Page 566 - Handbook of Electrical Engineering

Appendix G

Worked Example for the Calculation

of Volt-drop in a Circuit Containing

an Induction Motor

G.1 INTRODUCTION

The following example explains how volt-drop calculations can be carried out. Initially the subject
is approached from a rigorous standpoint. Subsequently various simplifications are introduced, their
results compared and their appropriateness discussed. The calculation sequence is:-

• Rigorous solution, see a) to p).
• Simplified solution, see q) to t).

• Formulae method based on kVA ratings, see u).
• Graphical estimation, see v).

Figure G.1 is a simplified one-line diagram of a power generation and distribution system
that would be suitable for most oil industry power systems that have their own power generating
plants, e.g. an off-shore production platform. Figure G.2 is the equivalent diagram showing the basic
symbols and configuration needed for the volt-drop calculation process. The example data are given
below:-

i) Generator data.
3 generators, each rated at 3.125 MVA at 0.8 PF lagging.
Rated voltage = 13.8kV, X d = 25%, R a = 2%.

ii) Switchboard data.
Rated voltage = 13.8kV
Standing load = 900 kW at 0.9 PF lagging (cos φ og ).

iii) Transformer data.
Rated at 3.15 MVA.
Rated voltage ratio = 13.2: 4 kV. X tpu = 6%, R tpu = 0.7%.

Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry. Alan L. Sheldrake
 2003 John Wiley & Sons, Ltd ISBN: 0-471-49631-6