Detailed Electromagnetic Simulations Research Articles

Transient Voltage and Current Stresses Estimation of MMC-MTDC System via Discrete-Time Analysis

Transient voltage and current stresses are essential for the design and protection of multi-terminal DC (MTDC) transmission systems, especially under the short-term dynamics after severe grid faults. In this paper, we propose a discrete-time model for the modular multilevel converter based MTDC (MMC-MTDC) system considering the DC-side disturbances in the system. Based on the piecewise linear modeling, the state variables such as the transient voltage on the sub-module (SM), arm current, etc., can be calculated analytically for the critical short term (within 10 ms) after the severe DC fault while the system protection is yet to be activated. Parameter sensitivity analysis is also performed to give a better system design guideline. The estimation results are validated by detailed electromagnetic (EMT) circuit simulations and the scaled-down experimental laboratory prototype.

Understanding the HERA Phase I receiver system with simulations and its impact on the detectability of the EoR delay power spectrum

ABSTRACT The detection of the Epoch of Reionization (EoR) delay power spectrum using a ‘foreground avoidance method’ highly depends on the instrument chromaticity. The systematic effects induced by the radio telescope spread the foreground signal in the delay domain, which contaminates the EoR window theoretically observable. Applied to the Hydrogen Epoch of Reionization Array (HERA), this paper combines detailed electromagnetic and electrical simulations in order to model the chromatic effects of the instrument, and quantify its frequency and time responses. In particular, the effects of the analogue receiver, transmission cables, and mutual coupling are included. These simulations are able to accurately predict the intensity of the reflections occurring in the 150-m cable which links the antenna to the backend. They also show that electromagnetic waves can propagate from one dish to another one through large sections of the array due to mutual coupling. The simulated system time response is attenuated by a factor 104 after a characteristic delay which depends on the size of the array and on the antenna position. Ultimately, the system response is attenuated by a factor 105 after 1400 ns because of the reflections in the cable, which corresponds to characterizable k∥-modes above 0.7 $h\,\,\rm {Mpc}^{-1}$ at 150 MHz. Thus, this new study shows that the detection of the EoR signal with HERA Phase I will be more challenging than expected. On the other hand, it improves our understanding of the telescope, which is essential to mitigate the instrument chromaticity.

Analysis of Wideband Oscillation of Hybrid MMC Interfacing Weak AC Power System

The wideband oscillation of the hybrid modular multilevel converter (MMC)induced by excessive power infeed under weak ac power system integration is analyzed in this article. A closed-loop state-space-based time-domain small-signal model is firstly established to investigate the instability problem. Different from the findings in two-level voltage-source-converters (VSCs) or half-bridge MMCs, the root locus analysis and participation factor analysis in this article reveal that the oscillation frequency and involved control loops are highly related to the operation status. When hybrid MMC operates as a rectifier, a low-frequency oscillation is observed with the d-channel control loop mainly participated. In contrast, a high-frequency oscillation occurs with a q-channel control loop mainly involved when the hybrid MMC operates as an inverter. This wideband oscillation phenomenon is explored with the aid of two simplified loop-gain-based s-domain models, which are derived referring to the selective modal analysis approach. To suppress the oscillation, sensitivity analysis regarding the impact of parameters on the phase margin is conducted to recognize effective parameter adjustment methods. The analysis results are validated by detailed electromagnetic simulations.

Analysis of stable operation region of the hybrid AC/DC distribution systems

Hybrid alternating current (AC)/direct current (DC) distribution is a promising technology emerging for future power systems owing to its advantages, such as high controllability and compatibility. The stability of grid-connected converters is essential for the operation of an AC/DC distribution system. This study analyses the stable operation requirements of converters under different control modes based on the short-circuit ratio (SCR). Then, a method to calculate the critical SCR (CSCR) of a specified converter is proposed. Based on the CSCR of each converter, the stability criterion to judge the stability of multi-terminal DC (MTDC) distribution networks is proposed. To verify the accuracy and effectiveness of the proposed method and criterion, detailed electromagnetic simulations using PSCAD are conducted. The simulations indicate that the analysis results obtained using the proposed method and criterion are accurate, and the presented stable operation requirements of the converters can help to enhance the controllability of hybrid AC/DC distribution systems.

