HIGH POWER UPS SELECTION METHODOLOGY AND INSTALLATION GUIDELINE FOR HIGH RELIABILITY POWER SUPPLY

Transcription

1 HIGH POWER UPS SELECTION METHODOLOGY AND INSTALLATION GUIDELINE FOR HIGH RELIABILITY POWER SUPPLY A. Sudrià 1, E. Jaureguialz 2, A. Sumper 1, R. Villafáfila 1 and J. Rull 1 1 Centre fr Technlgical Innvatin in Static Cnverters and Drives CITCEA, Universitat Plitècnica de Catalunya Av. Diagnal, 647. Pavelló A; Barcelna (Spain) phne: , fax: , sudria@citcea.upc.es; 2 SGS C. Rera Palau 11-2; Barcelna (Spain) phne: enrique.jaureguialz@retevisin.es Abstract. This paper presents the mst cmmn UPS technlgies fr high pwer available n the market and cmpares their functinal parameters. Als it presents the mst imprtant UPS parameters that must be cnsidered t design the high reliability facility. Different cnfiguratins and the influence f the parameters n the UPS peratin are discussed. All steps f this selectin prcess were put int practice in rder t select UPS systems fr installatins f RETEVSION, ne f the mst imprtant telecmmunicatin cmpanies in Spain. The peratin behavir f the installed UPS was cnfirmed by n site measurements. Key wrds UPS, Pwer Quality, Benchmarking, UPS technlgies; Reliability 1. Intrductin Ideal pwer supply t a custmer at lw vltage is at 230/400V at 50 Hz. The lcal netwrk peratr cannt keep supply exactly at the ideal due t a range f disturbances utside its cntrl and attempts t maintain its vltage within specified ranges. In the case f sme electrical lads even a shrt break in pwer supply may lead t enrmus material lsses r even cause hazard t peple s lives. Examples include, prtectin and autmatic circuits in pwer industry, cntrl systems f autmatic prductin lines in ther industrial sectrs (e.g. chemical, precise machining), sme equipment in hspitals, airprts, als in military sectr. First f all, the users with these critical applicatins have t assume investments t guarantee the required level f pwer quality. In rder t achieve high reliability in pwer supply, the mst cmmn slutin is uninterruptible pwer supplies (UPS). [1] The electrnic lads are highly vulnerable t vltage disturbances and therefre, a high reliability level is requested. That s why UPS are used in rder t supply reliable pwer t thse lads. Actually, UPS are indispensable in industrial prducing prcesses, telecmmunicatins, data-centers, financial business etc, where high pwer supply reliability is demanded. Mrever, they are easy t install, due t their cmpact design (apart frm the batteries) and the utput pwer can reach frm sme kw t WM. But nt all UPS technlgies are similar. Althugh the purpse f this equipment is the same, each UPS manufacturer develps its wn technlgy. 2. UPS Technlgies UPS available n the market can be classified int three grups, cnsidering their structure and the peratin mdus. Tw f them, the duble cnversin UPS (fig.1) and the stand-by UPS (fig. 2) are well knwn. The third technlgy, the delta cnversin (fig. 3) is an evlutin f the interactive UPS structure All these technlgies have a static switch by-pass (tyristrs) in rder t pass the lad t an alternative pwer supply in the case f a malfunctin f the UPS [2]. It is als well knwn that mst UPS manufacturers are emplying duble cnversin technlgy, which means RE&PQJ, Vl. 1, N.3, March 2005

