Document Sample

n° 160 harmonics upstream of rectifiers in UPS Jean Noël Fiorina Joined Merlin Gerin in 1968 as a laboratory technician in the ACS (Alimentations et Convertisseurs Statiques) department where he participated in the performance setting up procedures for static converters. In 1977 he obtained his ENSERG engineering degree following a 3 year evening course and rejoined the ACS department. Starting as development engineer he was soon afterwards entrusted with projets. He became later responsible for design projets in EPS department (Electricity Power Supplies). He is in some ways the originator of medium and high power inverters. At present he is with the Supplies Division where, as responsible for innovations he works on the preparation on new UPS designs of tomorrow. E/CT 160 first issued december 1993 glossary P1 cos j1 factor of phase shift = S1 ∞ ∑ Yn2 n=2 D% global distortion rate = 100 Y1 Yn Hn % individual rate of harmonics = 100 Y1 P l power factor = S P1 active power of fundamental component S1 apparent power of fundamental component λ n distortion factor = cos ϕ1 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) Cahier Technique Merlin Gerin n° 160 / p.2 harmonics upstream of rectifiers in UPS UPS, like most static converters draw energy from an A.C. mains network through rectifiers. Often, these rectifiers fitted with thyristors are generators of harmonics. Merlin Gerin, manufacturers of UPS equipment are well acquainted with this problem and consequently have decided to share their knowledge in this summary «Cahier Technique». In this treatise, the author highlights first the need for a standardized co- 1. Harmonics in supply networks Consequences due to harmonic existence between polluting and currents p. 4 polluted equipments. Need for standardization p. 4 He then recalls, which harmonic currents and voltages are produced by 2. Thyristor Graetz bridge rectifier Harmonic currents generated conventional (classic) rectifiers by a Graetz bridge rectifier p. 6 (thyristor Graetz bridge rectifier) and Influence of source impedance p. 7 proceeds to offer various solutions Current distortion rate p. 10 designed to minimize harmonics. 3. Minimisation of harmonic Insertion of inductance at rectifier Finally, in his conclusion, he alludes to disturbances input p. 11 the appearance in the near future of Use of double bridge rectifier p. 12 non-polluting UPS equipment and of Rectifier circuit with more than de-polluting converters. two bridges p. 15 Note: harmonic problems occuring Utilization of a passive filter for downstream of rectifiers which supply harmonics p. 16 non-linear loads are fully discussed in 4. Conclusions and prospects p. 18 Cahier Technique No 159. The latter provides also definitions and 5. Bibliography p. 20 mathematical formulae relating to harmonics. In the present booklet, only the principal definitions and formulae are listed in page 2. Cahier Technique Merlin Gerin n° 160 / p.3 1. harmonics in supply networks consequences due to disturbances produced by some It should be noted that for a same level consumers. of current disturbance, the voltage harmonic currents To achieve this, it is necessary to distortion ratio, at the point of Harmonic currents generated by certain define: connection, is dependent on the equipment, such as static converters, s first, a maximum distortion rate network impedance at that point. discharge lamps, arc furnaces etc.. allowing correct functioning of most (providing there are many or providing A solution that is fair, is to authorise installations (level of compatibility), disturbance causing power sources they have of higher power rating s second, a maximum disturbance rate, compared with the power of the source) that are proportional to the power for each user so that the cumulative contracted for by each user and for can adversely affect the operation of effects of various disturbances thus other equipment connected to the same each range of voltage i.e. LV, MV and generated do allow an operational HV. Emission levels must be network. compatibility between all the considered in domestic and industrial The effects of these harmonic currents installations (connected to the same applications. are discussed in Cahier Technique network); all must operate correctly. No 152 «Harmonic disturbances in Thus’ if this compatibility is needed to s domestic applications industrial supply networks and their exist between subscribers, it is also In the LV range, where the energy treatment». needed to exist within the installation distributor is unable to control the units of individual subscribers. situation, disturbance levels which Let’s recall the adverse effects of have to be observed in equipment units harmonic currents: The end user is therefore burdened are set in accordance with standards. s they cause additional heating with a level of disturbances induced by especially in line conductors, As an example, standard specification equipment units that he has installed IEC 555-2 referring to «Disturbances transformers and condensers, himself. That is why it is important that s they induce