Load Bank Sizing Calculations – Part Six



Today, we will explain the Steps for Selection/Calculation of the Proper Load Bank and Standard Load Bank Testing Procedure.



The (14) Steps for Selection/Calculation of the Proper Load Bank





There are several factors involved in the selection and sizing of the proper load bank used for testing a power source. Inadequately sized load bank may shorten its operating life or cause overloading failures.
These factors can be used to determine if the load bank can handle its operating load and conditions or not. The factors are indicated in the selection/calculation steps for the proper load bank.

Step#1: Assign the Proper Load Bank Type Based on the Type of the Application

Application
Generator Capacity
Load Bank Type
Portable, Small Generators and UPS System (120/208, 240 VAC, 60 Hz)
< 100 kVA
Resistive Only
Small Generators and UPS Systems (480VAC, 60Hz)
< 200 kVA
Resistive
Only
Single Units (480VAC, 60 Hz)
>200 kVA
combined
Single Units (480/5kV/15kV, 60 Hz)
1MVA – 6MVA
combined
Multiple Units Combined
(480/5kV/15kV, 60 Hz)
>6 MVA
combined
Small UPS and Battery systems (DC voltage)
Resistive Only
Rotary UPS , large UPS and Battery systems (DC voltage)
- Resistive Only
- combined

Step#2: Assign the Load Bank Type Based on the Power Source Voltage and Frequency

The following features of the power source affect the selection of the load bank type:
  • AC or DC voltage,
  • Single or three phases,
  • Voltage Rating.


Note:
The operating voltage of the load bank must match the voltage of the Generator or power source.
  
Applied Voltage Type
Voltage Rating
Load Bank Type
Low voltage AC systems
120/240v, single-phase, 2 or 3 wire
- Resistive
- Reactive
- Combined
Low voltage AC systems
208-240/416-480/575-600v , three-phase, 3 or 4 wire
- Resistive
- Reactive
- Combined
Medium voltage AC systems
5kV or 15kV class
- Low voltage (LV) load bank with a step-down transformer
- Containerized Solution (Integrated Load Bank and Transformer Package)
- Direct Connect  Solution
DC voltage systems
12/24/32v, 125/250v, or 350/700v
- Resistive
- Combined

Step#3: Assign The Load Bank’s Additional Features Based on Your Needs

Sometimes, you need to use the same load bank to test different power sources with different voltages and frequencies, so you need to select the load bank in this case to have the following features:
  • Single or multiple Voltages,
  • Single or multiple Frequencies.


Voltage Range
Selections
Single Voltage
Designed for a specific voltage, typical of exercise load banks up to 15KV.
Multiple Voltages
Capable of dual or quad voltages; (Typical voltages—60HZ: 120/240, 208-240, 416-480; 50HZ: 190-220, 380-420; DC: 12-60, 125, 250) to 15KV.

Frequency Range
Selections
Single Frequency 
Designed for a specific frequency.
Multiple Frequencies
Capable of multiple frequency or AC/DC operation (Typical frequencies: 50-60-400HZ, DC)


Step#4: Assign the Load Bank Type Based on the Power Source Capacity


Generator Capacity
Load Bank Type
Application
< 100 kVA
Resistive Only
Portable, Small Generators and UPS System (120/208, 240 VAC, 60 Hz)
< 200 kVA
Resistive
Only
Small Generators and UPS Systems (480VAC, 60Hz)
>200 kVA
combined
Single Units (480VAC, 60 Hz)
1MVA – 6MVA
combined Single Units (480/5kV/15kV, 60 Hz)
>6 MVA
combined
Multiple Units Combined
(480/5kV/15kV, 60 Hz)


Step#5: Calculate the Load Bank Capacity Based on the Power Source Capacity and Load Bank Type

Load Bank Type
Load Bank Capacity
Power Factor
Resistive - AC
Load bank capacity = 80% of power source capacity
1
Resistive - DC
Load bank capacity = 100% of power source capacity
-
Combined
Load bank capacity = 100% of power source capacity
0.8
Inductive or capacitive load = 75% of the combined resistive load
Radiator-mounted load banks with depth not more than 13 inch.
Load bank capacity = 50-60% of power source capacity

Radiator-mounted load banks with depth more than 13 inch.
Load bank capacity = 61% - 80% of power source capacity



Step#6: Assign the Required No. of Steps and Load Step Resolution

Load bank type
feature
Load step Resolution Value
Single Block
Entire capacity of load bank applied, on/off, in one step increment.
Equal Load bank KW
Multiple Steps

Note:
the load profile of the building will guide you to select the proper no. of steps and Load step Resolution Value

Load bank capacity is divided into discrete increments for application in steps; defined in terms of Load Step Resolution, which is the smallest step increment of load control.
Steps can be coarse (50%, 33%, 25%) or fine (5KW, 1KW).

