Electrical Power and Energy Systems 55 (2014) 486–496

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Electrical Power and Energy Systems

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Parallel operation of converter interfaced multiple microgrids

0142-0615/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijepes.2013.09.008

⇑ Corresponding author. Tel.: +46 738 150002.E-mail address: ritwik.majumder@se.abb.com (R. Majumder).

Ritwik Majumder ⇑, Gargi BagABB Corporate Research Center, Forskargrand 7, Vasteras, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 May 2012Received in revised form 10 September2013Accepted 24 September 2013

Keywords:MicrogridsBack to back convertersStabilityCommunication

This paper proposes methods to control utility connected multiple microgrids. Microgrids with renew-able energy sources and autonomous operations do not have high reliability while the smart grid visiondemands a highly reliable and efficient grid. Thus the connections of the microgrids to the main grid haveto be smart itself. A back to back (B2B) converter connection can provide a reliable interface and also canprovide isolation between utility and microgrids. However, with multiple microgrids connected throughB2B converter connections can lead to system instability. In this paper, a method to coordinate the B2Bconnections without losing the possibility of autonomous operation of the microgrids is proposed. Thesystem stability is improved first with a decentralized control of the B2B converters. Then with smart gridscenarios in mind, an application of communication is proposed and simulated. The proposed controlstrategy demonstrates stable system operation with multiple microgrids connected to utility throughB2B converters. The mathematical model of the system with analysis and closed loop simulation of powernetwork and communication network are presented to validate the claim.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Microgrid can generally be viewed as a cluster of distributedgenerators connected to the main utility grid [1], usually throughVoltage Source Converter (VSC) based interfaces. The power man-agement in a microgrid is mainly based on voltage-droop charac-teristic, voltage regulation, and load reactive powercompensation [2]. The real and reactive power sharing can beachieved by controlling two independent quantities – the powerangle, and the fundamental voltage magnitude [2–4]. The systemstability during load sharing has been explored by many research-ers [3,5,6]. Transient stability of power system with high penetra-tion level of power electronics interfaced (converter connected)distributed generation is explored in [3]. While [6] explores themicrogrid testing, [7] define converter control strategy for micro-grid operation. In [8], a study for a Smart Energy Management Sys-tem (SEMS) to optimize the operation of the microgrid ispresented. The SEMS consists of power forecasting module, EnergyStorage System (ESS) management module and optimization mod-ule. A knowledge-based expert system is proposed for the schedul-ing of an energy storage system installed in a power system [9].

Different converter and storage controls for microgrid operationare discussed in [10–14]. While a virtual synchronous generatoraspect is addressed in [10], the converter control for DC/DC opera-tion is investigated in [14]. An economic sizing of storage formicrogrid is proposed in [11] and storage control for frequency

support, primary reserve and peak shaving is discussed in[12,13]. The proposed method is successfully tested to two actualisolated microgrid different sizes and with different generationpattern.

As the microsources are not reliable source of power and so amicrogrid operates both in autonomous or grid connected modeto balance the demand supply. In general, a microgrid is interfacedto the main power system by a fast semiconductor switch calledthe Static Switch (SS). However, in a smart grid scenario, it is notadvisable to connect a highly reliable and efficient utility tomicrogrids in an uncontrolled way. For a better power manage-ment, it is important to achieve control over the power flow. Avoltage and frequency isolation between the grid and the micro-grid will always ensure a safe operation and a B2B converter canprovide both the power flow control and isolation.

Smart utility grid has been proposed to upgrade the present gridstructure through integration of information technology, commu-nication network, automatic control and distributed intelligent de-vices [15]. It is understood that the vision for a more secure, morerobust electrical grid can only be achieved by better communica-tion. In [16], it is examined how this information can be utilizedmore effectively for real-time operation as well as for subsequentdecision making.

A new methodology for coordinated voltage support in distribu-tion networks with large integration of distributed generation andmicrogrids is proposed in [17]. Given the characteristics of the LowVoltage (LV) networks, it is shown that traditional control strate-gies using only reactive power control may not be sufficient in or-der to perform efficient voltage control. A stage by stage

R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496 487

development for network power flow with DG (Distributed Gener-ators) output control is described in [18]. A hybrid power systemcomponent with communication ports allowing communicationwith a supervisory controller and a communication network be-tween each component is considered in [19], to prepare a founda-tion for further development in the intelligent adaptablesupervisory controller. The tests were conducted that confirm thecapability of this concept to use in hybrid power system.

It is very important to control the power flow between the gridand microgrid and maintain voltage isolation between the two sys-tems. The need of power quality improvement while achievingeconomic optima in a distributed generation is addressed in [20].A new concept related to the revitalized microgrid concept andthe vision of the smart grid is presented in [21].

In this paper, the connection and coordinated control of multi-ple microgrids to utility grid is proposed.

Fig. 2. Back-to-back connections.

