Large Voltage Fluctuations Research Articles

An adaptive power control method for soft open points based on virtual impedance

The fluctuations in power output from distributed power sources are rapid and dramatic, causing voltage fluctuations in the distribution network that threaten the safety of electricity consumption. Soft open points (SOPs) can replace traditional contact switches and are expected to suppress voltage fluctuations. However, traditional power regulation methods based on the reference values are unable to address rapid and large voltage fluctuations. Therefore, this paper proposes an adaptive power control method for SOPs based on virtual impedance. The SOP is modeled as a series link of adjustable impedance and a voltage source. Then, the voltage difference component is used to calculate the reference for the DQ domain to regulate the power flow in real time automatically. By doing so, the proposed method can smooth the voltage fluctuations in the distribution network. Additionally, the virtual impedance is also optimized to minimize the power loss. Finally, the method is validated through simulation and experiments, demonstrating that this method can automatically regulate power and significantly reduce voltage fluctuations.

Power Advance Prediction to Improve the Energy Utilization Efficiency of Motor-Driven System Considering Multilink Time-Delay

In the motor-driven system, the utilization of a supercapacitor energy storage unit contributes to energy saving. However, the multi-link time-delay in the system makes the supercapacitor unable to immediately track and respond to the sharp change of motor power. This will cause the large DC bus voltage fluctuation, the peak current at the power grid side, and unnecessary energy loss. In this paper, a power advance prediction control strategy considering multi-link time-delay is proposed. The multi-link time-delay mechanism in the system is analyzed, in particular for the different conditions of motor power change. Afterward, with the motor speed, torque, and the inverter acceleration/deceleration rate obtained in advance, the motor steady power and power change rate are predicted. In the dynamic process, by adjusting the tracking rate in real time, the motor reference power function is derived. The reference current of the supercapacitor is given online, so as to match the power between the motor and the energy storage unit. Both simulation and experimental results verify the rationality and validity of the proposed control strategy. In the dynamic process, the system stability and energy utilization are successfully improved in comparison with multi-parameter collaborative power prediction control and double closed-loop control.

Adaptive predetermined performance control of modular charging system for electric vehicles

In this paper, a predetermined performance controller is proposed for a modular charging system for electric vehicles to solve the problem that the entering and leaving of electric vehicles will cause large voltage fluctuation, which will affect the service life of the battery. A barrier Lyapunov function is introduced and combined with the backstepping method to design the controller to ensure that the output voltage of the system can be limited to a set range. The entering and leaving of electric vehicles will result in changes in the system's load, however, this change is unknown, so a finite-time load estimator is designed to estimate the load variation. Then the estimated load is introduced into the controller to improve the adaptive ability of the controller.

Integrated Control and Performance Analysis of High-Speed Medium-Voltage Drive Using Modular Multilevel Converter

The modular multilevel converter (MMC) is highly attractive to develop high-speed medium-voltage variable frequency drives (MV VFDs) due to its cascaded design, high-quality output, and strong reliability. However, high-speed MV VFDs have demanding requirements in capacitor voltage stabilization, current regulation, dynamic response, etc. The instability and distorted voltage/current could easily occur if MMCs are used as high-speed MV VFDs. This paper presents a high-speed VFD with SiC-based MMC feeding a permanent magnet synchronous machine. A new integrated control method is specifically proposed to address the aforementioned issues covering the low-/middle-/high-speed range. Performance analysis is conducted in terms of efficiency, power density, output current quality, and speed range. Steady-state and dynamic conditions are evaluated by a full-scale simulation model and a high-power SiC-based MMC testbed. Results show that the proposed method effectively removes the large capacitor voltage fluctuations, resonant circulating current, and saturated output voltage from zero to the highest speed. Compared with existing MMC VFDs of limited speed (< 4000 rpm), the SiC-based MMC VFD has three times higher speed (15000 rpm), improved efficiency (99.43%), high power density, and low current harmonic distortions.

