Real Time Digital Simulator Research Articles

Robust Motion Planning in Robot-Assisted Surgery for Nonlinear Incision Trajectory

In the era of digital OTs (operating theatres), the developments in robot-assisted surgery (RAS) can greatly benefit the medical field. RAS is a method of technological advancement that uses robotic articulations to assist in complicated surgeries. Its implementation improves the ability of the specialized doctor to perform surgery to a great extent. The paper addresses the dynamics and control of the highly non-linear 3DOF surgical robot manipulator in the event of external disturbances and uncertainties. The integration of non-linear robust SMC (sliding mode control) with a smoothing mechanism, a FOPID (fractional-order proportional integral derivative) controller, and a fuzzy controller provides a high degree of robustness and minimal chatter. The addition of fuzzy logic to the controller, named intelligent fuzzy-SFOSMC (smoothing fractional order sliding mode controller) improves the system’s performance by ruling out the disturbances and uncertainties. The prototype model is developed in a laboratory and its outcomes are validated on OP5600, a real-time digital simulator. Simulation and experimental results of the proposed fuzzy-SFOSMC are compared with conventional controllers, which illustrates the efficacy and superiority of the proposed controller’s performance during the typical surgical situations. The proposed fuzzy-SFOSMC outperforms conventional controllers by providing greater precision and robustness to time-varying nonlinear multi-incision trajectories.

A novel hybrid methodology to secure GOOSE messages against cyberattacks in smart grids

IEC 61850 is emerging as a popular communication standard for smart grids. Standardized communication in smart grids has an unwanted consequence of higher vulnerability to cyber-attacks. Attackers exploit the standardized semantics of the communication protocols to launch different types of attacks such as false data injection (FDI) attacks. Hence, there is a need to develop a cybersecurity testbed and novel mitigation strategies to study the impact of attacks and mitigate them. This paper presents a testbed and methodology to simulate FDI attacks on IEC 61850 standard compliant Generic Object-Oriented Substation Events (GOOSE) protocol using real time digital simulator (RTDS) together with open-source tools such as Snort and Wireshark. Furthermore, a novel hybrid cybersecurity solution by the name of sequence content resolver is proposed to counter such attacks on the GOOSE protocol in smart grids. Utilizing the developed testbed FDI attacks in the form of replay and masquerade attacks on are launched and the impact of attacks on electrical side is studied. Finally, the proposed hybrid cybersecurity solution is implemented with the developed testbed and its effectiveness is demonstrated.

Grounding electrode line fault location method based on simulation after test and deduction

• Fault characteristics of the grounding electrode line are weak. • Fault location by injecting signals into the line. • The initial pulse signal is eliminated after linear mode transformation. • Using simulation after test and deduction to obtain fault distance. • No need to calibrate the arrival time of the wave head. When a high-voltage direct current (HVDC) transmission system with the grounding electrode line (GEL) is in the balanced operation mode, the unbalanced current in the GEL and the neutral potential of the system are extremely small. In the GEL, after a fault incident, the additional fault component will be small, and fault characteristics will be weak. As the traditional passive fault location methods are challenging to apply, a GEL fault location method based on simulation after test and deduction (SATD) is proposed in this paper. Firstly, the pulse signal is injected into the GEL. Then, the mutation point of the pulse response signal is projected along the line to form a virtual fault point matrix. Based on the mentioned matrix, the fault location function is constructed, and the fault distance is obtained by the catastrophe point of the function. In this method, the bottleneck of precisely identifying the traveling wave (TW) arrival time in the time domain is broken, and the automated single-ended TW location of the GEL can easily be implemented. Many real time digital simulator (RTDS) tests validate the high accuracy and strong robustness of the method.

