GR-17-11: Medium Voltage Multi-Port Solid State Transformer Design and Implementation for Microgrids and Distribution Systems.- Saban Ozdemir

Many converter systems, which are connected to grid or special loads, contain a line frequency transformer (LFT) that provides some benefits such as galvanic isolation etc. However, it brings several disadvantages including increased size, weight, cost and decreased power density. Researchers have introduced high frequency and compact solid-state transformers (SSTs) instead of the bulky line frequency transformers for battery, wind and PV to grid application, and energy transmission systems. This topology has superior advantages such as being compact, cost effectivness and galvanic isolation. The increase in power level may require an increase in system voltage level and connection to Medium Voltage (MV). In this case, it requires a step up/down transformer between the inverter and grid and design of the SST according to grid voltage level. Also, connecting multiple sources to grid may requires one SSTs for each device and grid connection. This project proposes a Medium Voltage Multi-Port Solid State Transformer (MV MPSST) which combines four solid state transformers in one common transformer core. The MV MPSST system consists of three low voltage ports and one medium voltage port that allows connection to the medium voltage grid without any additional step-up line frequency frequency transformer. Different ports can use the same core even if their voltage levels are very different. Thus, in addition to reducing cost, size and weight, galvanic isolation of each source is ensured, and control action can be implement with single controller. According to the this topology, desired ports can operate bidirectionally and they can share power consumption/production via regulating port voltage/current value. The energy flow will be controlled by shifting the phase angles of the H bridge converter legs in PV, battery and MV ports. MV port has 4160 VAC grid compatible DC bus voltage level. Simulation studies is performed with Ansys-Maxwell for transformer design and MATLAB/Simulink for converter design. Hardware design involves medium voltage medium frequency transformer design, heat transfer, and converter design, hardware prototype implementation and testing.

PI: Adel Nasiri, UW-Milwaukee
Sponsor: GRAPES
Budget: $43,000.
Period: 2019 – 2019.

Executive Summaries

GR-18-03: Highly Efficient and Compact Medium Voltage Solar PV Inverter- Necmi Altin

Many studies have been presented on various inverter topologies and control algorithms to obtain high efficient, compact and cost effective inverter designs for grid-connected PV systems. However, these studies are mainly focus on low voltage grid systems. The proposed inverters are usually used with a low frequency transformer (LFT) in order to connect to medium voltage grid. The LFT increases the size, volume and cost and decreases the efficiency of the system. The proposed project will develop a novel inverter architecture for solar PV systems, which enables direct connection of the inverter to the Medium Voltage (MV) grid without any LFT. The proposed system has a DC-DC converter stage and an inverter stage. It employs a H-bridge inverter, a high frequency transformer (HFT), a full-bridge diode rectifier and an unfolder inverter circuit. The DC-DC converter stage is capable of reaching MV voltage with appropriate transformer turn ratio. In the proposed system the DC-DC converter stage is controlled to generate rectified sine wave voltage and current at the secondary side DC bus. Thus, the inverter employed at the secondary side (unfolder circuit) operates at line frequency and only inverts the rectified sinewave voltage and current to AC. This design and operation at line frequency reduce the grid side transients and improves efficiency. Replacing the LFT with the HFT provides significant improvements in terms of size, volume, cost and efficiency. In addition, by employing the LLC resonant converter, which can achieve zero voltage switching (ZVS) and zero current switching (ZCS), in the DC-DC converter stage total switching losses can be decreased, and the efficiency of the system is increased. The proposed single-phase resonant converter cells operating in buck-boost mode also enables much wider Maximum Power Point Tracking (MPPT) range. In addition, the proposed system is able to provide Volt-VAR support for the electric distribution systems.

GR-19-02: Development of a Compact Direct Medium Voltage Battery Charger- Saban Ozdemir

Due to the increasing need for low voltage DC supply for high power applications such as electric vehicle charge station, oil and gas fields, and other electrification needs, studies on medium voltage AC to low voltage DC converter are required. The main problem in developing this type of converter is lack of commercial availability for efficient medium voltage switches. While Si based switches cannot generally meet the efficiency and high frequency switching requirements, SiC-based switches are not available at medium voltage scale. Therefore, a step-down transformer is typically required to reduce the voltage for a DC-DC conversion. An Input Series Output Parallel (ISOP) multilevel structure with an additional DC/DC stage allows using low voltage semiconductor switches at medium voltage level, as well as reducing the EMI and harmonics. Major research needs to develop this topology are development of a high-power high frequency transformer and advanced controls to ensure high efficiency. This research project develops a novel architecture for medium voltage AC to low voltage DC converter offering several improvements over the existing state-of-the-art technologies including compact size, modular structure, scalability to different voltage and power level, and high efficiency. The proposed converter structure is selected as series input parallel output multilevel converter with Dual Active Bridge (DAB). The design performs for a MW scale AC-DC-DC converter with commercially available SiC switches that requires 11 modular AC-DC-DC converter structures. Due to time and budget constraint, a three-level single-phase prototype is developing instead of 11 levels. This provides the necessary knowledge, expertise, and experience to ultimately develop the three-phase MV converter. The contribution of this work will include: the robust controller design and development for the multi-level converter stage to create balanced DC voltage and to apply power sharing, design of a control algorithm to reduce power losses by minimizing RMS current in DABs while transferring the same power and development of an optimized HF for the DAB.

PI: Adel Nasiri, UW-Milwaukee
Sponsor: GRAPES
Budget: $61,000.
Period: 2019 – 2021.