Harmonics also can travel up to the feeder power system supplying the drives. Inverters did not allow input current to flow as a regular sine wave. Instead, current to the rectifier came in bursts, and harmonics moved around the entire connected power-distribution system. These harmonics were confined by isolating the power system for mechanical loads from the rest of the facility. Placing inductance in the input circuit to a VFD also can control harmonic proliferation on power systems and is used routinely.
Isolation transformers are an even better approach, but cost more and require more space.
Another problem was displacement power factor. Capacitors installed to correct for low, lagging power factor on motor drives had a good operating and application history, both in preserving system capacity and in satisfying electric-utility requirements. But capacitors, shunt-connected to correct motor loads, could not work in the rich harmonic environments around VFDs. Together with a steadily rising VFD load, concerns over harmonic levels spurred the development of standards and recommendations for control of these phenomena, notably Standard 519 from the Institute of Electrical and Electronics Engineers. Ever since, the industry has focused on measuring, developing, and meeting a structure of limiting values for harmonics on power distribution systems.