Showing papers on "Inertia published in 2017"

Virtual Inertia: Current Trends and Future Directions

Abstract: The modern power system is progressing from a synchronous machine-based system towards an inverter-dominated system, with large-scale penetration of renewable energy sources (RESs) like wind and photovoltaics. RES units today represent a major share of the generation, and the traditional approach of integrating them as grid following units can lead to frequency instability. Many researchers have pointed towards using inverters with virtual inertia control algorithms so that they appear as synchronous generators to the grid, maintaining and enhancing system stability. This paper presents a literature review of the current state-of-the-art of virtual inertia implementation techniques, and explores potential research directions and challenges. The major virtual inertia topologies are compared and classified. Through literature review and simulations of some selected topologies it has been shown that similar inertial response can be achieved by relating the parameters of these topologies through time constants and inertia constants, although the exact frequency dynamics may vary slightly. The suitability of a topology depends on system control architecture and desired level of detail in replication of the dynamics of synchronous generators. A discussion on the challenges and research directions points out several research needs, especially for systems level integration of virtual inertia systems.

A Self-Adaptive Inertia and Damping Combination Control of VSG to Support Frequency Stability

Abstract: In the virtual synchronous generator (VSG) field, the traditional methods, such as the constant parameters control method and the self-adaptive inertia control method, always neglect the effect of the damping factor. This letter proposes a self-adaptive inertia and damping combination control method to improve the frequency stability with an interleaving control technique. Tests on the MATLAB/Simulink VSG model demonstrate the effectiveness of the proposed method.

Optimal Placement of Virtual Inertia in Power Grids

Abstract: A major transition in the operation of electric power grids is the replacement of synchronous machines by distributed generation connected via power electronic converters. The accompanying “loss of rotational inertia” and the fluctuations by renewable sources jeopardize the system stability, as testified by the ever-growing number of frequency incidents. As a remedy, numerous studies demonstrate how virtual inertia can be emulated through various devices, but few of them address the question of “where” to place this inertia. It is, however, strongly believed that the placement of virtual inertia hugely impacts system efficiency, as demonstrated by recent case studies. In this paper, we carry out a comprehensive analysis in an attempt to address the optimal inertia placement problem. We consider a linear network-reduced power system model along with an $\mathscr {H}_2$ performance metric accounting for the network coherency. The optimal inertia placement problem turns out to be non-convex, yet we provide a set of closed-form global optimality results for particular problem instances as well as a computational approach resulting in locally optimal solutions. Further, we also consider the robust inertia allocation problem, wherein the optimization is carried out accounting for the worst-case disturbance location. We illustrate our results with a three-region power grid case study and compare our locally optimal solution with different placement heuristics in terms of different performance metrics.

Inertia Emulation in AC/DC Interconnected Power Systems Using Derivative Technique Considering Frequency Measurement Effects

Abstract: Virtual inertia is known as an inevitable part of the modern power systems with high penetration of renewable energy. Recent trend of research is oriented in different methods of emulating the inertia to increase the sustainability of the system. In the case of dynamic performance of power systems especially in Automatic Generation Control (AGC) issue, there are concerns considering the matter of virtual inertia. This paper proposes an approach for analyzing the dynamic effects of virtual inertia in two-area AC/DC interconnected AGC power systems. Derivative control technique is used for higher level control application of inertia emulation. This method of inertia emulation is developed for two-area AGC system, which is connected by parallel AC/DC transmission systems. Based on the proposed technique, the dynamic effect of inertia emulated by storage devices for frequency and active power control are evaluated. The effects of frequency measurement delay and phase-locked loop effect are also considered by introducing a second-order function. Simulations performed by MATLAB software demonstrate how virtual inertia emulation can effectively improve the performance of the power system. A detailed eigenvalue analysis is also performed to support the positive effects of the proposed method.

