مجله مهندسی برق و الکترونیک ایران
سال شانزدهم شماره 3 (پاییز 1398)

Considering simplicity of the practical implementation, application of the sliding mode control in power electronics converters has increased in recent years. Equivalent control approach can be used to stabilize the converter switching frequency in sliding mode control of the converters. In this approach, controller gains are tuned by trial and error and hence, stability of the controller cannot be guaranteed in a wide range of operation. To cope with this problem, a novel approach for sliding mode controller design as well as gains selection in flyback switch mode power supplies is developed in this paper. Also, adaptive backstepping and sliding mode nonlinear controllers are compared considering different criteria according to experimental tests in a wide operating range.
Evaluation criteria are steady-state error, converter dynamic response during start-up, robustness of the controller against load and line changes and finally, ease of practical implementation. Using Texas instruments digital signal processor (DSP-TMS320F2810), practical responses of the developed controllers are presented. Also, simulations are accomplished using MATLAB/Simulink software.

In this paper a quasi z-source DC-DC converter is proposed that provides soft switching condition for all semiconductor devices with only two resonant paths without the use of any auxiliary circuitry, as well as any auxiliary switch. Based on the proposed structure, only one switch is used in the circuit, which greatly simplifies the performance of this converter in comparison with the similar structures. Also, the non-isolated structure and the lack of use of transformers and even coupled inductors to increase the voltage gain have led to the simplicity and high efficiency of the proposed converter. The converter efficiency is 95.8% at full load range. The control technique of the converter is simple and the pulse width modulation (PWM) is used to trigger the switch. Due to use of high switching frequency, the total size and volume of the converter especially for passive elements in its structure has decreased. The converter is completely analyzed and simulated in the software environment. Also, a 40-360V 50W laboratory prototype operating at 100 kHz is implemented. The experimental results confirm the simulation results and theoretical analysis.

Nowadays, linear induction motors have been employed in different industries and in applications with linear motion. Different topologies can be considered for these motors; one of them is based on their secondary construction. Linear induction motors have two secondary types: sheet and ladder. In this paper, first, the analysis and modeling of the sheet and ladder-secondary linear induction motor are discussed and their formulation is expressed. Then, considering the effect of the end-effect phenomenon, these two types of motors have been designed and optimized by using the particle swarm optimization algorithm and choosing the efficiency, power factor, weight and end-effect braking force as the objective function. The results show significantly improvements of the objective function. To confirm the results of the design, Finite Element Method has been used. According to the results, it can be seen that the ladder-secondary linear induction motor produces more thrust than sheet-secondary linear induction motor, which can be used to improve starting conditions and motor performance.

In this paper, chaotic pulse width modulation (CSPWM) is introduced by applying the chaotic map theory. Influences of the CSPWM on common and differential modes of the electromagnetic interference and bearing voltage are analysed. The fractal dimension of the CSPWM also is used for evaluating the level of chaotic behavior in the CSPWM. Finally, by obtaining some experimental and numerical results, it can be observed that the CSPWM decreases the common mode electromagnetic interference with minor effects on the bearing’s voltage.

Electrical machines are one of the most important elements in power systems. To design and optimize electrical machines and attached drivers, real-time simulators with hardware-in-loop capability can be employed. Real-time simulator is a hardware module which interacts with other physical systems in real-time, and therefore, can replace the electrical machine in performing the required experiments even before assembling the first prototype. This saves time, money and enables optimal design. This paper proposes a real-time simulator for three-phase induction machines. The techniques presented here can be applied to other machines as well. The proposed solution incorporates novel mathematical methods in solving the corresponding differential equations in real-time as well as novel FPGA-based architecture, and as a result, solves the equations in 32-bit floating-point in 110 ns time-steps.

