Control of a DC MicrogridMarko Gulin

University of Zagreb, Faculty of Electrical Engineering and ComputingDepartment of Control and Computer Engineering

Unska 3, 10000 Zagreb, Croatiamarko.gulin@fer.hr

Abstract—A microgrid is a part of a distribution networkembedding multiple distributed generation systems (mostly non-conventional renewable energy sources like photovoltaic panels,small wind turbines etc.) and storage systems with local loads,which can be disconnected from the upstream network underemergency conditions or as planned. The microgrid conceptnaturally arose to cope with the penetration of renewable energysources, which can be realistic if the final user is able to generate,store, control and manage part of the energy that it will consume.The power connection between microgrid components can bedone through a direct current (DC) link or an alternating current(AC) link. In this paper we describe operation modes (grid-connected, islanded) and control methods of a DC microgrid.

Index Terms—Distributed generation and storage, DC micro-grid, Microgrid operation modes, Active load sharing, Droopcontrol methods

I. INTRODUCTION

Around the world, conventional fossil-fuelled power systemis facing problems of gradual depletion of fossil fuel resources,poor energy efficiency and environmental pollution [1]. Theseproblems have led to a new trend of generating power locallyat distribution voltage level by using small-scale conventionalbiomass-fuelled energy sources like gas and diesel micro-turbines, together with non-conventional renewable energysources like photovoltaic panels, wind turbines etc., and othernon-conventional sources like fuel cells. This type of powergeneration is termed as distributed generation and the involvedenergy sources are termed as distributed generation sources.Distributed generation can offer considerable social and eco-nomic benefits, including reduced power network losses andthe exploitation of renewable energy resources [2].

The integration of renewable energy sources poses a chal-lenge because their output is intermittent and variable andin principle requires an energy storage to enable time-shiftbetween energy production and consumption. If only one re-newable energy source is considered, the integration is simple– for stand-alone use the source is connected with a storageand load, while in the grid-connected case the source injectsthe power directly into the power network, whereas the issuesof power balancing are left to be handled by distribution and/ortransmission system operators. Considerable improvementsmay however be achieved when heterogeneous distributedenergy sources are used in a bulk for local or grid powersupply, like easier grid integration and smart power manage-ment, with benefits both locally and on the grid-side. Therequired power and information communication infrastructure

to enable it is called a microgrid. Most commonly used energystorage devices in a microgrid are batteries, supercapacitors,flywheels, and fuel cells with electrolyser (EL). This type ofenergy storage is termed as distributed storage and the energystorage devices are termed as distributed storage devices.

Microgrid is defined as a cluster of distributed generationsources, distributed storage devices and distributed loads thatoperate so as to improve the reliability and quality of thelocal power supply and of the power system in a controlledmanner [3]. The microgrid concept naturally arose to copewith the penetration of renewable energy sources, which canbe realistic if the final user is able to generate, store, controland manage part of the energy that it will consume [4].The power connection between microgrid components, i.e.distributed generation sources, storages and loads, can be donethrough a direct current (DC) link or an alternating current(AC) link. In this paper a DC link microgrid is considered,with emphasis on its control and power management in grid-connected and islanded operation mode.

Microgrid control must insure that: (i) new distributedgeneration and storage systems can be added or removed fromthe microgrid seamlessly, (ii) equal and stable current sharingbetween parallel power converters (i.e. sources) is enabled, (iii)output voltage fluctuations can be corrected, and (iv) desiredpower flow from/to the microgrid together with technicallyand economically viable operation is enabled. There is a fairlylarge number of methods for paralleling power converters(PCs). From the viewpoint of the operating mechanism tocurrent sharing and output voltage level management, controlmethods are classified into two basic categories: (i) active loadsharing, and (ii) droop control methods.

The report is structured as follows. In Section II a micro-grid concept is introduced. In Section III a microgrid powermanagement in grid-connected and islanded operation modeis described. In Section IV a commonly used DC microgridcontrol methods are described.

