Grid-following Inverters Research Articles

Optimal Controllers to Improve Transient Recovery of Grid-Following Inverters Connected to Weak Power Grids

Leading to the enormous growth in renewable and power electronics technologies and the global drive towards environmental friendliness and sustainability, a significant number of renewable energy sources are being connected to the power system via inverter-based systems. The inverter-based generations (IBG) have no stored energy and less fault current injection capability compared to the conventional synchronous machines. Consequently, a large penetration of IBG creates challenges to maintaining the stability of the power system, especially the transient stability. The weaker the power system, the higher the significance of instability. Few solutions exist in the literature to improve the fault recovery of IBG connected to weak power systems. This paper considers the method of storing energy in sub-module capacitors of the Modular-Multi-level Converter (MMC) along with temporarily boosting the inverter’s current limit. Conversely, increasing the ratings of the inverter will result in high manufacturing costs. Hence an optimization strategy is proposed in this paper, for obtaining a robust set of inverter control parameters that enhances fault recovery without excessively increasing the manufacturing cost of MMC. A frequency scanning technique supplemented with Generalized Nyquist criteria is incorporated into the optimization methodology to constrain the search space for the optimization algorithm. This enables the optimization algorithm to converge to an acceptable solution with a reasonable computing time. Furthermore, validation of the resultant set of parameters for different system conditions is presented. Finally, IBG with optimized fault recovery controllers is integrated into a simplified real-world power system, and the applicability of the proposed optimized controllers is illustrated.

Flexible Control Strategy for Enhancing Power Injection Capability of Three-Phase Four-Wire Inverter During Asymmetrical Grid Faults

A three-phase four-wire inverter is highly suitable for low-voltage distributed grids owing to its zero sequence control; however, essential aspects of grid-following inverters during faults, such as the power oscillation control flexibility and power injection capability, are rarely discussed in detail. Thus, by introducing zero sequence instantaneous power into a four-wire system, this article proposes a flexible control strategy that enhances the power injection capability of the inverter in the photovoltaic generation system during asymmetrical grid faults. First, a generalized reference current generation with flexible control of positive, negative, zero sequence current (FPNZSC), and power oscillations is presented. Second, a maximum allowable power injection (MAPI) algorithm based on the actual value constraints of power oscillations and peak current is presented according to the requirement. The proposed MAPI-FPNZSC strategy further extends the existing fault ride through methods in three-wire inverter systems, which is also suitable for single line-to-ground and double line-to-ground faults and can be easily realized in the natural or stationary frame. The effectiveness of the proposed strategy is verified by comparing the provided three typical objectives with the traditional method, and tested in a real-time hardware-in-the loop experimental platform.

Power-Synchronized Grid-Following Inverter Without a Phase-Locked Loop

This paper proposes a power-synchronized control strategy for grid-following inverters (GFLIs) to regulate their power exchange with the grid without any need for sensing/regulating the point of connection voltage. Contrary to conventional GFLIs, which rely on phase-locked loops for grid synchronization and have difficulties in weak grid conditions, the proposed strategy is power synchronized and utilizes the inverter terminal voltage for power control leading to its seamless performance in ultra-weak grids. Additionally, since the proposed approach does not require any voltage regulation at the point of connection to the grid, contrary to grid-forming inverters, it can reliably operate in stiff and/or series-compensated grids as well. The proposed approach benefits from a decoupling control structure that is tuned using a loop-shaping method. Compared to conventional GFLIs, this approach does not need any extra hardware; hence, it can easily be retrofitted into the existing large fleet of GFLIs. The performance of the proposed controller is evaluated in Matlab/PLECS for both weak and stiff grids, and the findings are experimentally validated in a scaled-down setup.

Grid Forming Inverter Modeling, Control, and Applications

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.

Identifying DQ-Domain Admittance Models of a 2.3-MVA Commercial Grid-Following Inverter via Frequency-Domain and Time-Domain Data

In this paper, we present two methods to identify the admittance of a 2.3-MVA commercial grid-following inverter through frequency-domain data and time-domain data. In addition to the well-known harmonic injection method to obtain the admittance at frequency points followed by vector fitting, we adopt a step response-based method where the step response data of the converter are collected and their s-domain expressions are obtained. In turn, the s-domain admittance model of the converter is found. The two methods can be used for cross validation. Step responses and frequency-domain responses obtained from the two methods are compared. Results show that the time-domain data-based method leads to an admittance comparable with that from the frequency-domain measurement in the range less than 60 Hz. In addition, two insights are obtained from the frequency-domain admittance measurements. First, when the dq-frame is aligned to the point of common coupling (PCC) voltage, the per unit DQ-domain admittance directly reflects operating conditions. Second, existence of negative-sequence control can be detected via sequence-domain admittances.

Grid-Forming Frequency Shaping Control for Low-Inertia Power Systems

As power systems transit to a state of high renewable penetration, little or no presence of synchronous generators makes the prerequisite of well-regulated frequency for grid-following inverters unrealistic. Thus, there is a trend to resort to grid-forming inverters which set frequency directly. We propose a novel grid-forming frequency shaping control that is able to shape the aggregate system frequency dynamics into a first-order one with the desired steady-state frequency deviation and Rate of Change of Frequency (RoCoF) after a sudden power imbalance. The no overshoot property resulting from the first-order dynamics allows the system frequency to monotonically move towards its new steady-state without experiencing frequency Nadir, which largely improves frequency security. We prove that our grid-forming frequency-shaping control renders the system internally stable under mild assumptions. The performance of the proposed control is verified via numerical simulations on a modified Icelandic Power Network test case.

