Maximum Power Limit Research Articles

Energy-Efficient Joint User Association and Power Allocation in a Heterogeneous Network

Heterogeneous networks provide flexible deployments for operators to improve spectrum efficiency and increase coverage. However, driven by new generation wireless devices, the exponential increase of data traffic has triggered new challenges of wireless networks to meet the green communications requirement. Therefore, energy-efficient design has emerged as a promising technique in heterogeneous networks. In this paper, we investigate the energy efficiency maximization problem for downlink transmissions by jointly considering user association and power allocation in a two-tier heterogeneous network with multiple small cells. We first consider a system model without the co-channel interference between small cells. The energy efficiency maximization problem is formulated under certain prescribed quality-of-service requirement and maximum power limit constraint. The original optimization problem is non-convex and NP-hard, and it involves integer programming. We first relax the formulate problem into a continuous one and decouple it into user association and power allocation subproblems. A gradient-based algorithm is used to solve the power allocation problem. Then, an iterative joint user association and power allocation algorithm is proposed to achieve the maximum energy efficiency. Moreover, we consider a more sophisticated system with the limited bandwidth resource, in which multiple small base stations require to share the same frequency band to serve users, and the co-channel interference is introduced. Inspired by the original Dinkelbach method, we use a lower bound approximation and the Lagrangian approach to derive a closed-form expression of power allocation, which reduces the computational complexity. Simulation results show that the proposed algorithms have improved energy efficiency when compared with other the existing schemes.

Outage Probability in the Uplink of Multitier Millimeter Wave Cellular Networks

In this article, using the stochastic geometry, we develop a tractable uplink modeling framework for the outage probability of the multitier millimeter wave (mmWave) cellular networks. Each tier’s mmWave base stations (BSs) are randomly located and they have particular spatial density, antenna gain, receiver sensitivity, blockage parameter, and pathloss exponents. Our model takes account of the maximum power limitation and the per-user power control. More specifically, each user, which could be in line-of-sight (LOS) or non-LOS to its serving mmWave BS, controls its transmit power such that the received signal power at its serving BS is equal to a predefined threshold. Hence, a truncated channel inversion power control scheme is implemented for the uplink of mmWave cellular networks. We derive closed-form expressions for the signal-to-interference-plus-noise-ratio (SINR) outage probability for the uplink of the multitier mmWave cellular networks, which we later degrade to the single-tier network. Furthermore, we analyze the case with a dense network by utilizing the simplified model, where the LOS region is approximated as a fixed LOS disk. The results show that imposing a maximum power constraint on the user significantly affects the SINR outage probability in the uplink of mmWave cellular networks.

A Control Approach Integrating Electric Vehicle Charger and Dynamic Response of Demand-Side Resource Aggregators

Regardless of whether level 1, level 2, or dc fast charging, several methods are under development or available as an aftermarket option for Plug-in electric vehicles (EVs). While confining the charging behavior of EVs to low-load periods is difficult, it leads to significant and uncertain load peaks and valleys in the utility power network. Therefore, this paper proposes a new method for an electric vehicle charger. The proposed charging method involves power supervisory characteristics on the basis of the paradigm of a constant-current and a constant-voltage (CC–CV) charger so that the external commands from the demand-side resource aggregator in the smart grid can be executed. In the method, first, a feedback loop is included in the controller to formulate the relationship between battery voltage and charging power as a quadratic concave downward polynomial. The controlled output power at the beginning of constant voltage charging is then set to the maximum value of the quadratic polynomial because the charging power requirement near this point is closed to the highest. Hence, if the charging power is lower than the allowable maximum power, the proposed charger is determined to have followed the traditional CC–CV charging steps. On contrast, the proposed circuit launches the power control loop when the actual charging power is beyond the maximum power limitation of aggregator. The proposed charger can keep the power consumption inside an allowable range of aggregator. That is the power consumption at the EV charging sites becomes directly programmable by their aggregators. The proposed method is examined by conducting an interactive test of a remote energy management system and the proposed controller. The results revealed that the proposed method provide an easy method to dynamically control consumption pattern of the EV charging for demand-side power management.

