Large-scale Photovoltaic Power Plants Research Articles

Analysis and Suppression of Active Power Backflow of Three-Phase Common DC-Bus Cascaded H-Bridge PV Grid-Connected Inverter During LVRT

Featured with the expandable modular structure, three-phase isolated cascaded H-bridge (CHB) inverters are capable of directly connecting to medium voltage power grid without bulky and heavy line-frequency transformer, which has outstanding advantages applied in large-scale photovoltaic (PV) power plants. However, different from traditional PV inverters, three-phase CHB topology is essentially composed of three single-phase CHB inverters. Affected by negative-sequence grid voltage, the active powers of some phases may inversely flow from ac grid to PV inverter during low voltage ride through (LVRT), resulting in that dc-bus voltages of all H-bridges in these phases continue to rise, and that system will shut down due to over-voltage protection. Aiming at this issue, this article studies the mechanism of active power backflow during LVRT, and deduces the quantitative relationship between active current and reactive current to be necessarily injected under different types of voltage faults and different degrees of voltage sags. In addition, combined with the relevant standard of LVRT, the feasibility of suppressing active power backflow by positive-sequence active power current injection is analyzed in detail. The validity of the proposed method is verified by simulation and experimental results.

Resonance mechanism analysis of large-scale photovoltaic power plant

To analyze the resonance mechanism of a photovoltaic (PV) power plant, a simplified impedance model of the PV power plant is first established. The structure of the PV power plant is then introduced, and the reason for the resonance is obtained by analyzing the on-site situation and measured data of the PV power plant. Finally, a simple and effective solution is proposed based on the structure of the PV power plant and its existing facilities. The results of the engineering experiments and the stable operation of the PV power plant verify the effectiveness of the proposed method.

Performance Analysis of 14 MW Grid-Connected Photovoltaic System

Many Libyan authorities proposed to investigate the possibility of utilizing a suitable terrain in Libya to add generation capacity of large-scale photovoltaic power plants. In this paper, the first grid-connected PV plant of 14 MWp which will be executed in Hoon city and supported by the Renewable Energy Authority of Libya (REAOL) is presented. To understand and improve the operational behavior of PV system, a comprehensive study including the plant design and detailed performance analysis under a local climate conditions is performed. Using polycrystalline silicon technology, the first year energy yield is estimated and the monthly system output for this plant is calculated. The performance ratio and various power losses (temperature, irradiance, power electronics, interconnection, etc.) are determined. The PV system supplied 24964 MWh to the grid during the first year giving an average annual overall yield factor 1783 kWh/kWp and average annual performance ratio of the system of 76.9%.

Performance Analysis of 14 MW Grid-Connected Photovoltaic System

Many Libyan authorities proposed to investigate the possibility of utilizing a suitable terrain in Libya to add generation capacity of large-scale photovoltaic power plants. In this paper, the first grid-connected PV plant of 14 MWp which will be executed in Hoon city and supported by the Renewable Energy Authority of Libya (REAOL) is presented. To understand and improve the operational behavior of PV system, a comprehensive study including the plant design and detailed performance analysis under a local climate conditions is performed. Using polycrystalline silicon technology, the first year energy yield is estimated and the monthly system output for this plant is calculated. The performance ratio and various power losses (temperature, irradiance, power electronics, interconnection, etc.) are determined. The PV system supplied 24964 MWh to the grid during the first year giving an average annual overall yield factor 1783 kWh/kWp and average annual performance ratio of the system of 76.9%.

