Line-Interactive UPS for Microgrids

1292 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 3, MARCH 2014

Line-Interactive UPS for MicrogridsMohammad A. Abusara, Josep M. Guerrero, Senior Member, IEEE, and Suleiman M. Sharkh

Abstract—Line-interactive uninterruptible power supply (UPS)systems are good candidates for providing energy storage within amicrogrid to help improve its reliability, economy, and efficiency.In grid-connected mode, power can be imported from the grid bythe UPS to charge its battery. Power can be also exported whenrequired, e.g., when the tariffs are advantageous. In stand-alonemode, the UPS supplies local distributed loads in parallel withother sources. In this paper, a line-interactive UPS and its controlsystem are presented and discussed. Power flow is controlled usingthe frequency and voltage drooping technique to ensure seamlesstransfer between grid-connected and stand-alone parallel modesof operation. The drooping coefficients are chosen to limit theenergy imported by the UPS when reconnecting to the grid and togive good transient response. Experimental results of a microgridconsisting of two 60-kW line-interactive UPS systems are providedto validate the design.

Index Terms—Distributed generation (DG), line-interactive un-interruptible power supply (UPS), microgrid.

I. INTRODUCTION

TO INCREASE reliability, energy storage systems withina microgrid are essential. Energy is stored while in grid-

connected mode, when the microgrid’s distributed generation(DG) systems produce excess power, to be used later to supplycritical loads during power outages. In stand-alone mode, theycan be used to boost the power supplied by the microgrid ifthe DG systems cannot meet the expected level of power. Tomeet these demands, the energy storage system needs to be ableto work in grid-connected and stand-alone modes. In the lattermode of operation, the system needs to operate in parallel withother DG systems to meet the variable power demand of theload. More importantly, it needs to switch seamlessly betweenthe two modes.

Line-interactive uninterruptible power supply (UPS) systemsare good candidates for providing energy storage within micro-grids as they can be connected in parallel with both the maingrid and local load. The classical topology of line-interactiveUPS systems [1], [2] is simpler, cheaper, more efficient, andmore reliable than that of the online double-conversion UPS.This topology, however, does not provide voltage regulationto the load. Voltage regulation is possible as in series–parallel

Manuscript received March 27, 2013; accepted April 22, 2013. Date ofpublication May 16, 2013; date of current version August 23, 2013.

M. A. Abusara is with the Renewable Energy Research Group, University ofExeter, Penryn TR10 9EZ, U.K. (e-mail: [email protected]).

J. M. Guerrero is with the Department of Energy Technology, AalborgUniversity, 9220 Aalborg, Denmark (e-mail: [email protected]).

S. M. Sharkh is with the Electro-Mechanical Engineering ResearchGroup, University of Southampton, Southampton SO17 1BJ, U.K. (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2013.2262763

Fig. 1. Microgrid structure.

or delta conversion line-interactive UPS [3]–[6] at the expenseof lower efficiency and extra size and cost due to the useof an extra inverter and a bulky transformer. However, thistopology is still more efficient than that of classical onlinedouble conversion UPS because the complementary inverterhas to supply only 10%–20% of the UPS nominal power [7].

There are a number of publications on the control of line-interactive UPS systems [8]–[15]. Tirumala et al. [8] proposeda control algorithm for grid-interactive pulsewidth modulation(PWM) inverters to obtain a seamless transfer from grid-connected mode to stand-alone mode and vice versa. When it isconnected to the grid, the inverter operates in current-controlledmode regulating the current injected into the point of commoncoupling (PCC). In stand-alone mode, however, the inverteroperates in voltage-controlled mode regulating the output volt-age across the load. Similar approaches were also reported in[9] and [10]. The main disadvantage of these systems is that,after the grid fails and before starting the transitional period(from current-controlled to voltage-controlled mode), the loadvoltage depends on the current demand and the load. This mightresult in a very high or very low voltage across the load duringthe transitional period. A notch filler that is used to mitigatethese voltage transients was proposed in [11]. Kim et al. [12]proposed a controller for truly seamless transfer from grid-connected mode to stand-alone mode. The controller has aninner voltage control loop that regulates the output voltage.During grid connection, the power angle of the output voltage(i.e., the angle between the inverter voltage and the voltageat the PCC) is set by an outer control loop depending on therequired injected grid current. When the grid fails, the powerangle is saturated to a maximum predefined value. In the systemreported in [13], the power angle between the UPS voltage andthe grid voltage is measured by a phase detector and adjusted tocontrol the power. The line-interactive UPS system proposedin [14] has the voltage controller and the current controllerworking in parallel in order to achieve seamless transfer be-tween the two modes. Although the controllers reported above

