Real Time Digital Simulator Research Articles

Measurement of impedance angle for anti-islanding protection in multi-source distribution network

With the increase of the proportion of distributed generators (DGs) access in the distribution network, it transforms from a traditional radial network to a multi-terminal active network, which brings many challenges to relay protection. If part of the distribution network is disconnected from all upstream networks, it can create unplanned islands that can compromise personal safety, power quality, and out-of-phase reclosing. As a result, it is essential to equip DGs with anti-islanding protection. Currently, anti-islanding protection based on impedance measurement is of great interest because it has the lowest risk of rejection and misoperation. However, in multi-source distribution networks, signal interference can lead to protection failure. In light of this, we proposed an impedance angle measurement anti-islanding protection based on voltage synchronous injection, where each DG uses the bus voltage as a reference to achieve synchronous injection of the islanding detection signal. The measured impedance angle was then used to identify the state of islanding operation. Combining the analyses of line impedance, injected signal frequency and other relevant factors, an impedance angle based islanding identification criterion is proposed. Furthermore, the realization method of signal injection and impedance angle detection in combination with the DG control strategy was presented. Finally, real-time digital simulation (RTDS) experiments show that the proposed method can accurately identify islands under zero power mismatch conditions and independent of grid disturbances.

Исследование точности определения места повреждения воздушных линий по данным от устройств синхронизированных векторных измерений различных классов и производителей

The technology of synchronized phasor measurements has been widely used in the Russian power system to analyze the parameters of steady-state electrical power modes and to record electro-mechanical transient processes. Issues on fault location based on synchronized phasor measurements have mainly been discussed in foreign publications. A significant drawback of most research papers on this issue is simplified modeling of both overhead lines and current and voltage measurement channel as well as digital filters of phasor measurement units (PMUs). The goal of this research is to develop a fault location algorithm based on synchrophasor measurements and to study its accuracy by using specialized equipment including a real-time digital simulator (RTDS), PONOVO current and voltage amplifiers and production-grade PMUs. A PMU-based double-ended fault location algorithm is developed using long-line equations and well-known electromagnetic transient theory concepts. Currents and voltage oscillograms of both overhead transmission line ends are modelled in the MATLAB/Simulink software package. These oscillograms are saved as COMTRADE files and played back using the RTDS hardware and all-in-one software package RSCAD. In addition, the study uses two production-grade PMUs, the first one is TPA-02, and the second one is a merging unit ENMU acting as a PMU, thus imitating PMUs at the line terminals. To time-align all the measurements and to aggregate PMU data frames, various auxiliary software such as PMU Connection Tester, and hardware is used. A double-ended fault location (FL) method utilizing synchrophasors under a fault has been developed. The method is based on an overdetermined system of nonlinear equations that describes physical processes in an overhead power line and can be applied under various fault types. An integrated study of the efficiency of the developed FL method has been conducted. FL errors have been computed using production-grade PMUs ENMU and TPA-02, along with the RTDS and other equipment. The authors have considered the case of configuring PMUs of different classes at the overhead line terminals, and different phasor reporting rates as well. The conducted experiments make it possible to reveal that the fault location errors do not exceed the thresholds imposed by the standard STO 56947007-29.120.70.241-2017 in 88 % of all the analyzed fault scenarios. The developed FL method makes it possible to achieve the accuracy required by regulatory guide in calculating the distance to the short circuit point in most of the cases examined. Based on the results of numerical experiments for various types of fault cases, it can be concluded that the PMU class and phasor reporting rate do not have a significant impact on the FL accuracy, provided that the fault duration is enough for the PMU filter to approach a steady output.

A robust damping control for battery energy storage integrated power systems to mitigate inter-area oscillations

This paper presents the effect of a Battery Energy Storage System (BESS) on the power system inter-area oscillations under changing load conditions. The dynamic interaction between the BESS control modes and synchronous generators, along with BESS control parameters, influence the damping of prominent inter-area oscillations under different loading conditions. Furthermore, H∞ mixed sensitivity scheme-based Wide-Area Damping Controller (WADC) is proposed for BESS. This controller is designed using the Linear Matrix Inequality (LMI) framework to mitigate the prominent inter-area oscillation of the power system. Sensitivity to control parameter variation and the geometric measure of the observability are utilized to select the location and feedback signal for the WADC. The performance of the proposed WADC is compared with an Energy Storage System-based stabilizer and WADC proposed in an existing work on a modified New England ten-machine benchmark system. The Real-Time Digital Simulator (RTDS) platform is used to assess the real-time viability of the proposed WADC. The simulation results demonstrate that the proposed WADC can effectively diminish several inter-area modes. Additionally, it offers robust performance under varying load conditions, contingencies and with communication delays in feedback signals. The proposed controller is also effective under the system integrated with Doubly Fed Induction Generator-based Wind Turbine System.

