Energies

Research

15 pages, 872 KiB

Open AccessArticle

An Accurate Power Flow Method for Microgrids with Conventional Droop Control

by Fang Lu and Hongda Liu

Energies 2022, 15(16), 5841; https://doi.org/10.3390/en15165841 - 11 Aug 2022

Cited by 7 | Viewed by 1248

With conventional droop control, the droop relationship between the voltage and reactive power is not purely linear because of the filter reactance. This paper focuses on the theoretical analysis to account for this characteristic and presents a precise power flow method for conventional [...] Read more.

With conventional droop control, the droop relationship between the voltage and reactive power is not purely linear because of the filter reactance. This paper focuses on the theoretical analysis to account for this characteristic and presents a precise power flow method for conventional droop control. The proposed method is universal, which can handle not only conventional droop control, but also other control strategies, such as robust droop control, constant power control, and constant voltage–frequency control. It can also handle frequency-dependent active and reactive power loads and is adapted for islanded and grid-connected systems. The proposed method extends the applicability of conventional power flow methods to microgrids so that the framework of the method is generic; any conventional power flow algorithm can be adapted to this framework. Compared with the time-domain simulation method, the proposed method is accurate, simple, and easy to implement for industrial applications. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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15 pages, 982 KiB

Open AccessArticle

Stochastic Analysis-Based Volt–Var Curve of Smart Inverters for Combined Voltage Regulation in Distribution Networks

by Dongwon Lee, Changhee Han and Gilsoo Jang

Energies 2021, 14(10), 2785; https://doi.org/10.3390/en14102785 - 12 May 2021

Cited by 4 | Viewed by 1934

Abstract

The proliferation of renewable energy resources (RES), especially solar photovoltaic (PV) generation resources, causes overvoltage and line overloading in distribution networks. This study proposes a two-level volt–var control method based on multiple timescales. The on-load tap changer (OLTC) operates on an hourly timescale, [...] Read more.

The proliferation of renewable energy resources (RES), especially solar photovoltaic (PV) generation resources, causes overvoltage and line overloading in distribution networks. This study proposes a two-level volt–var control method based on multiple timescales. The on-load tap changer (OLTC) operates on an hourly timescale, to regulate the voltage on the secondary winding. In the 15-minutes timescale, PV-connected smart inverters and static var compensators (SVCs) are obliged to compensate the reactive power for the voltage control at the point of common coupling. In the multi-timescale voltage control framework, this study proposes a new multi-sectional volt–var curve (MSVVC) of a PV inverter. The objective of the MSVVC is to minimize the energy loss in the network, improve the voltage profile, and obtain the operational margin of other reactive power compensation devices. In the process of determining the optimal parameters of the MSVVC, stochastic modeling-based load flow analysis is utilized to consider the intermittency and uncertainty of RES generation. The effectiveness of the proposed method is verified on the IEEE 33-bus system in comparison with the conventional volt–var curve cases. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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17 pages, 4215 KiB

Open AccessArticle

Modeling and Investigation of Demand Response Uncertainty on Reliability Assessment

by Jen-Hao Teng and Chia-Hung Hsieh

Energies 2021, 14(4), 1104; https://doi.org/10.3390/en14041104 - 19 Feb 2021

Cited by 1 | Viewed by 1677

Abstract

Demand Response (DR) provides an opportunity for customers to reduce their loads during times of high prices and therefore to shave the peak loads. The power outputs of large-scale generator units can be predicted and controlled easily; therefore, the pricing and reliability of [...] Read more.