Origin of selective enhancement of sharp defect emission lines in monolayer WSe2 on rough metal substrate

The defect states in atomically thin layers of transition metal dichalcogenides are promising candidates for single photon emission. However, the brightness of such quantum emission is often weak and is accompanied by undesirable effects such as spectral diffusion and strong background emission. By placing a monolayer WSe2 directly on a rough gold substrate, here, we show a selective enhancement of sharp defect-bound exciton peaks, coupled with a suppressed spectral diffusion and strong quenching of background luminescence. By combining the experimental data with detailed electromagnetic simulations, we reveal that such selective luminescence enhancement originates from a combination of the Purcell effect and a wavelength dependent increment of the excitation electric field at the tips of tall rough features, coupled with a localized strain-induced exciton funneling effect. Notably, insertion of a thin hexagonal boron nitride sandwich layer between WSe2 and the Au film results in a strong enhancement of the background luminescence, obscuring the sharp defect peaks. The findings demonstrate a simple strategy of using monolayer WSe2 supported by a patterned metal film that offers a possibility of achieving quantum light sources with high purity, high brightness, and suppressed spectral diffusion.

Analysis of the Harmonic Transmission Characteristics of HVDC Transmission Based on a Unified Port Theory Model

The application of power electronics can introduce flexibility into power systems, but it can also cause harmonic problems. As a typical application of power electronics, high-voltage direct current (HVDC) transmission exhibits very prominent harmonic problems. In recent years, many HVDC protection maloperation events caused by harmonic problems have occurred in the field. The phenomena of harmonic transfer and amplification in HVDC transmission have an undeniable impact on the overall operation of a power system. Previous studies of these phenomena have relied mainly on detailed electromagnetic transient simulations. However, these simulations normally suffer from a large computational burden and low efficiency due to the complexity of the models and the small simulation time steps used. Therefore, an efficient analysis model of harmonics for various HVDC operating modes is presented in this paper. This model is based on port theory; every part of the HVDC transmission system is represented by an equivalent two-port component to construct a unified equivalent circuit. The proposed model is simple to calculate, and only the connections of the two-port components need to be changed to model different operation modes. The proposed two-port model is applied for the harmonic analysis of an HVDC transmission system, and the validity and accuracy of the model are confirmed by detailed electromagnetic simulations. Moreover, the model can be easily applied to HVDC systems for harmonic transfer and amplification, and the effects of the HVDC transmission power and smoothing reactor on harmonic transfer and amplification are analyzed.

Positive sequence voltage source converter mathematical model for use in low short circuit systems

In this study, modifications are proposed to the state-of-the-art positive sequence generic converter model in order to capture the small-signal and large-signal oscillatory instabilities associated with converters interfaced at low short circuit locations. The performance of the positive sequence model is validated against detailed electromagnetic simulations to lend confidence in this proposed model. Additionally, validation of performance of the model against observed field measurements is also carried out.

