A Solid State Transformer model for power flow calculations

A Solid-State Transformer as Enabling Technology for Microgrids for Integration into the Power Distribution System: An Overview

Conventional Magnetic Transformers are key components in a distribution system as it performs several functions like voltage transformation and isolation. However due to increase in penetration of renewable energies into distribution system, existing conventional transformers are expected to be replaced by a solid-state transformer (SST) which offers several advantages as reported in literature. SST can also be used to connect AC or DC grids because it can be operated at a much higher frequency and voltage which results in reduction in the size of conventional transformers as size of transformer is inversely proportional to frequency. In this paper, design and analysis of a 3-phase PWM Inverter using half - bridge topology for 11kV/381V, 100kVA solid-state transformer (SST) is presented for power distribution system application. The mathematical model of SST is developed using analytical calculations and then implemented in MATLAB simulation program. Finally it is implemented in hardware in a laboratory for a half-bridge topology. The main system has been virtually created in order to actualize the conversion from DC to AC.

Solid state transformers (SSTs) have found widespread power applications in traction locomotives, distribution systems, smart grids, etc., where they are going to replace conventional transformers and associated power enhancement equipments to provide better power quality features and functionalities. An eminent structure of SST comprises of three power conversion stages: input (AC-DC), isolation (DC-DC), and output (DC-AC). Multilevel converters are incorporated in several SSTs for operating at high voltage levels. This paper presents a three phase multilevel converter based SST (MCSST) which contains Cascaded H Bridge (CHB) multilevel converters in its input stage. Most of the reported topologies of MCSST have a common low voltage DC (LVDC) bus and a common control block for the entire SST structure. The presented MCSST consists of three independent phase modules and each phase module is controlled separately. Therefore, the outputs of these modules can be applied to both three phase loads as well as three independent single phase loads. Such a configuration can also be used to operate effectively under unbalanced load conditions. The discussed analysis of the reported work is verified using PSCAD/EMTDC simulation results.

Distribution systems have traditionally received the least attention in the bulk picture of power systems; they are yet the most "critical component" as they directly impact customers’ perception and utilities’ reputation. While transmission systems have seen abundant literature, innovations, and applications of power electronic-based devices to optimize their performance, distribution systems still require more attention toward a flexible and dynamic scheme to cope with volatile energy consumption and production. As a replacement or enhancement of the traditional ones, Solid-State Transformers can play a central role in the energy management of distribution systems towards this goal, in addition to the mere voltage conversion. This manuscript, developed within the IEEE Task Force on Solid-State Transformer integration in distribution grids, offers a critical review of the Solid-State Transformer potential, services, and technical challenges for its integration in distribution systems. Particular attention is given to scientific trends and open points in the current research that must be addressed before utilities extensively integrate the Solid-State Transformer in distribution systems.

The recent innovations in the field of power electronic industries have led to advancements in high power converters and their usage in the modern grid. This trend focused on the interconnection of power converters with solid-state transformer (SST). Since it is capable of controlling power flow in the power system, it is seen as a potential technology which can replace the conventional transformers that are used today. SST consists of power electronic switches interconnected with a high frequency transformer (HFT). The usage of HFT reduces the size and volume of the SST. A suitable controller can upgrade the performance of SST under different operating conditions. This paper proposes a novel control strategy for SST with a reduced number of converter stages and also discusses the design of the HFT. The results show that the SST will enhance power flow from the source to the grid.KeywordsSolid-state transformerHigh frequency transformer designRenewable source integrationControl strategy

Solid-State Transformers (SST) have been growing importance in the upcoming power distribution system. Currently the available circuit topologies and the maximum voltage rating supported by available power devices are the key limits in the usage of Solid-State Transformers into the Medium Voltage (MV) distribution systems. To overcome these limitations, a novel solution denoted as ROMAtrix converter has been proposed and needs to be explored. Such a configuration is able to preserve all the benefits of existing SSTs topologies, such as bidirectional power flow, isolation barrier with a medium frequency transformer and modularity. In addition, the ROMAtrix converter enables connections to a wider range of MV networks by halving the voltage drop on each power switch. However, the power flows management abilities, which would enable this structure to operate in smart grids system, has not yet been proved. To this aim, an active and reactive control strategy has been described in this paper to demonstrate the power control capabilities of the ROMAtrix structure. In order to highlight the features of the proposed topology, significant results, achieved in MATLAB/Simulink and PLECS co-simulation environment, have been shown and discussed.

