Showing papers on "Automatic frequency control published in 2013"

Decentralized Vehicle-to-Grid Control for Primary Frequency Regulation Considering Charging Demands

Abstract: Vehicle-to-grid (V2G) control has the potential to provide frequency regulation service for power system operation from electric vehicles (EVs). In this paper, a decentralized V2G control (DVC) method is proposed for EVs to participate in primary frequency control considering charging demands from EV customers. When an EV customer wants to maintain the residual state of charge (SOC) of the EV battery, a V2G control strategy, called battery SOC holder (BSH), is performed to maintain the battery energy around the residual SOC along with adaptive frequency droop control. If the residual battery energy is not enough for next trip, the customer needs to charge the EV to higher SOC level. Then, a smart charging method, called charging with frequency regulation (CFR), is developed to achieve scheduled charging and provide frequency regulation at the same time. Simulations on a two-area interconnected power system with wind power integration have shown the effectiveness of the proposed method.

A robust PID controller based on imperialist competitive algorithm for load-frequency control of power systems.

Abstract: A new PID controller for resistant differential control against load disturbance is introduced that can be used for load frequency control (LFC) application. Parameters of the controller have been specified by using imperialist competitive algorithm (ICA). Load disturbance, which is due to continuous and rapid changes of small loads, is always a problem for load frequency control of power systems. This paper introduces a new method to overcome this problem that is based on filtering technique which eliminates the effect of this kind of disturbance. The object is frequency regulation in each area of the power system and decreasing of power transfer between control areas, so the parameters of the proposed controller have been specified in a wide range of load changes by means of ICA to achieve the best dynamic response of frequency. To evaluate the effectiveness of the proposed controller, a three-area power system is simulated in MATLAB/SIMULINK. Each area has different generation units, so utilizes controllers with different parameters. Finally a comparison between the proposed controller and two other prevalent PI controllers, optimized by GA and Neural Networks, has been done which represents advantages of this controller over others.

An Intelligent Droop Control for Simultaneous Voltage and Frequency Regulation in Islanded Microgrids

Abstract: Voltage and frequency of microgrids (MGs) are strongly impressionable from the active and reactive load fluctuations. Often, there are several voltage source inverters (VSIs) based distributed generations (DGs) with a specific local droop characteristic for each DG in a MG. A load change in a MG may lead to imbalance between generation and consumption and it changes the output voltage and frequency of the VSIs according to the droop characteristics. If the load change is adequately large, the DGs may be unable to stabilize the MG. In the present paper, following a brief survey on the conventional voltage/frequency droop control, a generalized droop control (GDC) scheme for a wide range of load change scenarios is developed. Then to remove its dependency to the line parameters and to propose a model-free based GDC, a new framework based on adaptive neuro-fuzzy inference system (ANFIS) is developed. It is shown that the proposed intelligent control structure carefully tracks the GDC dynamic behavior, and exhibits high performance and desirable response for different load change scenarios. It is also shown that the ANFIS controller can be effectively used instead of the GDC. The proposed methodology is examined on several MG test systems.

Load Frequency Control in Power Systems via Internal Model Control Scheme and Model-Order Reduction

Abstract: The large-scale power systems are liable to performance deterioration due to the presence of sudden small load perturbations, parameter uncertainties, structural variations, etc. Due to this, modern control aspects are extremely important in load frequency control (LFC) design of power systems. In this paper, the LFC problem is illustrated as a typical disturbance rejection as well as large-scale system control problem. For this purpose, simple approach to LFC design for the power systems having parameter uncertainty and load disturbance is proposed. The approach is based on two-degree-of-freedom, internal model control (IMC) scheme, which unifies the concept of model-order reduction like Routh and Pade approximations, and modified IMC filter design, recently developed by Liu and Gao [24]. The beauty of this paper is that in place of taking the full-order system for internal-model of IMC, a lower-order, i.e., second-order reduced system model, has been considered. This scheme achieves improved closed-loop system performance to counteract load disturbances. The proposed approach is simulated in MATLAB environment for a single-area power system consisting of single generating unit with a non-reheated turbine to highlight the efficiency and efficacy in terms of robustness and optimality.

