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Current improvement of a grid-connected photovoltaic system under unbalanced voltage conditions
Abstract
This paper discusses the performance of a large-scale grid-connected photovoltaic system (GCPS) under unbalanced voltage sag conditions in the grid. By applying the droop control to address the low-voltage-ride-through (LVRT) capability, the output currents are no longer sinusoidal due to the oscillations produced in the current references. To deal with this problem a moving average filter (MAF) technique is proposed. Beside this application, there are other advantages of applying the MAF which are described in this paper. Selected simulation results using MATLAB/Simulink for a 1-MVA GCPS confirm the effectiveness of the proposed method.
... To avoid this, constant power into the grid can be achieved by unbalancing the currents, i.e. including a proper negative-sequence into the grid currents. When the currents are measured and transformed into dq-components by rotating frames in opposite directions (dq + and dq − frames), both sequences interact with each other producing ripples at twice the fundamental frequency [19], i.e. 100 Hz when the grid frequency is 50 Hz. Therefore, a filtering technique is needed to remove these ripples. ...
... in which T w is the window width of the MAF. Under unbalanced grid voltage conditions, an MAF with T w = T /2, T being the grid period, can remove all the second-order harmonics from the dq transformed components [19]. The filtering process introduces a delay, regardless of the filtering technique implemented. ...
... Once k pc is defined, k ic can be calculated from (19), obtaining k ic = 1608 V/As. These values are used to define the parameters of the PR controller as k p = 2k pc = 0.4 V/A and k r = 2k ic = 3216 V/As. ...
This paper discusses the control of large-scale grid-connected photovoltaic power plant (GCPPP) operating under unbalanced grid voltages. The positive and negative sequences of the grid currents need to be controlled to regulate the power injected into the grid during unbalanced grid voltages. This paper shows that the use of conventional proportional-integral-based controllers compromises stability and dynamic performance of the inverter. The reason is the delays introduced by the filters needed to extract the sequences of the transformed grid currents. Because of such delays, there is a strong restriction on choosing the parameters for the current and voltage controllers, which forces the GCPPP to perform slowly. This can be improved by using resonant controllers instead, which avoid the need for filtering the transformed grid currents. Additionally, a new overcurrent protection is proposed for the GCPPP when it is providing grid voltage support during voltage sags. Simulation and experimental results are presented to evaluate and compare the performance of the GCPPP when operating with the different controllers.
... To avoid this, constant power into the grid can be achieved by unbalancing the currents, i.e. including a proper negative-sequence into the grid currents. When the currents are measured and transformed into dq-components by rotating frames in opposite directions (dq + and dq − frames), both sequences interact with each other producing ripples at twice the fundamental frequency [19], i.e. 100 Hz when the grid frequency is 50 Hz. Therefore, a filtering technique is needed to remove these ripples. ...
... in which T w is the window width of the MAF. Under unbalanced grid voltage conditions, an MAF with T w = T /2, T being the grid period, can remove all the second-order harmonics from the dq transformed components [19]. The filtering process introduces a delay, regardless of the filtering technique implemented. ...
... Once k pc is defined, k ic can be calculated from (19), obtaining k ic = 1608 V/As. These values are used to define the parameters of the PR controller as k p = 2k pc = 0.4 V/A and k r = 2k ic = 3216 V/As. ...
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.
... The reference currents given by (8) relates to lagging power factor and those from (9) corresponds to a leading power factor. Reference currents in (8)-(9) must comply with (6) In order o have realistic reference currents (without imaginary components), pref and qref should fulfil the following condition: ...
