160-Harmonics Upstream of Rectifiers in UPS

n 160harmonicsupstream ofrectifiers in UPS

E/CT 160 first issued december 1993

Jean Nol Fiorina

Joined Merlin Gerin in 1968 as alaboratory technician in the ACS(Alimentations et ConvertisseursStatiques) department where heparticipated in the performancesetting up procedures for staticconverters. In 1977 he obtained hisENSERG engineering degreefollowing a 3 year evening courseand rejoined the ACS department.Starting as development engineer hewas soon afterwards entrusted withprojets. He became later responsiblefor design projets in EPS department(Electricity Power Supplies). He is insome ways the originator of mediumand high power inverters.At present he is with the SuppliesDivision where, as responsible forinnovations he works on thepreparation on new UPS designs oftomorrow.

Cahier Technique Merlin Gerin n 160 / p.2

glossary

cos j1 factor of phase shift =P1S1

D % global distortion rate = 100Yn2

n = 2

Y1

Hn % individual rate of harmonics = 100 YnY1

l power factor = PS

P1 active power of fundamental componentS1 apparent power of fundamental component

n distortion factor = cos 1

Y1 effective value of fundamental (current or voltage)Yn effective value of harmonic of order n; for current: In (IniN according to standard spec. IEC 146-4)Zn impedance value for harmonic n (Un = Zn In)


harmonics upstream of rectifiers in UPS

summary

1. Harmonics in supply networks Consequences due to harmoniccurrents p. 4Need for standardization p. 4

2. Thyristor Graetz bridge rectifier Harmonic currents generatedby a Graetz bridge rectifier p. 6Influence of source impedance p. 7Current distortion rate p. 10

3. Minimisation of harmonic Insertion of inductance at rectifierinput p. 11Use of double bridge rectifier p. 12Rectifier circuit with more thantwo bridges p. 15Utilization of a passive filter forharmonics p. 16

4. Conclusions and prospects p. 185. Bibliography p. 20

UPS, like most static converters drawenergy from an A.C. mains networkthrough rectifiers.Often, these rectifiers fitted withthyristors are generators of harmonics.Merlin Gerin, manufacturers of UPSequipment are well acquainted with thisproblem and consequently havedecided to share their knowledge in thisCahier Technique.In this treatise, the author highlightsfirst the need for a standardized co-existence between polluting andpolluted equipments.He then recalls, which harmoniccurrents and voltages are produced byconventional (classic) rectifiers(thyristor Graetz bridge rectifier) andproceeds to offer various solutionsdesigned to minimize harmonics.Finally, in his conclusion, he alludes tothe appearance in the near future ofnon-polluting UPS equipment and ofde-polluting converters.Note: harmonic problems occuringdownstream of rectifiers which supplynon-linear loads are fully discussed inCahier Technique No 159. The latterprovides also definitions andmathematical formulae relating toharmonics.In the present booklet, only theprincipal definitions and formulae arelisted in page 2.

disturbances


1. harmonics in supply networks

consequences due toharmonic currentsHarmonic currents generated by certainequipment, such as static converters,discharge lamps, arc furnaces etc..(providing there are many or providingthey have of higher power ratingcompared with the power of the source)can adversely affect the operation ofother equipment connected to the samenetwork.The effects of these harmonic currentsare discussed in Cahier TechniqueNo 152 Harmonic disturbances inindustrial supply networks and theirtreatment.Lets recall the adverse effects ofharmonic currents:n they cause additional heatingespecially in line conductors,transformers and condensers,n they induce vibrations and noises inelectromagnetic equipment,n they can cause interference withcommunication and low currentprotection/signalling circuits.A distorted voltage can, in addition,upset the operation of some receiverssuch as regulators, static converters(when the crossing through zero of thevoltage waveform becomesindeterminate).Thus, one of the factors highlighting thequality standard of electricity supply isits voltage distortion rate.