Efficient Computation of Sensitivity Coefficients of Node Voltages and Line Currents in Unbalanced Radial Electrical Distribution Networks

Abstract: The problem of optimal control of power distribution systems is becoming increasingly compelling due to the progressive penetration of distributed energy resources in this specific layer of the electrical infrastructure. Distribution systems are, indeed, experiencing significant changes in terms of operation philosophies that are often based on optimal control strategies relying on the computation of linearized dependencies between controlled (e.g., voltages, frequency in case of islanding operation) and control variables (e.g., power injections, transformers tap positions). As the implementation of these strategies in real-time controllers imposes stringent time constraints, the derivation of analytical dependency between controlled and control variables becomes a non-trivial task to be solved. With reference to optimal voltage and power flow controls, this paper aims at providing an analytical derivation of node voltages and line currents as a function of the nodal power injections and transformers tap-changers positions. Compared to other approaches presented in the literature, the one proposed here is based on the use of the [Y] compound matrix of a generic multi-phase radial unbalanced network. In order to estimate the computational benefits of the proposed approach, the relevant improvements are also quantified versus traditional methods. The validation of the proposed method is carried out by using both IEEE 13 and 34 nodes test feeders. The paper finally shows the use of the proposed method for the problem of optimal voltage control applied to the IEEE 34 node test feeder.

Decentralized Sliding Mode Load Frequency Control for Multi-Area Power Systems

Abstract: Based on the decentralized sliding mode control, a load frequency controller is designed in this paper for multi-area interconnected power systems with matching and unmatched uncertainties. The proportional and integral switching surface is constructed for each area to improve system dynamic performance in reaching intervals. The robust controller is proposed by the reaching law method to assure that frequency fluctuation converges to zero after a load and operation point variation. A three-area interconnected power system is studied to illustrate the effectiveness of the proposed decentralized sliding mode control scheme.

An Improved Repetitive Control Scheme for Grid-Connected Inverter With Frequency-Adaptive Capability

Abstract: The power quality of grid-connected inverters has drawn a lot of attention with the increased application of distributed power generation systems. The repetitive control technique is widely adopted in these systems, due to its excellent tracking performance and low output total harmonic distortion (THD). However, in an actual system, the ratio of the sampling frequency to the grid frequency cannot always maintain an integer, and then, the resonant frequencies of the repetitive control technique will deviate from the real grid fundamental and harmonic frequencies. This will degrade the performance of the system, particularly when the grid frequency varies. Even if the ratio is a fixed integer, the auxiliary function for stabilization in the conventional repetitive control technique will also increase the steady-state tracking error and THD of the system. In this paper, an improved repetitive control scheme with a special designed finite impulse response (FIR) filter is proposed. The FIR filter cascaded with a traditional delay function can approximate the ideal repetitive control function of any ratio. The proposed scheme varies the FIR filter according to varied grid frequency and maintains its resonant frequencies matching the grid fundamental and harmonic ones. Finally, the simulation and experimental results show that the improved repetitive control scheme can effectively reduce the tracking error and compensate harmonics of the inverter systems.

A New Frequency Regulation Strategy for Photovoltaic Systems Without Energy Storage

Abstract: To maximize the revenue from selling energy, photovoltaic systems (PVs) in general operate in the so-called maximum power point tracking mode. However, the increasing penetration of renewable energy sources in power systems has motivated the design of innovative control to provide ancillary services. The focus of this paper is to develop a new control strategy that enables PVs to adjust the active power outputs and provide frequency regulation to power systems. In this strategy, two different modes are designed: 1) the frequency droop control mode for PVs to provide primary frequency support to power systems, and 2) the emergency control mode to prevent system frequency collapse and, therefore, to prevent too much generation tripping after fault. Based on a detailed PV dynamic model, simulation results show the effectiveness of the proposed control strategy in improving the frequency stability.

Fuzzy Control of Distributed PV Inverters/Energy Storage Systems/Electric Vehicles for Frequency Regulation in a Large Power System

Abstract: This paper presents a fuzzy based frequency control strategy by the Megawatt (MW) class distributed PV systems and electric vehicles (EVs). The frequency control is proposed from the view point of the frequency fluctuation problem produced by the large penetration of PV power and sudden load variation. The fuzzy based frequency control has three inputs: average insolation, change of insolation and frequency deviation. Following these three inputs, a frequency control system for the distributed PV inverters is proposed. For the case of different insolations in the different areas of the power system, a coordinated control method of the distributed PV inverters, energy storage systems (ESSs) and EVs is presented. The proposed method is simulated by considering dual power and information flows between supply and demand sides in a large power system and is found satisfactory to provide frequency control and to reduce tie-line power fluctuations.

