Power Quality Levels Research Articles

Simplified Frequency Estimation for Unit Scheduling Criteria for Grids With High Wind Penetration

The current unit scheduling criteria on Penghu Island entail applying net load demand to determine the numbers of online diesel units. The net load may underestimate the number of required online units in a system with high wind power penetration, especially under off-peak load and high wind conditions, both of which engender insufficient operating reserves. Therefore, this study proposes new unit scheduling criteria that integrate the system equivalent speed droop with the M factor by considering different loads and wind conditions. This study also identified the maximum allowable power loss and the required size of the operating reserve by considering system reliability. Finally, this study determined the allocation modes for units and the required generation distributions for various loads. In accordance with the requirements of system loads at specific times, the proposed unit scheduling criteria could provide system operators with corresponding unit operating modes, as well as generation distributions, thereby sustaining power reliability and quality levels.

Active and Reactive Power Control of ESS in Distribution System for Improvement of Power Smoothing Control

This paper proposes a new control technique of energy storage system (ESS) for smoothing the active power of renewable energy sources (RES) such as photovoltaic and wind turbine generation. As the penetration of RES into grid increases, it is difficult to maintain the permissible level of power quality, that is, voltage and frequency fluctuation in power systems. To solve this problem, ESS control methods using low pass filter (LPF) have been proposed for mitigating the fluctuation of RES output. However, those have a lot of drawbacks which need to be supplemented. Hence, this paper presents the improved active power control with additional reactive power control for maintaining power quality properly. The proposed method minimizes the capacity of ESS to be required for smoothing RES output fluctuation through mitigation of phase delay problem in LPF. In addition, the voltage regulation improves by using additional reactive power control. The proposed method was verified through simulation analysis using PSCAD/EMTDC.

New Multistage and Stochastic Mathematical Model for Maximizing RES Hosting Capacity—Part I: Problem Formulation

This two-part work presents a new multistage and stochastic mathematical model, developed to support the decision-making process of planning distribution network systems (DNS) for integrating large-scale “clean” energy sources. Part I is devoted to the theoretical aspects and mathematical formulations in a comprehensive manner. The proposed model, formulated from the system operator's viewpoint, determines the optimal sizing, timing, and placement of distributed energy technologies (particularly, renewables) in coordination with energy storage systems and reactive power sources. The ultimate goal of this optimization work is to maximize the size of renewable power absorbed by the system, while maintaining the required/standard levels of power quality and system stability at a minimum possible cost. From the methodological perspective, the entire problem is formulated as a mixed integer linear programming optimization, allowing one to obtain an exact solution within a finite simulation time. Moreover, it employs a linearized ac network model which captures the inherent characteristics of electric networks and balances well accuracy with computational burden. The IEEE 41-bus radial DNS is used to test validity and efficiency of the proposed model, and carry out the required analysis from the standpoint of the objectives set. Numerical results are presented and discussed in Part II of this paper to unequivocally demonstrate the merits of the model.

Using Smart Impedance to Transform High Impedance Microgrid in a Quasi-Infinite Busbar

Isolated microgrids, due to the high values of the output converter impedances, suffer power quality problems, such as harmonic distortion and resonances. These impedances are normally the LCL output filter needed to suppress the switching ripple of the converters. As complex impedances, they are dependent on the frequency, therefore very sensitive to harmonic content of load current. This leads to a significant microgrid voltage distortion which results in undesirable power quality levels. Hybrid active power filters are a reasonable choice for harmonic compensation allowing good filtering performance at reduced cost. This paper describes the use of Smart Impedance, which is a new application of a hybrid active power filter, to change the microgrid bus impedance forcing it to behave as a quasi-infinite bus, without increasing active power capacity of the microgrid. The implementation can improve microgrid power quality by mitigating harmonic currents and harmonic voltage resonance. It allows connecting more nonlinear load in the bus keeping the microgrid voltage quality. Experimental results are presented to demonstrate the effectiveness of the proposed approach.

