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Unformatted text preview: ower stations can either supply electricity directly to customers, or through the transmission network. 39 In South Africa a number of municipalities and large customers have their own power stations. The end-use load profiles can change significantly, depending on how those power stations are operated. The utility’s generation pattern (a schedule of the maximum power station capacities and imports from neighbouring countries available in the following years) have to be studied to ensure the power stations are included in the data files for power flow studies. Furthermore, a number of other aspects have to be considered. The availability of water to hydro-electric power stations depends on droughts and irrigation quotas. Low fuel reserves at power stations can also reduce the expected net sent outs. It also has to be ensured that the new power stations planned for the future are modelled and the old power stations to be decommissioned are excluded. The different generator base loads (minimum generating conditions - based technical specifications and design criteria) have to be documented. The net sent from each unit is determined by: Net Sent Out = Base + (Max − Base)* p (3.2.3) where Base minimum generating load depending on the technical specifications and design criteria. Max maximum unit rating p A percentage depending on the generation cost From equation (3.2.3), when p equals 0 %, then the net pow er output equals the base load and when p equals 100 %, then the net power output equals the maximum unit rating. In most cases power stations with high generation costs have lower p values. For a power station, equation (3.2.3) becomes Net Sent Out = number of units ∑ i =1 Basei + (Maxi − Basei )* pi (3.2.4) 40 Network constraints have limitations on the net power outputs from power stations and therefore it is important that all network topology changes are correctly modelled. In the early stages of the research, chaos theory was been studied to predict the generation pattern. The study was stopped, because it is not required for network studies as mentioned above. Chaos theory is about explaining apparent disorder in a very ordered way. Chaos theory states that things are not really random, just complex. Some events that look random can be represented by a simple computation. After a number of iterations, the simple computation will then produce the required complex results. [12 – 16] 3.2.3 Power Stations Any power station requires some auxiliary services to generate electricity. Some of those auxiliary services are located outside the power station’s main building. The electrical auxiliary supplies are normally connected to the station’s electrical boards, which are again electrically connected to the unit boards inside the power station’s main building. In most cases the station transformer supplies all the electricity required by the station’s electrical boards. The high voltage side of the station transformer is either connected to a distribution substation, or to a transmission substation. The power station loads supplied from the transmission network are classified as transmission loads. The utility has two pumped-storage power stations. A pumped-storage power station generates during peak conditions (as hydro electric power station) and pumps water to its storage dams during off-peak conditions. The two pumped -storage power stations generate at the time of maximum system demand and will therefore not take the pumping loads into account. The total power station loads are small compared to the system peak of 27 MW (see Chapter 5). 41 3.2.4 Transmission Losses The electrical losses in an electrical network is defined as P loss = I2R where I is the root mean square (rms) current in the conductors and R is the ac resistance of the conductors. The losses are modelled as 3.2 % of the maximum system load see (Chapter 5). [17] To determine the exact electrical losses will complicate the balancing algorithm and slow down the computational time. The figure assumed for electrical losses can change for different generation patterns, or large loads commissioned far from the power pool, or when new transmission lines are built, etc. The question that always remains is what is more important: a feasible solution (not the best, but acceptable for the purpose), or an optimal solution (the best but maybe long computational times). Computational time is of essence as long as the results are feasible. This principle is followed throughout this chapter and in Chapter 4. The rule of thumb (3.2 % of the system peak) is therefore acceptable and will be adjusted if necessary. 3.2.5 International Customers The end-use customers outside the South African borders are called Internatio nal Customers. Although small compared to the maximum system load, approximately four percent (Chapter 5) are very difficult to determine and to allocate to a supply point in the transmission network. For example, the power flow on a 400 kV line can be...
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