Non-critical Loads Research Articles

Distribution system restoration using graph theory after multiple faults

This study presents a novel graph theory-based approach to restore the distribution systems (DSs) after multiple simultaneous faults due to extreme weather conditions. The algorithm promptly finds the switching sequence to energise as many healthy zones as possible in an unbalanced DS. All the resulted topologies are later checked for operating constraints such as line flows and bus voltages. Hierarchy based load curtailment is proposed to remove any existing violations by shedding minimum non-critical loads. The proposed algorithm also gives the order in which the faults have to be cleared, depending on the priority of the disconnected load and inconvenience caused to the public. OpenDSS is used for unbalanced power flow analysis. Studies on the IEEE 37-bus system and the IEEE 123-bus system compared the proposed algorithm with other existing methods in the literature.

A Mixed Integer Linear Programming Based Load Shedding Technique for Improving the Sustainability of Islanded Distribution Systems

In recent years significant changes in climate have pivoted the distribution system towards renewable energy, particularly through distributed generators (DGs). Although DGs offer many benefits to the distribution system, their integration affects the stability of the system, which could lead to blackout when the grid is disconnected. The system frequency will drop drastically if DG generation capacity is less than the total load demand in the network. In order to sustain the system stability, under-frequency load shedding (UFLS) is inevitable. The common approach of load shedding sheds random loads until the system’s frequency is recovered. Random and sequential selection results in excessive load shedding, which in turn causes frequency overshoot. In this regard, this paper proposes an efficient load shedding technique for islanded distribution systems. This technique utilizes a voltage stability index to rank the unstable loads for load shedding. In the proposed method, the power imbalance is computed using the swing equation incorporating frequency value. Mixed integer linear programming (MILP) optimization produces optimal load shedding strategy based on the priority of the loads (i.e., non-critical, semi-critical, and critical) and the load ranking from the voltage stability index of loads. The effectiveness of the proposed scheme is tested on two test systems, i.e., a 28-bus system that is a part of the Malaysian distribution network and the IEEE 69-bus system, using PSCAD/EMTDC. Results obtained prove the effectiveness of the proposed technique in quickly stabilizing the system’s frequency without frequency overshoot by disconnecting unstable non-critical loads on priority. Furthermore, results show that the proposed technique is superior to other adaptive techniques because it increases the sustainability by reducing the load shed amount and avoiding overshoot in system frequency.

An Optimal Placement Model for Electric Springs in Distribution Networks

The electric spring (ES) is a type of demand-response technology that improves the voltage quality of critical loads (CLs) in a distribution network. This paper, proposes an optimal placement model to determine the optimal placement locations and power ratings of ESs in a radial distribution network. The total voltage deviations with respect to the buses are proposed as the placement criteria. The non-critical loads (NCLs) with ESs are extended to power-electronics-based loads, and they are modeled as constant-power-type loads. The operation characteristics of the individual ESs are investigated and linearized. On the basis of the linearized Distflow equations, an optimal placement model is proposed for ESs in a radial distribution network. The proposed model is solved as a mixed integer linear program problem. As per the case-study results, the installation locations and power ratings of the ESs are mainly associated with the network topology, weight coefficients of the voltage deviations on different buses, and bus loads in the radial distribution network.

Automatic Load-Shedding Protection at a Coal-Chemical Plant: Examining and Designing Effective Low-Frequency Backup Protection Methods

This article examines and studies a coal-chemical plant through a dynamic computer model as well as designs an automatic load-shedding protection. The plant has on-site generation to support partial plant loads. In the past, disconnecting from the utility grid had caused frequency and voltage decays in the plant electrical system to dropout critical loads. A complete system computer model is constructed, including generator dynamics, excitation/automatic voltage regulators (AVRs), and prime mover/governor control characteristics. Transient stability simulations of various loading and generation conditions were carried out to understand and study system frequency and voltage behaviors and patterns of change under a sudden islanding case. This was accomplished by analyzing frequency versus time and rate of change of frequency versus time curves. Further, through carefully evaluating plant load priorities as well as load dependencies, a feasible load-shedding control scheme with backup strategies was configured and simulated by computer modeling and study. Simulation results verified that the proposed loadshedding scheme can correctly respond to frequency drop and automatically initiate the shedding of noncritical and independent loads to prevent further system frequency drop, thereby exceeding the allowable deviation. The suggested solution is automatic and feasible and has been successfully implemented at the plant.

