Frequency Excursions Research Articles

Research on a Fundamental and Harmonic Detection Method Based on D-Q Rotating

The widespread use of power electronics in power systems has led to increasingly serious power quality problems in power systems. Active power filters play an important role in governing the harmonics of power systems. The fast and accurate detection of fundamental and harmonic currents in power systems is one of the key aspects of power filters to manage harmonics. In this paper, a new detection method for the fundamental or the nth harmonic component of the fundamental by presetting the frequency of the d-q conversion matrix is proposed. Analysis of simulation results shows that the detection method is not affected by the voltage distortion or asymmetry of the three-phase circuit, and that the results for fundamental and harmonic detection. The ability to accurately reflect changes in frequency and amplitude excursions.

Detection of modulated rippled noise

Bill Yost's study of modulation detection interference (MDI) was in part motivated by his earlier work with temporally modulated rippled noise (RN) which indicated difficulty monitoring a single auditory channel in the presence of cross-channel modulation. The present work further evaluated parameters in the processing of dynamic spectral profiles. In the first experiment, gain in RN generation was sinusoidally modulated. If the gain sign remained unchanged during modulation, modulation resulted in a roughly constant increment in ripple-detection thresholds across change in modulation rate from 2 to 128 Hz. However, cross-sign gain modulation led to a lowpass result with thresholds increasing with modulation rate. A similar lowpass result was obtained in the second ripple-detection experiment in which delay in RN generation was modulated. The third experiment measured thresholds for detecting delay modulation. Thresholds, in terms of Weber ratios, rose with increasing modulation rate from 2 to 8 Hz. Across experiments, the lowpass results contrast with MDI in which interference decreases with increasing modulation rate. With dynamic spectral profiles, there is significant envelope modulation of individual auditory channels due to stimulus frequency excursions. Results suggest a limitation imposed by cross-channel envelope incoherence in utilizing this envelope modulation.

A Novel SSA Tuned PI-TDF Control Scheme for Mitigation of Frequency Excursions in Hybrid Power System

ABSTRACT In this work, a novel Salp Swarm Algorithm (SSA) tuned centralized hybrid proportional integral-tilt derivative with filter (PI-TDF) control scheme is proposed for mitigation of frequency excursions in hybrid power system (HPS) subjected to varying operating conditions due to variations of load and output powers of renewable energy systems (RESs). To bring out the eminence of SSA over other evolutionary algorithms implemented in this study, the comparative performance analysis is carried out. Further, frequency regulation performance of SSA tuned PI-TDF controller is compared against SSA tuned conventional PI/PID/TID/TIDF/P-TIDF controllers. The proposed control scheme is effective even with rate constraint non-linearity in the system; subsystems being switched on/off at different time instants; and with parametric changes introduced in one of the subsystems. The proposition, thus, is a viable and effective control scheme for frequency regulation in HPS while being robust and less sensitive.

Monolithic piezoelectric control of soliton microcombs.

High-speed actuation of laser frequency1 is critical in applications using lasers and frequency combs2,3, and is a prerequisite for phase locking, frequency stabilization and stability transfer among optical carriers. For example, high-bandwidth feedback control of frequency combs is used in optical-frequency synthesis4, frequency division5 and optical clocks6. Soliton microcombs7,8 have emerged as chip-scale frequency comb sources, and have been used in system-level demonstrations9,10. Yet integrated microcombs using thermal heaters have limited actuation bandwidths11,12 of up to 10kilohertz. Consequently, megahertz-bandwidth actuation and locking of microcombs have only been achieved with off-chip bulk component modulators. Here we demonstrate high-speed soliton microcomb actuation using integrated piezoelectric components13. By monolithically integrating AlN actuators14 on ultralow-loss Si3N4 photonic circuits15, we demonstrate voltage-controlled soliton initiation, tuning and stabilization with megahertz bandwidth. The AlN actuators use 300nanowatts of power and feature bidirectional tuning, high linearity and low hysteresis. They exhibit a flat actuation response up to 1megahertz-substantially exceeding bulk piezo tuning bandwidth-that is extendable to higher frequencies by overcoming coupling to acoustic contour modes of the chip. Via synchronous tuning of the laser and the microresonator, we exploit this ability to frequency-shift the optical comb spectrum (that is, to change the comb's carrier-envelope offset frequency) and make excursions beyond the soliton existence range. This enables a massively parallel frequency-modulated engine16,17 for lidar (light detection and ranging), with increased frequency excursion, lower power and elimination of channel distortions resulting from the soliton Raman self-frequency shift. Moreover, by modulating at a rate matching the frequency of high-overtone bulk acoustic resonances18, resonant build-up of bulk acoustic energy allowsa 14-fold reduction of the required driving voltage, making it compatible with CMOS (complementary metal-oxide-semiconductor) electronics. Our approach endows soliton microcombs with integrated, ultralow-power and fast actuation, expanding the repertoire of technological applications of microcombs.

