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With the increasing penetration of renewables, the modern power system is facing emerging stability issues. A prime example of such issues is the subsynchronous oscillation (SSO) phenomenon observed in large-scale wind farms. It has been studied that hydrogen production equipment (HPE) has the potential to smooth the fluctuation in generated power. However, it is still unclear whether the HPE can be used to mitigate dynamic stability problems induced by wind generators. This study is aimed at filling the gap by exploring the capability of HPE to damp the emerging SSO. Firstly, the overall configuration of an MW-scale HPE is presented. Then its electromagnetic transient model is established for system dynamics studies. Next, a supplementary subsynchronous damping control (SSDC) of HPE is proposed to modulate its power absorption at the subsynchronous frequency, which can improve system damping and stabilise SSO. Finally, the effectiveness of the proposed SSO mitigation strategy is verified on the simulation model of a practical wind power system that suffered actual SSO incidents. The results demonstrated that the HPE with properly designed SSDC, along with suitable capacity and location, can efficiently mitigate the SSO, thus offers a new option to improve the dynamic stability of renewable power systems.

Recently, emerging sub-synchronous oscillation (SSO) issues caused by the interaction between power electronic converters and weak AC grids have provoked serious stability concerns. Impedance-based modelling approaches have been widely employed for stability analysis of such interactions. In this study, a sequence-domain frequency-coupled impedance model (FCIM) and the corresponding stability criterion are proposed. First, a fast identification method of the FCIM is proposed, in which the impedance-frequency curves of the FCIMs at multiple frequencies are measured, followed by identification of their transfer function models. Next, the proposed method is applied to model a practical system, where a SSO event once occurred due to the interactions between direct-drive wind turbines and a weak AC grid. The individual FCIMs of wind turbine generators, steam turbine generators and static var generator in this system are derived. After that, an aggregated FCIM is formed by combining the FCIMs without extra coordinate transformation. Then, the aggregated FCIM-based stability analysis is carried out to investigate the interaction between the wind farms and weak AC grids. In the end, the results of FCIM-based stability analysis are verified by time-domain simulations.

The electric vehicle, if accessed in the distribution network disorderly and extensively, will influence the safe, stable, and economic operation of the network, due to the characteristics of its randomness, intermittency, and duality with source and load. Here, the power demand models of different electric vehicles were established by analysing the behaviour of electric vehicle loads attributed to different users, and the spatial-temporal distribution pattern of various electric vehicle loads in the distribution network was predicted with the Monte–Carlo method. Based on this regular pattern, an evaluation method for the adaptability of the distribution network to electric vehicles was proposed. Finally, the influence of electric vehicle loads under different access modes on the static voltage stability margin was tested with the IEEE33 node distribution network.

Background Diabetes mellitus (DM) is a major risk factor for coronary artery disease (CAD), and the complications of CAD are the leading cause of deaths among people with DM. Herein, this study aims to identify the common genes and pathways between diabetes and myocardial infarction (MI) to provide more clues for the related mechanism studies. Methods Differentially expressed genes (DEGs) were identified using the cutoff (|log2(fold change)|>0.45 and P value Results PI3, ACSL1, MMD and MMP were altered in both T1DM and MI, and they were highly related to “regulation of cellular protein metabolic process”. Meanwhile, six genes were identified in both T2DM and MI, which are ADM, NFIL3, PI3, SLPI, ACSL1 and MMP9 and significantly related to “negative regulation of endopeptidase activity”. And the expression of these genes were validated. Conclusions In summary, we identified the common DEGs and pathways between T1DM or T2DM and MI, and further validated the changes of those DEGs, providing some clues for mechanism study and potentially therapeutic targets.

