مجله مهندسی برق و الکترونیک ایران
سال سیزدهم شماره 3 (پاییز 1395)

Due to widespread presence of distributed energy resources (DERs) in current power systems and need to a two-way communication system between the system operator and distributed generators, storage units and controllable loads, presence of a smart distribution system with proper communication infrastructure is required. Considering complication of smart grid operation in presence of DERs, there is various operating method where most of them aim to minimize the operation cost. In this paper, a more precise model of operating cost of energy storage systems has been proposed where the life cycle, times of charge or discharge and depth of discharge of the storage units have been taken into account. This model has been applied on operation cost function in presence of distributed generators and responsive loads. In order to study the system constraints the proposed model has been applied on a 69 buses distribution system with 5 fuel cell units, 5 micro turbine units, 3 lead-acid battery units, and 10 responsive loads. The PSO algorithm has been employed so as to achieve optimum solution. The simulation results validate the proposed model performance.

Recently connection of some Distributed generation and load has created Microgrids (MGs). This MG can operate in grid connected mode or disconnects from main grid. Maintaining stability of MG When it is disconnected from main grid, is an important problem. Frequency stability is one of aspects of stability. Using of energy storage with appropriate control strategy can be effective in this subject. In this paper A flywheel energy storage system (FESS) in detailed is described and is used for frequency control in islanding mode. Two control strategies for DGs inverters are stated and suitable method for each DG and FESS is selected. Simulation is carried by MATLAB/Simulink. Two case studies are examined. In two cases, FESS effect on frequency in islanding process is discussed. Results of simulation verify appropriate operation of FESS and confirm effectiveness of this application.

In this paper¡ at first¡ the non-detection zones (NDZ) of under/over voltage relay have been determined for constant current inverter based DG and then¡ an islanding detection method is presented using this relay. In the presented method¡ the DG direct current reference has a fixed value in normal condition¡ if the point of common coupling (PCC) voltage changes¡ Idref is determined as a linear function of PCC voltage. In this method¡ Idref is determined in terms of Idref0 and local load direct current axes adaptively. The proposed method is evaluated in islanding and non-islanding conditions¡ using PSCAD/EMTDC and MATLAB software. Simulation results show that the proposed method has a proper operation in the islanding and non-islanding conditions.

One of the basic components of future distribution networks are renewable energy resources (RER). The uncertainty in power production of renewable resources such as wind and solar as well as load is another characteristic of such networks. Conventional power flow methods as a basic tool for power system studies may not be suitable for active distribution networks such as microgrids. In this paper a heuristic load flow method considering the effects of intermittent behavior of RERs and load is modeled in probabilistic load flow algorithm. The method is suitable for both radial and weakly meshed distribution networks with RER for operation and planning of microgrids. Imperialist competitive algorithm (ICA) as heuristic based optimization algorithm is applied to formulate and solve the Probabilistic Load Flow (PLF). Based on PLF technique calculated parameters of the system such as bus voltages and feeders'' current¡ are extracted as random variables. A modified version of conventional IEEE 33-bus test system with RER is used as a weakly meshed network to evaluate efficiency and capability of the algorithm. Results are compared with Monte Carlo Simulation (MCS) method. The PDF and CDF of bus voltages¡ power of feeders'' and network total loss are compared. Based on the results the presented approach can solve the PLF problem regardless of the type of distribution network.

Energy is one of the most important requirements in development of societies. In the last decade¡ widespread attention has been paid to renewable energies. Despite the obvious advantages of renewable energy resources¡ their dependence on climate change is one of the most fundamental problems facing to the development of them. Optimal design of hybrid renewable energy systems is a major issue in power systems. In this paper¡ a new unit sizing method is considered for stand-alone hybrid renewable energy systems in order to maximize the electricity match rate between demand and supply intervals in lowest cost. This work is undertaken with triple objective function: inequality coefficient (IC)¡ correlation coefficient (CC)¡ and annualized cost of system (ACS). The reliability of the system is investigated by using the Loss of Power Supply Probability (LPSP) function. Weather data and load demand are collected from a small village in southeastern Iran¡ as a case study. A suitable control algorithm has been designed for the system which can decide to start the diesel generator or use the battery power when the total output power from the system cannot meet the load demand. The results of this study are valuable for implementation of future hybrid renewable energy system in any location; making best use of resources and maximum economic efficiency.

In this paper¡ a hybrid model-based nonlinear optimal control method is used to compute the optimal power distribution and power management in parallel hybrid electric vehicles. In the power management strategy¡ for optimal power distribution between the internal combustion engine¡ electrical system and the other subsystems¡ nonlinear predictive control is applied. In achieving this goal¡ a hierarchical control structure is utilized. This type of control structure consists of three levels of monitoring¡ coordinating and local level. Nonlinear modeling and performance index in the proposed problem¡ shall be formulated at the regulatory level of the controller. Discrete dynamic mode of operation (motor-generator) in hybrid electric vehicle¡ requires us to use a dual-mode switch model and to define an alternative expression of performance index for the optimal control problem. In the performance index¡ tracking error penalty¡ fuel consumption¡ friction losses and the maintenance of the battery charge has been included. Optimization of performance index is performed by the theory of nonlinear model predictive control using numerical methods.