Battery Power Control Strategy for Intermittent Renewable Energy Integrated Modular Multilevel Converter-Based High-Voltage Direct Current Network

Modular multilevel converters (MMC) play a dominant role in integrating remotely located renewable energy resources (RER) over the high-voltage direct current (HVDC) transmission network. The fault ride-through capabilities of the MMC-HVDC network during low-voltage faults and the power fluctuation due to RER intermittency are the major obstacles to the effective integration of renewable energy. In response, this article proposes a local voltage-based combined battery energy control scheme for a PV-wind-battery connected MMC-HVDC system to regulate the HVDC-link voltage during low-voltage faults at the point of common coupling of alternating current grids and to reduce the intermittent RER power fluctuation. The proposed technique removes the dynamic braking resistor from the HVDC-link and smoothly integrates the RER without active power reduction of renewable energy under low-voltage faults. Symmetrical and unsymmetrical low-voltage faults have been conducted to validate the effectiveness of the proposed control scheme for the battery in mitigating surplus energy in the HVDC-link. Additionally, wind speed, solar radiation, and temperature have been changed to confirm the improved performance of the battery energy management system. The complete systems have been simulated and tested in a real-time digital simulator (RTDS) and using dSPACE-based controller hardware in a loop setup.

A Control Method for Converter-Interfaced Sources to Improve Operation of Directional Protection Elements

The traditional fault control strategy of converter-interfaced renewable energy sources (CIRESs) may bring about a lower sensitivity level or misoperation of fault component-based directional elements. To overcome this problem, a new control scheme is proposed to adjust sequence impedance angles of CIRESs by computing suitable current references of the CIRES controller. Meanwhile, these current references are maximized by an iterative algorithm to make full use of the short-circuit capacity of CIRESs. The proposed control scheme is applicable to various faulty conditions such as different fault types, power factors, weak grids, and larger fault resistances. Compared with the new directional elements that need to update protection algorithms, the proposed control strategies can make CIRESs compatible with the existing directional elements whilst the necessary fault ride-through (FRT) requirements can still be satisfied. Furthermore, all the controller parameters are not required to be revised based on the detected fault type, even with only local measured data collected. The associated PSCAD simulations, real-time digital simulator (RTDS) testing and the downscale hardware experiment verify the proposed method.

A Capacitive-Coupling Winding Tap Injection DSTATCOM Integrated With Distribution Transformer for Balance and Unbalance Operations

The intelligent substations should be developed to perform multifunction operations, including power flow control, voltage regulation, and cost-effective power quality (PQ) improvement. In this article, an intelligent substation by a distribution transformer integrated with a capacitive-coupling winding tap injection static synchronous compensator (CWTI-DSTATCOM) is proposed for balance and unbalance operations. Compared with the state-of-the-art approaches, CWTI-DSTATCOM can operate at a low dc voltage with a small active inverter rating, which also can provide distributed voltage, reactive, and unbalanced current compensation. The power rating reduction mechanism is mathematically revealed via balance and unbalance compensation analysis, which is further used to determine the optimal inverter power rating of unbalance operation via an optimal <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filter design method. Furthermore, a comprehensive compensation control strategy is developed to coordinate the transformer taps and the active inverter under balanced and unbalanced operations. Simulation and real-time digital simulator hardware-in-the-loop experiments are performed to prove the validity and effectiveness of the proposed topology over inverter rating reduction and PQ enhancement.

Intelligent Fractional Order Sliding Mode Based Control for Surgical Robot Manipulator

In the era of digital OT (Operating Theatre), the developments in Robot-Assisted Surgery (RAS) can greatly benefit the medical field. RAS is a method of technological advancement that uses surgical robots to assist complicated surgeries. Its implementation improves the ability of the specialised doctors to perform surgery to a great extent. This paper addresses the dynamics and control of the highly non-linear 3DOF surgical robot manipulator in the occurrence of external disturbances and uncertainties. The integration of non-linear robust SMC (Sliding Mode Control) with smoothing mechanism, FOPID (Fractional-Order Proportional Integral Derivative) controller and fuzzy controller provides a high degree of robustness and minimal chatter. The addition of type-2 fuzzy logic to the controller, named intelligent T2F-SFOSMC (Type-2 Fuzzy-Smoothing Fractional Order Sliding Mode Controller), improves the system’s performance by ruling out the disturbances and uncertainties. The prototype model is developed in a laboratory and its outcomes are validated on OP5600: a real-time digital simulator. The simulation results and experimental results of the proposed T2F-SFOSMC are compared with conventional controllers, which illustrates the efficacy and superiority of the proposed controller’s performance during the typical situation of surgery. The proposed T2F-SFOSMC outperforms conventional controllers by providing greater precision, stability and robustness to time-varying nonlinear multi-incision trajectory.

