Download - Schneider Electric

More documents

Recommendations

Info

8 Example of the analysis of a simplified networkThe diagram in figure 26 represents a simplifiednetwork comprising a 2,000 kVA six-pulse rectifier,injecting a harmonic current spectrum, and thefollowing equipment which will be consideredconsecutively in three different calculations:c a single 1,000 kvar capacitor bank;c anti-harmonic reactor-connected capacitorequipment rated 1,000 kvar;c a set of two filters comprising a resonant shunttuned to the 5 th harmonic and a 2 nd orderdamped filter tuned to the 7 th harmonic.Note that:c the 1,000 kvar compensation power is requiredto bring the power factor to a conventional value;c the harmonic voltages already present on the20 kV distribution network have been neglectedfor the sake of simplicity.This example will be used to compare theperformance of the three solutions, however theresults can obviously not be applied directly toother cases.Network20 kVIsc 12.5 kA2,000 kVADisturbing equipment20/5.5 kV5,000 kVAUsc 7.5 %Pcu 40 kW5,5/0.4 kV1,000 kVAUsc 5 %Pcu 12 kWCapa.Load500 kVA at cosϕ = 0.9Motor560 kWReactor+capa.Resonant shuntand2 nd order damped filterFig. 26 : installation with disturbing equipment, capacitors and filters.8.1 Capacitor bank aloneThe network harmonic impedance curve (seefig. 27 ), seen from the node where the harmoniccurrents are injected, exhibits a maximum (antiresonance)in the vicinity of the 7 th currentharmonic. This results in an unacceptableindividual harmonic voltage distortion of 11% forthe 7 th harmonic (see fig. 28 ).The following characteristics are also unacceptable:c a total harmonic voltage distortion of 12.8% forthe 5.5 kV network, compared to the maximumpermissible value of 5% (without considering therequirements of special equipment);c a total capacitor load of 1.34 times the rmscurrent rating, exceeding the permissiblemaximum of 1.3 (see fig. 29 ).The solution with capacitors alone is thereforeunacceptable.Z (Ω)7.7538.2Fig. 27 : harmonic impedance seen from the nodewhere the harmonic currents are injected in a networkequipped with a capacitor bank alone.HCahier Technique Schneider Electric no. 152 / p.22
V (V & %)I (A)35011 %823 5 7 9 11 13 HFig. 28 : harmonic voltage spectrum of a5.5 kV network equipped with a capacitorbank alone.3 5 7 9 11 13 HFig. 29 : spectrum of the harmonic currents flowing inthe capacitors for a network equipped with a capacitorbank alone.8.2 Reactor-connected capacitor bankThis equipment is arbitrarily tuned to 4.8 f 1 .Harmonic impedance (see fig. 30 )The network harmonic impedance curve, seenfrom the node where the harmonic currents areinjected, exhibits a maximum of 16 ohms (antiresonance)in the vicinity of harmonic order 4.25.The low impedance, of an inductive character, ofthe 5 th harmonic favours the filtering of the 5 thharmonic quantities.Voltage distorsion (see fig. 31 )For the 5.5 kV network, the individual harmonicvoltage ratios of 1.58% (7 th harmonic), 1.5%(11 th harmonic) and 1.4% (13 th harmonic) maybe too high for certain sensitive loads. Howeverin many cases the total harmonic voltagedistortion of 2.63% is acceptable.For the 20 kV network, the total harmonicdistortion is only 0.35%, an acceptable value forthe distribution utility.Z (Ω)Capacitor current load (cf. fig. 32 )The total rms current load of the capacitors,including the harmonic currents, is 1.06 times thecurrent rating, i.e. less than the maximum of 1.3.This is the major advantage of rea ctorconnectedcapacitors compared to the firstsolution (capacitors alone).V (V & %)190.6 %501.58 %481.5 %451.4 %3 5 7 9 11 13 HFig. 31 : harmonic voltage spectrum of a 5.5 kVnetwork equipped with reactor-connected capacitors.15.6I (A & %)3424 %~ 4.254.8H35 7 9 11 13 HFig. 30 : harmonic impedance seen from the nodewhere the harmonic currents are injected in a networkequipped with reator-connected capacitors.Fig. 32 : spectrum of the harmonic currents flowing inthe capacitors for a network equipped with reactorconnectedcapacitors.Cahier Technique Schneider Electric no. 152 / p.23
Page 1 and 2: Collection Technique ..............
Page 3 and 4: no. 152Harmonic disturbances innetw
Page 5 and 6: Harmonic disturbances in networks,a
Page 7 and 8: Expression of the distorted waveAny
Page 9 and 10: Heating due to additional losses in
Page 11 and 12: 4 Harmonics generatorsIn industrial
Page 13 and 14: 4.3 Arc furnacesArc furnaces used i
Page 15 and 16: 5 Networks comprising disturbing eq
Page 17 and 18: If a harmonic current I n of order
Page 19 and 20: 6 Anti-harmonic reactorsAn anti-har
Page 21 and 22: (note that the subscript 1 refers t
Page 23: Figure 24 compares the impedance of
Page 27 and 28: 9 ConclusionStatic power converters
Page 29: c Filtrage d'harmoniques et compens

Download - Schneider Electric

Create successful ePaper yourself

Delete template?

Save as template?