Reactive Power Fluctuations Research Articles

Using the proportional dual integral strategy to improve the characteristics of the indirect field-oriented control of DFIG-based wind turbine systems

In this work, a new indirect field-oriented command (IFOC) for wind systems based on a doubly-fed induction generator (DFIG) is designed to get better energy and stream quality. Also, overcoming the problems of IFOC. It is proposed to use proportional dual integral (PDI) controllers in order to replace traditional controllers and augment the robustness of the studied wind turbine system. The proposed IFOC-PDI is a modification of the IFOC, where the basic idea behind the designed technique is to combine a proportional-integral (PI) regulator with an integral term. This technique has been applied to the machine inverter, where the IFOC-PDI aims to calculate reference voltage values. The pulse width modulation (PWM) is used to translate voltage reference values ​​into pulses to run the machine inverter. The main objective of this article is to compare the competence of the IFOC-PDI to that of the IFOC-PI in different tests, including two different wind speed profiles and DFIG parameters variation. The major benefit of this command is a decrease in harmonic distortion of the currents, and active and reactive power (Ps and Qs) fluctuations. The results using MATLAB demonstrated that the IFOC-PDI improved the IFOC-PI in the minimization of stream harmonic distortion (6.63%, 10.42%, and 17.86%) and notable minimization of fluctuations in both Ps (32.08%, 46.24%, and 12.10%) and Qs (45.95%, 66.88%, and 14.49%). Also, the overshoot value of Ps was minimized compared to the IFOC-PI in all tests by ratios estimated at 57.79%, 12.41%, and 33.96%. The steady-state error of Ps was minimized compared to the IFOC-PI in all tests by ratios estimated at 29.43%, 85.88%, and 76.36%. The IFOC-PDI is also very resistant in the case of DFIG parameters variation.

Nonlinear MPPT techniques to control hybrid power systems

In recent years, grid-connected multifunctional photovoltaic (PV) systems have proven to be highly efficient. This system integrates PV panels with a DC–DC boost converter (DC–DC–BC) and the electrical distribution grid (DEG). Linking the PV to the AC-DEG is accomplished through a three-level multifunctional voltage source inverter (MVSI). The DC–DC–BC component in this study is engineered to perform maximum power point tracking (MPPT) irrespective of normal or abnormal conditions. The conventional MPPT technique poses several challenges and constraints on system usage. Hence, the suggestion is to adopt synergetic control (SC) and sliding mode control (SMC) to enhance the MPPT technique's performance within the proposed system framework. Moreover, predictive direct power control is applied to the MVSI-based shunt active power filter, utilizing a phase-locked loop technique to achieve multiple objectives: minimizing energy fluctuations, injecting active power, correcting power factors, compensating reactive power, and mitigating harmonic currents. To implement the proposed system, the MATLAB is used for this purpose, with several tests used to study the behavior of the controls proposed in this work. Numerical results indicate significant reductions in active and reactive power fluctuations, with estimated rates of 38.46% and 15.30%, respectively, compared to traditional strategies. Moreover, the total harmonic distortion (THD) of the source current after filtering is reduced by 31.88% under solar irradiation of G = 1000 Wm2. Before filtering, the THD of current experiences a reduction estimated at 97.65%. These findings underscore the superior performance of the proposed control technique across all evaluated aspects.

