Control of Line-Interactive UPS Connected in

Parallel Forming a Microgrid

Josep M. Guerrero*, Nestor Berbel, Jose Matas, Jorge L. Sosa, and Luis Garcia de Vicufia

Escola Universtaria d'Enginyeria Tecnica Industrial de BarcelonaSustainable Distributed Generation and Renewable Energy research group

Dept. Automatic Control Systems. Comte d'Urgell, 187. 08036 BarcelonaEmail: josep.m.guerrero@upc.edu URL: http:Isepic.upc.edu

Department of Electronic Engineering. Universitat Politecnica de Catalunya (Spain)

Abstract In this paper, the control strategy for an overallmicrogrid is presented. The control has two main levels: i) thedroop control of the inverters, and ii) the control of the staticbypass switch, and the droop settings for the management of thepower flow. Thus, a flexible microgrid is obtained to operate ineither grid-connected or islanded mode. Results from line-interactive UPS inverters connected in parallel forming amicrogrid are presented, showing the feasibility of the presentedsolution.

Index Terms Microgrids, Uninterruptible Power Systems,Distributed Generation, Droop Method.

I. INTRODUCTION

MA ICROGRIDS are becoming a reality in a scenario inwhich renewable energy, distributed generation (DG),

and distributed storage systems can be conjugated and alsointegrated into the grid. These concepts are growing up due tonot only environmental aspects but also due to social,economical, and political interests. The variable nature of somerenewable energy systems such as photovoltaic or wind energyrelies on natural phenomenon like the sunshine or the wind.Consequently, it is difficult to predict the power that we canobtain through these prime sources, and the peaks of powerdemand do not coincide necessarily with the generation peaks.Hence, storage energy systems are required if we want tosupply the local loads in an uninterruptible power supply(UPS) fashion [1], [2]. Some small and distributed energystorage systems can be used for this purpose, such as: flowbatteries, fuel cells, flywheels, superconductor inductors, orcompressed air devices.

The distributed generation (DG) concept is takingimportance, pointing out that the future utility line will beformed by distributed energy resources and small grids(minigrids or microgrids) interconnected between them. Infact, the responsibility of the final user is to produce andstorage part of the electrical power of the whole system.

This work was sponsored by the Spanish Ministry of Science and Technology:CICYT DPI 2003-06508-C02-01.

This change of paradigm let the microgrid export andimport energy to the utility through the point of commoncoupling (PCC). And, when there is a utility failure, themicrogrid still can work as an autonomous grid [3]. As aconsequence, these two classical applications: grid-connectedand islanded operations can be used in the same application. Inthis sense, the droop control method is proposed as a goodsolution to connect in parallel several inverters in island mode[4]-[7]. However, although it has been investigated andimproved, this method by itself is not suitable for the comingflexible microgrids.

In this paper, a control scheme for UPS connected inparallel forming a microgrid is proposed. The UPS invertersuse a droop control function in order to avoid criticalcommunication between the modules. The droop function canmanage the output power of each UPS in function of thebattery charge level. The inverters, as opposed to theconventional methods, act as voltage sources even when theyare connected to the grid. In this situation, they are able toshare power with the grid in function of its nominal power.Finally, the intelligent static bypass switch connects ordisconnects the microgrid and sends proper references to thelocal UPS controllers by means of low bandwidth.

Fig. 1. Diagram of a photovoltaic microgrid.

1-4244-0755-9/07/$20.00 (C2007 IEEE 2667

II. THE MICROGRID STRUCTUREFig. 1 shows the schematic diagram of a microgrid. In our

example, it consists of several photovoltaic strings connectedto a set of line interactive UPSs forming local ac microgrid,which can be connected to the utility mains through theintelligent bypass switch (IBS). The IBS is continuouslyobserving its both sides: the mains and the microgrid. If thereis a fault in the mains, the IBS will disconnect the microgrid,creating an energetic island. When the mains is restored, allUPS units are advice by the IBS to synchronize with the mainsand to properly manage the energy.

