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Based on Contract Project M/s. JSW Steel Limited
Prep. INABB/PA-MME/ AC Date:2008.02.18 7
Proj. 3.5mtpa 2250mm width HSM#2
M/s.
JSW 2250mm Hot Strip Mill
Sizing Calculation
CONTENTS 1 General ... 3 2 Applicable standards... 3 3 General Description: ... 3 4 Design Considerations... 3 5 Design basis ... 4 6 Load Details ... 4
1 General
The purpose of this document is to provide project specific design criteria for the sizing of the battery and battery charger.
2 Applicable standards
IEC 60896 Stationary lead acid batteries.
IEEE 485-1983 IEEE recommended practice for sizing lead acid batteries
3 General Description:
In this project all the DC loads shall be fed from 110V battery. The calculation for sizing of the battery and its charger are shown below. The battery is to supply the DC power requirements during the following conditions:
a) Load on dc system exceeds the maximum output of the battery charger. b) Output of the battery charger is interrupted.
c) AC power is lost
Some Definitions:
Temperature derating factor: The operating temperature affects the available capacity of a cell.
The standard temperature for stating cell capacity is 25 deg C. If the lowest expected electrolyte temperature is below standard a cell large enough to have the required capacity is selected. The capacity deration factor for the same is known as temperature factor.
Design Margin: As a prudent design we are to provide a capacity margin to allow for unforeseen
addition to the DC system.
Aging Factor: The performance of a lead acid battery is relatively stable throughout most of its
life, but begins to decline at the later stage of its life. To ensure that the battery is capable of meeting its design loads throughout its service life, the battery’s rated capacity should be at least 125% of the load expected at the end of its service life ie an ageing factor of 1.25 is considered.
4 Design Considerations
1. The battery is sizing is done for a duty cycle of 30 minutes.
2. The calculated battery size is to be corrected for design margin, aging compensation and minimum temperature.
3. The batteries considered are sealed, maintenance free lead acid type with a 10 hour discharge rate i.e. C10 batteries.
4. 2Volt batteries with an end cell voltage of 1.75volts are considered.
5. K factors corresponding to 1.75 end cell voltage and temperature correction factor for the lowest ambient temperature are taken from Exide batteries ltd. (power stack series of VRLA batteries).
6. This calculation is done for battery nominal voltage of 110V D.C., which is the control supply voltage.
5 Design basis
1. The loads on the battery are as follows: a) 6.6kV HT switchboard control supply
b) 33kV HT switchboard Control supply 2. It is required to be identified which all loads are instantaneous and which all loads are
continuous. For example an 'on' lamp in a breaker feeder is a continuous load, while a tripping relay is an instantaneous load coming in at the time of a trip.
3. A design margin of 5% may be allowed for unforeseen additions or future expansion as well as for below optimum operating conditions.
4. Battery capacity is referred to at a nominal temperature of 20 deg.celcius. (The factors given by Exide are at 20deg C). A temperature correction factor for the lowest temperature is considered while determing the AH capacity of the battery.
5. The momentary load such as coil closing /tripping shall be considered as one minute loads. 6. The voltage at the load terminals shall never be outside +10% and –15% of the nominal voltage. 7. The spare transformer and motor feeders are not considered in the feeder list for the total load
calculation of the battery
8. An overall diversity of 50% is considered for the total load calculations. (Simultaneous operation of all the breakers does not happen together. In case of any fault condition in an outgoing the respective breaker shall trip or the incomer shall trip in case of any bus fault) 6 Load Details
The individual loads supplied by battery during the duty cycle maybe classified as continuous or non-continuous. Non-continuous loads lasting for 1 minute or less are designated as momentary loads.
Continuous loads: Loads, which are energized through out the duty cycle, are known as continuous loads. These loads are normally carried by the battery charger and are initiated at the inception of the duty cycle.
Some of the continuous loads are: Indication lamps, (ON, TCH etc)., continuously energized coils , annunciator loads etc.
Momentary loads: These are non continuous loads which can occur one or more times during the duty cycle but are of short duration ie less than 1 minute. Although the momentary loads may exist for only a fraction of second, it is common practice to consider that each load will last for one full minute because the battery voltage drop after several seconds.
Some of the typical momentary loads are: switchgear coil operations (trip and close), etc. The AH capacity of the battery is calculated as:
AH =AH1* K1 * K2 * K3
Where K1 = Ageing Factor
K2 = Design Margin
K3= Temperature correction factor (For the lowest temperature of 10deg C)
AH1 = max of Σ (Ap – A (p-1) )*K
Where p=1 to S (S is the section of the duty cycle being analyzed).
K= Capacity rating Factor which is obtained from the Manufacturer for a given end cell voltage and discharge capacity.(annexure-A)
The load cycle of the battery is as indicated below.
A3 – A2 A2 - A1 A1 A3 A2 M1 M2 M3 S1 S2 S3 Time in Minutes In this project,
S1 = 1 minute, S2=28 minutes & S3=1 minute
7.1 Load list
The power consumption of the various connected loads in watts is listed below.
