Active Power Oscillations Research Articles

Efficient Peak Current Limit Strategy and Active Power Oscillation Reduction in a Three-Phase Grid-Interfaced PV-Battery System (GIPVBS) with Low Voltage Ride-Through Control

Efficient Peak Current Limit Strategy and Active Power Oscillation Reduction in a Three-Phase Grid-Interfaced PV-Battery System (GIPVBS) with Low Voltage Ride-Through Control

A Transient Damping Improvement Strategy for Enhancing Grid-Connected Active Power Response Performance of VSG

The given power and grid frequency disturbances can cause transient oscillations and steady-state deviations in the output power of a virtual synchronous generator (VSG), which can be effectively addressed by adding transient damping. However, this approach may result in significant power overshoot. This article proposes an improved VSG control strategy based on transient electromagnetic power feedback compensation and a small-signal model reduction scheme. Firstly, the grid-connected active closed-loop small-signal models of typical VSG control and transient damping VSG control are established, respectively. The transient oscillation suppression mechanism of active power is revealed through root locus and frequency response analyses, and the power overshoot characteristics of the two control strategies are analysed by combining them with the system of zero points. Secondly, the active transient feedback compensation method and the small-signal model reduction design method are introduced in detail. Finally, comparative analysis experiments are conducted using the Matlab/Simulink and hardware-in-the-loop experimental platform. It is verified that the proposed control strategy can suppress transient oscillations in active power, prevent steady-state deviations, and effectively mitigate the power overshoot problem of the system.

Improving the performance of grid‐connected inverters during asymmetrical faults and unbalanced grid voltages

AbstractThe increasing penetration of the distributed energy resources (DER) in the power grid, which, while having significant advantages, also pose significant challenges. The behaviors of DERs differ from those of synchronous generators, particularly in abnormal conditions. For this reason, the power grid enforces grid codes to ensure that DERs perform properly in different conditions. For instance, short circuit faults and unbalanced grid voltage are severe transient events that inverters need to be able to pass through without disconnecting from the grid. Furthermore, the inverters are required to support the grid voltage by regulating the active and reactive power injections. This article proposes a voltage support control scheme to support grid voltage during asymmetrical voltage drop by utilizing an optimization problem. In this optimization problem, the active and reactive powers injected into the grid will be obtained optimally by considering constraints such as instantaneous active and reactive power oscillation magnitudes and peak current limitation. To aid in this purpose, the corresponding mathematical formulations such as instantaneous active and reactive power oscillation magnitudes will be obtained by using the currents and voltages in stationary reference frame. The proposed scheme will be verified by simulating it in MATLAB/Simulink under three different scenarios and tested on a real‐time experimental Opal‐RT platform.

Elimination of power and frequency oscillations for AC microgrid with parallel virtual synchronous generator and synchronous generator

The integration of conventional synchronous generators (SGs) and virtual synchronous generators (VSGs) in microgrids (MGs) is increasingly common due to the growth of renewable energies. However, demand fluctuations and grid faults can pose significant challenges to the stability of the MG, causing system frequency and active power oscillations. This study proposes control approaches for an isolated/islanded AC MG that consists of an VSG and an SG to mitigate power and frequency oscillations. The proposed methods utilize the VSG’s adjustable damping coefficient, which is determined by intelligent controls. The proposed strategies significantly reduce power fluctuations by 53% and frequency deviations by 75%, thereby improving system stability and reliability. The suggested controllers use short-term large damping to effectively amplify the system’s AC frequency dynamic response and enhance power delivery across different power-sharing modes. The controllers do not require additional communication infrastructure and rely solely on the local AC frequency for feedback. Furthermore, a novel synchronization mechanism with positive effects on system stability is presented.

Finite Set-Model Predictive Control for Modified quasi Switched Z Source Level 3 Off-board Charger for Active Power Oscillations Damping in Weak Grids

Finite Set-Model Predictive Control for Modified quasi Switched Z Source Level 3 Off-board Charger for Active Power Oscillations Damping in Weak Grids

Adaptive Control for Virtual Synchronous Generator Parameters Based on Soft Actor Critic.

