### Generators Sizing Calculations – Part Thirteen

Today, we will continue explaining in detail the applicable procedures for Generators Sizing Calculations for Existing and New Installations.

 Fourth: Applicable Procedures For Generators Sizing Calculations

 Step#7: Adjust the Generator Rating According To Transient Voltage Dip

 Generator Voltage Dip Calculation We have two methods to calculate/determine the Generator Voltage Dip as follows:   By using equations, By using generator manufacturers’ performance data.   1- By using equations   The transient reactance (X'd) is used to describe generator performance during transient events such as large load applications. reactances provide an agreed standard and consistent way to compare one generator to another, regardless of the manufacturer. The magnitude of the voltage dip at a generator's terminals, following load switching, is a direct function of the sub-transient and transient reactances of the machine. It can be calculated from the following equation:     Or by this equivalent equation:         Notes:   The equation above can be used to show that for a given machine rating (same kVA, same voltage, same frequency), the lower the X'd, the lower the amount of voltage dip for an applied load. This is based on physics and holds true for all generator set manufacturers. Many manufacturers’ software programs calculate the voltage dip easily.

 The Maximum Allowable Voltage Dip   The maximum allowable voltage dip is 30%.   Notes:   Choosing lower allowable voltage dip requires a larger generator set.  As you reduce the maximum allowable frequency dip, you increase the size of the generator set needed. To reduce the dip, reduced voltage starters can be used. Engine transient response will add to this dip, but the engine response is usually much slower than the generator. This assumes a very low starting power factor.   The relation between the generator size and voltage dip% is shown in Fig.1:

 If   calculated voltage dip ˃ Maximum Allowable Voltage Dip   You must increase the value of the generator effective load used in the calculation of the voltage dip. Then   Generator KW adjusted  = K x Generator KW effective   where factor K ˃1 by a percentage sufficient to minimize the voltage dip below the Maximum Allowable value   Example:   2000 kW @ 0.8 PF with X’D = 18.2% starts a 750 HP code F motor – What is the approximate voltage dip?   Solution:   Rated kVA = 2000 kW / 0.8 PF = 2500 KVA Starting kVA = 750 HP × 5.6 kVA / HP = 4200 KVA   % dip = 100 x (0.182) / (0.182 + (2500 / 4200)) = 23.4%   Calculated voltage dip (23.4%) ˂ Maximum Allowable Voltage Dip (30%)   Then the size of the selected generator is suitable for the application.

 Step#8: Adjust the Generator Rating According To Site Conditions

 1- Ambient Temperature   Where the temperature of the ventilating air to the generator exceeds 40°C (104°F), derating of the generator may be necessary.   The de-rating percentage will differ from a manufacturer to another. However, an average de-rating percentage can be as follows:   An engine’s power rating assumes a nominal altitude of less than 1000 feet, ambient temperature less than 104°F, and humidity less than 75%. Manufacturers detail the percentage reduction in available power for ambient conditions that exceed those assumed for the nominal rating.     2- Altitude   The higher the altitude, the lower the air density.  The altitude/temperature derating chart found in TMI can be used for proper derating information for generators and each specific engine. Figure .2 shows a sample engine TMI de-rate chart. (TMI: Technical Marketing Information).     Fig.2: TMI Sample     The following Altitude / Ambient De-rate Chart can be used to determine the de-rating factor for Altitude / Ambient approximately:       Fig.3: Altitude / Ambient De-rate Chart

 Step#9: Adjust the Generator Rating According To Fuel Type

 The selection of natural gas, diesel, or LPG fuel will affect generator set availability and sizing. Often, generator sets running on gas or LP must be oversized due to de-rating.   The standard de-rating formula states that for every 1000 ft above sea-level, the following de-rating must be applied:   Gasoline, diesel, or liquid propane generators usually should be de-rated by 2–3% of its standard output. Natural gas generators, the de-rating factor is typically closer to 5% of its standard output.

 Step#10: Adjust the Generator Rating According To 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 Factor (Kf) 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

 Step#11: Adjust the Generator Rating According To Power Factor

Lower PFs require larger alternators or generator sets to properly serve the load. Usually Three-phase generator sets are rated for 0.8 PF loads and single-phase generator sets for 1.0 PF loads.

For example, the following data in Table-2 for the Siemens AG type 1FC6 generators typify the de-rating factors for power factor that need to be applied.

 Lagging power factor De-Rating factor 0.8 to 1.0 1.0 0.7 0.97 0.6 0.91 0.5 0.89 0.4 0.87 0.0 0.84

Table-2: Typical derating factors

The following generator’s kW and kVA vs % lagging PF Chart can be used to determine the de-rating factor for lagging PF approximately:

Fig.4: generator kW and kVA vs % lagging PF Chart

 Step#12: Calculate the Adjusted Generator Rating

 From step#5, article, after application of demand and diversity factors to each category’s effective load, we calculate the Effective Load to Generator in step#6 in the same article as follows:   Effective Load to Generator KW G-effective = Σ KW effective of all Load categories   Based on this KW G-effective, we can make a preliminary selection of the generator. Then, we must adjust the selected generator rating to comply with Transient Voltage Dip rule explained in step#7 to get at the end the KW G-accepted   After that, Site Conditions de-rating factors, Fuel Type de-rating factor, Future Needs factor and Power Factor de-rating have to be applied as explained in steps# 8, 9, 10 & 11 respectively.   Finally, we can say that the Adjusted Generator Rating KW G-adjusted can be calculated as follows:   KW G-adjusted = KW G-accepted x K site conditions x K fuel type x K future needs x K pf

 Step#13: Select Generator Rating from Standard Sizes/Manufacturers Catalogs

 We can use the different Manufacturers Catalogs to assign the proper generator standard rating that will be next higher to KW G-adjusted calculated above.       You can download the most famous Manufacturers Catalogs from the following links:     After selecting the standard rating from above step, the following rules must be taken into consideration:   In a three-phase system, the Phase voltage unbalance must be ≤ 2% Single phase loads must be ≤ 10% of the generator set three-phase kVA capacity   Otherwise, you will need to move to the next higher size till you comply with the above two rules.

 Step#14: Assign Required Number Of Steps/Starting Sequence

In the next article, we will explain the rules of thumbs and solved examples for Generators Sizing Calculations. So, please keep following.

The previous and related articles are listed in the below table: