Monday, March 18, 2013

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IEC 60287-1-2-3       Electric Cables – Calculation of The Current Rating
IEC 60364-5-523      Electrical Installation at Building - Selection and Erection of Electrical Equipment and Current-Carrying Capacities in Wiring
IEC 228_1978           Conductor of Insulated Cables
IEC 60092-352          Electrical Installation in Ships, Part 352: Choice and Installation of Electrical Cable
IEC 60949                 Calculation of Thermally Permissible Short Circuit Currents
IEC 60364-4-43        Electrical Installation at Building – Protection For Safety – Protection Against Over Current
The basic criteria for the cable sizing calculation are as follows:
a)         Full Load Current
b)         Cable Ampacity
c)         Allowable Voltage Drop
d)        Derating factor due to cable grouping, ambient temperature and type installation of the cables
e)         Short Circuit Level
f)          Lock Rotor Current
Full load current to be given formula:
 ,   for 3-phase

,             for 1-phase
In        = Full Load Current (Ampere)
P        = Rating Capacity (kW)
VL-L    = Line to Line Voltage (Volt)
VL-N    = Line to neutral Voltage (Volt)
pf        = Power Factor (%)
eff       = Efficiency (%)
As Preliminary For cable ampacity resistance and reactance refer to GT KABEL/Kabel Metal Manufacturer, for the data see attachment-3.0. This data base will be updated after selection of Cable Manufacture finalized.
The maximum voltage drop does not exceed as the following limits:
Generator to MV Switchgear/MCC
2 %
MV Switchgear to Transformer
1 %
Motor at Steady State
5 %
Motor during Starting
15 %
LV Switchgear/MCC to Distribution Board
2 %
LV Switchgear/MCC to Lighting Panel
2 %
DC Distribution to Load Terminals
5 %
The cables are sized to the short circuit protection in accordance with IEC 60364-4-43 (2001). The formula to be applied is:
Where :
Isc    = effective short circuit current (A)
t      = duration of short circuit (second) (for maximum time is 5 seconds)
S     = cable cross section (mm2)
k      = 143 for copper conductors, XLPE insulated

For cables protected by short circuit limiting circuit breakers, the cable cross sections are calculated based on the limited short-circuit current by ensuring that the let through energy of the current limiting circuit breaker is less than the energy withstand capability of the cable.
As preliminary for lock rotor current refer to Siemens Motors Manufacture, data as shown on attachment 3.0.

Formula determining voltage drop for AC three phase systems (line to line) is:

Formula determining voltage drop for AC single phase systems (line to neutral) is:



VD%      = Voltage Drop (%)
VL-L       = Line to Line Voltage (Volt)
VL-N       = Line to Neutral Voltage (Volt)
I             = Load Current (Ampere)
R            = Resistance of Cable (W/km)
X            = Reactance of Cable (W/Km)
           = Power Factor Angle
L            = Cable Length (m)
n            = Number of Cables Run in Parallel

The permissible voltage drop at motor starting condition considered for different types of cable branch circuits have been based on the details given in (Electrical Design Philosophy). The voltage drop for AC system during motor starting is given by the following formula:
Formula determining voltage drop at motor starting for AC three phase systems is:

Formula determining voltage drop at motor starting for AC single phase systems is:



Vd start %          =  Motor Starting Voltage Drop (%)
VL-L          = Line to Line Voltage (Volt)
VL-N           = Line to neutral Voltage (Volt)
Is                      = Motor Starting Current (Ampere)
                    = Motor Starting Power Factor Angle
L                      = Cable Length (m)
n               = Number of Cables Run in parallel
(For calculation the value of cos  is assumed 0.5 for safety consideration)        
Determine for cable Short Circuit Rating
According IEC 60364-3-43 (2001), the formula of cable short circuit capability is :
            Isc            = Cable short circuit capability (kA)
     K     = 143 for XLPE cable
     S       = Selected conductor size (mm2)

     T       = breaker clearing time (second)
                 (In the calculation, breaker clearing time is assumed 0.1 second, except breaker clearing time for GTG is 0.4 seconds)
     n       = number of cable run
Determine for Load End Short Circuit Fault Current (ILF)
Formula to be given:
  V                = Nominal voltage
  ZT    = Total impedance,


In this project mostly power cable as a feeder or incoming/outgoing cable shall be run on the perforated tray and/or ladder. This type of installation gives a contribution for de-rating of cable current carrying capacity. The de-rating factor of cable current carrying capacity could be determined as below:
1.      Above Ground Installation
·      According to IEC 60364-5-523, table 52-D1 (refer to attachment-2.0) for 40o C temperature on air installation, using XLPE insulated cable, the correction factor, k1 = 0.91.
·      Correction factor in Table 52-E4 for installation method E in Tables 52-C7 to 52-C12 (see attachment 2.0) with zero cable to cable clearance (cable touching) for multi-core cables, correction factor is:
a)      Horizontal arrangement (assumed on ladder), k2’ = 0.75 (Assumed 6 circuits)
b)      Vertical arrangement (assumed on perforated tray), k2”= 0.9 (Assumed 2 circuits/layers)
Correction factor for cable arrangement on perforated tray/ladder is:
k2 = k2’ x k2”
= 0.675
Overall correction factor for air installation is k = k1 x k2 = 0.614

2.      Under Ground Installation
·      According to IEC 60364-5-523 table 52-D2 (refer to attachment-2.0), for 30o C ground temperature, using XLPE insulated cable, the correction factor, k1 = 0.93.
·      Reduction factor in Table 52-D2 for installation method D in Tables 52-C1 to 52-C4 (see attachment 2.0) with zero cable to cable clearance (cable touching) for multi-core cables, correction factor is k2 = 0.75 (assumed number of circuits is 2).
Overall correction factor for underground installation is k = k1 x k2 = 0.70

Cable Tag                                : SK81G01A-001-P
Cable Route                            : From Gas Turbine Generator to Switchgear (item no #1 in attachment 1.1 MV & LV Switchgear)
Number conductor per phase  : 2
Length                                     : 138 meter
Rating                                      : 6500 kVA (5200kW)
Allowable Voltage Drop         : 2 %
Full Load Current                   : 568.6 Ampere
Power Factor                           : 0.8
Voltage                                    : 6600 Volt
Load carried by cable,
Selected cable size is 3/C x 185 mm2 (ampacity = 493 A) 2 runs for 3 phase.
ICCC =    Cable Corrected Current (A)
        ICRC  =          Cable rated current (see attachment 3.0)
        k1   = Ambient temperature correction factor
        k2   =             Cable arrangement correction factor
        n    = Number of run

ICCC > In, therefore 2 runs 3/C x 185 mm2 is acceptable.
Cable Resistance (R) = 0.128 ohm/km
Cable Reactance (X)  = 2 π f l  (l = inductance of cable)
                                    = 2 x 3.14 x 50 hz x 0.277 10-3 H/km
 = 0.087 ohm/km (See on attachment-3.0)


                             (as calculated in spreadsheet)
Since 0.16% < 2%, therefore cable 2 runs 3/C x 185 mm2 for 3 phase is acceptable.

Cable short circuit rating:
Assume breaker clearing time = 0.4 sec
,      K = 143 for XLPE cable
83.6 kA

Load end short circuit fault current ILF:
Refer to “Electrical Power System Study Report”, GL-RP-E-004, clause 8.0 “Conclusion” that interrupting capacity on switchgear bus is = 12 kA.
0.328 ohm,
23.40 kA
Since ILF < ISC, therefore cable 2 runs 3/C x 185 mm2 for 3 phase is acceptable.


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