This paper introduces a model-free optimization method based on reinforcement learning (RL) aimed at resolving the issues of active power and frequency oscillations present in a traditional virtual synchronous generator (VSG). The RL agent utilizes the active power and frequency response of the VSG as state information inputs and generates actions to adjust the virtual inertia and damping coefficients for an optimal response. Distinctively, this study incorporates a setting-time term into the reward function design, alongside power and frequency deviations, to avoid prolonged system transients due to over-optimization. The soft actor critic (SAC) algorithm is utilized to determine the optimal strategy. SAC, being model-free with fast convergence, avoids policy overestimation bias, thus achieving superior convergence results. Finally, the proposed method is validated through MATLAB/Simulink simulation. Compared to other approaches, this method more effectively suppresses oscillations in active power and frequency and significantly reduces the setting time.

Conjoint Enhancement of VSG Dynamic Output Responses by Disturbance-Oriented Adaptive Parameters

Virtual synchronous generator(VSG) is an encouraging concept to ensure provision of inertia, following large integration of renewable sources in system. However, apart from VSG contribution in improving system's frequency stability, it is also important to ensure long term robust and stable operation of VSG itself. In this regard, active power loop(APL) of VSG constituting variable control parameters is analyzed subsequent to contingency scenarios. It is observed that dynamics of APL outputs (virtual frequency and active power) is critically sensitive and prone to intrinsic oscillations. Further, during disturbance in system, utilization of constant control parameters of APL illustrates contradictory effects in dynamics of its outputs. As a solution to above issues, novel adaptive inertia and adaptive damping coefficient is proposed. The work provides distinct approach in following manner- 1) judicious selection of different operation range of natural undamped frequency( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ω_{n}$</tex-math></inline-formula> ) and damping ratio(ζ) of APL as compared to existing strategies, along with 2) reduced computational complexity and improved accuracy of controller, 3) thereby reduction in over(-under)shoots and intrinsic oscillations in virtual frequency and output active power of APL during transient period. This strengthens classical VSG design and contributes in its robust long term industry/real-time application. The efficacy of proposed strategy is showcased by real-time simulations along with validation at PHIL level.

Consolidation of LVFRT capabilities of microgrids using energy storage devices

The performance and dependability of distribution networks may be enhanced by the incorporation of microgrids (MGs). However, it is necessary to enhance low voltage fault-ride-through (LVFRT), which has the capacity to prevent abrupt grid disconnections during LV occurrences under problematic circumstances. In this study, a control strategy for energy storage elements (ESDs) which includes batteries and supercapacitors is proposed to enhance LVFRT under balanced and unbalanced faults. The MG comprises wind farms and/or photovoltaic arrays. Based on the dynamic simulations using MATLAB/SIMULINK, the ESDs can enhance LVFRT capability. A comparison of the conventional crowbar scheme and ESDs is realized, and the latter has a better performance than the former in retaining the DC-link voltage within satisfactory bounds. For the purpose of maintaining the DC-link voltage at a reference level, the battery stores extra power in the DC-bus of three systems. LVFRT is improved by the crowbar circuit, however the resistance consumes the extra power. Super capacitors (SCs) prevent DC voltage fluctuations, reduce active power oscillations, and hasten system stabilization when present. At an advanced stage of this effort, the coot bird optimizer (CBO) is applied to generate the best gains of bi-directional converter PI-controller and the ESDs ratings to have minimum ripples in the DC-bus voltage and to boost the LVFRT capability of the MGs. The viability of the proposed method based on the CBO’s results is indicated with further validations under different operating scenarios.

An Active Power Dynamic Oscillation Damping Method for the Grid-Forming Virtual Synchronous Generator Based on Energy Reshaping Mechanism

The grid-forming virtual synchronous generator (GFVSG) with large virtual inertia can provide a friendly grid-connected operational mode for power electronic converters, but it may also introduce the active power dynamic oscillation problems similar to traditional synchronous generators. In view of this, the dynamic equivalent circuit model of the GFVSG grid-tied active power-angle is established firstly, and, then, the understanding of the GFVSG active power oscillations under variable disturbances is revealed from the perspective of circuit energy flow in this paper. On this basis, an active power dynamic oscillation damping method based on an energy reshaping mechanism for the GFVSG is proposed, and a parameter design method using the second-order equivalent reduced-order control model is given. The MATLAB 2016a simulation as well as experimental test platforms of a 100 kV·A GFVSG grid-connected system are established, then, both the feasibility and effectiveness of the proposed active power dynamic oscillation damping method are verified by using the simulation and experimental comparison results.