1][2][3] Assistant professor [1] Thirumala Engineering College [2][3] Vignanabharathi Engineering College Abstract-The proposed reactive power control is intended to regulate the maximum and minimum phase voltages at the point of common coupling within the limits established in grid codes for continuous operation. This paper presents a control strategy for a grid connected photovoltaic (PV) system aiming to regulate the active and reactive power injected to the electric system during asymmetrical voltage faults. Fuzzy controller is advance controller which is mostly suitable for the human decision making mechanism which also provided the operation of an electronic system with the expert decision. The active power reference is obtained from a Maximum Power Point Tracking (MPPT) algorithm. The proposed control strategy generates the required reference currents to be imposed by the grid-tied inverter from the desired active and reactive power and the measured supply voltage. In unbalanced voltage sags, positive and negative sequence reactive powers are combined to flexibly raise and equalize the phase voltages; maximum phase voltage is regulated below the upper limit and the minimum phase voltage just above the lower limit. The scheme is validated for a single stage PV system where the inverter currents are regulated via predictive control. By utilizing the fuzzy controller for a nonlinear system which allow the reduction for the uncertain effect in the system which control and perfectly improve the efficiency. Results showing the performance of the strategy are presented during unbalanced voltage sags and swells.
... Furthermore, less voltage and current sensors are required for the implementation of this method. A PI with droop control is proposed in [6] for grid-connected photovoltaic systems under an unbalanced grid. With the help of a moving average filter (MAF), the sinusoidal output current can be achieved without affecting the system significantly. ...
In this paper, a robust dual-current control is proposed for a three-phase grid-tied inverter under unbalanced grid voltage. To compensate the unbalanced grid current rooted by the grid voltage dips or faults, an additional negative sequence compensator is considered in this proposed controller. The current control for both positive and negative sequence utilizes state feedback with integral control to archive fast reference tracking without steady-state error. The robust control gain is computed by solving a linear matrix inequality-based optimization problem to obtain fast convergence rate in the presence of the uncertainty of the system. The positive sequence current reference is used to regulate the amount of current flow from DC source to the grid, and the zero negative sequence reference is used to compensate the unbalanced grid current. To verify the efficacy of this control method, a comparison between the proposed robust dual-current control and robust current control without negative sequence compensator is conducted using simulation.
... In general, extraction of the positive and negative sequence components of the measured currents must be performed so they may be regulated. A simple technique for achieving this is proposed in [10] based on the moving average filter (MAF). The MAF is defined as: ...
... The instantaneous reactive power is obtained as:|[ The reference currents needed to inject a certain active and reactive power to the grid is obtained as: [7], [8], [10], [11], [12], [13], [14]. ...
This paper presents a predictive current control strategy for Active Front End (AFE) rectifier supplying active and reactive power to the electric system during unsymmetrical voltage fault conditions without the use of a Phase Locked Loop (PLL) or Frequency Locked Loop (FLL). For a given active and reactive power and supply voltage, the strategy generates the required reference currents. A predictive current control strategy is used to impose those currents by the AFE rectifier. It is shown that the amount of active and reactive power that can be injected into the grid will depend on voltage unbalanced level. Simulations and experimental results showing the performance of the strategy are presented during unbalanced voltage sag and swells.
A linear matrix inequality (LMI)-based robust stabilizing control is proposed in this paper for a three-phase grid-connected inverter (GCI) with L-filtered output. Previous research, such as MPC, required high computational power and precise modeling in order to obtain offset-free performance. Achieving optimal performance in the case of PI control poses a persistent challenge in terms of gain tuning. This proposed control strategy effectively addresses the afore-mentioned issues by the utilization of systematic control design, incorporating integral action to mitigate the presence of offset error. The set of state feedback and integral gain is obtained by solving the LMI-based optimization problem to maximize the convergence rate to a steady state in the presence of uncertainty in the L-filter. The mentioned uncertainties are represented by potential ranges of the inductor values. Output power delivery can be simply regulated by a computed reference state using a given power reference and measured grid current and voltage. The effectiveness of the proposed method is verified through simulations. The proposed robust control method demonstrates a significant decrease in ripple, with a reduction of 86.66% when compared to the conventional PI control approach.
