High Priority Loads Research Articles

Impact of demand-side management on the sizing of autonomous solar PV-based mini-grids

Solar PV-based autonomous mini-grids represent an economically affordable and robust electrification option for rural communities. However, the initial investment cost for renewable energy technologies such as solar PV remains high for rural communities. Implementation of demand-side management (DSM) could increase the cost-efficiency of mini-grids in rural areas. This requires demand-side knowledge, but little is still known of electricity demands in recently electrified areas and, in particular, of how DSM implementation could impact mini-grids. The few studies available focus either on systems or on appliance levels while this study aims to determine cost-efficiency impacts of DSM implementation at a category level. A shifting strategy is applied based on classification into high priority loads and low priority loads. Autonomous rural mini-grid components sizing for four different load categories and load flexibility are carried out using particle swarm optimization. The results show that different load category combinations result in large variations in terms of possible levelized energy cost reductions and, thus, in terms of the cost-optimal sizing of the mini-grid components. The DSM implementation on the household and productive use categories have the largest capacity of reducing the levelized energy cost, by 45.8% and 20.7%, respectively, compared to the no demand-side management case.

Stability Analysis of Microgrid with Load Shedding Scheme using RTDS

A microgrid system was simulated using the powerful RTDS simulator, the modelling was done on the RSCAD software, and the microgrid comprised of a diesel generator, a photovoltaic (PV) system and a Doubly-Fed Induction Generator (DFIG) wind turbine system as DERs. The paper comprises the study on stability analysis of the microgrid in grid-connected and islanded modes of operation, along with a successful load shedding scheme which was implemented based on the loads distinguished as low priority and high priority loads in order to keep the system balanced during faults.

Transactive Energy Rationing in Islanded Electric Power Systems

Extreme weather events, many of which are climate-change related, are occurring with increasing frequency and intensity and causing catastrophic outages. One of the major modern-day concerns of utilities is dealing with such extreme outages and consequently, its repercussions on the lives of people in society and social aspects. Traditionally, operators have been using rolling blackouts as a contingency plan to serve critical loads during such events when electricity supply is scarce. However, such blackouts practices are executed on a last-minute mandatory basis, depriving customers having low-capacity high-priority loads (i.e. refrigeration, water, telecommunication, etc) that should also be serviced if at all possible. Historically, policymakers have often adopted quota-based regulatory actions or rationing for other commodities (such as gasoline, butter, sugar) to handle scarcity situations. Motivated from such quota-based systems, our contribution presented in this work is an alternative transactive rationing mechanism that would provide some minimum level of service to all of the customers and serve the critical loads using market-based control during such scarcity-based contingencies. This is in contrast to the state-of-art TE mechanisms that allocate resources to customers solely based on their willingness-to-pay. The effectiveness of the proposed mechanism is demonstrated through simulation-based evaluation on two real-life use cases having feeder-level and microgrid-level configurations respectively. Simulation results clearly demonstrate the capability of the rationing scheme to serve customers' high-priority loads through prolonged outages even during extreme scarcity scenarios.

Resilience enhancement of active distribution networks in the presence of wind turbines and energy storage systems by considering flexible loads

In recent years, global warming has led to a significant increase in the occurrence of disastrous conditions. These events always have devastating effects on distribution systems and lead to massive load shedding. Therefore, the structure of distribution systems must have sufficient strength and resilience to deal with these events. Hence, this paper presents a mixed-integer linear programming (MILP) framework for optimal placement of sectionalizing switches and backup distributed generators (DGs) to enhance resilience under emergency conditions. In the proposed model, low priority, medium priority and high priority loads are considered and a demand response (DR) program is also implemented. In order to evaluate the performance of the system during the extreme weather conditions, 15 scenarios have been generated by the fragility function and the outage rate of lines in each scenario has been calculated. The proposed model is implemented on a modified IEEE 33-bus distribution system and is solved in the form of five case studies by the CPLEX solver in GAMS software. The results demonstrate that optimal placement of switches leads to a 14.32% reduction in the amount energy not served (ENS) under emergency conditions. Also, the results illustrate that the optimal placement of DGs along with the possibility of topology change leads to a 57.03% reduction in the amount of ENS. Finally, the results show that applying DR program leads to a 26.7% reduction in load shedding of high priority loads.