Demand Response (DR) provides an opportunity for customers to reduce their loads during times of high prices and therefore to shave the peak loads. The power outputs of large-scale generator units can be predicted and controlled easily; therefore, the pricing and reliability of conventional power utilities can be assessed straightforwardly. However, the customer loads are very variable and difficult to predict and control; therefore, the integration of DR might cause uncertainty issues on pricing and reliability and is essential to be further investigated. A novel uncertainty model for load reduction is proposed in this paper. The probability intensities of load reduction are first estimated from the measured load reduction variations. A multi-state probability model is then proposed for load reduction and the Markov process is used to calculate the state probabilities. A stochastic analysis scheme using Monte Carlo simulation for pricing and reliability taking the DR uncertainty into account is then investigated. Several cases are designed to compare the effects of DR uncertainty. Simulation results show that the proposed uncertainty model can be integrated into conventional economic dispatch to precisely evaluate the DR uncertainty on system operation and reliability. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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17 pages, 4272 KiB

Open AccessArticle

Decentralized V2G/G2V Scheduling of EV Charging Stations by Considering the Conversion Efficiency of Bidirectional Chargers

by Jian-Tang Liao, Hao-Wei Huang, Hong-Tzer Yang and Desheng Li

Energies 2021, 14(4), 962; https://doi.org/10.3390/en14040962 - 11 Feb 2021

Cited by 22 | Viewed by 3277

Abstract

With a rapid increase in the awareness of carbon reduction worldwide, the industry of electric vehicles (EVs) has started to flourish. However, the large number of EVs connected to a power grid with a large power demand and uncertainty may result in significant [...] Read more.

With a rapid increase in the awareness of carbon reduction worldwide, the industry of electric vehicles (EVs) has started to flourish. However, the large number of EVs connected to a power grid with a large power demand and uncertainty may result in significant challenges for a power system. In this study, the optimal charging and discharging scheduling strategies of G2V/V2G and battery energy storage system (BESS) were proposed for EV charging stations. A distributed computation architecture was employed to streamline the complexity of an optimization problem. By considering EV charging/discharging conversion efficiencies for different load conditions, the proposed method was used to maximize the operational profits of each EV and BESS based on the related electricity tariff and demand response programs. Moreover, the behavior model of drivers and cost of BESS degradation caused by charging and discharging cycles were considered to improve the overall practical applicability. An EV charging station with 100 charging piles was simulated as an example to verify the feasibility of the proposed method. The developed algorithms can be used for EV charging stations, load aggregators, and service companies integrated with distributed energy resources in a smart grid. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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23 pages, 6430 KiB

Open AccessArticle

Technical Loss Calculation in Distribution Grids Using Equivalent Minimum Order Networks and an Iterative Power Factor Correction Procedure

by Carlos Eduardo Moreira Rodrigues, Maria Emilia de Lima Tostes, Ubiratan Holanda Bezerra, Thiago Mota Soares, Edson Ortiz de Matos, Lázaro Serra Soares Filho, Elaine Cristina dos Santos Silva, Michel Ferreira Rendeiro and Carlos Jeferson da Silva Moura

Energies 2021, 14(3), 646; https://doi.org/10.3390/en14030646 - 27 Jan 2021

Cited by 8 | Viewed by 2522

Abstract

Energy losses are a fundamental issue in the electricity distribution sector, being an inevitable consequence of transporting energy from supplying sources to consumers’ installations and are becoming one of the factors to be considered in planning and operation of electrical distribution networks. So, [...] Read more.

Energy losses are a fundamental issue in the electricity distribution sector, being an inevitable consequence of transporting energy from supplying sources to consumers’ installations and are becoming one of the factors to be considered in planning and operation of electrical distribution networks. So, electrical distribution losses must be continuously monitored so that they are kept within acceptable levels to ensure the business profitability as well as the good power quality of supplied energy. In this context, this work introduces a modified methodology for technical losses calculation with the application of the concept of reduced equivalent networks, via definition of an Equivalent Operational Impedance, taking as a starting point the electrical network modeling in the Open Distribution System Simulator (OpenDSS). The losses calculation also considers customer’s energy billing measurement data, measurements of injected energy and power factor at the feeder’s coupling bus at the substation, also considering measurement campaigns to characterize the load consumption profiles for working days, Saturdays and Sundays. The proposed methodology disaggregates energy injections in billed energy, technical and non-technical losses parcels, and presents, as the results have demonstrated, a good precision in the proposed calculation procedures. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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12 pages, 4124 KiB

