Frequency Control Loop Research Articles

PV Module Integrated Dual Boost Isolated dc-dc Converter Fed Three-Phase Micro-Inverter

Abstract This paper presents a photovoltaic (PV) array-integrated dual-boost isolated dc-dc converter-fed three-phase micro-inverter for the grid-connected applications. The coupled inductor and high-frequency DC link-based dc-dc converter offer more advantages over the conventional dc-dc converters used in the grid-tied PV system. A controller with an inner voltage control loop and frequency control loop is proposed to operate the converters of a micro-inverter to transfer the maximum power generated on the PV array to the grid. The inner voltage control loop generates the required control signal for the proposed high step-up dc-dc converter to extract the maximum possible power from the PV array during sunny days. It eliminates the need of cascaded connection of dc-dc converters. High maximum power point tracking (MPPT) accuracy, high voltage gain, and high energy conversion efficiency can be attained with the proposed system. The principles of operation and design considerations of the dual-boost isolated dc-dc converter-fed micro-inverter are presented. The MATLAB/Simulink simulation results are presented to validate the performance of the proposed three-phase micro-inverter.

Control Strategy for Dispatchable Distributed Energy Resources in Islanded Microgrids

This paper proposes a multi-input muti-output control strategy for dispatchable power electronic based distributed energy resources (DERs) in islanded mode operation of microgrids. The controller is designed based on the dq model of the DER unit. Cascade, feedforward, and internal model control principle, incorporating the theory of integral control and repetitive controller, are used. The proposed controller regulates the voltage at the output of DERs under any load (balanced, unbalanced, and harmonic) variation and/or fault condition. The stability of the closed-loop system has been investigated. Enhanced phase-locked loop (EPLL) is used to determine the system variables in the dq frame under unbalanced (and balanced) operating conditions. The structure of the EPLL is further modified to regulate the microgrid frequency and a stability analysis of proposed frequency control loop is presented. The performance of the proposed controller is verified through test case studies, carried out on an autonomous microgrid with single and multiple DER units.

Impact Analysis of a Bias Injection Cyber-Attack on a Power Plant

This paper addresses the concept of a bias injection cyber-attack on the load frequency control loop of a single-area power plant. The system evolves in the discrete-time domain and a convex and compact set of polyhedral state constraints represents a valid domain of safe operation under the effect of a stabilizing output-feedback controller. Whenever the safety constraints are violated, an alarm alerts the control center to a security breach. An attacker gains access to frequency sensor measurements and corrupts the data transferred to the automatic generation control unit. The objective of the adversary is to drive the electrical frequency to a safety-critical steady-state value without triggering an alarm. An analysis based on robust invariance concepts assesses the vulnerability of the system on this particular scenario and determines the maximal impact that such an attack can have without activating an alarm. Simulation studies highlight the effect of the cyber-attack on the physical plant.

Optimized coordinated control of frequency and voltage for distributed generating system using Cuckoo Search Algorithm

Abstract The distributed generating system may employ hybrid power system consists of diesel and wind power generating units based on synchronous and induction generator (IG), respectively, to supply small isolated load. Frequency and voltage controls are major problems for such system as smaller synchronous generator (SG) offers lesser inertia and IG draws reactive power for its operation. In this paper, the voltage control loop governed by automatic voltage regulator of SG has been integrated with frequency control loop to yield optimized transient responses for frequency and voltage deviations. The linearized model of hybrid system with coordinated control of voltage and frequency has been developed. The dynamic responses of frequency and voltage deviations are compared for different active and reactive load disturbances. The gains of controller of SG and Static Var Compensator (SVC) at the terminal of IG have been optimized with Cuckoo Search Algorithm to minimize frequency and terminal voltage deviations.