Recently, an emerging subsynchronous resonance (SSR) issue was observed in series-compensated systems comprising multiple wind farms. However, only simplified single-machine-infinite-bus system models were adopted to investigate the characteristics of this issue in the previous work. As a result, it is unable to reflect the impact on SSR from some critical factors, such as the network topology, the spatial distribution of wind farms, the diversity of wind turbine generators, and the distributed wind speed. To fill in this gap, this paper proposes an impedance network model (INM) based SSR stability analysis method. The impedance models of each wind farm and transmission line are established, and then interconnected according to system topology to form the whole INM. Such INM is further aggregated into lumped impedance. By analyzing the impedance-frequency features of the latter, a new stability criterion is developed to quantify the stability of SSR. The application of the criterion is demonstrated on a practical power system containing multiple wind farms. Both field measurements and time-domain simulations have been presented to validate its effectiveness and accuracy. The proposed method is of great potential in analyzing the SSR issue for very large-scale wind power systems.

The wind speed of high uncertainty has an important impact on the emerging subsynchronous resonance (SSR) caused by the interaction between doubly-fed induction generator (DFIG)-based wind farms and series-compensated transmissions. This paper proposes a piecewise probabilistic collocation method (PPCM) for assessing the probabilistic stability of SSR in terms of the random wind speed. The PPCM can tackle the inherent nonlinearity resulting from the switching among different operational modes of DFIG. Using the proposed PPCM, the more accurate probability density function (PDF) of the damping can be obtained with a small computation burden. Furthermore, for the probabilistic stability assessment of SSR, this paper also develops the Weibull probabilistic model of the wind speed using the two-year statistical data. The results obtained with the proposed method show consistence with those of the Monte Carlo method (MCM). Finally, field data of SSR events in the practical wind farms are also presented to validate the effectiveness of the proposed method and its produced results.

Recently, emerging sub-/super-synchronous oscillation (SSO) issues occurred in large-scale wind power systems, which are caused by the interactions between the power electronic converter based wind turbine generators (WTGs) and the ac/dc grids. In previous studies, the practical system is usually simplified to equivalent system models, which consist of a (several) large-capacity WTGs connected to a radial equivalent line. However, such equivalent models can hardly represent the dynamics of the practical large-scale wind systems. Besides, the impacts on SSO characteristics from such important factors as network topology, steam turbine generators, and high-voltage direct currents have not been clarified yet. To deal with these challenges, this paper proposes a systematic impedance network modeling and quantitative stability analysis method. All system components of the large-scale wind systems are represented with impedance models (IMs) in a unified reference frame, and then, they are interconnected based on the system topology to form the impedance network model (INM) of the whole system. With the INM reduced to an aggregated IM, a stability criterion is developed to assess SSO stability just by analyzing the frequency characteristics of the determinant of the aggregated IM, and a quantitative analysis method is proposed to quantify the damping, frequency, and sensitivity of the SSO mode. The electromagnetic transient simulations on a practical wind system, which suffered actual SSO issues, have validated the effectiveness and accuracy of the proposed method. This paper also conducts comparative analysis on four types of popular stability analysis methods to show the advantages of the proposed method.

Recently, sustained power oscillation at subsynchronous frequency was captured in direct-drive permanent magnetic synchronous generator (PMSG) based wind farms in Xinjiang Uygur Autonomous Region, China. This new type of subsynchronous interaction (SSI) detected in practical systems has never been reported and analyzed before. Therefore, its mechanism and characteristics are not yet clearly clarified. In this paper, a simplified but representative system model with multiple PMSGs interfaced with AC networks is established first based on the actual system and the PMSG model provided by the manufacturer. Then, small-signal eigenanalysis, time-domain simulation, and impedance model analysis are carried out to investigate the interactive dynamics between them. The results show that such interaction between direct-drive PMSG wind farms and weak ac grids characterized by low short-circuit ratio would cause negative-resistance effect for the SSI mode, leading to unstable oscillation. In such cases, the controller of PMSG would saturate soon, resulting in sustained power oscillation in the system. If unfortunately the oscillation frequency matches the torsional mode of any nearby turbogenerator, severe torsional vibration would be excited on the shaft of the latter. The analysis results are finally validated with field measurements of an actual SSI event. To address the problem, a supplementary subsynchronous damping control loop is attached to the controllers of PMSGs to reshape the impedance and thus to stabilize the SSI.

