Frequency Stability Requirements Research Articles

Twin-Field Quantum Key Distribution with Local Frequency Reference.

Twin-field quantum key distribution (TFQKD) overcomes the linear rate-loss limit, which promises a boost of secure key rate over long distance. However, the complexity of eliminating the frequency differences between the independent laser sources hinders its practical application. We analyzed and determined the frequency stability requirements for implementing TFQKD using frequency-stabilized lasers. Based on this analysis, we proposed and demonstrated a simple and practical approach that utilizes the saturated absorption spectroscopy of acetylene as an absolute reference, eliminating the need for fast frequency locking to achieve TFQKD. Adopting the 4-intensity sending-or-not-sending TFQKD protocol, we experimentally demonstrated the TFQKD over 502, 301, and 201km ultralow-loss optical fiber, respectively. We expect this high-performance scheme will find widespread usage in future intercity and free-space quantum communication networks.

An integrated planning framework for optimal power generation portfolio including frequency and reserve requirements

AbstractElectricity system decarbonisation poses several challenges to network stability and supply security, given renewables' intermittency and possible reduction of system inertia. This manuscript presents a novel integrated system framework to determine optimal generation investments for addressing decarbonisation challenges and achieving cost‐effective electricity systems while ensuring frequency stability and reserve requirements are met at the operational level in a net‐zero system. The novel planning framework is a mixed‐integer bilinear programming problem accurately modelling clustered variables for the on/off status of generation units and seconds‐timescale frequency requirements at an operational and planning level. The benefits of the decision framework and effects of dispatch decisions in a year are illustrated using the Great Britain case study. The results provide optimal trade‐offs and cost‐effective investment portfolios for including detailed modelling of unit‐commitment and frequency stability constraints versus not including them in the planning model. Making investment decisions for a net‐zero electricity system without these constraints can lead to very high system costs due to significant demand curtailment. Although the model's computation burden was increased by these constraints, complexity was managed by formulating them tightly and compactly. Non‐convex quadratic nadir constraints were efficiently solvable to global optimality by applying McCormick relaxations and branching techniques in an advanced solver.

Emergency coordinated control of multiple VSC‐HVDC for improving power system transient frequency and steady‐state frequency

AbstractWith the penetration of the high proportion of power electronics interfaced energy resources, the frequency stability issue in power systems is becoming increasingly prominent. This paper proposes a coordinated control strategy of multiple voltage source converter based high voltage direct current transmission (VSC‐HVDC) to simultaneously improve the transient frequency and steady‐state frequency after a severe disturbance. Firstly, a modified average system frequency response (MASFR) model is constructed with reserve power limitation, which takes into account the frequency responses of different types of generators (thermal, hydro and new energy resources). Secondly, according to the frequency stability requirements, the corresponding constraints, which consider the overload capacity of VSC‐HVDC, the AC transmission capacity and the frequency of AC grids on both sides of VSC‐HVDC, are formulated. Thirdly, for the transient frequency control, the coordinated optimization control model of multiple VSC‐HVDC is established, which aims to minimize the control energy considering the transient frequency threshold. For the steady‐state frequency control, the coordinated optimization control model is fomulated, which aims to minimize the control cost while considering the steady‐state frequency threshold. The two control schemes achieve seamless and smooth switching. Finally, the correctness and effectiveness of the proposed control strategy are verified in the improved IEEE 39 bus system.

Analysis of Minimum Rotational Inertia Requirements for Power System Frequency Stability

With the advancement of energy transformation and new power systems, large-scale integration of new energy to replace synchronous generators is leading to insufficient inertia in the system, threatening stable frequency operation. Evaluating the minimum inertia requirements of power systems is beneficial for improving the integration capacity of new energy and ensuring secure and stable operation. This paper establishes a mathematical model of frequency dynamics after a fault and advances an approach to assess the minimum rotational inertia requirements. The proposed approach is tested on the modified WSCC-9 bus system to validate its efficacy.

Inertia and Primary Frequency Response Requirement Assessment for High-Penetration Renewable Power Systems Based on Planning Perspective

In order to ensure the sustainable development of energy, the development of new power systems with a high penetration of renewable energy has become a key research direction in the field of power systems. This paper studies the system frequency response process and key indicators from the perspective of high-penetration renewable power systems and proposes an inertia and primary frequency response requirement assessment method for power system planning under high renewable penetration. First, by analyzing the frequency dynamic response process, the key parameters affecting frequency stability are determined, and the evolution trend of system inertia with increasing renewable penetration is analyzed. Second, based on the real-system data, the inertia and primary frequency response parameters for each generator are obtained. With the planning generation mix and load as the goal, whether the synchronous generators in the target system can meet the frequency stability requirements is determined. Finally, with the system inertia demand under the maximum rate of change of frequency (RoCoF) constraint as the starting point, we iteratively increase inertia and the primary frequency response capacity until the minimum matching configuration is found. The simulation results verify the correctness of the proposed assessment method. This method considers various processes in frequency response and multiple influencing factors, providing a practical evaluation tool for the inertia and primary frequency response requirements of high-penetration renewable power systems.

