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SIZI
NG
CA
LCU
LATI
ON
10Prior to selecting the UPS, it is necessary to
determine the need. UPS may be needed for a
variety of purposes such as lighting, startup
power, transportation, mechanical utility systems,
heating, refrigeration, production, fire protection,
space conditioning, data processing, communica-
tion, life support, or signal circuits.
Some facilities need an UPS for more than one
purpose. It is important to determine the
acceptable delay between loss of primary power
and availability of UPS power, the length of time
that emergency or backup power is required, and
the criticality of the load that the UPS must bear. All
of these factors play into the sizing of the UPS and
the selection of the type of the UPS
10-1
Single phase power is used in most homes and small
businesses and adequate for running lights, fans, 1 or
2 ACs, some computers and motors up to about 5
horsepower; a single phase motor draws significantly
more current than the equivalent 3-phase motor,
making 3-phase power a more efficient choice for
industrial applications
SELECTION OF UPS3 PHASE OR 1 PHASE
Single-phase
Volt
age
1
0.5
0
-0.5
-10 90 180 270 360
TIME
Figure-1 With the waveform of single-phase power, when the wave passes
through zero, the power supplied at that moment is zero. The wave cycles
50 times per second
3-phase power is common in large businesses, data
centers, as well as industry and manufacturing around
the globe. While it is expensive to convert to three
phase from an existing single phase
installation, 3-phase allows for smaller, safer and less
expensive wiring.
Figure-2 3-phase power has 3 distinct wave cycles that overlap. Each
phase reaches its peak 120 degrees apart from the others so the level
of power supplied remains consistent
3-phase1
0.5
0
-0.5
-10 90 180 270 360
Time
Volt
age
Most consumers of electricity in India have a three
phase mains connection if the total load is more
than 5-7 KW. Only if expected load is below 5-7KW,
then the consumer gets a single phase connection.
Even when the consumer has a three phase
connection, the choice of three phase or single
phase UPS depends on several factors like the
loads to be connected to UPS and also electrical
distribution within the facility from the building
incomer, electrical switchgear and distribution units
to the room the loads to be protected are within.
This not only builds up a complete picture of the
electrical circuits on-site. It also helps to determine
whether to offer a three phase or single phase UPS
system.
UPS Systems – Input and Output Phases
In UPS there are three potential phase
configurations available. This is because a 3
phase mains or generator supply actually consists
of three single phase supplies (and a neutral) with
a 120 degree phase orientation between the them.
A 3 phase supply can deliver more electrical
power than a single phase supply.
The laws of physics and Ohms Law also come into
play, meaning that cable sizes also increase in
diameter as amperages rise. A 10KVA output is
generally the largest single phase UPS system
available. This is due to the output amperage and
cable requirements. 10KVA=10,000VA / 230Vac =
43.5Amps.
In the world of UPS, it is common to refer to a single phase UPS only by its KVA/KW rating i.e. 5KVA. However for a three phase UPS it is common to refer to the KVA/KW rating along with the number of phases i.e. 20KVA 3/1 or 100KVA 3/3.
10-2
© Copyrights Reserved
Single phase UPS systems up to 2KVA can be
supplied with a plug or with covered terminals for
hardwired installation. At 3KVA, the power
required means that the UPS will be supplied as
either a hardwired system or with a 16A plug.
Above 5KVA to the largest single phase UPS
system available (typically 10KVA) the UPS will
require a hardwired installation and should also
include an UPS maintenance bypass switch.
10-3
SELECTION OF UPS3 PHASE OR 1 PHASE
3 Phase UPS Systems (3/3 and 3/1)
Most datacentres, commercial and industrial
buildings will have a 3 phase electrical incomer that
connects them via a local distribution transformer to
the Mains. Three phase circuits may be required
throughout the building to carry the large amounts of
electrical power required for large KVA three phase
This is a generalisation as many environments can
include both single and three phase loads of course.
From a UPS systems perspective, if we are to
connect the UPS to a three phase supply we require a
UPS with a 3/x configuration. If the loads are three
phase as well, then we require a 3/3 configuration. If
the loads are single phase we may need a 3/1
configuration.
Using a three phase UPS system can simplify a power
continuity plan and allows a site to adopt a centralised
power protection plan, where one large UPS is used to
protect a complete building or critical circuits and
operations within it. This is in contrast to a
decentralised power continuity plan using a number
of smaller UPS dispersed to protect clusters of loads
like computers and lower power equipment (<10KVA)
within a facility.
Single Phase UPS Systems (1/1)
The wall sockets that we typically plug into are single
phase supplies rated at 230Vac 50Hz in India. Typical
examples would include ATMs, small lab equipments,
desktop computers, file servers, switches, routers,
hubs and telecoms systems.
