Application of a subharmonic protection relay

With the increased use of renewable wind energy, wind farms are being built in remote areas, often far from load centers. The required long transmission lines are often constructed with series capacitive compensation to electrically shorten the lines. Events in the transmission system can cause interactions between the series capacitors and the system's inherent impedance which can result in sub-synchronous resonance frequencies. Modern wind turbine systems use advanced electronic control and converter systems which can generate harmonics and inter harmonics. Mechanical system interactions (tower-toblade) can also generate sub-harmonics. Wind generator speeds vary continuously depending on the availability of wind at any particular time. This particular operating characteristic of wind farms introduces the challenge of predicting when and how much harmonics and sub-harmonics are being introduced to the power system. Synchronous generators are often built with shaft torsional modes that are in the same range as system sub-synchronous resonance frequencies. These torsional modes can interact with transmission network resonances to produce damaging oscillations that can damage generators and transformers, and cause damage at points of common coupling in the electrical grid. This paper discusses an engineering process that can be followed for the application of a new sub-synchronous relay that provides protection for transmission lines, particularly those with serial compensation and wind farm interconnections, and how it can be used together with digital fault recorders to monitor the power system to determine the existence of sub-synchronous resonance phenomenon. Moreover, it gives the reader criteria to determine if the levels of SSR obtained by means of DFRs are considered critical. The paper goes further, providing possible settings for the sub-harmonic protection relay including the protection elements associated to sub-harmonics as well as providing overcurrent and other protection functions.