Microgrids: Integrating Renewable & Distributed Energy Resources

Micro grids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. Nowadays electrical and energy engineers have to face a new scenario in which small distributed power generators and dispersed energy storage devices have to be integrated together into the grid [9]. The new electrical grid, also named smart-grid (SG), will deliver electricity from suppliers to consumers using digital technology to control appliances at consumer's homes to save energy, reducing cost and increase reliability and transparency.

In this sense, the expected whole energy system will be more interactive, intelligent, and distributed. The use of distributed generation (DG) makes no sense without using distributed storage (DS) systems to cope with the energy balances [1]. In this sense, new power electronic equipment will dominate the electrical grid in the next decades. The trend of this new grid is to become more and more distributed, and hence the energy generation and consumption areas cannot be conceived separately [1] by using the droop method, the power sharing is affected by the output impedance of the units and the line impedances.

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Hence, those virtual output impedance loops can solve this problem. In this sense, the output impedance can be seen as another control variable. [3] Another important disadvantage of the droop method is its load-dependent frequency and amplitude deviations. In order to solve this problem, a secondary controller implemented in the micro grid central control can restore the frequency and amplitude in the micro grid [1] .The conventional secondary control approach relays on using a Micro Grid Central Controller (MGCC), which includes slow controls loops and low bandwidth communication systems in order to measure some parameters in certain points of the MG, and to send back the control output information to each DG unit[3] Micro grid control is designed to facilitate an intelligent network of autonomous units.

Micro grids have an interface switch, DER units and loads. The interface switch has the ability to autonomously island the micro grid from disturbances such as faults, IEEE 1547 events or power quality events [23]1.2 FACTORS OF REDESIGNThese factors are involve in redesign of power control system and power grid system The increasing number of participants whose different goals in the operation of the power grid require access to different sets of real-time (or historical) data. The emerging power markets, which produce a completely new set of real-time data. The increase, by several orders of magnitude, in transactions between buyers and sellers that not only require the tracking of more data but also produces completely unanticipated loading of the grid. The operation of the power grid in unanticipated operating conditions that requires more sophisticated monitoring by the regional transmission operator/independent system operator (RTO/ISO) to guarantee secure operation, where the RTO/ISO has to not only guarantee operational security but fair transmission access rules that are transparent. The recent blackouts and concerns with terrorism that have made it clear that security must be guaranteed against both natural (acts of God) contingencies and malicious acts; The new-generation technologies that are changing the generation mix to one with a large number of smaller units (such as wind units and micro turbines) operating in concert with the traditional large generation plants [21]1.3 MICRO GRID DECENTRALIZED CONTROL METHODSA key feature of micro grids with distributed energy sources is that the sources are dispersed over a wide area The decentralized control of the individual interfaces should address the following basic issues. The interfaces should share the total load (linear or nonlinear) in a desired way. The decentralized control based on local measurement should guarantee stability on a global scale. The inverter control should prevent any dc voltage offsets on the microgrid. The inverter control should actively damp oscillations between the output filters. [1]Microsource controls need to insure that new microsources can be added to the system withoutmodification of existing equipment, set points can be independently chosen, the microgrid can connect to or isolate itself from the grid in a rapid and seamless fashion, reactive and active power can be independently controlled, and can meet the dynamic needs of the loads. Each micro source controller must autonomously respond effectively to system changes without requiring data from the loads, the static switch or other sources. [22]1.3.1 Voltage vs. Reactive Power (Q) DroopIntegration of large numbers of microsources into a Microgrid is not possible with basic unity power factor controls. Voltage regulation is necessary for local reliability and stability. Without local voltage control, systems with high penetrations of microsources could experience voltage and/or reactive power oscillations. Voltage control must also insure that there are no large circulating reactive currents between sources. With small errors in voltage set points, the circulating current can exceed the ratings of the microsources. This situation requires a voltage vs. reactive power droop controller so that, as the reactive power generated by the micro source becomes more capacitive, the local voltage set point is reduced. Conversely, as Q becomes more inductive, the voltage set point is increased. Figure 1.1. Networked controlled MG system.1.3.2 Power vs. Frequency DroopWhen the micro grid is connected to the grid, loads receive power both from the grid and from local micro sources, depending on the customer's situation. If the grid power is lost because of IEEE 1547 events, voltage sags, faults, blackouts, etc., the Micro grid can autonomously transfer to island operation.[22]