Generators Sizing Calculations – Part Eight

Subject of Previous Article
Glossary of Generators – Part One Generators Sizing Calculations – Part One
Glossary of Generators – Part Two Generators Sizing Calculations – 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
Generators Sizing Calculations – Part Three
Third: Selection Factors Used For Generators Sizing Calculations
3- Location Considerations,
4- Fuel Selection Considerations,
5- Site Considerations,
Generators Sizing Calculations – Part Four
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,

Generators Sizing Calculations – Part Six
Third: Selection Factors Used For Generators Sizing Calculations
10- Percent And Type Of Loads To Be Connected – Part One
Generators Sizing Calculations – Part Seven

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:
  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.

10- Percent And Type Of Loads To Be Connected – Part Two


Why analysis and categorization of generator loads is very important?

Loads have different electrical characteristics. When developing a load analysis, it is helpful to analyze and categorize generator set loads into groups with common characteristics to assure proper consideration of their power demand because A generator set is a limited power source, sometimes referred to as a “limited bus”. The limited bus does not have the reserve capability of a utility grid.
There are no rigid standards for categorizing loads.


Loads Information Used In Generator Sizing Calculations
So, the following loads requirements must be determined before Sizing a generator:
  1. Knowledge of the customer’s loads,
  2. Knowledge of load management strategies,
  3. Knowledge of starting requirements.

First: Knowledge Of The Customer’s Loads
A generator’s electrical loads can be classified into various categories according to various factors as follows:
  1. According To Load Nature-1
  2. According To Load Nature-2
  3. According To Load /phase distribution
  4. According To Load Operation Time
  5. According To Load Importance
  6. According To Load Function
We explained the first five categories in article “Generators Sizing Calculations – Part Seven”.

6- According To Load Function
The most important step in sizing a generator set is to identify every type and size of generator loads. It is necessary to segregate these loads in different application categories after gathering a reasonably accurate load schedules.
Typical Electrical Loads
Lighting loads –linear
Incandescent lamps,
Lighting loads – nonlinear
Fluorescent lamps,
High-intensity Discharge (HID) and arc.
Resistor Ovens,
Convection Ovens,
Dielectric Heating,
Induction Heating,
Arc Furnaces.
Resistance Welding,
Arc Welding,
Induction Welding.
Critical loads
Medical imaging loads,
Data Centers/Computers,
Communications Equipment.
DC Motors,
Induction Motors,
Synchronous Motors.
Adjustable /Variable Speed Drive (VSD),
Rectifiers and solid-state controllers,
Uninterruptible Power Supply (UPS),
Battery charger loads.
Regenerative loads

A- Lighting loads
They will be divided to linear and non-linear lighting loads as follows:
A.1- Linear Lighting Loads
Linear loads are loads that draw current in a sinusoidal waveform. There are many examples of linear loads, but lighting is the most common. Example of the Linear Lighting Loads is:
  • Incandescent Lighting: very large blocks of tungsten lighting load may cause a momentary voltage dip transient to occur.
A.2- Non-Linear Lighting Loads
Non-linear loads are AC loads in which the current is not proportional to the voltage. Non-linear loads create harmonics, or additional sine waves, that are a multiple of the generated frequency in the current waveform. This will influence the generator size. Examples of the Non-Linear Lighting Loads are:
  • Fluorescent Lamps,
  • Gas Discharge Lamps.

B- Heating loads
Most of these loads are single phase loads and it is necessary to know whether they are connected between phase and neutral or between phase and of the most important heating loads are the furnaces.
Furnace loads
  • Generally, furnace loads are single phase loads for connection between lines. If such furnaces are provided with a balancing network, they impose less severe duty on AC generators. The power factor of the furnace and the transient load imposed are the other aspects to be considered. Where furnaces are controlled by thyristors, they present additional harmonics problems.
  • These loads fluctuate within wide limits. These fluctuations cause fluctuations in output voltage and frequency of the generator, which might affect other connected loads. Further, such loads need careful and detailed study before deciding the set rating. Normally and oversized generator which is rated 30% to 40% higher than the estimated steady continuous load, functions satisfactorily.

C- Welding Loads
Welders draw erratic fluctuating current. These current fluctuations produce voltage waveform distortion due to relatively high-load source impedance. Generator sets may require significant de-rating with welder loads.

