PV Curves Research Articles

Impacts of Large-Scale Offshore Wind Power Plants Integration on Turkish Power System

In this paper, the impacts of large-scale OWPPs penetration on the Turkish power system is addressed. The grid compliance analyses for the large-scale OWPP integration are carried out by using the grid connection criteria defined in the Turkish grid code. PV and QV curves are obtained to assess the effect of OWPP on the static voltage stability limit. Eight scenarios are conducted to analyze the effect of the OWPP on the static and dynamic characteristics of the power grid. To observe the large-scale OWPP impact on the voltage and frequency stability, transient events such as the outage of conventional power plants and three-phase to ground faults are applied. The results of the voltage and frequency stability analysis reveal that the Turkish grid remains stable after the integration of an 1800 MW OWPP. Furthermore, the Turkish system remains stable even in the event of the outage of the international transmission lines to Bulgaria and Greece.

Improving performance of transmission networks using facts through continuation power flow method

Over the past 50 years, modern electrical systems have become more complex, as they overrun the geographical boundaries of neighboring countries. The problem is that the power system faces many challenges, because it is exposed to difficult operating conditions. The phenomenon of voltage instability is the most frequent phenomenon, and this can lead to the collapse of the power system. To avoid power outages in the system (especially in blackout situations), the power system must be analyzed in order to maintain voltage stability in the expected difficult operating conditions. The main objective is to determine the maximum load capacity of the system and the causes of voltage instability. The voltage instability problem is related to the nature of nonlinear loads, so different load characteristics must be taken into consideration when analyzing voltage stability. This study aims to discover the maximum load capacity required by using the continuous power flow method (CPF) in the studied network. Then, the performance of this network using a Flexible Alternating Current Transmission System (FACTS) will be utilized. FACTS systems present a promising solution in improving the voltage stability by improving the power transmission capacity and controllability of the parameters of the existing power networks. This study will be conducted on a reference network platform under normal working conditions, then installation of one of the FACTS systems will show its effect on improving voltage stability. The continuous power flow method will be used to find PV curves, which in turn will help to determine the conditions of maximum loading while maintaining stability, and identify the bus bar with the smallest voltage, on which the flexible AC systems will be installed. The software environment MATLAB/PSAT will be used for modeling and simulation.

A Novel FFHE-Inspired Method for Large Power System Static Stability Computation

This article proposes a novel method termed HEAP for fast and reliably computing nose points, tracing the entire PV curve, and assessing static stability limits. Here, the prefix “HE” in HEAP refers to (fast and flexible) <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</u> olomorphic <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</u> mbedding, and “AP” is short for <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</u> rc-length <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</u> arametrization. This novel method uses an arc-length parametrization and piecewise approximants that enable it to have the favorable feature of Continuation Power Flow that can pass through the nose point without numerical difficulties. Another outstanding feature of the proposed method HEAP is its computational efficiency by enabling the accurate approximation over a long interval of the parameter (i.e., a large step size). This is because of the large convergence region of HEAP. The existing methods for similar tasks usually suffer from several issues such as empirical criteria for detecting nose points, small convergence regions of correctors, numerous intermediate points, and large memory space consumption. These issues are well addressed by HEAP. To examine its numerical performance, HEAP is tested on various power system models with as many as 70,000 buses, and numerical results substantiate the outstanding features of HEAP. Depending on intended applications (e.g., only locating the nose point or fully tracking the curve), two tailored approaches are devised to effectively derive numerical solutions. In comparison with the continuation-based power flow method, HEAP attains speedups of 3x to 300x and notably attains speedups of at least 19x for all large test cases with 10,000 or more buses. The continuation-based power flow method fails in several large test cases, however, all of which are successfully solved by HEAP. After enforcing the generator reactive power limit, HEAP also produces the same accurate results as the continuation-based power flow method.