UPS System Load Sizing
When sizing UPS it is important to know the phase
configuration required by both the mains supply
and the loads, in addition to the overall load size.
Electrical consultants and electrical contractors
will often state both load size and phase
configuration. An example would include ‘120KVA
three phase’. This refers to a 120KVA load run
from a three phase 415Vac, 50Hz supply. In terms
of load sizing, this means that each phase (of the
3 phase electrical supply) will deliver upto 40KVA
(or 174Amps at 230Vac). If the statement was
120KVA per phase then we would be looking at
3×120KVA per phase = 360KVA UPS load. The
need for a 120KVA three phase UPS could be met
with three single phase output 40KVA UPS
provided the connected loads are single phase
loads. These would be 3/1 configured and
installed one per phase. However, the overall
capital, installation and energy efficiency costs
just rose by a factor of 3 compared to a single
120KVA UPS system installation. 3/1 UPS upto
60KVA are also used in office environment where
the loads are single phase and this removes the
need to balance the load connections in each of
the three phases. Larger 3/1 UPS even upto
200KVA are typically required for DCS and
SCADA loads in heavy industries like Power Plant,
Steel Plant etc.
© Copyrights Reserved
1 Phase 1 Phase 1/1 230/230Vac, 50Hz 400VA-10KVA
3 Phase 1 Phase 3/1 415/230Vac, 50Hz 5 - 200KVA
3 Phase 3 Phase 3/3 415/415Vac, 50Hz 10KVA – 4.8MVA
Input Output Nomenclature Mains Voltage Typical UPS Sizes
Steady State Loading Conditions
As like any other power source, UPS is a limited power
supply and the capacity of the UPS is defined in KVA
(apparent Power) and KW (real power).
To arrive at the capacity of UPS and the configuration of
UPS, the following steps needs to be followed
UPS SIZINGSTEADY STATE LOADCONDITIONS
• Step 1 Need of Load
• Step 2 Configuration of UPS
• Step 3 Check on the KVA & KW demand
supplied by the UPS
Step 1: Need of Load
Tabulate the need of load as shown in the below table
and arrive at the load demand of the loads expected to
be connected to the UPS.
(Note: The load power factor has to be measured at the site or can be
assumed based on the past experience)
Step 2: Configuration of UPS
The criticality, of the loads will determine the necessary
availability of the UPS. Based on the criticality the UPS
capacity or configuration can be selected
Where N is the no of UPS, required to support the Load.
For critical load with 66% redundancy N>2, where a
minimum of 2 UPS is required to support the load and
1 UPS for redundancy.
Load KVA Demand Load Power Factor KW Demand
Load 1 KVA1 PF 1 KVA x PF1
Load 2 KVA2 PF2 KVA x PF2
Load 3 KVA3 PF3 KVA x PF3
Load n KVAn PFn KVA x PFn
Total Load KVA KW/KVA KW
Non-Critical Load 0% N
Critical Loads 66% N+1
Critical Loads 100% N+N
10-4
Total Load in KW (From Step 1) UPS Capacity in KW = ------------------------------------------ = > Total UPS in KW N (from Step 2)
Total Load in KVA (From Step 1) UPS Capacity in KVA = ------------------------------------------ =>Total UPS in KVA N (from Step 2)
Step 3: Selecting the required UPS capacity
Based on the total demand and the configuration
of UPS, the capacity of UPS is selected. The total
load in KVA and KW derived in step 1 will have to
divided by N as selected in step 2 to arrive the
UPS capacity.
© Copyrights Reserved
Configuration of UPS
Redundancy Level
Type of Load
Critical Loads 100% 2 NFault Tolerant System
The sizing of UPS for loads which are dynamic in nature
is a complicated subject, but with the recorded
information as shown below, the optimised UPS
capacity can be derived based on
• Inrush Current-Nature & Duration
• Peak Process Current–Nature & Duration
• Number of Loads, sequence of their operation
• Load Power Factor
• KVA and KW Demand of the UPS
Inrush Current
Input surge current or switch-on surge is the
maximum, instantaneous input current drawn by an
electrical device when first turned on. The inrush
current can be omitted in the selection calculation if the
load is switched on only once and run continuously till
the next shutdown of the plant as we can switch the
loads in manual bypass and once the loads reach the
steady state current, the loads can be transferred to the
UPS.
If the loads are switched on & off repetitively then the
UPS selection should include the inrush current also.
DYNAMIC LOADINGCONDITIONS
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.
