Handbook of Electrical Engineering

CALCULATION OF VOLT-DROP IN A CIRCUIT CONTAINING AN INDUCTION MOTOR 563

Convert to system base values.

MCC load kVA=Sol=

MCC load power

MCC load power factor

=

Pol

cosφol

=

500 × 1000

0. 85

= 588. 235 × 103 VA

MCC load kVA/phase=Solp=

588. 235 × 1000

3

= 196. 078 × 103 VA

MCC load power/phase=Polp=

500 × 1000

3

= 166. 667 × 103 VA

MCC load reactive power/phase=Qolp=

√

(Solp^2 −Solp^2 )

= 103. 29 × 103 VAr

Ohmic resistance per phase=Rolp=

(

Phase voltage

Phase active power

) 2

=

=

(

Vol

Polp

) 2

=

4160 × 4160

3 × 166. 67 × 103

× 106

= 34 .61 ohms per phase

Ohmic reactance per phase=Xolp=

(

Phase voltage

Phase reactive power

) 2

=

(

Vol

Qolp

) 2

=

41 , 600 × 4160

3 × 103. 29 × 103

× 106

= 55 .848 ohms per phase

Convert to the system base impedance values.

System impedance in per-unit=Ohmic impedance=

(load base kVA) (system base kVA)

(system base voltage)^2 (load base kVA)

Hence

Rol=

( 34. 61 × 588. 235 × 1000 )( 3125 × 1000 )

( 4160 )^2 ( 588. 235 × 1000 )

= 6 .2498 pu

and

Xol=

( 55. 848 × 588. 235 × 1000 )( 3125 × 1000 )

( 4160 )^2 ( 588. 235 × 1000 )

= 10 .085 pu

These are the parallel elements of the load in per-unit at the system base.

f) Motor feeder cable. Convert to system base.

A70mm^2 three-core 5 kV cable has an ohmic impedance of 0. 343 +j 0 .129 ohms per kilo-

metre per phase and a current rating of 250 A. Hence the total ohmic impedance is 0. 5145 +

j 0 .1935 ohms per phase.