1.1. Purpose

Aim of this paper is to propose a method for connecting theexisting microgrids to utility with B2B converter connections. Sys-tem stability with multiple B2B connections is investigated withmathematical model and time domain simulations. A coordinatedcontrol method for the B2B connections is proposed to improvethe system stability. First a decentralized control method is pro-posed. Then an application of communication is simulated. Theclosed loop simulations of power network and communicationshow improvement in system stability and maintain the high reli-ability of the grid connected with multiple microgrids.

1.2. Organizations

Section 2 describes the system structure and operation scenar-ios. The proposed control techniques are given in Section 3. Simu-lation results are presented in Section 4.

2. System structure and operation

A simple power system model with B2B converters, connectedto multiple microgrids is shown in Fig. 1. Each of the microgridshas their own micro-sources and loads. The utility loads and lineimpedances (z) are also incorporated as the connection of theB2Bs are not very close to each other.

A simple microgrid with two sources and one load connectedthrough the B2B connection is shown in Fig. 2. The B2B converters(VSC-1 and VSC-2) are supplied by a common DC bus capacitorwith voltage of VC. The converters can be blocked with their

Fig. 1. The microgrids a

corresponding signal input BLK1 and BLK2. DG-1 and DG-2 areconnected through VSC to the microgrid.

Although the proposed control is demonstrated with first twoand then four microgrids, it can be applied to any number ofmicrogrids connected to the utility as shown in Fig. 1. The controlschemes are described in the next sections.

3. Control and reference generation

In this section the control scheme and reference generation forthe VSCs are described. The control can work in two differentmodes. In mode one, with low power demand and contractual sce-nario with utility, the utility can supply a fixed amount of power tothe microgrid while the microgrid DGs shares the rest of the powerdemand through droop control. In mode two, with a high load de-mand, the microgrids DGs supply their maximum power and theutility provides the deficit power through B2B to the microgrid.

nd utility system.

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It is assumed that, in case of a very high power demand in themicrogrid, loads are switched out. In either case control of VSC-1(Fig. 2) is same and described first with the proposed control meth-ods. The control of VSC-2 and the DGs are described subsequently.

3.1. VSC-1 reference generation and proposed capacitor voltagecontrol

Both the VSCs of B2B are supplied from a common capacitor ofvoltage VC as shown in Fig. 2. First the fixed capacitor voltage con-trol VSC-1 is described and then the proposed controls are shown.

3.1.1. Control-1: Fixed capacitor voltage referenceIn this case reference angle for VSC-1 is generated as shown in

Fig. 3a. First the measured capacitor voltage VC is passed through alow pass filter to obtain VCav. This is then compared with the refer-ence capacitor voltage VCref. The error is fed to a Proportional Inte-gral (PI) controller to generate the reference angle dref. VSC-1reference voltage magnitude is kept constant, while angle is theoutput of the PI controller. The instantaneous voltages of the threephases are then derived from the references.

This controller works well with a microgrid connected to theutility, however with multiple microgrids connected as shown inFig. 1, it can lead to system instability (shown in simulation sec-tion) due to low system damping. This low system damping arisesfrom the connection of the parallel B2B converters. The B2B pro-vides the much needed isolation between the utility and microgridfor superior control but has some disadvantages. The inherent sys-tem damping due to utility line resistance is lost at the microgridside due to B2B connection. The output voltage angle control ofthe second VSC (VSC-2) through droop also create system stabilityproblem similar to parallel operation of converters. Moreover, thefixed value control of capacitor voltage from utility side converterleads to large power circulation in the utility side line during asmall disturbance (as the DC capacitor voltage is used for outputvoltage angle calculation of the converter).

Fig. 3. Angle c

3.1.1.1. Mathematical model and eigen value analysis. A mathemati-cal model of the B2B converter is developed as shown in AppendixA. A system with four B2B connections (Fig. 1) is considered for theanalysis. The effect of DC voltage controller gains (VSC-1) anddroop gains (VSC-2) are shown in Fig. 4 at nominal operating pointwith different number of B2B connections. Fig. 4 shows the eigenvalue trajectory of the dominant modes with change in power con-troller gain with two, three or four parallel B2B connections. It canbe seen that increased number of connections lead the system toinstability for the same value of the feedback gains. A selectionof lower value of controller gain would result in an inferior DC volt-age dynamics (VSC-1) and slower droop characteristic (VSC-2). Inthis paper control methods are proposed to improve the systemstability with acceptable feedback gains.

3.1.2. Control-2: Drooping capacitor voltage referenceAs the DC capacitor supplies VSC-1 and VSC-2, regulating the

capacitor voltage will result in improving dynamic response ofthe B2B converters. The reference capacitor voltage is then droopedfrom the rated value based on the power flow from utility to micro-grid. The control scheme is shown in Fig. 3b. However, this control-ler fails to ensure a stable system condition with two close B2Bconnections and /or reverse power flow (from microgrid to utility)in some of the B2B connections.

3.1.3. Control-3: Improving damping with voltage angle measurementThe system damping can be further improved by adding a sec-

ondary loop based on the measured voltage angle difference asshown in Fig. 3c. The measured angle is passed through a low passfilter and the output is used to rectify the angle reference as shownin Fig. 3c. However the angle information can be derived from thereal and reactive power flow which is discussed in the next section.