Cyberattack-Resilient Secondary Frequency Control Scheme for Stand-Alone Microgrids

This article proposes a cyberattack-resilient secondary frequency control (SFC) scheme for a stand-alone microgrid (MG) system. The proposed scheme is based on a hybrid system that employs an unknown input observer for state estimation, cyberattack detection and reconstruction, and a Type-2 fuzzy logic system for minimization of frequency deviation that may be caused by cyberattacks on the MG system. A cyberattack on the MG’s frequency measurement disrupts the operation of the SFC system and eventually compromises the frequency stability of the MG. The proposed approach shows an accurate estimation of the MG states and maintains the frequency of the MG within operating limits during various cyberattack scenarios. The practicability of the proposed hybrid scheme is validated using the Speedgoat real-time digital system simulator.

Harmonic Injection Based Distance Protection for Line With Converter-Interfaced Sources

Converter-interfaced renewable energy sour- ces are directly integrated into the power system using power electronics, so unique fault characteristics may lead to the maloperation of traditional distance relays. In this article, to deal with this issue, a harmonic injection- based distance protection method is proposed. The controllable harmonic current is injected without revising the controller structure, and the harmonic frequency and magnitude are determined according to the grid feature. Since the proportional integral cannot track the sine component without the static error, the actual frequency response will be deduced using the transfer function. On this basis, the harmonic impedance measured at the relay point can accurately reflect the fault distance due to the disappearance of the remote infeed, so the proposed method has a strong ability to withstand the fault resistance. Meanwhile, it only needs a few modifications to the phasor extraction method in original protection devices, so it is easy for engineering applications. Furthermore, typical fault ride through requirements can be still satisfied. Simulation results in PSCAD and real-time digital simulator verify the proposed method in different fault scenarios.

Interoperability of classical and advanced controllers in MMC based MTDC power system

This paper presents the modular-multilevel-converter (MMC) control interoperability (IOP) and interaction within the High Voltage Direct Current (HVDC)-based power system. IOP is a crucial issue in the large-scale HVDC grid with different suppliers. To accommodate future multi-vendor HVDC grids developments, this article comprehensively investigates the MMC control IOP issue. Firstly, the most commonly adopted proportional-integral (PI) control and other non-linear controllers, e.g., model predictive control (MPC), back-stepping control (BSC), and sliding mode control (SMC), are constructed for MMC. Then, the IOP simulations are carried out in a multi-terminal direct current (MTDC) system in real-time digital simulator (RTDS) environment. The most frequent transients of the practical projects, e.g., power flow changing, wind speed changing, and DC/AC grid faults, are simulated with eight different scenarios. Each scenario presents different control capabilities in maintaining system stability, more precisely, the scenarios with non-linear controllers show faster settling time and fewer DC voltage and power variations. Controller switchings are also achieved without bringing large system oscillations. This paper provides the optimal allocation strategy of controllers to cope with system transients.

Fault current control of MMC in HVDC-connected offshore wind farm: A coordinated perspective with current differential protection

The modular multilevel converter (MMC) based high-voltage dc (HVDC)-connected offshore wind farm (OWF) is power electronic converter dominated power system, where the conventional current differential protection may not operate effectively. Based on the basic operation principle of the current differential relay, this paper points out that the phase difference (ϕdiff) of fault currents from MMC and OWF should be limited within a specific range, i.e. (-ϕdiffmax, ϕdiffmax), to guarantee the reliable tripping of the current differential relay. Yet, the highly control-dependent fault currents from the MMC and OWF could yield an arbitrary ϕdiff that might beyond (-ϕdiffmax, ϕdiffmax), which leads to the possible malfunction of the current differential relay. To tackle this challenge, the coordinated control method of MMC is proposed to align the phase angle of its fault current with that of the OWF, such that ϕdiff ∈ (-ϕdiffmax, ϕdiffmax) can be always guaranteed, so as the reliable operation of the current differential relay. Finally, the proposed coordinated control is verified by the electromagnetic transient (EMT) simulations in PSCAD and the real-time digital simulator (RTDS).