Design of a High Frequency Stability Control Algorithm for Flexible DC Systems

Abstract The suppression scheme of flexible DC high-frequency oscillation needs to be based on model construction. Related literatures have established a high-order linearization model of converter station, gradually revealing the influence of control system and its parameters on impedance characteristics. In order to ensure the full utilization of renewable energy, it is necessary to deeply study the interaction characteristics between multi-terminal VSC-HVDC (Voltage Source Converter based High Voltage Direct Current Transmission) and the receiving power grid, and design a grid-friendly control strategy. In this paper, a VSC-HVDC high-frequency stability control algorithm is established, and a double-ended VSC-HVDC bidirectional VSC stable switching controller is constructed. The switching sequence generated by the controller replaces the control sequence generated by the previous PWM (Pulse width modulation) modulation, so as to weaken the influence of the inner loop current control parameters on the stable operation of the system. The simulation results show that the reactive power fluctuation amplitude of PI control on the inverter side is 0.031 p.u, and the stabilization time is 0.43s. The reactive power fluctuation amplitude of bidirectional VSC stable switching control is less than 0.012 p.u, and the stability time is less than 0.2s. When the single-phase grounding wire is short-circuited, the bidirectional VSC regulator switch has better control performance than the PI regulator switch. Compared with the traditional PI controller, the proposed controller has better stability. The system can quickly enter a stable and non-overshoot operation state, and achieved good control effect.

Research on intelligent auxiliary regulation technology of large power grid section based on artificial intelligence

In the modern era, large-scale renewable energy systems are integrated with advanced power systems and provide efficient operations. Also, optimized power systems require accurate energy generation and effective control systems to manage and ensure a stable power supply. Nevertheless, uncertainties are the intermittent balance of supply and high electricity demand. In addition, conventional power sources are not applicable for this challenging task and increase electricity costs. Therefore, an efficient Neuro Fuzzy Single phase Unified Power Quality Conditioner with Maximum Power Point Tracking (NF-SP UPQC -MPPT) strategy is developed for enhancing the grid-connected power systems. Here, Neuro-fuzzy logic is used as dynamic reactive power compensation in the grid. Also, this logic can efficiently handle the Energy storage system (ESS). Then SP UPQC was used to enhance power quality in electrical distribution systems. It combines both series and shunt compensators to mitigate various power quality issues such as voltage sags, harmonics, and unbalance. After that MPPT was utilized to extract the maximum power from the grid system. Model Predictive Control (MPC) controller to determine the overall stability and performance of the system. Moreover, the developed model was implemented on the MATLAB platform and performance is analyzed in terms of voltage deviation, grid current, reactive power fluctuations and Total Harmonic Distortion (THD).

Operation Assessment of a Hybrid Distribution Transformer Compensating for Voltage and Power Factor Using Predictive Control

Hybrid Distribution Transformers (HDTs) offer a compelling alternative to traditional low-frequency transformers (LFTs), providing auxiliary services in addition to standard functionalities. By integrating LFTs with power converters, HDTs enhance the operational capabilities of the system. The specific configuration in which converters are connected to the transformer allows for the provision of multiple services. This can not only prevent network failures but also extend the lifespan of its components, an outcome that is highly desirable in a distribution grid. This article discusses an HDT developed to mitigate voltage fluctuations in the grid and to decrease the reactive power drawn from the secondary side of traditional LFTs. A finite-control-set model predictive control (FCS-MPC), in conjunction with linear controllers, is utilized for the effective management of the HDT converters. Two separate control loops are established to regulate voltage and reactive power on the secondary side of the transformer. Results from Hardware-in-the-Loop (HIL) testing affirm the proficiency of HDT in reducing grid voltage variations by 15% and in cutting reactive power consumption by up to 94%. The adopted control strategy and topology are demonstrated to be effective in stabilizing voltage and reactive power fluctuations while concurrently facilitating the charging of the converters’ DC link directly from the grid.

ANFIS Controlled MMC-UPQC to Mitigate Power Quality Problems in Solar PV Integrated Power System

