Load Controller and Var Compensator Design for a Micro Grid System

1.      Introduction

The micro grid system is as shown in the Fig.1.The recent concept of micro grid system is to group a cluster of loads and parallel DG system within a certain local area. In remote areas when two or more DGs are interconnected formed micro grid system. Micro grid system can operate in both the grid connected mode or islanding modes.

Proper operation of these modes requires the implementation of high performance power flow control and voltage regulation algorithms [1,2].

Fig. 1. Micro grid System

Three critical components play major roles in the proper operation of micro grid in both modes [3]. They are

· Local micro source controller

· System optimizer

· Distribution protection

2.      Plant Design

Micro source controller is an important component of the micro grid system. It provides a basic control of real and reactive power using droop characteristics. All the sources are connected to the point of common coupling (PCC) after dc voltage sources are converted to an acceptable ac sources using voltage source inverter (fig.1). Energy manager provides the system optimization. Energy manager uses the information on local load needs, power quality requirement, demand side management request etc to determine the amount of power that micro grid should draw from the main grid. Based on the information it receives, some of its key functions are to provide the individual power and voltage set points for each micro source controller, provides logic and control for islanding and reconnecting the micro grid during events.

Protection is necessary in the event of fault in the micro grid system. If the fault occurs in the main grid then it should be isolated from the main grid as rapidly as possible. If the fault occurs within the micro grid system the protection coordinate should isolated the smallest possible section of the feeder to eliminate the fault.

Induction motor is used as an induction generator when electricity is generated from micro hydel units (fig. 2) or from wind turbines. Induction generators have many advantages over synchronous generator such as ruggedness, less maintenance requirements, absence of dc excitation and inherent short circuit protection when working as a stand alone low cost energy conversion schemes. On the other hand an induction generator has some drawbacks, as it requires reactive power to improve the voltage regulation. In addition to this induction generator used in micro hydel scheme, converts all available mechanical energy into electrical energy to eliminate the controller for the turbine.

3.     Micro Hydel Units

Controller on turbine side is avoided to make the system simple and cost effective. Thus, whenever the consumer load is reduced, the surplus generated energy from induction generator must be dumped somewhere else so as to maintain constant voltage and frequency in the system. These drawbacks can be overcome by using an electronic load controller and VAR compensator. Already many different types of electronic load controller exist for the stand-alone system. When several induction generators involve in a micro grid system, reactive power compensation required by these generator become more significant and high performance power flow control and voltage regulation algorithms are required.

Fig.2 Micro Hydel units for stand alone

System

On the other hand consumer load is fluctuating and varies continuously. This type of load can be named as uncontrolled load. Whenever micro grid system is operating in an islanding mode the unbalanced surplus electrical energy must be dumped to maintain the input essentially constant. This can be achieving by using load controller, which diverts the surplus energy to dump load placed parallel to uncontrolled load.

References

[1] S. S. Murthy, R. Jose, and B. Singh, "Experience in the development of microhydel grid independent power generation scheme using induction generators for Indian conditions," presented at TENCON '98. 1998 IEEE Region 10 International Conference on Global Connectivity in Energy, Computer, Communication and Control, 1998.

[2] Y. Li, D. M. Vilathgamuwa, and P. C. Loh, "Design, analysis, and realtime testing of a controller for multibus microgrid system," Power Electronics, IEEE Transactions on, vol. 19, pp. 1195-1204, 2004.

[3] R. H. Lasseter, "MicroGrids," presented at Power Engineering Society Winter Meeting, 2002. IEEE, 2002.

[4] B.Palle,M.G.Simoes, “Dynamic Simulation and Analysis of Parallel Self-Excited Induction Generators for Islanded Wind Farm Systems,” Industry Applications, IEEE

Transactions on, vol. 41, pp. 1099-1105, 2005.