Further Results on Delay-Dependent Stability of Multi-Area Load Frequency Control

Abstract: Further to results reported by Jiang , this paper investigates delay-dependent stability of load frequency control (LFC) emphasizing on multi-area and deregulated environment. Based on Lyapunov theory and the linear matrix inequality technique, a new stability criterion is proposed to improve calculation accuracy and to reduce computation time, which makes it be suitable for handling with multi-area LFC schemes. The interaction of delay margins between different control areas and the relationship between delay margins and control gains are investigated in details. Moreover, usage of delay margins as a new performance index to guide controller design is discussed, including tuning of the controller for a trade-off between delay tolerance and dynamic response, choosing the upper bound of the fault counter of communication channels and the upper bound of sampling period of a discrete realization of the controller. Case studies are carried out based on two-area traditional, two-area and three-area deregulated LFC schemes, all equipped with PID-type controllers, respectively. Simulation studies are given to verify the effectiveness of the proposed method.

System Frequency Support Through Multi-Terminal DC (MTDC) Grids

Abstract: Control of the converter stations in a multi-terminal DC (MTDC) grid to provide frequency support for the surrounding AC systems is the subject matter of this paper. The standard autonomous power sharing control loop for each converter is modified with a frequency droop control loop. The objective is to minimize the deviation from nominal AC system frequency and share the burden of frequency support among the converter stations of the MTDC grid. The effectiveness of the frequency support is demonstrated through nonlinear simulation of a test system consisting of three isolated AC systems interconnected through an MTDC grid with four converter stations. An averaged model of the MTDC grids is developed to carry out modal analysis of combined multi-machine AC-MTDC systems. Modal analysis is used to characterize and substantiate the time domain behavior in presence of frequency droop control. It is established that appropriate droop control loop for the MTDC grid converters could be effective in reducing the deviation from nominal AC system frequency provided the sensitivity of the system eigen-values to changes in control parameters (e.g., droop coefficients) is accounted for a priori through modal analysis.

“SRF Theory Revisited” to Control Self-Supported Dynamic Voltage Restorer (DVR) for Unbalanced and Nonlinear Loads

Abstract: The protection of the sensitive unbalanced nonlinear loads from sag/swell, distortion, and unbalance in supply voltage is achieved economically using the dynamic voltage restorer (DVR). A simple generalized algorithm based on basic synchronous-reference-frame theory has been developed for the generation of instantaneous reference compensating voltages for controlling a DVR. This novel algorithm makes use of the fundamental positive-sequence phase voltages extracted by sensing only two unbalanced and/or distorted line voltages. The algorithm is general enough to handle linear as well as nonlinear loads. The compensating voltages when injected in series with a distribution feeder by three single-phase H-bridge voltage-source converters with a constant switching frequency hysteresis band voltage controller tightly regulate the voltage at the load terminals against any power quality problems on the source side. A capacitor-supported DVR does not need any active power during steady-state operation because the injected voltage is in quadrature with the feeder current. The proposed control strategy is validated through extensive simulation and real-time experimental studies.

Stability Analysis of Grid-Connected Inverter With LCL Filter Adopting a Digital Single-Loop Controller With Inherent Damping Characteristic

Abstract: The stability problem of the grid-connected inverter with LCL filter adopting a digital single-loop controller without extra damping resistance is analyzed. The digital single-loop control based on the grid-side current feedback can attenuate the resonance introduced by LCL filters due to the inherent damping characteristic embedded in the control loop. The stability analysis is performed by means of a Nyquist diagram in the discrete domain and to provide a general design direction for filter and controller. The quantitative relationship of ratios of LCL resonance angular frequency to control frequency and controller parameters on the system stability is investigated. It is derived that the system would obtain preferable stability margin when the ratio is located at a certain range. The theoretical analyses are validated by experimental tests based on a 5-kW single-phase inverter laboratory prototype.

Application of plug-in electric vehicles to frequency regulation based on distributed signal acquisition via limited communication

Abstract: For application of a large number of plug-in electric vehicles (PEVs) to system frequency regulation, a distributed acquisition approach based on consensus filtering is proposed, where frequency-measuring function is performed only at distribution substations. Limited communication between neighboring PEVs/distribution substations can facilitate consistent and accurate acquisition of frequency deviation signals for all PEVs. Considering the battery charging/discharging characteristics, a dynamic PEV model with feedback control is further proposed and integrated with frequency regulation based on distributed acquisition. Asymptotical stabilities of distributed acquisition and system frequency regulation are analyzed. Simulation results demonstrate that the proposed approach can provide consistent and accurate control signals for a large number of PEVs and obtain better regulation than general decentralized control in terms of eliminating noise, improving robustness and reducing device costs.

