*2.2. Power Electronic Converters Used in Micro-Grids*

**Figure 2.** A comparison between the shares of RES in electricity, heat and transport for 2017 and 2023. RES: Renewable Energy Sources. Micro-grids deal with a wide range of topics, such as power electronics, power systems, RES generation and storage, and ICT (Information and Communication Technology). The conventional utility grid works in a passive mode absorbing energy from the network and delivers it to customers. This approach is well-known, well-developed, and presented in many research papers, but the modern micro-grids/smart grids need state-of-the-art technology, including a bi-directional power flow and big-data processing/cloud computing. Modern micro-grid systems should provide more flexibility, reliability, sustainability, security, and two-way communication services, as shown in Figure 3. In particular, the integration of RES, EVs and DGs into power systems is achieved in an efficient

The modern micro-grid concepts incorporate multiple DERs, power electronic converters, and

photovoltaic.

**2. Renewable Energy Source Trends for Micro-Grids**

*2.2. Power Electronic Converters Used in Micro-Grids*

*2.1. Renewable Energy and Micro-Grids*

RES: Renewable Energy Sources.

gusts.

way in micro-grid systems, based on power electronics with wide band-gap devices (IGBT (Insulated Gate Bipolar Transistor)s and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)s), such as gallium nitride on silicon (GaN-on-Si) technology. at around 70% of global energy generation will grow in the next five years. Distributed solar PV is counting for almost half of total solar PV grows over 2019-2024 [25]. The shares of RES in electricity, heat and transport for 2017, with an estimation for 2023 can also be seen in Figure 2. utility grid works in a passive mode absorbing energy from the network and delivers it to customers. This approach is well-known, well-developed, and presented in many research papers, but the modern micro-grids/smart grids need state-of-the-art technology, including a bi-directional power

significant power generation over the next five years, providing 30% of power demand in 2023 from 24% which was in 2017, as shown in Figure 2. This means that the power capacity expansion reaching

generation and storage, and ICT (Information and Communication Technology). The conventional

*Appl. Sci.* **2019**, *11*, x FOR PEER REVIEW 3 of 18

**Figure 1.** Classic local grid where RES and EV charger nodes are only connected to the main grid and with unidirectional energy flows. RES: renewable-energy-sources; EV: full-electric vehicles; PV:

Renewable energy will have the fastest growth in the electricity sector for the next five to six years, and is the central stage of the transition to less CO<sup>2</sup> emissions and more sustainable energy. Renewables like wind power and solar power have grown very fast in the past 10 years, particularly because of their cost reduction, which has a 50% reduction target of 2030. Even if the electricity generated by Renewable Energy Sources-RES represents is only one fifth of the global energy consumption, the roles of renewables in the transport and heating sectors still remain a huge challenge in sustaining the energy transition. International Energy Agency-IEA [24] estimated in 2018

*Appl. Sci.* **2019**, *11*, x FOR PEER REVIEW 4 of 18

The final objective is to remove challenges to smart grids integration, remove reliance on high-speed communication and peer-to-peer architectures, as well as create a plug-and-play reliable system.

4 presents an alternative micro-grid architecture. Conclusions are finally drawn in Section 5.

Hereafter, Section 2 reviews trends in RES, power converters and controllers. Section 3 analyzes trends in battery energy storage and the relevant issues in battery management and charging. Section

**Figure 2.** A comparison between the shares of RES in electricity, heat and transport for 2017 and 2023. **Figure 2.** A comparison between the shares of RES in electricity, heat and transport for 2017 and 2023. RES: Renewable Energy Sources. through a DC-AC inverter and a DC-DC converter. In this case, the DC-DC converter connected to the DC-link/DC-bus can serve as a storage or as an electrical brake, for wind turbine during wind

**Figure 3.** Power electronic converters used in micro-grids. **Figure 3.** Power electronic converters used in micro-grids.

DC/AC inverters play an important role in frequency and voltage control, while AC/DC converters can isolate the micro-grid from the utility and properly match it with the DC loads in residential applications [9,16,17]. Inverters for utility connection can be broadly classified into two types: Switch-mode single-phase inverters and switch-mode three-phase inverters. The detection of islanding is much easier in a three-phase than a single-phase inverter. Although, inverters that are rated at a power below 5 kW are mostly connected to single-phase networks. Distributed Energy Resources (DER) components used in micro-grids, including PV systems, Wind Turbines (WTs), ESSs, EVs, require power electronics interfaces like DC/AC inverters, bi-directional DC/DC converters or AC/DC/AC converters, as can also be seen in Figure 3. A micro-grid can include many converter topologies to interface DG units and loads. For instance, the PV array in Figure 3 connects a number of PV modules to a one quadrant DC-DC converter interfacing the PV-generated power to the load via the common DC bus, when operates in stand-alone hybrid mode, together with a battery bank that is connected to the common DC bus via a bidirectional DC-DC converter. The wind turbine generator and the flywheel system are connected to the common DC bus through a DC-AC inverter and a DC-DC converter. In this case, the DC-DC converter connected to the DC-link/DC-bus can serve as a storage or as an electrical brake, for wind turbine during wind gusts.

DC/AC inverters play an important role in frequency and voltage control, while AC/DC converters can isolate the micro-grid from the utility and properly match it with the DC loads in residential applications [9,26,27]. Inverters for utility connection can be broadly classified into two types: Switch-mode single-phase inverters and switch-mode three-phase inverters. The detection of islanding is much easier in a three-phase than a single-phase inverter. Although, inverters that are rated at a power below 5 kW are mostly connected to single-phase networks.

The most important power interfaces between DG sources and loads (DG inverters) employed voltage source inverters (VSI), which can be classified in two categories [28–30]: current-controlled inverter (CCI) and voltage-controlled inverter (VCI). CCI could also be used as an active power filter to reduce/mitigate the THD (Total Harmonic Distortion) factor of the converter, improving in the same time the power quality of the hybrid micro-grid. Instead, VCIs serve for supplying the active and reactive powers as well as to provide voltage and frequency regulation for islanding operation of the micro-grid.

Hybrid micro-grids have the benefits of both AC and DC micro-grids in supplying conventional AC loads and AC sources on AC side (AC bus) and DC loads and DC sources on DC side (DC bus), in controlling the corresponding individual micro-grid (sub-grid) and then in coordinating and interconnecting the AC with DC micro-grids through an interlinking converter [31–34], in which the DC-side is dynamically decoupled from the AC-side. Interlinking converters play a key role in hybrid micro-grids regarding the grid-tied and islanding operations and the seamless transition and resilience to main grid perturbation (LVRT-Low Voltage Ride Through capability), active harmonic filtering capability and power-flow control.

Each converter type requires a proper control strategy/algorithm that is selected among numerous methods, based on specific requirements and applications. The control algorithms could be classified according to their loop either, in feedback or in feed-forward structure. For instance, the controller of DC/AC switch-mode inverters can contain six parts: Maximum power controller, ESS charge controller, voltage controller, harmonic controller, RMS voltage controller, and current waveform generator.

Many DER components provide or generate a DC current, which is part of an internal DC micro-grid to avoid losses and permits plug-and-play capabilities. The hybrid energy systems involve a well-optimized energy control and management infrastructure to achieve the highest possible energy level.

Moreover, control strategies and monitoring capabilities, automatic control, and configuration of the grid, and the active involvement of the consumers in energy production extend the importance of the micro/smart grids. All the advantages of smart grids can be achieved by power electronic converter integration and ICT technologies with the grid [10,26,27].
