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10-EA-E-41021 power cable sizing criteria (2)

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CUSTOMER
MATIX FERTILISERS AND CHEMICALS LIMITED
PLANT LOCATION
WEST BENGAL - INDIA
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3850 MTPD UREA PLANT INDIA
10-EA-E-41021
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POWER CABLE SIZING CRITERIA
2
1
DE ISSUED FOR FINAL
A.Sandhiya
K. Ramanujam N.S.Prasanna 13.07.2011
DE ISSUED FOR APPROVAL
S.Venkat
K. Ramanujam N.S.Prasanna 25.01.2011
0
DE ISSUED FOR COMMENTS
Rev.
Description
R.Naga Sundar K. Ramanujam N.S.Prasanna 12.11.2010
Prepared
Checked
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Approved
Date
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INDEX
1.
SCOPE
3
2.
REFERENCE DOCUMENTS
3
2.1.
3
3.
4.
5.
Codes and Standards
BASIC CRITERIA
3
3.1.
3.2.
3.3.
3.4
4
7
8
9
Continuous current-carrying capacity
Voltage drop
Protection against thermal withstand of short circuit current
Fault clearing time
EXAMPLE OF M.V. CABLE SIZING CALCULATION
9
4.1.
9
HP Ammonia Pump (10-MP-101A – 2850kW)
EXAMPLE OF L.V. CABLE SIZING CALCULATION
5.1.
5.2.
Urea Melt Pump (10-MP-108A – 160kW) Breaker operated motor
feeder.
Main Seal Water pump (10-MP-143A – 55kW) Switch fuse unit
operated motor feeder
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SCOPE
The purpose of this document is to give basic criteria for sizing of MV & LV cables
to be used for the MATIX Fertilizer Project - 3850 MTPD UREA PLANT at West
Bengal, India.
2.
REFERENCE DOCUMENTS
2.1.
Codes and Standards
• IS 1255- Code of Practice for Installation and Maintenance of Power Cables upto
and including 33kV Rating
• IS 7098- Code of Practice for cross linked Polyethylene Insulated PVC Sheathed
Cables
• IEC 60502- Power Cables with extruded insulation and their accessories for
rated voltages from 1kV to 30kV
• IEC 60364-5-52 –Electrical Installations on Buildings- Selection and Erection of
electrical equipments-Wiring System.
• IEC 60364-4-43- Electrical Installations on Buildings-Protection for SafetyProtection against over current.
• IEC 60287- Electrical cables-Calculation of current rating
• Indian Electricity Rules 1956
Contractor Specifications:
• 10-EA-E-41000- Electrical Design Guide Line
3.
BASIC CRITERIA
The cables are sized to meet the following electric and thermal conditions:
- Maximum Continuous load current.
- Continuous current-carrying capacity.
- Voltage drops under steady state and Transient state conditions.
- Dissipation of heat from the conductors during short circuit condition.
- Derating factors.
The sizing takes into account cables insulated XLPE based upon a conductor
continuous rating temperature of 90°C and a final conductor temperature at the end
of a short circuit as 250°C.
Besides, the sizing takes into account the environmental condition of installation. In
this project the cable routing shall take place almost in air through above head
cable trays. Here after all derating factors shall be listed to define the currentcarrying capacity considering the cables installed in air with the ambient air
temperature of 50°C.
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Continuous current-carrying capacity
The continuous current rating capacity and the derating factors of the cables are
taken from the CCI (Cable Corporation of India) cable manufacturer data which is
an approved manufacturer for MATIX project.
KT
“Temperature factor”
Is a rating factor to allow for a different ambient temperature from the one at which
the ampacities are given.
The cable manufacturer provided the current rating in their catalogue for an ambient
temperature of 40˚C. The suitable rating factor needs to be applied for other than
40˚C as per the table-1 given below from the approved CCI cable manufacturer,
Table 1 – Rating Factors for variation in Air Temperature
Air
20
25
30
35
40
45
50
temperature ˚C
Rating Factor
for cond temp
of 90˚C
1.18
1.14
1.10
1.05
1.0
0.95
0.89
55
60
0.84
0.78
Hence
for cables in air, irrespective of the method of installation for an ambient
temperature of 50°C the derating factor(KT)shall be 0.89.
KPN
“Grouping of cables factor”
Is a rating factor to allow for more than one working cable laid into the same
common routing and referred to “N” working cables.
The suitable derating factor shall be applied to derive the current rating at different
grouping of cables as per the table-2, 3 (Source: CCI Cable Catalogue) and table 4
(Source: IEC 60364-5-52) listed below.
Table 2 - Rating Factors for multi core cables laid on open rack in air touching
each other
No of cables per rack
No of racks
1
2
3
6
9
2
1.00
0.80
0.76
0.71
0.69
3
1.00
0.78
0.74
0.70
0.68
6
1.00
0.76
0.72
0.68
0.66
Table 3 - Rating Factors for multi core cables laid on open rack in air with ‘D’
(One dia of Cable) spacing
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No of cables per rack
No of racks
1
2
3
6
9
2
1.00
0.95
0.93
0.90
0.89
3
1.00
0.94
0.92
0.89
0.88
6
1.00
0.93
0.90
0.87
0.86
Table 4 - Rating Factors for single core cables in trefoil circuits laid on open
rack in air with 2D spacing
No of cables per rack
No of racks
1
2
3
1
1.00
1.00
1.00
2
0.97
0.95
0.93
3
0.96
0.94
0.9
Hence for
Multi core LV cables considering cables laid in 3 tiers, 9 cables per tier in touching
each other formation, the derating factor (KPN) shall be 0.68
Multi core MV cables considering cables laid in 3 tiers, 9 cables per tier with one ‘D’
(diameter of the cable) spacing, the derating factor (KPN) shall be 0.88
Single core LV & MV cable considering cables laid in 3 tiers, 3 circuits per tier in
trefoil formation with 2D spacing the derating factor shall be (KPN) shall be 0.90
(From Table A.52.21 of IEC 60364-5-52, 2001)
As explained in basic criteria that the cable routing is only in air, the derating factor
for the cable routed in ground is not discussed in this specification.
KR
“Thermal resistivity of soil factor”
Is a rating factor to allow for variation in thermal resistivity of soil from the one at
which the ampacities are given. To be used only for buried cables.
As explained in basic criteria that the cable routing is only in air, the involvement of
thermal soil resistivity of cable is void. Hence the derating factor (KR) is not
discussed in this specification.
KL
“Laying depth factor”
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Is a rating factor to allow for variation in laying depth from the one at which the
ampacities are given. To be used only for cable installed direct in the ground.
As explained in basic criteria that the cable routing is only in air, the derating factor
(KL) is not discussed in this specification.
KUN
“General Derating factor”
Is the general maximum derating factor permitted for each cable laid in group.
The general maximum derating factor (KUn) permitted is the correction factor which
shall be used to determine the continuous current carrying capacity of the cable (Iz)
and it’s calculated as: KUn= KT⋅ KL⋅ KR⋅ KPN.
Note that
So the standard rated current InC, used for cable sizing, is suitably derated taking
into account the temperature factor (KT), thermal resistivity of soil factor (KR), laying
depth factor (KL) and Grouping of cables factor (K PN)
Here in this project the as the cable routing shall be in air through cable trays, the
“Laying depth factor (KL)” and Thermal Resistivity Factor (KUN)” shall not be
considered.
In this way, the general derating factor KUn shall be calculated as :
KUn= (K PN) x (KT)
Hence for
MV cables installation,
Multi core cable installed in air the derating factor is,
KUn = 0.88 x 0.89 = 0.78
Single core cable installed in air the derating factor is,
KUn = 0.9 x 0.89 = 0.80
LV cables installation,
Multi core cable installed in air the derating factor is,
KUn = 0.68 x 0.89 = 0.61
Single core cable installed in air the derating factor is,
KUn = 0.90 x 0.89 = 0.80
As per Company requirements the minimum conductor sizes for cables shall be as
follows:
1.1 kV cables:
• 2.5 mm2 for power and current transformer circuits
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• 1.5 mm2 for control and indication voltage transformer circuits and lighting
equipments
In this way, even if the sizing of power cables requires a calculated cross-sectional
area of 1.5 mm2, the selected cable shall be 2.5 mm2 to comply with the above
requirement.
3.2.
Voltage drop
The voltage drop, as a percentage of system nominal line to line voltage, will be
confined within the following values.
Power and lighting distribution:
3.2.1.
Feeder to local power distribution panel
Feeder to local lighting distribution panel
Lighting branch circuits
2%
2%
3%
Motor terminals:
at full load operation
during the starting period
4%
15%
Voltage drop under running or steady state condition
The voltage drop under running or steady state condition is verified with the
following formula.
Vd% =
Ib ∗ Lc ∗ [(Rc ∗ Cosφ ) + (Xc ∗ Sinφ )] ∗ K ∗ 100
Vn * n
Where:
Vd%
Ib
Lc
Rc
Xc
K
=
=
=
=
=
=
Cosφ
Vn
n
=
=
=
Percentage voltage drop at running or steady state conditions (V)
Rated current at full load at running or steady state conditions (A)
Cable length (km)
Resistance of cable conductor (Ohm/km)
Reactance of cable conductor (Ohm/km)
Coefficient determined by the kind of system. I.e. 3 in case of
three-phase system and 2 in case of single-phase system
Power factor of the load at running or steady state conditions
Nominal voltage (V)
No of cable runs
In this sizing Rc, Xc values for electric cables are from CCI cable manufacturer data.
Group Derating Factors for Cables have been taken from para 3.1 of this
specification.
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The motor starting current / full load current ratio have been considered as per design
guideline SPC 10-EA-E-41000-rev.2.The cable lengths are estimated considering plot
plan drawings.
3.2.2.
Voltage drop under motor starting conditions
The voltage drop for three-phase motors under starting condition is verified with the
formula:
Vd,st % =
Ibst ∗ Lc ∗ [(Rc ∗ Cosφ st ) + (Xc ∗ Sinφ st )] ∗ K ∗ 100
Vn * n
Where:
Vd,st%
Ibst
Lc
Rc
Xc
K
Cosφst
Vn
n
3.3.
=
=
=
=
=
=
=
=
=
Percentage voltage drop at motor starting time (V)
Motor current at starting time (A)
Cable length (km)
Resistance of cable conductor (Ohm/km)
Reactance of cable conductor (Ohm/km)
Coefficient determined by the kind of system. (I.e. 3 )
Power factor at motor starting time
Nominal voltage (V)
No of cable runs
Protection against thermal withstand of short circuit current
Protective devices shall be provided to break any short-circuit current flowing in the
conductors before such a current could cause a danger due to thermal and
mechanical effects produced in the conductors and connections.
The cables shall have a minimum section, so that it will be able to support, without
being damaged all thermal stress derived from short-circuit current for all duration
time, to be identified as intervention time of the protection device.
According to IEC 60364-4-43 the cable sizing shall satisfy the following condition,
valid for short circuit.
K2 S2 ≥ I2 t
Whereas,
K = coefficient depending on the electrical and thermal characteristics of the
cables, beginning and final temperature. According to IEC 60364-4-43:
K
= 143 for XLPE insulated copper cables;
= 94 for XLPE insulated Aluminium cables
S
= cross-sectional area in square millimeters.
= effective short-circuit current in A expressed as r.m.s. value.
I
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= duration in seconds of the short circuit depending of protective device that
shall be provided to break the short circuit current flowing in the circuit
(tripping time)
For the circuit breakers, the minimum admissible cross-sectional area of the cable
expressed in mm2 is sized as follow:
I t
S2 ≥ I2 t / K2
hence: S ≥
K
3.4
Fault clearing time
For the calculation of the minimum cross-sectional area of the conductor, fault
clearing time has been considered as follows
6.6 kV cables feeding 6.6/0.415kV transformers( TML-11A/1B) from SWGR 10-01SBM-1
6.6 kV cables feeding motor feeders from SWGR 1001SBM-1
415V cables feeding motor feeders from switchgear 1001SBL-1 A/B, 2A/B
4.
0.25sec
0.25sec
0.2Sec
EXAMPLE OF M.V. CABLE SIZING CALCULATION
For M.V. cable it is necessary to size first the minimum cross-sectional area
complying with the short-circuit level in the MV switchgear. After that, the
compliance with maximum allowable voltage drop shall be verified.
4.1.
HP Ammonia Pump (10-MP-101A – 2850kW)
Calculation of the minimum cross-sectional area of the conductor
The Short-circuit current at 10-01SBM-1 switchgear is:
40 kA
Max. Tripping time of the protection device:
0.25 s
I∗ t
S≥
= 40000 ∗ 0.2 5 / 94 = 212.766 mm2 for Aluminium cable
K
Minimum section of Cable considered is 240mm2. Aluminium Cable
Calculation of the current-carrying capacity of the conductor
Taking into account that cables are laid directly in above head cable trays
Rated power of the motor:
Rated current (Ib):
Starting current ratio (Ibst/Ib)
Motor current at starting time (Ibst)
2850 kW
281.01 A
4.5
1264.54 A
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Efficiency at running condition (η)
0.954
Power factor at running condition (cosφ)
0.93
Power factor at starting time (cosφst)
0.16
General derating factor (KUN):
0.78
(Derating factor considered for power cables with a horizontal spacing of one
diameter)
A 3 Core Aluminium conductor, XLPE insulated, Lead Covered, PVC inner sheath,
SWA, PVC outer sheath cable with the following characteristic data is selected.
Size (on the base of current-carrying capacity):
Conductor material:
Voltage grade:
Standard rated current (Inc):
Current-carrying capacity (Iz=Inc∗KUN):
Length (Lc):
Resistance (Rc):
Reactance (Xc):
3Cx240mm2
Aluminium
6.6 kV
390 A
304.2 A
267 m
0.162 Ohm/Km
0.093 Ohm/Km
IZ = INC∗KUN = 390 ∗ 0.78 = 304.2 A →IZ is higher than circuit current IB (281.01 A)
Calculation of running/steady state voltage drop
Percentage voltage drop at steady state condition shall equal to:
Vd% =
Ib ∗ Lc ∗ [(Rc ∗ Cosφ ) + (Xc ∗ Sinφ )] ∗ K ∗ 100
Vn * n
= 281.01 * 0.267 * [(0.162 * 0.93) + (0.093*0.37)] * 1.732 * 100
6600 * 1
=0.36% (which is less than allowable voltage drop of 4%)
Calculation of starting / transient voltage drop
Percentage voltage drop at starting condition shall equal to:
Vd st % =
Ib st ∗ Lc ∗ [(Rc ∗ Cosφ st ) + (Xc ∗ Sinφ st )] ∗ K ∗ 100
Vn * n
= 1264.54 * 0.267 * [(0.162 * 0.16) + (0.093 * 0.99)] * 1.732 * 100
6600 * 1
= 1.04% (which is less than allowable voltage drop of 15%)
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Conclusion
The selected cable cross-section of 3Cx240 mm2 Aluminium cable satisfies the
minimum cross-section criteria for short circuit, continuous current-carrying capacity
& the voltage drop values. Hence the selected size is adequate.
5.
EXAMPLE OF L.V. CABLE SIZING CALCULATION
5.1.
Urea Melt Pump (10-MP-108A – 160kW) Breaker operated motor feeder.
Two runs of three core cables of 240 mm2, Al conductor, XLPE insulated, Lead
Covered, PVC inner sheath, SWA, PVC outer sheath, having the following
characteristic data is selected:
Calculation of the minimum cross-sectional area of the conductor
The Short-circuit current at 10-01SBL-1 switchgear is:
50 kA
Max. Tripping time of the protection device:
0.2 s
I∗ t
S≥
= 50000 ∗ 0.2 / 94 = 237.8mm2 for Aluminium cable
K
Characteristic data
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Rated power (PN)
Power factor from Electrical Load List (cosφ)
Starting power factor (cosφST)
Efficiency from Electrical Load List (η)
Starting current ratio (IBST/IB)
Nominal voltage 3PH (VN)
Frequency
Short-circuit current on LV SWGR 10-01SBL-1 (ISCB)
Max. steady state voltage drop (Vd)
Max. starting drop voltage (Vdst)
PN
Circuit current
IB =
3 ∗ VN ∗ cosφ ∗ η
Starting current IBST
Selected cable cross-section (S)
Cable length (L)
Resistance (Rc)
Reactance (Xc)
Standard condition rated current of each cable (INC)
General derating factor (KUN)
160 kW
0.9
0.35
0.942
5.5
415 V
50 Hz
50 kA
4%
15 %
B
262.6A
1444.09A
2(3Cx240 mm2)
0.304 Km
0.162 Ohm/Km
0.077 Ohm/Km
390A
0.61
Continuous current-carrying capacity of the cable (IZ)
This value is obtained with the following formula:
IZ = 2∗INC∗KU = 2∗390∗0.61=475.8A → IZ is higher than circuit current IB 262.6 A
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Calculation of steady state voltage drop
The calculation is verified with the following formula:
Vd% =
Ib ∗ Lc ∗ [(Rc ∗ Cosφ ) + (Xc ∗ Sinφ )] ∗ K ∗ 100
Vn * n
= 262.6* 0.304 [(0.162*0.9)+ (0.077* 0.44) * 1.732 *100
415*2
= 2.99 % (which is less than allowable voltage drop of 4%)
Calculation of starting voltage drop
The calculation is verified with the following formula:
Vd st % =
Ibst ∗ Lc ∗ [(Rc ∗ Cosφ st ) + (Xc ∗ Sinφ st )] ∗ K ∗ 100
Vn * n
= 1444.09 * 0.304* [(0.162 * 0.35) + (0.077 * 0.94)] * 1.732 * 100
415*2
= 11.80 % (which is less than allowable voltage drop of 15%)
The selected cable cross-section 2 runs of 3 core x 240 mm2 satisfies the minimum
cross section criteria during short circuit and continuous current-carrying capacity
and voltage drop values. Hence selected cable size is adequate.