D- Motor loads
For accurate generator sizing for motor loads we must calculate the motor loads’ starting and running requirements. Generally, we can characterize motor loads as high inertia or as low-inertia loads for the purpose of determining engine power needed to start and accelerate motor loads.
D.1- Low-inertia loads:
  • They include fans and centrifugal blowers, rotary compressors, rotary and centrifugal pumps.
D.2- High-inertia loads:
  • They include elevators, single- and multi-cylinder pumps, single- and multi-cylinder compressors, rock crushers, and conveyors.
  •  However, Motor loads will be discussed in detail in next article when explaining the special cases for generator sizing calculation. We need to focus here on the large motors loads which will be over 50 HP as follows:
Motors over 50 HP:
  • A large motor started across the line with a generator set represents a low-impedance load while at locked rotor or initial stalled condition. The result is a high inrush current, typically six times the motor rated (running) current.
  • The high inrush current causes generator voltage dip which can affect other systems. The manner in which generator voltage recovers from this dip is a function of the relative sizes of the generator, the motor, engine power (kW capacity) and generator excitation forcing capability.
  • Depending on the severity of the load, the generator should be sized to recover to rated voltage within a few seconds, if not cycles. Various types of reduced-voltage motor starters are available to reduce the starting kVA of a motor in applications where reduced motor torque is acceptable.
  • Reducing motor starting kVA can reduce the voltage dip, the size of the generator set, and provide a softer mechanical start. However, these starting methods should only be applied to low-inertia motor loads unless it can be determined that the motor will produce adequate accelerating torque during starting.

E- Miscellaneous Loads
E.1- Adjustable (Variable) Speed Drives
  • Electronic adjustable speed drives are used to control the speed of both AC and DC motors. Speed control allows application of motors, at their most efficient speed, to be matched to the demand created at a particular moment. Terms used to describe these drives include the following:
  • Variable Speed Drive (VSD)
  • Variable Frequency Drive (VFD)
  • Adjustable Frequency Drive (AFD)
  • These drives rectify incoming AC power to form DC power. The DC is used to either power a DC motor directly, or power an inverter that converts DC back into AC at a desired voltage and frequency for driving a motor at any speed at any point in time.
  • The rectification of AC with SCR’s distorts current waveforms and subsequently causes distorted voltage waveforms, which can have impact on other equipment connected to the same source.
  • VFD’s start at zero frequency and ramp up to a set point. Variable voltage drives start at zero voltage and ramp up to a selected point. Both are under a current or torque limit to avoid large inrush current.
  • Generally, when these drives represent more than 25% of the total load on the generator set, larger alternators are required to prevent overheating due to the harmonic currents induced by the VFD and to lower system voltage distortion by lowering alternator reactance. Larger generators have greater reduction in impedance of the generator; this reduces the effects of the harmonic current distortion. When Total Harmonic Distortion (THD) exceeds 15%, additional generator capacity may be needed. For example, VFD loads on a generator must be less than approximately 50 percent of generator capacity to limit total harmonic distortion to less than 15 percent.
E.2- Uninterruptible power supply (UPS) loads:
  • UPS system uses silicon controlled rectifiers or other static devices to convert AC voltage to DC voltage for charging storage batteries and are another type of non-linear load. Larger alternators are required to prevent overheating due to the harmonic currents induced by the rectifiers and to limit system voltage distortion by lowering alternator reactance.
  • Past problems of incompatibility between generator sets and static UPS devices lead to many misconceptions about sizing generator sets for this type of load. Most UPS manufacturers have addressed these issues and it is now more cost effective to require UPS devices to be compatible with the generator set than to significantly oversize the generator for the UPS. Use the full nameplate rating of the UPS for determining load to allow sufficient capacity for generator set battery charging and accommodating full UPS load capacity.
  • However, UPS loads will be discussed in detail in next article when explaining the special cases for generator sizing calculation.
E.3- Battery charger loads:
  • A battery charger is a non-linear load requiring an oversized alternator based on the number of rectifiers (pulses)—up to 2.5 times the steady-state running load for three pulse; to 1.15 times the steady-state running load for 12-pulse. These loads are typically found in telecommunications systems.

F- Critical Loads
Critical loads are loads that cannot tolerate voltage and frequency dips. Medical equipment, Data centers and communication equipment are examples of critical loads.
F.1- Medical Equipment:
  • X-Ray equipment typically needs very short duration, high voltage from power supplies. This need results in high current draw in short durations, which in turn results in low kW demand at near unity power factor.
  • Power source equipment should be selected to maintain x-ray quality.
  • As x-ray equipment is activated, voltage dip due to inrush should be within 10% or within the manufacturer’s recommended tolerance.
  • These loads generally represent only a small part of the generator load, so x-ray pictures are not normally affected.
F.2- Data Centers/Computers:
  • Data centers require a reliable power source. Power quality requirements should be considered prior to power system design. As a rule, avoid heavy SCR (Silicon Controlled Rectifier) loads, block switching loads and large motor kVA on data processing equipment power circuits.
F.3- Communications Equipment:
  • Communication equipment includes broad ranges of electronic devices for transmission of information.
  • Most common are radio and television broadcasting equipment, studio equipment, transmitters and telephone equipment. Generally, all devices pass their power supply through transformers. Therefore, power factor is slightly less than unity. Most equipment tolerates frequency variations of ±5%, except where synchronous timing from the power source is used.
  • Voltage variations of ±10% are usually acceptable since electronic circuits sensitive to voltage variations contain internal regulation circuitry.
  • Power for complex telephone systems is frequently supplied from building system mains. Voltage and frequency stability requirements are usually not severe, however solid-state battery charging equipment may be part of the load and create disturbance to a generator power source.