0278-0046 © 2013 IEEE

ABUSARA et al.: LINE-INTERACTIVE UPS FOR MICROGRIDS 1293

Fig. 2. Circuit diagram of the UPS system.

enable seamless transfer of a single unit from grid-connectedand stand-alone modes, the main disadvantage is that the UPSunits are not capable of operating autonomously in parallel withother DG units, and thus, they cannot form a microgrid.

Chandorkar et al. [15] proposed a line-interactive UPS sys-tem based on P − ω and Q− V droop control where theinverter frequency and voltage amplitude are drooped linearlywith the inverter output active and reactive power, respectively.UPS units can operate in parallel, and load sharing is achievedwithout the need for communication signals between the in-verters. In [16], a line-interactive UPS system that is capableof operating in a microgrid was proposed. The controller of theUPS inverter is based on two control loops, i.e., an inner voltagefeedback loop that regulates the output voltage and an outeractive and reactive power sharing loop that is implemented bydrooping control. Both systems in [15] and [16] provide trulyseamless transfer between grid-connected mode and stand-alone parallel mode and vice versa. However, the integrationof the battery and its dc/dc converter into the system was notstudied. In addition, the stability of the dc link voltage wasnot discussed. This becomes an important issue, taking intothe account the requirement to transfer seamlessly from batterycharging mode in grid-connected mode to battery dischargingin stand-alone paralleling mode.

This paper presents and discusses a line-interactive UPSsystem to be used within a microgrid, as illustrated in Fig. 1.The system can transfer practically seamlessly between grid-connected and stand-alone modes sharing the load within amicrogrid in parallel with other DG units. The control of thecomplete system, including the battery and its bidirectionaldc/dc converter, is considered. The main novel contributions ofthis paper are: 1) analysis, design, and experimental implemen-tation of a new control strategy of a line-interactive UPS systemwithin a microgrid, which enables seamless transfer betweengrid-connected and stand-alone parallel modes of operation;and 2) the design of a dc link controller loop that sets the activepower demand during battery charging mode, which allows fora smooth transition between battery charging and dischargingmodes.

II. SYSTEM OVERVIEW

The general overall structure of a microgrid (see Fig. 1)consists of DG units, UPS units, local loads, supervisorycontroller, and a static transfer switch (STS). The STS is used

TABLE IDC/AC CONVERTER PARAMETER VALUES

at the PCC to isolate the microgrid from the grid in case ofgrid faults and reconnect seamlessly to the grid when the faultsare cleared. Local loads are connected on the microgrid sideof the switch so that they are always supplied with electricalpower regardless of the status of the switch. The controllerof each DG system uses local measurements of voltage andcurrent to control the output voltage and power flow. However,there is still a need for low-speed communication betweenthe STS and the DG units to update them about the statusof the switch, i.e., whether it is opened or closed. Further-more, the supervisory controller measures the power at thePCC, receives information about the state of charge of the bat-teries from the UPS controllers, and sends active and reactivepower commands Pref and Qref to the other DG units based ontariffs, local load requirements, and efficiency of the units.

