Combined Transmission and Distribution Systems (CoTDS) simulation requires development of

algorithms and software that are numerically stable and at the same time accurately simulate

dynamic events that can occur in practical systems. The dynamic behavior of transmission

and distribution systems are vastly different. With the increased deployment of distribution

generation, especially power electronic inverters, the complexity is further increased. The

time scales of simulation can be orders of magnitude apart making the combined simulation

extremely challenging. This has led to increased research in applying coupled simulation (also

referred to as co-simulation) techniques for integrated simulation of the two systems.

In this thesis, two methods for co-simulation of CoTDS are proposed using parallel and series

computation with integration impact on numerical convergence. The proposed co-simulation

methodology is validated against commercial EMTP software. The results show the limits

and benets of applying co-simulation by using test transmission and distribution systems. A

detailed phasor domain Distribution Generation (DG) inverter model is developed for power

system dynamic simulation using which the effectiveness of the proposed co-simulation methodology

is demonstrated in dynamic studies.

The co-simulation method is then applied to model reduction where the CoTDS based

dynamic load modeling with distributed load serves as a guiding tool to calculate some of the

key aggregated WECC Composite Load Model (CLM) parameters. As a further addition, a

Reduced Distribution System Model (RDSM) is proposed with a new single-phase A/C motor

model for the WECC CLM with fractional stalling and recovery. Such a model can be used for

developing measurement based control schemes that can mitigate events such as fault-induced

delayed voltage recovery in distribution systems.

With the addition of DG in the WECC CLM, the co-simulation is applied for studying

the effect of high DG penetration on bulk transmission system dynamics in reference to the

recommendations of the IEEE 1547 standard for interconnecting distributed energy resources

with electric power systems.