Harmonic control optimisation model considering power generation of microgrid

In a microgrid with a large amount of photovoltaic (PV), the harmonic pollution problem is mostly caused by the non-linear load in the PV system and the microgrid. The multi-function grid-connected inverter (MFGCI) and the active power filter (APF) used to control the harmonic pollution is integrated into the framework of the bi-layer planning model in this paper. The upper layer model of the bi-layer optimisation is based on the cost of the microgrid power generation with PV as the objective function. Consider the maintenance costs of PV power generation and MFGCI equipment, line loss costs, purchase expectations and sales period expectations. The lower objective function is the minimum root mean square of the harmonic distortion rate of each node of the microgrid satisfying the MFGCI and APF capacity allocation constraint. The improved particle swarm optimisation algorithm and Latin hypercube technology are used to solve the bi-layer planning model. The results show that in the microgrid, the integrated optimisation method based on the bi-layer planning model is optimised for MFGCI and APF. It has a significant effect on the efficient operation of the microgrid and the improvement of power quality level of the microgrid.

Sensitivity Analysis for the Power Quality Indices of Standalone PV Systems

Standalone photovoltaic systems is a key technology for the increase of renewable energy sources share in electricity production worldwide. The power quality of those systems plays a fundamental role in avoiding volatile power supply. This calls for a concrete design in order to meet power quality specifications. In this context, a methodology has been developed in a previous work, in order to set the values of the parameters that optimize the power quality indices of the system. In this paper, an extensive sensitivity analysis is performed regarding the influence of the optimized parameters variation on the power quality indices. Throughout the outcomes of the sensitivity analysis a more insightful view of the system performance under the deviations of system parameters is revealed, which in combination with the aforementioned optimal design strategy becomes an essential tool that ensures high power quality level. The theoretical outcomes are validated by experimental results, highlighting the effectiveness of the proposed design strategy.

Power Quality Conditioning in LV Distribution Networks: Results by Field Demonstration

Power quality in LV distribution networks is already a concern in many European countries especially where there is a strong presence of renewable energy generation. Therefore there is a growing interest in new solutions able to improve the power quality level of such a system. Among them, an interesting solution is represented by the open unified power quality conditioner (Open UPQC) proposed within the present work. The system consists of a single or three-phase ac/dc power converter installed at customer’s premises and a main single or three-phase ac/dc power converter in the MV/LV substation. The paper discusses the design, simulation and implementation phases related to an Open UPQC installed in a real LV distribution grid in the city of Brescia (Italy) within the smart domo grid project, co-funded by the Italian Ministry of Economic Development. Results from the field installation show the effectiveness of the proposed solution to face power quality issues in distribution networks.

Past, Present, and Future Challenges of the Marine Vessel’s Electrical Power System

The evolution of the use of electricity in marine vessels is presented and discussed in this paper in an historical perspective. The historical account starts with its first commercial use in the form of light bulbs on the SS Columbia in 1880 for illumination, going forward through use in hybrid propulsion systems with steam turbines and diesel engines and then transitioning to the present with the first fully electric marine vessel based entirely on the use of batteries in 2015. Electricity use is discussed not only in the light of its many benefits but also of the challenges introduced after the emergence of the marine vessel electrical power system. The impact of new conversion technologies like power electronics, battery energy storage, and the dc power system on overall energy efficiency, power quality, and emission level is discussed thoroughly. This paper guides the reader through this development, the present and future challenges by calling attention to the future research needs, and the need to revisit standards that relate to power quality, safety, integrity, and stability of the marine vessel power system, which are strongly impacted by the way electricity is used in the marine vessel.

