Generators Sizing Calculations – Part Four


Subject of Previous 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
  1. Generator Power Ratings
  2. Application type

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

Third: Selection Factors Used For Generators Sizing Calculations

as we indicated before that  preliminary factors for selecting a generator for certain project, which will be as follows:
  1. Generator Power Ratings,
  2. Application type,
  3. Location Considerations,
  4. Fuel Selection Considerations,
  5. Site Considerations,
  6. Environmental Considerations,
  7. System Voltage and Phase,
  8. Acceptable percent of voltage & frequency dip,
  9. Acceptable duration of the voltage & frequency dip,
  10. Percent and type of loads to be connected,
  11. Load step sequencing,
  12. Future needs.

3- Location Considerations


One of the first design decisions will be to determine whether the location of the generator set will be inside a building or outside in a shelter or housing. Making a good generator placement decision can prevent problems and ensure operation of the unit(s) when they are needed. For example, determining whether an application would be best served by a single, large generator or a paralleled solution with multiple, smaller generators can impact the location decision significantly.
For both indoor and outdoor locations, you must take care of the following issues:
  • Generator set mounting,
  • Location of distribution switchboard and transfer switches,
  • Branch circuits for coolant heaters, battery charger, etc.,
  • Security from flooding, fire, icing, and vandalism,
  • Containment of accidentally spilled or leaked fuel and coolant,
  • Possible simultaneous damage to normal and emergency services,
  • Service access for general maintenance and inspections,
  • Access and working space for major work such as overhauls or component removal/ replacement,
  • Access for load bank testing when required for maintenance, proper exercise, or code.

1- Outdoor Location Considerations
For outdoor locations, you must take care of the following issues: 
A- Sound Considerations:
  • Increasing distance between the generator set and the noise sensitive area will decrease the perceived noise.
  • Acoustic housings are often available and may be required to meet customer expectations or local noise ordinances.
Sound Attenuated Enclosure    
B- Weather protective housing
  • Weather protective housing may be required, as their name suggests, for protection from weather but also may provide a certain level of security as well as aesthetic containment of the generator set.
C- Starting and accepting load
  • Starting and accepting load, and doing so within specific time constraints, in cold ambient temperatures may be an issue. Emergency systems as defined by codes require the ambient temperature around the generator to be maintained at minimum levels. Examples are NFPA110 which requires the minimum ambient temperature around the generator set to be 40°F (4°C), and CSA 282 which requires this minimum temperature to be 10°C (50°F). Maintaining these minimum temperature requirements in a “skin–tight” or other similar housing may be difficult or impossible. An insulated and perhaps heated housing may be required. A housing that is designed strictly for acoustic treatment will contain insulation material but may not provide sufficient heat containment. Single unit “drop over” housings or walk in enclosures are usually available with insulation, motorized or gravity louvers, and heaters if necessary.
D- Several auxiliary heating devices
  • Several auxiliary heating devices may be required for starting or improved load acceptance, even if the application is not an emergency system. Heaters for coolant, batteries, even oil may be necessary.
E- Fuel conditioning and heating
  • At cold ambient temperatures diesel fuel will become cloudy, clog filters and pumps, or not flow sufficiently. Blended fuels are often used to address this issue however; fuel heating may be required for reliable operation.
  • The salt air in coastal regions may cause corrosion issues on outdoor–installed steel generator enclosures, skid bases, and fuel tanks. The use of an optional aluminum generator enclosure and skirt, whenever offered by CPG, is considered to be proper installation practice due to the additional corrosion resistance and is thus required for outdoor applications in coastal regions, defined as locations 60 miles and closer to bodies of saltwater.
F- Service access
  • Service access for major repairs, component replacement (such as radiator or alternator), or overhaul, should be considered in the design of housings and placement of generator sets near other equipment or structures. If major work is required due to high hours of operation or major component damage/failure, access allowances will be critical. These allowances include access covers, removable housing walls, adequate spaces to nearby structures, and access of required support equipment.
G- Security fences and sight barriers
H- Property line distances
I- Engine exhaust must be directed away from vents and building openings.
J- Grounding – Electrodes or grounding rings may be required for separately–derived system and/or equipment grounding.
K- Lightning protection.

