Effect of reconstituted method on shear strength properties of peat

Peat is an organic soil contains more than 75% organic content. Shear strength of the soil is one of the most important parameters in engineering design, especially during the pre-construction and post-construction periods, since used to evaluate the foundation and slope stability of soil. Peat normally known as a soil that has very low shear strength and to determine and understand the shear strength of the peat is difficult in geotechnical engineering because of a few factors such as the origin of the soil, water content, organic matter and the degree of humification. The aim of this study was to determine the effective undrained shear strength properties of reconstituted peat. All the reconstituted peat samples were of the size that passing opening sieve 0.425mm, 1.000mm, 2.360mm and 3.350mm and were preconsolidated at pressures of 50 kPa, 80 kPa and 100 kPa. The relationship deviator stress- strain, σdmax and excess pore water pressure, Δu, shows that in both of reconstituted and undisturbed peat gradually increased when confining pressure, σ’ and pre- consolidation pressure, σc increased. As a conclusion, the undrained shear strength properties result obtained shows that the RS3.350 has higher strength than RS0.425, RS1.000 and RS2.360. However, the entire reconstituted peat sample shows the increment value of the shear strength with the increment of peat size and pre- consolidation pressure. For comparison purposes, the undrained shear strength properties result obtained shows that the reconstituted peat has higher strength than undisturbed peat. The factors that contributed to the higher shear strength properties in this study are segregation of peat size, pre- consolidation pressure, initial void ratio and also the physical properties such as initial water content, fiber content and liquid limit

Multiobjective H2/H∞ Control Design with Regional Pole Constraints

This paper presents multiobjective H2/H∞ control design with regional pole constraints. The state feedback gain can be obtained by solving a linear matrix inequality (LMI) feasibility problem that robustly assigns the closed-loop poles in a prescribed LMI region. The proposed technique is illustrated with applications to the design of stabilizer for a typical single-machine infinite-bus (SMIB) power system. The LMI-based control ensures adequate damping for widely varying system operating conditions. The simulation results illustrate the effectiveness and robustness of the proposed stabilizer

Design of wide-area damping control systems for power system low-frequency inter-area oscillations

The recently developed robust control theories and wide-area measurementtechnologies make the wide-area real-time feedback control potentially promising. Theobjective of this research is to develop a systematic procedure of designing a centralizeddamping control system for power grid inter-area oscillations by applying wide-areameasurement and robust control techniques while putting emphasis on several practicalconsiderations.The first consideration is the selection of stabilizing signals. Geometric measuresof controllability/observability are used to select the most effective stabilizing signals andcontrol sites. Line power flows and currents are found to be the most effective inputsignals. The second consideration is the effects of time-delay in the communication ofinput/output signals. Time-delays reduce the efficiency of the damping control system. Insome cases, large delays can destabilize the system. Time-delays should be modeled inthe controller design procedure so that the resulting controller can handle a range of timedelays.In this work, time-delays are modeled by Padé Approximations and the delayuncertainty is described by Linear Fractional Transformations (LFT). The thirdconsideration is the controller robustness. The synthesis of the controller is defined as aproblem of mixed H2/H∞ output-feedback control with regional pole placement and isresolved by the Linear Matrix Inequality (LMI) approach. The controller designed byrobust control techniques has satisfactory performance in a wide range of operatingpoints. The fourth consideration is the efficiency of the controller designed by lineartechniques in realistic nonlinear discrete environments. A tuning process and nonlinearsimulations are used to modify the controller parameters to ensure the performance androbustness of the controller designed with linear techniques. The last consideration is theselection of PMU data reporting rates. The performance of controllers designed in the sdomainis tested in digital environments and proper PMU data reporting rates are selectedwith consideration of the effects of time-delay.The design procedure of wide-area damping systems is illustrated by three studysystems. The first study system is a two-area four-machine system. The second one is theNew England 39-bus 10-machine system. The last one is a 29-generator 179-bus studysystem, which is a reduced order model of the Western Electricity Coordinating Council(WECC) system

Design of Observer-Based Robust Power System Stabilizers

Power systems are subject to undesirable small oscillations that might grow to cause system shutdown and consequently great loss of national economy. The present manuscript  proposes two  designs for observer-based robust power system stabilizer (PSS) using Linear Matrix Inequality (LMI) approach to damp such oscillations. A model to describe power system dynamics for different loads is derived in the norm-bounded form. The first controller design is based on the derived model to achieve  robust stability against load variation. The design is based on a new Bilinear matrix inequality (BMI) condition. The BMI optimization  is solved interatively in terms of Linear Matrix Inequality (LMI) framework. The condition contains a symmetric positive definite full matrix to be obtained, rather than the commonly used block diagonal form. The difficulty in finding a feasible solution is thus alleviated. The resulting LMI is of small size, easy to solve. The second PSS design shifts the closed loop poles in a desired region so as to achieve a favorite  settling time and damping ratio via a non-iterative solution to a set of LMIs.  The approach provides a systematic way to design a robust output feedback PSS which  guarantees good dynamic performance for different loads. Simulation results based on single-machine and multi-machine power system models verify the ability of the proposed PSS to satisfy control objectives for a wide range of load conditions

