Application of a Varying Elements Tracing Method to Partial Refactorization in an Electromagnetic Transient Analysis Program

The large number of switching elements in modular multilevel converters (MMCs) contribute a tremendous computational burden for electromagnetic transient (EMT) simulation programs. Detailed equivalent models (DEMs) and average value model (AVMs) are currently two major types of accurate and efficient model. However, the DEMs are still computationally inefficient for the simulation scenarios in large-scale MMC based high-voltage direct current (MMC-HVDC) grids, as the models represent all submodule (SMs) switching events, and memorize all individual capacitor voltages. Though the AVMs provide a faster simulation speed by using a single equivalent capacitor on the DC side, they have a relatively low simulation accuracy compared to DEMs, especially under blocked mode. This paper proposes an enhanced computationally efficient model based on arm equivalence (AEM), which can accurately represent the dynamic behaviors in both de-blocked and blocked modes. Compared to the DEMs, the proposed AEM is more efficient, with no loss of accuracy, and the simulation speed is irrespective of the SM number. The accuracy and computational efficiency of the proposed model were validated against the DEM and AVM through several simulation scenarios in a two-terminal MMC-HVDC system on the power systems computer aided design/ electromagnetic transient in DC system (PSCAD/EMTDC) program.

Detailed switching models (DSM) of ac–dc converters are available in many offline and real-time electromagnetic transient (EMT) simulation programs. However, such discrete models typically require small simulation time steps for accurately handling the switching events without interpolation. This paper proposes a new parametric modeling approach for line-commutated and 2-level voltage-source ac–dc converters. The proposed methodology is based on parametric functions that relate ac and dc variables in the instantaneous sense, which allows the reconstruction of the waveforms of voltages and currents (with the same of details as the switching models of converters) without topological changes in the converter circuit. Thus, it can operate at larger time steps without requiring interpolation, thereby enhancing simulation efficiency. The computational advantages of the proposed models over the conventional switching models are demonstrated in offline PSCAD and real-time RTDS NovaCor simulators in terms of maximum possible time step size and accuracy. It is shown that the proposed models can accurately run with much larger time steps (up to ∼200 μs for LCRs, and up to ∼50 μs for VSCs) compared to detailed switching models, which are limited to relatively small time steps of ∼10 μs.

To address the limited speed and scale of the simulation of modular multilevel converter (MMC) in electromagnetic transient simulation program (EMTP), this paper put forward a MMC fast electromagnetic transient model based on rotation transformation. The crucial advantage of the rotation transformation is to reduce the fundamental frequency of the original signal via the rotation transformation of coordinate system. With proposed method implemented in the EMTP of MMC, the simulation step size can be relatively increased, and the fast electromagnetic transient simulation can be achieved. In this paper, the equivalent model of MMC switch function model expressed by the state equation is derived. Based on this, the novel MMC model based on rotation transformation is further derived combined with the concept of rotation transformation. Then, the state equation of proposed novel MMC model is calculated in Matlab, and the simulation results and simulation time-consuming are compared with which of 51-level topological MMC electromagnetic transient model established in PSCAD/EMTDC. The results of comparative analysis show that the simulation duration of the MMC model based on rotation transformation is far less than the traditional MMC electromagnetic transient model without sacrificing accuracy.

This study presents a method to interface an electromagnetic transient (EMT) simulator with a dynamic phasor (DP) simulation program. The hybrid DP–EMT simulator offers flexibility in deciding the harmonic contents to be preserved in the DP domain; additionally, it offers significant reduction in computing time of large networks compared with EMT simulation. The proposed interfacing method is based on a generic transmission line, which uses specialised numerical techniques to transfer discretised waveform samples between the two simulators. These numerical methods produce the required mapping between EMT simulation samples and their counterpart DPs. This study describes the mathematical foundations of the DP–EMT interface and demonstrates them using an illustrative example. Several large networks are then studied to assess the interface's accuracy and performance in reducing computing time.

