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Performance of large-scale grid-connected photovoltaic system under various fault conditions
Abstract
The paper discusses the performance of a 10 MVA grid-connected photovoltaic system (GCPS) under various grid faults. The faults include the typical three-phase symmetrical fault (3LG), the single-phase to-ground fault (LG), two-phases-to-ground fault (2LG) and the phase-to-phase fault (2L). The system is studied with and without reactive power support in the case of voltage sags. The faults that may lead to the inverter protection triggering from both DC and AC sides are identified based on grid codes, fault-ride-through (FRT) capability and according to the datasheet of the inverter. The evaluation is performed through electromagnetic transients (EMT) simulations using DigSILENT PowerFactory software. The results indicate that the reactive power injection in asymmetrical faults triggers inverter protection systems apart from the AC over-current protection.
... Photovoltaic system is one of the best ways to produce electricity but it also cannot stop the fault to happen in the system [6]. The faults include the typical three-phase symmetrical fault, the single-phase-to-ground fault, two-phases-to-ground fault and the phase-to-phase fault [7]. ...
... Symmetrical faults have a higher impact on PV systems performance than asymmetrical faults, both at the PCC and inside the grid connected PV array [14]. In addition, according to [7] The performance of the PV system inverter under various types of faults is evaluated with a 10 MVA GCPS system. The results show that the reactive power injection in asymmetrical faults triggers other inverter protection systems apart from the AC overcurrent protection. ...
... The unbalanced power between grid side and PV side causes increasing of the DC-Link capacitor voltage which causes semiconductor devices damage [4]. Also, the excessive of DC voltage and excessive AC currents causing inverter disconnection [5]. To maintain the DC-link voltage constant, dc-chopper circuit between the inverter and the DC-Link is used in [6] to absorb the power at DC side and to maintain the power balance. ...
... The inverter control system of PV uses two PI controllers [16]. The PI controller is most widely and commonly used controller in the process industry because the number of parameter to adjust in PI controller is very small and many tuning methods and algorithms are available that can be implemented for parameter adjustment [5], [17], [18]. The features of PI controller are: The proportional term (k) immediately impacts controller bias or null value based on the size of the error signal at a particular time, also the past history and current trajectory of the controller error have no influence on the proportional term computation. ...
In modern electric power systems, the dependence on solar power is increasing. The grid connected applications are very important with the deficit in conventional power stations due to fuel shortage. This paper focuses on controller design and implementation in grid connected PV systems for low voltage rid through (LVRT) in distribution power systems, this paper proposes a control method to keep the dc-link voltage of PV system constant during low voltage rid through (LVRT). This is done by changing the operating point on the I/V curve of the PV cell from the MPPT point to another point, the active power output of the PV will decrease from the maximum to the needed value. The proposed control system is evaluated under different fault scenarios to test the validity of the control system. The results show that the proposed control system gives the expected performance. The PSCAD package is used for simulating this study.
... A short comparison between the grid codes of China, Germany and US for PVPPs connected to the distribution or to the transmission system, considering also the electrical standards IEEE 1547 and EN 50160 is developed in [2]. As Germany was the first technical grid code launched for PVPPs connected to 30 medium and high voltage transmission network, there are some publications analysing these requirements like [3] and [4]. These focus on the response of the PVPP when there are disturbances. ...
... This study shows that the larger capacity of the converter, the reactive current support is increased [61]. In [4] Power Factory R and several voltage sags are tested. The results show that during a fault, large amount of active power is lost and the reactive current injection allows a better performance of the PVPP at the PCC. ...
The installation of large scale photovoltaic power plants connected at transmission level has increased during the last years. There are some challenges that these power plants have to overcome regarding the operation and control while dealing with the solar energy variability and uncertainty. Today, few countries are aware of the importance of this source of energy as part of the utility system and how it can affect the operability. Thus, this paper discusses about the trend of large scale photovoltaic power plants around the world and the importance of the development of grid codes for its integration. Then, the paper addresses a comparison of the grid codes of Germany, US, Puerto Rico, Romania, China and South Africa considering: fault ride through capability, frequency and voltage regulation, as well as active and reactive power support. In addition, a broad discussion about the challenges that the large scale photovoltaic power plants have to overcome is presented together with the compliance technology and future trend.
... Several articles had been accomplished deal with issue of interaction between solar power plant and networks. Ref [1] had studied the impact of different types of grid faults on interconnected PV system. Some of articles used different optimization methods to adapt the control parameters of PV inverters. ...
Due to the large presence of solar power plant in electric power networks, it became obligatory to study the interaction between them and the associated networks. In this paper, a Distribution Static Synchronous Compensator (DSTATCOM) is used to improve the performance of a photovoltaic generation plant (PV) during three phase fault of interconnected grid. Artificial neural networks (ANN) based on Lightning attachment procedure optimization (LAPO) is used to tune the control parameters of D-STATCOM in targeted with PV plant. The LAPO is used for generating historical data needed for ANN. In order to point out the contribution of this paper, the performance of PV system without and with D-SATCOM is compared during three-phase fault and when D-STATCOM is tuned by LAPO only and by proposed ANN.
