Generators Sizing Calculations – Part Six

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
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,

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.


8- Acceptable Percent Of Voltage & Frequency Dip
9- Acceptable Duration Of The Voltage & Frequency Dip

Dips are usually happened when there is a transient condition applied to a generator.
Transient Response definition
Whenever a load is applied to or removed from a generator set, the engine speed rpm, voltage and frequency are temporarily changed from its steady-state condition. This temporary change is called transient response.
 Figure.1: Transient Response
So, the two cases cause transient response are:
Case#1: Applying a Load
When a significant load is applied, the engine speed temporarily reduces (generally referred to as frequency or voltage dip) and then returns to its steady state condition. See Figure.1
The degree of this dip depends on the following:
  1. The amount of active power (kW) and reactive power (kVAR) changes based upon the voltage regulator settings,
  2. The total capacity and dynamic characteristics of the generator set
  3. The electrical inertia of the other loads in the system.
Case#2: Removing a Load
On removal of load, the engine speed increases momentarily (generally referred to as overshoot), then returns to its steady-state condition. See Figure.1

Recovery time definition:
The time required for the generator set to return to its normal steady-state speed is called recovery time. See Figure.1.

Voltage Dip
Voltage dip is the dip in voltage that results when a load is added, occurring before the regulator can correct it, or resulting from the functioning of the voltage regulator to unload an overloaded engine-generator.
We must differentiate between the following two expressions: see Figure.2
Figure.2: Instantaneous &  Sustained voltage dip
1- Maximum instantaneous voltage dip:
  • This describes the largest decrease in voltage that happens the moment after a load is added to the electrical bus.
2- Sustained voltage dip:
  • This statement refers to the voltage level that the generator set recovers to sometime after a large load is applied.
  • The sustained voltage value is determined by applying stepped load increases until the alternator does not recover to 90% of sustained voltage. This is called by NEMA MG1 part 32, which states: “The generator set shall be capable of recovering to a minimum of 90% of rated no load voltage following the application of the specified kVA load at near zero power factor.”
  • The larger the voltage dip a generator set can tolerate, the smaller and perhaps more economical the generator set can be.
  •  The amount of voltage dip is independent of the level of load already carried by the generator, particularly where that load is of a mixed nature (i.e. it consists of heating, lighting, and general power).


The Maximum Allowable Voltage & Frequency Dip
  • The maximum allowable voltage dip is 30%.
  • The maximum frequency dip is about 25% but modern equipment is restricting this to tighter margins.
  • 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.
  • A frequency dip above 35% may cause the engine difficulty in recovering. Frequency dips are normally more tightly controlled than voltage dips because they are typically connected to more sensitive electrical equipment.


Typical Voltage Dip Limitations According To The Facility Type
Typical voltage dip limitations are found in Table-1 for various facilities.

Typical Voltage Dip Limitations

Permissible Voltage dip
Hospital, hotel, motel,
Apartments, libraries, schools, and stores.


Lighting load,
large Power
load, large
Flickering highly
2% Infrequent
Movie Theaters (sound tone requires constant frequency. Neon flashers erratic)
Lighting load, large Flickering objectionable.
3% Infrequent
Bars and resorts.
Power load, large Some flicker acceptable
5% - 10% Infrequent
Shops, factories, mills, laundries.
Power load, large Some flicker acceptable
3% - 5% Frequent
Mines, oil field, quarries, asphalt, plants.
Power load, large Flicker acceptable.
25% - 30% Frequent

Greater voltage fluctuations permitted with emergency power systems.

Table-1: Typical Voltage Dip Limitations



Transient Response Standards - ISO 8528
Four performance classes are designated in ISO 8528-1-7 to describe a generator in terms of voltage and frequency. Table-2 below lists the performance class and their criteria and application examples.
  • The performance class relevant for the application must be followed to be within the standard and achieve maximum performance.


Generator’s Performance Class

Performance Class Criteria

Application Examples



Required for applications where the connected loads are such that only basic parameters of voltage & frequency need to be specified.
General purpose
applications Lighting & electrical loads


Required for applications where the demand on voltage is very much the same as for the commercial power system.
When load changes, temporary deviations in voltage and frequency are acceptable.
Lighting systems, pumps, fans and hoists
Required for applications where the connected equipment may make severe demands on voltage and frequency and waveforms.
Required for applications where the demands on voltage, frequency, and waveform are extremely severe.
Data-processing &
Computer equipment

Table-2: Generator’s Performance Class

Table-3 shows the acceptance (dip) and rejection (overshoot) parameters identified by ISO 8528-5. Class G4 is reserved for limits that are unique and must be agreed upon by the manufacturer and customer. ISO 8528-5 also sets limits on recovery times for each class and identifies how recovery time is measured.


