Volume 18, Issue 4 -

Due to the importance of arm inductances in modular multilevel converters (MMC), it is necessary to propose an approach which provide the optimal inductor size considering the technical and manufacturing constraints. The investigations prove that utilizing the coupled inductor enhances converter performance and decreases the required value arm inductance and sub-module capacitor and the final converter volume and mass especially in high power applications. The analytical model of coupled inductors is consisted of circuit, electromagnetic and thermal model which will be combined with MMC circuit model to create the global MMC model. The analytical model of coupled inductor leads to fast convergence of optimization algorithm while it does not provide the accurate results. Another proposed approach is to put a finite element software directly in the optimization loop which intensely increases the optimization time and sometime made it hard to converge. In this paper, a novel hybrid correction loop has been proposed and developed to modify the analytical model parameters while the optimization is running. It effectively increases the results accuracy whereas the optimization does not increase so much. The proposed hybrid correction algorithm was employed in a global optimization loop which tries to minimize the total mass of a high power MMC converter according to the number of series sub-modules per arm. The results show that there is an optimal point which provides the maximum performance dependent to the load specifications, IGBT characteristics, goal function and manufacturing constraints. Also, in this research, the effect of coupled inductors on total converter mass and efficiency was investigated and compared to the state of using uncoupled inductors.

This paper presents the voltage performance analysis of the system with various types of wind turbine generating units (WTGUs). A detailed voltage performance analysis is carried out by considering the different PQ models used for computing the reactive power output of the WTGUs (fixed/semi-variable speed and variable speed WTGUs). The different PQ models of fixed/semi-variable speed WTGUs incorporated for the studies are voltage dependent model, voltage independent model, power factor based model, and PX model. In addition, the variable speed WTGUs are also considered in different fixed power factor mode of operation. Based on these models, a comparative analysis is presented. A modified 27-bus equivalent distribution test system with dispersed wind generation is considered for the studies. Further, the case studies have been carried out by considering the various wind power output levels of WTGUs to examine its impact on system voltage performance. From the comparative analysis, the power factor based model can be the best choice over the other models (which are based on voltages) for the system studies with fixed/semi-variable speed WTGUs.

A battery model that predicts the electrical performance with high accuracy is vital for circuit designing to control the battery usage and improve its runtime and safety in electrical systems such as electric vehicle (EV). In this paper, a relatively simple equivalent circuit based model is developed for modeling the performance of lithium ion batteries used in automotive applications. The model’s strategy adopted for characterization of the internal parameters is based on two main techniques: one is the electrochemical impedance spectroscopy (EIS) tests, where comparable circuit is set up as per EIS test perceptions and the other is the current pulses technique, this latter is carried out using a proposed current pulses profile. The extracted parameters are scheduled on the state-of-charge, temperature, and current direction. The comparison between experiment and simulation results demonstrates that with the optimally extracted performance parameters, the proposed approach can accurately predict the performance of differents batteries of various sizes, capacities, and materials. Eventually, this simple battery model can serve as a robust and reliable tool for predicting the battery’s I-V performance in electrified vehicle battery management systems.

In order to harmonic propagation phenomenon damping, active power filters are installed on distribution systems.The installation of active power filter on buses, may itself cause harmonic propagation. The purpose of this paper is analyse the harmonic propagation also discuss the installation effect of a voltage-detection-based shunt active power filter that acts as a harmonic terminator. In order to meet this target because of the uncertain and variable nature of characteristic impedance, Z c , It is necessary to use active power filter with automatic gain adjustment. According to voltage harmonics detected at its installation bus, the active filter can adjust its control gain, keeping the voltage harmonics lower than a specified level, even if the characteristic impedance of the distribution line is unknown. In this paper, an automatic gain adjustment for active filter is designed based on keeping and controlling of total harmonic distortion in the allowable limitation according to IEEE-519 Std. Other features and benefits of using automatic gain adjustment are reduction of compensating current and hence losses reduction that these two results in reduction of the cost of active power filter. Finally the advantage of this controller compared with similar studies is simplicity, and its remoteness from the usual complexity.

Wind power is a major contributor in the renewable energy sector but it faces some issues regarding modern grid-code compliance. Popular wind power systems based on Doubly-Fed Induction Generators (DFIG) need additional protection under grid voltage disturbances. They also need to support the grid voltage under such transient occurrences. This paper presents a novel performance enhancement scheme for DFIGs subjected to symmetrical and asymmetrical voltage sags and swells at the Point of Common Coupling (PCC). The scheme comprises a protection system and a reactive power management system working simultaneously under the command of a supervisory control system. The protection system protects the DFIG converter by limiting the overcurrent in the Rotor Side Converter (RSC) of the DFIG and keeping the dc-link capacitor voltage within an acceptable range; whereas, the reactive power management supports the grid voltage by either injecting or absorbing reactive power to reduce the magnitude of voltage sags and swells. It is found that the performance of the DFIG wind generation system improves significantly under the proposed scheme. A grid-connected 9-MW DFIG wind farm is used for simulation in MATLAB/Simscape Power Systems.

In this paper, the dynamic equilibrium of power market is analyzed by using the economics principle. In the assumption that the market demand is the nonlinear function with variable coefficients of the price forecast, and the market supply is the nonlinear function with variable coefficients of the spot price, first-order difference power market model with variable coefficient is established. The coefficients of power market model are made by the original time series, such as electricity price and demand, and supply elasticity. Besides, the paper also discusses the practical stability and instability of the power market model by means of Lyapunov function and difference inequality, getting a series of criteria for judging the practical stability and instability of the power market and estimation formula of the ultimate convergence boundary. Finally, the validity of these models and Lyapunov stability conditions are simulated by using the data of California.

