An auxiliary voltage controller is proposed for the fault ride-through (FRT) control of voltage-controlled inverters used in distributed energy resources. The proposed auxiliary controller can be added to different existing voltage control strategies with minimum or no need to alter the original structure of the voltage controller. The proposed controller prevents the transient overcurrent during grid faults and controls the exchange of real and reactive powers according to the governing grid standard. A fault detection algorithm is also implemented to detect the fault instant and determine the interval of fault during which the real and reactive power references are adjusted accordingly. Complete stability analysis of the proposed controller is performed through a new model. Successful performance of the proposed controller in achieving FRT requirements of German and Danish codes are illustrated by simulations. Extensive real-time and experimental results are also presented and discussed.

A universal controller for three-phase inverters (called UC3) is proposed to operate converters of a microgrid (MG) in grid connected (GC) and islanded modes and ensure seamless transition between these modes without reconfiguration of the control structure. These are desirable features for the flexible MG applications. UC3 provides control over real and reactive powers while offering voltage and frequency support. It emulates an inertia behavior that is grid supporting and stabilizing. Additional features of the proposed controller are summarized as follows: 1) it can be used in inverters with different output filters and is robust against their changes and uncertainties as well as the grid parameters; 2) it integrates the synchronization, which offers agile and smooth responses without causing stability issues; and 3) its structure is well suited for digital implementations. UC3 is the extension of the original single-phase work to three-phase. This paper presents the following contributions: 1) development of UC3 equations in a 2-D stationary domain; 2) derivation of a new quasi-linear model for UC3 in a GC operation; 3) a comparative stability analysis of the UC3-operated GC inverters using the developed quasi-linear model and the conventional method; 4) initial development of a new mathematical framework for the modeling and stability analysis of UC3-operated islanded inverters; and 5) comparative simulation and experimental results.

With the increased uncertainty in the power system operation due to growing penetration of highly intermittent energy sources such as wind power, the need for the impact assessment of the renewable penetration on system operating risk and the quantification of benefits of using energy storage technologies is more than ever. A recovery-risk-analysis based analytical framework for operating risk assessment of windintegrated bulk power system following a major contingency disturbance is presented in this paper. Two new indices that quantify the recovery risk profile of the bulk power system and its load delivery points following major disturbances are introduced in this work. The indices obtained from the proposed framework quantifies the impact of increasing wind penetration on the system operating risk and the reliability benefits of using fast-responding energy storage system such as flywheel energy storage systems. The proposed methodology is illustrated through several case studies carried out in a test system.

Synchronverter is an inverter equipped with proper control algorithms to mimic the operation of a synchronous generator. In this paper, a stability analysis of the synchronverter is performed and it is shown that the synchronverter's stability margins are directly related to the presence of an external passive resistance. This resistance compromises its efficiency as much as five percent in a typical low-power setting. A modified synchronverter structure is proposed where the passive resistor is removed from the power circuit and its impact is emulated in the control structure. This modification avoids degradation of the efficiency without compromising the stability. The analytical results are verified by digital and real-time simulations.

Integration of an energy storage system (ESS) into a distribution network not only affects the supply reliability of the customer, but also has distinct reliability implications and consequences to the utility. The reliability value associated with an ESS highly depends on the ownership, market and regulatory structures. This paper presents a probabilistic framework to evaluate the reliability value of ESS to the distribution system considering aforementioned factors. In this regard, a new probabilistic reliability model of ESS is developed and integrated into a sequential Monte Carlo based simulation framework. The developed ESS model consists of the Markov based component model and the mixed integer linear programming based formulation of operating strategies that incorporate different scenarios of ownership, market structures, and the ESS characteristics. The reliability/financial risk performance of the Distribution System Operator (DSO) with ESS under quality regulations are quantified. Furthermore, the developed ESS model is utilized to explore the prospect of investor-owned ESS providing supply recovery and distribution grid capacity services to the DSO. Case studies are conducted on a test distribution network to show the effectiveness of the proposed model. Finally, the paper presents discussions on important considerations for efficient utilization of ESS in active distribution systems.