A New Configuration for Uninterruptible Distribution Systems

8/10/2019 A New Configuration for Uninterruptible Distribution Systems

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New Configuration for U ninterruptible Distribution Systems

F. Muzi member,E E EDepartment of Electrical Engineering

University ofL Aquila67040 Poggio di Roio, LAquila, Italy

Abstract: In this paper a new architecture for unintermptibledistribution systems is presented. The single ac-no-break systemisoperated at medium voltage and usesa double-looped configurationmade of two redundant underground cables. The system is three-phase wth neutral ungrounded. An example of applicationconcerninga satellite telecommunication center is reported.ATPsimulation resultsof some significant transients are also shown.Inthe numerical simulations carried out, original models of differentunconventional componentshave beenused.

Keywords: Urintermptible power supply, no-break electric systems,powerquality,distribution systems.

I. INTRODUCTION

Many powerful strategic centers, such as satellitetelecommunication centers, usually require high continuitylevel of supply. Presently, the problem is solved usinga largenumber of spreadUPS apparatuses. Nevertheless, when thepower of the center is relevant, this solution, besides beingexpensive, does not enable to reach the high reliability levelsusually required. To overcome this problem, the use ofa singleno-break-MV sy stemis proposed. In sucha system, the mainproblem to solve is the elimination of voltage dipsat the MVlevel occurring when faultsare present in the supplied LVsystems. Actually,a voltage sag in the V system, due toafaulted LV component,c n cause disturbances also in LVsystems unaEected by fault. The problem has been solvedusing unconventional devices such s a new-generationrotating UPS able to sustain the voltage thanks to its kineticenergy.

0-7803-4403-0/98/$10.00 998IEEE

To considerably improve the availability of thesystem, all subsystemsare duplicated (100 redundant). Inaddition, to make the chanceof fault occurrence directly in the

V system negligible, the use of specialMV undergroundcables is proposed.

11. THE NO-BFU3AKSYSTEM

The uninterruptible distribution system uses twothreeg hase-M V loops, each one supplied bya rotating U P Sequipped with a Diesel motor. The specialUPS is able toeliminate lsturbances on the voltage and to provide powerwhen normal supply is not workmg. Italso enables toeliminate batteries, considered to bebulky expensive, andabove all costly to be kept in operation. Loads are suppliedthrough load centers connected to thetwo M V loops by meansof two separate connectionsand two redundant transformers.In this way the loads are supplied bytwo independent sourceswith 100 redundancy. In the case of satellitetelecommunications centers, the unintermptible loadsthemselves (constituted by apparatuses for special antemas)are 100 redundant.

Th e layout of the systeim is characterizedby a doublevoltage tra nsfo mti on: fromLV to V in correspondence ofthe rotatingUPS and from V to LV a t the load centers. Theproposed archtecture enables to reach high reliabilityperfonnances and high flexibility capabilitiess loads orsources can be indifferently connected tothe no-break system.Often the load nodesare distinct from the generation nodes;nevertlheless it i s possible to havelocal loads directly suppliedby the rotating UPS as shown in Fig. 1. In the case of Vexternal supply, it is necessaryit0 transform the voltage toLVlevel.

The rotatingUPS, which ensure an absolute continuitylevel of supply, are constantly maintained in rotation. Thechosen rotating UPS consist ofa common mechanical partshared by the synchronous motor and the synchronousgenerator. The stator is unique but the windings of the motorare selparated from those of the generator.Also the rotor isunique, DC-excited s a conventional generator. The energytransfer occurs almost com pletely under electromagnetic form;

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the only conversion from electric to mechanical energy is thatnecessary for damping and ventilation. The rotating UPS canbe equipped witha primary Diesel motor, whichc n in thiscase be considered s an independent and permanent powersource.

IIII

..

IROTATING

UPS

II

ROTATING-ri,I

Fig. 1 Single-line schemeof a generation nodesupplying also local loads.