3.1.4. Control-4: Improving damping with communicationIt can be seen in controller 3 that, modulating the angle would

result in a much better system damping. However we need the

ontroller.

Fig. 4. Eigen value trajectory with parallel back to back connections.

R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496 489

angle information referred to a common frame. Hence it would beeffective if we can use controller-2 (Fig. 3b) but modulate the Vcrated

based on the common angle information. The angle informationcan be derived from the real and reactive power flow in the lines.

To derive the angle information from the real and reactivepower flow, let us consider the system shown in Fig. 1. It is as-sumed that four microgrids with their own DGs and loads are con-nected through B2B and loads. The utility loads are representingthe main outgoing feeders. If the line impedance Zi has resistanceRi and reactance Xi.

The power flow equations over the line for small angle differ-ences can be written as

d11 � d22 ¼ �R11Q M1R þ X11PM1R � R22Q M2L þ X22PM2L ð1Þ

where R11 = R1/(V11V), R22 = R2/(V22V), X11 = X1/(V11V) and X22 =X2/(V22V).

Similarly, the Point of Common Coupling (PCC) voltage angle ofB2B3 and B2B4 can also be represented with respect to d22 as,

d33 � d22 ¼ �R22Q M2R þ X22PM2R � R33Q M3L þ X33PM3L

d44 � d22 ¼ �R33Q M3R þ X33PM3R � R44Q M4L þ X44PM4L

The output voltage angle with respect to a common reference is de-rived from the power flow and then used to control the capacitorvoltage as

VCref ¼ Vcr � KP:PG � KFL � ðddiff Þ

The reference voltages of the B2Bs are then derived as,

VCref 1 ¼ Vcr1 � KP1:PG1 � KFL1:ðd11 � d22Þ ð2Þ

VCref 2 ¼ Vcr2 � KP2:PG2 � KFL2:ðd22Þ ð3Þ

VCref 3 ¼ Vcr3 � KP3:PG3 � KFL3:ðd33 � d22Þ ð4Þ

VCref 4 ¼ Vcr4 � KP4:PG4 � KFL4:ðd44 � d22Þ ð5Þ

The controller is shown in Fig. 3d. The primary loop is shownwith PG while the secondary control based on (2) is shown as VCpq

. It is to be noted that in (1) the power flow data from one point ofthe network need to be communicated to other part. In smart gridscenarios, communication of these power flow data can be imple-mented. Here a switched Ethernet network is considered. It is to benoted that in (2)–(5), the first primary control loops based on localpower measurements PGi , and will ensure system stability in mostof the cases. The microgrids operations do not need any frequentchange of capacitor voltages and hence a low bandwidth commu-nication would be sufficient. As evident from Eqs. (2)–(5)(wherethe reference voltages of the B2B converter capacitor are gener-ated) the first term is a constant and the second term is derivedfrom the local power measurement. The communication is neededfor the last part. Hence the impacts of communication delay or er-rors are only in part of the reference generation. However a limiteris placed to limit the communicated measurement in case of com-munication error. In general, the B2B control performance is notsensitive to communication delay in most of the scenarios. How-ever, in multiple parallel operations with continuous change inpower requirement and power flow direction, a large delay can failto meet the control requirement. In that case, the particular B2Bcan be switched to fixed power supply mode, to improve the sys-tem stability.

490 R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496

During fault, both the converters of the B2Bs are blocked bytheir protection functions and the capacitor voltages are held. Dur-ing fault, the secondary control loop (with angle) is also bypassed.

The communication network is shown in Fig. 5. The communi-cation network is simulated in MATLAB–SIMULINK based commu-nication network simulator TrueTime as switched Ethernetnetwork [22]. The detail parameters are given in Table 1.

3.2. VSC-2 reference generations

VSC-2 of Fig. 2 is connected with PCC through an output induc-tance LG and controls the real and reactive power flow between theutility and the microgrid. As mentioned before the B2B can supplya fixed amount power to the microgrid or the power deficit.

Let us assume that in fixed power supply mode the referencesfor the real and reactive power be PTref and QTref respectively andthe VSC-2 output voltage be denoted by VT\dT and the PCC voltageby VP\dP. Then the reference VSC-2 voltage magnitude and its canbe calculated as

VT ¼V2

P þ QTref XG

VP cosðdT � dPÞð6Þ

dT ¼ tan�1 PTref XG

V2P þ QTref XG

!þ dP ð7Þ

Fig. 5. Communication network structure.

Table 1Communication network.

Wired Ethernet

Network type Switched EthernetData rate 1,000,000 bits/sMinimum frame size 512Total switch memory 80,000Switch overflow behavior Retransmit

In case of high power demand, the DGs supply their maximumavailable power while utility needs to supply any deficit in thepower requirement through B2B converters. Let the maximum rat-ing of the B2B converters are given by PTmax, QTmax. Then the voltagemagnitude and angle reference of VSC-2 is generated as

dT ¼ dTmax �mT � ðPT � PTmaxÞVT ¼ VTmax � nT � ðQ T � Q TmaxÞ

ð8Þ

where VTmax and dTmax are the voltage magnitude and angle, respec-tively, when VSC-2 supplies the maximum load.

