Page 19 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH001.3D ± 11 ± [1±30/30] 17.11.2001 9:43AM

Power electronic control in electrical systems 11

. as step-up transformers to increase the operating voltage from generating levels to
transmission levels;
. as step-down transformers to decrease the operating voltage from transmission
levels to utilization levels;
. as control devices to redirect power flows and to modulate voltage magnitude at a
specific point of the network;
. as `interfaces' between power electronics equipment and the transmission net-
work.
For most practical purposes, power transformers may be seen as consisting of one
or more iron cores and two or three copper windings per phase. The three-phase
windings may be connected in a number of ways, e.g. star±star, star±delta and
delta±delta.
Modern three-phase power transformers use one of the following magnetic core
types: three single-phase units, a three-phase unit with three legs or a three-phase unit
with five legs.
Reactive power equipment is an essential component of the transmission system
(Miller, 1982). It is used for voltage regulation, stability enhancement and for
increasing power transfers. These functions are normally carried out with mechani-
cally controlled shunt and series banks of capacitors and non-linear reactors. How-
ever, when there is an economic and technical justification, the reactive power
support is provided by electronic means as opposed to mechanical means, enabling
near instantaneous control of reactive power, voltage magnitude and transmission
line impedance at the point of compensation.
The well-established SVC and the STATCOM, a more recent development, is the
equipment used to provide reactive power compensation (Hingorani and
Gyugyi, 2000). Figure 1.9 shows a three-phase, delta connected, thyristor-controlled
reactor (TCR) connected to the secondary side of a two-winding, three-legged
transformer. Figure 1.10 shows a similar arrangement but for a three-phase
STATCOM using GTO switches. In lower power applications, IGBT switches may
be used instead.
Although the end function of series capacitors is to provide reactive power to the
compensated transmission line, its role in power system compensation is better
understood as that of a series reactance compensator, which reduces the electrical
length of the line. Figure 1.11(a) illustrates one phase of a mechanically controlled,
series bank of capacitors whereas Figure 1.11(b) illustrates its electronically con-
trolled counterpart (Kinney et al., 1994). It should be pointed out that the latter has
the ability to exert instantaneous active power flow control.
Several other power electronic controllers have been built to provide adaptive
control to key parameters of the power system besides voltage magnitude, reactive
power and transmission line impedance. For instance, the electronic phase shifter is
used to enable instantaneous active power flow control. Nowadays, a single piece of
equipment is capable of controlling voltage magnitude and active and reactive power.
This is the UPFC, the most sophisticated power controller ever built (Gyugyi, 1992).
In its simplest form, the UPFC comprises two back-to-back VSCs, sharing a DC
capacitor. As illustrated in Figure 1.12, one VSC of the UPFC is connected in shunt
and the second VSC is connected in series with the power network.