System Identification I

In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for use in wider interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the expected behaviour of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e. angle stability, fault ride-through (FRT) capabilities, and transition from islanded to grid connected mode are discussed. Perspectives on challenges and future trends are lastly shared.

/pdf/grid-forming-converters-control-approaches-grid-dwbqar458r.pdf

Grid-Forming Converters: Control Approaches, Grid-Synchronization, and Future Trends—A Review

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.

/pdf/transient-stability-of-voltage-source-converters-with-grid-1s0m24oerw.pdf

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

This paper surveys current literature on modeling methods, control techniques, protection schemes, applications, and real-world implementations pertaining to grid forming inverters (GFMIs). Electric power systems are increasingly being augmented with inverter-based resources (IBRs). While having a growing share of IBRs, conventional synchronous generator-based voltage and frequency control mechanisms are still prevalent in the power industry. Therefore, IBRs are experiencing a growing demand for mimicking the behavior of synchronous generators, which is not possible with conventional grid following inverters (GFLIs). As a solution, the concept of GFMIs is currently emerging, which is drawing increased attention from academia and the industry. This paper presents a comprehensive review of GFMIs covering recent advancements in control technologies, fault ride-through capabilities, stability enhancement measures, and practical implementations. Moreover, the challenges in adding GFMIs into existing power systems, including a seamless transition from grid-connected mode to the standalone mode and vice versa, are also discussed in detail. Recently commissioned projects in Australia, the UK, and the US are taken as examples to highlight the trend in the power industry in adding GFMIs to address issues related to weak grid scenarios. Research directions in terms of voltage control, frequency control, system strength improvement, and regulatory framework are also discussed. This paper serves as a resource for researchers and power system engineers exploring solutions to the emerging problems with high penetration of IBRs, focusing on GFMIs.

/pdf/grid-forming-inverter-modeling-control-and-applications-2a4eb2edb1.pdf

Grid Forming Inverter Modeling, Control, and Applications

The virtual synchronous generator (VSG) was proposed to emulate a synchronous machine's dynamics when integrating power electronic converter-based distributed energy resources to the power grid. However, the VSG's synchronization stability during grid faults is not fully explored. The underlying mechanism of loss of synchronization still needs to be further revealed due to VSG's nonlinear characteristics. In this article, a step-by-step analytical method based on combining the linear and nonlinear models is proposed to analyze VSG's dynamic behaviors during a large disturbance. The relationship between the linear and nonlinear models is first brought to light, showing that the linear model is suitable for qualitative analysis to give an intuitive physical insight. Simultaneously, the latter is adopted for quantitative analysis to assess stability after a grid fault. Moreover, to avoid the conflict of the synchronization stability and the inertia response, a transient damping method is added in the active power control loop, which can simultaneously improve the synchronization stability and frequency stability. Design guidelines are also proposed to identify the parameter of the transient damping with different inertia requirements. Finally, the experimental results verify the analytical method and the theoretical analysis.

Transient Damping Method for Improving the Synchronization Stability of Virtual Synchronous Generators

This paper addresses robust design of the active-power and dc-link control loops of power-synchronization control. Robustness is obtained by analytic gain selections, which give large enough stability margins. The proposed design allows robust stability irrespective of the grid strength and of the operating point, the latter with one exception. The proposed design is compared to design based on the principle virtual synchronous machine. Experiments show that the time-domain results correlate well with the frequency-domain results.

/pdf/robust-analytic-design-of-power-synchronization-control-32xla8zpgh.pdf

Robust Analytic Design of Power-Synchronization Control

In this letter, an enhancement of power-synchronization control is proposed, whereby pole–zero cancellation in the closed-loop system is achieved. An effect thereof is that step-response ringing and overshoot are eliminated. For strong grids, the closed-loop bandwidth increases, allowing a shorter step-response rise time.

/pdf/reference-feedforward-power-synchronization-control-3c5ixfngo8.pdf

Reference-Feedforward Power-Synchronization Control

This article presents a real-time identification method for LCL filters used with three-phase grid converters The method can be applied to identify both the inductance and capacitance values of the filter and the series resistance seen by the converter As a side-product, an estimate of the grid inductance seen from the point of connection is also obtained A wideband excitation signal is added to the converter voltage reference During the excitation, converter current and converter voltage reference samples are used for identification The samples are preprocessed in real time by removing dc biases and significant grid-frequency harmonics Parameters of two discrete-time models are estimated at each sampling instant with a recursive estimation algorithm Depending on the estimated model, the model parameter estimates are translated to either the resistance or the inductance and capacitance values of the system The method can be embedded to a control system of pulsewidth-modulation-based converters in a plug-in manner Only the dc-link voltage and converter currents need to be measured Simulation and experimental results are presented for a 125-kVA grid converter system to evaluate the proposed method

/pdf/real-time-identification-of-lcl-filters-employed-with-grid-3u5u848ctz.pdf

Real-Time Identification of LCL Filters Employed With Grid Converters

This article deals with discrete-time state-space current controllers for three-phase grid converters equipped with an LCL filter. The integral action in the controller can be implemented either using an integrator or a disturbance observer. The results show that the disturbance-observer-based and integrator-based controllers become mathematically equal if the feedforward gains are selected to be equal, the feedforward zero is placed to cancel the pole originating from the integral action, and the closed-loop poles are placed identically. The equivalent performance in both designs is verified by means of analyses and experiments. The equivalence is also shown for double-frequency current controllers.

/pdf/equivalence-of-the-integrator-based-and-disturbance-observer-53763l4v19.pdf

Equivalence of the Integrator-Based and Disturbance-Observer-Based State-Space Current Controllers for Grid Converters

This paper deals with state-feedback current control for power converters, which are equipped with an LCL filter and connected to a weak grid. The grid-side current is measured and other states needed by the current controller are estimated using a reduced-order observer. The control system is designed directly in the discrete-time domain. The gains of the control system are calculated using direct pole placement, assuming a strong grid. Recommendations for the nominal pole locations are given. The results show that the control system is robust against the unknown grid impedance, ranging from strong to very-weak grid conditions. The proposed design is validated by means of experiments.

/pdf/observer-based-current-control-for-converters-with-an-lcl-2j3sss2pkw.pdf

F. M. Mahafugur Rahman

Papers

Robust Analytic Design of Power-Synchronization Control

Reference-Feedforward Power-Synchronization Control

Real-Time Identification of LCL Filters Employed With Grid Converters

Equivalence of the Integrator-Based and Disturbance-Observer-Based State-Space Current Controllers for Grid Converters

Observer-Based Current Control for Converters with an LCL Filter: Robust Design for Weak Grids