Power electronics converters are utilized for interconnecting distributed energy storage systems into the grid. These converters can support the stability of the power system under abnormal grid conditions. In this work a new coordinated voltage-frequency support strategy is proposed for energy storage systems considering the reactance to resistance ratio of the grid impedance. Unlike conventional support schemes for transmission grids, where voltage and frequency support are decoupled, the proposed strategy considers the coupling between voltage and frequency due to the resistive characteristics of the grid impedance in low voltage distribution grids. An advanced frequency support scheme is also developed considering both droop-based frequency support and virtual inertia control for improving frequency stability. The proposed strategy ensures a fair compensation between voltage and frequency support by utilizing an adaptive gain that is online calculated according to the short-circuit fault characteristics. Simulations and experimental tests are carried out using a laboratory setup to validate the proposed strategy.

The effective operation of the grid side converter is crucial for the reliability of renewable energy systems. Grid side converters are responsible for converting the energy produced by renewable energy sources into a form that can be injected into the grid. A grid side converter controller manages the conversion using feedback from voltage and current sensor measurements from both the alternative current (ac) and direct current (dc) sides of the converter. However, these sensory devices may fail or diverge from their normal operation, which can affect the effective operation of the converter. This article proposes a model-based fault diagnosis scheme for sensors located in the dc and ac sides of the grid-tied converter, taking into account the effects of fault propagation due to the physical interconnection between the ac and the dc sides. First, a discrete-time, nonlinear model of the grid side converter is formulated that considers the presence of ac current and dc voltage sensor faults. Second, a nonlinear estimation scheme is designed by considering the effects of sensor noise and modeling uncertainty. The estimator design utilizes the estimator gain matrix that is obtained by solving the linear matrix inequalities problem. Finally, a sensor fault diagnosis scheme is designed based on the analytical redundancy relations considering the estimations of the nonlinear estimation scheme. The performance of the proposed fault diagnosis scheme is validated through both simulation and experimental results, demonstrating its effectiveness in timely detecting and isolating sensor faults.

Alternating current (AC) microgrids are expected to operate as active components within smart distribution grids in the near future. The high penetration of intermittent renewable energy sources and the rapid electrification of the thermal and transportation sectors pose serious challenges that must be addressed by modern distribution system operators. Hence, new solutions should be developed to overcome these issues. Microgrids can be considered as a great candidate for the provision of ancillary services since they are more flexible to coordinate their distributed generation sources and their loads. This paper proposes a method for compensating microgrid power factor and loads asymmetries by utilizing advanced functionalities enabled by grid tied inverters of photovoltaics and energy storage systems. Further, a central controller has been developed for adaptively regulating the provision of both reactive power and phase balancing services according to the measured loading conditions at the microgrid’s point of common coupling. An experimental validation with a laboratory scale inverter and a real time hardware in the loop investigation demonstrates that the provision of such ancillary services by the microgrid can significantly improve the operation of distribution grids in terms of power quality, energy losses and utilization of available capacity.

Low voltage distribution grids face substantial challenges in terms of power quality, efficiency and grid utilization that can affect both utilities and consumers. This work proposes a controller for grid-tied photovoltaic (PV) inverters, enhanced with advanced functionalities, to compensate current asymmetries and harmonics in low voltage distribution grids. The proposed method relies on the estimation of the grid impedance that is utilised to approximate the current imbalance and harmonics caused by nearby loads. As a result, the need to measure the asymmetries and harmonics of nearby loads is alleviated, allowing a plug-and-play operation of PV inverters for the provision of phase balancing and harmonic compensation services. The effectiveness of the proposed method is experimentally validated in a laboratory-scale low voltage distribution grid with prototype inverters, in which PV inverters estimate and compensate current imbalance and harmonics of nearby loads. The experimental results indicate benefits in terms of power quality, reduced energy losses and effective utilization of distribution grid capacity.

A Renewable Energy Source (RES), enhanced with Fault Ride Through (FRT) capability, can provide proper voltage and frequency support to the power grid under several disturbances. This paper investigates how a realistic dynamic power system with high penetration of RES reacts under different operating conditions. More specifically, the power system operation when RES operates according to the current grid regulations of the Cyprus TSO is demonstrated. For the purposes of this investigation, a RES is interconnected with the IEEE 9-bus test system, reinforced with an appropriate directional over-current protection scheme, and dynamic electromagnetic transient simulation results are provided.

A grid-connected power electronics based distributed generator can act as an active element by providing support to the grid under various disturbances. In this paper, a voltage support scheme dedicated for distributed generators installed in the low voltage distribution network is proposed. The voltage support considers the resistive characteristics of the distribution line grid impedance in order to maximize its effectiveness. Therefore, the optimal active and reactive current references are estimated based on the resistance to reactance ratio (R/X). Moreover, a droop curve is suggested such that the intensity of voltage support will be defined according to the voltage drop conditions. The proposed voltage support concept is applied on a photovoltaic system to validate its operation. The effectiveness of the proposal is confirmed by means of simulations in a realistic low voltage distribution network.