A Numerical Convex Lens for the State-Discretized Modeling and Simulation of Megawatt Power Electronics Systems as Generalized Hybrid Systems

Modeling and simulation have emerged as an indispensable approach to create numerical experiment platforms and study engineering systems.However,the increasingly complicated systems that engineers face today dramatically challenge state-of-the-art modeling and simulation approaches.Such complicated systems,which are composed of not only continuous states but also discrete events,and which contain complex dynamics across multiple timescales,are defined as generalized hybrid systems(GHSs)in this paper.As a representative GHS,megawatt power electronics(MPE)systems have been largely integrated into the modern power grid,but MPE simulation remains a bottleneck due to its unacceptable time cost and poor convergence.To address this challenge,this paper proposes the numerical convex lens approach to achieve state-discretized modeling and simulation of GHSs.This approach transforms conventional time-discretized passive simulations designed for pure-continuous systems into state-discretized selective simulations designed for GHSs.When this approach was applied to a largescale MPE-based renewable energy system,a 1000-fold increase in simulation speed was achieved,in comparison with existing software.Furthermore,the proposed approach uniquely enables the switching transient simulation of a largescale megawatt system with high accuracy,compared with experimental results,and with no convergence concerns.The numerical convex lens approach leads to the highly efficient simulation of intricate GHSs across multiple timescales,and thus significantly extends engineers’capability to study systems with numerical experiments.

A Discrete State Event Driven Simulation based Losses Analysis for Multi-terminal Megawatt Power Electronic Transformer

At present,power electronic transformers(PETs)have been widely used in power systems.With the increase of PET capacity to the megawatt level.the problem of increased losses need to be taken seriously.As an important indicator of power electronic device designing,losses have always been the focus of attention.At present,the losses are generally measured through experiments,but it takes a lot of time and is difficult to quantitatively analyze the internal distribution of PET losses.To solve the above problems,this article first qualitatively analyzes the losses of power electronic devices and proposes a loss calculation method based on pure simulation.This method uses the Discrete State Event Driven(DSED)modeling method to solve the problem of slow simulation speed of large-capacity power electronic devices and uses a loss calculation method that considers the operating conditions of the device to improve the calculation accuracy.For the PET prototype in this article,a losses model of the PET is established.The comparison of experimental and simulation results verifies the feasibility of the losses model.Then the losses composition of PET was analyzed to provide reference opinions for actual operation.It can help pre-analyze the losses distribution of PET,thereby providing a potential method for improving system efficiency.

An Improved Submodule Unified Pulse Modulation Scheme for a Hybrid Modular Multilevel Converter

This paper presents an improved submodule unified pulse width modulation(SUPWM)scheme for a hybrid modular multilevel converter(MMC)composed of half-bridge submodules(HBSMs)and full-bridge submodules(FBSMs).The proposed SUPWM scheme can achieve an output voltage of(2N+1)(where N is the number of submodules in each arm)levels,which is the same as that of the carrier-phase-shifted PWM(CPSPWM)scheme.Meanwhile,the proposed SUPWM scheme can alleviate the uneven loss distributions between the left leg and right leg in FBSMs of the hybrid MMC.Moreover,the capacitor voltages of the sub-modules can be well balanced without complicated closed-loop voltage balancing controllers.The validity of the proposed SUPWM scheme is verified by both the simulated and experimental results.

Dual-Timescale Control for Power Electronic Zigzag Transformer

Power electronic zigzag transformer is an attractive solution for the flexible interconnection of smart distribution networks.It is constituted by slow-response and low-precision thyristor converters and fast-response and high-accuracy voltage source converters.This paper models its primary circuit and addresses its basic operation mechanism.Then a dual-timescale control scheme is investigated to realize the coordinated regulation of both types of converter.A simulation case is established in PSCAD containing interconnected mid-voltage distribution networks.Simulations with poor-and well-matched control timescales are both carried out.And accordingly,the power flow controllability under these conditions is compared.When the shorter control timescale is no more than tenth of the longer one,the power electronic zigzag transformer will operate with satisfying performances.

