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Conference paperTindemans SH, Strbac G, 2015,
Visualising risk in generating capacity adequacy studies using clustering and prototypes
, IEEE Power and Energy Society General Meeting, 2015, Publisher: IEEEGenerating capacity adequacy studies play a significantrole in long term capacity planning. Risks of capacitydeficits are usually reported in the form of one or more averagequantities, which cannot fully convey the nature of the risksbeing faced. Chronological Monte Carlo simulations may be usedto construct comprehensive multi-dimensional risk profiles, butsuch profiles tend to be difficult to interpret. This paper proposesthe use of a clustering method to partition the risk profile intoclusters of similar outcomes with associated probabilities. Theresults are presented in accessible tabular form, and prototypicalscenarios can be analysed in detail to provide further insight.
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Conference paperMylvaganam T, Astolfi A, 2015,
Control of Microgrids Using a Differential Game Theoretic Framework
, Conference on Decision and Control -
Journal articlePalladino M, Vinter RB, 2015,
Regularity of the Hamiltonian Along Optimal Trajectories
, SIAM Journal on Control and Optimization, Vol: 53, Pages: 1892-1919, ISSN: 1095-7138This paper concerns state constrained optimal control problems, in which the dynamic constraint takes the form of a differential inclusion. If the differential inclusion does not depend on time, then the Hamiltonian, evaluated along the optimal state trajectory and the co-state trajectory, is independent of time. If the differential inclusion is Lipschitz continuous, then the Hamiltonian, evaluated along the optimal state trajectory and the co-state trajectory, is Lipschitz continuous. These two well-known results are examples of the following principle: the Hamiltonian, evaluated along the optimal state trajectory and the co-state trajectory, inherits the regularity properties of the differential inclusion, regarding its time dependence. We show that this principle also applies to another kind of regularity: if the differential inclusion has bounded variation w.r.t. time, then the Hamiltonian, evaluated along the optimal state trajectory and the co-state trajectory, has bounded variation. Two applications of these newly found properties are demonstrated. One is to derive improved conditions which guarantee the nondegeneracy of necessary conditions of optimality in the form of a Hamiltonian inclusion. The other application is to derive new conditions under which minimizers in the calculus of variations have bounded slope. The analysis is based on a recently proposed, local concept of differential inclusions that have bounded variation w.r.t. the time variable, in which conditions are imposed on the multifunction involved, only in a neighborhood of a given state trajectory.
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Conference paperBoem F, Xu Y, Fischione C, et al., 2015,
A distributed pareto-optimal dynamic estimation method
, 2015 European Control Conference (ECC 2015), Publisher: IEEE, Pages: 3673-3680In this paper, a novel distributed model-based prediction method is proposed using sensor networks. Each sensor communicates with the neighboring nodes for state estimation based on a consensus protocol without centralized coordination. The proposed distributed estimator consists of a consensus-filtering scheme, which uses a weighted combination of sensors information, and a model-based predictor. Both the consensus-filtering weights and the model-based prediction parameter for all the state components are jointly optimized to minimize the variance and bias of the prediction error in a Pareto framework. It is assumed that the weights of the consensus-filtering phase are unequal for the different state components, unlike consensus-based approaches from literature. The state, the measurements, and the noise components are assumed to be individually correlated, but no probability distribution knowledge is assumed for the noise variables. The optimal weights are derived and it is established that the consensus-filtering weights and the model-based prediction parameters cannot be designed separately in an optimal way. The asymptotic convergence of the mean of the prediction error is demonstrated. Simulation results show the performance of the proposed method, obtaining better results than distributed Kalman filtering.
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Conference paperGiannelos S, Konstantelos I, Strbac G, 2015,
Option value of Soft Open Points in distribution networks
, IEEE Powertech, Publisher: IEEEWe propose a novel stochastic planning model thatconsiders investment in conventional assets as well as in SoftOpen Points, as a means of treating voltage and thermalconstraints caused by the increased penetration of renewabledistributed generation (DG) sources. Soft Open Points areshown to hold significant option value under uncertainty;however, their multiple value streams remain undetected undertraditional deterministic planning approaches, potentiallyundervaluing this technology and leading to a higher risk ofstranded assets.
