Enhance System Reliability Research Articles

Variable switching frequency hybrid PWM technique for switching loss reduction in a three-phase two-level voltage source inverter

Power loss reduction is one of the main objectives in power electronic system design for achieving higher efficiencies and enhancing system reliability. In this regard, this paper aims to develop a variable switching frequency hybrid pulse width modulation (VSF-HPWM) method to maximize loss saving while maintaining the same current total harmonic distortion (THD) of the conventional scheme. In doing so, this technique utilizes the modulation signal and the switching frequency simultaneously. Using a predefined mathematical representation of current ripple in every switching cycle, the modulating signal (either discontinuous or conventional space vector modulation) with lower RMS current ripple is applied. Meanwhile, the number of commutations under the constrains of constant RMS ripple is reduced by varying the switching frequency. The performance analysis is validated through MATLAB Simulink which shows that the proposed strategy can save up to 53% of switching losses with a similar THD of the conventional scheme.

A novel methodology in optimal setting of directional fault current limiter and protection of the MG

Integration of the Distributed Generators (DGs) in the MicroGrid (MG) provides an additional contribution to the fault current causing change of the short circuit current which could lead to the miscoordination problem (in the forms of maloperation and delayed operation) in the existing Over Current Relay (OCR) based protection system. Moreover, contribution of the MG can cause the overall fault current to exceed the designed capability level of distribution components, like Circuit Breakers (CBs). In this case, CBs are subjected to increased stresses and are thus more prone to failure to operate when desired. As a result of the above issues (miscoordination and CB failure) the system reliability degrades.Utilizing a Directional Fault Current Limiter (DFCL) between the MG and upstream network is an effective way to mitigate the above-mentioned issues. Nonetheless, setting of the DFCL parameters is a challenging task and should be done with the aim of restoring coordination among OCRs and also enhancing system reliability. In this paper, a novel methodology for optimal setting of DFCL parameters (R and X) is proposed therein the effect of DFCL on OCRs coordination and system reliability is considered simultaneously. For this purpose, a new objective function in terms of DFCL installation and reliability cost is introduced to minimize the total cost of DFCL utilization. Based on the proposed strategy, which will be evaluated through various scenarios, the optimal coordination between existing relays are restored and the whole MG would be protected without the need to use adaptive protection schemes or new relays.

A Novel Modular Multiport Converter for Enhancing the Performance of Photovoltaic-Battery Based Power Systems

This paper introduces a novel multiport power converter with modular architecture for photovoltaic (PV)-battery based power systems. Compared to conventional centralized multiport converters, the proposed converter significantly improves the utilization of PV available energy and battery capacity as it does not require a high number of series-connected PV and battery modules. The proposed converter also eliminates the need for additional battery cell/module equalizer circuitry by being able to implement directly energy management strategies that consider the different capabilities of battery modules to ensure charge/stress balancing. This makes it a promising solution for interfacing second-life batteries or for systems that utilize batteries with a high degree of mismatch. The modularity of the proposed converter enhances system reliability and fault tolerance and reduces the power/voltage ratings of the power electronic devices. The converter modes of operation, control strategy and design considerations are discussed. A 75 V/1 kW integrated PV-battery power system prototype is built and tested to validate the concept.

Allocation of Renewables, Switches, and Relays Considering Relays Actual Operation Time

System reliability enhancement of distributed generations (DGs) without aid of fast switches (FSs) is not possible. FSs are responsible for disconnecting unfaulted islands from faulty network to reduce energy not supplied (ENS). Directional overcurrent relays (DOCRs) are employed to send trip signals to FSs. In most of the DGs optimal allocation studies, the switches are assumed to operate correctly. This assumption might not be correct due to the delayed operation of the DOCRs. Few studies offer restricting minimum allowable size of DGs or allowable location of DOCRs to guarantee correct operation of DOCRs. These additional constraints limit the search space which causes nonoptimal answers. Here, the impact of actual operation of DOCRs is considered directly in calculation of ENS and hence in the final objective function. Also, a modified formulation for ENS in order to include cases where DGs capacity is not sufficient for supplying island loads is developed. Simultaneous allocation of DGs, FSs, and DOCRs to minimize cost of energy losses, ENS, and environmental costs are proposed. A multistate model that combines all possible operating conditions of DGs and load levels is utilized. Different cases are simulated in 33-bus, 69-bus, and 118-bus systems to evaluate the proposed method.