Increasing penetration level of modern generating units, in response to growing need for electrical energy and environmental issues, besides changing inherent of distribution systems and loads introduces new uncertainties into the power systems. In order to deal with such uncertainties, measurement-based control schemes are introduced in modern power systems. An attempt is made in this paper to propose a new measurement-based approach for tuning of excitation system parameters in large-scale power systems. For this purpose, Center of Gravity (COG) concept in corporation with fault propagation paradigm are introduced to reduce power system dimensions. In this framework, the power system dynamic behavior is represented by an equivalent model in which the geographical areas interact with the COG through unique fictitious interconnectors. Then, a new control scheme proposes in which utilizes post-fault system dynamics in the rotor angle-terminal voltage deviations portrait to design a coordinated Automatic Voltage Regulator (AVR)-Power System Stabilizer (PSS). Basic power system equations are employed to derive a case-independent algorithm. Effectiveness of the proposed method is examined for different fault scenarios on the 16 machines IEEE benchmark system.

Today, in order to use wind turbines in controlling the frequency of a power network, droop and inertial control methods are used for variable speed wind turbine. Adjusting the gains on the droop and inertia control loops is very influential on the performance of wind turbines, but because of varying the wind speed and power system conditions, the adjustment of the control coefficients that produce the best response in all situations seems to be impossible. In this paper, a new method for adaptive adjusting of droop and inertia control loops gains in DFIG is presented. In order to eliminate the defects and problems of power system and wind turbine modeling, it is also proposed to use the data-driven method which only executes based on the input and output of the system. To control faster and in order to prevent the frequency drooping, applying the second derivative of the error is used to adaptive adjusting of droop and inertia control loop gains. In the proposed control method, using the K-Vector Nearest Neighborhood, the output of the next moment is estimated, and then, using the Hessian matrix, the coefficients of the frequency control loops are adjusted adaptively. Simulation results demonstrate the proper performance of a data-driven-based adaptive approach to increase the frequency nadir (FN) and to decrease the frequency variations of the power system in a steady state and transient state.

Grid code compatibility of HVDC interfaced offshore wind farm is evaluated in this paper. DC coupling causes power system contingencies not to get reflected in the offshore wind farm. In this regard, DC link voltage level is modulated to reflect the status of the onshore power system. Accordingly, onshore power system contingencies are reflected in the offshore wind farm by means of DC link voltage measurement and appropriate exercise of the wind farm HVDC converter. It is to excite dynamic frequency response and Low Voltage Ride Through (LVRT) capabilities of the modern wind turbines. A unified control structure is also proposed for doubly fed induction generator-based offshore wind farms that can model and compare three communication based, frequency modulation based, and voltage modulation based LVRT approaches. Moreover, a hybrid frequency and voltage modulation based approach is proposed in which an appropriate discrimination is embedded between fast transients related to LVRT condition and slow transients related to frequency contingencies. Accordingly, voltage modulation and frequency modulation approaches are utilized under LVRT and frequency events respectively. Simulation results have revealed similar dynamic frequency response capability for all four approaches. Moreover, except frequency modulation approach, remaining approaches have been able to fulfill grid code requirements given agile communication speed. It is concluded that the proposed hybrid frequency and voltage modulation, as the superior approach, has advantages like minimum LVRT DC voltage profile increase and subsequent deviation from theoperating point after the fault and preservation of conventional control structure thanks to appropriate dissociation between slow and fast dynamics.

Circuit breakers (CBs) play an important role in protection of the components used in power systems such as transformers and generators. Therefore, the condition assessment of CBs are really important. The interruption chamber of high voltage CBs is one of the most prominent part including moving and fix arc and main contacts. The operation of CBs under short circuit currents or the high number operations under rated-current leads to the erosion of contacts due to the high temperature or abrasion. The assessment of this part due to difficulty in direct access to it is a main problem. Dynamic resistance measurement (DRM) is an indirect assessment method of contacts used in HVCB. This paper provides a comprehensive FEM-based simulations via COMSOL to assess the impacts of intentional failures in contacts on DRM profile. Subsequently, it has been attempted to develop a fuzzy method to evaluate the failure risk of CBs. The proposed algorithm has been verified through experimental results.

