Divan, Deepakraj M.
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ItemSiC-Based 5 kV Universal Modular Soft-Switching Solid-State Transformer (M-S4T) for Medium- Voltage DC Microgrids and Distribution Grids(Georgia Institute of Technology, 2021-03) Zheng, Liran ; Han, Xiangyu ; An, Zheng ; Kandula, Rajendra Prasad ; Kandasamy, Karthik ; Saeedifard, Maryam ; Divan, Deepakraj M. ; Georgia Institute of Technology. School of Electrical and Computing EngineeringMedium-voltage DC (MVDC) grids are attractive for electric aircraft and ship power systems, battery energy storage system (BESS), fast charging electric vehicle (EV), etc. Such EV or BESS applications need isolated bidirectional MVDC to LVDC or LVAC converters. However, the existing Si-based solutions cannot fulfill the requirements of a high-efficiency and robust converter for MVDC grids. This paper presents a 5 kV SiC-based universal modular solid-state transformer (SST). This universal current-source SST can interface either a LVAC or LVDC grid with a MVDC grid in single-stage power conversion, while the conventional dual active bridge (DAB) converter needs an additional inverter. The proposed SST module using 3.3 kV SiC MOSFETs and diodes is bidirectional and can serve as a building block in series or parallel for higher-voltage higher-power systems. The topology of each module is based on the soft-switching solid-state transformer (S4T) with reduced conduction loss, which features reduced EMI through controlled dv/dt, and high efficiency with full-range ZVS for main devices and ZCS for auxiliary devices. Operation principle of the modular S4T (M-S4T), capacitor voltage balancing control between the cascaded modules, design of components including a medium-voltage (MV) medium-frequency transformer (MFT) to realize a 50 kVA 5 kV DC to 600 V DC or 480 V AC M-S4T are presented. Importantly, the MV MFT prototype achieves very low leakage inductance (0.13%) and 15 kV insulation with coaxial cables and nanocrystalline cores. Here, the proposed universal modular SST is compared against the DAB solution and verified with DC-DC and DC-AC simulation and 4 kV experimental results. Significantly, the MV experimental results of a modular DC transformer with each module at MVDC are rarely covered in the literature and reported for the first time.
ItemNew power converter topologies for minimizing energy consumption of electronic appliances(Georgia Institute of Technology, 2011-06-01) Lambert, Frank C. ; Divan, Deepakraj M. ; Georgia Institute of Technology. Office of Sponsored Programs ; Georgia Institute of Technology. School of Electrical and Computer Engineering ; Georgia Institute of Technology. Office of Sponsored Programs
ItemStacked Low-Inertia Converter or Solid-State Transformer: Modeling and Model Predictive Priority-Shifting Control for Voltage Balance( 2021-01) Zheng, Liran ; Kandula, Rajendra Prasad ; Kandasamy, Karthik ; Divan, Deepakraj M. ; Georgia Institute of Technology. School of Electrical and Computing EngineeringThis paper presents control challenges of stacked low-inertia converter (SLIC) or cascaded reduced dc-link solid-state transformer (SST) and proposes a novel model predictive priority-shifting (MPPS) control with implicit modulator and a discrete-time large-signal model for voltage balancing and dc-link regulation. Low-inertia converters, featuring small electrolytic capacitor-less dc links, dramatically reduce cost, size, and weight compared to conventional solutions. However, without a large dc-link buffer, the input and output are tightly coupled, leading to significant control challenges. The control becomes even more challenging with these converters stacked input-series output-parallel (ISOP) for medium-voltage (MV) grid, which causes coupling between the modules besides the coupling within each module. This paper analyzes the multi-objective, multi-degree of freedom control problem, using the modular soft-switching solid-state transformer (M-S4T) as an example of the SLIC. First, distribution of control efforts under controller saturation is critical because multiple control objectives can be conflicting, especially when the module voltages are unbalanced and are being restored. The MPPS can shift the priorities to address this issue. Second, due to the low inertia and high dc-link ripple, classic space vector pulse-width modulation (SVPWM), average model with small-ripple assumption, and control design based on small-signal model cannot accurately modulate, model, and control the nonlinear reduced dc link. Therefore, a discrete-time large-signal model of the M-S4T is established to derive the predictive control in the MPPS. The MPPS and the PI control are compared in MV simulations to show the issue of applying the PI to the SLIC and the effectiveness of the MPPS for voltage balancing and dc-link regulation in a deadbeat manner. Finally, the proposed control is tested on a 5 kV ISOP SiC SST prototype to verify priority shifting to address controller saturation issue and fast and robust voltage balancing.