Power Control Strategy for a Ferry’s DC Power System Using Supercapacitors

Integrated power systems are gaining popularity in the field of power systems and DC integrated power systems are considered promising for electric propulsion ships due to their simple grid topology, low fuel consumption, and easy access to new energy sources. However, the dynamic response characteristics of the power plant can be compromised when a variable speed generator is used in a DC power system, despite achieving energy savings. In this research, we investigate the power control strategy of a specific type of a ferry’s DC power plant. We establish a mathematical model and a Matlab/Simulink-based simulation model to analyze the performance of the proposed strategy. The research utilizes the fast charging and discharging advantages of supercapacitor storage devices to compensate for the dynamic impact delay of the power output when using the variable speed generator set. Additionally, an improved DC bus voltage droop control method that incorporates voltage compensation is proposed to mitigate problems related to large bus voltage fluctuations under sudden load change conditions, enabling better load distribution between different power sources. The simulation results confirm the effectiveness of the proposed strategy in optimizing the speed-seeking method of the variable speed diesel engine sets matching with the supercapacitor, and its positive impact on the dynamic performance of the propulsion system is demonstrated under variable load conditions resulting from ferry operations.

Super Twisting Control Design for HSPMSG Voltage Stabilization Based on Disturbance Observation Compensation

For the purpose to solve the problem of poor anti-disturbance performance and large voltage fluctuations of the high-speed permanent magnet synchronous generator (HSPMSG), this paper proposes a compound control method. The method combines the super-twisting (STW) voltage outer loop controller with the super-twisting disturbance observer (STWO). First, in order to overcome the problem of poor anti-disturbance performance under the conventional PI control method, a voltage outer loop controller based on the STW is designed. Second, a load disturbance observer based on the STW is developed for further improving the anti-load disturbance capability of HSPMSG. The estimated value of the disturbance observer is feedforward into the STW controller as a compensation term. Finally, both simulations and experiments are performed on a HSPMSG to verify the effectiveness of the proposed control strategy.

Cascaded Modular Multilevel Converter and Cycloconverter Based Machine Drive System

Low-speed drive is one of the challenges for modular multilevel converters (MMCs) due to large capacitor voltage fluctuation. In this article, a cascaded MMC and cycloconverter (CCV) based machine drive system is proposed to ensure stable operation of medium-voltage and low-speed machine. The MMC provides medium-frequency ac voltage for the CCV, and the CCV converts the medium-frequency ac input to low-frequency ac output required by the machine. Detailed analysis about MMC's operation frequency, device current stress, and submodule (SM) capacitance are given in this article. Proposed drive system can operate at zero/low frequency under rated load torque, while the SM capacitance is much smaller than that in existing methods. Simulation and experimental studies are conducted, and the results verify the effectiveness of the proposed system.

Robust and reliable solid oxide fuel cells with modified thin film YSZ electrolyte

AbstractReduce electrolyte thickness can improve solid oxide fuel cell (SOFC) performance. However, thinner electrolyte often contains prominent defects and flaws, which may decrease the yield and increase operation risk. This work proposes a method to modify the thin film YSZ electrolyte, to improve cell reliability and durability. The as‐sintered anode supported half‐cell with screen printed YSZ electrolyte was immersed in precursor solution of Y(NO3)3·6H2O and Zr(NO3)4·5H2O, and being treated under hydrothermal condition of 150°C for 12 h. As a result, the modified cells show slight increase in the OCV values. Furthermore, the hydrothermal modification effectively promotes interface sintering between YSZ electrolyte and GDC barrier layer, yielding a smaller ohmic resistance of .142 Ω·cm2 (a decrease of ∼11%) and a higher peak power density of .964 W/cm2 (an increase of ∼18%) at 750°C, than pristine cell. Moreover, the modified cell operates stably over 300 h, while the pristine cell presents large and irregular voltage fluctuations. This work suggests that the hydrothermal modification is an effective and promisingly industrial applicable method for thin film electrolyte recovery in SOFCs.

Construction method of ancillary emergency backup service based on battery energy storage system

The power systems with a high proportion of clean energy face both greater uncertainty in power generation and the threat of stability brought by low inertia. After a fault, the rapid drop in power grid frequency, large voltage fluctuations, and insufficient reactive power support can lead to new energy generation units being disconnected from the grid. Fast active and reactive support will play an important role in suppressing fault propagation in low-inertia systems and preventing large-scale power outages. The reserve and frequency regulation market is a necessary means to deal with power grid failure events, but many blackout events occurred due to insufficient reserve capacity and ineffective market transaction mechanisms. As a flexible power regulation resource, BESS (battery energy storage system) has been incorporated into the power ancillary service market planning. In some engineering cases, the frequency regulation ancillary service provided by energy storage has also shown its advantages. However, there is a lack of in-depth research on how to gather BESS to take part in AEBS (ancillary emergency backup services) in the early stage after the failure. Therefore, in view of the dynamic change of power system risks and the different response speed of different backup resources, this paper proposes to establish a segmented combined backup mode to realize the combination of emergency backup at the initial stage of failure and frequency regulation backup during the fault recovery process. A related model of AEBS demand assessment and emergency backup service pricing mechanism is established. And considering the capacity, life loss and opportunity costs of energy storage for AEBS, a LMP (locational marginal price) method is used to calculate the electricity price of AEBS provided by BESS. It has proved the effectiveness by constructing a grid AEBS based on BESS used by the IEEE 30 node system.