From an electrical point of view, the machine can beconsidered asa special transformer since:

magneticflux can be regulated and thusthe secondary voltagecan be controlled;

- only the fundamental harmonic is present in the voltageoutput;

a reserve of active energy is guaranteed(also without a Dieselmotor, due to mechanical inertia);

a sou rce of reactive power is alw ays present.

The control system of the apparatus is completelydigitally operated and uses microprocessors to perform thedifferent capabilities, i.e. voltage regulation or powerregulation. Remote PC ca n controlthe whole system.

The Diesel motor version enables to eliminateexternal batteries. Inthis case the primary motor mustbe keptready to start in-ord er to supply the total load ina very fewseconds. During transients energy is supplied by the inertia offlywheels. Inthis situation, the electrical machine behaves as

an asynchronous generator, which operates an ACDCconverter supplying the static inverter, which transmits powerat rated frequency to the AC d istribution system. Th e completerotating-UPS layout in the version equipped by both externalbatteries and Diesel motor is shown inFig. 2. In this case, theDC converter supplies the batteries an d the static inverter; forhigh rated power, the rectifier is at 12 impulses to reduce the

amount of harmonics. The synchronous machine drives thethyristorsof the inverters; inthis way neither power capacitorsnor driven circuits for the tyristors are required. This, addedto the auto-ventilation and the reduced number of componen ts,greatly improves the system reliability level. Furthermore, t\erotating U P S protects the power static circuits against anydisturbances coming form the load (short circuits, reactivecurrents, voltage unbalances, harmonics). In Fig.2, twoversions of the system are shown: the UB-R version and theUB-RI3 version. In the former, the external distributionnetwork supplies the electric motor, whereas in the latter itsupplies the generator. This latter (usually not recommended)solution has a slightly bigger efficiency but is incapable tofilter the harmonics towards the load. Both versions use astatic switch anda decoupling inductance.

Automatic Bypass

Extemal balteq

Fig.2. Complete layoutof the static-rotatingunintermptible power generator.

Load+-

motor

111. ELECTROMAGNETICTRANSIENTS ANALYSIS

Owing to the innovative structure of the no-breakdistribution system, a preliminary simulation study wascarried out. The analysis refers to a satellitetelecommunication center[3]. The simulations were carriedout by means of the ATP with the aim to verify the effectscaused on the system by transients, due to faults o r importantchanges in the system layout.

The presence in the system of both unconventionaldevices and non-linear loads causeda number of problems indeveloping accurate simulation models. Asa matter of fact, forsome apparatuses such s rotatingU P S pecial limiter-fusesornon-linear loads, original models not reported in literaturewere developed131.

The modelused for simulatinga rotating UPS uses twoEMTP basic elements: the synchronousTYPE 59 machine andthe three-phase transformer. This model cannot be used whenthe U P S is working as an asynchronous machine. The ratedquantities of the simulated rotatingU P S are reported inTABLE 1, while the internal parameters are reported inTABLE 2.

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TABLE 1RATED UANTITIESOF ROTATING PS

R ,0.004

TIdO

A, RVA] I, [A] Current distortionFactor

X , X, X , X', X', XI', XI ,

0.042 0.875 0.39 0.141 0.35 0.063 0.069

TIqO T''dO X O Rn Xn

400/231I

1100I

1588I

2

VI, [V400

TABLE 2I NTERN L PARAMETERSF ROTATING PS

(The list of symbols is givenin the appendix)

K=V,,/V,, An[kVA] Z c c x

1 1100 18 1MQ

10.653 I 0.51 I 0.041 10.031 10.045 10.004 10.006 I

The simplified-ATP model used to simulate the rotatingUPS is shown inFig. 3.

EXTERNALSUPPLY

TRANSFORMERK=l / l I

MECHANICAL.TORQUE

Fig. 3. Simplifiedmodel of the rotatingU P S sed in ATP simulations.

The values of the main quantities of the virtualtransformer inFig. 3 are reported in TABLE 3.

