Mohamed Shawky Elmoursi

Stability Evaluation of Interconnected Multi-Inverter Microgrids Through Critical Clusters

This paper proposes the notion of “critical cluster” in inverter based droop controlled microgrids, thus representing the neighborhood of the distributed generation units that determines the small signal stability margin of the entire system. The clustering starts from the lowest impedance electrical connection between distributed generators, termed as “critical line”. At first, the systematic study of few similar microgrids by means of the eigenvalue analysis of their small signal models reveals the correlation between the location of their dominant low frequency modes and the individual connections between neighboring inverters. Second, the sensitivity analysis of active power droop gain with respect to network parameters confirms the previous findings and shows the dominating effect of the critical lines impedance. Simulations and small signal analysis studies assess the impact of the choice of various interconnection points when two individual droop controlled islanded microgrids are connected to form a single microgrid. At the same time, simplified models on the basis of critical clusters allow the effective approximation of the stability margin corresponding to the original systems. The results reveal that different interconnection points lead to strong variations in the performance of the coupled microgrid both in terms of transient response and small signal stability.

Adaptive secondary voltage control for grid interface of large scale wind park

Large scale, doubly-fed induction generator (DFIG) based wind power plants (WPP) are being designed to support grid voltage regulation. Based on the transmission level interconnection, this paper proposes a novel adaptive secondary voltage control (ASVC) strategy to improve network voltage profile and transient response performance. The control strategy combines algorithms for reactive current division, automatic gain controller and online voltage tracking scheme. The control strategy is tested in a system consisting of DFIG based Wind Park and conventional generation units, such as hydro and diesel synchronous generators, connected to a weak grid. Network voltage profile improvement achieved by ASVC is verified by simulation and compared with the conventional line-drop Secondary Voltage Control (SVC) and Primary Voltage Control (PVC). Since the system relies on the measurement and communication network, the time delay is modeled and considered. Improvement of the transient response of the controller is verified by simulation and comparison with a system without automatic gain control algorithm. Furthermore, the proposed control strategy is tested in both steady-state and under extreme system contingencies.

Enhancement of islanded droop-controlled microgrid performance via power filter design

This paper introduces a new method for tuning the power filter for enhancing the stability and transient response of a droop controlled microgrid system based on inverter-interfaced Distributed Generators (DGs). To tackle the stability issue of such a microgrid, faster and more robust droop controllers are required. Thus, the delay and phase shift imposed by the Low Pass Filter (LPF), which is incorporated in the power controller of inverter-based DGs, should also be minimized. In this context, the purpose of this paper is two-fold: to show that a tunable power filter with a cutoff frequency below the fundamental frequency can improve the performance of an islanded droop controlled DG microgrid; and to suggest a filter tuning method. A comprehensive study is carried out by means of small signal analysis in order to assess the impact of the power filter on the systems eigenvalues. The mathematical analysis shows that the proposed technique results in efficient damping of oscillations following system disturbances. The simulation results verify the superior performance of the proposed approach in terms of improving the transient response and the small signal stability of the islanded DG microgrid during the startup and load excursion operations.

Adaptive Voltage and Frequency Control of Islanded Multi-Microgrids

This paper introduces an adaptive voltage and frequency control method for inverter-based distributed generations (DGs) in a multi-microgrid (MMG) structure using distributed cooperative control and adaptive neural networks (ANN). First, model-based controllers are designed using the Lyapunov theory and dynamics of the inverter-based DGs. ANNs are then utilized to approximate these dynamics, resulting in an intelligent controller, which does not require a priori information about DG parameters. Also, the proposed controllers do not require the use of voltage and current proportional-integral controllers normally found in the literature. The effectiveness of the proposed controllers are verified through simulations under different scenarios on an MMG test system. Using Lyapunov analysis, it is proved that the tracking error and the neural network weights are uniformly ultimately bounded, which results in achieving superior dynamic voltage and frequency regulation.

An integrated system configuration for electric springs to enhance the stability in future smart grid

Electric spring (ES) has been reported recently as an alternative device for achieving the grid voltage stability in distribution system using demand side management. ES comprises of six switches in total and injects the voltage in quadrature with the non-critical load current for regulating the PCC voltage to a predefined value. The voltage across the non-critical load decreases/increases as a result of ES action. Since the ES can only perform reactive compensation it faces many operational limitations. Moreover, due to quadrature injection the injected voltage requirement becomes quite high even for a small variation in the PCC voltage. This also limits the compensation range of ES. To overcome these limitations this paper proposes a new configuration for ES comprising of nine switches in total. The proposed configuration allows the complete control over the phase angle of the injected voltage thereby controlling both active and reactive power exchanged between ES and grid. Various operating modes of the proposed nine-switch based ES configuration are analyzed. An appropriate control algorithm is developed and the validity of the proposed configuration is verified through extensive simulation under different operating conditions.