This paper focuses on the implementation of a Modular Multilevel Converter (MMC) based Unified Power Quality Conditioner (UPQC) in a grid-integrated Photovoltaic (PV) system. The integration of photovoltaic (PV) systems into the grid brings numerous benefits in terms of renewable energy utilization like sustainable energy generation, energy security, grid resilience, and economic growth. However, the voltage source converters (VSCs) employed in PV systems can introduce power quality (PQ) issues, including voltage fluctuations, harmonics, and reactive power imbalances and grid instability. The objective of this work is to enhance power quality by mitigating voltage and current fluctuations and harmonics generated due to VSC, compensating reactive power requirements by the load and PV system, and regulating the DC link voltage. For DC voltage regulation, an Adaptive Neuro-Fuzzy Inference System (ANFIS) controller is employed to control for MMC-UPQC. The proposed control strategy is simulated using MATLAB/SIMULINK software and the results are compared with conventional Proportional Integral (PI) and fuzzy controllers. Various dynamic conditions such as voltage sag and swell at the point of common coupling (PCC), changes in irradiance for the PV system, and load variations are considered. The simulation results demonstrate that the ANFIS-controlled MMC-based UPQC outperforms the PI and fuzzy controllers in terms of power quality improvement. It effectively suppresses harmonics, maintains a stable DC link voltage, and compensates for reactive power fluctuations. The proposed control strategy provides superior performance and achieves better power quality compared to conventional controllers. The findings of this work highlight the potential of using an ANFIS controller in MMC-based UPQC systems for grid-integrated PV systems.

Stochastic MPC based double-time-scale voltage regulation for unbalanced distribution networks with distributed generators

With the increasing penetration of renewable energy resources, the uncertainty of renewable energy resources output introduces challenges to voltage control in distribution networks (DNs). To solve these problems, this paper proposes a double-time-scale voltage control scheme for unbalanced DNs based on stochastic model predictive control (SMPC) to coordinate multiple voltage control devices. In fast-time-scale control, the active and reactive power outputs of distributed generators (DGs) are optimized to minimize voltage deviation, active power curtailment and reactive power fluctuation. In the slow-time-scale control, optimal tap changes of on-load tap changers (OLTC), step voltage regulators (SVR), and switched capacitor banks (CBs) are calculated to reduce voltage deviation and tap operations. Additionally, in the fast-time-scale control, the SMPC-based scheme adopts a scenario generation approach to represent the uncertainty of DG output. Considering the temporal correlation of the DG output, different prediction scenarios and scenario probabilities are obtained by the empirical cumulative distribution function fitted by the historical measurement data. In this paper, the effectiveness of the proposed double-time-scale voltage control scheme is verified by the unbalanced IEEE123 node system. The case study demonstrates that the proposed voltage scheme can effectively keep the three-phase voltages in unbalanced DN within the secure operation range and achieve better voltage profiles.

LCL Filter Design of STATCOM using Genetic Algorithm Scheme for SCIG Based Microgrid Operation

In a microgrid static Compensator (STATCOM) is the most prominent inverter circuit for stabilizing the bidirectional power flow requirements of the system. This inverter circuit is the primary source of harmonics when the supply current feeds from the microgrid to the main grid. Improved control strategy and proper filter design may give solution to these issues and so, there is a huge scope of research in the field of the converter control techniques and filter designing for such microgrid based power system. The key objectives of this paper are (i) to develop an adequate current control scheme for adjusting real and reactive power fluctuations produced by load time to time, and (ii) to reduce the harmonic level of output characteristics in terms of real and reactive power flow and current frequency. For this, an approach is presented to estimate the filter design parameters for current controlled STATCOM connected to squirrel cage induction generator (SCIG) based microgrid. A nature-inspired optimization namely, genetic algorithm (GA), is implemented to estimate the most suitable parameters for the LCL filter. Results obtained through GA are validated with a conventional mathematical method in terms of real and reactive power flow through microgrid along with harmonic-based studies.

Modelling and Performance Analysis of a Tidal Current Turbine Connected to the Grid Using an Inductance (LCL) Filter

