Environmental Engineering Reference
In-Depth Information
network. If the component were to inject or extract active power at the node, the node voltage
angle will shift to a more leading or lagging position respectively with respect to the voltages
of the adjacent nodes. If the component were to inject or extract reactive power at the node,
the node voltage magnitude will increase or decrease respectively compared to its original
value. It may be concluded that if a fl at network voltage profi le needs to be maintained, the
high voltage can be depressed or the low voltage augmented at a node by connecting induc-
tors or capacitors to that node respectively.
5.5 Reactive Power Management
The principle of conservation of reactive power described in Section A.13 requires that reac-
tive power is scheduled so that as the consumer demand of
Q
varies over a day, appropriate
Q
resources are available to provide it. In Chapter 3 it was explained that a discrepancy
between demand and supply of active power is manifested by rises or falls in frequency, a
parameter that is common to the whole interconnected power system. In contrast, reactive
power defi cits or excesses manifest themselves as local system voltage drops or rises respec-
tively. An overall defi cit of reactive power in a system will manifest itself as unacceptably
drooping voltage profi les. The reverse is the case for a surfeit of reactive power.
Ideally, reactive power should not be transported but generated locally where it is required.
This, of course, is the principle of power factor correction of consumer loads, an arrangement
described in Section A.14 and encouraged by utilities through tariffs. However, the vast
majority of domestic and commercial loads as well as some industrial loads are not power
factor corrected. Hence substantial amounts of reactive power have to be transported over
the system and/or generated locally by special utility equipment. The picture is complicated
by the fact that:
(a) High voltage overhead transmission lines overall generate
Q
when they are lightly loaded
due to their shunt capacitance, but absorb
Q
when heavily loaded due to their series
inductive reactance (Figure 5.8).
(b) Low voltage transmission lines have small shunt capacitance and overall are consumers
of
Q
.
(c)
Cables overall generate
Q
at all loads because of their substantial shunt capacitance.
(d)
Transformers always absorb
Q
as they consist basically of two inductively coupled coils
(Section 4.4.2 ).
5.5.1 Reactive Power Compensation Equipment
Utilities use the following devices to balance the
Q
fl ows in the network and therefore control
the bus voltages:
(a)
transformer tap changers;
(b)
synchronous generator AVRs;
(c)
shunt capacitors;
(d)
shunt reactors;