Advanced Control Strategies of PMSG-Based Wind Turbines for System Inertia Support

Abstract: This paper investigates two novel control strategies that enable system inertia supports by permanent magnet synchronous generator (PMSG) wind turbines during transient events. The first strategy seeks to provide inertia support to the system through simultaneous utilization of dc-link capacitor energy, and wind turbine (WT) rotor kinetic energy (KE). The second strategy supports system inertia through orderly exerting dc-link capacitor energy of WT and then WT rotor KE via a cascading control scheme. Both strategies can effectively provide system inertia support by fully utilizing WT's own potentials, while the second strategy distinguishes itself by minimizing its impacts on wind energy harvesting. Case studies of one synchronous generator connected with a PMSG-based WT considering sudden load variations have been studied to validate and compare the two proposed strategies on providing rapid inertia response for the system.

Inertia Provision and Estimation of PLL-Based DFIG Wind Turbines

Abstract: This paper presents an alternative inertial control method for doubly fed induction generator (DFIG)-based wind turbines by directly adjusting the phase locked loop (PLL) response. The synthetic internal voltage vector of the wind turbine-driven DFIG is defined in the electromechanical timescale to present the dynamic properties. The phase angle motion equation is further deduced to depict the relationship between the contributed inertial response and the defined internal voltage. Based on the developed motion equation, the equivalent inertia is estimated and quantified, and further found to be significantly determined by the PLL parameters. Moreover, the effect of both PLL and active power control on the defined internal voltage dynamics is also described during the inertial response. Simulated results on a modified 3-machine, 9-node test system were conducted to validate the feasibility of the proposed inertial control method and the correctness of the developed inertial characteristics.

Virtual Inertia Control-Based Model Predictive Control for Microgrid Frequency Stabilization Considering High Renewable Energy Integration

Abstract: Renewable energy sources (RESs), such as wind and solar generations, equip inverters to connect to the microgrids. These inverters do not have any rotating mass, thus lowering the overall system inertia. This low system inertia issue could affect the microgrid stability and resiliency in the situation of uncertainties. Today’s microgrids will become unstable if the capacity of RESs become larger and larger, leading to the weakening of microgrid stability and resilience. This paper addresses a new concept of a microgrid control incorporating a virtual inertia system based on the model predictive control (MPC) to emulate virtual inertia into the microgrid control loop, thus stabilizing microgrid frequency during high penetration of RESs. The additional controller of virtual inertia is applied to the microgrid, employing MPC with virtual inertia response. System modeling and simulations are carried out using MATLAB/Simulink® software. The simulation results confirm the superior robustness and frequency stabilization effect of the proposed MPC-based virtual inertia control in comparison to the fuzzy logic system and conventional virtual inertia control in a system with high integration of RESs. The proposed MPC-based virtual inertia control is able to improve the robustness and frequency stabilization of the microgrid effectively.

Piezoelectric Inertia Motors—A Critical Review of History, Concepts, Design, Applications, and Perspectives.

Abstract: Piezoelectric inertia motors—also known as stick-slip motors or (smooth) impact drives—use the inertia of a body to drive it in small steps by means of an uninterrupted friction contact. In addition to the typical advantages of piezoelectric motors, they are especially suited for miniaturisation due to their simple structure and inherent fine-positioning capability. Originally developed for positioning in microscopy in the 1980s, they have nowadays also found application in mass-produced consumer goods. Recent research results are likely to enable more applications of piezoelectric inertia motors in the future. This contribution gives a critical overview of their historical development, functional principles, and related terminology. The most relevant aspects regarding their design—i.e., friction contact, solid state actuator, and electrical excitation—are discussed, including aspects of control and simulation. The article closes with an outlook on possible future developments and research perspectives.