Fault Ride-Through Strategies for Synchronverter-Interfaced Energy Resources Under Asymmetrical Grid Faults

Synchronverters are inverters controlled to behave like synchronous generators. However, unbalanced faults or voltage dips may stimulate synchronverters to generate excessive currents and cause instantaneous active and reactive power oscillations at twice the grid's fundamental frequency. Therefore, this paper proposes three fault ride-through (FRT) strategies to remedy these power oscillations and the unlimited current generation during unbalanced faults. The first strategy is based on an instantaneous active and reactive control that significantly reduces power oscillations. The second strategy relies on an average active and reactive control that ensures the delivery of average active and reactive power and maintains sinusoidal phase currents. Finally, the third FRT strategy employs a comprehensive instantaneous active and reactive control to eliminate power oscillations and maintain sinusoidal currents simultaneously. The proposed FRT strategies are equipped with a power management controller that delivers the required active and reactive power during normal operation and limits the inverter's phase currents during faults. Further, the proposed FRT strategies maintain the intrinsic features of synchronverters and ensure seamless activation of the proposed FRT strategies. Comparative results confirm the efficacy of the proposed strategies to reduce power oscillations, limit current generation, maintain synchronverters' intrinsic features, and comply with grid codes.

An improved VSG control strategy based on transient electromagnetic power compensation

Virtual synchronous generator (VSG) not only increases the inertia of grid-connected system, but also brings the problem of active power oscillation under grid disturbance. Therefore, VSG control strategy and system model order reduction method with transient electromagnetic power compensation are proposed. The closed-loop active power small signal model of the system is established, and the influence of transient electromagnetic power compensation on the power stability of VSG is analyzed based on root locus method. By removing the items which have little influence on the stability of the system in the small signal model, the order is reduced to obtain the equivalent second-order model of the system. According to the second-order model, the quantitative design criteria of the system parameters are given. The proposed transient electromagnetic power compensation strategy not only increases the transient equivalent damping of the system, but also does not affect the primary frequency modulation characteristics and will not cause large overshoot of the output active power. The experimental results are consistent with the theoretical analysis, which testify the effectiveness and correctness of the system control strategy and the model reduction method.

A Robust Damping Control for Virtual Synchronous Generators Based on Energy Reshaping

Virtual synchronous generators (VSGs) have been proved to be important means to provide the inertia for future power systems. However, it suffers the issue of active power oscillation under various disturbances. In this paper, a robust damping control is proposed to mitigate the active power oscillation by reshaping the oscillation energy of VSGs. The paper first represents the power-angle dynamics of VSGs as an equivalent circuit and thus enables the understanding of oscillations from the circuit energy. It is revealed that the active power oscillation can be comprehended as an LC resonance and the damping provided by the traditional VSG is commonly insufficient. To tackle this issue, a robust damping method is proposed using interconnection and damping assignment passivity-based control (IDA-PBC). The theory of IDA-PBC is established based on the concept of energy reshaping, which guarantees the state tracking via its intrinsic energy dissipation characteristics. The IDA-PBC, when applied to VSGs, is a combination of the disturbance compensation via feedforward channels and the deviations regulation through feedback paths. Noticeably, the disturbance compensation is achieved with the support of an extended state observer (ESO), which can accurately estimate the lumped disturbance including the grid frequency variation and the model uncertainties. A guideline on the parameter selection is also provided through Bode-plot analysis. Finally, the effectiveness and merits of the proposed method is verified by hardware in the loop-based experiments with the comparison to the state of art work.