Open AccessArticle

Development of a Static Equivalent Model for Korean Power Systems Using Power Transfer Distribution Factor-Based k-Means++ Algorithm

by Bae-Geun Lee, Joonwoo Lee and Soobae Kim

Energies 2020, 13(24), 6663; https://doi.org/10.3390/en13246663 - 17 Dec 2020

Cited by 4 | Viewed by 1707

Abstract

This paper presents a static network equivalent model for Korean power systems. The proposed equivalent model preserves the overall transmission network characteristics focusing on power flows among areas in Korean power systems. For developing the model, a power transfer distribution factor (PTDF)-based k [...] Read more.

This paper presents a static network equivalent model for Korean power systems. The proposed equivalent model preserves the overall transmission network characteristics focusing on power flows among areas in Korean power systems. For developing the model, a power transfer distribution factor (PTDF)-based k-means++ algorithm was used to cluster the bus groups in which similar PTDF characteristics were identified. For the reduction process, the bus groups were replaced by a single bus with a generator or load, and an equivalent transmission line was determined to maintain power flows in the original system model. Appropriate voltage levels were selected, and compensation for real power line losses was made for the correct representation. A Korean power system with more than 1600 buses was reduced to a 38-bus system with 13 generators, 25 loads, and 74 transmission lines. The effectiveness of the developed equivalent model was evaluated by performing power flow simulations and comparisons of various characteristics of the original and reduced systems. The simulation comparisons show that the developed equivalent model maintains inter-area power flows as close as possible to the original Korean power systems. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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17 pages, 5430 KiB

Open AccessArticle

Lightning Performance and Economic Analysis of an Overhead 88 kV Power Delivery Network with Enhanced Protective Systems

by Eddie Singh, Innocent E. Davidson and Sindisiwe C. Malanda

Energies 2020, 13(24), 6519; https://doi.org/10.3390/en13246519 - 10 Dec 2020

Cited by 2 | Viewed by 1669

Abstract

Earthing and protective devices such as line surge arrestors (LSAs) play an important role in areas with high lightning occurrence for overhead HVAC lines’ performance. A lightning stroke of high magnitude can lead to back flash-overs, and the resultant power surge on the [...] Read more.

Earthing and protective devices such as line surge arrestors (LSAs) play an important role in areas with high lightning occurrence for overhead HVAC lines’ performance. A lightning stroke of high magnitude can lead to back flash-overs, and the resultant power surge on the phase conductor can cause instigate the line breaker operating to extinguish the power surge. This operation of the protective devices leads to consumer interruptions on the network, a loss of production, and negatively affects the economy. Studies have shown that reducing an earthing system’s values, which itself is costly, may not be sufficient to prevent back flashover and the associated customer production cost loss. A code was developed to determine the possibility of back flashover and the cost of various earthing schemes utilizing the MATLAB software analysis tool. This paper determines the possibility of a back flashover for various combinations of lightning strokes and earthing profiles. Tower Footing Resistances as low as 9.8 Ω can cause back flashover, provided the lightning stroke exceeds 12 kA. Furthermore, the paper presents and discusses an innovative hybrid power line protection scheme, which estimates and considers the high cost associated with establishing an earthing system; it examines the impracticality of re-engineering an earthing scheme for implementation and results obtained by the inclusion of lightning surge arrester’s (LSA). The cost-saving resulting from dips is also established over 25 years for an 88 kV line, and the breakeven point is established. The results showed that the best scenario would be to reduce the tower footing resistance to 29.1 Ω and install 11 LSA per phase. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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18 pages, 332 KiB

Open AccessArticle

FPAES: A Hybrid Approach for the Optimal Placement and Sizing of Reactive Compensation in Distribution Grids

by Diego José da Silva, Edmarcio Antonio Belati and Eduardo Werley Silva dos Angelos

Energies 2020, 13(23), 6409; https://doi.org/10.3390/en13236409 - 04 Dec 2020

Cited by 10 | Viewed by 1623

Abstract

Reactive power compensation with Capacitor Banks (CBs) is one of the most successful approaches used in distribution systems, mainly due to their versatility, long-term acceptance in the power industry, and reduced costs. Most allocation methods, however, lack specific strategies to handle the limited [...] Read more.