Sequential Operation of Load
When the loads are operated in sequence, the
UPS capacity is selected based on the summation
of rms currents of all the connected loads and the
maximum rms peak current of the load as shown
in the below formula
UPS Capacity in KVA =√3 X VX ((∑1N Irms)+ Imaxrms-peak)
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 =√3 X V X ∑1n(Irms+ Irms-peak)
10-5
Peak Process Current
It is the maximum current drawn momentarily by the
loads during the process time. This current can be
repetitive in nature. The peak current has to be part of
the UPS Sizing calculation irrespective of the nature
and duration.
© Copyrights Reserved
Number of Loads and Sequence of Operation
The UPS selection depends on the no of loads, 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
The purpose of the battery is to provide DC power to
the inverter of the UPS when the mains fail and
becomes an important component in the UPS system.
There are different technologies of battery available in
the market like Lead acid battery which is further
classified as Tubular battery, Sealed Maintenance
free(SMF,VRLA)Battery, Nickel Cadmium and Lithium
Ion battery.
Sealed Maintenance Free, Valve Regulated Lead Acid
(SMF VRLA Battery) is mostly used with the UPS
systems today.
A VRLA battery utilizes a one-way, pressure-relief
valve system to achieve a “recombinant” technology.
This means that the oxygen normally produced on the
positive plate is absorbed by the negative plate. This
suppresses the production of hydrogen at the
negative plate. Water (H2O) is produced instead,
retaining the moisture within the battery. It never
needs watering, and should never be opened as this
would expose the battery to excess oxygen from the
air.
• The nominal cell voltage of a battery cell is 2V, 6
cells are connected in series inside the battery
container to have a final voltage of 12V.
• The capacity of the battery is defined as
“Ampere Hour (AH)”.
• The batteries are connected in series to increase
the voltage of the battery bank and are
connected in parallel to increase the capacity of
the battery bank.
BATTERY SIZINGCALCULATION
By design, the battery has to be operated in a
controlled electrical and environmental conditions
and the critical elements affecting battery life are:
1. Under charge Charging of battery with a
lower voltage and current
2. Cycling Cyclic usage of battery
3. Overcharge Charging of battery with a
higher voltage or current which is above the
recommended conditions of the manufacturer
4. Temperature The ambient temperature
References
• IEEE 1184:2006 IEEE Guide for Batteries for
Uninterruptible Power Supply Systems
• IEEE 485:1997 IEEE Recommended
Practice for Sizing Lead-Acid Batteries for
Stationary Applications
• Datasheet’s of major battery manufacturer’s
10-6
Figure 3 Schematic of Battery in Series and Parallel
© Copyrights Reserved
By design, the battery has to be operated in a
controlled electrical and environmental conditions
and the critical elements affecting battery life are:
1. Under charge Charging of battery with a
lower voltage and current
2. Cycling Cyclic usage of battery
3. Overcharge Charging of battery with a
higher voltage or current which is above the
recommended conditions of the manufacturer
4. Temperature The ambient temperature
References
• IEEE 1184:2006 IEEE Guide for Batteries for
Uninterruptible Power Supply Systems
• IEEE 485:1997 IEEE Recommended
Practice for Sizing Lead-Acid Batteries for
Stationary Applications
• Datasheet’s of major battery manufacturer’s
LIFE EXPECTANCY OFSMF VRLA BATTERY
10-7
100
80
60
40
20
0
75 80 85 90 95 100 105 110
Exp
ecte
d Li
fe (P
erce
nt o
f Rat
ed)
Temperature ( ºF )
200 200 200 200 200 200 200 200
20
40
60
80
100
100% DoD 50%DoD 30%DoD
No. of cycles
Per
cent
of C
apac
ity A
vaila
ble
at 27oc
120
In simple terms, the battery will reach its end of life
when its capacity falls below 80% of its rated
capacity and warrants for immediate replacement.
Impact of temperature on life of battery
The battery is rated in watts/cell at an ambient
temperature of 25-27deg C. When the operating
temperature or battery is less the capacity of the
battery will be reduced and when the temperature is
higher than the design temperature, the capacity of
the battery increases.
Elevated temperature operation will shorten battery
life. A general rule of thumb for lead-acid batteries is
that the prolonged use at elevated temperatures will
reduce the battery life by approximately 50% for every
8 ºC above 25 ºC
Figure 4 Temperature vs Life Curve
Figure 5 Cyclic Life of Battery
Frequency and Depth of Discharge
The life of a battery is related to the frequency
and depth of discharges. A battery can provide
more short duration, shallow cycles than
long-duration, deep discharge cycles. Even
momentary fluctuations in the AC power to the
UPS may result in battery discharges for several
seconds or more. Frequent cycling of the UPS
battery, even for short durations, shortens
battery life.
Design Life of Battery
Design life is determined by the manufacturer and
takes into account cell design and battery ageing
under controlled conditions in the manufacturer’s lab.