This paper validates the modular multilevel converter (MMC) models, which are developed by two different institutes using PSCAD/EMTDC. The MMC uses a large number of power-electronic switching devices that operate independently. Modeling such converters with individual switches in an electromagnetic transient (EMT) simulation program leads to extremely slow runs, and hence seems to be impractical. To overcome this issue, MMC can be modeled as a detailed equivalent circuit. In this method, each MMC arm is represented by a Thévenin equivalent branch whose voltage and resistance are calculated at each time step, thus reducing the computational burden. The models have been developed by Yonsei University in South Korea and the Manitoba HVDC Research Centre in Canada based on this technique. The presented models include the detailed equivalent representation of each converter arm, corresponding controls, waveform generation, and cell capacitor balancing strategies. To validate overall performance and transient response of the model, a 7-level MMC-HVDC is modeled with individual switches and through the detailed equivalent circuit. As an example for a realistic system, a 201-level MMC-HVDC system is also simulated. The results demonstrate validity and compatibility of two MMC models.

Nowadays, there is quite high demand for electromagnetic transient (EMT) analysis programs and real-time simulators for power systems. In addition to the conventional demand such as overvoltage, over-current and oscillation simulations, the new demand that includes simulations of power-electronics circuits and power quality is increasing. With this background, development groups of EMT programs and real-time simulators have made progress in terms of computational performance and user experience. In Japan, Central Research Institute of Electric Power Industry has newly developed an EMT analysis program called XTAP (eXpandable Transient Analysis Program). This article overviews these international and domestic development trends of EMT analysis programs and real-time simulators.

The interior overvoltage in the extra-high voltage (EHV) power system determines the equipment insulation and safe operation of the system. At first, the paper expounds the problems brought from the application of traditional fixed high voltage shunt reactor to limit the overvoltage in our national EHV power system. Therefore this paper brings forward a proposition to solve these problems by adopting magnetically controlled shunt reactor. This paper mainly introduces a novel type of controlled shunt reactor: Magnetically controlled shunt reactor (MCR). Then its structure and principle of magnetically controlled shunt reactor is presented. The harmonic distribution characteristics, the voltage-current characteristics, control characteristics and response time of MCR are analyzed successively. Afterwards, this paper takes a 500 kV EHV transmission line from Sanyou to Jingzhou for an example, the line model and MCR model are built up. Based on Electromagnetic Transient Program (EMTP), analysis and numerical simulation has been performed to calculate frequency overvoltage and switching overvoltage in the EHV power system with magnetically controlled reactor. (7 pages)

Shifted-frequency based electromagnetic transient simulation for power system dynamics using two-stage singly diagonally implicit Runge–Kutta method

As more generators and loads are integrated by power electronic converters with complicated controls, electromagnetic transients (EMTs) simulation becomes an important tool for studying dynamic characteristics of large‐scale power systems. To accelerate system‐level EMT simulations, a fine‐grained parallel algorithm on graphics processing units (GPU) is proposed. By decomposing the computational models of the EMT simulation into heterogeneous, homogeneous and network solution computations, the simulations are mapped into three unified GPU kernels. To incorporate control signals and non‐linear features of electrical components, heterogeneous computations are formulated as layered direct acyclic graphs (LDAG) of primitive operations. An LDAG kernel is designed to carry out theses primitive operations efficiently by grouped threads. Then, homogeneous computations for state updates of electrical components are modelled as sets of fused multiply‐add (FMA) operations, which are concurrently processed by an FMA kernel. Moreover, a hybrid network solution kernel is designed to solve the network equations, which can adaptively select dense or sparse solvers. Large‐scale test systems are created and simulated on an NVIDIA K20x GPU. The results show that the proposed GPU‐based EMT simulations are accurate and achieve 10x speedups over the CPU‐based ones.

Considering electrical network as a continuum has become popular for electromechanical wave analysis. This paper reviews the concept of electromechanical wave propagation. Analysis of large number of generator ring system will be an easy way to illustrate wave propagation. The property of traveling waves is that the maximum and minimum values do not occur at the same time instants and hence the difference between these time delays can be easily calculated. The homogeneous, isotropic 10 generator ring system is modeled using electromagnetic transient simulation programs. The purpose of this study is to investigate the time delays and wave velocities using Power System Computer Aided Design (PSCAD)/Electromagnetic Transient Program (EMTP). The disturbances considered here are generator disconnections and line trips.

