1、升压升压变压器变压器输电线输电线降压降压变压器变压器知识点回顾知识点回顾有功功率、无功功率、视在功率有功功率、无功功率、视在功率能能 量量能能 量量能能 量量能能 量量能能 量量能能 量量能能 量量S=VI cosVIP sinVIQ SP视在功率有功功率 cos 1cos1cos cosVIVI 010cos1、unity power factor 整功率因数For purely resistive circuit 2、lagging power factor 滞后功率因数Load current lags voltage3、leading power factor 超前功率因数load c
2、urrent leads voltage例：例：已知某发电机的视在功率为已知某发电机的视在功率为1000KVA。（1）若）若1cos（2）若若7.0cos能发出能发出()KW的有功功率的有功功率只能发出只能发出()KW的有功功率的有功功率cos1cos0.6cos800kvarsinNN IVQ600kWcosNN IVP1000kWcosNN IVPAkV1000NNN IVS cosVIP )V/(PI cos(费电费电)设输电线和发电机绕组的电阻为设输电线和发电机绕组的电阻为 :r要求要求:(、V定值定值)时时 cosVIP cosVPI rIP2cos 日常生活中多为日常生活中多为感性
3、负载，感性负载，如电动机、日光灯。如电动机、日光灯。0.85cos0)()9090()90(1cos0.30.2cos0cos0co1s0.90.7cos0.60.5cosCIC VRLI1ICIC VRLI1Icoscos(1)电路的总电流电路的总电流 ，电路总功率因数，电路总功率因数Icos电路总视在功率电路总视在功率S1cos 不变不变感性支路的感性支路的功率因数功率因数不变不变感性支路的电流感性支路的电流I1,1II V2 1CI211sinsin IIIC CVIC 11sin I2sin I即即:2IICV sinsin11 1 并并C前前2 并并C后后CICIC VRLI1I22
4、VPVPCV sincossincos11 1II V2 1CI11sin I)tan(tan212 VPC2sin ICIExample4-3 For the overloaded generator problem of figure,a capacitance with kVAR160QC is added in parallel with the load as in figure.Determine generator current I.0QT kVA120ST A200V600kVA120I SolutionTherefore,Current drops from 333A to
5、 Example4-4 An industrial client is charged a penalty if the plant power factor drops below 0.85.The equivalent plant loads are as shown in figure.The frequency is 60Hz.a.Determine PT and QT.b.Determine what value of capacitance(in microfarads)is required to bring the power factor up to 0.85.c.Deter
6、mine generator current before and after correction.kW12P kVAR0Q kW54RIP2 kVAR72XIQ2 9.368.0arccosm kVARPQmmm609.36tan80tan kW146kW80kW54kW12PT kVAR132kVAR60kVAR720QT Solutiona.lights:Furnace:Motor:Total:(ind.)(ind.)(ind.)FVPC 305)tan(tan212 8.3185.0arccos2 1.42146/132arctan1 b.kVA8.196ST A328ESIT kV
7、A8.171ST A286V600kVA8.171I c.for the original circuitFor the corrected circuit）（1 A small electrical utility supplies a factory.The factory loads are shown in figure.The frequency is 50Hz.a.Determine PT,QT.and power factor b.To correct the circuit power factor to unity ,determine what value of capacitance is required.作作 业业
