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
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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
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:
Note:
The operating voltage of the load
bank must match the voltage of the Generator or power source.
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:
Step#4: Assign the Load Bank Type Based on the Power Source
Capacity
Step#5: Calculate the Load Bank Capacity Based on the Power Source
Capacity and Load Bank Type
Step#6:
Assign the Required No. of Steps and Load Step Resolution
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
Step#8:
Derate the Load Bank Capacity Based on the Altitude and Ambient Temperature
of the Installation Location
Step#9: Assign the
Load Bank Type Based On the Available Installation Form
Step#10: Assign the Cooling Methods for Load
Bank based on Load Bank Capacity and Space Limitations
Step#11: Assign the Method of Control (Mechanical
or Digital / Local or Remote/ Manual or Load Shed Control) For the
Required Load Bank
Step#12:
Assign Blower Power Source (Internal Or External)
Step#13: Assign Enclosure Type for the
Required Load Bank Based On Installation Location / Space Limitations
Step#14:
Assign the Required Load and Supply Connectors
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Standard Load Bank Testing
Procedure
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First:
Generator Load Testing Manually
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:
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
Second:
Testing A Load Bank With Autotest Software
A
sample of AutoTest Software report will be as follows:
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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
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Article
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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
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Types of Load Banks:
First: According to
the Load Element Type
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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.
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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.
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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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