Frequency %





Frequency %





Voltage %





Voltage %





Time Seconds





Table-3: ISO 8528-5 acceptance (dip) and rejection (overshoot) Limits



AMC:  Agreed between Manufacturer and Customer.


Voltage Regulators
The voltage regulator is a key component in determining the amount of voltage/frequency dips and recovery time.
There are several different types of regulators:
  • Constant,
  • Volts/Hertz,
  • 2 Volts/Hertz,
  • Digital Voltage Regulator (adjustable Volts/Hz).
1- Constant Voltage Regulator:
It attempts to maintain rated voltage as the load is applied. Since the generator is maintaining rated voltage, it is maintaining applied load (ekW).
The relationship between ekW and bkW is:
ekW = pf x kVA
bkW = Speed x Torque/lambda
bkW = ekW/ eff + Fan Demand
kVA = kVA output of generator
pf = power factor of connected load
ekW = electrical power (electrical kW)
bkW = engine power (brake kW)
eff = generator efficiency
  • Therefore, when a constant voltage regulator is used, it imposes increasing torque on the engine during frequency dips. Since most generator engines are not designed for increasing torque, significant amounts of frequency dip can occur. Engine speed rpm decreases as any load is imposed on a limited bus (generator). This causes frequency/voltage to dip accordingly.
  • Greater loads applied to the generator will impose a greater percentage of dip; greater loads will also require more time for the engine to recover.
2- The “Volts per Hertz” (1:1 Volts/Hz) regulator
  • It was designed to impose a decreasing torque on the engine during frequency dips. The Volts/Hz schedules voltage proportionally to speed.
  • If the speed dips 15%, the Volts/Hz regulator will cause the voltage to dip by 15%. This will reduce current flow into the load by 15%. The kW absorbed by the load is then 0.85 (85% of volts) multiplied by 0.85 (85% of current), or 72.25% of rated power. The engine only has to produce 72.25% of rated hp. mathematically, the formula for this concept is:
0.85 volts x 0.85 current = 0.7225 power = 0.85 speed x 0.85 torque
3- The 2 Volts/Hz regulator
  • Technology has allowed an increase in engine ratings. This increase creates the possibility of more severe transient loading. In some cases a Volts/Hz regulator cannot prevent excessive frequency dip because of large load changes; the torque has increased so much the deceleration on load is extreme. A 2 Volts/Hz regulator has been developed to address this situation.
  • The 2 Volts/Hz regulator decreases voltage at twice the rate of frequency dip. At 15% engine speed, the voltage would dip to 30% (2 x 15%) and the load would be reduced to 70%. The kW absorbed by the load is then 0.70 (70% of volts) multiplied by 0.70 (70% of current) or 49% of rated hp.
  • If the generator is large enough to carry the running load the 2 Volts/Hz  regulator may help in motor starting.
  • A reduced motor starter may not be required with the 2 Volts/Hz regulator when starting large motors depending on system design and applied load steps. However, the entire connected system would have reduced voltage, not just the motor.
4- Digital Voltage Regulator
  • The digital voltage regulator is a microprocessor-based voltage regulator. Its main purpose is to regulate output voltage of an engine generator set. It is designed to improve performance by allowing regulation characteristics to be modified that were previously not modifiable. This allows the engine generator set to function in a more efficient manner and to provide improved performance to the customer.
  • The digital voltage regulator can be software configured to optimize the transient response of any Caterpillar generator set package by changing the under frequency characteristics as well as the control loop gains.


Generator Voltage Dip Calculation
We have two methods to calculate/determine the Generator Voltage Dip as follows:
  1. By using equations,
  2. 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 subtransient and transient reactances of the machine. It can be calculated from the following equation:
  • Or by this equivalent equation:
  • 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.



 2- By using generator manufacturers’ performance data
  • Generator manufacturers supply performance data (usually in the form of curves supported by application notes) enabling determination of voltage dips for given impact loads.
  • Examples of such data are given in Figures.3 which shows typical 3-phase voltage dip characteristics, based on results using ultraviolet recorder measurements.
  • The performance curves are for a specific frame size. The terms 'minimum' and 'maximum' voltage refer to the bottom and top ends of the standard range of voltages available from the particular winding applied to the machine. Typically, these may be 346 volts and 480 volts, respectively. The 'maximum rated kVA of the frame' is its standard (industrial) continuous maximum rating.
  • In all instances, the impact current must never exceed the machine's declared overload capability.
Fig-3: Voltage dips related to impact load at low, lagging power factors

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



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