Krishna Mainali

Solid-State Transformer and MV Grid Tie Applications Enabled by 15 kV SiC IGBTs and 10 kV SiC MOSFETs Based Multilevel Converters

Medium-voltage (MV) SiC devices have been developed recently which can be used for three-phase MV grid tie applications. Two such devices, 15 kV SiC insulated-gate bipolar transistor (IGBT) and 10 kV SiC MOSFET, have opened up the possibilities of looking into different converter topologies for the MV distribution grid interface. These can be used in MV drives, active filter applications, or as the active front end converter for solid-state transformers (SSTs). The transformerless intelligent power substation (TIPS) is one such application for these devices. TIPS is proposed as a three-phase SST interconnecting a 13.8 kV distribution grid with a 480 V utility grid. It is an all SiC device-based multistage SST. This paper focuses on the advantages, design considerations, and challenges associated with the operation of converters using these devices keeping TIPS as the topology of reference. The efficiency of the TIPS topology is also calculated using the experimentally measured loss data of the devices and the high-frequency transformer. Experimental results captured on a developed prototype of TIPS along with its measured efficiency are also given.

A Transformerless Intelligent Power Substation: A three-phase SST enabled by a 15-kV SiC IGBT

The solid-state transformer (SST) is a promising power electronics solution that provides voltage regulation, reactive power compensation, dc-sourced renewable integration, and communication capabilities, in addition to the traditional step-up/step-down functionality of a transformer. It is gaining widespread attention for medium-voltage (MV) grid interfacing to enable increases in renewable energy penetration, and, commercially, the SST is of interest for traction applications due to its light weight as a result of medium-frequency isolation. The recent advancements in silicon carbide (SiC) power semiconductor device technology are creating a new paradigm with the development of discrete power semiconductor devices in the range of 10-15 kV and even beyond-up to 22 kV, as recently reported. In contrast to silicon (Si) IGBTs, which are limited to 6.5-kV blocking, these high-voltage (HV) SiC devices are enabling much simpler converter topologies and increased efficiency and reliability, with dramatic reductions of the size and weight of the MV power-conversion systems. This article presents the first-ever demonstration results of a three-phase MV grid-connected 100-kVA SST enabled by 15-kV SiC n-IGBTs, with an emphasis on the system design and control considerations. The 15-kV SiC n-IGBTs were developed by Cree and packaged by Powerex. The low-voltage (LV) side of the SST is built with 1,200-V, 100-A SiC MOSFET modules. The galvanic isolation is provided by three single-phase 22-kV/800-V, 10-kHz, 35-kVA-rated high-frequency (HF) transformers. The three-phase all-SiC SST that interfaces with 13.8-kV and 480-V distribution grids is referred to as a transformerless intelligent power substation (TIPS). The characterization of the 15-kV SiC n-IGBTs, the development of the MV isolated gate driver, and the design, control, and system demonstration of the TIPS were undertaken by North Carolina State Universitys (NCSUs) Future Renewable Electrical Energy Delivery and Management (FREEDM) Systems Center, sponsored by an Advanced Research Projects Agency-Energy (ARPA-E) project.

Design Considerations of a 15-kV SiC IGBT-Based Medium-Voltage High-Frequency Isolated DC–DC Converter

A dual active bridge (DAB) is a zero-voltage switching (ZVS) high-power isolated dc-dc converter. The development of a 15-kV SiC insulated-gate bipolar transistor switching device has enabled a noncascaded medium voltage (MV) isolated dc-dc DAB converter. It offers simple control compared to a cascaded topology. However, a compact-size high frequency (HF) DAB transformer has significant parasitic capacitances for such voltage. Under high voltage and high dV/dT switching, the parasitics cause electromagnetic interference and switching loss. They also pose additional challenges for ZVS. The device capacitance and slowing of dV/dT play a major role in deadtime selection. Both the deadtime and transformer parasitics affect the ZVS operation of the DAB. Thus, for the MV-DAB design, the switching characteristics of the devices and MV HF transformer parasitics have to be closely coupled. For the ZVS mode, the current vector needs to be between converter voltage vectors with a certain phase angle defined by deadtime, parasitics, and desired converter duty ratio. This paper addresses the practical design challenges for an MV-DAB application.

Closed loop D-Q control of high-voltage high-power three-phase dual active bridge converter in presence of real transformer parasitic parameters

Three-phase Dual Active Bridge (DAB) Y : Y/Δ composite topology offers advantage of nearly sinusoidal converter-currents without pulse-width modulation, which can be utilized for D-Q mode control implementation. D-Q control is smooth and regulates power-factor of DAB which ensures zero voltage switching (ZVS) operation of the DAB converter at wide-range loading conditions. A practical DAB high-frequency transformer has certain limitations like small leakage-inductance, limited magnetizing-inductance and unwanted parasitic-capacitances which distort the primary-side currents at the rated high-voltage because primary inter-turn capacitance is high in per-unit for a real 100kW transformerdesign. This problem can be solved by using secondary currents and estimated magnetizing current to emulate primary-currents for D-Q control. Parasitic are introduced in the LV TIPS set-up by adding lumped elements to emulate real HV-transformer with objective to test the controls in worst case scenario. This paper proposes the solutions for some of the practical implementation problems of the control algorithm for the DAB.

Design, measurement and equivalent circuit synthesis of high power HF transformer for three-phase composite dual active bridge topology

High voltage high frequency (HF) transformer provides the isolation between high and low dc link voltages in dual active bridge (DAB) converters. Such DAB converters are finding wide applications as an intermediate DC-DC converter in transformerless intelligent power substation (TIPS), which is proposed as an alternative for conventional distribution-transformer connecting 13.8 kV and 480 V grids. The design of HF transformer used in DAB stage of such application is very challenging considering the required isolation, size and cost. In this paper, the specification generation, design, characterization, test and measurement results on a 10kHz HF transformer are presented, highlighting the above challenges.

