Environmental Engineering Reference
In-Depth Information
larger than the inductive
Q
, reactive power is supplied to the grid. In the fi rst case the
L - C
combination is a net consumer of
Q
, in the second case it is a net generator.
These considerations are summarized by the following equations, which apply to any node,
bounded portion or the totality of a power system network under steady state conditions:
∑
P
−
P
−
P
=
0
⇒=
P
0
entering
leaving
loss
(A.29)
∑
QQQ
−
−
=
0
Q
=
0
entering
leaving
loss
The
P
loss
and
Q
loss
refer to any consumption of
P
and
Q
in the transmission system (see the
next section).
In Chapter 5, it is shown that the concepts underlying Equations (A.29) are vital in the
calculation of power fl ows in power system networks.
A.14 Effects of Reactive Power Flow - Power Factor Correction
Figure A.21 represents a basic circuit of energy transportation from a generator to a consumer
through a transmission line, which is simply represented by a series inductance and resistance.
The consumer is represented by an inductive resistive impedance. The reason for these rep-
resentations are explained in Chapter 5.
The consumer ' absorbs ' both
P
L
and
Q
L
. The generator or 'the utility' has to supply both
the
P
L
of the load and the transmission loss given by
I
2
R
t
, where
R
t
is the resistance of the
transmission line. It also has to supply the
Q
L
of the load plus the
I
2
X
t
absorbed by the reac-
tance of the transmission line. The transmission loss is therefore strongly infl uenced by the
transfer of
Q
L
.
To appreciate this, assume that the load consists solely of an inductor, which absorbs
Q
but not
P
. In this case the energy meter at the consumer premises, which records kilowatt-
hours (i.e. energy purchased), indicates zero but the fi nite current in the transmission system
results in
I
2
R
t
, which has to be supplied by the utility. Clearly this is a most undesirable
scenario which the utility endeavours to discourage, in the case of large consumers, through
special tariffs that penalize the absorption of
Q
.
a
b
X
t
R
t
Q
C
I
P
L
X
L
V
L
C
Q
L
R
L
Generator
Transmission Line
Consumer (load)
a
b
Figure A.21
Simple transmission system