However, the design life of battery can be only used
for reference as the real service life of battery depends
on the various factor like
• Operating Temperature
• Number of charge, discharge cycle
• Charging conditions
• Depth of discharge
© Copyrights Reserved
10-8
© Copyrights Reserved
Ageing factor captures the decrease in battery
performance due to age. The performance of a
lead-acid battery is relatively stable but drops
markedly at latter stages of life. The "knee point"
of its life vs performance curve is approximately
when the battery can deliver 80% of its rated
capacity. After this point, the battery has reached
the end of its useful life and should be replaced.
Therefore, to ensure that battery can meet
capacity throughout its useful life, an ageing
factor of 1.25 should be applied (i.e. 1 / 0.8).
There are some exceptions, check with the
manufacturer.
Temperature correction factor is an allowance to
capture the ambient installation temperature. The
capacity for battery cells are typically quoted for
a standard operating temperature of 25 deg C
and where this differs with the installation
temperature, a correction factor must be applied.
IEEE 485 gives guidance for vented lead-acid
cells (see table), however for sealed lead-acid
and Ni-Cd cells, please consult manufacturer
for recommendations. Note that high
temperatures, lower battery life irrespective of
capacity and the correction factor is for capacity
sizing only, i.e. you CANNOT increase battery life
by increasing capacity.
Design Margin
Design Margin is considered to provide a capacity
margin to allow for unforeseen additions of load to the
UPS system and less-than optimum operating
conditions of the battery due to improper maintenance,
recent discharge, or ambient temperatures higher than
anticipated, or a combination of these factors. A
method of providing this design margin is by adding
load of 10–15% to the battery sizing calculations.
% Life
100
95
90
85
80
10 20 30 40 50 60 70 80 90 100
%R
ated
cap
acity
% RATED CAPACITY
Figure 6 Capacity vs Life Curve
Load Profiling
Sizing a battery is important to ensure that the loads
being supplied or the power system being supported
are adequately catered for by the battery for a period
of time (i.e. autonomy) for which it is designed.
Improper battery sizing can lead to poor autonomy
times, permanent damage to battery cells from
over-discharge, and UPS shutdown due to low
voltage.
CONSIDERATIONS FORBATTERY SIZING
The load profiling has to be done based on
• Nature of Loads to be supported by the battery
• Continuous
• Non-Continuous
• Momentary
• Battery autonomy time
• Design Margin
• Ageing Factor
• Effects of temperature
Effects of Temperature
TEMPERATURE CORRECTIONFACTOR FORBATTERY SIZING
Note --- This table is based on vented lead-acid nominal 1.215 specific gravity. However, it may be used
for vented cells with upto a 1.300 specific gravity. For cells of other designs, refer to the manufacturer.
78
79
80
81
82
83
84
85
86
87
88
89
90
95
100
105
110
115
120
125
25.6
26.1
26.7
27.2
27.8
28.3
28.9
29.4
30.0
30.6
31.1
31.6
32.2
35.0
37.8
40.6
43.3
46.1
48.9
51.7
0.994
0.987
0.980
0.976
0.972
0.968
0.964
0.960
0.956
0.952
0.948
0.944
0.940
0.930
0.910
0.890
0.880
0.870
0.860
0.850
Electrolyte Temperature
25
30
35
40
45
50
55
60
65
66
67
68
69
70
71
72
73
74
75
76
77
-3.9
-1.1
1.7
4.4
7.2
10.0
12.8
15.6
18.3
18.9
19.4
20.0
20.6
21.1
21.7
22.2
22.8
23.4
23.9
24.5
25.0
1.520
1.430
1.350
1.300
1.250
1.190
1.150
1.110
1.080
1.072
1.064
1.056
1.048
1.040
1.034
1.029
1.023
1.017
1.011
1.006
1.000
Cell Size
correction
factor(oF) (oC)
Cell Size
correction
factor(oF) (oC)
Electrolyte Temperature
10-9
© Copyrights Reserved
10-10
Battery is connected to a DC-DC Converter and the
output of the DC-DC converter is connected as an
input to the UPS (refer figure 9)
In this case, the load on the battery is based on the
output load connected to the inverter, the losses of
the inverter bridge and also the losses of the
DC-DC Converter,which could increase the
required battery capacity.
UPS Efficiency And Power Factor
UPS power ratings are quoted in volt-amperes (VA)
and/or watts. The rating in watts is equal to the
rating in volts-amperes multiplied by the power
factor.
