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SOLUT IONS
— Adaptive Protections The Tmax XT adds a dual setting capability to the switching device to ensure continuous coordination
Application example A plant is connected to the MV utility by means of a MV/LV transformer. If the utility shuts down, the plant will become a microgrid supplied by a local generator G, which will feed priority loads by using the load shedding feature of the Tmax XT. In a grid-connected condition, the generator G is disconnected. With reference to Fig. 1:
Purpose User’s plants can work as a LV microgrid thanks to the energy produced by renewable and local power sources, in particular as a consequence of the lack of an utility power supply, e.g. due to a fault on the MV voltage side. In order to still guarantee a high level of selectivity and continuity of service, it is im- portant to take into account the variation of the short-circuit power when moving from on-grid to off-grid operation. Indeed, during grid connected conditions the fault current on a microgrid feeder is also supplied by the utility, thus resulting higher than the one sup- plied only by local generation during islanded con- ditions. As a result, it is desirable that several pro- tection thresholds of the units can be automatically changed during the transition to islanding condi- tions.
• Circuit-breaker A is closed • Circuit-breaker B is open
• Circuit-breakers at position C are closed. The pro- tection of the circuit-breaker at C that supplies the feeders at D are adjusted using "Set A" of the Tmax XT unit. • Circuit-breakers at position D are closed • Circuit-breaker E is closed • Molded case switch QS1 is closed • All loads are supplied. The circuit-breakers at position C are selectively co- ordinated with the upstream main circuit-breaker A, supplied by the utility, and the downstream load circuit-breakers at position D (see Fig. 2 at the fol- lowing page).
U
G
A
B
E
C
C
C
non priority loads
QS1
D
D
D
— Fig. 1
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