Page 12 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH001.3D ± 4 ± [1±30/30] 17.11.2001 9:43AM

4 Electrical power systems ± an overview

systems (FACTS) and Custom Power research (Hingorani and Gyugyi, 2000). In
high-voltage transmission, the most promising equipment is: the STATCOM, the
unified power flow controller (UPFC) and the HVDC Light. At the low-voltage
distribution level, the VSC provides the basis for the distribution STATCOM
(D-STATCOM), the dynamic voltage restorer (DVR), the power factor corrector
(PFC) and active filters.

1.3 General composition of the power network

For most practical purposes, the electrical power network may be divided into four
parts, namely generation, transmission, distribution and utilization. The four parts
are illustrated in Figure 1.1.
This figure gives the one-line diagram of a power network where two transmission
levels are observed, namely 400 kV and 132 kV. An expanded view of one of the
generators feeding into the high-voltage transmission network is used to indicate that
the generating plant consists of three-phase synchronous generators driven by either
hydro or steam turbines. Similarly, an expanded view of one of the load points is used
to indicate the composition of the distribution system, where voltage levels are
shown, i.e. 33 kV, 11 kV, 415 V and 240 V. Within the context of this illustration,
industrial consumers would be supplied with three-phase electricity at 11 kV and
domestic users with single-phase electricity at 240 V.
Figure 1.1 also gives examples of power electronics-based plant components and
where they might be installed in the electrical power network. In high-voltage
transmission systems, a TCSC may be used to reduce the electrical length of long
transmission lines, increasing power transfers and stability margins. An HVDC link
may be used for the purpose of long distance, bulk power transmission. An SVC or a
STATCOM may be used to provide reactive power support at a network location far
away from synchronous generators. At the distribution level, e.g. 33 kV and 11 kV, a
D-STATCOM may be used to provide voltage magnitude support, power factor
improvement and harmonic cancellation. The interfacing of embedded DC genera-
tors, such as fuel cells, with the AC distribution system would require a thyristor-
based converter or a VSC.
Also, a distinction should be drawn between conventional, large generators, e.g.
hydro, nuclear and coal, feeding directly into the high-voltage transmission, and the
small size generators, e.g. wind, biomass, micro-gas, micro-hydro, fuel cells and
photovoltaics, embedded into the distribution system. In general, embedded gener-
ation is seen as an environmentally sound way of generating electricity, with some
generators using free, renewable energy from nature as a primary energy resource,
e.g. wind, solar, micro-hydro and wave. Other embedded generators use non-renew-
able resources, but still environmentally benign, primary energy such as oxygen and
gas. Diesel generators are an example of non-renewable, non-environmentally
friendly embedded generation.