Previous studies have demonstrated that disharmony among voltage-source-controlled units (VSCUs) may occur on an alternating current (AC) transmission or distribution line under steady-state operating conditions (SSOCs) or quasi-static operating conditions (QSSOCs). As the studies on frequency disharmony have been expanded to multiple disharmonized VSCUs in the local power grid, its adverse effects on AC lines and equivalent load (EL) at the bus without active voltage control ability (non-active bus) need to be investigated further. Considering the locality of disharmony and common topological connections among VSCUs, this paper adopts a Y-type three-terminal local power grid (LPG) as the research object. The disharmony among the three VSCUs is discussed. Firstly, for the load at non-active bus, the formulas for single-phase instantaneous voltage, load current, load power, as well as average power under disharmony operating conditions (DOCs) are derived. The characteristic indicators of the above electrical quantities are defined, which can measure the amplification and reduction degrees of the above electrical quantities before and after disharmony. Secondly, for the line directly connected to VSCUs, the formulas for single-phase instantaneous line current and power and the average power under DOCs are derived. The characteristic indicators of power flow are defined, which can be used to quantify the peak amplification impact of oscillation before and after disharmony. Finally, the case study on the Y-type three-terminal LPG under the single-disharmony and the multi-disharmony switching scenarios indicates that the long-period power oscillation caused by disharmony may occur in the load flow at the non-active bus and the line flow. The oscillation causes a serious decrease in load capability and a significant amplification of the peak of line power oscillation.