vibrations and noises in caused in supply networks by electro- manufacturers clearly state the domestic appliances and similar electromagnetic equipment, disturbance levels produced by their s they can cause interference with equipment» prescribes limiting equipment. values of current for each harmonic communication and «low current» Standards are therefore needed to set protection/signalling circuits. (in appliances drawing an effective acceptable levels of harmonic current ≤ 16A - see table in fig. 2). A distorted voltage can, in addition, disturbances for the supply networks as upset the operation of some receivers s industrial applications well as for polluters. such as regulators, static converters In this sector, there are so far no Level of operational compatibility agreed international standards. (when the crossing through zero of the Levels of compatibility for Low Voltage However, a «consensus» appears to voltage waveform becomes (LV) public supply networks are defined emerge on the concept of stages. indeterminate). in Standard Specification IEC 1000-2-2 s stage 1: automatic acceptance Thus, one of the factors highlighting the of May 1990. As the levels retained in This acceptance is dependent on the quality standard of electricity supply is its voltage distortion rate. this standard specification are the same voltage level of network and applies to as those published in CIGREE equipment of low power as compared periodicals (Electra No 77 of July 1991 with the power contracted for need for standardization and No 123 of March 1989), it is (subscribed). For example, the rule at As electricity is today regarded as a probable that levels specified for «Electricité de France» (EDF) is to product (in particular in Europe Medium Voltages (MV) and for High have a disturbance causing power that following the directive of 25 July 1985 Voltages (HV) will also correspond to is inferior or equal to 1 % of minimum under reference 85/374/CEE), the these recommendations (see table in short-circuit power in a normal situation producer becomes fully liable for fig. 1). at the point of connection. damages caused by excess of Emission Levels This tolerance can be extended if the harmonics. Limits should be defined for each total disturbing power is inferior to: That is why electricity distributors in subscriber so as to avoid the necessity - 4 MVA in HV range, order to be able to guarantee a quality to carry out systematic controlled - 500 kVA in MV range, level acceptable to all consumers, do checks when the equipment is put into - 40 kVA in LV range. set or are compelled to set limits to service. Cahier Technique Merlin Gerin n° 160 / p.4 odd harmonics odd harmonics even harmonics non multiples of 3 multiples of 3 harmonic harmonic harmonic harmonic harmonic harmonic order 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 1.5 7 5 2 9 1.5 1 4 1 1 11 3.5 1.5 15 0.3 0.3 6 0.5 0.5 13 3 1.5 21 0.2 0.2 8 0.5 0.2 17 2 1 > 21 0.2 0.2 10 0.5 0.2 19 1.5 1 12 0.2 0.2 23 1.5 0.7 > 12 0.2 0.2 25 1.5 0.7 12.5 2.5 > 25 0.2 + 0.1 + n n Global rate of distortion: 8 % in LV and MV networks - 3 % in HV networks fig. 1: values indicative of levels (targets) of compatibility for harmonic voltages (in % of nominal voltage at fundamental frequency) in HV power networks (transports) and MV and LV networks (extracted from paper published in Electra No 123). s stage 2: acceptance with reservations harmonic max. permissible harmonic current When, in the case of a given user, the order (in Amperes) limits stated previously are exceeded, odd harmonics the energy producer generally prescribes a maximum distortion rate at 3 2.3 the point of connection. In cases where 5 1.14 these levels were likely to be exceeded, 7 0.77 the distributor/supplier would reserve 9 0.4 the right to ask for complementary 11 0.33 13 0.21 means of compensation to be resorted to if the distortion rate was exceeded. 15 15 ≤ n ≤ 39 0.15 x s stage 3: acceptance - exceptional and n provisional. even harmonics When the limits stated in stage 2 are exceeded but without, however, causing 2 1.08 the compatibility level to be exceeded - 4 0.43 due to the non generation of harmonics 6 0.30 by other users -, a provisional 8 8 ≤ n ≤ 40 0.23 x authorization permit may be granted. n Finally, in an effort to clarify the beha- viour of harmonic producing equipment, some standards are now in the process fig. 2: limits of harmonic components of current in domestic applications (In ≤ 16 A). of elaboration or modification. Cahier Technique Merlin Gerin n° 160 / p.5 2. thyristor Graetz bridge rectifier UPS equipments consist of an AC/DC converter (i.e. rectifier), a battery bank (which can be charged by the rectifier or with an appropriate current charger) charger and a DC/AC converter (i.e. inverter) rectifier inverter (see fig. 3). network utilization Generally as the input converter is expected to provide a charge or to maintain the charge of the battery at a constant voltage and to supply the required power to the inverter, it utilises usually thyristors arranged in the battery classic form of a Graetz bridge circuit. There are other types