DOBC-based frequency & voltage regulation strategy for PV-diesel hybrid microgrid during islanding conditions

The integration of renewable energy sources (RES) based distributed energy resources (DER) lead to reduction in overall system inertia which causes large voltage and frequency fluctuations during any intermittency. This necessitates the integration of fast and robust controllers for low-inertia microgrids. Therefore, this paper proposes disturbance observer based control (DOBC) approach for frequency and voltage regulation of microgrid consisting of solar PV and diesel generator (DG) based hybrid microgrid under islanding situations. The DOBC is included as feed-forward control to the synchronous generator-based DG, which handles the real-time power mismatches and regulates frequency and voltage of islanded microgrid. To demonstrate the operational resilience of the designed controller under real-time uncertainties in PV and load, the controller has been evaluated under the most extreme uncertainty scenarios. The suggested controller has been implemented on a MATLAB/Simulink platform, and results have been validated utilising the real-time simulator OPAL-RT. In addition, the efficacy of the suggested control strategy has been evaluated in the presence of communication delays and noisy load circumstances. Results demonstrate the controller's dynamic performance in regulating the frequency and voltage for low-inertia microgrids. Finally, the proposed control technique has been executed on laboratory-scale microgrid to demonstrate the potential of developed controller.

Model-Predictive Control for Modular Multilevel Converters Operating at Wide Frequency Range With a Novel Cost Function

Modular multilevel converters (MMCs) have been a promising topology in medium-voltage motor drive applications over the past few decades. The main challenge that hinders the widespread use of the MMC in medium-voltage motor drives is the large voltage fluctuation under low-speed conditions. In this article, we propose a model-predictive control (MPC) for the MMC operating in a wide frequency range. Unlike conventional MPC methods, a novel cost function has been designed to realize the output current control and the voltage balancing control for both low- and high-frequency operations. Compared with the high-frequency circulating current injection method, the proposed method can easily achieve a tradeoff between the modulation range and the amplitude of injected common-mode voltage under different operating conditions. In addition, due to the exclusion of the switching process between low- and high-frequency operations, the control structure of this method is more straightforward. Finally, the experimental results verify the effectiveness and superiority of the proposed method.

Research on Pre-Synchronization Control Strategy of Optical Storage VSG Off-Grid Switching

In the optical storage DC microgrid in island mode, in view of the large inrush current problem of the inverter controlled by the optical storage VSG when switching from off-grid to grid-connected operation mode, a pre-synchronization control strategy based on secondary frequency regulation and voltage regulation was proposed. This strategy can adjust the large frequency and voltage fluctuations caused by active and reactive load changes under the independent DC microgrid of the optical storage. At the same time, it can complete the effective tracking of the grid phase before connecting to the grid, and reduce the impulse current when the optical storage microgrid is isolated and connected to the grid. The real-time digital simulation platform (RTDS) is used to simulate and analyze the proposed control scheme, and the simulation results verify the feasibility of the proposed control strategy.

Operation and control schemes of a novel direct AC-AC modular multilevel converter

AC-DC-AC modular multilevel converter (MMC) and direct AC-AC MMC are two kinds of interfaces to integrating AC motors into power grid, and they are easily damaged from large capacitor voltage fluctuation with varying motor speed. Up to now, none of the methods could balance the reliability and power density of MMC-based motor drives system. To find a better solution to this problem, a novel direct AC-AC MMC topology is proposed in this paper. Operating principle of this topology is introduced and relative multi-loop feedback control method is designed for it. Then, simulation and experiments are conducted for verification. Compared with the traditional MMC with high-frequency injection method, the proposed topology has no common-mode voltage problem and the current stress is reduced by 38.5%. Compared with the half-bridge-based AC-DC-AC MMC, the number of capacitors and inductors of the proposed topology is reduced by 20.8% and 25%. In comparison with the direct AC-AC MMC (modular multilevel matrix converter), the number of power devices of the proposed topology is reduced by 28%. The proposed topology has higher reliability and power density.

