Improved transient stability of virtual synchronous generators for grid voltage cascading faults

Virtual synchronous generator (VSG) control, which can provide inertia output, damp power oscillations, and offer frequency and voltage support to power grids, has become a growing trend in the control field of wind power generation. As a new technology, there are still challenges that VSG control has not solved well, such as transient overvoltage suppression. A kind of transient overvoltage, which often occurs during the commutation failures of HVDC transmission systems, will trigger a mass of wind turbine generators (WTGs) disconnecting from grids. To reduce the grid-disconnection risk of the virtual synchronous generator control-based doubly fed induction generators (VSG-DFIGs), this paper first analyzes the mechanism of the automatic voltage regulation (AVR) control usually employed by VSG-DFIGs, then proposes measures to suppress the transient overvoltage. To solve the problem of the reactive power response lag issued by VSG-DFIGs, which will further aggravate the transient overvoltage in continuous low and high voltage faults, the time constant of the AVR control is switched. To fully exploit the potential of the DFIGs’ reactive power support, the droop coefficient of the AVR control is switched during the abnormal voltage stages. The switched droop coefficient will change the rotor excitation current magnitude, thus adjusting the internal potential of a DFIG, finally better supporting or suppressing the terminal voltage during the low or high voltage periods. Simulation results based on the DIgSILENT/PowerFactory platform demonstrate that the proposed method can effectively suppress the transient overvoltage that occurs in continuous low and high voltage events caused by the commutation failures of HVDC transmission systems, reducing the number of WTGs disconnecting from the grids.

As an important part of electric multiple units (EMU), the auxiliary power supply system plays an important role in the normal operation of the vehicle. In order to improve the reliability of the auxiliary power supply system, the virtual synchronous generator (VSG) technology is applied to the parallel control system of auxiliary inverter. The essence of VSG technology is to control the inverter to simulate the working principle of synchronous generator, so as to obtain similar operation characteristics of synchronous generator. The auxiliary inverter controlled by VSG technology can avoid the disadvantage of hard external characteristics of power electronic devices. Its rotating inertia can obviously prevent frequency fluctuation, and the system has high stability and reliability. In this paper, the frequency and voltage of the auxiliary inverters are controlled independently with VSG technology, so that the output voltage can be controlled stably and the power quality can be improved. Finally, the MATLAB simulation results show that the parallel control of auxiliary inverter based on VSG achieves the expected goals.

In recent years, virtual synchronous generator (VSG) technology has been more and more used in grid-connected inverters of PV power generation systems. Photovoltaic inverter based on VSG can improve the voltage support capacity of the power grid, but it increases the difficulty of the power grid elaborate modeling and simulation. This paper researched the modeling and parameter identification of the PV inverter based on VSG. In this paper, the PV inverter simulation model based on VSG technology was set up in PSCAD and the effective voltage support capability of VSG technology for the power grid was verified. The mathematical model of the inverter based on VSG was established in MATLAB and important parameters were identified based on the improved differential evolution algorithm. Finally, the performance of the modeling method and parameter identification method used in this paper were tested by considering various perturbations.

Single-line ground (SLG) faults in distribution networks may cause hazards such as fire, electric shock, and overvoltage. Active power electronic converters (PECs) can suppress the fault current and fault voltage. However, the application is limited due to the low suppression rate and high cost of additional power electronic equipment. This article proposes to employ distributed power electronics of active distribution networks to cooperatively eliminate fault. In this method, PECs are modeled as virtual zero-sequence synchronous generators (VZSGs), and the line-to-ground impedances (LGIs) are modeled as zero-sequence loads. A theory of distributed VZSGs supplying zero-sequence loads, which can realize fault current and voltage suppression, is proposed. The required supply current can be adaptively allocated between these VZSGs based on their reserve capacity ratio, and their supply current can be dynamically corrected based on the droop coefficients of VZSGs when the LGI changes. The fault current and voltage suppression rate is improved because the adaptive calculation of the VZSGs reference value combines the voltage- and current-based suppression methods. Simulation study and experimental validation results demonstrate that not only is the proposed method cost-effective, but it also efficiently improves the fault current and voltage suppression rate for various fault conditions.

In recent grid codes for wind power integration, wind turbines are required to stay connected during grid faults even when the grid voltage drops down to zero; and also to inject reactive current in proportion to the voltage drop. However, a physical fact, instability of grid-connected converters during current injection to very low (close to zero) voltage faults, has been omitted, i.e., failed to be noticed in the previous wind power studies and grid code revisions. In this paper, the instability of grid side converters of wind turbines defined as loss of synchronism (LOS), where the wind turbines lose synchronism with the grid fundamental frequency (e.g., 50 Hz) during very deep voltage sags, is explored with its theory, analyzed and a novel stability solution based on PLL frequency is proposed; and both are verified with power system simulations and by experiments on a grid-connected converter setup.

