Diesel Generator Sizing Work Sheet

Diesel Generator Sizing Voltage Dip Calculations Worksheet

Load summary

Largest Motor Connected to DG Data

Here you just add your load connected to DG bus. Make sure the load is in kVA, if it's in kW then divide it by system power factor. Make sure to also include largest motor kW in this.

Rated power Power factor Efficiency Full load current Starting current Starting power factor Rated voltage

Load kVA

1665

Total power consumption

kVA 2081.25 kVA @ 0.85 power factor

Starting capacity

Power consumption of all other loads

1424.262 kVA

tan θ2

1.252918

0.897192 51.40534 degree

A B C D

1210.623 210.2362 750.2766 1029.943

Minimum DG size

θ2

450 0.89 0.962 632 2 0.2 480

A times

632.187

V

1051.181 kVA Angels Section θ1 0.554811 31.78833 degree sin θ1 cos θ1 θ

2277.723 kVA

kW

θ2

0.526783 0.85

1.369438 78.46304 degree sin θ cos θ

0.979796 0.2

0.897192 51.40534 degree sin θ2 cos θ2

0.781579 0.623807

data to be entered by user calculated data

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11/16/2012

Diesel Generator Sizing Voltage Dip Calculations Worksheet

Generator terminal voltage Transit reactance X'd

480 0.25

initial pu current final pu current a b

0.26125 0.171958

Vg

0.905503 pu

Vdip

0.094497

V pu

0.625301 pu 0.95 pu

Voltage dip at DG bus formula:

(use 1.0 pu)

Calculations of motor per units currents or

9.449717 %

the above reading is the voltage dip at the main bus connected to the DG, which also will affect all other connected loads. 10% voltage dip due to starting of the largest motor while all other loads connected, is acceptable reading. Next step, should be to calculate the voltage drop at motor terminal during motor starting. And this reading should not exceed 20% of system rated voltage. Refer to worksheet #3 for the final step in the calculations.

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for full pu current you can always use 1.0 pu current, assuming that the system runs at full load at its rated kW. the initial pu current is the total load connected to the DG without including the largest motor. Initial power Initial current

1424.262 kVA 1713.121 A

Total power Total current

2277.723 kVA 2739.675 A

Initial pu Current

0.625301 pu

11/16/2012

Diesel Generator Sizing Voltage Dip Calculations Worksheet

Cable voltage drop at largest motor terminal: Cable length Cable resistance Cable Rreactance Vd

180 0.0495 0.045

m Ω/km Ω/km

0.044339 pu

Cable resistance data must be entered manually from manufacturer's catalogue. Cable reactance data must be entered manually from manufacturer's catalogue. or

4.433915 %

Vd is the voltage drop on the cable caused by motor starting current.

Total Voltage Drop at Largest Motor Starting

13.88363206

%

and this value should be at all times kept less than 20%, otherwise re-do the calculations again. Maybe try different cable size (larger cable, or double run per phase).

This worksheet is brought to you by Rajab. (http://thepiratebay.se/user/rajabmm/)

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11/16/2012