of rectifier circuits but the three-phase Graetz bridge fig. 3: circuit diagram of a charger rectifier. arrangement is the most commonly used, in particular in high powered UPS units; hence the following study of harmonic currents generated by the three-phase Graetz bridge with a fully λ Id regulated circuit and of the means of minimising them. T1 T2 T3 harmonic currents generated by a Graetz e1 Zs e'1 i1 bridge rectifier i2 e2 e'2 The rectifier in figure 4 is assumed to be connected to a high value i3 e3 e'3 inductance acting as filter to the DC current Id to ensure that the latter is perfectly smooth. Initially, the source T4 T5 T6 impedance is considered to be zero. The line currents I1, I2 and I3 assume in turn the value (and the shape) of the DC current Id. Each thyristor ensures current Graëtz bridge batteries conduction during 1/3 of a period. Having assumed a source impedance equal to zero, the current establishes itself instantaneously at its value Id as fig. 4: circuit diagram of charger rectifier. soon as one thyristor starts conducting. Cahier Technique Merlin Gerin n° 160 / p.6 Currents supplied by the source have a The voltage v is such that: rectangular waveform (see fig. 5). d . i1 d . i2 The spectrum is made up of current v = e1 + L . = e2 + L . , harmonics: dt dt Id I1 hence In = d . i1 d . i2 t n 2v = e1 + e 2 + L . + , where n = 6 k ± 1, k taking values 1, 2, dt dt i1 3... (whole numbers/integers) and I1 d . i1 d . i2 d( i1 + i2 ) t being the effective value of L. + = L. fundamental, i.e. I1 = 0.78 Id. dt dt dt For the first harmonics of current, the d . Id = L. = 0, amplitudes vary therefore in function of dt i2 I1 : t therefore: s I5 = 20 % of I1, 2v = e1 + e 2 , s I7 = 14 % of I1, or i3 s I11 = 9 % of I1, e1 + e 2 t s I13 = 8 % of I1. v = . 2 The global rate of distortion of this current is thus 30 %. The same phenomenon occurs later between T2 and T3, then between T3 The global rate of distortion of the and T1 and also in the negative polarity voltage is zero in this case, since the fig. 5: theoretical currents upstream of of the rectifier between thyristors T4, T5 rectifier with infinite dowstream filter source impedance has been assumed and T6. impedance and source impedance = 0. to be zero (i.e infinite power). influence of source impedance a) b) i1 Since the source is by nature inductive, i1 L T1 its inductance precludes any e1 Id instantaneous variations of current. The phenomenon of overlap Id When thyristor T2 (see fig. 6a) is gated t while thyristor T1 is conducting, current I2 establishes itself in thyristor T2 while v i2 i2 L T2 current I1 in Thyristor T1 decreases. e2 Id Inductances L oppose sudden sharp variations of these currents. e1 , e2: source voltage, phase-neutral During the time ∆t of commutation t v: voltage at + terminal of rectifiers (see fig. 6a) there is simultaneous with respect to neutral ∆t conduction in two thyristors (this L: line impedance representing phenomenon is also called «overlap»). source impedance ∆ t : commutation time The source is therefore in a state of interphase short-circuit (phases 1 and 2) limited only by the two fig. 6: overlap phenomenon. inductances L. Cahier Technique Merlin Gerin n° 160 / p.7 For an angle of lag α of 30° (which corresponds to a normal working point), voltages e'1, e'2 and e'3 obtained at the ω∆t input to the rectifier and also the line e'1 , e' , e' e'1 α e'2 e'3 2 3 current i1 are shown in fig. 7. The angle of lag α is used for the regulation of the DC voltage supplied by the rectifier. In the case of a rectifier/battery charger, this output voltage must be kept constant (see fig. 8) whatever the variations of the AC voltage or ωt whatever the charging conditions of the rectifier. The value of this DC voltage can be expressed by the approximate relation: 1 Ud = 1. 35 . Ueff . cos α − . L . ω . I1 2 where Ueff: effective value of «composed resultant voltages» i1 3 Id (Ueff = e1 . 2 if one refers to fig. 6a). ωt Disturbances due to overlap It is evident that during each half- period, each of the simple fundamental voltages is disturbed twice and exhibits: s a voltage drop when the fig. 7: overlap for a thyristor rectifier with an angle of lag α of 30°. corresponding thyristor is triggered into conduction, s an overvoltage when the current in lead storage cells open load nickel-cadmium this thyristor is turned off. "recombination" cells cells The higher the inductance of line L, the charging voltage longer is therefore the duration ∆t of (high rate) 2.30 < Ucharge <2.50 2.30 < Ucharge < 2.50 1.42 < Ucharge <1.65 these disturbances. (in V) As the line current no longer has a floating voltage perfectly rectangular shape, its (low rate) 2.23 < Ufloating < 2.30 2.18 < Ufloating < 2.25 1.38 < Ufloating < 1.50 harmonic content decreases (strong (in V) attenuation of harmonics of high orders). Sealed batteries (recombination) are generally charged at low rate charge only. Consequently, the resulting distortion of Open batteries are charged in two successive voltage steps. the voltage increases when the line impedance increases, but this increase fig. 8: charge of UPS