In recent years, the concept of solid-state transformers (SSTs) has evolved to be one of the front runners in the development and realization of a smart grid. The recent strides in the field of medium voltage wide bandgap semiconductor devices and the availability of 10 kV SiC MOSFETs and 15 kV SiC IGBTs have made it possible to build medium voltage SSTs using simple converter topologies. With the SST interfacing two grids, it forms a crucial juncture in the electrical distribution system which necessitates a high fault tolerance capacity of the SST during grid fault conditions. In addition, the grids need to be protected from any faults or abnormal operation in the SST which makes it imperative to implement different fault protection considerations on the SST. In this paper, these fault protection systems are studied in terms of their breaking time, breaking current/voltage, and fault coordination. The protection schemes are analyzed on a Gen3 10 kV SiC MOSFETs based Mobile Utility Support Equipment Solid-State Transformer (MUSE-SST) which is developed at FREEDM Systems Center aimed at interconnecting a 4.16 kV grid to a 480 V grid or load. Simulation results and are provided to evaluate the protection schemes. Initial experimental test results are also provided.

The large-scale integration of renewable energy sources (RESs) and the increasing number of energy -storage (FS) systems connected to the grid will create a big challenge for power flow management at the distribution level. In this new scenario, a solid-state transformer (SST) will be a highly efficient key element for providing highly efficient and controllable power flow in distribution grids. SSTs not only mimic the operation of conventional transformers, which scale the voltage level between primary and secondary terminals, but they also control bidirectional power flow, stabilize voltage, facilitate direct integration of ES, and mitigate harmonics and transients, which can cause the power quality of the grid voltage to deteriorate. This article surveys several proposed structures for SSTs in the literature based on different ways of interconnecting power converter topologies.

This paper analyzes the neutral current reduction performance of a three-phase four-leg solid state transformer (SST) under different degrees of unbalanced load. Several kinds of control strategies are presented, such as the neutral current elimination controls that rely on phase shifting, voltage amplitude, phase shifting, and voltage amplitude combination control. A neutral current minimization control, which ensures the SST output voltages complying with the EN 50160 output voltage unbalance standard, is also developed. These control approaches simply build on the balanced voltage control providing voltage references which slightly unbalance the voltage amplitude and phase angle or both. The effectiveness of the proposed strategies is validated through tests on a downscaled prototype. Simulation results for the neutral current minimization control of the SST applied to a real urban distribution network with distributed loads are presented. The results of this analysis show that overall the neutral current minimization results in an energy saving from both reduced losses in the distribution cables and reduced power consumption in the load.

The use of nonconventional power supplies featuring unstable quality of the electricity produced and fluctuations in the output power, on the one hand, and the availability of various consumers connected to power supply grids, on the other hand, are factors that give rise to phase unbalances in the grids. These circumstances complicate the operation of smart grids, especially in microgrids. One of possible ways to improve the quality of smart power supply grids is to use a three-stage solid-state transformer with galvanic isolation and two DC links, an arrangement that makes it possible to connect power supply sources and consumers, and to arrange backup storage of electricity in storage batteries. The aim of the study is to analyze the electric power quality at the output of a three-stage solid-state transformer powered by an unbalanced three-phase voltage source. The study was carried out by numerical modeling using the Matlab/Simulink software package. The modeling parameters are given, and the current and voltage waveforms at the solid-state transformer output are obtained. Phasors of the input and output voltages are constructed with expanding them into zero-, positive-, and negative sequence harmonic components. The harmonic distortion coefficients for current and voltage in each phase, and the voltage unbalance coefficients for the zero and negative sequences at the solid-state transformer input and output are determined using the method of symmetric components. It has been shown that by using solid-state transformers it becomes possible to obtain noticeably better quality of electric energy and eliminate the unbalance to the levels stipulated by the relevant state standard. It is shown that solid-state transformers are a promising technology as part of smart power supply grids, which opens the possibility to control and adjust power supply parameters, improve the power supply quality, distribute power flows between the sources and consumers, and integrate DC and AC grids of various voltage and power levels.