A Frequency Domain Decoupling Method and Multivariable Controller Design for Turbofan Engines

Advanced engine control, such as active control, often involves controlling engine with more control variables at a wider frequency range. When the conventional successive-loop-closure technology is applied to design controller with more objectives and controls, it will lead to multi-loop interleaving, and prevent the responses of the lower frequency control loops from meeting the requirements and hence exploiting the potential of the engine. A modified multivariable decoupling method in frequency domain is presented to improve the robustness and decoupling of the system and proved that it is equivalent to Rosenbrock’s modified method at a single frequency, and then extended to a weighted formulation on a certain frequency range. Finally, the modified method is used to design steady-state decoupling controller, and a multivariable gain-scheduling controller for large transient is formulated as the weighted function of a family of designed decoupling controllers, whose weights are chosen as transfer function of neural network optimized by BP algorithm. Computational results are presented for controlling the small and large transient of a military aircraft turbofan engine, and the simulations show that compared with standard Hawkins’s method, the resultant compensated system designed by the modified method is of better good decoupling and robustness.

Frequency regulation of multi‐area power systems with plug‐in electric vehicles considering communication delays

Dynamics of vehicle-to-grid service to frequency regulation may suffer from the uncertainties arising from the spatial and temporal diversities of the large-scale plug-in vehicles (PEVs). Communication delays incurred both in the aggregation of the PEVs and in load frequency control (LFC) loop also degrade the dynamics and even destabilise the system. This study designs the LFC scheme considering those uncertainties. First, a dynamic model of individual PEV based on equivalent circuit and an aggregated model for PEVs group considering driving behaviours, battery characteristics, and delays are developed. Then, a state-space model with time delays and uncertainties is constructed to model the closed-loop LFC scheme with PEVs in primary control loop. After constructing the relationships among control gains, robust performances, and delays, a robust proportional–integral–derivative-type LFC scheme design method is developed by using linear matrix inequality and particle swarm optimisation algorithm. Case studies based on a three-area LFC system demonstrate that the PEVs contribute to the frequency regulation under different load disturbances and that the proposed LFC scheme successfully suppresses frequency fluctuations in the presence of delays and provides robustness against the uncertainties arising from the PEVs.

A Single-Inductor Multiple-Output Auto-Buck–Boost DC–DC Converter With Tail-Current Control

A single-inductor multiple-output (SIMO) auto-buck–boost dc–dc converter with proposed tail-current control to achieve fast and robust reference tracking of inductor current is proposed in this paper. Moreover, the proposed backward– $V_{x1}$ control algorithm and frequency-control loop enable the proposed SIMO dc–dc converter to operate at a fixed switching frequency and achieve high power-conversion efficiency in both the buck-dominated and boost-dominated cases. The proposed SIMO auto-buck–boost dc–dc converter has four output channels and is implemented in a standard 0.35-μm CMOS process. Measurement results show that the proposed dc–dc converter achieves a peak efficiency of more than 91% at a total output power of about 0.5 W and load transient response time of less than 80 μs.

A novel optimal frequency control strategy for an isolated wind–diesel hybrid system with energy storage devices

This article presents the design of a new effective control strategy to enhance frequency stability of an isolated micro-grid-based wind–diesel hybrid system. The suggested control methodology involves load frequency control coordinated with battery energy storage systems. A recently developed meta-heuristic algorithm called multi-verse optimizer was applied to design an intelligent load frequency control scheme in the aim to handle the frequency fluctuation due to load changes and wind farm integration. The multi-verse optimizer algorithm was used to optimize the proportional–integral–derivative controller parameters for the load frequency control loop. The proposed controller was coordinated with two different kinds of storage system, which are redox flow batteries and electric vehicles. To demonstrate the effectiveness of the proposed control strategy, the simulation was performed under step load changes and then was extended with doubly-fed induction generator wind farm integration. Furthermore, to show the potential of multi-verse optimizer algorithm, a comparative study was done with other approaches available in the literature. In addition, robustness analysis was carried out. The obtained simulation results show that the proposed strategy is a very effective means for providing robust load frequency control controller and to avoid hybrid system instability. Furthermore, the system frequency can be improved using an optimal power management of the stored energy in both redox flow batteries and electric vehicles to compensate the load frequency control capability of the diesel groups, which allow to the possibility of integration of a large penetration of wind farms. In summary, the proposed control strategy may be helpful to identify the needed load frequency control capacity in the presence of dispersed generation’s units.

