جستجوی مقالات مرتبط با کلیدواژه « ریزشبکه ی جزیره ای » در نشریات گروه « برق »

Conventional droop-based techniques have widely been used in controlling Voltage Source Inverter (VSI) based on distributed generation units within a Micro-Grid (MG). Despite the simplicity and high reliability of the controller, the low-inertia nature of VSIs threatens the system stability against probable disturbances. Thus, from the stability viewpoint, it is essential to carefully monitor the system operating point to determine the stable region of the parameters affecting the micro-grid stability. To this end, a complete dynamic model for an inverter-based micro-grid has been derived. According to this model, states affecting the dominant eigenvalues of the system are identified using eigenvalues analysis and modal analysis. Then, bifurcation theory is used to predict the parameter stability margin and identify the type of bifurcation. In this paper, by introducing a bifurcation index, the appropriate virtual impedance X/R ratio is selected from the viewpoint of both small-signal analysis and bifurcation analysis. Finally, a multi-objective optimization framework is presented to prevent such a problem in parallel to other operational goals.

In this paper, the improvement of small signal stability of islanded inverter-based microgrids by selective dynamic gains of droop are discussed. An acceptable improvement must be reached by using the possible minimum number of voltage source inverters in order to adding the proper dynamic gains to their droop characteristic. First, using the linearized equations of inverter-based islanded microgrids, a state-space equation of islanded microgrids is presented. Then, a microgrid test system is considered and a sensitivity analysis is applyied to its low-frequency eigenvalues wih a low damping factor. Using the sensitivity analysis, the more effective voltage source inverters are identifiyed such that the addition of proper dynamic gains only to their droop characteristic can increase the damping of low-frequency power oscillations of microgrid. A simulation study is carried out in order to verify the effectiveness of selective dynamic gains of droop in PLECS software.

The conventional real power-frequency and reactive power-voltage droop characteristics are commonly employed to share the electric power among parallel distributed generation (DG) units. Despite some advantages such as easy implementation and no need for communication infrastructure, inaccurate reactive power sharing is one of the main disadvantages of conventional droop control. This paper presents a modified droop control scheme based on changing the y-intercept of the voltage droop characteristic. In this method, the initial control of the inverter-based DG units is performed using conventional droop characteristics. Then, the reactive power sharing error for each DG unit is determined by injecting a small real power disturbance and making a coupling between the real and reactive powers. Accordingly, the modified reactive power controller modifies the generated reactive power of each DG unit by changing the output voltage. As this process should be simultaneously implemented in all DG units and employment of the central controller and the communication link for activation of the modification procedure has some disadvantages, this paper presents a local activation mechanism. The proposed scheme operates based on a significant change of reactive power and ensures the execution of all stages of modified droop control and its reactivation to respond to the microgrid power changes. Several simulation case studies using a low voltage microgrid network verify the effectiveness of the proposed control scheme.

Nowadays distributed generation has been given considerable attention due to the increase in energy consumption and the need for using renewable energies. So, the concept of the micro grids for connecting and coordinating different distributed resources via the local power control systems has been introduced. A micro grid connects to the network at normal condition of the operation but in order to increase the system reliability and to promote the generated power quality to feed the sensitive loads, they can operate in the islanding mode too. In the islanding mode of operation in order to supply the whole load and to maintain the balance between the supply and demand, these resources use the conventional droop methods. Generally, using the conventional voltage droop control due to the difference in the impedance of the feeder connect to the resources and also the asymmetrical nature of the network, dose not lead to an appropriate reactive power sharing between the resources. In fact, using conventional droop control creates an intrinsic compromise between the accuracy of reactive power sharing and voltage regulation of micro grid buses. Therefore, in the proposed method of this paper, the coefficients of the voltage droops characteristics of the grid resources are determined by solving an optimization problem using the genetic algorithm (GA). Also in order to improve the reactive power sharing, a novel control strategy based on the adaptive virtual impedance is presented. In this method the ideal reactive power sharing is achieved by using the virtual impedance and compensating the voltage drop of the lines. The virtual impedance setting is done through the data of the voltage of PCC bus in microgrid and sending via a low bandwidth communicational link to the local controller of each resource. The proposed method is robust against the network structure changes, presence of local loads, and interruption in the communication link. Simulation results show the effectiveness of the proposed method.