Power oscillation suppression of multi-VSG based on both consensus and model predictive control

Under the circumstances of multi-VSG, virtual synchronous generator (VSG) control is liable to produce active power oscillation due to the absence of mutual damping, which may lead the power fluctuations more acute that may in turn cause instability problems. This study concentrates on the analysis of power fluctuations and the optimization of mutual damping within multi-VSG systems. By developing a predictive model for VSG, a mutual damping control strategy, which incorporates both Model Predictive Control (MPC) and a consensus algorithm, is proposed. The proposed method primarily employs the MPC model of VSG to calculate the increments active power for system interaction. Subsequently, a consensus algorithm is utilized to exchange the increments active power, which is then superimposed on each VSG to enhance the mutual damping. The stability of both the MPC and the proposed mutual damping control method is ensured using the Lyapunov stability criterion. Furthermore, a gain adjustment function is employed to accelerate convergence of the algorithm and maintain stability even under non-ideal communication. The effectiveness of the theoretical analysis and the proposed method is demonstrated through simulations.

A New Power-Frequency Oscillation Suppression and Power Allocation Control Method Based on MPC in Multi-VSGs Parallel System

The virtual synchronous generator (VSG) technology can provide frequency support for power systems that are dominated by renewable power sources. However, it also introduces the characteristic of active power oscillation, which can cause power-frequency oscillation between parallel VSGs when there are disturbances in the load power or command power. To address this problem, this paper proposes that the idea of equivalent comparison can be used to compare the power-frequency interaction of N-VSGs parallel systems with the current-voltage interaction of N-RLC circuits. This allows us to establish a mathematical model of the parallel VSGs system based on Kirchhoff’s equation of the circuit. Using this model, we designed a new power-frequency oscillation suppression and a power allocation control method based on the model predictive control (MPC) in multi-VSGs parallel systems. This method can effectively suppress power-frequency oscillation in each VSG branch after a disturbance occurs, and accurately allocate active power according to instructions. Finally, simulation results confirm the effectiveness of the controller.

Active Power Oscillation Suppression Based on Decentralized Transient Damping Control for Parallel Virtual Synchronous Generators

Similar to the traditional synchronous generator (SG), a drawback of the virtual synchronous generator (VSG) is the active power oscillation, especially in the system with parallel VSGs. In this paper, a decentralized transient damping (DTD) control strategy is proposed to suppress active power oscillation during load fluctuations in the parallel VSGs system, which is based on embedding the transient electromagnetic power compensation and transient damping power compensation (TDPCP) control into the original VSG active power controller, providing two new degrees of freedom for the flexible regulation of the system damping ratio without frequency measurement. In this way, the damping ratio of the parallel VSGs system can be adjusted more freely without affecting the steady-state characteristics of the system. Therefore, this flexible transient damping characteristic can facilitate the suppression of active power oscillation under load fluctuations. Moreover, the proposed method is immune to the parameter mismatch of the parallel VSGs. The design of key parameters and stability analysis are presented. Finally, the feasibility of the proposed control strategy is verified through the simulation and experimental results.

Phase-leg power equalization method of modular multilevel converter with APOD strategy under unbalanced fault condition

An active power oscillation damping (APOD) control is one of the representative unbalanced fault compensation strategies. Recently, various studies have been performed on the effect of the APOD with the internal power dynamic analysis of a Modular Multilevel Converter (MMC); however, most of the papers have considered only the active power component for analyzing the internal dynamics of the MMC. During the APOD operation, while reactive power is supplied to the grid for the voltage compensation, phase-leg power imbalance can be aggravated. To enhance stability and reliability, the phase-leg power imbalance phenomenon should be sufficiently considered at the development stage of the controller. This research discovers that the phase-leg power imbalance rate could be determined by the magnitude and phase of the negative sequence component of the grid voltage and the amount of reactive power. In addition, the unbalanced power component can be accurately calculated only based on the grid voltage information and the reactive power reference. Also, this paper proposes a phase-leg power equalization method (PPEM) using unbalanced power component estimation. Furthermore, an improved control structure is also proposed to achieve more accurate phase-leg power equalization using the internal leg-energy dynamics analysis. The effectiveness of the proposed methods is verified through high-fidelity PSCAD/EMTDC time-domain simulations using Point-to-Point (PTP) MMC-HVDC system and Hardware-in-the-Loop Simulation (HILS) with the MMC-MVDC system.