Grid Inertia Support Enabled by Smart Loads

The increasing penetration of renewable energy resources (RESs) leads to a continuous displacement of synchronous generators. Since the grid integration of RESs is usually achieved through inertialess power converters, the system inertia reduces and the power system frequency stability is compromised. Consequently, unexpected generator tripping, load shedding, and even blackouts may occur and pose a great threat to the system normal operation. In view of these challenges, this article explores the possibility of providing the inertia support from power-electronics interfaced smart loads. Specifically, a transformerless electric spring topology and the associated control strategy have been developed to provide a substantial inertia support from the noncritical load. As compared with existing inertia enhancement techniques, the proposed solution is cost-effective without the need of additional energy storage units. The smart load inertia provision capability is quantitatively analyzed and experimentally verified on a laboratory-scale test. In the testing results, 40% and 18% reductions of the rate of change of frequency and the maximum frequency deviation are, respectively, observed with a 12% smart load penetration ratio.

Small-Signal Stability Analysis and Active Damping Control of DC Microgrids Integrated With Distributed Electric Springs

Series DC electric springs (DCESs) are a state-of-the-art demand-side management (DSM) technology with the capability to reduce energy storage requirements of DC microgrids by manipulating the power of non-critical loads (NCLs). As the stability of DC microgrids is highly prone to dynamic interactions between the system active and passive components, this study intends to conduct a comprehensive small-signal stability analysis of a community DC microgrid integrated with distributed DCESs considering the effect of destabilizing constant power loads (CPLs). For this purpose, after deriving the small-signal model of a DCES-integrated microgrid, the sensitivity of the system dominant frequency modes to variations of various physical and control parameters is evaluated by means of eigenvalue analysis. Next, an active damping control method based on virtual RC parallel impedance is proposed for series DCESs to compensate for their slow dynamic response and to provide a dynamic stabilization function within the microgrid. Furthermore, impedance-based stability analysis is utilized to study the DC microgrid expandability in terms of integration with multiple DCESs. Finally, several case studies are presented to verify analytical findings of the paper and to evaluate the dynamic performance of the DC microgrid.

Energy Management in PV Based Microgrids Designed for the Universidad Nacional de Colombia

Stand-alone Electrical microgrids (MGs) require power management strategies to extend the life-time of their devices and to guarantee the global power balance of non-critical loads such as lighting of small sections of an university campus or individual air conditioning systems. This paper proposes an energy management strategy (EMS) for an isolated DC microgrid formed by a photovoltaic system (PVS), an energy storage system (battery), and a noncritical load. This configuration enables the photovoltaic system to control the power generation and ensures that the storage element does not exceed the safe limits of the state of charge. To control the generation of the photovoltaic system, two operating modes based on the perturb and observe (P&O) algorithm are implemented. The first one performs a maximum power point tracking (MPPT) action, while the second one regulates the power generated by the PVS to match the load requirement (power demand tracking, PDT). The management strategy also considers different operating states for ensuring the battery safety: normal operation, overcharge (at the maximum state of charge), and bulk charge (at the minimum state of charge); in those states the disconnection/connection of both the battery and the load is also considered. The main contribution of this work is to design and test a control strategy for an EMS aimed at regulating a standalone microgrid based on a PV system and an energy storage device. This solution is validated using detailed MG circuital simulations, which includes the PV source model (single-diode model), lithium-ion battery model, constant power load model and the DC/DC converters equations; moreover, realistic power generation and demand from Universidad Nacional de Colombia, located at Medellín-Colombia, are considered. The results obtained demonstrate the effectiveness of the energy management strategy, and in this way, enable to extend the battery lifetime and reduce the costs associated to the maintenance and disconnection of the microgrid in educational buildings or other applications focused on this type of DC microgrid.