A Novel Simulation Method for Power Electronics: Discrete State Event Driven Method

In the analysis of power electronics system,it is necessary to simulate ordinary differential equations(ODEs)with discontinuities and stiffness.However,there are many difficulties in using traditional discrete-time algorithms to solve such equations.Kofman and others presented the quantized state systems(QSS)algorithm in the discrete event system specification(DEVS)formalism.The discretization is applied to the state variables instead of time range in QSS.QSS is efficient to solve ODEs,but it is difficulty to be used when simulating actual power electronics systems with controller’s and other events.Based on the idea of this numerical algorithm and discrete event,a Discrete State Event Driven(DSED)simulation method is presented in this paper,which is fit for simulation of power electronics system.The method is developed to deal with non-linearity,stiffness and multi-time scale of power electronics systems.The DSED simulation method includes event definition,module seperation and modeling,event-driven mechanisms,numerical computation based on QSS,and some other operations.Simulation results verified the effectiveness and validity of the proposed method.

Message from Editors for the Special Section

AT present,the theories and technologies of the high power electronic system are in the rapid development.The corresponding manufacture and application are also at a crucial stage,facing three major challenges:to enhance the power conversion capability,optimize the system design,and improve the reliability.

Low-voltage ride through of multi-port power electronic trans-former

A low-voltage ride-through(LVRT)control strategy for the multi-port power electronic transformer(PET)based on power co-regulation is proposed.During the sag and recovery of the grid-side voltage of the medium-voltage ac(MVac)port,the grid-connected active power of the low-voltage ac(LVac)port,rather than the power from external renewable energy sources(e.g.,photovoltaic(PV)),is adjusted quickly to rebalance the power flowing across all ports,thereby preventing overcurrent and overvoltage.Moreover,a power-coordinate-frame-based LVRT mode classification is designed,and a total of six LVRT modes are classified to meet the LVRT requirements in all power configuration scenarios of the PET.In this way,the PET is endowed with the LVRT capability in both power-generation and power-consumption states,which is significantly different from traditional power generation systems such as PV or wind power.Furthermore,by optimizing the active power regulation path during LVRT transition,the overcurrent problem caused by the grid-voltage sag-depth detection delay is overcome.Finally,the effectiveness of the proposed control scheme is verified by experiments on a hardware-in-the-loop platform.

For better density and efficiency: Technique of novel high-voltage SiC based power converters

Medium-volage power converters have been widely used in diverse applications,such as electric grids,electric ship propulsion,and railway traction.Improving the efficiency and power densityof such converters is crucial for cost and availability concerns.Compared with power converters based on conventional sipower semiconductors,novel highvoltage SiC based power converters are expected to be employed in future application.However,the implementation of the later is limited owing to itshighvolage insulation and high electromagnetic interference.

Overview on Reliability of Modular Multilevel Cascade Converters

Multi-level converters have been used extensively in modern industry which calls for energy conversion with high-power and high-or medium-voltage.Because of its modularity and scalability,the multi-level converter with modular structure can be extended to different voltage levels and has a variety of forms in practical applications.It has attracted much attention from academia in the past decade,however,as a result of the numerous vulnerable power electronics sub-modules,significant challenges remain with regards to reliability.After summarizing the current research status of modular multilevel cascade converters,the main issues of reliability are reviewed in the paper.Firstly,the failure cases are thoroughly surveyed and classified,and the main failure causes are analyzed.Secondly,the reliability evaluation methods are reviewed and applied to the modular multilevel cascade converters.Thirdly,some promising measures to improve the reliability are presented and discussed,including parameter selection,redundancy design,fault-tolerant control and so on.Then,a complete reliability-oriented design procedure for the modular multilevel cascade converters is proposed.Finally,the challenges and opportunities to improve the reliability are concluded.