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Journal articleStrbac G, Konstantinidis CV, Moreno R, et al., 2015,
It's All About Grids
, IEEE POWER & ENERGY MAGAZINE, Vol: 13, Pages: 61-75, ISSN: 1540-7977 -
Conference paperCheng C, Evangelou SA, Arana C, et al., 2015,
Active Variable Geometry Suspension robust control for improved vehicle ride comfort and road holding
, American Control Conference (ACC), 2015, Publisher: IEEE, Pages: 3440-3446, ISSN: 0743-1619This paper investigates the design of robust ℋ∞ control for road vehicle Series Active Variable Geometry Suspension (SAVGS). The objective is to improve ride comfort and road holding, while guaranteeing operation inside existing physical constraints. The study utilizes a nonlinear quarter car model that represents accurately the vertical dynamics and geometry of one quarter of a high performance car with a double wishbone suspension. The control objective is to reduce the body vertical acceleration, tire deflection and suspension travel under the impact of road perturbations. Therefore, the selection of the weighting functions for a linear ℋ∞ control, designed for the linearized quarter car, is based on these objectives. The proposed controller is then applied to the nonlinear quarter car model and investigated by nonlinear simulation for a range of road disturbance inputs. The results show that the designed controller when applied on the SAVGS is effective in improving the vehicle ride comfort and road holding.
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Journal articleYe Y, Papadaskalopoulos D, Strbac G, 2015,
Factoring Flexible Demand Non-Convexities in Electricity Markets
, IEEE TRANSACTIONS ON POWER SYSTEMS, Vol: 30, Pages: 2090-2099, ISSN: 0885-8950 -
Journal articleFeng Z, Kerrigan EC, 2015,
Latching-Declutching Control of Wave Energy Converters Using Derivative-Free Optimization
, IEEE Transactions on Sustainable Energy, Vol: 6, Pages: 773-780, ISSN: 1949-3029We consider predictive control of a wave energy converter(WEC) that can switch between three modes: 1) powergeneration; 2) declutched with no power generation; or 3) latchedwith zero velocity. We propose a formulation that turns the optimalcontrol problem into a small dimensional discrete optimizationproblem, where the only decision variables are bounds onthe latching time and power take-off (PTO) time, whereas theobjective function is computed from the trajectory of a hybridsystem with linear dynamics in each sample interval. The optimizationproblem is solved using a novel derivative-free algorithmthat exploits the quantization of the decision variables in orderto reduce the number of function evaluations. Two closed-loopformulations are also studied within a receding horizon implementation:the first one uses past wave information and can double theenergy generation compared to the uncontrolled case, while thesecond formulation uses predictions of future waves and is able toresult in a further increase in energy generation. The benefits ofcodesigning the physical system and controller is compared to thesequential approach of first optimizing the physical system withoutcontrol, followed by controller design.
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Journal articleZhong WX, Hui SYR, 2015,
Maximum energy efficiency tracking for wireless power transfer systems
, IEEE Transactions on Power Electronics, Vol: 30, Pages: 4025-4034, ISSN: 0885-8993A method for automatic “maximum energy efficiency tracking” operation for wireless power transfer (WPT) systems is presented in this paper. Using the switched-mode converter in the receiver module to emulate the optimal load value, the proposed method follows the maximum energy efficiency operating points of a WPT system by searching for the minimum input power operating point for a given output power. Because the searching process is carried out on the transmitter side, the proposal does not require any wireless communication feedback from the receiver side. The control scheme has been successfully demonstrated in a two-coil system under both weak and strong magnetic coupling conditions. Experimental results are included to confirm its feasibility.
