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Saturation nonlinearities caused by control limiters will be activated during large disturbances, complicating the stability of grid-tied voltage source converters (VSCs). This letter investigates the large-disturbance synchronization stability of the grid-tied VSC considering two typical control limiters, i.e., the hard limiter of the reference current and the circular limiter used to avoid overmodulation. A unified differential equation (UDE) model is proposed to quantify the transient responses and stability boundaries of the system under different saturation conditions. It is revealed that the VSC behaves as a current source when the hard limiter is activated; while the interaction between the two limiters drives the VSC to work as a voltage source, making it immune to synchronization instability. [ABSTRACT FROM AUTHOR]

The interactions between permanent magnet synchronous generators (PMSGs) and weak grids may cause negatively damped or growing subsynchronous oscillation (SSO). The growing-SSO would evolve into a sustained-SSO when it is predominantly governed by the nonlinearities in the PMSG's converter control, such as current limiter. Most of the existing frequency-domain modeling and stability analysis methods are in the small-signal sense and thus unable to consider the impact of such nonlinearities. Besides, these methods can hardly give vital information to characterize the sustained-SSO, for instance, the magnitude. This paper presents a detailed characteristic analysis of the sustained-SSO while accounting for the effect of current limiter nonlinearity. First, a single-input-single-output (SISO) frequency-domain model of the PMSG is developed. This SISO model comprises the transfer functions of outer-/inner-loop controls and phase-locked loop (PLL). The nonlinear part, i.e., current limiter, is incorporated into the model using the describing function (DF) method. Then, the equivalent transformation on the system's closed-loop characteristic equation is used to separate the SISO model into independent linear and nonlinear parts. This separation enables the use of the DF-based Nyquist criterion for evaluating the system stability and determining oscillation attributes, such as the magnitude and frequency. Both the analytic results and time-domain simulations reveal that the current limiter has a significant impact on the characteristics of the sustained-SSO. [ABSTRACT FROM AUTHOR]

• The VSC is viewed as a three-port circuit with its port dynamics and couplings at ac and dc sides described by a three-dimensional admittance matrix (3DAM). • The principle and steps to transform the 3DAM to a single-input–single-output (SISO) equivalent form are introduced. The SISO model facilitates an intuitive stability analysis with a single Nyquist curve. • The 3DAM is applied to identify the dominate factor of oscillatory instability. In particular, the instability caused by ac-ac couplings and ac-dc couplings is quantitatively distinguished. The voltage-source converter (VSC) is physically a multi-port circuit consisting ac and dc terminals. The impedance/admittance models (IMs/AMs) of VSC are generally developed considering the frequency-couplings at its ac port, while couplings between ac and dc ports are paid less attention. The ac AM cannot directly reflect oscillatory risks between dc grid and VSC's dc port, and vice versa. This paper proposes a multi-port admittance modeling-based framework to simplify the above modeling and analysis procedure. First, a VSC is viewed as a three-port circuit modeled by a three-dimensional admittance matrix (3DAM). Both ac and dc port dynamics are incorporated into 3DAM with couplings among ports explicitly expressed by off-diagonal elements. Then, the dc and ac grids are easily modeled into 3DAM by sequentially terminating the VSC's three ports with grid impedances. In doing so, the three-port circuit is reduced into a one-port circuit. The principle and steps to transform the 3DAM to a single-input–single-output (SISO) equivalent form are introduced. The resulting SISO model preserves all coupling information in 3DAM and facilitates a more intuitive stability analysis procedure, where a single Nyquist curve suffices. Finally, the 3DAM and SISO model are applied to identify the dominant factors of oscillatory instability. Time-domain simulations validate the effectiveness of the proposed modeling and analysis methods. Index Terms : Admittance matrix, circuit theory, stability criterion, transfer function. [ABSTRACT FROM AUTHOR]