Integration of a semisubmersible floating wind turbine and wave energy converters: an experimental study on motion reduction

The integration of wave energy converters and a floating wind turbine has the potential to reduce the cost of energy, since they can share the mooring system and the infrastructure of the power grid. In this study, oscillating-water-column-type wave energy converters mounted on a semisubmersible-type floating wind turbine are presented. Water tank tests are carried out to illustrate that the wave energy converters can not only capture the wave energy but also help in reducing the motion of the floating platform when active control is applied. This study provides a new perspective on hybrid renewable energy application to widen renewable energy penetration by enhancing system reliability.

Optimal deployment of remote‐controlled switches in distribution networks considering laterals

Remote-controlled switch (RCS) placement problem is used to optimally deploy the switches in distribution networks to enhance system reliability. The candidate locations of RCSs are usually assumed at the main feeder while the possible locations on the laterals are ignored. This study studies the impact of considering laterals as potential switch locations and demonstrates that deploying RCSs in laterals would change the optimal number and location of the switches. To do so, this study proposes a mathematical model for the switch deployment problem by taking into account the laterals. The proposed model is formulated in mixed integer linear programming format which can be solved via commercial solver in an effective runtime. The effectiveness of the model is scrutinised through simulating a sample radial distribution feeder, a standard test system, and a practical distribution network. Also, to illustrate the superiority of the proposed model, the results are compared with those achieved via previous models.

A Modified $Q-\dot{V}$ Droop Control for Accurate Reactive Power Sharing in Distributed Generation Microgrid

The droop-control technology, which can achieve an autonomous power sharing among distributed resources interface converters, provides the advantages of enhanced system reliability and reduced complexity. However, the reactive power sharing accuracy of the conventional Q - V droop method can be degraded due to its dependence on the line impedances. The Q - V̇ droop-control method was proposed to reduce the influence of line impedance. The reactive power sharing accuracy is improved but still affected by the V̇ restoration operation. In this paper, a modified V̇ restoration mechanism is proposed. Its influence on the power sharing is theoretically explained and analyzed. Both simulation and experiment results are presented to verify the proposed method.

New level doubling architecture of cascaded multilevel inverter

This study presents a new topology of single-phase cascaded multilevel inverter (CMLI). The proposed topology offers an optimised DC source utilisation, reduced switch count and curtailment of active switches in the conduction path for minimising power losses. It can produce almost twice the number of output voltage steps in comparison to the cascaded H-bridge, hence named level-doubling architecture, and can be operated as both symmetric and asymmetric CMLIs. Identical modules of proposed CMLI precludes requirement of a variety of semiconductors and provides ease for spare management. The modular design also facilitates mass production and enhances system reliability. Furthermore, the proposed topology can be easily extended to high-voltage applications. The proposed design is tested for its practicability by simulations in MATLAB/Simulink and results are verified by experimental set up of a scaled prototype single-phase model.

Delay and energy-efficient data collection scheme-based matrix filling theory for dynamic traffic IoT

Data collection is the basic functions of the Internet of Things (IoT), in which the sensed data are concentrations from sensor nodes to the sink, with a timely style, so the smart response can be done for emergency. The goal of multi-modal sensor data fusion is to obtain simple and accurate data to enhance system reliability and fault tolerance. Energy efficiency and small delay are the most important indicators which govern the performance of IoT. Convergecast is a low-latency data collection strategy based on effective time division multiple access (TDMA), in which each sensor node generates a packet, and m packets can aggregate to a packet. However, in most practical networks, sensor nodes do not necessarily generate packets during each data collection cycle, but instead generate packets from time to time. In the previous convergecast strategy, each node was fixedly allocated a slot, which increased the delay and wasted energy. A delay and energy-efficient data collection (DEEDC) scheme-based matrix filling theory is proposed to collect data in a randomly generated WSNs with minimum delay and energy consumption. The DEEDC scheme uses a clustering approach. For each cluster, the number of slots required for transmission is calculated by matrix filling theory, not the number of nodes that actually generate data. This ensures that data can be collected in a network with randomly generated data (number of slots ≤ number of nodes), thereby avoiding the allocation of slots for each node and the acquisition of redundant data to lead to the wastage of time and energy. Based on the above, a mixed slot scheduling strategy is proposed to construct energy and delay-efficient, collision-free schedule scheme. After extensive theoretical analysis, by using the DEEDC scheme, the delay is reduced by about 50~80%, and the energy consumed is reduced by about 40~57%.