A Single-Inductor Multiple-Output Auto-Buck-Boost DC–DC Converter With Autophase Allocation

A single-inductor multiple-output (SIMO) converter with auto-buck-boost feature is presented in this paper. A seamless auto-buck-boost scheme for SIMO dc–dc converter using time-multiplexing control is proposed. Additionally, with the proposed first-order phase-locked loop (FOPLL) and autophase allocation, this circuit solves the problem of unbalanced loadings of different channels, and it also keeps the locking time of the inductor current sufficiently short to minimize average inductor current for attaining higher efficiency. By combining with all channel controllers, FOPLL not only allows fast load-transient response without degrading the power efficiency, but it also reduces controller order of the frequency control loop, attenuates the noise injected from charge pump, and achieves robust stability. The scheme enables the dc–dc converter to operate from wide input and output ranges. Implemented in a 0.35-μm CMOS technology, the chip area is 5000 μm × 1850 μm, including ESD test pads. The switching frequency is fixed at 0.25 MHz. The load-transient response time is less than 100 μs. The proposed converter achieves a peak efficiency of 89% and maximum output power up to 1.46 W with efficiency of 70%.

Autopilot for frequency-modulation atomic force microscopy.

One of the most challenging aspects of operating an atomic force microscope (AFM) is finding optimal feedback parameters. This statement applies particularly to frequency-modulation AFM (FM-AFM), which utilizes three feedback loops to control the cantilever excitation amplitude, cantilever excitation frequency, and z-piezo extension. These loops are regulated by a set of feedback parameters, tuned by the user to optimize stability, sensitivity, and noise in the imaging process. Optimization of these parameters is difficult due to the coupling between the frequency and z-piezo feedback loops by the non-linear tip-sample interaction. Four proportional-integral (PI) parameters and two lock-in parameters regulating these loops require simultaneous optimization in the presence of a varying unknown tip-sample coupling. Presently, this optimization is done manually in a tedious process of trial and error. Here, we report on the development and implementation of an algorithm that computes the control parameters automatically. The algorithm reads the unperturbed cantilever resonance frequency, its quality factor, and the z-piezo driving signal power spectral density. It analyzes the poles and zeros of the total closed loop transfer function, extracts the unknown tip-sample transfer function, and finds four PI parameters and two lock-in parameters for the frequency and z-piezo control loops that optimize the bandwidth and step response of the total system. Implementation of the algorithm in a home-built AFM shows that the calculated parameters are consistently excellent and rarely require further tweaking by the user. The new algorithm saves the precious time of experienced users, facilitates utilization of FM-AFM by casual users, and removes the main hurdle on the way to fully automated FM-AFM.

Harmony Search Algorithm-Based Controller Parameters Optimization for a Distributed-Generation System

This paper presents the utilization of a harmony search algorithm (HSA) to optimally design the proportional-integral (PI) controllers of a grid-side voltage source cascaded converter with two additional loops for smooth transition of islanding and resynchronization operations in a distributed-generation (DG) system. The first loop is the frequency-control loop which is superimposed on the real power setpoint of the cascaded controller of the voltage-source converter to minimize the frequency variation during the transition from the grid mode to islanding mode. The second loop is the resynchronization loop which reduces the phase shift of the ac voltages of the DG with the utility grid ac voltages during islanding operation leading to a successful grid reconnection event. The response surface methodology (RSM) is used to build the mathematical model of the system dynamic responses in terms of PI controllers' parameters. The effectiveness of the proposed PI control scheme optimized by the HSA is then compared with that optimized by both genetic algorithm and conventional generalized reduced gradient techniques. The HSA code is built using the MATLAB software program. The validity of the proposed system is verified by the simulation results which are performed using PSCAD/EMTDC.

