For a frequency diverse array (FDA) antenna, the combination of time-modulated and nonlinear distributed frequency offset is proposed to realize time-invariant spatial fine focusing beampatterns. The ...distribution of the nonlinear function is optimized by using artificial bee colony algorithm, and the resultant frequency offset is termed as time-modulated optimized frequency offset (TMOFO). In addition to design a TMOFOFDA in realistic situations, the interval-based tolerance analysis is introduced to predict the impact of frequency-offset uncertainties or errors on the time-invariant spatial focusing beampattern. The spatial focusing performance of the TMOFO-FDA is compared with an analytical FDA scheme based on a time-modulated logarithmically increasing frequency offset (TMLFO). Numerical examples of a ten-element linear FDA with λ 0 /2 element spacing in the presence of mutual coupling effects based on the TMOFO and the TMLFO are reported and compared. The comparative results between the ideal TMLFO and TMOFO show that, in range-angle plane the size of the focal spot for TMOFO-FDA is reduced by about 70% compared to that for TMLFO-FDA. The side lobe level is reduced about 49%. The comparative results from IA-based beampattern tolerance analysis also reveal the superiority of the TMOFO-FDA scheme.
Power systems are facing the displacement of conventional power plants by converter-interfaced generation, which does not inherently provide inertia; as a result, large frequency deviations can occur ...after a power imbalance, compromising the frequency stability. Energy storage systems (ESSs) are becoming key elements in improving the performance of both the electrical grid and renewable generation systems. They are able to store and release energy with a fast response time, thus participating in short-term frequency control. This letter proposes a strategy to minimize the frequency nadir in the event of a frequency disturbance using the energy stored in ESSs. An analytical procedure is presented to determine the optimal time to inject ESS power into the grid after a power imbalance. Different parameter scenarios and injected power waveforms are discussed.
This letter proposes a millimeter-wave (mm-wave) RF beamforming transmitter (TX) architecture that employs frequency multipliers (FMs) as frequency up-converters to avoid the challenges associated ...with the signal and local oscillator (LO) distribution networks that intensify with the increase of operating frequency. The proposed architecture incorporates a single-input- single-output (SISO) digital predistortion (DPD) scheme that is specifically devised to mitigate the nonlinear distortions exhibited by the cascade of FMs and power amplifiers (PAs). Proof-of-concept experiments conducted on a lineup, comprised of an off-the-shelf frequency doubler and an <inline-formula> <tex-math notation="LaTeX">8\times 8 </tex-math></inline-formula> array and operating at an output frequency of 28 GHz, confirmed the capacity of the proposed architecture to transmit 200 MHz orthogonal frequency division multiplexing (OFDM) signals with an error vector magnitude (EVM) and adjacent channel power ratio (ACPR) better than 1.6% and 45 dB, respectively, while achieving an effective isotropic radiated power (EIRP) of 46.3 dBm.
Integrated with a high share of Inverter-Based Resources (IBRs), microgrids face increasing complexity of frequency dynamics, especially after unintentional islanding from the maingrid. These IBRs, ...on the other hand, provide more control flexibility to shape the frequency dynamics of microgrid and together with advanced communication infrastructure offer new opportunities in the future software-defined microgrids. To enhance the frequency stability of microgrids with high IBR penetration, this paper proposes an optimal scheduling framework for software-defined microgrids which aims at combining the control design of the IBR dynamic frequency response and the steady-state economic optimization. This is achieved by utilizing the non-essential load shedding and dynamical optimization of the virtual inertia and virtual damping from IBRs. Moreover, side effects of these services, namely, the time delay associated with non-essential load shedding and potential IBR control parameter update failure are explicitly modeled to avoid underestimations of frequency deviation and over-optimistic results. The effectiveness and significant economic value of the proposed simultaneous and dynamic virtual inertia and damping provision strategy are demonstrated based on case studies in the modified IEEE 33-bus system.
Additional active power control (AAPC) of wind turbines (WTs) is essential to improve the transient frequency stability of low-inertia power systems. Most of the existing research has focused on ...imitating the frequency response of the synchronous generator (SG), known as virtual inertia control (VIC), but are such control laws optimal for the power systems? Inspired by this question, this article proposes an optimal AAPC of WTs to maximize the frequency nadir post a major power deficit. By decoupling the WT response and the frequency dynamics, the optimal frequency trajectory is solved based on the trajectory model, and its universality is strictly proven. Then the optimal AAPC of WTs is constructed reversely based on the average system frequency (ASF) model with the optimal frequency trajectory as the desired control results. The proposed method can significantly improve the system frequency nadir. Meanwhile, the event insensitivity makes it can be deployed based on the on-line rolling update under a hypothetic disturbance, avoiding the heavy post-event computational burden. Finally, simulation results in a two-machine power system and the IEEE 39 bus power system verify the effectiveness of the optimal AAPC of WTs.
