Grid Voltage Disturbances Research Articles

Decentralized optimal voltage control for wind farm with deep learning-based data-driven modeling

The random fluctuations of wind energy and external grid voltage disturbances can both lead to serious voltage fluctuations and voltage deviations in the wind farm (WF). Voltage/reactive power control is an effective way to improve the voltage stability of WF. The existing research is based on complex physical models with prior parameter information, and the accuracy and calculation speed of the WF model are difficult to guarantee. To address this issue, this paper explores a decentralized optimal voltage control method for WF with deep learning-based (DL) data-driven modeling (DL-DOVC). A hybrid convolutional neural network (CNN)-Transformer architecture is proposed to establish the data-driven model of WF, leveraging its enhanced capabilities for extracting time series correlation, to learn complex patterns and dynamics from time series data. Then, we develop a fully decentralized voltage control method for WF to regulate the terminal bus voltage of wind turbines (WTs) within a feasible range. A DL-based data-driven predictive controller is specially designed to solve the established data-driven voltage optimization problem, it eliminates the necessity for frequent manual model maintenance and is suitable for real-time application. Additionally, the difficulty of WF decentralized optimal voltage control arises from the inability of local controllers to fully consider the impact of all non-local WTs on the local WT states. By designing the DL-based auxiliary state-feedback controller, the effects of non-local WTs are implicitly considered in the auxiliary feedback control law. A WF with 32*6.25 MW WTs is used to test the proposed DL-DOVC method. Simulation results show that the proposed DL-based model can efficiently learn and predict the WF dynamics under different operating scenarios. The near-global optimal voltage control performance is achieved only with local measurements by employing the DL-DOVC method.

A novel RMPC strategy for three-phase inverters operating in grid-connected and standalone modes

One of the main features of microgrids is the capability of operating in both grid-connected (GC) and standalone (SA) modes. This paper presents a novel dual mode robust model predictive control (RMPC) strategy for a three-phase inverter with an LCL filter in both GC and SA operating modes under the filter’s parameter uncertainty. At first, a disturbance observer gain is obtained by solving a linear matrix inequality (LMI), which is determined to preserve the stability of the algorithm. The designed disturbance observer takes into account the polytopic uncertainty of system parameters. A performance index with two weighting matrices is then defined and solved in an infinite horizon by turning it into an optimization problem under LMI constraints. The performance of the control strategy highly depends on the weighting matrices. Hence, an optimization algorithm is formulated to ascertain the best matrices values for both GC and SA operation modes. Given the nonlinearity issue, particle swarm optimization (PSO) is employed to derive the optimal weighting matrices offline. To evaluate the proposed control strategy’s effectiveness, simulations and experiments are performed under several scenarios in both GC and SA operating modes. The results reveal the proposed control strategy’s powerfulness compared to other techniques in the presence of grid voltage and load current disturbances for both inverter operating modes.

Design of a reliable electrical photovoltaic generator based on power converters control

This work proposes a renewable energy generation system that operates with a similar behavior as a synchronous generator in terms of reliability in the power supply and inertial response properties against power changes and grid voltage disturbances. The main contributions of this work are: (1) the renewable energy variability managing by means of a battery energy storage strategy, whose charge and discharge are determined by a forecasted-based power adjustment algorithm, (2) the dynamical modeling of an inverter that includes physical inertia and its corresponding response to power and voltage variations, and (3) the synthesis of a nonlinear optimal control for the inertia-based inverter system where the reactive power injection is regulated. Simulation results for a photovoltaic-based generator are presented to demonstrate the effectiveness of the proposed methodologies.

Fast synchronization with enhanced switching control for grid-tied single-phase square wave inverter using FPGA

Research on grid synchronization has been conducted worldwide by researchers in conjunction with the development of innovative technologies, such as dedicated short-range communication (DSRC) and cellular vehicle-to-everything (C-V2X). However, grid-connected inverters face several challenges, mainly the mismatch in voltage amplitude, frequency, and phase angle, as well as grid voltage disturbance and grid faults. Thus, the control algorithm of this research mainly focused on a half-cycle algorithm to design an enhanced digital switching control for fast synchronization using an FPGA. The control algorithm was developed based on zero-crossing detection (ZCD) and digital phase-locked loop (PLL) modeling techniques using the hardware description language (HDL) and a combination of digital logic blocks in Quartus II software, where the proposed switching was applied using the square-wave switching technique through a 300-watt full-bridge experimental prototype. The performance of the proposed technique was studied, where the total harmonic distortion (THD) for voltage and current resulted in a percentage reduction of 89.29% and 78.05% for voltage and current, respectively, after filter implementation. Also, the resulting signal synchronized in every half cycle and matched the voltage amplitude, frequency, and phase angle of the grid signal in 10 ms.

