Modeling and control of microgrid: An overview
Introduction
The burden on the transmission network is increasing at an unexpected pace due to the increasing demand of power. Since updates to the transmission network are economically challenging, microgrids have evolved to become an economically viable alternative. In microgrids, generating units are commissioned within the scope of the conventional distribution network so that power can directly flow from the generators to the load without having to pass through the transmission network. The other advantage of using such an architecture is that loads can be served even if the transmission network is down due to a fault, increasing the overall reliability of the system. A Microgrid is generally known as the system consisting of small distributed generating stations along with the loads which is capable of going into islanded operation at times of need [1].
Among the many benefits of having a microgrid, one is that it facilitates distributed generation (DG) and high penetration of renewable energy sources [2], [3], [4]. They increase power quality and reliability of electric supply. A microgrid having renewable energy sources will help to alleviate some of the environmental issues related to burning fossil fuels. There is an extensive literature on the various challenges posed by microgrids. Despite having some benefits of microgrid architecture in the grid environment, there are some challenges related to this also. Implementation is an issue. Microgrid protection is also considered one of the most important challenges facing the implementation of microgrids. Once a microgrid is formed, it is important to assure that the loads, lines, and DGs on the island are protected because conventional unidirectional power flow protection method is no longer viable [5]. Solid regulatory base is another issue related to microgrids. It is known that energy related industries established policies and ‘solid regulatory base’ in place which became important for the growth in market. Government organizations should ensure that these regulatory policies should include guidelines and schemes to implement microgrid technologies
Control of the voltage and frequency during islanded operation of DGs is also a major challenge. A method for intentionally islanding a single DG to feed a local load was proposed in [6]. A much more complex and challenging task is to operate more than one DG on the island. With more than one DG on the island, it is necessary to regulate the voltage during microgrid operation, which could be achieved by using a voltage versus reactive power droop controller [7]. There needs to be an algorithm that should complete the resynchronization process once the grid is restored. A supervisory control mechanism will monitor the overall process and provide information to the local controller to respond accordingly.
The concept of power grid is based on the technology introduced around 120 years ago. It is facing lot of issues in keeping up with modern challenges. One of the main challenges is to guarantee electricity supply to customers and maintaining long-term energy security. Therefore increased reliability/efficiency is very much needed in today׳s world where the demand of electricity is ever-growing. Microgrids (MG) incorporate various distributed generator (DG) units into the utility grid and solve many problems of existing power systems. It is also the vital building block of the future Smart Grid [7].
On another front, the concept of system of systems (SoS) is gaining rapid interest in the field of research and can also possibly lead to a new branch of engineering known as SoS Engineering. This branch is related to systems engineering and deals with the optimization of a network of interacting systems. Any large scale integrated system or any complex system can be viewed as an example of SoS [8], [9]. A microgrid can be viewed as a system of system (SoS).
In this paper, motivation towards development of MG and an overview will be presented on the two key aspects, modeling and control, of MG. Recent developments in these two key aspects will be presented. A better control strategy, by viewing MG as a special case of SoS, will be discussed.
Section snippets
Distributed generation: applications and issues
The existing grid has small number of producers, long distribution ways and high maintenance cost, it is also difficult to achieve load balancing. Moreover, the depleting fossil fuels and the adverse effect on environment by its consumption have gain multi-national interest in reducing the excess use of nonrenewable energy resources and many nations are keeping tap on CO2 emissions [10].
The main concerns with the existing centralized power system grid are summarized below [11]:
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Increasing demand
Microgrid: definition and applications
A microgrid can be defined as, ‘A network of low voltage power generating units, storage devices and loads capable of supplying a local area such as suburban area, an industry or any commercial area with electric power and heat’. The components of Microgrid are interfaced through quick response power electronics and present itself as a single entity and therefore can be connected to traditional power grid or can also be operated in stand-alone mode as a self-sustained power system [7].
As stated
Microgrid: components and formation
A generalized structure of microgrid is shown in Fig. 1. The microgrid can be connected to the utility grid through single Point of Common Coupling (PCC). The isolating device is used to isolate the microgrid from the utility grid.
The Distribution Generation (DG) unit is responsible for generation of electricity. It consists of rotating type and inverter type generating devices. Rotating type includes IC engines, gas turbines, microalternator etc. whereas the inverter type includes
MG: modes of operation
Microgrid can operate autonomously and can also be connected to the utility/main grid. In case any fault occurs while operating in grid connected mode, microgrid has an ability to disconnect itself from grid and operate independently supplying its local load [25]. Therefore, the microgrid modes of operation can be classified into grid connected, islanded, transition between grid-connected mode to the islanded mode and vice-versa [26]. In any mode of operation, the heat generated by some of the
Microgrid: overview of modeling
A microgrid integration of various units. Basically, it consists of DG unit, energy-storage unit, controller unit and conventional load. The DG unit again compromises of various micro-generating devices. Therefore, microgrid modeling varies from one configuration to other depending on the components used. Various approaches for the modeling and control of microgrid can be found in the literature [27]. We will discuss the different models available in the literature.
A small signal dynamic
Microgrid – overview of control
The control strategies for microgrid depends on the mode of its operation. The aim of the control technique should be to stabilize the operation of microgrid. When designing a controller, operation mode of MG plays a vital role. Therefore, after modelling the key aspect of the microgrid is control. In this section we will discuss the various control paradigms.
MG – control in both modes
A novel control strategy which can be applied to MG in both the modes is introduced in [31]. This scheme of control allows a DG to control its real and reactive power components in grid-connected mode and to control the voltage and frequency in autonomous mode.
The proposed VSC frequency control is similar to that of a synchronous machine and is shown by the block diagram in Fig. 26. In grid-connected mode, the frequency at PCC (ωpcc) is equal to the grid frequency and hence has no impact on the
System of systems – introduction
To understand the concept of system of systems (SoS) or cyberphysical system (CPS), let us consider an airplane which is an example of large scale complex system. Various parts of airplane are operated by different systems but the plane flies only when all its systems operate collectively and does not fly if they operate individually. Therefore, a SoS is the large-scale integration of many systems that combine their capabilities together to form a more complex system offering more
SoS control – application to MG
The key issue of SoS, which is control, faces a main challenge of developing a comprehensive SoS model, analytically or by simulation. Availability of a proper model is necessary to design a controller. If a proper mathematical model is available then there are several available control strategies. Also control strategy for each system is not only dependent on its own sensory information but also on the communication links among its neighboring systems or components, this is another difficulty
Comparative analysis
After discussing the control techniques, it is worth performing the comparative analysing of the control techniques which are most commonly used. In this section, we will classify the control techniques considering vital aspects for the purpose of simplification and better understanding. Control strategies for MG are very vast and detailed comparison of each techniques with another is out of scope of this paper.
MG control (depending on architecture) can be generally classified into two main
Conclusions
The role of Microgrid in penetration of DG׳s in the present utility network is discussed. Modeling of microgrid is a key aspect and the recent developments in the modeling of microgrid are presented in both grid-connected and autonomous mode. The control techniques of microgrid available in the literature for various modes of operation are also discussed. The microgrid can be viewed as a special case of SoS. It can be concluded that using networked control system, a better control of microgrid
Acknowledgments
The authors would like to thank the Associate Editor and the reviewers for their constructive comments on our initial submission. This work is supported by the deanship for scientific research (DSR) at KFUPM through group research project RG-1316-1.
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