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\title{Controlling Voltage and Frequency of a Power Network with Microgrid Using Droop Method}
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             \author[1]{Yasser Rahmati  Kukandeh}

             \author[2]{Yasser Rahmati  Kukandeh}

             \author[3]{Mohammad Hossein  Kazemi}

             \affil[1]{  Shahedi University, Tehran, Iran}

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\date{\small \em Received: 19 August 2011 Accepted: 10 September 2011 Published: 22 September 2011}

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\begin{abstract}
        


 Microgrids (MG) Microgrids (MG) are new concept of electric power networks consisting of distributed  generation  (DG), ,  renewable energy resources and sensitive loads. The goal of microgrid  operation is to provide reliable and high-quality electric power regardless of faults or abnormal  operating conditions. Many control methods have been employed to regulate the output power  of  DGs   used in  MGs. This paper presents a control scheme for  DGs   of a power system,  especially with voltage source inverter  (VSI)  interface, for both grid connected and islanded  modes. A comprehensive study has been performed through PSCAD   simulations of the inverter- fed MG   behavior under both islanding and connected operation for different load conditions. The  simulation results prove the droop method efficiency in controlling the voltage and frequency of  the inverter output which is connected to the loads.        

\end{abstract}


\keywords{DG; Microgrid; Droop Method; Connected Mode; Islanding Mode.}

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\let\tabcellsep& 	 	 		 
\section[{I. INTRODUCTION}]{I. INTRODUCTION}\par
enewable energy emerges as an alternative way of generating clean energy. As a result, increasing the use of green energy benefits the global environment. This topic relies on a variety of manufacturing and installation industries for its development. As a solution, continuously small and smart grid energy systems appeared, including renewable energy resources, micro-generators, small energy storage systems and critical and non-critical loads have formed among them a special type of distributed generation system called the Microgrid (MG) \hyperref[b0]{[1,}\hyperref[b4]{5]}.\par
Advantages of MGs can be cited as: i) MG can operate independently without any support from the upper stream of the network. It is one of the advantageous as the MG would not be affected, but rather separated from the upper stream where the fault occurs, and thus in islanding mode \hyperref[b0]{[1]}. ii) MG has "plug-and-play" feature. This means that it can always be connected or disconnected to the medium-voltage network \hyperref[b2]{[3]}. iii) They are clean sources of energy that have very little environmental impact on the community compared to those conventional energy technologies \hyperref[b4]{[5]}.\par
The high penetration of DGs like fuel cell, photovoltaic, wind turbine, micro turbine and etc, along with different types of loads, always raises concern about coordinated control and power quality issues.\par
In microgrid, DGs are connected through VSIs as an interface to deliver the sinusoidal voltage to the network and loads. parallel DGs are controlled to deliver the desired active and reactive power to the system while local signals are used as feedback to control the inverters. The power sharing among the DGs can be achieved by controlling two independent quantities of frequency and fundamental voltage magnitude \hyperref[b8]{[9,}\hyperref[b9]{10]}.\par
A MG can operate in grid-connected mode or islanding mode, hence increases the reliability of energy supplies by disconnecting from the grid in the case of network faults. When the MG is connected to the main grid, if the produced power of the MG is lower than the power demand level, main grid provides this difference in power. On the other hand, MG can send the excessive generated power to the main grid \hyperref[b5]{[6]}.\par