Arctan Power–Frequency Droop for Improved Microgrid Stability

Abstract: The term microgrid is usually reserved for a modest sized, local distributed generation network that will largely operate standalone (i.e., without a grid connection.) The most common power flow control method utilized in a standalone microgrid is a technique known as power-frequency droop. This paper introduces the concept of utilizing an arctan function for the power-frequency droop profile. The use of this arctan function improves the small signal stability of the two-inverter microgrid, provides natural frequency bounding, and is flexible in its application. SABER simulations are performed to obtain the operating points about which the system is linearized for the stability analysis. Experimental results obtained from a dSPACE-controlled, low-voltage, two-inverter hardware system are presented to verify the theoretical and simulation results.

Generalized Optimal Pulsewidth Modulation of Multilevel Inverters for Low-Switching-Frequency Control of Medium-Voltage High-Power Industrial AC Drives

Abstract: A generalized optimal pulsewidth modulation (PWM) technique applicable to multilevel inverters for low-switching-frequency control of medium-voltage high-power industrial ac drives is presented. Proposed synchronous optimal PWM method allows setting the maximum switching frequency to a low value without compromising the harmonic distortion of machine currents. Low switching frequency reduces the switching losses of the power semiconductor devices, resulting in higher inverter power output and efficiency. The proposed optimization results in low harmonic distortion at low switching frequency. Experimental results of a five-level inverter drive using optimal PWM are presented.

Distributed vs. centralized power systems frequency control

Abstract: This paper considers a distributed control algorithm for frequency control of electrical power systems. We propose a distributed controller which retains the reference frequency of the buses under unknown load changes, while asymptotically minimizing a quadratic cost of power generation. For comparison, we also propose a centralized controller which also retains the reference frequency while minimizing the same cost of power generation. We derive sufficient stability criteria for the parameters of both controllers. The controllers are evaluated by simulation on the IEEE 30 bus test network, where their performance is compared.

Automated Design of a High-Power High-Frequency LCC Resonant Converter for Electrostatic Precipitators

Abstract: This work presents an automated design procedure for series parallel resonant converters (LCC) employed in electrostatic precipitator (ESP) power supplies, which reduces the designer effort significantly. The requirements for the power supplies in ESP applications and means to derive an accurate mathematical model of the LCC converter, such as the power loss from commercial insulated-gate bipolar transistors, are described in detail in this paper. The converter parameters, such as resonant tank elements, are selected in order to improve the overall efficiency of the system, when a typical ESP energization operation range is considered. The analysis comprises two different control strategies: the conventional variable frequency control and the dual control. Both control strategies are analyzed by comparing semiconductor losses of five commercial modules. Finally, the circuit operation and design are verified with a 60 kW LCC resonant converter test setup.

Power and energy capacity requirements of storages providing frequency control reserves

Abstract: Due to their fast response time and high ramp rates, storage systems are capable of providing frequency control reserves. However, the limit in energy capacity poses difficulties as frequency control signals are not unbiased. We describe a scheme to recharge or discharge the storage without impeding the quality of the provided service, and formulate an analyzing method to investigate the resulting size of the storage. We show that even small storage sizes are sufficient to provide continuous and reliable primary and secondary frequency control reserves to the grid.

Repetitive and Resonant Control for a Single-Phase Grid-Connected Hybrid Cascaded Multilevel Converter

Abstract: This paper investigates the use of adaptive repetitive and resonant control approaches for the control of a single-phase hybrid cascaded multilevel converter designed to interface with power grids of highly distorted voltage waveform. The proposed repetitive/resonant control (RPC/RSC) is used for the extraction of clean phase angle, frequency, and magnitude information of the fundamental grid voltage together with the control of the converter current and mitigation of current harmonics under distorted grid voltage and variable frequency. The RPC time period and the RSC resonant frequency are updated adaptively using the estimated grid frequency while interpolation is used to preserve the RPC rejection capability under noninteger number of time steps per period. Detailed modeling and experimental investigation of the proposed control scheme are carried out.

Sensitivity Analysis of Load-Damping Characteristic in Power System Frequency Regulation

Abstract: The smart grid initiative leads to growing interests in demand responses and the load models, especially the frequency-sensitive loads such as motors. The reason is that high-penetration controllable load may have substantial impact on system frequency response (SFR). However, the effect of the frequency-related load-damping coefficient is still not completely understood. This paper investigates the effect of frequency-sensitive load on system frequency using typical SFR model. Theoretic analyses based on transfer functions show that the frequency deviation under a different load-damping coefficient is relatively small and bounded when the power system is essentially stable; while the frequency deviation can be accelerated when a power system is unstable after disturbance. For the stable case, the largest frequency dip under a perturbation and the corresponding critical time can be derived by inverse Laplace transformation using a full model considering load-damping coefficient. Further, the error in evaluating the load-frequency coefficient gives the largest impact to frequency deviation right at the time when the largest frequency dip occurs. Multiple-machine cases and automatic generation control (AGC) are also included in the analyses with verifications by simulation studies. The conclusion can be useful for system operators for decision-making of load control or interruption.

