Operating Stability Analysis And Control For Microgrid

Features like low investment, environment-friendly, high reliability, flexibility turn DG (Distributed Generation) into an interesting and promising technological option. As power system sees an increasing DG penetration level, beside the advantages DG units may also bring problems to the utility grid like reverse power flow, voltage deviation, and voltage fluctuation. Microgrid which is developed by grouping a cluster of loads and parallel DG units in a local area is the practical answer to the problems caused by high DG penetration and makes large-scale application of DG system possible. Microgrid is connected to the utility grid at the Point of Common Coupling (PCC) and able to import and export energy from and to the grid flexibly by controlling the active and reactive power flow, When there is a fault in the utility grid or during its maintenance, microgrid disconnects from the utility gird and operates in islanding mode, in order to guarantee the power reliability for the critical localized load in the microgrid.In this paper the allowable maximum capacity of grid-connected DG in microgrid is calculated. The research model of calculating allowable maximum capacity of DGs is established after analyzing the influencing factors like location, capability, connection method of grid-connected DGs. Voltage, power flow, and DG capacity constraints are formed simultaneously. The mathematical model is realized based on Genetic algorithm and Newton-Raphson iterative method. The correctness is proved by example microgrid from a practical project which is supported by the International Energy Foundation.Generally, the typical DG operation mode is grid-connected, as in this case the produced power can be adjusted to the maximum efficiency. The control algorithms for these DGs have been developed and refined over time, being highly efficient for grid connected operation. Therefore, integrate extra functions to DG system by adding equipment in a microgrid is more efficient and practical. This work develops a microgrid stabilizer based on energy storage equipment, to provide DGs with the additional functionality of working in islanding mode without changing their control strategies for grid connected mode. In addition to this function, the microgrid stabilizer is important in grid-connected microgrid as well, especially if renewable energy sources which are characterized by having a stochastic and intermittent behavior. The exchange power at PCC should be smoothed by microgrid stabilizer; otherwise, output power fluctuations may cause problems like voltage fluctuation and large frequency deviation in main grid.First, DC capacity, DC voltage of the microgrid stabilizer value is calculated, IGBT and filter parameters are selected. Then, this paper described control circuit and its installation for microgrid stabilizer. Analysis of microgrid operating characteristics and mathematical models of the controller according to the operation mode are given after that. The microgrid stabilizer is expected to operate with high efficiency, low output current total harmonics distortion, and its output voltage waveform must be sinusoidal, with specified frequency and amplitude throughout all operation modes. In addition to this requirement, the microgrid stabilizer must be robust against disturbance, have good voltage regulation and fast dynamic and transient response for sudden changes.Sliding mode control (SMC) is one of the effective nonlinear robust control approaches since it provides system dynamics with an invariance property to uncertainties once the system dynamics are controlled in the sliding mode. In a practical control system, parameter variation, uncertainties, external disturbance and limitation of detection makes it difficult to acquire accurate mathematical model. Consequently, the traditional sliding mode control can hardly achieve the perfect result. Therefore, this work proposes an adaptive robust total sliding-mode control (ATSMC) system for the MSC, which colligate the advantages of adaptive control and sliding-mode control. The total sliding surface eliminates the control error and gets a sliding motion through the entire state trajectory. The pulse width modulation avoids unstable switching frequency.Microgrid operates in islanding mode, is a weak AC power system. Consequently, the DG output power variation or load disturbance is more likely to cause voltage fluctuation or voltage sag. These problems may disconnect DG unit or sensitive load from the ac line. Therefore, this paper improved the control system for the microgrid stabilizer based on SMC and proportion resonant (PR) control, which combines fast dynamic response and robustness of SMC and harmonics reduction of PR. Reaching law is designed to ensure the performance of this control system.The simulation and experimental results illustrate that the control system for the microgrid stabilizer is insensitivity to parametric uncertainty and external disturbances. The fast dynamic response and robust control performance of the microgrid stabilizer ensure the stable operation of microgrid in both modes. It improves controllability of the exchange power between microgrid and utility network, smooth DG output power while the microgrid working in grid-connected mode, provides voltage and frequency support when the microgrid working in islanding mode, meanwhile it ensure seamless transaction between the two operation modes.