... Germany developed the first technical GC specifically for PVPPs interconnected to medium-(MV) and high-voltage (HV) networks in 2008. These specifications were analyzed in some studies [6,7]. The focus of these requirements on the response of the PVPP is during failures. ...
Renewable energy systems (RESs), such as photovoltaic (PV) systems, are providing increasingly larger shares of power generation. PV systems are the fastest growing generation technology today with almost ~30% increase since 2015 reaching 509.3 GWp worldwide capacity by the end of 2018 and predicted to reach 1000 GWp by 2022. Due to the fluctuating and intermittent nature of PV systems, their large-scale integration into the grid poses momentous challenges. This paper provides a review of the technical challenges, such as frequency disturbances and voltage limit violation, related to the stability issues due to the large-scale and intensive PV system penetration into the power network. Possible solutions that mitigate the effect of large-scale PV system integration on the grid are also reviewed. Finally, power system stability when faults occur are outlined as well as their respective achievable solutions.
... Different studies have been conducted regarding the PV system's performance while connecting to grid or serving load as a stand alone system. In [5], the effects of different kind of grid faults on a 10 MVA PV system are discussed and different system responses to reactive power support during faults are investigated. In [6], different island mode control strategies are identified for PVs and batteries working in a droop-controlled medium as standalone distributed generation units (DGs). ...
The growing penetration of solar photovoltaic (PV) systems in power grids has increased the opportunity of these systems' exposure to different dynamics. The objective of this paper is to study and analyze the dynamics resulted from an accidental islanding event of a 30 MW grid connected PV system. The effect of voltage source converter (VSC) control switching and the influence of initial phase selection are discussed thoroughly. Different case studies are performed to test the system performance considering resistive and inductive loads. Moreover, the effect of the resulted dynamics on sensitive loads such as rotating machines is examined. Analysis are carried out based on electromagnetic transient (EMT) simulation results using MATLAB/SimPowerSystems.
... Similarly, Wieserman has shown the behviour of the inverter based DGs for Line to Ground (LG) fault in [8] but for only star-delta configuration of the transformer and also he has not analysed the other unsymmetrical faults. Similarly, Badi [9], Gonzalez [10] and Mirhosseini [11] have considered different faults but have not considered different transformer configuration for each type of faults. ...
... LVRT for grid-connected PV systems has been studied in the recent works [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], and is an actively researched topic. Numerous LVRT-enabled control techniques have been presented in the literature such as feedback linearization sliding mode control [8], model predictive control (MPC) [7], neural network (NN) control [5], proportional plus integral (PI) and proportional plus resonant (PR) control [4,[10][11][12]14,16,17], to name a few. ...
A robust control scheme with low-voltage ride-through ability is presented for grid-connected photovoltaic converters that operate under harsh conditions such as voltage sags and unknown disturbances and parameters. The proposed strategy allows for flexible active and reactive power injection into the grid during asymmetrical voltage sags without using a phase-locked loop or positive and negative sequence components. In addition, the same controllers are used under both normal operation and voltage sags, thus, lowering the control system's complexity. The scheme is conceived by combining uncertainty-and-disturbance-estimation compensation and repetitive control for the dc voltage and phase currents, respectively. Controller design is carried out systematically and closed-loop stability is proven through Lyapunov's analysis. Several simulation case studies are presented using the SimPowerSystemsTM toolbox of MATLAB/Simulink computing environment to demonstrate the performance of the proposed control system under voltage sags, unknown disturbances, abrupt changes in operating conditions, and parametric uncertainties.
... It is of main concern the resilience of the DPGS [9], the faultride-through within the normal operating limits of the grid voltage has to be considered in PV systems, otherwise, a power outage may occur [9]. For instance, in [10][11][12] the grid-connected PV-systems are proposed as reactive power support in case of sags and swells on the grid voltage, as well as unbalanced voltage conditions. Moreover, in [13] fault current limiters are proposed in order to allow the PV-system to operate under grid faults assuring grid sinusoidal currents. ...
This study describes a current controller, based on a Lyapunov control law and a limit cycle oscillator (LCO) to provide stability into the grid‐connected photovoltaic (PV) system during grid faults. The current controller also ensures an efficient current injection and active power regulation. The proposed controller offers a high degree of immunity and robustness against perturbation on the grid, due to the three‐phase LCO – frequency locked loop (FLL) synchronisation technique. The LCO‐FLL is used to compute the positive and negative sequence components of the grid, which are used by a Lyapunov control law in order to control the injected currents into the grid. Moreover, the inverter input voltage is considered in the controller design. The control configuration is proved by two different strategies, balanced injected currents and constant active power in the presence of an unbalanced grid voltage. Simulation and experimental test results are presented to demonstrate the proficiency and performance of the proposed technique in grid‐connected PV systems.
