Frequency Droop Research Articles

마이크로그리드에서의 데드 밴드와 회복 제어를 적용한 배터리 에너지저장시스템 주파수 제어

The grid-interconnected microgrid can be able to operate with and without the utility microgrid to supply electricity. when the microgrid operates in grid-connected mode, the frequency of the microgrid synchronizes with the system frequency. In this case, the frequency of the microgrid has small variation which is able to change the output of distributed generation with a droop controller. Thus, the small variation of frequency can make the distributed generation generate unnecessary electricity consistently. In this paper, we propose a frequency droop control with a dead band so as to prevent the distributed generations from generating unnecessary output while in grid-interconnected mode. In addition, a distributed generation can have a restoration control to restore the frequency changed by a droop control as a rated frequency. Also, we state the problem of restoration control with a dead band, and propose its solution when the microgrid operates in stand alone mode. We simulate the proposed droop control using PSCAD/EMTDC to verify the validity of the control.

Voltage and Frequency Droop Control in an Autonomous Microgrid

In this paper, a method for the parallel operation of inverters in an autonomous microgrid system is adopted. This paper presents the resistive output impedance control scheme that allows multiple voltage source converters (VSCs) to operate in parallel in a VSC fed microgrid. The control loops are taking into account the special nature of a low-voltage microgrid, in which the line impedance is mainly resistive. In contrast with the conventional droop-control method, the proposed controller uses a virtual resistance without communication signals to achieve good power sharing, which is insensitive to line-impedance unbalances.

An Enhanced Microgrid Load Demand Sharing Strategy

For the operation of autonomous microgrids, an important task is to share the load demand using multiple distributed generation (DG) units. In order to realize satisfied power sharing without the communication between DG units, the voltage droop control and its different variations have been reported in the literature. However, in a low-voltage microgrid, due to the effects of nontrivial feeder impedance, the conventional droop control is subject to the real and reactive power coupling and steady-state reactive power sharing errors. Furthermore, complex microgrid configurations (looped or mesh networks) often make the reactive power sharing more challenging. To improve the reactive power sharing accuracy, this paper proposes an enhanced control strategy that estimates the reactive power control error through injecting small real power disturbances, which is activated by the low-bandwidth synchronization signals from the central controller. At the same time, a slow integration term for reactive power sharing error elimination is added to the conventional reactive power droop control. The proposed compensation method achieves accurate reactive power sharing at the steady state, just like the performance of real power sharing through frequency droop control. Simulation and experimental results validate the feasibility of the proposed method.

Battery Energy Storage for Enabling Integration of Distributed Solar Power Generation

As solar photovoltaic power generation becomes more commonplace, the inherent intermittency of the solar resource poses one of the great challenges to those who would design and implement the next generation smart grid. Specifically, grid-tied solar power generation is a distributed resource whose output can change extremely rapidly, resulting in many issues for the distribution system operator with a large quantity of installed photovoltaic devices. Battery energy storage systems are increasingly being used to help integrate solar power into the grid. These systems are capable of absorbing and delivering both real and reactive power with sub-second response times. With these capabilities, battery energy storage systems can mitigate such issues with solar power generation as ramp rate, frequency, and voltage issues. Beyond these applications focusing on system stability, energy storage control systems can also be integrated with energy markets to make the solar resource more economical. Providing a high-level introduction to this application area, this paper presents an overview of the challenges of integrating solar power to the electricity distribution system, a technical overview of battery energy storage systems, and illustrates a variety of modes of operation for battery energy storage systems in grid-tied solar applications. The real-time control modes discussed include ramp rate control, frequency droop response, power factor correction, solar time-shifting, and output leveling.

Penetration impact of wind farms equipped with frequency variations ride through algorithm on power system frequency response

Focus of power system engineers is currently directed to the impact of wind power on system frequency. Research efforts concentrate on the ability of wind farms to contribute in the frequency droop events by injecting active power to the grid. This paper presents a detailed analysis for the effect of wind farm connection to a certain power system on the system frequency response. Most of wind farm parameters were considered, namely, the actual wind speeds, wake effects inside the wind farm, the different arrangements of wind turbines inside the wind farm and pitch control mechanism. Moreover, the major data of a h power system were included; different conventional generation technologies and their suitable speed governors. In addition, instant dynamic load variations were implemented. Five case studies were conducted through this system to test the system frequency attitude during normal operation and in case of sudden and large load changes. Three basic values are used to estimate the mentioned impact; the time needed to reach the safe margin after a certain droop, the RMS value of frequency deviation after the fault initiation by a given fixed time span and the maximum frequency drop for each event. All the previously stated studies are performed using MATLAB and Simulink simulations depending on real wind speeds data.