Electromagnetic transient (EMT) simulation is one of the routine tasks being executed in modern power systems. Typically, the most common EMT simulation method is the trapezoidal integration method, however, sustained numerical oscillations are possible to appear using this method, for instance, getting the voltage waveform across an inductance during a sudden current interruption. Two-stage diagonally implicit Runge-Kutta (2S-DIRK) method is a L-stable numerical method, which is able to avoid numerical oscillation. The 2S-DIRK method has been applied to solve simple RLC circuit simulations, in this paper, it is extended to system level EMT simulations considering the electromagnetic transient characteristics and nonlinearity for generators, transformers and loads. Case studies are performed to verify the effectiveness of the 2S-DIRK method in power system level EMT simulation, by comparing the accuracy and numerical stability with traditional methods.

Arcing horns interrupting fault current is a new type of arcing horns installed on transmission-line towers, which act to interrupt fault current independently within one cycle of commercial frequency. In this paper, we have developed an Electromagnetic Transients Program (EMTP) simulation model of the arcing horns interrupting fault current to ascertain the application effects and devise an installation strategy. The EMTP simulation model can consecutively simulate the entire process, from fault occurrence to fault clearing, by integrating two different kinds of macroscopic arc models, and is capable of dealing with the entire short-circuit current range by taking into account the characteristics of the arc parameters. Finally, we have verified the validity of the EMTP simulation model by comparing simulation and experimental waveforms, and have shown the simulation of current and voltage waveforms in the event of a system fault by using the Japanese Power System Model as an application example of the EMTP simulation model.

Parallel-in-time methods are emerging to accelerate the solution of time-consuming problems in different research fields. However, the complexity of power system component models brings challenges to realize the parallel-in-time power system electromagnetic transient (EMT) simulation, including the traveling wave transmission lines. This paper proposes a system-level parallel-in-time EMT simulation method based on traditional nodal analysis and the Parareal algorithm. A new interpretation scheme is proposed to solve the transmission line convergence problem. To integrate different kinds of traditional EMT models, a component-based EMT system solver architecture is proposed to address the increasing model complexity. An object-oriented C++ implementation is proposed to realize the parallel-in-time Parareal algorithm based on the proposed architecture. The results on the IEEE-118 test system show 2.30x speedup compared to the sequential algorithm under the same accuracy with 6 CPU threads, and a high parallel efficiency around 40%. The performance comparison of various IEEE test cases shows that the system's time-domain characteristics determine the speed-up of Parareal algorithm, and the delays in transmission lines significantly affect the performance of parallel-in-time power system EMT simulations.

A multiscale power system modeling methodology for the integrative simulation of electromagnetic and electromechanical transients is introduced, implemented and validated. It makes use of frequency-adaptive simulation of transients (FAST) in which the shift frequency appears as a new parameter in addition to the time step size. For fast electromagnetic transients, tracking of instantaneous waveforms as in the Electromagnetic Transients Program (EMTP) is performed. When slower electromechanical transients involving power oscillations prevail, the Fourier spectra of the waveforms are shifted by typically either 50 or 60 Hz to eliminate the ac carrier and enable envelope following as in phasor-based simulation. An algorithm for the automatic setting of both shift frequency and time step size is proposed. Multiscale models of transformers, synchronous machinery, and transmission lines based on FAST are developed. The line model is distinguished in that it bridges both lumped and distributed parameters to efficiently emulate scale-bridging phenomena from steady state to traveling waves. The overall methodology is thoroughly validated: first against a staged field test involving transients of line energization and recovery on the network of the Bonneville power administration (BPA); then through comparison with an EMTP-type simulator in modeling diverse transients on a four-machine two-area power system.

High Voltage Direct Current (HVDC) systems are increasingly popular in electric power grids. Control system of such devices can cause instabilities when interacting with other equipment in a power system. Depending on the control system dynamics, instability and resonance can occur in a wide range of frequencies (up to few kHz). Low and high frequency resonances have been registered in several power electronics based project. In this article, several realistic test cases where oscillations can occur between HVDC links and power system components are illustrated using electromagnetic transient (EMT) simulations. In order to analysis and identify the cause of such instabilities, this paper illustrates how to conduct a frequency domain analysis by means of EMT time domain simulation. As EMT-type models provided by the manufacturers are often inaccessible (or black-boxed), the method presented considers black-boxed HVDC link model.