A New Proposal for Power Quality and Custom

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 4, OCTOBER 2009 2107

A New Proposal for Power Quality and CustomPower Improvement: OPEN UPQC

Morris Brenna, Member, IEEE, Roberto Faranda, Member, IEEE, and Enrico Tironi

Abstract—Currently, the quality of supplied power is importantto several customers. Power quality (PQ) is a service and many cus-tomers are ready to pay for it. In the future, distribution system op-erators could decide, or could be obliged by authorities, to supplytheir customers with different PQ levels and at different prices.A new device that can fulfill this role is the OPEN unified power-quality conditioner (UPQC), composed of a power-electronic seriesmain unit installed in the medium-voltage/low-voltage (LV) sub-station, along with several power-electronic shunt units connectedclose to the end users. The series and parallel units do not have acommon dc link, so their control strategies are different than tra-ditional UPQC control techniques. This device can achieve generalimprovement in PQ, reducing the most common disturbances forall customers that are supplied by the mains (PQ) by using only theseries unit. Additional increments in PQ (i.e., mains power inter-ruptions), can be provided to the customers who need it (custompower) by the shunt units. Therefore, this new solution combinesan improvement in PQ for all end users, with a cost reduction forthose that need high quality power. The proposed solution has beenanalyzed and described, and a model of a 400-kVA LV grid is con-sidered a test network to evaluate the steady-state performance andfunctioning limits. The results obtained under steady-state con-ditions justify the configuration chosen and good device perfor-mance.

Index Terms—Active power conditioner, custom power, interfacedevices, OPEN unified power-quality conditioner (UPQC), powerquality (PQ), unified power quality conditioner (UPQC).

I. INTRODUCTION

P OWER QUALITY (PQ) is very important to certain cus-tomers. For this reason, many utilities could sell electrical

energy at different prices to their customers, depending on thequality of the delivered electric power.

Since most end users are connected to secondary distributionnetworks, at low voltage (LV), it could be important to monitorand compensate the main disturbances on the LV grid. Specif-ically, it has been reported in a survey [1] that, in the South-eastern region of the U.S., most monitored industrial customersand main end users did not suffer long outages. Rather, theyexperienced numerous short duration voltage sags and momen-tary interruptions. Therefore, local utility companies had to re-

Manuscript received May 16, 2008; revised May 14, 2009. Current versionpublished September 23, 2009. Paper no. TPWRD-00364-2008.

M. Brenna and R. Faranda are with the Department of Energy, Po-litecnico di Milano, Milan 20133, Italy (e-mail: [email protected];[email protected]).

E. Tironi is with the Electrical Engineering Department, Politecnico di Mi-lano, Milan 20133, Italy (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/TPWRD.2009.2028791

Fig. 1. Chronology of voltage sags occurring in the Southeastern states of theU.S. on December 4 and 5, 2002.

configure their systems to keep their most important customerson-line.

Fig. 1 shows the chronology of voltage sags occurring in theSoutheastern states of the U.S. on December 4 and 5, 2002. Ascan be seen, most sags take place around the 10%–20% level.

Various solutions are available to compensate for these distur-bances. One solution involves increasing the short circuit levelof the distribution network, i.e., revamping all the LV distribu-tion cables or raising the power of the MV/LV substation trans-former, thus increasing the power quality for all end users. Inthis way, an incoming disturbance from a load (i.e., harmonics)or from a fault in a line is reduced at the point of commoncoupling (PCC). Therefore, this solution effectively reduces thedepth of the voltage variations, but does not protect the loadsagainst transients and short interruptions. A second solution thatcan compensate any kind of disturbance, including interrup-tions, is installation [2] of on-line, off-line, line interactive andhybrid UPS systems. In all of these cases, only the end usersthat decide to install them are protected, while all of the othercostumers do not receive any improvement in PQ.

Often, these solutions cannot be adopted by the local utilitycompanies or by the end users, because they are too expensiverelative to the increase in power quality that they produce. How-ever, many cheaper solutions are available.