Identification of the flicker source in the power supply systems

THE PURPOSE. To consider issues related to the study of methods for determining the sources of violations of the quality of electrical energy in power supply systems, in particular consumers which create rapid voltage changes or voltage fluctuations that create rapid changes in the luminous flux in electric lighting installations. Show that electrical receivers with a sharply variable operating mode are sources of fluctuations in the consumed current and voltage in the electrical network of power supply systems, which in turn manifests itself in the form of unacceptable values of one of the indicators of the quality of electricity - flicker, defined as the subjective perception of fluctuations in the luminous flux of lighting devices. To study the possibilities of determining the places of occurrence of significant voltage fluctuations without the use of expensive and complex instruments for measuring flicker values in the electrical network. To develop a simplified method for determining the locations of electricity consumers that negatively affect the operation of electric lighting systems and, ultimately, the vision of personnel. Conduct a check on a computer model of a simplified method for determining the places of occurrence of voltage fluctuations that are inadmissible in their magnitude, which are expressed in the appearance of significant doses of flicker that exceed the standard values.METHODS. Solving the problem, a computer model was used, created in the Simulink extension package of the Matlab scientific and technical calculation system.RESULTS. The article describes the relevance of the problem, discusses the quality of electricity in power supply systems. It has been shown using experimental data that in electrical networks there is an excess of the standard values of flicker doses. The possibility of using interharmonic components of the mains voltage to determine the sources of unacceptable doses of flicker is considered. A simplified method for finding flicker sources based on obtaining the values of the derivatives of currents at various points of the electrical network is presented. Computer simulation of an electrical network diagram with electrical receivers, which have both calm and sharply variable nature of work, has been performed. It is shown that the use of the values of the derivatives of the current at various points of the electrical network makes it possible to identify the sources of the occurrence of unacceptable doses of flicker.CONCLUSION. The occurrence of significant rapid voltage changes in the electrical network negatively affects vision, which is inevitably accompanied by increased personnel fatigue and can lead to industrial injuries. The results obtained by the authors of the article show that it is possible to use it to determine the places of occurrence of large voltage fluctuations, which are manifested in unacceptable doses of flicker of the derivatives of currents. It should be taken into account when conducting surveys of power supply systems in order to use fairly simple hardware to identify sources of disturbance in the quality of electrical energy.

Adaptive Sliding Mode Control Method for Onboard Supercapacitors System

Urban rail trains have undergone rapid development in recent years due to their punctuality, high capacity and energy efficiency. Urban trains require frequent start/stop operations and are, therefore, prone to high energy losses. As trains have high inertia, the energy that can be recovered from braking comes in short bursts of high power. To effectively recover such braking energy, an onboard supercapacitor system based on a radial basis function neural network-based sliding mode control system is proposed, which provides robust adaptive performance. The supercapacitor energy storage system is connected to a bidirectional DC/DC converter to provide traction energy or absorb regenerative braking energy. In the Boost and Buck modes, the state-space averaging method is used to establish a model and perform exact linearization. An adaptive sliding mode controller is designed, and simulation results show that it can effectively solve the problems of low energy utilization and large voltage fluctuations in urban rail electricity grids, and maximise the recovery and utilization of regenerative braking energy.

Adaptive control strategy of solid state transformer with fast dynamic response and enhanced balance performance

Solid state transformer (SST) is designed as a power transmission device between AC grids and DC grids. However, the power fluctuation causes unbalanced power sharing among dual active bridge (DAB) converters and large voltage fluctuations on dc buses. Here, an adaptive virtual impedance control (AVIC) strategy is proposed to improve the balance performance of SST. Adaptive virtual impedance controller gets the same dynamic response for DAB converters, which matches dynamic impedance of DAB converters. Thereby the power balance can be achieved even the load changes. Meanwhile, an adaptive feedforward control scheme is proposed to reduce the influence of load fluctuation on DC bus voltage. The adaptive feedforward scheme also considers the parameter mismatch, thereby the balance of SST will not be destroyed. Moreover, the balanced power sharing achieved by adaptive controller eliminates the difference between actual transmission power and compensation power in each rectifier, which further suppresses the intermediate capacitor voltage fluctuations. Finally, simulation and StarSim experimental results verified the correctness and effectiveness of the proposed control strategy.