Solid State Transformer (SST) is widely used in distribution renewable energy resources. SST includes AC-DC rectifier, DC-DC Dual Active Bridge (DAB) converter and AC-DC inverter. In this paper, continuous time average model and dynamic phasor model for combined Rectifier and DAB converter (R-DAB) are developed. Control scheme for R-DAB model is proposed for dynamic phasor based model, in which rectifier and DAB are being individually controlled using PI controller. The effect of controller has been analysed with change in input grid voltage variation. Stability analysis has performed using small signal stability analysis and Lyapunov's criterion. Developed model is verified with simulation in MATLAB/Simulink. Simulation result verifies the proposed modelling technique and controller performance.

A major portion of a power system's total performance and energy loss is attributable to voltage profiles and power losses in electrical distribution networks (DNs). Enhancing voltage profiles and lowering network losses can result in a more efficient power grid. In the literature, reactive power support has been used with a number of devices to improve DN performance. Some examples are shunt capacitor banks, distribution static compensators (DSTATCOM), and universal power quality condensers (UPQC). A solid-state transformer (SST) that can be built to support the grid has recently attracted a lot of attention. In particular, the SST is being looked at as a possible replacement for the more common fundamental frequency transformer. This paper suggests using the SST's stage-I reactive power support (SI-RPS) function to reduce radial DN losses and improve the voltage profile of the network. With the competing goals of reducing network losses and improving the voltage profile, a planning strategy is given for putting SST installations in the right places and at the right sizes. The simulation results verified the viability and rationale of the suggested model and algorithm for the 69-bus RDN. The outcomes of a number of different situations are detailed in this work. The code for solving the mixed-integer non-linear programming model was developed in MATLAB R2020a. Increasing SST deployment demonstrated an improvement in the voltage profile of the network, along with cutting down on losses significantly.

The feasible equilibria of operation of distribution-level power system models interfaced with solid-state transformers (SST) are analyzed and presented through a set of analytical relationships. The active and reactive power balances in the SST are realized through control of power electronic converters with appropriate choices for voltage and current setpoints. These setpoints parameterize the nonlinear model of the SST. Therefore, choosing them appropriately in sync with the generation and load profiles in the system is critical for maintaining a feasible equilibrium. These equilibrium sets are derived by first considering a fundamental physics-based model of a single-SST system, and thereafter, by extending them to systems with multiple SSTs connected to a radial distribution feeder. Power sharing methods are developed by which multiple SSTs can share a given change in load by generating an appropriate set of feasible setpoints for their input stage rectifiers. A control architecture is proposed for executing these load-sharing methods for both instantaneous and predictive load commands. The algorithms are verified by simulations on a representative distribution test system with nine SSTs.

A solid-state transformer (SST) is one of the promising candidates to interface the renewable generation to a power distribution system. An emerging and attractive topology for the building block of the next generation SST is the multiple active bridge (MAB) dc-dc converter. The MAB converter technology has the inherent capability to reduce the number of the high-frequency magnetic links (HFMLs) and the overall complexity of the system, especially in multi-module-based SST configurations. However, an MAB converter suffers from the power unbalance issues due to the parameter mismatch of the HFML. As, several active bridges share a common HFML in an MAB converter, the controller architecture should ensure that the bridges share the same amount of power and the output port maintains a constant dc voltage. In this paper, an improved control architecture is proposed for the HFML-based MAB dc-dc converter for use in the SSTs. The simulation and experimental results show that the designed controller can effectively mitigate the power unbalance issues of the MAB converter due to the HFML parameter mismatch, while maintaining a constant dc voltage at the output.

Structural modularity is critical to solid-state transformer (SST) and solid-state power substation (SSPS) concepts, but operational aspects related to this modularity are not yet fully understood. Previous studies and demonstrations of modular power conversion systems assume identical module compositions, but dependence on module uniformity undercuts the value of the modular framework. In this project, a hierarchical control approach was developed for modular SSTs which achieves system-level objectives while ensuring equitable power sharing between nonuniform building block modules. This enables module replacements and upgrades which leverage circuit and device technology advancements to improve system-level performance. The functionality of the control approach is demonstrated in detailed time-domain simulations. Results of this project provide context and strategic direction for future LDRD projects focusing on technologies supporting the SST crosscut outcome of the resilient energy systems mission campaign.