8/13/2019 Voltage Drop Checking

1/6

VOLTAGE DROP CHECKING :

6.1.0 DATA:

Transformer rating : 1.6 MVA

Transformer impedance : 6.00%

Highest kW motor rating : 74.6 kW

Power factor : 0.85

Mode of start : DOL

Starting Current : 6 times the Full load current

Motor Starting kVA : 526.56 kVA

Total Base Load Except motor : 1043.6 kVA

Total load During Starting of Motor : 1570 kVA

Starting power factor : 0.2

Sin : 0.97

Base MVA : 1.6 MVA

Base Voltage on transformer primary voltage : 0.415 kV

Source faut kA : 31.5 kA

Source Voltage : 11 kV

Source fault MVA : 1.732X11X31.5

: 600.15MVA

Source Impendence in P.U : Base MVA /Source MVA

: 1.6 / (600.15)

: 0.00416 P.U

Cable size of transformer incomer : 1CX500 Sq.mm

8/13/2019 Voltage Drop Checking

2/6

Resistance of 11kV 500 sq.mm cable : 0.0485 ohms/km

Reactance of 11kV 500 sq.mm cable : 0.212 ohms/km

Length Considered : 30m

Number of Runs : 1

Cable resistance of transformer primary with 30 m length : 0.0485x (50/100)

: 0.02425 ohms

Cable reactance of transformer primary with 30 m length : 0.212x (50/100)

: 0.106 ohms

Cable impedance in Ohms : (0.02425)+ (0.106)

: 0.1087 ohms

Cable impedance in p.u : ohms X (Base MVA/kV)

: 0.001437 P.u

Transformer impedance : 6.00%

Transformer impedance in P.u : 0.06 p.u

Cable size of MDB to SDB incomer : 4RX3.5CX300Sq.mm

Resistance of 1.1 kV 300 sq.mm cables : 0.0769 ohms/km

Reactance of 1.1 kV 300 sq.mm cables : 0.0705 ohms/km

Length Considered : 100m

Number of Runs : 4

8/13/2019 Voltage Drop Checking

3/6

Cable resistance of motor incomer with 100 m length : 0.0769x (100/1000/4)

: 0.001925 ohms

Cable reactance of motor incomer with 100 m length : 0.0705x (100/1000/4)

: 0.0017625 ohms

Cable impedance in Ohms : (0.00192)+ (0.00176)

: 0.00260 ohms

Cable impedance in p.u : Ohms X (Base MVA/kV)

: 0.00260X (2.5/0.433/0.433)

: 0.00260 X 13.33

: 0.034 P.u

Cable size of SDB to MCC incomer : 2RX3.5CX300Sq.mm

Resistance of 1.1 kV 300 sq.mm cables : 0.0769 ohms/km

Reactance of 1.1 kV 300 sq.mm cables : 0.0705 ohms/km

Length Considered : 50m

Number of Runs : 2

Cable resistance of motor incomer with 50 m length : 0.0769x (50/1000/2)

: 0.001925 ohms

Cable reactance of motor incomer with 50 m length : 0.0705x (50/1000/2)

8/13/2019 Voltage Drop Checking

4/6

: 0.0017625 ohms

Cable impedance in Ohms : (0.00192)+ (0.00176)

: 0.00260 ohms

Cable impedance in p.u : Ohms X (Base MVA/kV)

: 0.00260X (2.5/0.433/0.433)

: 0.00260 X 13.33

: 0.034 P.u

Cable size of MCC to Motor incomer : 2RX3.5CX150Sq.mm

Resistance of 1.1 kV 150 sq.mm cables : 0.159 ohms/km

Reactance of 1.1 kV 150 sq.mm cables : 0.0716 ohms/km

Length Considered : 50m

Number of Runs : 2

Cable resistance of motor incomer with 50 m length : 0.159x (50/1000/2)

: 0.003975 ohms

Cable reactance of motor incomer with 50 m length : 0.0716x (50/1000/2)

: 0.00179 ohms

Cable impedance in Ohms : (0.00397)+ (0.00179)

8/13/2019 Voltage Drop Checking

5/6

: 0.00435 ohms

Cable impedance in p.u : Ohms X (Base MVA/kV)

: 0.00435X (2.5/0.433/0.433)

: 0.00435 X 13.33

: 0.05 P.u

Total Cable Impedance : 0.05+0.034+0.034+0.00024

: 0.1272

Total Impedance : Source impedance +Transformer

Impedance + Total cable Impedance

: 0.00416+0.0625+0.1272

: 0.1848 P.U

Total Voltage drop during Starting : (Total load during starting of motor in

MVA x Impedance x Sin)/Base MVA

: ( 2.079x0.91x0.1848)/ (2.5)

8/13/2019 Voltage Drop Checking

6/6

: 13.9%

Maximum Allowable Voltage Drop during Starting of Highest kW motor as per Is Standard is 15% which is

higher than our Actual Voltage drop 13.9 %. Hence 2.5 MVA transformer size is justified.