3.3. Reference generation for DG sources

In this section, the reference generation for the DGs is pre-sented. It is to be noted that in this paper the DGs are assumedas constant DC sources and the converters are controlled as VSC.The DG could be a fuel cell or Photovoltaic, and DC chopper main-tains the output DC voltage of the DG as [23]. The DG interfacingconverter can run in two different modes as stated in [24]. Onemode is PQ converter control where the converter is used to supplya given active and reactive power set-point. The other mode is VSCcontrol where the converter is controlled to ‘‘feed’’ the load withpre-defined values for voltage and frequency. In this paper, theinterfacing converters are controlled as VSCs. The voltage and fre-quency are then controlled with the load frequency control basedon the real and reactive power output.

The control strategy for both the DGs is the same and henceonly DG-1 reference generation is discussed here. It is assumedthat when the utility supplies a part of load demand in microgridby a fixed amount power through the B2B converters, rest of thepower demand in the microgrid is supplied and regulated by theDGs. The output voltages of the converters are controlled to sharethis load proportional to the rating of the DGs.

The power requirement can be distributed among the DGs, sim-ilar to a conventional droop by dropping the voltage magnitudeand angle as

d1 ¼ d1rated �m1 � ðP1 � P1ratedÞV1 ¼ V1rated � n1 � ðQ 1 � Q 1ratedÞ

ð9Þ

where V1rated and d1rated are the rated voltage magnitude and anglerespectively of DG-1, when it is supplying the load to its ratedpower levels of P1rated and Q1rated. The coefficients m1 and n1 respec-tively indicate the voltage angle drop vis-à-vis the real power out-put and the magnitude drop vis-à-vis the reactive power output.These values are chosen to meet the voltage regulation requirementin the microgrid.

In high loading condition, the DGs supply their maximum avail-able power and the utility supplies the deficit power. Let us denotethe available active power as P1avail. Then based on this and the cur-rent rating of the DG the reactive power availability Q1avail of theDG can be determined. Based on these quantities, the voltage ref-erences is calculated as

V1 ¼V2

P1 þ Q 1availX1

VP1 cosðdP1 � dPÞð10Þ

d1 ¼ tan�1 P1availX1

V2P1 þ Q 1availX1

!þ dP1 ð11Þ

The references for the other DGs are generated in a similar way.It is to be noted that there could be non-linear and unbalanced

loads. The reference for the DGs then needs to be compensating[25]. In [26], design aspects of various passive components andswitching dynamics of a voltage source inverter (VSI) for compen-sating unbalanced and nonlinear load are presented. A novel

Table 2System and controller parameters.

System Quantities Values

Systems frequency 50 HzSource voltage (Vs) 11 kV rms (L–L)Feeder impedance Rs = 3.025 X, Ls = 57.75 mH

LoadImpedance (Balanced) RL = 100.0 X, LL = 300.0 mHorInduction motor Rated 40 hp, 11 kV rms (L–L)

DGs and VSCsDC voltage (Vdc1, Vdc2) 3.5 kVTransformer rating 3 kV/11 kV, 0.5 MVA, 2.5% reactance (Lf)VSC losses (Rf) 1.5 XFilter capacitance (Cf) 50 lFInductances (L1, L2) 20 mH and 16.0 mHInductances (LG) 28.86 mHHysteresis constant (h) 10�5

Angle controllerProportional gain (Kp) �0.2Integral gain (KI) �5.0

Droop coefficientsPower-angle

m1 0.3 rad/MWm2 0.24 rad/MW

Voltage-Qn1 0.15 kV/MVArn2 0.12 KV/MVAr

Fig. 7. Power output of back-to-back converters.

R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496 491

method is proposed for VSI to track desired reference currentssmoothly. In case of a DSTATCOM (Distributed Static Compensa-tor), any change in the load affects the dc-link voltage directly[27]. A novel fast-acting dc-link voltage controller based on the en-ergy of a dc-link capacitor is proposed.

For a microgrid connection through B2B connection, we need tomodulate the DC capacitor reference voltage to improve the sys-tem damping. This has been the main focus of the proposed con-trollers and the performances of the controllers are described innext subsections.

It must be noted that parallel operation of converter interfacedsources has many challenges [25,28–30]. Different control tech-niques and synchronization method is proposed by many authors[25,28–30]. However the proposed method in this paper of B2Bconverter interfaced multiple microgrids is not directly related tothe number of microgrid microsources and can be extended tomore number of microgrid connection or DGs. The main reasonof taking a small test microgrid (Fig. 2) is the limitation of the com-puter and software capability. With the each B2B converter alsohas two VSC converters, for multiple microgrid case microsourcenumber in each microgrid is limited. A real microgrid will havemany more microsources and the proposed method can be appliedto such system as the main controllability issues are at the micro-grid PCC with voltage, frequency and power exchange. The gener-alized concept and control result of multiple converter sourcesmicrogrid with B2B converter interfaced is shown in [31]. In thispaper the control approach for parallel back to back connectionsis proposed.

