Grid-connected converters exposed to weak grid conditions and severe fault events are at risk of losing synchronism with the external grid and neighboring converters. This predicament has led to a growing interest in analyzing the synchronization mechanism and developing models and tools for predicting the transient stability of grid-connected converters. This paper presents a thorough review of the developed methods that describe the phenomena of synchronization instability of grid-connected converters under severe symmetrical grid faults. These methods are compared where the advantages and disadvantages of each method are carefully mapped. The analytical derivations and a detailed simulation model are verified through experimental tests of three case studies. Steady-state and quasi-static analysis can determine whether a given fault condition results in a stable or unstable operating point. However, without considering the dynamics of the synchronization unit, transient stability cannot be guaranteed. By comparing the synchronization unit to a synchronous machine, the damping of the phase-locked loop is identified. For accurate stability assessment, either nonlinear phase portraits or time-domain simulations must be performed. Until this point, no direct stability assessment method is available which consider the damping effect of the synchronization unit. Therefore, additional work is needed on this field in future research.

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An Overview of Assessment Methods for Synchronization Stability of Grid-Connected Converters Under Severe Symmetrical Grid Faults

With the increasing use of voltage source converters (VSCs) in power electronics dominated power systems, oscillation phenomena in DC-link voltage control (DVC) timescale (around 10 Hz) among multiple VSCs have occurred. Several studies have tried to analyze these oscillation problems, but all associated with the single VSC situation. To consider the dynamic interactions between VSCs in DVC timescale, especially in the weak grid condition, this paper presents a small-signal model to understand VSC external characteristics based on motion equation concept also featured in synchronous generator (SG). Comparisons of time-domain simulation responses and eigenvalues show that the proposed model can hold the main behaviors of concern. The form of the model is very similar to the rotor motion equation in SG, with which power engineers have been very familiar. In addition, by establishing the relationship between the unbalanced powers and state variables of internal voltage (viz., VSC output voltage), the modeling idea introduced in this paper can be applied to other power electronic devices.

Modeling of Grid-Connected VSCs for Power System Small-Signal Stability Analysis in DC-Link Voltage Control Timescale

The utilization of power electronic inverters in power grids has increased tremendously, along with advancements in renewable energy sources. The usage of power electronic inverters results in the decoupling of sources from loads, leading to a decrease in the inertia of power systems. This decrease results in a high rate of change of frequency and frequency deviations under power imbalance that substantially affect the frequency stability of the system. This study focuses on the requirements of inertia and the corresponding issues that challenge the various country grid operators during the large-scale integration of renewable energy sources. This study reviews the various control techniques and technologies that offset a decrease in inertia and discusses the inertia emulation control techniques available for inverters, wind turbines, photovoltaic systems, and microgrid. This study attempts to explore future research directions and may assist researchers in choosing an appropriate topology, depending on requirements.

Future low-inertia power systems: Requirements, issues, and solutions - A review

This paper investigates the impact of power grid strength and phase-locked loop (PLL) parameters on small signal stability of grid-connected doubly fed induction generator (DFIG)-based wind farm. Modal analysis of the grid-connected DFIG wind turbine under different operating conditions and various power grid strengths are investigated at first. Modal analysis results reveal that the DFIG connected to a weak grid may easily lose stability under the heavy-duty operating conditions due to PLL oscillation. The object of this paper is to identify the PLL oscillation mechanism as well as influence factors and propose a damping solution for this oscillation mode. A simplified linear system model of the grid-connected DFIG wind turbine is proposed for analyzing the PLL oscillation. Through the complex torque coefficients method and using this model, the oscillation mechanism and influence factors including the power grid strength and the PLL parameters are identified. To suppress this PLL oscillation, a mixed $H_2/H_{\infty }$ robust damping controller is proposed and designed for the DFIG. Electromagnetic transient simulation results of both single-DFIG system and multiply-DFIG system verify the correctness of the analysis results and effectiveness of the proposed damping controller.

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Impact of Power Grid Strength and PLL Parameters on Stability of Grid-Connected DFIG Wind Farm

Similar large signal synchronizing instability that is common in traditional power system also exists in voltage source converter (VSC) dominated power system, which is increasingly reported and investigated. In this paper, the large signal instability of phase locked loop (PLL) synchronized VSC connected to weak ac grid is investigated. First, the influence of high grid impedance on PLL's dynamics is explored and an additional feedback loop is found to be introduced, which deteriorates the performance of PLL. Then an analysis model that is similar as the rotor motion model of synchronous generator (SG) is developed. Based on the developed model, equal-area method is employed to carry out the large-signal stability analysis. The large signal instability is found to be mainly resulted by the following two factors. One is the nonexistence of equilibrium point, which is similar as that there does not exist intersection between the mechanical power input curve and electromagnetic restoring force curve in SG grid connected system. The other is related with the transition process resulted by insufficient decelerating area, which leads to that the system cannot stably go through the transition from initial point to the existing equilibrium point. The proposed analysis not only contributes to revealing the physical mechanism but also provides guidance for the future improvement measures.