When the grid is connected, the UPS can export power tothe grid or it can import power to charge its batteries. Forexample, the supervisory controller may charge the batteryduring the grid off-peak tariff and discharge it during peaktariff periods. When a grid fault occurs, the anti-islandingcontrollers embedded inside the controllers of the DG unitspush the voltage amplitude and/or frequency at the PCC towardtheir upper or lower limits. Once the amplitude or frequencyupper or lower limits are reached, the controller of the STSdecides that the grid is not healthy and opens the switch. Italso sends a signal to the DG units to update them about thestatus of the switch. The DG units carry on supplying power inparallel according to their ratings, sharing the power demandedby the distributed local loads. After the grid fault is cleared, theSTS reconnects the microgrid; it monitors the voltage signalson both sides and closes when these signals are in phase to


Fig. 3. DC/AC controller.

Fig. 4. DC/DC controller.

ensure transient-free operation. The microgrid considered inthis paper consists of two 60-kW line-interactive UPS systemseach powered by a 30-Ah (550–700 V) lithium-ion battery. Thecircuit diagram of each UPS system is shown in Fig. 2. The UPSsystem consists of a battery, a bidirectional dc/dc converter [17],and a bidirectional three-phase dc/ac converter with an outputLCL filter. The dc/ac converter parameters are given in Table I.When the power flows from the grid to the battery, the dc/dcconverter operates in buck mode and the boost insulated-gatebipolar transistor (IGBT) is held open. When the power flows inthe opposite direction, the buck IGBT is held open and the dc/dcconverter operates in boost mode regulating the dc link voltageto a suitable level in order to inject power into the grid. Fig. 3shows the controller of the bidirectional dc/ac converter. Anouter power flow controller sets the voltage demand for an innervoltage core controller loop. The core voltage controller (notshown) regulates capacitor voltage Vc. The response time for

the inner core controller is much faster than for the outer powerflow loop, and hence, it will be assumed (from the point of viewof the power flow controller) as an ideal voltage source withsettable magnitude and frequency. Fig. 4 shows the controllerof the bidirectional dc/dc converter. During battery chargingmode, the buck IGBT is pulsewidth modulated and, dependingon the battery voltage, the controller operates either in currentmode or voltage mode regulating the battery current or voltage,respectively. When the battery discharges, the boost IGBTis modulated to regulate the dc link voltage. During batterycharging, the dc link voltage is controlled by the three-phasedc/ac converter, which in this case operates as an active rectifier.When discharging, however, the dc link voltage is controlled bythe dc/dc converter, operating in boost mode as aforementioned.To decouple these two controllers, which control a commondc link voltage, the voltage demand for the active rectifierV ∗dclink_2 is set to be higher (800 V) than the voltage demand of


the boost V ∗dclink_1 (750 V). To understand how the controllers

react during a sudden transition between operating modes, thefollowing scenario is considered. Suppose that the UPS is incharging mode, the dc link in this case will be regulated by thedc/ac converter operating as an active rectifier and the dc/dcconverter will be charging the battery in buck mode. At the mo-ment when the grid fails and before the fault is detected by theSTS, the power flow through the three-phase converter changesdirection immediately and automatically as a consequence oflosing stiffness at the PCC. Thus, the power starts to flow fromthe dc link capacitor to the ac load causing the dc link voltage todrop from the V ∗

dclink_2 demand. Once the dc link drops belowV ∗dclink_1, the dc/dc controller recognizes the event and changes

its mode from buck to boost immediately and starts regulatingthe dc link voltage. In this scheme, the smooth transition fromgrid-connected charging mode to stand-alone mode is possiblewithout relying on external communication. When the grid faultis cleared, the STS closes and sends an update signal to theUPS units. If the power demand received from the supervisorycontroller during grid-connected mode is positive, i.e., Pref >0, the power flow controller sets the drooping controller demandto Pref such that P ∗ = Pref , as shown in Fig. 3. However, if thepower demand from the supervisory controller is negative, i.e.,Pref < 0, the dc link voltage controller, within the power flowcontroller, starts to raise the dc link voltage to V ∗

dclink_2. Theoutput of the dc link voltage controller will be a negative activepower demand to the power flow controller. The dc/dc converterwill stop operating in boost mode because the dc link voltage(regulated to V ∗

dclink_2) is higher than its demand V ∗dclink_1. It

will then start to operate in buck mode either in current mode orin voltage mode depending on battery voltage VB . If it operatesin current mode, the battery current demand is set to P ∗