In particular, several electronic devices have been developed,studied and proposed to the international scientific communitywith the goal of improving supplied power quality. In [3]–[10],various single apparatus are analyzed. Different connectiontopologies (series or shunt types) are used to realize thesedevices. The series devices are connected upstream of the pro-tected lines, while the shunt devices are connected in parallelto the sensitive loads. In general, both types of conditioningdevices increase the power quality level at the loads, as reportedin [3]–[7] for series devices and in [8]–[10] for shunt devices.

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2108 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 4, OCTOBER 2009

Other studies have been carried out to consider combina-tions of the previous single apparatus solutions (as UPS, UPLC,UPQC, etc.). The unified power quality conditioner (UPQC)compensator seems to be a particularly promising power con-ditioner device. This apparatus is constituted of a series and ashunt unit, with a common dc section through which power canbe exchanged. Its function is to improve the quality levels ofthe current absorbed at the mains and the load supply voltage[2], [11]. However, these devices do not allow local distribu-tors to guarantee different quality demand levels to the final cus-tomers, because they improve power quality for all the suppliedend users. The installation investments are also quite high rela-tive to the power quality level obtained. A solution that has sim-ilar performances and advantages, but also makes cost reductionpossible, is the proposed OPEN UPQC.

This new solution, analyzed in [12], [13], starts from theUPQC configuration, removes the common dc connection andsplits the shunt unit into several shunted devices. Therefore, thecontrol strategy is different than the traditional combined seriesand shunt converters, but the improvements to load voltage andnetwork current quality are quite similar. Above all, the OPENUPQC can stabilize load voltage, increase the network powerfactor, leading to keep load voltage and network current sinu-soidal and balanced as well.

The series main unit is installed in the MV/LV substation. Ina grid connected configuration, it can stabilize load voltage atthe LV busbar (PCC) as the series devices analyzed in [3]–[7].The shunt units do not affect the dynamic behavior of the seriesunit, because their dynamic responses are very slow under theseoperating conditions. In [6] and [7], the transient behavior of asingle dynamic voltage restorer) device was analyzed and simu-lated, and its working limits were determined. In particular, thedevice behavior in the presence of voltage sags (i.e. 10%–20%)is described.

The several shunt units are connected near the end users thatneed high power quality. If a storage system is present, they canexchange active power and nonactive power with the electricalsystem. Especially in a grid-connected configuration, nonactivepower can be exchanged with the mains in order to enhance theseries unit performance and extend its working limits. Other-wise, the users can disconnect themselves when the PCC voltageis out of the operating limits, and the load will be supplied inback-up mode.

II. THE OPEN UPQC

Most end user disturbances are characterized by short dura-tion and small amplitude, though they can still cause interrup-tions in production processes. As can be seen in Fig. 2, mostvoltage sags have small depth and short durations [14].

More than 95% of voltage sags can be compensated by in-jecting a voltage of up to 60% of the nominal voltage, with amaximum duration of 30 cycles. This information is primarilyused to evaluate a suitable size for the OPEN UPQC.

The series unit of the OPEN UPQC, sized to supply 60%of the LV network power and equipped with a small storagesystem, can compensate for most of the voltage disturbancesreported in Fig. 2. It has the same function as the DVR [6], [7],[15].

Fig. 2. Example of distribution of voltage disturbances reported in the EPRIevent coordination chart.

Fig. 3. Multiwire power diagram of the new proposed solution.

Each shunt unit is sized in relation to the supplied load power,and can protect its sensitive load against interruptions. The shuntunit’s function is similar to that of the UPS output stage [2],[16], but is less expensive because it only has one conversionstage and involves less power loss.

Fig. 3 shows the multiwire power layout of the device in athree-phase, four-wire distribution network.