Hurry Up and Get Out of the Way! Exploring the Limits of Muscle-Based Latch Systems for Power Amplification.

Animals can amplify the mechanical power output of their muscles as they jump to escape predators or strike to capture prey. One mechanism for amplification involves muscle-tendon unit (MT) systems in which a spring element (series elastic element [SEE]) is pre-stretched while held in place by a "latch" that prevents immediate transmission of muscle (or contractile element, CE) power to the load. In principle, this storage phase is followed by a triggered release of the latch, and elastic energy released from the SEE enables power amplification (PRATIO=PLOAD/PCE,max >1.0), whereby the peak power delivered from MT to the load exceeds the maximum power limit of the CE in isolation. Latches enable power amplification by increasing the muscle work generated during storage and reducing the duration over which that stored energy is released to power a movement. Previously described biological "latches" include: skeletal levers, anatomical triggers, accessory appendages, and even antagonist muscles. In fact, many species that rely on high-powered movements also have a large number of muscles arranged in antagonist pairs. Here, we examine whether a decaying antagonist force (e.g., from a muscle) could be useful as an active latch to achieve controlled energy transmission and modulate peak output power. We developed a computer model of a frog hindlimb driven by a compliant MT. We simulated MT power generated against an inertial load in the presence of an antagonist force "latch" (AFL) with relaxation time varying from very fast (10ms) to very slow (1000ms) to mirror physiological ranges of antagonist muscle. The fastest AFL produced power amplification (PRATIO=5.0) while the slowest AFL produced power attenuation (PRATIO=0.43). Notably, AFLs with relaxation times shorter than ∼300ms also yielded greater power amplification (PRATIO>1.20) than the system driving the same inertial load using only an agonist MT without any AFL. Thus, animals that utilize a sufficiently fast relaxing AFL ought to be capable of achieving greater power output than systems confined to a single agonist MT tuned for maximum PRATIO against the same load.

Power Capability of Hybrid Excited Synchronous Motors in Variable Speed Drives Applications

Power capability of synchronous machines, when operating in motoring mode, is discussed in this contribution. Maximum power limits and partial loads operations are both examined in the case of variable speed drives applications when the synchronous motors are driven with idealized inverters with limited volt–ampere ratings. This study is based on a per-unit ${d}$ - ${q}$ model normalized according to the base speed operation quantities. Different control strategies are thoroughly analyzed for different motors parameters. The use of per-unit quantities will help to draw some general conclusions regarding the effect of motors parameters on the drives performance. Since wound field (WF) excited and permanent magnet (PM) excited synchronous motors could be considered as particular cases of hybrid excited synchronous motors, the findings of this contribution would be applicable to all synchronous motors with a magnetic excitation source.

Effects of Tropical Deforestation on Surface Energy Balance Partitioning in Southeastern Amazonia Estimated From Maximum Convective Power

AbstractTo understand changes in land surface energy balance partitioning due to tropical deforestation, we use a physically based analytical formulation of the surface energy balance. Turbulent heat fluxes are constrained by the thermodynamic maximum power limit and a formulation for diurnal heat redistribution within the land‐atmosphere system. The derived turbulent fluxes of sensible and latent heat compare very well to in situ observations for sites with intact rainforest and soybean land cover in southeastern Amazonia. The equilibrium partitioning into sensible and latent heat flux compares well with observations for both sites, except for the soybean site during the dry season where water limitation needs to be explicitly accounted for. Our results show that tropical deforestation primarily affects the absorption of solar radiation and the water limitation of evapotranspiration, but not the overall magnitude of turbulent heat fluxes that is set by the thermodynamic maximum power limit.