7- Regenerative Power
  • Some motor applications, such as elevators, cranes and hoists, depend on motors for braking. If a mechanical load causes the motor to turn faster than synchronous speed, the motor will act as a generator and feed power back into the system. The term “regenerative power” is sometimes used to describe the power produced by these loads. If no other loads are connected to absorb this energy, these loads will cause the generator to act as a motor, possibly causing engine over-speed which can lead to engine failure/shutdown. Regenerative power is usually not a problem when the utility is supplying power because it can be considered as an infinite power source with many loads.
  • Only engine frictional horsepower can be relied on for braking. Exceeding frictional horsepower causes the generator set over-speed.
  • In calculating the ability of a system to overcome regenerative power, it is conservatively recommended that only engine friction horsepower be considered. Engine friction horsepower at synchronous speeds is available from the engine manufacturer.
  • Typically, a generator set will retard approximately 10% of its rating.
  • When combinations of connected load and engine frictional horsepower are not sufficient to restrain regenerative energy, load banks may be added to protect the generator from being affected regeneration.

Second: Knowledge of Load Management Strategies
Load Management Definition:
It is the deliberate control of loads on a generator and/or utility to have the lowest possible electrical costs.
Knowing the type of load management that is most economical to a facility can help determine the size of generator needed based on the load factor and application.
The Load Management strategy is used for the proper sizing of generator needed to operate within that strategy. However, there are two broad rating categories in terms of load management:
  1. Isolated from a utility,
  2. Paralleled with a utility.
1- Isolated from the utility
Under 500 Hours per Year
Over 500 Hours per Year
Output available with varying load for less than 6 hours per day
Output available without varying load for over 500 hours per year and less than 6 hours per day
Fuel Stop Power in accordance with ISO 3046/1, AS2789, DIN6271 and BS5514
Prime Power in accordance with ISO 8528.
Overload Power in accordance with ISO 3046/1, AS2789, DIN6271 and BS5514.
Typical Load Factor
60% or Less
60% to 70%
Typical Hours per Year
Less than 500 hours
More than 500 hours
Typical Peak Demand
80% of rated kW with 100% of rating available for duration of an emergency outage
100% of prime plus 10% rating used occasionally
Typical Application
Interruptible utility rates, peak sharing
Peak sharing or cogeneration
2- Paralleled with the Utility
Under 500 Hours per Year
Over 500 Hours per Year
Output available without varying load for under 500 hours per year
Output available without varying load for unlimited time.
Continuous Power in accordance with ISO 8528, ISO 3046/1, AS2789, DIN6271 and BS5514.
Typical Load Factor
60% to 70%
70% to 100%
Typical Hours per Year
Less than 500 hours
No limit
Typical Peak Demand
100% of prime rating used occasionally
100% of continuous rating used 100% of the time.
Typical Application
Peak sharing
Base load, utility, peak sharing, cogeneration, parallel operation.

Power utilities sometimes offer their customers power discounts if their loads do not fluctuate or exceed a certain limit. The most common are:
  1. Peak shaving,
  2. Base loading,
  3. Zero import/zero export control,
  4. Peak Sharing,
  5. Co-generation.
1- Peak Shaving
Figure.1 shows how a utility customer can qualify for a dis-counted rate by not allowing the power demand to be above 500 kW.
Fig.1: Peak Shaving
Any power generated over 500 kW is supplied by the customer’s generator. Thus, the customer “shaves” the peaks from the utilities’ responsibility.
Peak shaving can be very demanding on an engine; it must be able to start quickly and automatically parallel to the utility.
The response time of the engine is crucial because of the load fluctuations.
2- Base Loading
The least demanding power management type on an engine is base loading. The generator operates at a constant load and the utility imports power when the load exceeds the generator output.
Fig.2: Base Loading
The user can also export power to the utility if the load is below the output of the generator. Figure.2 shows a base loading system and indicates when power would be imported or exported. Since overloads are handled by the utility and the generator set is operating at a constant load, size and engine response time are not as crucial as in peak shaving.
3- Zero Import/Zero Export
The load management type in which the customer supplies all the electrical needs to the facility, while still paralleling with the utility is called Zero Import/Zero Export control. Refer to Figure.3. If the power requirements fluctuate widely, a series of generator sets can be used and brought on-line as required.
Since the customer remains paralleled to the utility, the demands made on the engines for this type are similar to base loading.
Reliability is the chief concern for these customers. Utilities will often invoke demand charge penalties each time they are called upon to supply power.
Fig.3: Zero Import/Zero Export
4- Peak Sharing
In a typical peak sharing arrangement, the customer installs and operates generators of specified capacity when directed to do so by the utility company.
Under many peak sharing contracts, utilities compensate the customer for each time they operate their onsite generators.
5- Co-generation
Co-generation is the term used to describe the load management system that produces electricity for lighting and equipment operations while at the same time it utilizes the waste heat produced in the exhaust for heating, cooling, or generation of process steam.
Co-generation plants can operate independently of the utility or in parallel so the co-generator can purchase from or sell power to the utility.