Multiobjective optimal power flow for static voltage stability margin improvement

Worldwide, utilities are aiming to increase the stability of modern power systems during system disturbances. Optimizing generation scheduling can improve system security in contingency and stressed conditions while lowering losses and generation costs. An efficient operating strategy for maintaining power system stability is proposed in this work. The paper focuses on incorporating a Voltage Collapse Proximity Index (VCPI) in the traditional optimal power flow problem for multiobjective optimization (MO). Different case studies are assessed to evaluate the impact on the control variables. A Preference Selection index (PSI) is utilized to determine the best-case study for optimal system operation. The effectiveness of the proposed approach is tested on standard IEEE 30-bus and IEEE 57-bus during normal, contingency, and stressed conditions using MATPOWER. During normal conditions, the MO voltage stability constrained optimal power flow (VSC-OPF) increases the system stability by 28.13 % higher than the single objective (SO) case. Furthermore, the transmission losses are lowered by 14.69% with the proposed MO approach. During line outage contingency conditions, the voltage stability enhancement and loss reduction are higher in the MO than in the SO case by 13.60% and 23.19%. However, the loss minimization and stability improvement during normal and contingency conditions come at a slightly higher generation cost of 5.05% in both systems. On the other hand, during stressed conditions, the SO performs better in voltage stability improvement (by 8.77%) and loss reduction (by 6.97%) than in the MO voltage stability constrained OPF. Additionally, PV Curve analysis for the two systems indicates that voltage stability in MO OPF problems provides a more significant margin enhancement of 9.00%, 118.95% in normal and contingency, respectively, higher than the SO case. However, the SO case increases the load margin by 12.36% more than the MO case in stressed conditions. Consequently, the PSI ranks the multiobjective optimization of the three objectives as the most optimal way for operating the systems in normal and line outage contingency conditions. However, during increased load conditions, the system performance is better if a singular objective function is considered. This is due to the lack of adequate reactive power generation during stressed conditions, and hence a singular objective focus is sufficient to assure system stability. Therefore, the proposed approach is an effective preventive control measure for possible voltage collapse in typical power systems. The resulting improvement also brings about a sufficient system stability margin, causing the system to become more secure.

A Novel Approach Based Incremental Conductance Method for MPPT Strategy of PV Systems Considering Partial Shading Conditions

The main advantages in terms of a simple algorithm, easy to use, fast maximum power point tracking (MPPT) speed of the incremental conductance (InC) and perturbation and observation (P&O) algorithms have been adopted in the literature. Besides, the oscillation phenomenon around the maximum power point (MPP) is a major drawback of these typical algorithms. More specifically, these traditional algorithms failed to find the global maximum power peak when the PV system was operating under partial shading conditions where multiple peaks appeared on the PV curve characteristic. Therefore, a new approach based on the conventional InC algorithm is proposed in this paper. By evenly dividing the search area at the early stage, this proposed method avoids the regional traps encountered by the traditional InC algorithm. Then select the regions where there is a high probability of finding the global MPP to implement the conventional InC approach. After finishing this process, the location with the best power is selected and the MPP search is further performed using the various step-size InC algorithm for the final stage. With this improvement, the steady-state oscillation amplitude of the proposed method is significantly reduced. The implementation process and simulation results show that the proposed method has key advantages in terms of simplicity, fast convergence speed, and reduced steady-state oscillation amplitude. Simulation and experimental results are also compared with the conventional P&O and PSO methods to demonstrate the effectiveness of the proposed method in both uniform illuminance and partial shading conditions.