II. A MICROGRID CONCEPT

Due to the ever-increasing demand for high-quality andreliable electric power, the concept of distributed generationand energy storage has attracted widespread attention in recentyears. Distributed generation and storage systems consist ofrelatively small-scale generation and energy storage devicesthat are interfaced with low- or medium-voltage distributionnetworks through power converters and can offset the local

power consumption, or even export power to the upstreamnetwork if their generation surpasses the local consumption.An upcoming philosophy of operation which is expected toenhance the utilization of distributed generation and energystorage is known as the microgrid concept.

The main benefits of microgrids are high energy efficiency,high quality and reliability of the delivered electric power,more flexible power network operation, and environmentaland economical benefits [5]. However, to achieve a stableand secure operation, a number of technical, regulatory andeconomic issues have to be resolved before microgrids canbecome commonplace. In this paper we deal with controlmethods for integrating distributed generation and energy stor-age systems into a microgrid, as well with power managementin grid-connected and islanded operation mode.

The main components of a microgrid are: (i) distributedgeneration sources such as photovoltaic panels, small windturbines, fuel cells, diesel and gas microturbines etc., (ii)distributed energy storage devices such as batteries, super-capacitors, flywheels etc., and (iii) critical and non-criticalloads. Energy storage devices are employed to compensatefor the power shortage or surplus within the microgrid. Theyalso prevent transient instability of the microgrid by providingpower in transient. The transient power shortage in a microgridcan be compensated for by fast energy storage devices in themicrogrid, or by the utility grid through a bidirectional powerconverter when operating in grid-connected mode.

The issue of the power quality in microgrids is an im-portant issue due to the presence of an appreciable numberof sensitive loads whose performance and lifespan can beadversely affected by voltage sags, harmonics and imbalances.In a microgrid, most distributed generation sources and storagedevices employ power converters which can rapidly correctindicated imperfections, even in the presence of nonlinear andunbalanced loads [5]. The selection of an appropriate powerconverter mainly depends on the generation source and storagedevice type, and on the used power connection between themicrogrid components.

The power connection between microgrid components canbe done through a DC link or an AC link. Many non-conventional energy sources generate low-voltage DC power,e.g. photovoltaic panels, fuel cells etc. Most of these sourcessupply power to an AC utility grid and require costly andinefficient power converters, even where the power mayultimately be delivered to a DC device. However, powertransmission through a low-voltage DC link produces morelosses than transmission through a high-voltage AC link.With development of a microgrid control methods alongwith cost-effective and efficient power converters, a DC linkmicrogrid can become a promising solution for integratingdistributed generation sources, storages and loads. Addingintelligence to a DC microgrid controllers further enablesconsumer engagement with utility grid through smart meteringand ultimately with dynamic demand management, and thiscould reduce costs associated with periods of high and lowpower consumption.

Figure 1 shows the schematic diagram of a microgrid whichembeds (i) distributed generation sources such as photovoltaicpanels, small wind turbine, and fuel cells, (ii) distributedstorage devices such as batteries, supercapacitors and flywheel,and (iii) distributed loads. Each distributed generation sourceand storage device is interfaced with a common link througha power converter. The microgrid is galvanically isolated fromthe utility grid and can be easily disconnected from the gridthrough the main switch for maintenance purposes. In a case ofpower shortage that can occur when utility grid is not available,non-critical loads can be disconnected from the microgridthrough an emergency switch. Worth noting, microgrid canalso embed combined heat and power systems that exploitwaste heat for domestic purposes where heat flows can bemanaged in addition to electrical energy flows.