Renewable Energy Power System Unit Commitment Considering Regional Frequency Stability Requirement

With the development of new power systems, the formation of a “double high” pattern in frequency has made frequency security a prominent problem. It is particularly important to formulate a unit power generation plan with frequency security constraints at the day-ahead scale. However, a unit commitment model with frequency security constraints based on a unified frequency for the entire network cannot be used because the frequency exhibits spatial variations. This problem is solved in this study by establishing a single-machine aggregation model for the frequency response of a partitioned power system. In the unit commitment model, the frequency support between individual regions is established based on the distribution of the disturbance power over individual subregions. The index of the initial frequency change rate is introduced for each subregion to establish an analytical expression for the relationship between the frequency security constraints and the unit commitment scheme. Finally, the effectiveness of the model and the solution algorithm is verified for the improved IEEE39 node system.

Frequency Stability Requirements in Quasi-Integer-Ratio Time-Expanded Phase-Sensitive OTDR

Time-expanded phase sensitive (TE-ϕ)OTDR is a recently reported technique for distributed fiber sensing that relies on the use of an electro-optic dual frequency comb (DFC) scheme. A distinctive feature of this approach is its ability to provide high spatial resolution (on the centimeter scale) with detection bandwidths orders of magnitude lower than those of conventional ϕOTDR systems. The stringent trade-off between resolution, range and sensing bandwidth that exists in TE-ϕOTDR has demonstrated to be substantially relaxed by implementing two frequency combs with quasi-integer-ratio repetition rates. However, employing very dissimilar line separations (with a ratio between them >100) is challenging due to the need of keeping the coherence over long sequences of interferograms, which eventually limits the attainable range. In this paper, we formulate the requirements for the frequency stability of the reference clock used in a quasi-integer-ratio DFC scheme. This analysis allows us to stablish the limits on the number of comb lines (i.e., on the number of available independent sensing points) for a particular reference clock. By using a rubidium atomic clock (with a relative frequency stability of ∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−13</sup> ), we demonstrate up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> sensing points along 2 km of fiber with tens of Hz sensing bandwidth.

EVALUATION OF THE VALUE OF EQUIVALENT ENERGY LOSSES DUE TO THE QUALITY OF FREQUENCY SYNTHESIS FUNCTIONING IN DIGITAL COMMUNICATION SYSTEMS WITH QUASI-COHERENT RECEPTION OF SIGNALS WITH QUADRATURE AMPLITUDE KEYPAD

Useful transmitted information when using digital transmission systems with quadrature amplitude shift keying lies in the change in the amplitude and phase of the signal. This circumstance causes significantly higher requirements for the magnitude of destabilizing factors, namely the phase noise of signal packages at the input of the demodulating device of signals with quadrature amplitude keying, which affect the useful signal, resulting in a decrease in the reliability of reception. In this article, we restrict ourselves to considering the requirements for short-term frequency stability of synthesizers of digital transmission lines, which use signals of multi-position quadrature-amplitude keying in conjunction with quasi-coherent reception. The carrier wave is extracted on the receiving side directly with the help of a carrier recovery device from the received QAM signal by demodulating it, followed by narrow-band filtering. The phase-locked loop in this case functions as a narrow-band tunable filter for extracting the carrier wave. An increase in the quality of demodulation and signal filtering processes leads to a decrease in the phase fluctuation of the restored carrier, a decrease in the level of additive noise, and a decrease in the magnitude of energy losses when using quasi-coherent reception compared to the ideal one, which requires the complete absence of a phase error of the selected reference signal. Such an approximation of the noise immunity of real coherent QAM demodulators to the theoretical one limits the phase noise of the signal at the demodulator input, which is associated with the non-ideal operation of frequency synthesizers on the transmitting and receiving sides of digital communication lines. The research methods used in the article are based on the theories of potential noise immunity, demodulator synchronization, and phase locked loop systems. For the carrier recovery device, the noise band value was chosen in the range from to (where T – is the symbol duration), since with the indicated ratios, the distribution law of the phase error can be considered normal. It was assumed in the work that the devices for automatic gain control and clock synchronization in the radio receiving system function ideally, and the frequency response of the channel corresponds to the Nyquist condition.