4. Simulation studies

Simulation studies are carried out in PSCAD/EMTDC (version4.2) with converter model for all the cases except case 4. In case4, the power network and communication network are simulatedin closed loop in MATLAB SIMULINK. The power network is mod-eled in SimPower, with VSC bridges while the communication net-work is modeled in TrueTime. The close loop simulation structureis shown in Fig. 6. Different configurations of load and its sharingare considered. The DGs are considered as inertia-less dc sourcesupplied through a VSC. The system data are given in Table 2.

4.1. Case 1: Control 1-fixed capacitor voltage

First it is assumed that only one B2B is connected to the utilityshown in Fig. 1. It is desired that B2B-1 supplies 50% of the loadpower requirement in microgrid-1 and rest of the power require-ment is supplied by the DGs. With the system running in steadystate, the load power demand is changed from 1 MW to0.525 MW at 0.1 s. The system response is shown in Fig. 7a.

It can be seen that B2B continues to supply 50% of the loadpower demand. Rest of the power requirement is shared by the

Fig. 6. Close loop simulating of power and communication network.

DG proportional to their ratings. At 0.35 s system is revert backto initial condition and DGs, B2B power go back to initial condition.This validates individual B2B operation.

To investigate the parallel operation of the B2Bs, B2B-1 andB2B-2 are then connected to each other at 0.1 s (an utility breakerbetween the two microgrids is used). The power flows through theB2Bs are shown in Fig. 7b. It can be seen that system oscillation in-creases and the system becomes unstable. The DC capacitor voltageis shown in Fig. 8a. The PCC voltage, DG output power and DG out-put current is also shown in Fig. 8. The DG output power startsoscillating and also PCC voltage. Then the system becomesunstable.

4.2. Case 2: Control 2-drooping capacitor voltage

To improve the system damping, the DC capacitor referencevoltage is drooped with the power flow through B2B as shown inFig. 3b. While both microgrids connected in parallel and utility issupplying, system is operated as case 1. Fig. 9 shows the systemresponse. The stable DC capacitor voltage is shown in Fig. 9a. The

Fig. 8. System instability.

Fig. 9. Capacitor voltage and DG output.

492 R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496

PCC voltage is also stable and the DG output powers share the loadas desired (case 1). DG output current shown in Fig. 9 also ensuresa stable system operation.

To further verify the controller ability and system stability, thepower flow through the B2B 2 is reversed and microgrid-2 startsupplying power to the utility at 0.1 s. The system response isshown in Fig. 10. The DC capacitor voltage and angle controlleroutput are also shown in Fig. 10. The voltage and angle oscillationslead the system to instability. Thus only drooping the DC capacitorvoltage is not sufficient to ensure stable system operation.

Fig. 10. Power flow through back-to-back converters and angle controller output:Unstable Case.

4.3. Case 3: Control 3-voltage angle measurement

To further improve the system stability, the PCC voltage angledifference is measured and passed through a low pass filter asshown in Fig. 3c. The system is simulated with this controllerin same operating condition as in previous case. Fig. 11 showsthe power flow through the B2Bs and it can be seen that a stablesystem operation is achieved. The B2B supplies (absorb) power to(from) microgrids as desired. The DC capacitor voltage and anglecontroller output are shown in Fig. 11b and c. A stable system

condition is apparent. Thus including the voltage angle informa-tion improves the system stability.

Fig. 11. Power flow through back-to-back converters and angle controller output:Stable Case.

Fig. 12. Power flow through the B2Bs and PCC voltage.

R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496 493

In the next section the angle information for a multiple micro-grid scenario is derived from the real and reactive power flow inthe network. Communication is used to carry the power flow mea-surement from one part of the network to the B2Bs.

Fig. 13. Network schedule and co

4.4. Case 4: Control 4-with communication

In this case, system shown in Fig. 1, with four microgrid con-nected through B2B connections are considered. The aim is to im-prove the system stability by modulating the DC capacitor voltagebased on the power flow through the B2B and PCC voltage angle. Tocalculate the angle with same reference, real and reactive powerflow in the network is used as (2)–(5). The communication net-work shown in Fig. 5 is used in this case. This is a TrueTime com-munication network, which has four send and four receive ports.The measured data are fed to send port and received at the receiveport as shown in Fig. 5. The data communications are controlledthrough the trigger signal. When a node tries to transmit a mes-sage, a triggering signal is sent to the network block on the corre-sponding input channel. At the end of the simulated transmission(in communication network) of the message is finished, the net-work block sends a new triggering signal on the output channelcorresponding to the receiving port. The transmitted message isput in a buffer at the receiving port. A message contains informa-tion about the sending and the receiving computer port, measure-ment signals or control signals, the length of the message, andoptional real-time attributes such as a priority or a deadline.

In this case, the power flow data at PCC of the B2B convertersare communicated to other B2B connections to calculate the con-verter output voltage angle difference as in (1)–(5). The networkparameters are given in Table 1.