Improve the voltage profile of the grid-integrated induction motor with a fuzzy-based voltage source converter

This article suggests installing a series compensator on grid-connected induction motors that allows for the ride-through of unbalanced voltage sag. Fuzzy logic controls the motor voltages in a standard three-phase voltage source inverter or voltage source inverter (VSI). An open-ended three-phase machine and the grid form a series connection for the VSI. The proposed system is well suited for uses where frequency variation is unnecessary, such as with large pumps or fans. There is no requirement for a direct current (DC) source or injection transformer when conducting an electric mutual coupling The severity of the sag in the grid voltage that will prevent EMC from shutting down is proportional to the load on the motors. An increase in the voltage of the DC link may be necessary if the voltage is not balanced. A 1.5 hp four-pole induction motor was used alongside grid voltage disturbances to test the proposed compensator's ride-through capability. Grid's current analysis demonstrates that the proposed system does not require the addition of a passive filter to achieve low levels of total harmonic distortion (THD). Additionally, the control strategy, component ratings, and analysis of the converter's output voltage operating principle and pulse width modulation technique are covered. The system's viability is shown via simulation and experimental results.

Extremely Low Frequency PS-PWM Based Technique for Cascaded H-Bridge Converters With Grid Voltage Compensation Capability

Cascaded H-Bridge Converters (CHB) are good candidates for many industrial applications. Among the modulation techniques that can be applied to CHB converters, the Phase-shifted PWM method (PS-PWM) provides very good harmonic quality being the most popular modulation technique at the expense of presenting high switching losses. On the contrary Block Switching technique as well as Selective Harmonic elimination/mitigation (SHE/SHM) techniques provides lower switching frequency and losses with a lower harmonic quality. Additionally with these techniques the converter operation becomes stiff. In this work a modification of PS-PWM based on an extremely low frequency carrier is proposed with the objective of achieving both low switching losses and an acceptable harmonic quality, simultaneously. The main advantage of the proposed solution is the capability of adjusting the phase-shift angle among the carriers to compensate for harmonic grid voltage disturbance. Several experimental results have been reported to demonstrate the good performance of the proposed modulation technique.

A Grid-Connected Inverter with Grid-Voltage-Weighted Feedforward Control Based on the Quasi-Proportional Resonance Controller for Suppressing Grid Voltage Disturbances

A grid-connected inverter (GCI) with LCL filters is widely used in photovoltaic grid-connected systems. While introducing active damping methods can improve the quality of grid-connected current (GCC), the influence of grid voltage disturbances can still significantly impact the quality of GCC, leading to stability degradation, especially in weak grid conditions. This paper proposes a grid-voltage-weighted feedforward control scheme based on the quasi-proportional resonance (QPR) controller. This scheme introduces compensatory terms with different proportional coefficients in the voltage feedforward, controlled by the QPR controller. Through a series of analyses, reasonable inverter parameters are first designed. Then, the proposed system model is built in Matlab Simulink. Through simulation experiments and comparisons with various types of operating conditions, the effectiveness of the proposed system scheme is validated. It minimizes the impact of grid voltage disturbances, suppresses the influence of grid harmonics on the control system, improves current quality, and enhances the stability of the GCI system.

Software phase lock loops for pulse width modulated rectifiers

Simulation and experimental results of the use of three SPLL (Software PLL) that differ by the method of the detection of symmetrical grid voltage sequences (positive and negative ones) in case of an unsymmetrical grid voltage system with harmonics and under usual disturbances and parameter changes of the grid are presented. The function of the DSC (Delayed Signal Cancellation) based SPLL with was experimentally verified under different grid voltage disturbances with satisfactory results.