batteries at constant voltage and limited current (according to Gimelec is not proportional to the impedance recommendations). since the harmonic content of current decreases. In addition, the time of commutation respect to the source impedance for The source impedance is represented decreases when the angle of lag α different angles of lag α. here by the term dXN which corres- increases which, as a result, brings The harmonic currents are represented ponds to the relative voltage drop on about - for the same inductance value - in relative value with respect to their the DC side. The latter is due to the an increase in the harmonic content of effect of total inductance of line. maximum theoretical value (IniN): current and the voltage distortion. For this rectifier: InN = Ieff of harmonic of order n Harmonic content of current 1 L .ω . I1 I1 d xN = . . 100 The figure 9 extracted from the IniN = 2 V1 standard specification IEC 146-4, n where V1: is the effective value of shows how harmonic currents vary with I1 = effective value of fundamental simple fundamental voltages. Cahier Technique Merlin Gerin n° 160 / p.8 InN InN . 100 . 100 IniN IniN 100 100 α = 90° α = 90° 90 30° 90 90° 80 20° 80 30° 10° 30° 70 70 5° 0° 20° 60 60 20° 5° 5° 50 0° 0° 10° 100 α = 90° 40 30° 10° 90 30 20° 10° 80 20 5° 0° 10 d XN d XN 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 a) harmonics of order: n = 5; n=7 b) harmonics of order: n = 11; n = 13 InN InN . 100 . 100 IniN IniN 100 100 α = 90° 90 90 80 80 90° α = 90° 70 70 30° 90° 60 60 30° 30° 50 5° 50 30° 20° 20° 40 0° 40 20° 0° 10° 5° 20° 30 30 5° 0° 10° 20 20 0° 5° 10° 10° 10 10 d XN d XN 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 c) harmonics of order: n = 17; n = 19 d) harmonics of order: n = 23; n = 25 fig. 9: amplitude variation of harmonic currents with respect to source impedance for various angles of lag α in a three-phase Graetz bridge circuit. Cahier Technique Merlin Gerin n° 160 / p.9 In a balanced three-phase circuit, dXN Power factor of rectifier As a first approximation, since the represents half the relative voltage drop s as the current drawn by the rectifier is overlap angle is small compared with of the line. highly distorted, the RMS current has the angle of lag, a phase shift equal to By calling U’cc this relative voltage drop therefore a value superior to that of the α can be retained, hence: that can be likened to a short-circuit fundamental. The effective value of cos ϕ1 = cos α . voltage, it is possible to write: current can be calculated by applying the basic formula: 1 d XN = . U' cc ∞ 2 Ieff = I 2 + ∑ (I 2 +I 2 ) direct current (downstream) 1 6k +1 6k −1 k =1 Id current distortion rate with a theoretical value of current Assuming a source impedance equal to (source being of infinite power) equal to: zero and a perfectly filtered DC current, I1 t I 6k ±1 = the effective value of each current 6k ±1 harmonic can be expressed as: hence line current (upstream) I1 In = 1 2 1 2 n Ieff = 1+ + + ...... i1 5 7 In this instance, the harmonic content is independent of α (∆t = 0). i.e. t The global rate of theoretical distortion Ieff = 1.05 . I1 is given by the expression: s in addition, the phase shift between the current and the voltage has a ∞ minimum value equal to α, to which ∑ (I 2 6k +1 +I 2 6 k − 1) k =1 must be added approximately half the fig. 10: factual currents upstream and D % = 100 . overlap angle ω ∆t. downstream of rectifier. I1 that is = 30 %. Note: in practice, for calculation voltage distortion purposes, the line current does not rate D (%) strictly assume the theoretical shape α = 50° taken as a basis for the calculations, 40° since perfect smoothing of DC current 30 cannot be achieved (see fig. 10). 30° As a result, the harmonic content of 20° current is slightly modified; in particular 10° it is observed that harmonics of order 0° 20 6 k - 1 are increased whereas those of order 6 k + 1 are decreased. Voltage distortion rate The figure 11 shows the variation of 10 the voltage distortion rate at the rectifier input with respect to the total source impedance referred to the short-circuit voltage U’cc and the angle of lag α set for thyristor control. It is 0 5 10 15 20 U'cc (5) (10) (d XN) clearly seen that this distortion rate increases very rapidly and that it is fig. 11: variation of voltage distortion rate with respect to source impedance for different values difficult as originally anticipated to of lag angle α . remain below a value of 5 %. Cahier Technique Merlin Gerin n° 160 / p.10 s allowing that the effective value of since: 1 voltage approaches very closely that of that is: λ = . cos α 1.05 the fundamental (which is true when P 3 . U1 . I1 . cos ϕ1 the distortion rate is low), the power λ = = S 3 . Ueff . Ieff Note: for more details refer to standard factor λ can be expressed with good IEC 146-4 § 424. approximation as: (U1 and Ueff. representing line to line λ = 0.95 . cos α voltages) 3. minimisation of harmonic disturbances Curves in figure 11 clearly show that It is, of course, possible to combine Theoretical calculation of distortion the voltage distortion rate at the