In recent years the grid modernization and rapid growth in distributed energy resourcesdue to environmental consciousness have resulted in distribution grids becoming more active which has led to significant interaction between transmission and distribution grids. In this dissertation, novel approaches in modeling and management tools are proposed considering integrated power transmission and distribution systems with Distributed Energy Resources (DERs). First, new power methods for power distribution system considering DERs is proposed in a single-phase, three-phase, and three sequence domain. Second, an integrated transmission and distribution (T&D) grid model where transmission and distribution systems are considered as a single unit is proposed. A coalescing Ybus approach is used to obtain the bus admittance matrix of the combined T&D system. Further, to successfully capture the effect of unbalances in the system at the same time reducing computational burden owing to the larger size, a three-sequence modeling framework is used for a unified system. A three-sequence-based multi-period power flow method is used to accurately capture the time-varying aspects of the system. Next, a three-sequence fault analysis method capable of conducting short circuit analysis on a DER integrated unbalanced distribution system is developed. All these sequence-based methods are then used for steady-state analysis of the integrated T&D system. Finally, a sensitivity-based coordinated voltage control scheme using reactive power support from DERs is proposed which can lead to reduced voltage regulator operations and tighter voltage profiles. The proposed methods have been validated using large-scale IEEE T&D feeders to prove the real-life implementation capabilities of the models and tools.