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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]

Recently, high-frequency resonance (HFR) incidents have occurred in several MMC-HVDC projects. The resonance frequency highly depends on system conditions, making mitigation methods designed for specific scenarios ineffective once the grid condition changes. To address the issue, this paper proposes a novel HFR mitigation method based on adaptive notch filters (ANFs). The embedding positions of ANFs in the MMC control are optimally determined through impedance-based analysis. With the HFR frequency estimated by the interpolated discrete Fourier transform, the parameters of ANFs are adaptively tuned. The ANFs improve the damping performance of the captured HFR mode without affecting the dynamic characteristics at other frequencies. As a result, the HFR under varying grid conditions can be mitigated effectively. The effectiveness of the proposed method is verified through electromagnetic transient (EMT) simulations of an actual MMC-HVDC system. [ABSTRACT FROM AUTHOR]

Identifying sources of subsynchronous resonance (SSR) is crucial for understanding and mitigating incidents. Recent studies have shown that SSR impedance and power are effective indicators that can identify an SSR source. However, the calculation of two indexes requires SSR voltage and current phasors as the inputs, which are not directly available from existing online monitoring systems. To address this issue, this paper presents a novel method to calculate SSR impedance/power based on synchrophasors provided by a wide-area monitoring system. As a result, the online detection of SSR sources is enabled. Through mathematical derivation, it is revealed that a synchrophasor under SSR is dominated by four modes: two from the fundamental frequency and two from the SSR frequency. The problem thus can be transformed to a modal parameter extraction (MPE). Two classic MPE algorithms, Prony analysis and the matrix pencil method, are employed in this paper to extract the SSR parameters and calculate the SSR impedance/power. Their performances are verified using synthetic signals and electromagnetic transient simulations. Furthermore, the method is applied to an actual SSR incident involving 23 wind farms. The findings suggest that 17 of 23 wind farms did not contribute negative damping to the oscillation. [ABSTRACT FROM AUTHOR]

A frequency-dependent network equivalent (FDNE) is essential to capture wide-band frequency dynamics in the hybrid simulation of large-scale modular multi-level converter based ac–dc grids. The FDNE model must be enforced to be passive, ensuring the numerical stability in time-domain simulations. However, existing passive enforcement techniques based on global optimization perturbations cannot guarantee convergence, accuracy, and efficiency simultaneously. To address the issues, a two-layer FDNE (T-FDNE) model is developed for the ac grids. The two layers, namely, detailed layer and equivalent layer, have their admittances derived by perturbation test and analytical approach, respectively. The passivity of the T-FDNE model is guaranteed by the proposed local passivity compensation technique using auxiliary rational functions. Since no optimization is required and the passivity is enhanced locally, the convergence, accuracy, and efficiency can be improved considerably. By incorporating the T-FDNE model into the interface model of transient (TS) and electromagnetic transient hybrid simulations, wide-band frequency interactions, especially those of very high frequency, can be reflected effectively. The performance (efficiency and accuracy) of the T-FDNE model as well as of the hybrid simulation method has been validated on a modified and practical ac–dc system in China. [ABSTRACT FROM AUTHOR]

Recently, emerging oscillation issues have been caused by subsynchronous interaction (SSI) between voltage source converters (VSCs) based renewable generators and ac/dc grids. To investigate the dynamics of SSI, several impedance models (IMs) of VSC have been developed in previous studies. They are established in different reference frames and consider the entire or just parts of the control systems. However, it is still an unanswered question whether or not the existing IMs are suitable for SSI analysis and to what extent the IMs with different modeling details affect subsynchronous dynamics. To fill in this gap, this paper, first, reviews the existing IMs of VSC and clarifies their relationship. Then, their frequency characteristics are examined so as to select the proper models for the studies of sub-/supersynchronous and high-frequency dynamics. Next, both zero-pole and IM-based SSI analyses are carried out on a simplified system and a real-world wind-farm system that suffered actual SSI incidents. The results indicate that the outer-loop control has a significant impact on the subsynchronous dynamics, and only those IMs taking full-scale controls and frequency-coupling effects into account can reflect SSI accurately. Finally, both modal analysis and electromagnetic transient simulations have been conducted to verify the theoretical results. [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]

• Proposed novel protection scheme against torque amplification for turbine generator. • Method of transient torque estimation to capture the impulsive torque accurately. • Guidance to tune tripping threshold to protect the shaft under various conditions. The torque amplification (TA) in turbine generators connected to a series-compensated network could cause excessive fatigue loss-of-life (FLoL) or even a sudden shaft failure, and thus poses a major threat to the shaft safety. This paper develops an instantaneous transient torque protection (ITTP) scheme to ensure shaft safety. It adopts the shaft speed to estimate transient torque in real-time and then judges whether to trip the generator by comparing the estimated transient torque with the pre-set tripping thresholds. As the key parameters of the proposed protection scheme, the tripping thresholds are delicately tuned based on the most severe circumstance, ensuring the inflicted FLoL less than the tolerable value and avoiding mal-operation under all possible circumstances. A prototype ITTP is developed and its performance is thoroughly verified through real-time digital simulator (RTDS)-based hardware-in-the-loop (HIL) tests on a practical series-compensated power system. The results under a variety of operating conditions and disturbances have well demonstrated the effectiveness of the developed ITTP in estimating transient torque and protecting the generator from damage caused by impulsive transient torque. [ABSTRACT FROM AUTHOR]

DC short-circuit fault is a crucial issue in DC grids because the DC fault current increases rapidly with accompanying high fault energy. Limiting the DC fault current is a promising solution to simplify the design of the DC breaker and DC protection. The majority of existing DC fault current limiting (FCL) approaches relies on extra devices, such as DC line inductors. However, the control ability of modular multilevel converters (MMCs) is not optimized. In this study, the submodule (SM) capacitor dynamics during the short DC fault procedure was considered a new control objective, and its effect on the limiting DC fault current was analyzed and revealed. The complex time-varying DC fault transient circuit was comprehensively represented by introducing two base circuit structures and one duty cycle index D. The averaged circuit model was proposed on the basis of the state–space averaging method, which can intuitively show the relation between SM capacitor and DC fault current. The analytical expressions of the DC fault current was given, which can guide the design of the DC FCL control of MMC. The duty cycle index D can represent the discharge of SM capacitors, and the increasing speed of the DC fault current declines with the decrease in D. Accordingly, a feasible FCL control strategy was proposed by choosing D as a tool for affecting and controlling the DC fault current. The proposed FCL control strategy can evidently reduce dc fault current, and avoid the drawbacks of the conventional block or bypass approaches. The simulation results validated the effectiveness of the proposed analysis method and corresponding conclusions. [ABSTRACT FROM AUTHOR]