Microgrid technology makes possible coordination and effective use of different energy resources for supplying loads. In order to have synchronous operation between inverter resources during the occurrence of islanding condition, the use of DROOP CONTROLLER structure would be beneficial. In this paper, the conventional DROOP CONTROLLER is modified to divide proportional power between resources and cause accurate voltage setting in output resources. By providing a model for connected inverter to the nonlinear load, a harmonic DROOP CONTROLLER has been designed. By DROOP CONTROLLER related to each harmonic, the harmonic voltages are calculated and add to the reference voltage. Therefore the quality of the output voltage is improved. Then the inverter voltage control loop would be modified with resistance impedance in the presence of non-linear loads, so that, in combination with harmonic DROOP CONTROLLER, THD of output voltage considerably reduced. Simulation results show the ability of suggested method in reduction of harmonic voltages in inverters parallel operation.

In this paper, a simpler implementation of the well-known DROOP method for the control of parallel Uninterruptible Power Supply (UPS) systems is presented. In this method, in the power-sharing control scheme, the output current is calculated by software without the need for a current sensor, resulting in a simpler and cheaper structure. By doing so, the number of feedback sensors is reduced from three to two. The paralleling strategy uses the DROOP method in which the control strategy is based on the drop in the inverter output frequency and amplitude. The application of Proportional-Resonant (PR) CONTROLLERs is also extended to parallel inverter and its superior performance over the well-known Proportional-Integral-Derivative (PID) CONTROLLER is shown. To show the performance of the proposed system, a system of two-parallel connected UPS is simulated, and two types of linear and non-linear loads are considered. The non-linear load is compliant with the IEC 62040-3 standard for class I UPS. The results show that the reduction of sensors results in no error, and the control system performance is quite satisfactory. To verify the proposed concept, a two-625VA UPS system is implemented. Several tests on both linear and non-linear loads are performed and the results, which are in good agreement with those of the simulations, are provided. The results indicate that the proposed parallel inverter control structure provides a better system in terms of performance parameters.

Load sharing, as an important challenge in microgrids (MGs), is realized commonly via a DROOP control method. Conventional DROOP control methods are not applicable in unpredictable renewable energy sources (RESs) like photovoltaic (PV) and wind turbines (WT) because their output power depends on the weather conditions and can be extracted only if these free sources are available. This paper, considers two operating modes for these types of sources as Maximum Power Point Tracking (MPPT) and DC-link Voltage Control (DCLVC). These power sources usually operate in the MPPT mode unless the load of the MG drops to a lower level compared to the maximum power generation by RESs, in which case the sources switch to the DCLVC operating mode. This study proposed a method based on enhanced DROOP control, which helps RESs to choose its control mode locally without communication and share the demand of the AC MG with other dispatchable sources besides supplying its maximum power. The proposed method focused on supplying MG load from RESs as much as possible and simplicity in implementation. MG frequency helps the proposed CONTROLLER to select its operation mode. Enhanced control for DC link voltage control is offered for inverter based RESs. The validity of the proposed method is approved by simulations in the MATLAB/SIMULINK environment.

The body freedom flutter phenomenon is one of the aeroelastic instabilities that occurs due to the coupling of the aeroelastic bending mode of the wing with the short-period mode in the flight dynamics of the aircraft. By using the aeroservoelastic model and applying closed loop control, this phenomenon can be suppressed in the operating conditions of the aircraft and the velocity of this event can be increased. The simplest model aircraft capable of displaying this instability includes the flexible wing and the planar flight dynamics model. For this purpose, the wing structure is modeled using the Euler-Bernoulli beam and, the theory of minimum variable state is used to model unstable aerodynamics to make the conditions suitable for modeling the system in state space. In the control section, the elevator is used as the control surface and LQR theory with Kalman filter is used to body freedom flutter suppression. Finally, the effect of adding a closed loop control to increase the body freedom flutter velocity and the limitations of this work are studied.

The penetration level of the photovoltaic (PV) systems is growing in the distribution networks throughout the world. On the other hand, the voltage drop across the feeder and the voltage imbalance are important issues in radial distribution networks. One of the most effective methods to deal with these problems is reactive power injection by PV-based multiple distributed static compensators (D-Statcom). Hence, a method based on the integral to DROOP line algorithm, which can regulate the reactive current injection for the voltage control by optimizing the DROOP coefficient and integral gain, has been proposed in this paper. Therefore, genetic algorithm (GA) is used to minimize the voltage deviation (VD) and voltage unbalanced factor (VUF). The proposed method has been simulated and evaluated on the typical low voltage (LV) 3-phase distribution network. The results indicate that the voltage profile along the feeder has been improved from a poor range to the acceptable range of 0. 95 to 1. 05, and therefore VUF’ s reach to under 0. 15. Hence, optimal use of PV-Dstatcom’ s capacity and validity of the mentioned method are obtained.

In order to solve the problem of voltage drop and voltage imbalance in the distribution systems, the injection of reactive power by multiple static compensators is used. The distributed generation such as photovoltaic systems could play a role of the static compensators by producing reactive power. In this paper, the integral to DROOP line algorithm is used to control the reactive power in busbars. The DROOP coefficient and integral gain are important parameters in this algorithm. The determination process of these coefficients is modeled as a nonlinear and multi-objective optimization problem. The objective function in this problem is defined to establish a tradeoff between the voltage deviation and voltage unbalanced factor considering stability condition, in which the salp swarm optimization algorithm is used to solve the problem. The above problem is solved by three other optimization algorithms, which the salp algorithm has a better performance than the three algorithms. Applying the obtained optimal gain coefficients to the compensators, not only the voltage profile is improved but also the voltage unbalanced factor is reduced. The proposed method is simulated on the IEEE-34 Node test feeder. To evaluate the stability of the system, the eigenvalue analysis is used. The results show the validity of the proposed method on the system stability.

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.