Full Order Averaged Modelling for Modular Solid State Transformer

This paper presents the design of new highfrequency transformer isolated bidirectional dc-dc converter modules connected in module-cascaded solid state transformer (SST). A phase-shift dual active bridge (DAB) converter is employed to achieve high-frequency galvanic isolation, bidirectional power flow, and zero voltage switching (ZVS) of all switching devices, which leads to low switching losses even with high-frequency operation. Bidirectional DC-DC converter is crucial to the power transmission in SST. The proposed DAB converter consists of two three-leg bridges and a high-frequency transformer with winding shunting taps. Furthermore, the commutation inductance connected in series with the transformer in the conventional DAB converter is integrated into the transformer windings, thus, enjoying smaller volume and higher power density. By changing the number of primary winding turns, leakage inductance, which is the key parameter in the energy transfer process of DAB converter, can be adjusted on a large scale, enabling the possibility of reducing loss over wide load range. Additionally, time-sharing circuit topology and operation mode are adopted to adapt to the output power by detecting the output current as the feedback. As a result, the efficiency at both light and heavy load can be significantly improved compared with the conventional DAB converter, and, therefore, high efficiency over wide load range and high power density can be achieved. Besides, the additional bridge arm structure along with a flexible control scheme provides possible high-level fault tolerance. Finally, the simulation results on a 2-kW DAB converter module switching at 10kHz are presented to validate the theoretical analysis.

Modular solid-state transformers (SSTs) are a promising technology in converting power from a 10kV three-phase medium voltage to a lower DC-voltage in the range of 100…400V to provide pure DC power to applications such as electrolyzers for hydrogen generation, data centers with a DC power distribution and DC micro grids. Modular SSTs which can be interpreted as modular multilevel converters with an isolated DC-DC output stage per module, are designed with redundant modules to increase reliability. Usually, each of the three arms operates independently, and therefore, only a fixed number of faulty modules can be compensated in each arm, even if all modules are operational in the remaining two arms. With the proposed zero-sequence voltage injection, up to 100% more faulty modules can be compensated in an arm by employing the same hardware. In addition, module power imbalances are nearly eliminated by utilizing a fundamental frequency zero-sequence voltage. A dominant 3rd harmonic zero-sequence voltage injection in combination with the 5th, 7th and several higher order harmonics with adaptive (small) amplitudes minimize the required arm voltages at steady-state. For nominal operation or symmetrical faults, the proposed technique is equivalent to the well known Min-Max voltage injection, which already reduces the peak arm voltage by 13.4% (to $\frac{{\sqrt 3 }}{2}$) compared to a constant star point potential. A statistical analysis proves, that the expected number of tolerable faulty modules of the 1MW SST increases by 12% without the need for additional hardware.

Solid-state transformers (SSTs) are promising in the flexible dc transmission and dc microgrid. However, the core equipment of SST, dual active bridge (DAB) converter, is only modeled by dc and 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> harmonic components, which is not precise enough for control design and stability estimation. Generalized averaging model considering the second harmonic component (SHC-GAM) for the DAB converter is proposed in the paper. The 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> harmonic component is considered to improve the switching dynamics of the DAB converter, and the small-signal model and control-to-output gain are studied. The magnitudes for the control-to-output transfer function of SHC-GAM and the detailed switching model are compared. The results are in good agreement. The SHC-GAM of the DAB converter for SST is accurate enough and promising for practical needs.

In this paper the results from a prototype low voltage DC input (<30 VDC) kilowatt class high frequency parallel-loaded resonant AC-link inverter is described. This system delivers utility grade voltage-source power at electronically selectable 50, 60, or 400 Hz. Cycle-by-cycle bidirectional current flow reduces the need for intermediate energy storage and allows for operation in a wide range of power factor with passive stability. The high frequency AC link and voltage boost stage is comprised of a zero voltage, zero current switching full-bridge inverter. This soft switching high frequency inverter is well suited for handling the large currents expected from a low voltage source. The output inverter is connected directly to this high frequency AC-link and zero voltage switching is employed in the output inverter to eliminate switching loss. A discretized sinusoidal carrier wave modulation (DSCWM) algorithm is used to control the high frequency boost inverter. The switching frequency of the DSCWM algorithm is fixed in relation to the frequency of the resonant high frequency AC link. The discretization of the modulation algorithm allows for the high frequency link to be discontinuous thus improving the zero voltage switching of the inverter. The elimination of a rectifier stage and reduction of the intermediate energy storage delivers a high power density design when compared to conventional dc-dc-ac methods. While switching loss has been greatly minimized, AC resistance due to high frequency operation and large peak currents present must be addressed in the interconnections and magnetic design.

One of the main disadvantages of the multi-level solid-state transformer (SST) system is the voltage imbalance on the output of the rectifier modules. This voltage imbalance can be caused by parameter mismatch of the active and passive components, different loads, and the floating structure of the high voltage DC-links. Some studies have been done to solve this voltage imbalance problem. A common way to avoid this imbalance is to balance the voltage of DC-links at the AC/DC conversion stage and balance the power between the modules at the DC/DC conversion stage. Most of these methods require a complex balancing controller or additional circuits. This paper proposes a novel dual active bridge (DAB) converter specialized in power balancing in a single-phase 5-level SST system. The proposed DAB converter does not require any additional balancing controllers or techniques for power balancing. The performance of the proposed DAB converter was verified by simulation and experiments using a 3 kW 5-level SST prototype system.