... The low voltage ride through (LVRT) is one of the most important features to be fulfilled by the gridconnected PV system. The LVRT capability implies the commitment of any power system to stay connected to the grid in case of grid failures, contributing to voltage support during and after any fault or disturbance [9], [10]. ...
Contribution of Photovoltaic (PV) systems is rapidly growing and great attention is given to the design of PV controllers to enhance both the performance of PV systems and the low voltage ride through (LVRT) capability during abnormal operational conditions. This article presents a novel application of the salp swarm algorithm (SSA) in order to optimally tune the PV controllers to enhance the LVRT of grid‐connected PV systems. Enhancement of LVRT is indicated in percentage undershoots or overshoots, settling time and steady‐state error of voltage response. A control strategy is applied to the DC‐DC converter to obtain a maximum power point tracking operation through a proportional‐integral (PI)‐based open fractional voltage control. The grid side inverter controls both the point of common coupling voltage and the DC‐link voltage through PI‐based cascaded‐voltage control. To get PI controller parameters that guarantee the optimum design of the controllers, the fitness function is optimized by using the SSA. The proposed optimal control scheme is tested under various fault scenarios and compared with other conventional optimization‐based PI controllers to examine its validity under PSCAD environment. The effectiveness of the optimal control scheme is verified by comparing the simulation results with the practical results of the PV system.
... Germany developed the first technical GC specifically for PVPPs interconnected to medium-(MV) and high-voltage (HV) networks in 2008. These specifications were analyzed in some studies [6,7]. The focus of these requirements on the response of the PVPP is during failures. ...
Renewable energy systems (RESs), such as photovoltaic (PV) systems, are providing increasingly larger shares of power generation. PV systems are the fastest growing generation technology today with almost ~30% increase since 2015 reaching 509.3 GWp worldwide capacity by the end of 2018 and predicted to reach 1000 GWp by 2022. Due to the fluctuating and intermittent nature of PV systems, their large-scale integration into the grid poses momentous challenges. This paper provides a review of the technical challenges, such as frequency disturbances and voltage limit violation, related to the stability issues due to the large-scale and intensive PV system penetration into the power network. Possible solutions that mitigate the effect of large-scale PV system integration on the grid are also reviewed. Finally, power system stability when faults occur are outlined as well as their respective achievable solutions.
... Similarly, Wieserman has shown the behviour of the inverter based DGs for Line to Ground (LG) fault in [8] but for only star-delta configuration of the transformer and also he has not analysed the other unsymmetrical faults. Similarly, Badi [9], Gonzalez [10] and Mirhosseini [11] have considered different faults but have not considered different transformer configuration for each type of faults. ...
... The nature of this kind of faults simply implies any abnormal condition which diverts current from its intended paths, which result in the flow of very high magnitude of current through the power system components which may be damaged if the fault currents persist even for a short time. There are various possible short circuit fault conditions; some of which are symmetrical such as all three phase to ground and the all three phase short circuited faults, while the remaining are phase to phase, single phase to ground, two phase to ground and phase to phase plus single phase to ground faults which come in unsymmetrical category [20,22,23]. Hence, this paper presents the impacts of all these possible conditions of short circuit fault on a proposed grid connected distributed micro-hydro system. ...
Distributed Generation (DG), deploying small
power sources of localized nature, has emerged as a well accepted
alternative to the conventional power generation due to its cost
effectiveness for peak sharing or increasing load capacity. It is
generally connected to the distribution system to work along with
the main grid to supply electric power but may operate in
islanding mode in case of a fault or any electrical contingency at
the main grid system. DG is mostly based upon clean renewable
energy technologies. However, amongst various options,
distributed micro-hydro generation is considered the most cost-
competitive to fossil fuels. In addition, it has high efficiency and
capacity factor, and reduced rate of change in comparison to the
same size of wind and solar based distributed generations.
Therefore, this paper engages micro-hydro as distributed
generation to analyze its response under various fault conditions
considering grid connected mode (gc-mode) and islanding mode
(i-mode) one by one. The proposed system is modeled and
simulated in SIMULINK to perform the fault analysis. The
results indicate that the magnitude of fault current under various
faults is comparable for gc-mode and i-mode. However, the
system becomes stable more rapidly for i-mode than the gc-mode.
... -24 Coefficients K p and K i inFigure 5are proportional gain and integral gain of PI control- ler, respectively, which verify global PI gain K f (s) in (10).22,28 ...