The emerging subsynchronous resonance (SSR) caused by the interaction of wind turbine generators (WTGs) with series compensation has aroused great concerns. For this particular issue, this paper is aimed to fill the gap between theoretical studies and actual observations. By analyzing the field data of 58 SSR events captured in a practical wind power system and examining the observed dynamics with previous theoretical results, the mechanism and characteristics of SSR are revealed in a more explicit and substantial way. The necessary conditions and dominant influential factors are identified and the underlying reasons are discovered. Theoretically derived as well as practically measured impedance models have demonstrated that the converter control of doubly fed induction generator (DFIG) produces negative resistance at the slip frequency and thus causes unstable SSR; while permanent magnet synchronous generators and self-excited induction generators are just passively engaged in those SSR incidents. The distribution of the oscillation frequency has also been examined with field measurements. It is discovered that WTGs at different locations participate into the same SSR mode and their frequencies are not fixed but keep changing with the time, the variation of grid topology, and the number of online generators.

Sub-synchronous control interaction (SSCI) is a stability problem caused by the interaction between the control system of doubly fed induction generator and a series-capacitor compensated electrical network. It, if not properly handled, could lead to system instability. Therefore, effective control measures must be taken to solve this problem. However, a practical control should be capable of depressing SSCI under varied operating conditions with different SSCI frequency. In this paper, the basic modeling and general characteristic of SSCI in Guyuan wind power system is firstly introduced. And the control-design task is formulated into a complex nonlinear optimization problem with many operation conditions. Then, a combination of genetic algorithm and simulated annealing, namely GA-SA, is proposed to solve the nonlinear optimization problem for multi-operation state. Next, simulations using RT-LAB are presented to demonstrate the effectiveness of our proposed control-design method. Finally, a brief conclusion is drawn that GA-SA offers an excellent approach to the optimal design of SSCI controllers.

A new type of subsynchronous resonance (SSR), namely, subsynchronous control interaction (SSCI), was recently observed in doubly-fed induction generators (DFIGs) interfaced with series-compensated power networks. In this paper, a more accurate method based on aggregated RLC circuit model is proposed to intuitively explain and quantitatively evaluate this type of SSR. For a practical power system containing multiple DFIGs and fixed series compensation, an improved impedance model (IM) is derived, which incorporates DFIG's full-scale control system. Around the series-resonant frequency, IM can be further represented with an aggregated RLC circuit model. Its equivalent parameters are worked out and then used for the quantitative assessment of potential SSR risk. The proposed method is applied for SSR analysis of a practical wind farm system in North China that experienced actual SSR incidents. The consistence between the obtained results and field measured data verifies its effectiveness very well. Further, its advantage in accuracy over existing impedance-based approaches is validated by both eigenvalue analysis and time-domain simulations. The method is also used to quantitatively investigate the impact on SSR stability from the various factors, including wind speed, number of online DFIGs and their control parameters.

To accurately evaluate the damage caused by subsynchronous resonance (SSR) to a turbo-generator shaft, we have developed a novel mechanical and electrical co-simulation method, wherein the continuous-mass-model-based mechanical system is simulated in a close-loop or coupled way with the electrical power system so that the detailed torque profile along the shaft can be obtained for further analysis of fatigue loss of life. Nonlinear mechanical damping is also modelled to reflect the torsional inter action more precisely. The co-simulation is implemented using commercial electromagnetic transient program and finite element analysis software. A special dynamic data exchange mechanism is designed to make it proceed very efficiently. As a case study, the proposed method has been applied for the root-cause analysis of a recent shaft failure incident in an India power plant. The extensive simulation results have fully demonstrated its effectiveness in acquiring fine-grained torques and its advantages over previous methods. Finally the direct cause of the shaft failure is identified to be the sustained torsional interaction (one type of SSR) between the turbo-generator and its connected series-compensated power system.

There are two critical issues in stabilizing subsynchronous resonance of practical power systems. One is to select an effective yet low-cost control scheme. The other is to optimize the controllers under all possible operating conditions and diversified system disturbances. To address these issues, this article proposes a combined control scheme and a coordinated control design method. The combined scheme is composed of a supplementary excitation damping controller and a voltage source converter based generator terminal subsynchronous damping controller deployed, respectively, on the rotor and stator sides of a generator to take full advantage of a supplementary excitation damping controller (very low cost) and generator terminal subsynchronous damping controller (fast response, flexibility in rating, and deployment). The coordinated control design is implemented through parameter optimization on the linearized system and performance verification with non-linear simulations. The former uses geneti...