Low voltage ride through control strategy for grid-tied solar photovoltaic inverter

Abstract This paper presents a low voltage ride through (LVRT) control strategy using an active power oscillations based reference current generation approach for grid tied solar photo voltaic (SPV) system under line-to-ground (LG) and double line-to-ground (LLG) faults. Proposed control strategy minimizes the harmonics injected into the grid, as well as oscillations in injected reactive power and DC-link voltage. Further this control strategy provides a dynamic control of active and reactive powers, the flow of sinusoidal currents into the grid, and avoids the disconnection between the utility grid and SPV system during faults. A proportional resonant (PR) current controller is developed in a stationary reference frame to achieve the power quality by fulfilling the grid codes. Simulation and experimental results are presented to highlight the merits of the proposed control scheme.

Small‐signal modelling and analysis of microgrids with synchronous and virtual synchronous generators

AbstractIn autonomous alternating current microgrids, the grid‐forming virtual synchronous generators can cooperate with the conventional synchronous generators to improve system inertia and frequency regulation capability. However, undesired active power oscillations between the synchronous generators and grid‐forming virtual synchronous generators may trigger their overcurrent protection and even result in a blackout. To explicitly reveal the oscillatory modes over all frequency bands, a high‐fidelity full‐order state‐space model is first developed. A potentially destabilising sub‐synchronous oscillation mode resulting from the interaction between grid‐forming virtual synchronous generators voltage controller and synchronous generators q‐axis damper winding is identified. Other modes reflecting the low‐frequency oscillation and frequency restoration dynamics are also assessed. Subsequently, to make a reasonable trade‐off between the accuracy and simplicity of system modelling, an enhanced quasi‐stationary model dedicated to low‐frequency oscillation evaluation is simplified from the full‐order type. The enhanced quasi‐stationary model features simplicity and low‐order benefits, which makes it more practical for multi‐generator system analysis. Moreover, by considering the dynamics of synchronous generators field winding and excitation system, the enhanced quasi‐stationary model significantly improves the low‐frequency oscillation characterisation accuracy compared with the existing quasi‐stationary model. The two developed models are comprehensively compared with the existing small‐signal models. Real‐time simulations based on RT‐LAB are conducted to verify the correctness of the theoretical analysis and the accuracy of the proposed small‐signal models.

A Multiobjective Control Strategy for Harmonic Current Mitigation With Enhanced LVRT Operation of a Grid-Tied PV System Without PLL Under Abnormal Grid Conditions

This article proposes a novel multiobjective control strategy (MOCS) for improving the performance of grid-tied photovoltaic (GTPV) system without phase-locked loop (PLL) operating under abnormal grid voltage conditions such as harmonically distorted voltages, grid faults, and weak grid. The proposed control strategy effectively mitigates the harmonics in grid injected current, eliminates the oscillations in grid injected power, and enhances the low-voltage ride-through (LVRT) capability simultaneously during abnormal grid conditions. The LVRT capability of the GTPV system is enhanced by incorporating an active power regulator (APR) with dynamic reactive power support (DRPS) controller in the proposed control strategy. The proposed MOCS is designed in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> stationary reference frame. Furthermore, the proposed MOCS consists of an advanced phase compensated multiresonant (APCMR) current controller and a phase-shifted reference current generator (PSRCG) integrated with the APR with DRPS controller. The APCMR suppresses the grid current harmonics under weak and distorted grid voltages and the PSRCG eliminates active and reactive power oscillations at the grid side caused by grid faults. The synchronization mechanism of the GTPV system is included in the PSRCG, and therefore, the requirement of PLL is avoided. Consequently, the associated computational burden on the digital controller is reduced. The effectiveness of the proposed control strategy is validated using the OPAL-RT digital simulator and offers superior performance compared to the conventional control strategy (COCS).

An Optimized Proportional Integral Derivative (PID) based Power System Stabilizer (PSS) for Damping of Active Power Oscillations

Rising load demand and possibilities of short interruptions either due to temporary faults or equipment switching may lead to a transient instability in terms of growing oscillations and eventually result into cascaded outages if not being damped properly and timely. To damp out such oscillations, the power system stabilizers (PSS) are invariably being installed to regulate the excitation of synchronous generators. In this research, a comparative analysis among the two design variants of PSS i.e., Lead Lag and PID is performed by comparing the overshoot and settling time of generator active power after distrubance, to observe the effect of each on the system characteristics and nominal conditions. The results obtained show a better performance of PID based PSS having better overshoot response and reduced settling time for symmetrical as well as unsymmetrical faults.