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

Current-limiting three-phase rectifiers

In this paper, a nonlinear controller is proposed for a three-phase rectifier so that its input current does not exceed a given limit. At the same time, the proposed controller can achieve accurate dc output voltage regulation and reactive power control independently from system parameters including the load during the normal operation. Using the generic dq transformation and the nonlinear model of the rectifier, the boundedness and the current-limiting property of the closed-loop system are proven using Lyapunov methods and the input-to-state stability theory. Moreover, an analytic framework for selecting the controller parameters is presented and the current limitation is proven for both the cases with L and LCL filters at the input of the rectifier. Different from existing approaches, the current-limiting property is achieved without external limiters, monitoring devices, or switches and is incorporated in the control dynamics, independently from the type of the load (linear or nonlinear). Extensive real-time simulation results are provided to verify the effectiveness of the proposed strategy

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

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

Design of a UDE Frequency Selective Filter to Reject Periodical Disturbances

In this paper a new filter design for the Uncertainty and Disturbance Estimator (UDE) is proposed to reject periodical disturbances when a limited bandwidth is required for the control output. The motivation comes from several applications where the system actuator may introduce a bandwidth limitation, as a result of internal delays, or when the actuator itself is a limited bandwidth closed-loop system. When the traditional UDE approach is applied in these systems, the stability requirements impose a limitation over the effective bandwidth of the UDE filter and therefore disturbances cannot be fully rejected by the filter. In the case where the expected disturbance is periodical with a known fundamental frequency, the proposed UDE filter is designed as a chain of filters to match selected bands of the expected disturbance spectrum and fully reject them while maintaining the desired stability margins. A design example of a power inverter application is investigated and extensive simulation results are provided to verify the proposed UDE filter design

PLL-less three-phase droop-controlled inverter with inherent current-limiting property

In this paper, a novel droop control method for three-phase grid-connected inverters is proposed to guarantee closed-loop system stability and an inherent current-limiting property without the need of a PLL. The inverter is connected to the grid via a filter and a line. Based on the synchronously rotating dq frame modelling and nonlinear ultimate boundedness theory, it is analytically proven that the proposed control scheme maintains the inverter current below a certain upper bound. This current limitation is guaranteed independently of the grid, line and filter parameters; thus increasing the controller robustness. In addition, asymptotic stability of the desired equilibrium point of the closed-loop system is guaranteed under different values of the proposed controller gain. To verify the effectiveness of the proposed nonlinear control strategy, extensive simulations are realized using Matlab/Simulink, where both the stability and the current-limiting property of the controller are validated

SRF-based current-limiting droop controller for three-phase grid-tied inverters

A nonlinear droop controller for three-phase gridconnected inverters that guarantees a rigorous current limitation and asymptotic stability for the closed-loop system is proposed in this paper. The proposed controller is designed using the synchronous reference frame (SRF) and can easily change its operation between the PQ-set mode, i.e. accurate regulation of real and reactive power to their reference values, and the droop control mode. Furthermore, nonlinear input-to-state stability theory is used to guarantee that the grid current remains limited below a given value under both normal and abnormal grid conditions (grid faults). Asymptotic stability for any equilibrium point of the closed-loop system is also analytically proven. The proposed control approach is verified through extended real-time simulation results of a three-phase inverter connected to both a normal and a faulty grid

Constrained control for microgrids with constant power loads

This paper analyses the reachability properties of Microgrids connected to constant power loads subject to input and state constraints. Constraint requirements in microgrids are either inherent, i.e. inverter modulation indices limitations, or imposed, such as current and voltage limitation which are safety critical. In this paper, we propose an analysis of the controllability properties of a microgrid using set theoretic notions; this analysis sheds light on the constraint admissibility properties of a microgrid with constant power loads in terms of constraint satisfaction and robustness to changes in power demands. Lastly, we provide a method of recasting the original nonlinear microgrid control problem into controlling a linear system subject to bounded additive disturbances and output constraints

Influence of fault-ride-through requirements for distributed generators on the protection coordination of an actual distribution system with reclosers

This paper analyses the existing protection scheme of a real distribution system with distributed generators, in Greece. Network protection utilizes three successive reclosers at the main trunk and fuses at the laterals. The generating units are protected by overcurrent and voltage/frequency relays. The analysis focuses on the fault-ride-through capability of the generating units and proposes the resetting of the generators and network protection relays so as to conform to the requirements imposed by distribution system operators and international standards. The proposed protection system guarantees selectivity for any short-circuits occurring inside or outside the distribution system, irrelative if the generating units are connected to the network or not. Meaningful conclusions are derived from the application of the proposed protection coordination principle

Current-limiting droop controller for single-phase inverters operating in island mode

In this paper, a current-limiting droop controller with nonlinear dynamics is proposed for the stand-alone operation of single-phase inverters. The proposed controller regulates the voltage and frequency of the load depending on the real and reactive power demand, as required in modern ac micro- grids. The dynamic performance of inverters equipped with the proposed control scheme is investigated under different load conditions (linear and non-linear loads) and their current-limiting property is analytically proven to hold at all times using nonlinear ultimate boundedness theory. Then, the closed-loop stability of a single-phase inverter operating in island mode is proven for the first time using both a resistive and a constant power load. The desired controller performance is experimentally validated on a testbed consisting of a single-phase inverter connected to a linear (resistive) and a nonlinear (diode rectifier) load, where the ability of the proposed controller to operate in the droop control mode while maintaining the desired current limitation is proven under various load changes