Forced Oscillation In Indian Grid -Case Study

Abstract

Historically Low frequency oscillations (LFO) had always been a menace in operating an interconnected grid. The challenge of mitigating LFO has only enhanced umpteen times with the grid becoming voluminous and complex. Hitherto, LFO could only be felt through hunting of rotating masses or fluctuation of voltage till recently, wide area monitoring mechanism through synchro phasors, decoded the phenomenon in real time. Any LFO oscillation especially of inter area type has its global footprint on hundreds of PMUs spread over thousands of kilometer. Signature generated thereby indicates the epicenter of oscillation or the constituent members of coherent resonance. Inherent modes, damping and severity can be elucidated through advanced applications. Root causes of LFOs are primarily attributed to switching or tripping in vulnerable weak interconnections in the grid. Such events leave footprints and these footprints recorded by PMUs are further analyzed. Based on the type of oscillation - whether inter area, local mode or intra plant mode, remedial actions such as tuning of damping controllers are taken. Sometimes the root cause of oscillations cannot be pinpointed due to all pervading & global nature of the phenomenon where footprints are left on PMUs without identified feet. Forced oscillations, however, are the response of system to an apparatus in a limit cycle–e.g. generator controller. As if a device is forcefully exciting the whole system at a particular frequency or a spectrum of frequencies. If the frequency of forced oscillation is very near to the inter area frequency mode or pre identified oscillation mode present in the grid, it can cause resonance; impact of which would be wide spread. The only way to damp these oscillations is to remove the source. This paper explains the difference of forced oscillation with natural low frequency oscillation and showcases some case studies of forced oscillation observed recently in Indian grid. It also explains how synchrophasors are used by real time grid operators for analysis and identification of these forced oscillations and mitigation in real time.