Reduced-Order Model for Inter-Inverter Oscillations in Islanded Droop-Controlled Microgrids

Representing a major technology toward the proliferation of sustainable energy resources, droop controlled microgrids have attracted numerous modeling approaches with a view to improving their operation and applicability. Acknowledging the low inertia of distributed generators and the network characteristics at the distribution level, this paper proposes a reduced order model for inter-inverter oscillations in autonomous droop controlled microgrids. At first, a simplified equivalent network on the basis of Kron reduction is suggested as an effective topology for the study of inter-inverter oscillating currents. The focus shifts toward low-frequency modes following the detailed small signal model and its simplification according to the singular perturbation theory with multiple time scales. Assuming accurate and instantaneous tracking of the voltage by the inner controllers on their local reference frame, the quasi steady state study results in a simple closed-loop model. This model is successfully benchmarked against the detailed model and other germane models over the full range of operational damping ratios.

Domain of Stability Characterization for Hybrid Microgrids Considering Different Power Sharing Conditions

The low cost of diesel generators have led to their increased presence in hybrid microgrids. The presence of low-inertia devices in such microgrids makes the stability of hybrid microgrids a pressing concern. This paper analyzes the impact of equal and unequal active power sharing conditions on the dynamic stability of a hybrid microgrid. The sensitivity of low-frequency eigenvalues to changes in active power droop gains is studied and a “domain of stability” region is proposed to assess the stability of hybrid microgrids under different power sharing conditions. The different operating regions within the domain of stability are defined. Based on the sensitivity analysis and the domain of stability, the optimal droop gain for the diesel generator to improve the stability of the microgrid is determined. Time-domain simulations are carried out in the MATLAB/Simulink software environment and are used to validate the small-signal stability analysis results.

Dynamic VAR planning for rotor-angle and short-term voltage stability enhancement

In the short-term timescale, separation between load-driven and generator-driven stability problems is rarely well-defined. Allocation of VSC-based VAR compensation, such as STATCOM, has the ability to boost system voltages in the fault-on condition, thus reducing generator power swing, in addition to circumventing delayed voltage recovery upon fault clearing, thereby reducing the risk of induction motor stalling. This paper proposes a multi-objective, hybrid static/dynamic VAR planning strategy incorporating distinct rotor angle and voltage stability indices found through time-domain simulations of full-order system models. The proposed method utilizes parallel high performance computing (HPC) capabilities combined with a genetic algorithm (GA) and is applied on the New England 39-bus system with assumed high penetration of induction machines. The study demonstrates that compared to voltage stability enhancement, improvement of rotor-angle stability through shunt dynamic VAR requires substantial additional capacity, the cost of which can be reduced using hybrid static/dynamic installations.

Dynamic analysis of OLTC and voltage regulator under active network management considering different load profiles

This paper provides a comparative study of coordinated voltage control (CVC) strategies for on-load tap-changers (OLTCs) and voltage regulator (VRs) under active and passive network management schemes (ANM and PNM). In PNM scheme, OLTC and VR are restricted to only regulate the secondary bus voltages at fixed set-points, while in ANM scheme, control of the secondary bus voltages within the statutory voltage range is allowed, thus providing area-based voltage controls. In this paper, the dynamic model of a 33kV 16-bus United Kingdom generic distribution system (UKGDS) is firstly implemented in PSCAD/EMTDC and validated with a steady-state model, using optimal power flow (OPF)-based technique, developed in GAMS software. Secondly, the OLTC and VR controls under ANM and PNM schemes are developed with discrete tap-changers, and tested considering different load profiles and dynamic loads. The results show that the ANM scheme allows considerable utilization of OLTC and VR compared to PNM scheme, but also may increase the loading of these devices depending on load levels and voltage profile in the power network.

Enhanced critical clearing time estimation and fault recovery strategy for an inverter-based microgrid with IM load

This paper analyzes an inverter-based microgrid with an induction motor (IM) load and proposes an enhanced method to calculate its critical clearing time (CCT) and a fault recovery strategy to increase its resiliency. The CCT calculation is improved by accounting for the transient electrical phase that deacelerates the IM immediatly after the fault. Results show that our method is able to accurately calculate this effect without adding much computational complexity and corrects the overestimation error obtained with previous methods. The fault recovery strategy increases the resiliency of the microgrid by avoiding IM stalling after a fault. It is achieved by commanding a reduction in the inverter source output frequency that increases the electrical torque available at low speeds leading to a larger CCT and expedited recovery. The accuracy and functionality of both the CCT estimation and the fault recovery strategy are validated with a microgrid experimental set-up.