Multiline Bus Bunching Control via Vehicle Substitution

Traditional bus bunching control methods (e.g., adding slack to schedules, adapting cruising speed), in one way or another, trade commercial speed for better system stability and, as a result, may impose the burden of additional travel time on passengers. Recently, a dynamic bus substitution strategy, where standby buses are dispatched to take over service from late/early buses, was proposed as an attempt to enhance system reliability without sacrificing too much passenger experience. This paper further studies this substitution strategy in the context of multiple bus lines under either time-independent or time-varying settings. In the latter scenario, the fleet of standby buses can be dynamically utilized to save on opportunity costs. We model the agency’s substitution decisions and retired bus repositioning decisions as a stochastic dynamic program so as to obtain the optimal policy that minimizes the system-wide costs. Numerical results show that the dynamic substitution strategy can benefit from the “economies of scale” by pooling the standby fleet across lines, and there are also benefits from dynamic fleet management when transit demand varies over time. Numerical examples are presented to illustrate the applicability and advantage of the proposed strategy. The substitution strategy not only holds the promise to outperform traditional holding methods in terms of reducing passenger costs, they also can be used to complement other methods to better control very unstable systems.

The JET upgraded toroidal Alfvén Eigenmode Diagnostic System

The Alfvén Eigenmode Active Diagnostic system (AEAD) has undergone a major upgrade and redesign to provide a state-of-the-art excitation and real-time detection system for JET.The new system consists of individual 4 kW amplifiers allowing for increased current, separate excitation and real time control of relative phasing between antenna currents. The amplifiers have a frequency range of 10–1000 kHz, divided into various frequency bands by external matching filters. Due to the varying transmission line impedance throughout the frequency range, the amplifiers were designed with a very high resilience to reflected power.The existing amplifier control electronics have been replaced with a digital control system incorporating a National Instruments platform and Field Programmable Gate Array (FPGA) modules for frequency, gain and phase control with a frequency and phase resolution of less than 1 kHz and 1 degree respectively. Complementing the digital control system is the Protection and Control System, which utilizes Field Programmable Analog Arrays (FPAAs) and an array of electronic devices to monitor and control the AEAD.New capabilities such as independent antenna current/phase control, allow for improved excitation control, better definition of antenna spectrum combined with enhanced system reliability. This paper will review the new AEAD system, its unique capabilities and improvements over the previous diagnostic system.

Event-Triggered-Based Distributed Cooperative Energy Management for Multienergy Systems

This paper investigates the issues of day-ahead and real-time cooperative energy management for multienergy systems formed by many energy bodies. To address these issues, we propose an event-triggered-based distributed algorithm with some desirable features, namely, distributed execution, asynchronous communication, and independent calculation. First, the energy body, seen as both energy supplier and customer, is introduced for system model development. On this basis, energy bodies cooperate with each other to achieve the objective of maximizing the day-ahead social welfare and smoothing out the real-time loads variations as well as renewable resource fluctuations with the consideration of different timescale characteristics between electricity and heat power. To this end, the day-ahead and real-time energy management models are established and formulated as a class of distributed coupled optimization problem by felicitously converting some system coordinates. Such problems can be effectively solved by implementing the proposed algorithm. With the effort, each energy body can determine its owing optimal operations through only local communication and computation, resulting in enhanced system reliability, scalability, and privacy. Meanwhile, the designed communication strategy is event-triggered, which can dramatically reduce the communication among energy bodies. Simulations are provided to illustrate the effectiveness of the proposed models and algorithm.