Low-secondary electron emission yield under electron bombardment of microstructured surfaces, looking for multipactor effect suppression

The reduction of the total secondary electron emission during electron-matter interactions is required to improve the maximum power limit of the vacuum RF equipments prone to multipactor discharge. In this work, a chemical deposition technique was developed to create the appropriate micron or submicron surface roughness to significantly improve the typical characteristics of the flat silver coatings used for high-power microwave devices for space applications. Rough silver coated RF filters with a high surface aspect ratio, >3, was achieved. It is found that the secondary electron emission yield (SEY) remains lower than 1 throughout the primary electron energy range and therefore multipactor electron avalanche is avoided. The influence of aging on the multipactor discharge is also drastically reduced in rough silver coatings. An experimental linear correlation was found between the multipactor power level and the minimum interaction energy for which the number of incident and emitted electrons is equal.

Accommodating Discharging Power With Consideration of Both EVs and ESs as Commodity Based on a Two-Level Genetic Algorithm

The emerging and booming of electric vehicles (EVs) and energy storages (ESs) endow power systems extra flexibility thanks to their ES capability. The charging and discharging activities of these facilities can be dispersed to perform demand response and benefit power grid operations. However, synchronous discharging from massive EVs and ESs may impose a huge power supply impact to potentially reshape the existing power markets. Unfortunately, this impact is always ignored by traditional research. To address the above-mentioned issues, discharging power from EVs and ESs is regarded as a kind of commodity in this paper. On such a basis, a pricing policy, where prices for discharging power poured into the power market and the user-side loads are regulated, is applied. The regulation strategy simultaneously incorporates the considerations of the system load condition, maximum power limit, aggregated discharging power from both the EVs and the ESs, as well as the user-side load in a fair manner. Besides, the battery degradation of EVs and EVs has also been considered. Furthermore, the price regulation obeys a hierarchical optimization procedure in which the operator acts as the leader to maximize its revenue, while the end appliances act as followers individually balancing their cost bill and comfort level. Also, the pricing policy is tested on a two-stage hierarchical market with a Genetic Algorithm-based hybrid algorithm. The outcome demonstrates that a prominent performance can be achieved in load shaping and economic benefit via the policy.

A Regulation Policy of EV Discharging Price for Demand Scheduling

The widespread penetration of electric vehicles (EVs) will cause an increase in energy demand. To achieve an effective demand response approach, the participation of EV users is a crucial factor. Nevertheless, the discharging price found in the literature is generally set in accordance with the spot electricity price. A fair pricing strategy is thus desirable to motivate EV users to regulate their charging behavior for less bill payment, and at the same time to achieve load regulation as well as benefit for power companies. The charging and discharging prices should be cohesively equilibrated to achieve the task. This paper presents a new V2G (Vehicle-to-Grid) pricing policy by incorporating the system load condition, maximum power limit, and price rate for user load in a fair manner. The price setting follows a hierarchical optimization procedure between the operator and end users with the aim to maximize operator's profit and balance end users' bill and comfort level. The optimization for the operator is achieved via genetic algorithm whereas the energy management of each user is decomposed as a single power optimization problem. Simulation results have confirmed the benefit of liberating the discharging price from the user load tariff and have verified the effectiveness of the proposed method.

Determination of Generator Steady State Stability Limit for Multimachine System based on Network Losses Concept

In the multimachine circumstances, it is difficult to analyze the steady state stability of each generator. In previous research, analysis of the steady state stability limit has been carried out but only look at the stability of the overall system. Therefore, to analyze the stability of each generator, the multimachine system must be changed into a Single Machine to Infinite Bus (SMIB) system by collecting all the loads into one central load in the infinite bus. The method to change from the multimachine system to SMIB system is presented in this paper. The multimachine system is converted into an equivalent impedance (req and xeq) and an equivalent load based on losses concept. After req and xeq is calculated, then by using steady state stability limit concept, the value of the maximum generation of each generator units can be determined. By means of maximum generation is the maximum output power limit that can be generated without causing unstability. ETAP simulation is used to validate the calculation results of the proposed method. The method was applied to units generator in Java Bali system 500 kV.

