actual load to full load is termed as demand factor. Demand factor will be applicable for each feeder. Estimate demand in kVA. Select the standard size of transformer which is closer to but above the estimated maximum demand. Lastly on transformer sizing, examine the substation growth rate. [inaudible]. In any case asserting appropriate design margin shall be built within the rating of the transformer sizing selected. Standard sizes of station auxiliary transformer available within the manufacturing range that are normally utilized for SAT include 250 kVA, 500 kVA, 800 kVA, 1000 kVA, 1250kVA, 1600kVA and 2000 kVA. Percentage impedance values as recommended in relevant IEC or IS standard, may be chosen in accordance with the kVA rating of the transformer. Sources of auxiliary power standby, DG set. The second power source may also be provided by a standby diesel engine driven generator, or DG set. However, there will be a brief interruption of power when main power supply is lost as diesel engine is required to be started and after root is ramped up to its rated speed, voltage built up at the alternator terminals is required to be adjusted to its rated voltage prior to switching on its circuit breaker. The whole process of sensing loss of main power supply until CB closure can be made automatic, and such a scheme is called AMF or Auto Mains Failure feature of the standby DG set. Several substation installations are provided with 2 into 100 percentage SAT in addition to stand by DG set to ensure high degree of availability of auxiliary power supply. Usually, such standby DGs are provided with black start facility that is capable of starting and providing standby power without depending on external power sources. Power supply scheme with standby diesel generator for essential loads. The emergency or essential loads are connected to the DG sets through the automatic transfer switch if normal source is unavailable. One-line diagram representation of automatic transfer switch appears on screen. Standby power generators versus emergency power generators. Standby generator and emergency generator are one and the same. If you use it to supply standby power, then it is a standby generator. If you use it to supply emergency power, then it is an emergency generator. A standby generator is a backup electrical system that can be designed to operate automatically on sensing the loss of main or normal power supply. A standby or emergency generator may be sized to provide backup for all essential and non-essential or just essential auxiliary loads. It is meant to run when normal power supply is not available for any reason. Standby generator is required to run for a short duration until the normal supply is restored. Normal power supply may not be available for a variety of reasons. For example, shutdown of distribution company, failure or maintenance of auxiliary transformer, failure of incoming power cables, et cetera. Stand-by generators are used in residential complexes, commercial houses, substations, and industrial areas. Typically, the standby power generators are required to run for a duration of one hour to eight hours, depending on the cause of non-availability of normal power supply. However, they are not designed to run on continuous basis for a long duration. Typically, they are designed for continuous run of eight hours. The fuel storage capacity, likely in such installations would also be limited for about eight hours of consumption only. It does not mean that a DG set designed for eight hours cannot be run for a duration longer than eight hours. If all auxiliaries of DG set and fuel storage permit for longer duration, the DG can be run but its load handling capacity will get reduced. Standby generators are necessary when continuous availability of power is required. A simple storm can cutoff power sources, but the generator can provide a solution for intervening duration until permanent power is restored. Emergency power supply scheme with standby diesel generator. When normal or main power supply is available, DG sets will be in off condition. In case normal source fails, then diesel engine will be started and its alternator will supply to the emergency loads using automatic transfer switch. In the switch gear, provision shall be made to connect an emergency diesel generator when required. The diesel generator incomer circuit breaker should have necessary interlock arrangement with main incomers and bus section CB. AMF panel or Auto Mains Failure is provided along with the DG sets to start the engine automatically on failure of the mains power supply. What happens if a station auxiliary transformer fails? An emergency power system is an independent source of electrical power that supports important electrical systems on loss of normal power supply. Diesel generators are by far the most preferable source of power for emergency power system. In the normal operation conditions, the AC power is normally supplied from the main source, that is, from the station auxiliary transformers. In case the main supply is out of service for any reason, the emergency standby diesel generator shall supply the critical or essential loads. Continued in next part.