Nowadays, integrating renewable energy sources, such as tidal power, into the existing power grids of turbines is crucial for sustainable energy generation. However, tidal turbine energy transforms the potential energy of moving water into electrical energy. When both nonlinear load and dynamic load harmonics are present, the tide speed variance causes serious power quality issues such as low power factor, unstable voltage, harmonic distortions, frequency fluctuations, and voltage sags. The integration of an LCL-filter-based connection scheme can address these challenges by improving power quality and the overall performance of the tidal current turbine grid system. This study shifts LCL filter research from its conventional wind energy emphasis to the emerging field of tidal stream generation systems. The LCL filter analysed in this paper is modelled to exhibit adequate mechanical, electrical, and hydrodynamic characteristics. This model accounts for tidal current variations, turbine speed control, and power extraction dynamics. The LCL filter is evaluated for its effectiveness in reducing harmonic distortions, voltage fluctuations, and reactive power fluctuations. This system is composed of a 1.5 MW/C, a 1.2 MW three-level inverter with a nominal voltage of 600 V, and an inductance (LCL) filter. The results show that the inverter produces a harmonic distortion of less than 0.5%, which demonstrates the effectiveness of the filter in improving total harmonic distortion, reactive power consumption, and voltage control.

Enhancement of Power Decoupling for Virtual Synchronous Generator: A Virtual Inductor and Virtual Capacitor Approach

The inertia of a power system decreases with the increasing penetration of renewable energy, and the stability of power systems is facing great challenges. A virtual synchronous generator (VSG) has become one of the effective schemes for the high permeability of distributed generation, due to its similar characteristics to a synchronous generator. However, the coupling of the VSG in the power transmission from the actual power output point to the grid-connected point may lead to a steady-state error and dynamic oscillations, which limits the power output capability of the grid-connected converter and the grid support ability. To solve this problem, first, the power coupling mechanism of VSG is studied. Second, a virtual-inductor and virtual-capacitor control strategy is proposed for power decoupling, which eliminates the power coupling and improves their capability in suppressing the reactive power fluctuation. Finally, simulation and laboratory experimental results verify the effectiveness of the proposed decoupling strategy.

Deadbeat Fault-Tolerant Control Scheme for Dual Three-Phase PMSG With High-Resistance Connection Fault

This article proposes a current deadbeat-based fault-tolerant control scheme for a dual three-phase permanent magnet synchronous generator with a high-resistance connection (HRC) fault. First, the voltage deviation caused by the HRC fault is deduced, and the postfault prediction model is established. Then, the instantaneous active power and reactive power are analyzed considering the negative-sequence voltage and current. To eliminate the fluctuation components and improve the steady-state performance, the fault-tolerant current references are designed under the condition of HRC fault. The deadbeat principle is employed to track the periodic <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> current references accurately. The two targets of eliminating output power fluctuations and stabilizing electromagnetic torque are achieved. Finally, the virtual voltage vectors are synthesized to generate the reference voltage and suppress the harmonic current. The proposed method effectively restrains the fluctuations of active power, reactive power, dc-link voltage, and electromagnetic torque. The experimental results verify the effectiveness of the proposed method.

Innovative Grid-Connected Photovoltaic Systems Control Based on Complex-Vector-Filter

The research presented in this paper explains how the complex-vector-filter (CVF) method can help in minimizing the current harmonic of a grid-tied photovoltaic system. In fact, the harmonic-free positive sequence (HFPS) load current is used to produce referential sinusoidal currents. This control stabilizes the grid’s currents under unbalanced load currents, as well as mitigates undesirable harmonic load currents, while feeding clean active power to the grid. Thanks to the proposed controller, the performance, such as robustness, as well as the stability and dynamics of the CVF are more effective compared to the proportional-integral (PI) with phase-locked-loop (PLL) controller. Moreover, the CVF ensures robustness and stability during the synchronization between the photovoltaic (PV) generator and the utility grid system. The PI&amp;PLL control presents higher active and reactive power fluctuations during synchronization. On the other hand, the CVF ensures the elimination of the reactive power fluctuations during synchronization. The performance of the proposed CVF is validated by simulation through MATLAB software. Under all conditions, the grid current, considering harmonics, is within the limits set by the IEEE-519 power quality standard, where a total harmonic distortion (THD) of 1.56% was achieved in the case of feeding a non-linear load.