Modeling of Type 3 Wind Turbines With df/dt Inertia Control for System Frequency Response Study

Abstract: A model for Type 3 wind turbines (WTs) with typical d f /dt inertia control is developed for studying frequency dynamics in power systems. A simplified small-signal Type 3 WT model with d f /dt control is first constructed based on the mass-spring-damping concept, such that the physical properties and frequency response of a Type 3 WT can be clearly understood, besides the frequency-domain expressions of the available inertia and the corresponding damping coefficient can be directly derived. The manifested inertia is apparently controllable and frequency-dependent, but differs from the constant inertia featured in a conventional synchronous generator (SG). Furthermore, the frequency response model of a generic two-machine system, composed of an SG and an aggregate Type 3 WTs, is established. The model synthetically considers the effects of the WTs’ different controller parameters, operating points, and the SG's governor response on system frequency characteristics. Then, time-domain simulations on the studied two-machine system are performed in MATLAB/Simulink. The simulated results verify that the proposed model is effective for analyzing system frequency dynamics, and that the test system frequency characteristics can be improved based on tuning the mass-spring- damping coefficients of Type 3 WTs.

Anthropomorphic Low-Inertia High-Stiffness Manipulator for High-Speed Safe Interaction

Abstract: In this paper, a manipulator is proposed for safe human–robot interaction at high speed. The manipulator has both low mass and inertia and high stiffness and strength. It is basically a cable-driven manipulator; nevertheless, by using a unique lightweight tension-amplification mechanism, the manipulator retains high stiffness and strength. The joint stiffness, which is strongly related to the motion control performance, is amplified by the quadratic order. Both 1-degree of freedom (DOF) and 3-DOF joint mechanisms using the tension-amplification mechanism are presented and combined to develop a 7-DOF anthropomorphic manipulator named LIMS. The mass and inertia beyond the shoulder were 2.24 kg and 0.599 kg·m2, respectively, which are lower than those of a human. The stiffness of the developed elbow joint was 1410 N·m/rad, which is approximately seven times higher than that of a human. Considering the ratio of stiffness to inertia, the manipulator is expected to show a control performance that is comparable to those of conventional industrial manipulators. Comprehensive experiments, including joint stiffness tests and high-speed interaction tests, were conducted to verify the feasibility of the developed manipulator.

Online Identification of Power System Equivalent Inertia Constant

Abstract: This paper proposes a closed-loop identification method to estimate the equivalent inertia constant of a power system at the connection bus. A microperturbation is first performed with a well-designed multisine signal probed through any power electronic devices in the internal system. Then, responses of frequency and active power measured by the phase measurement unit at the connection bus are used for the closed-loop identification. Compared to the conventional transient signal based method, the proposed method has simple implementation and minimum impacts on the system security, and thus could be carried out in real-time to identify the time-varying and nonlinear equivalent inertia constant in modern power systems with complex heterogeneous components. The effectiveness of the proposed method is validated in an 8-generator 36-bus simulation system and an actual power system.

Modeling and Design of $df/dt$ -Based Inertia Control for Power Converters

Abstract: Power systems with high penetration rates of inverter-based generation units exhibit reduced system inertia Faults like plant outages or fault-induced system splits then cause an increased rate of change of frequency and may lead to frequency instability One of the proposed schemes to provide synthetic inertia with power electronic converters is the frequency-derivative-based ( $df/dt$ ) approach Converters with this method inject an active current that is proportional to the derivative of the grid frequency to mimic the inertia of synchronous plants Specification of synthetic inertia is currently being discussed for future grid codes In this paper, we show that the frequency derivative in the $df/dt$ controller is subject to a severe bandwidth limitation Considerable low-pass filtering is indispensable to avoid instability due to the excitation of local oscillation modes in the harmonic frequency range We have found a first-order lag with a time constant around 1 s to be required for a robustly stable system with common parameters A new discrete-time linear model that accurately represents sampling effects is introduced and comprehensively described A $df/dt$ control design is proposed that leads to a robustly stable system Model and control design are validated by laboratory experiments, especially regarding mechanisms of instability The main finding of this paper—applying a $df/dt$ controller harbors the risk of merely shifting the initial danger of system-wide frequency instability to local stability problems—is important for power plant manufacturers as well as system operators