Background: With carotid artery stenosis, infarcts can occur in the cortical or internal watershed areas, or both. The timing of carotid artery stenting (CAS) after a cerebral watershed infarction (CWI) is not yet codified. In this retrospective study, we analyzed the safety and clinical effect of early CAS for CWI patients due to carotid artery stenosis. Methods: Between March 2011 and April 2014, 120 CWI patients with ipsilateral carotid artery stenosis were recruited. Of these 120 patients, 63 received CAS within 7 days of the symptom onset (group 1) and 57 received standard medical treatment at the symptom onset (group 2). Periprocedural complications were analyzed in group 1 to evaluate the safety of early CAS. Clinical effects were analyzed by evaluating National Institutes of Health Stroke Scale (NIHSS) score as well as modified Ranking Scale (mRS) score of pre- and post-treatment in 2 groups. Results: There was no significant difference in pre-treatment NIHSS score between the 2 groups (8.52 ± 2.46 and 7.84 ± 2.64, p = 0.15). However, group 1 had lower post-treatment NIHSS score as compared to group 2 (3.03 ± 1.44 and 3.84 ± 1.73, p = 0.006). In both groups, NIHSS score after treatment was significantly reduced (p < 0.05). Treatment effect in group 1 was larger compared to group 2 (-5.49 ± 2.12 and -4.00 ± 1.98, p < 0.05). Before the treatment, both groups had similar patient numbers with mRS score ≤2; however, after 30 days of surgery, group 1 had more number of patients with mRS score ≤2 than group 2. Conclusion: This study suggested that early CAS for CWI can be performed without significant risk. More importantly, early CAS for CWI can improve the prognosis.

Two main contributions of this paper are 1) the energy consumption of ships (ECS) in port is predicted, 2) reduction strategies for energy consumption of ships in port are discussed by the proposed prediction models considering green port. Firstly, 15 characteristics which have impact on the energy consumption of ships are collected by Jingtang Port in China and analysis is conducted. Then, five machine learning models including Gradient Boosting Regression (GBR), Random Forest Regression (RF), BP Network (BP), Liner Regression (LR) and K-Nearest Neighbor Regression (KNN) are developed and 15 features consisting of inherent property of ship and external features of ports are set as inputs. After then, k-folds cross validation is adopted to verify the effectiveness of models. Finally, the feature importance is calculated and the most important features are selected. Besides, experiments are conducted to find the effect of changing several features on energy consumption of ships, and two consumption reduction strategies are discussed. The results show that net tonnage, deadweight tonnage, actual weight and efficiency of facilities are the top 4 features for predicting the energy consumption of ships. In conclusion, when efficiency of facilities is doubled, the energy consumption of ships is reduced by 34.17% at berth and 8.41% in port. The finding of proposed methods and discussed strategies can give references to green port construction.

Torsional damping controllers, such as supplementary excitation damping controllers (SEDCs), are widely used to stabilise subsynchronous resonance (SSR) induced by torsional interactions (TIs) between turbo-generators and series-compensated power systems. However, because of the changeable operating situations of a power system, it is a great challenge to design them to guarantee torsional stability under all possible operating conditions. This study proposes a global optimal control-design procedure for tuning SEDCs to accommodate the variation of system conditions. Considering TI is a small-signal stability issue, the non-linear power system is converted into a family of linear parameter varying models and the parameter-tuning task for multiple SEDCs is formulated into a multi-model constrained non-linear optimisation problem. A global optimisation procedure based on genetic algorithm and simulated annealing is designed to efficiently solve this problem and obtain a set of robust or several sets of gain-scheduling SEDCs. The proposed method is applied to a multi-machine series-compensated power system. The results of both eigenvalue analysis and time-domain simulation have fully demonstrated the effectiveness of the optimised SEDCs in stabilising SSR under all possible operating conditions.

