Development of a smart transformer to control the power exchange of a microgrid

A smart transformer enables to control the power exchange between a microgrid and the utility network by controlling the voltage at the microgrid side within certain limits. The distributed generation units in the microgrid are equipped with a voltage-based droop control strategy. This controller reacts on the voltage change, making the smart transformer an element that controls power exchange without the need for communication to other elements in the microgrid. To build a smart transformer, several concepts are possible. In a smart transformer with continuous turns ratio, hereafter referred to as continuous smart transformer, the transformer's microgrid-side voltage can be controlled without voltage steps and the accuracy of the voltage control can be very high. The voltage control of a smart transformer with discrete turns ratio, hereafter referred to as discrete smart transformer, is less accurate, as the output voltage is regulated between several discrete values. In this paper, the development of a continuous and discrete smart transformer will be elaborated. Their validity will be proven by implementing these smart transformers in an experimental test setup. Also, some concepts to improve the control accuracy will be proposed

Contribution of a smart transformer in the local primary control of a microgrid

In order to enable an easy participation of microgrids in the electricity markets, the smart transformer (ST) concept has been developed. The ST controls the power exchange between a microgrid and the utility network by only controlling its microgrid side voltage, instead of the conventional arrangement where new set points are communicated to all microgrid elements. When the voltage-based droop (VBD) control is implemented in the DG units, loads and storage elements, all microgrid units automatically respond to this change of microgrid voltage by altering their power output or consumption. However, this reference value of power exchange is dependent on (day-ahead) predictions of both consumption and (renewable) power generation. Hence, when these predictions prove to be inaccurate, the ST will still control the power exchange, but with consequently large variations of the microgrid voltage from its nominal value. It is suggested to take the real-time microgrid voltage into account when determining the reference power of the ST. This is presented in this paper by extending the ST's control strategy with a VBD control, such that the ST can contribute in the primary control. Simulations are included to analyze this primary control of the ST combined with VBD control of the other microgrid elements

Robustness Analysis of Voltage Control Strategies of Smart Transformer

The increasing penetration of Distributed Generators (DG) in the modern electric distribution network poses high priority on the problem of the stability. In this article the Harmonic Stability of a Smart Transformer-fed microgrid is investigated under different control strategies. The considered microgrid is composed by a Smart Transformer and three Distributed Generators, considering the bandwidth of the DGs unknown. The robustness is evaluated analysing the eigenvalues as a consequence of a variation of the DGs bandwidth. The system is modelled as a Multi Input Multi Output System (MIMO); the eigenvalue based analysis is carried out to assess the stability and compare the robustness of the traditional double-loop PI and a state-feedback (SF) integral controller. The results show that the SF controller ensures a higher robustness than the traditional PI controller with respect to increasing bandwidths of the DGs

Smart microgrids and virtual power plants in a hierarchical control structure

In order to achieve a coordinated integration of distributed energy resources in the electrical network, an aggregation of these resources is required. Microgrids and virtual power plants (VPPs) address this issue. Opposed to VPPs, microgrids have the functionality of islanding, for which specific control strategies have been developed. These control strategies are classified under the primary control strategies. Microgrid secondary control deals with other aspects such as resource allocation, economic optimization and voltage profile improvements. When focussing on the control-aspects of DER, VPP coordination is similar with the microgrid secondary control strategy, and thus, operates at a slower time frame as compared to the primary control and can take full advantage of the available communication provided by the overlaying smart grid. Therefore, the feasibility of the microgrid secondary control for application in VPPs is discussed in this paper. A hierarchical control structure is presented in which, firstly, smart microgrids deal with local issues in a primary and secondary control. Secondly, these microgrids are aggregated in a VPP that enables the tertiary control, forming the link with the electricity markets and dealing with issues on a larger scale

