Mohamed Al Hosani

A Novel Approach to Solve Power Flow for Islanded Microgrids Using Modified Newton Raphson With Droop Control of DG

The study of power flow analysis for microgrids has gained importance where several methods have been proposed to solve these problems. However, these schemes are complicated and not easy to implement due to the absence of a slack bus as well as the dependence of the power on frequency as a result of the droop characteristics. This paper proposes simple and effective modifications to the conventional method (Newton Raphson) to compute the power flow for microgrids. The presented method provides a simple, easy to implement, and accurate approach to solve the power flow equations for microgrids. The proposed method is applied to two test systems: a 6-bus system and a 38-bus system. The results are compared against simulation results from PSCAD/EMTDC which validate the effectiveness of the developed method. The proposed technique can be easily integrated in current commercially available power system software and can be applied for power system studies.

Detecting defects in outdoor non-ceramic insulators using near-field microwave non-destructive testing

This paper presents a novel near-field microwave nondestructive testing technique for defect detection in non-ceramic insulators (NCI). In this work, distribution class 33 kV NCI samples with no defects, air voids in silicone rubber, cracks in the fiberglass core and small metallic inclusion between the fiber core and shank were inspected. The microwave inspection system utilizes an open-ended rectangular waveguide sensor operating in the near-field at a frequency of 24 GHz. The used inspection system is simple, safe and relatively inexpensive. A data acquisition system was used to record the measured data. The results showed that all defects were repeatedly detected with high sensitivity. Line scans of the samples were obtained revealing the presence of different defects and their location. The technique also demonstrated ability to detect thickness variations in the silicon rubber shank.

Development of Dynamic Estimators for Islanding Detection of Inverter-Based DG

In this paper, a new islanding detection method (IDM) is proposed to dynamically estimate islanding occurrence. The proposed dynamic estimators estimate amplitudes and phase angles of the current injected by the grid at the point of common coupling with the distributed generation (DG) in addition to the DGs bus voltage. A distributed two-level algorithm is proposed to detect an islanding condition for single and multi-DG configurations. Analytical design and transient analysis are carried out for the islanding detection problem to determine the nondetection zone (NDZ) of the proposed islanding detection algorithm. A local low-frequency meshed communication network is sufficient to achieve distributed islanding detection capability for a general multi-DG network with negligible NDZ. It is shown through simulations that the proposed IDM can successfully distinguish an islanding condition from other disturbances that may occur in power system networks.

A Transient Stiffness Measure for Islanding Detection of Multi-DG Systems

Islanding detection is important to ensure the reliability and safety of distributed generation (DG). In this paper, a new active islanding detection method (IDM) is proposed, and it depends on individually estimating an overall transient stiffness measure for any multi-DG system to establish a clear separation between prior- and post-islanding stiffness. For the multi-DG system to avoid spectrum overlapping, each of its DGs is required to perturb at distinct frequencies. By using this concept of perturbation separation, the proposed technique can be applied to multi-DG systems without requiring any communication among the DGs. Simulation results show that the proposed technique is scalable and robust against different loading conditions and variations of grid stiffness levels as well as with respect to the number of connected DGs and different types of DG controllers. It is also shown that the proposed technique can successfully distinguish islanding conditions from other disturbances that may occur in power system networks.

A Novel DC Fault Ride-Through Scheme for MTDC Networks Connecting Large-Scale Wind Parks

This paper proposes a novel fault ride-through protection scheme for multiterminal dc (MTDC) networks connecting large-scale offshore wind parks. The proposed scheme introduces an MTDC network with a hybrid fault current limiter (FCL), which consists of a high-temperature superconducting FCL (HTS-FCL) and series braking resistor (SBR) integrated with low-loss mechanical dc circuit breaker (DCCB). The aim of HTS-FCL is to provide fast quenching, limit the dc fault current within the interrupting capacity of the DCCB, and reduce the dc fault current to certain levels such that converters can sustain prolonged periods of operation without blocking. Also, SBR allows automatic reclosing of DCCBs following a temporary fault, thereby enhancing the network security and reliability. Mechanical DCCBs with the proposed hybrid limiter are installed at the terminating points of each dc cable. The proposed protection scheme is based on differential current approaches with fault clearing time of few milliseconds, which gives enhanced network selectivity and reliability. A four-terminal meshed MTDC network with the proposed hybrid FCL is modeled in PSCAD/EMTDC. The obtained simulation results confirm successful dc fault isolation and network recovery for different dc fault scenarios.