Allocation of Spinning Reserves in Autonomous Grids Considering Frequency Stability Constraints and Short-Term Solar Power Variations

Low-inertia, isolated power systems face the problem of resiliency to active power variations. The integration of variable renewable energy sources, such as wind and solar photovoltaic, pushes the boundaries of this issue further. Higher shares of renewables requires better evaluations of electrical system stability, to avoid severe safety and economic consequences. Accounting for frequency stability requirements and allocating proper spinning reserves, therefore becomes a topic of pivotal importance in the planning and operational management of power systems. In this paper, dynamic frequency constraints are proposed to ensure resiliency during short-term power variations due to, for example, wind gusts or cloud passage. The use of the proposed constraints is exemplified in a case study, the constraints being integrated into a mixed-integer linear programming algorithm for sizing the optimal capacities of solar photovoltaic and battery energy storage resources in an isolated industrial plant. Outcomes of this case study show that reductions in the levelized cost of energy and carbon emissions can be overestimated by 8.0% and 10.8% respectively, where frequency constraints are neglected. The proposed optimal sizing is validated using time-domain simulations of the case study. The results indicate that this optimal system is frequency stable under the worst-case contingency.

Research on Frequency Response Modeling and Frequency Modulation Parameters of the Power System Highly Penetrated by Wind Power

Renewable energy units led by wind power participate in diversified control primary frequency modulation, making the frequency response modes and the setting of frequency modulation parameters more complex. This paper proposes a frequency response model of the power system which is highly penetrated by wind power based on the two mainstream control strategies of wind power that participate in primary frequency modulation. The model considers the influence of wind capture devices, maximum power point tracking (MPPT), and other complex control strategies on system frequency response. Based on this model, the calculation formulas of the maximum change rate of dynamic frequency, the lowest point of dynamic frequency, and the maximum steady-state frequency deviation of the system after fault disturbance are derived in the frequency domain. The influences of wind power permeability and two typical frequency response control strategies on system frequency stability are analyzed. On the one hand, it is found that the proposed model can fit the system frequency response better than the traditional system frequency response model. Beyond that, two control strategies are mainly aimed at the different frequency stability requirements. On the other hand, under the condition of meeting the system’s stability requirements, the paper calculates the control parameters of frequency response of the doubly-fed induction generator (DFIG). The time-domain simulation model of the improved IEEE three-machine nine-node system and IEEE 39-node system with high permeability of wind power are built. Through the different fault scenarios, the simulation results verify the effectiveness of the proposed model and the accuracy of control strategy parameter calculation.

Stabilized laser system at 1550 nm wavelength for future gravitational-wave detectors

Proposed future gravitational wave detectors place high demands on their prestabilized laser system. We present a prototype for such a prestabilized laser system at $1550\text{ }\text{ }\mathrm{nm}$ wavelength with frequency and power stabilizations optimized for the needs of gravitational wave detectors. A power stabilization with shot noise limited operation below a relative power noise of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }{\mathrm{Hz}}^{\ensuremath{-}1/2}$ between 100 Hz to 100 kHz and an active frequency stabilization with a unity-gain bandwidth above 2 MHz were operated simultaneously. Out-of-loop measurements are performed to characterize the achieved stability and to analyze sensor noise limits. We find that nonlinear noise couplings at the spatial mode-filter cavity are of high relevance and lead to increased frequency stability requirements above 100 kHz . This prestabilized laser system can serve as the baseline for the Einstein Telescope gravitational wave detector [ET steering committee, Design report Update 2020 for the Einstein Telescope, Technical Report, Einstein gravitational wave Telescope, 2020.] and demonstrates stabilization concepts generally applicable to optical precision experiments.

Improving frequency characterization for power systems using the Allan variance and a GPS-controlled reference: Measurement procedure, test and validation

The interconnected modern smart grid has resulted in several new power quality (PQ) disturbances that affect the frequency stability requirements in industrial power systems. Ranging between 50±0.1 Hz for interconnected networks, and even within the European synchronous area, the frequency fluctuates mainly because of the primary control. In fact, owing to the development of distributed resources (e.g. renewable energies), not all generation units are accessible, and are controllable in a few network nodes. This variability, even between nearby locations, triggers the need for a specific measurement strategies, based on an absolute reference and also conceived for monitoring PQ disturbances. This work proposes an easy-to-implement GPS-traceable method that allows the measurement of frequency with better accuracy than that required by the current regulation for interconnected networks. This work explains the measurement methodology, characterising the uncertainty by using ISO JCGM 100. For the purpose of testing and validation, three experiments were conducted: computer-designed, instrument-triggered lab disturbances, and a comparison vs. PMU performance, using real-life signals. The results showed that these experiments offered better performance with improved uncertainty.