Generic microgrid controller with self-healing capabilities

Generic Microgrid Controller (GMC) specifications, developed at the University of California, Irvine (UCI) to support the standardization of microgrid controllers and guide IEEE standards were applied to the UCI Microgrid (UCIMG) for testing and deployment. While transition and dispatch capabilities of GMC were simulated and demonstrated, response of the controller to internal faults while islanded or to inadvertent trip of generation were not addressed previously. The present study adds self-healing capabilities to the GMC for islanded microgrid operations with the goal to enhance reliability of serving critical loads, and resiliency of the system. To verify and assess self-healing attributes as well as impacts on reliability and resiliency while islanded, the self-healing capability was simulated on the UCIMG model developed on MATLAB Simulink. The model was enhanced with recently deployed redundancies including two new switchyards and redundant lines, and was then divided into sections to (1) focus on critical and high priority loads, and (2) reduce the number of nodes and computational requirements. Our study explores the resiliency of a real system microgrid platform using the FLISR (fault location, isolation and service restoration) approach as the self-healing capability as part of the microgrid controller. This expert systems approach with precalculated and predetermined sequence makes it a practical and simple solution which can be implemented without having to deal with computational limits of the controller and time constraints. Results show that, even with a fault-induced circuit outage during islanded operation, critical loads can be served by interconnected circuits and rapidly dispatched switching, thereby minimizing the loss of critical and noncritical loads, and increasing reliability and resiliency.

Resilience enhancement of integrated electricity-gas-heating networks through automatic switching in the presence of energy storage systems

Multi-carrier energy systems meet the needs of customers in different forms of energy. Given that these systems cover a large number of customers, it is very important to improve their resilience in emergency conditions. Hence, this paper presents a mixed-integer non-linear programming (MINLP) framework to enhance the resilience of multi-carrier energy systems integrated with electricity-gas-heating networks. In this regard, residential, commercial and industrial energy hubs with different load supply priorities, wind turbines and power to gas (P2G) technologies are considered in the model. In the proposed model, the system operator is capable to enhance system resilience under emergency conditions through automatic switching and implementing the direct load control (DLC) program. In this model, the scenario-based method is utilized to deal with load demand, wind speed, and solar radiation uncertainties. The proposed model is implemented on both medium-scale and large-scale systems and is solved by DECOPT solver in GAMS software. The results indicate that automatic switching reduces the load shedding by 84.17% during the outage of the line. The results also illustrate that the DLC program has reduced load shedding at high priority loads by about 66%. Finally, the high impact of storage systems and P2G technology on enhancing system resilience has been proven.

Smart-UPS with a priority-based load management system

In this paper, a cost-effective Smart-UPS has been developed which will perform load shedding on a priority basis. This smart UPS will include an accessory installed along with the conventional UPS. This paper relates to the field of managing power distribution. In particular, it pertains to a system and method for power distribution among devices, based on priority. The objective of the present disclosure is to efficient supply to distribute power among requirement of devices, to provide cost efficient power management and to enhance output of the devices with limited power back up.The designed project will switch off lower priority loads according to Battery percentage to minimize the load suffered by the battery. The loads connected to supply load are divided into three sections, i.e. high priority (HP), medium priority (MP) and low priority (LP) loads. The project involves the development of a smart UPS system which will automatically switch off/on the high priority (HP) loads, medium priority (MP) and low priority (LP) loads (specified during installation of the UPS as specified by the user).

Internal Combustion Engine as a New Source for Enhancing Distribution System Resilience

Enhancing distribution system resilience is a new challenge for researchers. Supplying distribution loads, especially the residential customers and high-priority loads after disasters, is vital for this purpose. In this paper, the internal combustion engine (ICE) vehicles are firstly introduced as valuable backup energy sources in the aftermath of disasters and the use of this technology is explained. Then, the improvement of distribution system resilience is investigated through supplying smart residential customers and injecting extra power to the main grid. In this method, it is assumed that the infrastructure of distribution system is partially damaged (common cases) and it can be restored in less than one day. The extra power of residential customer can be delivered to other loads. A novel formulation for increasing the injected power of the smart home to the main grid using ICE vehicles is proposed. Moreover, the maximum backup duration in case of extensive damages in the distribution system is calculated for some commercial ICE vehicles. In this case, the smart home cannot deliver extra energy to the main grid because of its survivability. Simulation results demonstrate the effectiveness of the proposed method for increasing backup power during power outages. It is also shown that ICE vehicles can supply residential customers for a reasonable amount of time during a power outage.