Consequently, we can derive that the microgrid has twomain possible operation modes: grid connected and islandedmode. It is desirable that the transitions between both modesare as seamlessly as possible [8]-[12]. In addition, theconnection or disconnection of UPS modules should be madewithout perturbing too much the power quality of themicrogrid (hot-swap orplug'n 'play capability).

III. THE DROOP METHOD CONCEPTWith the aim of connect in parallel several inverters

without control intercommunications, the droop method, alsonamed wireless or autonomous control, is often proposed. Theapplications of such a kind of control are industrial UPSsystems [13] or islanding microgrids [14], [15]. Theconventional droop method is based on the principle that thephase and the amplitude of the inverter can be used to controlactive and reactive power flows. Hence, the conventionaldroop method can be expressed as follows [16]

= c -mP (1)E = E - nQ (2)

where E and V are the amplitudes of the inverter output voltageand the common bus voltage, + is the power angle.

The active and reactive power flown from an inverter to agrid through an inductor can be expressed as follows [17]

p <Vcos5-VcOO+ sinq5sinO (3)

EV V EsnV (4)Q = 0-cos0 --) sinO --sinO cosO

where E and V are the amplitudes of the inverter output voltageand the common bus voltage, 0 is the power angle, andX and 0are the magnitude and the phase of the output impedance.Notice that there is no decoupling between P - c and Q - E.However, it is very important to bear in mind that droopmethod is based on several assumptions:Assumption 1: The output impedance is pure inductive and, (3)and (4) become:

P= sino5 (5)xEV V (6

Q = ~~ cos _- (6)

This is often justified due to the large inductor of the filterinverter and the impedance of the power lines. However,inverter output impedance depends on the control loops, andthe impedance of the power lines is mainly resistive in low

voltage applications. This problem can be overcome by addingan output inductor, resulting in an LCL output filter, or byprogramming a virtual output impedance through a controlloop.Assumption 2: The angle q5 is small, we can derive that sin q5 0and cos 0q 1, and consequently

p EV 0

x

Q -(E-V)x

(7)

(8)Note that, taking into account these considerations, P and Q

are linearly dependents on q5 and E. This approximation is trueif the output impedance is not too large as in most practicalcases. On the contrary, you will need a large phase angle toexport the same amount of active power, thus becoming lesstrue the approximation.

The droop method uses the frequency instead of the phaseto control the active power flow. This is because the unit doesnot know the initial phase value of the other units. However,the initial frequency at no load can be easily fixed as CO. As aconsequence, the droop method has an inherent trade-offbetween the active power sharing and the frequency accuracy.This fact provokes frequency deviations.

Attempts to eliminate this frequency deviation wereproposed by using frequency restoration loops [18]. However,in general they are not practical, since the system becomesunstable, increasing the circulating current increases since thefrequencies of the DSP crystals are not exactly equals, and asteady-state phase droop is not able to compensate for thesedifferences.

IV. MICROGRID CONTROLTaking into account the features and limitations of the

droop method, we propose a control structure for a microgridwhich could operate in both grid-connected and islandedmodes. The operation of the inverters is autonomous as acontrary as other microgrid configurations, which use master-slave principles [8], [19]. Only low bandwidth communicationsare required in order to control the microgrid power flow andsynchronization with the utility grid.

A. Grid-connected operationIn this operation mode, the microgrid is connected to the

grid through the intelligent bypass switch (IBS). In his case, allUPS have programmed the same droop function [4], [18]

CO=CO - m(P - ) (9)E=E* -n(Q-Q*) (10)

where P* and Q* are the desired active and the reactive powers.Normally, P* should coincide with the nominal active

power of each inverter and Q* = 0. However, we have todistinguish between two possibilities: 1) importing energyfrom the grid or 2) exporting energy to the grid.

The first scenario, in which the total power load is notsupplied enough by the inverters, the IBS adjust P* by usinglow bandwidth communications to achieve that the gridsupplies the nominal power in the PCC. This is done with

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smalls increments and decrements of P* in function of themeasured grid power. The implementation can be done byusing a slow PI controller, as follows:

P* k(]< -Pg)+kJP, -Pgdt+ft (11)

being Pg and Pg* the measured and the reference active powerof the grid; and Pi the nominal power of the inverter i. his way,the UPSs with low battery level can switch to charger mode byusing P*<0. The second scenario occurs when the power of theprime movers (e.g. PV panels) is much higher than thoserequired by the loads, and the batteries are fully charged. Inthis case, the IBS will control that the power flowing to thegrid is less than those nominal value. In the same way, the IBSwill adjust the power references.

