Improve System Stability Research Articles

Real Time Swinging Up and Stabilizing a Double Inverted Pendulum Using PID-LQR

Abstract This study describes a method for swinging up and stabilizing a double inverted pendulum (DIP) in real-time utilizing a PID-LQR combined control system. Firstly, a dynamic model of the double inverted pendulum system is made up and the equations of motion are constructed. The pendulum then is moved from its unstable position to a stable one using a PID-LQR controller. A comparison of the PID-LQR controller’s output and suggestions for improving system stability is presented and is suggested combined control system.

Sustainable development and applications of variable frequency transformer (VFT)

Power system interconnection is achieved by incorporating phase shifting transformers (PST) and back-to-back high voltage direct current (B2B HVDC) systems. Incorporating PSTs in an electrical power system is highly undesirable owing to its step wise variation of power transition. On the other hand, B2B HVDC systems are rarely employed in weak grids, due to their massive consumption of reactive power for device commutation. The variable frequency transformer (VFT) is capable of resolving the concerns described above by delivering improved performance on account of lower reactive power consumption besides providing smooth controllable power. This paper presents a critical review of VFT depicting its sustainable development for power system interconnection. To start with, an overview of VFT is presented followed by a brief description of other power flow controllers. These power flow controllers are compared with VFT to highlight its advantages over other conventional power flow controllers. This paper further identifies role of VFT in improving system stability and suppressing low frequency oscillations. This study also identifies the possible gaps for incorporation of VFT in power systems that have not been reported in literature and intends to serve as a useful reference for researchers working in the field of power system interconnection.

Wideband Series Harmonic Voltage Compensator Using Look-Ahead State Trajectory Prediction for Network Stability Enhancement and Condition Monitoring

A wideband series harmonic voltage compensator (WSHVC) with an operating bandwidth of 10 kHz for microgrids with a plurality of grid-connected inverters (GCIs) is proposed. It generates a voltage to compensate for high-frequency harmonic voltage at the point of common coupling (PCC), resulting in a virtually zero high-frequency impedance at the aggregate output of the GCIs and, thus improving system stability. The generation of the harmonic voltage is based on predicting the state trajectory of the WSHVC after a switching period per half-switching cycle to determine the moment at which the switches in the WSHVC change their states. The PCC impedance and the equivalent impedance of the GCI network are extracted by analyzing the system response after injecting the entire system with particle-swarm-optimization (PSO) optimized multisine power disturbances. Therefore, system stability is predicted. The condition of passive components in the WSHVC is also monitored by estimating their values with the sampled state variables. A 500 VA, 1 MHz, FPGA-controlled, GaN-based prototype is evaluated on a 6.5 kVA testbed that consists of three commercial GCIs, nonlinear load, and adjustable grid impedance. A comparative study of the system performance with and without the WSHVC is conducted. Experimental results are favorably compared with theoretical predictions.

Improvement of power system stability in wind connected generator by using PSAT

Voltage stability in power systems is a major problem that receives international attention. An open and free source piece of software, Power System Analysis Toolbox (PSAT) is used in this study to create a model of modified IEEE 14 bus system. Models of two recently created wind turbines—the Squirrel Cage Induction Generator (SCIG) and Doubly Fed Induction Generator (DFIG)—are connected to a modified IEEE 14 bus system. The stability of the power system is examined in this work in relation to the effects of Squirrel Cage Induction Generator (SCIG) and Doubly Fed Induction Generators (DFIG). Here, both DFIG and SCIG, which have variable speeds, are regarded as wind generators. In this work, a small signal stability investigation using SCIG and DFIG wind turbine systems was carried out on a modified IEEE 14 bus system. The results of their simulation were analysed.

The Role of Slurry Reflux in a Corn Stalk Continuous Anaerobic Digestion System: Performance and Microbial Community.

