The electric power system is currently undergoing a period of unprecedented changes. Environmental and sustainability concerns lead to replacement of a significant share of conventional fossil fuel-based power plants with renewable energy resources. This transition involves the major challenge of substituting synchronous machines and their well-known dynamics and controllers with power electronics-interfaced generation whose regulation and interaction with the rest of the system is yet to be fully understood. In this article, we review the challenges of such low-inertia power systems, and survey the solutions that have been put forward thus far. We strive to concisely summarize the laid-out scientific foundations as well as the practical experiences of industrial and academic demonstration projects. We touch upon the topics of power system stability, modeling, and control, and we particularly focus on the role of frequency, inertia, as well as control of power converters and from the demand-side.

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Foundations and Challenges of Low-Inertia Systems (Invited Paper)

Driven by the large-scale integration of distributed power resources, grid-connected voltage-source converters (VSCs) are increasingly required to operate as grid-forming units to regulate the system voltage/frequency and emulate the inertia. While various grid-forming control schemes have been reported, their transient behaviors under large-signal disturbances are still not fully explored. This article addresses this issue by presenting a design-oriented transient stability analysis of the grid-forming VSCs. First, four typical grid-forming control schemes, namely, the power-synchronization control (PSC), the basic droop control, the droop control with low-pass filters (LPFs), and the virtual synchronous generator (VSG) control, are systematically reviewed, whose dynamics are characterized by a general large-signal model. Based on this model, a comparative analysis on the transient stabilities of different control schemes is then carried out. It reveals that the PSC and the basic droop control can retain a stable operation as long as there are equilibrium points, due to their noninertial transient responses, while the droop control with LPFs and the VSG control can be destabilized even if the equilibrium points exist, due to the lack of damping on their inertial transient responses. With the phase portrait, the underlying stability mechanism is explicitly elaborated, and the quantitative impacts of the controller gains and the virtual inertia are clearly identified. Subsequently, controller design guidelines are proposed to enhance the system damping as well as the transient stability. Finally, experimental results are provided to verify the theoretical analysis.

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Transient Stability of Voltage-Source Converters With Grid-Forming Control: A Design-Oriented Study

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

In a dc microgrid (DC-MG), the dc bus voltage is vulnerable to power fluctuation derived from the intermittent distributed energy or local loads variation. In this paper, a virtual inertia control strategy for DC-MG through bidirectional grid-connected converters (BGCs) analogized with virtual synchronous machine (VSM) is proposed to enhance the inertia of the DC-MG, and to restrain the dc bus voltage fluctuation. The small-signal model of the BGC system is established, and the small-signal transfer function between the dc bus voltage and the dc output current of the BGC is deduced. The dynamic characteristic of the dc bus voltage with power fluctuation in the DC-MG is analyzed in detail. As a result, the dc output current of the BGC is equivalent to a disturbance, which affects the dynamic response of the dc bus voltage. For this reason, a dc output current feedforward disturbance suppressing method for the BGC is introduced to smooth the dynamic response of the dc bus voltage. By analyzing the control system stability, the appropriate virtual inertia control parameters are selected. Finally, simulations and experiments verified the validity of the proposed control strategy.

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A Virtual Inertia Control Strategy for DC Microgrids Analogized With Virtual Synchronous Machines

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

Voltage problems are challenging in modern power systems with a high penetration of renewables integrated via power electronics. This paper extends the transient time-scale classification to identify voltage dynamics in modern power systems. Voltage dynamics in the current control time-scale is firstly proposed and then the mechanism of terminal voltage change is elaborated. After that the significant influences of the voltage source converter (VSC), current control (CC) loop and voltage feed forward (VFF) on the voltage dynamics in the current control time-scale are discussed. The VSC current control loop provides positive damping on the terminal voltage, while the VFF scheme results in an additional loop that deteriorates terminal voltage dynamic performance and stability. In addition, a sensitivity analysis was carried out to investigate the influence of critical parameters on stability. Finally, simulation results of a current-controlled VSC attached to different strength of AC grids (including a weak grid) are presented to validate the phenomenon and the influencing factors of voltage dynamics in the current control time-scale.