B. Islanding mode operationWhen the grid is not present, the IBS disconnects the

microgrid from the grid, starting the autonomous operation. Insuch a case, the droop method is enough to guarantee properpower sharing between the UPSs. However, we must take intoaccount the power sharing in function of the batteries chargelevel of each module. This way, m droop coefficient can beadjusted inversely proportional to the charge of the batteries, asshown in Fig. 2,

(12)mm =

being mmin the droop coefficient at full charge and a is the levelof charge of the batteries (1= fully charged and 0= empty).

CDst

a)*

w)i,

. batteries batteries30%0 charged . Fully charged

* <~~~~~~~~~~~~~1

phase errors between the grid and the microgrid.

V. UPS INVERTER CONTROLThe control of the UPS inverter is based on three kinds of

control loops [14]: 1) the inner voltage and current regulationloops, 2) the intermediate virtual impedance loop, and 3) theouter active and reactive power sharing loops. The innervoltage and current regulation loops can be done by usingconventional PI multiloop control or generalized integrators.

A. Virtual output impedance loopThis intermediate current loop is able to fix the output

impedance of the inverter by subtracting a processed portion ofthe output current (io) to the voltage reference of the inverter(Vref), as follows [13], [20]

V*= V, ZV (S) io (13)being v* the obtained voltage reference of the inner controlloops and Zr(s) the transfer function of the output impedance.In order to program different impedance over the currentharmonics, this harmonics can be extracted by using a discreteFourier transformation (DFT), and the following virtual outputimpedance can be programmed [15], [20]:

N

Z (s) sZLTh'ohh=lodd

(14)

being ioh the h-th harmonic current, and Lvh the impedanceassociated to each component. This way, we can adjust outputimpedance, sharing properly harmonic load currents, butwithout increase too much the voltage THD.

In addition, hot-swap operation, i.e. the connection of moreUPSs modules without cause large current disturbances, can beachieved by using a soft-start virtual impedance [15]. This canbe achieved by programming a high output impedance whenthe UPS is connected to the microgrid, and then, reduce itslowly to a proper value:

LD = Df + (LD -L)eLTD (15)

30o Pm,, P.Fig. 2. Droop characteristic in function of the batteries charge level.

C. Transitions between grid-connected and islanded operationWhen the IBS detects some fault in the grid, it disconnects

the microgrid from the grid. In such a case, the IBS canreadjust the power reference to the nominal values, but this isnot strictly necessary. In addition, the IBS can measure thefrequency and the amplitude of the voltage inside themicrogrid, and can move these set points (P* and Q*) in orderto avoid the frequency and amplitude deviation of the droopmethod. Fig. 3 shows the active power of a two-UPS microgridsharing power with the grid, the transition to islandedoperation, and the disconnection of the UPS #2.

When the microgrid is working in islanded mode, and theIBS detects that the voltage outside the microgrid (in the grid)is stable and fault-free, we have to resynchronize the microgridto the frequency, amplitude and phase of the grid, in order toreconnect seamlessly the microgrid. While amplitude isadjusting by moving up and down small steps of Q*, frequencyand phase is adjusted by small steps ofP* proportionally to the

where LDO and LDJ are the initial and final values of the outputimpedance, and TST is the time constant of the soft-startoperation. The soft-start operation proposed here consists inconnecting the inverter to the common bus using a high outputimpedance LDO and reducing it slowly towards the nominal

value LDf.

Fig. 3. Active power transients between grid connected and islanded modes.

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B. P/Qflow control loopsOnce the output impedance is fixed, the droop method can

be designed properly. In this case, we have included derivativeterms in order to improve the transient response of the system.

