Designing a central MIMO state feedback controller for a microgrid with multi-parallel VSGs to damp active power oscillations

Pole–zero placement based feed-forward damping control for virtual synchronous generators in power systems with inverter-based resources

In recent years, the increasing penetration of distributed generation in microgrids challenges the control and coordination of energy resources. Especially in microgrids with virtual synchronous generator (VSG)-controlled converters and conventional synchronous generators (SG), the inherent inertia difference (i.e., the VSG and SG) results in a poor transient performance when the VSG and/or loads are cut in/out. Thus, this paper explores the transient performance of microgrids with parallel VSG and SG systems. More importantly, a novel pre-synchronization control method is proposed to eliminate the phase jump while meeting the requirements in case of closures or re-closures of generation units. A small-signal dynamic model is presented, and accordingly, the VSG inertia and its damping can be designed considering the capacity ratio of VSG and SG units. In addition, with the power angle stability analysis, an active power provision strategy is introduced to suppress the transient power oscillation due to the inertia difference. Finally, the feasibility of the proposed methods is verified by simulations on a microgrid consisting of parallel VSG and SG units.

The present paper describes an Electric Vehicle (EV) charging station that provides ancillary services to the main AC grid, as for example frequency support and synthetic inertia. Due to the Direct Current (DC) nature of these loads and the easier integration of renewable energies, a DC Microgrid is considered to power the charging station. Targeting both DC grid voltage stability and the capability to support the main AC grid, nonlinear control laws based on Lyapunov theory and backstepping are developed for local and primary control levels (droop control). The proposed controllers are validated via MATLAB-Simulink simulations in a 25 kW DC Microgrid with PV production, Vehicle to Grid (V2G) mode of operation, fast and slow vehicle charge, ancillary services to the AC main grid and islanded mode operation.

The technology of Virtual Synchronous Generator (VSG), which can make the inverter connected to distributed generation work like traditional synchronous generator, plays an important role in improving the stability of the power system because of its good voltage and frequency supporting capability. In this paper, an improved VSG control method is proposed, which can ensure seamless transfer between grid-connected and stand-alone modes, and achieve accurate tracking control of active and reactive power. The control topology and mathematical model of VSG are constructed firstly based on the rotation equation and electrical equation of synchronous generator; then, the corresponding VSG active-frequency droop control, reactive-voltage droop control and inner loop voltage-current control are designed, and the influences of key parameters are analyzed through frequency domain method. Finally, two simulations with different operation modes and loads are conducted to verify the effectiveness in Matlab/Simulink environment, and the results show that the performance is well.

Virtual synchronous generator (VSG) is the concept of controlling the grid-connected inverters to mimic conventional synchronous generators (SGs), and it is considered to be a promising method to enhance the grid inertia and power regulation ability when the renewable energy has a high penetration rate in the grid. In this paper, the power frequency characteristic and the synchronization mechanism of the VSG are analyzed in detail, and a modified pre-synchronization method is proposed. Compared to the existing methods, the proposed method in this paper regulates the VSG frequency by resetting its active power reference according to the local load and eliminates the phase deviation by feedback control of the virtual power, thus the phase angle of the VSG in standalone mode is in accordance with the steady operation point in grid-connected mode and the dynamic power adjustment can be avoided when the VSG connect to the grid. A 30kV A VSG prototype was developed and tested in the laboratory, and the effectiveness of the proposed method is verified with the simulation results based on Matlab/Simulink results based on the VSG prototype.

The hybrid AC/DC microgrid gains a great attention in recent years as it can improve the energy efficiency and ensure the reliability of the system. This paper makes an investigation on the operation mode switching performance of an AC/DC combined hybrid microgrid based on a real system at Griffith University, Queensland, Australia. An improved virtual synchronous generator (VSG) control strategy is developed to optimize the switching performance from the grid-tied mode to the islanding mode. The basic VSG principle is described and the improved VSG model is introduced. Based on the improved model, a pre-synchronization method is proposed. Comparison of the microgrid performance between conventional droop control and the improved VSG control is carried out by simulation on Matlab/SIMULINK environment. Simulation results show the advantage of VSG control strategy and the validity of the pre-synchronization method.

The operation problem of a micro-grid (MG) in grid-connected mode is an optimization one in which the main objective of the MG operator (MGO) is to minimize the operation cost with optimal scheduling of resources and optimal trading energy with the main grid. The MGO can use incentive-based demand response programs (DRPs) to pay an incentive to the consumers to change their demands in the peak hours. Moreover, the MGO forecasts the output power of renewable energy resources (RERs) and models their uncertainties in its problem. In this paper, the operation problem of an MGO is modeled as a risk-based two-stage stochastic optimization problem. To model the uncertainties of RERs, two-stage stochastic programming is considered and conditional value at risk (CVaR) index is used to manage the MGO’s risk-level. Moreover, the non-linear economic models of incentive-based DRPs are used by the MGO to change the peak load. The numerical studies are done to investigate the effect of incentive-based DRPs on the operation problem of the MGO. Moreover, to show the effect of the risk-averse parameter on MGO decisions, a sensitivity analysis is carried out.

Simultaneous capacity optimization of distributed generation and storage in medium voltage microgrids

The new reform of power system promotes the market-oriented operation of microgrids. The ubiquitous power internet of things provide support in information, data, and computation to microgrids in market operation, energy management, and coordination interaction. This study takes the park microgrid with multi-stakeholder as the object, establishes a two-level optimisation model of microgrid bidding transaction based on multi-agent system. In the lower level optimisation, considering the deviation penalty of power generation and the previous round bidding results, the optimal bidding strategy model is established by the bidding unit agent to maximise its benefit. In the upper-level model, bidding strategies of distributed energy resources (DERs) as constraints, a multiple objective mixed-integer linear programming model was built to optimise the overall objectives of clearing price and imbalanced deviation, searching for the optimal clearing price and the generation plan of DERs. The results were fed back to the DERs to help them form the next round of strategies until the result reaches equilibrium. The proposed optimised operation mode is compared with the traditional operation mode in the case study, verifying that the proposed method can realise the optimal operation of the microgrid and the coordinated interaction with the main grid, increasing the benefit of stakeholders.