The HERA-19 Commissioning Array: Direction-dependent Effects

Abstract Foreground power dominates the measurements of interferometers that seek a statistical detection of highly-redshifted H i emission from the Epoch of Reionization (EoR). The chromaticity of the instrument creates a boundary in the Fourier transform of frequency (proportional to k ∥) between spectrally smooth emission, characteristic of the strong synchrotron foreground (the “wedge”), and the spectrally structured emission from H i in the EoR (the “EoR window”). Faraday rotation can inject spectral structure into otherwise smooth polarized foreground emission, which through instrument effects or miscalibration could possibly pollute the EoR window. For instruments pursuing a “foreground avoidance” strategy of simply measuring in the EoR window, and not attempting to model and remove foregrounds, as is the plan for the first stage of the Hydrogen Epoch of Reionization Array (HERA), characterizing the intrinsic instrument polarization response is particularly important. Using data from the HERA 19-element commissioning array, we investigate the polarization response of this new instrument in the power-spectrum domain. We perform a simple image-based calibration based on the unpolarized diffuse emission of the Global Sky Model, and show that it achieves qualitative redundancy between the nominally redundant baselines of the array and reasonable amplitude accuracy. We construct power spectra of all fully polarized coherencies in all pseudo-Stokes parameters, and discuss the achieved isolation of foreground power due to the intrinsic spectral smoothness of the foregrounds, the instrument chromaticity, and the calibration. We compare to simulations based on an unpolarized diffuse sky model and detailed electromagnetic simulations of the dish and feed, confirming that in Stokes I, the calibration does not add significant spectral structure beyond that expected from the interferometer array configuration and the modeled primary beam response. Furthermore, this calibration is stable over the 8 days of observations considered. Excess power is seen in the power spectra of the linear polarization Stokes parameters, which is not easily attributable to leakage via the primary beam, and results from some combination of residual calibration errors and actual polarized emission. Stokes V is found to be highly discrepant from the expectation of zero power, strongly pointing to the need for more accurate polarized calibration.

A System-Wide Protection Against Unstable SSCI in Series-Compensated Wind Power Systems

Unstable subsynchronous control interaction (SSCI) between wind farms (WFs) and series-compensated grid has posed a big threat to the safety of equipment and the stability of system. Many studies have been conducted on its analysis and control. However, very little research has been done on the protection. To fill the gap, this paper proposes a system-wide protection against unstable SSCI in wind power systems with fixed series compensations. The protection is composed of a centralized protection coordinator (CPC) and several distributed protection relays (DPRs). Local DPRs measure the subsynchronous impedances of WFs in real time. These impedances are next collected by the CPC to calculate the sensitivities of tripping for each WF. The sensitivities are then used to determine the candidate WFs and the number of WTGs to trip at those WFs, so as to stabilize the SSCI. The effectiveness of the proposed protection is verified with detailed electromagnetic simulations on a wind power system that had experienced actual SSCI incidents.

Experience of Severe Overvoltage in an Interconnected Transmission Subnetwork During Single-Phase Open Period

A single-phase trip and reclose scheme applied to a long transmission circuit, interconnecting a remote subnetwork to the grid, improves the interconnection reliability. However, the entire subnetwork loses supply in one phase should there be no generation connected in it during the open-phase condition. Under these conditions, BC Hydro experienced a hazardous temporary overvoltage on the open phase in a 287 kV subnetwork. Highly shunt compensated lines have experienced high open phase overvoltage but this incident revealed that transformers in the subnetwork having delta-connected secondary or tertiary windings can also cause this concern. Though it was not designed to do so, the transformer differential protection operated on excessive magnetizing current after the 5th harmonic blocking current dropped below the setting threshold (35%) and prevented damage to the surge arresters. This paper will use disturbance records to recreate the sequence of events and present a detailed waveform analysis. Expected differential relay response when the transformer is severely overexcited will be discussed. A steady-state analysis of the simplified system will be presented, followed by detailed electromagnetic transients simulations which successfully duplicated the disturbance. Based on further simulations, a mitigation plan was developed to avoid a reoccurrence of the problem.

Plasmon Modes and Hot Spots in Gold Nanostar–Satellite Clusters

Gold nanostars display strong electromagnetic field enhancement at their tips, and tip plasmon resonances can be tuned within the visible and near-infrared, which has been applied toward plasmonic molecular sensing. However, the sensitivity can be further increased by linking gold nanostars to other plasmonic nanoparticles and thereby inducing the creation of hot spots. We report the controlled formation of core–satellite assemblies comprising a central gold nanostar with gold spheres adsorbed to the tips via Raman active molecular linkers. Surface-enhanced Raman scattering (SERS) was recorded at the level of single gold nanostar–satellite clusters, and the experimental results were compared with detailed electromagnetic simulations.

