WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–24
Example
Calculate the voltage drop on a dc system where we have two 250 kcmil aluminum conductors
carrying 300 A to a load 200 ft away. The input voltage is 480 V. What is the voltage drop and power
loss?
From Table 2 in Appendix C, the resistance of 1000 ft of 250 kcmil aluminum conductor is 0.068
Ω. We have current flow through a total length of 400 ft (200 ft down and 200 ft back) with resistance
The voltage drop is
R = 400 (0.068) = 0.0272 Ω
1000
V drop = IR = 300(0.0272) = 8.16 V
The power loss in the line is
P loss = I 2 R = (300) 2 (0.0272) = 2448 W
The voltage drop is less because of the lack of inductive reactance, but not substantially less. The
drop due to resistance alone would not be a bad first estimate if Table 9.10 were not available.
7 LOSSES
Voltage drop (IZ) and resistive loss (I 2 R) are closely related concepts, but present two different
types of constraints. Voltage drop is a technical constraint. We want the voltage at the
wall receptacle to be between 114 and 122 V, for example, so the light bulbs have the proper
intensity and the electrical appliances work correctly. In the windfarm environment it would
not be hard to design for a voltage drop up to 10%, so this is not a significant constraint.
On the other hand, resistive loss is an economic constraint, at least for wire sizes adequate
to carry the desire current. The economic goal of a windfarm design is to minimize the ratio
of capitol cost to net energy production as measured at the windfarm boundary. The gross
energy production (the total energy produced by the turbines before losses are considered) is
a function of the skill of the turbine designer and of the wind resource at a particular site.
Once the turbine and site have been selected, the windfarm designer still has to select wire
sizes and other factors to minimize the cost per kWh delivered to the utility.
For example, a given turbine is rated at 220 A. The low voltage wire must have an ampacity
of 1.15(220) = 253 A, which is met by AWG 3/0. A larger wire size would be selected for
economic rather than technical reasons. A larger wire size increases the windfarm cost but,
by reducing the losses, also increases the energy supplied to the utility. For the wire costs
given earlier and for a typical windfarm layout, the cost per kWh hits its minimum at a wire
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001