need for standardizationAs electricity is today regarded as aproduct (in particular in Europefollowing the directive of 25 July 1985under reference 85/374/CEE), theproducer becomes fully liable fordamages caused by excess ofharmonics.That is why electricity distributors inorder to be able to guarantee a qualitylevel acceptable to all consumers, doset or are compelled to set limits to

disturbances produced by someconsumers.To achieve this, it is necessary todefine:n first, a maximum distortion rateallowing correct functioning of mostinstallations (level of compatibility),n second, a maximum disturbance rate,for each user so that the cumulativeeffects of various disturbances thusgenerated do allow an operationalcompatibility between all theinstallations (connected to the samenetwork); all must operate correctly.Thus if this compatibility is needed toexist between subscribers, it is alsoneeded to exist within the installationunits of individual subscribers.The end user is therefore burdenedwith a level of disturbances induced byequipment units that he has installedhimself. That is why it is important thatmanufacturers clearly state thedisturbance levels produced by theirequipment.Standards are therefore needed to setacceptable levels of harmonicdisturbances for the supply networks aswell as for polluters.Level of operational compatibilityLevels of compatibility for Low Voltage(LV) public supply networks are definedin Standard Specification IEC 1000-2-2of May 1990. As the levels retained inthis standard specification are the sameas those published in CIGREEperiodicals (Electra No 77 of July 1991and No 123 of March 1989), it isprobable that levels specified forMedium Voltages (MV) and for HighVoltages (HV) will also correspond tothese recommendations (see table infig. 1).Emission LevelsLimits should be defined for eachsubscriber so as to avoid the necessityto carry out systematic controlledchecks when the equipment is put intoservice.

It should be noted that for a same levelof current disturbance, the voltagedistortion ratio, at the point ofconnection, is dependent on thenetwork impedance at that point.A solution that is fair, is to authorisedisturbance causing power sourcesthat are proportional to the powercontracted for by each user and foreach range of voltage i.e. LV, MV andHV. Emission levels must beconsidered in domestic and industrialapplications.n domestic applicationsIn the LV range, where the energydistributor is unable to control thesituation, disturbance levels whichhave to be observed in equipment unitsare set in accordance with standards.As an example, standard specificationIEC 555-2 referring to Disturbancescaused in supply networks by electro-domestic appliances and similarequipment prescribes limitingvalues of current for each harmonic(in appliances drawing an effectivecurrent 16A - see table in fig. 2).n industrial applicationsIn this sector, there are so far noagreed international standards.However, a consensus appears toemerge on the concept of stages.nn stage 1: automatic acceptanceThis acceptance is dependent on thevoltage level of network and applies toequipment of low power as comparedwith the power contracted for(subscribed). For example, the rule atElectricit de France (EDF) is tohave a disturbance causing power thatis inferior or equal to 1 % of minimumshort-circuit power in a normal situationat the point of connection.This tolerance can be extended if thetotal disturbing power is inferior to:- 4 MVA in HV range,- 500 kVA in MV range,- 40 kVA in LV range.


fig. 1: values indicative of levels (targets) of compatibility for harmonic voltages (in % of nominal voltage at fundamental frequency) in HV powernetworks (transports) and MV and LV networks (extracted from paper published in Electra No 123).

harmonic max. permissible harmonic currentorder (in Amperes)odd harmonics

3 2.35 1.147 0.779 0.411 0.3313 0.21

15 n 39 0.15 x 15n

even harmonics

2 1.084 0.436 0.30

8 n 40 0.23 x 8n

fig. 2: limits of harmonic components of current in domestic applications (In 16 A).

odd harmonics odd harmonics even harmonicsnon multiples of 3 multiples of 3harmonic harmonic harmonic harmonic harmonic harmonicorder n voltage % order n voltage % order n voltage %

LV/MV HV LV/MV HV LV/MV HV

5 6 2 3 5 2 2 2 1.57 5 2 9 1.5 1 4 1 111 3.5 1.5 15 0.3 0.3 6 0.5 0.513 3 1.5 21 0.2 0.2 8 0.5 0.217 2 1 > 21 0.2 0.2 10 0.5 0.219 1.5 1 12 0.2 0.223 1.5 0.7 > 12 0.2 0.225 1.5 0.7

> 25 0.2 + 12.5n

0.1 + 2.5n

Global rate of distortion: 8 % in LV and MV networks - 3 % in HV networks

nn stage 2: acceptance with reservationsWhen, in the case of a given user, thelimits stated previously are exceeded,the energy producer generallyprescribes a maximum distortion rate atthe point of connection. In cases wherethese levels were likely to be exceeded,the distributor/supplier would reservethe right to ask for complementarymeans of compensation to be resortedto if the distortion rate was exceeded.nn stage 3: acceptance - exceptional andprovisional.When the limits stated in stage 2 areexceeded but without, however, causingthe compatibility level to be exceeded -due to the non generation of harmonicsby other users -, a provisionalauthorization permit may be granted.Finally, in an effort to clarify the beha-viour of harmonic producing equipment,some standards are now in the processof elaboration or modification.


