Nonlinear Current-Limiting Control for Grid-tied Inverters

A current-limiting controller with nonlinear dynamics is proposed in this paper for single-phase grid-tied inverters. The inverter is connected to the grid through an LCL filter and it is proven that the proposed controller can achieve accurate real and reactive power regulation. By suitably selecting the controller parameters, it is shown by using the nonlinear input-to-state stability theory that the inverter current remains below a given value at all times. This is achieved without external limiters, additional switches or monitoring devices and the controller remains a continuoustime system guaranteeing the boundedness of the system states. Guidelines for selecting the controller parameters are also given to provide a complete controller design procedure. Simulation results of a single-phase grid-tied inverter are presented to verify the desired power regulation of the proposed controller and its current-limiting capability

Current-limiting Droop Control with Virtual Inertia and Self-Synchronization Properties

In this paper a current-limiting droop control of grid-tied inverters that introduces virtual inertia and operates without a phase locked loop unit is proposed. The proposed controller inherits a self-synchronization function and can guarantee tight bounds for the inverter frequency. In addition, using nonlinear Lyapunov theory, it is analytically proven that the inverter current never violates a given maximum value. Compared to the original current-limiting droop controller, the maximum capacity of the inverter is utilized at all times using the proposed strategy, even under grid faults. It is also proven that the proposed controller significantly reduces the resonance problem of the LCL filter. Extended simulation results are presented to verify the performance of the proposed controller under normal and faulty grid conditions

PLL-less Nonlinear Current-limiting Controller for Single-phase Grid-tied Inverters: Design, Stability Analysis and Operation Under Grid Faults

A nonlinear controller for single-phase grid-tied inverters, that can operate under both a normal and a faulty grid with guaranteed closed-loop stability, is proposed. The proposed controller acts independently from the system parameters, does not require a phase-locked loop (PLL) and can achieve the desired real power regulation and unity power factor operation. Based on nonlinear input-to-state stability theory, it is analytically proven that the inverter current always remains below a given value, even during transients, independently from grid variations or faults (short circuit or voltage sag). The desired performance and stability of the closed-loop system are rigorously proven since the controller has a structure that does not require any switches, additional limiters or monitoring devices for its implementation. Therefore, nonlinear stability of a grid-tied inverter with a given current-limiting property is proven for both normal and faulty grid conditions. The effectiveness of the proposed approach is experimentally verified under different operating conditions of the grid

Current-Limiting Droop Control of Grid-connected Inverters

A current-limiting droop controller is proposed for single-phase grid-connected inverters with an LCL filter that can operate under both normal and faulty grid conditions. The controller introduces bounded nonlinear dynamics and, by using nonlinear input-to-state stability theory, the current-limiting property of the inverter is analytically proven. The proposed controller can be operated in the set mode to accurately send the desired power to the grid or in the droop mode to take part in the grid regulation, while maintaining the inverter current below a given value at all times. Opposed to the existing current-limiting approaches, the current limitation is achieved without external limiters, additional switches or monitoring devices and the controller remains a continuous-time system guaranteeing system stability. Furthermore, this is achieved independently from grid voltage and frequency variations, maintaining the desired control performance under grid faults as well. Extensive experimental results are presented to verify the droop function of the proposed controller and its current-limiting capability under normal and faulty grid conditions

Voltage support under grid faults with inherent current limitation for three-phase droop-controlled inverters

A novel nonlinear current-limiting controller for three-phase grid-tied droop-controlled inverters that is capable of offering voltage support during balanced and unbalanced grid voltage drops is proposed in this paper. The proposed controller introduces a unified structure under both normal and abnormal grid conditions operating as a droop controller or following the recent fault-ride-through requirement to provide voltage support. In the case of unbalanced faults, the inverter can further inject or absorb the required negative sequence real and reactive power to eliminate the negative sequence voltage at the PCC whilst ensuring at all times boundedness for the grid current. To accomplish this task, a novel and easily implementable method for dividing the available current into the two sequences (positive and negative) is proposed, suitably adapting the proposed controller parameters. Furthermore, nonlinear input-to-state stability theory is used to guarantee that the total grid current remains limited below its given maximum value under both normal and abnormal grid conditions. Asymptotic stability for any equilibrium point of the closed-loop system in the bounded operating range is also analytically proven for first time using interconnected-systems stability analysis irrespective of the system parameters. The proposed control concept is verified using an OPAL-RT real-time digital simulation system for a three-phase inverter connected to the grid

Current-limiting droop controller with fault-ride-through capability for grid-tied inverters

