There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Gallium nitride (GaN) power devices are an emerging technology that have only recently become available commercially. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This paper reviews the characteristics and commercial status of both vertical and lateral GaN power devices, providing the background necessary to understand the significance of these recent developments. In addition, the challenges encountered in GaN-based converter design are considered, such as the consequences of faster switching on gate driver design and board layout. Other issues include the unique reverse conduction behavior, dynamic $R_{\mathrm {{ds}},\mathrm {{on}}}$ , breakdown mechanisms, thermal design, device availability, and reliability qualification. This review will help prepare the reader to effectively design GaN-based converters, as these devices become increasingly available on a commercial scale.

Review of Commercial GaN Power Devices and GaN-Based Converter Design Challenges

Silicon carbide (SiC) switching power devices (MOSFETs, JFETs) of 1200 V rating are now commercially available, and in conjunction with SiC diodes, they offer substantially reduced switching losses relative to silicon (Si) insulated gate bipolar transistors (IGBTs) paired with fast-recovery diodes. Low-voltage industrial variable-speed drives are a key application for 1200 V devices, and there is great interest in the replacement of the Si IGBTs and diodes that presently dominate in this application with SiC-based devices. However, much of the performance benefit of SiC-based devices is due to their increased switching speeds ( di/dt, dv/ dt), which raises the issues of increased electromagnetic interference (EMI) generation and detrimental effects on the reliability of inverter-fed electrical machines. In this paper, the tradeoff between switching losses and the high-frequency spectral amplitude of the device switching waveforms is quantified experimentally for all-Si, Si-SiC, and all-SiC device combinations. While exploiting the full switching-speed capability of SiC-based devices results in significantly increased EMI generation, the all-SiC combination provides a 70% reduction in switching losses relative to all-Si when operated at comparable dv/dt. It is also shown that the loss-EMI tradeoff obtained with the Si-SiC device combination can be significantly improved by driving the IGBT with a modified gate voltage profile.

An Experimental Investigation of the Tradeoff between Switching Losses and EMI Generation With Hard-Switched All-Si, Si-SiC, and All-SiC Device Combinations

Recently, the world's first ever power electronic traction transformer (PETT) for a 15-kV 16.7-Hz railway grid has been newly developed, commissioned, and installed on the test locomotive, where the PETT is presently in use. The design and development of the PETT are described in two parts of this paper. The details related to the 1.2-MVA medium voltage (MV) PETT prototype, including the electrical design of the power circuit, the sizing and characterization of insulated-gate bipolar transistor (IGBT) devices, the design of the medium-frequency transformer, the cooling system design, insulation coordination, and protection, are presented in this part, while the preceding paper deals with the overall system requirements and control system design based on the low-voltage (LV) PETT prototype. The identical control hardware and software from the LV PETT prototype were used for the MV PETT prototype, thus reducing significantly commissioning time. The experimental results obtained during the testing are presented to demonstrate the performance of the developed MV PETT prototype.

Power Electronic Traction Transformer—Medium Voltage Prototype

A novel type of Model Predictive Direct Torque Control (MP-DTC) is proposed in this paper The future behavior of the plant is predicted using a discrete-time state-space model in the dq reference frame for a finite set of voltage vectors, which is pre-selected by a graph algorithm The preferable input is chosen based on a cost function Contrarily to conventional MP-DTC (and DTC), the stator flux is not explicitly controlled nor hysteresis bounds are used The cost function is designed to meet multiple demands: torque reference tracking, Maximum Torque per Ampere (MTPA) tracking for high electrical efficiency, and limitation of the states to their maximum admissible values

Model Predictive Direct Torque Control With Finite Control Set for PMSM Drive Systems, Part 1: Maximum Torque Per Ampere Operation

In this paper, an innovative sensorless two-degree-of-freedom current control scheme for the whole speed range is proposed. It consists of a model-based dynamic feedforward control to set the reference response and model reference tracking controllers providing disturbance rejection and the position error signals needed for sensorless control. For low- and zero-speed operation, alternating test current injection is used to gain a position error signal. A flatness-based test signal precontrol provides compensation for secondary saliencies like cross-saturation and higher harmonics. For mid- and high-speed operation, it is shown how the model-based dynamic feedforward control can be modified to obtain a high-quality position error signal from the tracking controller. No additional model-based estimator evaluating back-EMF information is needed. The effectiveness of the proposed method is proven by experimental results.

Sensorless Control of PMSM for the Whole Speed Range Using Two-Degree-of-Freedom Current Control and HF Test Current Injection for Low-Speed Range

In this paper a new kind of identification method for differential self and mutual inductances of permanent magnet synchronous machines (PMSMs) is proposed.

Identification of differential inductances of permanent magnet synchronous machines using test current signal injection

A converter system made of series-connected converter submodules based on medium frequency (MF)-dc/dc converters can replace the conventional traction transformer that works at frequencies of 16.7 Hz or 50 Hz saving both mass and energy. Engaging multilevel topology permits the connection to the HV catenary with a line choke. MF switching performance of typical and dedicated 6.5 kV IGBTs was characterized and is given herein. The impact of chosen switching frequency and resonance frequency on transformer mass and transformer efficiency was analyzed with a coupled electromagnetic, thermal and hydraulic transformer calculation.

High voltage IGBTs and medium frequency transformer in DC-DC converters for railway applications

In order to be able to optimize the performance of sophisticated model based AC machine control, precise models of the overall drive system are needed. This way compensation schemes for non-ideal characteristics can be derived based on the model inversion technique. Power electronic converters show the following non-ideal effects which have to be considered: turn-on delay time of the gate drive to prevent dc-link short-circuiting, non-ideal switching characteristics and forward voltages of the power electronic devices. In this paper models for these disturbances are derived and the respective compensation schemes are deduced. In constrast to conventional approaches for converter linearization, an analytic compensation law which allows considering different forward voltage-current characteristics of the diode and transistor is proposed. The effectiveness of this method is proven by means of experimental results.

Modelling and model based compensation of non-ideal characteristics of two-level voltage source inverters for drive control application

The modular multilevel converter (MMC) is becoming one of the most promising topology in multilevel converter series especially for high-voltage and high-power applications. This valuable features, nominates MMCs to interface high-voltage and high-power renewable energy resources into modern HVDC electric grids for more penetration of renewable energy. This paper investigates recent modelling, control, and dc-fault protection techniques that have been applied to MMCs.

Bernhard Piepenbreier

Papers

Sensorless Control of PMSM for the Whole Speed Range Using Two-Degree-of-Freedom Current Control and HF Test Current Injection for Low-Speed Range

Identification of differential inductances of permanent magnet synchronous machines using test current signal injection

High voltage IGBTs and medium frequency transformer in DC-DC converters for railway applications

Modelling and model based compensation of non-ideal characteristics of two-level voltage source inverters for drive control application

A survey on modeling, control, and dc-fault protection of modular multilevel converters for HVDC systems