- For small portable load banks (up to 50 KW), it may be selected as 0.25 or 0.5 or 1 KW.
- For Large portable load banks (50 to 100 KW), it may be selected as 5 or 10 KW.
-For Load bank from 100 to 400 KW, it may be selected as 5 or 10 or 25 KW.
-For Load bank more than 400 KW, it may be selected as 25 or 50 or 100 KW.



Step#7: Derate the Load Bank Capacity from Step#5 In Case Of Testing Power Sources with Lower Voltage Rating Than That of the Load Bank

Load banks are designed to provide a specific capacity at a rated voltage. They cannot be operated at a voltage higher than their rating without risking damage to the load bank. However, the load bank can be operated at lower voltages but its KW capacity must be derated. The derating is calculated as follows:

D= (Applied Voltage)2/(Rated Voltage)2

Where D is the Derating Factor

Derated Capacity = D X Rated Capacity KW

Example#1:

Can a 500 KW, 480 VAC Load bank fully load test a generator rated at 300 KW, 380 VAC?

Solution:

First: calculate the Derating Factor
D= (Applied Voltage)2/(Rated Voltage)2 = 3802/4802 = 0.627

Second: calculate the Derated Capacity
Derated Capacity = D X Rated Capacity KW = 0.627 x 500 = 313 KW

Yes, this load bank can test the 300 KW generator because it provides 313 KW load at 380 V which is higher than the 300 KW generator rating.

Also, the derating is affecting the value of load bank steps in the same way.

Example#2:

If we have a load bank rated at 80 KW, 400 V with the following steps 5, 10, 10, 25, 30 KW. Calculate:
The derated capacity of the generator,
The derated steps KW.
When the load bank used to test a generator rated at 380 V.

Solution:

First: calculate the Derating Factor
D= (Applied Voltage)2/(Rated Voltage)2 = 3802/4002 = 0.9

Second: calculate the Derated Capacity
Derated Capacity = D X Rated Capacity KW = 0.9 x 80 = 72 KW

Third: calculate The derated steps KW as follows

D = 0.9
Rated steps KW
5
10
10
25
30
Derated steps KW
4.5
9
9
22.5
27


Step#8: Derate the Load Bank Capacity Based on the Altitude and Ambient Temperature of the Installation Location

Altitude and ambient temperature condition
Required Derating
More than 3300 feet Altitude
(The thinner air at higher altitudes effect the proper cooling of the unit)
Derate the load bank capacity by (10-20%)
Ambient temperatures below -20°F or above 120°F
Use special  type “High Ambient” or “Arctic” load bank designs


Step#9: Assign the Load Bank Type Based On the Available Installation Form


Load Bank Installation Form
Load Bank Type
Radiator Mount
Resistive
Portable (DC or AC)
Resistive
Mobile/Static
Resistive / Capacitive / Inductive / Combined
Static Only
Resistive
Container
Resistive / Combined
Medium Voltage
Resistive / Combined










Step#10: Assign the Cooling Methods for Load Bank based on Load Bank Capacity and Space Limitations

Method Of Cooling
Application
Natural Convection Air load bank type
In case of Portable load banks rated up to a 20 KW maximum.
Integral Fan Forced Air load bank type



Horizontal Discharge:
Horizontal discharge units offer a low profile. By directing exhaust flows downward, their louvers prevent hot air flow from discharging directly towards nearby personnel
Large load banks (portable or stationary), indoor or outdoor, where there is no space limitations.
Vertical Discharge:
Vertical discharge units direct hot air upward, away from personnel. They offer a smaller footprint than comparable horizontal discharge units.
Radiator Airflow load bank mounted on the generator set’s radiator
In case of space limitations; Space not sufficient to install an Integral Fan Forced Air load bank type
Water Flow load bank
They used in case of indoor installations in controlled environments where an outdoor air-cooled unit would be impossible to install due to space or noise restrictions.