fig. 3: circuit diagram of a charger rectifier.

network utilization

battery

inverterchargerrectifier

2. thyristor Graetz bridge rectifier

UPS equipments consist of an AC/DCconverter (i.e. rectifier), a battery bank(which can be charged by the rectifieror with an appropriate current charger)and a DC/AC converter (i.e. inverter)(see fig. 3).Generally as the input converter isexpected to provide a charge or tomaintain the charge of the battery at aconstant voltage and to supply therequired power to the inverter, it utilisesusually thyristors arranged in theclassic form of a Graetz bridge circuit.There are other types of rectifier circuitsbut the three-phase Graetz bridgearrangement is the most commonlyused, in particular in high powered UPSunits; hence the following study ofharmonic currents generated by thethree-phase Graetz bridge with a fullyregulated circuit and of the means ofminimising them.

harmonic currentsgenerated by a Graetzbridge rectifierThe rectifier in figure 4 is assumed tobe connected to a high valueinductance acting as filter to the DCcurrent Id to ensure that the latter isperfectly smooth. Initially, the sourceimpedance is considered to be zero.The line currents I1, I2 and I3 assumein turn the value (and the shape) of theDC current Id.Each thyristor ensures currentconduction during 1/3 of a period.Having assumed a source impedanceequal to zero, the current establishesitself instantaneously at its value Id assoon as one thyristor starts conducting.

fig. 4: circuit diagram of charger rectifier.

i3

i2

e'3

e'2

e3

e2

I d

T1 T2 T3

T4 T5 T6

Zs i1e'1e1

Gratz bridge batteries


fig. 5: theoretical currents upstream ofrectifier with infinite dowstream filterimpedance and source impedance = 0.

t

t

t

t

I d

i1

i2

i3

Currents supplied by the source have arectangular waveform (see fig. 5).The spectrum is made up of currentharmonics:

In = I1n

where n = 6 k 1, k taking values 1, 2,3... (whole numbers/integers) and I1being the effective value offundamental, i.e. I1 = 0.78 Id.For the first harmonics of current, theamplitudes vary therefore in function ofI1:n I5 = 20 % of I1,n I7 = 14 % of I1,n I11 = 9 % of I1,n I13 = 8 % of I1.The global rate of distortion of thiscurrent is thus 30 %.The global rate of distortion of thevoltage is zero in this case, since thesource impedance has been assumedto be zero (i.e infinite power).

influence of sourceimpedanceSince the source is by nature inductive,its inductance precludes anyinstantaneous variations of current.The phenomenon of overlapWhen thyristor T2 (see fig. 6a) is gatedwhile thyristor T1 is conducting, currentI2 establishes itself in thyristor T2 whilecurrent I1 in Thyristor T1 decreases.Inductances L oppose sudden sharpvariations of these currents.During the time t of commutation(see fig. 6a) there is simultaneousconduction in two thyristors (thisphenomenon is also called overlap).The source is therefore in a state ofinterphase short-circuit (phases 1and 2) limited only by the twoinductances L.

The voltage v is such that:

v = e1 + L .d . i1dt

= e2 + L .d . i2

dt,

hence

2v = e1 + e2 + L .d . i1dt

+d . i2

dt,

L . d . i1dt

+d . i2

dt = L . d( i1 + i2 )

dt

= L . d . Iddt

= 0 ,

therefore:2v = e1 + e2, or

v = e1 + e2

2.

The same phenomenon occurs laterbetween T2 and T3, then between T3and T1 and also in the negative polarityof the rectifier between thyristors T4, T5and T6.

fig. 6: overlap phenomenon.

I d

e1

e2

v: voltage at + terminal of rectifierswith respect to neutralL: line impedance representingsource impedance

: source voltage, phase-neutral, e2e1

T1

T2

L

L v

a) b)

i2

i1

t

I di1

t

I di2

t

: commutation time t


t

I d

t

e'1 e'2 e'3

t

i1

e'1 , e'2 , e'3

fig. 7: overlap for a thyristor rectifier with an angle of lag of 30.

lead storage cells open load nickel-cadmium"recombination" cells cells

charging voltage(high rate) 2.30 < Ucharge


fig. 9: amplitude variation of harmonic currents with respect to source impedance for various angles of lag in a three-phase Graetz bridgecircuit.

