As we stated in the previous article “Evaluation Criteria for Selecting an UPS-Part One”, where we explained the (8) steps for Evaluation Criteria for Selecting an UPS and left Step#3: Determine the power requirements to be discussed later in detail. Today, we will do this.
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Stationary UPS Sizing Calculations |
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The sizing calculations include:
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1- The UPS sizing calculations |
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In
order to properly size the UPS, the following (10) steps must be followed:
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Step#1: List All the UPS Loads Make
a list of the equipment/load that needs to be supplied from UPS.
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Step#2: List for Each Equipment/Load, the Voltage, Number
of Phases, And Frequency In
existing installations: Two methods are available: Method#1: for each equipment/load, get the voltage, number of phases, and frequency from the equipment nameplate. Method#2:
for each equipment/load, get
the voltage by measuring it and you can easily determine the number of
phases, and frequency by experience. When the installation is in the planning stage or when the measurements cannot be taken: For
each equipment/load, get the voltage, number of phases, and frequency from
equipment manufacturers' data. At
the end of step#2, the table will be as follows:
Note: the load on any
single-phase must never exceed 33% of the total UPS loading. |
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Step#3: List the KVA for Each
Equipment/Load Beside each load write its KVA, if you don’t know the load KVA, you need to determine it as follows: In existing installations: Two
methods are available: Method#1: get the load KVA from the equipment nameplate. Method#2:
The load kVA should be determined by measuring the current with all equipment
operating. In three-phase
installations, the load current should be measured at each phase. Then, the
load kVA can then be estimated as follows: For
Single-phase loads kVA = VI/1000 Where: V
is the system voltage in volts I
is the measured current in amperes For
three-phase
loads kVA =1.73 VI/1000 Where: V
is the phase-to-phase voltage in volts I
is the highest measured phase current in amperes When
the installation is in the planning stage or when the measurements cannot be
taken: The individual load kVAs should be calculated from equipment manufacturers' data. An approximate but conservative estimate of the total load kVA may be obtained by arithmetically adding the individual load kVAs for single phase and three phase loads individually. At
the end of step#3, the table will be as follows:  |
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Step#4: Determine The UPS Voltage,
Number Of Phases, And Frequency. The load voltage, frequency and total load KVA requirements determine the UPS system output voltage and frequency as follows:
For more information about classification of ups according to No. of phases, please review the article “Classification and Types of UPS – Part One” |
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Step#5: Segregate the Loads (Non-Motor Loads & Motor Loads) At
the end of step#5, the table will be as follows:  |
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Step#6: Determining Load Power
Factor and KW Demand In
existing installations: The
load power factor should be determined by actual measurements using a power
factor meter. When the installation is in the planning stage or when the measurements cannot be taken: To calculate the load power factor, the kVA and power factor of the individual loads should be obtained by one of the following methods: Method#1:
From the owner‘s/operator’s manual available from the equipment manufacturer Method
#2: Estimated from the data in table-1 Method
#3: If the above methods not available, estimate it as 0.85  Table-1 Typical load power factors
and inrush requirements At the end of step#6, the table will be as follows:  |
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Step#7: Determining Load Inrush
Current/KVA. Definition
of Inrush /starting current: It is the maximum, instantaneous input current drawn by an electrical device when first turned on. Examples for equipment and loads have Inrush /starting current:
Important Notes:
In
existing installations: The
load inrush kVA or current in existing installations should be determined by
actual measurement using a high speed storage oscilloscope or oscillograph.
