**3. Switching to the backup power supply, in the case of low-voltage symmetrical smart microgrids in the modern residential buildings**

In the case of modern residential buildings, the automatic switching between the three-phase public grid and an electric generator can be achieved by the automatic changeover source, with automatic transfer switch (ATS) function, located in the low-voltage electrical station, **Figure 6**. The microgrid is symmetrical and uses two identically transformers, T1 and T2, one of the transformers being alternately in hot reserve state [1]. The users are divided into two categories: critical (b) and noncritical ones (a), **Figure 6**. When the users are supplied from the public grid by using one of the transformers, T1 or T2, correspondingly, one of the circuit breakers MP1 or MP2 is closed, while the "C" couple is also closed. The two circuit breakers (TN) NS250 types are closed, and the other two circuit breakers (TS) NS250 types are open so that the backup power supply path is maintained disconnected. In this case, all users are supplied, both (a) and (b) categories, from the three-phase public grid as shown in **Figure 6**. In a low-voltage grid, two distinct situations can occur:

The minimum voltage coils of the "MP1" or the "MP2" circuit breakers (as the case may be) and of the "C" couple, respectively, **Figure 6**, can determine their triggering, if the voltage drops well below the nominal limit. It achieves disconnecting the normal power supply path (the public grid), and for a time tATS, both consumers in group (a) as well as consumers of the group (b) will not be powered. Note that the ATS facility is composed in this case of two systems, "automatic changeover source" type, **Figure 6**. The ATS function of the automatic changeover source commands the two (TN) NS 250 circuit breakers, by opening them, and the other two (TS) NS 250, by closing them. Another important aspect is the existence of the "automatic changeover group" type, **Figure 6**, which is absolutely necessary for triggering a delay timing, tGROUP in starting the M1 electric power generator and the automatic closing of the KM 19 switch, only after the transient phenomena has been removed and a permanent regime of the electric generator has been set. Obviously tATS < tGROUP. The electric generator's services must be ensured at all times. If during timeout tATS the voltage on the bars is not found (allowing the ATS to operate), the electric power generator will be started. After the expiration of time tGROUP, the automatic closing of the KM 19 switch is achieved, the M1 electric power generator injects electric power on the base station bars and supplies power only the (b) group users, referred in **Figure 6** as safety power and emergency lighting. When the tension on bars returns, after "delay timing" tSTOP has been passing, the following processes take place: firstly, the automatic changeover group command stops of the M1 electric power generator and then the automatic opening of the KM 19 switch is achieved. Obviously tATS < tSTOP. Next, the ATS function of the automatic changeover source commands the two (TN) NS 250 circuit breakers, by closing them, and the other two (TS) NS 250, by opening them. Finally, closing the breaker MP1 or MP2 (as the case may be) and of the "C"

Unconventional Backup Structures Used in Smart Microgrids

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couple, respectively, it can also be done automatically or manually.

**photovoltaic panels**

from this grid.

**4. Switching to the backup power supply, in the case of a lowvoltage smart microgrids using an electric generator set and the** 

taic panels, the functions that are provided by the ATS regard the following:

In the case of modern residential buildings that use an electric generator set and the photovol-

**a.** When the three-phase public grid is available and connected, all the users are powered

**b.** When the three-phase public grid is not available, the electric generator will be connected.

**c.** When the three-phase public grid and the electric generator set are not available, the criti-

The automatic switching is necessary between the two types of users, critical and noncritical ones to the available sources, in this case the three-phase public grid, an electric generator set,

Both critical and noncritical users will be powered by the electric generator.

cal users are supplied by the photovoltaic panels' energy generation.

and photovoltaic panels that are shown in **Figure 7**.

**Figure 6.** Single-line diagram of force for switching to the backup power supply in the case of a low-voltage symmetrical grid using two transformers and an electric generator [1].


For the ATS facility to distinguish between these two situations, it must meet the following conditions simultaneously:


When the condition of minimum voltage on the station's bars is not satisfied in accordance to the abovementioned conditions, (a)–(d), the power supply is ensured by the M1 electric generator, **Figure 6**.

The minimum voltage coils of the "MP1" or the "MP2" circuit breakers (as the case may be) and of the "C" couple, respectively, **Figure 6**, can determine their triggering, if the voltage drops well below the nominal limit. It achieves disconnecting the normal power supply path (the public grid), and for a time tATS, both consumers in group (a) as well as consumers of the group (b) will not be powered. Note that the ATS facility is composed in this case of two systems, "automatic changeover source" type, **Figure 6**. The ATS function of the automatic changeover source commands the two (TN) NS 250 circuit breakers, by opening them, and the other two (TS) NS 250, by closing them. Another important aspect is the existence of the "automatic changeover group" type, **Figure 6**, which is absolutely necessary for triggering a delay timing, tGROUP in starting the M1 electric power generator and the automatic closing of the KM 19 switch, only after the transient phenomena has been removed and a permanent regime of the electric generator has been set. Obviously tATS < tGROUP. The electric generator's services must be ensured at all times. If during timeout tATS the voltage on the bars is not found (allowing the ATS to operate), the electric power generator will be started. After the expiration of time tGROUP, the automatic closing of the KM 19 switch is achieved, the M1 electric power generator injects electric power on the base station bars and supplies power only the (b) group users, referred in **Figure 6** as safety power and emergency lighting. When the tension on bars returns, after "delay timing" tSTOP has been passing, the following processes take place: firstly, the automatic changeover group command stops of the M1 electric power generator and then the automatic opening of the KM 19 switch is achieved. Obviously tATS < tSTOP. Next, the ATS function of the automatic changeover source commands the two (TN) NS 250 circuit breakers, by closing them, and the other two (TS) NS 250, by opening them. Finally, closing the breaker MP1 or MP2 (as the case may be) and of the "C" couple, respectively, it can also be done automatically or manually.