Summary: Based on the analysis of the short current of the fixed speed induction generator (FSIG) under symmetrical system faults, a strategy using the dynamic voltage restorer (DVR) is proposed to improve the low‐voltage ride through (LVRT) capability of FSIGs. During the system fault, LVRT‐DVR is connected in series between the grid and the generator by switching‐off the by‐pass bidirectional thyristors and provides a compensating voltage to isolate FSIG from the system fault. To increase the switch‐in speed of LVRT‐DVR, an antivoltage technique is used to shut the thyristors off immediately. After the fault is cleared, the phase‐shifting control is applied to avoid phase jumps of the FSIG terminal voltage. The PSCAD/EMTDC simulation and field tests have been carried out to prove the effectiveness of the proposed LVRT strategy, and related conclusions and comments are summarized. [ABSTRACT FROM AUTHOR]

Microgrids are highly compatible with photovoltaic (PV) sources because of their ability to internally aggregate and balance multiple renewable sources. Traditional grid-connected PV inverter control configurations are basically current sourced and cannot easily control ac voltage or frequency. The PV inverter using the Consortium for Electric Reliability Technology Solutions (CERTS) concepts can control ac voltage and frequency but have a major problem with load transients. During a load transient, the PV microsource becomes overloaded with the possibility of collapsing the dc bus voltage resulting in an ac voltage drop. This paper presents a PV inverter control strategy which enables PV to behave as a voltage source and is capable of maintaining dc bus voltage stability during load transient. With this PV inverter control configuration, it is shown that the PV microsource can operate as a voltage source in the CERTS microgrid. [ABSTRACT FROM AUTHOR]

This paper proposes a new three-phase software phase-locked loop (PLL) that operates fast and accurately in unbalanced, polluted, and frequency deviating circumstances. The proposed PLL consists of a frequency detector and an initial phase angle detector. In the synchronous reference frame, the initial phase angle detector tracks a ramp phase angle, which is generated from frequency deviation of the inputs, with steady error. This detection error is utilized to estimate the actual grid frequency. Frequency adaptive moving average filters (MAFs) are applied in this new PLL to eliminate noises, harmonics, and negative sequence components. In this paper, the effect of discrete sampling on the MAFs is analyzed, and a linear interpolation is employed to enhance the performances of the MAFs. The stability of the proposed PLL is also analyzed, a sufficient stability condition is identified, and the design procedures of the control parameters are also presented. Simulations and experiments verify the performances of the novel PLL. [ABSTRACT FROM AUTHOR]

Oscillations of grid-connected voltage source converter (VSC) usually reaches sustained state because of the limiters in controller. Describing function has been adopted to analyze the amplitude and frequency of the sustained oscillations, but high-order oscillation components caused by the limiter are ignored in previous literatures. This letter proposes an analysis method for sustained oscillations which considers high-order oscillation components. This method could obtain a more precise oscillation frequency and the amplitude of 1st, 2nd, and 3rd order components. The proposed method is verified by case studies and simulations in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Sub/Super-synchronous oscillations have occurred in wind farms consisting of doubly-fed induction generators (DFIGs) or permanent magnet synchronous generators (PMSGs). This paper investigates the impact of synchronous condenser (SC) on sub/super-synchronous oscillations in wind farms with different types of generator and compares it with STATCOM. Impedance model is adopted in this paper to explain the mechanism of oscillations in wind farms and its mitigation using SC. Compared to STATCOM, SC is more effective in oscillation mitigations of the two types of wind farms. Eigenvalue analysis and time domain simulations in PSCAD/EMTDC are used to check the influence on sub/super-synchronous oscillations from different parameters of SC, such as its capacity, leakage reactance and inertia constant. The results indicate that SC would improve the system stability while a larger capacity will induce a greater improvement. A smaller leakage reactance of SC also benefits the mitigation of oscillations while its inertia constant can hardly influence system stability. [ABSTRACT FROM AUTHOR]