Characterizing Current THD’s Dependency on Solar Irradiance and Supraharmonics Profiling for a Grid-Tied Photovoltaic Power Plant

The rapidly increasing distributed energy resources (DERs) in power systems are now getting interconnected to set community grid structures, where power quality will be a major concern. The grid-to-grid (G2G) bidirectional power transfer among the distribution microgrid will not be considered commercially feasible unless the upstream harmonics are under the limits. The aggregation of such harmonics, measured as total harmonic distortion (THD), is feared to be beyond tolerable limits with the progression of rooftop grid-tied PV-like installations. Hence, this THD needs to be characterized with DER generation end variables. In this work, the photovoltaic (PV) DERs’ dependency on environment variables such as irradiance was profiled in the context of generating and injecting harmonics into the grid. A mathematical model of a grid-tied three-phase PV DER was developed as part of this correlation characterization, matching the fundamental unit structure of a 1.4 MW solar canopy located on the Florida International University (FIU) Miami campus. To determine the qualitative association with produced THD patterns, the model was evaluated with various irradiance settings. A real-time digital simulation (RTDS) platform was used to verify it. Following this confirmation, sets of data from power quality meters at the point of common coupling and FIU field sensors were utilized to validate further the correlation model. The results showed that the grid current’s THD exhibited a high correlation with the irradiance profile and its variation over time. The early morning and late afternoon periods of the day, associated with a low irradiance, constantly had higher harmonics generated from the PV DER. The midday THD was rather rational with partial shadings, hence a geolocation-dependent factor. These findings were verified by an RTDS and validated by real field data. In quantifying the THD injected by a single DER at a high-frequency (2–150 kHz) supraharmonics (SH) level, a 3% peak increment in magnitude was observed from the high-fixed to the low-fixed irradiance profile. The correlation characteristics depicted that the hybrid microgrid suffered from a daytime-dependent harmonic insertion from the grid-tied DER. This is a global problem unless specific measures are taken to mitigate the harmonics. The electrically notorious higher-frequency SH was found to increase proportionally. The G2G power transfer can be limited because of the higher THD in the early morning and late afternoon, which will also worsen because the numbers of grid-tied PV DERs (i.e., rooftop solar and industrial solar) are likely to increase rapidly soon. The community grid structure can thus have a controlled harmonics filtration setup purposefully designed to address the findings of this work, which also fall within the scope of our future research.

Diagnosis of PV faults using digital twin and convolutional mixer with LoRa notification system

The amount of energy that is generated using photovoltaic (PV) arrays has been increasing rapidly over recent years. To avoid energy and financial losses due to PV faults, numerous methods for diagnosing PV faults have been proposed. Both digital twin (DT) and deep learning (DL) have been proven to be effective in solving detection/classification problems in various fields. This work develops a method of PV fault diagnosis that has the following three stages: (1) Detection of faults in a PV array using DT, (2) classification of the detected faults using ConvMixer, and (3) notification of detected and classified faults using a LoRa (long-range) system. DT is a virtual/digital model that is designed to reflect accurately the behavior and characteristics of a physical object. This work implements the model-based DC power generation of PV arrays using DT. In this study, a new DL method, called the convolutional mixer (ConvMixer) based on patch embedding and combining depthwise and pointwise convolutions to classify PV faults, is presented. The inputs of ConvMixer are 2D images generated from data on PV DC array power using a Markov transition field (MTF) transform. A LoRa notification system is very suitable for use in low-power wide-area networks (LPWANs), such as those needed in large PV farms. Simulation results demonstrate that the proposed ConvMixer outperforms other classical machine learning (ML) methods, such as decision tree, k-nearest neighbor, random forest, and support vector machine methods, as well as other classical CNN-based methods, such as AlexNet, ResNet50, VGG16, and VGG19. A real-time digital simulator (Opal-RT eMegasim) is used to verify the real-time applicability of the integration of DT, ConvMixer, and the LoRa notification system.