Batt/VB ,where P ∗

Batt is the battery charging power demand and is equalto P ∗

Batt = Ramp(|Pref |) (the absolute value is used becausethe Pref in this case is negative). The rate of change of theramp function needs to be slow enough so it does not disturbthe dc link voltage controller. For instance, if P ∗

Batt is suddenlychanged, the dc/dc converter will draw a large amount of powerfrom the dc link capacitor, which may result in a drastic drop inthe dc link voltage before the dc link voltage controller reactsto this drop. To avoid any unnecessary transient, any changesrequired to P ∗ and Q∗ values in the power flow controllershould also happen gradually via ramp functions.

III. POWER FLOW CONTROLLER

A. Small-Signal Analysis

The proposed drooping control is given by

ω =ω∗o − (kω + kω_I/s)(P − P ∗) (1)

V =V ∗o − (ka + ka_I/s)(Q−Q∗) (2)

where ω∗o, V ∗

o , kω , and ka are the nominal frequency reference,nominal voltage reference, proportional frequency drooping co-efficient, and proportional voltage drooping coefficient, respec-tively. The integral frequency and voltage drooping coefficientskω_I and kα_I are set to zero during the island mode, butthey are activated during the grid-connected mode to make sure

Fig. 5. UPS system equivalent circuit: (a) single-phase Thevenin equivalentcircuit; (b) three-phase grid-connected circuit.

that the power output follows the demand precisely, particularlywhen the grid voltage and frequency deviate from their nominalvalues. The values P ∗ and Q∗ are set to the demanded powerin grid-connected mode. In stand-alone mode, however, theyare set to nominal active and reactive power values Pn and Qn

to improve frequency and voltage regulation. Because this is abidirectional UPS system, Pn and Qn are set to zero.

The inverter can be modeled by a two-terminal Theveninequivalent circuit, as shown in Fig. 5(a). G(s) and Zo(s) rep-resent the closed-loop and output impedance transfer functions,respectively [18]. Fig. 5(b) shows an equivalent circuit diagramof the grid-connected UPS unit. Zo(s) has been replaced bysLo because the output impedance is predominantly inductivearound the fundamental frequency [18]–[20]. Inductance Lo

can be determined by the slope of Zo(s) around the fundamen-tal frequency, and it was measured to be 996 μH. The active andreactive power flow between the UPS unit and the grid can beshown to be given by

P =3V Vo sin δ/ωoLo (3)

Q =3(V Vo cos δ − V 2

o

)/ωoLo. (4)

Small changes in active and reactive power are given by

P =(3Vo/ωoLo)(sin δeq · V + Veq cos δeq · δ) (5)

Q =(3Vo/ωoL)(cos δeq · V − Veq sin δeq · δ) (6)

where δeq and Veq are the equilibrium points around which thesmall-signal analysis is performed. If the equilibrium points arechosen, such as δeq = 0 and Veq = Vo, (5) and (6) become

P =3V 2o δ/ωoLo (7)

Q =3VoV /ωoLo. (8)

By perturbing (1) and (2), we get

ω = ωo + (kω + kω_I/s)(P∗ − Pmeas) (9)

V = Vo + (ka + ka_I/s)(Q∗ − Qmeas) (10)

where Pmeas and Qmeas are small changes in the measured ac-tive and reactive power values, respectively. The grid frequencyand voltage reference points ω∗

o and V ∗o are fixed by the con-

troller. However, grid frequency ωo and voltage Vo may changeslightly, and hence, the notations ωo and Vo in (9) and (10)represent the small deviations in the grid frequency and voltagefrom their nominal values, respectively. The small-signal modelcan be represented by a block diagram, as shown in Fig. 6where the angle δo is the initial angle between the invertervoltage and the common ac bus voltage before connection. It


Fig. 6. Small-signal model: (a) active power; (b) reactive power.

is normally caused by measurement error, and it is representedas a disturbance in the block diagram. The transfer functionF (s) is the power measurement transfer function that relatesthe measured power to the actual instantaneous power. Averagepower can be calculated by integrating the instantaneous powerover one fundamental cycle T , i.e.,

F (s) = (1/Ts)(1− e−Ts). (11)

The time delay in (11) can by replaced by a second-orderPadé approximation such as

e−sT ≈(T 2/12s2−T/2s+ 1)/(T 2/12s2 + T/2s+ 1). (12)

Substituting (12) in (11) gives

F (s) ≈ 1/(T 2/12s2 + T/2s+ 1). (13)

Equation (13) represents a second-order low-pass filter with acutoff frequency of about 140 rad/s.

