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This paper investigates the transient stability of power systems co-dominated by different types of grid-forming (GFM) devices. Synchronous generators (SGs and VSGs) and droop-controlled inverters are typical GFM devices in modern power systems. SGs/VSGs are able to provide inertia while droop-controlled inverters are generally inertialess. The transient stability of power systems dominated by homogeneous GFM devices has been extensively studied. Regarding the hybrid system jointly dominated by heterogeneous GFM devices, the transient stability is rarely reported. This paper aims to fill this gap. It is found that the synchronization behavior of the hybrid system can be described by a second-order motion equation, resembling the swing equation of SGs. Moreover, two significant differences from conventional power systems are discovered. The first is that the droop control dramatically enhances the damping effect, greatly affecting the transient stability region. The second is that the frequency state variable exhibits a jump at the moment of fault disturbances, thus impacting the post-fault initial-state location and stability assessment. The underlying mechanism behind the two new characteristics is clarified and the impact on the transient stability performance is analyzed and verified. The findings provide new insights into transient stability of power systems hosting heterogeneous devices. [ABSTRACT FROM AUTHOR]

Accurate calculation of circulating current in stator transposition windings is a prerequisite for the design of stator windings in large synchronous compensators. Due to its special operating conditions, such as transient strong excitation and strong overload, the main field, magnetic saturation and transient variation should be considered in calculation of circulating current. For this, a 3-D full-domain non-linear transient electromagnetic problem considering the real transposition structure and rotor motion has to be solved, but existing methods cannot achieve. In this paper, a new electromagnetic network method (ENM) based on the equivalence of local magnetic field (ELMF) is proposed to solve the problem of calculation of the transient circulating current in stator transposition windings considering main field and magnetic saturation. Then, the proposed method is applied to an example of a 300Mvar synchronous condenser and verified by the indirect method combining experiment and FEM. The proposed method is shown to produce correct results, take less computation time and is suitable for different transposition structures. [ABSTRACT FROM AUTHOR]

With widespread application of voltage source converter (VSC) as a key energy-conversion power electronic device by using the phase-locked loop (PLL) technique for synchronization, the system dynamics has become much complicated. In this paper, the nonlinear dynamics and transient stability of the PLL-based VSC system are investigated, within a unified framework of the (normalized) generalized swing equation. It is found that there are three different types of bifurcation, including the generalized saddle-node, Hopf, and homoclinic bifurcations. Within the coexistence parameter region, the basin boundary of the stable equilibrium point shows either a closed-loop or a fish-like pattern. With the help of the equal area criterion (EAC), the transient stabilities under different transient disturbances including short circuit, voltage dip, and power rise are analyzed. Because the equivalent damping of the VSC is state-dependent, the theoretical results based on the EAC are examined. Furthermore, based on the analytical results from the generalized swing equation, the principle for all major transient stability enhancement methods is uncovered. All these findings are well verified by extensive electromagnetic transient simulations. Therefore, the generalized swing equation provides a deeper physical insight and plays a crucial role in transient stability problems in power-electronic-dominated power systems, similar to the swing equation in traditional power systems. [ABSTRACT FROM AUTHOR]

This paper focuses on the transient modelling of a specific type of permanent magnet linear synchronous machine designed for line-start operation. The machine uses conductive ladders embedded in the mover to realise a permanent-magnet machine capable of line-start operation. Transient modelling of this type of machine under line-start conditions is critical to optimising machine design, and electrical and mechanical performance prediction, which leads us to devise a novel set of matrices of electrical parameters first. These matrices are developed considering both the impact of the conductive ladders and the longitudinal end effect based on electromagnetic field analysis. The model is then suitable for numerical computation in typical simulation environments. This yields a sufficiently accurate transient model that is verified by experimental results. Hence our novel model, that includes the longitudinal end effects and the effects of conductive ladders, is proven effective in predicting the line-start transients and associated electrical and mechanical performance. The novel model assists in the design optimisation stage where a compromise between synchronised performance and line-start operation is required. The novel machine type can then be investigated as a competitive candidate topology in various applications including electromagnetic launch. [ABSTRACT FROM AUTHOR]

