Environmental Engineering Reference
In-Depth Information
10%) relies heavily
on the tap changer at the primary substation that feeds the 11 kV network. Because of its
crucial importance this tap changer is activated through an automatic voltage controller
(AVC). The allowable voltage range,
Thus, delivery of the correct voltage to domestic customers (230 V
±
10%, may seem generous, but in practice it is often
fully exploited, to allow for load variation between summer and winter.
±
(b) AVR s
The internal source voltage in the stator of a synchronous generator (Figure 4.7) can be con-
trolled by controlling the DC current (the excitation) in the fi eld winding on the rotor. This
control is often achieved by an automatic voltage regulator (AVR). This is an electronic
controller that monitors the voltage at the synchronous generator terminals, compares it to a
set value and if there is an error increases or decreases the excitation to nullify it. In the case
of a standalone synchronous generator, the AVR's role is straightforward: it monitors and
controls the terminal voltage.
Large conventionally fuelled generator units are connected to the grid through a
' unit ' transformer fi tted with a tap changer. Most auxiliaries that supply services to the
generator are connected to the generator terminals and require a fi xed voltage to provide the
appropriate service. The task of the AVR here is to maintain the generator terminal voltage
constant.
The level of reactive power injected to the grid by a generator connected to the grid through
a transmission line is controlled by the unit transformer tap changer as follows. By increasing
the nominal transformer ratio of, say, 22 kV : 400 kV by tapping - up, the transformer secondary
voltage increases with respect to the remote end voltage of the transmission line and more
reactive power is transported down the line. This extra reactive power fl ow through the syn-
chronous reactance of the generator results in a reduction of its terminal voltage. This is
sensed by the AVR which increases the excitation of the generator (hence the injection of
extra reactive power) to bring the voltage up to the nominal level.
Generators fed from RE sources are substantially smaller in rating and are, in general,
connected to the distribution rather than the transmission network. The effect on the system
of such generators is discussed in Chapter 6. However, if the RE source is truly large, for
example a large offshore wind farm or a tidal scheme, it could have an impact on the network.
In such cases, more conventional control schemes may have to be used. In fact, as penetration
levels by wind power increase, for example in Denmark where levels of 20% are no longer
uncommon, utilities require renewable generators to be designed to contribute positively to
power system voltage control.
(c), (d), (e) Static Compensators
At the transmission network, switchable capacitors and/or inductors are connected at crucial
nodes of the system to regulate the voltage under varying loading conditions. These devices
are controlled in steps and in the case of inductors are particularly lossy. For fi ner control,
static VAR compensators (SVCs) are used. These employ solid state power electronic devices
rather than mechanical switches to provide infi nitely variable control of
Q
from reactors and
capacitors.