Takagi and Sugeno have proposed a fuzzy model for a continuous system in which disturbances are also considered as a time-dependent variable function in the consequent part of the fuzzy model rules. To eliminate the disturbances effect on the output of a system with a Takagi and Sugeno fuzzy model, researchers have proposed a linear matrix inequality-based minimization problem in which system parameters are directly present in the problem constraints. In electromechanical systems, the existence of parametric uncertainties such as uncertainties in the moment of inertia matrices and inaccuracy of actuators parameters cannot be neglected in any way. In this paper, at first, by solving an optimization problem, the upper bound of the uncertainties in the system parameters is determined, and the fuzzy model with disturbances is extracted in the presence of uncertainties. In the following, in order to eliminate disturbances in the presence of parametric uncertainties using a stable fuzzy controller based on the parallel distributed compensation, a minimization theorem based on the new obtained fuzzy model is proposed and its validity is proven. Finally, to evaluate the performance of the proposed controller, a boat-mounted camera stabilizer is used as a case study. The simulations results well demonstrate the favorable efficiency of the proposed controller in eliminating disturbances by taking into account the parametric uncertainties.

One of the most important problems with the utilization of distance relays is wrong operation possibility during the power swing. The power swing is a transient phenomenon in the power system, which enters into the protective zones of distance relays and causes to mal-operation of them. In this paper, a new method needless to Fourier transform using Norm of sampled data from the current signal is proposed to proper detection of power swing and preventing the wrong operation of distance relays. Accordingly, a signal window is formed and begins to sampling from the current signal. Afterward, presence data in the window will form a matrix, which their Norm value will be calculated One of the advantages of the proposed method is independency from signal processing, fast detection of the power swing, detection of the simultaneous fault with power swing, and reduction of computations for ease of implementation. The proposed method has been evaluated on the standard 39-bus test system using the DIgSILENT and Matlab software. This method was also implemented by distance relay and tester device made by Amirkabir University of Technology (AUT) and Vebko Amirkabir Knowledge Base Company. Practical results of the proposed method are compared with the conventional methods which are used in protection relays especially Siemens protection relays. The results obviously show that the proposed method has better performance than the conventional industrial method.

In recent years, power system islanding is considered as the last resort to prevent network instability. The purpose of controlled islanding is to establish self-sustaining electrical islands to prevent a blackout and lead to ease of restoration. In this regard, an appropriate decision - making algorithm is required to determine the best separation points in a short time.
This paper proposes a novel methodology by developing the hierarchical spectral clustering algorithm. This method is computationally very efficient and determines an islanding solution with minimal power flow disruption while ensuring that each island contains only coherent generators. The proposed method also enables operators to constrain any branch to be excluded from the islanding solution. The method is tested using the models of the IEEE 39 and IEEE 118-bus test systems and comparative analysis with existing methods have been performed. The simulation results demonstrate that the methodology is more efficient than another existing approach and could be utilized in the real-time application of the power grid.

A hybrid optimization algorithm based on genetic algorithm and choatic hyper spherical search method is proposed. In the proposed method, in order to increase the efficiency of searching the optimal solution, chaos theory along with  genetic operators have been used. This, not only makes the results of the proposed algorithm definite and decreases their standard deviation, but also resolves the weakness of the hyper spherical search optimization algorithm based on chaos theory including the speed of convergence and the weak performance in some benchmark functions. The simulation results on the standard benchmark functions show that the proposed algorithm not only has faster convergence, but also acts as a more accurate search algorithm to find the optimal solution in comparison to the standard hyper spherical search algorithm, chaotic hyper sherical search algorithm, and some other optimization algorithms such as genetic, particle swarm, and harmony search algorithm.

A pseudo-dynamic approach solves a multistage transmission expansion planning (MTEP) problem as a sequence of single stage transmission expansion planning (STEP) problems. In this paper, the MTEP problem is reformulated. Based on the new formulation, a relation is obtained between an MTEP and STEPs local optima. Considering this relation, a new pseudo-dynamic algorithm is proposed. Unlike the ordinary pseudo-dynamic methods, in each stage, it applies a set of high quality local optimums of the STEP problems instead of just using the global optimum. The modification improves the search ability of the algorithm.  IEEE 24-bus, Colombian 93-bus, Bolivian 57-bus and Iranian 51-bus systems are used to evaluate the proposed and the simulation results have confirmed its effectiveness.