Small-signal Stability and Load-sharing Improvement of Autonomous Microgrids Using Auxiliary Loop

In this article, the problem of appropriate active load sharing, frequency regulation, and adequate damping in an autonomous voltage source converter based microgrid, which has a decentralized controller, is investigated. In decentralized autonomous voltage source converter based microgrids, proper load sharing needs a high gain frequency droop. However, this high gain has a negative impact on small-signal stability and frequency regulation. In order to eliminate this drawback, an auxiliary loop around the frequency droop method is proposed to increase oscillation damping and to decrease frequency deviation. Tuning of this auxiliary loop for each micro-source is formulated as an off-line fuzzy multi-objective optimization problem, which is solved using an evolutionary algorithm. The simulation results based on frequency-domain modeling and eigenvalue analysis show the oscillation damping improvement, frequency deviation reduction, and active power-sharing accuracy of the microgrid in the proposed methodology for wide range of operation conditions.

Optimal decentralized charging control algorithm for electrified vehicles connected to smart grid

Electrified vehicles (EV) and renewable power sources are two important technologies for sustainable ground transportation. If left unmitigated, the additional electric load could over-burden the electric grid. Meanwhile, a challenge for integrating renewable power sources into the grid lies in the fact their intermittency requires more regulation services which makes them expensive to deploy. Fortunately, EVs are controllable loads and the charging process can be interrupted. This flexibility makes it possible to manipulate EV charging to reduce the additional electric load and accommodate the intermittency of renewable power sources. To illustrate this potential, a two-level optimal charging algorithm is designed, which achieves both load shifting and frequency regulation. Load shifting can be realized through coordination of power generation and vehicle charging while reducing power generation cost and carbon dioxide emissions. To ensure practicality, a decentralized charging algorithm for load shifting is formulated by emulating the charging pattern identified through linear programming optimization solutions. The frequency regulation is also designed based on frequency droop that can be implemented in a decentralized way. The two control objectives can be integrated because they are functionally separated by time scale. Simulation results are presented to demonstrate the performance of the proposed decentralized algorithm.

Wind Farms With HVdc Delivery in Inertial Response and Primary Frequency Control

This paper develops a coordination control strategy for wind farms with line commutated converter-based HVdc delivery for participating in inertial response and primary frequency control. The coordination philosophy is to feedback the grid frequency and its derivative and adjust the delivery power of the HVdc link according to the feedback signals. The feedback loop employing the derivative of the grid frequency aims to improve the inertial response while the feedback loop employing the grid frequency deviation introduces a droop at the rectifier control loop. When the grid frequency is too high or too low, active power flow through the HVdc link will be ramped down or up. In turn, the wind generation will increase or decrease the blade angles to reduce or increase the captured wind power through pitch control. A case study demonstrates the effectiveness of the inertial enhancement and frequency droop in HVdc control. Simulation results in transient security assessment tool are given.

A Generalized Computational Method to Determine Stability of a Multi-inverter Microgrid

Microgrid-containing parallel-connected inverters, where each inverter is controlled by decentralized active power/voltage frequency and reactive power/voltage magnitude droop control laws results in flexible and expandable systems. These systems have been known to have stability problems for large values of active power/voltage frequency droop control gain. However, so far the stability analysis of multi-inverter systems has always been performed in a computationally intensive manner by considering the entire microgrid. In a practical microgrid, where the number of inverters may be large or the capacity of the units may differ, it becomes essential to develop a method by which stability can be examined without much computational burden. The system of differential algebraic equations has been simplified using justifiable assumptions to result in a final expression that allows the stability of the microgrid to be examined separately with respect to the droop control laws of each inverter transformed into an equivalent network. Moreover, the procedure allows taking into consideration the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> ratio of the interconnecting cables. Analysis of final expressions validate the stability results reported in literature. Experimental results on hardware show the stable operation of the microgrid.