It is assumed that utility is connected to microgrids 1, 2 and 3through the B2Bs and supply power. Microgrid 4 is connected tothe grid at 0.5 s. The system response (power flows through theB2Bs) is shown in Fig. 12a. PCC voltage and current through theB2B-4 are shown in Fig. 12b and c. It can be seen a stable PCC volt-age is maintained and the current becomes balanced within 2–3cycles. The power flow of the utility network is communicatedthrough the communication network as shown in Fig. 5.

The network has four send ports and four receive ports. Thescheduling of the network is shown in Fig. 13a. The power flowin the network, as shown in Fig. 5, is measured and communicated.Fig. 13b and c show the measured power and the power data re-ceived through the network. It can be seen that they are closeand moreover, the proposed control has the primary term basedon local power measurement ((2)–(5)). The control will work evenwith a much slower sampling of this secondary term. The data ratecould be chosen based on communication infrastructure available.

mmunicated power signals.

Fig. 14. Change inflow in B2B4 with different communication bandwidth.

494 R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496

In this paper, the coordination of B2B connections of multiplemicrogrids is demonstrated with switched Ethernetcommunication.

To investigate the effect of communication delay, the band-width is varied for this particular operating condition. It is foundthe control performance deteriorates significantly below 200 kbps.However, this would vary with measurement data precision, num-ber of port, switched memory and scheduling topology. The controlperformances with different bandwidth are given in Fig. 14, wherethe change in power flow for B2B4 is shown for various communi-cation bandwidths.

It must be noted that the communication of the power flowmeasurement for control is done with low bandwidth communica-tion network with acceptable system response as mentioned. Thefault data (converter blocking)/outage are communicated withhigh priority for shorter time delay and deactivation of the second-ary control.

4.5. Case 5: Control 4 and system expansion validation

In this case the expansion of microgrid is tested to ensure theproposed control can handle dynamic system growth. First threeparallel microgrid (Fig. 1 without microgrid 4) each having 4 DGsis simulated. The power sharing and stable system operation(Fig. 15a) validate the expansion of individual microgrid with theproposed control method. Then the microgrid in Fig. 1 is modified

Fig. 15. System expansion with the proposed control method.

with five parallel microgrids (for limitation in computing abilitynumber of DGs in each microgrid is limited to two). With the con-nection of microgrid 4, the power sharing is shown in Fig. 15b. Thestable system operation with expansion within a microgrid or anew microgrid verifies the dynamic system growth support ofthe proposed control method.

5. Conclusions

This paper demonstrates the possibility of connecting multiplemicrogrids to utility through B2B converters connections. The sys-tem stability with multiple B2B connections is addressed. The pro-posed decentralized controller modulates the DC capacitor voltagebased on power flow through B2B. A secondary control loop basedon real and reactive power flow in the network through communi-cation is also proposed. The power network and communicationnetwork is simulated in closed loop. The proposed control schemesshow stable system operations in different operating conditions.

Acknowledgement

The authors would like to thank ABB Corporate Research,Sweden.

Appendix A

The equivalent circuit of a B2B converter is shown in Fig. A.1.From equivalent circuit shown in Fig. A.1, the following equa-

tions are obtained for each of the phases of the three phase systemFor VSC-1.

di11

dt¼ �RT1

LT1i11 þ

ð�vcf 1 þ u1:VdcÞLT1

ðA:1Þ

dvcf 1

dt¼ ði11 � i21Þ

Cf 1ðA:2Þ

vcf 1 � v t1 ¼ Lf 1di21

dtðA:3Þ

For VSC-2

di12

dt¼ �RT2

LT2i1 þð�vcf 2 þ u2:VdcÞ

LT2ðA:4Þ

Fig. A.1. Equivalent circuit.

R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496 495

dvcf 2

dt¼ ði12 � i22Þ

Cf 2ðA:5Þ

vcf 2 � v t2 ¼ Lf 2di22

dtðA:6Þ

A.1. DC voltage

The power consumed by the DC side can be expressed as

Pdc ¼ VdcCdcddtðVdcÞ þ

V2dc

RdcðA:7Þ

In ac side the power can be expressed as (neglecting the resistance)

Pac ¼ ðv1di12d þ v1qi12qÞ ¼ ðvcf 1di21d þ vcf 1qi21qÞ ðA:8Þ

Equating the power we get,

VdcCdcddtðVdcÞ þ

V2dc

Rdc¼ ðv1di12d þ v1qi12qÞ

¼ ðvcf 1di21d þ vcf 1qi21qÞ ðA:9Þ

Taking Vdc2 as a state, we get,

ðV2dcÞ�¼ � 1

RdcCdcðV2

dcÞ þ1

Cdcðvcfdi21d þ vcfqi21qÞ ðA:10Þ

The Eqs. (A.1)–(A.6) are translated into a d–q reference frame ofconverter output voltages, rotating at system frequency x.