State–Space Modelling and Stability Analysis of Solid-State Transformers for Resilient Distribution Systems

Power grids are currently undergoing a significant transition to enhance operational resilience and elevate power quality issues, aiming to achieve universal access to electricity. In the last few decades, the energy sector has witnessed substantial shifts toward modernizing distribution systems by integrating innovative technologies. Among the innovations, the solid-state transformer (SST) is referred to as a promising technology due to its flexible power control (better reliability) and high efficacy (by decreasing losses) compared with traditional transformers. The design of SST has combined three-stage converters, i.e., the input, isolation, and output stages. The key objective of this design is to implement a modern power distribution system to make it a more intelligent and reliable device in practice. As the power converters are used in SST, they exhibit non-linear behavior and can introduce high-frequency components, making stability more challenging for the system. Besides, the stability issue can be even more complicated by integrating the distributed energy resources into the distribution system. Thus, the stability of SST must be measured prior to /during the design. To determine stability, state-space modeling, and its controller design are important, which this paper explains in detail. Indeed, the system’s stability is measured through the controllability and observability test. Further, the stability analysis is performed using frequency and time-domain diagrams: the Bode plot, Nyquist plot, Nichols chart, Root locus, pole-zero plot, and Eigen plot. Finally, the SST Simulink model is tested and validated through real-time digital simulation using the OPALRT simulator to show its effectiveness and applicability. The stability performance of the proposed SST is evaluated and shows the effectiveness of the controller design of each converter circuit.

Robust damping control for integrated wind turbine power networks during low inertia condition

AbstractThis paper presents a modal analysis‐based impact analysis of the change in system inertia due to the integration of the large‐scale doubly fed induction generator (DFIG)‐based wind turbine system (WTS) on the power system damping in inter‐area oscillation modes. The impact of replacement of a synchronous generator by WTS of same MVA capacity on system dynamics such as emergence of new critical modes of inter‐area oscillations affecting system stability is studied. Further, a double‐channel scheme‐based wide‐area damping controller (WADC) is proposed for WTS, which provides sufficient damping of prominent oscillation modes and enhanced stability margin to alleviate the negative impact of reduction in system inertia. The comparative assessment of the proposed controller with a PSS‐based WADC on a modified New England 10‐machine system validates the performance and is found to be more effective in mitigating inter‐area oscillation in the system. The real‐time feasibility of the proposed double‐channel WADC is evaluated on the Real‐Time Digital Simulator (RTDS) platform. The simulation results show that the proposed WADC can successfully mitigate various inter‐area modes simultaneously. Furthermore, it provides robust performance for a variety of contingencies, time delays in feedback signals, and uncertainties associated with different operating scenarios of WTS.

A Protection Principle of LCC–VSC Three-Terminal HVdc System Based on Instantaneous Boundary Impedance

In the field of hybrid multi-terminal HVdc (high voltage direct current) transmission, an inevitable problem is the short availability of fault information. Namely, only 1-ms complete fault information can be used to identify faults. To overcome this limitation, this study introduces the concept of instantaneous boundary impedance and proposes a dc line protection method. First, a dc transmission line is divided into three types of zones, namely internal, forward-external, and backward-external zones. Then, the instantaneous boundary impedance is analyzed and compared under normal operation and different types of faults. The results indicate that under normal operation, the instantaneous boundary impedance is zero; under a dc line fault, the instantaneous boundary impedance is less than zero; finally, under an external fault, the instantaneous boundary impedance is greater than zero. The proposed principle is validated using Wudongde LCC (line-commuted converter)-VSC (voltage source converter) three-terminal hybrid HVdc RTDS (real-time digital simulation) test system. The experimental results demonstrate that the proposed protection method is effective under different types, locations, and impedances of faults and signal-to-noise ratios within a period of 1 ms. Compared to the existing methods, the proposed protection method can achieve better performance in withstanding fault impedance (600 Ω) and has better robustness against noise interference (10 dB).