2 input pwer is rectified t dc vltage and supplied t a capacitr. T this cd bus, an inverter is cnnected and generates the utput ac vltage. This UPS structure is the mst reliable structure and is less vulnerable t disturbances cming frm the pwer supply. The structure f rectifier - dc-bus and inverter separates the input ac pwer frm the utput pwer. Batteries are cnnected t the dc-bus in rder t keep n supplying the utput pwer during a mains failure. The autnmy f this type f UPS is dependent n the number f batteries and can be adapted t the custmer s need. Hermetic lead batteries are the mst used type fr UPS systems. Nwadays, the inverts f all manufacturer f high pwer UPS are designed with IGBT s, using PWM r SVPWM cntrl algrithms at a cmmuting frequency between 4 khz and 20 khz. The differences between UPS manufactures can be fund in the type f rectifier, input and utput filter and the utput transfrmer. A new architecture is adding an IGBT bst after the rectifier, s the utput transfrmer can be remved. If we cmpare the structure f the duble cnversin in Figure 1 with the stand by cnfiguratin in Figure 2 we can bserve that the nly difference is that the latter ne is wrking in by-pass during the nrmal peratin. Mst manufactures f duble cnversin UPS are ffering an peratin mde f their equipment, that permits them t perate the duble cnversin UPS as an ff-line UPS. In ff-line UPS the rectifier unit is designed t rectify nly the charging current; the utput inverter is similar t the duble cnversin. One f the main advantages is the high efficiency f ffline UPS and the lw harmnic distrtin that the UPS is injecting t the installatin. The disadvantage f this type f UPS is that the cmmutatin time can between the nrmal peratin mde and the battery mde can be high. Fig 2 Fig. 1 The cnventinal input rectifier is a six r twelve pulse tyristr rectifier bridge, cmbined with a passive filter in rder t lwer the harmnic distrtin at the UPS input. Six pulse tyristr rectifier and active filter cmbinatin and six pulse dide rectifiers with an IGBT bst step are ther pssible input rectifier structures. Active rectifiers are als applied because f the advantage f lw harmnic distrtin and input pwer factr near ne. The input harmnic distrtin f an UPS is ne f the mst imprtant input parameter because it is decisive fr the electrical design f the input pwer circuit. Therefre, lw harmnic distrtins are requested by custmers t UPS systems in rder t avid an ver design f the electric installatin. Thus UPS manufactures are trying t reduce harmnic distrtin f the input currently by using different filter strategies. The ther imprtant factr fr the installatin design is the input pwer factr f the UPS. Installatins have t be designed t the ttal apparent pwer cnsumed by the UPS. If active rectifiers r active filters are applied, the input pwer has a pwer factr near the unity. Sme manufacturers incrprate special cntrl algrithm in rder better the input pwer factr. On the ther hand, n the utput we find the utput filter. The purpse f this filter is t smth the utput vltage frm the inverter and that s why lw harmnic distrtin in utput vltage can be achieved. Cnsequently, high capacity is cnnected t the utput and this capacity is als used in rder cmpensate inductive reactive pwer frm the lad. Thus manufactures specify the utput pwer f the UPS in apparent pwer with 0.8 inductive reactive pwer factr. Cnventinal three phase pwer UPS structures need a transfrmer in rder t reach the utput vltage f 400 V. The delta cnversin structure is a cmbinatin f the last tw structures. The main inverter is watching cntinually that the utput parameters are within defined tlerances and it ges int actin when a mains failure is ccurring. Meanwhile, this inverter charges the batteries and crrects the input pwer factr. The delta transfrmer is used t cmpensate small vltage variatin between +15% and -15%. Bth inverters are bi-directinal cnverters. Fig.3 The main inverter is designed fr the full utput pwer, while the delta inverter and the delta transfrmer are designed fr apprximately 20% f the utput pwer. Thus the efficiency f this UPS structure is in general higher than a duble cnversin UPS. The utput pwer factr is near the unity because f the main inverter, which is perating during the nrmal peratin as an active filter RE&PQJ, Vl. 1, N.3, March 2005

3 3. EN Standard Cmparing characteristics f high pwer UPS is nt an easy task because there is a set f different structures n the market and the cmmercial names f these technlgies can differ. In rder t prvide the user a clear and easy cmparable criteria fr the selectin f UPS, the CENELEC standard EN (June, 2001) was created. [3] This standard is establishing a classificatin fr UPS withut using the cmmercial terms like duble cnversin, ff-line, n line, etc. The fllwing classificatin cde was established: CLASSIFICATION 2 CLASSIFICATION 3 VFI SS 111 Dependency f UPS utput n the input pwer grid VFI: where the UPS utput is independent f input supply vltage and frequency variatins. VI: where the UPS utput is dependent f input supply frequency variatins, but supply vltage variatins are cnditined. VFD: where the UPS utput is dependent f input supply vltage and frequency variatins The vltage wavefrm f the UPS utput Tw characters crrelate the utput vltage wavefrm int a simple structure, accrding t the tw peratinal mdes, the first is nrmal mde, and the secnd is stred energy mde. S: sinusidal: ttal harmnics factr D<0.08 (IEC ) under all linear and under reference nnlinear lad X: nn-sinusidal: D>0.08 under reference nnlinear lad. Y: nn-sinusidal exceed the limits f IEC The dynamic tlerance curves f the UPS utput Three characters describe maximum allwable dynamic deviatins (frm a clean sine wave). First is fr change f perating mde perfrmance, e.g. nrmal mde stred energy mde bypass mde. Secnd is fr step linear lad perfrmance in nrmal r stred energy state (wrst case). Secnd is fr step nn-linear lad perfrmance in nrmal r stred energy state (wrst case). Three tlerance curves describe the utput vltage limits: CLASSIFICATION 1 4. Characteristics t cnsider After intrducing the state f art f static UPS, present standards and technlgies, it is imprtant that custmers can make an bjective evaluatin f the equipment which is mre interesting t ensure the suitable pwer quality fr the facility. [4] The analysis f the installatin must be the beginning. This must get all the expected parameters that UPS must carry ut, s, UPS specificatins are set. The list f parameters can be very large. If the different equipment is cmpared, it s nt pssible t take them all int accunt. Then, the characteristics used t evaluate the UPS shuld nly be the mst imprtant nes. These are listed next. The UPS descriptin is made with static and dynamic characteristics. The first reprts nrmal perfrmance mde withut any quick variatin. And the latter describes perfrmance with transients and change between mdes. Static characteristics The whle grup f static characteristics can be divided int three parts. The first set cnsists f thse whse value is practically independent frm the lad level. Standard EN classificatin Shrt Input filter Acustic nise RE&PQJ, Vl. 1, N.3, March 2005

