Due to its range-angle-dependent transmit beampattern, frequency diverse array (FDA) provides potential applications for joint range and angle estimation of targets and range-dependent interference ...suppression. However, a standard FDA using linearly increasing frequency increments will generate range- and angle-coupled S-shaped beampattern. This letter proposes a focused beampattern synthesis by optimizing the frequency increments with genetic algorithm. Both single-dot and multidot-shaped transmit beampatterns can be synthesized in this way. Numerical results show that the proposed method outperforms the existing method using logarithmically increasing frequency increments in suppressing undesired sidelobes, which implies that better target location performance can be achieved for the proposed method.
Temporally modulated metamaterials have attracted significant attention recently due to their nonreciprocal and frequency converting properties. Here, a transparent, time-modulated metasurface, which ...functions as a serrodyne frequency translator, is reported at <inline-formula> <tex-math notation="LaTeX">X </tex-math></inline-formula>-band frequencies. With a simple biasing architecture, the metasurface provides electrically tunable transmission phase that covers 360°. A sawtooth waveform is used to modulate the metasurface, allowing Doppler-like frequency translation. Modal analysis of an analogous time-modulated medium is performed to gain insight into the operation of the metasurface-based serrodyne frequency translator. Two such metasurfaces can be cascaded together to achieve magnetless devices that perform either phase or amplitude nonreciprocity.
The power system inertia is gradually decreasing with the growing share of variable renewable energy (VRE). This may jeopardize the frequency dynamics and challenges the secure operation of power ...systems. In this paper, the concept of frequency security margin is proposed to quantify the system frequency regulation ability under contingency. It is defined as the maximum power imbalance that the system can tolerate while keeping frequency within the tolerable frequency range. A frequency constrained unit commitment (FCUC) model considering frequency security margin is proposed. Firstly, the analytical formulation of system frequency nadir is derived while considering both the frequency regulation characteristics of the thermal generators and the frequency support from VRE plants. Then, the frequency security margin is analytically formulated and piecewise linearized. A novel FCUC model is proposed by incorporating linear frequency security constraints into the traditional unit commitment model. Case studies on a modified IEEE RTS-79 system and HRP-38 system are provided to verify the effectiveness of the proposed FCUC model. The impacts of VRE penetration on system frequency security are analyzed using frequency security margin.
To improve the frequency stability of power systems with a high penetration of converter-based generators, this paper proposes an Enhanced Frequency-Constrained Unit Commitment (E-FCUC) considering ...variable-droop-controlled Frequency Control (FC) from converter-based generators. Unlike previous studies emphasizing conventional and fix-droop converter-based generators participating in primary frequency response, the impact of converter-based generators with variable-droop FC (VD-FC) is investigated, modeled, and then incorporated into the UC problems. Firstly, the transfer function of variable-droop-controlled converter-based generators is modeled; the available primary power reserves bounding provided by converter-based generators is analyzed. Secondly, the frequency dynamic of power systems considering joint frequency control from conventional generators and variable-droop-controlled converter-based generators is derived based on an analytical aggregated System Frequency Response (SFR) model. Since the nonlinear non-smooth feature of obtained frequency dynamic function, the "max-affine" Piece-Wise-Linearization method (PWL) is adopted here to fit the calculated frequency function. The fitting function is subsequently reformulated into mix-integer linearized constraints participating in UC studies. Case studies based on a modified IEEE 6 bus test system and a modified IEEE 118 bus test system are carried out to verify the effectiveness of the proposed E-FCUC through comparisons of results with existing empirical models.
A 94-GHz highly efficient frequency octupler (×8) is presented, which consists of a 47-GHz frequency quadrupler followed by a 94-GHz push-push frequency doubler. To achieve high efficiency, the ...former adopts the current reusing technique of which transistors are biased to be in the class-C region. In addition, the core transistors and the input-matching network of the latter are optimized simultaneously by using a premade input matching network library, which yields a maximum conversion gain under a given dc power budget. It is implemented with a commercial 65-nm CMOS process, which generates -7.12-dBm output power with 0-dBm input signal, consuming only 1 mW of dc power. It operates in the frequency range from 84 to 98.4 GHz (15.3%) within 3-dB gain variation. The total efficiency (P OUT /(P dc + P IN )) is 9.69%, which is the highest among those of reported frequency octuplers.