An Electromagnetic Var Compensator Suitable for Wind Power Access and Its Control Strategy

As the proportion of large-scale wind farms and distributed wind power connected to the power grid increases annually, the effects of their intermittent and random characteristics on the active and reactive power fluctuations of the power grid are becoming increasingly evident, causing frequent voltage fluctuations at the grid-connected point. To solve these problems, this study proposes a new topology of an electromagnetic var compensator (EVC) based on a rotary phase-shifting transformer (RPST). The EVC can work under capacitive and inductive conditions to compensate for inductive and capacitive power, respectively. In accordance with the parallel steady-state mathematical model of the EVC, a double closed-loop control strategy with high precision and considerable robustness is proposed for the EVC on the basis of instantaneous reactive power theory. Finally, simulations show that the topology of the proposed EVC exhibits bidirectional and continuous adjustment capability that can meet the reactive power compensation requirements of power systems with a high percentage of wind power. Compared with the existing reactive power compensation device, the EVC exhibits the advantages of high voltage, large capacity, low cost, strong impact resistance, and good tolerance, imbuing it with great prospects for development.

Optimal Control of an Energy-Storage System in a Microgrid for Reducing Wind-Power Fluctuations

In conventional low-voltage grids, energy-storage devices are mainly driven by final consumers to correct peak consumption or to protect against sources of short-term breaks. With the advent of microgrids and the development of energy-storage systems, the use of this equipment has steadily increased. Distributed generations (DGs), including wind-power plants as a renewable energy source, produces vacillator power due to the nature of variable wind. Microgrids have output power fluctuations, which can cause devastating effects such as frequency fluctuations. Storage can be used to fix this problem. In this paper, a grid-connected wind turbine and a photovoltaic system are investigated considering the atmospheric conditions and wind-speed variations, and a control method is proposed. The main purpose of this paper is to optimize the capacity of energy-storage devices to eliminate power fluctuations in the microgrid. Finally, the conclusion shows that, in microgrids with supercapacitors, the optimal capacity of microgrid supercapacitors is determined. This method of control, utilizing the combined energy-storage system of the battery supercapacitor, in addition to reducing the active power volatility of the wind turbine and photovoltaic generation systems, also considers the level of battery protection and reduction in reactive-power fluctuations. In the proposed control system, the DC link in the energy-storage systems is separate from most of the work conducted, which can increase the reliability of the whole system. The simulations of the studied system are performed in a MATLAB software environment.

Adaptive Droop-Based Hierarchical Optimal Voltage Control Scheme for VSC-HVdc Connected Offshore Wind Farm

An adaptive droop-based hierarchical optimal voltage control (DHOVC) scheme is proposed for voltage-source converter high-voltage-direct-current (VSC-HVdc) connected offshore wind farms (WFs). The wind turbines (WTs) and WF side VSC (WFVSC) are coordinated to minimize the voltage deviations of buses inside the WF from the nominal voltage and mitigate reactive power (Var) fluctuations of WTs. The model predictive control is used to improve the performance of the DHOVC scheme during a certain predictive horizon. A hierarchical solution method based on the alternating direction method of multipliers is developed to reduce the calculation burden of the central controller while improving the information privacy protection. During the predictive horizon, the WTs and WFVSC are coordinated to achieve the near global optimal performance without global information. A WF with 32 × 5MW WTs was used in the MATLAB/Simulink to test the proposed DHOVC scheme.

Enhanced Control Scheme for a Three-Phase Grid-Connected PV Inverter under Unbalanced Fault Conditions

This paper presents an improved control strategy to cancel the double grid frequency oscillations in the active power, reactive power, and DC-link voltage of a three-phase grid-connected photovoltaic (PV) system under unbalanced grid condition. To achieve these goals, an enhanced positive–negative-sequence control (PNSC) to remove oscillations of active power and an instantaneous active–reactive control (IARC) to mitigate the fluctuations of active and reactive power, simultaneously, are suggested. These methods are also effective to reduce the oscillations of the DC-link voltage. To track the desired unbalanced or harmonic reference currents, improved proportional resonant (PR) current controllers have been designed using the Bode frequency analysis. Simulation studies are carried out via Matlab/Simulink® software to verify the effectiveness of the suggested control strategies.

