### Generators Sizing Calculations – Part Six

 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,

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.

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

 Introduction   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:   The amount of active power (kW) and reactive power (kVAR) changes based upon the voltage regulator settings, The total capacity and dynamic characteristics of the generator set 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.

 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.   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. 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

Facility
Application
Permissible Voltage dip
Hospital, hotel, motel,
Apartments, libraries, schools, and stores.
large Power
Flickering highly
Objectionable.
2% Infrequent
Movie Theaters (sound tone requires constant frequency. Neon flashers erratic)
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.
25% - 30% Frequent

# 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.

Note:
• The performance class relevant for the application must be followed to be within the standard and achieve maximum performance.

# Application Examples

G1
Required for applications where the connected loads are such that only basic parameters of voltage & frequency need to be specified.
General purpose
G2
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
G3

Required for applications where the connected equipment may make severe demands on voltage and frequency and waveforms.
Telecommunications
equipment
G4

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.

Class
G1
Class
G2
Class
G3
Class
G4
Frequency %
Acceptance

Frequency %
Rejection

Voltage %
Acceptance

Voltage %
Rejection

# Note:

AMC:  Agreed between Manufacturer and Customer.

 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 subtransient 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.

 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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