these methods so as to optimize the rate rectifier input grows rapidly in impor- results. For each harmonic of order n, there is a tance even when the source voltage component V’n at the point B, impedance is very low. It is therefore such that: necessary to reduce this rate of insertion of inductance at V' n = n . (Ls + LF ) . ω . In distortion so as to allow the use of rectifier input rectifiers of non-negligible power com- where ω: pulsation of fundamental. The circuit diagram corresponding to pared with the contracted for power. Voltage Vn measured at point A is: one phase is shown in figure 12. Since harmonic currents are responsible for the voltage distortion The insertion of inductance LF reduces Vn = n . Ls . ω .In the distortion rate of current. The Ls . when they flow across the source voltage distortion rate at point A Vn = V' n . impedance, reduction in their amplitude Ls + LF will bring about an improvement to the decreases. voltage waveform. Its value can be calculated from the By applying this reasoning for each value obtaining at point B. harmonic and calculating the total To achieve this, three classic methods Inductances Ls and LF form a divider distortion, it becomes evident that, if the are utilized: for harmonic voltages. voltage distortion rate measured at s the insertion of an additional inductance in the rectifier input in order to attenuate the amplitude of harmonics (especially those of higher Ls A LF B orders), e s the use of several rectifiers fed by voltages appropriately phase shifted. It is possible, with this method to eliminate - by combining currents - the most troublesome harmonics (that is D D' harmonics of the lowest orders for they have the highest amplitudes). LF: filtering inductance of rectifier s the retention of a single Graetz Ls: total inductance of source (generator + cabling) bridge rectifier to which is added a e: source of perfect voltage passive filter designed to eliminate the D, D': voltage distortion rates most troublesome harmonics and to reduce the amplitude of other fig. 12: harmonic separation (decoupling) through use of additional inductance. harmonics. Cahier Technique Merlin Gerin n° 160 / p.11 point B is D’, the voltage distortion rate Taking into account, the load rates of use of double bridge at A is: transformer and of the rectifier: rectifier Ls (see fig. 13) D = D' . Uccs becomes: 4 % x 265 = 1.7 % Ls + LF 630 This principle consists in utilizing a transformer with two secondary Remembering that: windings which supply voltages with a 265 UccL becomes: 12 % x = 9.1% phase displacement of 30° between Ls . ω . In 350 Uccs = them; each of those secondaries Vn supplies a Graetz bridge rectifier which LF . ω . In If, on average, the thyristors operate provides a six-phase rectification. UccF = with a lagging phase angle α of 20°, it Vn The rectifiers must supply identical DC is then possible to determine the where Vn = effective value of the currents to ensure that the AC currents distortion rate from figure 11: simple fundamental voltage. drawn from the transformer secon- As an example, if Ls is such that D = 18. 8 % daries have the same value. Uccs = 2 % and LF such that (α = 20°; Ucc = 10. 8 %) Under those conditions, there occurs a UccF = 6 %, then their sum recombining process between the hence a distortion rate across the harmonic currents generated by each Uccs + UccF = 8 %. transformer terminals amounting to: one of the rectifiers in the primary For an angle of lag α equal to 30°, the winding of transformer and calculations curve in figure 11 gives a distortion rate D = 18. 8 % x 1.7 % = 2.9 % show that harmonics of order 6 k ± 1 of 19 %. The distortion rate at point A is 10.9 % (k being an odd number) are eliminated. therefore: 2 D = 19 % x = 4.75 %. 8 Without the inductance LF in the circuit, I .R 1 + R1 the distortion rate would have been that referred to Ls alone, that is Uccs = 2 %, value which gives on the i11 curve in figure 11 i21 D = 10 %. i31 The insertion of inductance LF has made it possible, in this case, to reduce the voltage distortion rate by a factor greater than 2 compared with the rate J1 - R1 level in other utilizations. J2 Application + R2 I .R 2 An UPS unit, rated at 300 kVA, supplies J3 a load of 250 kVA with a cos ϕ = 0.8; its efficiency is 0.92 and the power factor i12 of its rectifier is λ = 0.82. i22 The apparent power drawn by the rectifier is therefore: i32 250 x 0. 8 = 265 kVA . 0. 92 x 0. 82 - R2 The rectifier is fed from a transformer rated at 630 kVA; Uccs = 4 % and is related to an inductance corresponding to a UccF value of 12 % and calculated fig. 13: basic diagram of a rectifier with two phase staggered bridges. for a rectifier power rating of 350 kVA. Cahier Technique Merlin Gerin n° 160 / p.12 This is, in particular the case of 5 th and 7 th harmonics whose theoretical I 11 amplitudes are the most important. 