Kartik V. Iyer

Winding design of a high power medium frequency transformer

The availability of improved magnetic materials and higher voltage rating semiconductor devices have made the design of high power medium frequency transformers feasible. The paper presents a simple design procedure to determine the core dimensions based on the area product method. A winding design procedure to use foil conductors in the high voltage side to achieve ac-to-dc resistance ratio close to 1 is also shown, providing a possible replacement to the litz wires having low window utilization factor. The paper demonstrates a design example of 13.8kV/690V, 10kHz, 50kVA transformer to validate the design methodology and the winding losses are validated using 2-D FEM.

Design and comparison of high frequency transformers using foil and round windings

High frequency transformers are widely used in Switched-mode power supplies and now are being proposed to be used with power electronic converters to replace line-frequency transformers. This paper presents a winding design procedure for minimizing the power losses using foils and solid round wires under sinusoidal excitation to limit the temperature rise. This paper derives the range from which the thickness of the layers can be chosen to obtain the minimum power loss. This thickness range is a function of the number of layers and does not include the “optimum” based on the previous literature. Using this design procedure, it is shown that interleaving is not necessary in foil-wound transformers to obtain the minimum loss. A comparison of winding losses between foil windings and round conductors is also given. The analytical results are verified by designing six different winding configurations for the same specifications using 2-D Ansys Maxwell finite element design package.

Multi-level converter to interface low voltage DC to 3-phase high voltage grid with medium frequency transformer isolation

This paper presents a single stage, bi-directional multi-level converter topology with a multi-winding medium frequency transformer (MFT) isolation to interface a low voltage DC with a 3-phase high voltage grid. The proposed topology can be used for integrating 3-ph utility grid with various low voltage renewables like solar photovoltaic, fuel cells etc. The transformer secondary windings in each phase have unequal turns ratio. This asymmetry introduced in the transformer turns ratio allows to generate more output voltage levels, using less number of semiconductor switches leading to a reduced footprint. The transformer secondary side has AC-AC converter modules which are pulse width modulated. This results in the elimination of the harmonics at multiples of grid frequency, resulting in reduced size of filter inductance. The paper also shows the winding procedure to interleave the windings resulting in lower leakage inductance to minimize the problem of leakage commutation. The proposed topology is simulated using MATLAB/Simulink.

A High-Frequency AC-Link Single-Stage Asymmetrical Multilevel Converter for Grid Integration of Renewable Energy Systems

Different power electronic converter topologies have been investigated to integrate renewable energy systems to the grid. Cascaded multilevel converters with a high-frequency link have emerged as a viable candidate for such applications. Electrical isolation can be provided using a compact high-frequency transformer connected in the link thus avoiding a bulky line frequency transformer. The use of cascaded modules allows the generation of a multilevel voltage having low total harmonic distortion but increases the overall system size. In this paper, a 15-level high-frequency ac-link single-stage asymmetrical multilevel converter for grid integration is proposed. The single-stage conversion approach eliminates the dc-link capacitors, resulting in a reduced footprint. The asymmetrical module voltages are generated by the multiwinding transformer having unequal turns on each of the secondaries. This allows the generation of 15 output voltage levels, using only three modules in each phase. A modulation strategy is proposed to generate multilevel output voltage. The effect of switch nonidealities on the output voltage is analyzed and two compensation techniques are developed to improve the voltage profile. A multiwinding high-frequency transformer is designed and characterized for the proposed converter. The presented concepts are verified by simulation and further validated experimentally on a three-phase 15-level converter prototype.

Determination of the optimal thickness for a multi-layer transformer winding

High frequency transformers (HFT) are needed along with power electronic converters to replace line frequency transformers in high power systems to increase power density. For the design of HFT, it is important to accurately estimate copper losses due to a duty-cycle modulated current waveform. The design also requires determination of the optimal thickness of winding layers, leading to a minimum AC power loss. This paper shows that the Fourier-series method for the loss computation requires the consideration of a large number of harmonics, leading to considerable computational time in the determination of the optimal thickness. A closed form approximate expression for the power loss is presented in this paper that obviates any need for a large series summation, resulting in a relatively simple computation of optimal thickness. Results are validated through numerical computations.