A simple and accurate approach to solve the power flow for balanced islanded microgrids

Power flow studies are very important in the planning or expansion of power system. With the integration of distributed generation (DG), micro-grids are becoming attractive. So, it is important to study the power flow of micro-grids. In grid connected mode, the power flow of the system can be solved in a conventional manner. In islanded mode, the conventional method (like Gauss Seidel) cannot be applied to solve power flow analysis. Hence some modifications are required to implement the conventional Gauss Seidel method to islanded micro-grids. This paper proposes a Modified Gauss Seidel (MGS) method, which is an extension of the conventional Gauss Seidel (GS) method. The proposed method is simple, easy to implement and accurate in solving the power flow analysis for islanded microgrids. The MGS algorithm is implemented on a 6 bus test system. The results are compared against the simulations results obtained from PSCAD/EMTDC which proves the accuracy of the proposed MGS algorithm.

Scheduled Perturbation to Reduce Nondetection Zone for Low Gain Sandia Frequency Shift Method

It is known that the choice of gain (K) in the Sandia frequency shift (SFS) scheme has direct impacts on the stability of a system with grid-connected distributed generations (DGs). In this paper, a scheduled perturbation technique is proposed to reduce the stability impact of K. In the proposed technique, chopping fraction (cf) is used to compensate for reduction in the value of K, where higher cf values are used to achieve zero nondetection zone (NDZ) under low gain SFS. It is shown by analysis that theoretical reduction of NDZ can be always achieved for a nonzero value of cf. Simulations for singleand multi-DGs systems are performed to verify the analytical analysis. It is shown that an appropriate design of scheduled signal duty cycle (d) is of critical importance to realize the proposed reduction in NDZ. While close synchronization of perturbation signals for multi-DG system is required, a delay of 0.33 s is shown to be tolerated for a two-DG system. Synchronization can be achieved either through locally synchronized timers or by limited communication among DGs. The proposed technique provides an attractive option for systems with high DG penetration by reducing the negative impact of K on stability.

Development of current dynamic estimator for Islanding Detection of inverter based Distributed Generation

Various Islanding Detection Methods (IDMs) have been developed within the last ten years due the tremendous increase in the penetration of Distributed Generation (DG) in distribution system. This paper proposes a new resident passive IDM in which a dynamic estimator is designed based on the system dynamics during islanding occurrence. The dynamic estimator estimates the amplitude of the current injected by the grid at the point of common coupling with the DG. The estimator can also determine the flow direction of grid current. The main goal of this paper is to provide transient analysis for the islanding detection problem and to approximate the Non-Detection Zone (NDZ) of the new passive IDM. Moreover, estimator sensitivity to grid amplitude, frequency and angle variations and disturbances will be studied and simulated.

Detecting damages in outdoor non-ceramic insulators using near field microwave non-destructive testing

This paper presents a novel near-field microwave nondestructive testing technique for defect detection in non-ceramic insulators (NCI). In this work, distribution class 33kV NCI samples with no defects, air voids in silicone rubber and small metallic particles between the fiber core and shank were inspected. The microwave inspection system utilizes an openended rectangular waveguide sensor operating in the near-field at a frequency of 24 GHz. The used inspection system is simple, safe and relatively inexpensive. A data acquisition system was used to record the measured data. The results showed that all defects were repeatedly detected with high sensitivity. Line scans of the samples were obtained revealing the presence of different defects and their location. The technique also demonstrated ability to detect thickness variations in the silicone rubber shank.

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.