A megawatt class compact power combiner for solid-state amplifiers

We propose a compact multiport two-stage combiner capable of handling peak power up to 10 MW at the UHF band and suitable for particle accelerator applications. The detailed electromagnetic and thermal simulations of the combiner operating at the ESS specifications of 400 kW at 352 MHz are presented. At the first stage, the power is combined to a 100 kW level by means of a non-resonant 12-way radial combiner, which is assumed to be fed by 8 kW solid-state amplifiers. At the second stage, a waveguide combiner with T-shape couplers separated by a half-wavelength of the fundamental waveguide mode is used in order to bring the combined power to the required level. The combiner is broadband and has a relative power non-uniformity less than 5% over a 10 MHz frequency band around the central frequency. The size of the proposed combiner is several times smaller than the existing ones. We also present low-power measurement results of a prototype of the radial combiner.

Kinetic simulation studies of laser-triggering in the Z gas switch

Advanced z-pinch accelerators require precise timing of multiple mega-ampere drivers to deliver terawatt power. The triggering of these drivers is now largely initiated by laser ionization of gas switches. In this paper, we discuss detailed fully kinetic simulation of the Z laser-triggered gas switch involving detailed finite-difference time-domain particle-in-cell Monte Carlo modeling of the trigger section of the switch. Other components of the accelerator from the Marx bank through the pulse-forming line are described as circuit elements. The simulations presented here build on a recently developed model of electro-negative gas breakdown and streamer propagation that included photons produced from de-excited neutrals. New effects include multi-photon ionization of the gas in a prescribed laser field. The simulations show the sensitivity of triggering to laser parameters including focal plane within the anode-cathode gap of the trigger section of the switch, intensity at focus, and laser pulse length. Detailed electromagnetic simulations of the trigger section with circuit modeling of the upstream and downstream components are largely in agreement with Z data and demonstrate a new capability.

Optimization of Microwave Packaging Structure for Directly Modulated Laser Modules Using Three-Dimensional Electromagnetic Model

In this paper, the packaging aspects of directly modulated laser (DML) modules are investigated using detailed electromagnetic (EM) simulations. The packaging influences of the laser module are specified in two parts as RF connector and optoelectronic subsystem. By carefully optimizing the package structure, the simulation results show that the insertion loss of the proposed module is less than 1 dB up to 40 GHz. Then, a DML module is packaged and measured based on the optimized package scheme. A resonance free bandwidth of approximately 20 GHz is obtained with a 3 dB bandwidth of about 15 GHz.

Improved Coherency-Based Wide-Band Equivalents for Real-Time Digital Simulators

This paper introduces an approach which enables very large power systems to be modeled on real-time electro-magnetic transients (EMT) digital simulators. This is achieved using an improved wide-band multi-port equivalent, which reduces a large power network into a small manageable equivalent model that preserves wide-band behaviors. The low-frequency or electromechanical transients are captured with a transient stability analysis (TSA) type electromechanical equivalent derived using coherency-based reduction techniques. The high-frequency behavior is accurately captured by placing in parallel with the TSA equivalent, a passive frequency dependant network equivalent (FDNE). The validity of the proposed technique is demonstrated by comparing the approach with detailed electromagnetic simulations of a modified version of the New England 39-bus test system that includes an HVDC infeed. The power of the method is demonstrated by the real-time electromagnetic transient simulation of a large 2300-bus 139-generator system.