This paper presents photovoltaic (PV) systems modeling and fault analysis with
solar energy fluctuation to discuss maximum fault current profiles. The modeled PV farm is arranged with series and parallel PV modules to offer 6.5 kV power. Two‐level 3‐phase voltage source inverter (VSI) and dc‐dc boost converter are used for all PV systems with LCL filter. A current control strategy using synchronous rotating frame method is implemented with park transformation approach. Average and aggregate models give inaccuracy, which this paper overcomes with accurate isolated single PV system, and PV‐dominated feeder. Pulse width modulation and sinusoidal pulse width modulation techniques are applied. Incremental conductance maximum power point tracking
technique is used to extract PV maximum power. Phase locked loop technique
for grid synchronization and proportional integral voltage control technique are
also applied. Here, the maximum fault current corresponds to the maximum
phase current at specified time. This simple and effective method is applied
on PSCAD software simulations to collect the data. Discussions on obtained
results showed short‐time irradiation fluctuation impacts on fault current profiles
and argue on related aspects. The data comments and explanations to
address analyzed protection challenges with PV penetration in distribution system
are very promising.
... Also, there have been many papers involving the performance of large-scale wind farms and their impact on the voltage and frequency of the system [5,6]. As for PV generation, the literature [7] investigated the performance of large-scale PV system under various fault conditions. The short-circuit current performance was focused on in the literature [8]. ...
Application of series compensation transmission lines to enhance the long‐distance transmission capability of photovoltaic (PV) plants is the most economical technical measure. To investigate the impact of series compensation on operating performance of large‐scale PV plants, the simulation research is performed in this study. The detailed model of large‐scale PV plants connected to the grid through series compensated lines is developed using power systems computer aided design (PSCAD)/electromagnetic transients inccluding DC (EMTDC) software. Using the detailed model, the performances of active and reactive power outputs, voltage and current profiles of PV plants are investigated for different levels of series compensation and strengths of AC system during short‐circuit faults. The simulation results show that series compensation and strong AC system can effectively improve the transient operation performance of the PV power plant. With the increase of the series compensation degree and the strength of AC system, the fault ride‐through performance of PV plants is improved.
... However, three phase electrical machines performances under various grid faults have been investigated in [8][9][10][11][12][13]. On the other hand, [14][15] discuss the performance of a grid-connected photovoltaic system under various grid faults. Reference [16] has proposed a method that can be developed from two matrices and used to solve the various types of single or simultaneous unsymmetrical faults with hybrid compensation for microgrid (MG) distribution systems. ...
This paper presents the impact study of using series Flexible AC Transmission System (FACTS) devices, Thyristor Controlled Series Capacitor (TCSC), GTO Controlled Series Capacitor (GCSC) and Thyristor Controlled Series Reactor (TCSR), on the short circuit parameters of a 400 kV single electrical transmission line in case of three phase fault. The case study is an electrical transmission line in the Algerian transmission network which is compensated by each of the three mentioned devices at its midpoint. Simulations results investigate the impact of the conduction angle of FACTS power electronic devices on the short circuit parameters of the line in case of three phase fault. The obtained results verify the presented theoretical analysis of short circuit calculations.
This paper presents a new intelligent control strategy to augment the low-voltage ride-through (LVRT) potential of photovoltaic (PV) plants, and the transient stability of a complete grid system. Modern grid codes demand that a PV plant should be connected to the main power system during network disturbance, providing voltage support. Therefore, in this paper, a novel fuzzy logic controller (FLC) using the controlled cascaded strategy is proposed for the grid side converter (GSC) of a PV plant to guarantee voltage recovery. The proposed FLC offers variable gains based upon the system requirements, which can inject a useful amount of reactive power after a severe network disturbance. Therefore, the terminal voltage dip will be low, restoring its pre-fault value and resuming its operation quickly. To make it realistic, the PV system is linked to the well-known IEEE nine bus system. Comparative analysis is shown—using power system computer-aided design/electromagnetic transients including DC (PSCAD/EMTDC) software—between the conventional proportional–integral (PI) controller-based cascaded strategy and the proposed control strategy to authenticate the usefulness of the proposed strategy. The comparative simulation results indicate that the transient stability and the LVRT capability of a grid-tied PV system can be augmented against severe fault using the proposed FLC-based cascaded GSC controller.