The series unit consists of a coupling transformer (TR), withthe primary circuit connected in series with the mains line and asecondary one supplying the reversible ac/dc power converter.The output stage of the pulsewidth modulation (PWM) voltagecontrolled converter contains passive RC shunt filters, to com-pensate for the harmonic currents at switching and multiple fre-quencies. Neglecting the active power to compensate the con-verter losses, the series unit is controlled to act as a purely re-active inductor when the supply voltage is within its operationlimits . This fact is of fundamentalimportance, because in this range the loads must be supplied by


BRENNA et al.: NEW PROPOSAL FOR PQ AND CUSTOM POWER IMPROVEMENT: OPEN UPQC 2109

the mains 95% of the time, as established by the IEEE Std 1159“IEEE Recommended Practice for Monitoring Electric PowerQuality” and European EN50160; therefore, the storage systemmust not discharge itself. Outside of this range, active powercan be used to compensate the disturbances, in the same way asthe usual series compensation devices [6], [7], when a storagesystem is present.

The shunt units consist of an ac/dc power converter, similar tothe one used in the series unit, connected to an energy storagesystem and a set of static switches (SS) [17]. The shunt unit,depending on the state of the network voltage, can supply eitherthe entire load, or a part of the load.

There are two different modes of OPEN UPQC operation:• compensator: when the PCC voltage is within its operation

limits, the SS are closed, the series unit works as a three-phase voltage generator and the shunt units work as currentgenerators;

• back-up: when the PCC voltage is outside of its operationlimits, the SS are open, decoupling the network and theload-compensator system. Each sensitive load is suppliedby its shunt unit, which acts as a sinusoidal voltage gen-erator, using the energy stored in the storage system as anenergy source.

III. OPEN UPQC PERFORMANCE

This section is focused on understanding the OPEN UPQCcompensation limits. The analysis will be carried out understeady state conditions, to evaluate the compensation capacity ofthe device in normal operation mode .

It is important to remember that the power absorbed by theloads and the shunt units influences the performance of the se-ries unit, and therefore of the whole OPEN UPQC. Therefore,when considering a particular set of load conditions, it is pos-sible to find operating conditions for the shunt units that increasethe compensating limits of the series unit.

Depending on whether or not storage systems are present, theseries and shunt units can exchange only nonactive power orboth nonactive power and active power with the mains. In thelatter case, as will be shown in the following, the OPEN UPQCcan better compensate for short duration disturbances.

In the following cases, all of the solutions will be analyzedunder the assumption that the voltages are sinusoidal and areconstituted of only the positive sequence component in the dif-ferent network buses.

It is important to emphasize that suitably coordinating the var-ious units of the OPEN UPQC allows for a wide compensationrange, comparable with the UPS, but more economical. This co-ordination requires a communication system (i.e., based on thecarrier waves) between the series unit and the shunt units, butthis system cannot be very fast. Moreover, in transient analysis,the communication between the series unit and the shunt unitscannot be included (the communication could be slow, could beout of order, etc.). Therefore, each unit necessarily works alone.The dynamic behavior of each individual device is described in[3]–[10], and [15].

Fig. 4. Voltage compensation, exchanging only nonactive power. Case (a1):it is possible to obtain a power factor equal to 1 in s section in low-voltagesituations. Case (a2): the power factor is always less than 1.

A. Nonactive and Power Exchange

The conditions under which all of the converters exchangeonly nonactive power must be confirmed in situations when thesystem voltage is near the contractual limits (normal opera-tion mode).

In normal operation mode, the maximum voltage drop inthe LV lines of the network must be less than 5% to maintainlow power loss. Therefore, if all the converter units are oper-ating to stabilize the voltage in the PCC at its nominal value(100%), the load voltage value will be at least 95% of the nom-inal voltage. This result allows an improvement of one of theaspects of the supply quality, the stability of the real value of thesupply voltage, for all customers. Therefore, the OPEN UPQCworks to stabilize the nominal voltage at the PCC. The phasordiagram of the OPEN UPQC is shown in Fig. 4.