A voltage source converter is necessary for micro-grid to maintain the appropriate voltage and frequency. A low-voltage ride-through strategy based on improved current droop is proposed in this paper, and this strategy can enable the converter to ride through voltage sag faults at the point of common coupling (PCC). Current droop is implemented by active current-frequency control and reactive current-voltage control without the need for power calculation and without an overcurrent beyond the active current. The reactive power support function is provided by regulating the reactive current. To resolve the problems of power-angle instability and slow response of current recovery after a fault, a power-angle-limiting controller is proposed. The lagging-loop controller is employed to constrain the power angle within a certain range around the prefault steady-state value so that the converter can ride through the transient state smoothly and recover to the prefault state rapidly. The proposed strategy is verified by simulations and experiments.
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.
This paper presents a control strategy for a grid connected photovoltaic (PV) system aiming to regulate the active and reactive power injected to the electric system during asymmetrical voltage faults. The active power reference is obtained from a Maximum Power Point Tracking (MPPT) algorithm. The proposed control strategy generates the required reference currents to be imposed by the grid-tied inverter from the desired active and reactive power and the measured supply voltage. The scheme is validated for a single stage PV system where the inverter currents are regulated via predictive control. Results showing the performance of the strategy are presented during unbalanced voltage sags and swells.
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.
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.
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.
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.
Large offshore wind power plants may have multi-MW wind turbine generators (WTG) equipped with full-scale converters (FSC) and voltage source converter (VSC) based high voltage direct-current (HVDC) transmission for grid connection. The power electronic converters in the WTG-FSC and the VSC-HVDC allow fast current control in the offshore grid. This paper presents a method of controlling the negative sequence current injection into the offshore grid from the VSC-HVDC as well as WTG-FSCs. This would minimize the power oscillations and hence reduce the dc voltage overshoots in the VSC-HVDC system as well as in the WTG-FSCs; especially when the offshore grid is unbalanced due to asymmetric faults. The formulation for negative sequence current injection is mathematically derived and then implemented in electromagnetic transients (EMT) simulation model. The simulated results show that the negative sequence current control mitigates the power oscillations and therefore limits the dc voltage excursions in the VSC-HVDC system during the asymmetric faults.
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.
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.
In this paper, a novel control system is proposed to control the active and reactive grid power flow in a three phase grid connected PV inverter. The control system not only controls the grid power but also reduces the grid current THD in the presence of typical non-linear loads in parallel with grid at the point of common coupling (PCC). The proposed control system also takes care of not only the grid voltage unbalance but also the unbalance in the connecting line side inductors. The stability of the proposed control system is ensured by the direct method of Lyapunov. The proposed control system is not only simple to implement in the digital form but also provides superior performance over the conventional multiple PI or resonant control methods. A new grid connected inverter modeling technique is also proposed to take care of unbalances in the inverter system. Experimental results are provided to show the efficacy of the proposed control system.
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.
This paper discusses the impact of balanced and unbalanced voltage dips on grid connected voltage source inverters, usually found in wind turbines. A Kalman filter synchronization method is used to provide an accurate and fast extraction of the voltage positive sequence under unbalanced voltage dips. Also, it minimize the transient effects due voltage-phase-jumps. In addition, two current controllers are compared e evaluated. Simulation results demonstrate that the conventional PI control has a poor performance during unbalanced faults and that the synchronization method play an important role to improve the VSI low voltage ride-through capability.
A control method for the compensation of unbalanced voltage supply that eliminates pulsating current in the DC bus of a voltage source inverter is investigated and analysed. Such a pulsating current can have detrimental effects on operating properties of the drive as well as on the quality of power supply in the electrical vicinity of the inverter. A feasible range of control parameters is determined based on the required level of immunity against unbalanced voltage supply and on the parameters of the system. The impact of the control method on the power factors and current amplitudes in individual input phases for the available range of control parameters is analysed and discussed. In particular, achievable power factor and current unbalance at the input of the front-end rectifier are addressed. Examples of numerical simulations and experimental measurements illustrate operation of the system under investigation, both with and without the use of the compensation method.
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