**2. Switching to the backup power supply, in the case of a lowvoltage symmetrical grid when using two frequency converters, one of which is alternately maintained in cold reserve**

In practice, the situations where electrical power systems need to be fed redundant can be solved through a backup of power supplies. Ensuring backup in this case must follow a logic sequence of power so that the consumer can be powered at any time from a single power source. Also, providing short circuits and overload protection is an important aspect. If an asynchronous motor with nominal active power P = 22 kW is powered by a single frequency converter, FR-A540-22 K Mitsubishi electric type, a low-voltage grid configuration can be used as in **Figure 1**. Switching to the backup power supply, in the case of a low-voltage symmetrical grid when using two frequency converters, one of which is alternately maintained in cold reserve, can only be done by microprogramming the two frequency converters. It is considered a low-voltage grid configuration using two frequency converters, **Figure 2**, FR-A540-22 K Mitsubishi electric type, in which one of the converters is alternatively cold reserve depending on the need. The only consumer is an asynchronous

can no longer satisfy the requirements of the consumers. The following will illustrate, in terms of the reserve supply, some possible topologies related to low-voltage electrical grids. First of all, it is considered the switching to the backup power supply, in the case of a low-voltage symmetrical grid when using two frequency converters, one of which is alternately maintained in cold reserve. Both frequency converters are of the FR-A540-22 K Mitsubishi electric [2] type. The only user is represented by an asynchronous motor with the short-circuited rotor, having the rated active power equal to P = 22 kW. The problem of ensuring continuity in power supply is all the more important if we refer to asynchronous motors powered by frequency converters that are used in applications such as gondolas and cable cars, the action of the water pumps within the drinking water abstraction and treatment plants, and the operation of large power compressors. The use of the methods described in the case of switching to the backup power supply, in the case of a low-voltage symmetrical grid when using two frequency converters, one of which is alternately maintained in cold reserve, could be a fair solution. In the second case, it is considered the switching to the backup power supply, in the case of low-voltage symmetrical smart microgrids, using in the modern residential buildings. This symmetrical low-voltage grid uses two transformers, one of them being alternatively in hot reserve. The users are divided into two categories, normal and safety ones. And for this case, one study on the ATS operation will be achieved. Also for the case of modern residential buildings, the ATS operation will be considered, in the event that they use both electric generator and photovoltaic panels. Finally a study about the implementation of recloser devices for autoconfiguration and automatic connecting/disconnecting decisions, in order to switch to the backup power supply, was initiated. Also, taking into account a series of scientific, technological, or socioeconomic aspects in terms of importance, the implementation of smart power grids is proposed [3]. This smart grids show auto-reconfiguration characteristics, by using recloser devices [4]. A very important aspect of the electrical systems is related to the safety in operation [5], lack of accidents, and extended damages. Starting from the necessity of a safe power supply system, more and more countries choose to use computerized and special telecommunications systems [6]. Moreover, in the field of electricity transmission, a high degree of operation safety is required. The characteristics of each considered microgrid determine a series of specific technical problems [7]. When considering the grid operation criteria and the specific needs of the users, the settings of the smart recloser equipment must be properly configured. Thus, the indication of the short circuits between phases, the indication of the faults between phases and earth, the signaling of defects, the signaling reset, the storage and the effective operation temperature, or the referral mode are of particular importance and may raise impor-

82 Smart Microgrids

tant shares of difficulty, in particular regarding the operation algorithm.

**of which is alternately maintained in cold reserve**

**2. Switching to the backup power supply, in the case of a low-**

**voltage symmetrical grid when using two frequency converters, one** 

In practice, the situations where electrical power systems need to be fed redundant can be solved through a backup of power supplies. Ensuring backup in this case must follow

**Figure 1.** Mitsubishi electric frequency converter FR-A540-22 K type, used to power an asynchronous motor with a short-circuit rotor with nominal active power P = 22 kW.

**Inputs Outputs Allocated** 

IN 1 — b1

**inputs**

, b<sup>2</sup> , b3 **Allocated outputs Input/output account**

fault

fault

, b<sup>4</sup> — 4 × HUPA, mushroom-type buttons NI, operated

Zero—right

Zero—right

inverter

inverter

brake

when faults are produced

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— OUT 1 — START/STOP MSB1 Enable/disable the Mitsubishi 1 inverter — OUT 2 — START/STOP MSB2 Enable/disable the Mitsubishi 2 inverter

— OUT 3 — h1 The signaling lamp 220 V~—Mitsubishi 1 inverter

— OUT 4 — h2 The signaling lamp 220 V~—Mitsubishi 2 inverter

— OUT 5 — C5 Activates the C5 switch coil which controls the brake

— OUT 6 — C1 Activates the C1 switch coil of the force circuit — OUT 7 — C2 Activates the C2 switch coil of the force circuit — OUT 8 — C3 Activates the C3 switch coil of the force circuit — OUT 9 — C4 Activates the C4 switch coil of the force circuit