Performance Evaluation of Microgrid Management System by using a Hareware-In-Loop-Simulation Method

This paper presents a real time digital simulator based test system as a new method for testing microgrid management system (MMS). This system is composed of a real time digital simulator and a developed communication emulator. Real time digital simulator runs a simulation case for microgrid model and distributed generation source model. Communication module emulates communication functions of micro-sources in microgrid by transmitting the simulated signals to MMS. The MMS controls operation of micro-sources to control the power flow at the point of common coupling (PCC) and the voltage and frequency of microgrid. A prototype MMS was tested for stand-alone and grid-connected operation to verify the validation of the developed hardware-in-the-loop simulation (HILS) system.

Impact of Plug-In Electric Vehicle Battery Charging on a Distribution System Based on Real-Time Digital Simulator

Although the electric vehicles (EVs) have many economic and environmental advantages as compared to the conventional cars, the increased adoption of these vehicles will bring up new concerns for the power distribution grid in terms of introducing new peaks into the system or causing voltage quality problems. This study investigates the impact of the electric vehicles battery charging on the distribution system in terms of voltage unbalance at various locations, transformers overloading, and shifting the peak of the system. In this research, a 12.47 kV real distribution network has been modelled using real time digital simulator, using real data from a power distributor. The study presents four different scenarios of uncoordinated EVs integration for two different charging times (evening and night) and two different charging rates (level I and level II) at different penetration levels ranging from 10% to 100%. Voltage unbalance at different locations is determined and transformer overloading is analysed. The influence of EVs charging on the daily load curve is shown.

Review of Synchronization Algorithms used in Grid-Connected Renewable Agents

This paper presents a review of several synchronization algorithms used in Grid-Connected Renewable Agents. Different synchronization algorithm structures will be shown and their advantages and disadvantages will be discussed using MATLAB/SIMULINK tool from The MathWorks, Inc. Some simulations will be shown firstly, and secondly, a Real Time Digital Simulator (RTDS) platform will be used for testing and debugging the Synchronization Algorithms.

A novel power control scheme for distributed DFIG based on cooperation of hybrid energy storage system and grid-side converter

Due to the uncertainty of wind energy, the wind power is difficult to be dispatched and may cause the voltage fluctuations for distributed network. Therefore, a novel power control scheme is proposed to manage the output power of the distributed doubly-fed induction generator (DFIG) by cooperating the hybrid energy storage system (HESS). In the proposed scheme, the grid-side converter of DFIG is controlled to manage overall output power of the DFIG/HESS hybrid system in accordance to the power dispatching command. The HESS is utilized to deal with the power difference between DFIG generation power and the power dispatching command. The HESS is embedded in the DC-link bus of DFIG and is composed of superconducting magnetic energy storage and batteries. Additionally, in order to avoid HESS from overcharging and over-discharging, the pitch angle control and power dispatching command are adjusted by considering the state of charge (SOC) of HESS. Finally, the simulation model of distributed DFIG and HESS is built on the real-time digital simulation (RTDS) platform, and the simulation results validate the effectiveness and reliability of the proposed power control scheme.

Real-Time Co-Simulation for the Analysis of Cyber Attacks Impact on Distance Relay Backup Protection

Smart Grid is a cyber-physical system that incorporates Information and Communication Technologies (ICT) into the physical power system, which introduces vulnerabilities to the grid and opens the door to cyber attacks. Wide area protection is one of the most important smart grid applications that aims at protecting the power system against faults and disturbances, which makes it an attractive target to cyber attacks, aiming at compromising the reliability of the power system. Understanding the interaction between the cyber and physical components of the smart grid and analyzing the damage that cyber-attacks can do to wide area protection is very important as it helps in developing effective mitigation approaches. This paper evaluates the impact of cyber attacks on a wide area distance relay backup protection scheme in real-time, through the development of a co-simulation platform based on Real Time Digital Simulator (RTDS) and network simulator 3 (NS3) and using the IEEE-14 bus power system model.

Theoretical Development and Experimental Evaluation of a Nonlinear Observer for sensorless WECS

This research paper introduces a novel sensorless wind energy conversion system (WECS) with a Vienna rectifiers-based high-gain observer. The main emphasis is on crafting a high-gain observer capable of providing real-time estimates for challenging-to-measure signals like rotor speed and load torque. Addressing the complexities arising from system nonlinearity and high dimensionality, the paper showcases the effectiveness of the proposed observer through simulations and theoretical analysis. Furthermore, an experimental validation on the RealTime Digital Simulator (RTDS) platform validates the observer’s performance and reliability, highlighting its potential for enhancing sensorless WECS technology.