B. Drooping Coefficients Selection

The selection of kω, ka, kω_I , and ka_I will be discussed inthis section. The coefficient kω is determined such that it givesgood dynamic response and at the same time limits the amountof energy that can be transferred from the ac bus to the UPS unitduring a grid connection transient. If the angle of the UPS out-put voltage lags the ac bus voltage by δo when it first connectsto the ac bus, active power will flow from the ac bus to the UPSand the energy transferred will cause the dc link voltage to rise.If the dc link voltage is higher than the maximum limitVdc_max, the protection system will trip. Therefore, the ob-jective is to select kω such that the maximum transient energyabsorbed by the converter (caused by the presence of an initialpower angle δo) does not cause the dc link voltage to riseabove the trip limit. If the demand power P ∗ is set to zeroin Fig. 6(a) and by ignoring the disturbance caused by thefrequency deviation, the block diagram that relates the outputenergy (integral of power) to the disturbance δo is shown inFig. 7. Table II shows the damping ratio and the overshoot in theoutput energy for different values of kω . Increasing kω not onlyreduces the overshoot in the output energy but also reduces thedamping ratio, which means that a compromise has to be made.The maximum transient energy that the converter can absorb isgiven by

Emax = 0.5Cdc

(V 2dclink_max − V 2

dclink_o)

(14)

where Vdclink_o is the initial dc link voltage before the transient,Vdclink_max is the maximum allowed dc link voltage, and Cdc

Fig. 7. Block diagram relating E to δo.

TABLE IITRANSIENT RESPONSE

is the dc link capacitance. According to the values in Table Iand knowing that when the inverter first connects to the ac bus,the initial dc link voltage is Vdclink_o = 750 V, the maximumenergy according to (14) is therefore Emax = 438 J. In orderto prevent the dc link voltage from rising to the maximumlimit of 1000 V, the overshoot transient energy should be lessthan Emax. The initial angle δo depends on the measurementerror in the synchronization method (phase-locked loop orzero-crossing detection). The synchronization in this design isperformed by detecting the zero crossing of the ac bus voltage.The error in this case is limited to one sampling period Ts

of the ac bus voltage, which is for the sampling frequencyof 16 kHz is equal to 0.02 rad. The drooping coefficient isset to kω = 1.5× 10−4. According to Table II, the maximumenergy overshoot is 8200× 0.02 = 164 J, which is less thanEmax calculated above. The damping ratio for the active powercontroller is 0.44. The voltage amplitude drooping coefficientka is set to ka = 3× 10−4, which gives a good system dampingratio of 0.6.

The integral drooping coefficients need to be selected to min-imize the effect of any variation in grid frequency and voltageon the output active and reactive power. Because the intentionis to minimize the power error resulting from the continuousdynamic deviations in grid frequency and voltage (not onlyagainst static deviations), in the analysis these deviations willbe modeled as ramp functions as follows:

ωo =Dω/s2 (15)

Vo =Da/s2 (16)

where Dω is the rate of change of the frequency deviation inradians per square second, and Da is the rate of change of the


TABLE IIICONTROLLER PARAMETER VALUES

voltage deviation in volts per second. From Fig. 6(a), PE , whichrepresents the error signal of the active power, is given by

PE =

(−3V 2

o /ωoLo

)sωo

T 2

12 s4 + T

2 s3 + s2 + kω

3V 2o

ωoLos+ kω_I

3V 2o

ωoLo

. (17)

