Electromagnetic Transients Program Research Articles

FPGA-Based Sub-Microsecond-Level Real-Time Simulation for Microgrids With a Network-Decoupled Algorithm

The real-time simulation based on the field programmable gate array (FPGA) is receiving more and more attention. However, up to now, the simulation scale for the power electronic system is not so satisfactory due to the real-time requirement and the FPGA resource limitation. This paper proposes a sub-microsecond level real-time simulation method for microgrids. The power converters are modeled with fixed-admittance models and simulated with a compact electromagnetic transients program (EMTP) algorithm. In the meanwhile, the distribution lines/cables are modeled with π-circuit models and simulated with a distributed circuit solution method, called the latency insertion method (LIM). As a result, the distribution generation (DG) systems are decoupled with each other and can be simulated in parallel. The case study shows that the proposed simulation method consumes much fewer FPGA resources compared with the traditional one. Besides, the time step can be much smaller and doesn't need to increase with the simulation scale. With these advantages, the real-time simulation of a microgrid consisting of three three-phase converters, three boost circuits and 21 three-phase lines can be achieved on the Xilinx Kintex-7 410T FPGA with a minmum time-step of 380 ns.

Exponential integration algorithm for large-scale wind farm simulation with Krylov subspace acceleration

Facing the increasingly complex power system transient characteristics, the electromagnetic transient simulation tools are gaining popularity, thanks to their detailed modeling of power system equipment and high fidelity simulation results. Consequently, the Electro-Magnetic Transient Programs are increasingly used in simulation studies of higher dimensions and wider range of scales, which challenges the circuit-based algorithmic design of these programs. This paper proposes to use explicit integration formulas based on the matrix exponential function in the transient simulations of large-scale wind farms, which possess both high efficiency of explicit methods and numerical stability of implicit methods. A Krylov subspace-based order reduction technique embedded into the integration formulas was developed to specifically address the simulation needs of high dimensional EMT model of detailed wind farms. Numerical studies are conducted based on a realistic large-scale wind farm, where the proposed state space modeling framework is established, and the matrix exponential-based algorithm is implemented and compared against a set of classical stiff ODE solvers. Simulation tests with different fault types and fault locations are presented, showing a superior computation speed for studies with both equivalent and detailed wind farm modeling. On the other hand, numerical errors of the new algorithm are shown to be at least one order of magnitude smaller than other solvers in the range of typical step sizes, under the same simulation setting. These properties ensure an efficient, reliable and highly scalable simulation capability for the computation challenges in the studies of large-scale wind farms.

Influence of Superconducting Fault Current Limiters on Traveling Wave Based Protection

Superconducting fault current limiters (SFCLs) have been used in electric power transmission systems to limit the current magnitudes when faults occur. Installation of SFCLs inevitably alters transmission line properties and thus may affect measurements in traveling wave based protection. In this paper, we investigate the impact of SFCLs on the performance of the traveling wave based protection schemes in high-voltage transmission systems. An SFCL and a 230 kV high-voltage transmission line are modeled into a simulation system in the electromagnetic transients program, version alternative transients program. The transmission line is represented by a frequency dependent model so that traveling wave characteristics can be effectively captured. We simulate faults on the transmission line, with the incoming and reflected traveling waves recorded and thoroughly analyzed. Also, the influence of SFCL stray capacitances on traveling wave shape is investigated. The results show that the traveling wave current magnitudes are attenuated by the SFCL and the shape of the traveling wave is slightly affected by SFCL stray capacitances. Possible consequences in protection performance are discussed and future research directions are suggested.

Impact of Smart HTS Transmission Cable to Protection Systems of the Power Grid in South Korea

In South Korea, Korea Electrotechnology Research Institute (KERI) has developed a 154 kV smart High Temperature Superconducting (HTS) cable system with fault current limiting function since May 2017. This project is funded by the Ministry of Trade, Industry, and Energy of Republic of Korea. It is very important to design the protective relay system for a successful application of the smart HTS cable system to a practical power grid. In general, the smart HTS cable can have a negative impact on the protective coordination in power transmission system because of the variable impedance of the cable. This paper reviews some protection problems which can be caused by the application of the 154 kV smart HTS cable to power transmission systems in Korea and proposes a protection scheme for a test power system. And then we conduct a basic study to find solutions for the problems using an electromagnetic transient program, PSCAD/EMTDC.