Reactive PowerControl of Grid-Connected Photovoltaic Power Generation

In order to solve the problem of grid-connected point voltage exceeding the limit caused by large-scale photovoltaic power stations connected to the grid, and to increase the penetration rate of photovoltaics in the grid, photovoltaic power stations should have more flexible reactive voltage regulation capabilities to provide reactive power support to the grid. To this end, this paper establishes an equivalent aggregation model for large-scale photovoltaic power plants. Under the premise that the photovoltaic power station is not equipped with reactive power compensation devices and the inverter reactive power output is zero, it is analyzed that due to the existence of line and transformer impedance, photovoltaic access reduces the grid the problem of voltage stability. Based on the above reasons, this paper proposes a three-layer reactive power control strategy for photovoltaic power plants from the perspective of the cooperation between the reactive power compensation device and the grid-connected inverter. This strategy coordinates the reactive power output between the reactive power compensation device and the photovoltaic power generation unit and between the inverters of a single photovoltaic power generation unit. Under this control strategy, the photovoltaic power plant can regulate the grid voltage more effectively, and the active and reactive power losses of the grid are minimized on the premise that the grid voltage is maintained within the required range. Combined with the photovoltaic array power reduction operation strategy, the stable operation of the power grid is ensured under the premise of a certain reactive power output capacity of the photovoltaic power station. Finally, an example is used to verify the correctness and effectiveness of the strategy.

Power Balancing in Cascaded H-Bridge and Modular Multilevel Converters Under Unbalanced Operation: A Review

Multilevel Voltage-Source Converters (VSC) based on modular structures are envisioned as a prominent alternative for grid and industry applications. Foremost among these are the Cascaded H-Bridge (CHB) and the Modular Multilevel Converter (MMC). In this context, depending on the application and the power conversion structure, unbalanced operating conditions can be asked to the converter. Previous investigations regarding the operation and the solutions for modular structures under unbalanced conditions have already addressed this topic, but information is dispersed over a wide number of sources. This paper identifies, classifies, and analyzes the intercluster active power balancing strategies for the adequate operation of the most commonly used modular structures in some typical unbalanced operating scenarios: the Static Synchronous Compensator (STATCOM) under unbalanced voltage and/or current conditions, the unequal power generation in large-scale photovoltaic (PV) power plants, and the uneven power distribution in a battery energy storage system (BESS). Each of the applications has been independently studied so as to provide a comprehensive analysis of the alternative techniques found in the specialized literature, clearly explaining their respective strengths and drawbacks. Several future challenges have been identified during the study, which will involve greater research effort in this key research topic.

Damping of Subsynchronous Control Interactions in Large-Scale PV Installations Through Faster-Than-Real-Time Dynamic Emulation

Large-scale photovoltaic (PV) power plant has witnessed a dramatic increase in the integration into transmission and distribution network, manifesting subsynchronous control interaction (SSCI) when the host grid is weak. In this work, the oscillation modes of a typical PV network are analyzed, and a faster-than-real-time (FTRT) emulation is proposed for predicting the SSCI and consequently mitigating its impacts on AC grid by taking the effective active/reactive power control action. The electromagnetic transient (EMT) simulation is utilized to model the PV panels and converter stations to reflect the actual dynamic process. Meanwhile, the AC grid undergoes transient stability (TS) simulation to obtain a high speed up over real-time, and consequently, a power-voltage interface is adopted for the coexistence of different simulation methods. The reconfigurability and parallelism of the field-programmable gate arrays (FPGAs) enable the EMT-TS co-emulation strategy to run concurrently. With a remarkable 122 FTRT ratio, the proposed hardware emulation can provide an effective solution before the SSCI causes serious disruption following its detection. The hardware emulation results are validated by the off-line simulation tool Matlab/Simulink® and TSAT® in the DSATools™ suite.

Over-frequency support in large-scale photovoltaic power plants using non-conventional control architectures

Large scale photovoltaic power plants must provide a frequency regulation service, which is defined in the grid codes. This service has commonly required a response time between 15 and 30 s. But some countries are now introducing more strict regulations and requiring response times below 2 s. The typical centralized control architecture of photovoltaic power plants for frequency regulation can present undesired oscillatory responses (or even become unstable) when tuning the controller to achieve these small time response requirements. The present article proposes an alternative solution based on a hierarchical control architecture. In the proposed solution, inverter controllers apply a local frequency regulation action and the central controller corrects active power errors at the point of connection, which can be caused by power losses or lack of irradiance in some inverters. Simulation models are used to study and test the response of this new control approach. The proposed hierarchical control architecture is compared with a fully centralized and a fully decentralized archirectures. Results show that the hierarchical control architecture is not only capable to obtain a fast and accurate response, but also is robust against communication failures. The proposed hierarchical control architecture advantages could be extrapolated to other services. So, further research is proposed to confirm this hypothesis.