InNIniN

. 100

InNIniN

. 100

InNIniN

. 100

InNIniN

. 100

20

a) harmonics of order: n = 5; n = 7

0 1 2 3 4 5 6 7 8 9

10

20

30

40

50

60

70

80

90

100

dXN

0 1 2 3 4 5 6 7 8 9

10

20

30

40

50

60

70

80

90

100

dXN0 1 2 3 4 5 6 7 8 9

10

20

30

40

50

60

70

80

90

100

dXN

302010

50

0 1 2 3 4 5 6 7 8 9

dXN

80

90

100

60

70

80

90

100= 90

9030

30

20

105

0

10

05

= 90

30

20

105

0

= 90

= 90

90 = 90

30

30

20

20

10

5

50

0

90

30

3020

20

10

0

5

0

5

10

10

c) harmonics of order: n = 17; n = 19 d) harmonics of order: n = 23; n = 25

b) harmonics of order: n = 11; n = 13


fig. 10: factual currents upstream anddownstream of rectifier.

t

t

I d

i1

direct current (downstream)

line current (upstream)

In a balanced three-phase circuit, dXNrepresents half the relative voltage dropof the line.By calling Ucc this relative voltage dropthat can be likened to a short-circuitvoltage, it is possible to write:

dXN = 12

. U' cc

current distortion rateAssuming a source impedance equal tozero and a perfectly filtered DC current,the effective value of each currentharmonic can be expressed as:

In = I1n

In this instance, the harmonic content isindependent of (t = 0).The global rate of theoretical distortionis given by the expression:

D % = 100 .(

k = 1

I 6 k + 12 + I 6 k 12 )

I1

that is = 30 %.

Note: in practice, for calculationpurposes, the line current does notstrictly assume the theoretical shapetaken as a basis for the calculations,since perfect smoothing of DC currentcannot be achieved (see fig. 10).As a result, the harmonic content ofcurrent is slightly modified; in particularit is observed that harmonics of order6 k - 1 are increased whereas those oforder 6 k + 1 are decreased.Voltage distortion rateThe figure 11 shows the variation ofthe voltage distortion rate at therectifier input with respect to the totalsource impedance referred to theshort-circuit voltage Ucc and the angleof lag set for thyristor control. It isclearly seen that this distortion rateincreases very rapidly and that it isdifficult as originally anticipated toremain below a value of 5 %.

fig. 11: variation of voltage distortion rate with respect to source impedance for different valuesof lag angle .

0 5 10(5)

15 20(10)

10

20

30

U'cc

voltage distortionrate D (%)

(d )XN

= 50

40

30

2010 0

Power factor of rectifiern as the current drawn by the rectifier ishighly distorted, the RMS current hastherefore a value superior to that of thefundamental. The effective value ofcurrent can be calculated by applyingthe basic formula:

Ieff = I 12 + (I 6 k + 12 + I 6 k 12 )

k = 1

with a theoretical value of current(source being of infinite power) equal to:I 6 k 1 =

I16 k 1

hence

Ieff = 1+ 15

2

+17

2

+ ......

i.e.Ieff = 1.05 . I1n in addition, the phase shift betweenthe current and the voltage has aminimum value equal to , to whichmust be added approximately half theoverlap angle t.

As a first approximation, since theoverlap angle is small compared withthe angle of lag, a phase shift equal to can be retained, hence:cos 1 = cos .


n allowing that the effective value ofvoltage approaches very closely that ofthe fundamental (which is true whenthe distortion rate is low), the powerfactor can be expressed with goodapproximation as: = 0.95 . cos

since:

= PS

=

3 . U1. I1. cos 13 . Ueff . Ieff

(U1 and Ueff. representing line to linevoltages)

that is: = 11.05

. cos

Note: for more details refer to standardIEC 146-4 424.

3. minimisation of harmonic disturbances

Curves in figure 11 clearly show thatthe voltage distortion rate at therectifier input grows rapidly in impor-tance even when the sourceimpedance is very low. It is thereforenecessary to reduce this rate ofdistortion so as to allow the use ofrectifiers of non-negligible power com-pared with the contracted for power.Since harmonic currents areresponsible for the voltage distortionwhen they flow across the sourceimpedance, reduction in their amplitudewill bring about an improvement to thevoltage waveform.To achieve this, three classic methodsare utilized:n the insertion of an additionalinductance in the rectifier input in orderto attenuate the amplitude ofharmonics (especially those of higherorders),n the use of several rectifiers fed byvoltages appropriately phase shifted.It is possible, with this method toeliminate - by combining currents - themost troublesome harmonics (that isharmonics of the lowest orders for theyhave the highest amplitudes).n the retention of a single Graetzbridge rectifier to which is added apassive filter designed to eliminate themost troublesome harmonics and toreduce the amplitude of otherharmonics.