Rapid measurement of the resonant frequency of quartz crystal (QC) devices plays a key role in applying QC-based dynamic monitoring, such as respiration monitoring and kinetic process in ...electrochemical reactions. This work develops an FPGA-accelerated QC measurement system without software-based curve fitting. A negative feedback loop based on the susceptance response collected by the quadrature demodulation circuit is constructed. Moreover, a proportional-integral-differential control and tracking strategy is applied to improve the measurement speed significantly. The test results show that the implemented system can track a frequency shift of 5 kHz bandwidth within 50 ms. The standard deviation of the measured frequency of a QC device under test is only 0.17 Hz, which indicates that the proposed system is high stability and high precision. The frequency measurement results have a relative error of 0.8 ppm compared with the Agilent 4294A impedance analyzer. It is demonstrated that the proposed QC measurement system has great potential for QC-based dynamic detection.
The intermode beat frequency stability of a 1 mm long quantum dash self mode-locked semiconductor laser emitting optical frequency combs centred at 1535 nm is improved by single-cavity optical ...self-injection. A novel all-fibre based cavity employing a piezo-stretchable fibre spool provides both macroscopic optical path cavity length of 76 m and microscopic fine-delay fibre length control. The maximum achieved intermode beat frequency tuning is 2 MHz. The intermode beat frequency line width indicating the optical frequency comb stability is improved from 67 kHz to below 900 Hz constituting an improvement by a factor of 74. An independent free-space stabilisation experiment and stochastic time-domain modelling confirm the demonstrated intermode beat frequency tuning range.
Charging electric vehicles wirelessly is promising because of its convenience as well as saving of charging cables. However, the existing wireless power transfer systems suffer from high resonant ...peaks and poor efficiency. Therefore, zero voltage switching (ZVS) operation of inverter should be achieved especially in high transfer power. Based on the variable frequency phase shift control strategy (VFPSC), this paper presents an optimal operation frequency range (OOFR) where the wireless power transfer (WPT) system can realize the required output and ZVS operation of inverter simultaneously without extra dc-dc converters. Meanwhile, based on OOFR, an optimized electrical parameter design method based on VFPSC is proposed with the multiple boundary conditions. Moreover, a novel three-loop control strategy (TLCS) is proposed to make the system operate at any points of OOFR. Especially, an implementation method of the TLCS is proposed, which can dynamically adjust frequency and phase shift to make the system always operate at the preset ZVS angle and realize the required output simultaneously. Finally, a 500-W WPT system is built to verify the correctness of theoretical analysis. The experimental results show that a very high overall efficiency is achieved in the whole charging process and the maximum efficiency can achieve 94.9% with <inline-formula><tex-math notation="LaTeX">k</tex-math></inline-formula> = 0.2.
Frequency drop due to loss of massive generation is a threat to power system frequency stability. Under-frequency load shedding (UFLS) is the principal measure to prevent successive frequency ...declination and blackouts. Based on traditional stage-by-stage UFLS scheme, a new continuous UFLS scheme is proposed in this paper to shed loads proportional to frequency deviation. The characteristic of the proposed scheme is analyzed with a closed-form solution of frequency dynamics. Frequency threshold and time delay are added to make the proposed scheme practical. A line-by-line scheme based on precise load control is introduced to implement the continuous scheme for systems without enough continuously controllable loads. The load shedding scale factor of the proposed scheme is tuned with an analytical method to achieve adaptability to different operating conditions. The adaptability of the proposed scheme is validated with 39-bus New England model and simplified Shandong Power Grid of China.
This paper proposes a torque limit-based inertial control scheme of a doubly-fed induction generator (DFIG) that supports the frequency control of a power system. If a frequency deviation occurs, the ...proposed scheme aims to release a large amount of kinetic energy (KE) stored in the rotating masses of a DFIG to raise the frequency nadir (FN). Upon detecting the event, the scheme instantly increases its output to the torque limit and then reduces the output with the rotor speed so that it converges to the stable operating range. To restore the rotor speed while causing a small second frequency dip (SFD), after the rotor speed converges the power reference is reduced by a small amount and maintained until it meets the reference for maximum power point tracking control. The test results demonstrate that the scheme can improve the FN and maximum rate of change of frequency while causing a small SFD in any wind conditions and in a power system that has a high penetration of wind power, and thus the scheme helps maintain the required level of system reliability. The scheme releases the KE from 2.9 times to 3.7 times the Hydro-Québec requirement depending on the power reference.
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