If the grid has to go in islanding mode due to fault or low-voltage, the network controllers must keep the system frequency and voltage in a desirable area leads to sending the qualified power to consumers. The MG should also recover the frequency and voltage in islanding mode rapidly. When MG goes to islanding mode, its last situation should be considered: if it was importing energy then after entering to the islanding mode, it has to increase the generation level to compensate the lost power; therefore in this mode the MG frequency will be decreased. On the other hand, if MG was exporting energy and afterwards it has gone to islanding mode, it should decrease the generation level so the grid frequency will rise \hyperref[b1]{[2,}\hyperref[b3]{4]}. Now a considerable research has been undertaken on the control strategy of the MG. Flexible and fast controls of active and reactive power are important requirements during transient and steadystate operation of a MG system in both grid-connected and islanding modes.\par
A MG when subjected to disturbances can experience angle instability and poor voltage quality due to the presence of DGs with slow response rotating machines and DGs with power electronic converters as the interface to the utility system. To ensure stable operation during network disturbances while maintaining power quality in the islanded mode of operation, a more sophisticated control strategy for MG needs to be developed, in order to warranty both quality of supply and ensuring power management supervising critical and non-critical loads \hyperref[b7]{[8]}. Also, the basic issue on small grids is the control of the number of micro-sources. MG1 VI ( F )\par
concept allows larger distribution generation by placing many micro-sources behind singles interfaces to the utility grid. Some key power system concepts based on power against. Frequency droop methodology, voltage control and can be applied to improve system stability, enhance active and reactive support. Flexible and fast controls of active and reactive power are important requirements during transient and steady-state operation of a MG system in both grid-connected and islanding modes \hyperref[b10]{[11,}\hyperref[b12]{13]}.\par
This paper is organized as follows. In Section II, the structure of the MG has been described. Section III explains the droop method to control the DGs in a power network with MG. To prove the efficiency of droop method some simulations have been done in different situations with connected and islanding modes and the results are discussed in section IV. The simulation results show that droop method can regulate the voltage and frequency of the PCC (point of common coupling) where inverter has been attached to the load or network both in connected and islanding modes. 
\section[{II. STUDY SYSTEM}]{II. STUDY SYSTEM}\par
Figure  {\ref 1} shows a single-line diagram of the system used to investigate typical MG operational scenarios. The basic system's con parameters were extracted from the benchmark system of the IEEE Standard 399-1997, with some modi to allow for autonomous MG operation. The system is composed of a 13.8 KV, three-feeder distribution subsystem which is connected to a large network through a 69 KV radial line. The 13.8 KV distribution substation is equipped with a three-phase 1.5 MVAr, fixed shunt capacitor bank. The 13.8 KV substation busbar is radially connected to the main grid through the substation transformer and a 69 KV line. The network at the end of the 69 KV line is represented by a 69 KV, 1000 MVA short-circuit capacity bus. Combinations of linear and nonlinear loads (L 1 to L 5 ) are supplied through three radial feeders of the subsystem. Loads L 2 to L 5 are composed of linear RL branches. Load L 1 is a three-phase diode recti aggregate of L 1 and L 2 constitutes a sensitive load within the distribution subsystem.\par
The system also includes two DG units, i.e., DG 1 (2.5 MVA) and DG 2 (5 MVA) on feeders F 1 and F 3 respectively. DG 1 utilizes a voltage-sourced converter (VSC) as the interface medium between its source and the power system. DG 1 Represents a dispatchable source with adequate capacity to meet the active/reactive power commands, within pre-speci limits, subsequent to disturbances. Such a dispatchable source may also include energy storage interfaced at the converter dc bus. DG 1 provides control on its output active and reactive power components independently. DG 2 is a synchronous rotating machine equipped with excitation and governor control systems. It may represent either a diesel-generator or a gas-turbinegenerator unit \hyperref[b6]{[7]}. BRK1 BUS1 0.0007[H] 0.00354[ohm] \#1 \#2 1. 
\section[{Global ( F )}]{Global ( F )}\par
Figure  {\ref 1} : Studied System 
\section[{III. MICROGRID CONTROL}]{III. MICROGRID CONTROL}\par
In a MG, the DG supply can be connected to a network using an inverter. An inverter can be tuned by frequency and voltage control.\par