Customer’s Loads Profiles
Assessing the customer’s load profiles is a key component to establishing their load management strategy and ultimately the size of generator needed to operate within that profile.
Definition of Load Profile Chart:
Chronological and duration curves used to illustrate the load profiles according to the nature of the load. Such curves are developed for a week, month, season or year.
Importance Load Profile Chart:
  1. It is used to analyze the load,
  2. It establishes the peak daily demand and an energy usage profile which can aid in the selection of an appropriately sized engine,
  3. It is used to operate a generator system at maximum efficiency,
  4. It is useful in programming generator units for economical operation.
1- Daily chart:
The daily chart represents the average kilowatt load for each day. A daily chronological load curve, illustrated in Figure.4, shows load demand throughout the day.
Fig.4: Daily Chronological Load Curve
2- Monthly/Yearly Chart:
The purpose of the monthly average number is to be able to graph a complete year and determine any seasonal variations. The twelve-month chart, shown in Figure.5, represents the average kilowatt load for each month.
For existing loads that are served by a utility, power bills or power consumption records will provide the needed data for a twelve-month period.
Fig.5:Average Monthly KW Load Curve
The average kW load can be determined from a Load Profile Chart by the following equation:
Average kW Load for Month = Total kW used in Month / Total Monthly Hours of Operation
Unless the load is known to be steady, this average cannot be used to establish the engine and generator requirements because the average will always be lower than the maximum kW demand.

Third: Knowledge of starting requirements
Starting Requirement Definition:
The time it takes to initiate a generator startup and when it is ready to accept load is defined as its starting requirement. Starting requirements will vary depending on the application. A typical starting requirement is 10 to 30 seconds.
Load Acceptance Definition:
Load Acceptance is the point at which breaker closure is initiated. This is considered to be 90% of rated frequency.

Ten-Second Start Rule
Ten-Second Start Rule refers to the ability of a non-paralleled generator set to start, accelerate to rated speed and be ready to accept load within 10 seconds after receiving a signal to start. For 10-second starting, the following conditions must exist:
  1. Cranking batteries must be adequately sized and fully charged.
  2. Combustion air must be a minimum of 21°C (70°F).
  3. A jacket water heater to maintain a minimum of 32°C (90°F) jacket water temperature.
  4. A readily available supply of clean fuel.
  5. The generator rotating inertia must not exceed that of the standard Generator.
Any variation in these conditions will affect the start time.
There is a difference for battery versus ambient temperature sizing. Also, in cases where air starting is used, the air system must supply the required air volume and maintain a 100 psi (689.5 kPa) minimum pressure.
Ten-Second Start Rule for Natural Gas Engines
  • Special engine conditioning is required for a natural gas engine to start in 10 seconds. Ten-second starting is only possible on specific gas engines.
  • Starting condition requirements for natural gas engines are the same as diesel engine starting requirements, except for one detail: The solenoid gas valve must be located as closely as possible to the carburetor or “A” regulator, depending on the fuel train consists. A maximum distance of 0.61 m (2 ft) is desired.
  • A customer’s starting time requirements should be determined before sizing the generator.
  • A quick starting application requires consideration of altitude, temperature and other factors that affect the engine starting to find the best solution.
Ten-Second Start Rule for Health Care Facilities as per NFPA 99
The standard NFPA 99 is written specifically for health care facilities.
3- states the following:
“The generator set(s) shall have sufficient capacity to pick up the load and meet the minimum frequency and voltage stability requirements of the emergency system within 10 seconds after loss of normal power.”
Customers requiring this capability can achieve it by using the correct generator system. Select an appropriate generator according to this standard for all health care facilities in the United States.

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



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