10 sizing calculation - fuji electric

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.


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.


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.


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


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


10-8


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


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



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


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



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


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


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


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


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


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)


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


• 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


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


10 sizing calculation - fuji electric

Documents