The high number of system simulations carried outenableld to evaluate the lst urb an ce s and the consequences onthe system, due to all possible electromagnetic transientscaused by faults, component energizations or switchingoperabions.

The faults are supposedto occur in both LV and MVsystems; the main faults examined were the ground-single-phase fault, the double-phase fault, andthe three-phas e fault.

The switching operations examined were theenergizations of both the transformers and the non-linearloads; some changes in the system layout were alsoinvestigated.

An important test was carried out to evaluate voltagedips, which occur in theM V no-break system whena fault ispresent in one LV system. The aim was to evaluate thedisturbances caused inLV systems n ot afected by fau lt beforefault e:limination.By way of example,Figs. 4 and 5 showsimulation results whena phase-to-neutral fault occurs in anLV system. In particular,Fig. 4 shows the currents calculated

in the LV sy stem affected by fault, w hile Fig.5 shows voltagesand currents in a n L V system unaffected by fault.

t [ E lFig.4 Plot of the currents calculated at the fault point; faulted

phase currentis higher than thoseof other phases.

As shown in Fig.5 during fault occurrence in an LVsystem, disturbances do not s i m c a n t l y affect the otheir LVsystems, due to the stabilizing effect of the rotatingU P S .Furthermore, Figs. 4 and 5 slhow that also in steady stateconditions the shapes of currents are non-sinusoidal. T h s isdue to the distorting effect of the non-linear load, whch was

already investigated ina previous study[3]. Fig. 4 shows that,during fault occurrence, this distortion remains in the currentsof non-faulted phases whereas it does not appear in the ,shortcircuit current of the faulted phase. This happens becauseduring fault occurrence the faulted phase supplies a linearimpedance (quasi-nil in value), whereas non-faulted phasessupply non-linear loads.

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3

150

0

-150

3 0 0

100 120 140 160 180 200 220 240t Imsl

i [AI

600

400

200

-200

-400

-600-...I-I

100 120 140 160 180 200 220 240t [ m l

Fig. 5 Plot of voltages and currents in a three-phase-LV systemunaffected by fault.The disturbance is negligible since the

fault is in another LV system.

Another simulation performed was the energizationof an MViLV transformer with the aim to venfy both thedisturbances caused in the V system and the possiblemalfunctioning of the adopted protection devices.In thesimulation a value of 1.5 T was assumed for magneticfluxdensity. With reference toa given phase, the w orst instant waschosen to close the breaker. The computed currents andvoltagesin the V side are shownin Fig. 6 and Fig. 7.

The simulation results show that during theenergization of $e chosen transformer, currents are lowerthan the rated ones and voltages at V level are notparticula rly affected bythis phenomenon.

In general, sim ulation results set th e correct behaviorof the system both in steady state and transient conditions.Nevertheless, to reduce the disturbance in the V systemduring transformer energizations, some precautions must beadopted. Particularly careful should be the choice of theM V LV transformers with respect to their inrush current,whlch must be sufficiently low. On the other hand, toattenuate voltage dips at V level - caused by faults occurring

in an LV system the internal impedance ofM V LVtransformers should be sufficiently high(>6 ).

i [AIc

8

6

4

2

0

2

-4

6

8

Fig. 6. Currents evaluated in the MV sideduring a transformer energization.

In the caseof satellite telecommunication centers, too,the loads can cause troubles during energization, due to theirnon-linear and high capacitive behavior.In these cases, theentire load should be subdivided into small, graduallyenergized loads to eliminate undesiredtrips [3].

20 4 0 60 80 100 120 140 160 180t Cmsl

Fig. 7 . Single-phase voltages calculated in the M Y sideduring a transformer energization.

The multiple-phase faults, which may occur in anM Y loop, usually lead to unacceptable situations.Todrastically reduce these occurrences, single-phase cables with

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special mechanical protections are used; these cables are thenplaced in separate concrete undergrou nd tubes.