Large Signal Synchronizing Instability of PLL-Based VSC Connected to Weak AC Grid

Instability phenomenon of type-4 wind turbine during grid faults is increasingly concerned by researchers and may result in failure of low-voltage ride through (LVRT), especially when connected to a high-impedance ac grid In order to carry out the stability analysis for type-4 wind turbines during deep voltage sag, dynamic model, which reflects the interaction between phase locked loop (PLL) and alternating current control (ACC), is first developed Eigenvalues and modal analysis show that there is a risk that a pair of poorly damped poles may become unstable, which is mainly dominated by PLL as well as the interaction between PLL and ACC The mechanism of small-signal instability during LVRT is identified as the insufficiency of damping Complex torque coefficient approach is further proposed to study the characteristic of the damping component, which is composed of the inherent one determined by PLL and the additional influenced by ACC Finally, simulated results are presented to verify the analytical results

Small Signal Instability of PLL-Synchronized Type-4 Wind Turbines Connected to High-Impedance AC Grid During LVRT

A model for Type 3 wind turbines (WTs) with typical d f /dt inertia control is developed for studying frequency dynamics in power systems. A simplified small-signal Type 3 WT model with d f /dt control is first constructed based on the mass-spring-damping concept, such that the physical properties and frequency response of a Type 3 WT can be clearly understood, besides the frequency-domain expressions of the available inertia and the corresponding damping coefficient can be directly derived. The manifested inertia is apparently controllable and frequency-dependent, but differs from the constant inertia featured in a conventional synchronous generator (SG). Furthermore, the frequency response model of a generic two-machine system, composed of an SG and an aggregate Type 3 WTs, is established. The model synthetically considers the effects of the WTs’ different controller parameters, operating points, and the SG's governor response on system frequency characteristics. Then, time-domain simulations on the studied two-machine system are performed in MATLAB/Simulink. The simulated results verify that the proposed model is effective for analyzing system frequency dynamics, and that the test system frequency characteristics can be improved based on tuning the mass-spring- damping coefficients of Type 3 WTs.

Modeling of Type 3 Wind Turbines With df/dt Inertia Control for System Frequency Response Study

Instability issues of the grid-connected doubly fed induction generator (DFIG) based wind turbines (WTs) during low-voltage ride-through (LVRT) have got little attention yet. In this paper, the small-signal behavior of DFIG WTs attached to weak ac grid with high impedances during the period of LVRT is investigated, with special attention paid to the rotor-side converter. First, based on the studied LVRT strategy, the influence of the high-impedance grid is summarized as the interaction between phase-looked loop (PLL) and rotor current controller (RCC). As modal analysis result indicates that the underdamped poles are dominated by PLL, complex torque coefficient method, which is conventionally applied in power system to study the interaction between mechanical and electrical subsystems of synchronous generator, is generalized to analyze how the PLL-RCC interaction influence the phase motion of PLL. Then, the concerned small-signal stability of the PLL-synchronized DFIG system can be discerned by the developed complex phase coefficients. Impacts of PLL's and RCC's parameters are highlighted, as well as the system's operating conditions during LVRT. Finally, the analytical result is validated by experiments.

Small Signal Dynamics of DFIG-Based Wind Turbines During Riding Through Symmetrical Faults in Weak AC Grid

With the continuous development of society and under the background of sustainable development and resource conservation, the proportion of renewable energy in the global energy structure is increasing. At the same time, wind power has been widely used in many regions of the world because wind power technology is more advanced and mature than other renewable energy sources. In addition, with a large number of wind turbines connected to the grid, it not only helps automatic generation control (AGC) of power systems but also brings new challenges and difficulties. In this study, a multi-source cooperative control model of wind power participating in AGC frequency regulation is established to solve the dynamic problem of power distribution from real-time total power command to different AGC units. This study presents an optimal AGC-coordinated control method based on the multi-objective mayfly optimization (MMO) algorithm, which makes the fitting degree of power command output and actual output curve high and the adjustment mileage payment minimum, so as to achieve the best AGC performance. Finally, the simulation results show that this method can effectively decrease the total power deviation and adjustment mileage payment in the multi-source-coordinated control of AGC.

https://www.frontiersin.org/articles/10.3389/fenrg.2022.848966/pdf

Multi-Objective Mayfly Optimization-Based Frequency Regulation for Power Grid With Wind Energy Penetration

The stator transient DC flux component emerges in the Double Fed Induction Generator (DFIG) following grid voltage dip, which results in an important influence on the controllability of rotor converter. In this paper, the research focuses on the responding time of the dynamic reactive current injection referring to DFIG with the crowbar protection under Zero Voltage Ride-Through (ZVRT). Firstly, theoretical analysis about the shortest quitting time of crowbar is carried out. Then, combined with crowbar protection and demagnetization control, ZVRT of DFIG is achieved as well as the fast injection of dynamic reactive current. Finally, the simulation results are presented to verify the theoretical analysis and the proposed control strategy. It is helpful to enhance fault ride through capability of DFIG.

Yongning Chi

Papers

Small Signal Instability of PLL-Synchronized Type-4 Wind Turbines Connected to High-Impedance AC Grid During LVRT

Modeling of Type 3 Wind Turbines With df/dt Inertia Control for System Frequency Response Study

Small Signal Dynamics of DFIG-Based Wind Turbines During Riding Through Symmetrical Faults in Weak AC Grid

Multi-Objective Mayfly Optimization-Based Frequency Regulation for Power Grid With Wind Energy Penetration

Analysis and control of doubly fed induction generator for zero voltage ride through