Directional Zone Selectivity in Low Voltage Networks
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- Institutt for elkraftteknikk 
In low voltage networks, radial network structure with circuit breakers in series and time-current selectivity are quite common to deal with the challenge of selectivity. This is an easy and well-known technique that works well for both short circuit and overload protection. In some electric installations it can be desirable to use ring or meshed network structures and in these situations the traditional time-current selectivity is not particularly suitable to use. ABB has developed the Emax 2 circuit breaker series communicating over bus or Ethernet which uses directional zone selectivity. This is a method that should be possible to use in both ring and meshed networks to achieve selectivity. The objective in this thesis was to simulate and analyze low voltage networks with directional zone selectivity and to consider different aspects of this selectivity technique. As a part of this the communication ability for those circuit breakers was reviewed to find possible solutions for remote control in overload situations. For this thesis, simulations of a hypothetical radial, ring and meshed network were performed. The traditional time-current selectivity method was used for the radial network, while directional zone selectivity was used as the selectivity method in the ring and the meshed network. Simulations of a hypothetical grid installation formed as a ring network in sparsely populated area with overhead lines and double feeding source were also performed. Simulations showed that it was possible to achieve selectivity in ring and meshed networks with the use of directional zone selectivity. When these network types and directional zone selectivity were combined, higher reliability in the installation was achieved. Because of the circuit breakers characteristic, it was most appropriate to use directional zone selectivity in the main distribution system of an electric installation or in the grid. To plan and design an installation with directional zone selectivity was experienced as more demanding compared to the traditional time-current selectivity. The complexity of the directional zone selectivity was increased with an increasing number of circuit breakers and the size of the electric installation. Currently there are few facilities that use directional zone selectivity, which means that experience with this selectivity method is limited. It is concluded that directional zone selectivity can be a possible solution for future use in low voltage networks, both for electric installations and in the grid. It is especially suitable for use in large electric installations at the main distribution level for electric power when ring or meshed network structures are being considered. The communication ability adds a new dimension for control of low voltage networks where an external control device can be used for remote load shedding in overload situations, remotely read out of measurements from the circuit breakers or be programmed with advanced customized functions.