In addition, in the very rare occ urrence of a fault inan M V cable, a particular comp uter relaying system basedonISM (Intelligent Switching Module) apparatusesis used inorder to quickly eliminate the faulted line branch. An ISMapparatus is a complete integrated module which is able tooperate control actions, breaker operations and protectionfunctions simultaneously and, at the same time, tocommunicate directly with the supervision control center ofthe power distribution system. The useof ISM apparatusesenables to eliminate the faulted branch of the V systemquickly and to drastically reduce the disturbance in theVloop.

1v. FUTURE

Recent studies demo nstrated the possibility to repla cethe rotatingUPS wt a digitally controlled static power unit,able to eliminate voltage dipsin real time [ l ] 2 ] [4]. Thesame unit equipped with external batteriesc n be used forremoving sudden supply interruptions. Of cou rse, to elim inatelong supply interruptions the use of traditional synchronousgeneratorsis required.

V. CONCLUSIONS

The new architecture proposed for no-breakdistribution systemsis particularly suitable to be adopted inimportant and powerful centers which require at the same timevery high levels of continuity, reliability, and flexibility. Theno-break system enables to eliminate spread staticU P S andbatteries thanks to the use of rotatingUPS of the newgeneration. ATP simulations demonstrate the correctperformances of the no-break system both in steady state andtransient conditions.

The possible use of special digitally controlledtransform ers isnow under investigation.

VI APPENDIX

List of symbols:

R a = Arma ture resistance,in per unit;

- X := Direct-axis (d-axis) subtransient reactance;- X := Quad rature-axis (q-axis) subtrans ient reactance;

- TIdO = Direct-axis open-circuit ransient time constant;

- TIgO= Quadrature-axis open-circuit transient time constant;-

- T qo = Quad rature-axis open-circu it subtrans ient time

- X, := Zero sequence reactance, in per unit;- R := The real part of th e neutral grou nding impedance;

- X , = The imaginary p a of the neutral grounding

- Z c 0 = Short-circuit mped ance of the transform erin %;- X, = Magn etization reactance of the transforme r.

= Direct-axis open-circuit subtransient time cons tant;

constant;

impedance;

[ l ] F. Muzi,R. Paggi, G.M. Veca, Trans form er regulated bymeans of flux shunt . Proceedings of the AMSEConference It Modeling & Simulation Sorrento (Italy)Sept.29 - Oct. 1 1986, Vol. 2.3 p. 17-27.

[2] A. DAngelo,F. Muzi, R. Paggi, A new finite-elementtechnique for simulating the voltage controlof atransformerMSC 1996 European Users ConferenceMun ich (Germany), September17-18, 1996.

[3] F. Muzi, AT P load model of an antenna for satellitetelecommunications, Proceedings of the ATP l34TP.Meeting 97 - Barcelon a, Spain, November9-1 1 1397.

141 F. Muzi, F. Panone, Optimal arrangement of spreadautomated centers for electrical distribution systems1998 ICHQP- IEEE International Conference- October14-16, 1998, Athens, Greece.

VIII. BIOGRAPHY

- X , = Arma ture leakage reactance, in per u nit;

- X = Direct-axis (d-axis) sy nchron ous reactance;

- X = Quad rature-axis (q-axis) synchron ous reactance;

- X, = Direct-axis (d-axis) transient reactan ce;- XIg = Quad rature-axis (q-axis) trans ient reactance;

Francesco Muzi M90) was born in LAquila Italy), on May 30,1955. In 1981 he graduated in Electrical engineering kom theUniversity of LAquila hons). In 1984 he was appointed Researcher inPower Systems at the University of LAquila and in 1991 AssistantProfessor of Electrical Distribution Systems. His main researchinterests are in the field of Power Systems Analysis, Power SystemsReliability, Electromagnetic Analysis using the Finite Element Methodand Power Quality indistribution systems.

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A New Configuration for Uninterruptible Distribution Systems