Decentralized voltage control in microgrids based on a novel structure of droop control
The renewable energy power generation combining with utility grid is considered as an alternative solution for economic and environmental issues of conventional power systems. The concept of microgrid is proposed for better renewable energy penetration into the utility grid. The microgrid can be defined as low voltage distributed power networks comprising various distributed generators, storage and controllable loads. Several main advantages of the microgrid can be given: improving renewable energy penetration level, better energy supply for remote areas, power balancing at local level with self-supplying possibility, and maintaining load supply during off-grid mode. In islanding mode, the load power in the MG should be properly supplied by multiple DG units. Usually, the droop control method which mimics the behavior of a synchronous generator in traditional power system is adopted, which does not need the use of critical communications. In the conventional droop control, the inverter’s output voltage droops with the reactive power and the output frequency droops with the active power. This behavior ensures the power sharing among inverters like occurs with generators and loads in electrical power systems. The droop method uses only local measurement and does not have a critical high bandwidth communication link among the DG units. Thus it achieves a higher reliability level and is flexible in terms of the physical location of the modules. However, the conventional droop method also has several drawbacks including a slow transient response, a trade-off between power-sharing accuracy and voltage deviation, unbalanced harmonic current sharing and a high dependency on the inverter output impedance. To overcome these drawbacks, modified droop control methods are proposed.