Dynamic Modeling of Sequential Service Restoration in Islanded Single Master Microgrids

This paper presents a methodology for generating restoration sequences in distribution systems and microgrids with emphasis on deriving optimal switching times for remote control switches. Microgrid architecture considered are assumed to be operating in islanded single master mode. Control methods for handling the unbalance inherent in distribution systems using single-phase controlled PQ inverters were developed and incorporated in the proposed method. The problem of considering simultaneously both energy optimization and dynamic stability of the system, which is lacking in existing methods, was addressed by studying the transient behavior of microgrids based on extended electromagnetic transients program (EMTP) simulations and then approximate and incorporate these behaviors into the optimization formulation for restoration. The approach is based on multi-objective optimization of final restored load and total restoration time. The methodology was studied on a modified IEEE 123 node test feeder. An EMTP simulation in PSCAD of the formed microgrids was used to validate the accuracy of the dynamic and steady-state characteristics of the resulting systems. By considering the transient behavior of distributed generators (DGs) during restoration, a better restoration sequence that does not trigger protection relays could be realized.

Analysis and Mitigation of Subsynchronous Resonance in a Korean Power Network with the First TCSC Installation

This paper presents a detailed analysis results of the effect of a thyristor-controlled series capacitor (TCSC) on subsynchronous resonance (SSR), which was first applied to a Korean power system. First, the TCSC parameters were calculated, the structure of TCSC with synchronous voltage reversal (SVR) controller was presented, and the torsional characteristics of Hanul nuclear power generator rotor were studied to investigate the natural frequency and mode shape. The test signal method was used to determine the electrical damping in the frequency range of SSR operation through an electromagnetic transient analysis program in various system configurations. The SSR phenomenon was analyzed by comparing the electrical and mechanical damping of a conventional fixed series capacitor (FSC), and the case of a TCSC installed, and the effectiveness of the TCSC without any risk of SSR was demonstrated. As a result, when installing FSC, SSR occurred under sensitive operating conditions, but SSR was prevented in the case of TCSC compensation with SVR. The results obtained in this study can be effectively applied to the installation of TCSC in real power systems.

Co-simulation of a Doubly Fed Induction Generator Connected to a Power Network: The use of DSOGI for Phasor Extraction

This work presents a hybrid simulation tool that combines fast analysis of transient stability programs with accurate results of electromagnetic transients ones to evaluate the dynamic behavior of transmission power systems with high penetration of power electronic-based devices. The interaction between the two programs is performed by controllable current and voltage sources, which are used to interface external and detailed systems. The double second order generalized integrator is used to extract the positive-sequence phasor to transfer information between the environments. A local network server controls the data communication between the two simulation environments by means of the TCP/IP protocol. In the present paper, the proposed tool is used to simulate the integration of a aggregate wind power plant, based on a doubly-fed induction generator, into a 29-bus electrical network. Results and computational timings are then compared with the ones obtained with an electromagnetic transients program, demonstrating the accuracy and speed of the proposed strategy.

A Modified IEEE 118-Bus Test Case for Geomagnetic Disturbance Studies–Part I: Model Data

Power grid test cases for geomagnetic disturbance (GMD) studies need particular data that are not provided by the typical IEEE transmission benchmarks. This two-part paper proposes a test case that contains required modeling details for time-domain simulation of a GMD within an electromagnetic transient program (EMT-type). Compared to existing load-flow-based GMD test cases, the proposed test case offers advantages, such as accurate representation of nonlinear transformer magnetization and interaction between the dc geomagnetically-induced currents and transformer saturation, additional var consumption, harmonics, and voltage regulation problems. Part I presents the test case system and parameter data, and Part II provides simulation results to enable software-to-software validation (future work) as a first step toward the establishment of the model as an official GMD benchmark.

A Modified IEEE 118-Bus Test Case for Geomagnetic Disturbance Studies—Part II: Simulation Results

This two-part paper proposes a modified version of the IEEE 118-bus benchmark, referred to as 118-GMD, for time-domain simulation of the impacts of a geomagnetic disturbance (GMD) on a power system within an electromagnetic transient program (EMT-type). The advantage of 118-GMD over existing GMD test cases in the literature is that it enables additional GMD studies including harmonic analysis, response of control and protection, and interaction between the dc geomagnetically-induced currents, transformer saturation, additional var consumption and voltage collapse due to a GMD. Part I has presented the network model and its parameters. Part II presents simulation results and identifies the influential modeling details that should be incorporated in an EMT-type GMD simulation study based on the type of simulation. The study includes modeling details such as line model, delta-winding resistance, over excitation limiter, on-load tap changer, and load model.