The mainstream researches focus on the linearised impedance/admittance model and its related coupling effects or stability problems. However, since various non‐linear components exist in control loops, their non‐linear dynamics shall be described to improve the modelling accuracy. To fill in this gap, this study develops a large‐signal impedance/admittance model (LSIM/LSAM) to describe the dominant non‐linearity of modulation saturation in a grid‐connected voltage‐source converter (VSC). The LSAM represents the non‐linearity with the equivalent saturation gains based on the describing function theory and then incorporates the derived gains into conventional small‐signal AM (SSAM) to obtain a partially non‐linear admittance. The proposed LSAM distinguishes itself from SSAM by reflecting the improvement in the magnitude‐ and phase‐response of admittance during non‐linear oscillations. Case studies on a grid‐connected VSC containing saturation non‐linearity are conducted to validate the model's effectiveness in stability analysis. It is found that the frequency deviation and sustained oscillation caused by non‐linearity, which can hardly be assessed using SSAM, can be well captured by LSAM. The results of parameter analysis verify that the modulation saturation dominates the system non‐linearity. [ABSTRACT FROM AUTHOR]

Under certain conditions, grid‐connected voltage source converter (VSC) may result in electromagnetic oscillations. Many coupling components with different frequencies could be observed in converter outputs when oscillation happens. The characteristics and mechanism of frequency coupling phenomenon in oscillations of VSC are studied. The magnitude relationship between first‐order components in the three‐phase domain is drawn by impedance model. The two first‐order components will result in higher‐order components due to the non‐linearity of VSC controller. There are mainly three coupling paths in a controller: phase‐locked loop, DC voltage fluctuation and overmodulation. The coupling mechanism and frequency distribution characteristics are investigated in this study. Theoretical results are validated by simulations with PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper proposes a novel multirate co-simulation method to improve simulation efficiency of large-scale AC plus multiterminal DC (MTDC) grids. In this method, the whole system is partitioned into different AC and MTDC subsystems. They each are simulated with different time steps according to their requirements of accuracy. Unlike the existing methods where simplified models are adopted, the proposed approach fully reserves both the detailed behavior of converters and the nonlinear dynamics of large-scale AC systems. To realize the coordination between different subsystems, updating and discretization of the interface models are significant to guarantee the overall numerical accuracy and stability of the co-simulation method. Accordingly, the interface models of the partitioned AC and DC networks are represented by time-varying Thevenin and Norton equivalents. To eliminate the aliasing or time-delay errors, their parameters are derived using moving-window prediction, stepwise correction, and averaging techniques. A numerical oscillation suppression algorithm is developed for the discretization of the interface model. Thus, numerical stability is well guaranteed. The variables inside each subsystem as well as the interfaces, which are derived by augmented network equations (ANEs), are calculated simultaneously. The rate ratio of large AC and MTDC systems is determined based on our proposed criterion of electromagnetic transient simulation accuracy. The effectiveness of the cosimulation method is validated on a practical system integrating large AC and modular-multilevel-converter-based MTDC grids. [ABSTRACT FROM AUTHOR]

The steady increase of power electronic devices and nonlinear dynamic loads in large-scale ac/dc systems desperately requires an efficient simulation method. However, the traditional hybrid simulation, which incorporates various components into a single electromagnetic-transient (EMT) subsystem, causes great difficulty in network partitioning and significant deterioration in simulation efficiency. To resolve these issues, a distributed hybrid simulation method is proposed in this paper. The key factor leading the success of this method is a distinct interfacing technique, which includes: 1) a new approach based on the two-level Schur complement to update the interfaces by taking full consideration of the couplings between different EMT subsystems; and 2) a combined interaction protocol to further improve the efficiency while guaranteeing the simulation accuracy. The improved performances of the proposed method in terms of efficiency and accuracy have been verified by the simulation studies on the modified IEEE 39 system as well as a practical ac/dc system, both of which consist of a two-terminal voltage-source converter HVdc and nonlinear dynamic loads. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel bipolar modular multilevel converter (MMC) topology with dc fault ride-through capability. The proposed bipolar MMC topology is a hybrid of two types of arms: the half-bridge submodule (HBSM)-based arms (HBSM-arms) which are connected to the ground pole, and the unipolar-voltage full-bridge submodule (UFBSM)-based arms (UFBSM-arms) which are connected to the positive or negative pole. During a pole-to-ground or pole-to-pole dc fault, the hybrid-arm design offers the proposed UFBSM-based hybrid-arm bipolar MMC (UHA-BMMC) the capability to provide additional reactive power to support the ac grid while blocking the dc fault current. The topology, operation principle, dc fault mechanisms, and dc fault ride-through scheme of the UHAB-MMC are presented in this paper. Compared with other possible bipolar MMC topologies, the UHA-BMMC provides good dc fault ride-through capability with a small cost of power-electronic devices. The validity and features of the proposed UHA-BMMC are verified by simulation results conducted in PSCAD/EMTDC. [ABSTRACT FROM PUBLISHER]