Frequency Stability Evaluation in Low Inertia Systems Utilizing Smart Hierarchical Controllers

The high penetration of the Renewable Energy Sources and other emerging technologies likely to be installed in future power grids will pose new operational challenges to grid operators. One of the main issues expected to affect the operation of the power grid is the impact of inverter-based technologies to the power system inertia and, hence, to system stability. Consequently, the main challenge of the future grid is the evaluation of the frequency stability in the presence of inverter-based systems and how the aforementioned technology can support frequency stability without the help of the rotating masses of the traditional power grid systems. To assess the above problem, this paper proposes a methodology to evaluate the frequency stability in a projection of the real distribution grid in Cyprus with the time horizon to be the year 2030. The power grid under investigation is evaluated with and without the presence of smart hierarchical controllers for providing support to the power system under disturbance conditions. The advanced controllers were applied to manage the available power resource in a fast and effective manner to maintain frequency within nominal levels. The controllers have been implemented in two hierarchical levels revealing useful responses for managing low-inertia networks. The first is set to act locally within a preselected area and the second level effectively supporting the different areas for optimal operation. After undertaking a significant number of simulations for time-series of one year, it was concluded from the results that the local control approach manages to minimize the frequency excursion effectively and influence all related attributes including the rate of change of frequency (RoCoF), frequency nadir and frequency zenith.

Learning adaptive fuzzy droop of PV contribution to frequency excursion of hybrid micro-grid during parameters uncertainties

This paper improves the performance of contribution of photovoltaic (PV) panels to the stabilization of a stand-alone hybrid microgrid during frequency excursions. Recent growth in converter-interfaced topologies and using renewable energy resources has led to decrease in inertia of isolated micro-grids (MGs) and presented some challenges to the frequency control and strength of hybrid ones. One of the effective measures to resolve mentioned problems is to deploy some potential energy resources like PV by considering a small proportion of solar panels’ generation as headroom. In addition, other distributed energy resources (DERs), which can be regarded as ancillary resources, can play an active role in the resiliency of MG’s performance in the low-inertia power systems. To this effective contribution considering some uncertainties of the microgrid parameters like the time constant of the micro-turbine, the time constant of the governor, the speed droop regulation constant as well as the load damping coefficient, an adaptive fuzzy mechanism can be proposed to determine the level of active power injection. Recurrent Adaptive Neuro Fuzzy Inference System (ANFIS) technique trains this non-linear adaptable droop to deal with uncertainties in a reasonable way. In order to ascertain parameters of membership functions in an intelligent way, this droop takes advantage of Artificial Bee Colony (ABC) algorithm based on a multi-objective. The Simulation results verify the robustness and reliability of this flexible fuzzy droop during different operating conditions as well as intermittent nature of renewable energy resources such as wind turbine generators in hybrid micro-grid.

Dimensioning Methodology of Energy Storage Systems for Power Smoothing in a Wave Energy Conversion Plant Considering Efficiency Maps and Filtering Control Techniques

This paper aims at presenting and describing a dimensioning methodology for energy storage systems (ESS), in particular for one based on flywheels, applied for the specific application of reducing power oscillation in a wave energy conversion (WEC) plant. The dimensioning methodology takes into account the efficiency maps of the storage technology as well as the effect of the filtering control techniques. The methodology is applied to the case study of a WEC plant in operation in Spain, using real power generation profiles delivered into the electric grid. The paper firstly describes the calculation procedure of the efficiency maps for the particular technology of flywheels, although it could be extended to other storage technologies. Then, the influence of using future data values in the dimensioning process and the control of the ESS operation is analysed in depth. A moving average filter (MAF) is defined to compensate for power oscillations, studying the difference between considering prediction and not doing so. It is concluded that a filtering control using future values based on a number of samples equivalent to a 4-min time order provides an important reduction in the energy storage requirements for a power generation plant. Finally, and based on the selection of storage modules previously defined, the efficiency maps, and the MAF used for delivering the power into the grid, an optimal operation strategy is suggested for the storage modules, based on a stepped switching technique. The selection of four flywheel energy storage system (FESS) modules achieves a reduction of 50% in power oscillations, covering 85% of the frequency excursions at the grid.