2- Indoor Location Considerations
Indoor Generator Installation
Indoor locations generally require a dedicated room with fire resistive construction.
Providing the required airflow to an interior room may be difficult. Fire dampers in ductwork to interior rooms are generally not permitted. Ideally the room will have two exterior walls opposite each other so that intake air flows over the generator set and is discharged out the opposite wall on the radiator end of the unit. For Indoor locations, you must take care of the following issues: 
A- Dedicated generator room
  • For emergency power systems, certain codes may require that the generator room be dedicated for that purpose only. Also consider the effect that large ventilating airflow would have on other equipment in the same room, such as building heating equipment.
B- Fire rating of room construction
  • Codes typically specify a 1 or 2–hour minimum fire resistance rating. Consult local authorities for applicable requirements.
C- Working space
  • Working space around electrical equipment is usually specified by code. In practice, there should be at least three feet (1 M) of clearance around each generator set. The alternator should be replaceable without removing the entire set or any accessories. Also, access for major work (such as overhaul or component replacement such as a radiator) should be allowed for in the installation design.
D- Type of cooling system
  • A factory–mounted radiator is recommended, however, the radiator fan can create a significant negative pressure in the room. Access doors should therefore swing into the room – or be louvered –– so that they can be opened when the set is running.
E- Ventilation involves large volumes of air
  • An optimum room design draws intake air directly from outdoors and discharges the air directly outdoors through the opposite wall. Room ventilation fans will be required for optional generator set cooling configurations that involve heat exchanger or remote radiators.
F- Engine exhaust
  • The engine exhaust outlet should be as high as practical on the prevailing down–wind side of the building and directed away from building intake vents and openings.
  • Fuel storage and piping Local codes may specify fuel storage methods inside buildings and restrict fuel storage amounts. Access will be required for refilling storage tanks.
G- Load Bank Connection
  • It is recommended that provisions be included in the electrical distribution system for connection of a temporary generator load bank.
H- Service access
  • Location within a building must allow for access both for initial product delivery and installation, and later for servicing and maintenance. The logical preferred location for a generator set in a building based on this is on the ground floor, near a parking lot or access driveway, or in an open parking ramp. Understanding that this is the premium building space, if forced to an alternative location, keep in mind that heavy equipment may be needed for placement or major service of the unit. Also, deliveries of fuel, coolant, oil, etc. are needed at various intervals. A fuel system will most likely be designed with supply tanks, pumps, lines, day tanks, etc. but lubricating oil and coolant changes can be difficult if the materials have to be hand carried in barrels or buckets.

3- Rooftop installations
Generator Rooftop installation
Rooftop installations, While common, require further planning and structural design consideration. Vibration and fuel storage/delivery may be problematic with rooftop installations. Table-1 lists the benefits and disadvantages for Rooftop installations
No air flow problems
Roof structure may have to be strengthened
No expensive ductwork
Large crane required
No lengthy exhaust runs
Possible road closure
No problems with exhaust fume emissions
Planning permission required
No noise
Longer cable runs
problem No space limitation problems
Limited fuel storage
Table-1: benefits and disadvantages for Rooftop installations


4- Fuel Selection Considerations

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. For example, The standard de-rating formula states that for every 1000 ft above sea-level, a gasoline, diesel, or liquid propane generator usually should be de-rated by 2–3% of its standard output. In case of generators using natural gas, the de-rating factor is typically closer to 5%. 

