Mohammad Amin Bahmani

Core loss behavior in high frequency high power transformers—II: Arbitrary excitation

High frequency high power transformers used in power electronic converters are frequently subjected to non-sinusoidal excitations. The main purpose of this paper is to study the effects of some general arbitrary waveforms on magnetic core loss in these transformers. First, using well-known empirical equations, general expressions were derived based on the parameters of the waveforms. Second, the impacts of different orders of voltage harmonics were investigated. Finally, capabilities of nanocrystalline and amorphous magnetic materials were compared. It is shown that the loss inside the core is highly sensitive to the rise time and duty cycle of trapezoidal and rectangular waveforms, respectively. Furthermore, although amorphous materials have higher saturation flux density, the total core loss inside the transformer designed using nanocrystalline material is considerably lower than the similar transformer with amorphous materials.

Core loss behavior in high frequency high power transformers—I: Effect of core topology

This two-part paper presents an overview of core loss computations performed in both time and frequency domains in order to evaluate their behavior in single phase transformers with different core topologies. Moreover, the effects of non-sinusoidal waveforms on well-known core loss calculation methods are investigated with both analytically and finite element calculations. Three well-known configurations of transformers utilized in high frequency high power applications are investigated, namely, the core type, the shell type, and the matrix transformer. Based on the results obtained from a large number of FEM simulations for different operating conditions, the efficiencies of the transformers are compared in terms of distribution of magnetic flux density, loss density, total core loss, and weight. The analysis shows that for lower range of frequency and power, the shell type core could be the favorable option, and on the other hand, core type seems to be an appropriate solution for higher values of the operating frequency and nominal power.

Design methodology and optimization of a medium frequency transformer for high power DC-DC applications

The high power medium frequency transformer (HPMFT) is one of the key elements of an isolated, bi-directional DC-DC converters in applications such as future all-DC offshore wind farms, traction and solid state transformers. This paper describes a design methodology taking into account the loss calculation, isolation requirements and thermal management. Incorporating this design methodology, an optimization process with a wide range of parameter variations is applied on a design example to find the highest power density while the efficiency, isolation, thermal and leakage inductance requirements are all met.

Performance and loss evaluation of a hard and soft switched 2.4 MW, 4 kV to 6 kV isolated DC-DC converter for a wind energy application

In this paper, a hard switched and zero-voltage switching full bridge converter for high power applications are compared in terms of their losses. The important issue in designing these kinds of converters is to manage the converter parasitics. Moreover, evaluating the losses at the high frequency transformer with square wave input is another critical point in designing such a high power density converter. The proposed full bridge converters are designed to convert 4 kV input voltage to 6 kV with rated power of 2.4 MW. The comparison is done for three different frequencies: 500 Hz, 1 kHz and 2 kHz. Also, the simulation results and loss calculations are presented.

Optimization and Experimental Validation of Medium-Frequency High Power Transformers in Solid-State Transformer Applications

High power isolated DC-DC converters are likely to provide solutions for many technical challenges associated with power density, efficiency and reliability in potential applications such as offshore wind farms, inter-connection of DC grids, MVDC in data centers and in future solid state transformer applications. The high power medium frequency transformer (HPMFT) is one of the key elements of such a converter to realize the voltage adaption, isolation requirements, as well as high power density. This paper describes a design and optimization methodology taking into account the loss calculation, isolation requirements and thermal management. Incorporating this design methodology, an optimization process with a wide range of parameter variations is applied on a 50 kW, 1 / 3 kV, 5 kHz transformer to find the highest power density while the efficiency, isolation, thermal and leakage inductance requirements are all met. The optimized transformers are then manufactured and will be presented in this paper.

A high accuracy regressive-derived winding loss calculation model for high frequency applications

When operating higher up in frequency, the copper losses in transformer windings will significantly rise due to enhanced skin and proximity effect. This leads to a high need to propose and develop new methods to accurately evaluate winding losses at higher frequencies. This paper investigates the effect of different geometrical parameters at a wide range of frequencies in order to propose a pseudo-empirical formula for winding loss calculation in high frequency transformers. A comprehensive analysis of the edge effect and AC resistance is done by performing more than 12300 2-D finite element simulations on foil and round conductors. Unlike previous studies which mostly focused on specific case studies with limited applications, this model provides very high accuracy, especially where the most common analytical models drastically underestimate the winding losses. Moreover the model has a wide-range applicability which could be of interest for designers to avoid time consuming FEM simulation without compromising with the accuracy.

Design considerations of medium-frequency power transformers in HVDC applications

High-power isolated DC-DC converters are likely to provide solutions for many technical challenges associated with power density, efficiency and reliability in potential applications such as offshore wind farms, inter-connection of DC grids and in future solid-state transformer applications. The medium-frequency power transformer (MFPT) is one of the key elements of such a converter to realize the voltage adoption, isolation requirements, as well as high-power density. This paper presents an overview of the most important design and optimization considerations of MFPTs taking into account the high-frequency magnetic losses and thermal management.

Efficiency analysis of 5MW wind turbine system in an all-DC wind park

This article presents the efficiency analysis of a 5 MW wind turbine generating system in an all-DC wind park comprising of a permanent magnet assisted synchronous reluctance generator (PMaSynRG) system as well as an IGBT equipped forced-commutated inverter. In this article, the focus is on the NPC converter and DC-DC converter with an optimized high-frequency transformer. The total efficiency through the AD-DC converter to DC-DC converter was found to be 96.72 % as safed operation.

Flexible HF distribution transformers for inter-connection between MVAC and LVDC connected to DC microgrids: Main challenges

Solid-state transformers or so called flexible high-frequency (HF) distribution transformers are likely to become a more efficient inter-connection between medium voltage (MV) AC grids and low voltage (LV) DC grids which can eventually be connected to DC microgrids. This paper addresses the main challenges in order to implement this technology. High-Frequency Transformers (HFTs) are one of the key elements of such converters which are the main contributor to realize the voltage adaption, isolation requirements, as well as high-power density. This paper addresses the essential design considerations taking into account the magnetic materials, leakage inductance integration as well as thermal management of such transformers. Moreover, the limitations of the currently available semiconductors are discussed and the applicability and performance of the latest generations of 10 kV SiC MOSFETs are discussed. The highlighted design considerations within both the transformer and HV SiC devices are then demonstrated using experimental results.

Grounding architectures for enabling ground fault ride-through capability in DC microgrids

Distributed generation in the power grid will result in considerable efficiency improvement and increase in reliability and stability of the grid. And DC microgrids have clear benefits such as higher reliability, higher efficiency, better compatibility with DC loads, expandability and etc., over their AC equivalent systems. Although DC microgrids have clear advantages over the AC microgrids, but there is not sufficient information available on their grounding. Realizing the grounding of DC systems would accelerate employing of these systems in the power grid. Grounding is a complex topic involving many design considerations and trade-offs and it is needed to ensure the safety of personnel and equipment as well as detection of ground fault in the system. Grounding of DC power system should be designed to 1) minimize the leakage current during normal operation, 2) maximize the safety of personnel and equipment under fault conditions. This work examines the different grounding methods and system architectures and discusses the design trade-offs in terms of safety, reliability, detection, mitigation, noise, and cost. We examine impedance grounding, isolation, and bi-polar architectures and discuss their benefits with respect to these criteria.