= -m(P p*)-M d(P ) (15)

£ E=* -n(Q - Q*)- n d(Q tQ) (16)

being md and nd the active and reactive derivative droopcoefficients.

VI. SIMULATION RESULTS

The aim of this section is to test the proposed controllerover a distributed UPS system in order to show its performanceand limitations in different scenarios.

A. Harmonic current sharingThe first step is to observe the effect of the proposed

controller over the harmonic current sharing, which is veryimportant when the system is supplying a nonlinear load.

Fig. 5 shows the feasibility of the harmonic current sharingloop (13), which consists in drooping the output voltageproportionally to the harmonic current terms, and, hence,providing an individual behavior for every harmonic term. Thisway, we can improve the harmonic current sharing withoutconsiderably increasing the output voltage distortion. With thisobjective, we have performed some simulations of a two-UPSparalleled system sharing a common nonlinear load.

Fig. 5(a) shows the steady-state and the transient responseof the output currents, without using the harmonic sharingloop. Consequently, fundamental currents are equal, but the 3rd,5th, and 7th harmonics are unbalanced. However, as shown inFig. 5(b), by using (13) with the harmonic current sharing loop,fundamental and harmonic current terms are properly shared.Note that we have included up to the 7th harmonic, althoughhigher harmonics can be added to this control loop ifnecessary.

The impedances of the lines connected between theinverters and the load were intentionally unbalanced, ZL] =

0.12 +j 0.028 Q, ZL2 = 0.24 +j 0.046 Q, ZL3 = 0.06 +j 0.014Q, and ZL4 = 0 + jO Q. Fig. 9 depicts the progressiveconnection of the four UPSs to the common ac bus: UPS#1 at t= 0 s, UPS#2 at 0.8s, UPS#3 at 1.6 s, and UPS#4 at 2.4 s. As itcan be seen, the UPS modules are seamlessly connected to thebus, increasing their output currents gradually. Fig. 6 illustratesthe disconnection of the UPS from the ac bus. There are noovercurrents in this case, and the control does not need to act ina special way. Droop functions of every UPS accommodate theoutput currents to the amount of load automatically with nocritical transients.

VII. EXPERIMENTAL RESULTS

Two 6-kVA single-phase UPS units were built and tested inorder to show the validity of the proposed approach. Eachinverter consisted of a single-phase IGBT full-bridge with aswitching frequency of 20 kHz and an LC output filter, withthe following parameters: L = 500 pfH, C = 100 tF, Vin = 400V, vo = 220 Vrms@,50 Hz. The impedance of the lines connectedbetween the inverters and the load were intentionallyunbalanced, ZL] = 0.12 +j 0.028 Q, and ZL2 = 0.24 +j 0.046 Q.The controllers of these inverters consist in on three loops: aninner current loop, an outer PI controller that ensures voltageregulation, and the power-sharing controller. The first twoloops were implemented by means of a TMS320LF2407Afixed-point 40 MHz digital signal processor (DSP) from TexasInstruments. The power-sharing controller was implementedby using a TMS320C6711 floating-point 200 MHz DSP (seeFig. 7). The connection between the two-DSPs was madethrough the Host Port Interface (HPI) of the '6711. The DSPcontroller also includes a PLL block in order to synchronizethe inverter with the common bus. When this occurs, the staticbypass switch is turned on, and the soft-start operation and thedroop-based control are initiated. The first set of experimentsconsiders a resistive load.

DiributedUtPS invciterB. Hot-swap operation

The second step consisted in testing the hot swapoperation of the UPS by using the proposed soft-startimpedance. We have to consider two main scenarios: theconnection and the disconnection of the UPS in the distributedsystem. When an additional UPS has to be connected to thecommon bus, the output voltage of the inverter synchronizedwith the bus by the PLL action. After that, the droop controlmethod kicks in together with the output impedance loop. Aswe stated above, at first, the output impedance has a highvalue, but later it goes down to the nominal value in a soft-startfashion. This way, initial current peaks are avoided.

We have simulated a four-UPS system sharing adistributed nonlinear load in order to see the effects ofconnecting and disconnecting these units. Fig. 4 shows theconfiguration setup used for the simulation results of theparalleled four-UPS system connected through its power linesto the common bus, which supplies the distributed local loads.