By substituting (15) in (17), the steady-state error in activepower PE_ss can be estimated using the final-value theorem

PE_ss = lims→0

(sPE) = −Dω/kω_I . (18)

Similarly, from Fig. 6(b) and (16), the steady-state error inreactive power QE_ss can be shown to be given by

QE_ss = lims→0

(sQE) = −Da/ka_I . (19)

The coefficients kω_I and ka_I are set to 5× 10−5 and 1×10−4, respectively. It has been found that at the test site, the gridcan drift by up to 0.05 Hz and 4 V in 5 min. This corresponds toDω = 0.001 rad/s2 and Da = 0.013 V/s. Therefore, accordingto (18) and (19), the steady-state errors in active and reactivepower caused by continuous dynamic deviation of the gridfrequency and voltage are only PE_ss = ±20 W and QE_ss =±130 VAR, respectively.

To validate the small-signal model, a detailed model of athree-phase half-bridge PWM inverter with an LCL filter asper Tables I and III was built in MATLAB/Simulink. Whenthe inverter connects to the grid, the grid voltage signals wereleading the inverter voltage signals (measured at the filtercapacitors) by 0.039 rad. The power demands were set to zero.Fig. 8 shows the response of the inverter transient absorbedenergy compared with that of the small-signal model producedusing Fig. 7. It is shown that the small-signal model provides areasonable prediction of the response of the system in terms ofovershoot and frequency of oscillations given the assumptionsmade to derive the small-signal model, such as assuming theinverter as an ideal voltage source.

Fig. 8. Energy transient response, actual model, and small-signal modelδo = 0.039 rad.

IV. DC LINK VOLTAGE CONTROLLER

The dc link voltage controller regulates the dc link voltageduring the battery charging mode. A PI controller is chosen,and the overall block diagram of the dc link voltage controller,including the small-signal model of the inner loop power flowcontroller (ignoring the frequency and angle disturbances), isshown in Fig. 9. The power drawn by the dc/dc converter isrepresented by PBatt. As shown, the model is nonlinear due tothe presence of the square root function. In order to be able touse linear control design techniques, the square root relation islinearized. Let x = V 2

dclink and y(x) = Vdclink =√x, a small

change in y is given by

y = x dy/dx∣∣x=xo

= x0.5/√x∣∣x=xo

(20)

where x is a small change in x, and xo is the point around whichthe linearization is performed. Given that the dc link voltagerange of interest is from 750 to 800 V, the linearization point ischosen to be xo = 7752; hence

y = mx m = 6.5× 10−4. (21)

The open-loop transfer function that relates Vdclink to P ∗ (as-suming PBatt = 0) is therefore given by

Gdc(s) =2mA

Cdc

×( (

(T 2/12)s2 + (T/2)s+ 1)(kωs+ kω_I)

(T 2/12)s5 + (T/2)s4 + s3 +Akωs2 +Akω_Is

)(22)

where A = 3V 2o /ωoLo.

The design of the dc link voltage controller now becomesstraightforward. The PI gains are chosen so that the dynamicresponse of the dc link controller is much slower than that of theinner power flow controller in order to decouple the two con-trollers. The proportional and integral gains kdc_P and kdc_Iare chosen to be 40 and 2000, respectively. Fig. 10 shows theclosed-loop poles of the complete system. The high-frequencypoles are the ones caused by the power flow loop, whereas thedominant lower frequency poles are caused by dc link voltageloop. The dynamics of the dominant closed-loop poles arewell decoupled from the dynamics of the power flow closed-loop poles. The simulated response of the controller when the


Fig. 9. DC link voltage controller.

Fig. 10. Closed-loop poles of the system.

Fig. 11. Step response of dc link voltage controller.

voltage is stepped up from the boost regulated value of 750 V tothe active rectifier demand value of 800 V is shown in Fig. 11.The rise and settling times are 0.05 and 0.3 s, respectively.