Decentralized Frequency Control of Battery Energy Storage Systems Distributed in Isolated Microgrid

The penetration and integration of renewable energy sources into modern power systems has been increasing over recent years. This can lead to frequency excursion and low inertia due to renewable energy sources’ intermittency and absence of rotational synchronous machines. Battery energy storage systems can play a crucial role in providing the frequency compensation because of their high ramp rate and fast response. In this paper, a decentralized frequency control system composed of three parts is proposed. The first part provides adaptive frequency droop control with its droop coefficient a function of the real-time state of charge of battery. The second part provides a fully decentralized frequency restoration. In the third part, a virtual inertia emulation improves the microgrid resilience. The presented results demonstrate that the proposed control system improves the microgrid resilience and mitigates the frequency deviation when compared with conventional ω -P droop control and existing control systems. The proposed control system is verified on Real-Time Digital Simulator (RTDS), with accurate microgrid model, nonlinear battery models and detailed switching models of power electronic converters.

A dynamic-adaptive load shedding methodology to improve frequency resilience of power systems

Frequency stability is one of the major concerns for secured operation of a power system. A severe disturbance causes rapid frequency decline, which may instigate power system blackout in an extreme case. In emergency situations, load shedding is applied to arrest frequency excursion. Therefore, a load shedding scheme needs to be sufficiently adaptive to tackle sudden generation loss. Note that conventional load shedding schemes may fail to provide satisfactory frequency response in certain cases. To address this challenge, a new dynamic-adaptive load shedding mechanism is proposed in this paper. The proposed methodology considers the actual contingency size, frequency sensitivity due to load damping, frequency response and ranking of load buses according to voltage stability index. The method ensures that the actual load cut is sufficiently close to the required load shedding to keep system frequency within the given acceptable limit. To do so, it ranks the load buses within a zone and dynamically selects appropriate feeders for load shedding. It also proportionally applies higher amount of load shedding to relatively weaker buses for taking care of the voltage stability. To examine the performance of the proposed scheme, it is applied to the IEEE 39 bus test system that is divided into six zones. It is found from simulations that for 600 MW, 700 MW and 900 MW generation loss events in zone-3, the required amounts of load shedding are 430 MW, 531 MW and 730 MW respectively to keep system frequency above 49.1 Hz. However, these quantities increase to 450 MW, 550 MW and 749 MW respectively to retain system frequency above 49.2 Hz. Furthermore, the impact of 400 MW generation loss in zone-1 is explored. It is found that after shedding additional loads in zone-1 and zone-2, the risk of tie line overloading is mitigated that eventually assures tie line security. In addition, performances of the proposed algorithm are compared to that of an adaptive technique and a semi-adaptive technique. It is noticed that the proposed methodology provides better results than the existing methods. Therefore, it can be articulated that the developed scheme ensures the correct amount of load shedding to maintain satisfactory frequency response and voltage stability.

Coordinated Control Using Backstepping of DFIG-Based Wind Turbine for Frequency Regulation in High Wind Energy Penetrated System

The expansion of renewable generation has raised some red flags in terms of power system stability, control, and management. For instance, unlike traditional synchronous energy sources, the doubly-fed induction generator- (DFIG-) based wind turbines (WTs) do not instinctively act against frequency deviations. In fact, the power electronics interfacing the generator, merely controlled to warrant maximum wind power conversion, make its output power and mechanical speed immune to the characteristics of the electric network frequency. Moreover, significant wind power penetration (WPP) promotes the retirement of many traditional generation groups, consequently curtailing the power system corresponding inertia and displacing the primary reserves that are essential to retain the frequency within an acceptable range of variation. This paper explores different control approaches, using backstepping, allowing DFIG-based WTs to engage actively in frequency regulation using a coordinated control of the rotor speed and pitch angle to regulate the system during both partial- and full-load operation modes. The first method momentarily discharges part of the kinetic energy stored in the WT spinning masses, and the second method follows a deloaded operation characteristic, so as to keep a specific power reserve that can be automatically activated at the events of frequency excursions. A study case considering an isolated power system that consists of synchronous generators, DFIG-based wind farm, static load, and a sudden frequency disturbance was performed. The simulation result in a Matlab/Simulink environment highlights the robustness and capability of the coordinated control scheme to furnish, under variant operation conditions, active power aid, consequently lifting the frequency nadir up to a superior level than that obtained with 0% wind power penetration in the system.