UPS output power rating in watts = UPS output in
volts-amperes × power factor
The battery load for sizing purpose is the UPS
output rating in watts divided by the efficiency of the
inverter. The efficiency should be based on rated
UPS output. UPS output power in kilo watts X1000Nominal battery load in W = Inverter efficiency
Nominal battery load in WNominal battery load in W/Battery = No of Batteries
Battery Sizing Calculation for UPS System
The inverter of UPS provides a constant voltage to
the loads connected to it. During a battery discharge
the battery supplies constant power to the inverter of
the UPS. The DC input voltage to the inverter
decreases during the discharge. To maintain a
constant power output, the battery discharge current
increases accordingly
There are different methods to connect the battery
with the inverter of UPS. Battery can be connected
directly to input of the inverter (refer Figure 8)
In this case, the load on the battery is purely based
on the output load connected to the inverter and the
losses of the inverter bridge.
Battery
Mai
ns S
uppl
y Recti�er InverterOutput to
Critical
Load
VOLT
S D
C
130
125
120 120
115110 110
105
100 100
95
90 90
85
MINUTES
AMPS
DC
130
VOLTAGE
AMPERES
POWER = V*A
15KW
14KW
153KW
12KW
Figure 7 Constant Power Discharge Characteristics
Figure 8 Battery connected to the DC bus
Battery
Mai
ns S
uppl
y Recti�er InverterOutput to
Critical
Load
BatteryCharger
BATTERY SIZINGCALCULATION FORUPS SYSTEMS
Figure 9 UPS with DC-DC Charger between the inverter and Battery
© Copyrights Reserved
BATTERY SIZINGCALCULATION
Adjusted Battery Load Calculation
The nominal battery load should be adjusted for ageing and
operating temperature conditions.
Battery Load in W/Battery = Nominal battery load in W/Battery
× ageing factor × temperature correction factor x design
margin
This final battery load in battery has to be cross referred with
the battery manufacturer’s discharge characteristics for a
specified battery autonomy time (sample table is shown in fig
10) with the required cutoff voltage to arrive at the capacity of
the battery required.
General Guidelines for Battery Selection
• Calculate the load in Watts-hours as accurate as
possible.
• Include system losses due to efficiencies of power
conditioning (inverter, battery charger - DC/DC
converters).
• Include the appropriate factors: Temperature,autonomy,
design margin, and depth of discharge (DOD), ageing
factor
• Consider shallow DOD (max 20% recommended) and
occasional deeper DOD (max 80%)
• Select highest battery capacities per unit to reduce the
number of battery strings in parallel for better charge
balance. The recommended maximum number of strings in
10-11
© Copyrights Reserved
Sample Calculation :15 mins backup on a 500KVA UPS with an output power factor of 0.9
UPS Rating (KVA) 500KVA Specified by Customer or Consultant
Actual Load on UPS (KVA) 500KVA Specified by Customer or Consultant
Output Power Factor 0.8 Specified by Customer or Consultant
Inverter Efficiency (n) 95% Based on UPS Manufacturer’s data
No of Batteries 50 Nos Based on UPS Manufacturer’s data
End Cell Voltage (ECV) 1.75V Specified by Customer or Consultant
Backup time required (in mins) 10 mins Specified by Customer or Consultant
Ageing Factor 1.25 Specified by Customer or Consultant
Design Margin 1 Specified by Customer or Consultant
Temperature Correction Factor 1 Specified by Customer or Consultant
CONSTANT POWERDISCHARGE RATINGWATTS PER BATTERY
Figure 10: Sample constant power discharge rating of battery
Constant power discharge rating watts per battery @ 27 OC*
ECV
1.60
1.65
1.70
1.75
1.80
10 min
3594
3441
3288
3135
2982
15 min
2801
2764
2727
2690
2570
20 min
2269
2223
2177
2127
2078
30 min
1817
1786
1755
1724
1693
60 min
1125
1100
1075
1050
1024
2 hrs
680
670
659
649
638
3 hrs
495
483
470
456
442
5 hrs
340
325
318
310
302
8 hrs
225
219
213
210
207
10 hrs
183
181
180
178
177
20 hrs
93
92
92
91
90
DURATION
10-12
Step 1:
Arrive UPS output power rating in watts = UPS output in volts-amperes × power factor
= 500 X 0.8 KW = 400KW
Step 2:
Arrive the nominal battery load in W
UPS output power in kW X1000 Answer of Step 1
Nominal battery load in W = =
Inverter efficiency Inverter efficiency
400 X 1000
= ------------------ = 421053 W
0.95
BATTERY SIZINGCALCULATION
© Copyrights Reserved
Step 3:
Arrive the nominal battery load in W per Battery
Answer of step 2 4721053
Nominal battery load in W/Battery = = = 8421 W/Battery
No of Batteries 50
Step 4:
Arrive at the adjusted battery power required by taking into consideration design margin,
ageing factor and TCF (Temperature correction factor)
Adjusted nominal battery load in W/Battery = Answer of Step 3 X Design Margin X Ageing Factor X TCF
= 8421.05 X 1 X 1.25 X 1
=10526 W/Battery
As the maximum available AH is 200AH Battery in 12V SMF VRLA battery, we need to parallel multiple
strings of battery to achieve the desired backup time.