Resonance frequency control for the KOMAC 100-MeV drift tube linac

A 100-MeV proton accelerator has been developed, and the operation and beam service started at the Korea Multi-purpose Accelerator Complex (KOMAC) in July 2013. The accelerator consists of a 50-keV proton injector, a 3-MeV radio-frequency quadrupole (RFQ) and a 100-MeV drift tube linac (DTL). The resonance frequencies of the DTL tanks are controlled by using the resonance frequency control cooling system (RCCS), installed at every DTL tank. Until now, the RCCS has been operating in the constant temperature mode. If the system is to be stabilized better, the RCCS must be operated in the frequency control mode. For this purpose, studies, including the relation between the resonance frequency and RCCS operation temperature, were done under various conditions. In this paper, the preparations for the frequency control loop of the RCCS are described.

Robust Frequency Control in an Islanded Microgrid: H∞ and μ-Synthesis Approaches

This paper addresses robust frequency control in an islanded ac microgrid (MG). In an islanded MG with renewable sources, load change, wind power fluctuation, and sun irradiation power disturbance as well as dynamical perturbation, such as damping coefficient and inertia constants, can significantly influence the system frequency, and hence the MG frequency control problem faces some new challenges. In response to these challenges, in this paper, ${H} _{ {\infty }}$ and $ {\mu }$ -synthesis robust control techniques are used to develop the secondary frequency control loop. In the proposed control scheme, some microsources (diesel engine generator, micro turbine, and fuel cell) are assumed to be responsible for balancing the load and power in the MG system. The synthesized ${H} _{ {\infty }}$ and $ {\mu }$ -controllers are examined on an MG test platform, and the controllers’ robustness and performance are evaluated in the presence of various disturbances and parametric uncertainties. The results are compared with an optimal control design. It is shown that the $ {\mu }$ -synthesis approach due to considering structured/parametric uncertainties provides better performance than the ${H} _{ {\infty }}$ control method.

Methods of Correcting Frequency Errors in Time-Type Analog-to-Digital Displacement Transducers

An optoelectronic time-type displacement transducer with different methods of correction of frequency errors is considered. It is shown that the use of an automatic frequency control loop represents the most promising method. A functional circuit of the device is developed and the basic functional dependences are presented.

A novel voltage and frequency controller for standalone DFIG based Wind Energy Conversion System

This paper presents a new speed-sensorless control strategy for a stand-alone doubly-fed induction generator supplying energy to an isolated load. The method is based on the root mean square (rms) detection scheme. The generated stator voltage is controlled via rotor currents. Amplitude of stator voltage and its frequency are controlled simultaneously. The output signals from the voltage controllers are the reference signals for the rotor current amplitude and frequency of the stator voltage is regulated with the help of frequency control loop. This developed direct voltage control method is applicable for both the balanced and unbalanced load and also for standalone and grid connected mode. The control pulses for the rotor side converter is supplied by the hysteresis controller which is operated on the error signal calculated between actual and reference rotor currents. This paper also provides the brief idea about the voltage and frequency control on modern Autonomous DFIG based Wind Energy Systems via single phase mathematical model of standalone DFIG system. A short state-of-the-art review on mechanical position/speed sensorless control schemes for autonomous DFIG based WESs is presented, which helps the present researcher and students working in this area. These include stator flux oriented control techniques; direct voltage control techniques; MARS observer based techniques for autonomous DFIG-based variable-speed WESs. Simulation results obtained from a 2MVA DFIG system, prototype in MATLab/Simulink, are presented and discussed in this paper.

