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:
 
  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
 

 

 
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:
 
  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
 

 

 
1- According To Load Nature-1
 
Loads according To their Nature can be classified to:
 
  1. Linear Loads
  2. Non-linear Loads
 
A- Linear Loads
 
  • 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.
 
 
B- Non-Linear 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.
 

  

 
2- According To Load Nature-2
 
Also, Electrical loads according To their Nature can be classified to:
 
  1. Non-reactive loads,
  2. Reactive loads.
 
A- Non-reactive loads
 
Non-reactive loads are purely resistive and include devices such as heater coils, filament lights, etc.
 
B- Reactive loads
 
  • Reactive loads are devices that use an electrical coil with a magnetic field such as electric motors, solenoids, transformers etc. Reactive loads can also be those including capacitors such as found in lighting ballasts and UPS systems.
  • They are called reactive because they produce a reactive current when current flows through them.
  • The inductances and capacitances in AC load circuits cause the point at which the sinusoidal current wave passes through zero to lag or lead the point at which the voltage wave passes through zero.
  • Capacitive loads, overexcited synchronous motors, etc. cause leading power factor, where current leads voltage (see figure.2). Lagging power factor, where current lags voltage, is more generally the case and is a result of the inductance of the circuit.
  • Power Factor is the ratio of kW to kVA and is expressed as a decimal figure (0.8) or as a percentage (80%).
 

Fig.2: A typical alternator curve of reactive power (kVAR) capability.
 

  

 
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
 

  

 
Notes for Leading Power Factor Loads
 
  1. Caution should be used whenever applying generator sets to leading power factor loads. Only slightly leading power factor can cause generator sets to lose voltage control.
  2. A reasonable guideline is that a generator set can carry up to 10 percent of its rated kVAR capability in leading power factor loads without being damaged or losing control of output voltage.
  3. Loading the generator set with lagging power factor loads prior to the leading power factor loads can improve stability.
 

  

 
3- According To Load /Phase Distribution
 
Electrical loads according To their Load /Phase Distribution can be classified to:
 
  1. Balanced Electrical Loads,
  2. Non-Balanced Electrical Loads.
 
A- Balanced Electrical Loads
 
  • If the electrical distribution system served by a three-phase generator set consists entirely of three-phase loads, the system is balanced.
  • The coils making up the generator’s three phases each supply the same amount of current to the load. Generators operate best with balanced loads. If the loads are unbalanced, the risk of overheating is probable. So, when sizing a generator all loads should be balanced.
 
 
B- Non-Balanced Electrical Loads
 
  • In three-phase power generation, a single-phase load is a load placed across one voltage phase of the generator.
  • If single-phase loads are added to the three-phase system, a condition of unbalance will exist unless the single-phase loads are equally distributed among each of the three phases of the generator set.
 

  

 
General Rule for Accepted Voltage Unbalance Percent
 
Tests have shown that phase voltage unbalance of more than 2% in three-phase will cause motor overheating if the motor is operating close to full load. Some electronic equipment may also be affected by an unbalance of more than 2%.
 
Calculation of the maximum single phase load which may be safely drawn from a generator set supplying single-phase and three phase power simultaneously
 
 
  • In many applications, balancing of single-phase loads may not be practical. If these loads are small (10% or less of the generator set three-phase kVA capacity), unbalanced single-phase loading is not cause for concern provided any of the three line currents do not exceed the generator set rated line current.
  • To determine the maximum single phase load which may be safely drawn from a generator set supplying single-phase and three phase power simultaneously, use Table-4 to help with calculations.
 

 
Table-4: KVA of AC Circuits
 
 

Fig.3: Examples#1 & #2
 
 
Example#1: (Closed Delta Generator)
 
Refer to Figure-3.A Find the amount of single-phase power which can be safely drawn from a three-phase 120/240 volt four-wire generator set rated to deliver 100 kW at a 0.8 power factor. The coil current rating of the generator set is 334 amperes.
Assume that the single-phase load is connected from one line-to-neutral, has an operating power factor of 0.9 lagging and that the generator set is supplying a three-phase load of 50 kW at a power factor of 0.08.
 