In order to avoid active power injections, the series voltagehas to be in quadrature with the mains current . The

value is reported in (1), and the grey areas in Fig. 4 indicate thefield of possible values

(1)



The current is primarily composed of the current of unpro-tected loads (whose phase difference with respect tocannot be varied) and the current of protected loads (whosephase difference with respect to can be changed by theshunt units) as reported in (2), where and are theactive and reactive power of the equivalent load , respec-tively, and are the active and reactive power lineslosses, respectively, and is the reactive power injected by allthe shunt units

(2)Therefore, the angle can oscillate between the upper

and lower limits and , obtained whenand respectively, in the area highlighted

in Fig. 4. The angle can be calculated by the equation shown atthe bottom of the page.

The current phasor can move along the black dotted line,varying the reactive power of the shunt units. In case (a1)in particular, it is possible to obtain a power factor equal to1 in the section in low voltage situations, because the line

intercepts the black dotted line. In case (a2), thepower factor is always less than 1.

The quantities and can be obtained with (4)and (5), as shown at the bottom of the page.

Assuming that , the range am-plitude can be obtained with (6)

(6)

It can be seen that the compensating range amplitudedepends on the value that the series

unit can inject, and on the nonactive power . The nonactivepower is susceptible to exchanges by the shunt units (lengthof the black dotted line, proportional to the loads apparentpower) and to the power factors of the equivalent loads and

.In normal operation mode, the compensation strategy can be

implemented in various ways. For example, power factor maxi-mization in the s section (corresponding to minimization of thecurrent ) is a compensation strategy that can be implementedby coordinating the series unit and the shunt ones. Therefore,communication between all the units is required. The simplestsolution is to employ a slow communication system that allows

Fig. 5. Compensation limits of the OPEN UPQC: with nonactive power ex-change only (light gray) and with also active power exchange (dark gray) by theshunt units.

the OPEN UPQC to stabilize the voltage at the PCC, maxi-mizing the power factor in normal operation conditions and in-creasing its compensation limits outside of normal operation.Obviously, in the case of large disturbances in that the se-ries unit cannot compensate, each shunt unit can supply the loadin back-up mode.

B. Nonactive and and Active Power Exchange

In this case, the series converter produces only nonactivepower, but the shunt units can exchange active and nonactivepower with the mains. This condition could be represented asan active network into which dispersed generations are inserted.Fig. 5 depicts the new phasor diagram of the OPEN UPQCunder the above operating conditions.

In order to avoid active power injections by the series unit,the voltage and the mains current have to be in quadraturewith each other. In Fig. 5, the light grey areas indicate the field inwhich can lay without active power exchanges by the shuntunits, and the dark gray areas indicate the possible values ofwith active power exchanges by shunt units. In this case, thecompensating range amplitude is greater thanwithout active power exchanges, but it is important to note thatthe difference is small. The phasor current can move inside ofthe gray dotted circle, varying the active and nonactive powerof the shunt units (movement on the black dotted line regardsonly nonactive power exchange).

(3)

(4)

(5)



Fig. 6. Compensation limits of the OPEN UPQC: with nonactive power ex-change only (light gray) and with also active power exchange (dark gray) by theseries unit.

C. Noncctive and and Active Power Exchange

In order to exchange active power with the mains, a storagesystem connected to the dc section of the series unit is needed.The storage system size does not need to be very large, be-cause little energy is required to compensate most of the dis-turbances For example, to compensate most of the voltage vari-ations reported in Fig. 2 (voltage sag 60% deep for 30 cycle)for a 400-kW load, an energy equal to 120 kJ is needed, cor-responding to a battery capacity of about 0.4 Ah at 96 V or acapacitor or supercapacitor bank of about 1.5 F at 400 V.

Given a storage system with twice the abovementioned ca-pacity, in order to allow bidirectional energy exchange with themains, it is possible to compensate voltage disturbances inthat are outside of the contractual limits.

In the case of mains interruptions lasting longer than 30 cy-cles, the SS of the shunt units switch off, and the loads are sup-plied in back-up mode.

