Telephone Interference From Solar PV Switching

The emergence of solar Photovoltaic (PV) generation has been one of the biggest changes in the Power Grid in the past decade. Such generation plants are generally inverter based and these devices are known sources of harmonics of the fundamental frequency and ‘supraharmonics’ (distortion in the frequency range 2 to $150 \,\mathrm {kHz}$ ). It has long been theorized that due to factors such as the frequency response of service transformers that interface solar PV plants to the grid, impedance of the power system at the point of common coupling and the impedance of devices connected near solar inverters, supraharmonics frequencies are localized and generally do not couple to the grid. Exceptions to this general hypothesis have been reported in Europe, where supraharmonics from PV and wind power plants have been shown to couple to the local three-phase three wire Medium Voltage (MV) system. This paper shows that in three-phase four wire multi-point grounded MV systems, such as the ones used in the United States (U.S.), application of conventional grounding schemes to PV plants can lead to the unintended consequence of coupling of supraharmonics to the grid through the neutral conductor and ground circuit. It describes a case study in which supraharmonics due to inverter switching led to telephone interference for customers located around a solar PV plant. To determine the mechanism by which the supraharmonics frequencies were coupling to the grid, an investigation of the emission from the plant was performed using conducted as well as radiated measurements. The novel setup used for performing these radiated measurements, the unique underlying mechanism by which inverter switching frequencies coupled to the grid and the lessons learned in this process regarding solar PV grounding and installation practices are described in this paper.