Subsynchronous resonance (SSR) was observed in wind farms located in North China. These wind farms prevailingly consist of doubly-fed induction generators (DFIGs) and are connected to series-compensated transmission lines. The observed resonant frequency is about 6 $\sim$ 8 Hz, which is much lower than that of the reported SSR occurred in Texas. The frequency varies during the occurrence and this phenomenon is observed for the first time. The output power is usually within a certain range, when SSR occurs. Based on the practical system, an equivalent simulation system has been established, in which wind farms are modeled as many identical low rating DFIGs. Then, the SSR event is reproduced by simulations. Analysis results indicate that SSR happens even when the equivalent transmission system compensation level seen from wind farms is only 6.67%. Eigenvalue analysis shows that this phenomenon is an electrical resonance, and could be affected considerably by wind speed, number and control of DFIGs. The number of in-service DFIGs has a nonlinear impact on the damping of SSR. An equivalent electric circuit is deduced to intuitively explain why SSR happens and how the above factors affect it. Considering its features, this phenomenon is recognized as DFIG control participated induction generator effect.

For large turbine generators connected to transmission lines compensated by series capacitors, potential subsynchronous resonance (SSR) poses a big threat to the safe and stable operation of both the generator and the system. To protect them against the harm of SSR, a novel two-level protection scheme is proposed in this paper. It consists of unit-level torsional stress relays (TSRs) and a plant-level coordinator called TMaster. TSR is equipped for each generator to determine the stability of its torsional modes and to estimate the fatigue loss-of-life caused by system disturbances. TMaster summarizes the judgments of modal stability from TSRs and decides if and which generator should be tripped to suppress SSR. The whole protection scheme provides three types of tripping functions, namely, modal instability trip logic, fatigue over-limit trip logic and instant trip logic. They can protect the generator shaft from damage caused by unstable SSR, accumulated fatigue, and intolerable transient torque, respectively. The SSR protection system was applied in a practical series-compensated system and its effectiveness in a real SSR event was demonstrated with field data recorded during the event.

Supplementary excitation damping controller (SEDC) has been widely acknowledged as one of the effective measures to stabilize subsynchronous resonance (SSR). However, how to quantify its stabilizing ability is still an open issue raising widespread concerns. There are few in-depth studies or conclusions in this aspect yet. This paper presents a quantitative measure of SEDC's ability to damp SSR, namely stabilizing ability index (SAI). It is defined as the percentage margin of the effective gain relative to the critical stability gain (CSG). The CSG is the minimum gain of SEDC to guarantee SSR stability, while the effective gain is the control gain of SEDC actually functioning following system disturbances. A practical system is used to investigate the various factors determining the effective gain, the CSG and thus the SAI, based on eigenvalue analysis as well as time-domain simulation. The dominant factors identified include: the total equivalent series-compensation degree of system, the parameters and operating status of the generator, the response time and ceiling voltage of the exciter, and the severity of the disturbance. Why and how these factors affect the control capability of SEDC is examined and thus a practical method is offered for the assessment of SEDC's ability to stabilize SSR.

With the rapidly growing penetration of wind power, photoelectricity and other renewables, the problems of subsynchronous resonances/oscillations resulting from the interaction between power-electronic converters and the transmission devices have become increasingly prominent. In these cases, there would be continuous subsynchronous power oscillations or sub- and supersynchronous voltage and current harmonics, which have serious impact on the safe and stable operation of the whole system. However, the existing synchrophasor technology based on fundamental frequency phasors failed to meet the requirement of monitoring sub- and supersynchronous oscillations. In this paper, a new method is proposed to track both fundamental phasor and sub- /supersynchronous oscillations. The method can automatically adapt to the change of the frequency of the signals, and is of high precision and fast response. The specific algorithm has been derived and analyzed theoretically, and the measurement accuracy and error characteristics have been investigated by testing signals. The results have verified the effectiveness of the proposed method.

