International Journal of Smart Electrical Engineering
Volume:3 Issue: 3, Summer 2014

A wind turbine transformer (WTT) is designed using a 3D wound core while the transformer’s total owning cost (TOC) and its inrush current performance realized as the two objective functions in a multi-objective optimization process. Multi-objective genetic algorithm is utilized to derive Pareto optimal solutions. The effects of inrush current improvement on other operating and design parameters of the transformer such as: losses, dimensions, and weights are investigated. An approach is presented to select one design from optimal Pareto solutions based on relative improvement of the inrush current performance.  Finally, this multi-objective optimum wind turbine transformer design is compared with an optimum transformer design obtained when just TOC is the objective function.

Unlike traditional power grids, smart grids have the advantage of bidirectional power flow and having distributed generations. Distributed generation systems are usually supplied by renewable sources which can cause unpredicted voltage fluctuations as a result of being intermittent. While traditional compensating devices deal with the problem of voltage fluctuation and reduced power quality with a long response time, inverter based photovoltaic systems can supply the required reactive power for the system to remain stable having a short response time and less thermal losses than typical compensators. When the insolation is weak or at nights, the reactive power capability of voltage source inverters can be used to improve the system efficiency. The challenge here is to keep the inverter DC link input voltage within an acceptable bandwidth for the inverter to be able to continue its operation when there is no active power provided by the PV cells. This paper proposes a controlling system to completely use the nominal apparent power capacity of the PV inverter that is to use the inverter capacity to provide reactive power when the active power is not nominally supplied by the PV system.  Simulation studies are carried out in MATLAB/SIMULINK to verify the effectiveness of the proposed method.

This paper presents a cooperative control which is applied to the secondary control of a microgrid controlled via a multi-agent scheme. Balancing power that leads to voltage and frequency stability in a microgrid is essential. The voltage and frequency regulations are limiting within the specified limits and conveying them to their nominal values. Limiting and conveying the voltage and frequency to their nominal values is done by the primary and secondary controls, respectively.  A Microgrid has both dispatchable and non-dispatchable sources. Dispatchable sources are controlled by the conventional P-ω and Q-E droop controls. A photovoltaic as a non-dispatchable source generates the active power according to weather conditions, but the reactive power is supplied using the E-Q droop method. The E-Q droop uses the idle capacity of the inverters in the reactive power supply. Distributed secondary control increases the stability due to good, accurate and reliable controls. The frequency is constant in the whole microgrid. Since line impedances are different, load terminal voltage control is necessary. The load is considered as another agent, who can request the desired voltage at its terminal bus.

This paper proposes a new configuration for solar energy conversion systems. One challenging issue of the photovoltaic (PV) systems is partial shading, and in this paper Module Integrated Converters (MIC) are used to overcome this problem in PV arrays. A few boost converters are employed as MICs to mitigate the shading effect. Furthermore, to reduce the cost and to increase the system performance the B4 inverter is used. This inverter is a four-switch, three-phase inverter. To validate the performance of the proposed system and simulation results, a typical prototype has been made and experimental results have been carried out. The results confirm the effectiveness of the circuit in mitigating the effects of partial shading.

Nowadays generation capacity in traditional grid depends on fossil fuels and contributes significantly to the increase of pollution emission. In deregulated grids in addition to using demand response programs (DRPs) to reducing the cost of electricity production, peak clipping and improve reliability use of green Plants such as hydro plant, wind plant become widespread. In a smart grid, end users according to their consumption, use one of the DRPs in reduction the cost of energy consumption as well as leads improvement in social welfare. DRPs change the normal pattern of end users consumption that these changes modeled in price elasticity matrix (PEM). The framework of this paper is reducing the cost of pollution generated by plants with a view to minimizing the overall system cost.  The objective function presented in this paper is the overall system cost that presented a new method for modeling the DRPs in PEMs, cost of energy produced by independent power producers units and pollution contribute to the plants. The numerical calculation of this paper calculated in a Low voltage residential network and consumers use time of use program (TOU) for load management. The Load Serving Entities (LSEs) aggregate the reduced load compared to normal pattern and participate in the pay as bid (PAB) Stackelberg competition market that called demand side bidding.

Single-phase squirrel cage and three-phase induction electromotors are the most popular electromotors used in industry from long ago. When an induction motor starts with direct connection method, it takes a lot of current from the power supply (several times the nominal current); so, several methods have been devised to start such electromotors. These methods either reduce the voltage of the stator or increase the value of rotor resistance. In this paper a new method is presented that reduces the value of the magnetic flux in stator core resulting in reduction of induction voltage in the rotor bars culminating in reduction of rotor and stator currents. This system reduces the electromotor loss and smartly protects itself against damages due to overload. Process functions are analyzed, the uses of the system are discussed and the operation of the electromotor is simulated in Matlab version 7.11.0.584 (R2010b) and Maxwell 16.0.

This paper presents a new centralized adaptive method under frequency load shedding. Sometimes, after initial frequency drop following severe disturbances, although the system frequency returns to its permissible value, however, the system might become unstable due to voltage problems. In this regard, the paper proposes a new centralized adaptive load shedding method to enhance the voltage stability margin during under frequency conditions. Selection of loads to be shed in the proposed method depends on the loads’ power factor, generators reactive power output as well as the location of the disturbance. The proposed method is implemented on the dynamic simulated model of the modified IEEE 30-Bus test system and is compared with the conventional approach to confirm the applicability and effectiveness.

Under the smart grid environments, Demand Response Resources (DRRs) as power system resources can effectively participate in and improve performance of electric systems. Congestion management is one of the technical challenges in which DRRs can play a significant role. Previously, congestion management is applied without considering the power system uncertainties. Therefore, a stochastic congestion management by means of a trade-off between DRRs and load shedding is proposed so that the outage of transmission lines and generating units are considered. In order to investigate the proposed framework, two types of Monte Carlo simulation methods, namely 1) ordinary and 2) lattice rank-1, are utilized and compared. Hence, Independent System Operator (ISO) can be able to relieve the existing transmission line congestion considering the uncertain network configuration. The proposed model is applied to the 24-bus Reliability Test System (RTS) and simulation studies are performed to examine the effectiveness and capability of the proposed framework.