Page 587 - Handbook of Electrical Engineering

580 HANDBOOK OF ELECTRICAL ENGINEERING

Therefore
0.2508 × 1.9701
| V |= × 100 = 24.42%
2.1158 × 0.9563
Which compares pessimistically with the simple case (19.6%) but closely with the rigorous
case (23.24 %).
v) Graphical estimation

This sub-section develops a simple graphical method for quickly estimating volt-drops for direct-
on-line starting situations. The following data forms the basis of the graphical results:
• The generator data.

Z g = 0.1, 0.15, 0.2, 0.25 and 0.3pu
R g = 0.0pu
S base = S gen = 10, 000 kVA
Rated power factor = 0.8 lagging

• The standing load data.

Rated power factor = 0.9 lagging

Load = 0.0, 50.0 and 80.0% of S gen
|Z l |= 1.25, 2.0 and infinity (no-load) pu

Where

Z l = 1.25(0.9 + j0.436) for 80% load
= 2.00(0.9 + j0.436) for 50% load
=∞(0.9 + j0.436) for 0% load

• Motor data.

Table G.1 shows the appropriate data for a range of four-pole high voltage motors.
Example. Consider a 630 kW motor and a 80% standing load.

|Z g |= 0.25 pu
0.2508
|Z m |= 0.1713 × = 2.2907 pu
747.81
Z l = 1.25(0.9 + j0.436) =|1.25| pu

|Z l + Z g |=|j0.25 + 1.25(0.9 + j0.436)|= |1.457| pu
Therefore
0.25 × 1.25
| V |= ×100 = 9.363%
1.457 × 2.2907