Time-of-use price will be an important demand side management method as a supplement of administrative strategy, which could encourage the electric vehicles to change the charging behaviors. In this paper, for parking garage charging stations with photovoltaic power generation, a pricing model is proposed to encourage different consumers to participate in demand response by providing them with a list of price plans. Firstly, we introduce the system structure of the photovoltaic-assisted charging station. Afterwards, the pricing and dispatch model for the charging station is proposed, aiming to achieve maximum profit. The formulated objective for this model is to maximize the profit of selling electricity to electric vehicles and back to the grid, feed-in-tariffs revenue and parking revenue or to minimize charging station’s cost of purchasing electricity, while considering various electric vehicles demands. The decision variables for charging station include the time-of-use price, the rebate on parking fee and the charging/discharging power. To solve this model, the distributed alternating direction method of multipliers is adopted to determine the operational strategies of charging stations via local decision making and limited communication between adjacent charging stations. Finally, according to the simulation results, the optimal time-of-use pricing strategy for charging stations can play an important role in shaving the peak loads. Besides, the interconnected charging stations coordinated operation can enhance the overall system operational flexibility and reliability.

The Photovoltaic systems have a slow dynamics of convergence toward the maximum power point (MPP), especially those equipped by the Boost converter based on the Conventional Incremental Conductance technique. This paper presents the simulation and hardware implementation of auto-scaling variable step-size MPPT in low cost and low power consumption μ-controller (PIC16F877A). Design and simulation of the proposed system are presented using ISIS PROTEUS. The two MPPT techniques, conventional IncCond method and variable step-size method are simulated in the ISIS PROTUES environment. The simulation results show the high tracking accuracy and fast speed convergence of the proposed method. Indeed, the experimental results are closely similar to those of simulation and also prove the advantages of the proposed technique, such as its reliability and accuracy in tracking the MPP. The suggested MPPT system reduces the complexity in implementation and cost of the photovoltaic systems.

Electrical machines are useful in nuclear power plants, military applications, domestic appliances and industrial utilities. The relevant literature indicates that the induction machine takes about ninety percent of all electrical machine globally used in the industry. The risk of the failure of this machine can be avoided if the proper diagnostic scheme is designed and implemented to detect failure/impending faults at an incipient stage. This paper describes Discrete Wavelet Transform (DWT) based diagnosis technique that analyses stator currents of induction machine under healthy and faulty conditions. This enables the extraction of harmonic components caused by each signal for further analysis. On one hand, the maximum energy for each signal gives information about the time a certain fault occurs in the machine and the time it starts deviating from the normal (healthy) state. On the other hand, a Fault Index (FI) is generated for each situation in order to classify the state of the machine into normal, medium or high. The colours; green, yellow and red are used to represent the normal, medium and high states respectively. Should the faults at medium state are not detected on time, it may lead a more serious fault (high state). Thus, this technique is robust to identify a fault of an induction machine at the incipient state with a very minimal time.

This paper addresses the transient stability problem in power systems of nonlinear character. A recursive nonlinear backstepping controller for improving the single machine infinite bus system’s dynamic behavior is proposed for the system global stabilization considering the network transfer conductances. Despite parameters uncertainties, nonlinear dynamics and/or disturbances, the feedback laws based on the backstepping approach are explicitly derived and the conservatism of the stability property is guaranteed for both lossy and lossless power system representations. Simulation results, via MATLAB ™ -Simulink, reveal that the proposed backstepping technique can be feasibly designed to ensure significant dynamic performance enhancements.

This paper proposes a hybrid method based on corrected kinetic energy to determine the critical clearing time. The proposed method structure has been implemented utilizing network preserving model to take details of power systems into consideration. To implement proposed method, the initial critical point is estimated using new concept of equal area criterion. Critical corrected kinetic energy is obtained using method which determines the amount of severity of generator contribution in a fault scenario. Due to the latter, the behavior of AVR and governor are taken into account. From initial and corrected kinetic energy of generators and consequently system, high precision critical clearing time is calculated. In order to validate the proposed method, some comprehensive case studies have been conducted on the IEEE9-bus, IEEE39-bus and IEEE68-bus test systems. Some comprehensiveness in considering the details, simplicity in implementation and low computational cost are the outstanding features of the proposed approach. Also, simulation results approve that the proposed approach can be used in real-time application without loss of any detail in transient stability assessment.

This paper proposes an efficient power smoothing control strategy for variable speed grid connected permanent magnet synchronous generator (PMSG) based wind turbine generator (WTG) with supercapacitor energy storage system (SCESS). As WTG installations are increasing, these systems generate a fluctuated output power as a result of varying wind speed and need to have a power smoothing capability to have a smooth output power profile. The optimal size of the SCESS is determined and a controller is proposed and implemented to continuously charge and discharge the SCESS to achieve its objectives. The SCESS is exploited to minimize the short term fluctuation to have a smooth power profile during normal operation. A bi-directional buck boost converter is used to integrate the SCESS with the system. Two back to back connected three level Neutral Point Clamped (NPC) converters are used for the power conversion. The control strategy and the system model have been developed for the NPCs, the buck boost converter and the variable speed WTG system. The Real Time Digital Simulator (RTDS) based results conducted on 2 MW/4 kV PMSG verify the effectiveness and superiority of the proposed controller.