The block diagram in Fig. 9 can be used to determine theramp rate of the power demand sent to the dc/dc converter(during battery charging mode) to produce P ∗

Batt (see Fig. 4).The ramp rate should be slow enough so it does not causeany major disturbance on the dc link voltage. For example,if sudden power is drawn from the dc link capacitor by thedc/dc converter, the capacitor voltage might significantly dropbefore the dc link voltage controller reacts to the disturbance.The power drawn by the dc/dc converter is modeled as a rampfunction

PBatt = DP /s2 (23)

where Dp is the rate of change in watts per second. From Fig. 9and by using the final-value theorem, the steady-state error in

Fig. 12. Experimental setup.

Vdclink as a result of the dynamic change in P ∗Batt can be shown

to be given by

Vdc_Ess = lims→0

(sVdc_E) = −DP /kdc_I . (24)

The ramp rate DP is set to 1 kW/s. Therefore, the steady-stateerror caused by ramping the dc/dc power demand up or down isonly Vdc_Ess = ±2 V.

V. EXPERIMENTAL RESULTS

A complete microgrid system was built and tested experi-mentally. The experimental setup is illustrated in Fig. 12. Itincludes two 60-kW line-interactive UPS systems and an STSwith a supervisory controller. In addition, a 60-kW resistiveload is used as a local load. The controller parameters ofthe converter are shown in Table III. The controllers wereimplemented using the Texas Instrument TMS320F2812 fixed-point DSP. The low communication link between the STS andthe UPS units was realized using the Controller Area Network(CAN) protocol. The power demand references were set bythe supervisory controller of the STS and sent via the CANbus. In addition, for the purpose of testing different scenarios,the power demand could be set by the user via the CAN-bus-connected computer. An update signal of the status of the STSis also sent to the UPS units via the CAN bus.

Fig. 13(a) shows the grid-connected-to-stand-alone modetransition of one UPS unit. The UPS unit was operating in grid-connected mode charging the battery at 1-kW rate (note thatthe current is 180◦ out of phase with respect to the voltage,which means that the UPS is importing power from the grid).When the grid fails, the UPS moves seamlessly to stand-alonemode and starts supplying the local load. It can be also observedfrom the figure that, at the moment when the grid fails, the dc


Fig. 13. (Current scale: 1 A/2 mV). (a) Grid-connected to stand-alone transition of one UPS unit. When in grid-connected mode, the UPS was charging thebattery at 1-kW rate. When in stand-alone mode, the UPS unit was supplying a local 60-kW load. (b) Grid-connected to stand-alone transition of two UPS units.When in grid-connected mode, each UPS was producing 0 kW. When in stand-alone mode, the two UPS units were sharing a local 60-kW load. (c) Grid-connectedto stand-alone transition of two UPS units. When in grid-connected mode, each UPS was charging its battery at 10-kW rate. When in stand-alone mode, the twoUPS units were sharing a local 60-kW load. (d) Stand-alone to grid-connected transition of one UPS unit. When in stand-alone mode, the UPS unit was supplying alocal 60-kW load. (e) Battery charging (10 kW) to discharging (60 kW) mode transition. (f) DC link voltage controller transient response when going from batterydischarging mode (the dc link voltage is controlled by the dc/dc converter) to battery charging mode (the dc link voltage is controlled by the ac/dc converter).

link voltage starts to drop from the active rectifier reference(V ∗

dclink_2 = 800 V) due to the change of power flow direction.However, when the dc link drops below the boost reference(Vdclink_1 = 750 V), the boost starts to regulate the dc linkvoltage and it raises it to 750 V. Fig. 13(b) shows the grid-connected-to-stand-alone mode transition of the two UPS units.Each unit was producing 0 kW as both batteries were fullycharged. At the moment when the grid fault occurs, the twounits move seamlessly to stand-alone parallel mode sharingthe 60-kW local load equally. The critical load does not seeany power interruption, as can be seen from the load currentsignal. The two UPS units behave exactly the same, as canbe seen from their current signals, which are placed on top ofeach other. In Fig. 13(c), the two UPS units were operating ingrid-connected mode charging their batteries at a 10-kW rate.When the grid fails, the two inverters move almost seamlesslyfrom grid-connected mode to stand-alone paralleling mode andstart sharing the critical load equally. The UPS charging currentlooks quite distorted, and this is because each unit generatesonly 17% of its rated power. According to the IEEE Standard1547 [21], the total harmonic distortion limit is defined basedon the unit’s maximum current, and the distortion at low poweris therefore acceptable. Fig. 13(d) shows the transition responsefrom stand-alone to grid-connected mode. During stand-alonemode, the microgrid was formed by one UPS unit supplyinga 60-kW resistive load. When the grid becomes available, theSTS closes and reconnects the microgrid seamlessly to the