30° 11 th and 13 th harmonics are retained but the 17 th and 19 th harmonics are t eliminated. The remaining harmonics are therefore of order 12 k ± 1 (k being a whole I 21 number). The figure 14 indicates the current t secondary drawn by the transformer primary and resulting from currents supplied by the I 31 two secondaries. The line current has a shape which is much closer to a sinusoidal waveform t than that of the current obtained with a single rectifier. The two rectifiers can be connected in series or in parallel (see fig. 15). I 12 When the two circuits are put in parallel and considering that the instantaneous t voltages delivered by each one of the two rectifiers are not equal (since they are displaced from each other by 30°), it I 22 is necessary to add an inductance with a centre tap in order to maintain a t continuous flow in each rectifier. secondary I 32 a) L1 + R1 +R t - R1 + R2 - R2 -R I12 − I 22 t I' 1 = b) 3 + R1 - R1 L2 +R λ + R2 t - R2 -R primary J1 = I11 + I' 1 L1 , L2 : inductances of DC current filtering λ: separation (decoupling) inductance with centre tap point fig. 14: shape of currents drawn by rectifier and resultant in primary of transformer with two fig. 15: connection in series (a) or in parallel secondaries. (b) of two rectifiers. Cahier Technique Merlin Gerin n° 160 / p.13 In the absence of this inductance, The theoretical rate of distortion is obtained with a two-rectifier circuit to conduction would be ensured at each thus: that obtained with a single rectifier is: instant only by that rectifier that delivers the highest voltage. ∞ 1 ≈ 0.7. There are several variants (patented by ∑ (I 2 ) + (I 12 k+ 1 2 12 k−1 ) 2 Merlin Gerin) of the circuit diagram k =1 D% = . 100 For a higher source impedance, the shown in figure 13 (see fig. 16) which I1 lead to the same result as regards level gain is more substantial since higher of harmonics. that is D ≈ 15 % which represents half order harmonics decrease rapidly as the value obtained with a single rectifier the source impedance increases. Distortion rate of current (see start of paragraph 2). However, the gain remains rather Assuming zero impedance upstream of rectifier and a perfectly smoothed DC Distortion rate of voltage «modest» and in practice a ratio of 0.5 current, the effective value of each The voltage distortion rate is dependent in favour of the double bridge circuit is current harmonic is of the following on the source impedance. to be retained. I1 For a very low source impedance, (sum Example: form: In = n of impedances upstream of rectifier(s)), s for an angle of lag α of 30°, the ratio where n = 12 k ± 1 the ratio between the distortion rates between the two distortion rates + + - - a) circuit connection with power transformer b) circuit connection with autotransformer A a A a A a α α α c c B c B C B C b C b b simple star double star polygonal c) various connection circuits for autotransformer fig. 16: circuit connections to obtain a phase shift of 30° and various connection methods for autotransformer. Cahier Technique Merlin Gerin n° 160 / p.14 amounts to 0.66 with U’cc = 8 % and 0.55 with U'cc = 16 %; s for angle α = 0, the ratios are 0.53 R1 + and 0.37 respectively. This ratio between the distortion rates - takes no account of the inductance of the phase shifting system. α1 R2 + rectifier circuit with more α2 - than two bridges (see fig. 17) The basic idea here is to increase the number of transformer secondaries with R3 + respective phase displacements α3 depending on the number of secondaries - retained for the purpose of eliminating other harmonics of current. Three rectifier circuit arrangement For this form of arrangement, the phase displacement must be such that: s α 1 = 0°, αn s α 2 = 20°, s α 3 = 40°. Rn + In this case, the only harmonics remaining are those of order 6 k ± 1 (where k = - multiple of 3) that is 18 k ± 1. The first harmonics of current are fig. 17: example using n rectifiers. therefore 17 th and 19 th followed by 35 th and 37 th harmonics. Four rectifier circuit arrangement α1=0 In this case, the phase displacement are as follows: s α 1 = 0°, s α 2 = 15°, α2 s α 3 = 30°, s α 1 = 45°. The only harmonics remaining are then those of order 24 k ± 1. The first harmonics are therefore the 23 rd and 25 th following by 47 th and 49 th. These arrangements are of interest in so far as they make it possible to obtain relatively low distortion rates of αn current and voltage. They have the disadvantage of being complex and expensive. Consequently, their utilization is reserved for equipment of high power fig. 18: principle of phase shifting. rating. For instance, aluminium electrolysis process, which utilizes DC current Special case of circuit connection between them and the currents drawn supplied from power sources of called «phase shifting» by each rectifier have identical several MW, requires circuit (see fig. 18) amplitudes. arrangements consisting of up to When several UPS units are operated It is then possible to supply the 72 phases! in parallel, they share the load current rectifiers from auto-transformers which Cahier Technique Merlin Gerin n° 160 / p.15 produce the required phase shifts according to the number or rectifiers connection number of rectifiers harmonics (instead of utilizing circuit arrangements type in service with transformers). H5 H7 H11 H13 H17 H19 H23 H25 The auto-transformers utilized can be 2 rectifiers 2 0 0 1 1 0 0 1 1 of the same type as those shown in 1 1 1 1 1 1 1 1 1 figure 16. 