To achieve accurate reactive power and harmonic currents sharing among inverter interfaced distributed generation (DG) units in islanded microgrids¡ a hierarchal control scheme consisting of primary and secondary levels based on instantaneous circulating currents is proposed in this paper. Firstly¡ fundamental and main harmonic components of each inverter output current are extracted at the primary level and transmitted to the secondary controller. Then¡ using this information¡ instantaneous circulating currents at different frequencies are calculated and to generate proper control signals are applied to the primary controller. Consequently¡ these signals are inserted as voltage references after passing control blocks. In contrast to the conventional virtual impedance schemes¡ where reactive power and harmonic currents sharing are realized at the expense of introducing additional voltages drop and harmonic distortions¡ the proposed control strategy effect on the amplitude and waveform quality of DGs’ voltage is negligible. Meanwhile¡ it is able to provide accurate harmonic currents sharing even if nonlinear load(s) is directly connected at the terminal of DG unit(s). Control system design is described in detail and simulation results for four voltage-controlled DG units are provided to demonstrate the effectiveness of the proposed control method.

In this paper¡ demand response modeled with unit commitment in a nonlinear optimization problem¡ for increasing system security and economic reasons. By incorporating the demand response in the market it should decrease cost reduction¡ market clearing problems¡ congested lines and make a flat load profile at peak hours. In this problem¡ loads are combined of fixed and responsive loads. When this loads participate in program¡ they could curtail or shift to another operating hours. By considering system security¡ Studies show system speed and higher accuracy in problem convergence. Also¡ demand response decrease network load peak¡ system operating cost¡ fuel consumption and line congestion. Numerical simulation shows the effectiveness of proposed method on 6- bus system.

With increasing influence of variable speed wind turbines in power systems¡ the equivalent inertia of network is reduced. Accordingly¡ after the occurrence of disturbance in system¡ the frequency fluctuations in power systems increase. To overcome this problem¡ a supplementary control loop is added to the converter of the variable speed wind turbine in order to share the inertia of this type of turbines in the network. But the appropriate rate of this contribution depends on the amount of load and must be suitably changed based on the load. In this paper¡ a Takagi–Sugeno (T–S) fuzzy system is designed to determine the coefficient of the contribution of the variable speed wind turbine after the onset of disturbance¡ whereas sharing the maximum value of their inertia to compensate the reduced production in the network. However¡ the turbines do not pass their minimum speed limit while sharing the maximum value of inertia¡ causing another disturbance in the network. In the proposed method¡ first¡ by using Particle Swarm Optimization (PSO) algorithm¡ the optimal values of contribution proportional to the amount of load is obtained¡ while the constraint of minimum speed is not violated. In the next stage¡ the initial T–S fuzzy system is extracted from the optimal values of contribution by using subtractive clustering algorithm. Moreover¡ Recursive Least Square (RLS) algorithm is used to adjust the consequent part of the T–S system. The efficiency of the proposed method is demonstrated through the simulation in the variable amount of load.

Nowadays multilevel converters have progressed in high power and renewable energy applications¡ because of their ability to generate multiple voltage levels. But the balancing of capacitor voltage¡ the existing of multiple DC sources and particularly protection issues in renewable energy application are the main problems of these multilevel converters. The proposed inverter in this paper¡ by modifying the structure of existing five-level multicell inverter with the new control method for this inverter¡ in addition to having the ability to maintain the capacitor voltage in the specified value¡ it also obviates the protection problems of them. Also the modified inverter has the ability of safety¡ connectivity of solar cells and obtaining maximum power. Finally¡ the operation of the proposed inverter and control method and mathematical analyzing is verified by simulation results with PSCAD/EMTDC software.

In this paper¡ has been proposed the collaboration between smart buildings as a solution for improving the energy efficiency and increasing the contribution of the consumers in energy consumption management of residential buildings integrated with micro-grid. In the proposed model¡ the scheduling of power consumption in smart building and the optimal operation of distributed energy resources through molding of the exchange energy among smart buildings is presented using mixed-integer linear programming (MILP). The smart buildings are equipped with building energy management system (BEMS)¡ distributed generation¡ and energy storage system. Being considered as a small micro-grid¡ each building is able to feed parts of its own electricity demand and all thermal demand. The proposed model is applied to the two smart buildings consisting of 30 units and 90 units. The simulation results indicate that the collaboration among smart buildings will lead to the more power trade options¡ the reduction of energy consumption costs¡ the reduced received power from the grid¡ and the balance between supply and demand.

This paper addresses the bidding problem faced by a virtual power plant (VPP) in energy¡ spinning reserve service¡ and reactive power service market simultaneously. Therefore¡ a non-equilibrium model based on security constraints price-based unit commitment (SCPBUC)¡ which is take into account the supply-demand balancing and security constraints of VPP¡ is proposed. By the presented model¡ VPP can provide spinning reserve and reactive power services to transmission system operator (TSO)¡ and reactive power service to distribution system operator (DSO) as well as it can participate in energy market simultaneously. The presented model is a nonconvex nonlinear mixed-integer optimization with inter-temporal constraints and solved by mixed-integer nonlinear programming (MINLP).