Slurry reflux is a low-cost slurry reduction technology, which can solve the problem that a large amount of slurry cannot be completely consumed in a biogas plant. Anaerobic digestion (AD) of corn stalks with slurry reflux and non-reflux was compared and evaluated in continuous anaerobic digestion to clarify the effects of slurry reflux on AD with organic loading rate (OLR) variation. It was found that slurry reflux increased cumulative methane production and improved system stability. The average methane yield of the slurry reflux group was 224.19 mL/gVS, which was 41.35% higher than that of the non-reflux group. High-throughput sequencing results showed that slurry reflux increased the microbial community richness. The dominant microorganisms in the reflux group were in phylum Bacteroidetes, which have the capacity to degrade polymers, and Methanothrix, which is an aceticlastic methanogen. The relative abundances of Bacteroidetes and Methanothrix were 32.41% and 41.75%, respectively. Clostridium III and Saccharofermentans, which are related to syntrophic acetate oxidation and hydrolysis, were increased in relative abundance in the slurry reflux system. The increase of the OLR altered the main methane-producing pathway from the acetoclastic methanogenic pathway to the hydrogenotrophic methanogenic pathway in the AD system, and the slurry reflux can delay this trend. This study provided an effective way for the reduction and utilization of slurry in a biogas plant.

Grid Inertia and Damping Support Enabled by Proposed Virtual Inductance Control for Grid-Forming Virtual Synchronous Generator

Inertia and damping are the core index of the estimation of power system stability. Existing analysis to the support of inertia and damping are focused on the improvement of system stability as well as parameters tuning of virtual inertia and virtual damping but rarely involved in the quantitative calculation of level of inertia and damping under a certain condition. Based on this background, this article puts forward a unified calculation method to identify the equivalence of inertia and damping under various ratios of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_g</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X_g</i> . In addition, the quantity of improved inertia and damping support enabled by virtual inductance control are estimated by the derived calculation model of equivalence. The calculated mathematical model of equivalence of inertia and damping is well verified by the overlapped eigenvalues distribution. Besides, the negative synchronous power will inevitably influence the aperiodic loss of synchronization (ALoS) of power system caused by the large virtual inductance. Therefore, a solution should be derived to make a compromise between oscillation damping and ALoS by tuning the value of virtual inductance. Finally, the simulation and experiment results in various cases verify the effectiveness of the proposed equivalent damping and inertia calculation method as well as virtual inductance control.

Stability assessment and parametric sensitivity analysis based on extended Gershgorin Theorem for multiple grid-connected converters

• A multiple grid-connected converters is modeled as a medium and high dimensional negative feedback system with a stable open-loop function in unified dq frame, which fills in the limitations of traditional impedance ratios for the small disturbance stability analysis of multiple grid-connected converters system. • In terms of estimating eigenvalues for the multiple grid-connected converters system, a stability assessment method based on extended Gershgorin Theorem is proposed in frequency domain. Moreover, the algebraic form of the criterion is further simplified to quantify the oscillatory stability, which has the lower complexity compared with generalized nyquist criterion. The proposed method is of great potential in analyzing the stability issue for large-scale new energy power systems. • The reshaped eigenvalue estimation area based on extended Gershgorin Theorem is smaller than traditional Gershgorin Theorem, which can reduce its conservatism to improve system stability margin. • Compared to observe the changed trend of system stability by the change of the related parameters, the parameter sensitivity analysis based on equivalent impedance network model is used to quantify the impact factors on stability, which can be given to guide the parameter optimization of the multiple grid-connected converters system. The large-scale integration of power electronic based systems is bringing new oscillatory stability issues, further threatening the safe and stable operation of power system. Early researches are focused on stability analysis of single grid-connected converters to simple networks. Instead, there have been growing interests in studying the small signal stability of multiple grid-connected converters nowadays, whose main method is to apply traditional stability assessment with high complexity and conservatism. To address this problem, a simplified criterion based on extended Gershgorin Theorem to assess the stability of multiple grid-connected converter. The proposed method has less conservatism than tradition Gershgorin Theorem-based criteria, and has the lower computational complexity with simple algebraic operation compared to generalized nyquist criteria. Moreover, the parametric sensitivity analysis is used for analyzing the impact factors to promote system stability based on the proposed criteria. Theoretical results as well as electromagnetic transient simulations have verified the validity. The proposed method has the potential to assess the high-order system, and provides guidance on system parameters optimization based on the sensitivity.

