Characterization of the voltage and electric field stresses in multi-cell solid-state transformers

Abstract-High-Frequency (HF) litz wires are extensively used for the windings of Medium-Frequency (MF) magnetic components in order to reduce the impact of eddy current losses that originate from skin and proximity effects. Literature documents different methods to calculate eddy current losses in HF litz wires, however, most of the computation methods rely on perfect twisting of the strands, which is often not present in practice. This paper analyzes the implications of imperfect twisting on the current distribution among the different strands of HF litz wires and the corresponding losses by means of a fast 2.5D PEEC (Partial Element Equivalent Circuit) method. The effects of different types of twisting imperfections (at the bundle-, subbundle-, or strand-level) are examined. It is found that imperfect twisting can lead to increased losses (more than 100 %). However, perfect twisting of the strands, which is difficult to achieve, is often not required, i.e. suboptimal twisting is sufficient. Analytical expressions are given for distinguishing between critical and uncritical imperfections. The experimental results, conducted with a 7.5 kHz / 65 kW transformer, reveal a reduction of the error on the predicted losses from 52 % (ideal HF litz wire model) to 8 % (presented model) and, thus, confirm the accuracy improvement achieved with the proposed approach.

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“…10. This transformer is used in a DC-DC converter composing a Solid-State Transformer (SST) [20], [21]. Tab.…”

Section: Measurements Of Twisting Imperfectionsmentioning

confidence: 99%

Litz wire losses: Effects of twisting imperfections

Guillod

Huber

Krismer

et al. 2017

2017 IEEE 18th Workshop on Control and Modeling for Power Electronics (COMPEL)

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“…Therefore, and for the sake of conciseness, these issues have not been considered here in detail. Instead, the interested reader is referred to the corresponding literature [2], [24]- [28]. Thus, the power density of the industrial IBE prototype discussed above is only about 0.4 kW/l (6.6 W/in 3 ) (overall) and 0.83 kW/l (13.6 W/in 3 ) for one of its converter cells [1], i. e., only about half the power density achieved with the presented IFE converter cell.…”

Section: Discussionmentioning

confidence: 99%

“…Note that no detailed isolation coordination such as in [2] is carried out, because the focus lies on evaluation of the IFE topology, not on the transformer design, which is covered by corresponding literature [2], [24]- [27].…”

Section: B Key Components Of the Realized Prototypementioning

confidence: 99%

Analysis and Cell-Level Experimental Verification of a 25 kW All-SiC Isolated Front End 6.6 kV/400 V AC-DC Solid-State Transformer

et al. 2017

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Abstract-Solid-state transformers (SSTs) could serve as interfaces between a medium-voltage (MV) AC grid and a low-voltage (LV) DC load or source, i. e., could be employed in applications with power supply character such as traction auxiliary supplies or rack-level power supplies in future datacenters. For handling the high input-side AC voltage and output side current, SSTs are typically realized as input-series output-parallel (ISOP) arrangements of multiple converter cells, whereby each cell comprises a medium-frequency isolation stage. This paper presents such a multi-cell 25 kW all-SiC MVAC-LVDC SST (6.6 kV AC to 400 V DC) based on the isolated front end (IFE) approach, which is an interesting alternative to the isolated back end (IBE) configuration mainly discussed in literature so far. The IFE concept is briefly explained, the main component stresses are derived, and a converter cell prototype is designed and tested. The 5 kW prototype cell features a power density of 1.5 kW/l (24.6 W/in3) and a measured peak efficiency of 97.5%. This is significantly higher than previously published data for IFEbased SSTs, and in the same range as what has been reported recently for industrial IBE-based SSTs. Thus, this paper confirms that the IFE approach can be a feasible and interesting alternative for realizing MVAC-LVDC SST systems with low complexity.

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“…So far, few papers discuss the CMV or CMI in EPTs. The literature [23,24] presents the CMV and CMI phenomenon in the SST with a solidly grounded neutral point on the high-voltage side and designs common mode chokes to suppress the CMI based on the CMI equivalent circuit. However, the generation mechanisms and distribution characteristics of the CMV in the EPT have not been discussed analytically in the literature.…”

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confidence: 99%

Modeling and Analysis of the Common Mode Voltage in a Cascaded H-Bridge Electronic Power Transformer

Yang¹,

Mao²,

Wang³

et al. 2017

Energies

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Electronic power transformers (EPTs) have been identified as emerging intelligent electronic devices in the future smart grid, e.g., the Energy Internet, especially in the application of renewable energy conversion and management. Considering that the EPT is directly connected to the medium-voltage grid, e.g., a10 kV distribution system, and its cascaded H-bridges structure, the common mode voltage (CMV) issue will be more complex and severe. The CMV will threaten the insulation of the entire EPT device and even produce common mode current. This paper investigates the generated mechanism and characteristics of the CMV in a cascaded H-bridge EPT (CHB-EPT) under both balanced and fault grid conditions. First, the CHB-EPT system is introduced. Then, a three-phase simplified circuit model of the high-voltage side of the EPT system is presented. Combined with a unipolar modulation strategy and carrier phase shifting technology by rigorous mathematical analysis and derivation, the EPT internal CMV and its characteristics are obtained. Moreover, the influence of the sinusoidal pulse width modulation dead time is considered and discussed based on analytical calculation. Finally, the simulation results are provided to verify the validity of the aforementioned model and the analysis results. The proposed theoretical analysis method is also suitable for other similar cascaded converters and can provide a useful theoretical guide for structural design and power density optimization.

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Characterization of the voltage and electric field stresses in multi-cell solid-state transformers

Cited by 61 publications

References 22 publications

Litz wire losses: Effects of twisting imperfections

Litz wire losses: Effects of twisting imperfections

Analysis and Cell-Level Experimental Verification of a 25 kW All-SiC Isolated Front End 6.6 kV/400 V AC-DC Solid-State Transformer

Modeling and Analysis of the Common Mode Voltage in a Cascaded H-Bridge Electronic Power Transformer

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