Distributed Restoration for Integrated Transmission and Distribution Systems With DERs

A resilient power system is capable of providing fast and efficient recovery from outages caused by natural or man-made attacks. After a major blackout, transmission, and distribution system operators (TSOs and DSOs) coordinate to restore the system in a timely and reliable manner. However, these two systems are usually operated separately without considering capabilities and limitations of each individual network, or their mutual impacts. This could prolong the restoration process and lead to unpractical outcomes. Therefore, a distributed strategy for integrated transmission and distribution systems restoration (ITDSR) has been developed in this paper. Based on the alternating direction method of multipliers (ADMM), the distributed algorithm is able to provide the coordinated restoration for both TSOs and DSOs by sharing limited information of boundary buses in an iterative procedure. The ADMM-ITDSR model includes convex ac power flow model and three-phase unbalanced branch flow model. The ITDSR strategy coordinates the operation of distributed energy resources in terms of serving high priority loads across different networks. The effectiveness and advantage of developed models and algorithms are validated and demonstrated through testing of the integrated IEEE test cases, such as 14-bus transmission with 13-node distribution systems, and 39-bus transmission with 123-node distribution systems.

Load shedding scheme with under‐frequency and undervoltage corrective actions to supply high priority loads in islanded microgrids

In the last few years, microgrids have become a potential solution to improve reliability and security in the supply of electricity to power systems. When a microgrid changes its operation mode from grid-connected to islanded, particular attention must be paid by the operation services. In the islanded mode, the microgrids' frequency and voltage may reach undesirable values, harming the security and quality of the operation. This study aims to propose global and local strategies of load shedding to preserve the energy supply to high priority loads within quality standards. The focus of this study is to consider a microgrid operating only with primary control. In this sense, a proper microgrid time-continuous load flow is applied, considering the constant verification of the frequency and the voltage of all buses. Finally, a Monte Carlo simulation is used to validate the proposal presented and to give some indices that quantify the load shed in each period. The results show the superior performance of the proposed strategy compared to a state-of-the-art load shedding solution that does not consider the priority of loads.

Energy management and performance evaluation of grid connected PV-battery hybrid system with inherent control scheme

To decrease the payback time of the photovoltaic (PV) system and make it financially attractive, we propose a Photovoltaic Customer Grid Supply System (PCGSS) with bi-directional power flow, so the surplus energy fully or partially is delivered of the grid, following its needs, while, for critical operating conditions, the grid supplies the loads directly. For this purpose, a new management algorithm is proposed to optimize the generated power and allows the system to act as a distributed source inside the utility grid. Two scenarios are considered; in the first, the grid absorbs all the power delivered by the distributed system, while in the second, the power is injected to the grid based on the its requirements. Furthermore, to meet the proposed objectives, the PV system is strictly sized to supply only the high priority loads, in addition, the system has to work at the Maximum Power Point (MPP) or at the Limited Power Point (LPP) modes to meet the management algorithm requests. To reach the LPP, a modified Perturbation and Observation (P&O) algorithm is used. Meanwhile, to sustain the functionality of the system and to assure the continues supply of energy, a battery system has been integrated with the PV system.

A Restorative Self-Healing Algorithm for Transmission Systems Based on Complex Network Theory

Restorative self-healing is one of the most important features of smart grids. The major purpose of a restorative self-healing control strategy is to steer a power system to secure operating states. In this paper, a self-healing transmission network reconfiguration algorithm based on the complex network theory is proposed. The capacities of generators and the amounts of important loads (i.e., the high priority loads), as well as the distribution and importance of each transmission line in the outage area are considered in the proposed method using the presented electrical “betweenness.” Next, the optimal restoration paths and the optimal restorative self-healing strategy can be attained automatically. Finally, the New England 10-unit 39-bus system and a part of the actual Guangdong power system in China are employed to illustrate the features of the proposed method.

