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 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

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

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

Enhanced current-limiting droop controller for grid-connected inverters to guarantee stability and maximize power injection under grid faults

Droop controlled inverters are widely used to integrate distributed energy resources (DERs) to the smart grid and provide ancillary services (frequency and voltage support). However, during grid variations or faults, the droop control scheme should inherit a current-limiting property to protect both the inverter and the DER unit. In this brief, a novel structure of the recently developed current-limiting droop (CLD) controller is proposed to accomplish two main tasks: i) guarantee current limitation with maximum power injection during grid faults and ii) rigorously guarantee asymptotic stability of any equilibrium point in a given bounded operating range of the closed-loop system for a grid-connected inverter. Since the maximum power of the DER unit can be utilized under grid faults with the proposed enhanced CLD, then inspired by the latest fault-ride-through requirements, it is further extended to provide voltage support to a faulty grid via the maximum injection of reactive power. This is achieved by simply adjusting the reactive power reference opposed to existing control schemes which require adjustment of both the real and the reactive power. Hence, a unified current-limiting control scheme for grid-connected inverters under both normal and faulty grids with a simplified voltage support mechanism is developed and experimentally verified in this brief

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