Photovoltaic power plants are obliged to maintain power quality standards when interconnected to the grid based on the grid codes (GCs). Variations in grid frequency can be one of the main sources of harmonic distortions in a generated currents of a grid-connected photovoltaic power plant (GCPPP) as long as it is incorporated with frequency-dependant controllers such as resonant controllers in the current control loops. In fact, either the controller itself or the reference currents can contribute to the harmonic components of the generated currents. Current references in turn may become distorted due to outer voltage controller or inaccurate phase angle detection by phase-locked-loop (PLL) technique. In this paper, a comprehensive analysis of a 3-phase GCPPP under grid frequency variations is performed with the focus on associated frequency-dependant controllers i.e. current controller and filtered-sequence PLL (FSPLL). The resonant controllers are selected in this study which have the capability to attenuate generated odd-order harmonics but their performance under frequency variations need to be investigated for both ideal and non-ideal controller types. FSPLL is an effective PLL, however its performance is highly dependent to the grid frequency which requires more research regarding frequency sensitivity. Afterwards, FSPLL is developed further to detect the instantaneous grid frequency to enhance the GCPPP performance. Finally, the GCPPP is equipped with an active power controller to contribute to the frequency control of the power system. Presented results within MATLAB/Simulink environment verify the enhanced capability of the GCPPP with frequency detector/controller satisfying GCs.
In grid-connected photovoltaic (PV) power stations, improving the life expectancy and long-term reliability of three-phase PV inverters is urgently needed to match the significantly higher lifetime of the PV modules. A key contribution towards such improvement is replacing the conventional electrolytic film capacitors inside the inverter with metallized polypropylene film ones. With more and more PV power generation systems being installed, the utilities require these generation systems to remain connected to the ac grid during grid faults / voltage sags to ensure the operating stability of the ac power system. This paper presents a detailed analysis of how the control of PV inverters deploying film capacitors can be improved to enable them comply with the low voltage ride through capability demanded by the stringent current grid codes in the events of voltage sags / grid faults. The simulation and experimental results obtained in this study confirm that this modified inverter configuration can meet the grid codes and retain longer lifetimes expected from the use of slim film capacitors.
Grid-connected distributed generation sources interfaced with voltage source inverters (VSIs) need to be disconnected from the grid under: 1)excessive dc-link voltage; 2)excessive ac currents; and 3)loss of grid-voltage synchronization. In this paper, the control of single- and two-stage grid-connected VSIs in photovoltaic (PV) power plants is developed to address the issue of inverter disconnecting under various grid faults. Inverter control incorporates reactive power support in the case of voltage sags based on the grid codes (GCs) requirements to ride-through the faults and support the grid voltages. A case study of a 1-MW system simulated in MATLAB/Simulink software is used to illustrate the proposed control. Problems that may occur during grid faults along with associated remedies are discussed. The results presented illustrate the capability of the system to ride-through different types of grid faults.
This paper discusses the control of the positive- and negative-sequence components of a large-scale grid-connected photovoltaic system (GCPS) under unbalanced voltage sag conditions in the grid. Some issues regarding stability and dynamic performance of the system occur when applying PI controllers in the current control loops. The reason is the delay that the filtering method imposes when extracting the current/voltage sequences. Because of such a delay, the dynamic response of the system becomes slower compared with the case when no filtering technique is needed. Furthermore, there is a strong restriction on choosing suitable parameters for the current/voltage loop controllers without compromising system stability. All these issues are discussed in this paper on a 1-MVA GCPV system using MATLAB/Simulink software.
In this paper the control of a single-stage grid-connected photovoltaic power plant (GCPPP) is developed to address the issue of inverter disconnection under various grid faults. There are three main reasons for inverter disconnection which are (i) excessive dc-link voltage, (ii) excessive ac currents and (iii) loss of grid-voltage synchronization. The control of the inverter incorporates reactive power support in the case of voltage sags based on grid code requirements to ride-through the faults and support the grid voltages. Accordingly, another requirement of the grid codes is to control the reactive current injection under unbalanced voltage sags so that the voltages in the non-faulty phases do not exceed the specified limitations. All these issues are discussed in this paper for a case study of 1-MVA GCPPP using MATLAB/Simulink. The results illustrate the capability of the developed GCPPP to ride-through different types of faults occurred on the grid side.
Ancillary services for distributed generation (DG) systems become a challenging issue to smartly integrate renewable-energy sources into the grid. Voltage control is one of these ancillary services which can ride through and support the voltage under grid faults. Grid codes from the transmission system operators describe the behavior of the energy source, regulating voltage limits and reactive power injection to remain connected and support the grid under fault. On the basis that different kinds of voltage sags require different voltage support strategies, a flexible control scheme for three-phase grid-connected inverters is proposed. In three-phase balanced voltage sags, the inverter should inject reactive power in order to raise the voltage in all phases. In one- or two-phase faults, the main concern of the DG inverter is to equalize voltages by reducing the negative symmetric sequence and clear the phase jump. Due to system limitations, a balance between these two extreme policies is mandatory. Thus, over- and undervoltage can be avoided, and the proposed control scheme prevents disconnection while achieving the desired voltage support service. The main contribution of this work is the introduction of a control algorithm for reference current generation that provides flexible voltage support under grid faults. Two different voltage sags have been experimentally tested to illustrate the behavior of the proposed voltage support control scheme.
