The influence of grid-forming loads on transient stability

With the development of science and technology, the cost of offshore wind power development and transmission has gradually decreased, making offshore wind power technology a research hotspot. The installed capacity has been increasing year by year. China has abundant offshore wind energy resources, which are much richer than those on land and are available for exploitation. Offshore wind speeds are stable and do not produce harmful substances. Therefore, studying the transient voltage stability of offshore wind power grid-connected systems to improve the utilization efficiency of offshore wind power and protect the ecological environment is critical. After large-scale wind power is connected to the grid, ensuring the stability of the system's transient voltage becomes crucial. This paper aims to deeply study the transient voltage stability of offshore wind farm grid-connected systems, providing theoretical support for the subsequent design and operation of offshore wind farms. A simulation model of a typical power system integrated with offshore wind farms was established using PSAT. Relevant transient voltage stability indices were calculated, and the impact of varying wind speeds on the transient voltage stability of the grid-connected system was investigated. Static Var Compensators (SVC) and Static Synchronous Compensators (STATCOM) were installed for comparative analysis. The results demonstrate that excessively low wind speeds degrade the system's transient stability, and the installation of SVC provides superior improvement in transient voltage stability compared to STATCOM.

The power transient stability analysis is one of the basis for determining the control strategy of power system security and stability. Considering the influence of power grid topology on the transient stability of power system, the transient stability evaluation model is constructed based on the graph attention neural network. The electrical components and their transient operation data are mapped to the graph data with the spatial topology characteristics of power system for model training, so as to improve the topological generalization performance of the model. The marginal contribution of input characteristics to the output of transient power angle stability evaluation model is quantitatively calculated based on Shapley additive explanation (SHAP), so as to improve the interpretability of data-driven method for transient power angle stability evaluation. The effectiveness of the proposed method is verified by the IEEE 39-bus system.

Transient voltage stability assessment of renewable energy grid based on residual SDE-Net

Stability of wind power is the long-concerned issue especially in the context of high grid-integration level. Different from traditional synchronous generators, the transient synchronization stability of the wind turbine is maintained by the phase-locked loop after being disturbed. This paper mainly discusses the transient stability of the doubly-fed wind turbine. Firstly, according to the principle of doubly-fed wind turbines connected to grid, a doubly-fed wind turbine model under the time scale of transient stability analysis is established. Then, by theoretically analyzing the dynamic process after being disturbed, the acceleration and deceleration area of phase-locked loop analogous to the synchronous generator is proposed. The critical stability region of the system is analyzed, and a calculation method for the transient synchronization stability margin of doubly-fed wind turbines is proposed. Finally, a detailed model of four-machine two-zone system is built on MATLAB/Simulink software, and the correctness of the conclusion is verified by analyzing the influence of key parameters on transient stability.

This paper investigates transient angle stability of parallel system of two virtual synchronous generators (VSGs) and hybrid system comprised by VSG and diesel generator (DG). Compared to single grid-connected VSG, the paralleled VSGs system and hybrid system are facing larger challenges on stability aspect during serious faults. Although there exist several publications on transient stability, the transient instability mechanism is rather ambiguous, especially for paralleled systems. Besides, swing equations of paralleled VSGs system and paralleled DG-VSG system are established to discuss the dynamic characteristics of the system during short-circuit fault. The increase of power angle between DG and VSG is caused by the different angular accelerations, which deteriorates the transient angle stability of paralleled DG-VSG system. Thus, paralleled DG-VSG system is more prone to instability than paralleled VSGs system, which can be illustrated by power angle (P-δ) characteristic curves. Finally, simulation results validate the correctness of theoretical analyses.

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.

Chapter 18 - Power System Stability

Transient stability analysis of a grid-connected type-4 wind turbine with grid-forming control during the fault

The integration of large-scale new energy (NE) into the grid has brought great problems to the transient stability of the power grid. Therefore, the transient angle stability improvement strategy of the power grid based on a grid-forming energy storage system (GFM-ESS) is proposed in this paper. First, the mechanism of GFM-ESS to improve the power grid’s transient stability is examined from the standpoint of regional power balancing in the power grid. Secondly, a hierarchical control strategy for GFM-ESS is proposed, including the control modules at the power station and converter level. The inertial control link is used to create the power station level control model, and the converter level control model takes into account the SOC constraints of the GFM-ESS. Finally, the 36-node (CEPRI-36) system of the China Electric Power Research Institute in PSASP is used for simulation verification. The simulation results show that the control strategy proposed in this paper is effective.

This paper investigates the influence of a high voltage direct current (HVDC) overlay grid on the transient voltage and rotor angle stability with respect to HVDC converter fault ride-through behavior. For this purpose, the studied meshed hybrid AC/DC transmission grid is established in MATLAB/Simulink with user defined dynamic models of voltage-source converters (VSC) considering fault ride-through requirements of the German Technical Connection Rules for extra high-voltage (VDE-AR-N 4130). The voltage profile during three-phase short-circuit faults as well as bus voltage relative entropy after fault-clearing as stability indices are analyzed. The influence of k-factor and power system stabilisers on transient stability is also investigated.