In the last years the microgrid are emerged as the key component able to increase the efficiency, reliability, and sustainability of traditional electrical infrastructures. Micro distribution systems aggregate small, modular renewable power source, distributed storage and local loads as autonomous entities that can exchange power with the traditional electricity if operating in connected mode. A prime task in microgrid operation is the dynamic balance of local supply and power demand due to the intermittent nature of renewable energy resource and the variability of load demand during the day. However the power transfer among each microgrid and the main grid is always associated with a cost due to the loss of power over the distribution line. In this paper, a multi-agent systems (MAS) for the optimal coordination of multiple distributed energy resources is presented. The agents, associated with each microgrid, implement a cooperative strategy to minimise the power loss over the distribution lines and to maximise the economic income by sharing the surplus of the generated power between the microgrids belonging to the same coalition. The simulation results show the effectiveness of the proposed control strategy demonstrating that the MGs payoff increases up to 30% when microgrids cooperate to gain the power balance.

Data-driven MIMO model-free reference tracking control with nonlinear state-feedback and fractional order controllers

In recent years, continuous efforts have been made on the modeling and stability analysis of converter-dominated grids (CDGs) to guarantee efficient, stable and resilient operations. The literature has tried to reveal the mechanism behind abnormal instability and resonances caused by the interaction between multiple time-scale control loops within a single converter, different types of converters and power networks. It is commonly assumed that CDGs are three-phase balanced systems and supply only passive loads. In reality, CDGs can experience imbalance caused by asymmetric networks, loads or faults. Moreover, induction motor (IM) loads, which exhibit highly nonlinear couplings between dynamics of power, voltage and frequency, typically account for a large portion of electric loads. Ignoring the impact of imbalance and dynamic loads in the modeling and stability analysis of CDGs can lead to unrealistic stability assessment results. To fill the gap, this paper presents a general small-signal modeling framework for CDGs in the presence of IM loads. Linear time-periodic (LTP) eigenvalue analysis is performed to investigate the impact of the interaction between IM loads, grid-following (GFL) converters and virtual synchronous generator (VSG) converters on the system stability. The time-domain simulation and experimental results validate the theoretical analysis.

This article presents an improved virtual synchronous generator (VSG) control strategy of doubly-fed induction generator (DFIG) during symmetrical voltage fault. Based on VSG control, DFIG can behave like a conventional synchronous generator (SG), which can support frequency and voltage in grid-connected mode. Moreover, when microgrid disconnects from grid, DFIG can provide voltage and power for local loads. However, during symmetrical voltage fault, the transient components in DFIG cannot be controlled properly by VSG. To solve this problem, the air-gap flux feedback is designed in VSG control to accelerate the decay of transient components, and rotor current limiting method is also introduced in the article. In the proposed method, there is no need to switch the control structure or control targets. DFIG always works in the VSG control. Therefore, DFIG can operate in the standalone mode when the connection to grid is cut off. The decay time constant, the voltage control, current limiting control and parameter variation of the improved VSG control are analyzed in detail. Finally, the simulation and experimental results in both normal and fault grid condition are given to validate the effectiveness of proposed VSG control strategy.

In an islanded microgrid supplied by virtual synchronous generator (VSG) based inverter-based resources (IBRs), inter-IBR oscillations pose a serious threat to frequency stability and power-sharing accuracy. While VSG control enables gridforming inverters (GFMs) to emulate inertia and improve frequency dynamics, it also introduces poorly damped modes. This paper proposes a distributed cooperative control strategy that enhances damping of inter-IBR oscillations without compromising key frequency performance metrics such as the rate of change of frequency (RoCoF) and frequency nadir. The proposed control introduces a dynamic state variable that adjusts the auxiliary frequency reference in real-time based on both local frequency error and a distributed feedback term derived from the mismatch in normalized active power among neighboring inverters, thereby enhancing damping and actively suppressing inter-IBR oscillations. In addition, the control incorporates normalized active power mismatch to achieve proportional load sharing, implemented using a sparse communication topology among neighboring IBRs. An eigenvalue analysis based on small-signal model is conducted for appropriate selection of control gains to achieve system stability. Simulation results under various IBR configurations demonstrate that the proposed control method effectively suppresses inter-IBR oscillations, ensures proportional power sharing, and achieves robustness. Accordingly, the applicability of the proposed control design is validated for both homogeneous and heterogeneous VSG systems.

Traditional microgrid lacks rotational inertia and it needs to operate in the economic state. A dynamic power optimization strategy is proposed for microgrid based on the virtual synchronous generator's frequency regulation characteristics in this paper. Load in microgrid is divided into planned load and unplanned load. The optimization model with the goals of minimizing the generation operation cost and the transmission power of the linking-up road is established to distribute the planned load. The genetic algorithm-simulated annealing algorithm is used to solve the optimization model. Active power generation plans for the VSGs are formulated in the light of calculation results. Central controller estimates the power fluctuation value when unplanned load fluctuations occur in the microgrid. VSG's output power is dynamically adjusted based on the principle of equal marginal cost to achieve dynamic power optimization of microgrid. The effectiveness of the proposed control strategy is verified by simulation.