Impact of wind turbine generators on network resonance and harmonic distortion

Distributed generators (DGs) are being increasingly connected at medium voltage level distribution networks. It is likely that different types of DGs, i.e. wind turbine generators (WTGs), photovoltaic solar systems, etc, may be connected on the same feeder. Most of the modern DGs are equipped with power electronic converters at their terminals, which act as sources of harmonic injection into the network. Any distribution feeder system will have its own network resonant frequencies. In this paper, the impact of WTG connections on resonant modes of system impedance and total harmonic distortion in the network are discussed in detail. First, a representative study system is devised where the impact of different elements of power system on system resonance frequencies is analyzed. Then the influence of system background harmonics on harmonic distortion is shown for the study system as well as for one of the actual Hydro One distribution feeder systems. It is shown that several scenarios are possible when these resonant frequencies align with harmonic frequencies that are likely to be injected by other power electronic based system equipment including DGs, thus causing unacceptable total harmonic distortion. The understanding will be useful to utilities for avoiding network resonance based harmonic amplification in distribution systems where wind farms and/or inverter based generation systems are integrated.