ItemDC-Link Current Minimization Control for Current Source Converter-Based Solid-State Transformer(Georgia Institute of Technology, 2022-05) Zheng, Liran ; Han, Xiangyu ; Kandula, Rajendra Prasad ; Divan, Deepakraj M. ; Georgia Institute of Technology. School of Electrical and Computing EngineeringThis article proposes a fast predictive control method and a small DC-link inductor to minimize the DC-link current in current-source converter (CSC)-based solid-state transformer. The DC-link current minimization can significantly reduce power loss and improve efficiency. The challenge of this problem is on improving both steady-state and dynamic performance. PI control methods and large DC-link inductors are conventionally used in the CSC but have limited dynamic performance. A model predictive control (MPC) method is proposed to achieve switching-cycle-level settling time, and the DC-link inductor is sized for 40% ripple to enable fast current change. Importantly, this article also proposes to minimize the DC-link current by varying the current even within a line cycle under single-phase load to improve the steady-state performance, in contrast with the reduction to a constant value in the literature. The proposed MPC features a constant switching frequency without weighting factors. The MPC does not have a high computational burden and is implemented in a regular digital controller for a prototype of soft-switching solid-state transformer (S4T) with reduced conduction loss. The effectiveness of the proposed method has been experimentally verified on the SiC S4T prototype during steady-state and dynamics under different multiport power flow conditions up to 2 kV peak. The DC-link current in the experiments is close to the minimum current with a short zero-vector duration, which further verifies the performance of the proposed method.
ItemRobust Predictive Control for Modular Solid-State Transformer with Reduced DC Link and Parameter Mismatch(Georgia Institute of Technology, 2021-06) Zheng, Liran ; Kandula, Rajendra Prasad ; Divan, Deepakraj M. ; Georgia Institute of Technology. School of Electrical and Computing EngineeringThis paper presents the analysis and implementation of a predictive control method for dc-link regulation and voltage balance in a cascaded modular reduced dc-link solid-state transformer (SST). Passive components like bulky dc links limit the power density of power converters, especially medium-voltage (MV) SST. Reduced dc-link or low-inertia converters can dramatically reduce the size, cost, and weight by tolerating larger dc-link ripples and improve the reliability with electrolytic capacitor-less dc link. However, a small dc link leads to tight coupling between the input and the output stages, which is a challenge for control design. In stacked low-inertia converters (SLIC), the low-inertia converter modules are stacked for MV applications, resulting in coupling between the modules and making the control more challenging. A new model predictive control method which can achieve deadbeat regulation on the dc link without weighting factors has been proposed to address this novel problem. This paper focuses on analyzing the condition of the low-inertia dc link up to 80% ripple, the robustness of the control under parameter mismatches, high-order terms, and important implementation issues such as model-based sampling and computation delay compensation. Significantly, the high-order terms are introduced because of the large dc-link ripple. These high-order terms are unique to the SLIC and negligible in conventional high-inertia converters. A discrete-time large-signal model is built to capture the dc-link’s nonlinear dynamics, and the eigenvalues of a small-signal Jacobian matrix are analyzed with Floquet theory to evaluate stability, using the modular soft-switching solid-state transformer (M-S4T) as an example of the SLIC. Simulation and experimental results of an MVDC M-S4T verify the analysis and the predictive control method. Finally, the general application of the predictive control to low-inertia converters is compared against a conventional PI controller using a reduced dc-link active-front-end (AFE) rectifier as an example.
ItemA Survey on Technologies for Implementing Sensor Networks for Power Delivery Systems(Georgia Institute of Technology, 2007-06) Lambert, Frank ; Yang, Yi ; Divan, Deepakraj M. ; Georgia Institute of Technology. National Electric Energy Testing, Research and Applications CenterThe task of monitoring asset status and optimizing asset utilization for the T&D industry, given millions of assets and hundreds of thousands of miles of power lines distributed geographically over millions of square miles, seems particularly challenging if not impossible. Given the traditionally high cost of sensing and communications, the grid has minimal ‘smarts’ with much of the intelligence located at major substations. Dramatic reductions in sensor, computing and communications costs, coupled with significant performance enhancements has raised the possibility of realizing widely and massively distributed sensor networks (SNs) to monitor utility asset status. Under NEETRAC funding, a survey was conducted to review existing sensor technologies and products, and to estimate the possibility of extending these to realize distributed SNs. Possible applications for such SNs were also explored, as was the issue of cost point at which such networks would become commercially viable. This paper provides an overview of the highlights from the detailed survey that was conducted, and identifies ‘gaps’ in currently available sensor technologies, both from a performance and cost point.
ItemPredictive Direct DC-Link Control for 7.2 kV Three-Port Low-Inertia Solid-State Transformer with Active Power Decoupling(Georgia Institute of Technology, 2022-05) Zheng, Liran ; Kandula, Rajendra Prasad ; Divan, Deepakraj M. ; Georgia Institute of Technology. School of Electrical and Computing EngineeringPromising for applications including renewable energy and electric vehicle fast charging, a medium-voltage (MV) solid-state transformer (SST) typically has multiple modules series stacked, which requires the modules to be single phase. One critical issue of the single-phase SST is the double-line-frequency power ripple. Traditionally, large passives are used to buffer the ripple, resulting in significantly increased volume and cost. This article for the first time proposes active power decoupling (APD) for the SST and a predictive direct DC-link control method, using a current-source single-stage SST as an example. An electrolytic capacitor-less buffer port is used to absorb the ripple, which tolerates up to a 30% voltage ripple for small capacitance. The APD SST control is challenging. First, the current-source SST realizes isolation, DC link, and multiport in a single stage. Second, different from conventional SSTs or APD converters, the low-inertia DC link in this SST has a 40% switching ripple, which is difficult to stabilize. To address the control challenge, a direct DC-link control architecture and a predictive control method are proposed. Simulation and experimental results verify the proposed control method on an MV SiC modular soft-switching solid-state transformer (M-S4T) prototype. The proposed APD reduces the SST volume by 53.8%. Importantly, the proposed concept is not limited to the M-S4T but is generic to other SSTs or APD converters.