A new technique for detection and classification of faults during power swing

The Lissajous figure of voltage and current signals of a transmission line follows a distinct pattern during power swing, although it deviates from its trend when a fault occurs. This feature of Lissajous figure is utilized in this article for detection and classification of faults during power swing. Autoregression technique is applied to obtain the trend of Lissajous figure and the deviation of actual Lissajous figure from this trend is calculated using Euclidean norm. Quarter-cycle moving window sum of Euclidean norm is used to obtain fault detection index for detection of faults during power swing. For identification of faulty phase, fault features are also calculated for three phases and ground using fault signals of only half-cycle duration after fault. The proposed method is tested on a hybrid transmission line consisting of an overhead line and an underground cable, simulated in electromagnetic transient program (EMTP). It can detect faults quickly and classify them with high accuracy irrespective of fault type, fault location, fault resistance, load angle, fault inception time and swing frequency. The method performs satisfactorily even under noisy condition upto 30 dB Gaussian noise. Finally, its performance is compared with one conventional and two novel methods to establish its efficacy.

Systematization of the Simulation Process of Transformer Inrush Current Using EMTP

An inrush current is generated when a transformer is energized. This current has a large magnitude and rich harmonics, thereby causing mal-operation of the protection relay. Therefore, the development of countermeasures against inrush current is necessary, and this study has been performed by computer simulations. However, it is difficult for a power system operator to perform a computer simulation as it is difficult to determine what data should be selected and entered. Therefore, this paper establishes the simulation process of transformer inrush current using the Electromagnetic Transients Program (EMTP). Two methods to simulate the transformer inrush current are described in detail. Based on the actual 154 kV transformer test report in Korea, the simulation results of the inrush current using the two methods are discussed.

Design and development of travelling‐wave‐frequency‐based transmission line fault locator using TMS320 DSP

The authors use a TMS320 digital signal processor (TMS320-DSP) to determine fault instants and estimate their location in real time in a laboratory environment. The fault instant is determined via examining the instantaneous differential changes in the line currents. After the fault is detected, the fault location is determined by processing the time-domain transient current waves. First, the travelling-wave frequency is determined by application of the fast Fourier transform to the positive-sequence-component line current after the fault, and subsequently, the fault location is estimated by utilising this frequency. The alternative transients programme (ATP)–electromagnetic transient programme is used to simulate the line currents and create short-circuit fault conditions. Furthermore, LabVIEW software and a National Instruments data acquisition board are used to transform the line currents obtained through the ATP programme into analogue signals. The TMS320-DSP determines the fault in real time and estimates the fault location using the completed software and analogue input signals. Their results indicate that the prototype device designed with the use of the TMS320-DSP is suitable for real-time fault detection.

Electromagnetic Transient Analysis of 500 kV XLPE Submarine Cable Interconnection Project from Zhenhai to Zhoushan

In Zhenhai-Zhoushan transmission project, 500kV XLPE submarine cable lines are used for the first time in the world. Using electromagnetic transient simulation program(EMTP), electromagnetic transient problems in this project are investigated including electrical parameter of submarine cable lines, power frequency overvoltages, secondary-arc currents and recovery voltages during the single-phase reclosing, power frequency resonance overvoltages, induced voltages and induced currents, switching surges and so on. On the basis, the recommended configuration scheme of HV shunt reactors and neutral reactors is proposed. In addition, it is suggested to install closing resistors of 400ohm on both sides of line breakers in order to restrict switching overvoltages and ensure safe operation of submarine cable lines.

An Efficient Phase Domain Synchronous Machine Model With Constant Equivalent Admittance Matrix

In this paper, a new synchronous machine model is developed for electromagnetic transients program type simulations. The stator circuit is expressed in the abc phase domain. The machine model is represented as a Norton equivalent with a current source in parallel with a constant Norton admittance. The machine equations are reformulated so that the computational effort required for the modeling of the machine is reduced. Test studies demonstrate the accuracy of the proposed synchronous machine model and show that the proposed model is computationally more efficient than the existing constant conductance phase domain model and voltage-behind-reactance model.

Measurement and Calculation of Voltage Stress in Industrial Motor due to Fast Surges

Large industrial motors fail occasionally due to stress on turn insulation. Fast surges resulting from switching on and off operations of industrial motors cause severe stress on insulation due to non-linear voltage distribution across the winding. In order to determine the voltage distribution in a typical 11 kV motor coil, theoretical analysis using electromagnetic transient program and experimental measurements using recurrent surge generator have been carried out. The terminal end coil of 11 kV motor is modeled by an equivalent electric network comprising the self and mutual inductances, series and shunt capacitances. Surge voltage of impulse type and other rise times are applied at the nose of the coil to determine the voltage stress in the inter turn insulation. The comparison is made between measured and computed results. This study is useful to industrial motor designers to design reliable motors.