In this paper, a multiscale induction machine model for the simulation of diverse transients in power systems is developed in the dq0 domain. The proposed model includes main flux saturation. The equations that describe the electrical performance of the induction machine are represented through analytic signals whose Fourier spectrum can be shifted. In addition to the time-step size, the shift frequency appears as a novel simulation parameter. When the shift frequency is set to zero, then the model processes instantaneous signals for the representation of natural waveforms. When the shift frequency is set to the frequency of the stator electrical quantities in the dq reference frame, the Fourier spectra of the analytic signals are shifted to become dynamic phasors that allow for efficient envelope tracking. The setting of the shift frequency is adapted to the transients observed during simulation. Test cases verify the proposed multiscale saturable induction machine model and show that it supports accurate and efficient simulation of both electromagnetic and electromechanical transients within a simulation run. [ABSTRACT FROM AUTHOR]

Failures in damper windings of synchronous machines operating in real facilities have been recently reported by several authors and companies. These windings are crucial elements of synchronous motors and generators, playing an important role in the asynchronous startup of these machines as well as in their stability during load transients. However, the diagnosis of failures in such elements has barely been studied in the literature. This paper presents a method to diagnose the condition of damper bars in synchronous motors. It is based on the capture of the stator current of the machine during a direct startup and its further analysis in order to track the characteristic transient evolution of a particular fault-related component in the time–frequency map. The fact that the damper only carries significant current during the startup and little or no current, when the machine operates in steady state, makes this transient-based approach specially suited for the detection of such failure. The Hilbert–Huang transform (based on the empirical mode decomposition method) is proposed as a signal-processing tool. Simulation and experimental results on laboratory synchronous machines prove the validity of the approach for condition monitoring of such windings. [ABSTRACT FROM PUBLISHER]

This paper presents a dynamical model for simulating combined cycle power plants to be used for assessing the plant behavior during large transients and for restoration procedures. The 80th dynamic order model considers the complex structure of the heat recovery steam generator and calculates the main variables that are critical during these transients. The model has the double purpose of studying both the plant behavior under the usual control systems as well as innovative logics and control strategies for increasing its performances in terms of regulation and emergency services. The possible use of combined cycle power plants for supplying a restoration area is finally investigated adopting different control strategies that also involve the regulating capacity of the steam turbine. [ABSTRACT FROM PUBLISHER]

This paper details the transient operation of a wind energy conversion system (WECS) used simultaneously as an active filter and power generator. This study is intended to address the system response to two types of transient phenomena: voltage dips (fast transients) and wind speed variations (slow transients). The system response to voltage dips is governed by the electrical system dynamics and control method and results in the evaluation of the WECS low-voltage ride through capability. The study of the system response to wind speed variations requires a complete mechanical model to be included. Simulation results are presented for a typical WECS, and a discussion is carried out dealing with the generalization of the present work to other configurations. [ABSTRACT FROM AUTHOR]

An improved mathematical model for transient simulations of commutator machines is presented in this paper. This model can simulate and predict the line current harmonics during healthy and faulty operation conditions. A lumped parameter model that considers every single coil in the stator and rotor as a separate electrical and magnetic circuit is proposed. Self and mutual inductances of the coils are estimated by means of the winding function approach (WFA), accounting for the most relevant space harmonics in the machine air gap. The commutation is modeled by using the real brush width and assuming that the brush-commutator contact resistance is a function uniquely of their contact area, which depends on the armature position. Short circuits between adjacent commutator bars are investigated. It is shown that the model achieves good accuracy, reproducing fairly well the armature current ripple for healthy and faulty operation of the machine. [ABSTRACT FROM AUTHOR]