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Conference paperSootla A, Oyarzun DA, Angeli D, et al., 2015,
Shaping Pulses to Control Bi-Stable Biological Systems
, American Control Conference 2015, Publisher: IEEE, Pages: 3138-3143In this paper, we present a framework for shaping pulses to control biological systems, and specifically systems in synthetic biology. By shaping we mean computing the magnitude and the length of a pulse, application of which results in reaching the desired control objective. Hence the control signals have only two parameters, which makes these signals amenable to wetlab implementations. We focus on the problem of switching between steady states in a bistable system. We show how to estimate the set of the switching pulses, if the trajectories of the controlled system can be bounded from above and below by the trajectories of monotone systems. We then generalise this result to systems with parametric uncertainty under some mild assumptions on the set of admissible parameters, thus providing some robustness guarantees. We illustrate the results on some example genetic circuits.
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Conference paperMylvaganam T, Astolfi A, 2015,
A differential game approach to formation control for a team of agents with one leader
, American Control Conference -
Journal articleJunyent-Ferre A, Pipelzadeh Y, Green TC, 2015,
Blending HVDC-link energy storage and offshore wind turbine inertia for fast frequency response
, IEEE Transactions on Sustainable Energy, Vol: 6, Pages: 1059-1066, ISSN: 1949-3029This paper explores the benefits of combining the dc-link energy storage of a voltage source converter-based high-voltage dc (VSC-HVDC) link and the kinetic energy storage from wind turbines to facilitate in fast primary frequency control and system inertia to an ac network. Alongside physical and analytical justifications, a method is proposed which blends the energy stored in the HVDC link with the power control capabilities of the wind turbines to provide frequency response that is fast while not requiring excessive volume of capacitance nor demanding performance requirements on the wind turbines.
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Conference paperChaffey G, Green TC, 2015,
Reduced DC circuit breaker requirement on mixed converter HVDC networks
, PowerTech Eindhoven 2015, Publisher: IEEERecently proposed meshed HVDC networks include both converters and DC circuit breakers, and the fault currents experienced and therefore the capacity requirement of circuit breakers are dependent on the topology of converters used on the network. This paper analyses the difference in fault currents seen in various network configurations utilising fault-feeding and fault-blocking converters. Results are presented showing the reduced fault currents seen in the regions of the DC network where fault current limiting converters have been implemented, which could have an impact on the topology, current rating and therefore size and cost of the circuit breaker.
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Conference paperJudge PD, Green TC, 2015,
Dynamic thermal rating of a Modular Multilevel Converter HVDC link with overload capacity
, PowerTech Eindhoven 2015, Publisher: IEEEThe power rating of Modular Multilevel Converter based HVDC has increased rapidly over the past decade, with individual links in the gigawatt power range now technically feasible and further power increases on the horizon. Such large links may be required to provide ancillary services such as fast frequency response or emergency power re-routing in the event of a system disturbance. Providing such services may require converters to be designed with overload capacity. This paper examines how the thermal aspects of semiconductor devices may impact the operation of such converters and how the exploitation of short-term thermal dynamics may lead to dynamic overload rating.
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Conference paperTindemans SH, Trovato V, Strbac G, 2015,
Frequency control using thermal loads under the proposed ENTSO-E Demand Connection Code
, PowerTech 2015, Publisher: IEEE, Pages: 1-6Thermal loads such as refrigerators and electric space heaters use temperature hysteresis controllers that are insensitive to small temperature fluctuations. This results in an ability to modulate their power consumption, thus providing cost-effective frequency support, balancing services and energy arbitrage. In order to partially realise these benefits, ENTSO-E has proposed a mandatory frequency support service for thermal loads in its Network Code on Demand Connection. This is to be implemented as a proportional shift of the setpoint temperature in accordance with frequency deviations. In this paper we argue that this implementation choice results in an unpredictable response that depends strongly on controller details. Furthermore, it restricts the flexibility to implement advanced controllers that deliver multiple services simultaneously. We present a case study that demonstrates very different frequency response patterns from three controllers that are each compatible with the proposed Code. Alternative implementations of the code and controllers are presented to illustrate the scope for improvement.
