As synchronous generators (SGs) are extensively replaced by converter-based generators (CBGs), modern power systems are facing complicated frequency stability problems. Conventionally, the frequency ...nadir and the rate of change of frequency (RoCoF) are the two main factors concerned by power system operators. However, these two factors heavily rely on simulations or experiments, especially in a power system with high-penetration CBGs, which offer limited theoretical insight into how the frequency response characteristics are affected by the devices. This paper aims at filling this gap. Firstly, we derive a formulation of the global frequency for a CBG-integrated power system, referred to as common-mode frequency (CMF). The derived CMF is demonstrated to be more accurate than existing frequency definitions, e.g., the average system frequency (ASF). Then, a unified transfer function structure (UTFS) is proposed to approximate the frequency responses of different types of devices by focusing on three key parameters, which dramatically reduces the complexity of frequency analysis. On this basis, we introduce two evaluation indices, i.e., frequency drop depth coefficient (FDDC) and frequency drop slope coefficient (FDSC), to theoretically quantify the frequency nadir and the average RoCoF, respectively. Instead of relying on simulations or experiments, our method rigorously links the system's frequency characteristics to the characteristics of heterogeneous devices, which enables an in-depth understanding regarding how devices affect the system frequency. Finally, the proposed indices are verified through simulations on a modified IEEE 39-bus test system.
Time-frequency analysis (TFA) technology plays a significant role in seismic signal processing. The time-frequency representation (TFR) calculated using the TFA method is helpful for the localization ...of time-varying frequencies within the signal. Nevertheless, limited by the Heisenberg uncertainty principle, traditional linear TFA methods always provide blurred TFRs, which makes them difficult to distinguish details of time-frequency structures. Recently, the synchrosqueezing transform (SST) was designed to improve the concentration of the TFR. The SST can provide a much concentrated TFR for the weakly frequency-modulated (FM) signal, but it is not effective for the interpretation of strongly FM signals, such as the thin interbed in seismic exploration. In this work, we propose a new tool by introducing the multi-synchrosqueezing operator to the frame of wavelet transform (WT). Employing an iterative operator to correct the frequency estimation of the original SST step-by-step, it thus can calculate a TFR with better concentration and robustness. Synthetic signals and a field seismic data are employed to verify the performance of the proposed method for characterizing the time-varying frequency features.
A fully integrated sub-terahertz (sub-THz) frequency synthesizer is proposed cascading a radio frequency subsampling phase-locked loop (SS-PLL) with millimeter-wave injection-locked frequency ...multipliers (ILFMs) and a sub-THz mixer for frequency extension. Enhanced third-harmonic and fourth-harmonic-extraction techniques are proposed for the ILFMs with different multiplication ratios. In addition, a frequency-tracking loop (FTL) with automatic frequency and amplitude calibration is embedded for the ILFMs. Finally, a distributed bias technique is employed to improve the linearity of the wideband magnetic-tuning sub-THz oscillator. Designed in a 65-nm CMOS process, the proposed prototype achieves an ultra-wide frequency tuning range from 61.2 to 100.8, 122.4 to 136.8, and 198.5 to 273.6 GHz with a phase noise at 1-MHz offset from −79.3 to −95.4 dBc/Hz and an integrated jitter from 124 to 159 fs over the tuning range. The synthesizer occupies a core area of 0.58 mm 2 and measures an output power of −11 dBm at 211.4 GHz with 49.5 mW, corresponding to a dc-to-radio frequency (RF) efficiency of 0.16%, an FoM from −171.4 to −177.7 dBc/Hz, and <inline-formula> <tex-math notation="LaTeX">{\mathrm {FoM}}_{T} </tex-math></inline-formula> from −177 to −190.5 dBc/Hz.