The solid-state transformer (SST) has the ability to operate with only two input phases, which can be called single-phase open-circuit (SPO) operation. With this ability, the SST can maintain operation under permanent single-phase-to-ground faults, where the fault phase must be isolated, providing a better power supply sustainability than that of the conventional low-frequency transformer. In the absence of research on the SPO operation of an SST, this paper presents a study of it. The influence of SPO operation on the grid and the SST itself is analyzed. Under the SPO state, the SST will inject the negative-sequence current to the grid; meanwhile, the maximum load power will decline; thus, the optimized power factor under the SPO state is unity. The maximum power of △-SST is higher than that of the Y-SST under the SPO state owing to the △ connection. A control strategy for the SST medium-voltage cascaded H-bridge stage is proposed. SPO current control can be achieved by controlling the positive-sequence current. A proper negative-sequence voltage can be injected to synchronize the cluster voltages to obtain a lower current total harmonic distortion and smaller dc voltage ripple. In the △-SST, a proper zero-sequence current can be injected to fully use the power rating of the double-cluster branch.

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.

As the size and the cost of power semiconductor switches are decreasing, converter topologies with high device count are starting to draw more attention. One such type of converter is the high frequency AC (HFAC) link converters. A popular control method for these converters is pulse density modulation (PDM). The HFAC link voltage of the converter in this paper is a high frequency, three-step, variable pulse-width (PW) square wave voltage waveform. A genetic algorithm approach is used to determine the PW to optimize the output voltage harmonic content.

The objective of this research is to investigate the feasibility of a high frequency AC link (HFACL) converter as a phase shifter in a power transmission system (PTS). The approach is to analyze its basic operation in a PTS and to develop a detailed model of the HFACL phase shifter for EMTP simulation study. The design method of the HFACL phase shifter and the control algorithm to implement a HFACL phase shifter as a power flow controller in the PTS are proposed. In the process of the simulation study, useful technical evaluations of the HFACL phase shifter are performed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A grid-tied multi-string PV inverter with a high-frequency ac (HFAC) link and soft switching operation is introduced. This single-stage topology can handle an arbitrary number of PV strings with different electrical parameters, locations, and orientations. Using a dedicated maximum power point tracker for each PV string, this inverter can harvest the highest possible power from each string independently, even though they might be at dissimilar irradiance levels and operating temperatures. The HFAC link is formed by an ac inductor and small ac capacitor without the use of bulky short-life electrolytic capacitors. The link is responsible for transferring the PV strings' power to the grid along with creating zero-voltage switching for the power devices. Therefore, the converter is expected to have high power density, high reliability, as well as high efficiency. If galvanic isolation is desired, a small-sized HF transformer can be easily employed in the HF link. After describing the control scheme and operating algorithm of the proposed multi-string inverter, a detailed analysis is carried out. Experimental results are also given to prove the effectiveness of the proposed PV topology.

Dual Active Bridge (DAB) Converter and PushPull Converters are mainly used for Conversion of power with large cycles per second. Dual active bridge converter consists of couple of full bridge converters linked through a transformer. Two way power flow capacity, high power density, high proficiency, soft switching, less number of passive elements and galvanic separation are the important characteristics of DAB pulling in industry. The push-pull converter is utilized as DC-DC converters are reasonable for low and medium power application. High efficiency, low switching loss and high power density are the important characteristics of this converter. By increasing the switching frequency, it results in the reduction of complexity of the circuit. However the switching loss increases as the switching frequency is increased. In this paper, the study is focused on a comparative analysis of DAB converter and Push-Pull converter.

In this paper, the design and comparison of single and three-phase transformers in Dual Active Bridge (DAB) Converter are carried out. DAB converter topology and operating schemes are introduced. And core-type transformers design optimization methods are proposed for both single and three-phase DAB converters (DAB1 and DAB3). Based on DAB topology assumptions, single and three-phase transformers are compared in volume with various design constraints. And analysis shows that comparison result varies at different cases of flux and thermal limits. Scaled-down DAB1 and DAB3 systems have been built and temperature rises of the transformers have been tested for verification.

This paper presents a thorough scope of a wide-used single-phase bidirectional battery charger which composes of a pulse-width modulation (PWM) converter in the front-end and a dual active bridge (DAB) converter for high power applications. The computer simulations of PWM converter and DAB converter are separately shown to support theoretical analyses. A prototype converter was built and tested at different operating conditions. Experiment results at 3.3 kW are given to verify theory and simulation results as well.

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.

The electrical distribution network is continuously changing due the fact that in recent years the distributed generation sources interacting with it have increased, this impacts direcly the energy consumption dynamics because the network does not just deliver energy but can also receive it. These new operating conditions requires new and modern conditioning stages and power flow control rules to ensure optimal system performance. One of the alternatives currently being studied and developed to face this challenge are Solid State Transformers (SST). These devices are composed by Power Electronic Converters and have diverse attractive features which make them a vital, world-class study case; one of his main requirements its to ensure a proper operation on the SST power flux direction. This paper describes a control proposal applied to a Dual Active Bridge (DAB) converter, which is part of an SST; the proposal serves as the starting point to establish new control rules that satisfy the basics criteria that a bidirectional converter needs to meet.