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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.

The high-voltage direct current (HVDC) circuit breaker is becoming popular with the rapid development of the flexible HVDC grid for efficient DC fault ride-through purposes. This paper proposes a novel module for reciprocating HVDC circuit breaker topology, whose branch connections are able to switch between series and parallel modes to limit the rising rate and interrupt the DC fault currents. Diode-bridge sub-modules (DBSMs) are used to compose the main branch for current interruption. Besides fault clearance, the proposed topology has the advantageous function of DC fault current limiting by employing DBSMs with bi-directional conduction capability. The topology can easily switch among branch connection modes through the assembled trans-valves, and their resistance and reactance are very small in the normal state when branches are in parallel and the values become promptly large in the transient state when the branches are series connected. With the modular design, it is easy to change the number of branches or sub-modules and the types of sub-modules to adapt to more specific needs. A 6-terminal modular multi-level converter (MMC) based HVDC grid is established in PSCAD/EMTDC, and various simulation scenarios are carried out to validate the proposed topology.

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.

With the increasing uptake of distributed renewable energy and low-carbon \ud technologies (e.g. energy storage and electric vehicle), the conventional AC \ud distribution network is transferring to hybrid AC/DC or pure DC. Power electronics\ud transformers (PETs), also known as solid-state transformers, are multifunctional of \ud integrating distributed generation, regulating bidirectional power flow, and \ud achieving grid interconnection, reactive power compensation, harmonic control, \ud etc. Therefore, PETs can serve as key interfaces for energy conversion in future\ud distribution networks.\ud At present, a number of medium- and low-voltage distribution networks using\ud PETs have commissioned in China, such as the Xiaoertai substation in Zhangbei\ud and Tangjia Bay three-terminal DC distribution network in Zhuhai. The off-line and \ud real-time electromagnetic transient (EMT) simulation studies are of great \ud significance for system-level analysis and prototype development of PETs, which \ud should be investigated timely.

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.

In order to significantly reduce the number of voltage sensors and computation boards in the modular multilevel converter (MMC), this paper proposes a diode-clamped half-bridge MMC with full capability of capacitor voltage parallel-balancing. The newer generation of MMC structure can be re-constructed from the traditional half-bridge MMC by integrating one clamped diode and damping resistor into each submodule (SM), and four auxiliary voltage balancing circuits across all three phases. The proposed MMC topology can naturally balance its SM capacitor voltages regardless of modulation and triggering techniques. This is due to the fact that all the SMs in the MMC belong to 6 Spontaneous Capacitor Parallel Behaviors (SCPBs), through which the voltages of SM capacitors are automatically balanced. The inserted SM capacitor is now parallel with the adjacent bypassed SM capacitor. By designing proper RC constant, the adjacent SMs in the proposed MMC will have equal capacitor voltage ripples. Although voltage balancing is naturally guaranteed, in order to average the switching losses, open loop nearest level modulation (NLM) and carrier phase shifted sinusoidal PWM (CPS-SPWM) techniques are used in the proposed MMC. The electromagnetic transient (EMT) simulations on PSCAD/EMTDC and the down-scaled prototyping results will validate the new topology.

Recently, nonlinear components such as metal oxide arresters (MOA) are widely used in MMC-HVDC systems. Equivalent models are proposed to accelerate the simulation of complex nonlinear components. However, it is hard to evaluate the similarity of the original model and equivalent model. In this paper, a novel method is proposed to calculate the similarity of similar waveforms after removing the time delay and DC offset, and is capable to give the degree of the accuracy of the equivalent model. We eliminate the influence of DC offset by differentiating and calculate the time delay by finding the most similar interval between two discrete series. And then the DC offset of similar waveforms is obtained by integrating in the fitting interval. Finally, based on the concept of the coefficient of determination in statistics, the two waveforms similarity are evaluated. Then waveforms obtained from the detailed model and the equivalent model of nonlinear component in MOA are analyzed with the method.

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.