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Conference paperCalvo JL, Ramirez Torrealba PJ, Tindemans S, et al., 2015,
Cost-Benefit Analysis of Unreliable System Protection Scheme Operation
, IEEE PowerTech 2015, Publisher: IEEESystem Protection Schemes (SPS) have the potential to greatly enhance the utilization of the network, often by automatically disconnecting generators in response to contingency events. However, malfunctions of such systems may expose the system to harmful blackouts. The operation of unreliable SPS is therefore subject to a cost-benefit balance between the benefits of increased system utilization and the risk of outages. This paper studies this trade-off in a year-round basis. The problem is firstly stated from a centralized perspective to probabilistically minimize the operational costs for a whole operating year. A case study based on a basic simple representation of the Great Britain system is considered. The results show great annual benefits from equipping the SPS with multiple generation disconnection systems, which are mainly associated with critical operating conditions. However, it is demonstrated that redundant SPS configurations do not necessarily reduce the levels of operational risk exposure.
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Journal articleAnagnostou G, Pal BC, 2015,
Impact of Overexcitation Limiters on the Power System Stability Margin Under Stressed Conditions
, IEEE Transactions on Power Systems, Vol: 31, Pages: 2327-2337, ISSN: 1558-0679This paper investigates the impact of the overexcitation limiters (OELs) on the stability margin of a power system which is operating under stressed conditions. Several OEL modeling approaches are presented and the effect of their action has been examined in model power systems. It is realized that, more often than not, OEL operating status goes undetected by existing dynamic security assessment tools commonly used in the industry. It is found that the identification and accurate representation of OELs lead to significantly different transient stability margins. Unscented Kalman filtering is used to detect the OEL activation events. In the context of stressed system operation, such quantitative assessment is very useful for system control. This understanding is further reinforced through detailed studies in two model power systems.
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Journal articleArana C, Evangelou SA, Dini D, 2015,
Series Active Variable Geometry Suspension application to chassis attitude control
, IEEE/ASME Transactions on Mechatronics, Vol: PP, ISSN: 1083-4435This paper explores the application of the recently introduced Series Active Variable Geometry Suspension (SAVGS) to the control of chassis attitude motions and the directional response of cars. A co-design methodology, involving a component dimensioning framework and a multi-objective control scheme, is developed to maximize the SAVGS control capabilities while respecting vehicle and actuator design constraints. The dimensioning framework comprises: a steady-state mathematical model based on the principle of virtual work; a parameter sensitivity analysis that sheds light on the dependencies that exist between the properties of the passive suspension, the SAVGS and the chassis; and an algorithm to size the main SAVGS components for any given vehicle and steady-state performance objectives. The general multi-objective control scheme is presented for general application, and the particular case of combined chassis attitude control and overturning couple distribution control is developed in detail. The proposed scheme is subsequently applied to a high performance sports car and a fully laden SUV and tested under a wide range of operating conditions through the simulation of standard open-loop maneuvers. Results demonstrate the SAVGS potential to favorably regulate the attitude motions and directional response in both vehicle classes.
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Conference paperUstinova T, Woolf M, Ortega Calderon JE, et al., 2015,
Analysis of Customers' Performance in Industrial & Commercial Demand Side Response Trials
, 23rd International Conference on Electricity Distribution (CIRED 2015) -
Conference paperSchofield J, Carmichael R, Tindemans S, et al., 2015,
Experimental validation of residential consumer responsiveness to dynamic time-of-use pricing
, 23rd International Conference on Electricity Distribution (CIRED)This paper describes the first analysis from the LowCarbon London (LCL), residential dynamic time-of-use(dToU) pricing trial that took place in the London areaduring 2013. High price induced peak reductions fornetwork constraint management are investigatedalongside the temporal availability of demand responsefor supply balancing. By examining both these use caseswe identify potential conflicts between network andsystem objectives. Demand response results are stratifiedby a ranking metric for engagement with the dToU tariffas well as household occupancy and socio-economicclassification.