Solution:
 
1. First, find the current drawn from each of the lines by the three-phase load.
P= (1.732 x I x pf x V)/1000
I= (P x 1000)/ (1.732 x V x pf) = (50x1000)/ (1.732x240x0.8) = 151 Amperes
 
2. Find the coil current capacity remaining for the single-phase load:
334 – 151 = 183 amperes
 
3. Find the single-phase power available:
P= (V x I x pf)/1000 = (120x183x0.9)/1000 = 19.8 KW
 
 
 
Example# 2: (Wye Generator)
 
Refer to Figure-3.B The generator set is rated to deliver 100 kW at a 0.8 power factor. It is a three-phase machine with a coil current rating of 334 amperes. The three-phase load to be supplied is 50 kW at 0.8 power factor. The single-phase load consists of both 120 and 208 volt circuits. The 120 volt load has a power factor of 0.9 and is connected from neutral to one leg.
This leg is common with one of the two supplying 10 kW at a 0.8 power factor to the 208 volt load.
 
Solution:
 
1. First, find the current drawn from each of the lines by the three-phase load.
P= (1.732 x I x pf x V)/1000
I= (P x 1000)/ (1.732 x V x pf) = (50x1000)/ (1.732x208x0.8) = 173.5 Amperes
 
2. Find the coil current capacity remaining for the single-phase load:
334 – 173.5 = 161 amperes
 
3. Find the 208 volt single-phase load current:
I = (P x 1000)/(V x pf) = (10x1000)/(208x0.8) = 60 Amperes
 
4. Find the coil current capacity remaining for the single-phase 120 volt load:
161 – 60 = 101 amperes
 
5. Find the 120 volt single-phase power available:
P= (V x I x pf)/1000 = (120x101x0.9)/1000 = 10.9 KW
 

  

 
  4- According To Load Operation Time
 
Electrical loads according To their Load Operation Time can be classified to:
 
  1. Base loads,
  2. Intermediate loads,
  3. Peak loads.
 
A- Base loads
 
  • Base Loads are the loads that keep running at all times (also referred as continuous load).
  • For residential installations, refrigerator and HVAC systems are examples of base demand.
 
B- Intermittent Loads
 
  • Equipment, such as furnaces and elevators, use power intermittently and are considered intermittent loads.
  • All loads that follow an intermittent load must consider the intermittent load as part of its total. This increases kVA requirements and a larger generator may be needed to account for intermittent loads.
 
C- Peak loads
 
  • Peak loads are caused by loads that cycle on and off. These peaking loads are often used for only shorter durations. Examples of peak demand loads for residential installations are microwave oven, toaster and television and for non- residential installations will be welding equipment, medical imaging equipment, or motors.
  • Taking cyclic loads into account can significantly increase the size of the recommended generator set despite efforts to place loads in a step starting sequence.
 

 

 
5- According To Load Importance
 
Electrical loads according To their importance can be classified to:
 
  1. Normal loads,
  2. Emergency loads.
 
But the most professional way is classifying them to:
 
  1. Non-critical loads,
  2. Critical loads.
 
A- Non-Critical loads
 
The loads that can be “shed” at any time and they don’t need a back-up generators. E.g.: sanitary water heating circuit. 
 
B- Critical loads
 
Critical loads can be subdivided to:
 
B.1- Low Criticality loads:
 
A power interruption causes temporary discomfort for the occupants of a building, without any financial consequences. Prolonging of the interruption beyond the critical time can cause a loss of production or lower productivity and it usually doesn’t need a back-up generators. E.g.: heating, ventilation and air conditioning circuits (HVAC). 
 
B.2- Medium Criticality loads:
 
A power interruption causes a short break in process or service. Prolonging of the interruption beyond a critical time can cause a deterioration of the production facilities or a cost of starting for starting back up and it usually needs a back-up generators. E.g.: refrigerated units, lifts. 
 
B.3- High Criticality loads:
 
Any power interruption causes mortal danger or unacceptable financial losses and a back-up generator is a must. E.g.: operating theatre, IT department, security department. 
 

  

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

 


 

 

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