How to Reliably Commission a Sub-Harmonic Protection IED?

Sub-synchronous Interactions (SSI) are a family of physical interactions which involve exchange of energy between a generator and a transmission system at AC frequencies below the system nominal frequency. A sub-harmonic protection relay protects against sub-harmonic oscillations by measuring the voltages and currents magnitudes of sub-harmonics with frequencies in the range of 5-45Hz for 50Hz systems or 5-55Hz for 60Hz systems. 

The advent of such protection devices has created a new challenge of testing and commissioning these devices. Commissioning sub-harmonic protection devices require a process that differs from the traditional testing and commissioning process of microprocessor devices. This paper presents the experiences of the authors during the commissioning of Sub-harmonic Protection Relays and describes the process using an advanced relay test system and the recommended process.