Load Frequency Control for Rural Distributed Generation

In rural low-voltage networks, distribution lines are usually highly resistive. When many distributed generators are connected to such lines, power sharing among them is difficult when using conventional droop control, as the real and reactive power have strong coupling with each other. A high droop gain can alleviate this problem but may lead the system to instability. To overcome this, two droop control methods are proposed for accurate load sharing with frequency droop controller. The first method considers no communication among the distributed generators and regulates the output voltage and frequency, ensuring acceptable load sharing. The droop equations are modified with a transformation matrix based on the line R/X ratio for this purpose. The second proposed method, with minimal low bandwidth communication, modifies the reference frequency of the distributed generators based on the active and reactive power flow in the lines connected to the points of common coupling. The performance of these two proposed controllers is compared with that of a controller, which includes an expensive high bandwidth communication system through time-domain simulation of a test system. The magnitude of errors in power sharing between these three droop control schemes are evaluated and tabulated.

Generalized Gain Scheduling for Deloaded Wind Turbine Operation

The ability to produce less power than what is available from a wind source, a condition known as deloaded operation, is needed for a wind turbine to reproduce synchronous machine behavior in terms of inertial response and frequency droop regulation. Deloaded operation requires the ability to regulate both power production and rotor speed under any wind speed conditions. In this paper, a novel controller for deloaded wind turbine operation is presented. This controller is made possible by a Cp table inversion procedure allowing generalized gain scheduling for linearization of the pitch response. After introducing the wind turbine models, a review of classical turbine control principles and the proposed deloaded wind turbine control architecture is presented. A discussion of wind turbine non linearity and linearization principles follows. Simulation results are shown for stability, immunity to icing and performance. The advantages of generalized gain scheduling over classical gain scheduling are demonstrated by simulation results.

An Accurate Power Control Strategy for Power-Electronics-Interfaced Distributed Generation Units Operating in a Low-Voltage Multibus Microgrid

In this paper, a power control strategy is proposed for a low-voltage microgrid, where the mainly resistive line impedance, the unequal impedance among distributed generation (DG) units, and the microgrid load locations make the conventional frequency and voltage droop method unpractical. The proposed power control strategy contains a virtual inductor at the interfacing inverter output and an accurate power control and sharing algorithm with consideration of both impedance voltage drop effect and DG local load effect. Specifically, the virtual inductance can effectively prevent the coupling between the real and reactive powers by introducing a predominantly inductive impedance even in a low-voltage network with resistive line impedances. On the other hand, based on the predominantly inductive impedance, the proposed accurate reactive power sharing algorithm functions by estimating the impedance voltage drops and significantly improves the reactive power control and sharing accuracy. Finally, considering the different locations of loads in a multibus microgrid, the reactive power control accuracy is further improved by employing an online estimated reactive power offset to compensate the effects of DG local load power demands. The proposed power control strategy has been tested in simulation and experimentally on a low-voltage microgrid prototype.

Wind Farms With HVDC Delivery in Load Frequency Control

This letter develops a coordination control strategy for wind farms with line commutated converter (LCC) based HVDC delivery to participate in load frequency control. The coordination philosophy is to let the HVDC rectifier sense the grid frequency. If the grid frequency is too high or too low, active power flow through the HVDC link will be ramped down or up by introducing a droop at the rectifier control loop. In turn, wind generation will increase or decrease the blade angles to reduce or increase the captured wind power. This will be done by wind generation pitch controllers. A case study demonstrates the effect of the frequency droop in HVDC control. Simulation results in TSAT are given.

Autonomous adaptive sensorless controller of inverter-based islanded-distributed generation system

A control method for inverter operation in an islanded-distributed generation system is presented. The controller provides sufficient active and reactive power to match the connected load, therein providing high power quality. Control in this mode is based on voltage and frequency droops. Detailed analysis, based on the concept of harmonic impedance, establishes the appropriateness of inductor current feedback against capacitor current feedback, with respect to inverter output power quality. Steepest descent adaptive control is embedded to estimate the output voltage for sensorless operation, which enhances system robustness and performance. To compensate the DSP calculation delay, a modified Kalman filter design is discussed. Simulation and experimental results demonstrate the validity of the proposed control method.

