EMT-type Simulations Research Articles

Control Solution to Over-Modulation of DFIG Converter for Asymmetrical Fault Ride Through

This letter proposes a control solution to mitigate the problem of converter over-modulation in doubly fed induction generator (DFIG)-based wind turbine generators (WTGs). Overmodulation is a critical reason in failing to ride through asymmetrical faults and inject positive- and negative-sequence reactive currents (I1R and I2R) per grid code requirements. Analytical expressions are first proposed to independently calculate the transient, positive- and negative-sequence current components. The I2R injection factor is then optimized to mitigate the problem of steady-state over-modulation. The transient components are eliminated in the inner-loop control to mitigate the problem of transient over-modulation. The proposed control solution is validated with EMT-type simulations. The results show that the transient performance and the fault ride through (FRT) capability are significantly improved.

Analysis of Cross-Bonded Cables Using Accurate Model Parameters

This paper presents a thorough investigation of frequency and electromagnetic transient (EMT) responses of cross-bonded cables. The study adopts newly developed Line/ Cable Data (LCD) Module in an existing EMT-type simulation software. The input impedances evaluated by positive sequence and various mode energizations on a typical cross-bonded cable (CBC) in frequency domain are investigated.In addition to cross-bonded, the solid-bonded cable (SBC) and core-transposed cable (CTC) are analyzed for characteristics of input impedance. The mixed propagation modes of CBC and CTC are studied. The influence of bonding methods and cable models on transient voltages is investigated. The results simulated by the new LCD are compared with the Classical Parameter Calculation (CPC) method and validated by recently proposed extended transmission line (ETL) approach.

MSEMT: An Advanced Modelica Library for Power System Electromagnetic Transient Studies

Electromagnetic Transient (EMT) simulation tools are typically developed using conventional procedural programming languages. On the other hand, modern high-level and equation-based programming languages, such as Modelica, are currently available. Modelica allows formulating models that are easy to develop, maintain and understand by expressing what needs to be computed without stating how it should be computed. This paper presents a Modelica-based simulator for electromagnetic transients. It is demonstrated that this approach offers significant advantages for developing sophisticated models. Computational performance and accuracy are compared to a conventional EMT-type simulation tool.

Flexible parameterizable grid-forming converter control by separated fast synchronization and slow inertia response control loops of Direct Voltage Control

As power systems, transit to a state of high renewable penetration with predominant converter-based grid interface instead of synchronous generators applied by conventional power plants, the dynamics of the grid significantly change, causing new challenges for transmission system operations and arising new opportunities, as converter-based generation is highly controllable in faster timescales. This paper discusses the extension of the direct voltage control as ‘a decoupled grid forming converter controller’, which effectively relies on an accurate and fast d/q-reference frame determination to control the frequency as well as its rate of change at the common coupling point with the grid. The proposed controller separates the task of synchronizing the converter to the grid and providing virtual inertia into two distinct control functions, which enables a specific tuning of both functions for their task avoiding the necessity to find any kind of trade-off between quality of synchronization and inertia provision. The functionality of the proposed control is explained and demonstrated via EMT-Type simulations in different study cases for system-spilt, islanding, and re-synchronization scenarios.

Simulation of electromagnetic transients with Modelica, accuracy and performance assessment for transmission line models

Electromagnetic Transient (EMT) simulation tools are typically developed using an imperative language. It is typically required to code the models using a specific numerical integration method and following existing solution algorithms. On the other hand, modern high-level, acausal and equation-based programming languages, such as Modelica, allow to formulate models that are easy to read, understand and develop by expressing what needs to be computed without stating how it should be computed. In this paper, the application of Modelica to EMT modeling is demonstrated and the benefits of the language for EMT-type simulations is established. This is done by using the declarative language concept for the development of two models, namely the Constant Parameter (CP) and the Wide-Band (WB) models. Two case studies are presented and used to validate the models and to compare accuracy and performance.

Detailed Equivalent and Average Value Models of Hybrid Cascaded Multilevel Converters for Efficient and Accurate EMT-Type Simulation

The emerging voltage source converters (VSCs) such as hybrid cascaded multilevel converters (HCMCs) are promising converter technologies which enable higher converter efficiency, compactness, and DC fault resilience for high voltage direct current (HVDC) transmission. However, the large number of switching events due to the converters' submodules introduces a heavy computational burden for electromagnetic transient (EMT) simulations. Efficient and accurate EMT-type converter models play a vital role in the control and design of HVDC converters. Several computationally efficient models for the HCMCs are proposed in this paper, including the detailed equivalent model (DEM) and several average value models (AVMs) of varying accuracy. Additionally, a combined model is proposed which allows the users to switch among the DEM and AVMs during dynamic simulations. The proposed models are validated against the DEM for dynamic transients. The simulation results demonstrate good modeling accuracy of the proposed models. The AVMs are also shown to significantly improve simulation efficiency compared to the DEM, which is very desirable especially for HCMCs with a large number of submodules.

Effect of frequency-dependent soil parameters on wave propagation and transient behaviors of underground cables

This paper investigates the influence of frequency-dependent (FD) soil parameters in comparison to the constant soil (CS) parameters on wave propagations and transient characteristics of underground cables. Various FD soil models are summarized, and the calculated results of the FD soil parameters are compared with independent measured FD soil data. Longmire / Smith (LS) model is found to show the best agreement with the measured results. The frequency responses of wave propagations on underground cables are studied by the recently proposed extended transmission line (TL) approach together with the CS and the LS FD soil models. Calculated transient responses show more attenuation with a large soil resistivity at 100 Hz in comparison with those with a small soil resistivity at 100 Hz. Further, the transient voltage responses of a cross-bonded cable are compared with the transients solved by the Cable Constants and the existing FD soil routine in an EMT-type simulation tool, and a significant difference is observed.

Advanced Hybrid Transient Stability and EMT Simulation for VSC-HVDC Systems

This paper deals with advanced hybrid transient stability and electromagnetic-transient (EMT) simulation of combined ac/dc power systems containing large amounts of renewable energy sources interfaced through voltage-source converter-high-voltage direct current (VSC-HVDC). The concerning transient stability studies require the dynamic phenomena of interest to be included with adequate detail and reasonable simulation speed. Hybrid simulation offers this functionality, and this contribution focuses on its application to (multiterminal) VSC-HVDC systems. Existing numerical interfacing methods have been evaluated and improved for averaged VSC modeling. These innovations include: 1) ac system equivalent impedance refactorization after faults; 2) amended interaction protocols for improved Thévenin equivalent source updating inside the EMT-type simulation; and 3) a special new interaction protocol for improved phasor determination during faults. The improvements introduced in this contribution lead to more accurate ac/VSC-HVDC transient stability assessment compared to conventional interfacing techniques.