A CASE STUDY ON HIGH POWER COMPENSATOR OF THE POWER GRID IRREGULARITIES FOR INDUSTRIAL APPLIANCES

The paper presents a case study on development of the high power compensator of the power grid irregularities. The compensator is dedicated for sensitive industrial installations protection against dips, sags and short duration interruptions in public power network. The energy for compensation will be stored in bank of high voltage stacked supercapacitors. The development of the new high power medium voltage compensator will be based on experience gained during the realisation in The Electrotechnical Institute Gdansk Branch a similar medium power low voltage device. The study includes: main power converter topology considerations with some preliminary simulation results including harmonic components discussion, the solution to the problem of fast identification of the grid voltage disturbances, the solution to the problem of compensator waveforms synchronization with the grid and the description of the main operational algorithm of the compensator.

A Hybrid Modulation Technique for Operating Medium-Voltage High-Power CHB Converters Under Grid Voltage Disturbances

Power converters, and particularly multilevel converters, are key elements in the current energy scenario. Among them, Cascaded H-Bridge Converters (CHB) are a popular and versatile solution for many industrial applications. Technical literature provides multiple modulation techniques for its operation. Among them, hybrid-PWM modulation techniques provide a good trade-off between switching loss and output voltage quality. In this work a hybrid modulation technique is presented which allows not only the fundamental block switching operation of some power cells but also a harmonic control over a specific harmonic distortion present in the grid. To validate the operation of the proposal, several experimental results have been reported to demonstrated the good behaviour of the proposed modulation technique.

A Comparative Study on the Performance of the Induction Motor with Fuzzy-Based Power Converters

This study investigates the influence of load conditions on grid voltage stability and the performance of a proposed compensator, encompassing a 1.5 hp 4-pole induction motor (IM) exposed to grid voltage disturbances. Through rigorous grid current analysis, our research demonstrates the effective mitigation of total harmonic distortion (THD) without necessitating passive filters. The comprehensive study covers multiple facets, including control strategy optimization, fine-tuning of control parameters, component ratings, output voltage analysis, operating principles, pulse width modulation, and the integral role of the fuzzy logic controller. Specifically, it underscores the significant impact of a Predictive Torque Control-based Diode Clamped Inverter in enhancing the dynamic and steady-state performance of the IM. The research delves into traditional PTC control methods and advanced techniques, such as fuzzy and evolutionary control, addressing the persistent challenge of reducing torque ripple under varying load conditions. By implementing model prediction control-based DCI, our method achieves rapid torque response without compromising the motor’s stator characteristics. Utilizing MATLAB’s environmental package, we thoroughly evaluate motor performance across diverse scenarios, encompassing stable, and rapid load changes. Despite inherent winding impedance challenges, our proposed approach consistently delivers satisfactory output responses, contributing to a more efficient, and resilient IM performance under diverse operating conditions.

Design and PIL implementation of a new robust backstepping based low-complexity voltage-oriented control method for four-leg rectifiers

This paper proposes a low computational complexity enhanced voltage-oriented control strategy (EVOC) based on robust backstepping controllers (BSC) in the synchronous rotating frame (dq0 frame) for a PWM four-leg voltage-source-rectifier (FLVSR). Unlike traditional VOC strategies, the suggested EVOC approach does not need any synchronization method and Park transformations, and it has the similar architecture and same features as the traditional VOC when it’s synchronization loop for grid voltage phase angle detection is properly operated. In addition to controlling the FLVSR, the suggested EVOC strategy is also used to develop the model of the three phase FLVSR in the dq0 frame using the concepts of direct instantaneous power control (DPC) theory, which combine the benefits of traditional DPC and traditional VOC with less computational complexity. The robust BSC is adopted for accurate regulation of FLVSR input currents and dc bus voltage simultaneously while taking into account the FLVSR parameter uncertainties. It produces sinusoidal currents with lower ripples and has high robustness against parametric variations. It is also simple to implement and has a relatively high grid voltage disturbance rejection ability. Finally, comparisons with both the traditional VOC strategy using PI controllers (VOC-PIC) and the EVOC strategy using PI controllers (EVOC-PIC) under various conditions demonstrate the superiority and benefits of the proposed EVOC-BSC strategy.