This study presents a metamaterial-based antenna using the concept of composite right–left-handed transmission-lines. The radiation-cells layouts are based on L/F-shaped slits, so these slits are ...engraved on the radiation-patches for establishing a series-capacitor effect. Moreover, the radiation cells including the spirals and via-holes for the shunt-inductors implementation. By cascading the proper number of cells, the desired antennas for very/ultra-high frequency-bands are designed. The first-antenna with four L-shaped cells isconstructed on the Rogers_RO4003 substrate with thickness of 0.8 mm, so that each of cells occupies the size of 2.3 mm × 4.9 mm. This antenna covers the experimental-bandwidth of 0.2–1.8 GHz, which corresponds to 160% feasible-bandwidth. This antenna resonates at frequencies of 600–850–1200–1550 MHz, so highest gain and efficiency, happened at 1550 MHz, are 3.4 dBi and 88%. For improving the antenna performances, the second-antenna is modelled with one-cell more than first-antenna and with changing in the slit configuration to F-shape and increasing in the same substrate thickness to 1.6 mm. F-shaped antenna has size of 14.5 mm × 4.4 mm × 1.6 mm, covering a measured-bandwidth of 0.11–2.1 GHz with five resonance frequencies at 450–725–1150–1670–1900 MHz, which corresponds to 180.1% practical-bandwidth. Maximum of the measured gain and efficiency of antenna are 4.5 dBi and 95%, which occur at 1900 MHz.
Sixth-generation (6G) wireless networks are envisioned to provide global coverage for the intelligent digital society of the near future, ranging from traditional terrestrial to non-terrestri-al ...networks, where reliable communications in high-mobility scenarios at high carrier frequencies would play a vital role. In such scenarios, the conventional orthogonal frequency division multiplexing (OFDM) modulation, that has been widely used in both the fourth-generation (4G) and the emerging fifth-generation (5G) cellular systems as well as in WiFi networks, is vulnerable to severe Doppler spread. In this context, this article aims to introduce a recently proposed two-dimension-al modulation scheme referred to as orthogonal time-frequency space (OTFS) modulation, which conveniently accommodates the channel dynamics via modulating information in the delay-Doppler domain. This article provides an easy-reading overview of OTFS, highlighting its underlying motivation and specific features. The critical challenges of OTFS and our preliminary results are presented. We also discuss a range of promising research opportunities and potential applications of OTFS in 6G wireless networks.
Nanomechanical sensors based on detecting and tracking resonance frequency shifts are to be used in many applications. Various open- and closed-loop tracking schemes, all offering a trade-off between ...speed and precision, have been studied both theoretically and experimentally. In this work, we advocate the use of a frequency counter (FC) as a frequency shift monitor in conjunction with a self-sustaining oscillator (SSO) nanoelectromechanical system (NEMS) configuration. We derive a theoretical model for characterizing the speed and precision of frequency measurements with state-of-the-art FCs. Based on the understanding provided by this model, we introduce novel enhancements to FCs that result in a trade-off characteristics which is on a par with the other tracking schemes. We describe a low-cost field-programmable-gate array (FPGA)-based implementation for the proposed FC and use it with the SSO-NEMS device in order to study its frequency tracking performance. We compare the proposed approach with the phase-locked-loop-based scheme both in theory and experimentally. Our results show that similar or better performance can be achieved at a substantially lower cost and improved ease of use. We obtain almost perfect correspondence between the theoretical model predictions and the experimental measurements.
Frequency response analysis (FRA) has become a widely accepted technique by worldwide utilities to detect winding and core deformations within power transformers. The main drawback of this technique ...is its reliance on the personnel level of expertise more than standard or automated codes. To establish reliable FRA interpretation codes, accurate high frequency transformer model that can emulate the frequency characteristics of real transformers in a wide frequency range is essential. The model can be used to investigate the impact of various winding and core deformations on the transformer FRA signature. The transformer equivalent high frequency electric circuit parameters can be calculated based on design data, which are rarely available, especially for old transformers. As such, this paper presents an artificial intelligence technique to estimate these parameters from the transformer FRA signature. The robustness of the proposed technique is assessed through its application on three, 3-phase power transformers of different ratings, sizes, and winding structures to estimate their high frequency electric circuit parameters during normal and fault conditions. Results show that the proposed technique can estimate equivalent circuit parameters with high accuracy and helps interpret the FRA signature based on the numerical changes of these parameters. The main advantage of this approach is the physical meaning of the model parameters facilitates reliable identification of various faults and hence aids in establishing reliable interpretation codes for transformer FRA signatures.