Reactive power compensation with Capacitor Banks (CBs) is one of the most successful approaches used in distribution systems, mainly due to their versatility, long-term acceptance in the power industry, and reduced costs. Most allocation methods, however, lack specific strategies to handle the limited discrete nature of CBs sizes seeking to improve the overall optimization and computational performance. We present an algorithm for the Optimal Placement of Capacitor Banks (OPCB) in distribution systems by means of a hybrid Flower Pollination Algorithm (FPA)–Exhaustive Search (ES) approach. The pollination process itself determines the sets of buses for placement, while CBs sizes and the final fitness values of each pollen are selected after a full-search is conducted in the sizing space. As the sizing phase works on the limited search space of predetermined discrete bank values, the computational effort to find the optimum CB capacity is greatly reduced. Tests were performed on distribution systems of 10, 34, and 85 buses with respect to the objective function, final losses, and voltage profile. The algorithm offers an excellent compromise between solution quality and computational effort, when compared to similar approaches. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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19 pages, 17898 KiB

Open AccessArticle

Voltage Regulation Using Recurrent Wavelet Fuzzy Neural Network-Based Dynamic Voltage Restorer

by Cheng-I Chen, Yeong-Chin Chen, Chung-Hsien Chen and Yung-Ruei Chang

Energies 2020, 13(23), 6242; https://doi.org/10.3390/en13236242 - 26 Nov 2020

Cited by 7 | Viewed by 1567

Abstract

Dynamic voltage restorers (DVRs) are one of the effective solutions to regulate the voltage of power systems and protect sensitive loads against voltage disturbances, such as voltage sags, voltage fluctuations, et cetera. The performance of voltage compensation with DVRs relies on the robustness [...] Read more.

Dynamic voltage restorers (DVRs) are one of the effective solutions to regulate the voltage of power systems and protect sensitive loads against voltage disturbances, such as voltage sags, voltage fluctuations, et cetera. The performance of voltage compensation with DVRs relies on the robustness to the power quality disturbances and rapid detection of voltage disturbances. In this paper, the recurrent wavelet fuzzy neural network (RWFNN)-based controller for the DVR is developed. With positive-sequence voltage analysis, the reference signal for the DVR compensation can be accurately obtained. In order to enhance the response time for the DVR controller, the RWFNN is introduced due to the merits of rapid convergence and superior dynamic modeling behavior. From the experimental results with the OPAL-RT real-time simulator (OP4510, OPAL-RT Technologies Inc., Montreal, Quebec, Canada), the effectiveness of proposed controller can be verified. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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21 pages, 6019 KiB

Open AccessArticle

Impedance Modeling and Stability Analysis of VSG Controlled Grid-Connected Converters with Cascaded Inner Control Loop

by Yunyang Xu, Heng Nian, Yangming Wang and Dan Sun

Energies 2020, 13(19), 5114; https://doi.org/10.3390/en13195114 - 01 Oct 2020

Cited by 9 | Viewed by 2606

Abstract

This paper develops the impedance models of grid-connected converters under the virtual synchronous generator (VSG) strategy with a cascaded inner control loop and analyzes the system stability of VSG controlled converters with different kinds of weak grid. Different from existing small-signal models with [...] Read more.

This paper develops the impedance models of grid-connected converters under the virtual synchronous generator (VSG) strategy with a cascaded inner control loop and analyzes the system stability of VSG controlled converters with different kinds of weak grid. Different from existing small-signal models with high dimensions, a single-in-single-out (SISO) impedance model with simple mathematical expression is obtained in this paper, which is applied to identify the influence of the cascaded control loop on impedance characteristics and system stability. It is found that the impedance characteristics of VSG controlled converters can become capacitive below the fundamental frequency, and it is mainly caused by the voltage controller in the cascaded control loop of the VSG strategy. Impedance-based stability analysis shows that the capacitive impedance characteristics can benefit the compatibility of converters operated with the series-compensated weak grid, but may deteriorate the system stability with the inductive weak grid, which can be avoided by increasing the proportional coefficients of the cascaded voltage and current controllers or applying a larger virtual resistor to reduce the negative resistance in the capacitive frequency range. Experiments based on the control-hardware-in-loop (CHIL) platform were carried out to verify the developed analytical models and possible system instable cases. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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23 pages, 8771 KiB