Load frequency regulation by de-loaded tidal turbine power plant units using fractional fuzzy based PID droop controller

This study presents a model to analyse and simulate tidal power generation for hybrid power system in the presence of highly infiltrated tidal units. The response of the hybrid system may be at risk without suitable frequency enhancement techniques. The complete load frequency model is a combination of conventional automatic generation control (AGC) and automatic voltage regulator (AVR). AVR is employed to keep the output voltage magnitude of conventional generator at a particular level. The purpose of integrating AGC with AVR is to maintain the equilibrium between system’s generation and load as well as to keep the frequency of system within suitable range. Owing to inclusion of nonconventional sources, the total inertia of the power system is eventually diminished. Conventional PID droop controller demonstrates inefficacy in diminishing the frequency deviations due to slow controlling action. This research proposes a fractional order (FO) fuzzy PID droop in de-loaded area to improvise the frequency excursion over fixed/Fuzzy PID/PID droop controllers. Imperialist competitive algorithm (ICA) is employed to tune the parameters of the controllers. ICA optimized fractional order PID droop control strategy unveils best performance (settling time = 11.65 s, undershoot amplitude = 0.26pu, performance index = 0.072e−6) over the integer order fuzzy PID control (settling time = 12.02 s, undershoot amplitude = 0.278 pu, performance index = 0.189e−6) and PID droop control (settling time = 30.68 s, undershoot amplitude = 0.95 pu, performance index = 0.215e−6). Examination of dynamic responses for abrupt changes in load request divulges the pre-eminence of proposed droop controller strategy with others controllers. The comprehensive study carried out in this paper implies that ICA optimized controller operates properly and has proven its robustness for ±10% variations in system parameters and physical constraints (Generation rate constraints, governor dead band, and time delay). This study also analyses the effect of tidal power plant contribution in the frequency regulation by inertia, primary and secondary frequency control.

A critical evaluation of grid stability and codes, energy storage and smart loads in power systems with wind generation

Existing power systems are facing new challenges in maintaining the security of the power system as the penetration of variable renewable energy technologies, such as variable speed wind turbines, increase. System non-synchronous generation replaces conventional generators as penetration of renewable generation increases. This affects system rotational inertia and limits the number of online thermal generators that can provide frequency stability services and system-wide areas voltage stability. This evolution has resulted in some changes to existing grid codes and new ancillary services. Furthermore, it could provide opportunities to address the security of the system utilizing modern smart technologies, e.g. smart loads, heat pumps and electric vehicles. The aim of this paper is to evaluate the impacts of large-scale renewable power generation on power system dynamics from the perspective of the power system operator. It focuses on the grid codes implications and challenges specifically. Synthetic inertia response opportunities from smart loads, electric vehicles and energy storage technologies and dispatching wind farms during frequency excursions are analyzed and thoroughly discussed. The key finding is that rethinking in the development of grid code requirements and market mechanisms are needed if a power system based on 100% power electronic renewable generation is to be achieved. This type of power system would need a range of technologies to provide the types of ancillary (i.e. system) services required, as none of the technologies alone can tackle all the challenges presented.