Phase-Locked-Loop (PLL) is one of the key technologies extensively used in grid connected power electronics system. A good PLL system can detect the grid phase angle and frequency fast and accurately, and additionally it can extract the positive sequence (or fundamental component for single phase system) exactly. In real applications, source signal (voltage or current) sensed for PLL usually includes harmonic distortion, unbalanced components, noises and frequency variations. Conventional PLL strategy cannot solve all the problems, especially the unbalanced and harmonic distortion. There is a trade-off between the dynamic response and phase angle tracking accuracy. Different PLL solutions are proposed in literature in recent years. The general considerations for these different approaches are to design positive sequence estimator to eliminate the negative sequence components and use filters to filter out the higher order harmonic distortions from the PLLs. In this paper, an adaptive quadrature filter based synchronous reference frame PLL (SRF-PLL) with positive sequence estimation feature is presented. The proposed PLL has good performances in filtering harmonic, eliminating unbalanced components and auto-adjusting frequency change. The simulation model is built in Matlab/simulink and the simulation results are given to verify the mathematical analysis.
Recently interest in photovoltaic (PV) power generation systems is increasing rapidly and the installation of large PV systems or large groups of PV systems that are interconnected with the utility grid is accelerating. To maintain the grid stability due to the huge penetration of photovoltaic power to the grid, much stricter grid codes are being imposed by the energy regulatory bodies. This paper discusses the detailed modeling and control strategies of a large scale grid connected photovoltaic system that can help to augment the low voltage ride thorough capability of DC based PV plant. It should be noted that grid side inverter plays an important role in low voltage ride through and therefore, overvoltage and undervoltage tripping of DC link of the grid tied inverter should be avoided, if possible. This study attempts to incorporate DC link over and under voltage protection in the control loop without increasing overall cost of protective device, which is another salient feature of this study. Furthermore, a comparative study with conventional inverter scheme is carried out. Different types of fault scenarios are analyzed to demonstrate the effectiveness of the overall control scheme, where recent grid code for distribution generation system is considered.
The current work focuses on two specific issues concerning grid-connected solar PV units, i.e. the fault ride-through capability, also called low voltage ride-through capability, and the voltage support function through reactive power injection during faults. With the first one the PV unit can actually provide some limited grid support, whereas with a defined reactive power characteristic it can give a complete dynamic grid support. These two requirements, already known for wind power generation but new for the PV, have been recently introduced in the German technical guidelines for connection to the MV grid. Scope of the paper is to implement these requirements in a large solar PV plant, modeled in DIgSILENT PowerFactory, in order to understand its operation, and to evaluate its behavior and impact on the grid, in terms of stability and voltage support during grid fault.
An increasing number of distributed generations (DGs) being connected to the utility grid has raised the need for new and more rigorous standards for power quality, safety operation and islanding protection of the grid interconnection. This paper focuses on the fault contribution of a distributed generation, specifically PV grid-connected system in the event of an unintentional islanding. An experimental setup is created using an off-the-shelf grid-connected inverter to emulate the event of a fault occurred on a distribution feeder. An extreme case scenario, according to the IEEE standard, that will result in the highest fault contribution of the PV grid-connected system is considered for conservative results. The experimental results have shown that, without violating the specified criteria in the standard, the grid-connected inverter takes times to detect an islanding condition and stop energizing. The experimental results are used to investigate the fault contribution in a distribution system. This study has shown that higher penetration level of the PV grid-connected systems results in higher fault current which unavoidably has effects on the protective devices. This observation introduces the possibility of selecting and/or upgrading protective devices in a distribution system.
This paper proposes a filtered-sequence phase-locked loop (FSPLL) structure for detection of the positive sequence in three-phase systems. The structure includes the use of the Park transformation and moving average filters (MAF). Performance of the MAF is mathematically analyzed and represented in Bode diagrams. The analysis allows a proper selection of the window width of the optimal filter for its application in the dq transformed variables. The proposed detector structure allows fast detection of the grid voltage positive sequence (within one grid voltage cycle). The MAF eliminates completely any oscillation multiple of the frequency for which it is designed; thus, this algorithm is not affected by the presence of imbalances or harmonics in the electrical grid. Furthermore, the PLL includes a simple-frequency detector that makes frequency adaptive the frequency depending blocks. This guarantees the proper operation of the FSPLL under large frequency changes. The performance of the entire PLL-based detector is verified through simulation and experiment. It shows very good performance under several extreme grid voltage conditions.
The interconnection of distributed generators (DG) to existing network may give rise to many technical problems. To ensure appropriate power quality and stability of the grid these problems should be recognized and solved. For this purpose, behaviour of such energy sources, under various disturbances, must be studied and known to the distribution grid operators. In this paper fault current contribution from large scale photovoltaic (PV) power plant as well as protection issues concerning such DGs are discussed. The mentioned problems were investigated in PSCAD model of 50 MW grid-connected PV power plant where various operating condition of the network and penetration levels of PV were considered.