Open AccessArticle

Modeling and Parameter Optimization of Grid-Connected Photovoltaic Systems Considering the Low Voltage Ride-through Control

by Li Wang, Teng Qiao, Bin Zhao, Xiangjun Zeng and Qing Yuan

Energies 2020, 13(15), 3972; https://doi.org/10.3390/en13153972 - 02 Aug 2020

Cited by 6 | Viewed by 2738

Abstract

The asymmetric faults often cause the power grid current imbalance and power grid oscillation, which brings great instability risk to the power grid. To address this problem, this paper presented a modeling and parameter optimization method of grid-connected photovoltaic (PV) systems, considering the [...] Read more.

The asymmetric faults often cause the power grid current imbalance and power grid oscillation, which brings great instability risk to the power grid. To address this problem, this paper presented a modeling and parameter optimization method of grid-connected photovoltaic (PV) systems, considering the low voltage ride-through (LVRT) control. The harmonics of the grid current under asymmetric faults were analyzed based on the negative-sequence voltage feedforward control method. The notch filter was added to the voltage loop to filter out the harmonic components of the DC bus voltage and reduce the harmonic contents of the given grid current value. The proportional resonant (PR) controller was added to the current loop. The combination of these two components could reduce the 3rd, 5th, and 7th harmonics of the grid current and the output power fluctuation. Then, the parameters of the inverter controller were identified by the adaptive differential evolution (ADE) algorithm based on the sensitivity analysis. The effectiveness of the proposed method was compared with two other strategies under the asymmetric grid faults. The suppression of DC bus voltage fluctuation, power fluctuation, and low-order harmonics of the grid current all had better results, ensuring the safe and stable operation of the PV plant under grid faults. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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14 pages, 826 KiB

Open AccessArticle

A Novel Probabilistic Power Flow Algorithm Based on Principal Component Analysis and High-Dimensional Model Representation Techniques

by Hang Li, Zhe Zhang and Xianggen Yin

Energies 2020, 13(14), 3520; https://doi.org/10.3390/en13143520 - 08 Jul 2020

Cited by 7 | Viewed by 1591

Abstract

Because the penetration level of renewable energy sources has increased rapidly in recent years, uncertainty in power system operation is gradually increasing. As an efficient tool for power system analysis under uncertainty, probabilistic power flow (PPF) is becoming increasingly important. The point-estimate method [...] Read more.

Because the penetration level of renewable energy sources has increased rapidly in recent years, uncertainty in power system operation is gradually increasing. As an efficient tool for power system analysis under uncertainty, probabilistic power flow (PPF) is becoming increasingly important. The point-estimate method (PEM) is a well-known PPF algorithm. However, two significant defects limit the practical use of this method. One is that the PEM struggles to estimate high-order moments accurately; this defect makes it difficult for the PEM to describe the distribution of non-Gaussian output random variables (ORVs). The other is that the calculation burden is strongly related to the scale of input random variables (IRVs), which makes the PEM difficult to use in large-scale power systems. A novel approach based on principal component analysis (PCA) and high-dimensional model representation (HDMR) is proposed here to overcome the defects of the traditional PEM. PCA is applied to decrease the dimension scale of IRVs and eliminate correlations. HDMR is applied to estimate the moments of ORVs. Because HDMR considers the cooperative effects of IRVs, it has a significantly smaller estimation error for high-order moments in particular. Case studies show that the proposed method can achieve a better performance in terms of accuracy and efficiency than traditional PEM. Full article

(This article belongs to the Special Issue Electric Power Systems Research 2020)

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