Batteries Flywheel

PV panels Wind turbine Fuel cells H2 tank

Utility grid

PC PC PC

Distributed generation systems

PC PC

PC

Distributed storage systems

EL

PC

Distributed loads

DC LINK

PC

FER building

Fig. 1. Schematic diagram of a DC microgrid

III. MICROGRID OPERATION MODES

A microgrid is connected into the utility grid through abidirectional power converter, that continuously monitors bothsides and manages power flow between them. If there is afault in the utility grid, the power converter will disconnect themicrogrid from the grid, creating an islanded energy system.The microgrid can continue to operate in the islanded mode,that is primarily intended to enhance system reliability andservice continuity, and it is typically unplanned. However, itcan also be introduced intentionally for maintenance purposesthrough the main switch. In some cases, islanded operation isthe only mode of operation, e.g. in off-grid remote electrifica-tion system. Concludingly, there are two operation modes fora microgrid: (i) grid-connected, and (ii) islanded mode.

Consider a DC microgrid that consists of (i) distributedgeneration sources such as photovoltaic panels, wind turbineand fuel cells stack with electrolyser, (ii) distributed storagedevices such as batteries and supercapacitors, and (iii) criticaland non-critical loads, all connected in parallel into the com-mon DC link through corresponding power converters. Thepower flow of the systems in the considered DC microgrid isshown in Figure 2.

PV WT FC

Generation systems

BAT SC

Storage systems Critical loads Non-critical loads

HT

Bidirectionalpower converter

AC grid

EL

PLOADPGRID

H2

Fig. 2. A DC microgrid power flow

The sum of the output power of the photovoltaic panels,the wind turbine and the fuel cells, i.e. distributed generationsources, is defined as:

PDG = PPV + PWT + PFC , (1)

where PPV , PWT and PFC are photovoltaic panels, windturbine and fuel cells output power.

The distributed generation systems supply unidirectionalpower to the DC microgrid and play a role as the main energysource. Since energy storage devices control the power balanceof a DC microgrid by charge and discharge, the power flowis bidirectional and the reference power for energy storagedevices is defined as:

PDS = PBAT +PSC +PEL = PGRID +PDG−PLOAD, (2)

where PBAT and PSC are batteries and supercapacitors charg-ing power, PEL is the electrolyser power, PLOAD is requiredpower of all loads connected into the DC microgrid, criticaland non-critical, and PGRID is the utility grid power.

The loads are assumed to demand unidirectional powerfrom the microgrid. According to a varying local demand,the distributed storage systems realize a power balance, andthus make a continuous high-quality power supply to theload possible [6]. In a case of power shortage that can occurwhen utility grid is not available, non-critical loads can bedisconnected from the microgrid.

In the following subsections, a simple algorithm of powermanagement for DC microgrid is described. It must be notedthat this is not the only option power management. In futurework, the problem of the optimal power management will behandeled by the control algorithm.

A. Grid-connected mode

In the grid-connected operation mode, the grid-tied powerconverter has control over the DC link voltage level. If theoutput sum of the power of the distributed generation systemsis sufficient to charge the storage devices, any excessive poweris supplied to the utility grid. If the sum of the power of thedistributed generation and storage systems is deficient withrespect to the load demand, the required power is suppliedfrom the utility grid. In the grid-connected mode, power man-agement is performed in a complementary manner betweenstorage devices and as a result a DC microgrid can operatesafely and efficiently.

B. Islanded mode

When a DC microgrid must be separated from the utilitygrid and switch to the islanded mode, the grid-tied powerconverter releases control of the DC link voltage level, and oneof the converters in the microgrid must take over that control.Since each converter of distributed generation sources is usedfor optimal control of its belonging source, only the convertersof the energy storage elements are free to regulate the DC linkvoltage level. During the islanded mode, the battery plays themain role in regulating the DC link voltage level, and thesupercapacitor plays a secondary role in responding of thesudden power requirement as an auxiliary source/sag, i.e. forpeak shaving during transients.