An Improved Power Controller For A Single Phase Grid Connected Inverter With Root Locus Analysis

This paper presents an improved controller for a single-phase inverter connected to the grid, which is based on frequency and voltage droops. A new feedback loop is introduced to increase the system damping. The small signal analysis and root locus plots are presented in order to show the connections between the feedback loop gains and the behavior of the system. Simulation results corresponding to the numerical solution of the nonlinear system equations are presented to validate the small signal analysis. A description of a laboratory prototype and the respective experimental results are also presented to confirm the theoretical analysis.

UPS Parallel Balanced Operation Without Explicit Estimation of Reactive Power—A Simpler Scheme

A novel scheme of the well-known technique for parallel operation of uninterruptible power supply systems, namely frequency and voltage droop control method, is presented. The innovation relies on active power estimation, together with the use of a piecewise-continuous nonlinear function, in order to eliminate the need for costly cascaded low-pass filters usually employed to estimate the reactive power. Moreover, the whole scheme is derived in the time domain on a state-space approach. As a consequence, models that are lower order and more amenable to nonlinear analysis are obtained. Simulated results for two UPS parallel operation connected to a resistive load are presented. The results indicate that the novel technique leads to a proper load sharing.

Stability of Voltage and Frequency Control in Distributed Generation Based on Parallel-Connected Converters Feeding Constant Power Loads

SummaryIn this paper stability and dynamic properties of voltage and frequency droop control of power electronic converters are investigated for a distributed generation system. Droop control is utilized to share active and reactive power among the source converters. The voltage and frequency controllers are designed so that stand-alone converter operation feeding constant power loads performs satisfactory. These controllers are adapted to mimic the behaviour of present rotating generators connected directly to the power system, for seam-less transfer between island and grid-connected modes of operation. The target system for the analysis is a stand-alone system to which three power electronic converters and one rotating generator could be connected (Fig. 1). Small-signal and switch-mode, time-domain simulation results of a three-converter distributed generation system facilitating stand-alone operation verify the operation. Simulation and experimental results of a three-converter stand-alone system, with and without a rotating generator present, are also included for verification.

A Voltage and Frequency Droop Control Method for Parallel Inverters

In this paper, a new control method for the parallel operation of inverters operating in an island grid or connected to an infinite bus is described. Frequency and voltage control, including mitigation of voltage harmonics, are achieved without the need for any common control circuitry or communication between inverters. Each inverter supplies a current that is the result of the voltage difference between a reference ac voltage source and the grid voltage across a virtual complex impedance. The reference ac voltage source is synchronized with the grid, with a phase shift, depending on the difference between rated and actual grid frequency. A detailed analysis shows that this approach has a superior behavior compared to existing methods, regarding the mitigation of voltage harmonics, short-circuit behavior and the effectiveness of the frequency and voltage control, as it takes the R to X line impedance ratio into account. Experiments show the behavior of the method for an inverter feeding a highly nonlinear load and during the connection of two parallel inverters in operation.

Frequency support from doubly fed induction generator wind turbines

An assessment on the capability of a doubly fed induction generator (DFIG) wind turbine for frequency regulation is presented. Detailed aerodynamic, structural and electrical dynamic models were used in this study. A control loop acting on the frequency deviation was added to the inertia contributing loop in order to enhance the inertia support from the DFIG wind turbine. The possibility of de-loading a wind turbine to provide primary and secondary frequency response was discussed. A frequency droop controller was examined where the droop is operating on the electronic torque set point below its maximum speed and is operating on the pitch demand at maximum speed. It is also shown that by reducing the generator torque set point the DFIG wind turbine can provide high frequency response.

Reduced pole placement method for cascaded frequency control via dispersed pulse inverters

For modular power systems, structures with parallel power inverters are favourable in view of both easy expandability and supply security. The inverters' embedded controllers are implementing voltage and frequency droops and the superimposed frequency control is coupled via fieldbus. This is a case where a superimposed control is acting via one or more locally dispersed subimposed control-loops. As the states of the subimposed loops are inaccessible, their feedback is no longer viable. The method of reduced pole placement allows reformulation of the design task as complete state feedback without employing a feedback of the single virtual state that just globally describes the one or more subimposed systems. Results are presented for a robust grid frequency controller acting via dispersed pulse inverters. Copyright © 2005 John Wiley & Sons, Ltd.