The design of power magnetic components for operation at high frequency (HF, 3-30 MHz) has been hindered by a lack of magnetic material performance data and by the limited design theory in that ...frequency range. To address these deficiencies, we have measured and present core loss data for a variety of commercially available magnetic materials in the HF range. In addition, we extend the theory of performance factor for appropriate use in the HF design. Since magnetic materials suitable for HF applications tend to have low permeability, we also consider the impact of low permeability on design. We conclude that, with appropriate material selection and design, increased frequencies can continue to yield improved power density well into the HF regime.
This paper proposes an adaptive dual droop control (ADDC) scheme, it can provide the disturbed onshore system with fast frequency support from both other undisturbed onshore systems, and voltage ...source converter-based multi-terminal direct current (VSC-MTDC) integrated offshore wind farms (OWFs). With conventional droop control at onshore converters, the DC voltage and power flow will change once frequency events occur, it will lead to frequency variations in other undisturbed systems. The proposed ADDC scheme firstly detects the disturbed and undisturbed systems, and then makes the undisturbed onshore system provide more frequency support power, while ensuring safe operation by settling the support power limitation and regulating the droop coefficients. Moreover, the offshore stations will estimate the onshore DC voltage as the control signal for fast frequency support. After that, the OWFs will recover their rotor speed with an asymptotic control scheme to reduce the second frequency drop. Case studies are carried out on 3-terminal and 5-terminal test systems, and the Opal-RT real-time simulation platform, respectively. Different control schemes are compared, and the parameter uncertainty and noise disturbance are considered to illustrate the performance and effectiveness of the proposed ADDC scheme.
This paper presents a new multiresonant frequency-adaptive synchronization method for grid-connected power converters that allows estimating not only the positive- and negative-sequence components of ...the power signal at the fundamental frequency but also other sequence components at other harmonic frequencies. The proposed system is called MSOGI-FLL since it is based on both a harmonic decoupling network consisting of multiple second-order generalized integrators (MSOGIs) and a frequency-locked loop (FLL), which makes the system frequency adaptive. In this paper, the MSOGI-FLL is analyzed for single- and three-phase applications, deducing some key expressions regarding its stability and tuning. Moreover, the performance of the MSOGI-FLL is evaluated by both simulations and experiments to show its capability for detecting different harmonic components in a highly polluted grid scenario.
Traditional radar waveforms are easily intercepted and interfered with by enemy's reconnaissance system with the time-frequency periodic pattern recognition. Frequency and pulse repetition frequency ...(PRF) agility is an effective approach to decrease interception probability and increase anti-jamming capabilities. On the other hand, the agility brings about high sidelobes and the difficulty of parameter estimation using the range-Doppler signal processing. In this article, an optimization and high-resolution imaging algorithm for sparse stepped linear frequency modulation waveform (SSLFMW) with frequency and PRF agility is developed. The range and Doppler 2-D autocorrelation function of the agile waveform is investigated to pave a way to find an optimization strategy for frequency and PRF to suppress range and Doppler sidelobes. Relied on the pulse trains of low Doppler sidelobes, we propose a method of cognitive transmitting and motion retrieval based on the maximum likelihood principle to eliminate the frequency and range coupling in velocity estimation. The 2-D sparse reconstruction with conjugate gradient solver is proposed to efficiently reconstruct the high-resolution range-Doppler image with the frequency and PRF agility waveform. Both simulated and real-measured data sets are used to verify the improved performance of the proposal.
For renewable energy penetration, a frequency control strategy is proposed to assure the multi-area hybrid power system stable operation. The load frequency control (LFC) and the battery energy ...storage system (BESS) are used together to design the frequency control strategy. Furthermore, in order to improve the control accuracy and reduce the battery capacity, the area control error and active power disturbance are divided into low-frequency components and high-frequency components. Therefore, LFC is designed by using sliding mode algorithm for the low-frequency deviation and BESS is adjusted by using the designed disturbance observer for the high-frequency disturbance. Finally, the effectiveness of the proposed frequency control strategy is proved by the real-time digital simulator which can optimise the frequency quality and reduces the capacity of the BESS.
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