In this paper, a static synchronous compensator (STATCOM) operation scheme of the modular multilevel converter adopting clamp double submodules (CDSM-MMC) during a pole-to-pole dc fault is proposed. By analyzing the current paths of a CDSM, this paper proposes three new operation states of the CDSM, which allow the CDSM to provide bipolar voltages when the arm current that flows through it is negative. Through control of the CDSMs to operate in these new states along with the blocked state, the arms of the CDSM-MMC can operate alternately in two different modes, that is, the conducting mode and blocked mode even when the dc-link voltage drops to zero. Thus, the CDSM-MMC is capable of working as a STATCOM during a dc fault. Moreover, a control strategy of the dc-fault STATCOM operation mode is designed to control the reactive power provided by the CDSM-MMC, to limit the dc fault current, and to reserve and balance the energies stored in capacitors. Simulation results conducted in PSCAD/EMTDC demonstrate the validity of the proposed STATCOM operation scheme and control strategy of the CDSM-MMC during a pole-to-pole dc fault. [ABSTRACT FROM AUTHOR]

Practical subsynchronous instability events have been observed in doubly fed induction generator‐based wind farms, which are radially connected to fixed series compensated transmission lines. As a special type of subsynchronous resonance, this phenomenon is described here as subsynchronous control interaction (SSCI). This study proposes a centralised SSCI mitigation scheme named as subsynchronous damper (SSD). In this scheme, an additional high‐power electronic voltage‐sourced converter is added to the collection bus of wind farms. It is designed to dissipate the electrical resonance power of SSCI. Thus, SSD provides positive damping for SSCI. Its basic principle is explained with equivalent electric circuit and the sufficient conditions for stabilisation are obtained mathematically. Controller design guidelines are given and have been verified first by eigenvalue analysis. Then a method to choose the capacity of power electronic converter is proposed. The effectiveness of SSD is finally demonstrated by extensive time‐domain simulations on a target system. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to analyze the mechanism of torsional interaction (TI) and its solution for series-capacitor-compensated power systems. The relationship between the oscillation of the generator shaft and the consequent electromagnetic torque is deduced by a time-domain analysis. It is revealed that the subsynchronous currents caused by torsional oscillations provide negative damping electromagnetic torque and are the cause of TI. A family of subsynchronous dampers (SSDs) is proposed, which is based on the controlled injection of supersynchronous and subsynchronous damping currents into the generator stator. The design procedure of subsynchronous damping controller (SSDC) of series SSD is elaborated. Eigenvalue analysis and simulations for the IEEE first benchmark model (FBM) have verified the effectiveness of the proposed SSDs in solving the multimodal subsynchronous resonance problem. [ABSTRACT FROM PUBLISHER]

