Could you please clarify what does a "TSO DSO Problem" imply? For instance, are you studying a small signal stability problem between the TSO and DSO?

Multi-area systems are used in the context of small-signal stability or voltage stability (voltage-control-areas) problems. In a small-signal stability problem, the synchronous machines in one area exhibit low frequency oscillations with respect to the other area. In Voltage stability problem areas are decoupled because of large electrical distances and the inability of reactive power to travel large electrical distances.

Srivats Shukla Thanks for your kind response. Well TSO-DSO means transmission system (TS) and distribution system (DS) coordination problem. Nowadays DSO need to coordinate with TSO for flexibility, area balancing, voltage stability, security etc. So my question is that normally transmission system and distribution system are taken as single area. Can they be considered as multi area problem as nowadays researchers are solving TS and DS problems by finding optimized coordination state between them.

Thanks for clarification Aamir. Your problem statement is clear, but there is still a point of ambiguity while taking the multi-area approach. The traditional multi-area systems involve weak coherency (loose coupling) due to large electrical distances. In contrast, the transmission and distribution systems are strongly coupled via substation transformers. Thus, due to a rather strong coherency, the transmission and distribution systems are taken as a single area.

I also agree with Srivats Shukla since electric power system (Transmission and Distribution) is the one entity. Due to economist and regulatory aspect they divide it in TSO, DOS, Production, Consumption...

New view in ETNSO is that together TSO and DSO solve problems of high penetration of RES on both levels!