Defining a state vector as for VSC-1 and VSC-2

xi1 ¼ ½i11d i11q i21d i21q vcf 1d vcf 1q�T

xi2 ¼ ½i12d i12q i22d i22q vcf 2d vcf 2q�T

The state equation in the d–q frame is given by (for VSC-1)

_xi1 ¼ Ai1xi1 þ B11ucdq1 þ B21v tdq1 ðA:11Þ

It is assumed here that the tracking is perfect and hence, in the lim-it, switching function u can be represented by uc. From (A.11), ucdqcan be expressed as

udq1 ¼ �k2i11dq þ ðk2 � k1Þi21dq � k3vcfdq1 þ k1i21refdq

þ k2icf 1refdq þ k3vcf 1refdq ðA:12Þ

The above equation can be written in matrix form as

ud1

uq1

� �¼ Gi1xi1 þ Hi1yrefdq1 ðA:13Þ

where

yrefdq1 ¼ ½i21refd i21refq icf 1refd icf 1refq vcf 1refd vcf 1refq�T

Substituting (A.13) into (A.11) we get

_xi1 ¼ ðAi1 þ B1G1Þxi þ B11Hi1yrefdq1 þ B21v tdq1 ðA:14Þ

The current references can be expressed in terms of voltage refer-ences as

icf 1refd

icf 1refq

� �¼

0 �xCf

xCf 0

� � vcf 1refd

vcf 1refq

� �

i21refd

i21refq

� �¼

0 1xLf

� 1xLf

0

24

35 vcf 1refd

vcf 1refq

� �þ

0 1xLf

� 1xLf

0

24

35 v t1d

v t1q

� �

The converter model can be written as,

_xi1 ¼ ACONV1xi1 þ BT1vcf 1refdq þ BBUS1v t1dq ðA:15Þ

where ACONV = Ai + B1Gi and BCONV = BiHi.

A.2. Converter output voltage controller

For VSC-1, the voltage magnitude is kept constant while voltageangle is controlled as (assuming tracking is perfect)

d1 ¼ dref ¼ ðVdcref � VdcÞ Kp þKi

s

� �

Assuming a state u, where

dudt¼ ðVdcref � VdcÞ

The converter voltage angle is

½d1� ¼ ½Ki�½u� þ ½Kp�½Vdcref �

We can write linearized state equation and output converter volt-age angle equation as,

½Du��¼ ½0�½Du� þ ½�1�½DVdc� þ ½1�½DVdcref � ðA:16Þ

½Dd1� ¼ ½Ki�½Du� þ ½Kp�½DVdcref � ðA:17Þ

For VSC-2, the converter output voltage magnitude is controlled as

vcf 2refd ¼ Vcf 2ref ¼ Vrated2 � n2 ðQ 2 � Q rated2Þ ðA:18Þ

where the reference voltage is aligned in d axis.The output angle is controlled as,

d2 ¼ d2ref ¼ d2rated �m2 ðP2 � Prated2Þ ðA:19Þ

From (A.18) and (A.19), the linearized voltage controller equationcan be written as

Dd2 ¼ �m2DP2

Dvcfrefd ¼ �nDQ 2

The power output is measured through a low pass filter and can bederived as,

P2 ¼xC

sþxCp2 ¼

xC

sþxCðvcf 2di22d þ vcf 2qi22qÞ

Q 2 ¼xC

sþxCq2 ¼

xC

sþxCðvcf 2di22q � vcf 2qi22dÞ

496 R. Majumder, G. Bag / Electrical Power and Energy Systems 55 (2014) 486–496

Linearizing we get,

D _P2

D _Q 2

" #¼�xC 00 �xC

� �DP2

DQ 2

� �þ BP

Dvcf 2dq

Di22dq

� �ðA:20Þ

A.3. Model of B2B converter

The model of the B2B converter is then derived by linearzing(A.10), (A.15) and then combining with (A.16) and (A.20) as

D _xib2b1 ¼ Aib2b1Dxib2b1 þ Bb2b1Dvcf 1refdq þ BBUSb2b1Dv t1dq

where

Dxib2b1 ¼ ½DV2dc Du Dxi1 Dxi2 DP2 DQ 2�

T

Considering there is Z B2B connection to the utility network, thetotal state space

D _xZ ¼ AZDxZ þ BZDv tZdq

where

AZ ¼ diag Aib2b1 Aib2b2 � � � Aib2bZð Þ

References

[1] Lasseter RH. MicroGrids. IEEE Power and Energy Society General Meeting;2002, p. 305–8.

[2] Katiraei F, Iravani MR. Power management strategies for a microgrid withmultiple distributed generation units. IEEE Trans Power Deliv2006;21(4):1821–31.

[3] Reza M, Sudarmadi D, Viawan FA, Kling WL, Van Der Sluis L. Dynamic stabilityof power systems with power electronic interfaced DG. In: IEEE power systemsconference and exposition; 2006, p. 1423–8.

[4] Dai M, Marwali MN, Jung JW, Keyhani A. Power flow control of a singledistributed generation unit with nonlinear local load. In: IEEE power systemsconference and exposition; 2004, p. 398–403.

[5] Guerrero JM, de Vicuna LG, Matas J, Castilla M, Miret J. A wireless controller toenhance dynamic performance of parallel inverters in distributed generationsystems. IEEE Trans Power Electron 2004;19(1):1205–13.