Since all loads are not normally started simultaneously, the inrush kVA or
current requirements should be determined by energizing the load with the
highest inrush kVA while all other loads are connected. The
installation is in the planning stage or when the measurements cannot be
taken: In cases where measurements cannot be taken or when the installation is in the planning stage, the load inrush requirements should be calculated. Data on individual load inrush kVA and duration should be obtained by one of the following methods: Method#1:
from equipment manufacturers or Method#2:
estimated from the data in table-1 or table-2 according to the starting
method.  Table-2 the starting current values
according to the starting method used At
the end of step#7, the table will be as follows:  |
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Step#8: Determine
Loads’ Sequence of Operation Definition of Peak Process Current It is the maximum current drawn momentarily
by the loads during the process time. This current can be repetitive in
nature. In motor loads, Peak
Process Current is equivalent to the inrush current. The UPS selection depends on the nos. of loads
as follows: 1- If there is only one load, then the
selection of UPS is simple and is based on the maximum peak Current. UPS Capacity in
KVA = √3 X V X Irms-peak 2- If there are multiple loads with a
combination of static and dynamic loading characteristic, then the UPS
capacity is selected based on the sequence of operation of the loads. And we
will have two cases as follows: Case#1 (the common Case): Sequential Operation of Load When the loads are operated in sequence, the
UPS capacity is selected based on the summation of the maximum starting
current of the load (or group of motors that working simultaneously at the
same time) and rms currents (FLC) of other connected loads and as shown in
the below formula: UPS Capacity in
KVA = 1.73*V*(maximum starting current + Ʃ FLC of other connected loads) Case#2 (Rare Case): Non-Sequential Operation of Loads When the loads are not operated in a
sequence, the UPS capacity is selected based on the summation of rms currents
of all the connected loads and the rms peak current of all the connected load
as shown in the below formula: UPS Capacity in
KVA = 1.73*V*(Ʃ starting current of all loads + Ʃ FLC of all loads) Therefore, the procedures for step#8 will be
as follows: 8.1 Calculate the full load current FLC for
each equipment/load 8.2 For motor loads determine the method of
starting 8.3 Calculate the starting/inrush current
for each equipment /motor load by using table-1 or table-2 8.4 Select the largest starting current for
the motor or group of motors that working simultaneously at the same time 8.5 Sum the full load currents for other
loads than that of point 8.4 8.6 Get the total current by Adding the current
from point 8.4 + the current from point 8.5 (note for case#2 the calculation
will be different) 8.7 Get the total KVA from the following
relations: For
Single-phase loads kVA = VI/1000 For Three-phase
loads kVA = 1.73 VI/1000 8.8 For UPSs in
today’s market, the levels and ranges of support time for 150% overload is Between
10 sec and 60 sec. so, divide the total KVA from point 8.7 by 1.5 8.9 Round the result from point 8.8 to
the next higher standard rating of UPS 8.10 Verify the peak KVA condition as follows:
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Step#9: Apply
the Derating Factors (If Any) The derating factors for an UPS will be as follows:
1- Derating For Ambient Temperature: A UPS conforming to IEC standard shall be able to operate under rated conditions in a minimum temperature range from 0 °C to + 40 °C except for indoor office ambient temperature range from +10 °C to + 35 °C. If UPS KVA rating is applicable at a higher
ambient temperature than specified 40 deg.c, you need to consider a derating
factor of at least 1.5%/deg.c above the 40 deg.c K1 = 1.5%/deg.c
above the 40 deg.c 2- Derating for Altitude: A UPS shall be designed to operate under rated conditions at a height up to and including 1 000 m above sea level. Note: The manufacturer can state on request a
necessary derating of equipment to be applied at a height exceeding 1 000 m.
The following table is provided for guidance.  Table-3
derating factors for use at altitudes above 1000 m K2 = value from
Table-3 3- Derating for power factor From the table of step#5, the total load
power factor can be calculated as follows: Total KVA Demand = (Ʃ three phase loads KVA
+ Ʃ single phase loads KVA/3) Total KW Demand = (Ʃ three phase loads KW +
Ʃ single phase loads KW/3) The total load power factor = Total KW
Demand/ Total KVA Demand Estimating the total load power factor is necessary since the kVA rating and performance parameters of most UPS system designs are guaranteed only at a power factor range of 0.8 lagging. Other power factors values will affect The UPS system kVA capacity and performance parameters. For example, Some UPS products have
difficulty handling leading PF loads like the active PFC circuits present in
IT devices which may cause instability when connected to a regulating source
such as a UPS or generator (see fig.1)  Fig.1 UPS with
leading power factor derating What are the solutions to leading PF loads? For installations that are already showing
symptoms of the problems created by leading power factor loads, there are
only two feasible solutions:
Option (2) above, could result in cost
increases about 30% over the base cost of the power protection equipment.
