### Generators Sizing Calculations – Part Seven

 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 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, Third: Selection Factors Used For Generators Sizing Calculations 8- Acceptable percent of voltage & frequency dip, 9- Acceptable duration of the voltage & frequency dip,

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.

 10- Percent And Type Of Loads To Be Connected

 Introduction       An electrical load is a device that uses electricity; lights, motors, heaters, welders and communications equipment are just a few examples of electrical loads.   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.   Note: There are no rigid standards for categorizing loads.

 General Rule For Minimum Generator Set Load/Capacity   Running a generator set under light load can lead to engine damage and reducing reliability. Most manufacturers don’t recommend running generator sets at less than 30 % of rated load. Load banks should supplement the regular loads when loading falls below the recommended value.

 Loads Information Used In Generator Sizing Calculations   So, the following loads requirements must be determined before Sizing a generator:   Knowledge of the customer’s loads, Knowledge of load management strategies, 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:   According To Load Nature-1 According To Load Nature-2 According To Load /phase distribution According To Load Operation Time According To Load Importance According To Load Function

Loads according To their Nature can be classified to:

• Linear loads are defined as alternating current (AC) loads which draw current proportional to voltage. See Fig.1
• The load may be resistive, inductive (lagging power factor) or capacitive loading (leading power factor). Regardless of the type, current drawn by a linear load, current drawn by a linear load will remain sinusoidal.

Fig.1: voltage and current waveforms for nonlinear loads.

An electrical load which changes or modifies the current or voltage waveform to one that is not sinusoidal is a non-linear load. See Fig.1

Table-1 provides examples of linear and non-linear loads.

 Current Drawn Voltage and Current Waveforms Examples Linear Proportional to voltage Sine wave Incandescent light bulbs Induction and synchronous motors Electromagnetic devices Resistance heaters Transformers (non-saturated) Non-Linear Non-proportional to voltage Pulses Silicon controlled rectifiers Variable speed drives Uninterruptible power supplies Battery chargers Fluorescent lighting Computing Equipment Transformers (saturated)

All of these Non-linear loads cause some distortion and harmonics to the applied source voltage. Non-linear loads in the system can cause problems for other loads.

Harmonics

As defined by ANSI / IEEE Std. 519, Harmonics are voltages or currents at frequencies that are integer multiples of the fundamental (60 Hz) frequency: 120 Hz, 180 Hz, 240 Hz, 300 Hz, etc. which called odd harmonics (3rd, 5th, 7th ,..., 25th,...). See Table-2.

Because three-phase generators are magnetically symmetrical, resulting in the cancellation of even harmonics, only odd harmonics are normally of any significance.

 Harmonic Frequency in Hz fundamental 60 3rd 120 5th 300 7th 420 9th 540 11th 660 etc. etc.

Table-2: Harmonics

Notes:

• In general, the higher the harmonic order, the lower the magnitude of the harmonic.
• Total Harmonic Distortion (THD) is the measurement of the sum of all harmonics. Most loads will continue to operate with THD at 15 to 20%.
• However, loads with sensitive electronic equipment can develop problems with THD greater than 5%.

 Generator Sizing Rule For Non-Linear Loads   In cases where non-linear loads cause increased generator heating, two techniques are typically used to compensate for the increased generator heating:   Method#1: Using Deration factors while sizing the generator. Method#2: using a generator with oversized kVA requirement.

Power Factor’s De-rating Rule Used in Generator Sizing Calculation

Generally for generators, the excitation current must be increased as the load power factor reduced to maintain rated voltage. This, in turn, increases the heat to be dissipated within the field windings. Therefore, Derating needs to be applied if the generator is required to operate for prolonged periods at low power factors i.e. 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-3 for the Siemens AG type 1FC6 generators typify the derating 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-3: Typical derating 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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