Transaction Mechanism Based on Two-Dimensional Energy and Reliability Pricing for Energy Prosumers

Prosumers, users that consume and produce energy, increase the diversity of energy system operations as distributed sources. However, they can reduce energy system reliability by increasing uncertainty. This study presents a novel transaction mechanism based on dynamic pricing for enhancing energy system reliability. The proposed dynamic energy-reliability pricing-based transaction mechanism (ERT) increases the controllability of prosumer uncertainties by a two-dimensional pricing mechanism based on time and reliability status, as compared to conventional time-based one-dimensional energy prices. Under the proposed ERT, utilities and prosumers exchange information about the utility price and the prosumer’s intent in order to ensure that demand is met. A two-way information infrastructure built for prosumer energy trading is used for this task. The utility enhances system reliability using this information, and the prosumer increases revenue through pricing selection. The practical implementation of the proposed ERT is described for both utilities and prosumers. A case study using practical renewable generation data revealed that the proposed ERT improves not only the reliability factor of the utility but also prosumer revenue as compared to a conventional energy-based dynamic pricing case. It is also shown that an economical optimum point that maximizes prosumer net revenue exists when electrical energy storage (EES) is applied to enhance performance. Increasing to the EES capacity provided room for uncertainty management, net revenue is improved, but the economic burden by the EES cost is increased. Under the proposed ERT, the optimal point results in greater EES capacity and higher net revenue enhancement than the conventional case.

Low operational cost distributed prioritised coordinated control for DC microgrids

This study presents a distributed prioritised coordinated control topology that has the capability to reduce the total generation cost and maintain the average DC microgrid voltage. The main advantage of this topology over the addressed ones is developing a prioritised normalised running cost methodology. In this methodology, the best economical combination of DGs is selected to fulfil the load demand and maintain the minimum required spinning reserve power. The developed coordinated controllers contain tertiary and secondary levels cooperating to guarantee economical load sharing between the dispatchable DGs through precise adjustment of the primary voltage levels. Simulation and experimental results verify the efficacy and feasibility of the proposed dispatch scheme compared to the conventional power-sharing techniques. In addition, system reliability enhancement during communication link failure and DG plug-n-play are investigated as well.

A green SLA constrained scheduling algorithm for parallel/scientific applications in heterogeneous cluster systems

Cloud services utilize Heterogeneous Clusters as the hardware infrastructure at the backend. Minimal energy consumption in Heterogeneous Cluster System (HCS) is an important but challenging job. It can bring numerous benefits such as drop in operational cost, enhanced system reliability and green environment. Scheduling on HCS, with the objectives of minimizing the energy consumption, is being addressed by the research community at large. This work proposes a scheduling algorithm with green Service Level Agreement (SLA) constrained execution of parallel applications. The green SLA assumes that the tasks are to be scheduled on Dynamic Voltage Frequency Scaling (DVFS) enabled HCS. The proposed algorithm acts in two phases: first, it minimizes the energy consumption for the slacks of the schedule using DVFS technique, and second, a green SLA is made between a customer and service provider to maintain a good energy-performance trade-off. With this strategy, makespan of the schedule is extended a bit but the energy consumption is minimized for the period of extended makespan. It is observed that the decrease in one level of voltage and frequency of high-speed Processing Element (PE) saves more energy compared to that of low-speed PE. To utilize this, jobs running on higher voltage and frequency level of the PE are prioritized first. The performance of the proposed algorithm is studied by simulation on several standard Directed Acyclic Graphs (DAGs) and few other well-known real-world scientific workflows. The comparative study exhibits the effectiveness of the proposed algorithm over state of the art on four real heterogeneous PEs.

Design of rice intelligent water-saving irrigation system based on agricultural internet of things

In order to realize the real-time remote monitoring and accurate management of the paddy field environment, the intelligent water-saving irrigation system based on the agricultural Internet of things was designed. The data packet consists of water level, soil moisture, soil fertilizer, crop height, real scene photos and so on, which are collected by the terminal monitoring equipment group, and is forwarded to the gateway through the main controller. The gateway transmits the processed data packets to the cloud server through GPRS, and then sends to the user app or the Internet web page. App and the internet can set the upper and lower threshold of the terminal node water level. According to the comparison results between the water level and the threshold set by the water level sensor, the node solenoid valve is controlled to open or close the gate of the rice field intake, and the automatic irrigation is realized. The system can set a fixed number for each monitoring node to find fault units quickly and enhance system reliability. The system gets the advantages of simple implementation, low power consumption, perfect function, and high reliability.