The Novel Current Control for Virtual Synchronous Generator

In recent years, power generation using renewable energy sources has been developed as a solution to the global warming problem. Among these power generation methods, wind power generation is increasing. However, as the penetration level of wind power generation increases, the low inertia and lack of synchronous power characteristics of the penetrated power system can have a significant impact on the transient stability of the grid. The virtual synchronous generator provides the ability of virtual inertia and synchronous power to interconnected inverters. The interconnected inverter with the virtual synchronous generator ability uses, in general, PI control based current controller. This paper proposes a new current-control method and compares it with conventional methods. The proposed current control is a method that follows virtual synchronous generator model that changes every moment by solving the discrete-time linear quadratic optimal control problem for each sampling time interval. The new method follows the conventional method, and therefore the reactive power fluctuation can be suppressed and the interconnected inverter will be downsized.

Applying analysis of power quality enhancement by using UPQC

The current power sector is dealing with the weak power quality issues and the main reasons hiding behind the weak power quality are harmonics, fluctuations of voltage, transients and reactive power consume. These problems are caused by arising trends in our consume. Nowadays, the share of supply from power electronic devices has greatly increased among our consume. Coupling of the grid to a symbiosis of wind farms and PV parks has also raised matter of weak power quality. There is no doubt that shunt and series compensation is efficient but if they are utilized as a single unit simultaneously, this will increase the efficiency of the device and become a greater beneficiary of the power sector, which together serve as UPQC (Unified Power Quality Conditioner). Therefore, to improve the power quality is the main task of UPQC and through compensation to the above problems proves that it has considerable advantages in improving power quality. So UPQC is considered to be an effective method to solve power quality problems. Since UPQC is used for both shunt and series compensators, it contains two inverters to provide this service. They are voltage source inverters with a common DC link, which are called shunt and series converters. These controllers can be controlled by various techniques such as PI controllers, Fuzzy controllers, Neural Networks etc. The series converter is in charge of the smoothing of the voltage, and the shunt converter compensates for the distorted load current.

Stability implications of bulk power networks with large scale PVs

With the shift in renewable portfolio standards, conventional fossil-fuel based generators are expected to be partially or fully replaced with renewable energy sources such as photovoltaics (PVs). However, replacing the conventional synchronous generators with PV generators may raise new stability concerns such as lack of inertia, insufficient reactive power and voltage fluctuations, which can jeopardize the reliability of the entire network. In this paper, various impacts of integrating utility-scale PVs and rooftop PVs and replacing the existing conventional generators on the stability of a power network are investigated. Detailed steady state, transient and small signal analyses are conducted on a large test system, namely Texas 2000-bus system for different PV penetration levels. System transients such as line faults, double-line faults, loss of generator, loss of PVs and cloud cover scenarios are simulated to study their impacts on the test system with different PV penetration levels. It is demonstrated from the impact analysis that the benefits of PVs are highly dependent on a spectrum of factors such as node criticality, type, location and penetration of PVs, and type of transients. Furthermore, the influence of battery energy storage system that are generally associated with PVs on the system stability is also discussed.

Coordinated‐control strategy of scalable superconducting magnetic energy storage under an unbalanced voltage condition

Modular multilevel converters (MMCs) have the advantages of high-power density and small-harmonic distortion because of their modularity and flexibility, thus providing a new avenue for research into scalable superconducting magnetic energy storage (SMES) in renewable energy generation. This study presents coordinated control for a three-phase four-wire (3P4W) MMC-based SMES system under unbalanced voltage applications. First, the positive- and negative-sequence mathematical models and the port-controlled Hamiltonian with dissipation model of the 3P4W MMC-SMES system are established by introducing an additional path for zero-sequence current. Second, a multi-objective passivity-based control strategy that can effectively improve the power quality and system robustness and eliminate both double-frequency active and reactive power fluctuations or double-frequency active power fluctuation and negative-sequence current is proposed. The simulation results based on MATLAB/Simulink demonstrate the effectiveness of the proposed topology of the SMES and its control strategy.