It is, of course, possible to combinethese methods so as to optimize theresults.

insertion of inductance atrectifier inputThe circuit diagram corresponding toone phase is shown in figure 12.The insertion of inductance LF reducesthe distortion rate of current. Thevoltage distortion rate at point Adecreases.Its value can be calculated from thevalue obtaining at point B.Inductances Ls and LF form a dividerfor harmonic voltages.

Theoretical calculation of distortionrateFor each harmonic of order n, there is avoltage component Vn at the point B,such that:V'n = n . (Ls + LF) . . Inwhere : pulsation of fundamental.Voltage Vn measured at point A is:

Vn = n . Ls . .InVn = V'n . Ls

Ls + LF.

By applying this reasoning for eachharmonic and calculating the totaldistortion, it becomes evident that, if thevoltage distortion rate measured at

e

Ls LA B

D'D

L : filtering inductance of rectifierLs: total inductance of source (generator + cabling)e: source of perfect voltageD, D': voltage distortion rates

F

F

fig. 12: harmonic separation (decoupling) through use of additional inductance.


point B is D, the voltage distortion rateat A is:

D = D' . LsLs + LF

Remembering that:

Uccs = Ls . . InVn

UccF = LF . . InVn

where Vn = effective value of thesimple fundamental voltage.As an example, if Ls is such thatUccs = 2 % and LF such thatUccF = 6 %, then their sumUccs + UccF = 8 %.For an angle of lag equal to 30, thecurve in figure 11 gives a distortion rateof 19 %. The distortion rate at point A istherefore:

D = 19 % x 28

= 4.75 %.

Without the inductance LF in the circuit,the distortion rate would have beenthat referred to Ls alone, that isUccs = 2 %, value which gives on thecurve in figure 11D = 10 %.The insertion of inductance LF hasmade it possible, in this case, to reducethe voltage distortion rate by a factorgreater than 2 compared with the ratelevel in other utilizations.ApplicationAn UPS unit, rated at 300 kVA, suppliesa load of 250 kVA with a cos = 0.8; itsefficiency is 0.92 and the power factorof its rectifier is = 0.82.The apparent power drawn by therectifier is therefore:

250 x 0.80.92 x 0.82

= 265 kVA .

The rectifier is fed from a transformerrated at 630 kVA; Uccs = 4 % and isrelated to an inductance correspondingto a UccF value of 12 % and calculatedfor a rectifier power rating of 350 kVA.

fig. 13: basic diagram of a rectifier with two phase staggered bridges.

Taking into account, the load rates oftransformer and of the rectifier:

Uccs becomes: 4 % x 265630

= 1.7 %

UccL becomes: 12 % x 265350

= 9.1%

If, on average, the thyristors operatewith a lagging phase angle of 20, itis then possible to determine thedistortion rate from figure 11:

D = 18.8 %( = 20; Ucc = 10.8 %)hence a distortion rate across thetransformer terminals amounting to:

D = 18.8 % x 1.7 %10.9 %

= 2.9 %

use of double bridgerectifier(see fig. 13)This principle consists in utilizing atransformer with two secondarywindings which supply voltages with aphase displacement of 30 betweenthem; each of those secondariessupplies a Graetz bridge rectifier whichprovides a six-phase rectification.The rectifiers must supply identical DCcurrents to ensure that the AC currentsdrawn from the transformer secon-daries have the same value.Under those conditions, there occurs arecombining process between theharmonic currents generated by eachone of the rectifiers in the primarywinding of transformer and calculationsshow that harmonics of order 6 k 1(k being an odd number) are eliminated.

I .R1+ R1

i32

i22

i12

I .R2+ R2

- R1

- R2

i31

i21

i11

J

J

J

3

2

1


fig. 15: connection in series (a) or in parallel(b) of two rectifiers.

fig. 14: shape of currents drawn by rectifier and resultant in primary of transformer with twosecondaries.

