So, your question seems to be based on one assumption; DGs are either connected to the sub-transmission system or DGs connected at distribution system would have some impact on the sub-transmission/ transmission system. Correct?
when you said reliability and stability, do you mean the reliability and stability of transmission system or distribution system? each system is different. If you are referring to the distribution system, stability issue may not be an issue. Stability is related to the response of the synchronous generator. So, if your distribution system does not have a synchronous generator, you won't have a stability issue, though you may have reliability issue.
it it suggest to focus on demand response of caused by the impact of DGs which will lay on reliability issues. it must be taken into account the temporal and spatial characteristics of the power grid as well of its generation and load dynamics. The problem is the frequency regulation design lends itself well to supporting, somewhat overview is required because current implementations draws specific structures and assumptions common to model aggregation in typical large-scale distribution systems. Anyway assumption should be clear in order to satisfy the proposed models and ensuring acceptable frequency response depend on the type of disturbances, electrical characteristic of interconnected systems then these assumption would lead to required power supply and demand balancing.
Follow this it might help !
Control and optimization methods for electric smart Grids
Author: Aranya Chakrabortty Marija D.Llc
I want to get some information (e.g thesis) about the impact of distributed generators in the sub transmotion - ResearchGate. Available from: https://www.researchgate.net/post/I_want_to_get_some_information_eg_thesis_about_the_impact_of_distributed_generators_in_the_sub_transmotion2 [accessed Jul 2, 2016].