Considering compensation of transient disturbances, such asvoltage sags, swells, etc., various compensation strategies areavailable for the OPEN UPQC, including minimizing the energyrequired by the storage system of the series unit. The new phasordiagram of the OPEN UPQC operation is shown in Fig. 6. In thelight gray areas, the series voltage and the mains currenthave to be in quadrature with each other, because active powerexchanges by the series unit are not allowed. In these areas, thebehavior of the OPEN UPQC is the same as that of the casesdescribed previously.

In the case of transient disturbances, the series unit can com-pensate the voltage over a very large range (the compensatingrange amplitude is ) compared with all thecases previously analyzed. Indeed, the series unit can exchangeactive power with the mains in the dark gray areas, but this isonly possible for transient disturbances due to the small size ofthe series unit storage system.

IV. CONTROL STRATEGY

The following describes a control strategy that can be em-ployed in normal operation mode ,under steady state conditions, and elucidates the device perfor-mance.

The dynamic response during transient events has not beenconsidered in this work, because it is described in detail in [15],[17]. For example, considering the dynamic behavior of the se-ries unit, it can be seen that the series unit cannot be affected bythe shunt units during a transient event. This is due to the factthat the communication system between them is slow, and doesnot allow a fast coordinated control strategy.

In order to compensate for the voltages in normal operation,the strategy that maximizes the power factor (corre-sponding to the current minimization) can be chosen. Withthis choice, it is possible to minimize the apparent power re-quired by the mains.

The mains current is reported in (2), and the compensatedvoltage is

(7)

Neglecting power losses and considering that the voltagehas to be in quadrature with the current , it is possible to writethe following relation:

(8)

where is the equivalent reactance of the series unit, giving avoltage proportional to . Solving (7) and (8) is not mathemat-ically easy due to the nonlinearity of the problem, and imple-menting them into a controller is not useful.

It is more convenient to implement two PI controllers: one toevaluate the voltage of the series unit, and another to evaluatea signal related to the nonactive power that all of theshunt units have to inject.

The conditions under which the series unit can exchange onlynonactive power can be obtained by applying the Park transformto the three phase currents , and , and calculatingthe two components and in a rotating reference frame,as reported in (9)

(9)

where the angle is equal to and is the mainsfrequency. Consequently, the components of the injectedvoltage have to be proportional to the components ofthe current , as follows:

(10)

For the constant to be independent of the load condi-tions, the previous expressions must be normalized with respectto the load current module

(11)



Fig. 7. Voltage control loop of the series unit and nonactive power control loopof the shunt units in the OPEN UPQC system.

The constant is obtained by a PI controller that keepsthe voltage at the output of the series unit equal to therated value , as reported in the block diagram of Fig. 7.

The second control loop acts to minimize the anglebetween the voltage and the current downstream of theMV/LV transformer, in order to maximize the power factor ab-sorption in the section. In this case, the PI controller producesa signal , which varies from 0 to 1, and is equal to the ratiobetween the desired nonactive power injectable by the shuntunits and the maximum injectable nonactive power. This signalis sent to all shunt units by the communication system. Thus,the injected nonactive power of the th shunt unit is equalto

(12)

where is the unit’s rated power. The total nonactive powerinjected by all the shunt units is

(13)

Obviously, this compensation strategy, which is useful for itsfast series unit response, requires nonactive power injection bythe shunt units to be capable of achieving a wide compensationrange. To enhance the entire system’s performance, the powerlosses and the voltage drops in the LV lines generally must in-crease. However, if the power factor at the is kept high( 0.8), these increments are negligible. Moreover, this incre-ment can be reduced by sending a different signal to eachshunt unit. This allows the closest shunt units to be used to injectmore nonactive power, avoiding useless nonactive power flows.

V. TEST NETWORK AND EVALUATION OF OPERATION LIMITS

Fig. 8 shows a simplified 400-kVA LV grid, used to validatethe OPEN UPQC.

Fig. 8. System compensation structure.