Analysis of the System Impact upon Thyristor Controlled Series Capacitor Relocation Due to Changes in the Power System Environment

Owing to geographical and political influences, Korea has an independent electric power system and the highest density of electric power facilities in the world. Large-scale base power generation complexes are located in non-metropolitan coastal areas, while the most expensive combined cycle power plants are operating or under construction in the metropolitan areas, which have the largest electricity demand. It has become difficult to secure a site for power plants, and the existing power transmission network is insufficient because of the additional construction of generators in existing power generation complexes, the increase in capacity of facilities, and the rapid increase in new and renewable energy. In particular, the East Coast region has a problem of transient stability for this reason, which is being addressed in advance through power generation restrictions. In addition, TCSC (Thyristor Controlled Series Capacitor) is installed and operated to expand the capacity of existing power transmission lines and improve stability in the failures of nearby high-voltage lines until new transmission lines that take more than 10 years are installed to resolve power generation restrictions. However, after the construction of a new transmission line, the efficiency of the existing TCSC is degraded, and for efficient use, it is necessary to rearrange the installation location to utilize the optimal TCSC according to changes in the configuration of the nearby power system. Moreover, a detailed analysis is needed on whether the TCSC designed according to the existing grid configuration exhibits accurate control performance even after the relocation and whether it interferes with nearby generators. In addition to the dynamic performance based on real-time simulations, it is necessary to study the control performance and interaction for this power electronic equipment. This study verified the need to change controller parameters, the interaction effect with nearby generators for stable operation, and the system effect of TCSC relocation using the same replica controller as the actual field controller, as well as RTDS (Real Time Digital Simulation) simulating the entire power transmission system.

Energy management in DC microgrid with an efficient voltage compensation mechanism

Direct current (DC) microgrid facilitates the integration of renewable energy sources as a form of distributed generators (DGs), DC loads, and energy storage system (ESS) devices. A new voltage compensation mechanism is presented in this study to resolve the control issues of DC microgrid in a distributed manner. In this mechanism, a fractional-order voltage compensation term is used in the outer controller loop which eliminates the voltage deviation in the steady-state condition. A detailed mathematical model is developed for the ESS along with the new voltage compensation controller to facilitate proper tuning of the control parameters. Since the proposed voltage compensation term guarantees autonomous bus voltage restoration, the supercapacitor state of charge (SoC) remains at nominal value without violation while it only buffers fluctuating power. However, the battery only compensates for the nominal power demand. The DG power control algorithm strictly maintains the battery SoC within lower and upper bounds. A DC microgrid comprising hybrid ESS, DC load, constant power load (CPL), and distributed generator is implemented with real time digital simulator (RTDS). The results show that the proposed controller is reliable, leading to excellent ESS performance and power management within the microgrid, without any DC bus voltage deviation.

Photovoltaic Controller Design Based on Adaptive Volt/Var Algorithm to Stand the Impact of Load Increase in Grid Tied Microgrid System

This paper presents an efficient design and real-time implementation of a controller for a large-scale grid-tied photovoltaic (PV) plant in a power system affected by disturbances. To direct the adequate amount of solar power, an Adaptive voltage and volt-ampere reactive (Volt-VAr) algorithm implemented in real-time using the Real Time Digital Simulator (RTDS) is proposed. An efficient and self-sufficient active and reactive P-Q power control scheme is proposed and implemented to transfer the generated power of the PV to the grid using a Voltage Source Converter (VSC). The proposed controller is implemented along with the PV system considered in RTDS to evaluate its effectiveness. By considering the impact of the disturbance caused by the load increase, the efficiency of the proposed power controller to transmit the generated power of the PV plant to the electrical grid system is investigated. It is observed that the hardware in the loop results verifies the advantage of the proposed method.

Fault Ride Through and Intermittency Improvement of Renewable Energy Integrated MMC-HVDC System Employing Flywheel Energy Storage