It is the general trend to increase the electricity production using distributed power system (DPGS), which is based on renewable energy sources such as wind, solar, fuel cell etc. These systems are to be properly controlled in order to provide reliable power to the utility network. For that power electronics converters are used as an interfacing device between DPGS and utility network. In case of unbalanced fault, the major problem in distributed power generation system is the phase unbalance which causes the power quality problem along with grid instability. This paper discusses the implementation of two different controllers namely Proportional - integral (PI) and proportional resonant (PR) controllers in order to obtain the control of grid side converter during single phase to ground fault. The analysis includes the grid current harmonic distortion along with the variation of active and reactive power during the fault condition. The system is simulated using MATLAB software and simulation results demonstrate the effectiveness of both the controller.
Nowadays, the majority of the photovoltaic (PV) power sources are connected to the public grid. One of the main connection problems occurs when voltage sags appear in the grid due to short circuits, lightning, etc. International standards regulate the grid connection of PV systems, forcing the source to remain connected during short-time grid-voltage faults. As a consequence, during the voltage sag, the source should operate with increasing converter currents to maintain the injection of the generated power. This abnormal operation may result in nondesired system disconnections due to overcurrents. This paper proposes a controller for a PV three-phase inverter that ensures minimum peak values in the grid-injected currents, as compared with conventional controllers. From the system analysis, a design method is presented in order to set the parameters of the control scheme. Selected experimental results are reported in order to validate the effectiveness of the proposed control.
The operation of distributed power generation systems under grid fault conditions is a key issue for the massive integration of renewable energy systems. Several studies have been conducted to improve the response of such distributed generation systems under voltage dips. In spite of being less severe, unbalanced fault cause a great number of disconnections, as it may produce the injection of unbalanced currents that can easily conduct to the tripping of the converter. In this paper a current control strategy for power converters able to deal with these grid conditions, without exceeding the current ratings of the converter is introduced. Moreover, a novel flexible algorithm has been proposed in order to regulate easily the injection of positive and negative currents for general purpose applications.
Voltage-source inverter (VSI) topology is widely used for grid interfacing of distributed generation (DG) systems. However, when employed as the power conditioning unit in photovoltaic (PV) systems, VSI normally requires another power electronic converter stage to step up the voltage, thus adding to the cost and complexity of the system. To make the proliferation of grid-connected PV systems a successful business option, the cost, performance, and life expectancy of the power electronic interface need to be improved. The current-source inverter (CSI) offers advantages over VSI in terms of inherent boosting and short-cir- cuit protection capabilities, direct output current controllability, and ac-side simpler filter structure. Research on CSI-based DG is still in its infancy. This paper focuses on modeling, control, and steady-state and transient performances of a PV system based on CSI. It also performs a comparative performance evaluation of VSI-based and CSI-based PV systems under transient and fault conditions. Analytical expectations are verified using simulations in the Power System Computer Aided Design/Electromagnetic Transient Including DC (PSCAD/EMTDC) environment, based on a detailed system model.
This paper will cover the solar PV inverter tests required for model development and validation including but not limited to voltage transients, frequency deviations, grid disconnection, short circuit, harmonics generation, and voltage oscillations. Additionally, it will cover the test results acquired from our tests of 3-phase 480VAC commercial solar PV inverters. Finally, it will cover proposed recommendations for solar PV inverter performance to accommodate high penetration of solar PV inverter generation.
In order to ensure the grid side converter and static reactive compensator in wind power systems run safely and reliably under unbalanced grid voltage, it is the basic requirement that the phase and frequency of the positive sequence fundamental component of grid voltage are obtained quickly and accurately. Because the dynamic behavior of the phase locked loop (PLL) based on the synchronous reference frame is dissatisfactory under unbalanced and distorted grid voltage, the improved PLL based on the synchronous reference frame is put forward to not only extract the phase of positive sequence component of unbalance voltage and eliminate the effect of unbalance voltages on phase detection, but also suppress the influence of harmonics on phase detection. The results of simulation in PSCAD/EMTDC and experiment in C8051F410 based set-up verify the feasibility and correctness of the improved PLL proposed in the paper.
This paper presents an improved three phase PLL technique to generate the reference signals for power electronic converters in a grid connected system. The synchronization scheme employed for phase information should provide a high degree of immunity to power system disturbances because its sensitivity affects the reference signal generated. The SRF PLL works well under balanced and undistorted conditions however the reference signal generated using SRF PLL is distorted under phase unbalancing and harmonics conditions. The performance of the SRF PLL can be improved by reducing the bandwidth but this increases the locking time. To overcome the above limitation an improved three phase PLL based on a linear phase finite impulse response (FIR) filter has been developed. The addition of FIR to the system eliminates the ripples due to the distortions in the utility. The improved PLL technique is simulated in MATLAB and results for abnormal grid conditions such as unbalance, harmonics, phase jump and dc offset are presented to demonstrate its phase-tracking ability.