The improved 2DoFs-PLL for MMC-HVDC transmission system

The performance of a modular multilevel converter (MMC) is highly related to the three-phase phase-locked loop (PLL). The presence of the DC component, the harmonic component, and the negative sequence component results in the poor dynamic and steady-state performance of the PLL, such as fundamental frequency oscillations in the phase estimated by the PLL. In order to suppress the influence of parameters variation and disturbance of the three-phase grid voltage, an improved PLL with a moving average filter with two degrees of freedom PID (2DoFs-PID) is proposed. Moreover, to improve the phase margin and gain margin, an extra compensator with a pole and a zero is introduced into the 2DoFs-PLL. The improved 2DoFs-PLL can track the frequency and phase with superior accuracy, stability, and rapidity. Furthermore, the methodology for the parameters design of the 2DoFs PID controller is introduced, based on the index of integrated time and absolute error. A simulated model and experimental platform are established to verify the performance of the improved 2DoFs-PLL. The results show that the improved 2DoFs-PLL can meet the needs of both steady-state performance and dynamic performance by employing the 2DoFs-PID controllers.

Model reference adaptive controllers with improved performance for applications in LCL-filtered grid-connected converters

This work presents a control strategy combining a direct Model Reference Adaptive Control (MRAC) with a robust state feedback for current control of grid-connected converters with LCL filters operating under grid impedance variations. The MRAC is based on a gradient law, providing properly current reference tracking and voltage disturbance rejection. The inclusion of a state feedback inner loop provides robust active damping of the filter resonance for an entire range of grid impedances. A systematic control design procedure is proposed, and the combination of both control actions in a multi-loop approach does not add significant computational complexity. The proposal allows to increase the MRAC gains adaptation dynamics, providing suitable responses under grid current reference variations and under typical grid voltage disturbances, including grid faults. Performance indexes as the grid-currents total harmonic distortion and the mean squared error are also improved. In addition, robust stability analysis based on Lyapunov functions are carried out for the inner control loop and for the outer control loop, considering the grid impedance uncertain and with unmodeled dynamics. The strategy is implemented in a commercial digital signal processor, and the compliance of the grid currents with the IEEE 1547 Std. is verified with controller hardware-in-the-loop and experimental results in a 5.4 kW prototype.

A General Diode Open-Circuit Fault Diagnosis Method for Autotransformer Rectifier Unit Considering Grid Voltage Disturbance

It is crucial for the reliable operation of the autotransformer rectifier unit (ATRU) to accurately diagnose diode open-circuit (OC) fault. However, grid voltage disturbance may lead to misdiagnosis by existing methods. This article takes the 18-pulse ATRU as an example and first analyzes its sag-based fault feature in the dc-link output voltage (DLOV) caused by diode OC fault under no grid voltage disturbance. Then, the reason for misdiagnosis under grid voltage disturbance is discussed. Subsequently, a general diode OC fault diagnosis method with strong immunity to grid voltage disturbance is proposed. In the proposed method, the DLOV waveform is divided equally according to the number of rectified pulses. Hausdorff distance (HD) algorithm is employed to extract the sag-based fault feature by calculating the similarity between the DLOV sampling sequence and the constructed template sequence. To be immune to grid voltage disturbance, the template sequence is adaptively generated as per the sampling sequence in each interval. Also, the generalized minimum failure mode is defined to normalize two sequences for good universality. With the help of the phase of grid single-phase voltage, OC faults can be detected and located according to the interval positions with the sag. Finally, simulation and experimental results confirm the strong immunity of the proposed method to grid voltage disturbance and its good universality to the different ATRUs.

Fault Detection and Ride Through of CHB Converter-Based Star-Connected STATCOM Through Exploring the Inherent Information of Multiloop Controllers

A simple detection and ride-through operation approach is proposed for star-connected cascaded H-bridge (CHB) converter-based STATCOMs with a single IGBT open circuit fault. It adopts a modified current regulator with high gains at both dc and fundamental frequencies. Then, the output signals of the current regulator branches and the dc voltage balancing regulator are synthesized as an inherent indicator to quickly determine the position of the open-circuit fault IGBT. With this approach, the controller can also be considered as an inherent system state monitor, but without any additional sensors or computing burdens. Afterwards, in the ride-through operation stage, the faulty H-bridge module can switch to half-bridge operation and the corresponding dc offset voltage is compensated by the remaining healthy modules. It has been validated that the proposed approach is accurate in various conditions, including grid voltage and load disturbances.