Modular multilevel converter (MMC)-based high voltage direct current (HVDC) transmission networks integrate remotely located distributed renewable energy resources (RER). The intermittent nature of RER and symmetrical and asymmetrical AC-side low-voltage faults produce operational difficulties. It reduces the MMC’s ability to transfer the rated power, which raises the voltage of the HVDC link. Therefore, the MMC-HVDC network’s fault ride-through (FRT) capabilities and the power fluctuation brought on by RER intermittency are potential challenges to the efficient integration of renewable energy resources. In response, this article suggests an energy control scheme for the flywheel energy storage of the PV-wind-MMC-HVDC system in order to regulate the HVDC-link voltage during low voltage faults at the point of common coupling (PCC) of the AC grids and to address the problem of power fluctuation caused by intermittent RER and sudden load changes. The suggested method eliminates the dynamic braking resistor (DBR) from the HVDC link and seamlessly integrates the RER without actively reducing renewable energy power during low voltage faults. To test the effectiveness of the proposed control method for the flywheel energy storage in reducing excess energy in the HVDC link, symmetrical and asymmetrical low voltage faults have been conducted. In addition, changes in wind speed, solar radiation, and temperature have been made to validate the flywheel energy management system’s performance. A real-time digital simulator (RTDS) and dSPACE-based controller hardware in loop (CHIL) configuration with a complete system have been used to simulate and evaluate the entire system.

Implementation of Novel Reduced-Order H∞ Filter for Simultaneous Detection and Mitigation of FDI-Attacks in AGC Systems

Recent developments in power systems to build a smart grid depend critically on communication networks which provide unprecedented advantages for comprehensive system visibility, monitoring, protection, and control. However, it increases the system complexity and vulnerability to cyberattacks that could jeopardize the seamless system operation. The attack must be detected effectively, and then proper measures to mitigate the effect quickly to restore normality. This work implements a reduced-order <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\infty }$ </tex-math></inline-formula> filter to simultaneously estimate the state variable and unknown inputs such as noise or attack signals in the automatic generation control (AGC) system. Further, the knowledge of the disturbances is utilized to mitigate the attack by appropriately compensating the sensor readings, such that the control decisions are based on the correct states, not the corrupted signals generated in the attack scenario. The stability criterion is provided based on Lyapunov functions to check the estimation error in the presence of the exogenous disturbance. Further, the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\infty }$ </tex-math></inline-formula> estimator performance is evaluated by comparing it with the Kalman filter to verify its effectiveness. The estimator’s performance is validated by developing a cosimulation platform using the server–client architecture of data exchange. Further, the performance is validated in a real-time digital simulator (RTDS) platform used as the power system interacting with an estimator deployed on Raspberry Pi board sensing the attacks.

Performance investigation of reweighted zero attracting LMS-based dynamic voltage restorer for alleviating diverse power quality problems

Dynamic voltage restorer (DVR) has proven to be the most coherent, effective and economic answer to voltage-based power quality problems. This work details the design, investigation and experimental validation of reweighted zero-attracting least mean square (RZA-LMS) based DVR addressing the issues like voltage sag, voltage swell, voltage harmonics, voltage imbalance and combination of these. The RZA-LMS based control algorithm is proposed for producing reference voltage signals. The features of proposed algorithm include finite-time convergence, simplicity and improvement in the robustness of the system against varying parameters. The viability of DVR controller is assessed by employing MATLAB/Simulink and Simpowersystem software. The performance of proposed DVR is experimentally investigated using digital real-time simulator.

Traveling Wave-Based Primary Protection and Fault Localization Scheme for MTDC Grid Considering IEC 61869-9 Measurement Standard

Traveling wave (TW) based protection and fault localization requires high sampling frequency measurements (500 kHz - 1 MHz) and are non-compliant to the IEC 61869-9 measurement standards, which allows maximum sampling frequency of 96 kHz for DC application. This work proposes a novel low sampling frequency (50 kHz) TW based primary protection and real-time DC fault localization technique for multi terminal high voltage DC (MTDC) grid which is compliant with the aforementioned measurement standard. In the proposed MTDC protection scheme, a TW arrival time (TWAT) is estimated under low sampling frequency measurement using cascaded combination of modified mathematical morphology gradient (MMG), sobel operator and linear regression tool. Subsequently, the TWAT information at both DC link terminals is communicated via dedicated communication channel compliant to IEC-61850-9-2 standard. And if the TWAT difference between DC terminals is less than the TW propagation threshold for the protected DC line, then a DC trip signal is issued to the associated hybrid DC circuit breaker (HDCCB). For fast power system restoration, the same TWAT information is also used for real-time fault localization in the faulted DC line, which is also compliant with IEC 61869-9 measurement standards. Finally, the practical feasibility of the proposed algorithm is demonstrated through control hardware-in-loop testing with real-time digital simulation setup and TI TMS320F28379d digital signal processor card.