MV Switchboard
ITEM Power consumption in watts
1 VCB 1.1 6.6kV CLOSING COIL 180 1.2 6.6kV TRIP COIL 180 1.3 33kV CLOSING COIL 310 1.4 33kV TRIP COIL 310 2 RELAYS: 2.1 REF542plus relay 40 2.2 REM543 Relay <50
2.3 Master trip relay RXPQ8 9
2.4 PSU14 65
2.5 Aux relay for trafo faults 4
2.6 Antipumping RXME 4
2.7 Aux relay for AC and DC fail 5
3 INDICATING LAMP (LED)
3.1 RED (ON) 2 3.2 GREEN(OFF) 2 3.3 AMBER(TRIP) 2 3.4 YELLOW (Y Phase) 2 3.5 WHITE (DC Fail,TCH) 2 3.6 BLUE(SPRING CHRG) 2
4 Annunciator per window 1.5
The load in amperes is calculated as:
A= W/ V*0.85 Where
V =110Volts, W = Power consumption in Watts.
An efficiency of 0.85 is considered, as the powers indicated above are the output power.(The power consumed by the device is also considered)
7.1 DC Load Details
ITEM SYSTEM LOAD (AMP)
VOLTAGE 0-1 MIN. 1-29 MIN. 29-30 MIN. CONTINUOUS
TOTAL of 33kV switchgear 110.00 30.66 4.55 30.73 4.55
TOTAL of 6.6KV switchgear sec 1 & 2 110.00 44.91 20.29 45.31 21.05
TOTAL of 6.6KV switchgear sec 3 & 4 110.00 45.09 20.47 45.32 21.03
TOTAL of 6.6KV switchgear sec 5 & 6 110.00 42.72 19.14 42.98 19.67
Total 163.38 64.44 164.34 66.30
7.2 Battery Capacity
The load changes seen by the battery is calculated as below:
The K factors used in the below calculation are the factors given by Exide at 20 deg C.
LOAD DETAILS
PERIOD LOAD TOTAL AMPS DURATION (MINUTES)
1 A1 163.38 M1=1
2 A2 64.44 M2=28
3 A3 164.34 M3=1
Sl.No. PERIOD CHANGE IN
LOAD AMPS DURATION OF LOAD AMPS TIME TO END OF SEC. K FACTO R TO 1.75EC V REQD. CAPACIT Y (AH) AT 1.75ECV
(a) MIN. MIN. (b) (a) X (b)
SECTION-A 1 1 A1-A0= M1=1 T=M1=1 0.74 120.9045 163.38 SECTION-B 1 1 A1-A0= M1=1 T=M1+M2 1.35 220.5691 163.38 =29 2 2 A2-A1= M2=28 T=M2 1.34 -132.5891 -98.95 =28 TOTAL 87.9800 SECTION-C 1 1 A1-A0= M1=1 T=M1+M2 1.36 222.2029 163.38 +M3 = 30
2 2 A2-A1= M2=28 T=M2+M3 1.35 -133.5785 -98.95 =29 3 3 A3-A2= M2=28 T=M3 0.74 73.9313 99.91 =1 TOTAL 162.5557
Highest capacity from the above three sections AH’1, AH1’’ & AH1’’’ is AH1’’’
Ie AH1= AH1’’’=162.55 AH
The final AH capacity is calculated after multiplying by the ageing factor, design margin and temperature correction factor.
Ie AH =AH1* K1 * K2 *K3
K1 ÆAgeing Factor =1.25
K2 Æ Design margin =1.05
Temperature correction factor (For the lowest temperature of 10deg C)= K3=1/(1+0.006(θ1 - θ2)
Where
θ1 =Lowest temperature seen by battery i.e. 10 deg C.
θ2 = Temperature at which the K factors are selected.
The K factors given by Exide are at an temperature of 20 deg. Hence θ2 = 20 deg C
=1/(1+0.006*(10-20)) =1.06
Required AH Capacity of the Battery = 162.55*1.25*1.05*1.06 =227 AH
A battery capacity of 240AH has been selected. The battery cells of 2 Volts are selected with an end cell voltage of 1.75V.
Float Charger Capacity
ICC =Continuous DC load current = 66.3 Amps
A design margin of 5 % is considered for unforeseen continues loads.
A trickle charging current Icc is as recommended by the manufacturer.(Usually 2%)
ITC = ICC *0.02
The total required capacity of the float charger shall be = ICC *1.05 + ITC
= 66.3 *1.05 + 66.3 *0.02
= 71 Amps
A float charger of 75 Amps had been selected.
Boost Charge capacity:
The boost charger shall be equal to maximum allowable battery charging current (generally 14% of the battery ampereage or the float charger capacity, which ever is higher.
As the float charger capacity is higher than the 14% battery charging current the boost charge capacity is selected as 75 Amps
The offered combination for the JSW Hot strip Mill is dual float cum boost chargers rated of 75 Amps with a 110V, 240AH, lead acid SMF batteries.( 55 cells of 2 Volts )
Revision Rev. ind. Page (P) Chapt.(C) Description Date Dept./Init. - - New Document 2008.02.18 – PAMME/AC A B C D E
