Use of a Sub Harmonic Protection Relay to Detect SSCI Conditions Associated with Type-III Windfarms and Series Compensated Transmission Systems

With the increased use of renewable wind energy, wind farms are being built in remote areas, often far from load centers. 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 natural frequencies of the series capacitors and the system's inherent impedance, which can result in SSR/SSCI conditions. Modern wind turbine systems use advanced electronic control and converter systems, which can generate harmonics and inter harmonics. Mechanical system interactions (tower-to-blade) can also generate sub-harmonics. Wind generator speeds vary continuously depending on the availability of wind at any particular time. Therefore wind farms now face a challenge to predict when (and to what extent) harmonics and sub-harmonics are being introduced to the power system.

Sub harmonic protection relays are now available in the market, capable of detection such sub-harmonic conditions and provide adequate protection.