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Journal articlePapadaskalopoulos D, Strbac G, 2015,
Nonlinear and Randomized Pricing for Distributed Management of Flexible Loads
, IEEE Transactions on Smart Grid, Vol: 7, Pages: 1137-1146, ISSN: 1949-3061Price-based management of distributed energy resources within microgrids is continuously gaining ground due to scalability and privacy limitations of centralized architectures. However, the concentration of flexible loads' response to the lowest-priced periods yields inefficient solutions. A previously proposed measure imposing a flexibility restriction on flexible loads might raise acceptability and feasibility concerns by the users. This paper develops a novel fully price-based approach where this hard restriction is replaced by a soft nonlinear price signal. This signal is customized to the operating properties of the different flexible load types by penalizing the square of the demand and the duration of cycle delay of loads with continuously adjustable power levels and deferrable cycles, respectively. This approach is shown to produce more efficient solutions than the flexibility restriction measure for both types of loads. For the latter type, randomization of the nonlinear prices brings additional benefits, especially in low operating diversity cases. These contributions are supported by case studies on a microgrid test system with electric vehicles and wet appliances used as representative examples of the above flexible load types.
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Journal articleTrutnevyte E, Strachan N, Dodds PE, et al., 2015,
Synergies and trade-offs between governance and costs in electricity system transition
, Energy Policy, Vol: 85, Pages: 170-181, ISSN: 1873-6777Affordability and costs of an energy transition are often viewed as the most influential drivers. Conversely, multi-level transitions theory argues that governance and the choices of key actors, such as energy companies, government and civil society, drive the transition, not only on the basis of costs. This paper combines the two approaches and presents a cost appraisal of the UK transition to a low-carbon electricity system under alternate governance logics. A novel approach is used that links qualitative governance narratives with quantitative transition pathways (electricity system scenarios) and their appraisal. The results contrast the dominant market-led transition pathway (Market Rules) with alternate pathways that have either stronger governmental control elements (Central Co-ordination), or bottom-up proactive engagement of civil society (Thousand Flowers). Market Rules has the lowest investment costs by 2050. Central Co-ordination is more likely to deliver the energy policy goals and possibly even a synergistic reduction in the total system costs, if policies can be enacted and maintained. Thousand Flowers, which envisions wider participation of the society, comes at the expense of higher investment and total system costs. The paper closes with a discussion of the policy implications from cost drivers and the roles of market, government and society.
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Journal articleAgalgaonkar Y, Pal BC, Jabr RA, 2015,
Stochastic Distribution System OperationConsidering Voltage Regulation Risks in thePresence of PV Generation
, IEEE Transactions on Sustainable Energy, Vol: PP, Pages: 1-10, ISSN: 1949-3037Variable over voltage, excessive tap counts andvoltage regulator (VR) runaway condition are major operationalchallenges in distribution network while accommodating generationfrom Photovoltaics (PVs). The conventional approach toachieve voltage control based on off-line simulation for voltage setpoint calculation does not consider forecast errors. In this work,a stochastic optimal voltage control strategy is proposed whileconsidering load and irradiance forecast errors. Stochastic operationalrisks such as overvoltage and VR runaway are definedthrough a chance constrained optimization (CCO) problem. Thisclassical formulation to mitigate runaway is further improvedby introducing a stochastic index called the Tap Tail Expectation.Operational objectives such as power losses and excessive tapcount minimization are considered in the formulation. A samplingapproach is proposed to solve the CCO. Along with other voltagecontrol devices, the PV inverter voltage support features arecoordinated. The simulation study is performed using a realisticdistribution system model and practically measured irradianceto demonstrate the effectiveness of the proposed technique.The proposed approach is a useful operational procedure fordistribution system operators. The approach can minimize feederpower losses, avoid voltage violations and alleviate VR runaway.