A Digital Current Control of Quasi-Z-Source Inverter With Battery

Abstract: This paper presents a fixed frequency operating sliding mode (SM) current control method with fast response and improved stability. Different from the conventional SM control with variable switching frequency, the fixed-frequency SM controller is proposed to control the modulation index and shoot-through duty ratio of the voltage-fed quasi-Z-source inverter (qZSI), which will not increase the passive components and filter design difficulty. A large-signal dynamic model of the system has been established, which can be used for the system stability control in a wide operating range. By using linear approximation, the system small-signal model is also obtained to analyze the control system stability and transient response. Compared with the conventional current mode controller, the proposed SM controller can achieve faster response, lower current ripple and better stability for qZSI when the supply and load variation is large. Experimental results are presented to demonstrate the validity of the theoretical design and the effectiveness of the proposed controller.

Comparison of Small Signal Characteristics in Current Mode Control Schemes for Point-of-Load Buck Converter Applications

Abstract: Different current mode controls have been widely adopted in commercial pulsewidth modulation controllers for point-of-load (POL) converter applications. To understand the unique properties of various current mode control schemes, and to study the differences between them, this paper investigates and compares four current mode control schemes (peak-current control, valley-current control, constant on-time control, and constant off-time control) for POL power converters and voltage regulator applications. Performance attributes under investigation include high-bandwidth voltage loop design, adaptability to a converter with a wide input voltage range, adaptive voltage positioning design, and audio susceptibility. The pros and cons of these schemes are identified and explained by the unified small signal equivalent circuit model. In terms of dynamic performance, this study provides a current mode control scheme selection criteria and feedback design guidelines. The theoretical analyses are verified by both simulation and experimental results.

Digital Adaptive Frequency Modulation for Bidirectional DC–DC Converter

Abstract: This paper introduces a digital adaptive control method for a bidirectional dc/dc charger/discharger, which is the core element for reliable and efficient energy storage systems. The proposed method achieves zero-voltage switching (ZVS) without the use of an auxiliary zero-crossing detection (ZCD) circuit. To satisfy ZVS conditions, proper switching frequency is determined through a digital calculation. It features soft switching over wide input and output ranges. Because this method does not require a ZCD circuit, it is easily implemented with bidirectional operation and reduces instability and noise susceptibility problems. To reduce conduction loss, a multiphase interleaving technique is applied. This interleaving method reduces the required capacitance by decreasing the current ripple. A phase shedding technique is also implemented to achieve higher efficiency over a wide load range. The operation of the proposed digital adaptive control method is analyzed. For experimental verification, a 200-W two-phase-interleaved bidirectional synchronous buck converter with 30-38-V bus voltage and 15-25-V battery voltage is implemented.

An Improved Three-Phase Variable-Band Hysteresis Current Regulator

Abstract: This paper presents an improved variable-band hysteresis current controller for a two-level three-phase voltage source inverter (VSI). The controller takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMF voltages, and uses these results to vary the hysteresis bands so as to maintain constant phase-leg switching frequencies. The switching frequency control process is then further refined by fine tuning the hysteresis band variations to synchronize the zero crossings of the phase-leg current errors with a fixed reference clock so as to achieve a nearest space vector switching sequence, which further ensures that the switched output spectrum has been optimized. Finally, a technique is proposed to replace the third phase-leg current regulator with a fixed-frequency open-loop pulse-width modulator, where its commanded reference is generated from the average switched output voltages of the other two phase legs. This avoids the hazard of the three independent hysteresis current regulators adversely interacting with each other in a conventional system, resulting from an overconstrained control problem with only two degrees of freedom. Additionally, this approach allows the linear modulation range to be increased by adding a common-mode third-harmonic component to the third phase-leg reference command signal.

Modeling, analysis, and design of a frequency-droop-based virtual synchronous generator for microgrid applications

Abstract: In this paper, a power-frequency (P-ω) controller is presented for voltage source converters (VSC). The approach is intended for multiple parallel VSCs forming a microgrid operating in both grid-connected and islanded modes. The proposed controller allows a VSC to mimic the operation of a synchronous generator (SG) by implementing the swing equation of SG with a primary frequency controller. In addition, a generalized model of the active power generation dynamics is developed in order to analyze the stability and to design the main control parameters. In contrast with the conventional droop control method, the proposed controller improves the close-loop system dynamic response without changing the frequency accuracy. The obtained results show the good performance of the proposed controller.