Non-recursive Discrete Fourier Transform-Based Frequency Estimation of the Power System

This paper suggests a frequency estimation method which consists of a combination of a revised 3-Level discrete Fourier transform (DFT) and a second derivative of each orthogonal component. The revised 3-Level DFT aims to counteract the harmful effect of harmonics and noises on frequency estimation accuracy by applying DFT to the input signal three times consecutively. Also, a cancelation technique of a tracking delay which is directly related to the data window length is proposed by adjusting the angle of voltage phasor. To evaluate the performance of proposed method, three different kinds of computer-generated-signals which are according to IEEE C37.118.1a 2014 PMU Standard requirements are considered. In addition, signals which are collected at the transmission system modeled by electro magnetic transient program (EMTP-RV) under the dynamic condition and the single line-to-ground fault are also used. The simulation results indicate that the proposed method can track the frequency of the signal accurately.

Novel Protection Scheme considering Tie Switch Operation in an Open-Loop Distribution System using Wavelet Transform

Loop power distribution systems are gaining increasing attention due to improvements in the reliability of the power supply and the connection of distributed generation. With loop distribution systems, there is the possibility of mal-operation of the protection relay because of the existence of the tie switch and bi-directional current injection. In this paper, we propose a novel protection scheme considering the tie switch operation in the open loop power distribution system using wavelet transform. We analyze the possibility of mal-operation of the protection relay as a result of the normal load current after tie switch operation and analyze the characteristics of the normal load current and fault current injection after tie switch operation. Using these results, a new index is proposed to distinguish the normal load current and fault current, and a novel protection scheme based on this new index is proposed. The proposed method is modeled using an electromagnetic transients program and MATLAB, and the various simulations are performed according to the tie switch position, the fault location, and the success or failure of the fault section separation. From the simulation results, we can confirm that the normal load current and the fault current after tie switch operation can be accurately distinguished and the protection relay can accurately operate at only fault conditions.

Fast fault detection during power swing on a hybrid transmission line using WPT

This paper presents a new technique for very high speed detection of fault during power swing on a hybrid transmission line consisting of an overhead line and an underground cable. The wavelet packet transform with the mother wavelet db1 is utilised to segregate the high-frequency components from three-phase current signals recorded at the relay point and a fault detection index is calculated using these components to detect faults during power swing. The proposed method is tested on a 400 kV double circuit transmission system and a 400 kV 3-machine 9-bus Western System Coordinating Council system simulated in electromagnetic transient program software for both symmetrical and asymmetrical faults, different fault resistances, load angles, and fault inception times at random locations on the transmission line. Its performance is also analysed for close-in faults, high dc offset current, and the faults on the series compensated transmission line. The method is very fast to detect faults during power swing irrespective of the variations of fault conditions, and a comparative assessment with the existing methods justifies its merit.

Circulating Current Reduction in MMC-HVDC System Using Average Model

Modular multilevel converters (MMCs) are quickly emerging as a suitable technology for a voltage-source converter-based high-voltage direct-current (VSC-HVDC) transmission systems due to its numerous advantages as reported in literature. However, for a large DC-network, MMCs require large numbers of sub-modules (SMs) and switches, which makes its modeling very challenging and computationally complex using electromagnetic transient (EMT) programs. Average Value Model (AVM) provides a relatively better solution to model MMCs by combining cells as an arm equivalent circuit. Circulating current is an important issue related to the performance and stability of MMCs. Due to circulating currents, power loss in a converter increases as root mean square (RMS) values of the arm current increases. The traditional method for inserting SMs in each arm is based on direct modulation, which does not compensate for the arm voltage oscillations, and generates circulating current in each leg of a three-phase MMC. This paper presents a new method for reducing the circulating current by adding 2nd and 4th harmonics in the upper and lower arm currents of an MMC. Less capacitor energy variations are obtained by the proposed method compared to traditional direct modulation methods. The proposed method is tested on a common symmetrical monopole (point-to-point) MMC-HVDC system using vector current control strategy in PSCAD/EMTDC software. Analytical and simulation results show the effectiveness of the new method in minimizing the circulating current and arm voltage oscillation reductions as compared to the direct modulation approach.

A Study on Overvoltages in Windfarm Caused by Direct Lightning Stroke

Wind turbine generators in a windfarm are connected by underground cables. Lightning sometimes strikes a nacelle or a blade. Lightning overvoltages due to a direct lightning strike to the system cause damages in the underground cable. Many parameters should be investigated to establish a rational lightning protection design for wind power generation system. Numerical simulation is a useful method to discuss the influence of the parameters on the lightning overvoltages. This paper uses an Electro-Magnetic Transients Program for the lightning surge analysis. This paper discusses the influence of grounding method of metallic sheath of the cable and grounding resistance on the lightning overvoltages for the direct lightning strike to the generation system.