To effectively mitigate the commutation failure (CF) of the line-commutated converter high-voltage direct current (LCC-HVDC) transmission system, a dynamic series voltage compensator (DSVC) scheme is proposed. The DSVC consists of three integrated gate-commutated thyristor (IGCT)-based full-bridge submodule (FBSM) chains to match the bulk power transmission of the LCC-HVDC. Inserted between the transformer and the AC port of the inverter, the DSVC assists the commutation process by superposing the transient voltage to the AC line voltage and increasing the commutation margin of the inverter valve. Operation and control strategies, which are two critical techniques for the DSVC, are designed based on the working states of the FBSMs, assisting the commutation process and balancing the DSVC capacitor dynamic voltages. The CF detection method is another critical technique of the DSVC control and is improved considering the reverse time of the LCC-HVDC thyristors. Compared with the topologies proposed in existing literature, the DSVC has better performances in safe and stable operation due to the urgent control strategy considering situations beyond its ability. A detailed EMT simulation shows that the DSVC can respond quickly to mitigate CFs and validate the effectiveness of the overall design and key techniques. Furthermore, the operation and control strategy can limit the current stress once it is beyond the capability of the DSVC, which guarantees its feasibility and practicability. [ABSTRACT FROM AUTHOR]

In this paper, a novel hybrid-arm modular multilevel converters (HA-MMCs) topology with DC low voltage operation and fault ride-through capability is proposed for unidirectional HVDC bulk power transmission. The HA-MMCs topology is a combination of two types of converter arms: the half-bridge sub-module based arm (HBSM-arm) and the diagonal bridge sub-module based arm (DBSM-arm). Replacing the traditional HBSM-MMC, the HA-MMCs are operated in current and reactive power control modes during normal conditions and can work as the rectifier or the inverter. Since the DBSM can provide bipolar voltages with unipolar currents, the HA-MMCs need not change its control strategy under DC low voltage conditions caused by the high-impedance faults. Under DC pole-to-pole or pole-to-ground faults, the HA-MMCs can both clear the DC fault current and work as the static synchronous compensator (STATCOM) to provide reactive power to support the AC grid. Compared with other MMC topologies with DC fault ride-through capability, the HA-MMCs topology is more economical for significantly decreasing the number of power devices. The operation states, control strategy and DC low voltage operation capability of the HA-MMCs are presented and simulation results of the LCC-HA-MMC hybrid system using PSCAD/EMTDC validate the characteristics and advantages of the HA-MMCs. [ABSTRACT FROM AUTHOR]

It is still vague for chronic hepatitis B (CHB) patients with normal or mildly increasing alanine aminotransferase (ALT) level to undergo antiviral treatment or not. The purpose of our study was to establish a noninvasive model based on routine blood test to predict liver histopathology for antiviral therapy. This retrospective study enrolled 258 CHB patients with liver biopsy from the First Hospital of Quanzhou (training cohort, n=126) and Huashan Hospital (validation cohort, n=132). Histologic grading of necroinflammation (G) and liver fibrosis (S) was performed according to the Scheuer scoring system. A novel model, ATPI, including aspartate aminotransferase (AST), total bilirubin (TBil), and platelets (PLT), was developed in training cohort. The area under ROC curves (AUC) of ATPI for predicting antiviral therapy indication was 0.83 in training cohort and was 0.88 in the validation cohort, respectively. Similarly, ATPI also displayed the highest AUC in predicting antiviral therapy indication in CHB patients with normal or mildly increasing ALT level. In conclusion, ATPI is a novel independent model to predict liver histopathology for antiviral therapy in CHB patients with normal and mildly increased ALT levels. [ABSTRACT FROM AUTHOR]

With the increasing integration of power electronic converters into the power system, the interactions between converters and their adjacent transmissions bring emerging oscillation issues. A new type of sub- and super-synchronous interactions (S $^2$ SI) between STATCOMs and the weak AC/DC grid were detected in China Southern Grid. As an oscillation incident never reported before, as far as the authors know, its characteristics were quite different from the previous ones. Field measurements as well as simulation studies have revealed that the various factors contributing to the S $^2$ SI include the number of online STATCOMs, the strength of the AC system (in terms of effective short-circuit ratio), the status of series compensation, and the loading level of the HVDC. The mechanism of such oscillation has been clarified and it is caused by the S $^2$ SI between STATCOMs and weak AC grids, where sub- and super-synchronous components are involved in a coupled way. To fully represent such a coupling effect, the concept of sub- and super-synchronous coupled impedance model (IM), or S $^2$ SC-IM, is proposed and developed in order to establish a stability criterion for the S $^2$ SI. Based on the IM-based method as well as the time-domain simulations, the impacts on S $^2$ SI from the various factors are quantitatively analyzed. The results show good consistence among theoretical analysis, electromagnetic simulations, and field measurements. Therefore, the effectiveness of the developed models and the stability criterion has been well verified. [ABSTRACT FROM AUTHOR]