UPS#]

)

ZJRU=,f ,XlJ Z

..

ws<.

; 2#isz

XW+j}?Z~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~1

UPS #3

Z()

I

II UPSW1II

. 1.

*)

ZJ4 I+XU Linc impedances

II SCoran bs

Distfbuted loads

Fig. 4. Configuration setup for simulation results of the microgrid.

2670

=R,L3 Lj+jYLJ

0 w20 OD4 OY7 O O 1

5=

0 a(1 0D2 f03 OS 0,,7 o , I.,C3i

0 a01 K0 040 0.05 GM3 C07 GM G09 1L

t~~~~(4

o Q01 00 0(X3 04 ODY _OM6 Q07 O1OG ) i

20o o0 uB ox.~ oow x o 0 1

0 aOl 0.02, 0,03 nA t O 7 Ot 0109 I

-0 -0 -i0 CLO ?D O .6 00 LB U9 O

0 0.01 0D OtX (A 00~ Am 0C O-3 0

(a) (b)Fig. 5. Harmonic decomposition extracted by using the bank ofband pass filters (top) and output currents (bottom) of a two-UPS system with highly unbalancedpower lines, sharing a non-linear load, (a) without and (b) with the harmonic current sharing loop.

_r 11IIIII II 11 IX Il I IuEs

O 05 1 1 5 2 25 3 35 410

-10o 05 1 1 5 2 2-5 3 35 4

10

0

-10o 05 1 1!5 2 25 3 3-5 4

10

0

-10o 0.6 1 1.5 2 25 3 as.5 4

t (s)

05 1 15 2 25 3 354EL-10 m| llm |iiiiR idil

105 1 1 5 2 2.5 3 a5 410

10-1005 1 1 5 2 2.5 3 35 4

10

-113W0.5 1 1.5 2 2.5 3 3-5 4

-10 rrrO.5 11 1 -5 2 2.5; 3 3-5 4

t$

(a) (b)Fig. 6. Output currents of the UPS#1, UPS#2, UPS#3, and UPS#4 in (a) soft-start operation (TST = 0.1 s and LD, = 80 mH), and (b) disconnection scenario.

Fig. 8 shows the output currents of every unit and thecirculating current (i,1 - i,2) for sudden changes from no loadto full load and vice versa. These results show an excellentdynamic response for the proposed controller. As it can beseen, the circulating current remains very small, even for noload conditions.

Secondly, the supply of a purely capacitive load of 40 tF isevaluated. Figs. 9 and 10 depict the steady-state and thetransient response of the output currents. The suddenconnection of the capacitor to the ac bus causes a high currentpeak during the instantaneous charge process. Nevertheless,the fast transient response of the controller enforces good equalcurrent sharing. The non sinusoidal waveforms of thesecurrents are basically due to the non idealities and parasiticelements of the capacitor. As it can be seen, the load sharingcapability is very good in their situation.

Fig. 7. DSP implementation of the power sharing controller.

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,>

,tqc

...AWl

. otuIuretxs:5.sdv A,

III Od~~~~~~~~~~~~~~~~~~~~~~~~~.. I I I~~~~~~~~~~~.....

Fi.9 teadyF-stt fth outpu curnt whe1n saringl 5a pur

Fr4itqiv OaHdIF,, e-2 Rh-s 1 39a.7.rh I 3 . 69" F7,q 5 [I. OOH2

-SdiY -1 diV Odiv - 1 diV -3di% OOi

Fig. 9. Steady-state of the output currents when sharing a pure40 pF capacitive load (X-axis: IO ms/div, Y-axis: 5 A/div).

(X-axis: 10 ms/div, Y-axis: 5 A/div).

VIII. CONCLUSION

In this paper, a control scheme for parallel connected UPSsystems forming a microgrid was proposed. The controlstructure was based on the droop method in order to achieveautonomous operation of each UPS module. An intelligentstatic bypass switch monitors the grid in order to send properlow bandwidth information to the modules. This way, themicrogrid can operate in both islanded and grid-connectedmodes.

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