Generators Sizing Calculations – Part Nine

 Subject of Previous Article Article Glossary of Generators – Part One Glossary of Generators – Part Two First: Reasons for having on-site generators Second: Applicable performance standards for generator sets Third: Selection Factors Used For Generators Sizing Calculations Generator Power Ratings Application type Third: Selection Factors Used For Generators Sizing Calculations 3- Location Considerations, 4- Fuel Selection Considerations, 5- Site Considerations, Third: Selection Factors Used For Generators Sizing Calculations 6- Environmental Considerations, 7- System Voltage and Phase, Third: Selection Factors Used For Generators Sizing Calculations 8- Acceptable percent of voltage & frequency dip, 9- Acceptable duration of the voltage & frequency dip, Third: Selection Factors Used For Generators Sizing Calculations 10- Percent And Type Of Loads To Be Connected – Part One 10- Percent And Type Of Loads To Be Connected – Part Two

Today, we will continue explaining other Selection factors used for Generators Sizing Calculations.

 Third: Selection Factors Used For Generators Sizing Calculations

 Here we will describe preliminary factors for selecting a generator for certain project, which will be as follows:   Generator Power Ratings, Application type, Location Considerations, Fuel Selection Considerations, Site Considerations, Environmental Considerations, System Voltage and Phase, Acceptable percent of voltage & frequency dip, Acceptable duration of the voltage & frequency dip, Percent and type of loads to be connected, Load step sequencing, Future needs.

 Loads Control Methods       There are many decisions, related to the method of loads control, must be taken by the designer which will affect the generator sizing process. The designer may select on or more loads control method from the following:   The Load prioritization order, The load acceptance order, The load shedding order, The unit starting order, The unit shut down order, and/or redundancy.   Using the one or more order from the above loads control methods will result in smaller size generators than a full-load system, and will have a significant savings in purchase price of the generators.

 4- The Unit Starting Order   Site personnel must determine the order in which generators will be started. If #1 generator is always started first, it will accumulate hours and maintenance expenses at a high rate compared to the remaining units. “Slobbering” of the generators may be another problem encountered. The last unit started may be lightly loaded and said to “slobber”. Slobber is a result of lubricating oil being drawn into the cylinder under low load conditions. This is not conducive to long life and good performance. A site may require a smaller supply of electricity on the weekend, when the loads are not as large. Generators of different sizes can be installed where the larger one(s) supply the week and the smaller the weekends.

 5- The Unit Shut Down Order, and/or Redundancy   Unit shutdown order is determined by analyzing which loads can be spared at certain times while other loads are still connected. Again, maintenance implications need to be taken into consideration if it is determined that the same generator will be shutdown first each time. Redundancy refers to the method of purchasing one or more generators than needed to operate at full load. This is especially important to loads where a major percentage of it is critical. With additional generators, down-time can be kept to a minimum because an additional generator is always free to be connected.       Fig.1: Redundancy order   Figure.1 shows generator #5 as a swing engine; this engine can be used to backup either critical load group if one of the engines requires service. Major maintenance can be performed on a generator while the additional one is connected. If two or more additional generators are present then maintenance can be done on two or more or have a backup for the system when one is being serviced.

 12- Future Needs

The customers’ future needs are to be taken into account when sizing the generator set. If the customer anticipates growth in their application due to increased volume or expanded needs, one of two design solutions can be applied:

1. Oversizing the generators or
2. Leaving room for another generator to be installed at a later date.

However, the projected load growth for any application should never be less than 10%.

Table-1 shows typical load growth over a period of 10 years for various applications.

 Load Growth Over 10 Years Application Typical Load Growth Bank 30 – 50% Medical Center 30 – 40% Church 10 – 30% School 50 – 80% Hospital 40 – 80% Warehouse 10 – 30%

Table-1: Load Growth over 10 Years for various applications

In the next article, we will start explaining the Applicable Procedures for Generators Sizing Calculations. So, please keep following.

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