4 Taken area fr pwer Accessible Output transfrmer Feed-back prtectin Overlad capacity Custm f batteries Figure 4 can give a first idea f what kind f equipment is better t use t prtect the lads. Fig. 4. UPS slutin accrd t EN Published by ZVEI: UPS Guide The secnd grup includes thse parameters whse value depends n lad level. This is because the equipment is designed fr 100% lad. Figure 5 gives an example f this variatin. S when UPS wrks away frm that pint, the values are lwer than expected. This situatin is imprtant if there is a parallel redundant cnfiguratin because UPS wrks under 50%. Input pwer factr Input THD i Efficiency Output THD u 5. Installatin recmmendatin Once UPS is selected crrectly, it is imprtant t pay attentin t the UPS installatin. This helps t avid prblems with UPS perfrmance. The mre imprtant pints t be cnsidered are the fllwing [1]: UPS installatin design Redundant cnfiguratin n+1 Dwnstream earthing system: TN-S if transfrmer is in wnership and TT if it is nt Neutral sectin equal r duble t phases Different prtectin t UPS grid cnnectin and by-pass cnnectin Over vltage prtectins dwnstream and upstream UPS Discnnecting switch at UPS utput t easy replacement Air cnditining system 6. Installatin case f an UPS: Neutral cmmutatin in UPS and generatin set installatins A lt f the installatins which use UPS systems, are supprted by an alternative supply thrugh a generatin set. In these cases, the cmmutatin between the grid cnnectin and the generatr is necessary fr the case f lng interruptins. Dwnstream f this cmmutatin unit usually the UPS set is installed and supplies the critical lad. In this sectin neutral schemes were analyzed in cmmutatin installatins with tw r mre different pwer supplies, supplying a UPS system with critical lad dwnstream. The tests explained in this sectin were carried ut in a telecmmunicatins centre. An electrical scheme is shwn in Figure 7. Fig. 5 Typical UPS efficiency curve The last grup is nt exactly static characteristics. They present an idea f manufacture infrastructure. Technical assistant Delivery term Fig. 7. Unifilar schema In the tested cases, the selected neutral system was the TN-S fr the whle installatin. The cmmutatin between grid and generatr set is dne by fur-ple breaker and therefre, the neutral wire is als sectined RE&PQJ, Vl. 1, N.3, March 2005

5 During the cmmutatin time between the surces, when ne breaker pens and the ther clses, the neutral wire scheme dwnstream f the cmmutatin is IT, because the neutral wire remains islated frm earth. These neutral wire cmmutatins prduce shrt duratin ver vltages and high vltage pulses with high frequency that can damage the lad. In the tested case, each cmmutatin caused a failure in the cntrl lgic in sme f the telecmmunicatin equipments cnnected t the UPS. Furthermre, the surges underwent dielectric stress and perturbatins in each cmmutatin that may affect the differential prtectins f the critical lads. Fig. 8 shws the neutral vs. earth vltage dwnstream f the UPS during the cmmutatin, in this specific case it was f three secnds. Fig. 9 and 10 shw the line vltage drp f the netwrk and the starting f the generatr set. During the tests imprtant phase-earth ver vltages appeared. The phase and neutral currents, and the line and line t neutral vltages (dwnstream f the UPS) d nt shw imprtant variatins NEUTRAL TO GROUND VOLTAGE DURING GE INSERTION (WITHOUT NEUTRAL COMMUTATION) 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 Fig. 11. Neutral t grund vltage during GE insertin withut neutral cmmutatin The tests were repeated with the neutral wire cnnected t earth (dwnstream f the cmmutatin), that means linking the cmmutatin neutral wires. A substantial reductin f the ver vltage peak was achieved. Figs. 11 and 12 shw the results f thse tests. 20 NEUTRAL TO GROUND VOLTAGE DURING MAIN LOSE (WITHOUT NEUTRAL COMMUTATION) Fig. 8 Neutral t grund vltage during cmmutatin NEUTRAL TO GROUND VOLTAGE DURING MAIN LOSE (NEUTRAL COMMUTATION) Fig. 9. Mains failure detail Fig. 12. Neutral t grund vltage during main lss withut neutral cmmutatin T slve the prblem f ver vltage there are the fllwing pssibilities: A. T change the neutral wire scheme by means f an islatr transfrmer. If the UPS has the transfrmer in the utput and it is pssible t be fed withut the neutral wire, adding a transfrmer in the by pass (static and dynamic) will prvide a new neutral scheme dwnstream f the UPS, independent f the upstream scheme. This system is technically ptimal, but expensive NEUTRAL TO GROUND VOLTAGE DURING GE INSERTION (NEUTRAL COMMUTATION) If the UPS needs a neutral wire in its cnnectin at the netwrk, the neutral wire in the UPS is nt cnnected internally. If there is n islatin transfrmer in the inverter utput, the transfrmer must be installed in the input r utput f the UPS, including the manual and static by pass. (See fig. 13) ,799 24,899 24,999 25,099 25,199 25,299 25,399 25,499 25,599 This scheme, althugh expensive, decreases the system s lsses by 3 % and 8 %, depending n the quality f the transfrmer and the amunt f the lad. Fig. 10. GE insertin detail RE&PQJ, Vl. 1, N.3, March 2005