ItemSoft-Switching Solid-State Transformer (S4T) With Reduced Conduction Loss( 2020-10) Zheng, Liran ; Kandula, Rajendra Prasad ; Divan, Deepakraj M. ; Georgia Institute of Technology. School of Electrical and Computing EngineeringSolid-state transformers (SSTs) are a promising solution for photovoltaic (PV), wind, traction, data center, battery energy storage system (BESS), and fast charging electric vehicle (EV) applications. Traditional SSTs are typically three-stage, i.e., hard-switching cascaded multilevel rectifiers and inverters with dual active bridge (DAB) converters, which leads to bulky passives, low efficiency, and high EMI. This paper proposes a new soft-switching solid-state transformer (S4T). The S4T has full-range zero-voltage switching (ZVS), electrolytic capacitor-less dc-link, and controlled dv/dt which reduces EMI. The S4T comprises two reverse-blocking current-source inverter (CSI) bridges, auxiliary branches for ZVS, and transformer magnetizing inductor as reduced dc-link with 60% ripple. Compared to the prior S4T, an effective change on the leakage inductance diode is made to reduce the number of the devices on the main power path by 20% for significant conduction loss saving and retain the same functionality of damping the resonance between the leakage and resonant capacitors and recycling trapped leakage energy. The conduction loss saving is crucial, being the dominating loss mechanism in SSTs. Importantly, the proposed single-stage SST not only holds the potential for high power density and high efficiency, but also has full functionality, e.g., multiport DC loads integration, voltage regulation, reactive power compensation, unlike traditional single-stage matrix SST. The S4T can achieve single-stage isolated bidirectional DC-DC, AC-DC, DC-AC, or AC-AC conversion. It can also be configured input-series output-parallel (ISOP) in a modular way for medium-voltage (MV) grids. Hence, the S4T is a promising candidate of the SST. The full functionality, e.g., voltage buck-boost, multiport, etc. and the universality of the S4T for DC-DC, DC-AC, and AC-AC conversion are verified through simulations and experiments of two-port and three-port MV prototypes based on 3.3 kV SiC MOSFETs in DC-DC, DC-AC, and AC-AC modes at 2 kV.
ItemMultiport Control with Partial Power Processing in Solid-State Transformer for PV, Storage, and Fast-Charging Electric Vehicle Integration(Georgia Institute of Technology, 2022-09) Zheng, Liran ; Kandula, Rajendra Prasad ; Divan, Deepakraj M. ; Georgia Institute of Technology. School of Electrical and Computing EngineeringThis article proposes a multiport control method to enable partial power processing (PPP) in a medium-voltage (MV) multiport solid-state transformer (SST). MV multiport SSTs are promising in integrating low-voltage DC sources or loads such as solar photovoltaic, energy storage, and electric vehicles into smart grids without bulky line-frequency transformers. Compared to voltage-source SST, current-source (CS) SST features single-stage isolated bidirectional AC/AC, AC/DC, or DC/DC conversion using an inductive DC link. For a multiport CS SST, it is revealed in this article that the PPP capability can be enabled through the proposed control without extra hardware, different from the case of voltage-source converters where special hardware architecture is required for the PPP. With the PPP, most power exchange between LV ports is processed by only a fraction of the entire conversion stage, leading to reduced DC-link current, volume, loss, and improved efficiency. The proposed multiport PPP control scheme is analyzed to verify the advantages across a wide voltage and power range against conventional full power processing (FPP) multiport control, using the soft-switching solid-state transformer (S4T) with reduced conduction loss as an example. Comparative experimental results based on a SiC three-port S4T prototype verify the effectiveness of the proposed PPP scheme against the FPP scheme under both steady state and dynamic conditions. The DC-link current reduction is measured to be more than 36%. Significantly, the proposed multiport PPP control scheme is generic and applicable to any hard-switching or soft-switching CS SSTs without extra hardware.
ItemPower line sensor nets for enhancing line reliability and utilization(Georgia Institute of Technology, 2010-07-31) Divan, Deepakraj M. ; Harley, Ronald G. ; Habetler, Thomas G. ; Georgia Institute of Technology. Office of Sponsored Programs ; Georgia Institute of Technology. School of Electrical and Computer Engineering ; Georgia Institute of Technology. Office of Sponsored Programs