Power-load management reduces energy-dependent costs of multi-aisle mini-load automated storage and retrieval systems

For economic and ecological reasons, the interest in the energy demand of material-handling systems is rising. As a result, the operators of these systems increasingly pay attention to the energy demand and the costs resulting from it. The energy demand of automated warehouses, with multi-aisle automated storage and retrieval systems, is volatile with uncontrolled power-peaks. These power-peaks result in high energy and hardware costs. In this paper, the effect of a power-load management on the throughput of the material-handling systems is investigated. We assume that the peaks of energy consumption can be significantly reduced by delaying tasks, without having an impact on the throughput. The goal is to find out the interdependence between the electrical power-limits of the power-load management (mean power demand in a period and maximum power demand) and the throughput of the warehouse. The results show that, with a maximum power-limit and a mean power-limit, the peaks in energy consumption can be avoided with only a slight loss of throughput. Load management is an effective method to reduce the energy peaks of an automated warehouse, thereby lowering the costs of automated warehouses.

An explanation for the different climate sensitivities of land and ocean surfaces based on the diurnal cycle

Abstract. Observations and climate model simulations consistently show a higher climate sensitivity of land surfaces compared to ocean surfaces. Here we show that this difference in temperature sensitivity can be explained by the different means by which the diurnal variation in solar radiation is buffered. While ocean surfaces buffer the diurnal variations by heat storage changes below the surface, land surfaces buffer it mostly by heat storage changes above the surface in the lower atmosphere that are reflected in the diurnal growth of a convective boundary layer. Storage changes below the surface allow the ocean surface–atmosphere system to maintain turbulent fluxes over day and night, while the land surface–atmosphere system maintains turbulent fluxes only during the daytime hours, when the surface is heated by absorption of solar radiation. This shorter duration of turbulent fluxes on land results in a greater sensitivity of the land surface–atmosphere system to changes in the greenhouse forcing because nighttime temperatures are shaped by radiative exchange only, which are more sensitive to changes in greenhouse forcing. We use a simple, analytic energy balance model of the surface–atmosphere system in which turbulent fluxes are constrained by the maximum power limit to estimate the effects of these different means to buffer the diurnal cycle on the resulting temperature sensitivities. The model predicts that land surfaces have a 50 % greater climate sensitivity than ocean surfaces, and that the nighttime temperatures on land increase about twice as much as daytime temperatures because of the absence of turbulent fluxes at night. Both predictions compare very well with observations and CMIP5 climate model simulations. Hence, the greater climate sensitivity of land surfaces can be explained by its buffering of diurnal variations in solar radiation in the lower atmosphere.

Techno-Economic Collaboration of PEV Fleets in Energy Management of Microgrids

This paper develops a generalized energy management approach for smart microgrids (MGs) with the collaboration of plug-in electric vehicle (PEV) charging facilities in both active and reactive powers exchange. In addition to the monetary saving and economical benefits, this feature reveals a promising opportunity for power quality improvement in modern MGs. To do this, an efficient multi-objective framework is proposed for the optimal operation scheduling of MGs. The optimization model incorporates the full potential benefits of PEV fleets in balancing both active and reactive powers supply and demand. The multi-objective formulation lies within a mixed-integer linear programming fashion, which is tackled by $\varepsilon $ -constraint method. A quadruple linear operating area is envisioned for PEV fleet inverters caped with the maximum apparent power limit. A suitable voltage-dependent load model, with negligible error, is deployed to tailor the proposed approach for real-world applications. The new methodology is successfully applied to a 33-bus test MG with illustrative case studies. The obtained results are reported in terms of voltage profile improvement, released capacity of generating units, moderated operation of under-load tap changers, and MG operation cost reduction.