Potential and Impacts of Smart Transformer in Green Harbours

Harbour grids are undergoing rapid transformation due to the increased interest in green harbour initiatives such as ship cold ironing, renewable energy integration, battery-powered marine vessels, etc. In this scenario, better controllability over the power flow is important to maintain the voltage and current quality within the grid-code specified limits and ensure a stable and efficient power supply. This paper aims to explore the potential of the smart transformer (ST) in providing various support features to green harbours. The features include the integration and control capability of ST in accommodating renewable energy sources, electric vehicle charging stations and storage. In addition, the impact of the ST in the green harbour is analyzed with the focus of addressing the key issues and challenges such as voltage variations, peak loads and poor power factor

Smart transformer: a revolutionary paradigm toward sustainable power grids

Electrical power grids are evolving technologically from different perspectives, specifically, aiming to guarantee the sustainability of the power grid itself, the introduction of new and emerging technologies for the production and storage of energy, advanced communication systems, as well as higher levels of power quality for all sectors of activity (from production to consumption). Particularly, with special focus over the last two decades, power grids are undergoing a depth transformation, moving from a centralized and unidirectional architecture to a decentralized and bidirectional architecture, mainly due to the massive incorporation of new electrical engineering technologies. This change also presents an important aspect for the entire power grid: the possibility of energy storage and management according to the real-time needs. In this context, within the scope of this paper, the sustainability of power grids is considered, focusing on the new paradigms offered by the smart transformer and hybrid AC/DC power grids, including all the added value that can be established in terms of power management. Encompassed in a smart transformer context, the contextualization of the conceivable arrangements of solid-state transformers, and the various configurations of smart hybrid transformers, are evaluated from the point of view of offering advantages of improved efficiency and power quality. In addition to a theoretical introductory context, the paper presents computational validations and a comparison regarding the various configurations that can be obtained

A Multi-port Smart Transformer for Green Airport Electrification

Green transportation and renewable energy production have attracted a global attention due to the needs of decreasing the environmental impact and still sustain increased energy needs. In this framework, the aircraft and airports are facing a profound renovations towards green technologies, among which the electrical ones are playing a central role. This paper explores how a Smart Transformer can upgrade the existing airport power system, enabling an efficient interface for renewable energy, electric vehicles and the future hybrid/electric aircraft, substituting the ground power units and enabling a smarter behavior of the electrical grid

Voltage-based control of a smart transformer in a microgrid

For the islanded operation of a microgrid, several control strategies have been developed. For example, voltage-based droop control can be implemented for the active power control of the generators and the control of the active loads. One of the main advantages of a microgrid is that it can be implemented as a controllable entity within the electrical network. This requires the ability of the utility grid to control or influence the power exchange with the microgrid by communicating with only one unit. However, little research has been conducted on controlling the power transfer through the point of common coupling. This paper addresses this issue by introducing the concept of a smart transformer (ST) at the point of common coupling. This unit controls the active power exchange between a microgrid and the utility grid dependent on the state of both networks and other information communicated to the ST. To control the active power, the ST uses its taps that change the microgrid-side voltage at the PCC. This voltage-based control of the ST is compatible with the voltage-based droop control of the units in the microgrid that is used in this paper. Hence, the microgrid units can automatically respond to changes of ST set points and vice versa. Several simulation cases are included in this paper to demonstrate the feasibility of the ST concept

A smart transformer-rectifier unit for the more electric aircraft

In the framework of the More Electric Aircraft (MEA), an efficient and flexible power distribution system is of paramount importance. Considering the presence of both AC and DC loads at multiple voltage levels, the distribution system of the most modern aircrafts is intrinsically hybrid. In this scenario, the different buses are connected by AC/DC converters. The simplest approach is to use a Transformer-Rectifier Unit (TRU) based on a low-frequency transformer followed by passive rectifiers to perform the AC/DC conversion. This solution, however, is intrinsically uni-directional, introduces current harmonics in the AC side and can have a considerable size. This paper proposes the use of a Smart-TRU, based on a Cascaded H-Bridge topology and a multi-port DC/DC converter, to solve the issues of the traditional TRU, increasing the controllability of the system. Experiments show how the proposed STRU is resilient to faults in the AC side