Optomechanical (OM) cavities enable coupling of near‐infrared light and GHz‐frequency acoustic waves in wavelength‐scale volumes. When driven in the phonon lasing regime, an OM cavity can perform ...simultaneously as a nonlinear mixer and a local oscillator—at integer multiples of the mechanical resonance frequency—in the optical domain. In this work, this property is used to demonstrate all‐optical frequency down‐ and up‐conversion of MHz‐bandwidth orthogonal frequency division multiplexed signals compliant with the IEEE 802.16e WiMAX wireless standard at microwave frequencies. To this end, a silicon OM crystal cavity (OMCC), supporting a breathing‐like mechanical resonance at fm≈3.9 GHz and having a foot‐print ≈ 10 μm2, which yields frequency conversion efficiencies better than −17 dB in both down‐ and up‐conversion processes at mW‐scale driving power, is employed. This work paves the way toward the application of OMCCs in low‐power all‐photonic processing of digitally modulated microwave signals in miniaturized silicon photonics chips.
All‐optical frequency down‐ and up‐conversion of MHz‐bandwidth OFDM signals compliant with the IEEE 802.16e WiMAX wireless standard is demonstrated by using an optomechanical cavity. The cavity, built on a silicon chip and supporting a breathing‐like mechanical resonance at 3.9 GHz, performs simultaneously as nonlinear mixer and local oscillator when operated in the phonon lasing regime.
Grid-connected power converters should employ advanced current controllers, e.g., proportional resonant (PR) and repetitive controllers (RC), in order to produce high-quality feed-in currents that ...are required to be synchronized with the grid voltage. The synchronization is actually to detect the instantaneous grid information (e.g., frequency and phase of the grid voltage) for the current control, which is commonly performed by a phase-locked loop (PLL) system. Hence, harmonics and deviations in the estimated frequency by the PLL could lead to current tracking performance degradation, especially for the periodic signal controllers (e.g., PR and RC) with a fixed sampling rate. In this paper, the impacts of frequency deviations induced by the PLL and/or the grid disturbances on the selected current controllers are investigated by analyzing the frequency adaptability of these current controllers. Subsequently, strategies to enhance the frequency adaptability of the current controllers are proposed for the power converters to produce high-quality feed-in currents even in the presence of grid frequency deviations. Specifically, by feeding back the PLL estimated frequency to update the center frequencies of the resonant controllers and by approximating the fractional delay using a Lagrange interpolating polynomial for the RC, respectively, the frequency-variation-immunity of these periodic current controllers with a fixed sampling rate is improved. Experiments on a single-phase grid-connected system are presented, which have verified the discussions and the effectiveness of the frequency adaptive current controllers.
The undergoing transition from conventional to converter-interfaced renewable generation leads to significant challenges in maintaining frequency stability due to declining system inertia. In this ...paper, a novel control framework for Synthetic Inertia (SI) provision from Wind Turbines (WTs) is proposed, which eliminates the secondary frequency dip and allows the dynamics of SI from WTs to be analytically integrated into the system frequency dynamics. Furthermore, analytical system frequency constraints with SI provision from WTs are developed and incorporated into a stochastic system scheduling model, which enables the provision of SI from WTs to be dynamically optimized on a system level. Several case studies are carried out on a Great Britain 2030 power system with different penetration levels of wind generation and inclusion of frequency response requirements in order to assess the performance of the proposed model and analyze the influence of the improved SI control scheme on the potential secondary frequency dip. The results demonstrate that the inclusion of SI provision from WTs into Unit Commitment (UC) can drastically impact the overall system costs.
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