IV. MICROGRID CONTROL METHODS

Microgrid control must insure that: (i) new distributedgeneration and storage systems can be added or removed fromthe microgrid seamlessly, (ii) equal and stable current sharingbetween parallel power converters (i.e. sources) is enabled, (iii)output voltage fluctuations can be corrected, and (iv) desiredpower flow from/to the microgrid together with technicallyand economically viable operation is enabled. There is a fairlylarge number of methods for paralleling power converters.From the viewpoint of the operating mechanism to currentsharing and output voltage level management, control methodsare classified into two basic categories: (i) active load sharing,and (ii) droop control methods. It is also possible to designa hybrid control method combining good aspects of activeload sharing and droop control method, but this will not befurther discussed. A microgrid control is often implemented ina hierarchical manner, with three control loops: (i) tertiary loopmanages the power flow from/to the microgrid, (ii) secondaryloop corrects output voltage fluctuations, and (iii) primary loopperforms current sharing control between power converters.

Figure 3 shows the equivalent circuit of two DC powersupplies connected in parallel sharing a common load throughresistive output impedances. If there is some voltage differencebetween sources, this will circulate a current between DCsources, and in order to reduce the circulating current aprimary control loop is applied.

+−V1

I1

R1 R2

+−V2

I2

IL

Vo

Fig. 3. Two parallel-connected DC power supplies

The output voltage Vo, i.e. the DC link voltage, can beexpressed as:

VoRp

=V1R1

+V2R2

− IL, (3)

where IL is the total load current, and Rp is the parallelresistance defined as:

1

Rp=

1

R1+

1

R2. (4)

In a case of sudden rise of the load current IL, the outputvoltage Vo drops. In order to restore its nominal voltage level,a secondary control loop is applied.

V. ACTIVE LOAD SHARING

The first category of control methods, named the ac-tive load-sharing technique [7]–[26], need intercommunicationlink. Although these links limit the flexibility of the microgridand degrade its redundancy, both tight current sharing andlow-output-voltage fluctuations can be achieved. The followingsection provides a review of the existing active load sharingcontrol methods for parallel converters available in literature[4]. The active load sharing control methods can be classifiedinto three different types: (i) centralized control, (ii) master-slave control (MS), and (iii) circular chain control (3C).

A. Centralized control

The centralized control, shown in Figure 4(a), consists ofdividing the total load current iL by the number of modules(MODs) N , so that this value becomes the current referencei∗k of each module k:

i∗k =iLN, k = 1, 2, . . . , N. (5)

The current reference value is subtracted by the currentof each module, obtaining the current error ∆ik, which isprocessed through a current control loop (CL). An outercontrol loop in the centralized control, i.e. voltage controlloop (VL), adjusts the load voltage. Using this approach, itis necessary to measure the total load current iL, so it cannotbe used in a large distributed systems. Consequently, a centralcontrol board (CCB) is necessary.

B. Master-slave control

In the master-slave control, the master module regulatesoutput voltage. Hence, the master current im fixes the currentreferences of the rest of the modules (slaves) as:

i∗s = im, s = 2, . . . , N. (6)

Consequently, as shown in Figure 4(b), the master acts as avoltage source converter, whereas the slave works as a currentsource converter. If the master unit fails, another module willtake the role of master in order to avoid the overall failureof the system. There exist different variants of this controlmethod, depending on the role of the master: (i) dedicated,where the master is one fix module, (ii) rotary, where themaster is arbitrarily chosen, and (iii) high-crest current, wherethe module that brings the maximum current automaticallybecomes the master.

C. Circular chain control

The circular chain control, shown in Figure 4(c), consistsof the current reference of each module taken from the othermodule, forming a control ring. Note that the current referenceof the first unit is obtained from that of the last unit to form acircular chain information. This strategy can be expressed as:

i∗k =

{iN , k = 1,ik−1, k = 2, . . . , N.