For controlling the inverters usually the drop method is used. The active and reactive power of inverter can be controlled by means of this method in order to control the frequency and to produce voltage in a specific level. Active and reactive power has influence on voltage frequency and amplitude, so the control method can be described as follows: a) Active power control versus frequency In this mode, a frequency change is affected by changing in active power. Figure \hyperref[fig_0]{2} shows the droop of power versus frequency changes. According to figure \hyperref[fig_0]{2} it can be deduced that whenever frequency decreases in a network, controllers make the supplies to increase their production and it means the production level changes from P 0 to P 1 . This state will happen to a MG when a fault occurs and it changes the status into islanding mode. The MG reconnects to main grid meanwhile the frequency increases then controllers make the DG to decrease the production level. In this mode, changes in voltage have an interaction with changes in reactive power. According to figure \hyperref[fig_2]{3}, this controlling mode is represented by Eq. 2: Where E is the amplitude of the inverter output voltage; Q is the reactive power of the inverter output, Q * and E * are the reactive power and voltage amplitude at no-load, respectively; and n is the proportional droop coefficient.* * .( ) E E n Q Q (2) 
\section[{IV.}]{IV.} 
\section[{SIMULATION AND RESULTS}]{SIMULATION AND RESULTS}\par
To verify the proposed method some simulations have been done in PSCAD. To cases have been considered and explained as follows:\par
C Case study1 : In this state the load of L3 in 0.5 seconds is disconnected. Then Micro Grid in the second 1 turns into the island state and the system acts as two separate Micro Grids. As in Figure  {\ref 4} is observed, in the 0.5 seconds, DG1, in addition to producing the amount of MG1, exports the excess energy to L3, while MG2 only produces the which meets its own need. After the segregation of load L3 at 0.5 seconds due to the reduction in the amount of demand, the rate of the production of the units is decreased. In a run-time between 0.5 and 1 second DG1 supplies some portions of. MG1`s energy need, but since the production in MG1 occurs in the first 0.5 second, its frequency is decreased. It is worth noting that since MG1 is importing, its frequency remains more than nominal amount. On the other hand, since the MG2 is importing energy, the frequency of it drops below the nominal value. In second 1, both Micro Grids are in an island state. According to the state before going into an island state, in which MG1 was importing and MG2 was exporting energy,, after going to a new state, (islanded mode), MG1 production levels set frequency to provide for themselves and 1.8 MW to reduce their production levels and MG2 increases to 2.3 MW.\par
As in the Figure  {\ref 5} is shown, when the Micro Grids at second 1 go to an island state, due to the reduction in the amount of required reactive capacity in the network, the voltage is increased. Therefore, the unit which controls voltage for increasing the capacity of the reactive, issues an order for the reduction of voltage. After adjusting for this transient condition, the voltage returns to its stable value and is set.\par
Case study 2 : in this case the Microgrid issues energy to the main grid until 0.5 th second and afterwards, the loads L3 and L5 are disconnected simultaneously. MG1 is also in this island state. As it is shown in figure \hyperref[fig_4]{6}, Since the power required in MG1 is 1.8 MW, DG1 reduces its production level to this amount. In other part of system, DG2 and L4 are responsible for the 0.8 MW by DG2 and 0.7 MW by the main supply network. At second 1, L3 and L5 are also disconnected mode. DG1 and DG2 at this time to meet the demands, they change their production levels.  These results show that the droop method could fix the main grid generation, thus to supply the load changes, it orders the Microgrid to produce the amount of power that is required due to islanding mode and load variations. 
\section[{VI ( F )}]{VI ( F )}\par
V. 
\section[{CONCLUSION}]{CONCLUSION}\par
In this paper the effect of drop method in voltage and frequency control has been discussed. As shown in Section III .in result of disconnection between network and microgrid, the active power generation level increases, if microgrid received energy before disconnection. If microgrid sends power to the network then active power generation level will reduce.\begin{figure}[htbp]