Optimal Power Flow in the Presence of HVDC Lines Along With Optimal Placement of FACTS in Order to Power System Stability Improvement in Different Conditions: Technical and Economic Approach

Optimal Power Flow in the Presence of HVDC Lines Along With Optimal Placement of FACTS in Order to Power System Stability Improvement in Different Conditions: Technical and Economic Approach

Stochastic dynamics of a discrete-time car-following model and its time-delayed feedback control

In this paper, a discrete-time optimal velocity model (DOVM) is presented by discretizing continuous car-following model into a difference equation. Considering the influences of stochastic disturbance on DOVM, the stochastic stability is studied by using Z-transform and Routh criterion. The theoretical expressions of the velocity oscillation amplitude and stability conditions are derived from the expected variance of the velocity variable. To stabilize the unstable traffic flow in DOVM, the time-delayed feedback control strategies are proposed by considering velocity difference and displacement–velocity–acceleration difference, respectively. Then, the stochastic stability of controlled DOVM and the choose of control parameters are provided. The numerical simulations for different traffic scenes indicate that the proposed control strategies can improve system stability and suppress traffic jams effectively. Based on the actual traffic data provided by NGSIM and quantum particle swarm algorithm, the parameters in DOVM are calibrated to optimize the car-following model. Furthermore, the proposed control methods are verified through the actual measured traffic data.

Multimachine stability improvement with hybrid renewable energy systems using a superconducting magnetic energy storage in power systems

Energy storage systems (ESS) have played a vital role in modern power systems to improve system stability and reliability in recent years. This paper describes the role of SMES in improving the power system stability of a multimachine interconnected with hybrid renewable energy systems (RES) such as wind and solar PV. It studies the transient stability of the system by creating a symmetrical fault in a multimachine system at various locations. The hybrid RES simulates an equivalent aggregated 75 MW photovoltaic array and a wind turbine with a capacity of 300 MW PMSG (permanent magnet synchronous generator). It couples a common DC link via a voltage source and dc/dc boost converters. A voltage source inverter connected with a step-up transformer and transmission line coupled to a multimachine system. The voltage source converter and inverter use a model predictive controller (MPC) for a better output voltage profile and to improve the system stability. A SMES is connected through a dc/dc converter with PID-SDC (Proportional Integrating Derivative Supplementary Damping Controller). It effectively suppresses power oscillations and smoothening during generation fluctuations. It reduces the low-frequency oscillations at the multimachine side during three-phase fault conditions at various locations in the system. The system's stability is improved by 3.36 s after clearing the fault. The effectiveness of the entire system is studied in the time domain simulations using MATLAB/SIMULINK software.

A Bio-Thermal Convection in a Porous Medium Saturated by Nanofluid Containing Gyrotactic Microorganisms Under an External Magnetic Field