you must take care of the following issues:
1- Diesel Fuel
  • Diesel fuel is recommended for emergency and standby applications. ASTM D975
  • No. 2–D Grade diesel fuel is recommended for good starting performance and maximum engine life.
  • On–site fuel storage must be provided, however the tank should not be too large.
  • Diesel fuel lasts up to two years in storage, so the supply tank should be sized to allow for fuel turnover based on scheduled exercise and testing in that time period. A microbicide may need to be added if fuel turnover is low, or if high–moisture conditions promote the growth of fuel microbes. Microbes in the fuel can clog fuel filters and disable or damage the engine.
  • Cold climates –– Premium No. 1–D Grade fuel should be used when ambient temperatures are below freezing. Fuel heating may be required to prevent fuel filters from clogging when temperatures fall below the cloud point of the fuel –– approximately 20° F (–6° C) for No. 2–D and –15° F (–26° C) for No. 1–D.
2- Biodiesel Fuel
  • Biodiesel fuels are derived from a broad variety of renewable sources such as vegetable oils, animal fats, and cooking oils. Collectively, these fuels are known as Fatty Acid Methyl Esters (FAME). When used in diesel engines, typically smoke, power, and fuel economy are all reduced. While smoke is reduced, the effect on other emissions varies, with some pollutants being reduced while others are increased. Biodiesel fuel is a substitute fuel, meaning the performance and emissions of the engine cannot be warranted when operated on this fuel.
  • A blend of up to 5% volume concentration biodiesel fuel with quality diesel fuel should not cause serious problems. Above 5% concentration serious operational problems should be expected.
3- Natural Gas
  • No on–site fuel storage is required for most sites.
  • Natural gas may be an economical fuel choice where available, at required flow rates and pressure.
  • An on–site backup LPG fuel supply may be required for emergency power supply systems.
  • Field natural gas can be used with certain generator sets. However, fuel analysis and consultation with the engine manufacturer are required to determine potential power de-rating and whether fuel composition will lead to engine damage due to poor combustion, detonation, or corrosion.
  • Detonation and engine damage may result when some utilities occasionally add butane to maintain line pressure. Natural gas engines require clean, dry, pipeline– quality gas to generate rated power and ensure optimal engine life.
  • Frequency stability of spark–ignited engine generator sets may not be as good as diesel engine generator sets. Good frequency stability is important when supplying
  • UPS loads.
  • Cold climates –– In ambient temperatures below 20° F (–7° C) spark–ignited engines generally start easier and accept load sooner than diesel engines.
4- LPG (Liquefied Petroleum Gas)
  • The local availability of LPG should be investigated and confirmed prior to selecting an LPG–powered generator set.
  • On–site fuel storage must be provided. LPG can be stored indefinitely.
  • Frequency stability of spark–ignited engine generator sets may not be as good as diesel engine generator sets. This is an important consideration when supplying UPS loads.
  • Cold climates –– Either the LPG storage tank must be sized to provide the required rate of vaporization at the lowest ambient temperature expected, or liquid withdrawal with a vaporizing heater must be provided.
5- Gasoline
  • Gasoline is not a suitable fuel for stationary standby generator sets due to volatility and shelf life of gasoline fuel.
6- Substitute Fuels
In general, diesel engines may be run on substitute fuels with acceptable lubricity during periods when the supply of No. 2–D diesel fuel is temporarily limited. Use of substitute fuels may affect warranty coverage, engine performance, and emissions. The following substitute fuels are generally within prescribed limits:
  • 1–D and 3–D diesel fuel
  • Grade No. 2 fuel oil (heating fuel)
  • Grade Jet A and Jet A–1 aviation turbine fuel (commercial jet fuel)
  • Grade No. 1 GT and No. 2 GT non–aviation gas turbine fuel
  • Grade No. 1–K and No. 2–K kerosene