An Edge Computing-Oriented Islanding Detection Using Differential Entropy and Multi-Support Vector Machines

This paper presents an edge computing-oriented islanding detection paradigm using multi-support vector machines (SVM) and differential entropy. The proposed methodology can detect islanding events and locate which microgrid was disconnected in a distribution system with microgrid cluster. The proposed method employs multi-SVMs in the cloud to detect and locate potential islanding events in power distribution systems. The accuracy and reliability of the results of SVMs are enhanced by introducing differential entropy for quantitative evaluation. Additionally, the method decomposes the SVM inference process at the edge, enabling edge devices with low computational power and limited resources to support data-driven algorithms without compromising accuracy and speed. An extensive study is performed on the modified IEEE123 distribution system established in real-time digital simulation. Simulation results elucidate that the proposed method can accurately and promptly detect system islanding operations with zero non-detection zone. Comparison with other methods illustrates the superiority of the proposed method in terms of discrimination accuracy, detection time, and reliability.

Stability-Constrained Contingency Analysis for Modern Power Systems

Modern power systems comprise diverse nonlinear components, and an increasingly large number of low inertia renewable power sources necessitate the modernization of conventional power system security measures. Contingency analysis (CA), a routine process for power system operators, ensures grid security under unforeseen circumstances by identifying potential issues and enabling proactive measures for uninterrupted power flow. Electromechanical oscillations (EMOs) in the power system that are a threat to stability must be regularly monitored and mitigated. An online hierarchical EMO index integrating time and frequency response analysis can be utilized for system stability assessment. The integration of an EMO index threshold into contingency analysis is presented in this paper to enhance system security. This new approach is referred to as the stability-constrained contingency analysis (SCCA). Typical results for a modified two-area, four-machine power system with large solar photovoltaic plants simulated on a real-time digital simulator (RTDS) are presented. These results demonstrate that SCCA flags potential issues that can arise from EMOs for certain contingencies, whereas traditional CA does not, as it solely considers bus voltage limits and line ratings.

EMT Model of 20-MW Wind Generator for Real-time Simulation of HVDC Networks

As modern trends demand efficient methods to massively and promptly exploit clean energy, there is an urgent need of solutions that can help the ambition of accelerating the development and integration of larger scale energy systems with the capacity to extract, store, and utilize huge amounts of renewable energy sources. In view of this, the rated power output of the wind generation technology is expected to significantly increase in the near future to enable multi-gigawatt roll outs, e.g. the European plans for Offshore infrastructure development in the North-Sea. Besides, gigantic energy systems shall transmit the power to onshore interconnected systems via HVDC transmission. This paper introduces an EMT model of a 20 MW wind generation system with the conventional back-to-back converter replaced by a combination of an AC to DC converter and a DC to DC converter. While the need of such systems is acknowledged in the literature, the design and performance analysis of high power implementation of such systems have not been addressed yet. The proposed system operates at a voltage level of 100 kV, which can directly transfer power through HVDC interconnection. The designed control aspects are outlined and the resulting dynamic response obtained from realtime digital simulation (RTDS) are discussed. The presented wind generator concept can help in reducing the size of a planned energy system by requiring lesser system components. Also, the system efficiency can be enhanced by decreasing the number of power conversion stages.

A Decentralized Power Management and Sliding Mode Control Strategy for Hybrid AC/DC Microgrids including Renewable Energy Resources

This paper presents a new decentralized robust strategy to improve small and large-signal stability and power-sharing of hybrid AC/DC microgrids and improve its performance for nonlinear and unbalanced loads. In addition to the sliding mode controller for DC/DC converters, for the sake of improving power sharing and regulating active and reactive powers injected by distributed energy resources, and moreover, controlling harmonic and negative-sequence current in the presence of nonlinear and unbalanced loads, two separate controllers for positive sequence power control and negative sequence current control are designed based on the sliding mode control and Lyapunov function theory, respectively. The theoretical concept of the proposed robust control strategy, including mathematical modeling of microgrid components, basic theorems, controller design procedure, and robustness and closed loop stability analysis are outlined. Also, this direct power/current/voltage control scheme is governed by a new hybrid AC/DC hierarchical control scheme that exploits a harmonic virtual impedance loop and voltage compensation scheme. To show the effectiveness of the proposed robust control scheme, offline time-domain simulations are done on a hybrid AC/DC wind/photovoltaic/fuel-cell microgrid with nonlinear and unbalanced loads in MATLAB/Simulink environment, and the results are experimentally verified by OPAL-RT real-time digital simulator.

