Reserved-Energy-Aided Control in Solid-State Transformers

The solid-state transformers with input-series output-parallel structure are considered for various applications where MVAC to LVDC conversion is needed. However, due to the inherent second harmonic pulsating power in the single-phase AC system, large DC link capacitors are required to smooth the voltage ripple of all floating cells, which reduces achievable power density. The active power decoupling method can shift away the pulsating power from the DC link capacitors, leading to a significant reduction to the DC link capacitance and resulting voltage ripple. So far, the application of active power decoupling is limited to single-level converters, and little research on the impact in modular converters has been done. This paper investigates the feasibility of applying active power decoupling to solid-state transformers. The active power filter with dc-split-capacitor circuit is selected to minimize the DC link capacitance. Simulation results show that the active power filter can significantly reduce the second harmonic voltage ripple and the total DC link capacitance of the SST. A prototype of the active power filter has been built and tested to verify the design.

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 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.

In this paper, a new linear-based closed-loop control method for a Solid-State Transformer (SST) has been proposed. In this new control method, individual current and voltage loops for each of the power conversion stages (AC-DC, DC-DC, DC-AC) are implemented. The feedback between the input and output control signals for each loop is achieved through the voltage on the DC link capacitors and the current transferred between the converters. This enables the SST to be controlled easily in a linear-based closed-loop manner without the need for complex computations. In order to evaluate the performance analysis of the proposed control system, a simulation of an SST with approximately 10 kVA apparent power was performed. Based on the obtained simulation results, the response time of the proposed control method for dynamic load variations was proved to be in the range of 40 milliseconds, and it has been observed that this method allows electrical power to be transferred from the load to the grid. The power factor value of SST under inductive load is measured to be approximately 99%, and the overall system efficiency is 96% and above, indicating that this proposed new control method has very high performance.

Solid State Transformer (SST) is expected to be a key component in future smart grid systems, which integrates MVAC grid with LV DC/AC grid. For a three stage SST, resonant converter is a potential candidate as the DC-DC stage due to high conversion efficiency. However, it exhibits an oscillation of 100Hz (double line frequency) in the high frequency (HF) link current, which increases device current stress and also increases conduction loss. This paper proposes an analytical model which determines the magnitude of the double line frequency oscillation in terms of input line current, input power factor, modulation index of AC-DC stage, and DC bus capacitors. The obvious solution to minimize the double line frequency oscillation is to keep a high value of capacitor at the MVDC bus. However, it reduces the power density of the system. Therefore, this paper proposes an improved solution with a combination of DC-link inductor and capacitor at LVDC bus, which reduces the double line frequency current oscillation in HF link without the need of high MVDC bus capacitors. The design is verified in a laboratory prototype of 8 kVA, 1.65kV/300V solid state transformer.

This article proposes a Distributed-control Architecture (D-CA) and an operation sequence with start-up strategies for a Digital Signal Processor (DSP)-based Solid-State Transformer (SST). Although various control techniques for SSTs have been reported in earlier studies, there is still a lack of research covering comprehensive content, including hierarchical control architectures and operation sequences with start-ups considering the implementation of DSPs. Therefore, this article addresses the following factors of SST. First, the D-CA is described for the design of the hierarchy between control boards. With the D-CA, because sub-boards are in charge of their corresponding DC-link voltage balancing control individually, the computational burden on the master board can be reduced. Second, the operation sequence of the SST system is explained based on the SST with D-CA. The step of DC-link voltage balance is considered throughout the entire operation sequence for safe driving. Furthermore, the PWM start-up strategies for a Cascade H-bridge Multilevel (CHM) converter and Dual Active Bridge (DAB) converter are proposed to prevent switching pulse errors caused by DSP operating characteristics. These start-up strategies reduce the current surges. The validity of the proposed D-CA and operation sequence with start-up strategies are verified by experimental results.

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 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.

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.

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.

A solid state transformer (SST), as a kind of energy router in the Energy Internet, provides a unified access point for AC or DC distributed power subjects. However, the DC-link capacitors inside the SST will suffer huge voltage fluctuations when the output power of the microgrid changes dramatically. With respect to this problem, caused by the random and intermittent characteristics of distributed generation (DG), a hierarchical coordinative control strategy is proposed. Compared with the common independent control, the proposed method not only makes full use of the regulation capacity of super capacitors, but also enhances the dynamic power tracking speed and reduces the speed difference between different stages of an SST. The dynamic voltage response under the proposed method is analyzed in frequency domain and compared with the independent control. To validate the effectiveness of the coordinative control strategy, a simulation model of an SST-based grid-connected DC microgrid system is established, and the topology of the SST is improved. The voltage stability of the DC bus is compared under different control strategies, and the coordinative control strategy is also verified, effectively under transition conditions.

Multiport converters are used in interfacing of distributed energy sources with grid/load. Isolated converters are needed in applications where converter gain is high and there is a requirement of isolation. Dual transformer asymmetric triple active bridge offers the advantage of reduced circulating current. However, the operating range is low for variation in load and source voltage. In this paper duty ratio modulation technique is proposed to regulate the load voltage and control the power flow in both the directions. As a result of the new gating scheme, the converter switches operate with ZVS, irrespective of variations in load power and source voltage. The converter is designed to ensure high switch utilization. The control technique is validatedthrough simulation of a 1kW three port DC-DC converter. It was observerd that the load voltage was regulated for wide range of variation in load power and source port voltages. The single input dual output mode was also verified.

In the future AC-DC hybrid distribution system, solid state transformer (SST) will take place of conventional transformer, as it could reduce the weight and volume of transformer, and achieve voltage and current regulation, power flow control, power quality enhancement. MMC-based SST is a novel SST topology, which could provide the MVDC port. However, the excessive bulky submodule capacitors limits this topology into practical application. This paper proposes a fluctuation power control strategy for MMC-based SST to reduce the submodule capacitor voltage oscillation, which could optimize the capacitance and reduce the volume and cost of capacitor. The feasibility of this proposed method is verified by MATLAB simulation.

The soft-switching solid-state transformer (S4T) employs only 12 main active devices and an auxiliary resonant circuit to implement a bidirectional solid-state transformer, with an attractive feature of achieving a full range of zero-voltage-switching conditions for all the main devices. This paper covers detailed design of the power stage, auxiliary resonant circuit, and control of the S4T. The high-frequency transformer is an essential element for the S4T, and it has a unique feature of dc-biased flux. Design of such a high-frequency transformer is also discussed in detail in this paper. Soft startup, shutdown, and fast dynamic response under load transients are also attractive behaviors because of the low inertia of the S4T. Experimental results from a 208-V/10-kV·A S4T unit are presented.

This paper has the purpose to investigate the variation of electric field inside a HVDC cable insulation, and consequent potentiality of partial discharge inception in a defect upon load current variation. Simulations carried out by COMSOL Multiphysics show that load variations can incept partial discharges during cable operation at a constant voltage, depending on insulating material characteristics, defect size and location, and, in particular, the temperature and field dependence of electrical conductivity. Different values for temperature and field dependency of electrical conductivity, as well as of thermal conductivity and heat capacity are considered and their effect on the field distribution inside cavities near inner and outer semiconducting layer analyzed. For this purpose, a new analytical model to predict the behavior of PD amplitude and repetition rate from cable energization to steady state, including load variation, is introduced. The likelihood of PD inception during load transients, and not only voltage transients as previously demonstrated, poses further attention and constraints to HVDC cable design.