Types Of Microgrid Control Techniques
Microgrids are a new form of power systems that are gaining popularity in industry to increase efficiency. A microgrid consists of a collection of distributed generation (DG) units, each of which produces some form of renewable energy. These renewable energy sources provide energy to the energy storage systems (ESS) that are contained in the microgrid and generate, store, and distribute energy to a utility grid. Microgrids can be DC microgrids, AC microgrids or Hybrid microgrids. New control techniques were needed to make sure these three different kinds of microgrids were properly manipulating, receiving, and optimizing the use of the energy provided.
There are different control schemes depending on which type of microgrid is being used and for which purposes the microgrids are used for. The microgrid originated with AC microgrids and they are typically used less due to hardware and software complications. AC microgrids are used less compared to other types of microgrids so the control techniques are less advanced compared. Control structure of AC microgrids are used to fulfil six different roles including: Regulating voltage and frequency, proper load sharing and DG coordination, resynchronization between the microgrid and the main grid, optimizing the operating cost, and proper handling of transients and switching between modes.
The AC microgrid achieves these six goals using a hierarchical control structure consisting of a primary, secondary and tertiary level generally. The primary level is used when the microgrid needs voltage or frequency stabilization, when the DGs are switched, and to limit damage due to circulating currents. The primary level utilizes many different algorithms such as droop control, virtual output impedance, or signal injection method to control voltage and frequency levels. The block diagram of the primary level is usually a PI controller and a small signal model usually consists of an integral block and the transfer function in the feedforward loop and the control variable Dp in the negative feedback loop. The secondary level is used to compensate for deviations caused by the primary control and it acts as a centralized controller to the microgrid. Deviations are compared and fixed in the secondary level by observing the error signal in individual controllers and returning the values to the primary controller. The tertiary controller is the last control level of the microgrid and it is used to monitor power flow and optimal efficiency of the microgrid using PI controllers and feedback from the other levels (Bidram).
The DC microgrid is the more common used microgrid among the uses of microgrids, typically because the output of many renewable energy sources is in DC rather than AC. A DC microgrid’s control technique is dependent on three criteria that are required for operation: reliability - operational stability and safety; function ability - maximum power generation, energy storage and power supply connectivity; and optimization – concerns optimal but not necessary processes to performance. Using these criteria, an appropriate control technique can be chosen to suit the needs of the system. DC microgrids can be autonomously controlled within the microgrid. Autonomous control is achieved by dividing the operational voltage range into several bands that activate parts of the microgrid at certain voltage bands.
Another DC microgrid control technique is like the AC control technique and it is also called hierarchical control. For a DC microgrid, hierarchical control is used to limit static voltage variation that occurs in the other control techniques that are typically used. This hierarchical control technique consists of an autonomous control layer, a local central control layer, and a remote-control layer. The autonomous control layer usually does the most work in this microgrid control technique because it typically uses instant control feedback to monitor voltage and current in a matter of milliseconds. The local central controller usually monitors the processes performed by the autonomous layer and uses either a PI controller or a P and lead-lag controller combo to manage the voltage and current using a technique such as droop control. The remote controller layer is designed to allow the system operator to monitor and manage the energy management strategy being employed by the autonomous controller currently.
AC/DC hybrid microgrids are a new technology that is becoming more widely used. These microgrids were created to primarily to reduce the amount of conversions and reversing of these conversions present in purely AC or DC microgrids. AC/DC hybrid microgrids also present more control challenges due to their broader usage. Some of these challenges are that no global variable can be used for regulation in the microgrid, so the control is more complicated; conventional droop control cannot be used in the autonomous mode; and there is a tradeoff between reactive power sharing and voltage regulation. Several control techniques have been created to manage these challenges. For example, central primary control is used to create accurate load sharing, but it is not optimized for unbalanced loads.
Hybrid AC/DC droop with a dead zone was created and is easy to implement, but it comes with the drawback that the voltage deviations are load dependent. Many other control techniques have been created to help utilize an AC/DC hybrid microgrid, but a lot of the methods have clear tradeoffs which can make their use situational .