This article investigates the phase-locked loop (PLL) synchronization transient stability (PLL-STS) issue of grid-tied multiple voltage source converters (MVSCs) systems, specifically focusing on transient stability evaluation and transient interaction analysis. First, a novel PLL-STS assessment method for MVSCs systems has been proposed, which considers angular frequency jumps, nonlinear damping, and transient interactions among VSCs. Compared with previous studies, this method can accurately evaluate PLL-STS of MVSCs system since the acceleration and deceleration areas of VSCs can be accurately determined by combining equal-area criterion and numerical integration method. Moreover, it can identify the dominant converters which lead to transient instability. Second, by introducing static and dynamic interaction coefficients, influence of static and dynamic interaction on the transient stability of individual converters is quantitatively analyzed. Finally, the theoretical analysis results are verified by PSCAD/ EMTDC time-domain simulation results and RT-LAB hardware-in-the-loop experimental results.

Nowadays, integration of large-scale wind farms (WFs) into power systems is experiencing rapid growth. As this rapid integration can affect transient stability significantly, employing doubly fed induction generator (DFIG)-based wind turbines, which have shown better behavior regarding system stability, has attracted much attention. This research contributes to the literature by investigating the transient stability of the power system with increasing penetration of DFIG-based WFs. In the proposed framework, the current-balance form is utilized for the network equations, and in this way, transient stability is performed using time-domain simulation. According to the simulation results, when the rate of wind power generation exceeds 0.7 per-unit, the increasing trend of the critical clearing time (CCT) is reversed and the CCT decreases greatly with the increased wind power penetration. In addition, the reactive power compensation by DFIG, the gearbox ratio, the power system strength, and DFIG parameters are comprehensively investigated as effective parameters on transient stability. Since the rated rotor speed of DFIG significantly impacts the electrical torque and machine currents, the reduction of the rated rotor speed due to the change of the gearbox ratio has been investigated as one of the effective factors to improve the transient stability. The simulation results demonstrate the effectiveness of the proposed approach in improving power system transient stability.

Transient stability, which is an important aspect in maintaining power system operation, was the focus of this research. Critical clearing time calculation, popular for assesing the transient stability by observing the generator rotor angle performance, was utilized as the assesment method. Wind turbine effect in transient stability is added as the specific problem representing the current condition in Indonesian power system. Three study cases were observed, namely observing the effect of wind farm integration, observing the effect of thermal generator loading, and observing the effect of wind farm penetration level on transient stability. The research is conducted in modified IEEE 9 Bus system with wind turbine modeled as doubly fed induction generator using DIgSILENT 15.1 environment. The result showed that the effect o wind farm integration and the greater the loading of a thermal generator in the system worsen the transient stability performance. Increasing the penetration level of the wind farm in the system also worsen the system transient stability even if the energy cost is zero.

Affected by the current limitation, grid-forming inverters (GFMs) will transition to a current source under grid faults, which makes the inverter-based microgrid lose the voltage support and then be switched to a current source interconnection microgrid (CSIM). It may deteriorate the system's transient synchronization stability (TSS). This issue has not been studied previously. To this end, this article proposes an analysis and enhancement method of TSS of CSIM. First, a large signal synchronization model of CSIM is derived, which takes into account the effect of the current limitation strategy of GFMs and the dynamic interactions between GFMs and grid-following inverters (GFLs). Second, by constructing the Lyapunov energy function, the criterion of TSS is obtained and the impacts of parameters on stability are investigated. Based on the stability criterion, the feasible region of current references of GFLs is clearly described. Therefore, a dynamic current control method of GFLs is proposed. It can well address the absence of equilibrium points (EPs) and strengthen the transient stability. Meanwhile, a novel antiwindup strategy of GFMs considering synchronization stability constraint is proposed, which can contribute to the fault recovery of GFMs. Finally, simulation and experimental results verify the effectiveness of the proposed methods.

Braking resistor is a very powerful tool to improve transient stability in a power system. Usually, a fixed value of the braking resistor is considered for the transient stability analysis. However, when the braking resistor is in operation, temperature of the resistor material rises above ambient temperature which ultimately causes the resistance value to increase. This paper analyzes the effect of the temperature rise of the fuzzy logic-controlled braking resistor on the transient stability in a multimachine power system. The performance of the braking resistor scheme with fuzzy controller is compared to that of with conventional proportional-integral-derivative (PID) controller. Simulation results of both balanced and unbalanced faults at different points in the system indicate that the temperature rise of the fuzzy logic-controlled braking resistors has little or almost no effect on the transient stability of the multimachine power system. Moreover, it is found that the performance of fuzzy logic controller is better than that of conventional PID controller. Thus, the proposed fuzzy control strategy provides a simple and effective method of transient stability enhancement.