A Method for the Computation of PV Curve using a Novel Voltage Control Method in Power Systems

A Method for the Computation of PV Curve using a Novel Voltage Control Method in Power Systems

Modifikasi Firefly Algorithm Untuk Partial Shading pada Photovoltaic

Shadows of buildings or trees can partially cover the PV resulting in differences in solar radiation reception. Partial shading occurs when the PV module receives different solar radiation due to shadows of buildings or trees or clouds. This condition causes the output power of the PV array to decrease. Based on the PV curve, partial shading has a direct effect, so that a decrease in voltage or current causes a decrease in the output power of the PV. Because this requires a good control device. In this study, the method was compared without control (Uncontrolled), conventional PID (PID), PID tuned by Firefly Algorithm (PID-FA), and PID tuned by Modified Firefly Algorithm (PID-MFA). From the simulation results, it is found that the best voltage is obtained from the PID-MFA controller, the best voltage obtained from the PID-MFA controller is 1.2755 pu, the best current on the PID-MFA is 4,313; 3.67; 2,551 pu, and the maximum power is 3,253 pu. Thus it can be concluded that the best controller is PID-MFA. This research can later be used as a reference and other controllers are used to obtain an optimal controller.

A generalized voltage stability indicator based on the tangential angles of PV and load curves considering voltage dependent load models

The majority of traditional techniques for voltage stability assessment do not account for load power-voltage characteristics and assume a constant PQ load model, which is proven unreliable especially if the system load contains a large percentage of induction motor loads. This work presents a generalized voltage stability index which has two important features. First, it is applicable to general load models including ZIP, exponential, and induction motor loads. Second, it can be used for both measurement-based and model-based voltage stability assessment. The key idea of this index is that at the collapsing point, the system P-V curve and the load P-V curve share the same tangent line. Thus, the tangential angle between the two curves is an appropriate indicator of proximity to voltage collapse when voltage-dependent loads like ZIP and motor are considered. The performance of the proposed index is tested on both the IEEE 14-bus system and the 181-bus WECC system. The results show that the proposed index gives a good and reliable assessment of system stability. In addition, it demonstrated a consistent behavior, always reaching a value of 0.0 at the stability limit which enables the definition of a threshold for taking corrective measures.

Research on power system static voltage stability considering geomagnetic disturbances effect

AbstractThe reactive power losses caused by the power grid Geomagnetically Induced Current (GIC) severely disturb the static voltage stability of power systems, and may even trigger voltage collapses. In this paper, the 1 V/km uniform induced geoelectric field is adopted to analyze how greatly geomagnetic disturbance (GMD) can influence the static voltage stability of power systems, given the features of GMD disasters hitting the power grids of China. Considering the GIC reactive power losses, a method that combines second‐order sensitivity adaptive variable step size and local curve fitting is put forth to calculate the critical point of the active power and voltage nodes (PV) curve. With the increment ratio of the curve slope set at discretion, the method enables the curve to approach its critical point with great strides, a design managing to reduce the quantity of power flow calculations remarkably. By taking into account the reactive power violation of PV nodes in the acquisition of load increment step size, the method calculates the critical point of the PV curve in a much more accurate way. The method applied to the Northwest 750 kV power grid, the calculation results suggest that the 1 V/km induced geoelectric field markedly undermines the static voltage stability, thus posing grave threats to the above grid.

Non-invasive measurement of leaf water content and pressure\u2013volume curves using terahertz radiation

In this paper we describe a non-invasive method of measuring leaf water content using THz radiation and combine this with psychrometry for determination of leaf pressure–volume relationships. In contrast to prior investigations using THz radiation to measure plant water status, the reported method exploits the differential absorption characteristic of THz radiation at multiple frequencies within plant leaves to determine absolute water content in real-time. By combining the THz system with a psychrometer, pressure–volume curves were generated in a completely automated fashion for the determination of leaf tissue water relations parameters including water potential at turgor loss, osmotic potential at full turgor and the relative water content at the turgor loss point. This novel methodology provides for repeated, non-destructive measurement of leaf water content and greatly increased efficiency in generation of leaf PV curves by reducing user handling time.

