Power Quality Improvements in Wind Based DG Systems using Solid State Transformer

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

This paper presents a wind-solar hybrid system using solid state transformer (SST) for reactive power compensation to achieve power quality. The proposed hybrid system using SST can effectively suppress the voltage fluctuation. This work proposes a new modelling approach of a combined Photovoltaic and Wind power system. The controller has been designed with fuzzy logic and sliding mode along with SST. This approach enables the design and implementation of efficient controllers for distributed energy resource (DER) hybrid systems. The simulation of proposed work is carried out using MATLAB.

Solid State Transformer (SST) is a new type of power transformer based on power electronic converters and high frequency transformers. The SST realizes voltage transformation, galvanic isolation, and power quality improvements in a single device. In the literature, a number of topologies have been introduced for the SST. In this work, employing a modular multilevel converter, a new SST topology is introduced which provides not only high-voltage AC (HVAC), low-voltage AC (LVAC) and low-voltage DC (LVDC) ports, but also high-voltage DC (HVDC) port. Besides, the proposed topology is easily scalable to higher voltage levels.

Solid state transformer (SST), as one of the most important technologies for energy internet application, will play a crucial role in the future especially with the the development of semiconductor technology. In this paper, a simplified 5.96 MVA three-phase SST system based on 15kV silicon carbide (SIC) power semiconductor devices is shown in order to solve some disadvantages of the three-phase cascaded modular SST. The simplified topology has a great practical prospect, such as urban rail transit power supply of most cities in china. The two stages of SIC-SST, front-end rectifier and DC/DC converter are both designed in closed loop, and then a novel control strategy under unbalanced grid voltages by suppressing the negative-sequence currents at the grid side of SST is presented. Simulation results in PSIM software have verified the effectiveness of the proposed control strategy.

This paper represents a review of the dynamic voltage restorer for power quality improvement in the electrical distribution system. Over the past 50 years, issues concerning power quality have steadily increased, to prevent the effect of the voltage disturbances, some of the devices are put as a solution to these problems such as distribution static compensator (D- STATCOM), solid-state transformer (SST), uninterruptible power supply (UPS), dynamic voltage restorer (DVR). The DVR is one of the economic solutions to overcome the voltage disturbances like voltage sag/swell and harmonics. It is widely used to mitigate the voltage disturbances in the power distribution system, especially in the medium and low distribution networks. This paper aims to review the implementation of the DVR in the system integrated with renewable energy resources. This is important because the future of electricity business is moving towards renewable energy and also provides a thorough discussion of the typical components, controllers, compensation methods, and the application of DVR. The extensive review of the technology aims to ease and speed up the development and the advancement of the DVR in the near future.

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.

Due to its high functionality, the solid state transformer (SST) represents an emerging technology with huge potential to replace the conventional low-frequency transformer (LFT) in a wide range of applications, including railway traction, smart grids, and others. On the other hand, model predictive control (MPC) has proven to be a highly promising control approach for several power electronics systems, especially those based on multiple power converters. Considering these facts, over recent years, different MPC techniques have been proposed for different types of SSTs. In addition to that, numerous MPC strategies have also been investigated for various power converters topologies that can be used in SSTs. However, a paper summarizing and discussing MPC strategies in the framework of SSTs has not yet been proposed in the literature, being the main goal of this work. In this paper, all the existing MPC techniques in complete SST topologies will be presented and discussed. In addition, for the sake of the example, an overview of MPC strategies in converter topologies typically used in SSTs will also be presented.

Solid state transformers (SSTs) are at the forefront of emerging technologies, enhancing distribution and transmission networks. These transformers facilitate the development of highly controllable and robust smart grids, seamlessly integrating renewable energy sources and accommodating connections for both alternating current and direct current components/lines at medium or low voltage levels. This research paper introduces an innovative SST incorporating the inductive power transfer (IPT) technology, currently under development within SSTAR, a Horizon Europe project. This novel device operates at a frequency of 50 kHz and provides with an output power of 50 kW. The IPT system is submerged in an ester-based, biodegradable dielectric fluid, providing effective cooling and insulation characteristics. The primary focus of this research is to highlight the guidelines for the optimal design of the forced liquid cooling system of the SST module. The analysis employs a multi-faceted simulation strategy to calculate the cooling aspects of the transformer, utilizing the commercial software ANSYS. This involves 3D coupled Electromagnetic (EMAG) - Computational Fluid Dynamics (CFD) simulations, as well as stand-alone CFD and thermal FEA simulations, ensuring robust cross-verification of the obtained results. This novel SST exhibits improved thermal performance, achieving a hot-spot temperature of 58 °C, the lowest value found in similar research studies. The findings underscore the efficiency and robustness of the proposed design, marking a significant step towards the realization of active and environmentally-friendly electrical networks.