Electromagnetic transient (EMT) and transient stability hybrid simulations are predominantly used to analyze the interactions between HVDC systems and the ac grids. However, the dynamics of the converters will be greatly affected by the waveforms of adjacent ac systems. Waveform distortion as well as time delay caused by interfacing can significantly increase interface errors, resulting in the decrease of the overall accuracy of the simulations. To solve such problems, a dynamic phasor based interface model (DPIM) is proposed in this paper to improve the accuracy of interfaces, especially when the fault occurs near the converters. In doing so, the whole system is partitioned into three parts: the transient stability (TS) subsystem, the EMT subsystem, and the DPIM. During each iteration, the interfaces between the TS subsystem and DPIM are represented by their Norton equivalents at the fundamental frequency and Thevenin equivalents in the dynamic phasor form. Similarly, the interfaces between DPIM and EMT subsystems are represented by their three-phase Norton equivalents and Thevenin equivalents in dynamic phasors, respectively. The effectiveness concerning the accuracy and the efficiency of the proposed method is validated by simulating a practical HVDC Project. [ABSTRACT FROM AUTHOR]

When investigating voltage and current stresses in critical main circuit components during faults of the converter, a detailed equivalent model capable of representing the balancing control strategies of the capacitor voltages on the submodule level, along with blocking and delocking, is always necessary. Among previously proposed equivalent models of the modular multilevel converter (MMC), only submodule averaged models (SAMs) can capture interested inner dynamics inside each arm. However, the simulation accuracy of SAMs is not always satisfactory, especially when the time step is larger than 10 $\mu$s. In order to further improve the simulation accuracy with guaranteed simulation efficiency, the shifted frequency modeling of the half- and full-bridge hybrid MMC is proposed in this paper. Therein, each submodule is represented by Thévenin equivalents derived by submodule dynamic phasors. The arm of the MMC is represented by Norton equivalents to guarantee the efficiency, considering both normal and dc-blocking conditions. The effectiveness of the proposed model in terms of accuracy and efficiency is validated by simulating an MMC-based HVdc transmission. [ABSTRACT FROM AUTHOR]

The dynamics of load, especially induction motors, are the driving force for short-term voltage stability (STVS) problems. In this paper, the equivalent rotation speed of motors is identified online and its recovery time is estimated next to realize an emergency-demand-response (EDR) based under speed load shedding (USLS) scheme to improve STVS. The proposed scheme consists of an EDR program and two regular stages (RSs). In the EDR program, contracted load is used as a fast-response resource rather than the last defense. The estimated recovery time (ERT) is used as the triggering signal for the EDR program. In the RSs, the amount of load to be shed at each bus is determined according to the assigned weights based on ERTs. Case studies on a practical power system in China Southern Power Grid have validated the performance of the proposed USLS scheme under various contingency scenarios. The utilization of EDR resources and the adaptive distribution of shedding amount in RSs guarantee faster voltage recovery. Therefore, USLS offers a new and more effective approach compared with existing under voltage load shedding to improve STVS. [ABSTRACT FROM PUBLISHER]

Short‐term voltage stability (STVS) issue poses a significant threat to receiving‐end power systems with high penetration of induction motors. To improve STVS of a receiving‐end system in a limited geographic area, this study proposes a local‐area STVS control system (LVCS). LVCS consists of phasor measurement units (PMUs), a local‐area centralised controller (LCC), generator‐level actuators (GLAs) and high‐speed communication networks. The PMUs or GLAs are pre‐deployed according to the observability or controllability of STVS. LCC receives and processes voltage measurements from PMUs to generate supplementary excitation control signals (SECSs). The SECSs are next forwarded to and executed by GLAs. Simulation studies on a practical receiving‐end system in China Southern Power Grid demonstrate that: once the target system suffers a serious fault, LVCS can 'feel' the voltage sag at critical buses and enforce the excitation control of generators to inject more dynamic var into the receiving‐end system. As a result, the increasing var demand is met locally and STVS can be improved considerably. Time delay caused by synchrophasor transmission has also been considered in the design of LVCS to make it feasible in practice. Thus, the proposed LVCS offers a new and effective solution to the STVS problem. [ABSTRACT FROM AUTHOR]