[6] Lasseter RH, Paigi P. Microgrid: a conceptual solution. In: IEEE powerelectronics specialists conference; 2004, p. 4285–90.

[7] Yunwei L, Vilathgamuwa DM, Poh CL. Design, analysis, and real-time testing ofa controller for multibus microgrid system. IEEE Trans Power Electron2004;19(5):1195–204.

[8] Chen C, Duan S, Cai T, Liu B, Hu G. Smart energy management system foroptimal microgrid economic operation. IET Renew Power Generat2011;5(3):258–67.

[9] Ross M, Hidalgo R, Abbey C, Joós G. Energy storage system scheduling for anisolated microgrid. IET Renew Power Generat 2011;5(2):117–23.

[10] Bevrani H, Ise T, Miura Y. Virtual synchronous generators: a survey and newperspectives. Int J Electric Power Energy Syst 2014;54:244–54.

[11] Aghamohammadi MR, Abdolahinia H. A new approach for optimal sizing ofbattery energy storage system for primary frequency control of islandedmicrogrid. Int J Electric Power Energy Syst 2014;54:325–33.

[12] Serban I, Marinescu C. Battery energy storage system for frequency support inmicrogrids and with enhanced control features for uninterruptible supply oflocal loads. Int J Electric Power Energy Syst 2014;54:432–41.

[13] Sigrist L, Lobato E, Rouco L. Energy storage systems providing primary reserveand peak shaving in small isolated power systems: an economic assessment.Int J Electric Power Energy Syst 2013;53:675–83.

[14] Kumar L, Jain S. A multiple source DC/DC converter topology. Int J ElectricPower Energy Syst 2013;51:278–91.

[15] Wang Y, Li W, Lu J. Reliability of wide area measurement system. IEEE TransPower Deliv 2010;25(3):1483–91.

[16] Bose A. Smart transmission grid applications and their supportinginfrastructure. IEEE Trans Smart Grid 2010;1(1):11–9.

[17] Madureira AG, Pecas Lopes JA. Coordinated voltage support in distributionnetworks with distributed generation and microgrids. IET Renew PowerGenerat 2009;3(4):439–54.

[18] Jupe SCE, Taylor PC. Distributed generation output control for network powerflow management. IET Renew Power Generat 2009;3(4):371–86.

[19] Abdulwahid U, Manwell JF, Mcgowan JG. Development of a dynamic controlcommunication system for hybrid power systems. IET Renew Power Generat2007;1(1):70–80.

[20] Soultanis NL, Papathanasiou SA, Hatziargyriou ND. A stability algorithm for thedynamic analysis of inverter dominated unbalanced LV microgrids. IEEE TransPower Syst 2007;22(1):294–304.

[21] Hadjsaid N, Caire R, Raison. Decentralized operating modes for electricaldistribution systems with distributed energy resources. IEEE Power & EnergySociety General Meeting; 2009. p. 1–4.

[22] TrueTime 2.0 beta 5 – Reference manual, Department of Automatic Control,Lund University, Sweden; June 2010.

[23] Shahnia F, Majumder R, Ghosh A, Ledwich G, Zare F. Operation and control of ahybrid microgrid containing unbalanced and nonlinear loads. Electric PowerSyst Res 2010;80(8):954–65.

[24] Pecas Lopes JA, Moreira CL, Madureira AG. Defining control strategies formicrogrids islanded operation. IEEE Trans Power Syst 2006;21(2):916–24.

[25] Majumder R, Ghosh A, Ledwich G, Zare F. Load sharing and power qualityenhanced operation of a distributed microgrid. IET Renew Power Generat2009;3(2):109–19.

[26] Mishra MK, Karthikeyan K. An investigation on design and switching dynamicsof a voltage source inverter to compensate unbalanced and nonlinear loads.IEEE Trans Ind Electron 2009;56(8):2802–10.

[27] Mishra MK, Karthikeyan K. A fast-acting DC-link voltage controller for three-phase DSTATCOM to compensate AC and DC loads. IEEE Trans Power Deliv2009;24(4):2291–9.

[28] Xiaonan L, Guerrero J, Teodorescu R, Kerekes T, Kai Sun S, et al. Control ofparallel-connected bidirectional AC–DC converters in stationary frame formicrogrid application. IEEE Energy Convers Cong Expos (ECCE) 2011:4153–60.

[29] Sao CK, Lehn PW. Control and power management of converter fed microgrids.IEEE Trans Power Syst 2008;23(3):1088–98.

[30] Majumder R, Ghosh A, Ledwich G, Zare F. Control of parallel converters for loadsharing with seamless transfer between grid connected and islanded modes,IEEE power and energy society general meeting – conversion and delivery ofelectrical energy in the 21st century; 2008. p. 1–7.

[31] Majumder R, Ghosh A, Ledwich G, Zare F. ‘Power management and power flowcontrol with back-to-back converters in a utility connected microgrid. IEEETrans Power Syst 2010;25:821–34.