PEV’s Smart Charging Strategy Based on Individual State of Charge

Themain contribution of this work is a smart-charging strategy based on the state of charge analyses to avoid the undervoltages caused by plug-in electric vehicles into the distribution system. The work uses power hardware-in-the-loop simulations, where a modified IEEE 34 bus system, with five groups of electric vehicle stations, is modeled in a real-time digital simulator. The number of electric vehicles charging and the initial state of charge (SoC) are generated randomly. The first analyses identify the undervoltage problems due to the increased number of electric vehicles connected during peak hours load. After, a smart-charging solution is necessary to solve this power quality problem. So, if the voltage profile decreases under certain limits, defined by a hysteresis band, the control occurs, and the smart-charging is applied. The proposed strategy based on individual state of charge priority reduces the electric vehicle recharge power at virtual stations, by comparing its value with the mean of state of charge values from each station group. The experimental results presented show that, by varying the recharge current, the voltage profiles do not reach the limit for sags determined by the Brazilian standard, proving the performance and solving the problem even if not reducing the recharge current of all electric vehicles equally.

Optimal single settings based relay coordination in DC microgrids for line faults

A novel time–current-rate-based inverse characteristic curve for relays in a DC microgrid is proposed in this paper. Line current rise rate is used as actuating quantity, ensuring quick line fault clearing (relay operating time is in order of a few μs). The advantage of using line current rise rate as actuating quantity is that for a line short-circuit fault, it does not vary significantly for grid-connected and islanded modes of operation and varying network topologies of DC microgrid. Consequently, using the proposed characteristic, a single set of optimal relay settings is obtained using an optimization solver in MATLAB. The obtained settings ensure reliable and selective coordination between primary and backup relays for various pole-to-ground and pole-to-pole line faults under different operating conditions with multiple sources in two different 4-bus 400V low voltage DC microgrids. The maximum relay operating time for a high resistance fault with the proposed curve is 394.9μs for the first considered low voltage DC microgrid. Simulations in the Real Time Digital Simulator and comparisons with previous schemes (one comparison on the second considered DC microgrid), conventional standard inverse, and extremely inverse curves for DC microgrid protection indicate the effectiveness of the proposed scheme.

A Data-Driven Finite-State Machine-Based Control for Hybrid Parallel Multiconverters: Fusion Topology for High-Power Applications

In this paper, the concept of fusion topology is proposed to tackle the development trend and control issue of hybrid parallel multi-converter applications and to achieve better performance in transient and steady-state operations. Compared to previously fixed frequency compensation strategies, data-driven finite-state machine (FSM) control can determine which voltage source converter (VSC) dominates the operation or whether two VSCs operate simultaneously rather than giving the specific target operation to each VSC alone. The results are verified by a real-time digital simulator (RTDS) with the hardware-in-the-loop (HIL) experiments. The 20kVA RTDS experimental results show that the transient time is shorter by 53.8%− 85.7%, and power loss is smaller by 28.6%− 78.3%, respectively, compared with the previous methods.

Analytical voltage sensitivity-based distributed Volt/Var control for mitigating voltage-violations in low-voltage distribution networks

Integrating solar photovoltaic system in low-voltage distribution networks leads to significant voltage violations. This issue can be alleviated using the cutting-edge control techniques (Volt/Var control, Volt/Watt control, etc.) of smart inverters. Moreover, the smart inverters absorb/inject the necessary amount of reactive power from/into the substation to alleviate the over/under voltages. This increases the reactive power burden on the substation and deteriorates its power factor. Therefore, this paper proposes a novel distributed Volt/Var control technique to reduce the dependency on substations for reactive power support and improve its power factor. The proposed technique is based on analytical voltage sensitivity analysis. By developing suitable relations, the feeder nodes are categorized into two groups, one operating in lagging mode, while the other in leading mode. This ensures proper utilization of reactive power capability of the inverters. An IEEE 13-node low voltage distribution network with the solar photovoltaic system at different load conditions is used to test the proposed and other existing Volt/Var control techniques. Compared to other distributed Volt/Var control techniques, the proposed method requires a minimum reactive power support to regulate the feeder voltage. Additionally, the obtained results by the proposed method are closer to those by the optimal reactive power support approach using GAMS software and are validated on a real-time digital simulator platform.