Wide-area oscillation damping in low-inertia grids under time-varying communication delays

Wide-Area Control (WAC) can be efficiently used for oscillation damping in power systems. However, to implement a WAC, a communication network is required to transmit signals between the generation units and the control center. In turn, this makes WAC vulnerable to time-varying communication delays that, if not appropriately considered in the control design, can destabilize the system. Moreover, with the increasing integration of renewable energy resources into the grid, usually interfaced via power electronics, power system dynamics are becoming drastically faster and making WAC more vulnerable to communication delays. In this paper, we propose a design procedure for a delay-robust wide-area oscillation damping controller for low-inertia systems. Its performance is illustrated on the well-known Kundur two-area system. The results indicate that the obtained WAC successfully improves the oscillation damping while ensuring robustness against time-varying communication delays

Stability analysis and robust control of power networks in stochastic environment

The modern power grid is moving towards a cleaner form of energy, renewable energy to meet the ever-increasing demand and new technologies are being installed in the power network to monitor and maintain a stable operation. Further, the interactions in the network are not anymore localized but take place over a system, and the control centers are located remotely, thus involving control of network components over communication channels. Further, given the rapid integration of wind energy, it is essential to study the impact of wind variability on the system stability and frequency regulation. Hence, we model the unreliable and intermittent nature of wind energy with stochastic uncertainty. Moreover, the phasor measurement unit (PMU) data from the power network is transmitted to the control center over communication channels, and it is susceptible to inherent communication channel uncertainties, cyber attacks, and hence, the data at the receiving end cannot be accurate. In this work, we model these communication channels with stochastic uncertainties to study the impact of stochastic uncertainty on the stability and wide area control of power network. The challenging aspect of the stability analysis of stochastic power network is that the stochastic uncertainty appears multiplicative as well as additive in the system dynamics. The notion of mean square exponential stability is considered to study the properties of stochastic power network expressed as a networked control system (NCS) with stochastic uncertainty. We develop, necessary and sufficient conditions for mean square exponential stability which are shown in terms of the input-output property of deterministic or nominal system dynamics captured by the mean square system norm and variance of the channel uncertainty. For a particular case of single input channel uncertainty, we also prove a fundamental limitation result that arises in the mean square exponential stabilization of the continuous-time linear system. Overall, the theoretical contributions in this work generalize the existing results on stability analysis from discrete-time linear systems to continuous-time linear systems with multiplicative uncertainty. The stability results can also be interpreted as a small gain theorem for continuous-time stochastic systems. Linear Matrix Inequalities (LMI)-based optimization formulation is provided for the computation of mean square system norm for stability analysis and controller synthesis. An IEEE 68 bus system is considered, and the fragility of the decentralized load-side primary frequency controller with uncertain wind is shown. The critical variance value is shown to decrease with the increase in the cost of the controllable loads and with the rise in penetration of wind farms. Next, we model the power network with detailed higher order differential equations for synchronous generator (SG), wind turbine generator (WTG). The network power flow equations are expressed as algebraic equations. The resultant system is described by a detailed higher order nonlinear differential-algebraic model. It is shown that the uncertainty in the wind speed appears multiplicative in the system dynamics. Stochastic stability of such systems is characterized based on the developed results on mean square exponential stability. In particular, we study the stochastic small signal stability of the resultant system and characterize the critical variance in wind speeds, beyond which the grid dynamics becomes mean square unstable. The power fluctuations in the demand side and intermittent generation (from renewables) cause frequency excursions from the nominal value. In this context, we consider the controllable loads which can vary their power to achieve frequency regulation based on the frequency feedback from the network. Two different load-side frequency controller strategies, decentralized and distributed frequency controllers are studied in the presence of stochastic wind. Finally, the time-domain simulations on an IEEE 39 bus system (by replacing some of the traditional SGs with WTG) are shown using the wind speeds modeled as stochastic as well as actual wind speeds obtained from the wind farm located near Ames, Iowa. It can be seen that, with an increase in the penetration of wind generation in the network, the network turns mean square unstable. Furthermore, we capture the mean square unstable behavior of the power network with increased penetration of renewables using the statistics of actual wind analytically and complement them through linear and nonlinear time domain simulations. Finally, we analyze the vulnerability of communication channel to stochastic uncertainty on an IEEE 39 bus system and design a wide area controller that is robust to various sources of uncertainties that arise in the communication channels. Further, the PMU measurements and wide area control inputs are rank ordered based on their criticality