A land-based HF transmitter for ionospheric propagation studies using SuperDARN radars

Purpose The purpose of this paper is to design, build and test a low power high frequency (HF) transmitter that can be received by the Super Dual Auroral Radar Network (SuperDARN) radar installed at SANAE IV, the 4th South African National Antarctic Expedition Station. It is proposed that it may be possible to do propagation studies using the radar and the fixed frequency, ground-based HF transmitter beacon. Interpretation of the measurements can be used to study the ionosphere, especially Travelling Ionospheric Disturbances, which are signatures of atmospheric gravity waves. Design/methodology/approach In the absence of the actual deployment of the HF transmitter beacon in Antarctica, extensive simulations have been done to evaluate the expected performance of the transmitter in relation to the SuperDARN. A field trial has been executed between Hermanus (34.4241° S, 19.2247° E) and Pretoria (34.0558° S, 18.4589° E) in South Africa. In future, the beacon will be placed at the South Pole with its antenna radiating towards SANAE IV. Findings The HF transmitter conforms to the power and frequency stability requirements both during propagation tests conducted between Hermanus and Pretoria, as well as when the device was exposed to temperatures that ranged from +40°C to −45°C in a thermal chamber. Propagation in Antarctica is expected to differ from the field tests conducted due to the differences in density and dynamics of the polar ionosphere, compared to the mid-latitude ionosphere. Originality/value Space weather research, including forecasting atmospheric gravity waves and determining the expected electron density profile of the ionosphere, is of great scientific interest. The data received from the HF beacon can be used to study and characterize the ionosphere of the region between the South Pole and SANAE IV. Parameters of the ionosphere, such as electron density, geomagnetic storm effects, ionospheric motions and sky wave propagation paths will be better understood from analysing the signal received from this transmitter after it has been reflected and refracted by the ionosphere.

Optimal Power Flow Incorporating Frequency Security Constraint

Optimal power flow (OPF) minimizes the production cost, while satisfying the constraints on the system, such as real and reactive power balance, equipment capability constraints, and voltage limits. This article presents a method of incorporating a frequency security constraint within the framework of the OPF problem. Increasing displacement of conventional generation by renewable resources that contribute little or no frequency regulation capability may necessitate the enforcement of such a constraint to meet frequency security requirements. The system frequency is required to be maintained within a safe limit, thus indicating the balance between generation and consumption. Hence, the solution for optimization of power flow should not only present a minimum cost of generation within the operating conditions, but also ensure frequency stability. In order to obtain this solution, the requirement of frequency stability is introduced as a new constraint of the power dispatch problem and is represented by the maximum frequency deviation limit. This new constraint is constructed as a nonlinear function of system inertia and the frequency regulation constant, since frequency deviation is highly sensitive to these factors. It is shown that the inclusion of this constraint causes the OPF to preferentially select higher inertia generators as necessary, to satisfy the frequency security requirement. A genetic algorithm is utilized as the optimization tool in this article. The IEEE RTS-79 test system is used to demonstrate efficacy of the proposed method.

Assessment of inertial and primary frequency control from wind power plants in the Mexican electric power grid

AbstractLarge‐scale integration of converter‐based renewable energy sources into power systems, such as wind generation, can lead to frequency stability issues due to the variable nature and lack of inertia of these technologies in combination with the gradual replacement of conventional generating units. However, wind turbine generators (WTGs) can be exploited to provide frequency support and keep system frequency stability requirements. A case study considering the Mexican Electric Power System is presented in this study to highlight both the impact of large‐scale deployment of inverter‐interfaced wind energy generation on system frequency response, and the participation of WTGs in inertial and primary frequency control (IPFC) as a mitigation approach. By incorporating synthetic inertia and droop control functions into the active power control loop of WTG converters, IPFC by wind generation is assessed for several wind shares, different active power modulation strategies based on the rate of change of frequency and frequency deviation, and several IPFC contribution levels under critical contingencies for generation outage. Simulation results show the combination of increasing share of wind energy generation in the study system and retirement of conventional generation has a clearly negative impact on system frequency dynamics. However, the incorporation of IPFC functions into wind power generators of the sample system, with appropriate control gain values, may contribute to effectively achieve an improved system performance in terms of grid frequency response under high wind power penetration scenarios.This article is characterized under: Wind Power &gt; Science and Materials Wind Power &gt; Systems and Infrastructure Energy Research &amp; Innovation &gt; Science and Materials