Voltage Dynamics of Current Control Time-Scale in a VSC-Connected Weak Grid

The electromagnetic stability issues of the grid-connected doubly fed induction generator (DFIG) system are usually overlooked This paper presents a reduced order small-signal model that can be used to analyze the stability of DFIGs dc-link voltage control system, especially under weak ac grid conditions This model neglects DFIG flux and fast current control dynamics However, the effects of operating points, grid strengths and control loops interactions on system dynamic performance are taken into account An eigenvalue comparison shows the proposed model holds dominant oscillation mode featured by the detailed model and is suitable for stability analysis of dc-link voltage control system of DFIG Influence coefficients reflecting control loops interactions are also presented Application studies of the proposed model show it is suitable for illustrating the effect of grid strength on dynamic performance of the DFIGs dc-link voltage control system Meanwhile, phase-locked loop (PLL) and rotor-side converter (RSC) active power control (APC)/reactive power controls (RPC) effect on system stability are also explored

Modeling of Grid-Connected DFIG-Based Wind Turbines for DC-Link Voltage Stability Analysis

This paper is to investigate the inertial dynamics of virtual-synchronous-controlled (VSynC) doubly fed induction generator (DFIG)-based wind turbines (WTs). VSynC, different from the conventional synchronization method based on phase-locked-loop (PLL) synchronizing technique, makes DFIG-based WT synchronize with power grid via the active power control (APC), and thus provide the desired inertial support to power grid. Further, an effective approach for describing the inertial dynamics of DFIG-based WT with VSynC is proposed by establishing the WT's electromechanical motion equation. The approach synthetically considers the impacts of the WT's different controller parameters, operating points, and, in particular, the variations of mechanical power caused by the rotational speed or pitch angle changes during the inertial response period. It also makes the essential of DFIG-based WT's inertia clearer, which, as a matter of fact, is controllable and manifests frequency-dependent characteristics, and noticeably differs from the fixed inertia time constant featured in synchronous generator (SG). The impacts of different controller parameters and operating points on single WT's frequency response characteristics are studied. Simulated results also validate the superiority of VSynC on inertial support capability and operation stability to the typical PLL-based vector control (VC), especially for weak grid conditions. Finally, the frequency response on wind power plant (WPP) level is initially explored and further research to improve VSynC is discussed.

On Inertial Dynamics of Virtual-Synchronous-Controlled DFIG-Based Wind Turbines

With the wide application of voltage source converters (VSCs) in power system, dc-bus voltage control instabilities increasingly occurred in practical conditions, especially in weak ac grid, which poses challenges on stability and security of power converters applications. This paper aims to give physical insights into the stability of dc-bus voltage control affected by ac-bus voltage control in VSC connected to weak grid. The concepts of damping and restoring components are developed for dc-bus voltage to describe the stability of dc-bus voltage control. The impact of ac-bus voltage control on dc-bus voltage control stability can be revealed by investigating the impact of ac-bus voltage control on damping and restoring components essentially. Furthermore, the detailed analysis for the impact of ac-bus voltage control on damping and restoring components is presented considering varied ac system strengths, operating points, and ac-bus voltage control parameters. The simulation results from 1.5-MW full-capacity wind power generation system are demonstrated which conform well to the analysis. Finally, the experimental results validate the analysis.

Xiaoming Yuan

Papers

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

Voltage Dynamics of Current Control Time-Scale in a VSC-Connected Weak Grid

Modeling of Grid-Connected DFIG-Based Wind Turbines for DC-Link Voltage Stability Analysis

On Inertial Dynamics of Virtual-Synchronous-Controlled DFIG-Based Wind Turbines

DC-Bus Voltage Control Stability Affected by AC-Bus Voltage Control in VSCs Connected to Weak AC Grids