Microgrid 101: Definition and Overview
This article will give a quick microgrid definition and introduction about microgrids, basic layout and overview of it. A quick overview and definition according to characteristics explained in literature.
One thing i think i must write in beginning is that there is no standard and well elaborated specific definition of microgrid is available in literature. Multiple times microgrids are defined in literature according to improvement and addition in basic electric grid. First we will define the microgrid using the common factors most definition used, in literature:
Definition of Microgrid:
A microgrid has multiple distributed sources and local load centers with low voltage distribution network with or without backup storage components. It can work with or without collaboration of main grid to feed the local load demand. Two way power can flow when main grid is connected with it. For control and management of all components and power flow in microgrid, single or multiple intelligent control unit(s) are there. Minimization of toxic gasses emission from thermal generation and maximization of profit (if taking part in energy market), are also taken as objectives of intelligent controller of microgrid.
So, from above definition, we can say that microgrid :
- have distributed sources
- Works inside or outside energy market
- Have intelligent control and management unit(s).
Basic Structure and Elements in Microgrid:
As stated in start that microgrid is home for a lot of components according to requirements of stakeholders (consumer, investor, market). All components join together to form structure of microgrid. From the definition, we can conclude that basic elements in microgrid are:
- Distributed Generating units
- Load Centers
- Distribution network
- Control Unit(s)
- Electric Power Storage (optional)
The distributed generation units could be conventional, non-conventional (renewable or alternate sources) or combination of both. Main objective of these generating units is to meet the demand of local load centers. Load centers can also have own generation, partially dependent to the supply or total demand is fulfilled by the generating units through distribution network.
Control in Microgrid:
Control unit is one of major component of microgrid. The flow of power from generation to the load centers should be monitored, controlled and managed properly. Even before, the generation of electric power must have controller to maintain power quality (voltage, frequency and sin wave within limit).
Synchronization and control of single type of generation in microgrid is relatively easy and less complex. And this can be controlled by single central controller. But microgrid can have multiple type of generation (by its nature) at single place to feed the single load center. So, the control of such diverse type of generation becomes very complex and difficult to handle for single central controller.
So, basically two main classes are there for microgrid controller:
- Centralized Controller
- Distributed Controller
- Hybrid Controller
Centralized Controller of Microgrid:
In the centralized scheme of microgrid, single controller is whole responsible for controlling all the generators, power flow, power quality, meeting the load demand and/or selling/buying power to/from the market. For this purpose, commonly used controller is supervisory control and data acquisition (SCADA) centralized control unit. It takes input of all kind and control and manage accordingly.
This controller works fine for simple systems but when complexity increases, due to increase in number of components, tasks and constraints to handle, the communication and computation burden also increases exponentially. Also, if the central control unit fails, whole system is destined to collapse.
Distributed Control Structure of Microgrid:
In distributed control of microgrid, each component have its local controller and responsible for control of that specific component locally, according to all the constraints. Controller of generators maintains the power quality and main objective remain the same i.e. meeting the load demand collectively.
In this scheme, communication burden is reduced on single controller and computational time is reduced and system works efficiently. The cost of whole system may increase due to multiple control units.
Hybrid Control Structure of Microgrid:
The combination of both central controller and distributed control is also purposed in many research articles. Researches tried to combine the positive aspects of both types of controllers to form a hybrid controller. These controllers reduces the communication and computation burden on single controller by somewhat decentralizing the controller (not completely).
Storage in Microgrid:
Microgrid can have storage as backup while operating in stand-alone mode of operation. When there is renewable source of power, excess power from renewable (after the load demand is fulfilled), can be stored in batteries. This can then be used as backup when needed.
Structure of Microgrid:
From a brief introduction of components of microgrid, a visualization of microgrid can be viewed as following fiugure:
This figure has multiple load centers (commercial and residential) and multiple type of generation (conventional and non-conventional). The microgrid has distributed sources and local controllers also whole microgrid is monitored by centralized control unit. There is also a battery bank for storage of excess renewable power.
This is stand-alone operation of a microgrid.
Mode of Operation of Microgrid:
Microgrid can work with the main power supply from the utility to feed the load and provide power to the main grid when it has excess power. It can work as island of load units and generation when other parts of whole power system is under maintenance or have bug in it. Control unit is also responsible of successful connection and disconnection of microgrid from main grid. and operation must not be disturbed by this.