A rule-based approach for improvement of autonomous operation of hybrid microgrids

The work presented in the article proposes a new rule-based energy management algorithm to improve the autonomy of hybrid (AC/DC) microgrids consisting of higher quantum of heating loads and renewable energy sources. The objective of the algorithm is to make the microgrids more autonomous by reducing the power import from the utility grid, during generation deficits. The heating or lighting loads in this work are considered as non-critical loads, and all other voltage sensitive loads are classified as critical loads. The presented strategy makes the AC and DC non-critical loads operate in power saving modes during generation insufficiency by controlling the electric springs based on the voltage produced by the renewables. Usage of electric springs in the microgrids consisting of AC and DC loads and a couple of renewable sources to improve the autonomous operation of microgrids is probably the first attempt. The advantage of using the suggested methodology is that it operates the voltage insensitive loads, based on the prevailing power generation condition without completely curtailing them. Further, the effectiveness of the algorithm is tested by developing suitable simulation models in MATLAB and also a scaled down laboratory setup is developed using dSPACE 1104 real-time operating system. The simulation and experimental results prove that the algorithm is effective enough in reducing the power import from the main grid.

Smart Boilers: Grid Support Services from Non-Critical Loads

The aim of this paper is to present the concept of a simple and cheap upgrade for electric water boilers, allowing them to provide power quality services to the distribution grid. The upgrade requires only minimum additional hardware and it is easily installable. “Smart Boilers”, as the upgraded boilers are named, perform precise active and reactive power control, but most significantly mitigate line current harmonics. Αctive and reactive power control is implemented by appropriate regulation of the modulation sinewave amplitude and phase, respectively. This type of control is easily customizable in order to accommodate a variety of power quality targets, depending on the required level of services and available grid monitoring equipment. The method used for line current harmonic compensation is based on the injection of mirror harmonics created at the modulation stage of the converter. It is indifferent of harmonic source: it is equally successful at mitigating harmonics caused by the power electronic converter of the Smart Boiler, other sources of current harmonics or loads. The achieved grid services are clearly beyond the “on/off” operation of electric boilers, currently implemented by Demand Side Management (DSM) in order to shift load away from peak hours. It has been demonstrated through simulations, that Smart Boilers can assist voltage regulation at terminal buses, compensate reactive power and suppress harmonic currents at lines.

Resilience-Driven Modeling, Operation and Assessment for a Hybrid AC/DC Microgrid

High-impact and low-probability extreme events can cause severe damage to power systems, especially for distribution systems. Microgrids (MGs) with distributed generation resources provide a viable solution for local load survivability via islanding schemes during extreme events. Recently, much research has focused on resilience-driven modeling and operations of MGs. In this paper, a resilience-driven operational model incorporating two operational modes (grid-connected and islanded) and detailed technical characteristics, such as voltage-related operational constraints, is developed for the resilience enhancement of a hybrid AC/DC MG. A detailed AC optimal power flow (OPF) algorithm is employed to model operational constraints and the power exchange between AC and DC subgrids. Preventive power importing is utilized for better preparedness before extreme events and demand response is employed to reduce load shedding during emergency mode. Existing literature on resilience assessment is reviewed and a modified multi-phase curve is proposed to fully represent the influence of limited generation resources and uncertain event duration on resilience. Extensive case studies capturing the distinction of critical loads and non-critical loads and two types of contingencies (multiple line faults and interrupted connection between AC subgrid and DC subgrid) are conducted to demonstrate the effectiveness of the proposed resilience strategy on protecting critical loads and reducing total load shedding. Particularly, a sensitivity analysis considering different event occurrence time has been simulated to capture, in a simple but rather effective way, the effect of the uncertainty surrounding event occurrence.

Techno‐economic potential gains of electric springs in distribution networks operations

The electric spring (ES) is a reactive compensator which is series with a noncritical load. This paper utilizes the ES in the distribution networks and examines the underlying performance operationally and economically. A virtual generator model for the electric spring is proposed to ease the study of power systems. An algorithm based on the backward/forward sweep method is proposed for solving the optimal power flow. Then, with a 24-hour real price and load data of ISO-NE market, the performance of ES is evaluated operationally and economically in two distribution systems. The ESs may exchange reactive power between each other and this raises active and reactive power losses. To cope this problem, the feeders' voltages of the middle load demand are set as the reference voltages of ESs in each feeder so, each ES has a different reference voltage to cease the ES power exchanges. In this work, indexes such as voltage profile, active power loss, reactive power loss, power factor, electric bill payment, net present value, internal rate of return and the payback year are measured. The results of case study confirm the ES improves power loss, voltage profile, power factor, and stability while it reduces the operation cost.