Simple Continuous Assessment of Transmission-Network Flicker Levels Caused by Multiple Sources

Transmission system operators (TSO) should be able to assess power quality levels in a network in order to properly address the mitigation of power quality problems. High flicker levels might be a serious power-quality problem in transmission networks and are usually caused by arc-furnace operation. TSO can assess flicker levels in real time only at nodes where power-quality measurements are available. As for the rest of the system, a TSO could calculate flicker levels, if a reliable and relatively simple method is available. Besides, proper summation of flicker levels originating from several flicker sources remains an issue to tackle. The paper proposes a simple method for the calculation of flicker levels at all nodes of a transmission network based on the known flicker values at multiple flicker-sources and based on the known network topology. Different flicker summation coefficients are compared and the one that gives the smallest error is chosen. The proposed method enables a TSO to continuously assess flicker levels at all network nodes and to investigate the consequences of network changes on flicker levels. The method was verified on the Slovenian transmission network where the results of flicker calculation were compared with available flicker measurements.

Passive Filter Aided by Shunt Compensators Based on the Conservative Power Theory

Passive filters are widely used in electrical system for power quality improvements. Their first installations from 1940 s and their advantages make them an attractive and standard solution up to nowadays. However, passive filters have their filtering characteristics deteriorated due to parameter variation caused by aging or temperature. In addition, a capacitor bank for power factor correction is designed for specific loads and may not supply the right amount of reactive power when loads keep being added or changed. When these issues make the passive filter and the capacitor bank incapable to keep the system operating within acceptable level of power quality, an inconvenience arises and a solution must be provided. A common one is to replace both of them either by new elements or by active power compensators. However, replacing the passive filter and the capacitor bank may not be economically feasible, because they belong to a past investment. This paper presents a solution to overcome such inconvenience keeping the passive filter and the capacitor bank installed and unchanged. It consists of installing two shunt compensators specially designed for performing what the passive filter and the capacitor bank are incapable to do. The result is a reduced processed power in the compensators. The generation of the references is based on conservative power theory (CPT). A case study is presented to prove the compensators’ efficacy and the power quality improvement.

Research on demand side management programs and analysis of their usage efficiency

Modern trends in Smart Grid systems aimed on intellectualization of existing electricity supply networks, energy supply and creating local Microgrid systems ensure a high level of reliability and power quality. As part of the Smart Grid concept demand side management applications play an important role in solving technical and technological problems arising in the practical implementation of this concept. The network must implement a catena of demand side management programs by providing various services according to the situation, requirement contracts, forecasting of consumption/demand and collect information about energy savings.An important need in demand side management programs implementation is an adequate indicator for actual power consumption to relatively optimal, as a variant of this indicator should be used an indicator based on Frieze power, as it takes place on uneven terms of processes even in the absence of reactive elements in the network.The concept of Frieze power Qф for the grid period is distributed, which helped to get value for optimal assessment processes taking into account voltage and current deviations and ripple coefficients. Variant of usage of Qф/U12·I12 indicator is an illustration of the real power consumption to optimal, because it takes place even in absence of reactive elements in grid.

Implementation of Fractional-order Integral-plus-proportional Controller to Enhance the Power Quality of an Electrical Grid

This article integrates a suitable controller to the direct power control algorithm for a shunt active power filter. The conventional proportional-integral (PI) controller is commonly used in DC bus regulation loops because of its advantages in the steady state, but it is limited in the dynamic state. Thus, this article contributes an appropriate controller, the fractional-order–integral-plus-proportional controller. The controller under study extends the integration order of the controller from integer to real order based on fractional calculus theory and Oustaloup continuous approximation. The fractional-order–integral-plus-proportional controller offers a short response time, withstands parameters variations, and deals with external disturbances. To investigate the efficiency and accuracy of the proposed fractional-order–direct power control algorithm considering all robustness tests, an experimental setup was made. The experimental results confirm its high performance in both steady and dynamic states and demonstrate its feasibility and effectiveness. Therefore, the shunt active power filter improves the power quality, corrects the power factor, and reduces the harmonics injected by non-linear loads. Thanks to its high level of power quality, the fractional-order–direct power control could become an interesting alternative for active power filtering.