Step 5
Watts/Per battery required (Answer of step 4)
No of strings required =
Watts the battery can deliver
(from battery manufacturer datasheet)
A 160AH battery can deliver 3552 W at end cell voltage of 1.75V/Cell for 10 mins
10526 W
= = 2.96 strings = 3 strings
3552W
Hence in this scenario, 3 strings of 160AH battery with 50 battery in each string will provide 10 mins backup
at end cell voltage of 1.75V/Cell.
SAMPLECALCULATION
10-13
© Copyrights Reserved
SELECTION OFCABLES
Selection Of Cables
The cross section of cables depends on:
• Permissible temperature rise
• Permissible voltage drop
For a given load, each of these parameters results in a
minimum permissible cross section. The larger of the
two must be used.
When routing cables, care must be taken to maintain
the required distances between control circuits and
power circuits, to avoid any EMI disturbances caused
by HF currents.
Temperature Rise
Permissible temperature rise in cables is limited by
the withstanding capacity of cable insulation.
Temperature rise in cables depends on:
• Type of core (Cu or Al)
• Installation method
• Number of touching cables type of cable, the
maximum permissible current.
Voltage Drops
The maximum permissible voltage drops are:
• AC circuits (50 or 60 Hz)
• If the voltage drop exceeds 3% (50-60 Hz),
increase the cross section of conductors.
• DC circuit
• If the voltage drop exceeds 1%, increase the
cross section of conductors.
Special Case For Neutral Conductors
In three-phase systems, the third-order harmonics
(and their multiples) of single-phase loads add up in
the neutral conductor (sum of the currents on the three
phases).
For this reason, the following rule may be applied:
neutral cross section = 2 x phase cross section in
Sq mm
Output Cables
To arrive at the cross section of the cable, the
output current needs to be calculated using the
below formula
KVAX1000
Rated Current in A(I) =----------------------
√3 X Vph-ph
using the cable manufacturer’s datasheet and the
conditions linked with routing and bunching of
cables, the required cable can be selected.
As thumb rule, we can consider 2A/sq mm to arrive
the cross section of the required cables.
Rated Current in A(I)
Cross Section of Cables in sq mm = --------------------------
10-14
© Copyrights Reserved
2
Input Cables
The cross section of cables required for the input of
the UPS can be derived using the same formula like
output cables, but the input power in KVA needs to
be derived based on the
• Connected Load
• Efficiency of the Inverter
• Battery charging Power
• Efficiency of Rectifier
• Input power factor of rectifier
• Minimum operating Voltage of Rectifier
Step 1: Arrive at the input power of Inverter
Capacity of UPS in KVA X Output Power Factor X 1000
Inverter Input Power= ------------------------------------------------------------
Inverter Efficiency
Step 2: Calculate the battery charging power in W
Battery Charging Power = 2.2VX No of Cells X Charging Current
The charging current is typically 10% of AH Capacity
Step 3: Calculate the Input power of Rectifier in W
Inverter Input Power + Battery Charging Power
Rectifier Input Power = --------------------------------------------------------
Efficiency of Rectifier
Step 4: Calculate the input current drawn
The rectifier input power calculated in step 3 needs
to be converted to KVA by taking into consideration
the input power factor
INPUT, OUTPUT ANDUPS TO BATTERY CABLES
UPS to Battery Cables
The inverter of UPS provides a constant voltage to
the loads connected to it. During a battery
discharge the battery supplies constant power to
the inverter of the UPS. The DC input voltage to the
inverter decreases during the discharge. To
maintain a constant power output, the battery
discharge current increases accordingly.
The selection of UPS to battery bank cables has to
be based on the current at minimum discharge
voltage, which can be derived based on the below
formula
UPS Capacity in KVA X Power Factor X 1000
Current Idc in A = ---------------------------------------------------------------
No of Cells X End Cell Voltage X Inverter efficiency
Uninyvin cables are generally preferred for cables
between UPS & battery due to high current carrying
capacity and smaller cross sectional area.