\noindent\textbf{2}\includegraphics[]{image-2.png}
\caption{\label{fig_0}Figure 2 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{}\includegraphics[]{image-3.png}
\caption{\label{fig_1}}\end{figure}
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\noindent\textbf{3}\includegraphics[]{image-4.png}
\caption{\label{fig_2}Figure 3 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{415}\includegraphics[]{image-5.png}
\caption{\label{fig_3}Figure 4 : 1 Figure 5 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{6}\includegraphics[]{image-6.png}
\caption{\label{fig_4}Figure 6 :}\end{figure}
 			\footnote{© 2011 Global Journals Inc. (US)} 			\footnote{© 2011 Global Journals Inc. (US) Production levels for DG1 and DG2 reach to 1.7 MW and 1.6 MW respectively.} 			\footnote{November} 		 		\backmatter  			 
\subsection[{REFERENCES RÉFÉRENCES REFERENCIAS}]{REFERENCES RÉFÉRENCES REFERENCIAS}
\begin{quote}
Output\end{quote}
			 			  				\begin{bibitemlist}{1}
\bibitem[Kim et al. (2002)]{b12}\label{b12} 	 		‘A novel droop method for converter parallel operation’.  		 			J W Kim 		,  		 			H S Choi 		,  		 			B H Cho 		.  	 	 		\textit{IEEE Trans. Power. Electron}  		Jan. 2002. 17  (1)  p. .  	 
\bibitem[-R et al. (2008)]{b8}\label{b8} 	 		‘Adaptive decentralized droop controller to preserve power sharing stability of paralleled inverters in distributed generation microgrids’.  		 			Y A I -R 		,  		 			E F Mohamed 		,  		 			El-Saadany 		.  	 	 		\textit{IEEE Trans. Power Electron}  		Nov. 2008. 23  (6)  p. .  	 
\bibitem[Marwali et al. (2004)]{b9}\label{b9} 	 		‘Control of distributedgeneration systems-Part II: Load sharing control’.  		 			M N Marwali 		,  		 			J W Jung 		,  		 			A Keyhani 		.  	 	 		\textit{IEEE Trans. Power Electron}  		Nov.2004. 19  (6)  p. .  	 
\bibitem[Lasseter and Piagi (20040051387)]{b1}\label{b1} 	 		‘Control of small distributed energy resources’.  		 			R H Lasseter 		,  		 			P Piagi 		.  	 	 		\textit{National Electrical Manufacturers Association (NEMA), Motors and Generators MG}  		20040051387. p. .  	 	 (United States Patent Application) 
\bibitem[Lopes et al. (2006)]{b10}\label{b10} 	 		‘De operation’.  		 			J A P Lopes 		,  		 			C L Moreira 		,  		 			A G Madureira 		.  	 	 		\textit{IEEE Trans. Power Syst}  		May 2006. 21  (2)  p. .  	 
\bibitem[Guerrero et al. (2007)]{b11}\label{b11} 	 		‘Decentralized control for parallel operation of distributed generation inverters using resistive output impedance’.  		 			J M Guerrero 		,  		 			J Matas 		,  		 			L G De Vicuña 		,  		 			M Castilla 		,  		 			J Miret 		.  	 	 		\textit{IEEE Trans. Ind. Electron}  		Apr. 2007. 54  (2)  p. .  	 
\bibitem[Piagi and Lasseter (2001)]{b0}\label{b0} 	 		\textit{Distributed Energy Resources Integration Industrial Application of Microgrids},  		 			P Piagi 		,  		 			R H Lasseter 		.  		October 200.1.  		 			Power System Engineering Research Center, University of Wisconsin-Madison 		 	 	 (Consortium for Electric Reliability Technology Solutions) 
\bibitem[Katiraei and Iravani (2005)]{b6}\label{b6} 	 		‘Micro-Grid autonomous operation during and subsequent to islanding process’.  		 			F Katiraei 		,  		 			M R Iravani 		.  	 	 		\textit{IEEE Trans. Power Delivery}  		Jan. 2005. 20  (1)  p. .  	 
\bibitem[Hatziargyriou et al. (2007)]{b5}\label{b5} 	 		\textit{Microgrids},  		 			N Hatziargyriou 		,  		 			H Asano 		,  		 			R Iravani 		,  		 			C Marnay 		.  		Jul./Aug. 2007. p. .  	 	 (IEEE power \& energy magazine) 
\bibitem[Lasseter (2007)]{b4}\label{b4} 	 		‘Microgrids and distributed generation’.  		 			R H Lasseter 		.  	 	 		\textit{Journal of Energy Engineering}  		March, 2007. 133 p. .  	 
\bibitem[Blaabjerg et al. (2006)]{b3}\label{b3} 	 		‘Overview of control and grid synchronization for distributed power generation systems’.  		 			F Blaabjerg 		,  		 			R Teodorescu 		,  		 			M Liserre 		,  		 			A Timbus 		.  	 	 		\textit{IEEE Trans. Ind. Electron}  		Oct 2006. 53  (5) .  	 
\bibitem[Katiraei and Iravani (2006)]{b7}\label{b7} 	 		‘Power management strategies for a Microgrid with multiple distributed generation units’.  		 			F Katiraei 		,  		 			M R Iravani 		.  	 	 		\textit{IEEE Trans. Power Systems}  		Nov. 2006. 21  (4)  p. .  	 
\bibitem[Marnay and Bailey (2004)]{b2}\label{b2} 	 		\textit{The CERTS Microgrid and the Future of the Macrogrid},  		 			C Marnay 		,  		 			O Bailey 		.  		August 2004.  	 	 (LBNL-55281) 
\end{bibitemlist}
 			 		 	 
\end{document}