Impact of PV/Wind/Diesel Hybrid System on the Distribution Networks – Fault Currents

The short circuit analysis secures the electrical power systems protection scheme. Thus, in turn ensures the power system reliability, safe mode of operation, and uninterruptable power supply. The impact of distributed generation on the power systems stability, power quality, and short circuit level is still unclear and uncertain. The different characteristics of each distributed generation type, such as synchronous generators, induction generators, power electronics inverters make it more complicated to assess. Studying the effect of the penetration of distributed generation on fault current level of existing distribution networks is crucial. This paper will examine the impact of implementing distributed generation mix, including wind turbine generators, photovoltaic, and diesel generators on the fault current level of the distributed network under study. The short circuit analysis is carried out on the IEEE 13-bus distribution test system using ETAP software. The simulation results are presented in a comparison between four cases, comparing the impact of the distributed generation mix on the fault current level at different levels of penetration. Moreover, a comparison showing the contribution of each distributed generation type separately as a percentage is presented. The selection of distributed generation technology and location has a significant impact on the fault current.

Power Quality Comprehensive Assessment Based on Fuzzy Backward Cloud

In order to improve the power quality comprehensive assessment of objectivity and accuracy, the fuzzy backward cloud generator that combines fuzzy theory and backward cloud model is proposed. Expectation of the backward cloud model is the qualitative concept that reflects the level of power quality best. The fuzzy backward cloud model using the fuzzy method to determine the numerical characteristics of cloud model for qualitative concepts of each power index, the weight of each index is determined by the combination weighting method, and then the power quality assessment is achieved. This approach is compatible with the influential factors of various levels of power quality indicators. The simulation results show the proposed method is more objective and accurate compared with the conventional method.

Online voltage estimation and control for smart distribution networks

The increasing deployment of Distributed Generation (DG) technologies introduces power quality challenges to the grid, in particular steady state voltage rise at the connection point for DG units. In most distribution networks, control and monitoring of grid parameters is missing, as well as system security is at risk. Smart grid technologies have the capability to realize the real-time measurements and on-load voltage controls. With the steady implementation of smart grid technologies throughout the existing distribution networks, the online voltage control can be achieved ensuring the power quality and voltage levels within the statutory limits. This study presents a methodology for the estimation of voltage profile in a smart distribution network with DG for the online voltage control, taking into account different line X/R ratios and laterals. This method is based on maximum and minimum voltage estimation by remote terminal units (RTUs) placed only at DG connected bus and at capacitor connected bus. Voltage regulation is carried out based on RTUs estimated values. This work is tested on two radial distribution networks with/without DGs and laterals. Comparative results for voltage magnitudes estimated with different methodology are presented. The reported simulation results show that the method presented is capable of estimating the voltage profile along the distribution network with DGs for the online voltage control, considering different line X/R ratios and laterals.

Power Quality Analysis in Microgrid: An Experimental Approach

Microgrid (MG) integrated with Distributed Generation (DG) provides several benefits like reliable, secure, and high efficient of energy supply, while minimizing power loss, deferring expansion of power distribution infrastructures, and reduced carbon emission of energy supply etc. to the communities. Despite of the several benefits, there are several challenges existing due to the integration of different characteristics and technology of DG sources in MG network. Power Quality (PQ) issue is one of the main technical challenges in MG power system. In order to provide improved PQ of energy supply, it is necessary to analyse and quantify the PQ level in MG network. This paper investigates the detail of PQ impacts in a real MG network carried out through an experimental analysis. Voltage and frequency variations/deviations are analysed in both on-grid and off-grid mode of MG operation at varying generation and varying load conditions. Similarly unbalance voltage and current level in neutral are estimated at unbalanced PV generation and uneven load distribution in MG network. Also current and voltage THD are estimated at different PV power level. Finally the results obtained from the analysis are compared to that of Australian network standard level.