10-15
Rectifier Input Power in W
Input Power in VA = ---------------------------------------
Input Power Factor
VA
Rated Current in (A) = ----------------------
3 X Vph-ph
where Vph-ph is the minimum operating Voltage of
rectifier
Rated Current in A(I)
Cross Section of cables in sq mm = -------------------------
2
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Uninyvin Size(area) Conductor Overall Diameter Conductor Max
Cable Diameter “Max” “Max” Resistance Current Rating
at 20°C “Max” “Amps”
Core “Sq. mm” “mm” “mm” “ Ω/ 900m” BS-G-177
22 0.347 0.838 2 49.66 11
20 0.566 1.04 2.3 30.95 14
18 0.966 1.32 2.5 17.82 18
16 1.17 1.55 2.8 14.7 21
14 2.05 1.95 3.4 8.41 31
12 3.22 2.43 3.8 5.35 43
10 5.33 3.15 5 3.23 61
8 8.76 4.24 6.3 1.97 87
6 13.3 5.54 7.5 1.3 115
4 21.5 6.9 9.3 0.802 160
2 33.3 8.76 11 0.517 200
1 40.7 9.75 12.2 0.423 220
0 53 11 13.7 0.325 240
0 68.3 12.4 15.4 0.252 270
0 84.2 13.9 16.9 0.204 300
0 109 15.6 18.7 0.158 350
CABLE DATASHEET
10-16
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Ambient Tem. °C 40 45 50 55 60 65 70 75 80 85 90 95 100
Derating Factors 1 0.96 0.92 0.88 0.83 0.78 0.75 0.73 0.68 0.62 0.53 0.48 0.3
12 3.22 43 30 22 15
10 5.33 61 47 36 25
8 8.76 87 65 49 36
6 13.3 115 87 65 -
4 21.5 160 120 92 -
2 33.3 200 155 120 -
1 40.7 220 165 130 -
0 53 240 185 168** -
0 68.3 270 210/240* 190** -
0 84.2 300 235/265* 210** -
0 109 350 270/305* 245** -
(*Denotes two cables only, ** Denotes five cables only)
10-17
CABLE DATASHEET
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Maximum Continuous Rating Amperes in Free Air
When a short circuit happens on any one
the distribution system on the output of the
UPS, the current increases significantly. If
the fault is not cleared within milliseconds,
we might risk the uptime of other loads
connected to the same UPS as the UPS or
the upstream protection of the UPS will trip
resulting in downtime of all the connected
loads.
In practice, for a given prospective
short-circuit current value, the minimum i2t
let-through of the upstream device must
higher be than the maximum i2t let-though
of the downstream device.
For protection of short circuit on the
downstream, the UPS will be based under
two conditions
• Shortcircuit current with bypass
source available
• Shortcircuit current without
bypass source
• Shortcircuit current with
downstream transformer in PDU or
global output of UPS
SELECTION OFPROTECTIONS (CIRCUIT BREAKERSOR FUSES)
Moulded Case Circuit Breakers are electro mechanical
devices, which protect a circuit from Overcurrent and Short
Circuit.
Their primary functions are to provide a means either to
manually open a circuit and automatically open a circuit under
overload or short circuit conditions. The overcurrent, in an
electrical circuit, may result from short circuit, overload or faulty
design.
MCCB is an alternative to a fuse since it does not require
replacement once an overload is detected. Unlike fuse, an
MCCB can be easily reset after a fault and offers improved
operational safety and convenience without incurring
operating cost.
Moulded case circuit breakers generally have a
• Thermal element for overcurrent and
• Magnetic element for short circuit release which has to MCCBs are now available with a variety of releases or
operating mechanisms and these are given below
• Thermal Magnetic Release
• Electronic Release
• Microprocessor Release
Protections Against Short Circuit
UPS is a limited power source, that is short circuit withstand
capacity is also limited based on the selection of components.
One of the features that must be carefully evaluated when
choosing a UPS is its capability to properly withstand a short
circuit current on its output for a certain amount of time. This
capability depends on whether the output short circuit current is
withstood solely by the inverter or by the source through the
static bypass.
10-18
In the first case, the capability strictly
depends on the UPS design and in the
second case it is based on the i2t
characteristic of the SCR selected in the
bypass path or fuse (if present in UPS)
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Short Circuit Current with bypass
Short Circuit Current without bypassMCB 8 MCB 7
MCB 6MCB 5
MCCB 4MCCB 3
MCCB 1 MCCB 2
Fault
Short circuit current without bypass
When the bypass is disabled or if the bypass
source is not available and if short circuit
happens downstream the UPS the inverter of
UPS will support for a short duration before it
trips because of electronic protections.
In this scenario, the i2t MCCB3> i2t MCB6>
i2t MCB7 For the magnetic setting of MCCB’s
& MCB’s has to be coordinated with inverter
S.C current.
Short circuit current with Transformer in PDU or
global output of UPS
When a transformer is used either at the
global output of the UPS or in a PDU,the
transformer changes the short circuit
discrimination of the downstream circuit. Now
the UPS short circuit current has no relevance
to fault discrimination.
The fault circuit current or the let though
energy will purely depend on the impedance
of the transformer.