Robust Adaptive Control for Large-scale Inverter-based Resources with Partial and Complete Loss of Inverters

This article proposes an approach to address the current and the aggregated active power control challenge for large-scale inverter-based resources subjected to partially or completely loss of inverters and grid voltage variations. To address this problem, a distributed active power mechanism is proposed which generates desired inverters current. Then an adaptive mechanism distributes the voltage control input automatically between inverters in response to the partial or complete loss of inverters. An L2-gain-based controller is designed for each inverter to track the desired current and rejects the grid voltage disturbance. Simulation results show a significant robust tracking for collective active power and current.

Control and Performance of 240-Clamped Space Vector PWM in Three-Phase Grid-Connected Photovoltaic Converters Under Adverse Grid Conditions

This article evaluates the performance of a relatively new pulsewidth modulation (PWM) method i.e., 240 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> -clamped space vector PWM (240CPWM), in three-phase grid-connected photovoltaic (PV) converters under various grid voltage conditions. 240CPWM is a minimum switching loss PWM method that reduces the switching loss by 85% at unity power factor and has better total harmonic distortion (THD) as compared to conventional space vector PWM. A unique six-pulse dynamically varying dc-link voltage is required for 240CPWM. Typically, a three-phase grid-connected PV system consists of a dc–dc stage followed by a dc–ac stage. In this article, the dc–dc stage is operated in closed-loop control to shape the dynamic dc-link voltage required for 240CPWM. The dc–ac stage is operated in the current control mode using a proportional–integral controller along with a harmonic compensator to ensure sinusoidal grid currents along with maximum power point tracking. The modulation index of the dc–ac stage is uncontrolled and fixed at the maximum value, and the primary control mechanism changes the magnitude, phase, and wave shape of the dc-link voltage. The coordinated control of dc–dc and dc–ac stages is ensured for better dynamic performance and grid support functions. Finally, the performance of the grid-connected PV converter with 240CPWM is validated using a three-phase 3-kW 208-V hardware prototype under nonunity power factors, voltage unbalance, and voltage sag/swell conditions for the first time. The implemented control with 240CPWM achieves smooth operation under nonunity power factor and grid voltage disturbance conditions with THD as low as 3.5% and peak combined efficiency of dc–dc and dc–ac stages as 96.4%.

Virtual Voltage Control of VSG for Overcurrent Suppression Under Symmetrical and Asymmetrical Voltage Dips

This article proposes a novel and easily implementable overcurrent suppression control for voltage-controlled virtual synchronous generators (VSGs). The VSG is vulnerable to grid voltage disturbances, such as voltage dip/recovery, because it consists of voltage source inverters (VSIs). In the worst case, the overcurrent caused by the voltage disturbance damages the VSI. The proposed VSG control can suppress the overcurrent caused by not only symmetrical but also asymmetrical voltage dip/recovery. The key idea is to use novel virtual grid voltages for the VSG reference output voltage. The proposed VSG control requires no complicated analysis/tuning for the implementation and no additional equipment, such as a fault-current limiter. Because of these advantages, the overcurrent in the VSG can be suppressed without increasing the cost and the tuning effort. The effectiveness of the proposed method is verified through simulations and experiments.

Research on Modeling, Stability and Dynamic Characteristics of Voltage-controlled Grid-connected Energy Storage Inverters Under High Penetration

With the increasing penetration of renewable energy, the power grid is characterised by weak inertia and weak voltage support. Some current-controlled inverters have been modified to voltage-controlled inverters and are gradually being used in distributed systems, thus constituting a multi-inverter hybrid operation mode system, which brings more severe challenges to the system's stable operation. In this paper, a framework consisting of three main parts of this particular voltage-controlled energy storage inverter is built. Each part's small-signal transfer function matrices are established by their control and structure. Based on this, the sensitivity of the SCR (short-circuit ratio) variation and droop coefficients of grid-connected inverter systems to stability is analysed in both dq domain and pos-neg sequence domain. The coupling of the inverter output active and reactive power and the effect of grid voltage disturbances are analysed under SCR variations in dq domain. Finally, the accuracy of the proposed model, the stability and dynamic response are verified by simulation and experimental platform of a 20 kW energy storage inverter system.