A new stochastic gain adaptive energy management system for smart microgrids considering frequency responsive loads

Islanded microgrids as flexible, adaptive and sustainable smart cells of distribution power systems should be operated in accordance to both techno-economic purposes. Motivated by this need, the microgrid operators are in charge to elevate the active accommodation of both demand-side and supply-side distributed energy resources. To that end, in this paper, a new flexible frequency dependent energy management system is proposed through which distributed generators have time varying droop controllers with a gain-adaptive strategy. Besides to cope economically with uncertainty arise frequency excursions, a new, comfort-aware and versatile frequency dependent demand response program is mathematically formulated and conducted to the energy management system. It is aimed to co-optimize the microgrid energy resources such a way the day-ahead operational costs are managed subject to a secure frequency control portfolio. The presented model is solved using a two-stage stochastic programming and by a tractable efficient mixed integer linear programming approach. The simulation results are derived in 24-h scheduling time horizon and implemented on a typical test microgrid. The effectiveness of the proposed hourly gain assignment and frequency responsive load management program has been verified thoroughly by analyzing the results.

Frequency control analysis based on unit commitment schemes with high wind power integration: A Spanish isolated power system case study

The relevant integration of wind power into the grid has involved a remarkable impact on power system operation, mainly in terms of security and reliability due to the inherent loss of the rotational inertia as a consequence of such new generation units decoupled from the grid. In these weak scenarios, the contribution of wind turbines to frequency control is considered as a suitable solution to improve system stability. With regard to frequency response analysis and grid stability, most contributions introduce wind control discuss generation tripping for isolated power systems under arbitrary power imbalance conditions. Frequency response is then analyzed for hypothetical imbalances usually ranged between 5% and 20%, and assuming averaged energy schedule scenarios. In this paper, a more realistic framework is proposed to evaluate frequency deviations by including high wind power integration. With this aim, unit commitment schemes and frequency load shedding are considered in this work for frequency response analysis under high wind power penetration. The Gran Canaria Island’s isolated power system (Spain) is used for evaluation purposes. Results provide a variety of influences from wind frequency control depending not only on the wind power integration, but also the generation units under operation, the rotational inertia reductions as well as the available reserves from each resource, aspects that have not been addressed previously in the specific literature to evaluate frequency excursions under high wind power integration.

Optimised PI‐4VPI current controller for three‐phase grid‐integrated photovoltaic inverter under grid voltage distortions

This study aims to propose optimised and robust current controller for the transformerless grid-integrated photovoltaic inverter. Challenge of ensuring improved power quality while minimising the DC-component under non-ideal grid condition is at the forefront in the photovoltaic system. In this context, an advanced current controller is designed and developed in the stationary reference frame with proportional-integral (PI) plus four-vector PI (4VPI) controllers. The PI-controller modulates the shape of fundamental current to follow its reference induced from outer DC-link loop. Meanwhile, 4VPI controllers are tuned at the multiple of grid frequencies including third-(), fifth-(), seventh-(), and ninth-() to introduce infinite gain and thereby alleviating stationary errors in the current loop of the photovoltaic inverter. The most distinguishing feature of the proposed controller is its inherent control robustness, and therefore, it operates satisfactorily against control delay time and frequency excursion. Traditionally, cumbersome trial-and-error techniques have been largely employed to adjust the PI-VPI controller gains. Nonetheless, tuning of these parameters has made many practitioners switch-off from employing the proposed controller. Given this, the particle swarm optimisation algorithm is proposed. Finally, the practicality of the proposed controller is verified through comprehensive set of simulation and experimental tests conducted on a laboratory-developed prototype using dSPACE-1104.

Estimation of solar photovoltaic energy curtailment due to volt–watt control

The widespread deployment of autonomous inverter-based solutions for mitigating voltage and frequency excursions caused by high-penetration photovoltaic (PV) systems has drawn increased attention due to their potential impact on PV production. It is now important to quantify the amount of solar energy curtailed as a result of the activation of inverter-based grid support functions (GSFs). This study proposes a methodology for estimating the impact of volt–watt on customer PV energy curtailment using smart meter voltage data. This method estimates maximum possible curtailment for a given volt–watt curve based on the customer smart meter voltage during the time period of interest. This study compares the proposed methodology with field measurements using irradiance and customer inverter data from Hawaii as well as with results from a previous simulation-driven study on the impact of advanced inverter GSF activation on PV energy curtailment. Results show that the proposed method for estimating lost PV production caused by volt–watt control aligns reasonably well with field measurements and computer simulations for hundreds of customers. The proposed method could be used to estimate customer energy curtailment, which could inform future compensation mechanisms for utilities leveraging customer-sited resources to mitigate high voltage and defer infrastructure upgrades.