5.2.
Main Seal Water pump (10-MP-143A – 55kW) Switch fuse unit operated motor feeder
One run of three core cables of 150 mm2, Al conductor, XLPE insulated, Lead
Covered, PVC inner sheath, SWA, PVC outer sheath, having the following
characteristic data is selected:
Characteristic data
•
•
•
•
•
•
•
•
•
Rated power (PN)
Power factor from Electrical Load List (cosφ)
Starting power factor (cosφST)
Efficiency from Electrical Load List (η)
Starting current ratio (IBST/IB)
Nominal voltage 3PH (VN)
Frequency
Short-circuit current on LV SWGR 10-01SBL-1 (ISCB)
Max. steady state voltage drop (Vd)
B
• Max. starting drop voltage (Vdst)
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55 kW
0.85
0.40
0.92
5.5
415 V
50 Hz
50 kA
4%
15 %
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• Circuit current
•
•
•
•
•
•
•
IB =
14
PN
1
2
97.8A
3 ∗ VN ∗ cosφ ∗ η
Starting current IBST
Selected cable cross-section (S)
Cable length (L)
Resistance (Rc)
Reactance parallel (Xct)
Standard condition rated current of each cable (INC)
General derating factor (KUN)
0
538.2A
(3Cx150 mm2)
0.280 Km
0.266 Ohm/Km
0.078 Ohm/Km
290
0.61
Continuous current-carrying capacity of the cable (IZ)
This value is obtained with the following formula:
IZ = INC∗KU = 290∗0.61=176.9 A → IZ is higher than circuit current IB 97.8A
Calculation of steady state voltage drop
The calculation is verified with the following formula:
Vd% =
Ib ∗ Lc ∗ [(Rc ∗ Cosφ ) + (Xc ∗ Sinφ )] ∗ K ∗ 100
Vn * n
= 97.8* 0.280 [(0.266*0.85) + (0.078* 0.53) * 1.732 *100
415*1
= 3.06% (which is less than allowable voltage drop of 4%)
Calculation of starting voltage drop
The calculation is verified with the following formula:
Vd st % =
Ibst ∗ Lc ∗ [(Rc ∗ Cosφ st ) + (Xc ∗ Sinφ st )] ∗ K ∗ 100
Vn * n
= 538.2 * 0.280* [(0.266 * 0.40) + (0.078 * 0.92)] * 1.732 * 100
415*1
= 11.19 % (which is less than allowable voltage drop of 15%)
Protection against thermal withstand of short circuit current
This value is obtained from the following formula
K2 * S2 = 942 * 1502 = 198810000 A2 s
The I2t of the chosen protective device is 150000A2 s (from ABB coordination table)
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14
0
1
2
this is less than A2 s value calculated for the cable. Hence the chosen cable cross
section is safe.
Hence the selected cable cross-section 1 run of 3 core x 150 mm2 size is adequate.
Form code: MDT.GG.QUA.0004 Sh. 01/Rev. 3.96 File code: normal.dot
Data file: 41021.doc
This document is the property of SAIPEM S.p.A. who will safeguard their rights according to the civil and penal provisions of the law.
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