Step#11: Assign the Method of Control (Mechanical or Digital / Local or Remote/ Manual or Load Shed Control) For the Required Load Bank


Control Type
Description
Features
Mechanical
Manual
Local
Toggle Switches
Basic manual switch for applying and rejecting load locally.
Simple Load Control
Decade Switches
Basic switches similar to toggle switches but provides synchronous load changes.
Synchronous load changes.
Digital Toggle
Switches


Toggle switches with the features of electrical control including synchronous load step changes and digital instrumentation.
Synchronous load changes, digital instrumentation.
Digital
Remote
Hand-Held
Remote Control



Load control and instrumentation that can be viewed on a hand-held control.
No pre-calculations, synchronous load changes, digital instrumentation, graphical display, networking, voltage correction, variable power factor testing.
Automatic
PC Software
Feature rich control for testing that requires detailed instrumentation and reporting.
No pre-calculations, synchronous load changes, digital instrumentation, graphical display, networking, voltage correction, data capture, automatic testing, testing to predetermined standards.
BMS Integration
Integrate the load banks into an existing BMS system through PLC.


No pre-calculations, synchronous load changes, digital instrumentation, graphical display, networking, voltage correction, data capture, automatic testing, testing to predetermined standards, customized test parameters.


Step#12: Assign Blower Power Source (Internal Or External)

Blower Power Source
Rating
Internally from load bank bus bars
The voltage rating of the blower is usually specified the same as the load bank
External power source which is the preferred method which will allow the load bank blower motors to operate if main voltage supply is lost.


Step#13: Assign Enclosure Type for the Required Load Bank Based On Installation Location / Space Limitations

Installation Location
Enclosure Type
NEMA Definition
Indoor
the load bank can be enclosed in a NEMA-1 cabinet
Enclosures constructed for indoor use; to provide a degree of protection to personnel against incidental contact with the enclosed equipment and to provide a degree of protection against falling dirt
outdoor
the load bank can be enclosed in a NEMA-3R cabinet
Enclosures constructed for indoor or outdoor use; to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet and snow; and that will be undamaged by external formation of ice on the enclosure


Step#14: Assign the Required Load and Supply Connectors

Load And Supply Connectors Type
Application
Cable Set (Cam-Lok plugs, lug style connectors or via bare braded wire ends)
Used with portable units
Terminal Block
Used with stationary and duct mounted units
Plugs (To match commercial wiring devices, aircraft plugs, MIL-SPEC plugs, cam-lock connections)
Used with Wide range of load bank application





Standard Load Bank Testing Procedure





First: Generator Load Testing Manually

  • The following is the basic step-by-step procedure for performing load-bank testing on a generator-set by utilizing a portable resistive load-bank unit. Duration of actual test varies based on type of test (NFPA-110) and customer preference.
The following items are performed prior to operating the unit and with the unit in its normal operating position (estimated duration 30-90 minutes depending on unit size / cable run):


1- Remove breaker / switchgear access panels and determine suitable cable termination point. (Note: Ideal location is after the generator-set breaker at a point where the load cables can remain connected to the generator. This allows a seamless application of facility load in the event of a loss of utility power. Load-bank equipment is designed to shut itself down if it senses AC voltage / frequency lower than set parameters. In the event the customers load cables must be disconnected in order to terminate temporary cables customer will be notified immediately of additional unit down time related to hook-up and disconnect of cables in the event of a loss of utility power.)

2- Calculate necessary cable run based on distance between portable load-bank unit and cable termination point.

3- Calculate number of cable runs necessary based on total amperage load that is to be applied to unit and amperage rating of temporary cables.

4- Roll out necessary cable based on above calculations, phase by phase to prevent cross phase connections.

5- Terminate cabling at portable load-bank.

The following steps are performed prior to operating the unit and with the unit off-line, circuit breaker open (if connection point is beyond the generator-set circuit breaker) (estimated duration 15-45 minutes):

6- Land temporary load-cables, phase by phase, at connection point designated in step #1. If customer load cables are disconnected from the system ensure that cable ends are properly taped up to prevent fraying and that cables are secured to prevent rubbing against temporary load-cables or breaker enclosure.

7- Visually verify that no phase to phase connections have been made.