Investigating Fourteen Countries to Maximum the Economy Benefit by Using Offline Reconfiguration for Medium Scale PV Array Arrangements

Over the past few years, electricity demand has been on the rise. This has resulted in renewable energy resources being used rapidly, considering the shortage as well as the environmental impacts of fossil fuel. A renewable energy source that has become increasingly popular is photovoltaic (PV) energy as it is environmentally friendly. Installing PV modules, however, has to ensure harsh environments including temperature, dust, birds drop, hotspot, and storm. Thus, the phenomena of the non-uniform aging of PV modules has become unavoidable, negatively affecting the performance of PV plants, particularly during the middle and latter duration of their service life. The idea here is to decrease the capital of maintenance and operation costs involved in medium- and large-scale PV power plants and improving the power efficiency. Hence, the present paper generated an offline PV module reconfiguration strategy considering the non-uniform aging PV array to ensure that this effect is mitigated and does not need extra sensors. To enhance the economic benefit, the offline reconfiguration takes into account labor cost and electricity price. This paper proposes a gene evolution algorithm (GEA) for determining the highest economic benefit. The proposed algorithm was verified using MATLAB software-based modeling and simulations to investigate fourteen countries to maximize the economic benefit that employed a representative 18-kW and 43-kW output and the power of 10 × 10 PV arrays in connection as a testing benchmark and considered the electricity price and workforce cost. According to the results, enhanced power output can be generated from a non-uniformly aged PV array of any size, and offers the minimum swapping/replacing times to maximize the output power and improve the electric revenue by reducing the maintenance costs.

Performance of Communication Network for Monitoring Utility Scale Photovoltaic Power Plants

The grid integration of large scale photovoltaic (PV) power plants represents many challenging tasks for system stability, reliability and power quality due to the intermittent nature of solar radiation and the site accessibility issues where most PV power plants are located over a wide area. In order to enable real-time monitoring and control of large scale PV power plants, reliable two-way communications with low latency are required which provide accurate information for the electrical and environmental parameters as well as enabling the system operator to evaluate the overall performance and identify any abnormal conditions and faults. This work aims to design a communication network architecture for the remote monitoring of large-scale PV power plants based on the IEC 61850 Standard. The proposed architecture consists of three layers: the PV power system layer, the communication network layer, and the application layer. The PV power system layer consists of solar arrays, inverters, feeders, buses, a substation, and a control center. Monitoring parameters are classified into different categories including electrical measurements, status information, and meteorological data. This work considers the future plan of PV power plants in Saudi Arabia. In order to evaluate the performance of the communication network for local and remote monitoring, the OPNET Modeler is used for network modeling and simulation, and critical parameters such as network topology, link capacity, and latency are investigated and discussed. This work contributes to the design of reliable monitoring and communication of large-scale PV power plants.

Aerial infrared thermography for low-cost and fast fault detection in utility-scale PV power plants

The uptime of utility-scale solar photovoltaic (PV) power plants is of utmost importance for meeting contractual energy yields and expected capacity factors. Aerial Infrared Thermography (aIRT) is a non-destructive, no-downtime, fast and cost-effective method for monitoring large-scale PV power plants and assisting in fault detection. The use of aIRT techniques aims at increasing the quality and service life of PV plants especially in sunny and developing countries such as Brazil, where there is a shortfall of specialised workforce and the costs for a detailed supervisory system of utility-scale PV power plants are high. This paper presents an analysis of an aIRT flight campaign over four utility-scale PV plants in the northeast of Brazil. Two types of measurement equipment have been tested and compared, resulting in more stability and efficiency using a commercially available solution. This solution was also equipped with a RGB camera that accelerated the inspections, since it helped to differentiate defects from hot spots caused by soiling and vegetation over the modules, which were common. Different methods for fault detection were also tested and it was concluded that post-flight image analysis provides faster results. The most common faults that can happen in the early operation of PV power plants and how operators should address and prevent them were also discussed. The most common defect detected during the campaign were disconnected cell substrings. However, disconnected strings had the most impact on the power plants energy performance.