Solar photovoltaic distributed generation (PV-DG) systems are one of the fastest-growing types of renewable energy sources being integrated worldwide onto distribution systems. Many North American utilities, governed by state or provincial incentives and/or mandated by green-generation portfolio requirements, are facing installations of large PV plants with capacities in the order of several megavoltamperes (MVAs) that are owned either by the utility or by private power producers.
This paper deals with a fundamental aspect in the control of grid-connected power converters, i.e., the detection of the positive-sequence component at fundamental frequency of the utility voltage under unbalanced and distorted conditions. Accurate and fast detection of this voltage component under grid faults is essential to keep the control over the power exchange with the grid avoiding to trip the converter protections and allowing the ride-through of the transient fault. In this paper, the systematic use of well known techniques conducts to a new positive-sequence voltage detection system which exhibits a fast, precise, and frequency-adaptive response under faulty grid conditions. Three fundamental functional blocks make up the proposed detector, these are: i) the quadrature-signals generator (QSG), ii) the positive-sequence calculator (PSC), and iii) the phase-locked loop (PLL). A key innovation of the proposed system is the use of a dual second order generalized integrator (DSOGI) to implement the QSG. For this reason, the proposed positive-sequence detector is called DSOGI-PLL. A detailed study of the DSOGI-PLL and verification by simulation are performed in this paper. From the obtained results, it can be concluded that the DSOGI-PLL is a very suitable technique for characterizing the positive-sequence voltage under grid faults.
This paper presents modeling guidelines and a benchmark system for power system simulation studies of grid-connected, three-phase, single-stage Photovoltaic (PV) systems that employ a voltage-sourced converter (VSC) as the power processor. The objective of this work is to introduce the main components, operation/protection modes, and control layers/schemes of medium- and high-power PV systems, to assist power engineers in developing circuit-based simulation models for impact assessment studies, analysis, and identification of potential issues with respect to the grid integration of PV systems. Parameter selection, control tuning, and design guidelines are also briefly discussed. The usefulness of the benchmark system is demonstrated through a fairly comprehensive set of test cases, conducted in the PSCAD/EMTDC software environment. However, the models and techniques presented in this paper are independent of any specific circuit simulation software package. Also, they may not fully conform to the methods exercised by all manufacturers, due to the proprietary nature of the industry.
The power quality of a three-phase photovoltaic (PV) inverter drastically deteriorates in the presence of grid faults with unbalanced voltages. A ripple in the injected power and an increase in the current harmonic distortion are the main noticeable adverse effects produced by this abnormal grid situation. Several grid-fault control schemes are nowadays available for operating under unbalanced grid voltage. These control schemes usually have extreme power quality characteristics. Some of them have been conceived to completely avoid power ripple during unbalanced voltage sags, but at an expense of high current harmonic distortion. With other schemes, the harmonic distortion is totally eliminated but at an expense of high ripple in the injected power. This paper further explores the performance of PV inverters under unbalanced voltage sags. It has three theoretical contributions: 1) a generalized control scheme, which includes the aforementioned grid-fault controllers as particular cases; 2) a control strategy based on the use of continuous values for the control parameters. This original approach gives adjustable power quality characteristics that cannot be achieved with the previous control schemes; 3) three different control algorithms for calculating the continuous values of the control parameters. These contributions are experimentally validated with a digital signal processor-based laboratory prototype.
Use of an environmentally clean photovoltaic (PV) power generation system will become more widespread in the future due to anticipated cost reduction in PV technology. As the capacity of PV systems increases, the stable output of the grid-connected PV system becomes important. However, the output variation of PV systems is dependent on the faults and transients of the power system. This paper analyzes the dynamics of a PV system at fault conditions. A 3 MW PV system is modeled by Matlab/Simulink, and then the distribution system with the PV system is modeled. This paper simulates the various fault types, and analyzes the output of the PV system at fault conditions. The results show that output of the PV system depends on the fault type.
The analysis and design of the phase-locked loop (PLL) system is
presented for the phase tracking system of the three phase utility
interface inverters. The dynamic behavior of the closed loop PLL system
is investigated in both continuous and discrete-time domains, and the
optimization method is considered for the second order PLL system. In
particular, the performance of the three phase PLL system is analyzed in
the distorted utility conditions such as the phase unbalancing,
harmonics, and offset caused by the nonlinear load conditions and
measurement errors. The tracking errors under these distorted utility
conditions are also derived. The phase tracking system is implemented in
a digital manner using a digital signal processor (DSP) to verify the
analytic results. The design considerations for the phase tracking
system are deduced from the analytic and experimental results
New german grid codes for connecting PV systems to the medium voltage power grid
- E Troester