Frequency Tuning of Work Modes inZ-Axis Dual-Mass Silicon Microgyroscope

Frequency tuning of work modes in the silicon vibratory gyroscope is studied by the theoretical, numerical, and experimental methods in this paper. First, the schematic structure and simplified kinematics model of the gyroscope were presented for deducing the natural frequencies. Then, the width and length of support beams were optimized to tune work frequencies at their designed value. Besides, the frequency difference was experimentally tested and manually tuned by varying the voltage applied on the tuning capacitors. The test on a prototype showed that the difference could be localized between −55.8 Hz and 160.2 Hz when the tuning voltage limit is 20 V. Finally, a frequency control loop was developed to automatically tune the sense frequency toward the drive frequency. Both the theoretical analysis and numeric simulation show that the difference is stabilized at 0.8 Hz when no Coriolis force or quadrature coupling force is applied. It is proved that the frequency difference is successfully tuned by modifying the size of support beams before fabrication as well as the voltage applied on the tuning capacitors after fabrication. The automatic tuning loop, used to match the work modes, is beneficial to enhance the performance of the gyroscope as well as its resistance to environment disturbances.

Study on Stability Improvement of Wind Farm Integrated to Weak Grid by VSC-HVDC

Reactive and active power variation of large scale wind farms always lead to the instability of voltage and frequency. To solve this problem, this paper proposed a method of joining with frequency and voltage control loop of Voltage Source Converter HVDC (VSC-HVDC). Control loop of VSC-HVDC converter WFVSC (Wind Farm Side VSC) and GSVSC (Grid Side VSC) were detailed designed. By the frequency loop of WFSVSC, transient stability is greatly improved. Finally, based on the DIgSILENT/Power Factory simulation, the results show its validity and effectiveness.

Mixer-First FSK Receiver With Automatic Frequency Control for Body Area Networks

We present a low-power (382 ) body area network receiver operating in the 900 MHz band. The wideband FSK receiver supports bit rates up to 625 kbps. To save power, the power consuming phase-locked-loop (PLL) is replaced by an energy efficient digital automatic frequency control (AFC) loop. The AFC acts as a low-bandwidth frequency-locked-loop (FLL), using the FSK demodulator as frequency detector; the measured frequency offset is fed back to the on-chip digitally controlled oscillator. To further decrease the power consumption, the LNA is removed, the passive mixer being the first circuit in the receiver front-end. The mixer-first topology increases the linearity compared to injectionlocked and envelope detector based receivers. Additionally, analytical passive mixer transducer power gain and noise figure models are presented which are used to obtain an optimal mixer-first design. We achieve a -81 dBm sensitivity at a bit rate of 12.5 kbps.

A Coordinated Voltage and Frequency Control of Inverter Based Distributed Generation and Distributed Energy Storage System for Autonomous Microgrids

Distributed generation and distributed energy storage systems can be controlled to improve power system stability. This article proposes control strategies for adopting distributed generation units to autonomous microgrids. The proposed distributed generation interface is utilized not only to control the active power flow, but also to provide essential ancillary services, such as voltage regulation and frequency control, to the autonomous microgrid. A coordinated voltage and frequency control of the inverter-based distributed generation and distributed energy storage system is presented. Moreover, a frequency estimation algorithm based on orthogonal filters is adopted for the frequency control loop. One advantage of the proposed control system is its unified structure for the different operating modes. Extensive simulations of the proposed coordinated control of the inverter-based distributed generation and distributed energy storage system are conducted to evaluate the dynamic performance of the proposed microgrid.

Resonance Tracking of Nonlinear MEMS Resonators

In this paper, the frequency control loop performance of nonlinear microelectromechanical resonators is investigated. A phase-locked loop (PLL) controller is used to track the resonance frequency of a nonlinear resonator and to compensate for the changes in the natural frequency. Based on the harmonic balance approach, governing equations of the dynamics of a nonlinear resonator are obtained, and using the mathematical model of the PLL, a new stability criterion for the closed-loop system is proposed. The relationship between the amplitude of the external driving force command and the stability of the control loop is elaborated for both hardening and softening nonlinearities. A nonlinear gain control strategy is proposed for the frequency-tracking loop, and its effectiveness is shown through simulation studies.