Estimation of the largest expected photovoltaic power ramp rates

Photovoltaic (PV) systems are prone to irradiance variation caused by cloud shadows leading to fluctuations in generated power. Since these fluctuations can be harmful to the operation of power grids, there is a need to restrict the largest PV power ramp rates (RR). This article proposes a method to estimate the largest expected PV power RRs. The only inputs of the method are the minimum PV system dimension and the measurements of point irradiance and cloud shadow velocity. Since cloud shadows cause the largest power RRs for well-designed large-scale PV power plants, the relation between the largest RRs in irradiance and power during partial cloud shading events was studied based on irradiance measurements. The largest RRs in PV power are estimated from RRs in the average irradiance across the PV system. The proposed method was validated using measured data of 57 days from two PV systems. It showed superior performance compared to an existing method enveloping the RR in the measured power over 99.99% of the time. The method can be used in design and component sizing of PV power plants.

A review of energy storage technologies for large scale photovoltaic power plants

Energy storage can play an essential role in large scale photovoltaic power plants for complying with the current and future standards (grid codes) or for providing market oriented services. But not all the energy storage technologies are valid for all these services. So, this review article analyses the most suitable energy storage technologies that can be used to provide the different services in large scale photovoltaic power plants. For this purpose, this article first summarizes the different characteristics of the energy storage technologies. Then, it reviews the grid services large scale photovoltaic power plants must or can provide together with the energy storage requirements. With this information, together with the analysis of the energy storage technologies characteristics, a discussion of the most suitable technologies is performed. In addition, this review also discusses how to locate the energy storage within the photovoltaic power plant. The results show that (i) the current grid codes require high power – medium energy storage, being Li-Ion batteries the most suitable technology, (ii) for complying future grid code requirements high power – low energy – fast response storage will be required, where super capacitors can be the preferred option, (iii) other technologies such as Lead Acid and Nickel Cadmium batteries are adequate for supporting the black start services, (iv) flow batteries and Lithium Ion technology can be used for market oriented services and (v) the best location of the energy storage within the photovoltaic power plays an important role and depends on the service, but still little research has been performed in this field.

Deep learning based automatic defect identification of photovoltaic module using electroluminescence images

The maintenance of large-scale photovoltaic (PV) power plants is considered as an outstanding challenge for years. This paper presented a deep learning-based defect detection of PV modules using electroluminescence images through addressing two technical challenges: (1) providing a large number of high-quality Electroluminescence (EL) image generation method for the limit of EL image samples; and (2) an efficient model for automatic defect classification with the generated EL image. The EL image generation approach combines traditional image processing technology and GAN characteristics. It can produce a large number of EL image samples with high resolution using a limited number of samples. Then, a convolution neural network (CNN) based model for the automatic classification of defects in an EL image is presented. CNN is used to extract the deep feature of the EL image. It can greatly increase the accuracy and efficiency of PV modules inspection and health management in comparison with the other solutions. The proposed solution is assessed through extensive experiments by using the existing machine learning models, VGG16, ResNet50, Inception V3 and MobileNet, as the comparison benchmarks. The numerical results confirm that the proposed deep learning-based solution can carry out efficient and accurate defect detection automatically using the electroluminescence images.

A Photovoltaic Array Fault Diagnosis Method Considering the Photovoltaic Output Deviation Characteristics

There are a large number of photovoltaic (PV) arrays in large-scale PV power plants or regional distributed PV power plants, and the output of different arrays fluctuates with the external conditions. The deviation and evolution information of the array output are easily covered by the random fluctuations of the PV output, which makes the fault diagnosis of PV arrays difficult. In this paper, a fault diagnosis method based on the deviation characteristics of the PV array output is proposed. Based on the current of the PV array on the DC (direct current) side, the deviation characteristics of the PV array output under different arrays and time series are analyzed. Then, the deviation function is constructed to evaluate the output deviation of the PV array. Finally, the fault diagnosis of a PV array is realized by using the probabilistic neural network (PNN), and the effectiveness of the proposed method is verified. The main contributions of this paper are to propose the deviation function that can extract the fault characteristics of PV array and the fault diagnosis method just using the array current which can be easily applied in the PV plant.

