THE ELECTRICITY SUPPLY SYSTEM ASPECT

Technical issue

Electrical grid

National electricity supply system is a complex formation to ensure electricity to everyone in the country who actually need it. Power plants and electrical grid are present in the system. Generation and distribution of electricity are part of the market and they are subjects of trades. But electricity is a good without any possibilities to store. Due to this fact every hour generation and consumption have to be balanced. On this purpose both the energy market and electricity supply system have mechanism to adjust to specificity of the electricity. The energy market is a financial matter and its operation for nuclear power plant is not different than for fossil fuels, because item of a trade is still electricity. But in case of electricity supply system, nuclear power plant can caused additional, restrictions and requirements that must be fulfilled.

Nuclear power plant means very high power concentration in one unit. For example gen III+ reactor being under construction in Finland and France has power capacity of 1600 MWe. Assuming its operation in national electricity supply system, it determines the system to ensure every means to keep it online. One of the measures are primary, secondary regulation power and spare capacity reconstruction power. Capacities for regulations and reconstructions are provided by additional units. Those units generate electricity when electricity supply system imbalance occurs or when any basic power plant falls out or is not able to produce more electricity for the moment (when voltage or frequency in the system is changing out of range). To conclude: additional regulation units are to keep all system working properly and to prevent system from blackout. Generation from those units and theirs maintenance are expensive. Although power capacity of units operating in the regulation or in the stand-by mode are limited theirs capacity should be at least as high as the largest single unit operating in the electricity supply system. In case nuclear power plant with the reactor of 1600 MWe, system regulation and reserves should be able to cover gap when the reactor falls out. And this is serious problem, especially for small systems. In Poland regulation and reserve capacities are not high enough for this moment. Adjusting this requirement entails additional significant costs. [69], [70]

The European UCTE has defined electricity supply system susceptibility factor, which describes what is response to disturbance occurring in the system. The factor for all UCTE system is 18000 MW/Hz. Disturbance of 18000 MW power change, causes change of frequency of 1 Hz. Sudden and significant frequency drop is very dangerous and can lead to avalanche frequency and as a consequence to system blackout. In the view of all Europe, high power nuclear power plant is not a problem, but the same factor for Poland is 1091 MW/Hz, what suggest than Poland should not have single units with power capacity above 1091 MW. [69]

Small electricity supply system is not suitable for nuclear power plant also because of small amount of energy generated. Then nuclear power plant will reduce flexibility of the system and it is unacceptable. For Poland, with current electricity demand only one unit will fit the system (according to [71] 3200 MWe unit). As a consequence it can lead to limited number of reactors in Poland, but it deteriorates economics of nuclear power plant. Costs of the first reactor in country without nuclear experience are average 30% higher. [71]

Nuclear power plants reduce level of energy security of consumers due to less reliability caused by high power concentration in one nodal point. Such condition increases blackout risk. [71]

Nuclear power plant have to be protected from sudden disconnection from the system, because then the reactor must be intensively cooled by other system. During normal operation it is cooled by working medium in the primary circuit. When system connection is cut, the turbine can not be driven. Steam drop is possible but only at emergency, because it generates losses, and if the reactor is one circuit only, like BWR is, it will cause radioactive emission. The problem is to solve by system similar to after-heat removal system. However such system need very large amount of water. However the most preferred solution is to ensure large power take off for electricity generation structure of nuclear power plant, like large engines or pumps. If these devices operates in pumped-storage water power plant, it is perfect, because pumped water accumulates the energy, that can be used in peak load. Ensuring large power take off brings next significant costs. [72]

Every element in electrical grid (power plants, substations with transformers, power lines etc. ) have to fulfil n-1 criterion. Falling out any single element should not endanger electricity supply system operation. Elements working with nuclear power plant have to fulfil n-2 criterion. Power lines from nuclear power plant have to be doubled, and adequate, strong grid is necessary, preferably at high voltage (400 kV). In Poland establishing network infrastructure is a challenge and increases total nuclear power plant investment cost. [73] [74]

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