Nonlocal Gravity

Abstract: The analysis of measurements of accelerated observers in Minkowski spacetime has led to the development of nonlocal special relativity theory Inertia and gravitation are intimately connected in accordance with the principle of equivalence We therefore seek a nonlocal generalization of the theory of gravitation such that in the new theory the field equations are integro-differential equations for the local gravitational field We show that it is possible to develop a nonlocal generalization of Einstein's theory of gravitation via the introduction of a scalar "constitutive" kernel in the teleparallel equivalent of general relativity The resulting nonlocal theory is essentially equivalent to Einstein's theory plus "dark matter" That is, nonlocality simulates dark matter by introducing a new source term into general relativity In the linear approximation for the nonlocal modification of Newtonian gravity, we recover the theoretical basis for the phenomenological Tohline-Kuhn modified gravity approach to the explanation of the astrophysical evidence for dark matter

Evolution of Motions of a Rigid Body About its Center of Mass

Abstract: The book presents a unified and well-developed approach to the dynamics of angular motions of rigid bodies subjected to perturbation torques of different physical nature. It contains both the basic foundations of the rigid body dynamics and of the asymptotic method of averaging. The rigorous approach based on the averaging procedure is applicable to bodies with arbitrary ellopsoids of inertia. Action of various perturbation torques, both external (gravitational, aerodynamical, solar pressure) and internal (due to viscous fluid in tanks, elastic and visco-elastic properties of a body) is considered in detail. The book can be used by researchers, engineers and students working in attitude dynamics of spacecraft.

Graph Spectra Based Controlled Islanding for Low Inertia Power Systems

Abstract: The inertia of modern power systems is decreasing and becoming more variable as more inverter-connected renewable energy sources and loads are integrated. This leads to a low and time-varied inertia power system that is more sensitive to disturbances and may not be robust enough to survive large disturbances. How to protect such a low and time-varied inertia system from blackouts is in question. This paper tries to answer this question using a graph spectra-based controlled islanding method. Eigenvector sensitivity, with respect to inertia, is used to identify the impact of reduced inertia on the spectral properties of power system graphs and, thus, coherent generator grouping. Constrained spectral clustering is then used to find the islanding boundary with minimal power-flow disruption to island low inertia systems. Simulation results, obtained using the IEEE 9-bus and 118-bus test systems, validate the effectiveness of the proposed algorithm in the case of low inertia systems.

Modeling and Experiments of a Hydraulic Electromagnetic Energy-Harvesting Shock Absorber

Abstract: Hydraulic electromagnetic energy-harvesting shock absorbers (HESAs) have been proposed recently, with the purpose to mitigate the vibration of vehicle suspensions and also recover the vibration energy traditionally dissipated by oil dampers. This paper designs an HESA prototype for heavy vehicles and creates a dynamic modeling to study its characteristics. The model shows that the HESA's output force can be decomposed into the electric damping force, friction damping force, the inertia force, and the accumulator force. Based on the modeling, the counteracting effect between the accumulator force and the inertia force is explained, and the influences of the parameters are analyzed. Simulations are conducted to investigate the effects of the high-pressure accumulator and the inertia on the regenerative voltage. Experiments are also carried out to study the characteristics of vibration damping and energy harvesting. Results show that the damping coefficient of the proposed HESA ranges from 32 to 91 kNs/m, which covers most of the damping range of 25–50 kNs/m for typical heavy-duty trucks and 15–80 kNs/m for railway freight vehicles. The average regenerative power reaches 220 W and the corresponding hydraulic efficiency reaches 30%, at a vibration input of 3 Hz frequency and 7 mm amplitude. Moreover, the influences of different hydraulic components on the hydraulic efficiency are also experimentally studied in order to guide the future design of HESAs.