TABLE IMV/LV TRANSFORMER PARAMETERS

TABLE IILV CABLE PARAMETERS

The protected loads are grouped into the equivalent load ,so all of the shunt units are represented by means of an equiv-alent unit. In the same way, all of the unprotected loads aregrouped in the equivalent load .

All of the parameters of the three-phase MV/LV transformerused for the simulations are reported in Table I.

The LV cables used for the following analysis, with differentpower factors and loads and , are reported in Table II.

In each analysis, the correct 200-m cable is chosen as a func-tion of the current needed to supply the equivalent load with avoltage drop of less than 3%, without considering the OPENUPQC. In this way, it is possible to neglect power loss and thevoltage drop on the LV grid.

In this study, all of the converters are represented as idealcontrolled voltage or current sources. Moreover, the series unitis not equipped with a storage system. For these reasons, theOPEN UPQC limits are evaluated mainly in the normal oper-ation mode in the following. Therefore, the series unit cannotexchange active power with the mains.

The following figures and tables report the power factorand the mains current in the section as functions

of the network voltage . Each diagram is represented for afixed load power factor and , and is parametric in . This



Fig. 9. Power factors of the system and maximum line currents in case 1, fordifferent � values. The maximum voltage of the series unit is equal to 0.6 p.u.

parameter indicates the ratio between the apparent powers ofthe total loads of shunt units and the total apparent powerof loads

(14)

With a fixed , and therefore fixed , it is possible to calcu-late as a function of the power factors of the loadsand

(15)The reference current is expressed in per unit (p.u.), as the

ratio between the power reference and the voltage refer-ence.

Since the network cables are correctly designed and theirparameters are constant, the voltage drop variation when theOPEN UPQC is present can be neglected under maximum loadconditions when the load power factor is equal to 0.9 and it isconnected at the end of the line.

The operation limits reported in Figs. 9 and 10, which allowthe voltage to be fixed at the nominal value, were obtainedby assuming the above hypothesis and that the maximum in-jectable voltage by the series unit is equal to 0.6 p.u..

The following results for the proposed solution were obtainedby converting the vector diagrams of Fig. 4 into geometricalequations.

Fig. 10. Power factors of the system and maximum line currents in case 2 fordifferent � values. The maximum voltage of the series unit is equal to 0.6 p.u.

In the following, the maximum nonactive power injected byall the shunt units can reach the apparent power .

In this case, if the control strategy can keep the voltageequal to the nominal value, then these relations have to be true

(16)

(17)

The operation limits given by (16) and (17) were obtained byusing (3) and (15).

The figures reveal that:1) the OPEN UPQC is well-adapted when the power factor

of the load is low. Fig. 9 shows the interval that can becompensated by exchanging only nonactive power whenthe power factor of the load is equal to 0.8. In this case,the OPEN UPQC produces excellent voltage stabilization,especially when the parameter is greater than 0.4;

2) the OPEN UPQC is not well-adapted when the powerfactor of the load is high. Fig. 10 shows the intervalthat can be compensated by exchanging only nonactivepower when the power factor of the load is equal to1. In this case, the OPEN UPQC does not produce goodvoltage stabilization, because it is too limited. It is possibleto obtain voltage stabilization in normal operation mode



TABLE IIIMAXIMUM AND MINIMUM MAINS VOLTAGE RANGE VARIATION THAT THE

SYSTEM CAN COMPENSATE, AS A FUNCTION OF THE MAXIMUM NONACTIVE

POWER THAT THE SHUNT UNIT CAN INJECT, WITHOUT CONSIDERING THE

MAINS POWER FACTOR

range (between 0.9 p.u. and 1.1 p.u.) only with a highvalue.

From Figs. 9 and 10, it is possible to estimate the power ofthe series unit, given the maximum current value. This valueis equal to the product between the maximum injectable voltage(equal to 0.6 p.u.) and the maximum line current (equal to 1.1p.u. when and as shown in Fig. 9). There-fore, with slight over-sizing of the series unit, good stabilizationof the mains voltage is possible. The usual working conditionspresent an interesting case, when the power factor of load isbetween 0.9 and 1, and the mains voltage is inside of the con-tractual limits (normal operation). The distribution power lossesshould be estimated, in order to understand the energy cost as-sociated with this solution.