A systematic data-driven Demand Side Management method for smart natural gas supply systems

Advanced sensor and communication technologies can make natural gas supply systems smarter than ever before, in both system management and operation. This paper presents the development of a novel data-driven Demand Side Management, whose framework includes demand forecasting, customer response analysis, prediction of dynamic condition of the gas network, quick supply reliability evaluation, multi-objective optimization and decision-making. The aims of this DSM method are to smooth load profiles, improve company profit and enhance system reliability, by means of a dynamic pricing strategy. To verify the effectiveness of the developed framework, a case study is considered, concerning the management of a relatively complex gas supply system, wherein four different pricing periods are introduced for comprehensively testing. The results in the case study show that the DSM framework is able to effectively achieve the targets of peak shaving and valley filling. Besides, it can significantly and stably improve the system efficiency and reliability, for different pricing periods. Finally, pricing period determination is discussed in relation to the features of performance.

Versatile decentralised control of the DC microgrid

This study proposes a versatile decentralised control strategy for the direct current (DC) microgrid based on the DC bus signalling method. Performance of the proposed control strategy is investigated under various generation and load disturbances in both grid-connected and islanded operation modes of the DC microgrid. The studies are conducted using a realistic DC microgrid study system simulated in the PSCAD/EMTDC software. The study results indicate that the proposed control strategy: (i) effectively maintains the power balance in the DC microgrid, (ii) effectively limits the DC bus voltage deviations under large disturbances, (iii) improves stability and dynamic response, (iv) does not curtail renewable power generation unless it is necessary, (v) does not require excessively large grid-tied converter and battery energy storage systems, and (vi) enhances system reliability, flexibility, modularity, and expandability.

Optimal placement of switching and protection devices in radial distribution networks to enhance system reliability using the AHP-PSO method

This paper presents a new method to determine the optimal number and locations of autorecloser and sectionalizer switches (AR/S) in distribution networks. The costs of AR/S investment, switch maintenance, and undistributed energy as well as reliability coefficients are considered in the objective function. Reliability parameters such as SAIFI, SADI, MAIFI, and ENS are evaluated in the case study system. As a new method, the weights of the reliability parameters are obtained by decision-makers using the analytical hierarchy process (AHP). The optimal size, type, and location of automatic switches are determined by minimizing the objective function using the particle swarm optimization (PSO) algorithm. The effectiveness of the proposed method is tested by implementing it on two feeders of Isfahan's electric distribution network. The results indicate the efficiency and applicability of the proposed method.

Adaptive Protection Coordination Scheme Using Numerical Directional Overcurrent Relays

Dynamically changing distribution system operating conditions compromises the coordination of the existing protection schemes. This paper proposes an online adaptive protection coordination scheme by utilizing numerical directional overcurrent relays (DOCRs) and commercial A Mathematical Programming Language (AMPL) based Interior Point OPTimization (IPOPT) solver. The proposed scheme also utilizes intelligent electronic devices and a communication channel to obtain real-time system information and to update relays settings. The presented approach is able to handle different operating conditions of the system, including loss of loads, generations, and lines. Further, the miscoordination arising due to injections of distributed generations (DGs) is also handled. The proposed approach has been tested on the IEEE 14-bus system (with and without DG), under various operating conditions. Overall system reliability enhancement using the proposed approach has been analyzed in terms of the amount of energy not supplied. Further, the optimum settings of DOCRs obtained using AMPL based IPOPT solver has been compared with those obtained by General Algebraic Modeling System based Sparse Nonlinear OPTimizer solver, OPTI Toolbox based IPOPT solver, MATLAB based interior point method, and metaheuristic genetic algorithm and differential evolution. It has been found that the proposed coordination approach gives good results in reasonably small time and thus, is suitable for the adaptive protection scheme.