Islanding Detection With Positive Feedback of Selected Frequency for DC Microgrid Systems

The oscillation frequency of the point of common coupling (PCC) voltage is proposed as a new detecting freedom for dc-based islanding events in this article. In the presented method, the voltage positive feedback with a frequency selection is applied to a distributed generator to make the PCC voltage oscillate at the selected frequency when islanding events occur. With the frequency information, islanding events can be identified by the small-amplitude oscillation superimposed on the normal dc voltage without the necessity of voltage amplitude to be shifted out of the normal range. Thus, the islanding events can be effectively distinguished from other abnormal conditions that lead to large voltage fluctuations, and so, the accuracy of islanding detection is improved. Also, the power quality at islanding events is enhanced, which provides important benefits for seamless transfer from the unintentional islanding conditions to the controlled islanding conditions. Moreover, the frequency selection is designed to only transfer the voltage signal at the sensitive frequency band of the islanding condition. As a result, the deterioration of stability caused by the voltage positive feedback can be relieved under the grid-connected condition compared to the existing methods. Finally, the effectiveness of the proposed method has been verified by both time-domain nonlinear simulation and experimental results.

An improved control strategy for current source converter-based HVDC using fundamental frequency modulation

When pulse width modulation (PWM) is applied in a high-voltage current source converter (CSC) based on fully controlled devices, there are some concerns arising, such as large DC voltage fluctuations, high switching losses, and a narrow power operation range. To solve these problems, the CSC-based HVDC using fundamental frequency modulation (FFM-CSC) is proposed. First, the topology of the CSC is introduced. Then, an independent control strategy for FFM-CSC is presented, which can be used to realize univariate control of DC voltage or DC current. The mathematical model of CSC is also analyzed, and on this basis, a coordinated control strategy is proposed for FFM-CSC to decouple the active and reactive power. An outer loop power controller and an inner loop current controller are designed, and the power operation range is investigated. Finally, an FFM-CSC simulation model is built in PSCAD/EMTDC. The simulation results show that there are smaller DC voltage fluctuations, lower switching losses, and a wider power operation range under the proposed FFM-CSC control strategy.

Intelligent light-driven flexible solar thermoelectric system

Solar thermoelectric generation (STEG) is an excellent and environmentally-friendly way to convert thermal energy into electricity by utilizing Seebeck effect of thermoelectric material. However, how to overcome the rigidity and large output fluctuation of traditional STEG is the key to realize its large-scale application in self-powered devices. Herein, a smart light-driven flexible STEG system was designed and constructed through simple layered structure of VO2 flexible film and carbon nanotube (CNT) thermoelectric modules. Based on the optical control characteristics of VO2 flexible film due to the thermal crystal phase transformation, and the light capture, photo-thermal-electric conversion capabilities of CNT flexible film, the smart light-driven flexible STEG system exhibits good flexibility, light-thermal-electrical conversion and stable voltage output. In the above system, CNT, polyvinylidene fluoride (PVDF) as raw material and polyethyleneimine (PEI) as electron donor, p- and n-type flexible thermoelectric films and modules were constructed; VO2 ultrafinepowder was dispersed in polyvinylpyrrolidone(PVP), and the optical control VO2 flexible film was obtained by simple spin coating on the flexible substrate. The system is so flexible that can be folded or bended to different shapes to adapt to different application environments. Furthermore, the intelligent light-driven flexible STEG system can obtainrelatively stable voltage to prevent the damage of equipment caused by large voltage fluctuation and has great application potential in civil micro-self-powered devices.

Predictive Coordinated and Cooperative Voltage Control for Systems With High Penetration of PV

This article proposes a predictive coordinated and cooperative voltage control method in a power distribution system with high penetration of photovoltaic (PV) units. First, an integrated coordinated voltage control of voltage regulators (VRs) tap positions and cooperative distributed control of the reactive power output from PV inverters are used to maintain system voltages within an appropriate bandwidth. Next, solar power forecasting is applied to predict voltage changes, which are used to set the VR tap positions and capacitor switch status to prevent large voltage fluctuations. The fine tuning of voltage adjustment is then achieved by cooperative control of PV inverters to maintain a uniform voltage profile across the system. The proposed method is tested on a modified IEEE 123-node test feeder with high penetration of PVs using real measurement data and compared with the base case. Simulation results demonstrate the effectiveness of the integrated voltage control, as well as the enhancement from the predictive control through solar power forecasting-enabled voltage change estimates. Comparison to previous work in the literature shows significant improvement in terms of voltage deviation and reduction in excessive tap changes.