Distributed virtual inertia based control of multiple photovoltaic systems in autonomous microgrid

Abstract: The large inertia of a traditional power system slows down system U+02BC s frequency response but also allows decent time for controlling the system. Since an autonomous renewable microgrid usually has much smaller inertia, the control system must be very fast and accurate to fight against the small inertia and uncertainties. To reduce the demanding requirements on control, this paper proposes to increase the inertia of photovoltaic U+0028 PV U+0029 system through inertia emulation. The inertia emulation is realized by controlling the charging U+002F discharging of the direct current U+0028 DC U+0029-link capacitor over a certain range and adjusting the PV generation when it is feasible and U+002F or necessary. By well designing the inertia, the DC-link capacitor parameters and the control range, the negative impact of inertia emulation on energy efficiency can be reduced. The proposed algorithm can be integrated with distributed generation setting algorithms to improve dynamic performance and lower implementation requirements. Simulation studies demonstrate the effectiveness of the proposed solution.

Relativistic theory of magnetic inertia in ultrafast spin dynamics

Abstract: The influence of possible magnetic inertia effects has recently drawn attention in ultrafast magnetization dynamics and switching. Here we derive rigorously a description of inertia in the Landau-L ...

Inertia estimation of the GB power system using synchrophasor measurements

Abstract: A novel procedure for estimating the total inertia of the Great Britain (GB) power system is presented Following an instantaneous in-feed loss, regional variations in the estimate of inertia are obtained from measured frequency transients using installed synchronised phasor measurement units (PMUs) A method is proposed to first detect a suitable event for analysis, and then filter the measured transients in order to obtain a reliable estimate of inertia for a given region of the GB network The total inertia for the whole system is then calculated as a summation, with an estimate also provided as to the contribution to inertia from residual sources, namely synchronously connected demand and embedded generation The approach is first demonstrated on the full dynamic model of the GB transmission system, before results are presented from analyzing the impact of a number of instantaneous transmission in-feed loss events using phase-angle data provided by PMUs from the GB transmission network and also devices installed at the domestic supply at 4 GB universities

A Unified Robust Motion Controller Design for Series Elastic Actuators

Abstract: Series elastic actuators (SEAs) have several mechanical superiorities over conventional stiff and non-back-drivable actuators, e.g., lower reflected inertia at output, greater shock tolerance, low-cost force measurement, energy storage, safety, and so on. However, their applications generally suffer from performance limitations, particularly in position control, due to insufficient controller designs. This paper proposes a unified active disturbance rejection motion controller for the robust position and force control problems of SEAs by using differential flatness and disturbance observer. It can suppress not only matched but also mismatched disturbances. Robust state and control input references are systematically generated in terms of a fictitious design variable, namely differentially flat output, estimations of disturbances and their successive time derivatives. The proposed robust motion controller improves the performance of SEAs when they suffer from internal and external disturbances, such as friction, inertia variation and external load, in real implementations. Experimental results are given to validate the proposal.

Failure of double friction pendulum bearings under pulse-type motions

Abstract: Summary Although the behavior of friction sliding bearings is well understood, the failure behavior has not been thoroughly investigated. However, predicting and understanding the failure of bearings is an important key in designing isolated structures to minimize their collapse in extreme events, and thus, this study is critical. Because of its relative simplicity and particular availability in certain markets, the failure of the double friction pendulum (DFP) bearing at its physical displacement limit is investigated. The bearing is modeled with a rigid body model including inertia for each of the bearing components. A nonlinear viscoelastic impact model is included to simulate the impact between bearing components. As isolation systems are particularly vulnerable to long-period excitations, analytical pulses are used as input excitations to investigate the influences of pulse parameters on the failure of DFP. The influences of DFP design parameters are investigated as well. To confirm that the response to the analytical pulses correctly represents the behavior under long-period ground motions, wavelet analysis to is performed on 14 pairs of pulse-type ground motion records to extract their pulses, and the failure prediction made from the extracted analytical pulse is compared with the failure from the real ground motions. It is found that using the extracted pulses provides a good estimation for the failure prediction of the ground motions. Copyright © 2016 John Wiley & Sons, Ltd.