General composition of the power network

For most technical purposes, the electrical power system may be divided into four parts, namely generation, transmission, distribution and utilization. The four parts are illustrated in Figure 1. 1.

This figure provides the one-line diagram of a power network where two transmission stages are observed, namely 400kV and 132kV. An expanded view of one of the generators feeding into the high-voltage transmission network is used to indicate that the generating plant involves three-phase synchronous generators based on either hydro or steam turbines. Likewise, 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 representation, industrial consumers would be provided 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 generators, 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 generation 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-renewable 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.

Fig. 1.1 Power network.

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