5- Site Considerations

An engine requires a certain quantity of air for combustion to achieve its rated power. Altitude, temperature and humidity will all affect the air density.
The environment or site conditions also impact an engine or generator rating, thus impacting the rating of the entire generator. These Conditions include:
  1. Ambient Temperature,
  2. Altitude,
  3. Corrosive Atmospheres,
  4. Humidity,
  5. Dust.
An engine requires a certain quantity of air for efficient combustion to achieve its rated power. Altitude, temperature and humidity will all affect the air density. Therefore, a de-rating of the engine must occur in high altitude and/or high temperature conditions in order for the generator to meet performance expectations.
1- Ambient Temperature
  • The National Electrical Manufacturers Association (NEMA) generator ratings are based on the generators “temperature rise” limit.
  • Outside the United States, the International Electrotechnical Commission (IEC) has limits on temperature rise for generators; these may be found in IEC 6034-22 (1996) and IEC 6034-1 (2004).
  • Temperature rise is the increase in winding temperature above the ambient temperature. Ambient temperature is the temperature of the cooling air as it enters the ventilating openings of the machine.
  • This temperature rise occurs because of the flow of current in the windings and internal losses that occur in the machine during operation.
  • The most common classes of generators are the “F” and “H” class. For all classes, NEMA assumes operation at 40°C ambient or lower. The temperature rise limits also allow for a 10°C margin for hot-spots. A hot-spot is the spot in stator windings with the highest temperature. Table-2 shows temperature rise for F and H class generators at various ratings.
Generator Class
Rise °C
Genset Package Rating
Table-2: Generator Class Vs. Temperature Rise
For prime power:
The F class has a 105°C rise or total temperature limit of 155°C (40°C ambient temperature + 10°C hot-spot margin + 105°C temperature rise) or less.
The H class allows for a 125°C rise or total temperature limit of 175°C or less: 40°C + 10°C + 125°C
For standby power:
The F class has a 130°C temperature rise limit or 180°C total temperature: 40°C + 10°C + 130°C
The H class has a 150°C temperature rise or 200°C total temperature limit: 40°C + 10°C + 130°C
  • Where the temperature of the ventilating air to the generator exceeds 40°C (104°F), de-rating 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:
  1. 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%.
  2.  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 de-rating chart found in TMI can be used for proper de-rating information for generators and each specific engine. Figure .1 shows a sample engine TMI de-rate chart. (TMI: Technical Marketing Information).
Fig.1: TMI Sample
A site with 200kW generator at 2,500ft and 122˚F, what is the de-rated rating of this generator? 
According to manufacturer’s data, this unit de-rates 2% per 1,000’ above 1,000’ above sea level (ASL) and 3% per 10˚ over 77˚F.
At our given site, conditions with these deration factors, we have 1,500’ of altitude and 45˚F to account for and properly de-rate.
1,500’ requires 3% of deration and 45˚F requires 13.5% deration for a total deration factor of 16.5%.
The de-rated power rating = 200 x (100-16.5)/100 = 167 KW
This means the 200kW unit you were expecting will only make 167kW at these site conditions. Typically, the only way to overcome this is to oversize the generator set accordingly.
3- Corrosive Atmospheres
  • Salt and other corrosive elements can cause damage to the winding insulation which can lead to failure of the generator. Protection from these elements includes additional coatings of insulation on the windings during the manufacturing process and epoxy compounds as a final winding overcoat.
4- Humidity
  • Condensation resulting from humidity will present a problem for all generators unless they are fully enclosed. Temperature rise of the machine and the circulation of cooling air with sufficient load operation will usually prevent condensation. Space heaters should be used to raise the temperature to 5°C above the ambient temperature to prevent condensation in high humidity areas.
5- Dust
  • Conductive or abrasive dust drawn in through the cooling fan can be very harmful to the generator.
  • Examples of abrasive dust are: cast iron dust, carbon dust, sand, powdered graphite, coke dust, lime dust, wood fiber, and quarry dust.
  • When these foreign particles blow through the generator, they act as sandpaper scraping away the insulation. These abrasions can cause an electrical short within the generator. An accumulation of these materials in the crevices of the insulation system will act as an insulator or as a moisture attractor.
  • Filters which fit over the unit’s intake air openings or enclosure ventilation openings can prevent damage. When using filters, it is important that they be regularly changed so as not to impede airflow. The use of a generator air filter will cause the generator to be de-rated due to higher temperature rise resulting from reduced cooling airflow. Differential pressure switches may be available as an option on many generator sets.

In the next article, we will continue explaining other Selection factors used for Generators Sizing Calculations. So, please keep following.

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