The short circuit current of the transformer is
the ratio of full load current of the transformer
and its impedance. If we have transformer
with a rated current of 200A and with an
impedance of 5%, the short circuit current of
transformer will be 4KA.
Figure 10 Short circuit current with bypass source available
SELECTION OFPROTECTIONS (CIRCUIT BREAKERSOR FUSES)
10-19
When a short circuit happens it will downstream the UPS,
and the UPS will transfer the short circuit immediately to the
static bypass as the static bypass will have a higher
let-through energy(i2t).
In this scenario, let through energy(i2t) of the MCB 7 has to
be lower than that of the breakers present in the upstream in
to have a proper discrimination of the short circuit. If the
MCB 6 has a lower let through energy(i2t) when compared
with MCB 7, then we risk to lose all the loads connected to
MCB6.
The let through energy(i2t) of MCCB2 is very important. If the
let through energy of MCCB 2 is higher than what the SCR
can handle, then the SCR will fail.
To protect the loads, SCR and to have the proper
discrimination of short circuit, the following rule has to be
respected
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• i2tSCR> i2tMCCB2
• i2tMCCB3> i2tMCB6>i2tMCB7
PROTECTING BATTERYFROM SHORT CIRCUITS
The following figure shows the curve of the short-
circuit current delivered by a stationary lead-acid
battery; as it can be seen in the figure, after the
time, and this is the time necessary to reach the
peak, and the short circuit value decreases to the
quasi steady-state short circuit current.
Figure 12 Curve of Short circuit current in a battery
The short circuit current of battery can be
calculated by using the Ohms Law(V=IR).
V
The short circuit of the battery Isc = --------
R
Where V Open Circuit Voltage of the battery
R Internal Resistance of the battery
Figure 13 Pole Configuration of Battery bank based on Operating Voltage
Short Circuit Protection in Battery Path
Battery is one of the vital components in an UPS
system and its main purpose is to provide DC power
to the inverter of the UPS when the mains fail and get
charged through the rectifier when the mains return.
Like any other power source, battery will also
contribute to the fault current when there is fault on the
battery. The main parameters which contribute to
magnitude of the current are battery’s internal
resistance (this depends on plate surface area,
internal plate spacing and electrolyte type) and its
external circuit resistance. The short circuit current will
vary based on the condition and the age of the
battery.
10-20
IK
ttpB
ipB
iB
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Short Circuit Current of Battery Bank
The short circuit current of the battery can be
calculated based on the standard “IEC 61660-1,
“short circuit currents in DC auxiliary installations in
power plants and substations – part1: Calculation of
short circuit currents”.
+ -
LOAD
+ -
LOAD
+ -
LOAD
+ -
LOAD
+ -
LOAD
≤ 250 ≤ 500 ≤ 750Rated voltage(Un)
Protection+
isolationfunction
Selection of Battery Breaker Capacity and its Trip Unit
The selection the battery breaker depends on
parameters like
Operating Voltage of the Battery Bank: Generally most
of the breakers are designed with an voltage of
250V/Pole and based on the operating voltage of
the battery bank, the poles has to be connected in
series to achieve the desired voltage level as shown
in fig.13
Nominal Discharge Current of the Battery Bank: This is
the current which passes through the breaker under
normal conditions of battery discharge
Short Circuit Current of the Cattery Bank: Most of the
breakers have a thermal and a magnetic trip unit.
While the thermal setting is used for overload
protection, the magnetic setting is used for short
circuit protection. When we discuss about battery
protection, the magnetic setting of the breaker is
used to disconnect the battery from the circuit when
there is a short circuit. It is important to select the
breaker with the right trip unit so that the battery is
isolated when there is an fault.
Note: When an AC breaker is used for a DC
applications a derating is applicable on the trip
settings of the breaker.
Coordination of Battery Breaker with the Battery Fault
Current
Now that we have selected the right breaker for the
battery protection, the most important task which
lies ahead is to coordinate the battery breaker with
the short circuit current of the battery.
As we said earlier, the short circuit current depends
on the voltage and the internal resistance of the
battery. The internal resistance increases with the
ageing of battery under these conditions and the
short circuit current decreases. If this short circuit
current is less than the pickup value of the magnetic
setting of the breaker the principle objective of using
the breaker is defeated as the breaker will not trip.
To overcome this issue, the magnetic pickup of the
breaker trip unit is set at 70% of the nominal short
circuit current so that even at low voltage or when the
battery reaches the end of life, the battery breaker
will do its job of “protecting the battery “
The magnetic setting(Im) of the breaker is < 70% of Isc
of Battery
CO ORDINATION OF BATTERY BREAKER
KV
LR
10-21
Figure 14 Schematic of a DC Circuit
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