Research On Method Of Harmonic Controland Reactive Power Control In Microgrid Of Large-scale Enterprises

Microgrid is a special grid which can provides power and heat to nearby load by optimalconfigurations of local distributed micro sources or traditional ways of small and medium-sizedpower generation. Appearing as a whole unit, microgird can be incorporated into grid smoothly.Since the capacity of microgrid is small, and plenty of power electronics are used for network,its operation is easily affected by harmonics. Harmonic resonance, which is divided into seriesand parallel resonance, is a common problem in power system. Series resonance is often causedby faulty or abnormal operation and posing overcurrent damages. Parallel resonance excited bycurrent source will lead to overvoltage damages and great harm to capacitors, which will causeworse results. When microgrid contains nonlinear devices such as converting devices or electricarc furnace, the load will generate lots of harmonic current that can be injected into power gridin grid-connecting condition, and will cause voltage distortion and power quality decline. Thiswill also affect the safety and economic operation of the distributed power generationequipment.In the microgrid system that contains large power rectifying devices, continuous and stableproduction is required to satisfy high quality requirements for power supply. But due to the useof rectifying devices, the harmonic situations have become complicated. Especially for a fewlarge electrical equipments, such as transformers and compensating capacitors, the switch willchange the harmonic resonance situation in the electric system.In this paper, harmonic impedance method is adopted，which is a harmonic analysis suitable forenterprise microgrid. The principle is explained and the method is used to analyze harmonicresonance in the electrical system of Qinghai Rui He Aluminium Corporation. Combined withactual measurement data, harmonic impedances in various operation conditions are calculated,for example, system in grid-connected and islanding condition, switching of different filter branches.etc. According to the frequency scan of the system impedance by PSCAD/EMTDC,the ZvsF (impedance vs frequency) curve can be got, the curve will directly show whether theparallel resonance occurs in the system. The reasonable design of install base of reactivecompensation and parameters of passive filter branches has been made to help the system avoidharmonic resonances in all operational conditions. The technical solutions proposed in thispaper have been proved to be effective with harmonic impedance method. The simulationresults show that the design basing on the changes of the system impedance can improve theoperational reliability.