Smart Energy Management and Power Flow Control for Multi-Microgrids Interfacing with Utility Grid
Authors: Bayhan Sertac , Abu-Rub Haitham 
Published: July 2019

Nowadays, new trends have made possible to reconfigure the traditional power systems in a more efficient way while world energy consumption is being continuously increased. To meet future energy demands, a more flexible, smart and configurable power system is required. To create such systems, microgrids are emerging and becoming a more attractive solution. The microgrid is a weak grid formed with different energy sources (renewable and conventional), energy storages, power electronics, power control systems and different loads. The microgrids are also particularly suitable for communities and regions where adequate renewable energy sources are available such as in Qatar. Therefore, the energy cost can be significantly decreased, energy security ensured, and energy production become environmental friendly with lower carbon footprints.

The energy sources are the major part of the microgrid systems. As a result of increasing environmental awareness and as a consequence of the exhaustible nature of fossil fuels, renewable energy sources (RES) are playing an important role in modern microgrid systems. The RES based power generation systems have several advantages compared to the conventional power generation systems. Some of these advantages are sustainability, pollution-free operation and the possibility of being installed closer to the end users. In the last decades, especially the wind and photovoltaic (PV) based power generation systems have become more popular than other RESs. However, intermittent and stochastic nature of the wind and solar affect the stability, reliability and power quality of the microgrids. For instance, the PV based system cannot produce energy at night or during cloudy conditions, and wind-based systems generate energy that depends on the wind condition. To overcome these limitations, two or more (hybrid) RES, in addition to proper storage technologies, are needed to provide reliable, stable and continuous power to the customers. However, using different types of renewable energy sources in the same microgrid leads to complex control structure because these sources have different dynamic characteristics and need different control structures. Hence, a well-designed energy management and power flow control systems are essential to ensure the extraction of maximum power from these energy sources.

Another part of the microgrid systems is an energy storage system (ESS) that plays a vital role to maintain stability and robustness as well as to improve the power quality of those systems. For these reasons, an effective ESS must characterize high power density as well as high energy density. In recent years, various types of battery technologies are used for energy storage systems. In spite of their maturity and variety, batteries still have limited lifecycle and poor power density, which is an important element for balancing the renewable based power generation systems. Thus, to support and improve the battery performance, lifetime and system cost, hybrid energy storage systems (HESS) can be suggested while comprising supercapacitors (SCs) and batteries. SCs have a number of advantages related to high efficiency (95%), high power density (up to 10000 W/kg),

tolerance for deep discharges, and long life-cycle (500000 cycles at 100% depth-of-discharge). The combination of SCs and batteries allows to have the advantages of both solutions by obtaining high energy density, high power density, high life-cycle, high efficiency HESS and ensuring better power stability when interfacing with the grid. However, batteries and SCs have different charge and discharge characteristics. Therefore, a well-designed energy management and power flow control system is essential for that system to provide efficient operation and long life cycle.

In the microgrid system, power flow should be bi-directional. For example, in case of insufficient energy, the utility grid can support the microgrid, vice versa, in case of exceed energy, microgrid can inject this energy to another microgrid(s) and/or to the utility grid. Therefore, in such systems, power electronic converters are important to allow the power flow between energy sources, energy storage devices, loads, and the utility grid. These power electronic converters not only allow to connect different electric devices together (whether they are loads, generators or storage devices), but also to provide suitable control for optimizing and protecting the whole system. Furthermore, to ensure the power connection between these different units, a direct current (dc) microgrid or an alternating current (ac) microgrid can be used. However, as mentioned above, most of these units are controlled by power converters, and each of these converters requires a dc-link. For this reason, one common dc-link can obtain appreciable savings for such systems.

To ensure power flow between energy storage devices, energy sources, loads, and the utility grid (if needed), energy management algorithm is essential. A well-configured energy management algorithm increases energy efficiency, system stability, and battery life cycle. Therefore, the energy management algorithm and control structures must be defined properly according to system requirements. Several research activities focus on the energy management algorithm for HESS and power flow control algorithm. Types of the developed algorithms depend on the system power, storage techniques, types of energy sources, and operating modes such as grid-connected and/or standalone. However, most of these studies focused on only one microgrid and its control. In reality, more than one microgrid in the same region and different types of distributed generation units in these microgrids are common. Typically, the energy management control structure can be divided into three categories; centralized, distributed, and multi-level control structures. In all three cases, each energy sources and energy storage devices are controlled by the local controller to determine the optimal operating point locally. To increase the impact of the microgrids, the microgrids should be controlled by the same centralized controller. The microgrids also should have monitoring facilities to observe and reconfigure the energy consumption of the consumers.

The main goal of this study is to design, develop and implement novel smart energy management and power control strategy for two grid-connected microgrids. The presented two microgrids have different charactersitics in terms of renewable energy sources and energy storage technologies. Thus, by the proposed smart energy management and power control strategy, the two different microgrids operate at their best efficiency points regardless of different conditions. They can also operate in bidirectional with each other and/or utility grid. For example, if there is exceed power in one of the microgrids, this energy will be transferred to the other microgrid and/or the utility grid, vice versa if there is insufficient power for the local loads, microgrids request power from the utility grid. This innovative feature of the study will be an effective solution for growing microgrids toward securing the increased power demand. Furthermore, this study presents condition monitoring to adjust and reconfigure the energy consumption of the consumers.

To verify the proposed smart energy management and power flow control system, two laboratory-scale microgrids are designed and implemented. As shown in Fig. 1, the proposed prototype consists of two microgrids connected to the utility grid through the grid interactive inverters. These microgrids can also connect to each other through isolated bi-directional converter. In addition to these, each microgrid consists of four subsystems. (1) Wind energy conversion subsystem, (2) PV energy conversion subsystem, (3) Hybrid energy storage subsystem, and (4) Power electronics interface for AC load. Each subsystem has own local controller that can communicate with centralized controllers in order to increase system efficiency. Furthermore, whole system is controlled by a central controller to ensure optimal power flow between Microgrid I, Microgrid II and utility grid. The results of the study will not only benefit the energy management of multi microgrids but will also benefit the power grid operation especially the distribution system creating positive environmental impacts paving the road for future large-scale integration of the smart grid flexible load technology in Qatar.