6 Fig. 14. Three phase cmmutatin C. Changing the neutral system. Fig. 13. Islate transfrmer installatin If the transfrmer is installed upstream, the system lsses are higher fr the whle system, but the UPS better prtected. The advantage f a dwnstream installatin f the USP is that the lsses f the UPS nt included in the transfrmer pwer. The disadvantage is that the UPS is nt prtected in case f disturbances in the main supply. B. Three-phase sectining in the cmmutatin. If the cmmutatin is in three phases, the UPS and the lad will nt be discnnected frm neutral, and ver vltages will be remarkably decreased as was verified during the tests. Hwever, sme difficulties were fund: a) Impedes the differential prtectin upstream f the cmmutatin, because the unbalanced neutral currents will returns by the transfrmer branch as well as by the generatr set. This is because bth wye pints are linked t earth, in the case f TNS- scheme. TN-C system t the UPS and keeping (dwnstream) a scheme TN-S is an easy realizable slutin. It can be dne by means f the cnnectin f the neutral wire in the UPS t earth. Disadvantages f this slutin are the impssibility f the use f differential prtectins upstream frm the UPS and, basically, the neutral currents will return t the transfrmer (r t the generatr set) acrss all the earthing pints f the installatin causing interferences and ptentials between thse pints. UPS with transfrmer utput and three phase cnnectin a pssible slutin is the cnnectin t earth f the neutral pint f the UPS by means f a switch in such a way s that when the by pass is cnnected, the switch must pen. The result is t have an independent neutral scheme while the UPS wrks n line (mst f the time) and being unified with the thers when the systems wrks in by pass r static. This requires mre cmplex maneuver prcedures and changes between neutral schemes during the maneuver (see fig. 15). b) In the TN-S scheme, bth neutral pints f the transfrmer and generatr set are linked t earth, nrmally, in the transfrmer statin. In this statin is cnnected the grund prtectin cnductr (see fig. 14) During the break an induced vltage in the lad appears between neutral and earth because f the phase current variatin and the cupling with the neutral wire and earth lp. These vltages may be lw due t the means f ver vltages prtectin cnnected between the neutral pint and earth. Fig Changing the neutral system by switches RE&PQJ, Vl. 1, N.3, March 2005

7 7. Cnclusin Selectin and installatin f UPS fr highly reliable installatin require a gd knwledge f UPS structures and technlgies. Thus manufactures are applying their wn technical terms and technical parameters, which are nt easy t cmpare, an evaluatin f the best suitable technlgy fr the applicatin is a cmplex task. Therefre, the standard EN shuld be applied in rder t cmpare the characteristics f UPS frm different manufactures. On the ther hand, the installatin f an UPS cmbined with a generatr set implies that ver vltages can ccur. In rder t avid these ver vltages, three slutins were presented: changing the neutral wire scheme by means f an islatr transfrmer, changing the neutral system and using three-phase sectining in the cmmutatin. References [1] IEEE Orange Bk: IEEE Recmmended Practice fr Emergency and Standby Pwer Systems fr Industrial and Cmmercial Applicatins; IEEE Clr Bk Series, 1987 [2] A. Sudrià, J. Fnt, A. Fernández: Sistemas de alimentación interrumpida de ptencia; Autmática e Instrumentación, juli 1989, nº193, pp [3] CENELEC, Eurpean Cmmittee fr Electrtechnical Standardizatin: EN Methd f specifying the perfrmance and test requirements; June 2001 [4] CEMEP, Eurpean Cmmittee f Manufacturers and Pwer Electrnics: UPS Eurpean Guide; Paris, December RE&PQJ, Vl. 1, N.3, March 2005