I' 1 =I12 I22

3

J1 = I11 + I' 1

a)+ R1

+ R2

+ RL1

- R1

- R- R2

L1, L2 : inductances of DC current filtering: separation (decoupling) inductance withcentre tap point

b)+ R1

+ R2

+ RL2- R1

- R- R2

This is, in particular the case of 5 th and7 th harmonics whose theoreticalamplitudes are the most important.11 th and 13 th harmonics are retainedbut the 17 th and 19 th harmonics areeliminated.The remaining harmonics are thereforeof order 12 k 1 (k being a wholenumber).The figure 14 indicates the currentdrawn by the transformer primary andresulting from currents supplied by thetwo secondaries.The line current has a shape which ismuch closer to a sinusoidal waveformthan that of the current obtained with asingle rectifier. The two rectifiers can beconnected in series or in parallel(see fig. 15).When the two circuits are put in paralleland considering that the instantaneousvoltages delivered by each one of thetwo rectifiers are not equal (since theyare displaced from each other by 30), itis necessary to add an inductance witha centre tap in order to maintain acontinuous flow in each rectifier.

30

t

t

t

I 32

t

I 22

t

I 12

t

I 31

t

I 21

t

I 11

secondary

primary

secondary


fig. 16: circuit connections to obtain a phase shift of 30 and various connection methods for autotransformer.

In the absence of this inductance,conduction would be ensured at eachinstant only by that rectifier that deliversthe highest voltage.There are several variants (patented byMerlin Gerin) of the circuit diagramshown in figure 13 (see fig. 16) whichlead to the same result as regards levelof harmonics.Distortion rate of currentAssuming zero impedance upstream ofrectifier and a perfectly smoothed DCcurrent, the effective value of eachcurrent harmonic is of the following

form: In = I1n

where n = 12 k 1

The theoretical rate of distortion isthus:

D% =(I

12 k+ 12 ) + (I

12 k12 )

k =1

I1

. 100

that is D 15 % which represents halfthe value obtained with a single rectifier(see start of paragraph 2).Distortion rate of voltageThe voltage distortion rate is dependenton the source impedance.For a very low source impedance, (sumof impedances upstream of rectifier(s)),the ratio between the distortion rates

obtained with a two-rectifier circuit tothat obtained with a single rectifier is:

12

0.7.

For a higher source impedance, thegain is more substantial since higherorder harmonics decrease rapidly asthe source impedance increases.However, the gain remains rathermodest and in practice a ratio of 0.5in favour of the double bridge circuit isto be retained.Example:n for an angle of lag of 30, the ratiobetween the two distortion rates

+

-

a) circuit connection with power transformer b) circuit connection with autotransformer

+

-

c) various connection circuits for autotransformer

A a

b

BC

c

simple star

A a

B

b

c

C

A a

B

b

c

C

polygonaldouble star


fig. 17: example using n rectifiers.

fig. 18: principle of phase shifting.

amounts to 0.66 with Ucc = 8 %and 0.55 with U'cc = 16 %;n for angle = 0, the ratios are 0.53and 0.37 respectively.This ratio between the distortion ratestakes no account of the inductance ofthe phase shifting system.

rectifier circuit with morethan two bridges(see fig. 17)The basic idea here is to increase thenumber of transformer secondaries withrespective phase displacementsdepending on the number of secondariesretained for the purpose of eliminatingother harmonics of current.Three rectifier circuit arrangementFor this form of arrangement, the phasedisplacement must be such that:n 1 = 0,n 2 = 20,n 3 = 40.In this case, the only harmonics remainingare those of order 6 k 1 (where k =multiple of 3) that is 18 k 1.The first harmonics of current aretherefore 17 th and 19 th followed by35 th and 37 th harmonics.Four rectifier circuit arrangementIn this case, the phase displacementare as follows:n 1 = 0,n 2 = 15,n 3 = 30,n 1 = 45.The only harmonics remaining arethen those of order 24 k 1.The first harmonics are therefore the23 rd and 25 th following by 47 th and49 th.These arrangements are of interest inso far as they make it possible toobtain relatively low distortion rates ofcurrent and voltage.They have the disadvantage of beingcomplex and expensive.Consequently, their utilization isreserved for equipment of high powerrating.For instance, aluminium electrolysisprocess, which utilizes DC currentsupplied from power sources ofseveral MW, requires circuitarrangements consisting of up to72 phases!