Microwave engineering of plasma-assisted CVD reactors for diamond deposition

The unique properties of CVD diamond make it a compelling choice for high powerelectronics. In order to achieve industrial use of CVD diamond, one must simultaneouslyobtain an excellent control of the film purity, very low defect content and a sufficientlyrapid growth rate. Currently, only microwave plasma-assisted chemical vapour deposition(MPACVD) processes making use of resonant cavity systems provide enough atomichydrogen to satisfy these requirements.We show in this paper that the use of high microwave power density (MWPD)plasmas is necessary to promote atomic hydrogen concentrations that are highenough to ensure the deposition of high purity diamond films at large growth rates.Moreover, the deposition of homogeneous films on large surfaces calls for theproduction of plasma with appropriate shapes and large volumes. The production ofsuch plasmas needs generating a fairly high electric field over extended regionsand requires a careful design of the MW coupling system, especially the cavity.As far as MW coupling efficiency is concerned, the presence of a plasma load represents amismatching perturbation to the cavity. This perturbation is especially important at highMWPD where the reflected fraction of the input power may be quite high. This mismatchcan lead to a pronounced heating of the reactor walls. It must therefore be taken intoaccount from the very beginning of the reactor design. This requires the implementation ofplasma modelling tools coupled to detailed electromagnetic simulations. This is discussed insection 3.We also briefly discuss the operating principles of the main commercial plasma reactorsbefore introducing the reactor design methodology we have developed. Modelling results fora new generation of reactors developed at LIMHP, working at very high powerdensity, will be presented. Lastly, we show that scaling up this type of reactorto lower frequencies (915 MHz) can result in high density plasmas allowing forfast and homogeneous diamond deposition on up to 160 mm diameter surfaces.

A Wideband and Scalable Model of Spiral Inductors Using Space-Mapping Neural Network

A wideband and scalable model of RF CMOS spiral inductors by virtue of a novel space-mapping neural network (SMNN) is presented. A new modified 2-pi equivalent circuit is used for constructing the SMNN model. This new modeling approach also exploits merits of space-mapping technology. This SMNN model has much enhanced learning and generalization capabilities. In comparison with the conventional neural network and the original 2-pi model, this new SMNN model can map the input-output relationships with fewer hidden neurons and have higher reliability for generalization. As a consequence, this SMNN model can run as fast as an approximate equivalent circuit, yet preserve the accuracy of detailed electromagnetic simulations. Experiments are included to demonstrate merits and efficiency of this new approach.

Efficient modeling of RF CMOS spiral inductors using generalized knowledge-based neural network

Efficient modeling of RF CMOS spiral inductors by virtue of a novel generalized knowledge-based neural network (GKBNN) is presented. Prior knowledge of on-chip inductors is used for constructing the GKBNN. This new modeling approach also exploits merits of the iterative multi stage algorithm. This GKBNN has much enhanced learning and generalization capabilities. Comparing with the conventional neural network or the knowledge-based neural network, this new GKBNN model can map the input---output relationships with fewer hidden neurons and has higher reliability for generalization. As a consequence, this GKBNN model can run as fast as an approximate equivalent circuit model yet generate results as accurate as detailed electromagnetic simulations. Experiments are included to demonstrate merits and efficiency of this new approach.

The HYPERcollar: A novel applicator for hyperthermia in the head and neck

The purpose of this work was to define all features, and show the potential, of the novel HYPERcollar applicator system for hyperthermia treatments in the head and neck region. The HYPERcollar applicator consists of (1) an antenna ring, (2) a waterbolus system and (3) a positioning system. The specific absorption rate (SAR) profile of this applicator was investigated by performing infra-red measurements in a cylindrical phantom. Mandatory patient-specific treatment planning was performed as an object lesson to a patient with a laryngeal tumour and an artificial lymph node metastasis. Comfort tests with healthy volunteers have revealed that the applicator provides sufficient comfort to maintain in treatment position for an hour: the standard hyperthermia treatment duration in our centre. By phantom measurements, we established that a central focus in the neck can be obtained, with 50% iso-SAR lengths of 3.5 cm in transversal directions (x/y) and 9–11 cm in the axial direction (z). Using treatment planning by detailed electromagnetic simulations, we showed that the SAR pattern can be optimised to enable simultaneous encompassing of a primary laryngeal tumour and a lymph node metastasis at the 25% iso-SAR level. This study shows that the applicator enables a good control, and sufficient possibilities for optimisation, of the SAR pattern. In an ongoing clinical feasibility study, we will investigate the possibilities of heating various target regions in the neck with this apparatus.