The advantages, such as the mature control method, less volume of the converter and generator, make the doubly fed induction generator system prevalent in the wind power industry. Nevertheless, the doubly fed induction generator is more susceptible to grid faults and disturbances. The instantaneous high voltage fault may occur due to the excessive local reactive power after the doubly fed induction generator system achieving low voltage ride through, and then the wind turbine might be disconnected again. Hence, not only the low voltage ride through but also high voltage ride through capability should be required for the doubly fed induction generator system to meet the grid code requirements. A comprehensive review of the state of the art low voltage ride through and high voltage ride through technologies for the doubly fed induction generator system is presented. Firstly, different types of common low voltage ride through and high voltage ride through techniques are classified according to their features, i.e. auxiliary hardware, linear or nonlinear control strategies etc. The pros and cons of different low voltage ride through and high voltage ride through techniques are given. Furthermore, the latest developments of low voltage ride through and high voltage ride through technologies are introduced. Finally, the future trends of both the low voltage ride through and high voltage ride through technologies are discussed.

Transient virtual inertia optimization strategy for virtual synchronous generator based on equilibrium point state assessment

This paper examines a completely novel method for improving the transient stability of multi virtual synchronous generator (VSG) grids. In this method, it has been tried to control the oscillation of VSGs with respect to the center of inertia frequency (COI-frequency) of the grid, during short-circuit. It is also well demonstrated here that the proposed method is able to prevent the increase in irreversible distance angle of VSGs relative to each other, which is the main factor in preventing VSGs reaching to instability point. The method can enhance both the synchronizing and damping terms efficiently. Detailed analyzing of the direct Lyapunov method illustrates the non-linear proof of the method. Also, non-linear algebraic equations for a two-VSG system have been derived to prove the possibility of corroborating simulation results. Equal area criterion is also used to clarify this method further. One interesting feature of the method is that there is no conflict between the requirements of the low voltage ride-through and transient stability improvement.

Currently, compression ignition engine technology is becoming less competitive compared to spark ignition engine technology due to emission standards. Lean combustion strategies are preferable for spark ignition engines, but they require enhanced ignition strategies compared to current stoichiometric spark ignition engines. In this paper, we lay physical foundations for such enhanced ignition strategies that can support lean combustion strategies in spark ignition engines. In this theoretical study, we conduct a preliminary foray into the possibility of controlling the species composition of a cold-plasma kernel as a precursor for an ignition process. We consider a multi-pulse electric source system coupled to a DBD (Dielectric Barrier Discharge) electrode immersed in a dry air medium. The coupled system is designed such that the medium in the electrodes’ gap is subjected to a pulse structure which is composed of a peak voltage followed by a secondary low-voltage pulse. Use is made of a comprehensive mathematical model that includes the electric field, potential and current, with the relevant conservation equations for 24 species and 168 kinetic reactions, while considering different energy modes (rotational, vibrational, electronic excitation, dissociation, and ionization) within the electrodes’ gap. The model was validated against independent published results, and was used to understand the mechanism of energy conversion to species that play a central role in terms of the ultimate ignition dynamics. Various pulse repetition frequencies and different pulse constructions were considered. Special attention was given to the amount of deposited energy, and to the energy channeled to known ignition supportive modes and species. The present results show that the energy deposition can be divided into two main stages which are characterized by high and low voltage levels, respectively. We propose that for more successful ignition, the amount of energy deposition during the second (low voltage) stage should be higher than that during the first (high voltage) stage. During the second stage, the deposition of energy into specific modes can be fine-tuned by setting appropriate voltage levels. Based on these findings, we demonstrate how control of the sequence of voltage pulses can further increase enhancement of ignition and combustion promoting processes.

This study constructs a novel virtual synchronous generator system based on a transfer function, and optimizes the parameters of the model by using the improved whale algorithm to improve the frequency control ability of virtual synchronous generator. Virtual synchronous generator technology helps to solve the problem that the integration of large-scale renewable energy generation into the power system leads to the deterioration of system frequency stability. It can maintain the symmetry of grid-connected scale and system stability. The virtual synchronous generator technology makes the inverter to have the inertia and damping characteristics of a synchronous generator. The inverter has the inertia characteristics and damps to reduce the frequency instability of high penetration renewable energy power system. The improved whale algorithm is efficient to find the best combination of control parameters and the effectiveness of the algorithm is verified by microgrid and power system. The results show that the proposed frequency coordination control scheme suppresses the frequency deviation of power system and keep the system frequency in a reasonable range.

Aiming at the difference between conventional power electronic interface converter and traditional synchronous generator in microgrid, it cannot provide inertial support and damping effect for microgrid. In this paper, a converter based on virtual synchronous generator technology is used to design the power outer loop and the electromagnetic equation, so that the power electronic converter has some external characteristics of the synchronous generator while providing some damping and inertia for the microgrid, It is clarified that sag control is essentially a kind of virtual synchronous generator control. Finally, the feasibility and correctness of the method are verified by MATLAB simulation.