Integration of Flexible Loads and Electric Spring Using a Three-Phase Inverter

In this article, a new structure of the electric spring (ES) system using a three-phase inverter is proposed. Noncritical or flexible loads associated with ES can be turned into a smart load. The smart load consumes power adaptively to achieve quick demand response in the future smart grid. Compared to previous ES topologies, the new structure of ES eliminates the battery storage and isolation transformer. The benefit of size and weight reduction results in further improvement on power density of the system. The control strategy is developed and the optimal operating condition for the new ES system is explicitly studied. Power inverter prototype has been built and tested to demonstrate not only the functionality but also the practicality of the new proposal. At the end of the article, the power capability of the new ES system is investigated and compared with previous ES to highlight its features.

A Load control method with reactive power control capability and enhanced flexibility based on smart load

To promote the real‐time supply–demand balance of power grids and to improve the economy and security of power systems, the load control technology of power systems is becoming increasingly important. Electric spring (ES), first proposed in 2012, is a power electronic device used in power systems to achieve power consumption that can vary with power generation. Based on a smart load (SL), which comprises an ES and a noncritical load (NCL), a new load control method is proposed here, aiming to solve the problems of weak flexibility and uncontrollable reactive power in the traditional method. First, the power control model of an SL is established taking into account its basic principles. Next, the control method of a single SL is proposed, and the control loops are designed. Subsequently, the cooperative control strategy of multiple SLs is proposed using the particle swarm optimization (PSO) algorithm. Finally, our simulation and experimental results prove that both the active power and reactive power of SL can be controlled flexibly and smoothly using the method proposed here. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Identification and Control of Electric Elasticity Limit for Electric-Spring-Based Flexible Loads

By comprehensively analyzing the effective working voltage range of electric springs (ESs), this paper proposes a new control strategy to improve the absorption capacity of ES-based flexible loads for mains voltage fluctuations; thus, enhancing the stability of the microgrid fed by intermittent renewables. Analogous to a mechanical spring, the effective working voltage range of ESs is also restricted by the electric elasticity limit (EEL). In specific circumstances, the ES-based flexible load is incapable of dealing with excessive fluctuations of mains voltage caused by intermittent renewable energy sources. Accordingly, this paper investigates the impact of key parameters on the EEL of ESs, such as the distribution line, the voltage reference, and the loads. Besides, by adjusting the proportion between the critical and the noncritical loads, this paper proposes and implements an EEL controller to regulate the absorption capacity of ES-based flexible loads for mains voltage fluctuations; thus, effectively ensuring the voltage stability of critical loads. Finally, the simulated and experimental results are both given to verify the theoretical analysis for ES-based flexible loads and the feasibility of the proposed EEL controller.

Decentralized Control of DC Electric Springs for Storage Reduction in DC Microgrids

Series dc electric springs (series ESs) can tackle the intermittency of the renewable generations (RGs) with reduced storage capacity in dc microgrids (MGs). They can actively manipulate the power profiles of the serially connected noncritical loads (NCLs) to follow the fluctuating RG. In the process of regulating the dc bus voltages, the power rating of an NCL can affect the power of its corresponding series ES. If multiple series ESs with differently rated NCLs can be operated in a coordinative way, the total storage capacity of the series ESs can be potentially reduced. In this article, a decentralized controller is proposed to coordinate the operations of multiple series ESs such that the required storage capacity of the series ESs can be reduced. The conditions for achieving the minimum battery charging and discharging power are analytically derived. The stability analysis of the system as well as the design of the decentralized controller is provided. Experiment results on a 120-V dc grid confirmed that the proposed controller can effectively reduce the total storage capacity of the series ESs in dc MGs by $\text{7.5}\%$ .

