A Solid-State Transformer as Enabling Technology for Microgrids for Integration into the Power Distribution System: An Overview
Authors: Ellabban Omar 
Published: July 2019

In the present architecture of the electric power grids, energy is generated in large-scale that is remotely located from consumers and transmitted using passive high-voltage transmission networks to substations. The electric power is then delivered to consumers via medium and low-voltage distribution systems. Line frequency transformers (LFTs) enable high-efﬁciency and long-distance power transmission by stepping up the voltage on the transmission side. On the distribution side, the voltage is stepped down for industrial, commercial, and residential consumers. Furthermore, the traditional electric power system is characterized by hierarchical control structures with minimal feedback, limited energy storage, and passive loads. In recent years, the electric power system has been facing increasing stress due to fundamental changes in demand growth, technology change, and consumer preference. There has been a paradigm shift from centralized generation and control to distributed generation, storage and local control; in particular, there has been a global growth of investments in Renewable Energy Sources (RES) such as wind and solar. These RES generate fluctuating electric power and therefore they require Energy Storage Systems (ESS) to enable a time-shift between energy production and consumption. The high penetration of RES and other Distributed Generation (DG) types such as fuel cells (FCs) and Micro-Turbines (MTs) impose significant challenges on the operation of power system and coordination of supply and demand in real time. At the same time, the power system will need to react to events for which it was not designed for. Microgrids (MGs), being controllable small scale power networks, have become popular in distributed systems by facilitating an effective and smooth integration of distributed energy resources, loads, and energy storage devices into existing power systems. They help reduce expenditure by reducing network congestion, line losses and line costs and thereby higher energy efficiency. Power electronics converters are used in microgrids to control the routing of electricity and also provide flexible distributed generation sources interfaces to the distribution network to ensure stable and secure operation of the grid. These power converters rely on the type of microgrid (AC or DC), as well as on other features of the devices (voltage levels, power flow direction, etc.). In addition, they usually include a transformer in order to obtain galvanic isolation. The solid state transformer (SST), which is also called Electronic Power Transformer (EPT), Active Power Electronics Transformer (APET) and Intelligent TRansformer (ITR), has been proposed to replace the traditional line frequency distribution transformers. The basic operation of the SST is first to change the 50Hz AC voltage to high frequency (normally in the range of several to tens of kilohertz), then this high frequency voltage is stepped up/down by a high frequency transformer with significantly decreased volume and weight, and finally shaped back into the desired 50Hz voltage to feed the load. The SST is a power electronics circuit that provides a flexible method for interfacing microgrids with the existing Medium-Voltage (MV) power distribution system. It achieves voltage transformation via a high-frequency isolated transformer, therefore the mass and volume of the system can be reduced. Current and voltage are independently controlled via power semiconductor devices which enables grid support functions not present in the traditional LFT, including voltage and power factor regulation, fault isolation and limitation, filtering harmonic, reactive power compensation, power flow control and voltage and load disturbance rejection. However, at the present time, the commercial success of SSTs has been limited. Efforts have been made to design and implement SSTs with satisfactory performance, as well as explore its use at the distribution system level. The SST can functionally replace the traditional LFT and some power electronics converters, thus indicating a potentially more integrated and compact system. Although the concept of the SST can be seen straight forward, its implementation is a challenge. The SST combines the high voltage, high power, and high frequency operation, which make its design and operation a real challenge. To implement the SST, different power devices can be combined with different circuit topologies in different configurations. In addition, different core materials and transformer structures may be considered to build the high frequency transformer with different phase connections. Furthermore, due to power rating limitation of the semiconductor devices and magnetic components, SST topologies may have several SST cells connected in series or in parallel. Numerous degrees of freedom for the SST modularization are available, which lead to a vast amount of possible arrangements depending on the different levels of modularization in the three modularization axes: degree of power conversion partitioning; degree of phase modularity and number of levels or series/parallel connected cells. Since the level of modularity in each of these different directions is independent, these three axes can be considered to be orthogonal to each other and each element represents a specific design with a certain degree of modularization in each of the axes. The SST can be used as an enabling tool for interfacing microgrids (AC and DC) to medium voltage distribution network. For a SST-based microgrid, the performance of the microgrid mainly depends on the control algorithms of the SSTs. Generally, there are three SST control levels: system level, equipment level, and switch level. For a well-designed SST, the equipment level and firing level control strategies are well defined. However, the system level control strategy can be changed to fit different applications. In order to operate a microgrid properly and reliably, SST- based power control strategies should be developed to accommodate the modes of operation: grid-connected mode and standalone mode. That is why, the development of a controller that manages active and reactive power flow between the DC microgrid, the AC microgrid and the main distribution network is one of the most important research topic for the SST. Furthermore, investigating the effect of grid faults on the stability and performance of the SST is another hot research topic in the same area. Also, the SST can be used in different applications, such as: oil and gas and electric transportation applications. To give an overview of the SST, this work will present a review of the different levels of modularization, all possible control algorithms for the different control levels. Furthermore, stability and reliability analysis for grid connected SST will be reviewed. Finally, all possible application of the SST will be presented.