Resistive switching (RS) devices based on Nb:SrTiO3 (NSTO) hold significant potential for next-generation data storage due to their tunable resistance, stability, and low power consumption. The majority of existing research studies have predominantly focused on metal/NSTO structures that utilize single-crystal NSTO. Although this configuration exhibits superior performance, it is costly and cannot be easily integrated. Another important point is that epitaxial structures can enhance device stability because of their excellent interfaces. However, epitaxial growth of metals on NSTO poses significant challenges due to inherently high structural mismatches, which has led to inconsistent results reported by various researchers employing the same metal/NSTO structure. In this study, NSTO films with different Nb doping amounts were prepared on YBa2Cu3O[Formula: see text][Formula: see text] (YBCO) layers to form NSTO/YBCO heterostructures. YBCO serves as both the electrode and the epitaxial heterolayer material for NSTO. The photogenerated voltage and RS properties of epitaxially grown NSTO/YBCO structures were investigated in detail. At a low Nb doping amount, NSTO cannot produce sufficient photogenerated carriers, while excessive Nb doping results in a low interface barrier height, causing the photogenerated open-circuit voltage of the NSTO/YBCO structure to initially increase and then decrease with the increase in Nb doping amount (the peak value is observed at a Nb doping concentration of 1%). On the other hand, the electrical conductivity of NSTO increases with Nb doping, leading to an increase in the photogenerated short-circuit current. Further, the RS characteristics of the NSTO/YBCO structure were investigated, and clockwise and counterclockwise RS behaviors under low and high bias voltages were observed, respectively. At low voltages, the clockwise RS effect is primarily attributed to the increased interfacial barrier resulting from the migration of oxygen vacancies in YBCO. Conversely, at high voltages, the migration of oxygen ions within NSTO leads to a reduction in the interfacial barrier, thereby inducing a counterclockwise RS effect. In addition, at low bias voltages, the resistance ratio between the high-resistance state and the low-resistance state under [Formula: see text] 1/[Formula: see text] 2 V bias voltages first increases and then decreases as the Nb doping amount increases, with the highest resistance ratio (>100) observed at 1% Nb doping. These findings provide novel insights into the RS mechanism, and it is envisaged that they will promote the application of conductive oxides in RS devices and the development of multilevel memory devices.

A large number of distributed generations (DGs) connected to the distribution network have a significant impact on the stability of the power system due to their low inertia. Meanwhile the DGs are generally linked to the end of the distribution network, where power quality problems are very common. This paper applies virtual synchronous generator (VSG) technology in three-phase four-leg inverter. A strategy of harmonics compensation is utilized to reduce the harmonic pollution caused by unbalanced and nonlinear load. A parameter selection method based on the dynamic response characteristics of VSG control is proposed. The inner current control loop is used so that the inverter output current could track the reference value accurately. The validity and correctness of the control strategy are proved by the simulation results.

A closed form equation is derived that describes the stable manifold for each of the n-1 type-1 unstable equilibrium points (UEPs). These stable manifolds can be obtained without having to compute the postfault stable equilibrium points for each of the stable manifolds. The equation describing the stable manifold for each UEP can be written knowing only the postfault stable equilibrium point. The n-1 stable manifolds for the UEPs constrain the power across the cutset of branches that encircle each of the n-1 generators that are not assumed to be the swing machine. For a point to lie on the stable manifolds of a specific UEP, the sum of power flows across all branches in the cutset that surround the generator that corresponds to a specific UEP must equal the sum of the power flows across this cutset at the postfault equilibrium point. These results are intuitive since the equal-area criterion could be used to characterize the region of stability for a single-machine infinite-bus system in the exact same manner. >

Since the wide application of virtual synchronous generators (VSGs), the power grid faces great challenges in the safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability. During transient period, for example, a fault occurs in the transmission line, the VSG may lose the transient angle stability and provoke the current hard limit. Even if the fault is cleared by tripping of line, it still faces the problem of instability and voltage dips. To address this problem, in this paper, the post-fault large-signal model of VSG is derived first via the travelling waves based fault information acquisition. Subsequently, with the effect of both active and reactive power loops taken into account, a two-stage simultaneous control scheme is proposed for improving the transient stability of VSG, while considering the current limitation during fault state and voltage support after fault clearance. This method is fulfilled by mode switching and an additional feedback control based on the fault signal. Finally, the effectiveness of the proposed method under both symmetrical and asymmetrical faults is verified. Moreover, the application of the proposed method in a multiple VSGs system is also verified. Besides, the robustness to parameter mismatch and the feasible operating region for the method are discussed.