Non-linear control of the output stage of a solar microinverter

ABSTRACTThis paper presents a proposal to control the output stage of a two-stage solar microinverter to inject real power into the grid. The input stage of the microinverter is used to extract the maximum available power of a photovoltaic module enforcing a power source behavior in the DC-link to feed the output stage. The work here reported is devoted to control a grid-connected power source inverter with a high power quality level at the grid side ensuring the power balance of the microinverter regulating the voltage of the DC-link. The proposed control is composed of a sinusoidal current reference generator and a cascade type controller composed by a current tracking loop and a voltage regulation loop. The current reference is obtained using a synchronized generator based on phase locked loop (PLL) which gives the shape, the frequency and phase of the current signal. The amplitude of the reference is obtained from a simple controller regulating the DC-link voltage. The tracking of the current reference is accomplished by means of a first-order sliding mode control law. The solution takes advantage of the rapidity and inherent robustness of the sliding mode current controller allowing a robust behavior in the regulation of the DC-link using a simple linear controller. The analytical expression to determine the power quality indicators of the micro-inverter's output is theoretically solved giving expressions relating the converter parameters. The theoretical approach is validated using simulation and experimental results.

On electric warship power system performance when meeting the energy requirements of electromagnetic railguns

This paper investigates the ability of a gas turbine alternator to provide power to an electromagnetic (EM) railgun in surface combatants whilst simultaneously maintaining acceptable levels of power quality for other consumers. Following the justification of the investigation and a description of the research methods, which includes a proposed EM railgun charging control system, this paper discusses simulated results obtained from a validated power system model for both single shot and salvo operations and identifies factors that limit the rates of fire in each case. The findings from the investigations suggest that a large gas turbine alternator is able to maintain quality of power supply within acceptable limits for the majority of EM railgun operations. However, the transient quality of power supply limits was found to be exceeded for heavy firing rates but would remain within tolerable limits as defined for exceptional loads by North Atlantic Treaty Organization (NATO) standardization agreement (STANAG) 1008 Edition 9. A method to increase the maximum rate of fire whilst maintaining the quality of the power supply within acceptable limits is investigated by increasing the size of the energy storage device and retaining a residual charge. The paper concludes by suggesting that the EM railgun system could be integrated into the ship's electrical system without the need for any additional prime movers to achieve rates of fire commensurate with a surface combatant's requirements.

Day-ahead Optimal Scheduling of Loads and Dispatchable Resources in a Hybrid AC/DC Microgrid of an Industrial System

Industrial systems are usually characterized by high energy consumption and presence of sensitive loads which require high levels of Power Quality. Based on the grid architecture of an actual Italian industrial facility, this paper proposes the installation of a hybrid AC/DC microgrid. The hybrid microgrid includes dispatchable and non-dispatchable renewable generation units, battery storage systems and controllable loads. An optimization model is formulated to solve the problem of the day-ahead optimal scheduling of the microgrid. The model aims to optimize the operation of dispatchable resources, controllable loads and storage systems minimizing the daily costs of the energy imported from the AC grid and the costs required by the dispatchable generators while satisfying operational constraints such as those related to the production process of the industry facility. Different case studies are investigated in order to show the feasibility and effectiveness of the proposed procedure.

Some advanced parametric methods for assessing waveform distortion in a smart grid with renewable generation

Power quality (PQ) disturbances are becoming an important issue in smart grids (SGs) due to the significant economic consequences that they can generate on sensible loads. However, SGs include several distributed energy resources (DERs) that can be interconnected to the grid with static converters, which lead to a reduction of the PQ levels. Among DERs, wind turbines and photovoltaic systems are expected to be used extensively due to the forecasted reduction in investment costs and other economic incentives. These systems can introduce significant time-varying voltage and current waveform distortions that require advanced spectral analysis methods to be used. This paper provides an application of advanced parametric methods for assessing waveform distortions in SGs with dispersed generation. In particular, the Standard International Electrotechnical Committee (IEC) method, some parametric methods (such as Prony and Estimation of Signal Parameters by Rotational Invariance Technique (ESPRIT)), and some hybrid methods are critically compared on the basis of their accuracy and the computational effort required.