Advanced control of direct-driven PMSG generator in wind turbine system

Abstract The paper presents the advanced control system of the wind energy conversion with a variable speed wind turbine. The considered system consists of a wind turbine with the permanent magnet synchronous generator (PMSG), machine side converter (MSC), grid side converter (GSC) and control circuits. The mathematical models of a wind turbine system, the PMSG generator and converters have been described. The control algorithms of the converter systems based on the methods of vector control have been applied. In the advanced control system of the machine side converter the optimal MPPT control method has been used. Additionally the pitch control scheme is included in order to achieve the limitation of maximum power and to prevent mechanical damage of the wind turbine. In the control system of the grid side converter the control of active and reactive power has been applied with the application of Voltage Oriented Control (VOC). The performance of the considered wind energy system has been studied by digital simulation. The results of simulation studies confirmed the good effectiveness of the considered wind turbine system and very good performance of the proposed methods of vector control and control systems.

Full-Core Evaluation of Uranium-Based Fuels Augmented with Small Amounts of Thorium in Pressure Tube Heavy Water Reactors

Thorium, a fertile nuclear fuel that is nearly three times as abundant as uranium, represents a long-term energy source that could complement uranium and eventually replace it. With the expected refurbishment and new construction of pressure tube heavy water reactors (PT-HWRs) within the international community, there is an opportunity to gain experience with thorium-based fuels and to start the transition toward the use of thorium as part of the nuclear fuel cycle.This paper presents an evaluation of fuel types that could be implemented in the near-term to transition into thorium-based fuels in current PT-HWRs. The near-term fuel consists of small amounts of thorium (in a traditional 37-element fuel bundle that is mostly filled with natural uranium or slightly enriched uranium). In addition, a modified 37-element fuel bundle type comprised of slightly enriched uranium fuel (1.2 wt% 235U/U or less), a thorium central element, and the mass equivalent of 1-cm thorium end pellets was studied. Both lattice physics depletion simulations and full-core time-averaged neutron diffusion simulations were carried out to evaluate the performance and safety characteristics of the different studied full-core configurations.The results demonstrate that adding small amounts of thorium into the fuel of a 37-element bundle is feasible, through enrichment, without reducing power in the reactor or incurring a severe burnup penalty. The most viable core configuration is a core filled with modified 37-element fuel containing slightly enriched uranium dioxide with 1.2 wt% 235U/U. Even with the addition of 1.2 kg of thorium metal to the bundle, significant gains are achieved, including an increased margin to maximum bundle power limit of 40 kW and a 50% increase in fissile utilization.

Broad climatological variation of surface energy balance partitioning across land and ocean predicted from the maximum power limit

AbstractLongwave radiation and turbulent heat fluxes are the mechanisms by which the Earth's surface transfers heat into the atmosphere, thus affecting the surface temperature. However, the energy partitioning between the radiative and turbulent components is poorly constrained by energy and mass balances alone. We use a simple energy balance model with the thermodynamic limit of maximum power as an additional constraint to determine this partitioning. Despite discrepancies over tropical oceans, we find that the broad variation of heat fluxes and surface temperatures in the ERA‐Interim reanalyzed observations can be recovered from this approach. The estimates depend considerably on the formulation of longwave radiative transfer, and a spatially uniform offset is related to the assumed cold temperature sink at which the heat engine operates. Our results suggest that the steady state surface energy partitioning may reflect the maximum power constraint.

MIMO Cognitive Relay Networks With Correlated Antennas Over Rayleigh Fading Channels

In this paper, we examine orthogonal space–time block coding with receiver maximal ratio combining (OSTBC/MRC) and selection combining (OSTBC/SC) in multiple-input–multiple-output (MIMO) decode-and-forward (DF) relay networks with underlay spectrum sharing, considering optimal and suboptimal cases. The secondary network under consideration is equipped with multiple correlated antennas at the source, the relay, and the destination. On the other hand, the primary network is composed of $L$ primary users (PUs), each of which is equipped with multiple correlated antennas. For the considered underlay spectrum sharing, the transmit power condition is related by an interference limit on the primary network and the maximum transmission power in the secondary network. In particular, new exact expressions for the outage probability of the adopted system models are obtained. Moreover, simple asymptotic expressions for the outage probability are provided to enable the characterization of the achievable diversity orders and coding gains. To enhance the secondary network performance, optimal power allocation (PA) between the secondary source and the secondary relay is obtained based on the asymptotic outage probability under the constraints of total allowable transmit power at the secondary user (SU) and maximum allowable interference power limit at the PU. The derived analytical formulas herein are supported by numerical results to clarify the main contributions.