Stability and bifurcations in low inertia PV rich power networks

The present trend of inserting increasingly more solar photovoltaic (PV) sources into the electricity grid leads to a significant reduction in mechanical inertia. Inertia represents energy reserve in the grid, that inherently and instantaneously supports frequency stability. Therefore, recent studies have re-examined the frequency stability of the grid. Most of these consider scenarios where the participation from grid-following inverters remains relatively low. More importantly, these studies include an infinite bus in their analyses. The infinite bus acts as a very stiff voltage source which has a significantly stabilising influence, the more so as all control refers back to the infinite bus. Here, the frequency stability of a grid with significant generation from grid-tied PV inverters is considered without reference to an infinite bus. In the proposed simplified grid model, all the synchronous generators (SGs) are lumped as one large SG complete with classically operating controls. Similarly, all the PV generators are lumped as one large, quasi-instantaneous, non-linear power source. In this simplified network, the feasible operating region is identified using bifurcation techniques. It transpires that the stable operating region is bounded by a locus of Hopf bifurcations linked (inter-alia) to the fraction of power generated by grid-tied PV inverters.

An Optimal Power Control Strategy for Grid-Following Inverters in a Synchronous Frame

This work proposes a power control strategy based on the linear quadratic regulator with optimal reference tracking (LQR-ORT) for a three-phase inverter-based generator (IBG) using an LCL filter. The use of an LQR-ORT controller increases robustness margins and reduces the quadratic value of the power error and control inputs during transient response. A model in a synchronous reference frame that integrates power sharing and voltage–current (V–I) dynamics is also proposed. This model allows for analyzing closed-loop eigenvalue location and robustness margins. The proposed controller was compared against a classical droop approach using proportional-resonant controllers for the inner loops. Mathematical analysis and hardware-in-the-loop (HIL) experiments under variations in the LCL filter components demonstrate fulfillment of robustness and performance bounds of the LQR-ORT controller. Experimental results demonstrate accuracy of the proposed model and the effectiveness of the LQR-ORT controller in improving transient response, robustness, and power decoupling.

Reduced-order modeling of inverter-based generation using hybrid singular perturbation

This paper contributes to the field of inverter modelling for large-scale simulations by introducing a novel Model-Order Reduction (MOR) method based on singular perturbation. Motivated by the timescale separation between the fast and slow dynamics in an inverter-based power system, the proposed nonlinear MOR concept extends on the existing zero- and first-order reduction methods by combining the low computational burden of the former approach with the higher accuracy of the latter one. As a result, such hybrid MOR technique preserves the slow system dynamics of the full-order model, while simultaneously capturing the impact of the removed fast states on slow variables. Moreover, we introduce several improvements to the existing first-order MOR in order to make it tractable and more efficient when applied to a realistic full-order inverter model. The novel hybrid approach is applied to both grid-forming and grid-following inverter control schemes, and compared against existing reduction methods from the literature. The results showcase a better time-domain performance of the hybrid method during transients, while having a negligible increase in computational requirements compared to the traditional zero-order approach.

Modeling of Grid-Forming and Grid-Following Inverters for Dynamic Simulation of Large-Scale Distribution Systems

Historically, distribution system planning studies mainly focused on steady state and quasi-steady state analysis, with limited attention paid to dynamic analysis. This paper develops three-phase, electromechanical models for both grid-forming and grid-following inverters, and integrates them into an open source, three-phase distribution network solver, thereby enabling dynamic simulation of large-scale, three-phase unbalanced distribution systems with high penetration of inverter-based DERs. The proposed inverter models are validated against electromagnetic simulation and field test data from the CERTS/AEP microgrid testbed, and simulated in an islanded 5252 node distribution system in the GridLAB-D simulation environment. Simulation verifies the effectiveness of the proposed inverter models for large-scale distribution system analysis. Study results show that compared to traditional grid-following inverters, the high penetration of grid-forming inverters can improve the voltage and frequency stability of islanded distribution systems.

Control of Grid-Following Inverters Under Unbalanced Grid Conditions

This paper proposes a new control scheme to eliminate the 3rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup.

Grid-Forming Inverters: A Critical Asset for the Power Grid

Increasing inverter-based sources reduces the system’s inertia resulting in possible frequency stability issues. Understanding low-inertia systems and their stability properties is of crucial importance. This article introduces fundamental ways to integrate high levels of renewable energy (RE) and distributed energy resources (DERs) in the power system while creating a more flexible power system. Using RE and DER in the distribution system has many advantages such as reducing the physical and electrical distance between generation and loads, bringing sources closer to loads contributes to the enhancement of the voltage profile, reduction in distribution and transmission bottlenecks, improved reliability, lower losses, and enhances the potential use of waste heat. A basic issue for high penetration of DER is the technical complexity of controlling hundreds of thousands to millions of inverters. This is addressed through autonomous techniques using local measurements eliminating the need for fast control systems. The key issues addressed in this article include using inverter damping to stabilize frequency in systems with low or no inertia, autonomous operation, methods for relieving inverter overload, energy reserves, and their implementation in photovoltaics (PV) systems. This article provides important insight into the interactions between inverter bases sources and the high-power system. The distinction between grid-forming (GFM) inverter and grid-following (GFL) inverter is profound. GFM inverters provide damping to frequency swings in a mixed system, while GFL inverter can aggravate frequency problems with increased penetration. Rather than acting as a source of inertia, the GFM inverter acts as a source of damping to the system. On the other hand, the application of inverters in the power system has two major issues. One is the complexity of controlling hundreds of thousands to millions of inverters. This is addressed through autonomous techniques using local measurements. The other is the potential of high overcurrent in GFM inverters and techniques for explicitly protecting against overloading. To exploit the innate damping of GFM inverters, energy reserves are critical.