Numerical oscillation is a fatal problem caused by the trapezoidal (TR) algorithm in nodal analysis‐based electromagnetic transient (EMT) programmes. Traditional techniques of suppressing numerical oscillation involve switching among different algorithms or adding damping elements, which would undermine the flexibility and accuracy of the overall algorithm. In this study, a new algorithm based on rational‐fraction approximations to exponential functions is proposed to address the issue. Furthermore, it is combined with TR algorithm to form a hybrid method (HM) for EMT simulation. Analytical as well as numerical analyses have been conducted to compare the HM with other popular methods in terms of stability, accuracy and efficiency. The results from either theoretical analysis or case studies on an RLC circuit and a practical power system have concluded that the proposed HM gains advantages over classical methods in suppressing numerical oscillation without sacrificing accuracy and efficiency. [ABSTRACT FROM AUTHOR]

To make better use of local generators' dynamic var reserve to improve short-term voltage stability (STVS), this paper proposes an integrated high side var-voltage control (IHSV^2C) for power plants in receiving-end power systems. The IHSV^2C consists of a plant-level multi-machine var coordinator (MMVC) and several unit-level high side voltage controllers (HSVCs). MMVC coordinates the reactive power output among generators and provides control parameters for HSVCs, while HSVCs can maintain the voltage of a pre-defined voltage control point (VCP) by regulating the voltage reference of the excitation control of each generator. Therefore, when the system suffers a serious fault, IHSV^2C can drive the generators to provide stronger var support. Consequently, the voltage stability can be improved. A conceptual model of the receiving-end power system in China Southern Power Grid (CSG) is established, and the IHSV^2C has been validated through a simulation analysis on the system. Both time-domain simulation results and the voltage sag severity index (VSSI) have fully demonstrated its performance. Therefore IHSV^2C offers a new and effective approach to improve STVS of receiving-end power systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• A two-stage available transfer capacity evaluation framework is proposed considering flexible demand response. • An integrated dispatch model of generation and demand side resources is formulated. • A demand side bidding scheme is developed to enable flexible demand response programs. • A real-world power system in Sichuan, China is tested to validate the effectiveness of the proposed method. Available transfer capability (ATC) has been widely adopted as a crucial measure to guarantee free and reliable power trading. The existing literature generally evaluates the ATC of a power grid considering inelastic load. However, the impacts of responsive load have not yet been systematically explored, which underestimates the ability of demand side resources in ATC enhancement. Therefore, we propose a two-stage ATC evaluation framework in this paper integrating flexible demand response (FDR). In the first stage, i.e., a day-ahead market, a demand side bidding scheme is developed in which electricity users can make flexible choices to participate in demand response (DR) programs according to their preferences. In this paper, FDR is categorized into three typical patterns: (i) deferrable DR, (ii) switchable DR and (iii) adjustable DR. The objective is to minimize the total generation and dispatch costs, while respecting the constraints for FDR, the minimal accommodation of renewable energy, the inter-area power trading, etc. Then based on generation and demand side bidding, a day-ahead unit commitment model is formulated to schedule thermal generators, interchange power and FDR. In the second stage, a real-time ATC evaluation model based on the aforementioned unit commitment results is proposed to quantify the contributions of FDR on ATC enhancement. Case studies based on IEEE 14-bus system and a provincial power grid in China demonstrate that FDR can effectively improve real-time ATC by load shifting and peak shaving, and facilitate the accommodation of renewable energy. [ABSTRACT FROM AUTHOR]