Search

The extensive application of the power electronic transformers (PETs) in power systems poses a challenge as the accurate and high-speed electromagnetic transient (EMT) simulation of PET has been a critical issue. The computational time of the detailed EMT simulation of PET on EMT programs is unacceptable due to the large electrical node count, microsecond-range simulation steps, and high switching frequency of the devices and transformers. This article proposes a hierarchical modeling scheme for PET. Unlike the existing modeling methods, the proposed technique recursively decreases the dimension order of the admittance matrix to obtain the generalized Norton equivalent of each phase leg. The final admittance matrix overlaid onto the external system admittance matrix has a dimension order of magnitude remarkably smaller than that of the unreduced structure. By comparison with a detailed EMT model of a medium-voltage dc system, the performance of the proposed scheme has been assessed in PSCAD/EMTDC under various working conditions. With negligible loss of accuracy, approximately one to two orders of magnitude speedup over a straightforward EMT program is achieved.

The high-speed accurate electromagnetic transient (EMT) simulation of the power electronic transformers (PET) has become a challenge, due to the microsecond-range time steps and the large number of high-frequency semiconductor switches and isolating transformers. Taking the input-series-output-parallel (ISOP) connected cascaded H-bridge (CHB) type dual active bridge (DAB) based PET as an example, this paper proposes a general high-speed equivalent EMT modelling method of PET. First, the isolating transformer within each DAB is discretized into two-port Norton circuits. Second, each CHB-DAB is equivalent to two single-port circuits by eliminating the internal nodes. And the input-side is represented by a Thevenin circuit and the output-side is represented by a Norton circuit. Third, all the CHB-DABs are equivalent to a two-port circuit with all the internal node information preserved. Fourth, the implementation of PET blocking is also considered for startup and fault protective actions. Fifth, the stability of the proposed discrete decoupling method of the PET is verified that it does not introduce specific limitation on the simulation step size. Finally, the developed PET models on PSCAD/EMTDC are shown to be two orders of magnitude faster than the currently available fully-detailed models with negligible loss of accuracy.

Accurate and efficient electromagnetic transient (EMT) simulation of various types of solid-state transformers (SST) is extremely time-consuming due to the complex module structure, flexible topology connections, large number of electrical nodes and simulation time-steps limited in the range of micro-seconds. Therefore, it is urgent to develop the EMT equivalent modelling and fast simulation of SSTs for system level studies. Taking the modular multilevel converter (MMC) based SST as an example, this paper proposes an accelerated EMT model which focuses on the equivalence of the dual active bridge (DAB) based high-frequency link (HFL) in the SST. Compared with the existing algorithms, two critical factors of the proposed method that contribute the most to the efficiency improvement are the preprocessing of the nodal admittance equation and the conversion of the short-circuit admittance parameters. The proposed model is verified in PSCAD/EMTDC by comparing it with the detailed EMT model. The results show that the accelerated model is one to two orders of magnitude faster than the detailed model without sacrificing the accuracy. The experiment validation also confirms the validity of the proposed model.

This paper proposes a high-speed modeling architecture for fast electromagnetic transient (EMT) simulation of PETs. Unlike the existing modeling methods, the proposed technique recursively decreases the dimension order of the admittance matrix to obtain the generalized Norton equivalent of each phase leg. The final admittance matrix overlaid onto the external system admittance matrix has a dimension orders of magnitude remarkably smaller than that of the unreduced structure. By comparison with a detailed EMT model of a medium-voltage dc (MVDC) grid, the performance of the proposed scheme has been assessed in PSCAD/EMTDC under various working conditions. With negligible loss of accuracy, approximately one to two orders of magnitude speedup over a straightforward EMT program is achieved.

Accurate and efficient electromagnetic transient (EMT) simulation of various types of power electronic transformers (PET) is extremely time consuming due to the complex module structure, flexible topology connections, large electrical node count and microsecond-range simulation time-steps. Therefore, the EMT equivalent modelling and fast simulation of PET topologies face very urgent needs for research and practical development. Taking the modular multilevel converter (MMC) based PET as an example, this paper proposes an enhanced EMT model which focuses on the equivalence of the dual active bridge (DAB) based DC/DC converter in the PET. Two critical factors of the proposed method that contribute the most to the efficiency improvement are the preprocessing of nodal admittance equation and the conversion of the short-circuit admittance parameters. The proposed model is verified in PSCAD/EMTDC by comparing with the detailed EMT model. The results show that the accelerated model is one to two orders of magnitude faster than the detailed model with negligible loss of accuracy.

With the rapid development of dc power grid, nonlinear components represented by metal oxide arrester (MOA) are becoming an important part of the grid. However, MOA electro-magnetic transient (EMT) model has complicated nonlinear characteristics and is difficult to be integrated with other components to establish the integrated EMT model, which remarkably influences the ease of application of nonlinear models. An EMT modeling method for nonlinear components is proposed in this paper. It gives out the method to simplify complex circuit that contains nonlinear components. The method is validated by PSCAD/EMTDC simulation.

The high-performance carbon fiber reinforced poly(phenylene sulfide) composites were continuously fabricated using thermoplastic prepregs in a double-belt press. The effects of process velocity on the composite consolidation quality and mechanical properties were investigated. It is found that the tensile and interlaminar shear properties of composites prepared using the double-belt press are comparable to that of compression-molded composites when the process velocity is no more than 0.20 m·min −1 . The composite fracture morphologies also show different failure mechanisms between different samples and indicate that the interfacial adhesion strength may play a vital role in the mechanical properties of CF/PPS composites. Furthermore, experimental results show that the heating time above 330 °C should be over 440 s and the void content should be lower than 2.38% in order to obtain high performance CF/PPS composites.

The commingled glass fiber/polypropylene (GF/PP) composites were fabricated using a double-belt press and the influence of temperature and velocity on the consolidation quality and mechanical performance of the composites were investigated. The contents and distribution of the voids in the composites were changed with varying the level of the processing parameters, and the composite tensile and flexural properties were dramatically decreased when the void contents were beyond 3.6%. Reducing the viscosity of the matrix or increasing the uniformity of fibers in the commingled yarns was important to improve the consolidation quality. But the production efficiency was not improved as hoped by increasing the velocity at higher temperatures because of the weakened interfacial bonding. The crystal form and crystallinity degree had no obvious changes under different processing conditions, but the composite mechanical performance was enhanced when the degree of order in the crystal structures was decreased, which might be caused by more effective stress transferring or less initial cracks. (C) 2014 Elsevier Ltd. All rights reserved.