The continuously increasing demand for electric power and the economic access to remote renewable energy sources such as off-shore wind power or solar thermal generation in deserts have revived the interest in high-voltage direct current (HVDC) multiterminal systems (networks). A lot of work was done in this area, especially in the 1980s, but only two three-terminal systems were realized. Since then, HVDC technology has advanced considerably and, despite numerous technical challenges, the realization of large-scale HVDC networks is now seriously discussed and considered. For the acceptance and reliability of these networks, the availability of HVDC circuit breakers (CBs) will be critical, making them one of the key enabling technologies. Numerous ideas for HVDC breaker schemes have been published and patented, but no acceptable solution has been found to interrupt HVDC short-circuit currents. This paper aims to summarize the literature, especially that of the last two decades, on technology areas that are relevant to HVDC breakers. By comparing the mainly 20+ years old, state-of-the art HVDC CBs to the new HVDC technology, existing discrepancies become evident. Areas where additional research and development are needed are identified and proposed.

HVDC Circuit Breakers: A Review Identifying Future Research Needs

This document is a summary of a report prepared by the IEEE PES Task Force (TF) on Microgrid Stability Definitions, Analysis, and Modeling, IEEE Power and Energy Society, Piscataway, NJ, USA, Tech. Rep. PES-TR66, Apr. 2018, which defines concepts and identifies relevant issues related to stability in microgrids. In this paper, definitions and classification of microgrid stability are presented and discussed, considering pertinent microgrid features such as voltage-frequency dependence, unbalancing, low inertia, and generation intermittency. A few examples are also presented, highlighting some of the stability classes defined in this paper. Further examples, along with discussions on microgrid components modeling and stability analysis tools can be found in the TF report.

Microgrid Stability Definitions, Analysis, and Examples

Recently, sustained power oscillation at subsynchronous frequency was captured in direct-drive permanent magnetic synchronous generator (PMSG) based wind farms in Xinjiang Uygur Autonomous Region, China. This new type of subsynchronous interaction (SSI) detected in practical systems has never been reported and analyzed before. Therefore, its mechanism and characteristics are not yet clearly clarified. In this paper, a simplified but representative system model with multiple PMSGs interfaced with AC networks is established first based on the actual system and the PMSG model provided by the manufacturer. Then, small-signal eigenanalysis, time-domain simulation, and impedance model analysis are carried out to investigate the interactive dynamics between them. The results show that such interaction between direct-drive PMSG wind farms and weak ac grids characterized by low short-circuit ratio would cause negative-resistance effect for the SSI mode, leading to unstable oscillation. In such cases, the controller of PMSG would saturate soon, resulting in sustained power oscillation in the system. If unfortunately the oscillation frequency matches the torsional mode of any nearby turbogenerator, severe torsional vibration would be excited on the shaft of the latter. The analysis results are finally validated with field measurements of an actual SSI event. To address the problem, a supplementary subsynchronous damping control loop is attached to the controllers of PMSGs to reshape the impedance and thus to stabilize the SSI.

Subsynchronous Interaction Between Direct-Drive PMSG Based Wind Farms and Weak AC Networks

This task force paper summarizes the state-of-the-art real-time digital simulation concepts and technologies that are used for the analysis, design, and testing of the electric power system and its apparatus. This paper highlights the main building blocks of the real-time simulator, i.e., hardware, software, input-output systems, modeling, and solution techniques, interfacing capabilities to external hardware and various applications. It covers the most commonly used real-time digital simulators in both industry and academia. A comprehensive list of the real-time simulators is provided in a tabular review. The objective of this paper is to summarize salient features of various real-time simulators, so that the reader can benefit from understanding the relevant technologies and their applications, which will be presented in a separate paper.

Real-Time Simulation Technologies for Power Systems Design, Testing, and Analysis

This paper presents a compendious summary of power hardware-in-the-loop (PHIL) simulations that are used for designing, analyzing, and testing of electrical power system components. PHIL simulations are an advanced application of real-time simulations that represent novel methods, which conjoin software and hardware testing. This contribution outlines necessary requirements for the implementation of PHIL simulations, which are defined by the nature of the digital real-time simulator, the power amplifier, and the power interface (PI). Fundamental characteristics, such as the input/output systems, PI, interface algorithm, and system stability considerations, are discussed for PHIL setups, in order to illustrate both flexibility and complexity of this compound simulation method. The objective of this work is to elaborate an understandable overview of PHIL simulation for electrical power systems and to constitute a contemporary state-of-the-art status of this research area.