Gravitational distribution of regional opening and closing pressures, hysteresis and atelectrauma in ARDS evaluated by electrical impedance tomography

BackgroundThe physiological behavior of lungs affected by the acute respiratory distress syndrome (ARDS) differs between inspiration and expiration and presents heterogeneous gravity-dependent distribution. This phenomenon, highlighted by the different distribution of opening/closing pressure and by the hysteresis of the pressure–volume curve, can be studied by CT scan, but the technique expose the patient to radiations, cannot track changes during time and is not feasible at the bedside. Electrical impedance tomography (EIT) could help in assessing at the bedside regional inspiratory and expiratory mechanical properties. We evaluated regional opening/closing pressures, hysteresis and atelectrauma during inspiratory and expiratory low-flow pressure–volume curves in ARDS using electrical impedance tomography.MethodsPixel-level inspiratory and expiratory PV curves (PVpixel) between 5 and 40 cmH2O were constructed integrating EIT images and airway opening pressure signal from 8 ARDS patients. The lower inflection point in the inspiratory and expiratory PVpixel were used to find opening (OPpixel) and closing (CPpixel) pressures. A novel atelectrauma index (AtI) was calculated as the percentage of pixels opening during the inspiratory and closing during the expiratory PV curves. The maximal hysteresis (HysMax) was calculated as the maximal difference between normalized expiratory and inspiratory PV curves. Analyses were conducted in the global, dependent and non-dependent lung regions.ResultsGaussian distribution was confirmed for both global OPpixel (r2 = 0.90) and global CPpixel (r2 = 0.94). The two distributions were significantly different with higher values for OPpixel (p < 0.0001). Regional OPpixel and CPpixel distributions were Gaussian, and in the dependent lung regions, both were significantly higher than in the non-dependent ones (p < 0.001). Both AtI and the HysMax were significantly higher in the dependent regions compared to the non-dependent ones (p < 0.05 for both).ConclusionsGravity impacts the regional distribution of opening and closing pressure, hysteresis and atelectrauma, with higher values in the dorsal lung. Regional differences between inspiratory and expiratory lung physiology are detectable at the bedside using EIT and could allow in-depth characterization of ARDS phenotypes and guide personalized ventilation settings.Graphic abstract

Reconfiguration Method in Photovoltaic Array for Nonuniform Shading Condition

The output of the photovoltaic array is directly related to the external environment and solar irradiation. Photovoltaic arrays are affected by partial shading which deviates the VI and PV curve from ideal state. This phenomenon influences the operating efficiency of Photovoltaic arrays, as generated power is reduced and hotspots are formed. One possible way to get rid of such negative phenomenon is to reconfigure the Photo voltaic arrays and another method is to operate Photo voltaic panels in peak voltage, which includes global maximum power (GMP) but it has few drawbacks like high complexity, time-consuming. This paper proposes a new methodology of reconfiguring series-parallel (SP) photovoltaic array, which is implemented on MATLAB Simulink. The scheme extracts maximum power by electrically connecting all the shaded or underperforming photovoltaic panels in a common series connection, such that the restricted flow of electricity due to shaded panels is only restricted through one part of the series-parallel photovoltaic array. This method is unlike some interconnection method as it utilizes a reconfigurable part to normalize the fixed part, it is completely adaptable. The obtained results from simulation prove the reliability and overall efficiency under partially shaded array configuration.

Optimización Multi-objetivo de Potencia Activa y Reactiva para crear la Curva PQ en las Barras del SEP

En este trabajo, se propone un problema de optimización multiobjetivo de potencia activa y reactiva en las barras del sistema, el cual considera los límites operativos, la capacidad de generación, las reservas de potencia reactiva, capacidad de potencia activa, las restricciones de transmisión, y los límites de voltaje del sistema eléctrico de potencia (SEP).&#x0D; El problema de optimización multiobjetivo propuesto se basa en una combinación de la función objetivo1: margen de potencia reactiva y la función objetivo 2: potencia activa, en la barra sujeta a analisis del margen del SEP. Del problema multiobjetivo se obtiene la Frontera de Pareto, que representa la curva PQ (proyección de la curva PV y QV sobre el plano) de la barra en análisis. El modelo del problema de optimización es aplicado a un sistema de prueba para su validación y análisis de resultados. De esta manera se proporciona una medida confiable para evaluar la estabilidad de voltaje y la seguridad del sistema en redes eléctricas considerando los limites máximos de operación del SEP.