The following steps are performed with the unit on-line and breaker closed (estimated duration 2 hours 10 minutes to 4 hours 10 minutes):

8- Start unit up at control panel. Allow unit to come up to speed.

9- Power up load-bank and check proper load-bank cooling fan rotation. Adjust as
necessary.

10- After 5-minute warm up period take base readings.

11- After initial readings are taken apply load accordingly based on type of test (ex: Standard annual NFPA-110 test consists of a 2-hour test; 30-minutes at 25% unit rating, 30- minutes at 50% unit rating and 60-minutes at 75% unit rating).

12- Readings are taken every 15 minutes and are recorded.

13- Readings throughout the load-bank test consist of the following:

  • Time -Frequency (Hz)
  • Battery Voltage (DC) -Voltage (AC-L1, L2, L3)
  • Oil Pressure (PSI) -Amperage (AC-L1, L2, L3)
  • Coolant Temperature -Power Factor (1.0)
  • Ambient Temperature -kW
  • Exhaust Temperature -Load Percentage


14- Upon completion of test load is removed from unit and a 5-minute cool-down period is allowed for both the unit and load-bank.

15- Upon completion of cool-down period load-bank is powered down and unit is turned to the “OFF” position with the circuit breaker open.

The following steps are performed with the unit off-line and the circuit breaker open (estimated duration 15-45 minutes):

16- Disconnect temporary load cables from designated connection point.

17- Terminate customer load cables (if disconnected). Verify connections are secure and properly phased.

18- Secure cover panels.

19- Return unit to its normal operating positions, unit in “AUTO” circuit breaker closed.

The following steps are performed with all controls in their normal operating positions (estimated duration 30-90 minutes depending on unit size / cable run):

20- Roll up temporary load cables and secure to load-bank.

21- Re-verify all controls in normal operating position and secure customer site.


Observations on the Test Results

  • If the exhaust has not cleared (smoke-free) during the test period, engine repair will be required. Extensive smoke is most often caused by rings that have never seated properly, a fuel pump that needs to be rebuilt or bad fuel.
  • If the temperature is not within the manufacturer’s recommended range, the cooling system may need to be flushed to remove any restriction, or there could be a faulty thermostat
  • or defective water pump.
  • If the oil pressure drops below the manufacturer’s recommended range, the oil pump may need to be rebuilt or low pressure may be an indication of excessive engine wear that could signal the need for an engine overhaul.



Second: Testing A Load Bank With Autotest Software

  • Many load banks come now with their own AutoTest Software which provide automated load-test routines, pass-fail analysis, data collection and report generation.
  • AutoTest is equally applicable to field testing, performance proving, commissioning and acceptance of generator sets.
  • AutoTest is intended for use with resistive and resistive/reactive load banks, either as new equipment or as a field upgrade.
  • AutoTest can be user configured for automated test per specific generator rating, model, serial number or bar-code scan. Test routines can be user defined as KW versus time, KW @ power factor versus time, percent load versus time, etc. Each routine can be saved to memory.
  • Typical pass-fail criteria are voltage error and frequency error under transient and steady-state loads.


A sample of AutoTest Software report will be as follows:









 

This article is the end of this course “Load Bank Sizing Calculations Course”.

The previous and related articles are listed in below table:

Subject Of Pervious Article
Article
What is a Load Bank?
Why we don’t use the actual facility loads to test the power source?
Wet Stacking Problem
Load Bank Applications
Applicable standards for Using load banks with emergency power generating systems



Types of Load Banks:
First: According to the Load Element Type

Second: According To Portability,
Third: According To Cooling Method,
Fourth: According To Method of Control,
Fifth: According To Operating Mode,
Sixth: According To Application,
Seventh: According to no. of Load Steps,
Eighth: According to Load Bank Voltage and Frequency.


Load Bank Basic Components:
Enclosure,
Load elements,
Controls & Instrumentation,
Cooling system,
System Protections,
Load and supply Connectors.

The load bank essential circuits:
Control Circuit,
Cooling Circuit,
Load Element Circuit.
The Relationship of the Control, Cooling, and Load Element Circuits.


the Load Bank Power Connections:
a- Three Phase Connection
b- Making connections for single-phase operation
c- Making connection for Using Multiple Load Banks

An example for Specification, Installation details, Power and Control Wiring Diagrams for A Load Bank.




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