Power system fundamentals - Wind Power Integration: Connection and System Operational Aspects - page 44

2.5 Power transmission

The basic process of power transmission is that current flows round a loop consisting
of source, line and load. The current flow is impeded by line resistance and loop
inductance. There is also capacitance between the lines. Thus the line parameters are:

Series resistance

●

Series inductance

●

Shunt capacitance.

●

In the power transmission relevant to wind farm connection, the significant
parameters are overhead line resistance and inductance. Shunt capacitance cannot
be ignored for underground cables. However, the capacitive effect of cabling within
a wind farm is small, and will be ignored here.

2.5.1 Line parameters
The following discussion will be based on single-phase power transmission. This
simplification is justified here - our objectives are merely to estimate resistance
and inductance at a given voltage level and, in particular, to appreciate the factors
that determine their relative magnitudes. The extension to three-phase transmission
is fairly intuitive. A sound derivation of line parameters from the line's physical
dimensions may be found in the well-established power systems textbook by
Grainger and Stevenson (1994).

2.5.1.1 Line resistance
Consider the single-phase line shown in Figure 2.23, consisting of parallel con-
ductors of length l and cross-sectional area a in air. The resistance of each con-
ductor is R ¼r l / a where r is the conductor resistivity. The resistance is often
quoted per unit length:

R ¼ r= a ð 2 : 19 Þ

2.5.1.2 Line inductance
The line loop inductance may be found by considering the flux through the ele-
mental rectangle shown in Figure 2.23, co-planar with the conductors, x from
conductor A and of width d x . The total flux linking the current path may then be

A B

x d x

d

r

Figure 2.23 Single-phase line