IN 2 — b<sup>5</sup> — The ND contact—left selector key on the left

IN 3 — b6 — The ND contact—right from the key selector is left

IN 4 — b7 — Commands the state START/STOP of Mitsubishi 1

IN 5 — b8 — Commands the state START/STOP of Mitsubishi 2

IN 6 — b9 — Auxiliary ND of the C5 switch which controls the

IN 7 — b<sup>10</sup> — The pin NI of the heat block from the break circuit

IN 8 — b11 — Auxiliary ND of the C3 switch IN 9 — b<sup>12</sup> — Auxiliary ND of the C4 switch IN 10 — b13 — Auxiliary ND of the C1 switch IN 11 — b<sup>14</sup> — Auxiliary ND of the C2 switch IN 12 — b<sup>15</sup> — Pin for the S1proximity sensor

IN 13 — b16 — Fault removal

**Table 1.** The assignment of the inputs and outputs for microprogrammable automaton.

**Figure 2.** Switching to the backup power supply, in the case of a low-voltage symmetrical grid using two frequency converters managed by a programmable microautomaton [1].

motor with a short-circuit rotor with nominal active power P = 22 kW. When considering redundant backup drives, using two frequency converters in order to ensure the power supply continuity, it is desirable that the interlocking management as well as the activation/deactivation commands be achieved by means of a programmable micro-automat, with software-established logic. The digital inputs of the two frequency converters, such as the frequency converter activation/deactivation commands, are performed via OUT 1 and OUT 2 outputs, **Table 1**, and are managed by a microprogrammable automaton, [8, 9] Alpha Mitsubishi, AL 2–24 MR-D type, **Figure 3**. When using two frequency converters, one of which is alternately maintained in cold reserve, the output interlock management of the frequency converters is very important. The interlock aspect is achieved by C3 and C4 switches, **Figure 2**, activated by the OUT 8 and OUT 9 output of the microprogrammable automaton Alpha Mitsubishi, AL 2–24 MR-D type, **Table 1**.

By a key selection, b5 and b6 , respectively, **Figure 4** [1], it is decided which of the converters will supply the load, represented by the M1 asynchronous motor with the short-circuited rotor.


**Table 1.** The assignment of the inputs and outputs for microprogrammable automaton.

motor with a short-circuit rotor with nominal active power P = 22 kW. When considering redundant backup drives, using two frequency converters in order to ensure the power supply continuity, it is desirable that the interlocking management as well as the activation/deactivation commands be achieved by means of a programmable micro-automat, with software-established logic. The digital inputs of the two frequency converters, such as the frequency converter activation/deactivation commands, are performed via OUT 1 and OUT 2 outputs, **Table 1**, and are managed by a microprogrammable automaton, [8, 9] Alpha Mitsubishi, AL 2–24 MR-D type, **Figure 3**. When using two frequency converters, one of which is alternately maintained in cold reserve, the output interlock management of the frequency converters is very important. The interlock aspect is achieved by C3 and C4 switches, **Figure 2**, activated by the OUT 8 and OUT 9 output of the microprogrammable

**Figure 2.** Switching to the backup power supply, in the case of a low-voltage symmetrical grid using two frequency

supply the load, represented by the M1 asynchronous motor with the short-circuited rotor.

, respectively, **Figure 4** [1], it is decided which of the converters will

automaton Alpha Mitsubishi, AL 2–24 MR-D type, **Table 1**.

and b6

converters managed by a programmable microautomaton [1].

By a key selection, b5

84 Smart Microgrids

**Figure 3.** Microprogrammable automaton Alpha Mitsubishi, AL 2–24 MR-D type.

Also, the enabling/disabling command management for the frequency converters is achieved by the OUT 1 and OUT 2 programmable micro-automaton outputs, **Table 1**.

In **Figure 5**, it is presented the flowchart where the logic of switching as well as electrifications can be seen (it is preferable to study together with **Figures 2**, **4**, and **Table 1**).

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

low-voltage grid, two distinct situations can occur:

**Figure 5.** Flowchart, the logic of switching and electrifications. **\***

potentiometer, **Figure 2**.

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

by actuating the P1 potentiometer, **Figure 2**. **\*\***There is the possibility to obtain a variable frequency by actuating the P2

There is the possibility to obtain a variable frequency

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**Figure 4.** Connecting the inputs/outputs of the microprogrammable automaton alpha Mitsubishi, AL 2–24 MR-D type [1].

Also, the enabling/disabling command management for the frequency converters is achieved

In **Figure 5**, it is presented the flowchart where the logic of switching as well as electrifications

**Figure 4.** Connecting the inputs/outputs of the microprogrammable automaton alpha Mitsubishi, AL 2–24 MR-D

type [1].

86 Smart Microgrids

by the OUT 1 and OUT 2 programmable micro-automaton outputs, **Table 1**.

**Figure 3.** Microprogrammable automaton Alpha Mitsubishi, AL 2–24 MR-D type.

can be seen (it is preferable to study together with **Figures 2**, **4**, and **Table 1**).

**Figure 5.** Flowchart, the logic of switching and electrifications. **\*** There is the possibility to obtain a variable frequency by actuating the P1 potentiometer, **Figure 2**. **\*\***There is the possibility to obtain a variable frequency by actuating the P2 potentiometer, **Figure 2**.