Special case of circuit connectioncalled phase shifting(see fig. 18)When several UPS units are operatedin parallel, they share the load current

between them and the currents drawnby each rectifier have identicalamplitudes.It is then possible to supply therectifiers from auto-transformers which

1

n

2

R1

-

+

R2

-

+

3

R3

-

+

Rn

-

+

n

1 = 0

2


fig. 19: variation of harmonic content of current in principal connection systems.

fig. 20: basic circuit of passive filter for harmonics.

fig. 21: equivalent circuit diagram of filter forharmonics.

connection number of rectifiers harmonicstype in service H5 H7 H11 H13 H17 H19 H23 H252 rectifiers 2 0 0 1 1 0 0 1 1

1 1 1 1 1 1 1 1 13 rectifiers 3 0 0 0 0 1 1 0 0

2 1/2 1/2 1/2 1/2 1 1 1/2 1/24 rectifiers 4 0 0 0 0 0 0 1 1

3 1/3 1/3 1/3 1/3 1/3 1/3 1 1

filter and this harmonic no longeraffects other users.n as regards 7 th harmonic, because ofits proximity to the tuned frequency, theparallel impedance is still low andconsequently a large proportion of thisharmonic is also eliminated.n finally, as regards harmonics ofhigher orders, the parallel impedance ofthe filter is very close to that of itsinductance Lp: the filter thus functionsas a current divider.For harmonics of higher orders:

IHn = I' Hn . LpLp + Ls + L' F

if Lp is chosen so thatLp Ls + LF then

IHn = 12

. I' Hn

Global distortion rate of voltageDetailed calculations of the voltagedistortion rate obtained at the sourceoutput, are beyond the terms ofreference for this technical booklet.Lets however consider an example:n if LF = LF with UccF = 12 % andn if Lp corresponds to Uccp = 15 %,

I H Ls L'F I'H LF

I"H

Lp

Cp

I H Zsn I' H

I" H

Zpn

n

n n

produce the required phase shiftsaccording to the number or rectifiers(instead of utilizing circuit arrangementswith transformers).The auto-transformers utilized can beof the same type as those shown infigure 16.The polygonal circuit arrangement ismostly utilized for economic reasons.The principal disadvantage of thissystem is due to the fact that harmonicrates increase when one of the UPSunits is shut down.Table in figure 19 gives the harmoniccontent in principal circuit connectionsin which all the rectifiers - except one -are operational.

utilization of a passive filterfor harmonicsThe filter is tuned to a particularfrequency.Its effectiveness is highest at thisfrequency, but several filters areneeded to strongly attenuate severalharmonics.The introduction of passive filters isalways critical because of risk ofresonance.

(Refer on this subject to CahierTechnique No 152: Les perturbationsharmoniques dans les rseauxindustrielles, et leur traitement).Filter utilized by Merlin Gerin forUPS units of high power ratingThe figure 20 shows the equivalentbasic circuit for one phase.The parallel arm of the filter consists ofa circuit tuned to the 5 th harmonicwhich is the most important. The seriesarm of the filter comprises an induc-tance whose function is to achieveseparation of the parallel arm from thesource.

Calling Zpn and Zsn the impedances ofparallel and series arms of the filtertuned to harmonic of order n andassuming that the current generated bythe rectifier for this order is IHn, thenthe current supplied by the source is:

IHn = I' Hn . ZpnZpn + Z sn

(see fig. 21).n as regards 5 th harmonic, the parallelimpedance is equal to zero.All the current of 5 th harmonic flowsthus through the parallel arm of the

the gain following the insertion of induc-tance LF alone, is at least 3 whateverthe value of the source impedance.The figures 22 and 23 illustrate theshape of line currents with and withoutthe presence of a filter, as well as thespectra of these currents for a rectifiercomprising an input inductance and afilter inductance such that:L'F = LF with UccF = 10 %The rectifier is supplied from a sourcesuch that Uccs = 2 %.For a current harmonic of order n, thevoltage VHn developed across thesource impedance is:

VHn % = Uccs % . n . I HnI1


current spectrumorder IHn / I1H5 33 %H7 2.7 %H11 7.3 %H13 1.6 %H17 2.6 %H19 1.1 %H23 1.5 %H25 1.3 %global rateof distortionof current 35 %

fig. 22: line current of rectifier without harmonic filter.

current spectrumorder IHn / I1H5 2.1 %H7 1.4 %H11 3.6 %H13 0.7 %H17 1 %H19 0.7 %H23 0.6 %H25 0.5 %global rateof distortionof current < 5 %

fig. 23: line current of rectifier with harmonic filter.