A novel smart grid theory for optimal sizing of hybrid renewable energy systems

Smart grid enables interaction between the generation and load to optimally deliver energy based on the operating conditions. Although smart grid solves many of the contemporary problems, it gives rise to new control and optimization problems especially with the growing role of renewable energy sources. Load shifting technique has been applied in this paper by dividing the load into two categories, high priority load (HPL) and low priority load (LPL). HPL is that must be supplied whatever the generation conditions, but LPL can be supplied when the generation from renewable sources is available. This paper introduces a new proposed design and optimization program for techno-economic sizing of standalone hybrid PV/wind/diesel/battery energy systems under smart grid theory for the lowest cost of generated energy at highest reliability. An accurate methodology for pairing between five Saudi Arabia sites and ten wind turbines (WTs) from different manufacturers to maximize energy production and minimize the price of kWh generated has been introduced. The new proposed program is implemented in flexible fashion which is not available in many market available programs. Many valuable results can be extracted from the proposed program that could help researchers and decision makers.

Shipboard power restored for active duty

Network reconfiguration for restoration in a military shipboard power system is a very important task that is performed in order to restore loads as a result of battle damage or system faults. This article features an expert-system-based reconfiguration methodology for load restoration in shipboard power systems. The power system considered in the present study is based on the layout profile of a US surface combatant ship. The methodology developed will determine whether the loads that lost supply are restorable. When considering loads, it gives precedence to high priority loads. It also determines if there is any violation of current constraints of any cables and voltage constraints at load nodes. If the load is restorable, as suggested by an expert system, and there is no violation of any constraints, then the load is said to be restorable and the operation sequence required in restoring that load would be given as the output. A CAD drawing of the power system and a database of the system information has been developed using a geographic information system (GIS). A software tool incorporating failure assessment and expert system restoration methodology was developed using Visual C++, Multilogic Exsys Developer, Alternate Transients Program (ATP), and Microsoft Access. Microstation has been used for GIS. Various fault cases were designed, and the developed tool was used to run them and perform expert system restoration.

Wide-area protection against voltage collapse

A wide-area protection system against long-term voltage collapse was designed, implemented, tested, modified, and commissioned. The system is based on the present SCADA system in southern Sweden, which has been complemented with input signals and equipment to execute action orders from the protection system. Bus voltages from the transmission system, reactive power output from generators connected to the transmission system, and current limiter information from main generators were used as input signals to the wide-area protection system. Actions from the protection system are shunt reactor disconnection, shunt capacitor connection, start of gas turbines, emergency power request from the HVDC connection to Germany, low priority load disconnection, and finally shedding of high priority load.

A priority media access control protocol for video communication support on CSMA/CD LANs

Real-time multimedia communication applications demand performance requirements which differ significantly from conventional data communication applications. Current local area networks (LANs) provide efficient transport for bursty data traffic; however, they cannot necessarily provide quality of service guarantees for real-time communications. In this work we introduce and investigate an experimental priority protocol for supporting real-time communication on Ethernet, a popular implementation of multiple-access broadcast bus LANs. We examine the new protocol, known as Priority Mode-CSMA/CD (PM-CSMA/CD), providing a high priority (HP) class with CSMA/CD employed for standard priority (SP) traffic. PM-CSMA/CD performance is examined through computer simulation of videotelephony workstations operating over the shared bus LAN. Over all observed traffic conditions, the priority protocol provides performance satisfying real-time packet transport requirements of audio and video streams. The primary advantage of our protocol over similar priority schemes is its physical layer compatibility with standard CSMA/CD. The protocol also provides improved channel utilization with increasing high priority load, with no penalty in SP class performance at low to medium network loads.

Single-Stage Dual Priority Regulator for photovoltaic systems

In this paper the design and application of a voltage regulator for use in Photovoltaic Systems with two separate loads of different priority is presented. The Single-Stage Dual Priority Regulator (SSDPR) charges two separate battery systems for high and low priority loads. The controller always charges the main battery first. It toggles between the two batteries when the main battery is nearly charged, and reduces the charge duty cycle of the main battery as it approaches full charge. The SSDPR utilizes the PV more efficiently by supplying low priority loads without jeopardizing the reliability of the critical load. Both the main and auxiliary voltage settings are adjustable, allowing the use of different types of batteries. The SSDPR is being tested in the University of Lowell Vaccine Refrigerator Laboratory. A dual priority regulator is useful in any stand-alone system with multiple loads. Uses include photovoltaic powered homes, medical clinics, and recreational vehicles.