Subsynchronous resonance (SSR) issue, namely subsynchronous control interaction (SSCI), was observed in doubly-fed induction generators (DFIGs) based wind farms connected to fixed series compensation. This paper proposes a novel scheme to deactivate SSR by adding subsynchronous suppression filters (SSFs) to DFIG converter controllers. Based on the impedance model analysis, a quantitative index is defined and implemented to identify the “best” location of converter controllers for embedding a SSF. A specific SSF scheme is recommended for practical application, which is to embed a SSF into d-axis channel in inner current loop of rotor-side converter. This scheme has been applied to a practical series-compensated DFIG-based wind farm system located in North China, which has experienced real SSR events. Both impedance analysis and time-domain simulation have validated the effectiveness of the proposed scheme. This control scheme has a broad application prospect due to its easiness to implement and robustness to changeable operating conditions.

The aim of this study was to compare the clinical outcomes of early versus delayed carotid artery stenting (CAS) for symptomatic cerebral watershed infarction (sCWI) patients due to stenosis of the proximal internal carotid artery. We retrospectively collected clinical data of those who underwent early or delayed CAS from March 2011 to April 2014. The time of early CAS and delayed CAS was within a week of symptom onset and after four weeks from symptom onset. Clinical data such as second stroke, the National Institutes of Health Stroke Scale (NHISS) score, and modified Rankin Scale (mRS) score and periprocedural complications were collected. The rate of second stroke in early CAS group is lower when compared to that of delayed CAS group. There was no significant difference regarding periprocedural complications in both groups. There was a significant difference regarding mean NHISS score 90 days after CAS in two groups. Early CAS group had a significant better good outcome (mRS score ≤ 2) than delayed CAS group. We suggest early CAS for sCWI due to severe proximal internal carotid artery stenosis as it provides lower rate of second stroke, comparable periprocedural complications, and better functional outcomes compared to that of delayed CAS.

The interaction between controllers of doubly-fed induction generators (DFIGs) and fixed series compensation would induce subsynchronous resonance (SSR) problem. To address this issue, this paper proposes a novel control scheme to defuse such an interaction just by adding notch filters (NF) to DFIG controllers. Two specific schemes are developed. One is to embed a NF into its d-axis channel in the inner current loop of rotor-side converter (RSC) controller. The other is to add a NF to its q-axis channel. They are then applied to a practical series-compensated DFIG-based wind power system for performance verification. Eigenvalue analysis and time-domain simulation have been conducted to investigate their effectiveness. The results show that they both can significantly enhance the damping and guarantee SSR stability without affecting the normal operation of DFIGs. Moreover, it is discovered that the first scheme can provide better damping than the second one.

Interaction between fixed series compensation and doubly-fed induction generator (DFIG)-based wind farms can cause a new type of subsynchronous resonance (SSR) problem, which is also named as subsynchronous control interaction (SSCI). To explain its basic mechanism as well as to evaluate the risk, this paper proposes a small-signal impedance method, based on which, a practical series-compensated power system including DFIGs can be simplified into a second-order series RLC circuit around the operating point concerned and then the frequency and stability of SSR can be intuitively represented by the equivalent parameters of the circuit. The derivation of the circuit parameters is given and the asymmetry phenomenon of equivalent impedance matrix in dq0 reference frame is observed. Both Eigenvalue analysis and time-domain simulation have verified the effectiveness of the proposed method and its advantage in accuracy over the existing impedance-based method. The impacts on SSR due to wind speed, number of online DFIGs and their control parameters are also investigated.

The interaction between the converter controllers of doubly-fed induction generators (DFIGs) and fixed series compensation tends to cause a new type of subsynchronous resonance (SSR), or subsynchronous control interaction. To address this issue, this paper proposes a novel scheme to deactivate such an interaction by embedding subsynchronous suppression filters (SSFs) into DFIG's converter controllers. Its ability to stabilize SSR is validated by applying it to a practical wind farm system in North China, which has experienced many SSR events. The electromagnetic model incorporating multiple DFIG-based wind farms and series-compensated transmissions is established. Then its SSR characteristics are investigated and used to design SSF's parameters. Two SSF schemes are developed for the target system. The high-cost scheme has all DFIGs equipped with SSFs while a more cost-effective scheme is also designed, which only needs to add SSFs to an optimized number of DFIGs. Simulation results have verified the effectiveness of both schemes in guaranteeing the stability of SSR under all possible operating conditions. In practice, the latter scheme is preferred because of its low cost and sufficient performance.