since,

D % = VHn2n = 2

hence

D % = Uccs % . n . IHnI1

2

n = 2

By taking the values listed in figure 22,the distortion rates at the source outputare 4 % without filter, and 1 % with filterrespectively.For comparison purposes, it is to benoted that in the case of a two-bridgerectifier having the same inputinductance, harmonics of orders 5, 7,17 and 19 are eliminated, which resultsin a distortion rate at the source outputequal to 1.9 %.The harmonic filter is in this case,practically twice as effective comparedwith the use of a two-bridge circuitarrangement.Furthermore, this is a less costlysolution which can be resorted to afterthe equipment has been put in service.Additional characteristic ofharmonic filterThe presence of the parallel arm of thefilter tuned to 5 th harmonic causes theappearance of a capacitive current atfundamental frequency. This capacitivecurrent improves the power factor cos of the rectifier.

I 1

t

I 1

t


4. conclusions and prospects

Thyristor rectifiers of classic typesutilized in UPS equipments are sourcesof harmonic disturbances andadversely affect the power factor of theinstallation.These pollutions are acceptable as longas the power rating of an UPSequipment is low compared with theshort-circuit power rating of thenetwork.When the voltage distortion rateexceeds acceptable values (in theorder of a few %), corrective measuresmust then be taken.The simplest solution and the mostcommon consists in inserting a seriesinductance which achieves harmonicdecoupling.When this measure is found to beinsufficient, the use of phase staggeredrectifiers or passive filters makes it

possible to bring these disturbancesdown to an acceptable level.These solutions are nowadays perfectlymastered and widely applied.The figure 24 gives a synthesis ofadvantages and disadvantages forvarious solutions.In the not too distant future, themultiplication of polluting equipments,the changes in standards and therequirements of energy distributorsshould lead to the use of cleanrectifiers (this has already beenachieved in single phase equipmentthanks to the technique of sinusoidalsampling).Furthermore, a converter utilising thetechnique of PWM (pulse widthmodulation) can, by making use ofappropriately adapted regulationcontrol, behave as an active filter

intended to deal with a particularpolluting load or with the whole of theinstallation; this principle can becompared to the one adopted foreffecting acoustic de-pollution(i.e. emission of sounds in phaseopposition to the sounds to beneutralised).By utilizing a different regulationstrategy, the same converters can alsoachieve self compensation of the powerfactor cos of the installation.In order to make these devices, whichare technically feasible, available toindustry, it is necessary to ensure thattheir production costs are acceptablecompared with those of classicsolutions.The principles of such converters andof their possibilities will be developed ina future Cahier Technique.


circuit type diagram observations

a) no reducing interface n acceptable if power required is lowcompared with short-circuit powerof network

b) series inductance n simple, reliablen can be used in most cases

n inductance can be added after equipmenthas been put in service

n economic

c) double bridge and transformer n complicated (requires balancing of voltages,with two secondaries of Icc's, of currents in rectifiers)

n to be considered at design start

n expensive

d) double bridge with n the solution for parallel connection ofautotransformer UPS units in active redundancy

n compared with circuit Cnn same effectiveness and drawbacksnn smaller lossesnn more economic

e) passive filter n simple, reliablen best persorming

n can be inserted after equipment has been putin service

n more economic than solution with two rectifiers

fig. 24: comparison of anti-harmonics solutions.

Cahier Technique Merlin Gerin n 160 / p.20Ral. : Illustration Technique Lyon -DTE - 12/93 - 2500 - Imp. : Lostic Seyssinet-Pariset

5. bibliography

Standard specificationsn IEC 146-4: semi-conductorconverters part 4: Method for specifyingperformances of and test procedureswith UPS.n IEC 552-2: Disturbances caused insupply networks by electro-domesticequipment and similar equipment.n IEC 1000-2-2: Electromagneticcompatibility (CEM) part 2:Environment.Section 2: Levels of compatibility.

Merlin Gerin Cahier Techniquepublicationsn Les perturbations harmoniques dansles rseaux industrielles, et leurtraitement : Cahier Technique No 152by P. ROCCIA and N. QUILLON.n Inverters and Harmonics (casestudies of non-linear loads): CahierTechnique No 159 by J.N. FIORINA.

Other publicationsn Directive of 25 July 1985 underreference 83/374/CEE.n Electra No 77 - July 1991.n Electra No 123 - March 1989.

160-Harmonics Upstream of Rectifiers in UPS

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