(7)

The current limitation control is a variant of the circularchain control. In this case, the load voltage is controlled bythe master module, whereas the slave modules are only forsharing the load current. Except for the master module, thecurrent command of the slave is generated by its previousmodule and limited in amplitude. In this scheme, any modulecan be the master (dedicated, rotating, high-crest current).

MOD1

CL

MOD2

CL

MODN

CL CCB

Load

i∗1i1

i∗NiNiloadvo

vrefi∗2i2

(a) Centralized control of a DC microgrid

MODM

CL

MODS2

CL

MODSN

CL

Load

vo

VL

vref

im i∗sis2 isN

i∗s

(b) Master-slave control of a DC microgrid

MOD1

CL

MOD2

CL

MODN

CL

Load

i∗1

i1

i∗N

iN

i∗2

i2

(c) Circular chain control of a DC microgrid

Fig. 4. Active load sharing control methods

VI. DROOP CONTROL METHOD

The second category of control methods, named the droopcontrol method [27]–[34], is able to avoid critical communi-cation links. The absence of critical communications betweenthe modules improves the reliability without restricting thephysical location of the modules. The droop method is basedon a well-known concept in large-scale power systems, whichconsists of drooping the frequency of the AC generator whenits output power increases [4]. The droop method achieveshigher reliability and flexibility in the physical location of themodules since it only uses local power measurements.

A. Virtual output impedance

In the virtual output impedance control, shown in Figure 5,current at the module output is sensed and sent back to themodule input via virtual impedance RD, where is comparedwith the output voltage reference at no load:

v∗o = vref − ioRD, (8)

where io is the module output current, RD is the virtual outputimpedance, and vref is the output voltage reference at no load.

Load

CL

vref

VL

MOD1

voRD

δvo

io1 ioN

vMG

v∗MGCL

vref

VL

MODN

voRD

δvo

VL

Fig. 5. Droop control of a DC microgrid via virtual output impedance

This control loop has the inherent load-dependent voltagedeviation. To solve the problem of the voltage deviation, thevoltage level in the microgrid vMG is sensed and comparedwith the voltage reference v∗MG, and the error processedthrough a compensator is sent to all the modules to restore theoutput voltage. The controller can be expressed as follows:

δvo = kpeV + ki

∫eV dt, (9a)

eV = v∗MG − vMG, (9b)

where kp and ki are the control parameters of the microgridvoltage level compensator. Finally, equation (8) becomes:

v∗o = vref + δvo − ioRD. (10)

B. Series resistor

In the series resistor control, a resistor is placed in serieswith the module output to provide a voltage drop in the output.In this control method, all of the paralleled modules havean initial setting that, via a potentiometer, are made almostidentical. Obviously, the major disadvantage of this approachis the high power dissipation in the series resistor if the droopin output voltage is large. Because of added power dissipation,this method is used only for low-power linear post-regulators[35]. Microgrid voltage level deviation is corrected in the sameway as in virtual output impedance control method.

VII. CONCLUSION

A microgrid is a part of a distribution network embed-ding multiple distributed generation systems (mostly non-conventional renewable energy sources like photovoltaic pan-els, small wind turbines etc.) and storage systems with localloads, which can be disconnected from the upstream networkunder emergency conditions or as planned. The microgrid con-cept naturally arose to cope with the penetration of renewableenergy sources, which can be realistic if the final user is able

to generate, store, control and manage part of the energy thatit will consume. The power connection between microgridcomponents can be done through a direct current (DC) link oran alternating current (AC) link.

In this paper we describe operation modes and controlmethods of a DC microgrid. A microgrid can operate in a grid-connected mode or in an islanded operation mode. From theviewpoint of the operating mechanism to current sharing andoutput voltage management, control methods can be classifiedas an active load sharing and droop methods. The main differ-ence between aforementioned control methods is that droopcontrol methods do not require fast communication betweencomponents (i.e. generation sources and storage devices), thusimproving system reliability and flexibility at the cost of theDC link voltage level stability.

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