Medium voltage power converter design and demonstration using 15 kV SiC N-IGBTs

This paper summarizes the different steps that have been undertaken to design medium voltage power converters using the state-of-the-art 15 kV SiC N-IGBTs. The 11 kV switching characterization results, 11 kV high dv/dt gate driver validation, and the heat-run test results of the SiC IGBT at 10 kV, 550 W/cm2 (active area) have been recently reported as individual topics. In this paper, it is attempted to link all these individual topics and present them as a complete subject from the double pulse tests to the converter design, for evaluating these novel high voltage power semiconductor devices. In addition, the demonstration results of two-level H-Bridge and three-level NPC converters, both at 10 kV dc input, are being presented for the first-time. Lastly, the performance of two-chip IGBT modules for increased current capability and demonstration of three-level poles, built using these modules, at 10 kV dc input with sine-PWM and square-PWM modulation for rectifier and dc-dc stages of a three-phase solid state transformer are presented.

Design considerations of a 15kV SiC IGBT enabled high-frequency isolated DC-DC converter

The advent of the 15kV SiC IGBT device has made a single series stage medium-voltage (MV) and high-frequency (HF) DC-DC Dual Active Bridge (DAB) converter application viable. The Y: Y/Δ three-phase DAB is a high-power isolated DC-DC converter based on three-level neutral-point clamped (NPC) on the MV side. A MV/HF transformer used in the DAB, has significant parasitic capacitances, which cause ringing in the DAB current under high dV/dT switching. In addition, the converters need sufficient dead-time between complimentary switches to avoid possibility of any shoot-through. The length of the dead-time depends on switching characteristics. Both the dead-time and transformer parasitics affect zero voltage switching (ZVS) performance of the DAB. Thus, the DAB design has to be closely coupled with the switching characteristics of the devices and MV/HF transformer parasitics. For the ZVS mode, the current-vector needs to be between converter voltage vectors with a certain margins defined by dead-time, parasitics and desired duty ratio of three-level MV converter. This paper addresses these design challenges for the MV DAB application.

A MV intelligent gate driver for 15kV SiC IGBT and 10kV SiC MOSFET

This paper presents an Intelligent Medium-voltage Gate Driver (IMGD) for 15kV SiC IGBT and 10kV SiC MOSFET devices. The high voltage-magnitude and high dv/dt(> 30kV/μs) of these MV SiC devices, pose design challenge in form of isolation and EMI. This problem is solved by development of a <; 1pF isolation capacitance power-supply. But due to applied high stress, smaller short-circuit withstand time and the criticality of the application, these devices need to be monitored, well protected, active gate-driven and safely shut-down. This paper presents an EMI hardened IMGD built around a CPLD, sensing and optical interfacing unit. It provides advanced gate-driving, added protection and optically isolated state-monitoring features. The device operating conditions such as module temperature and Vds(on) can be data-logged. They can be used for diagnosis/prognosis purposes such as to predict failure and safely shut-down the system. This paper describes the functionality of different building blocks. The 15kV SiC IGBT has higher second switching slope above its punch-through level which is moderated without increasing losses by using digitally controlled active gate-driving. The shoot-through protection time can be reduced below withstand time by advanced gate driving. Soft turn-on and over-current triggered gate-voltage reduction helps reducing blanking time and quick turn-off reduces the protection response time. In this paper, the IMGD is high side tested at 5kV with device state monitoring on. The active gate-driving is tested at 6kV.

Medium voltage (≥ 2.3 kV) high frequency three-phase two-level converter design and demonstration using 10 kV SiC MOSFETs for high speed motor drive applications

High speed variable frequency motor drives are required for marine applications, compressors for oil and gas industries, wind energy generation systems etc. Traditionally, low voltage high speed motor drives are used in such applications. This results in large currents at high power levels leading to large copper loss in the motor winding. Therefore, medium voltage (MV) drives are being considered. The silicon (Si) based MV drives need gears to increase the speed due to low switching frequency operation of Si devices in the converter. Gears reduce both efficiency and power density. With the development of 10 kV SiC MOSFET, high switching frequency at MV is possible, which has enabled the scope of high power density MV direct drive variable speed controlled motors. In this paper, the design of a three-phase, 2-level, ≥ 2.3 kV MV, high frequency converter based on 10 kV SiC MOSEFT is explained. Performance analysis is presented along with experimental demonstration.

Design and evaluation of isolated gate driver power supply for medium voltage converter applications

The commercial gate drivers are available upto 6.5 kV IGBTs. With the advances in the SiC, power devices rated beyond 10 kV are being researched. These devices will have use on medium voltage power converters. Commercial gate drivers rated for such high voltages are not available. These power devices have very high dv/dts (30-100 kV/μs) at switching transitions. Such high dv/dts bring in challenges in the gate driver design. The isolation stage of the gate power supply needs to have very low coupling capacitance to limit the high frequency circulating currents from reaching the gate driver control circuits. Also, the isolation stage has to be designed with insulation several times higher than the peak system voltage level. In this paper, design, development and evaluation of the gate power supply for medium voltage level applications have been investigated. Several isolation transformer designs have been investigated and optimum design, with very low coupling capacitance ≈ 0.5 pF, has been identified and used in the gate driver design. Experimental characterization of the transformer has been done. The performance of the gate driver power supply has been evaluated in several MV power converters, using 10 kV SiC MOSFETs.