Regulation Performance of Multiple DC Electric Springs Controlled by Distributed Cooperative System

DC electric springs (DCESs) have been recently developed to improve the voltage stability of a DC microgrid. A lately proposed DCES topology is comprised of a DC/DC three port converter (TPC), a bi-directional buck-boost converter (BBC) and a battery, and is arranged as follows: The TPC input port is fed by a renewable energy source (RES) whilst the two TPC output ports supply a non-critical load (NCL) and a critical load (CL) separately; in turn, BBC together with the battery constitutes the DCES energy storage unit (ESU) and is connected in parallel to CL. In this paper, a set of DCESs with such a topology and with their CLs connected to a common DC bus is considered. The control of the DCESs is built up around a distributed cooperative system having two control levels, namely primary and secondary, each of them endowed with algorithms committed to specific tasks. The structure of the control levels is explicated and their parameters are designed. The control system is applied to a DCES set taken as a study-case and tested by simulation. The results of the tests show the excellent performance of the control system in both regulating the CL DC bus voltage and keeping the state-of-charge of the battery within predefined limits.

Pre-scheduled load shutdown in microgrid utilizing computerized surveillance system

The successful inclusion of microgrids for enhancement of the existing power system is laden with many challenges. Increase in demand and insufficient power generation results in power shutdown in several areas which comprises non-critical loads. The preface of Distributed Generation (DG) in power system has varied implications on the existing system. The concept proposed in this paper helps in time scheduling and load scheduling of a power system based on priority setting and time setting. The load management may also be done using IoT which enables load shedding from remote areas.

Adaptive PI control of Electric Springs For Voltage Regulation Under Dynamic Load Changes

Due to continuous distributed generation development technology, the accessibility of wind, solar and also the renewable energy sources tends to intensify. To suppress the voltage fluctuation caused by the distributed generation electric springs had been developed. In this article an adaptive control of Electric spring is proposed, in which the gains of the PI controller are optimised by TLBO to maintain constant voltage across critical load. The proposed strategy is tested for dynamic changes in the non-critical load. Simulation results show that, for voltage fluctuations caused by the DGs and also with the dynamic load changes, ES with adaptive controller stabilize the bus voltage effectively, over ES with Fuzzy Logic Control and traditional PI control.

Hybrid energy management strategy for residential consumers using virtual and actual storage systems

The paper presents a novel energy management algorithm for residential consumers. The strategy proposed in this work is a combination of the rule based approach considered by the authors and the genetic algorithm. The proposed hybrid algorithm is designed to effectively schedule the virtual and the actual storage facilities present in a residential complex powered by solar panels and main grid. The suggested methodology tries to make the residential complex more autonomous by making it less reliable on the main grid, which is possible by reducing the power import from the utility grid. Further, in this work the virtual storage units are fitted in series with noncritical loads and scheduled based on the instructions given by the consumer and hence there will be maximum consumer satisfaction. Also, the virtual storage units present in the residence can also support the grid by effectively handling the reactive power to a certain extent. The combined usage of virtual storage units like the AC electric springs and the actual storage facilities like the batteries to attain certain benefits for residential consumers is probably the first attempt. A 24 h load curve of a residence consisting of higher constant impedance loads and roof top solar modules is considered for testing the efficiency of the proposed algorithm. The simulation studies required to prove the robustness of the proposed strategy are conducted in MATLAB.

Voltage control of critical and non-critical loads in distribution networks with electric spring

The electric spring (ES) is a novel voltage compensator which is series with a non-critical load to regulate the critical load voltage. The voltage fluctuation is caused by wind speed fluctuation, load fluctuation, and generator tripping. In busbar voltage drop situation, the electric spring decreases the voltage of non-critical load in order to support the critical load (busbar) voltage. All the non-critical loads couldn’t work under any voltage (for example 0.5pu). In this paper, a control strategy founded on active and reactive power compensations has been proposed for voltage control of critical loads on a reference value while it controls the voltage of non-critical loads between an acceptable boundary. The proposed controller has two voltage control loops which adjusts active and reactive power of the electric spring. The experimental results from the case study show that the ES with the proposed control strategy can effectively mitigate double voltage control of both critical and non-critical loads while dynamically managing the demand response of the system at the same time.