대규모 태양광발전소의 개별접지 및 통합접지에 따른 안전전압(보폭전압 및 접촉전압) 비교 분석

대규모 태양광발전소의 개별접지 및 통합접지에 따른 안전전압(보폭전압 및 접촉전압) 비교 분석

Detailed modeling of large scale photovoltaic power plants under partial shading conditions

As the popularity of solar panels increases, significant research is being performed on maximizing the power extracted from photovoltaic systems. A phenomenon known as partial shading is a major source of power losses for photovoltaic plants. One way to reduce the effects of partial shading conditions is by using bypass–diodes. Typically any number of series and parallel–connected photovoltaic cells, known as photovoltaic cell arrays, are connected in anti–parallel with bypass–diodes. Due to this, it is advantageous to have an equivalent model for a photovoltaic cell array that is connected to bypass–diodes. In order to accurately develop this model, it is necessary to consider both series and parallel configurations of bypass–diodes, as well as the thermal behavior of bypass–diodes. The bypass–diode’s temperature model must be comprised of the constantly changing dissipated power, the effects of wind speed and the ambient temperature. This paper constructs an electro–thermal modular model for any size photovoltaic system and includes the effects of both the photovoltaic cells and bypass–diodes. A detailed analysis of the proposed model is performed and is then validated in both PSCAD/EMTDC and Matlab–Simscape. The proposed model is then further validated by the use of real measurements of ambient temperature, cell temperature, wind speed and then finally by a performance analysis of several maximum–power–point–tracking algorithms.

Accuracy Improvement Method of Energy Storage Utilization with DC Voltage Estimation in Large-Scale Photovoltaic Power Plants

In regard to electric devices, currently designed large-scale distributed generation systems require a precise prediction strategy based on the composition of internal component owing to an environmental fluctuating condition and forecasted power variation. A number of renewable resources, such as solar or marine based energies, are made up of a low voltage direct current (DC) network. In addition to actively considering a power compensation plan, these generation systems have negative effects, which can be induced to a connected power system. When a storage is connected to a DC-based generation system on an inner network along with other generators, a precise state analysis plan should back the utilization process. This paper presents a cooperative operating condition, consisting of the shared DC section, which includes photovoltaic (PVs) and energy storage devices. An active storage management plan with voltage-expectation is introduced and compared via a commercialized electro-magnetic transient simulation tool with designed environmental conditions. Owing to their complexity, the case studies were sequentially advanced by dividing state analysis verification and storage device operation.

Active and Reactive Power Control of a PV Generator for Grid Code Compliance

As new grid codes have been created to permit the integration of large scale photovoltaic power plants into the transmission system, the enhancement of the local control of the photovoltaic (PV) generators is necessary. Thus, the objective of this paper is to present a local controller of active and reactive power to comply the new requirements asked by the transmission system operators despite the variation of ambient conditions without using extra devices. For this purpose, the control considers the instantaneous capability curves of the PV generator which vary due to the change of solar irradiance, temperature, dc voltage and modulation index. To validate the control, the PV generator is modeled in DIgSILENT PowerFactory ® and tested under different ambient conditions. The results show that the control developed can modify the active and reactive power delivered to the desired value at different solar irradiance and temperature.

Research on modelling and simulation of converters for electromagnetic transient simulation in photovoltaic power generation system

Large-scale photovoltaic (PV) power plants connected to the grid leads to the complex calculations in the modelling and simulation of the power system, especially in the electromagnetic transient progress. Common averaging methods are studied for modelling a grid-connected converter to simplify the calculation and balance accuracy and efficiency in simulation. The piecewise technique, which combines the time segment with similar operating characteristic, is applied to different averaging models of converters. The advantages and disadvantages of different piecewise averaging models are compared. In addition, a new piecewise generalised state-space averaging (P-GSSA) model is derived and a multiple time scale modelling is achieved for the grid-connected converters in PV systems. The model error and accuracy of the P-GSSA model are analysed, the physical significance and the influence factors in the model error of P-GSSA model are discussed. The research shows that P-GSSA model is flexible to be used in a PVs system simulation, and it can accurately reflect the steady state and transient characteristics of a converter. The analysis of the P-GSSA model is verified by simulating a DC/AC converter and a PV system.