Modulation and Commutation of a Single Stage Isolated Asymmetrical Multilevel Converter for the Integration of Renewables and Battery Energy Storage System in Ships

In upcoming all electric ships, for the integration of low-voltage renewables and battery energy storage system with the high-voltage ac distribution, a single stage isolated asymmetrical multilevel converter is advantageous for the following benefits: 1) uses medium frequency transformer (MFT), thereby eliminating the bulky line frequency transformer; hence, resulting in a smaller footprint; 2) the asymmetry introduced in the MFT turns ratio generates high number of output voltage levels with less number of converter modules; and 3) single stage conversion removes the need for a dc-link capacitor, hence, improving the power density. In this paper, a modulation technique using triangular level shifted carriers based on the medium frequency link is investigated for this topology. Unlike previously used techniques, the proposed modulation shifts the dominant harmonics in the output voltage to the sidebands of multiples of twice the switching frequency, thereby reducing the output filter size. The incorporation of the switch nonidealities requires the implementation of a commutation strategy for the single stage conversion. A detailed circuit analysis showing different modes of operation to generate a specific output voltage level, considering the switch nonidealites is outlined in this paper. The analysis aids in the real-time implementation of the converter, and it also explains the distortion in the ideal output voltage profile. The analysis in general can be used for any isolated single stage converter. Simulation and experimental results are provided on a 15-level three-phase converter prototype with MFT isolation.

A Dual-Active-Bridge-Based Single-Phase AC to DC Power Electronic Transformer With Advanced Features

Power electronic transformers (PETs) offer the advantage of size and weight reduction compared to line-frequency transformers by operating at much higher frequencies than line frequency. In this paper, a push–pull-based ac/dc PET has been proposed and analyzed. The PET offers bidirectional power flow between single-phase ac and dc, using the dual-active bridge principle. Such a system may find applications in interfacing plug-in hybrid and electric vehicles to the grid. The proposed PET offers advantages of open-loop unity power factor operation, soft switching of secondary-side converter power switches for all operating points, high power density owing to use of a high-frequency transformer, and high utilization factor (UF), compared to previous work. Analysis has been done for power transfer, UF, and soft switching. Simulation and experimental results have been provided to demonstrate the operation of the PET.

A compact active filter to eliminate common-mode voltage in a SiC-based motor drive

This paper presents an active compensation device for common-mode (CM) voltage elimination in 3-phase space-vector pulse-width-modulated (SVPWM) inverters. The proposed device consists of a single-phase 2-level inverter (H-bridge) which supplies a compensating voltage to the inverter via a step-up common-mode transformer tied to all three phases at the output. The H-bridge active filter is supplied by a low voltage bus and switched several orders of magnitude faster than the inverter switching frequency. This device takes advantage of the direct knowledge of the switching pulses sent to the inverter to predict and generate the compensating voltage. A technique is employed to subtract the low frequency harmonics from the modulation of the H-bridge which allows for the size of the common-mode transformer to be reduced significantly. Small passive components are added to attenuate the active filters PWM frequency content and thus produce an effective compensating voltage. This paper will review existing common-mode voltage compensation techniques and demonstrate that the proposed method is a logical choice for certain drive applications. Design considerations are included to provide understanding and guidance for implementation of the device, as well as MATLAB/Simulink simulation results to demonstrate the operation of the active compensation device. Final validation is presented through experimental results from a hardware prototype.

Asymmetrical converters with medium frequency transformer isolation to interface low voltage DC to a medium voltage three phase grid

This paper presents an asymmetrical converter topology with medium frequency transformer (MFT) isolation to interface a low voltage DC source with a 3-phase medium voltage grid. The topology consists of asymmetrical converters connected in parallel across the DC input, followed by a MFT for isolation and a diode bridge fed current source inverter (CSI) connected to the MFT secondary. The asymmetrical converters connected in parallel share the high input currents and are modulated so as to generate a multi-level stepped waveform across the transformer secondary. The asymmetry is introduced in the MFT turns ratio. The power flow in DC sources like PV being unidirectional, the transformer secondary side is connected to diode bridge and pulse-width modulated (PWM) CSI which interfaces a medium voltage grid. The stepped voltage across the transformer secondary and the use of diode bridge results in a trapezoidal current through the transformer windings eliminating the problem of leakage commutation. The paper also determines analytically the transformer voltages based on the DC link current requirement of the CSI based on the grid active power. The proposed topology is simulated using MATLAB/Simulink.

Overview of series connected flexible AC transmission systems (FACTS)

Static series compensation devices provide additional flexibility in the operation of transmission lines. Many articles have been published in literature which demonstrate how static series connected FACTS devices can solve numerous power system problems. However, they are not widely utilized in the power industry. The complexity and cost of these devices make it hard to justify their installation. The paper presents the various applications of series connected FACTS devices and the controllers used in literature. It also presents a comparison between fixed and static series compensation device. The paper also explores the economic challenges of installing series connected FACTS devices on the electric power system and some possible economic solutions. A cost effective solution to implement static phase shifting at the neutral of power transformers is presented.