3 rectifiers 3 0 0 0 0 1 1 0 0 The polygonal circuit arrangement is mostly utilized for economic reasons. 2 1/2 1/2 1/2 1/2 1 1 1/2 1/2 The principal disadvantage of this 4 rectifiers 4 0 0 0 0 0 0 1 1 system is due to the fact that harmonic 3 1/3 1/3 1/3 1/3 1/3 1/3 1 1 rates increase when one of the UPS units is shut down. fig. 19: variation of harmonic content of current in principal connection systems. Table in figure 19 gives the harmonic content in principal circuit connections in which all the rectifiers - except one - IH Ls L'F I' H LF are operational. I" H utilization of a passive filter for harmonics Lp The filter is tuned to a particular frequency. Its effectiveness is highest at this frequency, but several filters are Cp needed to strongly attenuate several harmonics. The introduction of passive filters is always critical because of risk of fig. 20: basic circuit of passive filter for harmonics. resonance. (Refer on this subject to Cahier filter and this harmonic no longer I Hn Zsn I' H n Technique No 152: «Les perturbations affects other users. harmoniques dans les réseaux s as regards 7 th harmonic, because of I" H n industrielles, et leur traitement»). its proximity to the tuned frequency, the Filter utilized by Merlin Gerin for parallel impedance is still low and consequently a large proportion of this Zpn UPS units of high power rating The figure 20 shows the equivalent harmonic is also eliminated. basic circuit for one phase. s finally, as regards harmonics of higher orders, the parallel impedance of The parallel arm of the filter consists of the filter is very close to that of its fig. 21: equivalent circuit diagram of filter for a circuit tuned to the 5 th harmonic inductance Lp: the filter thus functions harmonics. which is the most important. The series as a current divider. arm of the filter comprises an induc- tance whose function is to achieve For harmonics of higher orders: the gain following the insertion of induc- separation of the parallel arm from the Lp tance L’F alone, is at least 3 whatever source. IHn = I' Hn . the value of the source impedance. Lp + Ls + L' F Calling Zpn and Zsn the impedances of The figures 22 and 23 illustrate the parallel and series arms of the filter if Lp is chosen so that shape of line currents with and without tuned to harmonic of order n and the presence of a filter, as well as the Lp ≈ Ls + LF then spectra of these currents for a rectifier assuming that the current generated by the rectifier for this order is I’Hn, then 1 comprising an input inductance and a IHn = . I' Hn filter inductance such that: the current supplied by the source is: 2 L' F = LF with UccF = 10 % Zpn Global distortion rate of voltage IHn = I' Hn . The rectifier is supplied from a source Zpn + Z sn Detailed calculations of the voltage such that Uccs = 2 %. distortion rate obtained at the source (see fig. 21). For a current harmonic of order n, the output, are beyond the terms of voltage VHn developed across the s as regards 5 th harmonic, the parallel reference for this technical booklet. source impedance is: impedance is equal to zero. Let’s however consider an example: All the current of 5 th harmonic flows s if L’F = LF with UccF = 12 % and I Hn VHn % = Uccs % . n . thus through the parallel arm of the s if Lp corresponds to Uccp = 15 %, I1 Cahier Technique Merlin Gerin n° 160 / p.16 since, ∞ I1 current spectrum D% = ∑ VHn2 order IHn / I1 n = 2 H5 33 % hence H7 2.7 % H11 7.3 % ∞ 2 t H13 1.6 % IHn D % = Uccs % . n . ∑ I1 H17 2.6 % n = 2 H19 1.1 % H23 1.5 % By taking the values listed in figure 22, H25 1.3 % the distortion rates at the source output are 4 % without filter, and 1 % with filter global rate respectively. of distortion of current ≈ 35 % For comparison purposes, it is to be noted that in the case of a two-bridge fig. 22: line current of rectifier without harmonic filter. rectifier having the same input inductance, harmonics of orders 5, 7, I1 current spectrum 17 and 19 are eliminated, which results order IHn / I1 in a distortion rate at the source output H5 2.1 % equal to 1.9 %. H7 1.4 % The harmonic filter is in this case, H11 3.6 % practically twice as effective compared t H13 0.7 % with the use of a two-bridge circuit H17 1% arrangement. H19 0.7 % Furthermore, this is a less costly H23 0.6 % solution which can be resorted to after H25 0.5 % the equipment has been put in service. global rate of distortion Additional characteristic of of current < 5 % harmonic filter The presence of the parallel arm of the fig. 23: line current of rectifier with harmonic filter. filter tuned to 5 th harmonic causes the appearance of a capacitive current at fundamental frequency. This capacitive current improves the power factor cos ϕ of the rectifier. Cahier Technique Merlin Gerin n° 160 / p.17 4. conclusions and prospects Thyristor rectifiers of classic types possible to bring these disturbances intended to deal with a particular utilized in UPS equipments are sources down to an acceptable level. polluting load or