Characteristics and Design of Power Hardware-in-the-Loop Simulations for Electrical Power Systems

The closed-loop stability and the simulation accuracy are two paramount issues in power hardware-in-the-loop (HIL) simulation in regard to the operational safety and the experiment reliability. In this paper, the stability issue of the power HIL simulation is first introduced with a simple example. A stability analysis and accuracy estimation method based on the system's loop transfer function is later given. Five different interface algorithms are described, and their respective characteristics with respect to the system stability are compared. Through Matlab simulations and field experiments of two representative power HIL examples, it is revealed that certain interface algorithms exhibit higher stability and accuracy than the others under the given conditions. A recommendation for selecting appropriate interface algorith.ms is finally proposed at the end of this paper.

Improve the Stability and the Accuracy of Power Hardware-in-the-Loop Simulation by Selecting Appropriate Interface Algorithms

We report on the application of a 5-MW variable voltage source (VVS) amplifier converter for utilization in power hardware-in-the-loop (PHIL) experiments with megawatt-scale motor drives. In particular, a commercial 2.5-MW variable speed motor drive (VSD) with active front end was connected to a virtual power system using the VVS for integrating the drive with a simulated power system. An illustrative example is given, whereby a 4-MW gas turbine generator system, including various loads, is simulated and interfaced with the VSD hardware in the lab through the VVS using current feedback to the simulation. Mechanical loading is applied to the motor via an identical 2.5-MW dynamometer connected to the same shaft. This paper first describes the PHIL facility, illustrates the challenges of powering a motor drive from a controlled voltage source converter at the multimegawatt scale, and provides experimental results from dynamic simulations. While certain challenges remain with the accuracy of the interface, it is concluded that PHIL simulations at the megawatt power level are possible and may prove useful for validating models of drive systems in the future.

A Megawatt-Scale Power Hardware-in-the-Loop Simulation Setup for Motor Drives

The closed-loop stability and the simulation accuracy are two paramount issues in power hardware-in-the-loop simulation in regard to the operational safety and the experiment reliability. In this paper, the stability issue of the power HIL simulation is first introduced with a simple example. A stability analysis and accuracy estimation method based on the system's loop transfer function is later given. Five different interface algorithms are described and their respective characteristics with respect to the system stability are compared. Through Matlab simulations and field experiments of two representative power HIL examples, it is revealed that certain interface algorithms exhibit higher stability and accuracy than the others under the given conditions. A recommendation for selecting appropriate interface algorithms is finally proposed at the end of the paper.

This paper deals with the current state-of-the-art in interfacing issues related to real-time digital simulators employed in the simulation of power systems and power-electronic systems. This paper provides an overview of technical challenges encountered and their solutions as the real-time digital simulators evolved. Hardware-in-the-loop interfacing for controller hardware and power apparatus hardware are also presented.

Interfacing Issues in Real-Time Digital Simulators

Frequency-scan approaches for conducting Sub-Synchronous Control Interaction (SSCI) studies of wind farms is becoming a normal practice in the industry. Modern power electronics control used in many wind generators has a characteristic that couples two frequency components, similar to saliency effects of conventional generators. As a consequence, treating both wind farm and grid as single-dimension impedances could generate an inaccurate result. This paper describes a two-dimension transfer function approach that can be practically applied to ensure accuracy when evaluating SSCI aspects of power-electronic equipment such as renewable generation.

Wei Ren

Papers

Improve the Stability and the Accuracy of Power Hardware-in-the-Loop Simulation by Selecting Appropriate Interface Algorithms

A Megawatt-Scale Power Hardware-in-the-Loop Simulation Setup for Motor Drives

Improve the Stability and the Accuracy of Power Hardware-in-the-Loop Simulation by Selecting Appropriate Interface Algorithms

Interfacing Issues in Real-Time Digital Simulators

A Refined Frequency Scan Approach to Sub-Synchronous Control Interaction (SSCI) Study of Wind Farms