INCREASING OF THE CAPACITY OF LOW-VOLTAGE NETWORKS WITH HIGHER EXPLOITATION OF THEIR COMPONENTS INCREASING OF THE CAPACITY OF LOW-VOLTAGE NETWORKS WITH HIGHER EXPLOITATION OF THEIR COMPONENTS

ABSTRACT Problems related to the increased exploitation of lowvoltage networks are in every distribution company the subject of constant attention, above all with regard to the liberalisation of the electricity market. This paper's proposal deals with the following problem solving: -Increasing system reliability, -Decreasing operational costs, -Higher exploitation of network components, -System monitoring of the supply points in the network, -Targeted and optimised maintenance. The authors will attempt to solve the following partial problems: a) Maximal exploitation of transformers thanks to better and more precise protection using fuses with special characteristics for the full exploitation and parallel operation of networks makes possible the adjusting the optimal transformer capacity. b) New configurations and new ways of operation networks; parallel operation with weak couplings. c) Connecting dispersed generators with regard to low-power losses and the voltage profile in lowvoltage network (individual contributions) d) Co-ordination of system voltage control in distribution systems with regard to full exploiting voltage tolerance in low-voltage networks. This paper contains examples of realisation of provisions in distribution company and evaluations of individual contributions. This study is finished and particular rules started in operation. When planning the capacity of distribution transformers, HV/LV designers conform to maximal loading in low-voltage networks. The transformers designed this way have in operation small exploitation. Network monitoring is necessary the paper deals with the methods of protection of distribution transformers. The course of the distribution transformer loading is characterised by the maximum in the winter season functioned with electrical heating. To specify the overloading time we modify the actual daily diagram on the two level loading diagram in accordance with CSN IEC 354. The time of the overloading is determined to four hours. The maximal exploitation of transformer thanks to better and more protection using fuses with special characteristics for full exploitation and parallel operation of networks make it possible to adjust the optimal transformer capacity. The target of dimensioning studies is to find the fixed design policies in order to construct simplified technoeconometric models for low-voltage system. These models have as inputs network configurations, load models, any technical constraints, cost models for components, and models for supply interruptions. Results from this model-based study are used to determine the most appropriate electricity supply arrangements, optimal sizes of transformers, optimal standard sizes of cables and distribution lines. The most common network configuration is partially looped network. Under normal conditions, the network can be operated as a radial arrangement with a high degree of reliability. New configurations use under normal conditions totally closed-loop arrangement and for case of a fault is a system disconnected by a weak coupling fuses to radial network. This net configuration has a simple protection system and is inexpensive to recontruct while maintaining all the good qualities of a classical mesh grid. Its good qualities include a low voltage drop, high reliability, and high quality of electricity supply. In the case of old systems with relatively small loads we can use the co-ordination of voltage control in the MV system and changes of the tap on distribution transformers for keeping operation voltage in LV system within the voltage limits given by standards. Generating units connected to LV networks can permanently or temporarily inject power into the grid. Inversely, a client who is also a producer can absorb power at any time. The choice that he makes between these two operating modes depend on the economical approach he chooses for his electricity generating plant as well as for his heat producing unit. His analysis usually takes into account gas sales, electricity purchases and sales prices and his proper needs. The voltage profile along a LV feeder depends on the balance between the power absorbed from or injected into the system by clients and producers and the other clients' power demand. We must seek the optimal point of connection with the network to improve the voltage profile in the LV network without the construction constructing new (MV/LV transformer stations). Losses in low-voltage networks depend mainly on the load and the line material. Every generator affects load flow and loss characteristics. INCREASING OF THE CAPACITY OF LOW-VOLTAGE NETWORKS WITH HIGHER EXPLOITATION OF THEIR COMPONENT