In this paper, the recently proposed current-limiting droop (CLD) controller for grid-connected inverters is enhanced in order to comply with the Fault-Ride-Through (FRT) requirements set by the Grid Code under grid voltage sags. The proposed version of the CLD extends the operation of the original CLD by fully utilizing the power capacity of the inverter under grid faults. It is analytically proven that during a grid fault, the inverter current increases but never violates a given maximum value. Based on this property, an FRT algorithm is proposed and embedded into the proposed control design to support the voltage of the grid. In contrast to the existing FRT algorithms that change the desired values of both the real and reactive power, the proposed method maximizes only the reactive power to support the grid voltage and the real power automatically drops due to the inherent current-limiting property. Extensive simulations are presented to compare the proposed control approach with the original CLD under a faulty grid

Small-signal modeling of grid-supporting inverters in droop controlled microgrids

An approach to modeling externally controlled inverters in droop controlled microgrids is presented. A generic three-phase grid-tied inverter and control system model is derived in synchronous reference frame. The structure of this inverter is intended to be similar in composition to other three-phase inverters whose models and dynamics are well understood. This model is used as a starting point in the development of a more comprehensive model, which is capable of representing the coupling between complex power, bus voltage, and frequency that occurs in a microgrid. This new model is a combination of the generic inverter and an autonomous, grid-forming inverter with a local load. The accuracy of the new model is verified through comparisons of small-signal dynamic predictions, simulations, and experimental results from a microgrid testbed. The proposed procedure of modifying an existing small-signal model for use in a microgrid system retains the information of the original model while successfully enabling the prediction of dynamic interactions with other generating units in the microgrid. The process is scalable for any number of inverters at the same point of connection, allowing accurate predictions of full system dynamics during distributed control actions, such as black start or grid-resynchronization. Traditional linear control techniques may be used to improve the performance and stability of the microgrid system. This is a demonstrated in an analysis of the system\u27s eigenvalues. Drawing from the insights provided by this analysis, hardware and control parameters are selected to improve the response of the generic inverter --Abstract, page iii

Grid-Forming Converter Control Method to Improve DC-Link Stability in Inverter-Based AC Grids

As renewable energy sources with power-electronic interfaces become functionally and economically viable alternatives to bulk synchronous generators, it becomes vital to understand the behavior of these inverter-interfaced sources in ac grids devoid of any synchronous generation, i.e. inverter-based grids. In these types of grids, the inverters need to operate in parallel in grid-forming mode to regulate and synchronize their output voltage while also delivering the power required by the loads. It is common practice, therefore, to mimic the parallel operation control of the very synchronous generators that these inverter-based sources are meant to replace. This practice, however, is based on impractical assumptions and completely disregards the key differences between synchronous machines and power electronic inverters, as well as the dynamics of the dc source connected to the inverter. This dissertation aims to highlight the shortcomings of conventional controllers and derive an improved grid-forming inverter controller that is effective in parallel ac operation without sacrificing dc-link stability. This dissertation begins with a basis for understanding the control concepts used by grid-forming inverters in ac grids and exploring where existing ideas and methods are lacking in terms of efficient and stable inverter control. The knowledge gained from the literature survey is used to derive the requirements for a grid-forming control method that is appropriate for inverter-based ac grids. This is followed by a review and comparative analysis of the performance of five commonly used control techniques for grid-forming inverters, which reveal that nested loop controllers can have a destabilizing effect under changing grid conditions. This observation is further explored through an impedance-based stability analysis of single-loop and nested-loop controllers in grid-forming inverters, followed by a review of impedance-based analysis methods that can be used to assess the control design for grid-forming inverters. An improved grid-forming inverter controller is proposed with a demonstrated ability to achieve both dc-link and ac output stability with proportional power-sharing. This dissertation ends with a summary of the efforts and contributions as well as ideas for future applications of the proposed controller

Adaptive saturation system for grid-tied inverters in low voltage residential micro-grids

Provision of ancillary services, like power quality improvement is a key to attain higher utilization of multifunctional grid-tied inverter. However, the power quality improvement is mainly limited by the power capacity the grid-tied inverter. This paper explores integration issues of the next-generation intermittent power sources. In particular, two different strategies for enhancing power quality given the residual power capacity of the inverters are developed. One strategy aims to obtain the expected power quality exploiting the dynamic saturation of the inverter rated apparent power and another strategy is based on peak current detection. Both strategies offer the possibility to generate appropriate references for the inner current control loop. The two proposed strategies are compared in performance, and a discussion on their practical implementation for the best performance of the inverters is provided78478915th IEEE International Conference on Environment and Electrical Engineering (EEEIC