Fortunately, there are UPS products readily available which are designed to
handle these loads. K3 = 0.2 to 0.4 So, Derated UPS
Capacity in KVA = UPS Capacity in KVA from point 8.10.F at step#8 /
(K1*K2*K3) |
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Step#10: Calculate the Design UPS Load KVA There are two other factors that affecting the UPS Sizing calculation:
1- The Load Growth Factor (LGF) It is common to make considerations for
future load growth (typically between 5 and 20%), to allow future
loads to be supported. If no future loads are expected, then this contingency
can be ignored. 2- Design Margin Factor (DMF) Also, a design margin is used to account
for any potential inaccuracies in estimating the loads, less-than-optimum
operating conditions due to improper maintenance, etc. Typically, a design
margin of 10% to 15% is recommended, but this may also depend on Client
preferences. Therefore, The Design UPS Load KVA = Derated
UPS Capacity in KVA from step#9 * (1+LGF)*(1+DMF) |
In the
next Article, we will explain the Rectifier, Inverter & Battery sizing
Calculations. So, please keep following.
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Subject Of Pervious Article |
Article |
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Applicable Standards for UPS Systems What is a UPS? Why do we need a UPS? UPS Rating Classification of UPS: 1-Voltage range, 2-No. of phases, 3- Mobility, 4- Technological design, |
Classification and Types of UPS – Part One |
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5- Physical Size/capacity, 6- Form factor/ configurations: 6.1- “N” System
Configuration |
Classification and Types of UPS – Part Two |
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6.2- “N+1” System
Configuration, which includes:
6.3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1) |
Classification and Types of UPS –
Part Three |
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6.4- Parallel Redundant with STS Configuration
6.5- System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N |
Classification and Types of UPS – Part Four |
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7- According to UPS Topology 7.1 Off-line or Standby UPS, 7.2 Line Interactive UPS, 7.3 Standby-Ferro UPS, 7.4 Online Double Conversion UPS, 7.5 The Delta Conversion On-Line UPS. |
Classification and Types of
UPS – Part Five |
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8- According to UPS Distribution
Architecture 8.1 Centralized UPS Configuration, 8.2 Distributed (Decentralized) UPS Configuration, 8.2.1 Distributed UPS-Zonewise Configuration 8.3 Hybrid UPS Configuration. Conventional (Monolithic) Vs Modular
UPS System:
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Classification and Types of UPS – Part Six |
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Three Basic Configurations Of Mains And Bypass For A UPS System:
9-According to Use of transformers with the UPS
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Classification and Types of
UPS – Part Seven |
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Transformer Arrangements in Practical UPS Systems: 1-Transformer options for the “single mains” configuration 2-Transformer Options for the “Dual Mains” Configuration |
Classification and Types of UPS – Part Eight |
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3-
Transformer options for “single mains without bypass” |
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Components of Online Double Conversion UPS: 1- Rectifier, 2- Inverter, 3- Energy Storage system: 3.1 Battery |
Components of Online Double Conversion UPS– Part One
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3.1.1 Battery
Configurations
3.1.2
Battery Size and Location 3.1.3
Battery Transition Boxes 3.1.4
Battery Monitoring 3.2
Energy Storage System – Flywheel 3.3
Energy Storage system – Super Capacitors 3.4
Hydrogen Fuel Cells 4- Static
switch Earthing
Principles of UPS Systems |
Components of Online Double Conversion UPS – Part Two |
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Evaluation Criteria for Selecting an UPS: Step#1:
Determining the need for an UPS, Step#2:
Determining the purpose(s) of the UPS, Step#3:
Determining the power requirements, Step#4:
Selecting the type of UPS, Step#5:
Determining if the safety of the selected UPS is acceptable, Step#6:
Determining if the availability of the selected UPS is acceptable, Step#7:
Determining if the selected UPS is maintainable, and Step#8: Determining if the selected UPS is affordable. |
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Example:
Selecting an Uninterruptible Power Supply (UPS) UPS
System Ratings and Service Conditions:
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Evaluation Criteria for Selecting an UPS-Part Two |
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