with the whole of the of harmonic disturbances and These solutions are nowadays perfectly installation; this principle can be adversely affect the power factor of the «mastered» and widely applied. compared to the one adopted for installation. The figure 24 gives a synthesis of effecting «acoustic de-pollution» These pollutions are acceptable as long advantages and disadvantages for (i.e. emission of «sounds» in phase as the power rating of an UPS various solutions. opposition to the sounds to be equipment is low compared with the neutralised). In the not too distant future, the short-circuit power rating of the multiplication of polluting equipments, By utilizing a different regulation network. the changes in standards and the strategy, the same converters can also When the voltage distortion rate requirements of energy distributors achieve self compensation of the power exceeds acceptable values (in the should lead to the use of «clean» factor cos ϕ of the installation. order of a few %), corrective measures rectifiers (this has already been In order to make these devices, which must then be taken. achieved in single phase equipment are technically feasible, available to The simplest solution and the most thanks to the technique of sinusoidal industry, it is necessary to ensure that common consists in inserting a series sampling). their production costs are acceptable inductance which achieves harmonic Furthermore, a converter utilising the compared with those of classic decoupling. technique of PWM (pulse width solutions. When this measure is found to be modulation) can, by making use of The principles of such converters and insufficient, the use of phase staggered appropriately adapted regulation of their possibilities will be developed in rectifiers or passive filters makes it control, behave as an active filter a future Cahier Technique. Cahier Technique Merlin Gerin n° 160 / p.18 circuit type diagram observations a) no reducing interface s acceptable if power required is low compared with short-circuit power of network b) series inductance s simple, reliable s can be used in most cases s inductance can be added after equipment has been put in service s economic c) double bridge and transformer s complicated (requires balancing of voltages, with two secondaries of Icc's, of currents in rectifiers) s to be considered at design start s expensive d) double bridge with sthe solution for parallel connection of autotransformer UPS units in active redundancy s compared with circuit C s same effectiveness and drawbacks s smaller losses s more economic e) passive filter s simple, reliable s best persorming s can be inserted after equipment has been put in service s more economic than solution with two rectifiers fig. 24: comparison of anti-harmonics solutions. Cahier Technique Merlin Gerin n° 160 / p.19 5. bibliography Standard specifications Merlin Gerin Cahier Technique Other publications publications s IEC 146-4: semi-conductor s Directiveof 25 July 1985 under converters part 4: Method for specifying s Les perturbations harmoniques dans reference 83/374/CEE. performances of and test procedures les réseaux industrielles, et leur s Electra No 77 - July 1991. with UPS. traitement : Cahier Technique No 152 by P. ROCCIA and N. QUILLON. s Electra No 123 - March 1989. s IEC 552-2: Disturbances caused in supply networks by electro-domestic s Inverters and Harmonics (case equipment and similar equipment. studies of non-linear loads): Cahier Technique No 159 by J.N. FIORINA. s IEC 1000-2-2: Electromagnetic compatibility (CEM) part 2: Environment. Section 2: Levels of compatibility. Réal. : Illustration Technique Lyon - Cahier Technique Merlin Gerin n° 160 / p.20 DTE - 12/93 - 2500 - Imp. : Léostic Seyssinet-Pariset

DOCUMENT INFO

Shared By:

Categories:

Tags:
protection schemes, electrical substation, Electrical power system, ground cable, power electronic, over head, power transformer, Power System Protection, sf6 gas, power system, Transmission Lines, power plant, natural resonance, Electrical power system, power system, protection schemes, electrical substation, resonance frequency, ground cable, Power System Protection, Capacitor bank, Transmission Lines, power plant, power transformer, sf6 gas, over head, types of electric motors, ground cable

Stats:

views: | 21 |

posted: | 4/11/2011 |

language: | English |

pages: | 20 |

OTHER DOCS BY MohamedSheba

How are you planning on using Docstoc?
BUSINESS
PERSONAL

By registering with docstoc.com you agree to our
privacy policy and
terms of service, and to receive content and offer notifications.

Docstoc is the premier online destination to start and grow small businesses. It hosts the best quality and widest selection of professional documents (over 20 million) and resources including expert videos, articles and productivity tools to make every small business better.

Search or Browse for any specific document or resource you need for your business. Or explore our curated resources for Starting a Business, Growing a Business or for Professional Development.

Feel free to Contact Us with any questions you might have.