Under these conditions, it is always possible to compensatethe voltage , without considering the power factor in section, if is greater than or equal to 0.5, as reported in Table III.

In the case of smaller spread among the shunt units, it is always possible to compensate for the voltage by

decreasing the power factor in the section. However, the powerfactor will always be greater than 0.8. When the powerfactor of the load is equal to one, the power factor in the

section is always close to one, and the compensation limitspreviously mentioned can be maintained.

The mains voltage limits reported in Table III change to thosereported in Table IV when it is important to keep the mainspower factor between 0.9 and 1.

VI. COST EVALUATION

To evaluate the costs of power quality improvement and theeconomic convenience of the proposed solution, an analysis ofthe 400-kVA LV distribution network has been carried out.

It was supposed that the line represents an equivalent linein which all of the sensitive loads that make up the OPENUPQC are connected, while the line represents an equiva-lent line that supplies only the nonsensitive loads . Therefore,each load that belongs to the set needs to be protected against

TABLE IVMAXIMUM AND MINIMUM MAINS VOLTAGE RANGE VARIATION THAT THE

SYSTEM CAN COMPENSATE, AS A FUNCTION OF THE MAXIMUM NONACTIVE

POWER THAT THE SHUNT UNIT CAN INJECT TO KEEP THE MAINS POWER

FACTOR BETWEEN 0.9 AND 1 (LIMITATIONS OF THE VOLTAGE DROP

IN THE LINE)

disturbances and network interruptions, while the ones that be-long to set only require general improvement of the powerquality.

Several solutions are available for compensating each load. In the following description, only two possible solutions are

considered. The first solution consists of the installation of anUPS for each end user, while the second one is the installationof a shunt unit for each load.

Instead, in order to obtain general power quality improvementfor all loads, it is possible to rebuild the LV distribution systemto increase the short circuit level in the load connection pointor to install a series unit in the MV/LV substation. Therefore,three different methods for improving the power quality havebeen considered:

• installation of a UPS for each end user. In this case, it isnot possible to improve the power quality of the distribu-tion network. However, it is possible to compensate for allvoltage disturbances for the end users;

• revamping of all of the LV distribution cables. In this case,it is not possible to compensate for all voltage disturbances;

• installation of an OPEN UPQC. In this case, it is possibleto compensate for most of the voltage disturbances.

The last solution consists of the installation of a series unitsized for 66% of the total power loads supplied (264 kVA), whileeach shunt unit has an assumed size of 5 kVA. Moreover, eachUPS is assumed to have the same power, and the input stage ofeach UPS is composed of PFC rectifiers.

It is important to clarify that the storage systems cost forUPS and OPEN UPQC solutions in this analysis is not consid-ered, because it primarily depends on the technologies and au-tonomies required. The revamping cost considered here can befound in the economical analysis carried out in [17].

The total cost for each solution is reported in Fig. 11. Theyinclude only the devices and various materials; the installationcost is assumed to be 50% of the device cost.



Fig. 11. Comparison between installation costs of a new line, of the OPENUPQC and of the UPS, as functions of the compensated sensitive loads power.

Fig. 11 shows that to compensate for most of the disturbancesin the whole network, installing the series unit only is a bettersolution than revamping all of the LV distribution system. Tocompensate for the loads , it is necessary to install a UPS ora shunt unit close to them, which increases the total cost as afunction of their power.

Referring to Fig. 11, it can be seen that the UPSs are a goodsolution if only a few sensitive loads are present. However, if itis necessary to improve the power quality of the whole network,they become too expensive to use. They can only be more con-venient than the proposed OPEN UPQC if the total power of thesensitive loads is lower than about 80 kVA (20% of the totalload) when, and only when, it is not necessary to increase thePQ of the network.