grid. Only small fluctuations appear on the dc link, whichsettles within two fundamental cycles. The load current seesa small increase due to the fact that the grid voltage is slightlyhigher than the stand-alone UPS voltage because of the inheritcharacteristic of the voltage droop control. The UPS currentstarts to decrease and will eventually become equivalent to thepower command in grid-connected mode. Fig. 13(e) shows thetransient response when the active power demand Pref changesfrom −10 to 60 kW by the user interface. The controller rampsup the power demand gradually (to avoid any unnecessarytransient as discussed earlier), and thus, the current changesamplitude and phase gradually. The dc link voltage dropsfrom 800 V (controlled by the ac/dc active rectifier) to 750 V(controlled by the dc/dc converter). Fig. 13(f) shows the dc linkvoltage controller transient response during transition from bat-tery discharging mode (the dc link voltage is controlled by thedc/dc converter) to battery charging mode (the dc link voltageis controlled by the ac/dc converter). The dc link shows goodtransient response similar to the simulated response shown inFig. 11.

Fig. 14 shows the starting sequence of one UPS unit in grid-connected mode. Initially, the inverter voltage is controlled tohave the same magnitude and frequency as the grid voltage, andit is also synchronized so that the power angle is minimized tobe virtually zero. When the STS controller is satisfied that thetwo voltage signals across its terminals are healthy and in phase,it closes the STS so the microgrid is connected to the main grid.


Fig. 14. UPS starting in grid-connected mode.

VI. CONCLUSION

A line-interactive UPS system to be used in microgrids hasbeen presented. The control strategy that is based on a voltageand frequency drooping technique was experimentally demon-strated to be capable of achieving seamless transfer betweengrid-connected and stand-alone parallel modes of operation. Adc link voltage controller that sets the active power demandduring battery charging mode was demonstrated to be effectivein facilitating smooth transition between battery charging anddischarging modes.

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Mohammad A. Abusara received the B.Eng. de-gree from Birzeit University, Birzeit, Palestine, in2000 and the Ph.D. degree from the University ofSouthampton, Southampton, U.K., in 2004, both inelectrical engineering.

From 2003 to 2010, he was with Bowman PowerGroup, Southampton, responsible for research anddevelopment of control of power electronics fordistributed energy resources, hybrid vehicles, andmachines and drives. He is currently a Senior Lec-turer in renewable energy at the University of Exeter,

Penryn, U.K.

Josep M. Guerrero (S’01–M’04–SM’08) receivedthe B.S. degree in telecommunications engineering,the M.S. degree in electronics engineering, and thePh.D. degree in power electronics from the TechnicalUniversity of Catalonia, Barcelona, Spain, in 1997,2000, and 2003, respectively.

He is a Full Professor in the Department of EnergyTechnology, Aalborg University, Aalborg, Denmark.His research interests include power electronics con-verters for distributed generation and distributed en-ergy storage systems, control and management of

microgrids and islanded minigrids, and photovoltaic and wind power plantscontrol.

Suleiman M. Sharkh received the B.Eng. and Ph.D.degrees in electrical engineering from the Universityof Southampton, Southampton, U.K., in 1990 and1994, respectively.

He is currently the Head of the Electro-Mechanical Research Group, University ofSouthampton.

Dr. Sharkh is a member of The Institution of En-gineering and Technology and a Chartered Engineer.He was the recipient of The Engineer Energy Innova-tion Award for his work on rim-driven thrusters and

marine turbine generators in 2008.

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Line-Interactive UPS for Microgrids