Environmental Engineering Reference
In-Depth Information
Table 5.1
Transmission line parameters
k V
Typical X / R ratio
400
16
275
10
132
6
33
2
11
1.5
at one end of the line compared to the other, for a given active and reactive power transfer
over the line.
Moreover, the same equation can be used to estimate the voltage change that will occur at
the point of common coupling (PCC) when a new generator (or load) is connected to a system.
In this case, R and X need to represent the Thévenin equivalent impedance (Appendix) of the
system as a whole, but more on this later.
A second approximation is applicable in transmission networks and the higher layers of
distribution where, due to the geometry and size of the conductors, the impendence Z is
dominated by the reactance X . Table 5.1 shows typical X / R ratios for high to low voltage
overhead transmission lines. When the X / R ratio is high, R = 0 in Equation (5.6) with little
loss in accuracy. This gives the very useful rule of thumb:
(5.7)
Δ
VQ
In Figure 5.10 it can be seen that the angular displacement between V A and V B is to a large
extent proportional to the imaginary component of the V AB phasor. The imaginary part of
Equation (5.5) gives a second rule of thumb:
P
(5.8)
δ ∝
where
is the load angle between V A and V B , the same angle that was encountered in Chapter
4 when the operation of the synchronous generator was under study. Equations (5.8) and (5.7)
match Equations (4.8a) and (4.8b) .
These two relationships provide an important insight into the fl ow of complex power in
transmission networks where X
δ
>>
R :
Network voltages are determined largely by reactive power fl ow. Turning round this state-
ment it can be affi rmed that voltage magnitude differences between the ends of a transmis-
sion line is the primary driver of reactive power fl ow.
Phase angles are determined largely by active power fl ow and are largely independent of
reactive power fl ows. Turning round this statement as well it can be affi rmed that angular
differences between the voltages at adjacent nodes is the primary driver of active power
fl ow.
These insights can be extended further. Consider that a power system component, capable of
absorbing or injecting active or reactive power, is connected to a node of the transmission
 
Search WWH ::




Custom Search