A Fast and Accurate Analytical Method for Parameter Determination of a Photovoltaic System Based on Manufacturer’s Data

Modeling and simulation of a photovoltaic solar system play a significant role in understanding its behavior in various environmental conditions. Utilization of the datasheet information in modeling and simulation of the PV system correlates the experimental data and the theory that instigate the mathematical predictions of an actual system. A single-diode model gives a simple, fast, and straightforward way of depicting the PV system performance. We have developed a new approach of determining the five unknown parameters of a single-diode model using manufacturer’s data at three main points: the open circuit point (OCP), short circuit point (SCP), and the maximum power point (MPP) of the IV and PV curves. The ideality factor (A) and the diode saturation current (Io) are the key unknown parameters that greatly affect the reduplication of the three main points. The purpose of this study is to evaluate the ideality factor using simple calculation procedure starting from its optimal value (Ao) and other values within the proximity of Ao. The optimal value is obtained by assumptions of negligible series resistance (Rs) and very large shunt resistance (Rsh). Therefore, the choice of the other ideality factors in the neighborhood of its optimal value gives rise to different values of Rs, Rsh, and Iph that are more realistic in an experimental setup. Positive values of Rsh and Rs have been iteratively obtained by utilizing data at maximum power point combined with open and short circuit data. The five unknown parameters have been determined in the proximity of Ao and have been used to plot the PV curve with accuracy and precision of less than 0.5% error of maximum power and less than 0.1% error of Voc of manufacturer’s data. The proposed method has been implemented using fast, simple, and accurate procedures using GNU Octave programming software to calculate Ao, Io, Rs, Rsh, and Iph and to execute both Rs-Rsh and PV characteristic equations of BP3235T, KC200GT, BP-SX 150, and MSX60 PV modules. The reduced steps employed in the algorithm improve execution speed, thereby reducing the computation time.

Voltage stability assessment algorithm to predict power system loadability margin

In this study, a novel voltage stability assessment algorithm (VSAA) is proposed, which can accurately detect the stability stiffness of power systems. To this purpose, VSAA uses only three measured samples of a bus voltage and the power flowing out of the bus to detect the maximum loadability limit of that bus. Although VSAA has a low computational burden and uses only locally measured data, it can consider the impact of small and/or large disturbances that change the Thevenin equivalent network (i.e. PV curve) seen from the target bus and is able to accurately assess the stability status. To examine the performance of VSAA, it was applied and tested on a 4-bus, IEEE 39-bus, Nordic32 and IEEE 33-bus distribution test systems using DIgSILENT PowerFactory software. The static and dynamic simulation results show the advantages of VSAA over other stability indices proposed in the literature and verify its efficiency and accuracy for online applications.

Optimal Placement, Sizing and Coordination of FACTS Devices in Transmission Network Using Whale Optimization Algorithm

Flexible AC Transmission Systems (FACTS) play an important role in minimizing power losses and voltage deviations while increasing the real power transfer capacity of transmission lines. The extent to which these devices can provide benefits to the transmission network depend on their optimal location and sizing. However, finding appropriate locations and sizes of these devices in an electrical network is difficult since it is a nonlinear problem. This paper proposes a technique for the optimal placement and sizing of FACTS, namely the Thyristor-Controlled Series Compensators (TCSCs), Shunt VARs Compensators (SVCs), and Unified Power Flows Controllers (UPFCs). To find the optimal locations of these devices in a network, weak buses and lines are determined by constructing PV curves of load buses, and through the line stability index. Then, the whale optimization algorithm (WOA) is employed not only to find an ideal ratings for these devices but also the optimal coordination of SVC, TCSC, and UPFC with the reactive power sources already present in the network (tap settings of transformers and reactive power from generators). The objective here is the minimization of the operating cost of the system that consists of active power losses and FACTS devices cost. The proposed method is applied to the IEEE 14 and 30 bus systems. The presented technique is also compared with Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). The findings showed that total system operating costs and transmission line losses were considerably reduced by WOA as compared to existing metaheuristic optimization techniques.