Multi-agent supervisory control for optimal economic dispatch in DC microgrids

The deepening impact of global warming has stimulated numerous researchers to come up with alternative energy sources and transportation facilities that create fewer emissions. DC distribution systems are attractive candidates for adapting high penetration of distributed generation and electric vehicles. Accordingly, this work proposes a distributed power management scheme to enhance the reliability and economic performance of isolated dc microgrids. In the presented approach, the dc microgrid is assumed to contain both renewable and nonrenewable generating units. At lower loading conditions, the proposed scheme stimulates a power-sharing strategy among renewable sources to increase system reliability, whereas nonrenewable sources are kept operating at their lower power limits. Alternatively, renewable sources are allowed to operate at their maximum power limits while maintaining optimum economic operation among nonrenewable sources to meet higher loading conditions. To achieve these operational criteria, both renewable and nonrenewable DG units apply a consensus-based algorithm in a supervisory control level. Each DG unit exchanges information with its neighbors, thereby locally updating the no-load voltage setting of its primary control according to system voltage sensitivity analysis. The incorporation of DG droop-based primary control renders the proposed algorithms fully distributed with a reduced number of agents. The proposed algorithm has the additional advantage of restoring system voltage to its nominal value. The stability and convergence of the algorithm are analytically addressed, and real-time OPAL-RT simulations are performed in a hardware-in-the-loop (HIL) application to verify its effectiveness.

Математическое моделирование процессов автоматического регулирования частоты и мощности в ОЭС с солнечными электростанциями

The unstable nature of solar power plant (SPP) power output in the network determines the need for a comprehensive study of the relevant control processes and their impact on the operation of the power system, especially from the point of view of influence of processes of generation capacity in the SPP on the frequency, and on the relevant processes of automatic frequency and power control (AFPC). The mathematical model of the generation processes on the SPP included in the AFPC model allows to investigate the processes of regulation in the AFPC systems with traditional regulators (generators), and in the AFPC systems with fast controllable loads. On the example of interconnected power system (IPS) of Ukraine presents the results of experiments with the mathematical model of the SPP given in the article. It is established that for generators-regulators is a key limitation on the speed of the power set/reset, unlike controllable loads for which key is a maximum power limitation. It is revealed that under the condition of maximum power SPP equal to the maximum power of the generator-regulator of the power plant (3 GW) speed of the power set/reset of hydroelectric power provides effective regulation for the permissible frequency deviation of 0.02 Hz. Ceteris paribus, neither coal-fired nor oil-gas thermoelectric power station is not able to hold the frequency deviation even in the range of 0.2 Hz.

A nonlinear control algorithm for a wind turbine

Purpose – The purpose of this paper is to propose a new nonlinear control algorithm to control a wind turbine based on permanent magnet synchronous generator (PMSG) connected to the grid via a back-to-back converter. The control algorithm is composed of a flatness-based method for the machine side convertor (MSC) and a voltage-oriented method for the grid side converter (GSC). Design/methodology/approach – For the MSC control, the output variable is chosen properly to prove that the system is flat at first. Then, the appropriate reference trajectories are planned on its components. The reference trajectories are such designed that the system operates in maximum power point tracking (MPPT) mode. Finally, state feedback regulators are used to force the system output to follow its reference. To control the GSC, a classical voltage-oriented control method is used. Findings – The simulation results obtained with a random wind speed are presented in order to prove the validity of the proposed control algorithm. These results show that the system is controlled successfully while it operates in the MPPT mode or in its maximum power limitation mode. Originality/value – In this paper, a new algorithm based on flatness property is presented to control a variable speed wind turbine based on a PMSG. The proposed control method allows the system to operate in optimal operating modes.