An Acoustic Characterization of Prosodic Differences in Autism Spectrum Disorder and First-Degree Relatives

This study examined prosody through characterization of acoustic properties of the speech of individuals with ASD and their parents, during narration. A subset of utterances were low-pass filtered and rated for differences in intonation, speech rate, and rhythm. Listener ratings were minimally related to acoustic measures, underscoring the complexity of atypical prosody in ASD. Acoustic analyses revealed greater utterance-final fundamental frequency excursion size and slower speech rate in the ASD group. Slower speech rate was also evident in the ASD parent group, particularly parents with the broad autism phenotype. Overlapping prosodic differences in ASD and ASD Parent groups suggest that prosodic differences may constitute an important phenotype contributing to ASD features and index genetic liability to ASD among first-degree relatives.

Coordination between Demand Response Programming and Learning-Based FOPID Controller for Alleviation of Frequency Excursion of Hybrid Microgrid

In recent years, residential rate consumptions have increased due to modern appliances which require a high level of electricity demands. Although mentioned appliances can improve the quality of consumers’ lives to a certain extent, they suffer from various shortcomings including raising the electricity bill as well as serious technical issues such as lack of balance between electricity generation and load disturbances. This imbalance can generally lead to the frequency excursion which is a significant concern, especially for low-inertia microgrids with unpredictable parameters. This research proposes an intelligent combination of two approaches in order to alleviate challenges related to the frequency control mechanism. Firstly, a learning-based fractional-order proportional-integral-derivative (FOPID) controller is trained by recurrent adaptive neuro-fuzzy inference (RANFIS) in the generation side during various operational conditions and climatic changes. In the following, a decentralized demand response (DR) programming in the load side is introduced to minimize consumption rate through controllable appliances and energy storage systems (ESSs). Furthermore, parameters uncertainties and time delay, which are generally known as two main concerns of isolated microgrids, are regarded in the frequency plan of a low-inertia microgrid including renewable energy sources (RESs), and energy storage systems (ESSs). Simulation results are illustrated in three different case studies in order to compare the performance of the proposed two methods during various operational conditions. It is obvious that the frequency deviation of microgrid can be improved by taking advantage of intelligent combination of both DR program and modern control mechanism.

Practical Framework for Frequency Stability Studies in Power Systems With Renewable Energy Sources

The transition from power systems dominated by synchronous machines to systems based on converter-based generation technologies (CGTs), is weakening currently robust power systems by reducing system inertia with the replacement of synchronous generators with low-inertia CGTs. From a frequency stability viewpoint, this is resulting in faster frequency dynamics and more frequent and larger frequency excursions after system contingencies, thus significantly affecting the stability of power systems dominated by CGTs, requiring detailed stability assessments to ensure the secure integration of CGTs. In this paper, a practical framework is presented for frequency stability studies based on time domain simulations of power systems with CGTs. A fundamental part of the proposed approach is the use of a filter to first identify worst-case scenarios among various possible system operating conditions. Once these worst-case scenarios are identified, a clustering technique is used to select representative worst-case operating conditions to evaluate the frequency stability of the system using time-domain simulations. The effectiveness of the proposed framework is demonstrated on the Chilean Northern Interconnected System (NIS), where it is shown that the proposed filter is able to quickly identify worst-case scenarios for further study. Moreover, we show that the selected representative operating conditions cover a wide-range of worst-case frequency responses, demonstrating the effectiveness of the proposed tool for frequency stability analyses.

Technical and Economic Impact of the Inertia Constraints on Power Plant Unit Commitment

The whole interconnected European network is involved in the energy transition towards power systems based on renewable power electronics interfaced generation. In this context, the major concerns for both network planning and operation are the inertia reduction and the frequency control due to the progressive decommissioning of thermal power plants with synchronous generators. This paper investigates the impact of different frequency control constraints on the unit commitment of power plants resulting from market simulations. The market outputs are compared in terms of system costs, and of frequency stability performance evaluated on the basis of the rate of change of frequency and the maximum frequency excursion. The best compromise solution is found using a multiple-criteria decision analysis method, depending on the choice of the decision maker’s weighting factors. The proposed approach is tested on a real case taken from one of the most relevant future scenarios of the Italian transmission system operator. The results show how the best compromise solution that can be found depends on the decision maker preference towards cost-based or frequency stability-based criteria.