Non-iterative MPPT Method: A Comparative Study

The presented work is a contribution to maximum power point tracking problem with improved performance. The analysed and discussed method is based on mathematical model of a PV panel. The output power of PV panel is dependent on the load as well as the almost unpredictable behaviour of the environment. It has a non-linear implicit behaviour on the load due to the weather parameters dependency. Due to different conditions of PV curve, it may have several local maxima. Existing MPPT techniques are mainly based on iterative method which are more time consuming and complex in nature considering the sense of comparative techniques. The most used approach is based on P&O algorithm with gradient comparison. The proposed technique improves the performance on the basis of time and computational complexity. During a low changing environmental condition this method achieves good result on the way to reach the overall point for maximum power. Taking into account the data sheet values of the panel along with the usage of existing knowledge from the datasheets, this technique is possible to implement and flexible for digital signal processing platform. An experimental setup is also done to verify the accuracy, robustness and simplicity of the introduced algorithm. It is found that the proposed technique is less complex and can be coupled with other method too.

A regulating capacity determination method for pumped storage hydropower to restrain PV generation fluctuation

The variable speed and constant frequency pumped storage hydropower (PSH) unit can strongly support the friendly complementation and joint power supply of cascaded hydropower and PV plant. Its fast response capability has provided a feasible solution for the rapid power and voltage regulation caused by real-time fluctuations of PV. However, there is a lack of research on precise evaluation on regulation capability and regulating capacity configuration for PSH to restrain the real-time fluctuations nowadays. In this paper, a cascaded hydro-PV-PSH complementary joint power system (CHPP) is studied, and a rule-based method for regulating capacity determination is proposed. A combined statistical technique is introduced to analyze the initial estimated regulating capacity of PSH. A continuous cyclic revision method is adopted to renew the ideal PV curve using the main operating constraints repeatedly until an optimal regulating capacity of PSH matching PV generation scale is achieved. The results of the case study verified the feasibility and effectiveness of PSH for restraining the fast fluctuations of PV in real-time, and the configuration between PV and PSH regulating capacity is obtained with real-time application requirements. Finally, analyses including weather conditions, curtailed energy and electricity shortage, the sensitivity analysis, state transition frequency are presented to demonstrate the robustness of the study.

Voltage Stability of Power System using PV Curve and PMU data

This paper describes a voltage stability indicator that can be used to determine the proximity to system collapse by power system operators. The proposed PV curve method to determine voltage instability is compared with the existing voltage collapse proximity index (VCPI). VCPI is indicative of critical transmission lines whereas the PV curve analysis is indicative of critical buses. Phasor Measurement Unit (PMU) data facilitates in quick calculation and presentation of the indices to the system operators. The drawbacks of VCPI and how PV curve method is better than VCPI are presented. The simulations have been carried out in Real Time Digital Simulator (RTDSTM) using the New England IEEE 39 bus system. The indices accurately quantify the closeness of the power system towards instability. A slope monitoring method has been used to take restorative actions for the system to return to secure operating conditions.

Assessment of Grid-Feeding Converter Voltage Stability

This letter applies voltage stability analysis to grid feeding converters in the presence of the converter stability versus the grid state and its operation. By applying this analysis, it is shown that the converter may become unstable if the converter reference power or current exceeds the line capacity. This letter proposes to use a conventional PV curve to determine the stability of the dynamic response of grid-feeding converters considering both power and current limits.