VII. CONCLUSION

The OPEN UPQC apparatus is a good compensation systemif wide installation of shunt units is needed. An increase in thepercentage of the protected load enhances the voltage stabi-lization interval over which the OPEN UPQC can significantlyimprove the power quality, especially if the load power factortakes a high value. If the power factor of load is less than one,the power factor in section increases, to avoid nonactive powerabsorption from the mains.

For low values of the parameter, the OPEN UPQC be-comes expensive if there are few shunt units. In this case, it isbetter to install other compensation device typologies (as UPS,UPQC, etc.) near the sensitive loads, and a nonactive compen-sator system near the nonsensitive loads if necessary.

It is possible to conclude that installation of the series unit is acost-effective way for distributors to improve the power qualitylevel in the distribution networks in order to achieve the stan-dards imposed by the authorities. Compensation improvementfor the sensitive end users can be achieved by installing ashunt unit near them, instead of the more expensive UPS device.The OPEN UPQC working conditions are reported in Table V.

At this moment, the OPEN UPQC study is still under investi-gation. The dynamic behavior, considering changing operatingmodes, of a 5 kW prototype shunt unit is developed and experi-mental results are presented in [18]. As shown in Fig. 11, largeinvestments are needed to analyze the completed solution, and

TABLE VOPEN UPQC UNITS ACTIONS

the � injection is possible only for few time and if necessary.

availability of electrical distribution operators for an infield testwill be required.

ACKNOWLEDGMENT

The authors would like to thank the referees for their usefulremarks, which helped to improve the paper.

REFERENCES

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[14] M. Buschmann, G. Linhofer, P. Maibach, and O. Suter, “Voltage sourceconverter based power quality solutions,” in Proc. Asia Pacific RegionalPower Quality Seminar, Putrajaya, Malaysia, Mar. 28–31, 2005.

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[17] A. Agustoni, E. Borioli, M. Brenna, G. Simioli, E. Tironi, and G.Ubezio, “LV DC distribution network with distributed energy re-sources: Analysis of possible structures,” presented at the 18th Int.Conf. Electricity Distribution CIRED 2005, Turin, Italy, Jun. 6–9,2005.

[18] R. Faranda, F. Castelli Dezza, I. Mazzucco, P. Redi, and E. Tironi,“An interface converter for DG/storage system able to improve powerquality of the load,” presented at the IEEE Power Eng. Soc., Montreal,QC, Canada, Jun. 18–22, 2006.

Morris Brenna (M’06) received the M.S. and Ph.D. degrees in electrical engi-neering from the Politecnico di Milano, Milan, Italy, in 1999 and 2003, respec-tively.

Currently, he is an Assistant Professor with the Department of Energy, Po-litecnico di Milano. His research interests include power electronics, distributedgeneration, traction systems, and electromagnetic compatibility.

Dr. Brenna is a member of the Italian Electrical Association (AEIT) andItalian Railways Engineering Association (CIFI).

Roberto Faranda (M’06) received the Ph.D. degree in electrical engineeringfrom the Politecnico di Milano, Milan, Italy, in 1998.

He is an Assistant Professor with the Department of Energy, Politecnico diMilano. His areas of research include power electronics, power system har-monics, power quality, power system analysis, and distributed generation.

Dr. Faranda is a member of the Italian Standard Authority (CEI), the ItalianElectrical Association (AEI), and the Italian National Research Council (CNR)Group of the Electrical Power System.

Enrico Tironi received the M.S. degree in electrical engineering from the Po-litecnico di Milano, Milan, Italy, in 1972.

He then joined the Dipartimento di Elettrotecnica, Politecnico di Milano,where he is a Full Professor. His areas of research include power electronics,power quality, and distributed generation.

Dr. Tironi is a member of the Italian Standard Authority (CEI), the ItalianElectrical Association (AEI), and the Italian National Research Council (CNR)group of the Electrical Power System.


A New Proposal for Power Quality and Custom

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