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Journal articleScarciotti G, Praly L, Astolfi A, 2015,
Invariance-like theorems and “lim inf” convergence properties
, IEEE Transactions on Automatic Control, Vol: 61, Pages: 648-661, ISSN: 1558-2523Several theorems, inspired by the Krasovskii-LaSalle invariance principle, to establish “lim inf” convergence results are presented in a unified framework. These properties are useful to “describe” the oscillatory behavior of the solutions of dynamical systems. The theorems resemble “lim inf” Matrosov and Small-gain theorems and are based on a “lim inf” Barbalat's Lemma. Additional technical assumptions to have “lim” convergence are given: the “lim inf”/“lim” relation is discussed in-depth and the role of some of the assumptions is illustrated by means of examples.
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Conference paperTrovato V, Tindemans SH, Strbac G, 2015,
Designing effective frequency response patterns for flexible thermostatic loads
, 2015 IEEE 15th International Conference on Environment and Electrical Engineering (EEEIC), Publisher: IEEE, Pages: 1003-1008Future power systems will have to integrate large amounts of wind and solar generation to drastically reduce CO2 emissions. Achieving this goal comes at the cost of a reduced level of the system inertia and an increased need for fast response services. Previous research has shown the effectiveness of thermostatically controlled loads (TCLs) providing frequency response, and the ability to accurately control the aggregate power consumption of TCLs. In this paper, we explore the design space of frequency response patterns of flexible TCLs. Two distinct frequency response implementations are presented. The first makes the TCLs' power consumption a linear function of system frequency and/or its rate of change; in the second, TCLs respond to a frequency event tracking a pre-programmed reference power profile. Computer simulations illustrate strengths and weaknesses of the proposed implementations in the context of the GB 2020 Gone Green scenario.
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Journal articleMueller MA, Angeli D, Allgoewer F, 2015,
On Necessity and Robustness of Dissipativity in Economic Model Predictive Control
, IEEE Transactions on Automatic Control, Vol: 60, Pages: 1671-1676, ISSN: 1558-2523In this paper, we study a dissipativity property which was recently used in several results on economic model predictive control to ensure optimal operation of a system at steady-state as well as stability. In particular, we first investigate whether this dissipativity property is not only sufficient, but also necessary for optimal steady-state operation. In the most general case, this is not true; nevertheless, under an additional controllability assumption, we show that dissipativity is in fact necessary. Second, we provide a robustness analysis of the dissipativity property with respect to changes in the constraint set, which can result in a change in the considered supply rate.
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Journal articleChen HT, Tan S-C, Hui SYR, 2015,
Analysis and Modeling of High-Power Phosphor-Coated White Light-Emitting Diodes With a Large Surface Area
, IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol: 30, Pages: 3334-3344, ISSN: 0885-8993- Author Web Link
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Conference paperKwan CH, Kkelis G, Aldhaher S, et al., 2015,
Link efficiency-led design of mid-range inductive power transfer systems
, Pages: 1-7 -
Journal articleChen B, Pin G, Ng WM, et al., 2015,
A parallel prefiltering approach for the identification of a biased sinusoidal signal: theory and experiments
, International Journal of Adaptive Control and Signal Processing, Vol: 29, Pages: 1591-1608, ISSN: 1099-1115The problem of estimating the amplitude, frequency, and phase of an unknown sinusoidal signal from a noisy-biased measurement is addressed in this paper by a family of parallel prefiltering schemes. The proposed methodology consists in using a pair of linear filters of specified order to generate a suitable number of auxiliary signals that are used to estimate—in an adaptive way—the frequency, the amplitude, and the phase of the sinusoid. Increasing the order of the prefilters improves the noise immunity of the estimator, at the cost of an increase of the computational complexity. Among the whole family of estimators realizable by varying the order of the filters, the simple parallel prefilters of orders 2 + 2 and 3 + 3 are discussed in detail, being the most attractive from the implementability point of view. The behavior of the two algorithms with respect to bounded external disturbances is characterized by input-to-state stability arguments. Finally, the effectiveness of the proposed technique is shown both by comparative numerical simulations and by a real experiment addressing the estimation of the frequency of the electrical mains from a noisy voltage measurement.
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