Methane microwave optical master oscillator for fountain references

We have demonstrated a microwave master oscillator operating on optical principles and based on a He – Ne/CH4 optical frequency standard (λ = 3.39 μm) and femtosecond fibre laser system ( λ = 1.54 μm). The output signal spectrum of the oscillator has the form of an equidistant frequency comb in the range 60 MHz to 10 GHz with a 60-MHz step. Comparison of the frequencies of two master oscillators shows that the short-term output frequency instability for the comb components in the range 0.8 – 1.5 GHz is under 1 × 10−14 at an averaging time of 1 s. We have tested microwave signals from the oscillator using apparatus in the reference facility at the State Time and Frequency Service, All-Russia Research Institute of Physical and Radio Engineering Measurements. A signal at a nominal frequency of 100 MHz synthesised from one comb component was compared to signals from two hydrogen masers with enhanced short-term stability in the reference facility. The short-term frequency stability of the synthesised signal has been shown to be twice better than the stability of the masers and to be limited by the intrinsic instability of the commercial synthesiser of a 100-MHz nominal frequency. Our experiments confirm that the proposed microwave optical master oscillator is potentially attractive for use in systems with increased requirements for short-term frequency stability, in particular in time and frequency fountain references.

A ± 1.55ppm Stable FBAR Reference Clock with Oven-Controlled Temperature Compensation

We present an oven-controlled FBAR oscillator that achieves a frequency stability of +/-1.55ppm from -5°C to 85°C. The highly integrated system includes a 0.64mm2 FBAR chip with integrated heater and sensor resistors and a 3 mm2 CMOS chip with the control electronics. The oscillator achieves an Allen deviation of 4ppb enabled by a temperature-to-digital converter (TDC) with a 150uK resolution. It corresponds to a 1.68JK2 FOM. The heater consumes a power of 14mW at -5°C and the oscillator consumes only 0.25mW. The ovenized oscillator meets the stringent frequency stability requirements (2 ppm) of GPS applications.

A Frequency Control Strategy Considering Large Scale Wind Power Cluster Integration Based on Distributed Model Predictive Control

With large scale wind integration and increasing wind penetration in power systems, relying solely on conventional generators for frequency control is not enough to satisfy system frequency stability requirements. It is imperative that wind power have certain capabilities to participate in frequency control by cooperating with conventional power sources. Firstly, a multi-area interconnected power system frequency response model containing wind power clusters and conventional generators is established with consideration of several nonlinear constraints. Moreover, a distributed model predictive control (DMPC) strategy considering Laguerre functions is proposed, which implements online rolling optimization by using ultra-short-term wind power forecasting data in order to realize advanced frequency control. Finally, a decomposition-coordination control algorithm considering Nash equilibrium is presented, which realizes online fast optimization of multivariable systems with constraints. Simulation results demonstrate the feasibility and effectiveness of the proposed control strategy and algorithm.

Compact Optical Clock with 5×10 −13 Instability at 1 s

We present a compact optical atomic clock design based on a two-photon transition at 778 nm in rubidium that is capable of a frequency instability of 5 x 10^-13 at 1 s. The simple architecture occupies less than 30 L volume and currently uses commercial off-the-shelf components making this system an attractive, appealing, and competitive candidate to meet GPS-III spacecraft frequency stability requirements.

Iodine frequency references for space

Optical frequency references are a key element for the realization of future space missions. They are needed for missions related to tests of fundamental physics, gravitational wave detection, Earth observation and navigation and ranging. In missions such as GRACE follow-on or LISA the optical frequency reference is used as light source for high-sensitivity inter-satellite distance metrology. While cavity-based systems are current baseline e.g. for LISA, frequency stabilization on a hyperfine transition in molecular iodine near 532 nm is a promising alternative. Due to its absolute frequency, iodine standards crucially simplify the initial spacecraft acquisition procedures. Current setups fulfill the GRACE-FO and LISA frequency stability requirements and are realized near Engineering Model level. We present the current status of our developments on Elegant Breadboard (EBB) and Engineering Model (EM) level taking into account specific design criteria for space compatibility such as compactness (size iodine spectroscopy EM: 38 × 18 × 10 cm3) and robustness. Both setups achieved similar frequency stabilities of ∼ 1 · 10−14 at an integration time of 1 s and below 5 · 10−15 at integration times between 10 s and 1000 s. Furthermore, we present an even more compact design currently developed for a sounding rocket mission with launch in 2017.