One promising topology for solid state transformer (SST) is a modular multilevel cascaded converter, in which submodule is composed of dual active bridge (DAB) and H-bridge. For SST application in PV system, the efficiency could be severely affected especially for DAB due to the wide voltage and power range of PV panels. Thus, the motivation of this paper is to deal with the control strategy to improve DAB efficiency inside SST for PV application. Instead of utilizing time-domain based analysis method, which requires complex modeling process, this paper models DAB under frequency domain by fully considering the effect of both fundamental and harmonic frequency component. To achieve high efficiency operation, a multiorder reactive-current suppression (MRS) strategy is proposed by controlling phase shift angle as well as the duty cycles of primary and secondary side H-bridges. In terms of DAB controller design inside SST, the small signal model under MRS is established and a notch filter is implemented to suppress the second order line frequency fluctuation in the control loop. Finally, a 10 kV/1 MW SST prototype is introduced along with the system control structure and implementation method. The experiment of the submodule and SST confirms the effectiveness of proposed method.

The research in semiconductor technology and power electronics gives scope to replace a conventional low frequency transformer by power electronic converter based transformer which is known as solid state transformer (SST). Reliability issue is one of the major drawbacks of SST because as it contains power electronic switching devices, the chances of occurring open and short circuit faults are more. Therefore, it requires suitable fault tolerant operation. The basic SST topology consists of AC-DC-DC-AC conversion stages. This paper presents fault tolerant operation of third stage of SST (DC-AC converter) for open circuit faults. In this paper, a novel topology of fault tolerant multilevel inverter (FTMLI) is proposed, which can sustain single switch and some specified multi switch faults. The component count also taken into consideration here and the balancing of capacitors also properly maintained. The proposed topology is giving better results in terms of efficiency, cost and THD under fault tolerant operation. The proposed circuit topology is simulated using MATLAB/SIMULINK tool.

Along with the DC grid distribution issue, the Solid State Transformer (SST) is increasingly emerging in the field of electric power electronics, and the Hybrid SST (HSST) study is also emerging between the current AC-based Grid and future DC grid distribution. At present, several SST and HSST studies have discussed ways to implement the same topology in series to create high DC voltages. These cascade-style topologies not only make it difficult to implement controllers and designing gate drivers but also require a lot of elements, which drives up the overall system’s price. Meanwhile, IGBT and MOSFET series connections are actively being studied for high voltage topology, and various techniques are being studied to solve simultaneous turn on/off problems. This paper proposes by altering the PFC circuit used in existing LVs to switch device series-connected base for HSST and SST. In this paper, circuits were constructed using balancing resistance and passive component for voltage balancing of series-connected switches, and validity is verified through proper controller selection and formulas.

Conventional voltage regulation strategies that use reactive or active power injection are ineffective in active distribution networks (ADNs) used as conventional power systems. This paper proposes a new way to operate solid-state transformers (SSTs) to improve voltage quality in active distribution networks (ADNs). The proposed control method uses the unused capacity of SSTs to regulate voltage in ADNs without adding any new hardware or overloading the SST. Since SSTs can participate in voltage regulation via reactive and active power control with the proposed method, they can be operated as “enhanced smart inverters” rather than smart inverters that provide voltage regulation service by reactive power regulation. In this paper, the fundamentals of the proposed method are explained and its effectiveness is confirmed with simulation results obtained from an IEEE 13 node test feeder modeled in MATLAB/SIMULINK.

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 solid state transformer (SST) is a power electronic transformer that combines power electronic conversion technology with high frequency power conversion technology based on the principle of electromagnetic induction. Modulation strategies based on the input stage of modular multilevel solid state transformers currently have the following types: nearest level modulation (NLM), carrier phase shift modulation (CPS) and hybrid modulation. This paper focuses on the advantages and disadvantages of the nearest level modulation and hybrid modulation. Considering comprehensively, the hybrid modulation method is adopted in the input stage of the modular multilevel solid state transformer to realize the control of the solid state transformer rectifier stage. The simulation results show that the hybrid modulation method reduced the harmonic content of the grid side output current and the arm circulation to a certain extent.

A Solid State Transformer model for power flow calculations