The study of thermal convection in porous media saturated by nanofluid and microorganisms is an important problem for many geophysical and engineering applications. The concept of a mixture of nanofluids and microorganisms has attracted the interest of many researchers due to its ability to improve thermal properties and, as a result, heat transfer rates. This property is actively used both in electronic cooling systems and biological applications. Thus, the purpose of this research is to study biothermal instability in a porous medium saturated by a water-based nanofluid containing gyrotactic microorganisms in the presence of a vertical magnetic field. Given the presence of an external magnetic field in both natural and technological situations, we were motivated to perform this theoretical research. Using the Darcy-Brinkman model, a linear analysis of the convective instability has been considered for both-free boundaries, taking into account the effects of Brownian diffusion and thermophoresis. The Galerkin method was used to perform this analytical study. We have established that heat transfer is accomplished by stationary convection without oscillatory movements. In stationary convection regimes, metal oxide nanofluids (Al2O3), metallic nanofluids (Cu, Ag), and semiconductor nanofluids (TiO2, SiO2) are analyzed. Increasing the Chandrasekhar and Darcy numbers improve system stability significantly, but increasing porosity and modified bioconvection Rayleigh-Darcy number speed up the beginning of instability. To determine the transient behavior of heat and mass transports, a non-linear theory based on the representation of the Fourier series method is applied. In small time intervals, the transitional Nusselt and Sherwood numbers exhibit an oscillatory character. The Sherwood numbers (mass transfer) in the time interval reach stationary values faster than the Nusselt numbers (heat transfer). This research might help with seawater convection in the oceanic crust as well as the construction of biosensors.

Impact of Wind Power Penetration on Wind–Thermal-Bundled Transmission System

The wind–thermal-bundled transmission system is a feasible way to transmit wind power generation; however, the stability of the system should be paid more attention under high wind power penetration. In this article, the impact of wind energy penetration on torsional vibration and electrical characteristics is investigated for wind–thermal-bundled transmitted by high-voltage direct current system. The torsional vibration responses of generator shaft with different wind–thermal-bundled ratios are analyzed by time and frequency methods. The electrical characteristics of wind–thermal-bundled transmission system are disclosed under the influence of the wind farm penetration. The analysis illustrates that the penetration of doubly fed induction generation makes the torsional vibration of turbo generator and wind turbine shaft present different mode characteristics, and the risks of different modes are different. The current and voltage of wind–thermal-bundled transmission system show regular evolution with the change of wind–thermal-bundled proportion. Analytical results provide the value for the design and control of the wind–thermal-bundled transmitted system, which contributes to the improvement of power system stability.

Flushing control strategies to improve the stability of a biogas slurry drip irrigation system: Behavioral characteristics and mechanisms

Biogas slurry drip irrigation can mitigate environmental pollution and reduce the use of chemical fertilizers to enable sustainable development. However, the stability of the biogas slurry drip irrigation system (BSDIS) is disrupted by emitter clogging; hence, it is essential to explore the flushing control strategy of BSDIS. By means of combining actual measurement and simulation, this study investigates the BSDIS stability based on the three technical parameters of the flushing control strategy. Appropriate flushing control strategies can improve system stability and cause spatial differences on the drip irrigation tape. Under various flushing control strategies, the system stability primarily undergoes delays, sensitivity, and ineffectiveness of flushing with time. Compared with the without flushing and emitter outlet downward-oriented treatment, the optimal flushing combination (the high frequency flushing + emitter outlet upward-oriented treatment) reduces the emitter clogging content by approximately 70.97% and increases system stability by 189.1%. In the internal hydrodynamics, the laying direction of emitters does not change the movement characteristics of water flow, although the clogging particles do not completely follow the water flow, with some particles settling owing to gravity, thereby clotting the emitters. When clogging occurs, the increase in flushing speed is conducive to the increase in turbulent kinetic energy on the inlet surface of the emitter, which facilitate the flushing of clogged substances. This study proposes optimal flushing strategy parameters along with a new management mode for the waste liquid represented by biogas slurry.

