مجله کیفیت و بهره وری صنعت برق ایران
سال نهم شماره 1 (پیاپی 18، بهار 1399)

One of the main tasks of the control systems in the wind turbines is to maintain the power of the wind when its wind speed proceed its nominal value. Because the failure to maintain the power in its nominal value in the region of the turbine curve damages the turbine and increases the mechanical stress. This object is obtained by controlling the pitch angle in the third region of the turbine curve. In this study, a fuzzy fractional-order PID controller is designed to regulate pitch angle in the wind turbines and its efficiency is compared with conventional PID and fractional-order PID controllers. The proposed control method is applied to a 100kW wind turbine under a variable wind speed. The simulations results show that the value of the root mean squre error for the proposed control method is less than the conventional PID and fractional order PID controllers, and also the proposed control scheme results in smoother and better control signals, output power and generator speed in third region of performance.

The safe operation of power system depends on its stability and security supply in all times. The dynamic instability (small signal instability) is one of phenomena that results in power system instability and has been discussed as a challenge in power system control and operation from a long time ago. Commonly the dynamic instability appears as undamped low frequency electromechanical oscillations or with insufficient damping. In order to damp these oscillations and improve dynamic stability, the use of Power System Stabilizer (PSS) in excitation systems is the most effective option. In this paper the impact of PSSs of various units of a real power plant on Iran grid dynamic stability is investigated and its performance is improved. In this way first the excitation system and PSS model of the power plant are identified and verified, and then the main parameters of PSSs are optimally retuned using the proposed method based on Particle Swarm Optimization (PSO) algorithm. In the following, power plant operation is considered in different disturbances with respect to the retuned PSSs, so that, the effect of optimal parameters on local and inter-area oscillation modes damping is taken into account, not only within time domain analysis, but also within frequency domain analysis. The results of proposed flexible and comprehensive analysis display the suitable performance of retuned PSSs.

Photovoltaic systems (PVs), despite their many advantages, may have effects such as power quality issues (voltage and harmonics increase), short-circuits level increase, protection issues, and transient stability for network. Some of these effects are due to the high PV penetration, which encounters the network with the over-voltage and harmonics problems. In this paper, the location, size and optimal operation of batteries and the passive filters are proposed separately and simultaneously to provide a practical solution to overcome the over-voltage and harmonic problems. In the economic objective function, the fixed and operation costs of batteries and passive filters are considered and constraints are defined to limit the total harmonic voltage and effective voltage of the network buses below the standard ranges. Taking into account the profit of energy arbitrage and reduction of network losses in the objective function, it is concluded that the use of the filter alone has the lowest cost of 736 $ (but results in an effective voltage increase beyond the permissible limit). Also, simultaneous use of battery and filter resulted in the highest loss reduction of 4.1 kWh, with the total harmonic voltage and effective voltage of the network buses are within the 5% range. Also, cables, overhead lines, and transformer are modeled by frequency-dependent characteristics. The studied network is composed of a real low voltage feeder including two PVs; however, in simulations, the PV penetration is increased to study a more challenging situation. Simulations are carried out by using DIgSILENT and MATLAB software and their interface.

Transformer oil is one of the most important materials which are used in electrical and transformer industrial. The main function is protection and insulation of transformer core components, as well as heat transfer from its core to environment. Improving the performance of this material has been one of the researchers' concerns. The effect of different nanoparticle on the electrical breakdown voltage and heat transfer of this oil is investigated in this research. The nanofluids have made by various six nanoparticles suspended in base transformer oil. New suspension is tested under the ultrasonic condition. Thermal investigation also carried out by KD2 instrument. Electrical mode analyzed by Breakdown voltage meter. The experiments carried out in tow temperature 25oC and 55oC. Results show that, using Nano particles in transformer oil causes improve heat transfer and decrease Electrical breakdown Voltage, respectively. Diamond nanofluid showed the best result as 83% enhancement in thermal conductivity

One of the most critical and complex issues in long-term planning of distribution networks is the optimal placement of distribution transformers. In this paper, the optimal placement of distribution transformers was investigated based on a complete and multi-objective function. In the proposed method, location, optimal capacity, and the service area are determined by minimizing costs (investment, operation, reliability) and improving reliability and power quality parameters (voltage drop) due to electric restrictions (radial structure, voltage drop and equipment capacities). The planning of distribution networks is complicated due to the existence of numerous and multiple-criteria decision-making variables in objective functions; hence in this paper, the symbiotic organisms search (SOS) algorithm was used as a new heuristic optimization algorithm to solve the problem of long-term planning of distribution networks. The proposed method is tested on a real distribution network to evaluate the performance of the proposed algorithm,  Comparing the results of the proposed algorithm with the results of studies with genetic algorithms (GA), biogeography-based optimization (BBO) and imperialist competitive algorithm (ICA), indicates the performance and advantages of the proposed algorithm over other algorithms.

This paper studies the effects of static synchronous compensator (STATCOM) on synchronous generator conventional loss of field (LOF) protection. To accomplish a comprehensive study, a typical and realistic excitation system is considered for the generator by using the phase-domain generator model available in the real-time-digital-simulator. Using such a system, LOF phenomenon is realistically studied in accordance with IEEE Standard C37. 102-2016, and the generator dynamics along with the LOF relay performance are investigated through different types of complete and partial LOF failures. It is shown that STATCOM presence can cause two major impacts on the LOF protection; i) It imposes an additional time delay on the relay performance to detect the LOF failures, while the amount of such a time delay is dependent on the generator initial loading and the type of LOF failure, and ii) It might lead the generator to reach a new stable operating point (NSOP) during change-over from AVR to MVR mode. Both impacts and the related consequences are discussed in the paper and some solutions are suggested to deal with the probable negative aspects. The solutions comprises 1) employing an overloading relay to avoid thermal impacts on the stator windings and 2) employing an alarming scheme to avoid the undesirable effects, that the generator may experience during prolonged under-excitation operation in the probable NSOPs.