Assessing Database Contribution via Distributed Tracing for Microservice Systems

Microservice architecture is the latest trend in software systems development and transformation. In microservice systems, databases are deployed in corresponding services. To better optimize runtime deployment and improve system stability, system administrators need to know the contributions of databases in the system. For the high dynamism and complexity of microservice systems, distributed tracing can be introduced to observe the behavior of business scenarios on databases. However, it is challenging to evaluate the database contribution by combining the importance weight of business scenarios with their behaviors on databases. To solve this problem, we propose a business-scenario-oriented database contribution assessment approach (DBCAMS) via distributed tracing, which consists of three steps: (1) determining the importance weight of business scenarios in microservice system by analytic hierarchy process (AHP); (2) reproducing business scenarios and aggregating the same operations on the same database via distributed tracing; (3) calculating database contribution by formalizing the task as a nonlinear programming problem based on the defined operators and solving it. To the best of our knowledge, our work is the first research to study this issue. The results of a series of experiments on two open-source benchmark microservice systems show the effectiveness and rationality of our proposed method.

Development of Voltage Control Algorithm for Improving System Stability in Korean Distribution Grids

Problems with overvoltage and reverse energy flow arise as the percentage of distribution energy resources (DERs) in distribution system rises. This paper implements a MATLAB/Simulink linked model that communicates with the OpenDSS engine. The point of common coupling (PCC) voltage typically mitigates using the inverter outputs control. However, case overvoltage issues occur in distribution networks and the influence of the volt–var control effect is insufficient to reduce PCC voltage to a stable range. To improve the reactive control effect, this paper proposes active power control and voltage error compensation methods that maximize the reactive power control effect. The DER management system (DERMS) platform was constructed to analyze the effect of changing DER output values. The DERMS data are organized based on real distribution line (D/L) system data where overvoltage occurred in Korea and the control effect of the real system is analyzed. This control method analysis can be used to develop an inverter control scheme for the over voltage issues described in this paper.

Power System Stability Improvement of FACTS Controller and PSS Design: A Time-Delay Approach

The existence of low-frequency oscillations in power systems is the cause of power angle instability, limiting the transmission of maximum tie-line power. One of the effective ways to improve the stability limits is by installing a power system stabilizer and supplementary excitation control to augment with an automatic voltage regulator (AVR) supplemental feedback stabilizing signal. This paper proposes a new strategy for simultaneously tuning the power system stabilizer (PSS) and FACTS controller, considering time delays. The design of the proposed controller is modeled as an optimization problem, and the parameters of the controller are optimized through the grasshopper optimization algorithm (GOA). The suggested controller’s efficacy is evaluated for both single-machine infinite bus systems and multi-machine power systems under various disturbances. It also investigated the performance of the proposed controller with variations in signal transmission delays. The results obtained from GOA optimized proposed controller are compared with those obtained from the differential evolution algorithm, genetic algorithm, and whale optimization algorithm. In this context, the proposed GOA optimized controller reduced the objective function value by 16.32%, 14.56%, and 13.72%, respectively, in the SMIB system and 1.41%, 9.98%, and 13.31%, respectively, for the multi-machine system compared with the recently published WOA, and the well-established GA and DE. Further, the proposed controller is found to be stable and effectively increases stability even under small disturbances.

Power system stability improvement considering drive train oscillations of virtual synchronous generator‐regulated type‐4 wind turbines

The type-4 wind turbine generators (WTGs) can provide inertial frequency response by implementing the virtual synchronous generator (VSG) concept. However, unstable torsional oscillations (TOs) would be induced in the multi-mass-spring drive-train which can destabilize the entire power system. In this study, participation factor analyses are performed for a power system inclusive of a thermal unit and aggregated WTG system. The system's eigenvalues are characterized and the possible detrimental impacts of TOs on the inertia provision process and overall power system stability are demonstrated and the related stability limits are identified. A comprehensive active torsional oscillations damper (CA-TOD) and a supercapacitor-based energy storage system (ESS) are presented and discussed to implement the CA-TOD. A state of charge (SoC) regulator is also employed to set the reference speed difference for the turbine-generator. Small-signal studies and electro-magnetic transient (EMT) simulations are performed to evaluate the functionality of the CA-TOD and SoC controller and estimate their impacts on the system's dynamics. The robust performance of the CA-TOD is compared to the band-pass filter-based TOD by sweeping key control parameters of the VSG and emulating drive-train parameter uncertainty. Key results are cross-verified by EMT simulations of a multimachine power system.

Can digestate recirculation promote biohythane production from two-stage co-digestion of rice straw and pig manure?

Digestate recirculation is often considered an important way to improve system stability (system acidification, ammonia inhibition, hydrolysis limitations, etc.) and gas production performance. However, it is not clear how the promotion of biohythane production works in anaerobic co-digestion with digestate recirculation of rice straw (RS) and pig manure (PM). Two sets of laboratory-scale two-stage continuous stirred tank reactors were operated continuously for 95d to investigate the performance of biohythane production in the first/second phase under mesophilic (M)/thermophilic (T) and digestate recirculation conditions. Firstly, biohythane was not produced by PM with RS under digestate recirculation. The main reasons were: 1) Digestive recirculation promoted the growth of hydrogenotrophic methanogenic bacteria; and 2) limitations in hydrolysis. Secondly, digestate recirculation has positive effects on the removal rates (removal rates of TS, VS, polysaccharide, protein and TCOD increased by 30.4%, 22.3%, 9.9%, 31.4%, and 11.9%, respectively) and energy yield (up to 68.7%). Finally, there was a higher abundance of hydrogen-producing bacteria (Fervidobacterium [44.9%] and Coprothermobacter [18.8%]) in T2, accounting for >80% of the total, and of which the huge hydrogen production potential cannot be ignored. The results provide new ideas for alleviating the energy crisis and developing green energy in the future.

High-solids anaerobic co-digestion performances and microbial community dynamics in co-digestion of different mixing ratios with food waste and highland barley straw

High-solids anaerobic co-digestion (HSACoD) performances and microbial community dynamics were investigated. Food waste (FW) and highland barley straw (HBS) were used as co-digestion substrates with mixing ratios of 1:1, 1:3, 1:5, 5:1, and 3:1 (FW: HBS) in an anaerobic digestion (AD) system with 20% total solids. The results showed that the highest methane yield was 509.5 mLN/gVS for FH51, which was 129.8% higher than that of the single HBS. Synergistic effects contributed 20.4%–30.9% to improving methane yields in the first 25 days. The HSACoD system displayed excellent buffer capacity. Fastidiosipila, Gallicola, Proteiniphilum, Aminobacterium, Syntrophaceticus, and W5053 were dominant genera. Methanobacterium, Methanosphaera, Methanoculleus, and Methanosarcina were the key methanogens, displaying high relative abundance. Functional profiling indicated a relative abundance of metabolic pathways of 36.9%–39.6%, with the next most prevalent pathways being cellular processes and signaling, and information storage and processing. Therefore, HSACoD of FW and HBS can enhance AD speed in the early phase of AD and improve system stability.

Power system transient stability improvement with FACTS controllers using SSSC-based controller

Early power systems were rudimentary and local in nature. Power consumption has risen quickly in recent years, resulting in more complicated systems. However, expansion of transmission and generating is limited due to limited resource availability and tight environmental regulations. As a result, today's power systems are increasingly loaded and running under stress, putting them close to their transient stability limits. The study discussed in this paper focuses on employing contemporary PSS to improve power system reliability. The work presented in this thesis mainly focuses on power system stability improvement using modern PSS. In modern power systems trend of using wide area damping controllers and wide area signals are increasing. Also Flexible AC Transmission Systems (FACTS) based controllers are most suitable for stability enhancement and power control. This method proposed tie line power as a broad area control signal for the Static Synchronous Series Compensator (SSSC) based controller based on the geometric measure of joint index value. Kundur's two-area, four-machine system is controlled by this controller. For this system, the effectiveness of the selected signal is applied to an SSSC-based controller. The results reveal that the picked signal performs better than the non-selected signal (rotor speed deviation).